A multiphysics model for biogenic gas extraction from coal seams

Approximately 20% of global natural gas resources, including coalbed methane (CBM), could be microbial in origin. This discovery has attracted great interests in extracting biogenic gas in coal seams. Previous studies have demonstrated that this could be achieved through injecting nutrients solution, either with or without microbes, into a coal reservoir. The injected nutrients transport in coal, stimulate the growth of microbes, and enhance their metabolic activities. Through these complex processes, the organic components in coal are biodegraded into methane gas. The generated biogenic gas may be either in free phase or adsorbed on coal grains and can be extracted as an integral part of the gas-in-place. Although this bio-stimulation technique has been proposed for decades, generating and extracting additional biogenic methane in coal seams is still in a stage of conceptual development. In this study, we develop a modelling tool to validate this concept under the laboratory conditions and upscale to reservoir conditions. The model consists of a complete set of partial differential equations (PDEs) to define: (1) coal deformation, (2) water and gas flow, (3) multispecies reactive transport, and (4) microbial growth/decay and adsorption/desorption. All these processes are coupled through coal porosity and permeability model that links hydrological, mechanical, chemical and biological processes together. The multiphysics model is verified against laboratory coal bioconversion data. The verified model is applied to simulate practical operation in which nutrients solution is continuously delivered into coal seams. Simulation results capture all important processes involved and validate the effectiveness of coal-to-methane bioconversion and its extraction.