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 integrity of wellbore cement is an important guarantee for the long-term safety and effectiveness of carbon dioxide geological storage. During the process of CO2 capture, utilization, and storage, construction factors will cause changes in temperature and pressure distribution, leading to changes in the pore structure of cement and a decline in compressive strength, which can easily cause the failure of cement ring integrity. To provide theoretical guidance and analysis methods for evaluating the mechanism of cement strength performance degradation and optimizing injection parameters, in this study, we conducted the following research based on relevant studies: (1) The fracture theory was revised based on the characteristic factors of oil-well cement; (2) a pore structure model was established to analyze the failure process of cement, clarifying the relationship between pore structure and the compressive strength of cement; (3) the parameters of the pore structure model were determined and analyzed, considering the influence of cement content and total porosity on the model, and finally, the regression parameter K value was determined to be 1600. This article can provide a useful reference for the research on the failure of cement rings in the CO2 injection process and other related studies.

Gas storage wells in eastern China have been in operation for nearly 30 years. Fatigue damage may have occurred in the cement sheath after years of injection-production cycles. If the risk of integrity failure of the cement sheath can be predicted accurately, optimization or remedial measures can be implemented over time to further prolong the life of gas storage wells. In this paper, we established a mechanical equivalent method to restore the wellbore load borne by the cement sheath to the self-developed wellbore simulation device, and carried out the cyclic load tests of 376, 141 and 54 rounds at the injection-production differential pressure of 5, 10 and 15 MPa, respectively. Based on this method and the twin-shear unified strength theory, the integrity failure mechanism of the cement sheath was analyzed. We found that after cyclic loading, the tensile strength of the cement sheath decreased. Fatigue-tensile failure was the main reason for the integrity failure of the cement sheath in gas storage wells. Residual tensile strength decreased with the increase in cycle time and injection-production differential pressure. Taking a gas storage well in eastern China as an example, it was found that if the well maintains the current injection-production differential pressure, there is a risk of cement sheath failure. In this regard, the optimization measures of reducing injection-production pressure difference were proposed to prolong wellbore life. At the same time, the suggestion of using a modified cement system in gas storage wells was also proposed. The research results can provide an important reference for predicting gas storage well life and designing mechanical properties of cement sheath.