Experiment and failure mechanism of cement sheath integrity under development and production conditions based on a mechanical equivalent theory

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.