A Model for Changes in Coalbed Permeability During Primary and Enhanced Methane Recovery
- Ji-Quan Shi (Imperial College of Science) | S. Durucan (Imperial College)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- August 2005
- Document Type
- Journal Paper
- 291 - 299
- 2005. Society of Petroleum Engineers
- 4.3.4 Scale, 5.4 Enhanced Recovery, 5.8.6 Naturally Fractured Reservoir, 5.5.8 History Matching, 4.6 Natural Gas, 1.2.3 Rock properties, 5.5 Reservoir Simulation, 5.4.2 Gas Injection Methods, 5.8.3 Coal Seam Gas, 5.1.5 Geologic Modeling, 4.1.5 Processing Equipment, 1.2.2 Geomechanics, 5.10.1 CO2 Capture and Sequestration
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The natural fracture network of a dual-porosity coalbed reservoir is made upof two sets of orthogonal, and usually subvertically oriented, cleats. Coalbedpermeability has been shown to vary exponentially with changes in the effectivehorizontal stress acting across the cleats through the cleat-volumecompressibility, which is analogous to pore compressibility in porous rocks. Aformulation for changes in the effective horizontal stress of coalbeds duringprimary methane recovery, which includes a Langmuir type curve shrinkage term,has been proposed previously. This paper presents a new version of the stressformulation by making a direct link between the volumetric matrix strain andthe amount of gas desorbed. The resulting permeability model can be extendedreadily to account for adsorption-induced matrix swelling as well as matrixshrinkage during enhanced methane recovery involving the injection of an inertgas or gas mixture into the seams. The permeability model is validated againsta recently published pressure-dependent permeability multiplier curverepresentative of the San Juan basin coalbeds at post-dewatering productionstages. The extended permeability model is then applied successfully to historymatching a micropilot test involving the injection of flue gas (consistingmainly of CO2 and N2) at the Fenn Big Valley, Alberta, Canada.
Over the past 2 decades, coalbed methane (CBM) has become an importantsource of the (unconventional) natural gas supply in the U.S. On the basis ofthis experience, CBM has attracted worldwide attention in recent years as apotential clean energy source. Current commercial CBM production occurs almostexclusively through reservoir-pressure depletion, which is simple butconsidered to be rather inefficient, with an estimated total recovery ofgenerally around 50% (this figure appears to be pessimistic; mature coal playsin the U.S. have now seen recovery of 60 to 80%) of the gas in place. In recentyears, enhanced CBM (ECBM) recovery techniques have been proposed as a moreefficient means for the recovery of a larger fraction of methane inplace.
There are two principal variants of ECBM recovery, namely N2 and CO2injection, which use two distinct mechanisms to enhance methane desorption andproduction. Unlike the primary recovery method, ECBM allows the maintenance ofreservoir pressure. The mechanism used in N2 injection is somewhat similar toinert gas stripping because nitrogen is less adsorbing than methane. Injectionof nitrogen reduces the partial pressure of methane in the reservoir, thuspromoting methane desorption without lowering the total reservoir pressure. Onthe other hand, CO2 injection works on a different mechanism because it is moreadsorbing on coal compared with methane. Carbon dioxide ECBM recovery thus hasan added benefit that a potentially large volume of greenhouse gas can besequestrated in deep coal seams globally.
|File Size||545 KB||Number of Pages||9|
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