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Optimization of Hydraulic Fractures in CBM Wells

Optimization of Hydraulic Fractures in CBM Wells. Outline. Conductivity requirements in CBM Understanding fluid flow in fractures Field results Other factors to consider. Conductivity Requirements for CBM Fractures. Which well requires higher permeability proppant?. Gulf of Mexico

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Optimization of Hydraulic Fractures in CBM Wells

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  1. Optimization of Hydraulic Fractures in CBM Wells

  2. Outline • Conductivity requirements in CBM • Understanding fluid flow in fractures • Field results • Other factors to consider

  3. Conductivity Requirements for CBM Fractures • Which well requires higher permeability proppant? Gulf of Mexico 10 MMcfd Shallow CBM 0.2 MMcfd 50 times more production from high rate well…

  4. Darcy’s Law vs. Forchheimer Equation •  P/L = v / k • Pressure drop is proportional to fluid velocity • Applicable only at low flowrates •  P/L =  v / k + v2 • Pressure drop is proportional tosquareof fluid velocity • Applicable at realistic fracture flowrates

  5. Consider Downhole Conditions Compressed 165-fold Compressed 9-fold

  6. Consider Downhole Conditions Cross-sectional area of fracture is 13x greater in GOM Gas velocity is over 4-5x higher in CBM well Superficial v = 6 in/sec. Assuming 33% porosity = 18 in/sec. Gas travels around 800 grain hemispheres per second! With flowpath arc (π/2), interstitial velocity ~ 2 feet per second

  7. Often must compromise • Before we even consider CBM issues such as embedment, coal fines plugging, and multiphase flow, there is reason to suspect that our propped fractures have inadequate conductivity. Options to Increase Fracture Conductivity • Increase fracture width • Reduce gel damage • Increase proppant permeability

  8. Sieve Distribution

  9. Proppant Shape Most ceramics Most sands API RP60, From Stratigraphy and Sedimentaion, Krumbein and Sloss

  10. 27% more porosity (9 porosity units) Pack PorosityStim-Lab, 2 lb/sq ft, 20/40, 5e-6 psi core

  11. 92% improvement Permeability at Low Stresses Stim-Lab PredK 6.57, Feb 2002

  12. 60% improvement Permeability at Low Stresses

  13. 33% improvement 58% improvement Beta Factor Comparison Stim-Lab PredK 6.57, Feb 2002, CBM Well, 1000 psi stress, 100F, 50% gel damage

  14. Intermediate Strength Ceramic20X Photomicrographs Stim-Lab @ 4000 psi @ 8000 psi @ 10000 psi

  15. Resin Coated Sand20X Photomicrographs Stim-Lab @ 4000 psi @ 8000 psi @ 10000 psi

  16. Field Results

  17. Coal Bed Methane, San Juan Basin SPE 77675 • Restimulations of CBM Southern Ute 12-2; 32-9

  18. Coal Bed Methane, San Juan Basin SPE 77675 • Restimulations of CBM Southern Ute 18-2; 32-8

  19. SPE 77443 Fig 6, Stutz (Anadarko)Helper Federal B-10 Restimulation, Utah 2001: Re-frac, 330,000 lb 16/30 sand 1999: Initial frac, 60,000 lb 16/30 sand 15-fold increase

  20. SPE 77443 Fig 4, Stutz (Anadarko)Helper Federal 1999 Drilling Program, Utah Average gas rates for 19 wells, during 1st 9 months of production Gas Rate MCFD (scale 0 to 1200) All wells in 1999 16/30 sand with 25#XLG Cumulative Gas, MMCF (scale = 0 to 160)

  21. SPE 22395 Fig 16, Palmer, AmocoCedar Cove Field, Black Warrior Basin, Alabama

  22. CBM Field Results • Analysis of 900 Virginia CBM wells: Production problems caused by low fracture conductivity – SPE 72380 • Propped fractures in Australia CBM are superior to under-reaming in cost and performance. Some wells produce 5 MMSCFD – SPE 64493 • “Very high fracture conductivity is needed to ensure rapid dewatering” – SPE 21292 • Ultimate gas recovery from CBM depends on maintaining fracture conductivity – SPE 51063 • High fracture conductivity “is more important than heretofore recognized.” – SPE 22395 • “High fracture conductivity is paramount”–SPE 18253

  23. Other Factors to Consider • CBM wells are more sensitive to fracture conductivity than traditional reservoirs. In CBM, desorption is driven by Fickian diffusion, which is highly pressure-dependent. – SPE 51063, 52193 • A high conductivity frac will reduce the flowing pressure over a larger area, and initiate dewatering and desorption in a greater portion of the CBM reservoir • High conductivity fractures distribute pressure drop over larger area, reducing mobilization of coal fines – SPE 18253

  24. Multiphase Flow in Proppant Packs Increased Pressure Drop due to Mobile Liquid in Proppant Packs 60 50 40 Multiplier of Total Pressure Drop 30 0.75 MMCFD 20 0.25 MMCFD 10 Trend - 0% 5% 10% 15% Fractional Flow of Liquid Source: Stim-Lab Proppant Consortium, Feb. 2001. 2.8 lb/sq ft CarboLite at 4000 psi stress, 550 Darcy reference perm. Multiplier is incremental to total pressure drop under non-Darcy conditions with dry gas. Equivalent rates from 50’ frac height at 2000 psi BHFP.

  25. dP to Initiate Cleanup @ 4.0 lb/sqft + YF130LG +Breakers @ 150 Deg F and 2000 psi Closure Stress

  26. Post-Cleanup Conductivity@ 4.0 lb/sq. ft. + YF130LG + Breakers 150ºF and 2000 psi Closure Stress Source: Stim-Lab 12/97

  27. % Retained Conductivity@ 4.0 lb/sq. ft. + YF130LG + Breakers 150ºF and 2000 psi Closure Stress Source: Stim-Lab 12/97

  28. Other Factors to Consider • Multiphase flow • Coal Fines Plugging / Flowback • Coal compaction with high treating pressures • Erosion of coal frac faces during treatment by angular sand is likely more severe than with round ceramic. Erosion may contribute to width, but also “contaminates” pack with fines. – SPE 48886 • Low reservoir energy to cleanup gel residue. LWC clean up easier than sand. • Embedment • Additives

  29. Conclusions • The conductivity needs of low pressure CBM wells are often underestimated • For rapid dewatering and ability to handle multiphase flow, superior fracture conductivity is needed • Many frac gels are extremely damaging to coals. It is desirable to use low damage fluids but maintain conductivity • Light weight ceramic proppants provide superior productivity

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