The mechanical damage and failure of cement sheaths in gas storage wells under cyclic loading have been studied extensively. However, because the test device cannot restore the wellbore condition, most studies have been theoretical or regular experimental. If the load‐bearing mode and stress environment in a test device differ from those in a wellbore, then the damage and failure modes will deviate from what occurs in the actual wellbore. Therefore, it is necessary to explore a method that restores the wellbore condition and design a wellbore simulation device that reveals the deformation and damage of the cement sheath. In this study, the development laws of microannulus and microcracks in the cement sheath were studied by triaxial cyclic loading, low‐field nuclear magnetic resonance imaging, and scanning electron microscopy. A device to evaluate cement sheath integrity was then developed. A stress equivalence method was proposed, based on the principle that the stresses at the first and second interfaces of the device are equal to those of the wellbore cement sheath. This method was used to design material, size, and experimental conditions to simulate the load‐deformation law of a cement sheath in a gas storage well. The damage failure mechanism was investigated using the simulation results and computed tomography. Micropores and microcracks in the cement sheath increased continuously under cyclic loading. Damage accumulated, causing strength failure as the period of the cyclic loading increased. Plastic deformation of the cement sheath occurred under cyclic loading, but interfacial peeling did not. The reason is that the cement sheath is under compressive stress during loading and unloading, and the interface between the cement sheath and the outer wall is not damaged. This study provides a new method for studying the mechanical failure of a cement sheath under complex wellbore conditions.

During their life cycle, high‐pressure gas wells experience circulating working fluid, acid fracturing, blow off, and other development and production conditions. This may lead to the failure of the cement sheath integrity and result in sustained casing pressure (SCP). Therefore, we explored the failure types and mechanisms of the cement sheath using different wellbore operating procedures. In this study, we used the downhole packer as the demarcation point; the integrity of the cement sheath at the upper and lower parts of the packer was tested through a self‐developed wellbore simulation device, which is based on the equivalent theory of cement sheath interface differential pressure. Results showed that the lower part of the single‐layer cement sheath underwent compressive strength failure due to the pressure drop in the wellbore before perforation. Plastic deformation of the cement sheath occurred during acid fracturing. In addition, the cooling effect caused by the acid fracturing led to the bonding failure at the cement sheath's second interface. The double‐layer cement sheath's inner‐layer cement sheath was subjected to tensile failure – attributed to high‐pressure – and the outer‐layer cement sheath maintained its integrity under the pressure changes. Considering the risk factors associated with integrity failure, we propose an engineering optimization plan in the study. The retest results showed that reducing the internal casing pressure and temperature and controlling the annular pressure in the production stage was beneficial in ensuring the integrity of the cement sheath at the lower and upper parts of the packer, respectively. The research results provided an important reference for ensuring the integrity of the cement sheath of high‐pressure gas wells.

The plastic deformation and interface micro annulus of oil-well cement during acid fracturing are key reasons for the failure of the wellbore seal and sustained casing pressure. However, most of the existing research ignores the influence of the wellbore cooling effect during acid fracturing, owing to which, the design conditions in the theoretical analysis may be inconsistent with those of the actual wellbore, and the calculation results may be biased. In this study, a novel elastoplastic mechanical model of the cement sheath was established. This model can analyze the yield state of the cement sheath under the influence of three-dimensional principal stress and consider the effect of the differential temperature stress on the interface debonding of the cement sheath from the beginning to the end of acid fracturing. Moreover, the generation mechanism and development law of the interface micro annulus were clarified. The findings indicated that the influence of the intermediate principal stress cannot be ignored; otherwise, the elastoplastic analysis results of the cement sheath may be conservative. During acid fracturing, the casing–cement sheath interface is influenced by the differential temperature stress, and the interface is debonded; however, a micro annulus is not generated. The debonding of the cement sheath–formation interface and micro annulus occurs only when the cement sheath is completely plastic. After acid fracturing, the interface micro annulus is likely to be generated at the casing–cement sheath interface, and the presence of the differential temperature stress may increase the formation risk of the interface micro annulus. The research results can provide theoretical guidance for the prediction of oil-well cement sheath interface seals under acid-fracturing conditions.