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Modern FracturingEnhancing Natural Gas Production

Michael J. Economides

University of Houston

Tony Martin BJ Services

ET Publishing

Houston,TX



© BJ Services Company 2007

BJ Services Company PO Box 4442 [772104442]4601 Westway Park BlvdHouston, X 77041

Graphic design and production: Jay Clark Production manager: Alexander M EconomidesCopy Editor: Stephanie WeissCover Art: Armando Izquierdo

Published by: Energy ribune Publishing Inc820 Gessner RdSte 920Houston, X 77040

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All rights reserved No part o the publication may be reproduced, stored in a retrievalsystem, or transmitted, in any orm or by any means, except under the expressedpermission o BJ Services Company, Designs and Patents Act 1988

ISBN 978 1 60461 688 0

Printed and bound by Gul Publishing Co



Contents

Preace XV

Foreword XV

Contributing Authors XV

Acknowledgements XX

Chapter 1ntroduction to this Book

1-1 ntroduction 3

1-2 Natural Gas in the World Economy 31-3 Russia: A Critical Evaluation o its Natural Gas Resources 5

1-3.1 The Resource Base 7

1-3.2 Russian Natural Gas Production 8

1-4 Alaska, its Natural Gas Resources and their mpact on US mports 8

1-4.1 Alaskan Reserves and Production 9

1-4.2 The Uncertain Destiny o the North Slope o Alaska Natural Gas 10

1-4.3 Alaska in the Context o the United States and Canadian Natural Gas 11

1-5 Qatar Natural Gas 12

1-5.1 North Field Characteristics and Development 131-6 Fracturing or the Ecient use o Existing

Resources and or ncreasing Recovery Factor 13

Chapter 2Natural Gas Production

2-1 ntroduction 19

2-2 diosyncrasies o Dry Gas, Wet Gas and Gas Condensates 19

2-3 nfow rom Natural Gas Reservoirs 20

2-3.1 Fundamentals o Non-Darcy Flow in Porous Media 20

2-3.2 Transient Flow 20

2-3.3 Steady State and Pseudosteady State Flow 21

2-3.4 Horizontal Well Flow 22

2-4 Eects o Turbulence 23



2-4.1 The Eects o Turbulence on Radial Flow 23

2-4.2 Perorated and Cased Well in a High-Rate Gas Reservoir 24

2-5 Production rom Hydraulically Fractured Gas Wells 25

2-5.1 Unique Needs o Fracture Geometry and Conductivity 26

2-5.2 Turbulence Remediation in High- and Low-Permeability Wells 26

2-5.3 Multi-ractured Horizontal Gas Wells 28

2-6 Well Deliverability, PR and Well Flow Perormance 33

2-7 Forecast o Well Perormance 34

2-7.1 Gas Material Balance and Forecast o Gas Well Perormance 34

2-8 Correlations or Natural Gas Properties 35

2-8.1 Pseudocritical Pressure, p pc

and Pseudocritical Temperature, T pc 35

2-8.2 Gas Viscosity 35

2-8.3 Gas Deviation Factor, Z 35

Chapter 3Gas Well Testing and Evaluation

3-1 ntroduction 41

3-2 Background Theory 42

3-3 Radial Flow Solutions 44

3-4 Superposition 45

3-5 Model Development 46

3-6 Hydraulically Fractured Wells 473-7 Specialized Plots 48

3-8 Type Curves and the Log-Log Derivative Plot 49

3-9 Flow Regime dentication 51

3-10 Derivatives – A Few Cautionary Remarks 54

3-11 PTA nterpretation Methods 56

3-12 Dierence Between High and Low Permeability Analysis Techniques 57

3-12.1 High-Permeability Wells 57

3-12.2 Low-Permeability Wells — Pre-Treatment Evaluation 59

3-12.3 Example 3-1, PD Test 60

3-12.4 Low-Permeability Wells — Post-Treatment Evaluation 61

3-12.5 Example 3-2, Low-Permeability Well, nnite-Conductivity Fracture 62

3-12.6 Example 3-3, Low-Permeability Well, Finite-Conductivity Fracture 65

3-13 Non-Darcy Flow 66

3-13.1 Example 3-4, Non-Darcy, High-Permeability Well, Finite-Conductivity Fracture 68

3-13.2 Example 3-5, Non-Darcy, Low-Permeability Well, Finite-Conductivity Fracture 69

3-14 Production Analysis 70



3-15 Heterogeneity 76

3-15.1 Dual Porosity 76

3-15.2 Anisotropy 76

3-16 Multiphase Flow 77

3-16.1 Gas Condensates 78

3-16.2 Fracture Fluid Cleanup 79

3-16.3 Example 3-6, Fracture Fluid Cleanup Case 79

3-17 Closure Analysis 81

3-18 Deconvolution 86

Chapter 4Hydraulic Fracture Design or Production Enhancement

4-1 ntroduction to Hydraulic Fracturing 934-1.1 Brie History o Fracturing and Qualitative Description o Process 93

4-1.2 High Permeability vs. Low Permeability 94

4-1.3 Near-Well Flow Enhancement vs. Reservoir Stimulation 94

4-1.4 Acceleration vs. ncrease o Reserves 95

4-2 Description o the Process 95

4-2.1 One o the Most Energy- and Material-ntensive ndustrial Activities 95

4-2.1.1UnderstandingtheSignicanceoPressure 96

4-2.1.2DierentTypesoPressure 96

4-2.1.3NetPressure 97

4-2.1.4EectsoTortuosityandPerorationFriction 98

4-2.1.5FluidLeakoandSlurryEciency 101

4-2.1.6DimensionlessFractureConductivity 102

4-2.1.7Nolte-SmithAnalysis–PredictingFractureGeometryromPressureTrends 103

4-2.1.8StepRateTests 104

4-2.1.9Miniracs 106

4-2.2 The Role o Advanced Technology in Design, Execution and Evaluation 109

4-2.2.1RecentAdvancesandBreakthroughs 109

4-2.2.2PressureMatching 112

4-2.2.3GettingClosertoUnderstandingFractureGeometry 115

4-2.2.4Real-TimeAnalysis 115

4-2.3 From Fracturing a Single Vertical Well to Complex Well-Fracture Architecture 116

4-3 Rock Mechanical Characteristics 116

4-3.1 Basic Denitions 116

4-3.1.1StressandStrain 116

4-3.1.2ThePoisson’sRatio 116



V

4-3.1.3Young’sModulus 117

4-3.1.4OtherRockMechanicalCharacteristics 118

4-3.1.5Hooke’sLaw 119

4-3.1.6FailureCriteriaandYielding 119

4-3.2 n-Situ Stress and Fracture Orientation 121

4-3.2.1OverburdenStress 121

4-3.2.2HorizontalStresses 121

4-3.2.3TheEectoPorePressure 122

4-3.2.4FractureOrientation 122

4-3.2.5StressAroundaWellboreandBreakdownPressure 123

4-3.3 Fracture Shape 125

4-3.3.1Two-Dimensional(2-D)FractureGeometry 125

4-3.3.2EllipticalFractureGeometry 125

4-3.3.3LimitationstoFractureHeightGrowth 126

4-3.3.4ComplexFractureGeometry 127

4-3.4 Fracture Propagation, Toughness and Tip Eects 127

4-3.4.1LinearElasticFractureMechanics 127

4-3.4.2SignicanceoFractureToughness 129

4-3.4.3ComplexityattheFractureTip 130

4-3.5 Measuring Rock Mechanical Characteristics 132

4-3.5.1Introduction 132

4-3.5.2MethodsoMeasurement 132

4-3.5.3CoreSelection/SamplePreparationConsiderations 134

4-3.5.4DeducingElasticPropertieswithoutCore 135

4-4 Fluid Rheological Characteristics 137

4-4.1 Viscosity 137

4-4.1.1ShearRate,ShearStressandViscosity 137

4-4.1.2MeasurementoViscosity 137

4-4.2 Fluid Behavior 138

4-4.2.1NewtonianFluids 138

4-4.2.2Non-NewtonianFluids 138

4-4.2.3ApparentViscosity 139

4-4.3 Flow Regimes 140

4-4.3.1Plug,LaminarandTurbulentFlow 140

4-4.3.2Reynold’sNumber 140

4-4.4 Fluid Friction 141

4-4.4.1TheInfuenceoFlowRegime 141

4-4.4.2PredictingPressureLossduetoFriction 141

4-5 Optimum Treatment Design 141

4-5.1 Dimensionless Productivity ndex and Dimensionless Fracture Conductivity 143



V

4-5.2 Optimum Dimensionless Conductivity 144

4-5.3 Optimum Length and Width 144

4-5.4 Treatment Sizing and Proppant Placement Eciency 145

4-5.5 Taking nto Account Operational Constraints 145

4-5.6 Using Fracture Propagation Models 146

4-5.6.1Heightcontainment 146

4-5.6.22-Dmodels 147

4-5.6.33-Dmodels 149

4-6 Predicting Production ncrease 150

4-6.1 Pseudo-radial Concepts: Equivalent Wellbore Radius, Fracture Skin 150

4-6.2 Finite Reservoir Concepts, Folds o ncrease 150

4-6.3 Combining Productivity ndex and Material Balance 151

4-6.3.1Pseudo-steadystate 151

4-6.3.2Combinedtransientandstabilizedfow 151

4-6.4 Reservoir Simulation and Nodal Analysis 152

4-7 Fracturing Under Specic Circumstances 153

4-7.1 Tight Gas 153

4-7.1.1TheImportanceoInfowArea 154

4-7.1.2EectivevsActualProppedLength 154

4-7.2 High-Rate Gas Wells 155

4-7.2.1Non-DarcyFlow 155

4-7.2.2WellboreConnectivity 155

4-7.3 High-Permeability Wells 155

4-7.3.1TheImportanceoFractureConductivity 156

4-7.3.2TheTipScreenout 156

4-7.4 Unconsolidated Formations 156

4-7.4.1Re-StressingtheFormation 156

4-7.4.2TheFrac-PackTreatment 157

4-7.5 Skin-Bypass Treatments 157

4-7.6 Condensate Dropout 158

4-7.6.1DescriptionoPhenomena 158

4-7.6.2MitigatingtheEectoDropout 158

4-7.7 Shale Gas and Coal Bed Methane 158

4-7.7.1GasShales 158

4-7.7.2CoalBedMethane 158

4-7.8 Acid Fracturing 159

4-7.8.1DescriptionoProcess 159

4-7.8.2EstimatingFractureConductivity 159

4-7.8.3UseoDiversionTechniques 160



V

Chapter 5Well Completions

5-1 Wellbore Construction 169

5-1.1 Eects o Uncertainty in Reservoir Description 1695-1.2 Fitting Well Design to the Reservoir Potential 169

5-1.3 Well Design 170

5-1.4 Other Well Equipment 171

5-1.5 Well ntegrity 171

5-2 Gas Well Cementing 172

5-2.1 General Objectives or Gas Well Cementing Operations 172

5-2.2 Gas Well Zonal solation 173

5-2.3 Review o Fundamental Cement Placement Practices 174

5-2.4 Predictive Wellbore Stress Modeling 1745-2.5 Cement Slurry Criteria or Hydraulically Fractured Gas Wells 176

5-2.5.1SlurryCriteriaorOptimizedPlacement 176

5-2.5.2SlurryCriteriaorAnti-GasMigration 177

5-2.5.3SlurryCriteriaorLong-TermZonalIsolation 178

5-2.6 Fracturing Constraints Required to Maintain Long-Term Zonal solation 179

5-3 dentiying Gas Pays, Permeability and Channels 179

5-3.1 Pay and Water Zone Logging Methods 179

5-3.2 Eect o Formation Clays and Micro-porosity 180

5-3.3 Wellbore Deviation and Resultant Logging and Flow Problems 1815-3.4 Completion Considerations or Naturally Fractured Reservoirs 181

5-3.5 Formation Characterization or Well Completions 182

5-4 Sizing the Completion 183

5-4.1 nitial Design Considerations 183

5-4.2 Flow Factors or Tubing Design 184

5-4.3 Tubing Selection 185

5-4.4 Multi-Phase Flow and Natural Lit 185

5-4.5 Multiphase Flow and Flow Correlation Options 186

5-4.6 Critical Lit Factors 1875-4.7 Liquid Hold-up and Back Pressure 188

5-4.8 Lit Options or Gas Wells 188

5-5 Completion Design or Flow Assurance 188

5-5.1 Completion Design or the Prevention o Gas Hydrates 188

5-5.2 Formation Damage in Gas Wells, Completion Damage and Scales 190

5-5.3 Organic Deposits and Condensate Banking 190

5-5.4 Eects o H2S and CO

2on Corrosion 191



V

5-6 Sand Control or Gas Wells 192

5-6.1 Why is the Sand Flowing? 192

5-6.2 s Sand Flow All Bad? 192

5-6.3 Establishing and Monitoringa Sand-Free Rate 193

5-6.4 Sand Control Methods or Gas Wells 194

5-6.5 Reliability o Sand Control Completions 194

5-6.6 Repairing and Restoring Productivity in Wells hat Flow Sand 194

Chapter 6Fracture-to-Well Connectivity

6-1 ntroduction 201

6-2 Completion Techniques and Their mpact on Well Connectivity 202

6-2.1 Cased-Well solation Techniques 202

6-2.2 Open-Hole Completions 205

6-2.3 Open-Hole and Uncemented Liner Fracture Treatment Diversion 205

6-3 Perorating in General 206

6-4 Perorating or Fracturing 206

6-4.1 Oriented Perorations 206

6-4.2 Deviated and Horizontal Well Perorating 208

6-4.2.1ProductionImpairmentromInecientFracture-to-WellboreContact 209

6-4.3 Underbalanced vs. Extreme Overbalanced Perorating 211

6-5 Near-Wellbore Fracture Complexity 2136-5.1 Near-Wellbore Complexity 214

6-5.2 Diagnosing and Quantiying Near-Wellbore Complexity (Tortuosity) 215

6-5.3 Minimizing the Eects o Tortuosity 217

6-6 Mid- and Far-Field Fracture Complexity 218

6-6.1 An ntroduction to Complex Fracture Growth 219

6-6.2 Evidence o Complex Fracture Growth 220

6-6.3 Consequences o Complex Fracture Growth 220

Chapter 7Fracturing Fluids and Formation Damage

7-1 ntroduction 227

7-2 Fracturing Fluid Function 228

7-2.1 Fracture nitiation 228



V

7-2.2 Proppant Transport 229

7-3 Fracturing Fluid Rheology 230

7-3.1 Pressure Loss Gradient in the Fracture 232

7-3.2 Rheology in the Presence o Proppant Material and its Relation to Settling 234

7-3.3 mpact o Fluid Rheology on Fluid Loss 235

7-3.4 Calculation o Pressure Loss in the Wellbore Using Rheological Parameters

and the Virk Maximum Drag Reduction Asymptote 235

7-3.5 Advanced Rheology 235

7-3.6 Foam Rheology 236

7-3.7 Eect o Proppant on Rheology 237

7-3.8 Laboratory Rheology Measurements 239

7-4 Types o Fracturing Fluids 242

7-4.1 Water-Based Fluids 243

7-4.1.1Low-ViscosityFluids 243

7-4.1.2CrosslinkedFluids 243

7-4.1.3BorateCrosslinkedFluids 244

7-4.1.4MetallicIonCrosslinkedFluids 244

7-4.1.5Delayed-CrosslinkSystems 245

7-4.1.6FunctionoBreakersinWater-BasedFluids 246

7-4.1.7Water-BasedFluidsinGasWells 246

7-4.2 Oil-Based Fluids 247

7-4.3 Energized fuids 248

7-4.4 Foams and Emulsions 249

7-4.5 Unconventional Fluids 250

7-4.5.1ViscoelasticSuractantFluids 250

7-4.5.2ViscoelasticSuractantFoams 251

7-4.5.3EmulsionoCarbonDioxidewithAqueousMethanolBaseFluid 251

7-4.5.4CrosslinkedFoams 251

7-4.5.5Non-AqueousMethanolFluids 252

7-4.5.6LiquidCO2-BasedFluids 253

7-4.5.7LiquidCO2-BasedFoamFluid 254

7-4.6 Acid Fracturing Fluid 254

7-5 Fracturing Fluid Additives 254

7-5.1 Additives or Water-Based Fluids 254

7-5.1.1FrictionReducers 254

7-5.1.2GellingAgents 255

7-5.1.3Biocide 257

7-5.1.4Buers 259

7-5.1.5Crosslinkers 259

7-5.1.6Breakers 260



X

7-5.1.7ClayStabilizers 262

7-5.1.8Suractants 262

7-6 Fluid Damage to Fractures and Sources o Productivity mpairment 262

7-6.1 Example Calculation o Productivity mpairment rom Fracture Damage 264

7-6.2 Formation Damage rom Saturation Changes 265

7-6.2.1FluidRetention 265

7-6.2.2Rock/FluidInteractions 267

7-6.2.3Fluid/FluidInteractions 267

7-6.2.4WettabilityAlterations 267

7-6.3 Formation Damage rom Production 268

7-7 Fracturing Fluid Selection 268

7-7.1 Mineralogical Evaluation 269

7-7.1.1X-RayDiraction(XRD)Analysis 269

7-7.1.2ScanningElectronMicroscopy(SEM) 270

7-7.1.3ImmersionTesting 271

7-7.1.4CapillarySuctionTimeTesting 271

7-7.1.5CoreFlowAnalysis 271

7-8 Selection o Fracturing Fluids or Applications in Gas Wells 273

Chapter 8Proppants and Fracture Conductivity

8-1 ntroduction 2838-1.1 Overview 283

8-1.2 The Evolution o Proppants 283

8-1.3 Fracture Conductivity 285

8.2 Conductivity mpact on Fractured Well Production Potential 286

8-2.1 How a Propped Fracture Benets Well Flow Rate 287

8-2.2 Steady-State Solutions 288

8-2.3 Transient Solutions 288

8-3 Proppants 289

8-3.1 Sands 289 8-3.1.1OttawaSands 290

8-3.1.2BradySands 290

8-3.2 Ceramic Proppants 291

8-3.2.1SinteredBauxite 291

8-3.2.2IntermediateStrengthCeramicProppant 291

8-3.2.3LightweightCeramicProppant 292

8-3.3 Resin-Coated Proppants 292



X

8-3.4 Ultra-Lightweight Proppants 294

8-4 Proppant Properties, Testing Protocols, and Perormance Considerations 295

8-4.1 Proppant Testing Procedure Standards 295

8-4.2 Proppant Sampling 296

8-4.3 Grain Size and Grain Size Distribution 297

8-4.3.1ProppantSizeTesting 297

8-4.4 Proppant Shape 298

8-4.4.1ProppantShapeTesting 299

8-4.5 Proppant Bulk Density and Apparent Specic Gravity 299

8-4.5.1ProppantBulkDensityandSpecicGravityTesting 300

8-4.6 Proppant Quality 300

8-4.6.1AcidSolubilityTesting 300

8-4.6.2TurbidityTesting 301

8-4.7 Proppant Strength 301

8-4.7.1ProppantCrushandFinesGeneration 302

8-4.7.2CrushTesting 302

8-4.8 Proppant Concentration 303

8-5 Proppant Placement 305

8-5.1 Eects on Fluid Rheology 305

8-5.2 Convection 305

8-5.3 Proppant Transport 305

8-6 Fracture Conductivity 308

8-6.1 AP “Short-Term” Testing Procedure 308

8-6.2 SO “Long-Term” Testing Procedure 309

8-6.3 Non-Darcy Flow Testing 310

8-6.4 Multiphase Flow Tests 311

8-6.5 Gel Damage 312

8-6.6 Other Factors 313

8-7 Proppant Flowback 314

8-7.1 Proppant Flowback Control 314

8-7.2 Curable Resin-Coated Proppant 315

8-7.3 Proppant Flowback Control Additives 315

8-7.3.1Tackiers 315

8-7.3.2Fibers 315

8-7.3.3DeormableParticles 315

8-8 Proppant Selection 316

8-8.1 Productivity Potential 317

8-8.2 Flowback Control 317

8-8.3 Availability 317

8-8.4 The Cost-Value Proposition 318



X

Chapter 10Fracturing Horizontal Wells

10-1 ntroduction 363

10-2 Production rom Transversely Fractured Gas Horizontal Wells 36510-2.1 A Calculation or Transversely Fractured Gas Horizontal Wells 366

10-3 Open-Hole Horizontal Well Completions 369

10-3.1 Perorating 370

10-3.2 Zonal solation 370

10-4 Open-Hole Fracturing 371

10-4.1 Acid Fracturing Execution 372

10-4.2 Proppant Fracturing Execution 372

10-4.3 Cleanup 373

10-5 Cased-Hole Completions 37310-5.1 Cementing Horizontal Wells 373

10-5.2 Perorating Cemented Completions 374

10-5.3 Zonal solation in Cased Completions 375

10-6 Fracturing o Cased-Hole Completions 376

10-6.1 Acid Fracture Execution 376

10-6.2 Proppant Fracturing Execution 377

10-7 Rationale and Conditions o Fracturing

Horizontal Wells in Gas Formation 377

Chapter 11Unconventional Gas


11-2 Description o Unconventional Reservoirs 383

11-3 Production Mechanisms 385

11-3.1 CBM (Coalbed Methane) 385

11-3.2 Shale Gas Reservoirs 385

11-3.3 Shale Gas Reserves 386

11-4 CBM Reservoirs 387

11-4.1 Coalbed Description 387

11-4.2 CBM Fractured Systems 388

11-4.3 Adsorption/Desorption 390

11-4.4 Stimulation Techniques 391

11-4.5 Alternate Completions and Enhanced Production Techniques 393



X

11-4.6 Fracture Modeling o CBM Wells 396

11-4.7 Fracturing Treatment Evaluation o CBM Wells 397

11-4.8 Estimation o Reserves and Production Data Analysis 398

11-5 Shale Gas 400

11-5.1 Shale Description 400

11-5.2 Thermogenic and Biogenic Systems 401

11-5.3 Ft. Worth Basin Barnett Shale 402

11-5.3.1BarnettShaleSlickwaterTreatmentDesignConsiderations 404

11-5.4 Barnett and Woodord Gas Shale, Delaware Basin 406

11-5.5 Fayetteville Shale in Arkansas 409

11-5.5.1TreatmentDesignConsiderationsFayettvilleShale 409

11-5.6 Woodord/Caney Shale, Arkoma Basin 410

11-5.7 Floyd Shale/Conasauga Shale, Black Warrior Basin (Alabama) 412

11-5.8 Mancos and Lewis Shales 412

11-6 Shale Treatment Design and Evaluation 413

11-6.1 Stimulation and Treatment Design or Shale Reservoirs 413

11-6.2 Fracture Modeling 416

11-6.3 Summary 416

Chapter 12Fracturing or Reservoir Development

12-1 ntroduction 42712-2 mpact o Fracturing on Reservoir- or Drainage-Wide Production 428

12-2.1 Example Application o neld Drilling and Fracturing o Gas Wells 429

12-2.2 Transient Flow o Fractured Gas Wells 430

12-3 Forecasting Natural Gas Well Perormance and Recovery 431

12-3.1 A Case Study or Reservoir Recovery Using Unractured and Fractured Wells 431

12-3.2 Field Development Strategy 432

12-4 mpact o Fracture Azimuth on Well Planning 434

12-4.1 Determination o Fracture Azimuth 435

12-4.2 Considerations Regarding Directional Permeability in the Reservoir 435

12-4.3 Barnett Shale Case Study 437

12-5 Data Mining Techniques 441

12-5.1 Purpose o Data Mining 441

12-5.2 Data Sources 441

12-5.3 Data Preparation 442

12-5.4 Selected Data Mining Tools 442

12-5.5 Data Mining Case History 443



XV

Chapter 13Technologies or Mature Assets


13-1.1 Denition o a Mature Asset 45513-1.2 Minimum Cost & Maximum Value 456

13-1.3 Motivation or Fracturing 457

13-1.4 New Technologies/Approaches 458

13-1.5 Reducing Treatment Costs 462

13-2 Candidate Selection 464

13-2.1 Regional Considerations 464

13-2.2 Neighborhood Considerations 465

13-2.3 Localized Considerations 466

13-2.4 Risk Ranking and Data Manipulation 46713-2.5 Case Histories and Results 468

13-3 Fracture Design in Mature Fields 469

13-4 Depletion Considerations 470

13-4.1 Pore-Pressure Considerations 470

13-4.2 Fracturing Fluid Selection 472

13-4.3 Proppant Selection 473

13-4.4 Cleanout and Flowback 474

13-4.5 Mechanical Deployment 476

13-5 Re-Fracturing Operations 47913-5.1 Re-Fracturing Case Histories 480

13-5.2 Candidate Selection or Re-Fracturing 481

13-5.3 Re-Fracture Re-Orientation 481

13-5.4 mproved Treatment Design 483

Nomenclature 491

ndex 503



XV

PreaceIt is with great pleasure that I welcome you to Modern Fracturing: Enhancing Natural Gas Production BJ Services

Company is proud to be involved in developing and publishing this work We hope you nd this book to be

instructive, inormative and interesting

Tis book is intended or use by all industry proessionals, not just those who are already amiliar with the engineering concepts and eld practices o hydraulic racturing Te pages within comprise a stateotheart engineering manual

or planning, preparation, perormance and evaluation o hydraulic racture treatments in natural gas reservoirs We

envision industry proessionals throughout the world beneting rom the inormation in this book

Hydraulic racturing is already the completion method o choice or most natural gas wells in North America As

global dependence upon natural gas increases, it seems likely the application and popularity o this completion

method will only increase urther and spread arther Te techniques described within this book are applicable to all

gas reservoirs, not just to the low permeability ormations typically developed in North America We rmly believe

racturing is the best possible completion technique or each and every gas reservoir throughout the world

A wide range o knowledgeable authors rom throughout the industry have come together to produce this book On

behal o BJ Services, I want to thank them or their sharing their experience and knowledge, as well as or their hard

work and dedication in completing such an ambitious project We eel certain that in the years to come, each author

will continue to be proud o his or her involvement in this undertaking We also trust that readers like you will

continue to improve “best practices” in developing natural gas resources worldwide with the insights derived rom

this signicant work

Dave Dunlap

Executive Vice President and Chie Operating Ocer, BJ Services



XV

ForewordI was very pleased when my riend Michael Economides asked me to write the Preace to his new book BJ Services

Company should be complimented or sponsoring this eort and or attracting some o the world’s top experts to

contribute I know many o the contributors, and I am sure the result will be lasting and useul or years to come

I am even more pleased that this specic book is put together or three reasons Te rst is that natural gas will shortly become the premier uel o the world economy Second, hydraulic racturing, already the most important production

enhancement technique or oil wells, is absolutely indispensable or natural gas wells Tird, the existing knowhow and

skill sets o the racturing community are dreadully inadequate, especially in management

Fracturing in the petroleum industry is no longer an experimental or daring activity by some hotshot, brash engineers,

oten working against the established old thinking and even worse, conservative managers who still believe that economics

equal cost reduction, ignoring the benet rom improved well perormance When enhanced production and injection

perormance is the motivation, nothing can compete with properly integrated racturing

Oten, people are conused about the real impact rom this well completion and stimulation technique Most oten, any improvement in production compared to what a well did beore racturing is considered a “success” In reality, we already

know how much a well should be producing ater racturing by using the concept o maximizing the J D , the dimensionless

productivity index Anything less than that should be considered a perormance gap and managed as such We have to

push the limits and manage the completion and execution community to deliver what we know can be done

All activities in a company must be integrated with hydraulic racturing We are by denition “cando” people So the idea

that ultrahigh production targets are “unrealistic and theoretical” should be replaced by developing and implementing

the knowhow and skill sets to deliver maximum perormance

Consider this: When my associates and I (including Michael) were working in Russia, in a veyear period we managed

to double a company’s production, increasing by 20% per year to almost 2 million barrels per day while shutting-in 50%o the original well stock Most o this success occurred by pushing the limits o hydraulic racturing and integrating the

other parts o the production system And despite this success, we were constantly enhancing materials and increasing job

sizes to push the calculated perormance limits We established two management rules:

1 All new wells and workovers must be ractured unless top management approves otherwise

2 All rac jobs must be designed and executed to perorm at the peak o the NPV bell curve unless top management

approves otherwise

Te point is that many companies require approval to do it right but delegate enough nancial authority, no approval

required, to do it wrong We reversed this by giving enough authority (no approval required) to do it right and requiredtop management approval to do it wrong

It is not so dicult to reproduce the same perormance everywhere else Just look at current worldwide well perormance,

and one can easily see huge gaps, including the largest and bestknown multinational oil companies Fracturing can go a

long way to correct this obvious problem Not only will the benet to companies be immediate and large, but silly talk

about “peak oil” and “twilight in the desert” will go away

Joe Mach February 2007



Contributing Authors

Editors

Michael J. Economides, University o HoustonTony Martin, BJ Services

Authors

Bob Bachman, Taurus Reservoir Solutions Steve Baumgartner, BJ Services Harold Brannon, BJ Services

Andronikos Demarchos, Hess Corporation Michael J. Economides, University o Houston John Ely, Ely & Associates , Inc .Satya Gupta, BJ Services Robert Hawkes, BJ Services Barry Hlidek, BJ Services George King, BP Randy Lafollette, BJ Services

David Mack, Marathon Oil Mark Malone, BJ Services Tony Martin, BJ Services

C. Mark Pearson, Golden Energy, LLC David Ross, InTuition Energy Associates Ltd.Martin Rylance, BP Gary Schein, BJ Services Peter Valkó, Texas A&M University Leen Weijers, Pinnacle Technologies Xiuli Wang, BP Don Wolcott, Aurora Oil and Gas

XV



XX

AcknowledgementsFirst and oremost, the editors would like to express their sincere gratitude to JC Mondelli, who has been the

champion o this book within BJ Services rom its initial conception, all the way through to printing and publication

Without his perseverance and vision, this publication would never have come about We would also like to thank

the senior management o BJ Services or providing unding and, especially, or allowing a great number o highly

dedicated people to put their time and energy into writing chapters, in spite o their busy schedulesOur thanks to Joe Mach or gracing the book with his Preace and endorsement, and who also, in his unique

style, reminded all o us why doing this book mattered in the rst place

Writing this book was an added task both or our BJ Services colleagues and those rom other companies and

institutions, and the result is a testament to their dedication and proessionalism Putting together a multiauthored,

multiedged book is never an easy task and to no small measure, the authors deserve particular praise or persevering

and having to respond to suggestions and editorial intererence by two admittedly highly demanding and opinionated

Editors Compliments and credit are deserved by all o them, without whom this project would not have been

possible

Special thanks go to Greg Salerno who shepherded many o the logistical tasks and kept a levelheaded approach

on the daytoday management o the project Tanks also to Garth Gregory and Margaret Kirick or their invaluablehelp with the organisation and administration o this undertaking

Te copyeditor Stephanie Weiss served a key role in the nal version o the book She is a highly experienced

and exceptional technical copy editor, a ormidable “vacuum cleaner” or cleaning up deciencies, omissions and

errors Her work reminded all that adherence to detail and perection are essential in elevating a proessional book to

a dierent level She was a rare nd

Alexander M Economides and his sta in the Energy ribune, headed by Jay Clark and the publication assistants

Alex Lewis and George Song, did a spectacular job in producing the book Tey deserve special praise

Michael J. Economides and Tony Martin September 2007



Michael J. Economides is a proessor at the Cullen College o Engineering, University o Houston, and themanaging partner o a petroleum engineering and petroleum strategy consulting rm His interests includepetroleum production and petroleum management with a particular emphasis on natural gas, natural gastransportation, LNG, CNG and processing; advances in process design o very complex operations, andeconomics and geopolitics He is also the editorinchie o the Energy ribune Previously he was theSamuel R Noble Proessor o Petroleum Engineering at exas A&M University and served as chie scientisto the Global Petroleum Research Institute (GPRI) Prior to joining the aculty at exas A&M University,Economides was director o the Institute o Drilling and Production at the Leoben Mining University

in Austria Beore that, he worked in a variety o senior technical and managerial positions with a majorpetroleum services company Publications include authoring or coauthoring 14 proessional textbooks andbooks, including Te Color O Oil, and more than 200 journal papers and articles Economides does a widerange o industrial consulting, including major retainers by national oil companies at the country level andby Fortune 500 companies He has had proessional activities in over 70 countries

Tony Martin is business development manager or international stimulation at BJ Services Company Sincegraduating rom Imperial College, London, with an honors degree in mechanical engineering and a master'sdegree in petroleum engineering, Martin has spent 17 years in the oil industry and has completed engineeringassignments around the world Martin's primary interest has been hydraulic racturing and stimulation, andhe has been involved in production enhancement projects in more than 25 countries He teaches racturingacidizing and sand control both inhouse and externally A constant theme in this teaching is the need

to demystiy the world o hydraulic racturing, in an attempt to make the process more accessible andless intimidating He is the author or coauthor o numerous SPE papers and has served on the technicalcommittees or several SPE events He is also the author o BJ Services’ Hydraulic Fracturing Manual



3

Chapter 1ntroduction to this Book Michael J Economides, University o Houston and

ony Martin, BJ Services

1-1 ntroduction

Tis is a book about enhancing natural gas production

using one o the most important and widespread well

completion technologies — hydraulic racturing

Te book addresses the way that natural gas is

produced rom natural reservoirs (Chapter 2) and then

describes diagnostic techniques that can pinpoint whether

the well is producing as it should or whether intervention

should be undertaken (Chapter 3), which is the centraltheme o this book

Hydraulic racturing is introduced as the solution

o choice, showing the idiosyncratic nature o natural

gas wells compared to oil wells (Chapter 4) Te

subsequent two chapters address important peripheral

issues whose successul or ailed resolution may aect

the well perormance with equal or even more serious

consequences than the racture treatment itsel Tese

issues include well completions (Chapter 5) and the

extremely important welltoreservoir (and racture)

connectivity (Chapter 6)Te next two chapters deal with materials or

racturing: fuids and proppants (Chapters 7 and 8) Teir

selection is essential to the successul execution o the

treatment Te execution itsel becomes the next chapter,

and practical issues are addressed there (Chapter 9)

Ten some modern applications are described

One chapter deals with racturing horizontal wells,

increasingly an important option among reservoir

exploitation strategies (Chapter 10) Not only new

well architecture but also newer reservoir targets areopening up, and natural gas demand points towards

unconventional sources, namely coalbed methane

(CBM), shale gas and very lowpermeability ormations

echnology makes their exploitation possible, and this

is the subject o the next chapter (Chapter 11)

Finally, two issues round out the book:

Fracturing is employed in the ull development o

reservoirs (Chapter 12); and how mature elds, a

mainstay o the developed world such as the United

States and Europe, can be revitalized through

this process (Chapter 13)

Beore the technical issues are addressed it is

essential to look at natural gas in the world economy,

why it is becoming increasingly important and what

are the reasons or all the excitement surrounding itsenhanced production

1-2 Natural Gas in the World Economy

Although natural gas, with some 23% o all world energy

demand in 2005, is still slightly behind coal (256%) as

the world’s thirdlargest source o primary energy (oil

still dominates at 38%), it is poised to move up because

o signicantly emerging new trade Member countries

in the Organization or Economic Cooperation andDevelopment (OECD) and the USA, specically,

consume about 51% and 22% respectively o global

natural gas, now comprising about 103 c (29 Bm3) per

year (Energy Inormation Administration, EIA, 2007)

Figure 1-1 The top 12 holders o natural gas reserves:

Russia, Iran and Qatar dominate (EIA, 2006, BPStatistical Review, 2006, ET, 2007)

Tere are several obvious benets to the use o

natural gas First, it is the cleanestburning ossil uel

and produces ewer emissions and pollutants than either

oil or, especially, coal Second, the resource is becoming

increasingly diverse Since the early 1970s, world reserves

o natural gas have been increasing steadily, at an annual



Modern Fracturing

4

rate o some 5% Similarly, the number o countries

with known reserves has also increased rom around 40

in 1960 to about 85 in 2005 Te distribution among

those countries, dominating the global proved reserves o

natural gas, is shown in Fig 11 and able 11

One reason or anticipated increase in demand or

natural gas is the public concern over environmental

issues Furthermore, orecasts o rapid increase in

natural gas demand over the next two decades, in thebiggest market o all, the United States, have been

exacerbated by orecasts o declining production

Declining production orecasts have been extended to

Canada, a reliable provider to the US thus ar (EIA,

Annual Energy Outlook, 2007)

Although natural gas demand is expected to

increase, such an increase in the near uture will

be driven by additional demand rom current uses,

primarily power generation Tere is yet little overlap

between the use o natural gas and oil in all large

markets However, certain developments on the

horizon, including the electriying o transportation,

will push natural gas use to ever higher levels

Although potential natural gas supplies abound

throughout the world, acilities and inrastructure

to receive and distribute the product to market are

expensive to build, and their development can easily behindered by geopolitics Tese reasons have historically

inhibited natural gas rom reaching its ull potential in

the world’s energy markets Natural gas is transported

either by pipeline (73% o internationally traded gas

in 2005, EIA 2007), mainly across land masses, and

by liqueed natural gas (LNG) transportation across

the oceans (the remaining 27%) Te rapid expansion

o LNG inrastructure worldwide in the past decade is

Table 1-1 Top 25 Countries Ranked According to Proved Natural Gas Reserves and identiying the proved

reserves-to-production ratio (R/P) or each country

Proved Natural Gas Reserves at January 1, 2006

CountryTrillion Cubic

Feet (Tcf)

Trillion Cubic

Meters (Tm3)Share of Total

Cumulative

Share of Total

Reserves /

Production (R/P)

Years

1 Russian Federation 1688 47.8 26.6% 26.6% 80.0

2 Iran 944 26.7 14.9% 41.5% >1003 Qatar 910 25.8 14.3% 55.8% >100

4 Saudi Arabia 244 6.9 3.8% 59.6% 99.3

5 United Arab Emirates 213 6.0 3.4% 63.0% >100

6 USA 193 5.5 3.0% 66.0% 10.4

7 Nigeria 185 5.2 2.9% 68.9% >100

8 Algeria 162 4.6 2.5% 71.5% 52.2

9 Venezuela 152 4.3 2.4% 73.9% >100

10 Iraq 112 3.2 1.8% 75.6% >100

11 Kazakhstan 106 3.0 1.7% 77.3% >100

12 Turkmenistan 102 2.9 1.6% 78.9% 49.3

13 Indonesia 97 2.8 1.5% 80.5% 36.3

14 Australia 89 2.5 1.4% 81.9% 67.9

15 Malaysia 88 2.5 1.4% 83.2% 41.416 Norway 85 2.4 1.3% 84.6% 28.3

17 China 83 2.4 1.3% 85.9% 47.0

18 Egypt 67 1.9 1.1% 86.9% 54.4

19 Uzbekistan 65 1.9 1.0% 88.0% 33.2

20 Canada 56 1.6 0.9% 88.8% 8.6

21 Kuwait 55 1.6 0.9% 89.7% >100

22 Libya 53 1.5 0.8% 90.5% >100

23 Netherlands 50 1.4 0.8% 91.3% 22.3

24 Azerbaijan 48 1.4 0.8% 92.1% >100

25 Ukraine 39 1.1 0.6% 92.7% 58.7

Total World 6347.79 179.82 100% 65.1

Sum of Top 25 Countries 5885 166.7 92.7%

Rest of World 463 13.1 7.3%



Chapter 1 ntroduction to this Book

5

enabling natural gas to penetrate many more markets

through the development o many remote reserves once

considered to be stranded and uneconomic to develop

Ongoing construction and plans to expand and build

new LNG receiving terminals in North America

(Canada, Mexico and the United States) are opening

up rapidly growing gas imports, destined to supportmany new LNG supply chains worldwide European

and Asian markets are also hungry or LNG

But beyond the usual energydemanding markets,

China and India have both emerged rom the developing

world to become globally signicant economies in their

own right, both requiring massive energy imports to

sustain uture economic growth But their approaches are

very dierent; China is ocused on manuacturing, India

more on services However, both have large populations

with aspirations to lead highenergy consuming liestylesogether, they are promoting globalization that is putting

pressure on the world’s energy resources and existing

supply chain, traditionally directed to serving the OECD

world Te rapid growth in China and India over the

last ew years has precipitated huge increases in demand

or all energy sources, because o their lack o sucient

indigenous energy resources Tis has let the rest o

the world scrambling or the same sources o energy,

including natural gas Te US is hampered by the myriad

permit approvals required and public opposition to siting

o LNG receiving terminals Nevertheless, major UScompanies and others are investing heavily in building

new LNG liqueaction inrastructure in Qatar, several

countries in West Arica and Russia’s Sakhalin Island

ransportation is an essential aspect o the gas

business because gas reserves are oten quite distant

rom the main markets Gas is ar more cumbersome

than oil to transport, and the majority o gas is

transported by pipeline Tere are welldeveloped

networks in Europe and North America and a relatively

adequate one in the ormer Soviet Union However,in its gaseous state, natural gas is quite bulky – or the

same time, a highpressure pipeline can transmit only

about oneth o the amount o energy that can be

transmitted in an oil pipeline o the same size, even

though gas travels much aster When gas is cooled to

–160 °C it becomes liquid and much more compact,

occupying 1/600 o its standard gas volume Where

long overseas distances are involved, transporting gas

in its liquid state becomes economic But the supply

chain consists o expensive and specialized acilities

both upstream and downstream, and generally requires

dedicated marine vessels

Te LNG industry is set or a large and sustained

expansion as improved technology has reduced costs

and improved eciency along the entire supply chainduring the past decade Tis shit in the dynamics o

the natural gas market will urther commoditize and

diversiy the natural gas globally New LNG carriers

are 1000 t long and require a minimum water depth

o 40 t when ully loaded Te global feet o LNG

carriers reached 217 by the end o 2006 (Wood et al.,2006) with more than 11 million tons o LNG capacity

Te order book or new LNG marine carriers to 2010

is some 120 rm and 32 proposed, meaning the uture

feet may exceed 370 vessels by the end o 2010 Te feet was just 90 vessels in 1995 and 127 vessels in 2000 Te

current feet transports more than 140 million metric

tons o LNG every year (converted to 7 c), about

23% o gas trade internationally and about 65% o

total gas consumed worldwide

Below is a discussion o the state o natural

gas in three o the most important countries/

regions o the world which, or dierent reasons,

are dening the present and uture o natural

gas in the world economy

1-3 Russia: A Critical Evaluation o its

Natural Gas Resources

Te dissolution o the Soviet Union in 1991 and its

replacement by the Commonwealth o Independent

States (CIS), prominent among which was the Russian

Federation, was a signicant geopolitical event, aecting

the subsequent development o Russian resources –

particularly natural gas Contrary to widely held belies,

i current trends continue, Russia likely will have a severenatural gas shortall by 2010 (Moscow Institute o Energy

Research, 2007) Tis prediction is astonishing, given

that Russia has more gas reserves than any other country,

and one o the largest reservestoproduction ratios

One o the reasons or the looming gas shortall is

that over the past several years, Russia has not invested

suciently and lacks the technology to develop new gas

elds to replace its rapidly depleting ones



6

F i g u r e 1

- 2

R u s s i a n

g a s r e s o u r c e s ,

i n f r a s t r u c t u r e , p i p e l i n e s a n d

f u t u r e

p l a n s




7

Tere are complicated reasons behind

the state o Russia’s natural gas industry A

thorough understanding o the industry and its

history is required beore we can discuss its uture

(see Section 132)

Next, we examine Russia’s natural gas reserves,

production and transportation

1-3.1 The Resource Base

Russia has the world’s largest proven natural gas

reserves, estimated at 1,680 c (EIA, 2007), about

double those o Iran, the next largest Russia is also the

largest gas producer and exporter In 2004, Russia’s gas

production exceeded 224 c and exports totaled 71

c In addition, the gas industry plays a signicant

role in the Russian economy, contributing about26% o total GDP in 2004 (E, 2007) Fig 12 is an

annotated map o Russia with all important natural

gasrelated inormation (EIA, 2007, wwwGazprom

com, and BP Statistical review, 2006)

0

50

100

150

200

250

300

350

400

450

N o r t h D o m e , 1 , 2 0 0 ( T c f )

U r e n g o y *

Y a m b u r g

O r e n b u r g

S h t o k m a n

Z a p o l y a r n o y e

K h a r a s e v e y

B o v a n e n k o

M e d v e z h ’ y e

H a s s i R ’ M e l

S o u t h P a r s

P a n h a n d l e - H u g o t o n

*Urengoy had been the world’s

largest gas field for years until the

North Dome was discovered.

Rank Field Reserves Location1 North Dome 1,200 Qatar/Iran2 Urengoy 275 Russia3 Yamburg 200 Russia4 Orenburg 200 Russia5 Shtokman 200 Russia6 Umm Shaif/Abu el-Bukush 175 Abu Dhabi7 Zapolyarnoye 150 Russia8 Kharasevey 150 Russia9 Bovanenko 125 Russia

10 Medvezh’ye 100 Russia11 Hassi R’Mel 100 Algeria12 South Pars 100 Iran13 Panhandle-Hugoton 80 U.S.A.

U m m S

h a i f / A b u e l - B u k u s h

Table 1-2 The World’s Largest Natural Gas

Reservoirs (EIA, 1994-2004, Interfax,

2005,www.gazprom.com, ET, 2007)

Figure 11 compares Russian gas reserves with those

o the other major gas producing countries able 12

lists the 13 largest gas elds in the world As is shown,

Russia owns twothirds o them (E, 2007, EIA, 2007,

wwwGazpromcom, and BP Statisitcal review, 2006)

Gazprom, tracing its origins to the Soviet Gas

Ministry, is the dominant gas company in RussiaFig 13 shows Russia’s total gas production and

consumption and Gazprom’s contribution rom 2000

to 2005, which accounts or about 80% Gazprom

is not only Russia’s largest gas producer, it also owns

the entire gas pipeline inrastructure in Russia – all

155,000 km o it, along with the compressor stations

In addition, Gazprom controls the sole means o

getting gas to domestic and export markets

12

14

16

18

20

22

24

2000 2001 2002 2003 2004 2005

Total Production

Gazprom's Share

Total Consumption

T c f / y e a r

Figure 1-3 Russian gas production and consumption and

Gazprom’s contribution (EIA, 2004-2006, Interax, 2005,

www.gazprom.com, ET, 2007)

Te reason that Russia has given Gazprom control

over its natural gas is the socalled “social obligation”

Trough Gazprom, the Russian government subsidizes

its inecient domestic industries with lowpriced

natural gas Gazprom sells most o its gas to domestic

customers at a considerable discount Te wholesale

price o 1,000 m3

o gas or a Russian household isaround $1590 (about $045/Msc) For industrial

users, gas costs around $2420 ($069/Msc) By

comparison, in the European Union, household taris

range rom Finland’s $159 ($450/Msc) to Denmark’s

$735 ($2082/Msc, E, 2007) Clearly, Gazprom

is losing large amounts o money on domestic sales,

compared to international market prices, and must rely

on export revenues or the dierence



Modern Fracturing

8

Gazprom’s major challenge is the aging o its major

producing gas elds Production rom these elds is

declining and studies project steep declines in Russia’s

overall natural gas output between 2008 and 2020

According to projections rom the Moscowbased Institute

o Energy Research (2006), Russia will ace a gas shortall

o about 100 Bm3 by 2010 Considering that Russia ownsthe largest gas reserves in the world and one o the largest

reservestoproduction ratios (815 years compared to

Algeria’s 554 and Canada’s 88, or example, rom EIA,

2007, calculated by E, 2007), the uture o Russian

natural gas production eorts is important globally

1-3.2 Russian Natural Gas Production

Gazprom holds about onethird o the world’s natural

gas reserves and produces about 80% o Russia’snatural gas Te remaining percentage comes rom

independent producers Te company operates 155,000

km o natural gas pipeline and 43 compressor stations

As the world’s largest producer and exporter, Russia

is also a huge consumer o natural gas Te country

produces an annual 21 c, consuming 145 c and

exporting the rest (2002 numbers rom EIA and E,

2007) Despite the country’s huge reserves, natural

gas production has remained essentially fat over the

past several years, with a mild production increase

(13%) orecast or 2008 In contrast to the natural gasstagnation, oil production has fourished

Te immediate uture o natural gas production

in Russia does not allow or much optimism Te

overall production decline orecast or Gazprom is

quite steep, as shown in Fig 14 (Moscow Institute o

Energy Research, 2006)

Considering that Russia’s domestic consumption

is increasing by 25% annually, the current demand

in Europe, urkey and the Commonwealth o

Independent States (CIS) or up to 325 Bm3

(E,2007), and China’s demand or 38 Bm3 (Moscow

Institute o Energy Research, 2006) it’s clear that

additional sources o natural gas must be ound i

Russia wants to play a major role in the uture natural

gas market It’s equally clear that the problem o Russia’s

looming gas shortage can only be solved by optimizing

existing elds and through the rapid development and

production o major elds such as Yamal, Shtokman

and Sakhalin Obviously, implementing these solutions

will require a substantial investment that Gazprom has

not yet been able to make

One scenario or the potential contribution

o independent producers shows a net increase o

100 Bm3 per year by 2010 (Moscow Institute o

Energy Research, 2006)

0

100

200

300

400

500

600

2004 2010 2015 2020

C o m b i n

e d B c m / y e a r

Zapadno-TarkosalinskoyeKomsomol'skoyeZapolyarnoyeMedvezhye

Aner'yakhinskoyeKharvutinskoye

YabburgskoyeEn-Yakhinskoye

OthersOrenburg

AstrakhanUrengoyskoye(achimov)

Ety-Purovskoye Yuzhno-Russkoye Vyngayahinskoye

Pestsovoye Yubileynoye Urengoyskoye

G a z p r o m ’ s f o r e c a s t p r o d u c t i o n d e c l i n e

Figure 1-4 Gazprom’s production decline orecast (Moscow

Institute o Energy Research, 2007)

1-4 Alaska, its Natural Gas Resources

and their mpact on US mports

It has been known or many decades that Alaska hasprolic hydrocarbon resources, rst with the discovery

o oil in the south central part (Cook Inlet) in the

1960s and then with the 1969 discovery o Prudhoe

Bay, the US’s largest eld Oil has been successully

commercialized in Alaska since the 1970s construction

o the ransAlaskan pipeline that stretches rom the

North Slope to Southern Alaska From there, oil is

shipped to the lower 48 states




9

Despite the success o Alaskan oil production,

and although it is widely known that natural gas

exists in large quantities in the state, two important

questions have always arisen: 1) in what kind and

size o reservoirs is the gas trapped and 2) how can

it be commercialized? Furthermore, ater 30 years o

Alaskan oil production and almost 15 years ater itsproduction peak, substantial natural gas exploitation

rom the state is still not orthcoming

We are convinced that Alaska has a very large

natural gas resource base, larger than commonly

accepted Beyond the conventional gas reserves

on the North Slope (about 100 c) and Cook

Inlet (at least 30 c), perhaps as much as 1000

c are in the orm o coalbed methane and, at

least, 500 c as natural gas hydrates (Anchorage

Chamber o Commerce, 2005)Economic and technical obstacles abound Te

cost or exploiting conventional reserves, with or

without government subsidies, has been a hindering

actor, but other actors such as the emerging

large LNG trade are having an impact Te most

important question is whether Alaskan gas will be

commercialized any time in the oreseeable uture,

and we shall discuss this issue in detail Tis has major

implications on the uture o the state, the USA and

the natural gas trade into the country

1-4.1 Alaskan Reserves and Production

Tere are two major hydrocarbon producing areas

in Alaska today: the Cook Inlet region in south

central Alaska and the Prudhoe Bay complex on the

North Slope Te proved gas reserves or the Cook

Inlet and the North Slope are 2 c (6% o total)

and 27 c (94% o the total), respectively (EIA,

2007) Currently all the gas produced on the North

Slope is reinjected or pressure maintenance exceptor the gas needed to maintain eld operations

and uel the local villages

Figure 15 shows the historical production and

the prediction o natural gas production to 2025

As can be seen, the 2006 production rom the two

areas is approximately 490 Bc per year o gas and

is expected to decrease to 240 Bc per year by 2025

(Alaska Department o Natural Resources, 2006)

Clearly, Cook Inlet gas production is on decline while

North Slope gas production remains stable – with its

market limited to the local market without a natural

gas export pipeline to larger markets

0

100

200

300

400

500

600

1 9 5 8

1 9 6 4

1 9 7 0

1 9 7 6

1 9 8 2

1 9 8 8

1 9 9 4

2 0 0 0

2 0 0 6

2 0 1 2

2 0 1 8

2 0 2 4

B c f / Y e a r

Cook Inlet North Slope

Figure 1-5 Historic and orecast gas production (Alaska

Department o Natural Resources, 2006)

Te orecast in Fig 15 is only or the current proved

reserves o natural gas I we consider the unconventional

resources in Alaska, the natural gas resource base grows

much larger However the technology and economics

or developing the unconventional resource base are

major blockers Te two main unconventional gas

reservoirs that capture a lot o attention are coalbed

methane and natural gas hydrates

It is estimated that coalbed methane is prevalent

in the northern and southern parts o the state, shown

on the map in Fig 16 (Alaska Department o NaturalResources, 2006)

1

1

Baja California Sur

SonoraGuadalupe

Huata

Ciudad Constitucion

Ciudad Obr

L

Guaymas

Her

L

Bering Sea

Pacific Ocean

Ca

Russia

Alaska

Bituminous & Higher Rank

Subbituminous

Lignite

Rual Sites with Sufficient

Data for Drill Testing

of Coalbed Methane

Potential

Barrow

Fort Yukon

Nome

Cordova

Fairbanks

Anchorage

Figure 1-6 Location o potential coalbed methane

reservoirs (Alaska Department o Natural Resources, 2006)



Modern Fracturing

10

Alaska’s estimated coal resources exceed 55

trillion tons and may contain up to 1,000 c o gas

(Alaska Department o Natural Resources, 2006)

In 1994 the Alaska Div o Oil and Gas drilled the

state’s rst coalbed methane test well near the town o

Wasilla, located in the northern portion o Cook Inlet

Basin Te well was drilled to a total depth o 1245 t;coal was continuously encountered, with the thickest

seam measuring 65 t and a net coal thickness o 41

t Tirteen seams were sampled or gas content Te

results were encouraging, but as elsewhere they are

likely to suer rom the standard CBM problems:

low permeability, water disposal and dicult and

expensive application o hydraulic racturing and

horizontal well technologies

Our current assessment o the total resource base

or natural gas in Alaska, derived rom a number o reerences, is shown in Fig 17

North Slope

Hydrates,

529, 32%

Cook Inlet

Conventional,

30, 2%

North Slope

Conventional,

100, 6%

CBM, 100

60%

Figure 1-7 Natural gas resource base in Alaska (Williams

et al., 2005, Meyers, 2005, Hite, 2006, and Korneld, 2002)

It is clear the 2006 resource assessment shows

the majority o potential reserves are locked in

unconventional reservoirs For these plays to be

developed, investment and technology hurdles will

need to be overcome

1-4.2 The Uncertain Destiny o the North Slope o

Alaska Natural Gas

Methods to deliver natural gas to market rom the North

Slope o Alaska have been studied and proposed or over

30 years Te various schemes can be grouped into three

major categories, with variations in each (Anchorage

Chamber o Commerce, 2005) See Fig 18

• A gas pipeline rom the North Slope through Canada

to the Lower 48 states

• An All-Alaska gas pipeline rom the North Slope to

Valdez, where the gas would be converted into LNG

and taken to markets outside Alaska in LNG tankers

• A “spur line” to take natural gas rom one or more

otake points on the main gas pipeline (whicheverroute it takes) and deliver that gas to customers and

users in Alaska

1

15

Baja California Sur

Sonora

Sinaloa

pe

Navojoa

Huatabampo

Empalme

Ciudad Constitucion

Ciudad Obregon

Los Mochis

Guasave

Guaymas

Hermosillo

La Paz

ring Sea

Pacific Ocean

Canada

Alaska

Anchorage

Juneau

“All Alaska”

LNG shipped from “All Alaska”

“Y-Branches from ”All Alaska”

Northern Route

Southern Route Barrow

Fort YukonNome

Unalaska

Cordova

Fairbanks

PrinceRupert

Figure 1-8 Potential Alaskan natural gas pipeline routes

Tere are two variations on the gas pipeline to the

Lower 48 states proposal: the Northern Route and the

Southern Route Te Northern Route , also reerred to as the

ARC over-the-top route (ARC is or the Artic Resources

Company that rst proposed such a gas pipeline in

the early 1980s), would start rom Prudhoe Bay, move

oshore into the Beauort Sea and run parallel to the

coastline eastward into Canada to the Mackenzie River

Delta, where up to 20 c o natural gas reserves are just waiting to be produced From there, i Canadians have

already built a pipeline to transport the Mackenzie River

reserves to Alberta, the Alaskan Northern Route would

simply reach and merge with it

On the other hand, i Canadians haven’t started

yet to exploit the Mackenzie Delta reserves and

a pipeline to Alberta is not available, the NorthernRoute pipeline would be extended to Alberta, and




11

in all probability it would still oer the prospect to

carry also the Mackenzie gas along with the North

Slope gas Ater the Alaskan natural gas is delivered in

Alberta, there still would be the open issue o how to

carry it down to the rest o the United States

Te two systems currently discussed and proposed

to accomplish this goal are:

• Rerouting Prudhoe Bay natural gas in the Canadian

pipelines network that currently delivers Alberta gas

to markets in Canada and the Lower 48 states

• Building a dedicated pipeline that would transport

Prudhoe Bay natural gas straight to the Northern

Midwest pipeline network

Te Southern Route , also known as the Alaska

Natural Gas Transportation System or as the Foothills , would also start rom Prudhoe Bay, but it would go

south halway the length o Alaska, just south o

Fairbanks, and then cross into the Yukon and north

eastern British Columbia

Te All-Alaska route, also known as the YukonPacic LNG Proposal , would start rom Prudhoe Bay, run

or 805 miles parallel to the ransAlaska oil pipeline to

Valdez and then turn to the east to Anderson Bay

A nal (but tremendously important) part o the

All-Alaska proposal would be the construction o a

liqueaction and shipment plant in the Anderson Bay, toenable shipping as LNG the natural gas coming rom the

North Slope to Asian markets (Japan mainly) and potential

terminals along the Canadian and US West Coast

1-4.3 Alaska in the Context o the United States and

Canadian Natural Gas

Te current situation o the oil and gas industry in

Canada adds substantial reasons or considering the

over-the-top Northern Route (the green line on Fig18) the most suitable option or the whole North

American continent

Canada has been a net exporter o natural gas

or many years, and all o that exported gas has been

imported into the United States Tis gas comprises

about 90% o the natural gas imported into the

US and about 17% o the total US natural gas

consumption Although this relationship has been

successul or many years, Canada can no longer

be relied upon to singlehandedly secure the uture

o US natural gas supply

A declining conventional natural gas resource

has pushed Canada into investing in arctic, CBM and

tight gas plays o date however, those unconventional

resources have contributed a very small percentage tothat country’s overall production o natural gas

As is apparent in Fig 19, the conventional

natural gas supply in Canada is predicted to

decline by roughly 35% rom 2005 to 2020, while

the production o unconventional/stranded gas is

expected to increase dramatically by 2012 (CAPP,

2006a) Tis assumes in part the construction o the

Mackenzie pipeline to get arctic gas to the south as

well as an expectation that CBM will be economic

to produce within the next two decades

0

5

10

15

20

25

2 0 0 5

2 0 0 6

2 0 0 7

2 0 0 8

2 0 0 9

2 0 1 0

2 0 1 1

2 0 1 2

2 0 1 3

2 0 1 4

2 0 1 5

2 0 1 6

2 0 1 7

2 0 1 8

2 0 1 9

2 0 2 0

P r o d u c t i o n ,

B c f / D

Mackenzie Delta

Coalbed Methane

Nova Scotia

Conventional Gas

Figure 1-9, Canadian natural gas production orecast

(CAPP, 2006a)

Te amount o gas Canada will have let over

to export to the US remains in question, and this

is what may push the building o the North Slope

pipeline Te rst issue is that Canadian natural gas

consumption is expected to increase by 16% per year

Tis equates to a demand o almost 12 Bc per day by

2020 (Stringham, 2006) However, this consumption

does not include the gas that will be needed to producethe Canadian tar sands

Tat Canada expects to be producing about 4

million barrels a day by 2020 (CAPP, 2006b, Fig 1

10) means more o Canada’s natural gas will be used

or this purpose In act, the 05 Mc o gas needed to

process each barrel o this crude equates to at least 2

Bc per day natural gas needed to meet the production

orecast or Canada’s oil sands



Modern Fracturing

12

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

2 0 0 1

2 0 0 3

2 0 0 5

2 0 0 7

2 0 0 9

2 0 1 0

2 0 1 2

2 0 1 4

2 0 1 6

2 0 1 8

2 0 2 0

P

r o d u c t i o n ,

M b b l / D

Conventional Oil Sands

Figure 1-10 Prediction o Canadian heavy oil sands

growth (CAPP, 2006b)

Tis, o course, causes some concern because the

total natural gas production rom Canada in 2020 is

expected to be about 18 Bc per day, and Canada will

be using 14 Bc per day or its needs Tis leaves 4 Bc per day suitable to be exported to the US However,

the demand in the US over the next several years is ar

greater than what Canada can provide

Te over-the-top Northern Route is surely not the

ultimate solution to the constantly growing hunger or

natural gas in North America Te over-the-top pipeline

may never be built because o competition rom LNG

imports, which are expected to boom in the next several

years i additional terminals can be built

Our assessment o the Alaskan gas resources, and

in particular the North Slope basin, indicates someopportunities to develop a sustained market or natural

gas with the US Lower 48 states and Eastern Asian

destinations (mainly Japan, South Korea and aiwan)

via LNG shipments Tis motivates all the projects

proposed by several groups o advocates or transporting

the natural gas produced in the North Slope into the

Lower 48 states market, as well as Eastern Asia

Nevertheless, a wide set o reasons leads us to believe

that these projects cannot even be considered marginally

competitive to LNG, especially when compared to theeconomically superior LNG shipped rom the recently

developed elds and acilities in countries such as Qatar,

Russia, Australia and Indonesia In act, as is usual or

large construction projects, the technical easibility o

North Slope natural gas exploitation must be weighed

against the inexorable balance o the economics Tis

is the bottleneck where all the advocated Alaskan gas

pipeline schemes become dicult to justiy

1-5 Qatar Natural Gas

Qatar is a small, independent nation on the western

coast o the Persian Gul Te country has good

relationships with its Middle Eastern neighbors

like Iran, and it has been leading the region

in democratic reormsBeore the discovery o its vast hydrocarbon

reserves, dominated by natural gas, Qatar was a poor

country However, by 2006 Qatar had achieved one o

the world’s highest per capita gross domestic products

(Central Intelligence Agency, 2006)

Figure 111 shows that compared to its neighbors

in the Middle East, Qatar is a leader in natural gas

reserves Iran and Qatar have comparable amounts o

gas reserves Tis is because Qatar’s super giant North

Field and Iran’s super giant South Pars Field overlie onthe broad Qatar arch Te Qatar arch subdivides the

Khu ormations into two basins located northwest

(North Field) and south east (South Pars) Te North

Field reservoir boundary is the political boundary

between Iranian and Qatari waters as shown in Fig

112 (Note: Te names o the elds in Fig 112, at

times cause conusion Qatar’s North Field is north o

Qatar but south o the Iranian demarcation boundary

Te Iranian eld known as South Pars is actually

in southern Iranian waters but north o Qatar’s

North Field Te two elds constitute essentially a single geological structure, one o the largest gas

accumulations in the world)

0

100

200

300

400

500

600

700

800

900

1000

P r o v e n

R e s e r v e s ,

T c f

S y r i a

Y e m e n

O m a n

K u w a i t

E g y p t

I r a q

U A E

S a u d i A

r a b i a

Q a t a r

I r a n

Figure 1-11 Dominant natural gas producers in the

Middle East (ater EIA, 2006)




13

Al Manamah

Doha Persian Gulf

North Field

South Pars

Figure 1-12 The North Field extends o the coast o

Qatar and is divided rom Iran’s South Pars Field by a

political boundary

1-5.1 North Field Characteristics and Development

Te North Field is the largest nonassociated gas

eld in the world with estimated reserves o 900

c o gas AlSiddiqi and Dawe (1999) explain that

the North Field produces rom our intervals in the

Khu ormation Tese zones are Permian dolomite

carbonates located at depths o 10,000 to 13,000 t

with thickness ranging rom 1,300 to 2,000 t Te

gas produced is rich in condensates

Given the tremendous size o natural gas reserves,major investments or the production and transportation

o natural gas have ollowed

QatarGas was ounded 13 years ater the North

Field was discovered Eight years later, in 1992, the rst

customer, Chubu Electric o Japan, signed a sales and

purchase agreement (SPA) with QatarGas or 4 million

metric tons per year (Mta) o LNG wo years later,

Chubu Electric and other buyers signed a second SPA

or 2 Mta o LNG wo years later, in January 1997,

the rst LNG ship delivered gas to Japan Ecientproduction, processing, rerigeration, storing, loading

and shipping processes or LNG established by

QatarGas have allowed it to deliver 100 loads o LNG

to Japan every year since 1997 (EIA, 2007)

In October o 2002, BP signed an SPA with

QatarGas or 075 Mta o LNG to deliver to Spain

o exploit the tremendous demand or natural gas

in Europe, ExxonMobil signed an agreement with

QatarGas to deliver 15 Mta o LNG to the UK market

A year later, in June 2005, Shell signed a SPA or 78

Mta o LNG or Europe and North America Te

contracts or LNG have been progressively getting

bigger and bigger since the rst SPA with Japan

RasGas was ounded in 1993 In 1995, an SPA

with KOGAS, a Korean company, was agreed uponwo years later the SPA was increased to 49 Mta, and

in April o 1999 the rst LNG cargo let or Korea

Te delivery time o LNG to KOGAS was our years,

like the 4year delivery time between QatarGas and

Chubu Electric Also, an SPA with Petronet o India

was signed to deliver 5 Mta o LNG Te delivery

time or this order was ve years, and the rst LNG

cargo let or India in 2004 RasGas also signed a 25

year SPA or 35 Mta o LNG with Edison Gas o

the United States Te SPA agreement was altered toincrease the LNG volume to 46 Mta in 2003 RasGas

signed an agreement with ExxonMobil to deliver 156

Mta o LNG to the United States In February o

2005, an SPA with Distrigas o Belgium was signed to

deliver 207 Mta o LNG (EIA, 2007)

It is interesting to note the disparity in development

between Qatar and Iran Qatar and Iran have comparable

gas reserves Despite its sizeable gas reserves, Iran remains

a net importer o natural gas According to Wood et al (2006), Iran’s surging internal demand or natural gas and

sti gas market competition rom Russia and Azerbaijan will present Iranian leadership with dicult hurdles to

overcome in order to externally market those reserves

While Iran is relatively isolated politically, Qatar has

been busy orging relationships with the major natural

gas consumers such as Japan, the United Kingdom, and

the United States Te Qatari civil reorms, natural gas

resource development, and good political relationships

have culminated in its enormous success

1-6 Fracturing or the Ecient use o Existing Resources and or ncreasing

Recovery Factor

Since its advent in the 1950s, hydraulic racturing has

proven to be a very robust technology, lending itsel

to many dierent types o reservoirs Additionally,

although racturing is a very complex process, it

remains – or the most part – extremely orgiving o the



Modern Fracturing

14

industry’s overall general lack o expertise Tese two

actors have led to racturing becoming the most widely

used completion process

Fracturing has its roots rmly planted in the gas

production industry Even with the widespread use o

racturing or oil and injection wells, gas well racturing

is still the largest sector o the industry, by a wide margin(see Fig 113) Te majority o gas reserves in North

America are only produced as a result o hydraulic

racturing However, apart rom a ew specic locations

(such as China, Argentina, Australia and – to a lesser

extent – Russia), the global gas industry has ailed to

embrace this technology to even a raction o the extent

it is used in North America (see Fig 114)

Tight Gas

42%

Unconventional Gas

28%

Oil

25%

Other

5%

Figure 1-13 Targets o Fracture Treatments Perormed in

the USA in 2006 (BJ Services, 2006)

USA

70%

Canada

17%

Rest of the World

(excl. China) 13%

Figure 1-14 Estimated Proportion o Fracturing

Treatments Perormed in the USA and Canada,

compared to the Rest o the World, excluding China

(BJ Services, 2007)

One reason or this is the relative size, immaturity

and prolic productivity o the gas reservoirs outside

North America (see earlier discussions in this Chapter)

Another reason is that the USA is the only country in the

world where the landowners oten own the mineral rights

under their land In every other country, the government

controls the mineral resources and decides how they areexploited Consequently, in the US there is oten a very

ragmented approach to the depletion o a reservoir,

habitually concentrating on wellbore tactics, whilst

elsewhere gas companies are more inclined towards the

“big picture,” allowing more ocus on eld development

strategies Canada sits somewhere in the middle, having

inherited the British system o Crown ownership o all

mineral rights, while at the same time being heavily

infuenced by the activities o the US gas industry In

any case, small operators, eager to maximize shorttermcash fow, have always been the driving orce behind the

popularity o racturing in the US

Outside the US, Canada, China, Argentina and –

possibly – Russia, racturing has ailed to reach the “critical

mass” that has allowed the easy exploitation o its potential

in these countries Operating companies oten complain

that service companies do not have the inrastructure

and expertise necessary or the costeective execution o

racturing operations in a specic geographic area At the

same time, service companies complain that operators do

not provide enough work to economically justiy building up suitable equipment and personnel resources Tis is a

“Catch22” situation that can only be overcome by a)

eld development projects that are large enough to justiy

the introduction o a complete racturing operation,

and b) having an operating company (or companies)

with sucient condence in the racturing process to

proceed with racturingdependent eld development

Outside the abovementioned countries, there are very

ew companies with sucient institutional condence

in the racturing process to make this happen Evencompanies based in North America with considerable

experience in racturing seem to be unable to translate

this condence internationally

However, condence in the racturing process is

required i many countries and companies are to ully

exploit their gas resources It is hoped that the processes

and experiences described in this book will help

signicantly with this process




15

Ultimately, producing hydrocarbons rom a reservoir

comes down to ecient management o the pressure in

the reservoir Pressure, which is stored energy (or more

accurately, energy per unit volume), lies at the heart o

everything we do Te basic principle o hydrocarbon

production is the act that liquids and gases will move

rom a region o high energy (or pressure) to a region o low energy, i a fow path exists When we drill a well,

we are creating a region o low pressure at the wellbore,

and the conductive path is provided by the ormation’s

permeability I we are lucky, there is sucient energy let

in the liquids and gases to reach the surace, once they

have arrived at the wellbore In many cases, however, extra

energy has to be supplied via pumps or gas lit systems,

in order to achieve fow to the surace Ultimately, the

ecient production o a reservoir is all about getting the

maximum amount o oil and gas out, while using theminimum energy to do so

In gas reservoirs, it is dicult to provide extra energy

ater the gas reaches the wellbore Although the density o

the gas means that ar less energy is required to reach the

surace, oten there is insucient energy to produce the

gas at sucient rates

In its most basic orm, racturing can be thought o

as a process that minimizes the energy required or the

gas to reach the wellbore Tis has several benets:

1 It leaves more energy available or bringing thegas to the surace

2 It can reduce the minimum energy (ie pressure)

required in the reservoir to achieve economic fow

to the wellbore, thereby extending production

beyond reserve levels that might otherwise be

considered “depleted” In gas reservoirs, pressure

is reserves, and so minimizing energy losses during

production can signicantly increase the ultimate

recovery rom the reservoir

3 It minimizes secondary pressuredependent eectssuch as water production (and associated problems

such as scale deposition, nes migration and hydrate

ormation), retrograde condensation within the

reservoir, and nonDarcy fow

Fracturing eectively allows the wellbore to achieve

a signicant size in comparison with the reservoir

Tis allows the wellbore’s localized depletion to spread

ar urther into the reservoir, providing much greater

depletion at the drainage perimeter Tis eect can

be maximized i the racture azimuth is known Wells

can be drilled urther apart in the direction o racture

propagation and closer together in the perpendicular

direction, allowing maximum depletion o resources

Such a strategy signicantly reduces the localizedor “pinpoint” depletion caused by the wellbores

and spreads the eects o the depletion much more

evenly across the reservoir

Finally, it must be remembered that although

racturing can be very eectively used to redevelop a

mature eld (see Chapter 13), it reaches maximum

eectiveness when applied to a new reservoir:

1 Ater the racture azimuth has been obtained,

the placing o wells can be planned to allow orincreased drainage eciency in the direction o

racture propagation Tis could easily result in the

need or ewer wells

2 Wellbores can be planned to acilitate racturing

As discussed in Chapter 5, the wellbore can be

completed in such a ashion as to make racturing

easy and reliable (whereas the completion oten

does just the opposite) In addition, perorations

can also be planned to maximize the eectiveness

o racturing operations (see Chapter 6) O all

the things under our control, the perorations willhave the single biggest eect on the outcome o any

individual treatment Finally, multiple intervals can

be more eectively and eciently stimulated on new

wells than on existing wells (see Chapter 9)

3 Surace acilities also can be planned to acilitate

racturing, especially with regard to fuid recovery

and handling o returned proppant

4 Longterm relationships can be built between

operating companies and service providers Tis

allows or building and retaining experienceand expertise in both operational and technical

personnel Tis also improves project economics

due to eciencies o scale and a greater ability to

plan or the long term

Hydraulic racturing o gas wells is no longer a

luxury – instead, it is now a necessity For economic,

environmental and political reasons, operating



Modern Fracturing

16

companies and national operating companies have

an obligation to maximize the recovery rom their

resources, while doing this as eciently as possible

Tere is no question that hydraulic racturing will

continue to be a major tool or achieving these goals

Fracturing will only increase in importance as reserves

become more depleted and harder to exploitHydraulic racturing remains an inherently complex

process, and as a result is viewed with suspicion by many

resources owners and asset managers However, the

reality is that racturing is no more complex than any

number o widely accepted practices, such as drilling

deviated wellbores, perorming pressure transient

analysis, studying petrophysics and stimulating the

reservoir Yet these techniques are widely practised

and trusted throughout the world, whereas hydraulic

racturing remains a largely unexploited techniqueoutside o North America

Consequently, the authors o this book hope its

publication will have two proound eects First, we

hope this book will help to improve the techniques

and practices employed by those who are already

amiliar with hydraulic racturing Secondly, we hope

this book will increase the utilization o racturing

technology in reservoirs and geographic areas that

have hitherto ailed to appreciate the potential o this

reservoir development technique

Reerences

“Alaska Oil & Gas Report,” Alaska Department o

Natural Resources, Div o Oil and Gas, Anchorage,

Alaska (May 2006)

AlSiddiqi, A, and Dawe, RA: “Qatar’s Oil and

Gaselds: A Review,” Journal o Petroleum Geology

(October 1999) 22, 4, 417 Anchorage Chamber o Commerce: “Natural Gas and

Alaska’s Future,” 2005

BJ Services Company: Internal Marketing Inormation

(2006)

BJ Services Company: Internal Marketing Inormation

(2007)

BP Statistical Review, 2006

Canadian Association o Petroleum Producers (CAPP):

“Canadian Natural Gas, A stable Source o Energy

Supply,” 2006a

CAPP: “Canadian Crude Oil Supply and Forecast

20062020,” 2006b

Central Intelligence Agency: Fact Book, 2006

Energy Inormation Administration: Annual Energy

Outlook, 2007

Energy Inormation Administration, 2007 http://wwweiadoegov/pub/international/ieal/table18xls

Energy Tribune , Various articles, February, 2007

Hite, DM: “Cook Inlet Resource Potential ‘Missing

Fields’ Gas (and oil) Distributive/Endowment A

LogNormal Perspective,” presented at the South

Central Alaska Energy Forum, September 2006

Korneld, S: “Alaska North Slope Gas ask Force,”

Presentation to the US Department o Energy,

April 2002

Meyers, MD: “Alaska Oil and Gas Activities,”presentation to Te House Special Committee on

Oil and Gas, January 2005

Moscow Institute o Energy Research: “Russia’s Natural

Gas Future,” 2006 (in Russian)

Stringham, G: “Canadian Natural Gas Outlook,”

presentation by CAPP, October 2006

Williams, E, Millheim, K, and Liddell, B: “Methane

Hydrate Production rom Alaskan Permarost,

Final Report,” (March 2005)

Wood, D, Mokhatab, S, and Economides, MJ:

“Iran Stuck in Neutral,” Energy Tribune (December 2006)

Wood, D, Mokhatab, S, and Economides, MJ:

“Global rade in Natural Gas and LNG Expands

and Diversies,” Hydrocarbon Processing , 2007

wwwinteraxcom, 2006

wwwGazpromcom, 2007



Dr. Xiuli Wang is a petroleum engineer with BP in Houston, currently unctioning as a completion engineer with worldwide responsibilities She serves as the project leader o a major companywide project in injection well completions and sand control She has more than seven years o service with BP, rom work as areservoir engineer to ulleld modeling work She supported the completion team as a petroleum engineerdeveloping fux models and guidelines or minimizing erosion o producer well screens Finally, she was thelead production engineer or a major eld in the continental shel Beore immigrating to the United States

Wang earned a MS degree rom China’s premier technical university, singhua University, ollowed by sixyears o work with one o China’s major petroleum companies, Sinopec She joined BP ater earning a PhDin chemical engineering, with a number o proessional publications in the undamentals o multiphaseand complex fow through porous media She was recently eatured in a major journal as an exemplaryrepresentative o Chineseborn engineers employed by the US based petroleum industry In 2007, she wasnamed the US 2007 Asian American Engineer o the Year

Michael J. Economides is a proessor at the Cullen College o Engineering, University o Houston, and themanaging partner o a petroleum engineering and petroleum strategy consulting rm His interests includepetroleum production and petroleum management with a particular emphasis on natural gas, natural gastransportation, LNG, CNG and processing; advances in process design o very complex operations, andeconomics and geopolitics He is also the editorinchie o the Energy ribune Previously he was theSamuel R Noble Proessor o Petroleum Engineering at exas A&M University and served as chie scientisto the Global Petroleum Research Institute (GPRI) Prior to joining the aculty at exas A&M University,Economides was director o the Institute o Drilling and Production at the Leoben Mining University

in Austria Beore that, he worked in a variety o senior technical and managerial positions with a majorpetroleum services company Publications include authoring or coauthoring 14 proessional textbooks andbooks, including Te Color O Oil, and more than 200 journal papers and articles Economides does a widerange o industrial consulting, including major retainers by national oil companies at the country level andby Fortune 500 companies He has had proessional activities in over 70 countries



19

Chapter 2Natural Gas ProductionMichael J Economides, University o Houston and

Xiuli Wang, BP

2-1 ntroduction

Te natural gas we use in everyday lie - as a

source o space heating ater combustion, or power

generation even as industrial eedstock - is primarily

methane Such fuid has been stripped o higherorder

hydrocarbons Tis is not how natural gas appears just

one or two steps beore its ultimate use

At the present time there are two main sources or

natural gas as a petroleum production fuidFirst, gas is ound in association with oil Almost

all oil reservoirs, even those that insitu are above their

bubble point pressure, will shed some natural gas, which

is produced at the surace with oil and then separated

in appropriate surace acilities Te relative proportions

o gas and oil produced depend on the physical and

thermodynamic properties o the specic crude oil

system, the operating pressure downhole, and the

pressure and temperature o the surace separators

Te second type o gas is produced rom reservoirs

that contain primarily gas Usually such reservoirs areconsiderably deeper and hotter than oil reservoirs We

will deal with the production characteristics o these

reservoirs in this chapter

Tere are other sources o natural gas, one o which

(coalbed methane desorbed rom coal ormations) is

already in commercial use Tis process is described

in relative detail in Chapter 11 o this book In the ar

uture, production rom massive deposits o natural

gas hydrates is likely, but such eventuality is outside

the scope o this book

2-2 diosyncrasies o Dry Gas, Wet Gas

and Gas Condensates

Petroleum fuids ound in nature, are always multi

component mixtures o hydrocarbons Characterizing

these fuids is dicult both rom a scientic/laboratory

point o view and in production operations Tus,

petroleum engineers have traditionally examined oil

eld hydrocarbons in the context o phase behavior,

separating the mixture into liquid and gas Fig 21

shows a twophase envelope with a pseudocritical

point (C) separating the bubblepoint curve (AC)

rom the dew point curve (BC) at a constant

composition Emanating rom the pseudocriticalpoint are equal saturation quality curves (DC,

EC) inside the twophase envelope o the right o

the pseudocritical point is the maximum possible

temperature, called the cricondentherm

Natural gas reservoirs whose pressure and

temperature lie to the right o the cricondentherm

are known as “dry gas” reservoirs I fuids rom

these reservoirs stay outside o the twophase

envelope in traversing a pressure and temperature

path rom the reservoir to the wellhead,they will produce only dry gas

I the path rom reservoir to surace carries

the fuid into the twophase envelope – below the

cricondentherm – “wet gas” is produced

Figure 2-1 Phase diagram showing regions o retrograde

condensate

Between the critical point and the cricondentherm,

liquid emerges as the pressure declines below the dew

point value (at a constant temperature) rom point 1 to

point 2, shown in Fig 21 As pressure decreases rompoint 2 to point 3, the amount o liquid in the reservoir

increases Further pressure reduction causes liquid to re

vaporize Tis is the region o retrograde condensation

(McCain, 1973) Many natural gas reservoirs behave in

this manner During production rom such reservoirs, the

pressure gradient ormed between the reservoir pressure

and the fowing bottomhole pressure may result in liquid

condensation near the wellbore (Wang, 2000)



Modern Fracturing

20

One way to prevent condensate ormation is

to maintain the fowing well bottomhole pressure

above the dew point pressure Tis is oten not

satisactory because the reservoir pressure drop may

not be sucient to achieve economic production

rate An alternative is to allow condensate to orm

but occasionally to inject methane gas into theproducing well Te gas dissolves and sweeps the

condensate into the reservoir Te well is then put

back in production Tis approach is repeated several

times in the lie o the well It is known as gas cycling

(Sanger and Hagoort, 1998)

2-3 nfow rom Natural Gas Reservoirs

2-3.1 Fundamentals o Non-Darcy Flow

in Porous Media

Fluid fow is aected by the competing inertial and

viscous eects, combined by the wellknown Reynolds

number whose value delineates laminar rom turbulent

fow In porous media the limiting Reynolds number is

equal to 1 based on the average grain diameter (Wang

and Economides, 2004)

Because permeability and grain diameter are

well connected (Yao and Holditch, 1993), or small

permeability values (eg, less than 01 md) the production

rate is generally small, fow is laminar near the crucialsandace and it is controlled by Darcy’s Law:

− =

dp

dx k v

g

g

g

µ, (21)

where x represents the distance, p the pressure, v g

the gas velocity, μ g

the gas viscosity, k g

the eective

permeability to gas A small amount o connate water

is almost always present besides the gas Te water

saturation is oten small and it does not aect the gas

permeability signicantly Tereore, k g is oten equalto k , the singlephase permeability

NonDarcy fow occurs in the nearwellbore

region o highcapacity gas and condensate reservoirs

as the fow area is reduced substantially, the velocity

increases, inertial eect becomes important, and the

gas fow becomes nonDarcy Te relation between

pressure gradient and velocity can be described by the

Forchheimer (1914) equation:

− = +dp

dx k v v

g

g

g g g g

µρ β 2 , (22)

where ρ g

is the gas density and β g

is the eective non

Darcy coecient to gas Te condensate liquid may

fow i its saturation is above the critical condensate

saturation,S cc (Wang and Mohanty, 1999a) Additionalcondensate dropout because o the urther reduced

pressure will aggravate the situation Tereore,

two phenomena emerge NonDarcy eects and a

substantial reduction in the relative permeability to

gas Because o the radial nature o fow, the near well

bore region is critical to the productivity o a well

Tis is true in all wells, but it becomes particularly

serious in gascondensate reservoirs

Forchheimer’s equation describes highvelocity,

singlephase fow in isotropic media Many naturallyoccurring porous media are, however, anisotropic

(Wang et al , 1999) A direct understanding o

multiphase nonDarcy fow behavior in porous media

that are anisotropic at the porescale is studied elsewhere

(Wang, 2000, Wang and Mohanty, 1999b)

2-3.2 Transient Flow

o characterize gas fow in a reservoir under transient

conditions, the combination o the generalized Darcy’s

law (rate equation) and the continuity equationcan be used Tus:

φρ

ρ µ

∂

∂= ∇ ∇

t

k p , (23)

where φ is porosity, and in radial coordinates:

φρ

ρ µ

∂

∂=

∂

∂

∂

∂

t r r

k r

p

r

1. (24)

Because gas density is a strong unction o pressure (in

contrast to oil, which is considered incompressible),the real gas law can be employed:

ρ = =

m

V

pM

ZRT , (25)

and thereore

φµ

∂

∂

=

∂

∂

∂

∂

t

p

Z r r

k

Z rp

p

r

1.

(26)



Chapter 2 Natural Gas Production

21

In an isotropic reservoir with constant permeability,

Eq 26 can be simplied to:

φ

µk t

p

Z r r

p

Z r

p

r

∂

∂

=

∂

∂

∂

∂

1.

(27)

Perorming the dierentiation on the righthandside o Eq 27 - assuming that the viscosity and gas

deviation actor are a small unctions o pressure -

and rearranging gives:

φµ

kp

p

t

p

r r

p

r

∂

∂=∂

∂+

∂

∂

2 2 2

2

21.

(28)

For an ideal gas, c g

= 1/ p and, as a result, Eq 28 leads

to:

∂

∂ +

∂

∂ =

∂

∂

2 2

2

2 21 p

r r

p

r

c

k

p

t

φµ

.

(29)

Tis approximation looks exactly like the classic

diusivity equation or oil Te solution would look

exactly like the solution o the equation or oil, but

instead o p, the pressure squared, p2, should be used,

as a reasonable approximation

AlHussainy and Ramey (1966) used a ar more

appropriate and exact solution by employing the real

gas pseudopressure unction, dened as:

m p p Z

dp p

p

o

( ) ,= ∫ 2µ

(210)

where po

is some arbitrary reerence pressure (usually

zero) Te dierential pseudopressure, Δm( p),

dened as m( p) – m(

p

w ), is then the driving orce

in the reservoir

Using Eq 210 and the chain rule:

∂

∂=∂

∂

∂

∂

m p

t

m p

p

p

t

( ) ( ). (211)

Similarly,

∂

∂=

∂

∂

m p

r

p

Z

p

r

( ).

2

µ(212)

Tereore, Eq 29 becomes

∂

∂+

∂

∂=

∂

∂

2

2

1m p

r r

m p

r

c

k

m p

t

t ( ) ( ) ( ).

φµ (213)

Te solution o Eq 213 would look exactly

like the solution or the diusivity equation

cast in terms o pressure Dimensionless time is

(in oileld units):

t kt

c r

D

t i w

=

0 0002642

.

( )

,

φ µ

(214)

and dimensionless pressure is

pkh m p m p

qT D

i wf =

−[ ( ) ( )].

1424

(215)

Equations 213 to 215 suggest solutions to

natural gas problems (eg, well testing) that are

exactly analogous to those or an oil well, except

now it is the real gas pseudopressure unctions

that needs to be employed Tis unction is

essentially a physical property o natural gas,dependent on viscosity and the gas deviation

unction Tus, it can be readily calculated or

any pressure and temperature by using standard

physical property correlations

By analogy with oil, transient rate solution under

radial innite acting conditions can be written as:

qkh m p m p

T

i wf =

−[ ( ) ( )]

1638

t

k

c r st i w

+ − +

log log ( ) . .3 23 0 872φ µ

−1

,× (216)

where q is gas fow rate in Msc/d, pi

is reservoir

pressure, pw

is the fowing bottomhole pressure, φ is

porosity, c t is the total compressibility o the system,

and s is the skin eect

Equation 216 can be used to generate transient

IPR (Infow Perormance Relationship) curves

or a gas well

2-3.3 Steady State and Pseudosteady State Flow

Starting with the well known Darcy’s law equation

or oil infow,

qkh p p

Br

r s

e w

e

w

=−

+

( )

. [ln( ) ]

,

141 2 µ

(217)

and recognizing that the ormation volume actor,



Modern Fracturing

22

B, varies greatly with pressure, then an “average”

expression can be used as shown by Economides et al. (1994):

B g . ZT

( p p )/ e wf

=+

0 0283

2. (218)

With relatively simple algebra, and introducing thegas rate in Msc/d, Eq 217 and 218 yield:

p pq Z T

p p khe wf

e wf

− =+

141 2 1000 5 615 0 0283

2

. ( / . ) ( . )

[( ) / ]

µ

r

r

e

w

[ln( )) ],+ s×(219)

and, nally:

p p

q ZT

kh

r

r se wf

e

w

2 2 1424− = +

µ

[ln( ) ], (220)

which rearranged provides the steadystate

approximation or natural gas fow, again showing a

pressure squared dierence dependency

A similar expression can be written or pseudo

steady state:

qkh p p

ZT r

r s

wf

e

w

=−

+

( )

[ln(.

) ]

.

2 2

14240 472

µ

(221)

All expressions given thus ar in this chapter haveignored one o the most important eects in natural

gas fow: turbulence

One o the simplest and most common

ways to account or turbulence eects is

through the use o the turbulence coecient,

D , which is employed by adding a component

to the pressure drop, as shown below or the

steadystate equation:

) ] p p

ZT

kh

r

r s qe wf

e

w

2 2 1424

− = +

µ

[ln(

+µ ZTD

khq21424

, (222)

which rearranged, provides the wellknown:

qkh p p

ZT r

r s Dq

e wf

e

w

=−

+ +

( )

[ln( ) ]

.

2 2

1424µ (223)

Similarly, the same coecient can be employed

to the more rigorous expression using the realgas

pseudopressure As an example, or pseudosteady

state with q in Msc/d:

qkh m p m p

T r r s Dq

wf

e w

=−

+ +

[ ( ) ( )]

[ln( . / ) ].

1424 0 472 (224)

2-3.4 Horizontal Well Flow

Analogs to Eq 223 (or steady state) and 224 (or

pseudosteady state) can be written or a horizontal

well Allowing or turbulence eects, the infow

perormance relationships or a horizontal well in a

gas reservoir are or the steady state:

q k h p p

ZT AI h

L

I h

r I Dq

H e wf

aani ani

w ani

= −

++

+

( )

ln( )

2 2

14241

µ

,(225)

where

Aa a L

La =

+ −

ln( / )

/,

2 22

2

and or pseudosteady state:

qk h p p

ZT A I h L

I hr I

Dq

H wf

aani ani

w ani

=−

++

− +

( )

ln( )

2 2

14241

34

µ

,

(226)

where I ani

is a measurement o verticaltohorizontal

permeability anisotropy given by:

I k

k ani

H

V

= .

(227)

In Eqs 225 and 226, a is the large halaxis o the

drainage ellipsoid ormed by a horizontal well o length L Te expression or this ellipsoid is

aL r

L

eH = + +

2

0 5 0 252

40 5

. ./

.

0 5.

< 0 9. ,for L

2r eH (228)

where r eH

is the equivalent radial fow drainage radius




23

2-4 Eects o Turbulence

Te eects o turbulence have been studied

by a number o investigators in the petroleum

literature, pioneer and prominent among which

have been Katz and coworkers (Katz et al , 1959;

Firoozabadi and Katz, 1979; ek et al , 1962) Intheir work they suggested that turbulence plays

a considerable role in well perormance showing

that the production rate is aected by itsel: Te

larger the potential rate, the larger the relative

detrimental eect would be One interesting means

to account or turbulence was proposed by Swit

and Kiel (1962), who presented Eq 222, which

when rearranged gives Eq 223

Equation 223 is signicant because it suggests

that turbulence eects can be accounted or by a ratedependent skin eect, where the turbulence (at times

reerred to as the non-Darcy ) coefcient, D, has the

units o reciprocal rate One o the implications is

that in testing a highrate gas well, a calculated skin

eect must be construed as “apparent,” rather than the

real damage skin Among the procedures suggested

or testing test gas wells are multirate testing with

subsequent determination o apparent skins at

each rate, and straightline construction graphing

o s+Dq vs q Te graph allows eld determination

o s, the skin not aected by turbulence, romthe vertical axis intercept, and D rom the slope

(Economides et al , 1994)

2-4.1 The Eects o Turbulence on Radial Flow

Katz et al. (1959) have presented an explicit

relationship or the radial fow o gas into a well, using

natural gas properties and by providing correlations

or the coecient, β :

) ] p p ZT kh

r r

s qe wf e

w

2 2 1424− = +

µ [ln(

+

ZT r r

h

g

w e

123 16 101 1

−− βγ . ( ) ( )

22

2q , (229)

where

β =2 33 10 10

1 201

. ( ).

.k (230)

For an isotropic ormation, k equals the horizontal

permeability For an anisotropic ormation, k is dened

as the equivalent permeability,

k k

k

k

k k eq

V

H

V

H

H = −[ log( )]( ) ,11

3 (231)

where k V is the vertical and k H the horizontalpermeability

o demonstrate the eects o turbulence on

natural gas production, a number o calculations are

shown here, using the Katz et al (1959) approach or a

range o permeabilities able 21 contains the well and

reservoir data; able 22 presents the results

Table 2-1. Well and Reservoir Characteristics

pe

3000 psi Case 1 Case 2

r e

660 ft pwf

1500 psi 2500 psi

r w

0.359 ft μ 0.0162 cp 0.0186

h 50 ft Z 0.91 0.9

T 710˚R

γ g

0.7

Table 2-2 Turbulence Eect at Dierent Permeabilities

and Dierent Drawdowns

k,

md

Case 1: ∆ p = 1500 psi

q ( β =0, s=0)

MMscf/d

q ( β >0, s=0)

MMscf/d

q ( β >0, s<0)

MMscf/ds

1 3.0 2.9 8.1 -5.7

5 15.1 13.0 24.6 -5.1

25 75.3 51.9 71.7 -4.3

100 301.2 151.2 179.1 -3.7

k,

md

Case 2: ∆ p = 500 psi

q ( β =0, s=0)

MMscf/d

q ( β >0, s=0)

MMscf/d

q ( β >0, s<0)

MMscf/ds

1 1.1 1.1 3.7 -5.7

5 5.4 5.1 12.2 -5.1

25 27.0 23.0 37.9 -4.3

100 108.1 75.5 100.1 -3.7

Te rst two columns o able 22 show

the expected production rates or two fowing

bottomhole pressures, or laminar and turbulent

conditions, and or permeabilities rom 1 to 100 md

At low permeability, as expected, the rate reduction is

negligible; however, at 100 md and pw

= 1500 psi the

reduction is almost 50%



Modern Fracturing

24

urbulence eects, viewed as an apparent skin, result

in values o 03, 12, 34 and 75, or the 1, 5, 25 and

100md cases, respectively (and pw

= 1500 psi)

Because the range o 5 to 25 md is perhaps the

most likely to be encountered in emerging natural gas

elds, the ratio o actual to ideal (without considering

turbulence) rates is perhaps the most telling For the twodierent drawdowns, these ratios are 086 and 094 (or

the 5md case) and 068 and 085 (or the 25md case)

Tese results, plotted on Fig 22, show the eect o both

the permeability value and the drawdown Te ratio

between turbulenceaected production and production

calculated under the assumption o laminar fow declines

precipitously as reservoir permeability and drawdown

(and, hence, production rate) increase

1 10 100

q

a c t u a l

/

q

i d e a l

Permeability, md

0.4

0.5

0.6

0.7

0.8

0.9

1.0

% p = 500 psi

% p = 1500 psi

Figure 2-2 Turbulence eects or a permeability range

and dierent drawdowns

An additional interesting issue is the question

o the negative skin eect Tis will become even

more pronounced in a later section o this chapter

presenting the expected production rate rom

hydraulic racturing For now, a hypothetical negative

skin eect is used to represent, or example, matrix

acidizing o carbonate rock In able 22, the listed

negative skin eects result rom racturing What

will become apparent is that hydraulic racturing in

natural gas wells has a much larger eect than merely imposing a negative skin because o the extraordinary

reduction in turbulence eects

When turbulence eects are insignicant, the

negative skin eect is very large In the 1md case with

1500psi fowing bottomhole pressure, the production

ratio between the negative skin (57) and the zero skin

is nearly 3 Conversely, when the turbulence eects are

great, as in the 100md case, the production ratio between

the negative skin (37) and the zero skin is ar less (12),

but again, these production ratios do not paint the true

eect o racturing, which will be addressed later

2-4.2 Perorated and Cased Well

in a High-Rate Gas Reservoir

Te previous section deals with the fow reduction in

an openhole well and could also be considered as a

reasonable approximation or a slotted liner

For a cemented and perorated well, in the absence o

turbulence, a conguration skin eect can be envisioned

and added to the denominator o the deliverability

relations Karakas and ariq (1988) have published a

method to calculate this skin eect which depends on

the length o the peroration tunnel, the peroration

diameter, the phasing (degrees among adjoining planes o perorations) and, especially, the peroration density, ie,

how many perorations per unit net thickness, measured

in shots per oot (SPF) Tey also quantied the eect

o verticaltohorizontal permeability anisotropy: Te

lower the vertical permeability, the larger the value o

the skin eect would be Finally, they showed that i the

peroration tunnel lengths end outside a damage zone,

rather than inside, the composite damage/peroration

skin eect is substantially reduced

Using the Karakas and ariq (1988) model, one

important conclusion is that in a permeabilityisotropicormation without nearwell damage, 4 SPF o typical

tunnel length and diameter result in a peroration skin

eect equal to zero; ie, this conguration may be

construed as openhole equivalent

Ichara (1987) used a similar approach, constructing

a numerical model or a perorated natural gas well

and accounting or turbulence eects He showed that

perorations add a production impediment because o the

increase in turbulence Fig 23 presents some o Ichara’s

results, which show the eect o permeability anisotropy and peroration tunnel length One observation is that

long perorations are useul, making a well with reasonable

peroration density (4 SPF) near the perormance o an

openhole well (still aected by turbulence) From Fig

23 it can be concluded that a gas well with 4 SPF and

a typical 8in tunnel length in a sandstone reservoir

(k v /k

h= 01) will perorm at about 85% o an openhole

well Fig 23 is or 0˚ peroration phasing




25

0.6

0.7

0.8

0.9

1.0

1.1

1.2

0 3 6 9 12 15 18 21

Perforation Length, in

P r o d u c t i v i t y R a t i o

4 SPF

Δ p = 1500 psi

0º Phasing Angle

k v / k

h

1.0

0.10.01

Open Hole

Figure 2-3 Productivity ratio vs. peroration length or

k V / k

H= 0.01, 0.1, and 1.0 (ater Ichara, 1987)

Figure 24 (or an isotropic ormation) suggests

that improving the peroration phasing to 90˚ and,

especially, increasing the peroration density to 8 or

12 SPF may render the cased and perorated wellan even better perormer than an open hole Tis

is because o the penetration o the fow channels

beyond the sand ace In short, high peroration

density o longpenetrating tunnels will reduce

turbulence eects For example, rom Fig 24,

or 8 SPF o 18 in peroration tunnels, the well

perormance would be about 10% larger than that

o an openhole well

(Note: Ichara’s work assumes that all perorations

are open and undamaged Tis is o course rarely true

and the results presented here should be consideredan upper limit urbulence eects would be enhanced

with damaged or partly open perorations)

0.6

0.7

0.8

0.9

1.0

1.1

1.2

0 3 6 9 12 15 18 21

Perforation Length, in

P r o d

u c t i v i t y R a t i o

Open Hole

90º

Phasing Angle

0º

SPF

12

12

8

8

4

4

Figure 2-4 Eect o shot density and phasing angle on

productivity ratio or k V /k

H= 1

2-5 Production rom Hydraulically

Fractured Gas Wells

Hydraulic racturing has been established as the

premier production enhancement procedure in the

petroleum industry For the rst 40 years since its

inception, hydraulic racturing has been or primarily lowpermeability reservoirs; in the last two decades

it has expanded into medium to highpermeability

ormations through the tip screenout (SO) process

(see section 4732) For natural gas wells, a reservoir

above 05 md should be considered a medium

permeability reservoir Above 5 md it should be

considered a highpermeability ormation In all

highpermeability cases, the racture should be a SO

treatment (Economides et al , 2002a) Even in many

mediumpermeability ormations with relatively smallelastic moduli, SO is the indicated method

Valkó and Economides and coworkers as in Romero

et al (2002) introduced a physical optimization technique

to maximize the productivity index o a hydraulically

ractured well Tey call it the Unied Fracture Design(UFD, Economides et al , 2002a) approach Tey

introduced the concept o the dimensionless Proppant

Number, N prop

, given by:

N I C k x w

kx

k x w h

kx h prop x fD

f f

e

f f p

e p

= = =2

2 2

4 4

=

k V

kV

f p

r

2,

(232)

where I x

is the penetration ratio, C D

is the

dimensionless racture conductivity, V r is the reservoir

drainage volume, V p

is the volume o the proppant inthe pay (the total volume injected times the ratio o the

net height to the racture height), k

is the proppant

pack permeability, k is the reservoir permeability,

x e is the well drainage dimension, h is the ractureheight and h is the reservoir thickness Te proppant

permeability or gas wells will have to be adjusted

because o turbulence eects Tis adjustment will be

shown in a later section

Valkó and Economides also ound that or a

given value o N prop

there is an optimal dimensionless

racture conductivity at which the productivity index

is maximized



Modern Fracturing

26

(233)

2-5.1 Unique Needs o Fracture

Geometry and Conductivity

At “low” Proppant Numbers, the optimal conductivity,

C D

= 16 Te absolute maximum dimensionless

productivity index (see Section 451), J D

, is 6/π = 1909

(the productivity index or a perect linear fow in a square reservoir) When the propped volume increases

or the reservoir permeability decreases, the optimum

dimensionless racture conductivity increases somewhat

Valkó and Economides (1996) also presented

correlations or the maximum achievable dimensionless

productivity index as a unction o the Proppant

Number (see Eq 233 at the bottom o the page)

Similarly, correlations were presented or

the optimal dimensionless racture conductivity

or the entire range o Proppant Numbers (seeEq 234, at bottom)

Ater the optimal dimensionless racture conductivity

is known, the optimal racture length and width can be

readily determined rom:

xk V

C khw

C kV

k h fopt

f f

fD opt

.

opt

fD opt f

f

=

=

,

,

0 5

and

0.5

. (235)

where V

is the volume o one propped wing,

V

= V p / 2.

Te idea o using the maximized dimensionlessproductivity index to 1) design a hydraulic racture

treatment and 2) to evaluate the subsequent well

perormance against a benchmark and indeed any

other well conguration allows a generalized approach

to production engineering It becomes important to

rationalize substandard perormance and a constant

eort to “push the limits” (Economides et al , 2001)

2-5.2 Turbulence Remediation

in High- and Low-Permeability Wells

In the case o a potentially highrate natural gas well, the

eective proppant pack permeability used to calculate

the Proppant Number and the dimensionless racture

conductivity depends on the production rate because o the nonDarcy fow eects

Economides et al (2002b) presented an iterative

procedure combining the UFD method with the Gidley

(1990) adjustment to proppant pack permeability and

the Cooke (1973) correlations or fow in ractures

Te procedure starts with correcting the eective

permeability using the insitu Reynolds number by:

k k

N f e

f n

,

,

Re

,=+1

(236)

where k ,n

is the nominal racture permeability

First a Reynolds number is assumed A good rst

value is Reynolds number equal to zero Ten rom

Eq 232 and the adjusted proppant pack permeability

the Proppant Number is calculated rom which the

maximum J D

(Eq 233) and optimum dimensionless

conductivity (Eq 234) are calculated Te latter

allows the determination o the indicated racture

dimensions using Eq 235

From the dimensionless productivity index and

drawdown, the actual production rate is calculated, which in turn is used to obtain the Reynolds number

Te procedure ends when the assumed and calculated

Reynolds numbers are close enough

Te Reynolds number or nonDarcy fow

is given by

N k f n

Re

,,=

β νρ

µ(237)

J N N

D prop

prop

max

. . ln( )= −

1

0 990 0 5

N p−

−6 0 423 0 311exp

. .

π

r rop prop

prop prop

N

N N

−

+ +

0 089

0 667 0 015

2

2

. ( )

. . ( )1

if

if

N prop ≤ 0 1.

N prop > 0 1.

(234)C N fD opt prop, ( )=

1.6

N prop+

− +

+1 6

0 583 1 48

1 0. exp

. . ln

.1142ln N prop

N prop N

if N prop < 0 1.

10 N prop≤ ≤if 0.1

if prop >10




27

where k ,n

is the nominal permeability (under Darcy

fow conditions) in m2, β is in 1/m, v is the fuid

velocity at reservoir conditions in m/s, μ is the

viscosity o the fuid at reservoir conditions in Pa.s

and ρ is the density o the fowing fuid in kg/m3 Te

value o β is obtained rom:

β = ×(( )

,)

,

1 108 b

k f n

a(238)

where a and b are obtained rom Cooke (1973) Some

values are given in able 23

Table 2-3 Constants a and b

Prop Size a b

8 to 12 1.24 17,423

10 to 20 1.34 27,539

20 to 40 1.54 110,470

40 to 60 1.60 69,405

Te velocity, v, is determined as the volumetric fow

rate in the racture near the well divided by the racture

height times the racture width (both determined rom

the design in each iteration) For a detailed approach

and example see Economides et al (2002b)

able 24 presents the results or the racture

designs and expected production rates or the our

permeabilities used earlier or the nonractured wells

presented in able 22 Tese designs assumed sand as

a proppant with k = 60,000 mdTere are some very important implications

in comparing the results in ables 22 and 24

At 5 md the nonractured well would deliver 13

MMsc/d (with pw

= 1500 psi) I the racture

induced skin o 51 is assumed the production

rate would be 246 MMsc/d, approximately a two

old increase (see able 22) Tis production ratio

increase would be expected in an oil well fowing

under laminar conditions However, the implicit

reduction in turbulence eects (because o the fow prole modication in going rom converging radial

fow to racture fow) leads to a considerable urther

increase in the production to (in this example)

435 MMsc/d, a more than threeold increase

(see able 24) For higherpermeability wells, the

resulting olds o increase are similar, albeit in

actual production rates the achievable results are

spectacular (see Fig 25)

Table 2-4 Results rom Hydraulically Fractured Well

( k f = 60,000 md)

k, md sCase 1: p

wf = 1500 psi

q, MMscf/d k f,e

, md x f , ft

1 -5.7 13.1 9251 218

5 -5.1 43.5 7950 91

25 -4.3 160.3 6670 36

100 -3.7 524.0 5525 16

k, md sCase 2: p

wf = 2500 psi

q, MMscf/d k f,e

, md x f , ft

1 -5.7 5.8 12493 250

5 -5.1 18.9 10770 108

25 -4.3 69.2 8980 44

100 -3.7 224.0 7494 20

able 25 shows even more prolic ractured wells i

premium proppants are used (k

= 600,000 md),

“pushing the limits o hydraulic racturing” (Demar

chos et al , 2004)

Table 2-5 Results rom Hydraulically Fractured Well

( k f = 600,000 md)

k, md sCase 1: p

wf = 1500 psi

q, MMscf/d k f,e

, md x f , ft

1 -6.1 19.9 38300 375

5 -5.9 59.2 32050 182

25 -5.4 202.0 27110 75

100 -4.8 637.0 22410 35

k, md sCase 2: p

wf = 2500 psi

q, MMscf/d k f,e

, md x f , ft

1 -6.1 8.8 51600 456

5 -5.9 26.3 44150 211

25 -5.4 88.4 37020 91

100 -4.8 270.0 31720 41

In summary, turbulence aects are the dominant

eatures in the production o highpermeability (>5

md) gas wells urbulence may account or a 25 to 50%

reduction in the expected openhole production rate

rom such wells, i laminar fow is assumed Cased and

perorated wells may experience urther turbulence

induced rate declines, which can be alleviated somewhat

with longpenetrating peroration tunnels and large



Modern Fracturing

28

peroration densities (eg, 8 to 12 SPF) However,

nothing can compete with hydraulic racturing In

higherpermeability gas wells, the incremental benets

greatly exceed those o comparable permeability oil wells,

exactly because o the dramatic impact on reducing the

turbulence eects beyond the mere imposition o a

negative skin It is air to say that any gas well above5 md will be greatly handicapped i not hydraulically

ractured Indeed, pushing the limits o hydraulic

racturing by using large quantities o premium

proppants will lead to extraordinary production rate

increases (Wang and Economides, 2004)

1.0

10.0

100.0

1000.0

1 10 100

Permeability, md

q ,

M M s

c f / d

Fractured Well

(Premium) Fractured

Well

Negative

Skin Radial

Flow

Figure 2-5 Comparison o gas production rates rom non-

ractured wells, wells with negative skin and ractured wells

2-5.3 Multi-ractured Horizontal Gas Wells

As discussed in the previous section, in vertical gas wells

turbulence can be greatly reduced through hydraulic

racturing because the fow pattern (shown in Fig 2

6) through the hydraulic racture towards the well is

dierent than or radial fow (Wang and Economides,

2004) Te same is not necessarily true or transversely

ractured horizontal gas wells (see Section 102)

Because turbulence eects are enhanced in the latter

(due to the very small contact area between the well

and the racture), the conclusion is more nuanced Telimited communication between the transverse racture

and the wellbore generates an additional pressure drop

and a choking eect or all transversely ractured

horizontal gas wells Tis also increases turbulence,

which precludes application to essentially any well

whose permeability is 1 md or more and, perhaps, to

even much lower values o permeability, depending on

project economics (Wei, 2004)

Depending on the well orientation with respect to

the state o stress, either a longitudinal or a transverse

racture may be created in a horizontal well (Soliman

and Boonen, 1997; Mukherjee and Economides, 1991;

Soliman et al , 1999) Te longitudinal conguration

is generated when the well is drilled along the expected

racture trajectory Te perormance o such well isalmost identical to a ractured vertical well when both

have equal racture length and conductivity Tereore,

existing solutions or vertical well ractures can be applied

to a longitudinally ractured horizontal well (Economides

et al., 2002a; Soliman et al , 1999; Villegas et al , 1996;

and Valkó and Economides, 1996)

Radial ow

Hydraulically ractured vertical well

Figure 2-6 Confgurations o radial ow and ractured

vertical well

Almost all reported applications o ractured

horizontal wells are or transverse ractures (Crisby et al ,1998; Emannuele et al , 1998; Eirae and Wattenbarger,

1997; Minner et al , 2003; and Fisher et al , 2004) A

transverse hydraulic racture is created when the well

is drilled normal to the expected racture trajectory

(Valkó and Economides, 1996; Soliman et al , 1999;and Economides et al , 1994) Te conguration o a

transversely ractured horizontal well is demonstrated

in Fig 27 Te cross section o the contact between a

transverse racture and a horizontal well is 2 π r w w where

w is the width o the racture (which can be obtained by

using a design procedure such as the Unied Fracture

Design approach) and r w

is the radius o the horizontal

well In this case, the fow rom the reservoir into the




29

racture is linear; the fow inside the racture is converging

radial (Economides et al , 1994) Tis combination o

fows results in an additional pressure drop that can

be accounted or by a choke skin eect, denoted as s c

(Mukherjee and Economides, 1991) Te horizontal

well is assumed to be in the vertical center o a reservoir

(see Fig 27) and the fow is rom the reservoir into theracture and then rom the racture into the wellbore

(Mukherjee and Economides, 1991) Te produced fuid

enters the wellbore only through the racture, regardless

o whether the remaining part o the well is perorated

In this study, this assumption is also valid

Side view, uid ow rom reservoir to the racture

Top view, uid ow rom the racture to the wellbore

Figure 2-7 One transverse racture intersecting a

horizontal well

In the ollowing section, the theory and calculation

method or transversely ractured horizontal

gas well are described Ten some results and

discussions are presented

Calculation Method and Theory

or Transversely Fractured Gas Well

o study the perormance o a transversely ractured

horizontal gas well, it is essential to account or

turbulence eects, which are likely to be large

because o high gasfow velocity Economides et

al (2002b) have developed an iterative procedureto account or turbulence eects in a hydraulic

racture Te main steps and the correlations used

are described below

1 Assume a Reynolds number, N Re

, and

calculate the eective racture permeability k ,e

using Eq 236

2 Using k ,e

, calculate the Proppant Number, Nprop

, rom Eq 232

3 With N prop

, calculate the maximum productivity

index, J Dmax , and optimal dimensionless ractureconductivity, C Dopt

, rom Eq 233 and 234,

respectively

4 With C Dopt

, calculate the indicated optimum

racture dimension x opt

and w opt

rom Eq 235

5 With the known k ,e

and w opt

, calculate the choke

skin actor by:

skh

k w

h

r c

f w

=

−

ln .2 2

π

(239)

6 With the calculated J D,max

and s c , calculate the

dimensionless productivity index o transversely ractured horizontal oil well J

DTH (neglecting

turbulence eects or now), J DTH

:

J

J s

DTH

DV

c

=

+

1

1(240)

where J DV

is the dimensionless productivity

index o the ractured vertical well calculated

using the procedure described by Wang and

Economides (2004)7 With J DTH

and drawdown, the actual production

rate can be obtained using Eq 241 With this

production rate, a new Reynolds number N Re

can be calculated with Eqs 237 and 238, and

the fow velocity v obtained rom the cross

sectional area o fow

qkh p p

ZT J

wf

DTH =

−( ).

2 2

1424µ(241)



Modern Fracturing

30

8 Compare N Re

calculated in Step 7 with the

assumed N Re

in Step 1 I they are close enough,

the procedure can be ended I they are not, repeat

rom Step 1 until they are close enough

Te calculated results are optimum, which means

that at a given Proppant Number the dimensionless

productivity index is the maximum at the optimumdimensionless racture conductivity (Demarchos et al , 2004) However, this optimization oten must

be tempered by physical and logistical constraints

(Economides et al , 2002a)

o compare the perormance o ractured vertical

and transversely ractured horizontal gas well, the

Equivalent Number o Vertical Wells, X, is dened as:

X J

J

DTH

DV

= .

(242)

Assume the ormation permeability is the same

throughout and n transverse ractures are generated

intersecting a horizontal well (Fig 28) J DTHt

is the total

dimensionless productivity index (sum) or n transverse

ractures J DTH1

is the dimensionless productivity

index o one isolated zone or a transversely ractured

horizontal well Tereore:

J nJ DTHt DTH = 1. (243)

Figure 2-8 Multiple transverse ractures intersecting a

horizontal well

Results and Analysis or Formation Permeability rom 1 to 100 md

A case study is presented here or the multiple racturing

o a horizontal well in a gas reservoir with h = 50 t,

γ g = 07, reservoir pressure o 3000 psi and fowing

bottomhole pressure o 1500 psi

Assume a single transverse racture is generated in

the horizontal well and the mass o proppant is 150,000

lbm Proppantpack permeability, k

, is 600,000 md

Te details o the racture design are omitted here

What are presented are ractured well perormance

results, summarized in able 26

It should be noted that the skin choke eect,

s c

, (rom Eq 239) is inversely proportional to the

proppantpack permeability Tus, choosing highquality

proppant would decrease s c and benet the dimensionlessproductivity index, J

DTH (Eq 240), and the Equivalent

Number o Vertical wells, X (Eq 242)

Table 2-6 Results or k f = 600,000 md

k f = 600,000 md, 150,000 lbm mass,

single transverse fracture

k, md J DV

J DTH

w, in. sc

k f,e

1 0.739 0.121 0.35 4.64 1002

5 0.457 0.056 0.69 13.3 871

10 0.389 0.036 0.86 22.3 832

25 0.324 0.018 1.04 48.7 79450 0.288 0.013 1.48 69.1 783

100 0.255 0.009 2.07 100 774

Te results in able 26 show the value o J DTH

is

very small (compared to that o the vertical well, J DV

)

and decreases dramatically with increasing ormation

permeability It is obvious that turbulence eects

infuence the perormance o a transversely ractured gas

well so much that even with the most premium proppant

(permeability 600,000 md), the results are unacceptable

Te comparison o production between a racturedvertical well, a transversely ractured horizontal well and

laminar fow openhole well (the ideal case in Wang and

Economides, 2004) is summarized in Fig 29 Te top

solid curve (q v /q

ideal ) represents the ratio o the ractured

vertical well production to that rom a laminarfow,

openhole vertical well Te solid bottom curve shows

the ratio o a transversely ractured horizontal well (one

racture) with the same laminarfow, openhole vertical

well (q TH

/q ideal

) Results clearly show that because the

racture in the vertical well changes the fow pattern inthe nearwellbore area and alleviates the nonDarcy eect

the q v /q

ideal is considerably larger than 1 Conversely,

the q TH

/q ideal

is much smaller than 1 even at reservoir

permeability equal to 1 because o the choke skin and

nonDarcy eects Te dashed line in Fig 29 shows that

even with our transverse ractures, the productivity ratio

o a ractured horizontal well to an ideal openhole is still

less than 1 or permeability larger than 10 md




31

Would increasing the mass o proppant improve

the perormance? Te answer is no Te reason is

that the main actor that makes J DTH

so low is the

converging skin eect, s c , which cannot be reduced

by increasing the mass o proppant (see Eq 239)

For example, or the 1md ormation, doubling the

mass o proppant to 300,000 lbm (with all othervariables kept the same) increases the J

DTH only to

0122, almost the same as that or the 150,000lbm

mass case, where J DTH

is 0121

0

1

2

3

4

5

6

1 10 100

k, md

qv / q

ideal

4qTH

/ q ideal

qTH

/ q ideal

Open Hole

q

/ q

i d e a l

Figure 2-9 Turbulence eect on ractured vertical and

transversely ractured horizontal wells

Te conclusion rom this part o the study is thathydraulic racturing is essential or both stimulating

and reducing the strong turbulence eects in higher

permeability vertical gas wells, but the same is not

necessarily true or transversely ractured horizontal gas

wells ransversely ractured horizontal gas wells are not

attractive in terms o productivities or moderate and

higher ormation permeability (eg k > 1 md)

ResultsandAnalysis

orFormationPermeabilityrom0.01to10md A second study presents results or a much lower

permeability range (001 to 10 md) Designs assume the

use o 150,000 lbm mass o proppant with proppant

pack permeabilities o 60,000 md and 600,000 md

Drainage radius is 660 t

A single transversely ractured horizontal gas well is

calculated Te results are plotted in Figs 210 (a) and

210 (b) Te obvious trends rom these results are:

• Te J DTH

is smaller than J DV

when other parameters

are the same

• Te J DTH

decreases with increasing ormation

permeability regardless o proppantpack permeability,

as expected

• When reservoir permeability is less than 01 md,

proppantpack permeability has slight impact on s c • When reservoir permeability increases, s

c increases

and X decreases

Tese results urther suggest that or high and

even moderatepermeability reservoirs, a transversely

ractured horizontal gas well is not attractive because o

the production impediment rom turbulence eects and

converging skin eect For lowpermeability (k ≤ 05 md)

reservoirs, the results should be attractive i multiple

ractures intersecting a horizontal well are generated (andi the project economics are attractive)

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.01 0.1 1 10

k , md

J D V

a n d

J D T H

J DV

( k f =600,000 md)

J DTH

( k f =60,000 md)

J DTH

( k f =600,000 md)

J DV

( k f =60,000 md)

Figure 2-10 (a) JDV

, JDTH

, vs. k or dierent proppants

0.01 0.1 1 10 k , md

X

0

5

10

15

20

25

30

35

40

S c

X ( k f

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0

sc

( k f =60,000 md)

=60,000 md)

f c( k =600,000 md) s

X ( k f =600,000 md)

Figure 2-10 (b) sc, X vs. k or dierent proppants

Because J DV

and s c

are unctions o the mass

o proppant and proppantpack permeability, it is

worth perorming a parametric study to show the

eect o important reservoir and treatment variables

on J DTH

, J

DV and X



Modern Fracturing

32

mpact o Fracture Treatment Size

o nd the impact o the mass o proppant on J DTH

, a range

o proppant mass rom 75,000 to 300,000 lbm is used

Te proppantpack permeability used in this study is sand,

with permeability 60,000 md, and the drainage radius is

660 t Te results are summarized in able 27

Table 2-7 Impact o Mass o Proppant on X and JDTH

75,000 lbm

k, md J DTH

X sc

0.01 0.786 0.531 0.16

0.05 0.465 0.481 0.45

0.1 0.31 0.35 0.86

0.5 0.105 0.198 5.63

1 0.067 0.152 10.5

5 0.029 0.092 28.6

10 0.017 0.06 51.3

150,000 lbm

k, md J DTH X sc

0.01 1.075 0.589 0.08

0.05 0.345 0.487 0.43

0.1 0.323 0.294 0.91

0.5 0.106 0.162 5.66

1 0.067 0.127 10.6

5 0.029 0.08 28.7

10 0.018 0.058 51.3

300,000 lbm

k, md J DTH

X sc

0.01 1.42 0.755 0.19

0.05 0.314 0.518 0.46

0.1 0.332 0.235 0.95

0.5 0.107 0.138 5.691 0.068 0.116 10.6

5 0.029 0.071 28.9

10 0.018 0.053 51.3

It is apparent that increasing the mass o proppant

has impact on the results or the lowpermeability

(k ≤ 01 md) ormation but virtually no impact in higher

permeabilities Te reason is that increasing the mass

o proppant, while it may increase the dimensionless

productivity index, also increases the skin actor s c (see

able 27) Te one eect nullies the other Tus,there is no need to increase the mass o proppant A

modest treatment is sucient

mpact o the Number o solated Zones

on Equivalent Number o Vertical Wells, X

As mentioned earlier, or lowpermeability (k ≤ 05

md) reservoirs, racture stimulation results will not be

attractive unless multiple transverse ractures intersecting

a horizontal well are generated Tus, it is useul to study

how the number o isolated zones aects the Equivalent

Number o Vertical Wells

Assume the total drainage radius is 1320 t, the

proppantpack permeability k

is 60,000 md and mass

o proppant is 150,000 lbm Te number o isolated

zones and, thus, the number o transverse racturesintersecting a horizontal well vary rom 1 to 4

Te results, plotted in Fig 211, show that

when the number o transverse ractures is more

than our or low permeability (k < 05 md), X becomes more than 1, which makes transversely

ractured horizontal gas wells attractive Te lower

the ormation permeability is, the more attractive

the transverse racture conguration is (subject to

overall economic considerations) I the ormation

permeability is larger than 1 md, the transverseconguration does not appear attractive For

example, X is only 0280 or k = 10 md ormation

with our transverse ractures generated

X

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

1 2 3 4 5

n

k =0.01 md

k =0.05 md

k =0.1 md

k =0.5 md

k =1 md

k =5 md

k =10 md

Figure 2-11 Impact o number o ractures, n, on X

In summary, turbulence eects have a great

impact on transversely ractured horizontal gas wells

due to the small crosssection o the contact between

the well and the racture Although a vertical racturedgas well in the permeability range o 1 to 100 md

may perorm very well, turbulence eect procduce

in unacceptable results in transversely ractured

horizontal gas wells in the same permeability range

For low permeability (k < 05 md), the results are

attractive i a racture stimulation treatment generates

multiple ractures intersecting a horizontal well

However, i the permeability is larger than 05 md,

modern fracturing by uh

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