Development of a multiscale microbial kinetics coupled gas transport model for the simulation of biogenic coalbed methane production

Abstract

In this paper, we develop a multiscale model to simulate coalbed methane (CBM) production from reservoirs along with the inclusion of secondary biogenic gas generated by the continued anaerobic breakdown of coal. A two-step gas transport model is derived for this purpose, based on the assumption that coal porosity can be classified into two scales, macropores and micropores. The model assumes laminar gas flow in macropores and diffusive flow in micropores, driven by desorption. Surface diffusion of gas due to the Klinkenberg effect occurring at low permeability and pressure conditions is also considered. The transport model built for gas flow simulation in a 1D radial reservoir is non-dimensionalized and solved using the Levenberg–Marquardt method. The Morris OAT (one-at-a-time) method for global sensitivity analysis is used to identify important gas transport/storage parameters to perform model refinement for history matching of production data from the gas producing phase of Manville wells found in Alberta. The validated transport model is then combined with a suitably modified enzymatic kinetic model for coal bioconversion that was originally developed by us for lab-scale experiments. Finally, parametric investigations revealed that an order of magnitude increase in the methanogenesis rate can significantly improve biogenic gas recovery, and the next most significant parameter is the solubilization rate.