Optimization of Hydraulic Fractures in CBM Wells

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Optimization of Hydraulic Fractures in CBM Wells
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Outline

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

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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
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Darcys 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 to square of
  fluid velocity
• Applicable at realistic fracture flowrates

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Consider Downhole Conditions
Parameter Gulf of Mexico Shallow CBM
Gas Rate, MMSCFD 10 0.2
BH Pressure 3500 psi 150 psi
BH Temp 250 F 100 F
Compressed 165-fold
Compressed 9-fold
Gas Rate, MMACFD 0.06 0.02
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Consider Downhole Conditions
Parameter Gulf of Mexico Shallow CBM
Frac height, feet 100 30
Frac width, inch 0.8 0.2
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
(p/2), interstitial velocity 2 feet per second
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• 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

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Sieve Distribution
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Proppant Shape
Most ceramics
Most sands
API RP60, From Stratigraphy and Sedimentaion,
Krumbein and Sloss
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Pack PorosityStim-Lab, 2 lb/sq ft, 20/40, 5e-6
psi core
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Permeability at Low Stresses
Stim-Lab PredK 6.57, Feb 2002
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Permeability at Low Stresses
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Beta Factor Comparison
Stim-Lab PredK 6.57, Feb 2002, CBM Well, 1000 psi
stress, 100F, 50 gel damage
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Intermediate Strength Ceramic20X
Photomicrographs Stim-Lab
_at_ 4000 psi
_at_ 8000 psi
_at_ 10000 psi
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Resin Coated Sand20X Photomicrographs Stim-Lab
_at_ 4000 psi
_at_ 8000 psi
_at_ 10000 psi
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Field Results
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Coal Bed Methane, San Juan Basin SPE 77675

• Restimulations of CBM Southern Ute 12-2 32-9

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Coal Bed Methane, San Juan Basin SPE 77675

• Restimulations of CBM Southern Ute 18-2 32-8

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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
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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 25XLG
Cumulative Gas, MMCF (scale 0 to 160)
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SPE 22395 Fig 16, Palmer, AmocoCedar Cove Field,
Black Warrior Basin, Alabama
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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 paramountSPE
  18253

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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

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Multiphase Flow in Proppant Packs
Increased Pressure Drop due to Mobile
Liquid in Proppant Packs
60
50
40
Multiplier of Total
Pressure Drop
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0.75 MMCFD
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0.25 MMCFD
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Trend
-
0
5
10
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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.
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dP to Initiate Cleanup _at_ 4.0 lb/sqft YF130LG
Breakers _at_ 150 Deg F and 2000 psi Closure Stress
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Post-Cleanup Conductivity _at_ 4.0 lb/sq. ft.
YF130LG Breakers 150ºF and 2000 psi Closure
Stress
Source Stim-Lab 12/97
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Retained Conductivity _at_ 4.0 lb/sq. ft.
YF130LG Breakers 150ºF and 2000 psi Closure
Stress
Source Stim-Lab 12/97
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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

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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