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Subsurface Pressure and Stress Subsurface Stress Pressure Stress = force / area Units = Pascals, psi and Stress = N/m2 =kg/(m . s2) Force= 50 kg * 9.8 m/s2 Area = 1 cm2= 0.0001 m2 Stress = 4,900,000 Pascals Geol493K 686 psi How much is 1 Pascal? Stress/pressure • 1 Pa= 0.00014 psi •What is the difference? • Car tire ~230,000 Pa •Pressure - Scalar quantity • 1 Megapascal (1 MPa) = 1 million Pa •Stress - Tensor quantity = 1 x 106 Pa = 145 psi •Pressure – in fluids •Stress – in solid bodies On-in convention: Stress Tensor 2D Stress Tensor ON the x-plane xy IN the y-direction Normal and zz Shear components zx z xz xz xx xx zx zz x Lithostatic stress/ hydrostatic stress Lithostatic Stress • Due to load of overburden • Magnitude of stress components is the same in all •Lithostatic stress directions •Tectonic stress •Fluid Pressure gravity -Hydrostatic -Hydrodynamic Lith= ρ.g.z depth density Pressure/stress gradients Lithostatic Stress Gradient Lith= ρ.g.z Rock density= 2000 to 3000 kg/ m3 22 to 26 Mpa/km 1 psi/ft 3 2 Lith= 2500 kg/ m x 9.8 m/ s x 1000 m 10 Mpa/km ≈ 25 MPa/km 0.43 psi/ft ≈ 1 psi/ft What about horizontal stress? Horizontal Stress Hor= [ ρ.g.z If Hor ρ.g.z Rocks are elastic solids If Hor ρ.g.z Horizontal stress depends on rock compressibility Poisson’s Ratio e /e | yy xx Horizontal stress is about 1/3 of vertical if incompressible material (no volume loss) rocks are lithified Rocks = 0.1 to 0.3 Comparison of vertical and World Stress Map – direction horizontal stresses Hor ρgz ρgz Suppe, 1985 Vertical Min horizontal Borehole Wall Breakouts and Stress Pressure/stress gradients Horizontal Stresses Min Max 22 to 26 Mpa/km 1 psi/ft Max 10 Mpa/km Min 0.43 psi/ft Caliper log measures the shape of the borehole Overpressure = more than hydrostatic What about these? Why more than 0.43? Spindletop, 1901 Deepwater Horizon, 2010 Shale Porosity Shale Compaction vs. Depth Curve Shale compacts as water is expelled Schmoker and Halley, 1982 Over Pressure Zone Well 1 Well 2 Well 3 Pressure barrier Sand A Sand B Sand C Pressure seal inferred to exist in region of high fluid pressure Reservoir gradient Compartments Fluid pressure as a function of depth, Cook Inlet Alaska - Hunt, 1990 Controlling Reservoir Pressure Characteristics of Overpressure Zones •Drilling Mud • Under-compacted shale •Casing • Low density, low sonic velocity •Blow-out preventers • Rapid Drilling Rate • Low Thermal Conductivy, high T • Low Salinity Blow Out Preventers How to measure formation pressure Formation Tester Schlumberger How to measure reservoir pressure? Drill Stem Test (DST) Drill Stem Test 2. Initial Flow Period (IFP) removes effect of mud filtrate “supercharge” 3. Initial Shut-in is used to determine a reservoir Pressure (ISP) Structural Dip 1o Northeast Final Flow Period (FFP) is used to collect a fluid sample and create a pressure disturbance beyond any damaged zone. Final Shut-in is used to test permeability, production rate and well damage Initial Reservoir Drill Stem Test Pressure Results Pressure Return to Mud Leduc Fields, Reservoir Pressure Pres. Alberta (Canada) Open Valve Initial Shut In Pre-Flow (~1 hour) Flow Leduc #1 Gas flare Hydrodynamic System Phase Behavior for a single Phase Behavior of Gas-Oil Mix hydrocarbon Supercritical fluid Reservoir Supercritical Critical Point fluid Liquid Bubble Point Critical point 100% Liquid Separator 75% 50% Gas Liquid + Gas Pressure 25% 5% Pressure Stock Tank Gas Temperature Supercritical: density like a fluid, viscosity like a gas Temperature Phase Behavior of Wet Gas or Summary Condensate = ρ.g.z • Hydrostatic gradient = 0.43 psi/ft or 1 Mpa/km • Lithostatic gradient= 1 psi/ft or 25 MPa/km • Ovepressure forms when water cannot escape 100% Reservoir • Pressure gradient defines reservoir compartments Liquid conditions • Artesian aquifers can produce anomalous P 75% 25% • Methods to control P during drilling Gas 50% • Hydrodynamic reservoirs lead to tilted oil-water Pressure Separator contacts • Physical state of hydrocarbons depends on P-T regime • Bringing oil to the surface cools and depressurizes it Temperature.
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