Damage simulation of casing-cement interface of wellbore structure under non-uniform formation stress

In ultra-deep drilling operations, formations such as creeping mudstone and high-pressure salt layers are frequently encountered, generating non-uniform loads that pose severe challenges to casing design and wellbore integrity. The interfacial bonding behavior between the cement sheath and casing is a critical factor governing the long-term integrity of the wellbore. This paper develops a damage model for the double-layer casing-cement sheath interface under non-uniform loading based on peridynamic theory. The model numerically characterizes the evolution mechanism of interface damage under non-uniform stress. Laboratory tests were conducted using a Digital Image Correlation (DIC) system to capture the strain evolution on the casing-cement sheath surface during radial compression. The results indicate that the peridynamic simulation of damage at the double-layer casing-cement sheath interface under non-uniform loading is in strong agreement with the DIC strain measurements. From the perspective of strain accumulation and damage morphology, increasing the outer casing radius enhances the overall structural stiffness. Strain accumulation at the inner casing-cement interface accelerates, with damage concentrated at the interface. while damage within the cement sheath primarily propagates along the 90° and 270° loading directions. In contrast, an increase radius of the inner casing reduces the constraint on the cement sheath, making the casing more prone to “ovalization” deformation. This extension of the stress transfer path slows strain accumulation at the inner casing–cement interface, with damage propagating along the 0°, 90°, 180°, and 270° directions. Experimental results further indicate that the load-bearing capacity of the model with an inner casing is at least 2.5 times higher than that of the model without an inner casing. The study reveals the mechanical mechanism governing the sealing capacity of the cement sheath in double-layer casings, providing significant theoretical and engineering implications for ensuring wellbore integrity and stability in oil and gas operations.