Influence of the WHCP on cement sheath stress and integrity in HTHP gas well

Abstract

The wellhead casing pressure (WHCP) resulted from gas production, downhole operations, casing and oil tubing or packing leakage poses great challenges to the mechanical integrity of cement sheath in HTHP gas wells. This paper proposed an analytical model of cement sheath on the basis of the multi-layer thick wall cylinder theory and Mohr–Coulomb failure criterion. The pressure and temperature loads were both considered in the model and the calculated stresses are all in good agreement with those obtained by FEA method. Meanwhile, the influences of the WHCP on cement sheath stress and integrity have been investigated with the proposed model, also a safety factor diagram of cement sheath at the weakest casing interface has been drawn by considering WHCP ranged from 10 to 70 MPa and wellbore temperature change ranged from −60 to 60 °C. Results show that the greater the WHCP, the lower the safety factor and the smaller the influence of wellbore temperature change on it. If the WHCP is less than 40 MPa, wellbore temperature effect cannot be ignored. For a specific gas well, the safety factor diagram can be used to determine the reasonable maximum allowable WHCP, which is of great guiding significance for protecting the cement sheath integrity and improving the WHCP management level in HTHP gas well.