Scope and Application of Pressure Transient Tests in CBM and Shale Gas reservoirs

1. Scope and Application of Pressure Transient Tests in CBM and Shale Gas reservoirs Baijayanta Ghosh Reservoir Domain Champion Testing Services, Schlumberger

2. Agenda • Brief introduction to Pressure Transients • With reference to Shale Gas/CBM reservoirs • The bigger picture • where Pressure Transient Tests fit in unconventional gas exploitation/reservoir management • Techniques and applications of Pressure Transient Analyses in – conventional and the latest • Shale Gas reservoirs • Coal Bed Methane (CBM) reservoirs • Conclusion • Hardware enablers for better Pressure Transient tests

3. Specially with reference to Shale Gas and CBM reservoirs Brief Introduction to pressure transient tests

4. Physics of Gas flow in SG & CBM reservoirs Coal Bed Methane Shale Gas Reservoirs

5. Pressure Transient Analysis (PTA)Deconstructed • Determine • Permeability (k) • Initial Reservoir Pressure (Pi) • Skin (S, D|Q|) • Detect • Boundaries

6. Type Curve Analysis vs.

7. Derivative Analysis Gringarten, 2011

8. Where Pressure Transient Tests fit in unconventional gas exploitation/reservoir management Applications of Pressure Transient Tests The Bigger picture

9. Workflow to optimize FDPin SG Reservoirs

10. Adsorption Isotherm and CBM Reservoirs

11. Pre-frac PTA Diagnostic Formation Integrity Test, Perforation Inflow Diagnostic Test Post-frac PTA G-function based Mini-frac, Flow-back & BU Test Advanced Interpretation Models Coupled Geomechanical model, non-linear adsorption Shale gas

12. Case Studies – Shale Gas • Pre-frac evaluation • Important to determine virgin conditions and to eliminate incomplete frac fluid clean-up effects • DFIT – Diagnostic Fracture Injection Test • Perforation Inflow Diagnostic test • Recommended to perforate large section in extreme underbalanced and using deep penetrating charges • Post-frac flow and BU • Determining p, k, S, Drainage Area to compute EUR • 9-stage frac horizontal well • Experience: drainage area =f(k) • forecast model for multi-stage frack well = sum(combination of n no of PSS single stage frack well model) • Recalibration of PTA kh using Production Analysis (Declien Curve)

13. Case Studies – Shale Gas • Comparison between DFIT & Conventional Analysis • Log-log derivative approach- example: stage frac horizontal well – inadequate, only kmin and xf|max can be determined • DFIT– sleek water fracture and closure analysis using G-function & independent after closure IFOT • Advanced Interpretation Models • Type curves for shale gas reservoir response with non-linear adsorption and Klinkenberg effect • Coupled geomechanical & reservoir flow simulator to models • Change in pore volume and permeability • Failure near the fracture and away from wellbore

14. Pre-frac & Pro-frac PTA Injection Falloff Test, Slug Test Advanced Interpretation Models Pressure dependent reservoir properties Coal bed methane

15. Case Studies – CBM IFOT • Injection at pressure > frac pressure • Reservoir simulator that models frac growth must be used to match pressure transient due to • fracture growth • leak-off • Accounting for free gas (above Critical Desorption Pressure) • System compressibility = • pore volume compressibility + • fluid saturation X fluid compressibility + • apparent compressibility due to gas sorption on the surface of the coal • The effects of sorption compressibility are shown to have a significant impact on skin but not on permeability. Neglecting sorption compressibility will lead to a more (-)ve skin value

16. Case Studies – CBM IFOT • Recommended practice • Otherwise use numerical reservoir simulator capable of handling • Effect of injecting at a pressure higher than coal fracture pressure • Unstable pressure before start of injection

17. Advanced interpretation:Pressure dependent properties • Pseudo-pressure based approach to model stress dependent porosity & permeability – rock mechanical parameters regressed • Uses Palmer & Mansoori model • Effect of stress dependent permeability – magnifies skin effect [Fig. 13] • Injection Falloff Tests done in Colorado, US – SDPP model [Fig.16] • For boundary detection both DST & IFOT recommended – stress dependent change in trend of derivative is opposite directions [Fig. 17 & 18] • Modeling gas block formed at reservoir pressures below critical desorption pressure • Skin effect – Hawkin’s equation • Radial composite behaviour

18. Applications in Production data Analysis Type curve used for Production Data Analysis needs to be of the corresponding model as determined from the flow regime analysis of PTA

19. Conclusion • Pressure Transient Tests have become more integrated to the engineering workflow • We have the interpretation tools for sufficiently accurate transient analysis • We are getting better – more robust models that handle detailed complex phenomena • We have the hardware enablers • High resolution electronic gauges • Downhole shut-in devices

20. bghosh@slb.com Thank you