For processes such as water injection in deep geothermal production, heat transfer and fluid flow are coupled and affect one another, which leads to numerous challenges in wellbore structure safety. Due to complicated wellbore structures, consisting of casing, cement sheaths, and formations under high temperature, pressure, and in situ stress, the effects of thermo-hydro-mechanical (THM) coupling are crucial for the instability control of geothermal wellbores. A THM-coupled model was developed to describe the thermal, fluid, and mechanical behavior of the casing, cement sheath, and geological environment around the geothermal wellbore. The results show that a significant disturbance of effective stress occurred mainly due to the excess pore pressure and temperature changes during cold water injection. The effective stress gradually propagated to the far-field and disrupted the integrity of the wellbore structure. A serious thermal stress concentration occurred at the junction of the cased-hole and open-hole section. When the temperature difference between the injected water and the formation was up to 160 °C, the maximum hoop tensile stress in the granite formation reached up to 43.7 MPa, as high as twice the tensile strength, which may increase the risk of collapse or rupture of the wellbore structure. The tensile radial stress, with a maximum of 31.9 MPa concentrated at the interface between the casing and cement sheath, can cause the debonding of the cementing sheath. This study provides a reference for both the prediction of THM responses and the design of drilling fluid density in geothermal development.

Shale reservoir is the research hotspot in the development of unconventional oil and gas resources. The properties of bedding plane have an important influence on the mechanical behavior of shale. According to the fracture characteristics of layered shale under Brazilian splitting load, the effects of bedding angle, bond strength ratio and natural fractures on the tensile strength and fracture pattern of layered shale in Brazilian splitting test were studied by particle flow code. The evolution mechanism of meso–macro fracture of shale is analyzed by combining the dynamic change process of particle mesoscopic force chain and crack. The results show that three types of fracture patterns are observed: splitting tensile failure of shale matrix, splitting tensile failure along bedding plane and tensile–shear composite failure along shale matrix and bedding plane. Mesoscopic cracks initiate at the top and bottom loading points of the disk. After the peak load, mesoscopic cracks propagate through and appear dense force chains until macroscopic failure occurs. The bond strength ratio of bedding plane affects the failure pattern and peak strength of the specimen. With the increase in bedding angle, the tensile strength decreases gradually. Considering the influence of natural fractures, the tensile strength of shale is lower, and the crack propagation is more complex. The research model and results provide theoretical basis for the mechanical properties evaluation and fracture pattern analysis of shale.

The mechanical properties of deep shale are influenced by various factors, including high-temperature and high-pressure conditions, as well as its bedding structure. To investigate the evolution of mechanical properties of deep shale under different environmental temperatures, this study takes the Longmaxi Formation shale as an example, conducting high-temperature and high-pressure triaxial tests. Additionally, a thermal-mechanical coupled particle flow model for deep shale with a bedding structure is established from both macroscopic and mesoscopic perspectives. Based on X-ray diffraction experiments to determine the mineral composition and content of the shale, the model assigns the number of grouped mineral particles and their thermal expansion coefficients. Meanwhile, smooth joint contact is incorporated to characterize the influence of bedding structure, and the model’s mesoscopic parameters are calibrated using high-temperature and high-pressure triaxial test results. Based on the triaxial experiments and the thermal-mechanical coupled model, this study investigates the variations in mesoscopic thermal damage, rock mechanical properties, and failure modes of layered shale under different environmental temperatures. The results indicate that high temperature conditions weaken the mechanical properties of shale, while high confining pressure reduces the anisotropy of layered shale. As temperature increases, thermal damage initially occurs at the bedding interfaces. When the temperature exceeds 150°C, mineral particles undergo shear displacement, leading to a sparse distribution of mesoscopic force chains, and macroscopic thermal cracks expand extensively. In high-temperature and high-pressure environments, the thermal-mechanical coupling effect causes a reduction in the peak strength and an increase in peak strain, with the failure mode transitioning from brittle failure to plastic-ductile failure. Meanwhile, bedding cracks branch and gradually extend into the matrix, forming a composite failure mode involving both the shale matrix and the bedding structure.

The perforations play an important role in determining the near‐wellbore fracture geometry during the stimulation phase. To address the impact of perforations on fracture geometry in horizontal wells, a 3D coupled hydromechanical finite element model is developed and employed. Based on the theory of continuum damage mechanics, scalar damage variable governs the degradation of the stiffness of the solid. Damage affects the crack element modeling that is used to consider crack behavior and construct a crack‐tracking algorithm to simulate propagation. The model was validated against the analytical solutions and perforation fracture experiments. The results indicate that perforation can be used to control the fracturing pressure and propagation behavior of the initial fracture, which has a further effect on the fracture geometry of near‐wellbore region in horizontal wells. Optimizing perforation parameters can direct the propagation of the initial fracture toward the preferred fracture plane. The results demonstrate an improved capability to depict the 3D near‐wellbore fracture geometry and fracture propagation with a continuum damage model. The model enables the optimization of orientations and perforation parameters, so that most efficient perforating completions can be designed for hydraulic fracture stimulation.