Evaluation technology for isolation capacity of cement sheath in HTHP high-sulfur gas wells

As CNOOC develops offshore and overseas complex oil and gas fields vigorously, there are more and more HTHP (high temperature and high pressure) high-sulfur wells. And the annulus pressure or acidic gas leakage caused by the isolation failure of cement sheath is the bottle neck which restricts the safe operation and production of HTHP high-sulfur gas fields. In this paper, the main reasons for the isolation failure of cement sheath in HTHP high-sulfur wells were analyzed from the aspects of influence factors and failure mechanisms. It is indicated that the mechanical and structural change of set cement is one of the main factors influencing the isolation capacity of cement sheath under high temperature, high pressure and high sulfur. Then, based on the characteristics of HTHP high-sulfur wells, the evaluation idea on the isolation capacity of cement sheath was proposed, the evaluation method on the interlayer isolation capacity of cement sheath was established, and the technical measures to improve long-term isolation capacity of cement sheath were developed. In this way, it provides the new technical development direction and solution for improving the isolation integrity of cement sheath in HTHP high-sulfur wells and prolonging the service life of oil and gas wells.