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SubsurfacePressureand StressStressStress force / areaUnits Pascals, psi N/m2 kg/(m . s2)Force 50 kg * 9.8 m/s2Area 1 cm2 0.0001 m2Stress 4,900,000 Pascals686 psiGeol493KHow much is 1 Pascal? 1 Pa 0.00014 psiStress/pressure What is the difference? Car tire 230,000 Pa 1 Megapascal (1 MPa) 1 million Pa 1 x 106 Pa 145 psiOn-in convention: xyON the x-planeIN the y-direction Pressure - Scalar quantity Stress - Tensor quantity Pressure – in fluids Stress – in solid bodies2D Stress TensorStress Tensor zzNormal andShearcomponentsz zx xz xz xx xx zx zzx

Lithostatic stress/ hydrostatic stress Lithostatic stress Tectonic stress Fluid Pressure-Hydrostatic-HydrodynamicLithostatic Stress Due to load of overburden Magnitude of stress components is the same in alldirectionsgravity Lith ρ.g.zdepthdensityPressure/stress gradientsLithostatic Stress Gradient Lith ρ.g.zRock density 2000 to 3000 kg/ m3 Lith 22 to 26 Mpa/km1 psi/ft2500 kg/ m3 x 9.8 m/ s2 x 1000 m 25 MPa/km 1 psi/ft10 Mpa/km0.43 psi/ftWhat about horizontal stress?Horizontal Stress Rocks are elastic solidsHorizontal stress depends on rock compressibilityPoisson’s Ratio eyy/exx incompressible material (no volume loss) Rocks 0.1 to 0.3Hor [ ρ.g.zIf Hor ρ.g.zIf Hor ρ.g.zHorizontal stress is about 1/3 of vertical ifrocks are lithified

Comparison of vertical andhorizontal stressesWorld Stress Map – Hor directionρgzρgzSuppe, 1985VerticalMin horizontalBorehole Wall Breakouts and StressPressure/stress gradientsHorizontal Stresses Min Max22 to 26 Mpa/km1 psi/ft Max10 Mpa/km0.43 psi/ft MinCaliper log measures the shape of the boreholeOverpressure more than hydrostaticWhat about these?Why morethan 0.43?Spindletop, 1901Deepwater Horizon, 2010

Shale CompactionShale Porosityvs. Depth CurveShale compacts aswater is expelledSchmoker and Halley, 1982Over Pressure ZoneWell 1Well 2Well 3Pressure barrierSand ASand BSand CPressure sealinferred to exist inregion of highfluid pressuregradientReservoirCompartmentsFluid pressure as a function of depth, Cook Inlet Alaska - Hunt, 1990Characteristics of OverpressureZones Under-compacted shaleLow density, low sonic velocityRapid Drilling RateLow Thermal Conductivy, high TLow SalinityControlling Reservoir Pressure Drilling Mud Casing Blow-out preventers

Blow Out PreventersHow to measureformation pressureFormation TesterSchlumbergerHow to measure reservoir pressure?Drill Stem TestDrill Stem Test (DST)2. Initial Flow Period (IFP)removes effect ofmud filtrate“supercharge”3. Initial Shut-in is usedto determine a reservoirPressure (ISP)Structural Dip 1o NortheastFinal Flow Period (FFP)is used to collect a fluidsample and create apressure disturbancebeyond any damagedzone.Final Shut-in is usedto test permeability,production rate andwell damageDrill Stem Test Pressure ResultsReservoir PressureReturn to MudPres.Initial ReservoirPressureLeduc Fields,Alberta (Canada)Open ValveInitial Shut InPre-Flow ( 1 hour)FlowLeduc #1 Gas flare

Hydrodynamic SystemPhase Behavior for a singlehydrocarbonPhase Behavior of Gas-Oil MixSupercriticalfluidReservoirLiquidCritical PointBubble Point100%LiquidPressurePressureCritical pointGas75%50%25%Stock TankSupercriticalfluidSeparatorLiquid Gas5%GasTemperatureTemperatureSupercritical: density like a fluid, viscosity like a gasPhase Behavior of Wet Gas 5%25%50%GasSummary SeparatorTemperature ρ.g.zHydrostatic gradient 0.43 psi/ft or 1 Mpa/kmLithostatic gradient 1 psi/ft or 25 MPa/kmOvepressure forms when water cannot escapePressure gradient defines reservoir compartmentsArtesian aquifers can produce anomalous PMethods to control P during drillingHydrodynamic reservoirs lead to tilted oil-watercontactsPhysical state of hydrocarbons depends on P-T regimeBringing oil to the surface cools and depressurizes it

2D Stress Tensor x z xx xx zz zz xz xz zx zx. Lithostatic stress/ hydrostatic stress Lithostatic stress Tectonic stress Fluid Pressure-Hydrostatic-Hydrodynamic Lithostatic Stress Due to load of overburden Magnitude of stress components is the same in all

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