Gas storage and flow in coalbed reservoirs: Implementation of a bidisperse pore model for gas diffusion in a coal matrix

Unipore diffusion models are used widely to model gas transport in a coal matrix in conventional dual-porosity coalbed-reservoir simulators. The unipore models implemented in conventional coalbed-reservoir simulators assume that there is a negligible free-gas phase in the coal matrix and that gas exists only in an adsorbed state under hydrostatic pressure. In low-rank coals, however, a substantial amount of free gas may exist in the macropores of the coal matrix. There is strong laboratory evidence that many coals exhibit bi-or multimodal 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 an in-house 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 with 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.