Stress TestingStefano Cantini Reservoir Engineer16January 20142Agenda Stress Test (Microfrac): Background Info and Applications Basics of hydraulic fracturing Overview of acquisition methods Case Histories UGS Nuclear Waste Deposit Shale Gas (Barnett) Heavy Oil23Microfrac vs Minifrac Microfrac (MDT stress test): provides breakdown and stress over a limited interval of pay or shale / salt (< 1m). It is run by wireline with the MDT or equivalenttools. Injection rate of the order of 1 liter/minute, with total injected fluid around 10-20 liters (few gallons). Minifrac: stress tests performed by a hydraulic fracturing crew. It is a pre fracturing calibration treatment and includes determination of closure pressure" over the height of the pay 10 to 30 m (10s to 100 of bbl are used at fracturing rate of 5 to 20 bpm).4Stress Test with MDT (Microfrac) With MDT Stress Tests in-situ minimum stress is determined from closing/reopening of an induced fracture. Estimate of breakdown pressure is also provided. Minimum stress is on the horizontal plane(except for very shallow wells/active tectonic areas) In general a fracture grows perpendicularly to the direction of minimum stress, so it is vertical5Applications of Minimum Stress KEY INPUT FOR GEOMECHANICAL MODELS Hydraulic Fracturing design UGS (Underground Gas Storage) design and caprock integrity CO2 sequestration Waste disposal sites Gas or water flood reservoir management Coal Bed Methane Steam Injection Wellbore Stability Sand Production6Geomechanic Assessment from Logs/CoresDetermination of Dynamic Elastic Modulus from Sonic and Density Logs:Poissons RatioYoungs ModulusDynamic modulus are converted into static through calibration with core dataTc is the compressional delta timeTs is the shear delta time is the density7Geomechanic Assessment from Logs/Cores Poissons ratio Biots constant pp Pore PressurevOverburden Minimum Stress (estimatedfrom sonic and density logs)Biots Poro-Elastic Model Overburden( )p p v hp p + =18Geomechanic Assessment from Logs/CoresIn Situ Minimum Stress Measurement from MDT MicrofracCalibration ofStress Profile derived from sonic and density logs9it is the contrast in stressesbetween layers that causes fractures to be contained in the reservoirStress Applications: Hydraulic Fracturing Design10Stress Applications: UGS Design Geomechanics is essentialfor UGS , especially if overpressured (injection pressure exceeding discovery pressure) . If an existing fracture/defect is pressurized above the minimum stress, it openscompromising caprock integrity, with gas leakage and possible reactivation of faults causing induced earthquakes Injection and withdrawal cycles induce expansion and shrinkage of reservoir with consequent caprock movement. Need coupled modelling approach: petrophysical and geomechanical data should converge into a model able to simulate the reservoir fluid dynamics , the stress strain rock behaviour and their interrelations. Example of coupled modelling outputs: maximum safe injection pressure volume changes stress-dependent permeability saturation-dependent rock strengths subsidence11WorkflowConventional approach: focus on reservoir Petrophysical andgeomechanical focus on reservoir AND caprock/overburden12Agenda Stress Test (Microfrac): Background Info and Applications Basics of hydraulic fracturing Overview of acquisition methods Case Histories UGS Nuclear Waste Deposit Shale Gas (Barnett) Heavy Oil1213Hydraulic Fracturing/In Situ Stresses213minimum stressoverburden14Hydraulic Fracturing/In Situ StressPiPi213overburdenminimum stress15Hydraulic Fracturing Theoryflowratetimepressure316Hydraulic Fracturing Theoryflowratetimepressure3317Hydraulic Fracturing Theoryflowratetimepressure3318Hydraulic Fracturing Theoryflowratetimepressure33Fracture Propagation19Hydraulic Fracturing Theoryflowratetimepressureminimum in-situ stress320Interpretation MethodolgyflowratetimepressureCLOSURE PRESSUREISIP ISIPREOPENINGPROPAGATION21Agenda Stress Test (Microfrac): Background Info and Applications Basics of hydraulic fracturing Overview of acquisition methods Case Histories UGS Nuclear Waste Deposit Shale Gas (Barnett) Heavy Oil2122Methods to Measure Stress Magnitude and Direction Direct Methods: Measure Fracture Open/Close Pressures Standard Extended Leak-Off Tests (ELOTs) Step Rate Tests Creation Of Large Hydraulic Fractures Micro-hydraulic fracturing technique (MDT) Sonic Logs Allow Inference of Stress Properties: shorizontal = F(Vp/Vs,sv), sh /sH inferred from Shear Wave Anistropy Stress Direction from Image analysis and Calipers (borehole ovality, breakouts, fault slip, drilling induced fractures) Core relaxation techniques23Direct Stress Measurement Methods LOT Stress measurements seldom made in oil industry: Tools usually conveyed on pipe, costly Uncertainty of initiating fracture in the preferred layer Wellbore storage effects Only one test possible per run Often doing FIT (Formation Integrity Tests) withoutfracturing MDT (MODULAR DYNAMICS TOOL) Wireline conveyed Several Tests during the same trip in the hole Test interval is 1 m., with rigorous depth control Downhole Pump reducing wellbore storage Real time control of injected rates and pressures24MDT MicroHydraulic Fracturing Overview Pressure record during a micro-hydraulicfracturingtestFracture initiationBreakdown pressuresquare root of the shut-in timeInduced fractures detected and oriented on FMI image25 MDT Dual Packerpositioned across the perforated interval 1 to 2 ft Perforation with HSD Gun , deep penetrating charges, 12 spf, phasedMDT Stress Test in Cased Hole Ensure cement bonding is good: USIT-CBLVDL26MDT Stress Test Environment Vertical wellbore (less than 30o deviation) or wellbore aligned with a principle stress Permeability under 100 md. Higher permeabilityneeds viscous fluids to fracture Pressure Differential under 5500 psi and temperature < 350degF Hole size limitations: ovality < 20%, low rugosity and borehole diameter in the range 6 8.5 (12 1/4) Open and cased hole27Agenda Stress Test (Microfrac): Background Info and Applications Basics of hydraulic fracturing Overview of acquisition methods Case Histories UGS Nuclear Waste Deposit Shale Gas (Barnett) Heavy Oil2728Case History 1: UGS Evaluate on UGS (Underground Gas Storage) in South West of France: Maximum safe pressure at which gas can be reinjected (reservoir pressure is above original to displace water; is actual reservoir pressure of 1.5 bar/10msafe?) Overburden Integrity through different pressure regimesGas ReservoirNorthSouth Aquiferous SandsMolasse (shalylimestones) overburdenGWCGround surface29Case History 1: UGS 31 valid MDT stress measurements allowed detailed characterization of field stress profilebetween 520 and 150m: Well LUG 34 (Cased): 4 tests attempted, 3 successful Well LUG 201(Open): 12 tests , 10 successful Well IZA203 (Open):11 tests , 9 successful Well LUG 202 (Open):12 tests , 9 successful30Case History 1: UGS, Test Example31Case History 1: UGS, Test Example32Case History 1: UGS, Test Example33Case History 1: UGS, Results 31 representative minimum stress tests from MDT in UGS field overburden (4 wells) In situ minimum horizontal stress gradient is 2.19 Bar/10 m, giving a large safety ratio Input for 3D geomechanical model: gas overpressure is converted into displacement and strain in the reservoir sands where the vertical effective stress is drastically decreased upon gas injection. The overburden appears moving up and down elastically in one piece, without stress or pore pressure change. The vertical uplift of the overburden is estimated in 20 cm after first injection, and 10 cm during the following summer injections. Same values occur in subsidence during winter withdrawals. Neither shear nor tensile failure occurs during cycles2.19 Bar /10 mreservoir pressure 1.5 bar/10m34 The complete state of stress is required as a necessary input for building the caverns in the shales and to assess their long term stability Detailed investigation on 150 m. thickness shale (Callovo-Oxfordien)and surrounding carbonate formationsCase History 2: Nuclear Waste Storage Site Design 35Case History 2 : MDT Induced Fractures 3003504004505005506006506 8 10 12 14 16Magnitude (MPa)Sigma hWeight of sedimentshVWeight of sedimentsDepthm.LimestoneLimestoneShaleVertical fracture (limestone)Horizontal fracture (Shale)366 inchesMDT vertical hydraulic fractures mined back duringthe sinking of the shaftsTOP VIEWCase History 2 : MDT Induced Fractures 37Case History 2: Test Example Fracture Closure=Minimum Stress38Case History 3: Barnett Shale Gas Extensive MDT stress test measurements for fracturing designEllenberger is the major source of water production in wells where hydraulic fracture growth is allowed to expand into the aquifer39Need to avoid steam release at surface!!Case History 4: Heavy Oil 40Case History 4: Heavy Oil In order to contain the steam , need to asses geomechanic behaviour of caprock: stress profile requiredCanada: more than 80 MDT stress tests job performed to this purpose so far