A Coupled Elastoplastic Damage Model for Stress-Sensitive Permeability Evolution in Tight Sandstones Based on Mineralogical Plasticity Classification

Permeability stress sensitivity influences productivity evaluation and stimulation design in tight sandstone reservoirs. Lithic-rich tight sandstone cores from the Baijiantan Formation in the Junggar Basin were investigated using X-ray diffraction and staged effective-stress permeability tests at 25 °C. The plastic mineral index (PM), defined as the summed whole-rock contents of clay minerals, calcite, siderite, and pyrite, was used to classify the samples. Weakly plastic samples S1–S4 had PM values of 28.2–33.5% (PM < 35%), whereas strongly plastic samples S5–S7 had PM values of 37.9–48.3% (PM ≥ 35%); 35% was adopted as a dataset-specific engineering threshold. A complete coupled elastoplastic damage model was developed by incorporating plastic damage and elastic-modulus degradation into the traditional exponential model. The sum of squared errors (SSE) between the measured and predicted normalized permeability values was used to assess model fit; lower SSE values indicate better agreement. Weakly plastic samples were described by the traditional exponential model, with SSE values ranging from 6.0 × 10−5 to 1.39 × 10−3. Strongly plastic samples exhibited pronounced nonlinear permeability decline at effective-stress increments of 10–20 MPa. Across the full 0–20 MPa dataset, the coupled model yielded SSE values ranging from 0.00309 to 0.00498 and lower prediction errors than the traditional exponential model. Sensitivity analysis showed that initial fracture compressibility dominated the overall decline, whereas characteristic plastic strain and damage evolution rate mainly controlled nonlinear decline at effective-stress increments of 10–20 MPa. These results show that mineralogical plasticity classification can guide permeability-model selection for tight sandstones.