Experimental investigation of effects of CO2 injection on enhanced methane recovery in coal seam reservoirs

Enhanced coalbed methane (ECBM) recovery performed by injecting CO2 into coal reservoirs has been found to be a viable option for methane recovery, while CO2 injection into coal seams, especially unmineable ones, has great potential to sequestrate large volumes of CO2 to mitigate greenhouse gas accumulation in the atmosphere. This study was therefore initiated to investigate the effect of CO2 flooding on ECBM recovery and CO2 storage in coal. A series of CO2 core flooding tests was carried out on a high rank bituminous coal core with CO2 injection pressures from 6 to 10 MPa. CO2 injection rate, methane and CO2 production rates, methane displacement efficiency and the volumetric strains of the core were calculated and recorded to enable interpretation of the results. It was observed that, compared with natural methane production in which only around 51.73% of methane is recovered, CO2 injection provides excellent methane displacement efficiency (over 90%), especially for high injection pressures (greater than 8 MPa), which drive methane completely out of the coal sample. Although higher CO2 flooding pressure favours greater CO2 injection rates and methane production rates for any coal types, high rank coal shows faster and greater methane production and CO2 storage capacity. The observed rapid breakthrough of CO2 in the gas produced by elevated CO2 injection, especially for high injection pressures, may complicate the gas separation process. Adsorption-induced volumetric strain is approximately proportional to the CO2 storage capacity in coal which increases with CO2 pressure, and the strain reduces coal permeability and therefore CO2 injectivity and methane production.