Pore-Scale Evaluation of Dielectric Measurements in Formations with Complex Pore and Grain Structures
- Huangye Chen (Texas A&M University) | Zoya Heidari (Texas A&M University)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- Publication Date
- December 2014
- Document Type
- Journal Paper
- 587 - 597
- 2014. Society of Petrophysicists & Well Log Analysts
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- 249 since 2007
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Dielectric-permittivity measurements are typically used to estimate water-filled porosity. The dielectric interpretation methods, such as, complex refractive index model (CRIM) (i.e., volumetric techniques) are extensively used to correlate dielectric permittivity of fluid-bearing rocks to petrophysical properties such as water-filled porosity. However, volumetric techniques usually oversimplify the rock structure and do not take into account the impact of spatial distribution of solid and fluid components on dielectric properties of the rock. The lack of reliable rock-physics models to interpret dielectric-permittivity measurements can lead to significant uncertainty in estimates of water-filled porosity.
We applied a pore-scale numerical simulation method to quantify the impact of pore and grain structures and heterogeneity on dielectric-permittivity measurements, and introduced a new dielectric model to improve assessment of water-filled porosity in formations with complex pore/grain structure. A directional-tortuosity factor was introduced and calculated using pore-scale rock volumes to quantify the geometry of pore and grain networks.The introduced techniques were applied on 3D computed-tomography (CT) scan images of sandstone and carbonate rock samples as well as synthetic organic-rich mudrocks. We showed that the new model is more reliable for assessment of water-filled porosity compared to the conventional CRIM, in the 12 sandstone and carbonate rock samples evaluated in this paper. In the case of synthetic organic-rich mudrocks, we observed that (a) despite the change of the overall dielectric permittivity, the accuracy in estimates of water-filled porosity is not affected by the presence of kerogen, if the influence of kerogen is correctly taken into account by CRIM, and (b) the presence of pyrite and its spatial distribution significantly affect the dielectric permittivity of organic-rich mudrocks. Failure to consider the influence of pyrite and its spatial distribution on dielectric permittivity may cause large uncertainty in estimates of water-filled porosity.
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