Sealing failure mechanism and control method for cement sheath in HPHT gas wells

Abstract

The sustained casing pressure (SCP) caused by the sealing failure of cement sheaths has become one of the main threats to the safe and efficient production of high-temperature and high-pressure (HTHP) gas wells. An actual HTHP gas well was used as an example, and the temperature calculation models were established. Then, the temperature–pressure coupling finite element coupling models of casing-cement sheath-formation assembly for the whole well were established, and the factors affecting the integrity were examined. Finally, the maximum production and wellhead annular pressure based on the constraints of cement sheath integrity was analyzed. The results indicate that the natural gas would transfer energy to the wellbore assembly during production; the greater the production, the greater the temperature fluctuation of the cement sheath, which was unbeneficial to the integrity, the radial and tangential stresses of the cement sheath increase and decrease, respectively, with depth. The cement sheath near the ground was at risk of tensile failure, which could result in tangential tensile cracks. The cement sheath near the packer was susceptible to compression failure, resulting in radial compression cracks, reducing production and wellhead annular pressure were beneficial to relieve the stress state. Compared to the production, the wellhead pressure has a greater impact, it is necessary to control the wellhead pressure in a reasonable range to ensure integrity.