A fully coupling coal–gas model associated with inertia and slip effects for CBM migration

Abstract

Although inertia and slip effects of gas flow that may impose significant influences on Coalbed methane (CBM) well and reservoir performance have been widely confirmed and studied, and some mathematical models have also been proposed to quantitatively evaluate these two effects, the combined inertia and slip effects on CBM migration were seldom considered in the previous models. In this study, a fully coupled finite element (FE) model of non-Darcy gas flow and coal deformation process with sorption and Klinkenberg effects in the coalbed is developed to quantify CBM migration mechanism. The FE model is validated by comparison with available analytical and numerical solutions. The results indicate that the evolution of coalbed permeability and gas transport in CBM reservoir not only relate with the sorption-induced coal deformation and the pore pressure change, but also closely depend on the coupling inertia (non-Darcy) effect and slip (Klinkenberg) effects. Based on the simulation, it is found that CBM production can be significantly enhanced due to gas slip (Klinkenberg) effect. But when slip effect is significant, the inertia effect will be ignored. The simulation results can improve the understanding of the coal–gas interactions during underground CBM migration and provide a scientific basis for evaluation of the gas-drainage efficiency, design and optimization of drainage systems, etc. However, the different pressure and permeability thresholds corresponding to prominent response of inertia or slip effect should be further determined through experiments and theoretical models in the future.