Effect of dynamic expansion of interface micro-annulus on casing failure mechanism during multistage fracturing

Abstract

The cyclic loads generated by multistage fracturing often affect the sealing integrity of the cement sheath, which in turn causes casing failure. Due to the difficulty in accurately describing the stress state in the cement sheath, the interacting mechanism between the sealing integrity of the cement sheath and the failure of the casing is still unclear. In this work, the fatigue damage cohesion model under cyclic loads is constructed by introducing the fatigue damage calculation criterion into the cohesive zone model (CZM). The effectiveness of the fatigue damage cohesive zone model is verified by the simulation results of double cantilever beam (DCB). Subsequently, by comparing the model results with the existing literature results, it is confirmed that the constructed fatigue damage cohesive zone model can be used to study the casing failure caused by cement sheath sealing integrity. The influence of the dynamic evolution of micro-annulus on casing failure is analyzed from the two key factors of casing eccentricity and wear. On this basis, potential strategies to mitigate casing failure are proposed, focusing on aspects such as fracturing well section selection, construction pressure and cementing quality. A case analysis of a shale gas well in a cementing quality cooperation in Weirong block in southwest China is conducted using the proposed fatigue damage cohesive zone model. The findings of this study provide valuable insights into improving wellbore sealing integrity and preventing casing failure during multistage fracturing operations.