A review on recent advances in the numerical simulation for coalbed-methane-recovery process

The recent advances in numerical simulation for primary coalbed methane (CBM) recovery and enhanced coalbed-methane recovery (ECBMR) processes are reviewed, primarily focusing on the progress that has occurred since the late 1980s. Two major issues regarding the numerical modeling will be discussed in this review: first, multicomponent gas transport in in-situ bulk coal and, second, changes of coal properties during methane (CH4) production. For the former issues, a detailed review of more recent advances in modeling gas and water transport within a coal matrix is presented. Further, various factors influencing gas diffusion through the coal matrix will be highlighted as well, such as pore structure, concentration and pressure, and water effects. An ongoing bottleneck for evaluating total mass transport rate is developing a reasonable representation of multiscale pore space that considers coal type and rank. Moreover, few efforts have been concerned with modeling water-flow behavior in the coal matrix and its effects on CH4 production and on the exchange of carbon dioxide (CO2) and CH4. As for the second issue, theoretical coupled fluid-flow and geomechanical models have been proposed to describe the evolution of pore structure during CH4 production, instead of traditional empirical equations. However, there is currently no effective coupled model for engineering applications. Finally, perspectives on developing suitable simulation models for CBM production and for predicting CO2-sequestration ECBMR are suggested