A new numerical investigation of cement sheath integrity during multistage hydraulic fracturing shale gas wells

Volume fracturing of shale gas wells has been observed to lead to the failure of cement sheath and to sustained casing pressure (SCP). This paper presents a new numerical investigation aimed at understanding the failure mechanism of the cement sheath during volume fracturing. A wellbore temperature model was established to obtain the required input parameters for the dynamic temperature boundary. The numerical model considers the coupling of casing, cement sheath, and formation rock to calculate the radial and tangential stresses in the cement sheath. In addition to the cement mechanical properties, the influencing factors considered include the casing pressure, fracturing fluid displacement, and initial temperature. The cement sheath integrity was evaluated using the Mohr-Coulomb failure criterion. The results show that the temperature of cement sheath changes drastically during fracturing. The radial and tangential stresses in the cement sheath change continuously with time. Lowering the internal wellbore pressure can effectively reduce the radial and tangential stresses in the cement sheath. Reducing the fracturing fluid displacement can significantly lower the radial and tangential stresses in the cement sheath. Increasing the initial fracturing fluid temperature will cause the radial stress of cement sheath to increase and the tangential stress to decrease. The use of cements with low Young's modulus values can significantly reduce the radial and tangential stresses in the cement sheath. The use of cements with low Poisson's ratio values can lower the tangential stress. The results obtained using the model were verified by field data that demonstrated no occurrence of casing pressure when using a cement with a low elastic modulus, as suggested by the model.