Modeling Electrical Resistivity in Carbonates Using Micro-CT Scans and Assessing the Influence of Microporosity with MICP

Understanding the complex relationships between resistivity, porosity and pore structure is of fundamental importance to accurately inverse from down-hole log data to permeability and pore fluids. This is especially important in carbonates, where various depositional and diagenetic processes produce a wide range of pore sizes and complex pore structures. This heterogeneity also results in complex electrical behavior, especially in microporous rocks. The purpose of this study is to investigate the influences of microporosity and pore size and number on both electrical and also hydraulic behavior with the goal to better understand resistivity logs. These influences are assessed with two different approaches: (A) Micro-CT imaging of pore networks and subsequent resistivity modeling is used to evaluate the influence of the number of pores and (B) MICP measurements are used to investigate pore size controls.

(A) Two pairs of samples, each with similar porosities (2x 16% and 2x 27%) but different grain sizes and formation factors, were imaged using high-resolution micro-CT scans. The coarse grained samples were imaged at 23µm resolution and the microporous samples at 1.6µm resolution (voxel size). The resultant 3D-images (tomograms) were filtered and segmented, then used for pore network analysis and finite element resistivity modeling. The most interesting result is that resistivity decreases with increasing total number of pores per volume. This result corroborates the hypothesis by Verwer et al.
(2011) that the “apparent cross-sectional area”, which increases with pore count, is the second most important factor controlling resistivity and the cementation factor m, after the total porosity. The modeling results for formation factor and resistivity correlate well with laboratory measurements.

(B) Resistivities of 12 samples with low porosity (1-10%) and permeability (<0001-0.785mD) were measured under variable pressures. Mercury Injection Capillary Pressure (MICP) measurements were then used to assess the pore (throat) size distribution. The most arresting finding is the trend of increasing cementation factor with increasing average pore size. Rocks with smaller pore sizes (average < 0.1µm) show cementation factors between 1.4 and 2.1, whereas rocks with larger pore sizes (average > 0.1µm) have cementation factors of 2.4 and higher. This trend is in contrast to what has been proposed by previous modeling studies.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California