The cement sheath, serving as the primary element of well barriers, plays a crucial role in maintaining zonal isolation, protecting the casing from corrosion, and providing mechanical support. As the petroleum industry shifts from conventional to deep unconventional resources, the service environment for cement sheaths has become increasingly complex. High temperatures, high pressures, cyclic loading, and thermal stresses in downhole conditions have significantly increased the risk of cement sheath failure. A growing trend toward theoretical analysis of stress distribution, failure modes, and control mechanisms within the casing-cement sheath-formation system is evident. This paper comprehensively reviews theoretical research on cement sheath integrity from four key perspectives: (1) the concept of cement sheath integrity failure, (2) cement sheath constitutive models, (3) analytical models of the cement sheath-casing-formation system, and (4) numerical simulations of the cement sheath-casing-formation system. Through these discussions, this review provides profound insights into cement sheath integrity failure and offers valuable guidance for future research and practices.

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.

The cement sheath-formation interface is a weak link in wellbore isolation systems, which is prone to the failure of sealing integrity during fracturing, affecting the fracturing effect, and wellbore safety. In this study, we establish a new model to evaluate the failure mode and crack propagation of cement sheath-formation interface. Using the proposed model, we predicted the propagation direction and length of the interface and zigzag cracks caused by the interface stress, fracturing load, and stress intensity factor (SIF) under temperature and pressure in the shale oil well X in the eastern China oil field. The prediction results were consistent with the distribution law of measured microseismic events. In addition, we analyzed the influence of the internal casing pressure, mechanical properties of cement sheath, and formation on the sealing integrity failure and cracks. At the same time, we discussed the influence of these mechanical properties on the propagation length of interface and zigzag cracks and the propagation direction of the zigzag crack in detail, and proposed the optimal mechanical properties design to avoid the crack propagating to the cement sheath and causing its failure. The results can provide effective guidance for sealing integrity prediction and cement sheath mechanical performance design.