Shale gas well engineering practice demonstrates that volume fracturing easily leads to the integrity failure of cement sheaths, thus causing sustained casing pressure (SCP). According to statistics, the SCP wells account for as high as 79.52% among 166 brought in wells in China's Fuling shale gas field. The analysis has shown that the ratio of SCP is very low before fracturing, but it increases substantially after fracturing. The ratio of the first-grade casing head (between surface casing and intermediate casing) increases from 14.85% to 50.05%, meanwhile, that of the second-grade casing head (between intermediate casing and production casing) increases from 15.84% to 53.01%. This fully indicates that the influence of casing fracturing on sustained casing pressure of shale gas wells is relatively greater.
To address this problem, a wellbore temperature field model is established so as to obtain the required input parameters of the dynamic temperature boundary, considering the friction heat of liquid and the influence of fracturing fluid on heat exchange coefficient, and at the same time, a numerical model of casing-cement sheath-formation is proposed and analyzed. Sensitivity analysis is conducted on the influences of internal wellbore pressure, fracturing fluid displacement, initial temperature, Young's modulus as well as Poisson's ratio on the cement sheath stress. Furthermore, the cement sheath integrity is evaluated by using Mohr-Coulomb failure criteria.
The results show that: (1) during the fracturing of shale gas wells, the temperature of the cement sheaths change drastically with time. Besides, there exists a significant temperature difference between inner and outer walls, which increases at first and then decreases throughout the fracturing operation. (2) During the fracturing of shale gas wells, the radial and tangential stresses of the cement sheath continuously change over time. The radial stress decreases first and then increases. The tangential stress decreases first, then rises and finally decreases again. Judging from the Mohr-Coulomb failure criterion, cement sheaths are prone to tensile failure, and the initial stage of fracturing is the "risky phase" for the failure of cement sheaths. (3) Reducing internal wellbore pressure can effectively decrease the radial and tangential stresses of cement sheath. Besides, the reduction of fracturing fluid displacement decreases the radial and tangential stresses of the cement sheath, however, the decreases are not obvious. Elevation of initial fracturing fluid temperature can increase the radial stress of the cement sheath and decrease the tangential stress. In addition, reducing the Young's modulus significantly decreases the radial and tangential stresses of the cement sheath. When the Young's modulus reaches a certain value, the tangential stress is lower than the tensile strength. As well, the radial stress is lower than compressive strength. Elevation of the Poisson's ratio of the cement sheath can decrease the tangential stress.Based on the calculated results, a new cement slurry system was designed, which can decrease the Young's modulus of the cement sheath. Engineering practice demonstrates that there is no occurrence of SCP after multistage hydraulic fracturing, thus ensuring the safe production of shale gas wells. Besides, the results of this study can provide a reference for cement slurry design and wellbore integrity control during fracturing of shale gas wells.