Sealing Failure Mechanism and Control Method for Cement Sheath during Hydraulic Fracturing

This study focused on the sealing failure mechanism and control method for a cement sheath during hydraulic fracturing. Taking a shale gas well as an example, whole wellbore numerical models of the casing-cement sheath-formation assembly were established, failure modes of the cement sheath at different depths were clarified, and control methods were proposed based on the calculation results. The following conclusions were drawn. (1) The maximum radial/tangential stress of the cement sheath increased/decreased with an increase in the depth, and the cement sheath above the intermediate casing shoe posed the risk of tangential tensile failure, resulting in tensile cracks. The cement sheath below the intermediate casing shoe produced a micro-annulus under a cyclic casing pressure, and the tensile cracks and micro-annulus constituted passages for the sustained casing pressure. (2) The swelling stress of the expansion cement slurry could offset the circumferential tensile stress and increase the radial compressive stress. Because a cement sheath with a high Young's modulus usually exhibits high tensile and compressive strengths, it is recommended to use a high Young's modulus cement slurry system above the intermediate casing shoe and optimize the free expansion ratio. (3) In comparison with ordinary cement stone, low residual strain cement stone exhibited a larger elastic deformation interval. The cumulative residual strain caused by cyclic loading was smaller, and the Young's modulus demonstrated a lesser decrease. The results of an equivalent physical experiment demonstrated that an ordinary cement sheath lost its integrity after 13 loading cycles with a maximum casing pressure of 60 MPa. A low residual strain cement sheath could guarantee integrity after 30 loading cycles when the maximum casing pressure was 90 MPa, and sealing failure occurred after 11 loading cycles when the maximum casing pressure was 130 MPa. It is recommended to use a low residual strain cement slurry below the intermediate casing shoe to prevent a micro-annulus.