AFES Masterclass Overburden Correction. Evidence from Log Data

Transcription

1 AFES Masterclass Overburden Correction Evidence from Log Data Ken Russell Geomechanics & Sonic Scanner Support Europe Africa 19 th May 2010 Aberdeen

2 Mechanical Earth Model A numerical description of subsurface rock properties and stresses for use in predicting reservoir behaviour Elastic 10 0 Young s Modulus 100 Poisson s Ratio 1 Strength 0 Friction Angle70 20 UCS 400 Earth Stress & Pore Pressure Stress Direction h S Stress W N E MPa fault? PR E UCS F P p S h S H S V Regional Trend 2

3 Horizontal Stress Magnitudes A test that fractures the formation then measures Fracture Closure Pressure: Driller s Leak-Off Test Drill Stem Test mini-dst Direct Measurement Wireline Dual Packer mini-frac Breakdown pressure Propagation pressure Tensile strength Re-opening pressure Initial shut-in pressure Closure pressure σ h 3

4 Horizontal Stress Magnitudes Inferred from Image Data Minimum Stress Hydraulic Fracture σ θ max σ θ min σ r Pmud Pp Shear Failure Maximum Stress 4

5 HPHT- North Sea Central Graben Overpressure Mechanisms: Undercompaction - Trapped fluids - with increasing depth of burial Clay diagenesis Smectic to Illite - Illite occupies larger volume Hydrocarbon generation - Kerogen conversion to oil/gas with increase in temperature and pressure 5 Pressure Study of the North Sea Central Graben GeoPressure Technology

6 Effective Stress in HP Reservoirs σ x = σ x - α P p (α = Biot s constant) Normally pressured HP Reservoir Dept thσx Dept thσx P p σ x Pressure P p σ x Pressure 6 HP reservoirs have low initial effective stress

7 Compaction of weak carbonate (Ekofisk) Porosity Pore (fabric) collapse Threshold stress 1 MPa 10 MPa 100 MPa log(σ v ) 7

8 Acoustic Core tests Triaxial compression Jacket Acousto-elasticity: Non-linear relationship of stress to acoustic velocity Confining pressure Confining pressure 8

9 Earth Stresses Far-field vs. Wellbore σ h σ V σ t σ a σ H Effe ective Stresses (psi) SigT SigA SigR SigV SigH Sigh σ r Radius (in) 9 Far-field stresses: Vertical Min horizontal Max horizontal Wellbore stresses: Tangential Axial Radial After Tom Bratton

10 Stress Loading Acoustic Response Lab Stress Loading Wellbore Stress Loading Effective Stres sses (psi) SigT SigA SigR SigV SigH Sigh Shear Stress Radius (in) 10 After Tom Bratton

11 Calibration Points Dipole Radial profile of velocity Wellbore Stress Loading Effective Stress ses (psi) SigT SigA SigR SigV SigH Sigh Shear Stress Radius (in) 11 After Tom Bratton

12 Direct Estimate of S hmax & S hmin from sonic data Waveform data exhibiting crossover due to imbalanced stress is inverted for horizontal stress ratio. 12 Estimation of Formation Stresses Using Borehole Sonic Data Sinha et al, Paper F, SPWLA 49 th Annual Logging Symposium May 25-28, 2008.

13 Conclusion Understanding how changes in effective stress during production affect both porosity and permeability is essential to develop an effective drilling, completion and production strategy. 13 We can save 700 Lira by not taking soil tests

14 Compressibility from Core Phil McCurdy and Colin McPhee and from logs too

15 Why is compressibility important??? Hydrocarbon recovery Reservoir depletion causes increase in effective stress Pore volume compacts and adds energy to reservoir Pore volume compressibility used in material balance calculations Porosity and permeability reduction Reduction in porosity and permeability with increasing effective stress on depletion In-Situ Permeability Multiplier Productivity reduction in depleting reservoirs Inferred Reservoir Pressure, psi Compaction and subsidence (weak sands & HPHT) Compaction can lead to casing and tubular failures Compaction can lead to surface subsidence Compaction linked to compressibility 2

16 Compressibility terms and calculations Compressibility units 10-6 psi -1 referred to as microsips Grain compressibility, Cma or Cg Cg ~ microsips Bulk Modulus, K related to rock stiffness inverse of compressibility Bulk Compressibility, Cb K E 3(1 2ν ) Cbc constant pore pressure and changing confining pressure Vb Cbc = 1 Vb Pc Pp = Cbp - under constant confining pressure and changing pore pressure (depletion) Vb Cbp = 1 Vb Pp Pc Cb Cup for a microsip = 1 K Vb = bulk volume Pc= confining pressure Pp = pore pressure 3

17 Compressibility terms and calculations Bulk and Grain Compressibility Cbp = Cbc Cg As Cg is small in comparison, Cbc Cbp Pore Volume Compressibility, Cf (Dake) or Cp Cpc - isostatic pore volume compressibility under constant pore pressure and changing confining pressure Vp Cpc = 1 Vp Pc Pp Cpc Cbc φ Cpp isostatic pore volume compressibility under constant confining pressure and changing pore pressure (depletion) Vp Cpp = 1 Vp Pp Pc = Cg Cpp = Cpc Cg i.e. pore volume compressibility is 3 to 5 times higher than bulk compressibility As Cg is small in comparison, Cpp Cpc 4

18 Measurement Conditions Reservoir (Triaxial) three principal stresses uniaxial loading SCAL Labs isostatic loading radial stress = axial stress Rock Mechanics Labs biaxial loading radial stress < axial stress σ hmax = σ x Radial σ v = σ z σ hmin = σ y Axial 5

19 Compressibility terms and calculations Isostatic and Uniaxial Compressibility, Cpu uniaxial loading assumes reservoir formations behave elastically and are boundary constrained in horizontal direction assumes strain is entirely vertical assumes no tectonic strain during burial loading Reservoir has stiff lateral restraints Cpu defined as uniaxial pore volume compressibility under producing conditions (from Teeuw) α Cpu = Cpp 3 1 ( 1+ ν ) ( ) ν For example, Biot factor (α) = 1 and ν = 0.3 then Cpu = 0.62*Cpp 6

20 Typical Lab Presentation Cpu = α Cpp 3 1 ( 1+ ν ) ( ) ν Note neither α nor υ are measured! 7

21 Core Test Methods Direct Measure change in pore volume as a function of increasing effective stress σ ' iso = σ iso αp p Effective stress method SCAL labs Increase σ to increase σ Simulated depletion method SCAL labs Reduce Pp to increase σ Uniaxial (K0) Test Rock Mechanics labs Reduce p p to increase σ Instrument core to determine strains Indirect From E and υ from triaxial tests 8

22 Direct Measurements SCAL Lab Effective Stress Method SCAL lab method (porosity/ff at overburden) pore pressure constant, radial pressure increased effective stress increased by increasing confinement pore volume by squeeze-out 1 Cpc = Vp Vp Pc Pp 1 = Vp δvp δσ ' Simulated Depletion Method raise stresses and pore pressure to reservoir values total stress (Pc) constant Pp reduced depletion isostatic pore volume compressibility (SCAL) 1 Vp Cpp = Vp Pp Pc 1 δvp = Vp δσ ' 9

23 Uniaxial Ko Test Sample instrumented with axial and radial strain gauges Sample loaded to same total vertical (axial) and total horizontal (radial) stresses as in reservoir Pore pressure increased to reservoir value Pore pressure reduction vertical stress stays the same horizontal stress adjusted to maintain zero radial strain rock mechanics labs only uniaxial pore volume compressibility (K0) Cpu 1 = Vp Vp Pp σ h ε h = 0 ε radial= 0 Core Compaction p p 10

24 Example PV calculation SCAL data cf ( hyd ) 1 = Vp i ( Vp Vp ) i ( σ ' σ ' ) i d d Pore Volume (ml) Data Model Initial Reservoir Pressure Depleted Reservoir Pressure Effective Hydrostatic Pressure (psi) 11

25 Stress Hysteresis Effective Stress Method initial loading cycle microcracks in plug close higher pore volume reduction OK for φ stress correction Simulated Depletion Method extended loading cycle load to initial conditions (cracks close) GAUGE ROSETTE depletion stage (Cp from matrix pore volume compaction) more reliable pore volume compressibility data Uniaxial KO Method potentially most reliable data closest representation of stresses/pressures during depletion 12

26 Stress Hysteresis Example Pore Volume Compressibility (x10-6 psi -1 ) Suffix A: Effective Stress Method Suffix D: Simulated Stress Mrthod 1A 2A 3A 4A 5A 6A 7A 8A 1D 2D 3D 4D 5D 6D 7D 8D Effective Overburden Stress (psi) 13

27 Indirect Method Triaxial data Determine E and υ over equivalent deviatoric stress range associated with depletion Cbc Cg Cpc = φ Cbc = 1 K K = E 3(1 2ν ) 14

28 Compressibility from Logs DSI Logs DTS ( t s ), DTCO ( t c ) Obtain dynamic (elastic) moduli Poisson s Ratio, ν Shear Modulus, G (psi) Young s Modulus, E (psi) ( ts / tc ) 2 ( t / t ) 1 s c 1.34x G( 1+ν ) 1 ρ t b 2 s ρ b in g/cc t in µsecs/ft Bulk Modulus, Kb (psi) Bulk Compressibility, Cbc (psi -1 ) Pore Volume Compressibility, Cpc (psi -1 ) E K = 3(1 2ν ) 1 K b Cpc = Cbc φ Cg 15

29 Scaling Dynamic and Static Moduli Dynamic elastic and perfectly reversible Static (core) large strains irreversible Scaling static ε < dynamic ε E sta = E dyn ν sta = ν dyn 16

30 Compaction and Subsidence Compaction change in reservoir thickness (H res ) as a result of depletion (Geertsma) H = C m H res ( Pi Pfinal Compaction coefficient 1 1+ ν Cm = ( β )Cb ν β = Cg Cb Casing compressive strain ) Mud Line Depth, D Subsidence Compaction Thickness, H H ε = 0.5(1 + cos2θ ) c C m p S Subsidence (Bruno) = 2C (1 ν ) m [ ( ) ] H R + ( D + H ) + ( R + D ) p res res Reservoir Radius, R 17

31 Conclusions Common techniques for measuring compressibility and situations that they are most suited to 18

32 AFES Overburden Correction Masterclass Permeability Craig Lindsay Core Specialist Services Ltd.

33 Synopsis Permeability definitions Effect of overburden on permeability Other factors Examples -Unconsolidated core -Tight gas - High pore pressure Conclusions You can t squeeze blood from a stone or can you?

34 Permeability Definitions Fluid flow in porous media, units millidarcy(md) >50 md= good, 1 50 mdlow < 1m D = tight Routine Core analysis ambient or overburden air permeability (hydrostatic), e.g. Ka 75 md Special Core Analysis (SCAL) effective permeability at overburden (hydrostatic), e.g. Ko@Swi at 3000 psi OB Effective permeability single mobile phase e.g. Oil permeability at immobile water saturation (Swi) Relative Permeability 2 mobile phases

35 Permeability at Overburden O v e r b u r d e n P e r m e a b i l i t y Increased Decreased Ambient Permeability General expectation Increased overburden stress = reduced & more tortueous flow path = lower permeability Low ambient permeability = greater reduction at overburden

36 Permeability at Overburden including Fluid Effects O v e r b u r d e n P e r m e a b i l i t y Increased? Decreased?? Immobile fluid phase e.g. Swi, Sor, Sgr Endpoints, typically Ko@Swi Wettability Effect typically = > Overburden Variable and unpredictable effects Ambient Permeability

37 All Permeability's are Equal? BLAXTER SST. Average Ka 284 md Average Ko 269 md Average Kw 93 md

38 North Sea ClasticGas Reservoir In-situ Per rmeability, md Kw Combined impact of overburden & fluid system Non-linear relationships & Overburden >= Ka at ambient (Ka >1 md) Kw << low brine mobility Slip Boundary Condition In the presence of very small amounts of a wetting phase the permeability of a non-wetting phase can be > the absolute permeability of the rock. Ambient Air Permeability (Ka), md.

39 Special case: Unconsolidated core Shaken not stirred? Poor core handling at wellsite & during transportation Log porosity constant at 35% (+-1%). Overburden permeability ~ 80% ambient??

40 Special case: Tight Gas Rotliegend Sandstone Ambient Conditions: Average Ka = 0.1 md Average φ = 8% Overburden: Average Kg@Swi = md Average = 6.5% Cannot easily measured!

41 Special case: Tight Gas Rotliegend Sandstone Ambient Conditions: Average Ka = 0.1 md Average φ = 8% Overburden: Average Kg@Swi = md Average = 6.5% Cannot easily measured!

42 Special case: Tight Gas CER, Holditch & Assoc 1991

43 Special case: High Pore Pressure Gulf of Mexico: Depth > 20,000 ft Pore pressure > 20,000 psi Initial overburden 1,000 psi Permeability at e.g psi net stress & ambient pore pressure Pore pressure supports grains True permeability may be 5-25% (up to 50%) > Ambient (Shafer et al).

44 Conclusions Overburden has significant impact on permeability Overburden acts in conjunction with fluid effects must be accounted for too Effects variable and unpredictable must be measured not assumed Make sure lab is suitably equipped!

45 Thank you Any Questions?