Measurement of In-situ Fluid Density in Shales with Sub-Resolution Porosity Using X-Ray Microtomography

The use of X-ray microtomography (micro-CT) for quantitative characterization of fluid storage and transport behavior in unconventional reservoir rocks is of practical importance to aid reserves estimation and gas/oil recovery. However, challenges remain in directly measuring in-situ fluid density/concentration under dynamic processes using industrial CT scanners. Without the availability of dual-energy X-ray CT imaging (DECT), we propose a simplified total attenuation equation and calibration methodology for industrial CT scanners that enable accurate estimation of in-situ fluid densities within unconventional reservoir rocks from a single-energy image. Sixteen standard elemental and compound materials were imaged at a voltage of 200 keV, and the data were used to validate the proposed equation and calibration protocol. This was followed by blind tests of (1) densities of pure xenon gas at different pressure stages; (2) in-situ liquid diiodomethane (CH2I2) density in a Bakken shale plug and, (3) in-situ xenon gas density in a Bakken shale plug. Results showed that the measured densities of pure xenon and in-situ CH2I2 were consistent with their corresponding theoretical densities. The average error in measured xenon densities based on our proposed method was 1.9%, while the error of in-situ CH2I2 density was only 0.2%, thus validating the proposed methodology. In-situ measurements of xenon gas density in the Bakken shale plug demonstrated evidence of localized phase densification likely attributed to adsorption, capillary condensation, or confinement-induced supercriticality. The average xenon density in the Bakken shale sample was found to be 171.53 kg/m(3), while the theoretical free gas density of xenon is 130 kg/m(3) at the same pressure-temperature conditions, equivalent to a new 32% increase. Validation of the proposed equation and calibration protocol provides a robust method to capture temporal-spatial distribution changes in in-situ fluid density from sub-resolution CT images. This will therefore facilitate the investigation of fluid storage and transport behavior in unconventional reservoir rocks, such as shale and coal.