The cyclic injection and production of fluids into and from underground gas storage (UGS) may lead to caprock failure, such as capillary sealing failure, hydraulic fracturing, shear failure, and fault slipping or dilation. The dynamic sealing capacity of a caprock-fault system is a critical constraint for safe operation, and is a key factor in determining the maximum operating pressure (MOP). This study proposed an efficient semi-analytical method for calculating changes in the in situ stress within the caprock. Next, the parameters of dynamic pore pressure, in situ stresses, and deformations obtained from reservoir simulations and geomechanical modeling were used for inputs for the analytical solution. Based on the calculated results, an experimental scheme for the coupled cyclic stress-permeability testing of caprock was designed. The stability analysis indicated that the caprock was not prone to fatigue shear failure under the current injection and production strategy, supported by the experimental results. The experimental results further reveal that the sealing capacity of caprock plugs may remain stable. This phenomenon is attributed to cyclic stress causing pore connectivity and microcrack initiation in certain plugs, while leading to pore compaction in others. A comparison between the dynamic pore pressure and the minimum principal stress suggests that the risk of tensile failure is extremely low. Furthermore, although the faults remain stable under the current injection and production strategies, the continuous increase in injection pressure may lead to an increased tendency for fault slip and dilation, which can cause fault slip ultimately. The MOPs corresponding to each failure mode were calculated. The minimum value of approximately 36.5 MPa at capillary sealing failure indicated that the gas breakthrough in the caprock occurred earlier than rock failure. Therefore, this minimum value can be used as the MOP for the target UGS. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

This paper investigates the feasibility of a proposed underground gas storage facility. Based on S gas storage, a large-scale 2D hydromechanical coupling FEA model is established to explore the geo-mechanical properties of S gas storage under a multi-cycle alternating injection and production and validated by the interference logging test. To account for the damage development of fault damage area under the influence of seepage-stress coupling, the soil adopts the Mohr-Coulomb constitutive assumption. Additionally, a zero-thickness cohesive element is proposed as a mechanical model to simulate the fault gouge. The mechanical parameters of zero-thickness cohesive elements are verified by a ring shear test and a preliminary FE model. Thereafter, another refined conceptual finite element (FE) model considering the fault damage area, fault core, water-containing damaged area, overburden damaged area, and the contact model between different damaged areas of the fault and the fault core is developed and validated. The simulation results demonstrate that the initial seal ability of the caprock and faults remains intact. Specifically, (i) the maximum caprock and ground displacements are 8.5 cm and 5.4 cm, respectively. (ii) The most significant slip distance is 0.125 mm, indicating that, leakage under the action of multi-period alternating injection-production, the S aquifer structure had no fault activation and caprock. (iii) The risk of fault activation is higher for high-angle faults compared to low-angle faults. Low-angle faults are more susceptible to shear slip. Providing a scientific reference for the feasibility study of gas storage.