Abstract

Quantification of electrical rock-fluid interfacial interactions is of great importance for reliable formation evaluation, since borehole geophysical measurements, such as electromagnetic measurements, is significantly affected by electrical rock-fluid interfacial interactions. These interactions become even more important in unconventional formations. Cation Exchange Capacity (CEC) is commonly used to quantify these interactions and the influence of clay minerals on evaluation of electrical conductivity of clay-rich rocks and clay-bound water. We recently developed a new method for CEC assessment in pure clay minerals and shaly-sand formations by integrating N2 adsorption-desorption, nuclear magnetic resonance (NMR), and X-ray diffraction (XRD) measurements. In this paper, we

  • apply our recently developed multi-physics workflow for CEC evaluation in organic-rich mudrocks with complex mineralogy and presence of multiple types of clay minerals and

  • compare the effective CEC from the new method with directly-measured CEC from wet chemistry method in powdered organic-rich mudrock samples.

The introduced workflow for CEC estimation in mudrocks incorporates quantitative characterization of mineral composition and total organic carbon (TOC). We start with quantitative XRD measurements to obtain weight concentrations of mineral components of organic-rich mudrock samples and quantify TOC of the samples using pyrolysis. Then, we perform NMR, N2 adsorption-desorption measurements, and XRD measurements to obtain volume of hydration water, surface area, and interlayer spacing variations, respectively. We perform successive controlled NMR measurements and N2 adsorption-desorption measurements and propose an inversion algorithm to estimate volume of hydration water and surface area of each mineral in multiple samples, respectively. Then, we use these results as inputs for CEC estimation. Meanwhile, we use wet chemistry method to directly measure CEC of powdered organic-rich mudrock samples. Finally, we compare the estimates of CEC from the new method against those obtained from the wet chemistry method.

The proposed workflow is tested on twelve organic-rich mudrock samples from two formations. The estimated CEC from the new method falls within the upper and lower bounds of CEC, calculated based on the reported data for pure clay minerals. However, the results show significant overestimates of the CEC evaluation based on the wet chemistry method. The overestimation results from the salt precipitant in the rocks, which is inevitable due to the nature of wet chemistry method. The experimental results demonstrate the reliability of the introduced method for CEC assessment, which is promising for CEC quantification in formations with complex lithology.

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