Cement sheath sealing integrity evaluation under cyclic loading using large-scale sealing evaluation equipment for complex subsurface settings

摘要

Cement sheaths should provide zonal isolation and structural support through the entire service lifetime of wells. However, even if high-quality primary cementing was achieved, variations in pressure, for example, those due to staged fracturing in shale gas wells, are likely to lead to sealing integrity failure of the cement sheath. This work presents large-scale sealing evaluation equipment to assess the failure mechanisms of cement sheaths under cyclic loading. The equipment includes a borehole system, pressure system, and control and data acquisition system for automatic loading and unloading, gas channeling monitoring and pressure change and gas output recording. The pressure limit of the equipment is 120 MPa. A finite element model of the borehole system and sealing equipment was built to study the stress and strain under internal pressure. The failure mechanisms of the sealing integrity of the cement sheath were obtained by combining the test results with the finite element calculations. At a high internal pressure, plastic deformation was produced during loading due to the residual strain that arose after unloading of the cement sheath. Therefore, incompatible deformation occurred, and the tensile stress increased at the interfaces of the cement sheath. Under repeated loading and unloading, the plastic deformation increased, and the residual strain and interface tensile stress also increased. When the interface tensile stress exceeded the bonding strength, a microannulus formed between the cement sheath and casing. Based on these results, guidelines for the assessment of the long-term sealing integrity of a shale gas well cement sheath and for the selection of mechanical properties of the cement systems are given to mitigate the risk of zonal isolation failure.