Changes Of Multiscale Surface Morphology And Pore Structure Of Mudstone Associated With Supercritical Co2-Water Exposure At Different Times

CO2 enhanced coalbed methane recovery (CO2-ECBM) has been confirmed as an effective technology to improve coalbed methane (CBM) production; however, the injected CO2 and the reservoir water can react with the caprock of the coal seam, which changes its internal structure and increases the risk of CO2 leakage. To clarify the CO2-water-rock reaction process and the structural responses of the caprock, the roof mudstone samples of coal seams from Qinshui Basin were first selected to conduct the geochemical reaction experiment; then, scanning electron microscopy (SEM), N-2 adsorption, and mercury intrusion porosimetry were adopted to monitor the multiscale structure alteration of the samples. The results show that the rock sample has progressively deteriorated during the CO2-water-rock reaction process, and this effect is aggravated with the increase of the reaction time. The dissolution process consists of three stages: (1) lots of isolated and shallow holes are developed; (2) holes are connected and formed to the dissolution grooves; (3) dissolution grooves are widened, and the dissolution zone is expanded until all the soluble minerals immerse into the reaction solution. The pore volume (PV) and specific surface area (SSA) of small pores are reduced, while those of large pores are increased, which can be attributed to the pore-blocking effect caused by clay mineral swelling and mineral precipitation and the pore-enlarging effect caused by mineral dissolution, respectively. Additionally, the ScCO2-water-rock reaction varies the pore structure distribution and makes the PV distribution more heterogeneous and the SSA distribution more homogeneous. The changes in the internal structure of the roof mudstone promote gas diffusion and seepage, which may increase the potential threat of CO2 leakage during the long-term CO2-ECBM process to a certain degree. This study will provide an essential basis for the investigation and evaluation of the security of CO2-ECBM in Qinshui Basin, China.