Sustained casing pressure brings heavy routine maintenance and management workload, impedes stimulation treatments such as fracture and threatens safe production. This paper uses numerical model to analyze rising process of sustained casing pressure and study how cement sealed integrity is damaged, finally controlling sustained casing pressure by improving cement properties.

Cement sealed integrity collapse is one of the main reasons of sustained casing pressure, which distributes widely in high-pressure gas fields and gas storage reservoirs. First, numerical model is built according to volume consistency and mass conservation law to study rising process of sustained casing pressure. This model cam also clarify the relationship between sustained casing pressure and cement sealed integrity. And then a nonlinear equation set is established to study the mechanism of cement sealed integrity collapse. This equation set is based on Culon-Morper rule and stress-displacement continuity law. It can calculate micro annulus caused by wellbore pressure change. Lastly, a numerical model is built to predict the sustained casing pressure caused by micro annulus. Through this model, the influencing law of every factor is analyzed to find effective measures to mitigate sustained casing pressure and improve cement sealed integrity. The numerical calculation indicates that pressure rising period becomes shorter as cement micro annulus increases. Micro annulus happens when tensile force exceeds cement bonding strength. Optimizing the cement Poisson's ratio, and elasticity modulus can prevent micro annulus. Meanwhile, the casing internal pressure change should be controlled and cement bonding strength must be increased. It can inhibit micro annulus by using self-healing cement, casing sand adhesion and expansion casing, thus preventing sustained casing pressure. The optimization of cement top can permanently control sustained casing pressure.

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