Mechanical Response and Failure Characteristics of Granite Under In Situ High-Temperature and High-Pressure True Triaxial Conditions

The exploitation of deep oil and gas resources faces challenges posed by complex high-temperature and high-pressure environments. Understanding the mechanical behavior of reservoir rocks under such conditions is therefore critical for ensuring engineering safety. Most existing studies utilize reservoir rock samples at ambient temperature or subjected to high-temperature pretreatment for conventional triaxial tests. However, research on the mechanical response under the coupled conditions of real-time high temperature and in situ true triaxial stress remains limited. To address this gap, this study employed a self-developed in situ true triaxial testing system capable of simultaneous high-temperature and high-pressure loading. Systematic in situ true triaxial mechanical tests were conducted on granite under temperatures up to 200 °C and confining pressures up to 200 MPa. After fracturing, the three-dimensional crack morphology was obtained using CT scanning, and quantitative characterized based on the average crack width and fractal dimension, systematically investigating the effects of temperature, intermediate principal stress (σ2), and minimum principal stress (σ3) on the mechanical parameters and fracture characteristics of granite. In this study, the results indicate that over the temperature range of 25–200 °C, the peak strength and elastic modulus decrease by approximately 11–18% and 15–40%, respectively, while the peak strain increases by 4–20%. The failure mode transitions gradually from a tensile–shear composite fracture at room temperature to a predominantly shear fracture at elevated temperatures. The rock brittleness is reduced, thus the damage zone expands, and macroscopic fractures decrease. Correspondingly, the average fracture width decreases from approximately 0.66 mm to 0.55 mm, and the fractal dimension increases from about 2.28 to 2.38. An increase in σ2 leads to a 19–26% increase in peak strength and a 33–75% increase in elastic modulus, while also increasing the average fracture width and decreasing the fractal dimension. Also, the rock brittleness increases, and the failure mode shifts from tensile–shear composite fracture to shear fracture. An increase in σ3 results in an approximately 11% increase in peak strength, a 30% increase in peak strain, and a 21% decrease in elastic modulus, accompanied by a decrease in average fracture width and an increase in fractal dimension. This suppresses the formation of a single dominant fracture surface, consequently increasing the complexity of the fracture morphology. This research reveals the mechanical response and failure laws of deep granite under high-temperature and true triaxial conditions, providing important insights for understanding the mechanical properties of deep reservoir rocks and for the design of drilling and fracturing operations.