Integrity evaluation method for cement sheath of hydraulic fracturing wells

In hydraulic fracturing for fractured wells, the pressure of cyclic loading and unloading in the casing can easily lead to integrity failure of the cement sheath in the fracturing section, representing a production safety hazard. The integrity failure evaluation experiments of cement sheath were carried out for fractured wells under the condition of alternating loading. The cause of integrity failure was analyzed based on the fatigue cumulative damage theory and failure criteria. The integrity failure law was obtained for cement sheath under a confining pressure of 30 MPa and alternating loading. The results demonstrate the following findings: firstly, under alternating pressure, the internal structure of the cement is damaged, and its mechanical properties (such as compressive strength) decrease with increases in the number of alternating pressure cycles; the maximum decrease can be 40%. Secondly, the bond strength of the casing–cement interface is low, concentrated between 0.45 and 0.65 MPa, and the plastic accumulation of the cement sheath during fracturing leads to inconsistency in the interface strain between the casing and the sheath. The interface separates and forms a micro-annulus when the interfacial tensile stress exceeds the interfacial bond strength during pressure relief. Thirdly, under the condition of a confining pressure 30 MPa and high alternating pressure, micro-annulus formation is the primary mode of failure regarding cement sheath integrity. Increasing the elasticity and deformation ability of the cement sheath helps prevent this failure. The conclusions of this research may provide a technical reference for improving the cement slurry and fracturing operation of fractured wells and help ensure the integrity of the cement sheaths of fractured wells.