Application of X-Ray CT for Investigation of CO 2 and WAG Injection in Fractured Reservoirs

1. Application of X-Ray CT for Investigation of CO2 and WAG Injection in Fractured Reservoirs D. Chakravarthy, V. Muralidharan, E. Putra and D.S. Schechter Texas A&M University

2. CO2 for EOR Water flooding - secondary recovery CO2 Flooding – Both for secondary and tertiary recovery Understanding CO2 flow is complex Extreme heterogeneities like fractures

3. Martin Co Borden Co Midland Co Glasscock Co Upton Co Reagan Co Spraberry Trend Area Fractured Reservoir An NFR with extensive fractures OOIP 10B bbls Ultimate recovery < 12%

4. Martin Co Borden Co Midland Co Glasscock Co Upton Co Reagan Co CO2 Flood in Spraberry ET O’Daniel CO2 Pilot CO2 flood not successful in mobilizing oil Fluid flow through fractures Early breakthrough, Oil bypass

5. Understand physical mechanisms of oil bypass Improve prediction of sweep in NFR Improve modeling of tracer studies Significance Fracture Studies Better understand multiphase flow in fractures Better performance prediction and improved recovery!

6. Extreme heterogeneities Common heterogeneities Injection rates Fluid mobility Fluid flow in reservoirs Fluid Flow

7. X-Ray CT Scanner Objectives • How is a highly mobile fluid like CO2 affected by heterogeneity? • What happens in the presence of fractures? • How does injection rate affect oil recovery and sweep? • Can WAG improve sweep efficiency in a fractured system? What other solutions exist?

8. Workstation 3D CT image X-Ray source Digital detector Object Principles of X-Ray Tomography

9. X-ray CT scanner • CT number • Depends on density • Based on X-Ray attenuation • Every material has a characteristic CT number • E.g. CT for air = -1000

10. Experimental Setup

11. CO2Phase diagram Experimental Outline • Displaced Fluid - SoltrolTM Refined Oil • Doping agent – 1-iodohecadecane • Displacing Fluid - CO2 • Pressure - 800 psig • Temperature - 75° F • CO2 phase – Vapor

12. CO2 Phase diagram 1070 psi Critical Point Solid Liquid Pressure (Not to scale) Triple Point Temperature (Not to scale) 100 psi Vapor 89°F -70°F

13. CO2 Phase diagram Dense Vapor Solid Liquid Pressure (Not to scale) Liquefies Does not liquefy Temperature (Not to scale) Vapor

14. Immiscible displacement CO2 Phase diagram Solid Liquid Pressure (Not to scale) Temperature (Not to scale) Vapor

15. Unfractured Cores Fractured Cores Viscosified water Injection Rates Gel Experiments CO2 injection

16. Low density High density Unfractured Core (a) Dry core scans Red color indicating regions with higher CT numbers (b) Oil saturated core

17. Injection Rate = 1 cc/min 3 minutes of CO2 injection 5 minutes of CO2 injection 15 minutes of CO2 injection 60 minutes of CO2 injection

18. Uniform sweep Injection Rate = 0.3 cc/min 30 minutes of CO2 injection 60 minutes of CO2 injection 120 minutes of CO2 injection 165 minutes of CO2 injection

19. Reconstructions (1 cc/min) Heterogeneity Oil saturated core CO2 injection – 3 minutes CO2 injection – 5 minutes CO2 injection –15 minutes CO2 injection – 45 minutes CO2 injection – 60 minutes

20. Uniform sweep Reconstructions (0.3 cc/min) Heterogeneity Oil saturated core CO2 injection – 30 minutes CO2 injection – 60 minutes CO2 injection – 120 minutes CO2 injection – 150 minutes CO2 injection – 180 minutes CO2 injection – 300 minutes

21. Decrease in CT number CT Number Plots 100% Oil CO2

22. CO2 Saturation CO2 Saturation – 3 Minutes CO2 Saturation – 5 Minutes CO2 Saturation – 15 Minutes CO2 Saturation – 45 Minutes

23. Porosity and Saturation Porosity equation Saturation equation

24. Producer Injector Average saturation is 95% Early breakthrough Spatial variation of saturation

25. Average saturation is 96.7% Late Breakthrough Spatial variation of saturation

26. ≈ 300 minutes ≈ 60 minutes Oil recovery

27. Fractured Core • Fluid flows through fracture simultaneously displacing oil from the matrix • Counter current exchange mechanism Oil + gas Fracture Matrix Gas

28. Fracture filled with oil Fractured Core Oil saturated core

29. Irregular CO2 saturation due to heterogeneity Increase in CO2 saturation in fracture Breakthrough Scans

30. Final CO2 saturation • 2.1 PV of injection • Final recovery of about 58%

31. Water Alternating Gas (WAG) Alternate injection of water and gas Fairly homogeneous reservoirs In the presence of extreme heterogeneities?

32. Brine preparation Objective: Delay CO2 breakthrough! • Brine iodated with sodium iodide and potassium iodide • High mobility, early breakthrough • Add Xanthan to increase viscosity and reduce mobility • High injectivity • Viscosity insensitive to salinity

33. Higher density regions Oil saturated core Oil Saturated Core

34. Water Injection

35. Water Breakthrough

36. CO2 injection (0.4 pv) Large amount of CO2 is diverted into the matrix!

37. Analysis • Overall recovery ≈ 85% • Incremental recovery ≈ 4.5% Reason? • Core used was strongly water wet • Xanthan still in gelant form • Considerable amount of water “leakoff” into the matrix • Increase in viscosity gave similar results

38. Gel Experiment with gel • Injected directly into fracture • Seen as a yellow streak in the fracture

39. CO2 in matrix CO2 injection

40. CO2 Injection Block in grooves

41. Final recovery • Final Recovery obtained ≈ 95% • Gel effective in diverting CO2 into matrix

42. Highest Recovery Oil recovery

43. Reliable upscaling Better Model Match saturation and flow profile apart from other parameters

44. Conclusions Are low injection rates better? • Lower rates give better sweep and lesser utilization of CO2 but time taken is higher. • Rates need to be optimized

45. Conclusions Can WAG delay CO2 breakthrough from fractures? • Viscosified water can delay breakthrough but “leak off” prevention is required (particulate matter) • Performance might be better in oil-wet reservoirs • “Washout” problems

46. Conclusions Is gel treatment an effective solution? • Gels can prevent breakthrough and improve recovery • No “wash out” problems

47. Gel placement Injection Pressure Resistance factor and “screen out” Issues Gels for conformance control Gel type

48. High fracture k Fluid flow through fractures Early breakthrough, oil bypass Problems Low Matrix k Fractured System

49. Alternate injection of specific pore volumes of water and gas to reduce gas mobility. • Proven to be mostly effective in fairly homogeneous reservoirs. • Performance in the presence of extreme heterogeneities like fractures has not been adequately investigated.