Effect of thermal expansion annulus pressure on cement sheath mechanical integrity in HPHT gas wells

Highlights: • Cement sheath mechanical calculation model with thermal expansion annulus pressure considered is established. • Maximum production and thermal expansion annulus pressure are determined based on the cement sheath safety evaluation. • Thermal expansion coefficients of casing, cement sheath differ from each other greater, cement sheath safety factor is lower. • The increasing annulus thermal expansion pressure and temperature might prominently reduce the cement sheath safety factor. - Abstract: In HPHT gas wells, thermal expansion annulus pressure might threaten the mechanical integrity of the cement sheath. To analyze the effect of the thermal expansion annulus pressure on the cement sheath mechanical integrity, the cement sheath safety factor calculation model with thermal expansion annulus pressure considered was established. A HPHT gas well in the west part of the South China Sea was analyzed, the relative error of the calculated thermal expansion annulus pressure with field data is less than 8.0%. The increasing thermal expansion annulus pressure might prominently reduce the cement sheath safety factor. The cement sheath safety factor decreases quickly when the thermal expansion annulus pressure is under 18 MPa. For the case well, the maximum thermal expansion pressures of annulus A, B, C are 39 MPa, 24.4 MPa and 18.5 MPa respectively, the production shall not exceed 98.6 × 10⁴ m³ d⁻¹. The cement sheath safety factor is maximum when the thermal expansion coefficients of casing, cement sheath and formation are equal to each other. The iso-surfaces of the cement sheath safety factor under different thermal expansion annulus pressure and temperature are established to obtain the critical value of the relevant parameters including thermal expansion coefficients of casing, cement sheath and formation, elasticity modulus and Poisson ratio as well as the temperature which satisfy the mechanical integrity of the cement sheath.