Experimental study on dynamic sealing capacity and safe threshold of caprock in underground gas storages.

The sealing capacity of the caprock of a gas-reservoir-type underground gas storage (UGS) directly affects the safe storage and efficient utilization of natural gas, especially during a high rate of multicycle injection-withdrawal under different pressure ranges. In order to determine the safety threshold, we propose an in-situ stress-fluid coupling simulation and evaluation method to analyze the dynamic breakthrough pressure of caprock, and use the caprock samples from three UGSs to explore the evolution of the sealing capacity at alternating stress and the critical sealing capacity. We find that the fatigue effect caused by multicycle injection-withdrawal can change the pore-throat structure and enhance the in-situ sealing capacity of the caprock. Increasing gas injection pressure leads to the decline of breakthrough pressure, and the failure of capillary sealing occurs before mechanical failure. Our results show that the sealing capacity of caprock changes dynamically under in-situ effective stress during injection-withdrawal periods, and the dynamic sealing mechanism reveals the risk timing of the caprock, and guides to release the full potential of an UGS. • A novel method to evaluate dynamic sealing capacity of UGS caprock is proposed. • Safe threshold of caprock is found under in-situ stress-fluid coupling condition. • Sealing capacity of caprock changes dynamically during injection-withdrawal. • Capillary sealing failure is more likely to cause leakage than mechanical failure. [ABSTRACT FROM AUTHOR]