reservoir engineering aspects of unconventional reservoirs

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Presentation at SPE GCS Reservoir Study GroupHouston, TX (USA) — 27 October 2011

Reservoir Engineering Aspects of Unconventional ReservoirsTom Blasingame — Texas A&M University (27 October 2011)

Reservoir Engineering Aspects ofUnconventional Reservoirs

Tom BLASINGAMEDepartment of Petroleum Engineering

Texas A&M UniversityCollege Station, TX 77843-3116 (USA)

[email protected]


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+1.979.845.2292 — [email protected]


Brief Biography — Tom Blasingame


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Short Bio: Blasingame

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Start-Up





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Start-Up: Unconventional Reservoir Systems are DIFFERENT●Flow mechanisms

(…nano-scale flow mechanisms)●Fluids are often at critical state

(…uniqueness? cause?)●Overpressure is good

(…stresses are poorly understood)●Clean-up phase must be managed

(…no "choke cowboys")●Shales are generally dessicated

(…low water recovery)●Fracture patterns poorly understood

(…rock mechanics)●Hydrocarbon liquid production

(…artificial lift)●Simultaneous Exploration/Development

(…fast track)●"Completions can't Trump Geology"

(…but we try)●Unconventional Plays are "Statistical"

(…lots of wells)●Stimulation → Multi-Fracture Horizontal Wells

(…future?)

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Overview





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Overview:●Orientation●Nano-Scale Flow Behavior●Time-Rate Analysis: (aka Decline Curve Analysis)●Time-Rate-Pressure Analysis: (aka Production Analysis)●Practical Aspects●Challenge Points for Unconventional Reservoirs●Questions/Discussions

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Orientation





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Orientation: Reservoir Engineering Aspects of UR

●Where we want to be: (or so we think)■ Fit for purpose stimulation

(... oil/gas/condensate/geology)■ Effective reservoir monitoring

(... this is important!)■ Early EUR (months? years?)

(... prediction/correlation)■ Minimum Well Spacing

(... geology + PVT + modeling(?))

●How do we get there…■ Better understanding of flowback/dewatering

(... optimization)■ Pressure-dependent properties…

(... k, FcD, desorption?)■ Understanding of the pore-scale

(... what flows/when/how)■ Petrophysics

(... conventional petrophysics not adequate)■ PVT

(... oil/gas/condensate/water — HP/HT)

●Facts of life…■ Analogs

(... need to understand uncertainty (very high))■ EUR

(... minimum of 12-18 months for high confidence)■ IP

(... may be uncorrelated with EUR)■ Early Productivity

(... poor wells don't get better)■ Time-Rate Analysis

(... not representative? (chaotic operations))

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How Small is Small?(nano-scale pores)





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Pore-Scale: Nelson Pore/Molecule Size ChartQuestion(s):●How small are pores in shale gas? Note

that the size of the pores is on the order of 5-10 times the size of the fluid molecule.

AAPG Bulletin, v. 93, no. 3 (March 2009)Pore-throat Sizes In Sandstones, Tight Sandstones, and ShalesP.H. Nelson, USGS

Perspective:●The concept of pores and

pore throats begins to break down at these scales.

●The flow path can be as small as 10-20 molecular diameters (or less).

Issues:●How do the fluids move?

— Darcy flow?— Dispersion (gases)?— Knudsen flow?

●How are the fluids stored?— In the organic matter?— Adsorbed?— Another mechanism?

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Pore-Scale: Shale Pore Space (Barnett Example)Question(s):●Where is/are the gas/liquid stored?

There is porosity, often in the organic materials.

●Why is the phase behavior of many shales "near critical"? Nanopores?

J. Sedimentary Research, v. 79/12 (2009)Morphology, Genesis, and Distribution of Nanometer-scale Pores in Siliceous Mudstones of the Mississippian Barnett ShaleLoucks, R.G., R.M. Reed, S.C. Ruppel, and D.M. Jarvie

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a. Gas Slippage — Kundt, A. and Warburg, E.: "Über Reibung und Wärmeleitung verdünnter Gase, " Poggendorfs Annalen der Physik und Che-mie (1875), 155, 337.

Pore-Scale: Petrophysics/Permeability

c. Microflow model and correlation, "fully implicit" formulation.b. Knudsen "microflow" model (Modified from Karniadakis and Beskok, 2002).

SPE 107954Improved Permeability-Prediction Relations for in Low Permeability SandsF.A. Florence, Occidental Petroleum Corp., J.A. Rushing, Anadarko Petroleum Corp., K.E. Newsham, Apache Corp., and T.A. Blasingame, Texas A&M U.

vx

vy

Gas Flow

1 /11

41

1

1 4tan

15128

121

2121

0

0

4.0

01-

2

aa

m

aa

m

aa

m

a

kp

ak

pak

pa

kk

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Time-Rate Analysis





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Time-Rate: Schematic Production Performance Plot

●Discussion: Schematic Production Performance Plot ■ The schematic represents the most common approach to EUR.■ Used CAREFULLY, this should be valid (other methods needed).

Loga

rithm

of R

ate

Production Time

Hyperbolic Rate

Exponential Rate

"Switch Point"from Hyperbolicto Exponential Economic Limit (in rate — qlimit)

= Estimated Ultimate Recovery (EUR) [The area under the hybrid (hyperbolic- exponential rate curves]

Economic Limit (in time — tlimit)

(tlimit)

(qlimit)

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Time-Rate: Flow Regimes — Multi-Fracture Horizontal Well

Discussion:●Very high permeability case (30 md?) → blame graduate student.●Exhibits various flow regimes.●Note square-root time approximation … poor man's EUR (too high).

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1:4 Slope (low FcD)

1:2 Slope (high FcD)

1:1 SlopeDepletion

(SRV?)

Compound Linear Flow Regime

Formation Linear Flow Regime

TransitionRegime

Bilinear FlowRegime

EllipticalFlow Regime

Logarithm of Production Time

Loga

rithm

of P

rodu

ctio

n R

ate

])(1[/)( )/1( bii tbDqtq

Early-Time Regimes are HYPERBOLIC?

Discussion:●1:2 Slope → b=2 (HIGH fracture conductivity) .●1:4 Slope → b=4 (LOW fracture conductivity).●This is a schematic, it overly simplifies the system.

][1/ )( tatq LFLF ][1/ )( 4 tatq BLFBLF

Time-Rate: Flow Regimes — Multi-Fracture Horizontal Well

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Modified Hyperbolic Rate Relation:

Decline Function: D(t)

Hyperbolic Function: b(t)

Function: (t)

Time-Rate: Modified Hyperbolic Rate Relation

)( ]exp[

)( )1()(

limexp,

/1hyp,

t*ttDq

t*ttbD

q

tq

i

bi

i

dtdq

qtD 1)(

constant )(

1 )(

tDdtd

tb

Discussion:●qDb functions are DIAGNOSTIC.●D(t), b(t), and (t) are evaluated continuously (at all points).●This shale gas case exhibits b=2 behavior → q = a√t (Linear Flow).●Appears to be "hyperbolic," but this is just the Linear Flow portion.

)( 1)( tDtdtdq

qtt

Logarithm of Production Time

Loga

rithm

of P

rodu

ctio

n R

ate,

q(t)

Loga

rithm

of D

(t) a

nd

b(t)

q(t)

D(t)

b(t)

1:2 slope

b(t) ≈ 2

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PLE Rate Relation:

Decline Function: D(t)

Hyperbolic Function: b(t)

Function: (t)

Time-Rate: Power-Law Exponential Rate Relation

]ˆexp [ˆ)( nii tDtDqtq

)1(ˆ

1)(

nitDnD

dtdq

qtD

nn

i

i ttDDn

nDn

tDdtd

tb

2)1( ] ˆ[

)1(ˆ

)(1

)(

)1(ˆ ni tDnD

)( 1)( tDtdtdq

qtt Discussion:

●qDb functions are DIAGNOSTIC.●Power-law exponential relation is derived from:●No direct analog to hyperbolic case.●This is a "tight gas" reservoir case.

Logarithm of Production TimeLo

garit

hm o

f D(t)

and

b(

t)

q(t)

D(t)

b(t)

Loga

rithm

of P

rodu

ctio

n R

ate,

q(t)

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Discussion:●EUR is computed at each "time step" (cumulative or incremental).●EUR estimated for each model.●EUR comparison plots probably min/max trends.

d. CEUR master summary plot (all results).

Time-Rate: Continuous EUR

a. Continuous EUR (CEUR) process plots.

b. CEUR hyperbolic, PLE, and q-Gp summary plots.

c. CEUR governing equations.

])(1[/)( )/1( bigi tbDqtgq

]ˆexp [ˆ)( nigi tDtDqtgq

]/[ )( ig i DqGpGiDg iqtgq

[hyperbolic]

[PLE]

[qg(t) vs. Gp(t)]

From: SPE 132352 (2009) Continuous Estimation of Ultimate Recovery, S. Currie, D. Ilk, and T. Blasingame, Texas A&M U.

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Time-Rate-Pressure Analysis





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Theory:● Palacio and Blasingame [1993]● Mattar and McNeil [1997]● Agarwal et al [1999]

Advantages:● Straightforward and intuitive.● Shut-in pressures NOT required.● Direct estimation of contacted

gas-in-place.

Limitations:● Boundary-dominated flow regime

must exist.

"Flowing Material Balance" Plot:

a. The "Flowing Material Balance" (Normalized Rate-Cumula-tive Function Plot) formulation is derived using the solution for the diffusivity equation during boundary-dominated flow regime. This formulation provides a direct estimate of the contacted gas-in-place using time, flowing wellbore pres-sure, and flowrate data.

Time-Rate-Pressure: Flowing Material Balance Plot

JCPT (June 1997), 52-55.The 'Flowing' Gas Material BalanceL. Mattar and R. McNeil, Fekete Assoc.

Question(s):●What is the "Flowing Material Balance" plot? In

simple terms, pwf(t) data are "converted" to pavg(t) data using the pseudosteady-state flow equation, then plotted as a straight-line extrapolation function and "solved" for gas-in-place.

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b. "Transformed" data shows fractured well response at early times, very strong evidence of closed system at late times.

a. Raw (daily) rate and pressure data — bottomhole pressures are calculated, note the effect of liquid loading.

(1/4) )(

g

ppss,g

transg q

Gm̂

qpm

(1) )(

g

ppss,g

bdfg q

Gm̂

qpm

Boundary-DominatedFlow

TransientFlow

Question(s):●Can the well-reservoir model be inferred

from such data? Yes.● Is diagnosis sufficient? No, we must

also be able to model/history match data with a model (complete process).

Time-Rate-Pressure: Material Balance Time

? )(1 )/1( bi

gig

tbD

qq

SPE 25909 (1993)Decline Curve Analysis Using Type Curves — Analysis of Gas Well Production DataJ.C. Palacio and T. Blasingame, Texas A&M U.

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Time-Rate-Pressure: Flowback AnalysisSPE 135607A Comprehensive Workflow for Early Analysis and Interpretation of Flowback Data from Wells in Tight Gas/Shale Reservoir SystemsD. Ilk and S.M. Currie, Texas A&M U., D. Symmons, Consultant, J.A. Rushing, Anadarko Petroleum, N.J. Broussard, El Paso, and T.A. Blasingame, Texas A&M U.

a. Crossplot — All wells: qg versus qw.

c. Crossplot — All wells: p2/qg versus t.b. Crossplot — All wells: pcf versus t.

[DI]

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)ln()ln(

)(, dtdq

qt

tdqd

tcpq

Time-Rate-Pressure: Time-Rate DiagnosticsSPE 135616Hybrid Rate-Decline Models for the Analysis of Production Performance in Unconventional ReservoirsD. Ilk and S.M. Currie, Texas A&M U., D. Symmons, Consultant, J.A. Rushing, Apache, and T.A. Blasingame, Texas A&M U.

a. -derivative — Holly Branch Wells.

)ln()ln()(, dt

dqqt

tdqd

tcpq

)ln()ln()(, dt

dqqt

tdqdtcpq

b. -derivative — "Shale Gas Field C" Wells. c. -derivative — All Models (Holly Branch Well).

[DI]

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"Shale Gas Field C"

ˆ

ˆ nEUR

Time-Rate-Pressure: Integration of ResultsSPE 140556 (2011)Integration of Production Analysis and Rate-time Analysis via Parametric Correlations — Theoretical Considerations and Practical ApplicationsD. Ilk, DeGolyer and MacNaughton, J.A. Rushing, Apache, and T.A. Blasingame, Texas A&M U.

a. Correlation plot — kcal versus kmeas.

b. Correlation plot — EURcal versus EURmeas.

Horizontal well with multiple vertical fractures:

)1(ˆ1)( ni tDnD

dtdq

qtD

] ˆ e xp [ ˆ)( nii tDtDqtq

Power-Law Exponential Relations:

"Shale Gas Field C" ˆˆ d

ici

b qDnak

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Practical Aspects





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Practical Aspects: Stimulation

Discussion:● Maximizing SRV (Stimulated Reservoir Volume)

■Build Complexity →Slickwater■Build Conductivity →Hybrid/Gel Systems

● Future Stimulation Challenges:■Can we "rubblize" the reservoir?■Can we "pulverize" the reservoir?■Can we do this with little or no water?

"You only produce from what you frac …"Anonymous

Individual Fractures fromIndividual Perforation Clusters

Complex Fractures fromIndividual Perforation Clusters Project Rulison (1971)

Stimulation using Atomic Weapons

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Practical Aspects: Clean-Up

Discussion:● Build choke profile slowly (2/64ths every 2-3 days).● Hold choke constant once rates peak.● For HPHT systems, start at 10-12/64ths, build to 12-16 64ths.

100 10110-5

102 103

10-4

10-3

Total Fluid Material Balance Time, days

Tota

l Flu

id P

rodu

ctiv

ity In

dex,

STB

/D/p

si

1:2 slopeFormation Linear Flow

(very high fractureconductivity)

Clean-upBehavior

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Dimensionless Time

Dim

ensi

onle

ss P

ress

ure

Practical Aspects: Pressure Transient Analysis

Discussion:● Multiple BHP pressure transient test data sets (5 wells).● Apparent 1:4 slope → low conductivity vertical fractures.● Data are "normalized" (dimensionless) in pressure drop and time.

Apparent 1:4 slope for pressure drop andpressure drop derivative functions

LOW CONDUCTIVITY VERTICAL FRACTURES.

Nearby wellis being stimulated.

1:4 Slope

Nearby wellis being stimulated.

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Challenge Pointsfor Unconventional Reservoirs





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Challenge Points: "What Keeps Me Up at Night…"

●What we REALLY know…■ Tight gas is relatively easy

(... vertical wells, HP/HT, PVT)■ Gas shales are technically viable as a resource

(… economics?)■ Horizontal multi-fractured wells

(… (now) taken for granted)

●What we THINK know…■ The fracture geometry is

(... planar? complex? who cares?)■ The phase behavior

(… can be extremely complex)■ The ptf to pwf conversion(s)

(... early-time heavy water load?)

●What we may NEVER know…■ Distribution of natural fractures

(... impossible?)■ Transport of gas/liquids in shales

(... via organic matter?)

●What we SHOULD KNOW in the near future…■ Duration of data required for EUR

(... more is always better)■ Better understanding of phase behavior

(... not "conventional")■ Optimal well spacing/orientation/placement

(... do this early!)

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Challenge Points: "Conventional Wisdom…"●Estimated Ultimate Recovery (EUR)?

■ Early EUR?

(... can this be meaningful?)■ EUR = f(t)?

(... how do we incorporate this?)■ Well Spacing?

(... is this really the holy grail?)

●QUANTIFYING reservoir properties?■ Pressure Transient Analysis

(... ultra-low k ... issues?)■ Production Analysis

(... ptf may not be sufficient)■ Petrophysical analysis

(... theory ≠ application)

●Liquids-Rich Systems?■ Fluid-Flow Mechanisms

(... what is really flowing where?)■ PVT

(... near-critical fluids are not trivial)■ Improved Recovery

(... we all know this is coming)■ Fit-for-Purpose Stimulation

(... higher FcD, more complexity)■ Artificial Lift

(... fact of life)■ Recovery

(... low to extremely low primary recovery?)

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●Never-Ending Arguments…■ SRV

(... what is it, really?)■ Desorption

(... significance? timing? relevance?)■ Stimulation Fluids

(... where does it go? does it matter?)■ Microseismic

(... crystal ball, roulette wheel, or roadmap?)■ Pressure-Dependent Whatever

(... so what?)■ Natural Fractures

(... if/when/why/what?)■ Dual Porosity/Dual Permeability

(... what about the physics?)■ Well Placement/Effect of Layering

(... when does it matter?)

●Things that SHOULD help…■ Production Logs

(... but just a snapshot in time)■ Optimal Proppant Design/Placement

(... obvious, but)■ Stimulation Stages/Perforation Clusters

(... geology + logs)

●Things that DEFINITELY WOULD help…■ Measured pwf

(... yes, this is my favorite song)■ Downhole Fluid Sampling

(... sooner or later)■ Horizontal Core

(... why not?)

Challenge Points: "Everybody Has An Opinion"

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Challenge Points: "My Crystal Ball Tells Me…"●EUR:

■ Time-Rate Analyses

(... may not be sufficient)■ Time-Rate-Pressure Analyses

(... requires reservoir model)■ Constraints

(... e.g., 15 years seems reasonable)

●Reservoir Modeling: (i.e., simulators)■ Present

(... conventional models with modifications)■ Near-Future

(... fundamental flow kinetics, complex geometries)■ Distant-Future

(... pore-scale phenomena, nano-scale PVT, ?)

●Reservoir Engineering Tools:■ Material Balance Methods

(... not applicable at reservoir-scale)■ Pressure Transient Tests

(... surprisingly good in cases [need k])■ Production Analysis

(... very good in cases [need good ptf data])■ Reservoir Fluids

(... very complex, near-critical liquids)■ EOR

(... not sure where to start — CO2, lean gas, ???)■ Ad-hoc Tools

(... e.g., Linear flow analysis — lack resolution)

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End of Presentation



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reservoir engineering aspects of unconventional reservoirs

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