Analysis of the influence of cement sheath failure on sustained casing pressure in shale gas wells

摘要

Sustained casing pressure (SCP) in shale gas wells has been of growing interest in the industry, owing to its effects on the safety and efficiency of shale gas production. In terms of the current literature, this issue should be attributed to the elastic failure of the cement sheath under high inner pressure during the hydraulic fracturing process. However, the current calculation of cement sheath stress neglects the influence of the drilling and cementing processes and the dramatic temperature change of a well system during the hydraulic fracturing process. In this work, a novel elastic-plastic model incorporating the effects of drilling, cementing, and dramatic temperature change of the well system during the hydraulic fracturing is established to describe the stress of cement sheath. The failure mechanism of cement sheath is fully revealed. The proposed model for cement sheath integrity is successfully verified through its excellent agreement with field observations from the Fuling shale gas field, China. In addition, based on the proposed model, the impacts of the geometrical and material parameters of casing and cement are studied and analyzed. The results illustrate that the tensile failure more likely occurs at the inner face of the cement sheath, which turns out to be more severe as the true vertical depth decreases. The density of cement slurry is in favor of decreasing the tensile stress in the cement sheath. On the contrary, the thicker cement sheath would increase the risk of tensile failure in the cement sheath, which would lead to serious SCP. The extremely low elastic modulus and thermal conductivity of the cement sheath are beneficial for maintaining its integrity, which, unfortunately, is difficult to achieve in actual operations.