Study on brittle–plastic transition of tight sandstone under high confining pressure based on digital rock modeling

The mechanical behavior of deep tight sandstone reservoirs, particularly the brittle–plastic transition under high confining pressure, is critical for drilling safety and efficient reservoir development. However, under deep-reservoir confinement (typically >90 MPa), the micro–macro origin of the brittle–plastic transition and the phase-dependent damage pathway remain insufficiently quantified. This study investigates the intrinsic relationship between macroscopic mechanical response and microscale damage mechanisms of tight sandstone under high confinement. Tight sandstone from the Permian Jiamuhe Formation in the Junggar Basin was selected, and a digital rock model was constructed using a three-dimensional Voronoi polyhedron algorithm informed by mineral composition (XRD) and mechanical parameters, with pores and clay units incorporated to represent heterogeneity. The model parameters were calibrated, and the digital-rock scale was assessed by comparison with laboratory stress–strain curves at σ 3 = 0–60 MPa, and then extended to numerical triaxial compression tests under confining pressures of 0–200 MPa. Damage evolution was simulated using the equivalent modulus method to analyze stress–strain characteristics, damage progression, and equivalent stress distribution. The results show that: (1) At the macroscopic scale, compressive strength and Young's modulus increase significantly with confinement, reaching 1005 MPa and 86.1 GPa, respectively, at 200 MPa, while post-peak behavior shifts from brittle fracture to plastic softening. (2) At the microscale, confinement fundamentally alters the damage path. At 0 MPa, damage initiates in hard minerals (quartz, K-feldspar) due to localized stress concentrations (>133 MPa), coalescing into macroscopic cracks. At 200 MPa, the sequence reverses: softer minerals (albite, muscovite) yield plastically under stresses of 576–672 MPa (damage factor <0.8), whereas quartz, even under stresses up to 1152 MPa, remains intact because confinement suppresses crack propagation, acting instead as the load-bearing skeleton in the plastic stage. (3) The brittle–plastic transition is controlled by three cooperative mechanisms: stress-field homogenization, reversal of the damage path, and transformation of the damage mode.