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Summary The natural fracture network of a dual-porosity coalbed reservoir is made up of two sets of orthogonal, and usually subvertically oriented, cleats. Coalbed permeability has been shown to vary exponentially with changes in the effective horizontal stress acting across the cleats through the cleat-volume compressibility, which is analogous to pore compressibility in porous rocks. A formulation for changes in the effective horizontal stress of coalbeds during primary methane recovery, which includes a Langmuir type curve shrinkage term, has been proposed previously. This paper presents a new version of the stress formulation by making a direct link between the volumetric matrix strain and the amount of gas desorbed. The resulting permeability model can be extended readily to account for adsorption-induced matrix swelling as well as matrix shrinkage during enhanced methane recovery involving the injection of an inert gas or gas mixture into the seams. The permeability model is validated against a recently published pressure-dependent permeability multiplier curve representative of the San Juan basin coalbeds at post-dewatering production stages. The extended permeability model is then applied successfully to history matching a micropilot test involving the injection of flue gas (consisting mainly of CO2 and N2) at the Fenn Big Valley, Alberta, Canada. Introduction Over the past 2 decades, coalbed methane (CBM) has become an important source of the (unconventional) natural gas supply in the U.S. On the basis of this experience, CBM has attracted worldwide attention in recent years as a potential clean energy source. Current commercial CBM production occurs almost exclusively through reservoir-pressure depletion, which is simple but considered to be rather inefficient, with an estimated total recovery of generally around 50% (this figure appears to be pessimistic; mature coal plays in the U.S. have now seen recovery of 60 to 80%) of the gas in place. In recent years, enhanced CBM (ECBM) recovery techniques have been proposed as a more efficient means for the recovery of a larger fraction of methane in place. There are two principal variants of ECBM recovery, namely N2 and CO2injection, which use two distinct mechanisms to enhance methane desorption and production. Unlike the primary recovery method, ECBM allows the maintenance of reservoir pressure. The mechanism used in N2 injection is somewhat similar to inert gas stripping because nitrogen is less adsorbing than methane. Injection of nitrogen reduces the partial pressure of methane in the reservoir, thus promoting methane desorption without lowering the total reservoir pressure. On the other hand, CO2 injection works on a different mechanism because it is more adsorbing on coal compared with methane. Carbon dioxide ECBM recovery thus has an added benefit that a potentially large volume of greenhouse gas can be sequestrated in deep coal seams globally.

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In this short note we address the issue of numerical resolution and efficiency of high order weighted essentially nonoscillatory (WENO) schemes for computing solutions containing both discontinuities and complex solution features, through two representative numerical examples: the double Mach reflection problem and the Rayleigh-Taylor instability problem. We conclude that for such solutions with both discontinuities and complex solution features, it is more economical in CPU time to use higher order WENO schemes to obtain comparable numerical resolution.

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