Improved Interpretation of Electrical Resistivity Measurements in Mixed-Wet Rocks: An Experimental Core-Scale Application and Model Verification
- Chelsea Newgord (The University of Texas at Austin) | Artur Posenato Garcia (The University of Texas at Austin) | Ameneh Rostami (The University of Texas at Austin) | Zoya Heidari (The University of Texas at Austin)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- Publication Date
- October 2018
- Document Type
- Journal Paper
- 703 - 718
- 2018. Society of Petrophysicists & Well Log Analysts
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- 172 since 2007
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Interpretation of electrical resistivity measurements for assessing hydrocarbon saturation in mixed-wet and hydrocarbon-wet rocks often requires extensive recalibrations of resistivity models. In these conventional resistivity models, the impact of wettability is not reliably incorporated. Recently, we analytically derived a new resistivity model that incorporates parameters to account for wettability and complexity of pore structure. This new model requires experimental verification to enhance its applicability in core- and log-scale domains. In this paper we (1) experimentally quantify the influence of wettability on electrical resistivity measurements, (2) improved and verified our analytically derived resistivity model in rocks with different levels of wettability at different water saturations, and (3) demonstrated the physical meaning of the parameters related to wettability in the resistivity model. We altered the wettability of selected core samples from the same rock type. To prepare the core samples as mixed-wet, a surfactant solution was injected into the samples and then they were aged in decane. These approaches created a range of wettability states in the samples. To quantify the altered wettability of the samples, we used the combined USBM and Amott-Harvey method along with sessile drop contact angle measurements. Next, we used a centrifuge to vary the water saturation in the core samples. We then measured the electrical resistivity of each sample. Finally, we improved the recently introduced resistivity model and compared the estimates of resistivity index from the improved model against the experimentally measured resistivity indices.
We successfully verified the reliability of the improved resistivity model for mixed-wet carbonate core samples. The wettability of the core samples was altered to be in the range of 0.1 to 0.4 on the Amott wettability scale. We also demonstrated that all the coefficients required by the improved resistivity model are physically meaningful. One of the unique contributions of this paper is the introduction of a new interpretation diagram, called the wettability triangle. This diagram can potentially be used to quantify wettability from resistivity measurements, if combined with other geophysical measurements. The outcomes of this work are promising for reliable interpretation of resistivity logs in mixed-wet formations for improved assessment of hydrocarbon saturation with minimal calibration efforts.
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