Effects of acid–rock reaction on physical properties during CO2-rich industrial waste gas (CO2-rich IWG) injection in shale reservoirs

Abstract

"Carbon peaking and carbon neutrality" is an essential national strategy, and the geological storage and utilization of CO2 is a hot issue today. However, due to the scarcity of pure CO2 gas sources in China and the high cost of CO2 capture, CO2-rich industrial waste gas (CO2-rich IWG) is gradually emerging into the public's gaze. CO2 has good adsorption properties on shale surfaces, but acidic gases can react with shale, so the mechanism of the CO2-rich IWG–water–shale reaction and the change in reservoir properties will determine the stability of geological storage. Therefore, based on the mineral composition of the Longmaxi Formation shale, this study constructs a thermodynamic equilibrium model of water–rock reactions and simulates the regularity of reactions between CO2-rich IWG and shale minerals. The results indicate that CO2 consumed 12% after reaction, and impurity gases in the CO2-rich IWG can be dissolved entirely, thus demonstrating the feasibility of treating IWG through water–rock reactions. Since IWG inhibits the dissolution of CO2, the optimal composition of CO2-rich IWG is 95% CO2 and 5% IWG when CO2 geological storage is the main goal. In contrast, when the main goal is the geological storage of total CO2-rich IWG or impurity gas, the optimal CO2-rich IWG composition is 50% CO2 and 50% IWG. In the CO2-rich IWG–water–shale reaction, temperature has less influence on the water–rock reaction, while pressure is the most important parameter. SO2 has the greatest impact on water–rock reaction in gas. For minerals, clay minerals such as illite and montmorillonite had a significant effect on water–rock reaction. The overall reaction is dominated by precipitation and the volume of the rock skeleton has increased by 0.74 cm3, resulting in a decrease in shale porosity, which enhances the stability of CO2 geological storage to some extent. During the reaction between CO2-rich IWG–water–shale at simulated temperatures and pressures, precipitation is the main reaction, and shale porosity decreases. However, as the reservoir water content increases, the reaction will first dissolve and then precipitate before dissolving again. When the water content is less than 0.0005 kg or greater than 0.4 kg, it will lead to an increase in reservoir porosity, which ultimately reduces the long-term geological storage stability of CO2-rich IWG.