Deformation mechanism and displacement ability during CO2 displacing CH4 in coal seam under different temperatures

Abstract

Liquid CO₂ has the synergistic effect of low-temperature damage and displacing CH₄ after injecting into the coal seam, which can effectively improve coalbed permeability and promptly promote the adsorbed CH₄ to desorption state for preventing the coal and gas outburst disasters. The injected CO₂ gets adsorbed at the surface of the coal pores, which causes the coal swelling. In this work, we developed a triaxial experimental platform to explore the features of CO₂ displacing CH₄ under different temperatures. The variation of coal swelling and segmentation features of the displacing concentration was elucidated. The seepage ability and mechanism of gas mitigation in the coal seam during LCO₂-ECBM were revealed. The results showed that the coal samples displayed swelling deformation in the CH₄ adsorption and CH₄ displacement stages, and the strain curves can be divided into rapid and slow deformation phases. The strain in the CO₂ displacing CH₄ stage is notably larger than that in the CH₄ adsorption stage. Three dominant results were obtained during the CH₄ displacement: Free CH₄ driving by CO₂ injection, CH₄ self-desorption, and CO₂–CH₄ competitive adsorption. The displacing flow rate increased swiftly in the initial stage, and then decreased to a stable tendency. The cumulative displacement volume of CH₄ in different stages exhibited distinct functional relationships. The integrated contribution of the coal matrix shrinkage and thermal stress damage to gas seepage improvement was more prominent than that of the adsorption swelling to seepage inhibition in the coal seam.