Density-dependent relationship between changes in ultrasonic wave velocities, effective stress, and petrophysical-elastic properties of sandstone.

The present study aims to reveal the importance of density as a moderator variable in interpretation of possible relationships between variations in compressional and shear wave velocities (ΔVp and ΔVs), effective stress, and rock's petrophysical and elastic properties. Towards this end, fourteen subsurface sandstone samples were collected and analyzed through the measurement of ultrasonic wave velocities at standard and reservoir conditions within a Triaxial-testing cell. The results were interpreted for two groups, i.e., low density (LD) and high density (HD) samples, where greater ΔVp and ΔVs were observed for the HD group samples that have similar average porosity and permeability compared to samples from LD group. Effective stress exhibits better fit with ΔVp and ΔVs for the LD compared to HD group samples. Density was revealed to be well fitted with ΔVp of LD and ΔVs of HD samples. Porosity exhibits good fit with ΔVs of LD and permeability exhibits good fit with ΔVp of both LD and HD groups. Also, variations in estimated elastic limit (ΔEd) is well matched with ΔVs, while changes in estimated Poisson's ratio (Δν) manifests a good fitting with ΔVp. Finally, variation in deviatoric stresses from triaxial tests is in a good agreement with ΔVp. The results of this study provide convenient insights for conversion of wave velocities and elastic properties between standard and reservoir conditions.