Study on the Bonding Performance and Reinforcement Mechanism of the Second Interface of Cement Sheath

Abstract

To address the engineering challenge of debonding failure of the “second interface” (between the cement sheath and the formation rock) induced by alternating thermal-mechanical loads during geothermal well cementing, this study systematically investigated the reinforcement mechanism of a hybrid system of graphene and carbon nanotubes (CNTs) on the second-interface bonding performance of cement stone, using typical sandstone cores from the Ordos Basin as the substrate. The evolution of interfacial bond strength and failure mode under different CNTs contents (0%–0.12%) was evaluated by push-out shear tests. Combined with microstructural characterization techniques such as FE-SEM and EDS, the densification and toughening mechanisms of the interfacial transition zone (ITZ) were revealed from the perspectives of morphological evolution, element migration, and chemical bonding. The results showed that the 28-day second-interface bond strength reached 8.20 MPa, representing an approximately 15-fold increase compared to the baseline group (0.51 MPa). The shear stress–slip curve transitioned from a brittle “cliff-like” drop to a “pseudo-ductile” failure with serrated rebounds, indicating significantly enhanced fracture toughness. Microstructural analysis confirmed that graphene provided “template nucleation” and a two-dimensional skeleton, while CNTs played a “bridging–pull-out” energy dissipation role spanning cracks. Together, they constructed a “line-surface” three-dimensional interpenetrating network, reducing the interfacial Ca/Si atomic ratio from 0.59 to 0.43 and driving the interface bonding from physical van der Waals forces to chemical covalent bonds (Si–O–Si/Si–O–Ca). Excessive CNTs (≥0.12%) led to agglomeration, forming stress concentration sites and causing a decline in strength. This study provides a theoretical basis and material proportioning reference for the integrated design of “high thermal conductivity” and “strong interface” in geothermal well cementing.