Study on channeling-leakage pressure of casing-cement sheath-stratum assembly in thermal recovery wells

Abstract

The main purpose of the article is to study the critical leakage pressure of the casing cement sheath stratum assembly for cyclic alternating thermal effect in the thermal recovery wells, to provide technical measures for sealing integrity of thermal recovery wells. To achieve those objectives, a device for evaluating the sealing integrity of casing-cement sheath-stratum assembly in a high-temperature, high-pressure cyclic alternating environment is established in this paper due to wellbore safety risks of offshore thermal recovery directional wells, such as wellbore fluid channeling-leakage and sustained annular pressure caused by the alternating thermal effect of multi-round steam huff and puff. The device is adopted to simulate two working conditions: low-temperature production at 50 ℃ and ultra-high temperature curing at 350℃. The channeling-leakage curves of different gas-sealing pressures at cementing interfaces I and II are measured to obtain the critical channeling leakage pressure of the assembly. The influence rules of cyclic alternating thermal effect on the microstructure and pore characteristics of cement sheath are analyzed using industrial CT scanning technology. The study clarifies the equivalent diameter of pores inside the cement sheath and the dimensional variation of cracks. Moreover, the mechanism of cementing interface damage is revealed, and the wellbore’s sealing integrity failure under cyclic alternating thermal effect acting on thermal recovery wells is investigated. Technical measures are proposed to improve the wellbore’s sealing integrity of thermal recovery wells from three aspects: elastic-plastic parameters design of cement sheath, elastic-specific work optimization, and control of wellbore critical temperature and pressure. Thus, theoretical basis and technical support are provided for ensuring the wellbore’s entire-life sealing integrity of offshore thermal recovery wells.