The influence of closed pores and stacked coal grains on gas transport in CO2 injection enhanced CH4 recovery process

Abstract

CO2 injection enhanced coalbed methane (CH4) recovery technology can achieve the enhancement of CH4 production and the effective geological storage of carbon dioxide in coal seams. A coal seam is composed of matrix and fractures. Matrix has open pores and closed pores and fractures may have stacked coal grains. However, the influences of closed pores in matrix and stacked coal grains in fractures on CO2 injection enhanced CH4 recovery have rarely been studied. This paper develops a numerical simulation model to explore the hydraulic-mechanical coupling responses in CO2 injection enhanced coalbed methane (CO2-ECBM) recovery process. This model considers the binary gas displacement and transport in ternary pores, the deformation of coal with different porosity, and fractal stacked coal grains in fractures. After verification with the data available from literature, this model is used to simulate a field trial in the Qinshui Basin. The simulation results show that the contribution of gas in closed pores to the total CH4 production enhanced by CO2 injection is more than 60% in high-rank coal and is proportional to the volume fraction of closed pores. The stacked coal grains in fractures affect the effective roughness and can be expressed by three structural parameters. Higher effective roughness induces lower cumulative CO2 injection and lower cumulative CH4 production and the conical bottom diameter of cone-like element has the largest influence. These findings can promote our understanding for the influence of closed pores and stacked coal grains on gas transport in coalbed gas reservoirs from a multi-scale perspective.