A Laboratory Method for Evaluating the Bonding Tensile Strength of the Cement–Formation Interface Considering Multiple Factors

Due to the influence of surface roughness of wellbore walls, residual drilling fluids, and temperature perturbation, the cement–formation interface is the weakest link in the whole annulus sealing system. Additionally, tensile fracture is prone to occur and can lead to annulus sealing failure. The existing evaluation methods, however, can only determine the bonding shear strength of the cement–formation interface and cannot adequately evaluate its bonding tensile property. Therefore, a laboratory method was established to evaluate the bonding tensile strength of the cement–formation interface. This method took into consideration the main factors such as lithology, interface roughness, drilling fluid deposit, flushing fluid washing, and cement slurry. It systematically evaluated the bonding tensile strength of the cement–formation interface from multiple perspectives, including the flushing efficiency, the mesoscopic structure of bonding, tensile strength, and fracture morphology. Results from the research suggest that the proposed method is scientific and reasonable, and it reveals clear steps, simple application, and small discreteness of test results, and the test results are discriminative under different factors. Hence, the method improves the evaluation method for the bonding strength of the cement–formation interface and merits dissemination.