Experimental study on the softening effect and mechanism of anthracite with CO2 injection

The injection of supercritical CO2 into coal seam alters the mechanical properties of the coal, which should be paid close attention when considering security issues during CO2-Enhanced CoalBed Methane recovery (CO2-ECBM). In this study, the Qinshui Basin is considered as the research object, and mechanical parameters, such as the peak strength, elastic modulus, and Poisson's ratio of anthracites under the coupled influences of CO2 saturation pressure, CO2 saturation time, and water content were investigated. The results indicate a decrease in the peak strength and elastic modulus and an increase in the Poisson's ratio of the CO2-saturated coal. Increasing the CO2 saturation pressure, extending the CO2 saturation time, and increasing the water content contributes to the softening of the coal. The modified Langmuir equation and stretched exponential (SE) equation are used to accurately predict the peak strength and elastic modulus reduction with CO2 saturation pressure and CO2 saturation time, respectively. The failure patterns change from brittle shear failure for natural coal and transitional shear failure for supercritical CO2-saturated coal to the ductile non-dilatant barreling for the supercritical CO2+water-saturated coal. The softening effect of coal can be attributed to the chemical and physical reactions between coal and CO2, including the surface energy reduction of CO2-bearing coal, extraction and plasticization reactions, differential adsorption swelling and swelling stress, and dissolution of carbonate solution for the mineral filling pore/fracture. In view of the geo-mechanical changes caused by CO2 to coal, the optimization of the targeted reservoir is pivotal for the successful implementation of CO2-ECBM.