Responses of multi-scale microstructures, physical-mechanical and hydraulic characteristics of roof rocks caused by the supercritical CO2-water-rock reaction

Abstract The integrality of caprock decides the safety of CO2 enhanced coalbed methane recovery (CO2-ECBM), the inferior caprock may reduce the seal performance and induce gas leakage. However, the attribute of caprock can be modified through the supercritical CO2 (ScCO2)-water-rock reaction. To clarify the responses of multi-scale microstructures, physical-mechanical and hydraulic characteristics of caprock after the geochemical reaction, the rock samples of coal seam roof from Qinshui Basin was first collected to simulate geochemical reactions between ScCO2, water, and caprock, then the scanning electron microscope (SEM), low-temperature liquid nitrogen adsorption (LTLNA), mercury intrusion porosimetry (MIP), P-wave velocity, mechanical and permeability tests were conducted. Results show that the ScCO2-water-rock reaction promotes the formation of dissolution caves, which makes the surface rough and uneven and affects the pore distribution through enlarging the volume of transition pores, mesopores, and macropores and reducing the volume of micropores. The P-wave velocity of the sample is decreased nonlinearly with the increasing ScCO2-water-rock reaction time because of the pore structure damage induced by the chemical dissolution effect. The strain energy evolution is also influenced by the ScCO2-water-rock reaction, the long-time geochemical reaction promotes more elastic energy transform to dissipative energy. The peak strength, elastic modulus and Possion's ratio change regularly with increasing reaction time while the failure pattern is independent of the ScCO2-water-rock reaction. The varieties of mechanical parameters can be ascribed to the considerable increase of macropores and can be predicted by the logistic function. The ScCO2-water-rock reaction has not changed the primary permeability while it largely enhances the permeability peak and elevates the infiltration ability of the loaded rock sample. The main leakage form in the study area is the diffusive loss and the CO2-ECBM recovery is relatively safe, however, the CO2 leakage may be exacerbated if the ScCO2-water-rock geochemical reaction is coupled with the local stress concentration or connected with the pre-existing fractures in the roof rocks. This study deepens the understanding of the consequences of ScCO2-water-rock reaction on caprock and provides some help for the site selection and the preliminary safety evaluation of CO2-ECBM.