The maximum permissible fracturing pressure in shale gas wells for wellbore cement sheath integrity

Sustained Casing Pressure (SCP) has been recognized as a major problem in multi-fractured shale gas wells. The problem is attributed to the failure of cement sheath during the hydraulic fracturing process. Currently available mechanics models predict well integrity conditions with high error. We believe the reason is that the debonding at the casing-cement interface and plastic deformation of cement do not create significant axial channels for gas to migrate to the surface but the radial cracking of cement sheath is responsible for the gas migration resulting in the SCP. Although some mathematical models can predict radial cracking of cement sheath, they have limitations in applications because they do not account for the weakened cement sheath and reduced support from formation rock due to the low efficiency of cement placement in the annulus. An analytical model was derived in this work to predict the Maximum Permissible Fracturing Pressure (MPFP) due to radial cracking with consideration of the weakened cement sheath and the reduced support from formation rock. The results are in good agreement with observations in the Fuling shale gas field, China. Sensitivity analyses show that improving cement strength is not an effective means of increasing MPFP. The MPFP should be enhanced by increasing the thickness of the cement sheath using large wellbore size and improving the cement placement efficiency. This work also shows that cement sheath failure will occur before casing failure occurs in the hydraulic fracturing process. Therefore integrity of cement sheath should be considered in the hydraulic pressure design.