Pore-type partitioning for complex carbonates: Effective versus total porosity and applications to electrical conductivity

Abstract

The pore-scale characterization of complex carbonate rock is of considerable importance in the context of optimizing hydrocarbon recovery due to structural heterogeneity, resulting in complex spatial fluid distributions. Recent advances in micro-CT techniques allow imaging such pore-systems at various scales. Here we present a workflow to determine effective and total porosity for different pore-types and apply this knowledge to improve our understanding of electrical properties by integrating experiment and simulation in a consistent manner via integrated core analysis. Defining as micro-porosity voxels in tomograms containing porosity below voxel resolution, a pore-typing technique is introduced separating microporous rims of oolithes from other types of microporosity using Indiana Limestone; about 50% of the pore space falls into the micro-porosity category for the chosen sample size of diameter (resolution) 1 inch (11um). While the rims exhibit well connected porosity in the high resolution image, they appear as a particular micro-porosity type at low resolution. Various corners are lost due to partial volume effects and imaging noise in the low resolution data. The low-resolution pore-typing allows us to derive micro-porosity specific total- to effective porosity transforms. We utilized the latter regional transforms to establish regional Archie parameters. Experimentally measured formation factor and resistivity index at partial saturation are compared with direct-image based calculations considering both the case of globally and regionally defined Archie parameters. In particular, we find reasonable agreement with the experiments for higher water saturations using global parameters.