Tensile Strength Evolution and Microscopic Damage Mechanism of Shale under High-Temperature Conditions

In numerous rock engineering applications, such as explosion fracturing, the influence of high temperatures on the tensile strength of shale remains incompletely elucidated. To investigate the impact of elevated temperatures on the tensile strength of shale, Brazilian splitting tests were conducted on specimens under high-temperature conditions (25 °C–800 °C). Numerical simulations using PFC2D were performed to explore mesoscopic thermal damage mechanisms under high-temperature conditions. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed to characterize the micropore structure and to elucidate the mechanisms governing their thermal evolution. The results reveal that tensile strength initially increases with temperature, reaching a peak at 200 °C, and subsequently decreases with further temperature. The failure mode of shale transition progressively changes from brittleness to plasticity with increasing temperature. At temperatures above 600 °C, secondary fracture branches emerge, increasing the complexity of the fracture structure. Numerical results demonstrate that the tensile strength of the shale model at room temperature is 4.52 MPa, deviating by only 5% from the experimental result, thereby validating the reliability of the simulation approach. Due to the incompressible nature of shale particles, the tensile strength tends to decrease with increasing temperature. SEM and AFM analyses reveal that abundant primary pores and fractures are present within mineral grains, yet the overall structure remains compact and mechanically strong. With increasing temperature, the number of thermally induced fractures increases. The α–β phase transition of quartz at 573 °C and the decomposition of major carbonate minerals result in the formation of CO2-induced pores and intersecting fractures. Finally, this study analyzes the discrepancies between its findings and those of previous studies and highlights the potential applications as well as the associated challenges of high-temperature conditions in shale reservoir engineering.