The coupling mechanism of the thermal-hydraulic-mechanical fields in CH4-bearing coal and its application in the CO2-enhanced coalbed methane recovery

Abstract

The injection of CO2 into deep coal seams can not only contribute to the CO2 geological sequestration but also increase the CH4 recovery (CO2-ECBM). Aimed at revealing the coupling mechanism of the thermal-hydraulic-mechanical (THM) fields during the CO2-ECBM process, the numerical models of the THM fields were established. Using COMSOL software, the performances of direct exploitation and CO2-ECBM production are compared. The effects of CO2 pressure injected and production well temperature are provided. The CO2 injection has enhancement effects on gas pressure and CH4 cumulative output. Increasing the CO2 pressure injected can raise the gas pressure and energy in a short time, which improves the efficiency of CO2 injection and CH4 production, and has positive effects on improving CH4 cumulative output and CO2 cumulative storage. The convective heat transfer and solid heat transfer are not obvious due to the low thermal conductivity coefficient and the slow gas migration velocity. Although increasing the production well temperature can raise the CH4 cumulative output and CO2 cumulative storage, the increasing amplitude is small. The differential pressure between the reservoir and the boundaries, and the competitive adsorption of gas are the major factors affecting the permeability. When CO2 doesn't reach the production well, the permeability near the injection well and the production well is mainly affected by the gas competitive adsorption of gas, and the differential pressure, respectively. When CO2 spreads to the production well, the competitive adsorption effect will gradually replace the differential pressure effect on permeability near the production well.