Evaluation on the sealing capacity of caprocks in underground gas storage by measuring breakthrough pressure under complex stress paths

Natural gas plays a critical role in advancing national energy transitions. Increasing underground natural gas storage capacity is essential for ensuring energy supply security. The operation of underground natural gas storage results in cyclic stress variations in formation that can significantly affect the pore structures and sealing performance of caprocks. This study investigates the impacts of lithology, permeability, and cyclic loading on the sealing capacity of caprocks. Using a self-developed experimental device, the breakthrough pressure under alternating loads was measured. Results indicate that the breakthrough pressure varies by lithology in the following the order: rock salt > mudstone > shale > muddy siltstone (MS) > silty mudstone (SM) > volcanic rock (VS) > marlstone > sandstone. A functional relationship exists between rock permeability and breakthrough pressure. However, variations in rock composition cause notable variations in the parameters of this relationship. The breakthrough pressure increases linearly with the confining pressure. For example, when the confining pressure is increased from 5 MPa to 30 MPa, the breakthrough pressure of argillaceous siltstone rises from 0.73 MPa to 12.2 MPa, that of mudstone rises from 0.7 MPa to 9.5 MPa, and that of marlstone rises from 0.6 MPa to 5.5 MPa. The cyclic loading experimental results demonstrate that cyclic stress affects caprock sealing capacity. The dynamic breakthrough pressures increased by 12%, 14%, and 7% compared to values under static state, respectively. It highlights the impact of cyclic stress on caprock sealing and provide valuable insights for improving the safety and efficiency of underground gas storage systems.