Influence of supercritical CO 2 -H 2 O-caprock interactions on the sealing capability of deep coal seam caprocks related to CO 2 geological storage: A case study of the silty mudstone caprock of coal seam no. 3 in the Qinshui Basin, China

We conducted simulation experiments on supercritical carbon dioxide (ScCO 2 )-H 2 O-rock interactions for caprock samples obtained from the Qinshui Basin, China, to explore the influence of carbon dioxide (CO 2 ) injection on the sealing capability of deep unminable coal seam caprocks as it relates to CO 2 geological storage. This research focused on the changes in mineral composition, pore-fissure structure, permeability, and mechanical properties of the caprocks after ScCO 2 -H 2 O treatment. These results revealed that ScCO 2 -H 2 O-rock interactions lead to the formation of dissolution pores on the contact surface and a significant increase in macropores volumes and permeability, while the precipitation of secondary minerals can prevent CO 2 from entering the rock in the later reaction stage. The sealing capacity of intact caprock initially decreases and then changes slightly after being affected by ScCO 2 -H 2 O-rock interactions. The caprock integrity determines the security of the CO 2 geological storage. However, both natural and induced fractures provide channels for CO 2 leakage. The compressive strength of the rock was reduced by 36 %, and the toughness increased after 60 days of ScCO 2 -H 2 O treatment. Thus, the formation of new fractures and the reopening of pre-existing faults in caprock may occur before damage under stress changes and ScCO 2 -H 2 O-rock interactions. Fracturing and shedding of fault gouge and particles inside the fractures by high-pressure ScCO 2 -H 2 O fluid can expand the fractures in caprock significantly, and hence increase the risk of CO 2 leakage. Therefore, deep coal seams with thick and intact caprock should be selected to increase the safety of CO 2 geological storage. Furthermore, the CO 2 injection pressure must be considered to prevent the formation of additional fractures and occurrence of rock failure resulting from the softening effect of long-term ScCO 2 -H 2 O-interaction.