Gas permeability response of deep sandstones under triaxial loading in the Dongying Sag

To elucidate the full-stress-path evolution of gas permeability in deep sandstone reservoirs within the Fanjing block of the Dongying Sag, Shengli Oilfield, sandstone specimens were selected from burial depths of 1 970 to 3 375 meters, with axes oriented both perpendicular and parallel to the bedding. Systematic investigations were conducted using P-wave velocity measurements, X-ray diffraction, mercury intrusion porosimetry, and gas permeability tests to analyze permeability evolution across different stress stages and the underlying mechanisms. The results indicate that as burial depth increases, P-wave velocity rises while porosity and pore size diminish, suggesting a denser structure. Quartz is identified as the predominant mineral, with total clay content exhibiting a consistent decrease with increasing burial depth. The specimen at 2 601 meters displays a more developed pore structure and significantly higher initial permeability compared to specimens at other depths, highlighting the influence of local lithologic variations and structural complexity on permeability. During confining pressure loading, overall gas permeability decreases. In the context of triaxial compression, specimens with relatively higher permeability primarily undergo compaction, showing a continuous decrease in permeability, while denser specimens initially experience a decrease in permeability before increasing again as microcracks propagate under higher deviatoric stress. During the reconfinement phase, permeability decreases once more. The orientation of bedding has a pronounced impact on deformation and flow responses. This study reveals a stage-based mechanism characterized by confinement compaction, fracture-induced permeability enhancement, and recompaction during reconfinement, clarifying the influential roles of lithologic differences and pore structure on stress-flow coupling. These findings provide a crucial foundation for reservoir evaluation and the efficient development of deep hydrocarbon reservoirs in the Dongying Sag.