Abstract: During shale hydraulic fracturing, strain evolution exacerbates physical damage, leading to differentiated permeability changes across reservoir regions. Variations in mineral composition and microstructural heterogeneity in shale contribute to creep characteristics, leading to time-dependent reservoir deformation and reduced fracture conductivity. Previous creep experiments on shale cores mainly focus on their mechanical properties, with few studies investigating how creep effects influence permeability evolution. Although some international researchers have examined the relationship between shale permeability and time through experiments, comprehensive studies on cores with different flow capabilities and characteristics remain lacking. In this study, the reservoir near the wellbore was divided into three zones: propped fracture zone, unpropped fracture zone, and matrix zone. Using actual downhole shale cores, the reservoir characteristics of each zone were identified. A testing device and methodology for coupling creep effects and permeability were established. By analyzing the time-dependent evolution of core physical parameters, the mechanisms and variation patterns of permeability damage induced by creep effects in shale propped fractures, unpropped fractures, and the matrix were investigated. The results showed that the permeability of propped fracture cores, unpropped fracture cores, and matrix cores all exhibited an exponential decay with increasing effective stress duration, characterized by an initial rapid decline followed by a gradual slowdown. The permeability decay rate increased with effective stress, with the fastest decline in unpropped fracture cores, followed by matrix cores, and the slowest in propped fracture cores. Specifically, under an effective stress of 25 MPa for 108 h, the permeability of matrix, unpropped fracture, and propped fracture cores decreased to 44.07%, 4.21%, and 1.55% of their initial values, respectively. Under 45 MPa for the same duration, the corresponding values decreased to 9.28%, 3.81%, and 1.02%. The homogeneous pore structure (with highly uniform pore size, shape, and distribution) enabled the external effective stress to be evenly distributed throughout the core. Under low-stress conditions, this uniformity prevented pore collapse or fracture propagation due to local stress concentration. Consequently, the permeability attenuation in matrix cores was relatively minor under low effective stress conditions. Based on physical simulation, this study effectively reveals and clarifies the influence mechanisms of creep effects on permeability in different shale reservoir zones under varying stress conditions.

Key words: shale gas, stress effect, permeability dynamic evolution, creep effect, physical simulation experiment