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Assessing Control of Pore Network Geometry on Electrical Resistivity Using Integrated Multiscale Digital Image Analysis and Mercury Injection Capillary Pressure

Abstract

Microporosity has shown to have a strong impact on electrical flow properties of carbonate rocks but macroporosity also plays an important part. Understanding pore-structural controls on electrical resistivity hence relies on a multiscale approach incorporating both macro- and micropores. Quantification of micropores, however, remains difficult as their size is well below the resolution of conventional optical light microscopes. New flat surface preparation methods using broad-ion-beam (BIB) milling now produce true 2D surfaces that enable imaging micropores with scanning electron microscopes (SEM) at a nanometer scale resolution. Combining the quantitative parameters of the micropores from BIBSEM mosaics with those of the macropores from thin-sections enables a multiscale digital image analysis (MsDIA). In this study, the pore-body properties from MsDIA are also compared to pore-throat data from mercury injection capillary pressure (MICP) measurements in order to gain additional information about pore structural controls on electrical resistivity. The dataset consists of 54 MICP samples; 12 of them have been analyzed in more detail with the MsDIA approach. The samples cover a wide range of different depositional environments and diagenetic stages, hence the derived relations to electrical resistivity should be widely applicable. Results confirm previous studies that pore size, throat size, and pore density are the dominant factors controlling electrical flow in porous media. Extracted pore network parameters from MICP measurements [R35 (throat diameter at 35% mercury saturation) and Dc (critical diameter when mercury first spans the sample)] both show a good correlation with electrical resistivity. Smaller throat sizes result in lower resistivity. Pore geometry parameters from MsDIA [DOMsize (maximum pore size to comprise 50% of the optical porosity) and pore density] show very good correlation with electrical flow. Samples with a large number of small pores have a lower resistivity compared to samples with few, large pores but similar porosity. We conclude that microporosity can effectively be characterized by a few pore-geometric parameters that can be derived from different analytical techniques. Additionally, microporosity can be estimated from conventional thin-section digital image analysis due to fractal pore size distributions found in carbonate rocks.