Interpretation of electrical measurements for estimation of fluid content in organic-rich mudrocks is challenging. Rock components other than saline water can contribute towards the effective conductivity of the rock in organic-rich mudrocks and needs to be incorporated into rock physics models. Conventional petrophysical models have limited universal applicability due to the extensive requirement of core-based model calibration in different formations. In this paper, we introduce and test a universal physics-based resistivity model, which takes into account the contribution of all the conductive rock components and their spatial distribution. The developed model can be applied to unconventional and conventional reservoirs without requiring any model calibration.

We developed a resistivity model that incorporates the geometric model parameters, which are shape-related factors for each rock component present in the formation for the estimation of water saturation. Other inputs to the model are the volumetric concentration of rock components and their conductivity values. We estimated the water saturation in unconventional reservoirs with (a) different levels of thermal maturity of kerogen, (b) varying concentrations of clay and kerogen, and (c) varying concentrations of pyrite. We then estimated the water saturation using the same physics-based resistivity model in sandstone reservoirs and carbonate reservoirs to demonstrate how the model can be adjusted for different types of reservoirs.

We tested the introduced workflow in multiple wells in the Wolfcamp formation and in a well from the Haynesville formation. The estimates of water saturation using the introduced workflow were relatively improved by 44.4% and 26.1% in the Wolfcamp formation, and by 80.5% and 48.5% in the Haynesville formation compared to those obtained from Archie's and Waxman-Smits models (with default parameters of a=1, m=2, n=2), respectively. To show the universal application of this calibration-free model, we also applied it to sandstone and carbonate formations. The results in the case of the sandstone formation were comparable to those obtained using Archie's and Waxman-Smits models (after core-based calibration for both models). The improvement in water saturation assessment was more significant when the new model was applied to a carbonate formation.

The novelty of this resistivity model lies in its universal applicability. The resistivity-based model discussed in this paper can be applied to unconventional reservoirs such as organic-rich mudrocks as well as carbonate and sandstone reservoirs. This paper also quantifies the impacts of (a) volumetric concentration of various rock components and (b) their conductivity values on the effective conductivity of the rock and subsequently on the estimation of fluid saturation. Furthermore, the introduced rock physics model is physics-based and does not require core-based calibration for the determination of water saturation in any formation, which is the unique contribution of this paper.

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