Gas Storage and Flow in Coalbed Reservoirs: Implementation of a Bidisperse Pore Model for Gas Diffusion in Coal Matrix

Abstract

Abstract Unipore diffusion models are widely used for modelling gas transport in coal matrix in conventional dual-porosity coalbed reservoir simulators. The unipore models implemented in conventional coalbed reservoir simulators assume that free gas phase is negligible and gas exists only in adsorbed state in coal matrix. In low-rank coals, however, a substantial amount of free gas may exist in the macropores of coal matrix. There is strong laboratory evidence that many coals exhibit bi- or multi-modal pore structure. This paper describes the implementation of a bidisperse pore-diffusion model in a coalbed reservoir simulator. In the bidisperse model, gas adsorption is assumed to take place only in the micropores, with the macropores providing storage for free gas, as well as tortuous paths for gas transport between the micropores and cleats. Gas production performance from a sub-bituminous Powder River Basin coalbed reservoir has been studied using the in-house Imperial College coalbed reservoir simulator. The implementation of the triple-porosity formulation in the simulator overcame the reported inconsistency between field gas production rates and predicted rates obtained using conventional dual-porosity simulators. With the introduction of an appropriate storage volume of free gas in the macropores, the predicted increase in gas production rates are consistent with the published field data.