Influence of supercritical CO2-H2O-caprock interactions on the sealing capability of deep coal seam caprocks related to CO2 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 (ScCO2)-H2O-rock interactions for caprock samples obtained from the Qinshui Basin, China, to explore the influence of carbon dioxide (CO2) injection on the sealing capability of deep unminable coal seam caprocks as it relates to CO2 geological storage. This research focused on the changes in mineral composition, pore-fissure structure, permeability, and mechanical properties of the caprocks after ScCO2-H2O treatment. These results revealed that ScCO2-H2O-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 CO2 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 ScCO2-H2O-rock interactions. The caprock integrity determines the security of the CO2 geological storage. However, both natural and induced fractures provide channels for CO2 leakage. The compressive strength of the rock was reduced by 36 %, and the toughness increased after 60 days of ScCO2-H2O treatment. Thus, the formation of new fractures and the reopening of pre-existing faults in caprock may occur before damage under stress changes and ScCO2-H2O-rock interactions. Fracturing and shedding of fault gouge and particles inside the fractures by high-pressure ScCO2-H2O fluid can expand the fractures in caprock significantly, and hence increase the risk of CO2 leakage. Therefore, deep coal seams with thick and intact caprock should be selected to increase the safety of CO2 geological storage. Furthermore, the CO2 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 ScCO2-H2O-interaction.

Automated summary

The study found that supercritical CO2-H2O-rock interactions can lead to the formation of new fractures and reopening of existing faults in caprock, increasing the risk of CO2 leakage. To enhance the safety of CO2 geological storage, deep coal seams with thick and intact caprock should be chosen, and the CO2 injection pressure should be considered.