CO2 injectability and CH4 recovery of the engineering test in qinshui Basin, China based on numerical simulation

Abstract

An important CO2-ECBM (CO2 enhanced coalbed methane) engineering test will be carried out in #3 coal seam of Qinshui basin, China this year. This study is a preliminary work of CO2-ECBM engineering test with an aim at optimizing the engineering proposal and evaluating the effectiveness of CO2-ECBM. In this study, considering the competitive adsorption and diffusion of CO2 and CH4, seepage of gas and water, coal deformation, the dynamic evolution of coal seam porosity and permeability, and heat conduction and convection between coal and fluid, a fully coupled THM (thermo-hydro-mechanical) mathematical model for CO2-ECBM was established. The modelling was followed by a detailed discussion of the engineering well group’s CO2 injection and CH4 output characteristics under different CO2 injection pressures. The results confirm that CO2 injection has a positive effect on improving CH4 production. Within the simulated time, the CH4 recovery was enhanced by 4.26%–12.80%. Although the CO2 injectability of the engineering well group reduced significantly due to the drop in permeability near the CO2 injection well, the engineering well group has considerable CO2 storage potential. Under simulation conditions, the CO2 storage capacity for single CBM well is up to 0.73×105 - 2.54×105 m3 (standard temperature and pressure, STP). Increasing CO2 injection pressure can expand the coverage of CO2, thereby significantly improving the CH4 production rate and CO2 storage capacity. In addition, the permeability decrease caused by CO2 injection mainly affects the CO2 injection rate and storage capacity with weak influence on CH4 production and recovery.