Damage evolution of the casing-cement sheath interface during thermally assisted casing retrieval in abandoned onshore wells

To address the challenges of difficult recovery, low operational efficiency, and high engineering cost associated with conventional casing cutting and fishing methods during abandoned-well casing retrieval, this study proposes a thermally assisted casing retrieval method based on thermally induced interfacial failure. The method aims to weaken the casing–cement sheath bond and reduce the critical pull-out force. Composite casing–cement sheath–formation specimens were prepared, and interfacial damage experiments were conducted under internal casing-wall heating. Acoustic sensing was used to quantitatively evaluate interfacial failure. A finite element model of the wellbore composite system was further developed to investigate interfacial damage evolution and critical pull-out force under different heating temperatures. The results show that heating temperature is the dominant factor controlling retrieval performance. Increasing temperature intensifies thermal stress and deformation, promotes casing–cement interface damage, and markedly reduces the critical pull-out force. At approximately 800°C, interfacial damage becomes sufficiently developed, while further heating provides limited additional load reduction. Deformation mismatch caused by differences in thermal conductivity and thermal expansion between the casing and cement sheath is the fundamental mechanism inducing interfacial slip, local extrusion, and bond failure, with axial thermal deformation playing a dominant role.