Ji-Quan Shi (Imperial College of Science) | S. Durucan (Imperial College)
Unipore diffusion models are used widely to model gas transport in a coalmatrix in conventional dual-porosity coalbed-reservoir simulators. The uniporemodels implemented in conventional coalbed-reservoir simulators assume thatthere is a negligible free-gas phase in the coal matrix and that gas existsonly in an adsorbed state under hydrostatic pressure. In low-rank coals,however, a substantial amount of free gas may exist in the macropores of thecoal matrix.
There is strong laboratory evidence that many coals exhibit bi- ormultimodal pore structure. This paper describes the implementation of abidisperse pore-diffusion model in a coalbed-reservoir simulator. In thebidisperse model, gas adsorption is assumed to take place only in themicropores, with the macropores providing storage for free gas, as well astortuous paths for gas transport between the micropores and cleats.Gas-production performance from a sub-bituminous Powder River basin coalbedreservoir has been studied using an in-house coalbed-reservoir simulator. Theimplementation of the triple-porosity formulation in the simulator overcame thereported inconsistency between field gas-production rates and predicted ratesobtained with conventional dual-porosity simulators. With the introduction ofan appropriate storage volume of free gas in the macropores, the predictedincrease in gas-production rates are consistent with the published field data.