Multiphysics Modeling of Coal-to-Methane Bioconversion Considering the Impact of Microbial Clogging

In situ coal-to-methane bioconversion could be conducted by introducing nutrient solution, either with or without microbes, into coal seams. Under nutrient-rich conditions, the metabolic activities of these introduced or naturally occurring microbes are stimulated, and organic matter in coal can then be microbially decomposed into biogenic methane. In this process, some microbes may adsorb and grow on the fracture walls. The microbial adsorption and growth on the fracture walls can lead to microbial clogging of the fracture system. Once fracture clogging occurs, subsequent nutrient solution injection and biogenic methane extraction will be hindered. In this work, a multiphysics model at the core/near-wellbore scale is developed to investigate coal-to-methane bioconversion behavior under the influence of microbial clogging. A set of partial differential equations is built to describe the involved processes: (1) deformation of coal; (2) water–methane two-phase flow; (3) reactive transport of nutrients, microbes, and metabolic products; and (4) adsorption/desorption and growth/decay of microbes in the fracture system. The multiphysics model is validated by using lab microbial transport data as well as coal-to-methane bioconversion data. Then, the validated model is utilized to model various key processes in coal-to-methane bioconversion. The simulation results indicate that (1) nutrient intermittency delivery strategy can ensure long-term biogenic methane generation and extraction; (2) coal-to-methane bioconversion not only can generate additional biogenic methane but also has great potential to enhance gas flow capacity in coal seams; and (3) biogenic methane extraction efficiency, defined as the ratio of extraction rate to generation rate, gradually increases with the progress of coal-to-methane bioconversion.