Permeability and Rigidity of Green River Shale Before and After Exposure to Water

Abstract

Acoustic travel times through Green River shale samples both parallel and perpendicular to bedding are measured to investigate water interactions with Green River shale and the impact of pertinent factors including exposure durations, pore pressure, effective stress, and anisotropy. To assist these analyses, X-ray diffraction (XRD) and permeability are also measured. Understanding petrophysical and rock properties before and after exposure to water is essential to optimize stimulation design in shale reservoirs. The XRD shows that the samples are clay-poor and mainly consist of carbonate minerals. Bedding-parallel and bedding-perpendicular permeability to non-sorbing He are measured before exposure to tap water to analyze permeability evolution under different stress conditions using the pressure transient method. The samples record very low permeabilities at recreated confining stresses, indicating that the shale requires stimulation. The permeability decreases as confining stress increases, while the permeability increases with increasing pore pressure. Acoustic travel time measurements show that compressional and shear wave velocities, and elastic moduli of the samples increase as confining stress increases before samples are exposed to tap water. This results in more rigid samples which exhibit higher fracture conductivity. The samples are saturated with tap water in containers at atmospheric pressure after the initial acoustic travel time measurements. Further acoustic travel time measurements and chemical elemental analysis in tap water show that quartz dissolves from the pore structure of the Green River shale, reducing the acoustic velocities, and elastic moduli of the samples that then exhibit lower rigidity and fracture conductivity.