CO2 adsorption and swelling of coal under constrained conditions and their stage-change relationship

The adsorption of CO2 into coal will cause the volumetric swelling of coal body, which is a highly concerned issue for the CO2 enhanced coalbed methane recovery (CO2-ECBM). However, the research of adsorption and swelling of coal is not all-inclusive and their relationship is ambiguous. This paper conducted the experiments regarding the CO2 adsorption and corresponding swelling of coal influenced by multiple factors using the self-designed device. Results show that the adsorption capacity is promoted by the injection pressure and is restrained by confining pressure, temperature and water content, but the thermal-induced swelling strain and moisture-induced swelling strain can offset the swelling loss to some extent for the wet coal at a high temperature. There has a stage-change relationship between swelling strain and adsorption capacity of coal, and the adsorption swelling can be divided into three parts: the slow swelling stage, the rapid swelling stage and the stable swelling stage. Under the action of the external stress, the adsorption swelling develops towards to the interior of coal body first and then develops towards to the external when the swelling stress exceeds the external stress. Different extended directions of adsorption swelling are the main causes of the stage adsorption swelling. The study contributes to the deep understanding for the gas adsorption swelling mechanism and the attenuation-rebound of the coal reservoir permeability during the CO2-ECBM.