Pore and permeability properties of reservoir sandstone under a uniaxial compression CT test

For underground gas storage projects that use depleted gas reservoirs, cyclic gas injection will change the stress fields and cause rock deformation. In-depth knowledge of the stress-induced deformation of reservoir rocks is important for secure gas storage. In this study, we performed a uniaxial compression computed tomography (CT) test with a reservoir sandstone sample to investigate the influence of rock deformation on the pore and permeability properties. Based on scanned 3D images, we analyzed the axial strain evolution by using the digital volume correlation (DVC) technique and the variations in pore properties with the microstructures and permeability properties that were extracted by the pore network modeling method. The DVC analysis shows that strain localization develops gradually with increasing axial stress. The initial porosity distribution with sample height is inhomogeneous. The local porosity variations are not completely consistent with the global porosity variations under increasing axial stress. The level of inconsistency will decrease with increasing axial stress in both the volumetric compression and dilation stages. Increases in axial stress mainly induce the variations in pore numbers and pore volumes. The variations in pore connectivity are not significant. For the permeability evolution, X-ray CT scanning combined with pore network modeling can simulate the permeability decreases during volumetric compression. For high porosity sandstones (>15%), the permeability still decreases in the volumetric dilation stage due to grain size reductions and tortuosity increases. However, this technique has limitations in analyzing the permeability reductions in the volumetric dilation stage. The microcracks that are induced inside the grain particles are too small to be recognized. The variations in the relative gas and water permeabilities at different axial stresses are not significant.