Gas Shale Hydraulic Fracturing, Enhancement. Ahmad Ghassemi

Transcription

1 Gas Shale Hydraulic Fracturing, Stimulated Volume and Permeability Enhancement Ahmad Ghassemi

2 Tight Gas A reservoir that cannot produce gas in economic quantities without massive fracture stimulation treatments Fracture length 0.5 Drainage radius 2

3 Stimulation by Multiple Fractures Multiple horizontal wells are drilled to increase more contact area between the well and the reservoir Wells are stimulated in multiple stages (1-10 per well, depending on need) Frac lengths ft S H S v S h

4 Multiple Well Stimulation In a typical stage, water and sand are pumped p at a rate of bbls per min. Large volumes of pumped water per stage (100,000-1,000,000 gal) Horizontal Wells Stage 1 Stage 1 Stage 2 Stage 2 S v S h S H Stage 3 Stage 3 Z-Frac S-Frac

5 For Successful Stimulation Need to Consider Formation Characteristics Fabric & structure Rock permeability, strength & rheology, etc Pre-existing fractures & their properties Stress Regime Magnitude and orientation, distribution

6 Key Rock Parameters in HF Outcome Young s modulus Mineral composition, texture, Pressure & Temp dependent Poisson s ratio Natural fractures r r 0 2 L 0 L 2 6

7 Low Poisson ss Ratio, High Modulus Desirable C o III C Modulus, E 20.7 GPa; 3x10 06 psi Poisson s ratio, v /3C 0 II B C o = uniaxial compressive strength E B = yield point (very difficult to determine) Elastic Ductile Brittle I Irreversible changes Post-peak behavior (large perm. deformations) 7

8 Rock Failure Enhances Permeability (Brittle, Ductile) De Paola et al

9 In-situ Stress Controls fracturing: Fracture propagate parallel to S Hmax Orientation Height growth Complexity Stress variation With production impacts conductivity 9

10 Damage Propagation in Normal Regime Normal (Iso-surface of 20% damage) Normal Regime S H,max < S h,max < S v S v SH,max S h,min (minimum)

11 Damage Propagation in Thrust Regime Thrust (Iso-surface of 20% damage) Thrust Regime S v < S h,min < S H,max S v (minimum) S H,max S h,min

12 Barnett Shale, Very Successful HF H 43 h 39 MPa 56 V p 28 NF spacing 3 ft 8 hrs of pumping at 72 bbls/min Reservoir permeability, k (darcy) 10-4 Fluid density 3 f (kg/m ) 1000 Rock density r (kg/m 3 ) 2300 Fracture half height, H f (m) 60 Fracture half length, X f (m) 300

13 HF, NF, Stress Contrast 3 1 Bons (2000) 13

14 A portion of HF consisting of two sub-parallel branches Courtesy R. Jeffry

15 In Addition to Stress, Fracture Growth/Containment is Controlled by Discontinuities Frac fluid density Permeability Local fracture growth controlled by fabric Globalfracture l growth controlled by stress Bons (2000) 15

16 In Gas Shale Stimulated Volume Results from Network generation by pore pressure & stress change: Rock failure Critically-stressed fractures Need to stimulate large volume with minimum energy HF Spacing

17 Mechanical Fracture Interaction HFs propagate perpendicular to the least principal stress unless: Out of zone growth encouraged 17

18 3D Numerical Analysis of Stress, and Failure for Multiple HF- (Barnett Shale Simulation) Fracture aperture distribution (in cm) after 3 hours of pumping.

19 Pore Pressure Distribution p 28 M Pa Distribution ib ti of pore pressures (MPa) in the formation after 3hrs of injection 19

20 In-Situ Stress (S yy ) Increases S 39 MPa yy h S xx ; previously Max. 20

21 Effective Stress is Reduced, Promoting Rock Failure Distribution of the maximum and minimum (iii) principal effective stresses in the reservoir (MPa).

22 Shear/Tensile Failure Failure is caused by high stresses near the tip of the fractures, & by increased pore pressure by leakoff 22

23 Estimating Stimulated Volume MEQ s are believed to be caused by pore pressure induced shear slip around HF Accepting this, the SRV (volume of failed rock ) is assessed using the areal extent of the MEQ cloud This is based on the assumption that energy release is exclusively related to fluid penetration, which may not always hold true s 1 s 3 23

24 Prediction of Enhanced Permeability The usual methodology for predicting the permeability in the failed region is a trial and error procedure: (i) Guess a value for permeability (ii) Predict the failed rock volume (FRV) using the stress analysis for a selected net fracture pressure (iii) Vary perm until the FRV matches the extent shown by induced d seismicity i it for the given net fracturing pressures 24

25 Issues It is assumed Mode I fracture propagation dominates, but MEQ monitoring shows shear dominates Event location problems Feedback between failed rock and HF Aseismic deformation with significant slip and perm enhancement Rock heterogeneity and time dependent bh behavior of rock andfractures not considered d 25

26 Concluding Remarks HF is an effective method for producing gas from tight sand a and shale Sustaining fracture conductivity remain an important concern Improved understanding of the rock mechanics will improve the economics Water usage Sand usage Minimize seismicity 26

27 Acknowledgement Partial Financial Support by RPSEA is Appreciated 27