Numerical study on the injection of heated CO2 to enhance CH4 recovery in water-bearing coal reservoirs

A hydraulic-mechanical-thermal coupled numerical model for enhanced CBM recovery by injecting heated CO2 (CO2-ECBM) is established, which fully couples the gas-water two-phase flow, competitive adsorption, and temperature change as well as coal deformation. Through the finite element simulation software COMSOL Multiphysics, the model is used to investigate the effects of initial water saturation, CO2 injection temperature and pressure on the CO2-ECBM process from the perspectives of CO2 storage, CH4 production and the evolution of reservoir permeability. The coalbed water hinders the migration of gases in the reservoir. The CO2 migration distance is always negatively correlated with the initial water saturation in the reservoir, even after the coalbed water is discharged. Interestingly, in the early stage of drainage and recovery, the rapid decrease of pore water pressure facilitates the desorption of adsorbed CH4, especially near the producing well. The increase of CO2 injection temperature and pressure is conducive to the transport of CO2 in the reservoir, allowing more opportunities for CO2 to come into contact with coal, which promotes CO2 sequestration and thus enhances CH4 recovery. The coalbed water can delay the decline of permeability induced by CO2 adsorption by impeding CO2 migration. The increase of CO2 injection temperature and pressure can exacerbate the reduction of absolute permeability by inducing more CO2 to be adsorbed.