Thermal-Hydraulic-Mechanical Coupling Simulation of CO2 Enhanced Coalbed Methane Recovery with Regards to Low-Rank but Relatively Shallow Coal Seams

Abstract

CO (Formula presented.) injection technology into coal seams to enhance CH (Formula presented.) recovery (CO (Formula presented.) -ECBM), therefore presenting the dual benefit of greenhouse gas emission reduction and clean fossil energy development. In order to gaze into the features of CO (Formula presented.) injection’s influence on reservoir pressure and permeability, the Thermal-Hydraulic-Mechanical coupling mechanism of CO (Formula presented.) injection into the coal seam is considered for investigation. The competitive adsorption, diffusion, and seepage flowing of CO (Formula presented.) and CH (Formula presented.) as well as the dynamic evolution of fracture porosity of coal seams are considered. Fluid physical parameters are obtained by the fitting equation using MATLAB to call EOS software Refprop. Based on the Canadian CO (Formula presented.) -ECBM project CSEMP, the numerical simulation targeting shallow low-rank coal is carried out, and the finite element method is used in the software COMSOL Multiphysics. Firstly, the direct recovery (CBM) and CO (Formula presented.) -ECBM are compared, and it is confirmed that the injection of CO (Formula presented.) has a significant improvement effect on methane production. Secondly, the influence of injection pressure and temperature is discussed. Increasing the injection pressure can increase the pressure difference in the reservoir in a short time, so as to improve the CH (Formula presented.) production and CO (Formula presented.) storage. However, the increase in gas injection pressure will also lead to the rapid attenuation of near-well reservoir permeability, resulting in the weakening of injection capacity. Also, when the injection temperature increases, the CO (Formula presented.) concentration is relatively reduced, and the replacement effect on CH (Formula presented.) is weakened, resulting in a slight decrease in CBM production and CO (Formula presented.) storage.