This paper provides an extension of an existing elasto-plastic framework originally proposed by Gens & Nova (1993) for modelling the response of structured soils and soft rocks. The model is enhanced to reproduce not only the mechanical response of caprocks under standard monotonic triaxial loading, but also the effects of the environmental and hydraulic loading induced by modern energy applications, including gas/hydrogen storage and geological carbon storage. The novelty of these applications, compared to the more usual ones developed by the oil and gas industry over decades, lies in the complex pore fluid and stress pressure histories applied and in the strong geochemical interaction of the rock formations with non-native fluids. Cyclic pore pressure histories due to seasonal gas storage may result in a mechanical degradation of the caprock material, while chemical degradation may occur due to pore water acidification resulting from the rock-water-CO2 interaction. To cope with the cyclic mechanical degradation, the framework is first coupled with the extended overstress theory, so to satisfactorily reproduce the time-dependent behaviour of caprocks, which presents inelastic strains even within the yield surface. Such an extension is shown to be essential to reproduce the strong strain-rate dependence and the increase in the number of cycles to failure with the amplitude of cyclic loading observed in experimental data obtained on intact specimens of an Italian stiff carbonatic clay. The elasto-plastic model is then enhanced to account for chemical degradation, using the calcite mass fraction dissolution as a variable controlling damage evolution. Combined with a geochemical reactive transport model, this extension satisfactorily reproduces the progressive degradation of a Chinese shale due to CO2 exposure, showing the ability of the framework to model coupled geo-chemo-mechanical processes.

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/).