Stress-Dependent Permeability of Artificially Fractured Siliceous Rocks from Southern Sakhalin

Large hydrocarbon fields are being developed in northern Sakhalin, whereas southern Sakhalin contains prospective resources hosted in unconventional, low-permeability siliceous source rocks. Their development requires stimulation to create conductive fracture networks, whose long-term integrity is critical for production feasibility. This study investigates permeability changes in four artificially fractured specimens subjected to cyclic confining pressure; the specimens are treated as case studies rather than as a statistically representative formation-scale data set. Cylindrical cores, 30 mm in diameter and approximately 30 mm long, containing an induced axial fracture were hydraulically tested under biaxial cyclic confinement. The tests showed irreversible loss of fracture conductivity, with residual permeability after unloading amounting to 7.1–37.4% of the initial value. Direct application of laboratory data to field scale fracture longevity models is inappropriate because cylindrical specimens develop nonuniform circumferential stresses and heterogeneous closure. A procedure is proposed to transfer core scale measurements to a planar fracture subjected to uniform normal stress in the rock mass. Fracture aperture was quantified by X-ray computed tomography (CT) and incorporated into a cell-based contact–hydraulic model accounting for geometric and hydraulic aperture. A nonuniform closure function was used to reconstruct the closure field and correct the laboratory results. At maximum pressure, Kmass/Klab ranged from 0.034 to 0.93 for the three specimens reproduced with acceptable fit; sample 3–4 was retained only as a diagnostic case because of its high root-mean-square error (RMSE). These values are model-based single-fracture scenario estimates pending direct-normal-loading validation.