Abstract
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/m3, while the theoretical free gas density of xenon is 130 kg/m3 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.
Article Highlights
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Demonstrate a methodology for direct measurement of in-situ fluid density within unconventional reservoir rocks from a single-energy CT image.
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Measured in-situ fluid densities are in good agreement with their corresponding theoretical densities at the same temperature–pressure conditions.
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Offer an opportunity to capture temporal-spatial density/concentration distribution of fluids in unconventional reservoir rocks, thereby facilitating the investigation on storage capacity and diffusion process.
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adapted from Bushberg et al. (1995))
adapted from Chakraborty et al. (2020))
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Abbreviations
- \(A = \frac{\alpha }{{E^{l} }}\) :
-
Modified photoelectric constant
- \(E\) :
-
Applied voltage during a CT scan
- \(I\) :
-
Intensity of transmitted X-rays
- \(I_{0}\) :
-
Intensity of the incident X-rays
- \(k\) :
-
System-related constant for photon energy
- \(l\) :
-
System-related constant for photon energy
- \(t\) :
-
Sample thickness
- \(Z\) :
-
Atomic number of an elemental sample
- \(Z_{{{\text{CH}}_{{2}} {\text{I}}_{{2}} }}\) :
-
Effective atomic number of diiodomethane
- \(Z_{{{\text{eff}}}}\) :
-
Effective atomic number of sample material
- \(Z_{{{\text{fluid}}}}\) :
-
Effective atomic number of any given fluid
- \(Z_{i}\) :
-
Effective atomic number of any given i-th element in the compound
- \(Z_{{{\text{xenon}}}}\) :
-
Effective atomic number of xenon
- \(\beta\) :
-
Compton scatter constant
- \(\mu\) :
-
Linear attenuation coefficient.
- \(\mu_{{{\text{CS}}}}\) :
-
Compton scatter attenuation
- \(\mu_{{{\text{PE}}}}\) :
-
Photoelectric effect attenuation
- \(\mu_{{{\text{xenon}}}}\) :
-
CT number of free xenon gas
- \(\mu_{{{\text{matrix}}}}^{1}\) :
-
CT number of the rock matrix filled with the saturating fluid 1
- \(\mu_{{{\text{matrix}}}}^{2}\) :
-
CT number of the rock matrix filled with the saturating fluid 2
- \(\mu_{{{\text{free}}}}^{1}\) :
-
CT number of pure fluid 1
- \(\mu_{{{\text{free}}}}^{2}\) :
-
CT number of pure fluid 2
- \(\mu_{m} = \frac{\mu }{\rho }\) :
-
Mass attenuation coefficient
- \(\mu_{{{\text{air}}}}^{{\text{@Dry}}}\) :
-
CT number of free space at dry conditions
- \(\mu_{{{\text{matrix}}}}^{{\text{@Dry}}}\) :
-
CT number of each voxel of the rock matrix at dry conditions
- \(\mu_{{{\text{matrix}}}}^{{\text{@Saturated}}}\) :
-
CT number of each voxel of the rock matrix at saturated conditions
- \(\mu_{{{\text{fluid}}}}^{{@{\text{Saturated}}}}\) :
-
CT number of pure fluid at saturated conditions
- \(\mu_{{{\text{matrix}}\quad {\text{avg}}}}^{{\text{@Subtraction}}}\) :
-
Average of CT numbers of the rock matrix in the subtraction image dataset
- \(\rho\) :
-
Average material/fluid density
- \(\rho_{{{\text{xenon}}}}\) :
-
Pure xenon gas in a given voxel volume
- \(\rho_{{{\text{fluid}}}}^{{\text{@Saturated}}}\) :
-
In-situ fluid density of each voxel within porous media
- \(\rho_{{{\text{CH}}_{{2}} {\text{I}}_{{2}} }}^{{\text{@Saturated}}}\) :
-
In-situ CH2I2 density of each voxel within porous media
- \(\phi_{v}\) :
-
Porosity of each voxel
- \(\phi_{{{\text{avg}}}}\) :
-
Average of the rock porosity
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Acknowledgements
We thank Hess Corp for facilitating Bakken shale samples for this study.
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This work received funding support from the Hess Corp and the Energi Simulation Foundation. Their financial support is appreciated.
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NC and XL were principally responsible for the concept of this manuscript and for preparing the manuscript. NC and XL contributed equally to this work. KE helped with the data collection and analysis presented. ZK was the supervisor of the project and provided technical guidance. All authors contributed to reviewing the manuscript. All authors read and approved the final manuscript.
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Nirjhor Chakraborty and Xuanqing Lou are Co-first authors.
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Chakraborty, N., Lou, X., Enab, K. et al. Measurement of In-situ Fluid Density in Shales with Sub-Resolution Porosity Using X-Ray Microtomography. Transp Porous Med 141, 607–627 (2022). https://doi.org/10.1007/s11242-021-01738-4
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DOI: https://doi.org/10.1007/s11242-021-01738-4