Methodologies for improving spectral induced polarization measurements in low permeability rock cores

Abstract

SUMMARY Geophysical measurements such as induced polarization (IP) are invaluable for understanding the physical properties of rocks, including pore structure, hydraulic properties and mineral content. However, collecting reliable IP measurements from low-permeability rocks poses substantial challenges due to the difficulty of saturating their tight pore spaces. Additionally, IP measurements on rocks that are not cored to fit conventional sample holders, or are irregularly shaped, are particularly difficult to obtain. In this work, we address these challenges through (1) the use of reliable saturation procedures developed for low-permeability samples, and (2) a molding procedure designed to overcome the difficulties of measuring IP on irregularly shaped or broken rock cores. Core-scale gravimetric porosity measurements closely match values obtained from destructive mercury intrusion porosimetry (MICP) on rock fragments, confirming the effectiveness of the saturation procedure. Direct comparisons of IP measurements between molded and unmolded cores demonstrate that the molding process does not significantly alter the intrinsic electrical response of the samples. Fully saturated mudstones exhibit strong statistically significant relationships between the mean relaxation time (${{\tau }_{{\rm{mean}}}}$) and permeability (k), and between effective porosity (1/formation factor, F) and interconnected porosity ($\phi $) (Archie’s law). Conversely, partial saturation due to ineffective saturation methods introduces substantial scatter to these petrophysical correlations. Overall, these findings underscore the potential of these methods to enhance the reliability and accuracy of SIP measurements on challenging rock samples.