Experimental and Theory Study on Failure Mechanisms of Cement Sheath under Ultrahigh-Temperature Thermal Cycle

Summary With the aim of clarifying the failure mechanism of cement sheath (CS) integrity for solving sustained casing pressure and wellhead movement caused by ultrahigh-temperature thermal cycle (UTTC) in geothermal wells, we developed a full-scale experimental device of the “production casing-cement sheath-technical casing” system (PC-CS-TC system) based on the typical wellbore profile in geothermal wells, by which the interface mechanical properties (IMP) of CS under three different UTTCs were measured to describe the interface integrity. Based on experimental results, we established an analytical model of the PC-CS-TC system under UTTC, by which the failure mechanism of interface integrity and the evolution characteristics of microannulus under UTTC were revealed. The 3D simulation mechanical model of the PC-CS-TC system is presented based on the extended finite element method (XFEM). Furthermore, we investigated the damage behavior and crack propagation mechanism of the CS body using the concrete damage plasticity (CDP) damage model and by introducing the damage factor, and we investigated the interface damage and microannulus expansion behavior by combining the cohesive element method and bilinear traction separation criterion. The results show that the increase in the peak of alternating temperature and cycles can lead to damage to CS integrity, and the peak of temperature plays an important role in integrity damage, including radial cracking, yield, plastic deformation, interface detachment, and microannulus. The experimental results of IMP well describe the interface integrity of the CS, by which the chemical cementing strength reflects the interface cementing state and the friction force relates to the size of the microannulus. The analytical and simulation results show the formation of microannulus, and that interface detachment is primarily associated with the accumulation of plastic deformation caused by UTTC. The size of the microannulus increases with an increasing number of thermal cycles after interface detachment, leading to the decrease of friction force at the CS interface. The tensile failure and radial crack caused by fatigue damage under UTTC are the main failure modes of the CS body because the growth rate of tensile damage is higher than that of compressive damage. The microannulus and radial crack will provide channels for gas/liquid migration, causing gas channeling and sustained casing pressure. All the analytical, simulation, and experimental results have a good verification relationship with each other, confirming the reliability of the analytical and simulation models. Research results offer novel methods and concepts for exploring the failure mechanism of CS in geothermal wells.