Numerical Modeling Study on Mineral Alteration and Sealing Performance for CO2 Geological Sequestration with Enhancing Water Recovery in Hydraulic Fractured Shale Reservoirs

Recently, CO2 geological sequestration combined with enhancing deep saline water/brine recovery is regarded as a potential strategic choice for reduction of CO2 emissions. This technology not only achieves the relatively secure storage of CO2 which was captured during industrial processes but also can enhance the recovery of water for drinking, industrial, and agricultural utilization. However, the impact of CO2-water-rock reactions on the shale reservoir in the system is unclear and the sealing performance of mudstone caprock has not been investigated. For analyzing the mechanism of mineral alteration in the shale reservoir, a three-dimensional injection-production model in the double-fractured horizontal well pattern is established according to actual parameters of shale and mudstone layers. In addition, mineral alteration was characterized and caprock sealing performance was also assessed. Numerical results showed that the presence of CO2 can lead to the dissolution of k-feldspar, oligoclase, chlorite, and dolomite and the precipitation of clay minerals such as kaolinite, illite, and smectite (Ca-smectite and Na-smectite). Due to positive ion released by dissolved primary minerals, the precipitation of secondary carbonate occurs including ankerite and dawsonite, which induces the mineral sequestration capacity of the shale reservoir. The amount of CO2 sequestration by mineral is 51430.96 t after 200 years, which equals 23.47% of the total injection (219145.34 t). Besides, the height of the sealing gas column is used for evaluating the sealing performance of the shale-mudstone interface. Results show that the height of the sealing gas column at the interface above the injection well is lower but the maximum value of CO2 gas saturation is only 0.00037 after 200 years. The height of the sealing gas column at the interface is greater than 800 m, which can be classified as level II and guarantee the security of the CO2 storage. The analysis results provide reliable guidance and reference for the site selection of CO2 geological sequestration.