Investigation of Micro Scale Structural Variations and Current Flow Geometry Impacts on Resistivity Responses Utilizing Digital Core Technology
Authors: Mutairi Bandar , Wang Yanhui , Ferreira Enrico 
Published: November 2024

Abstract
Resistivity measurements play a fundamental role in accurately establishing water saturation through well logging. Typically, oil zones exhibit high resistivity but the presence of low-resistivity oil zones in numerous global oilfields complicates reservoir evaluation. This study focuses on examining the influence of electrical current flow geometry and rock heterogeneity at micro-structural scale on resistivity responses, with an emphasis on their implications for reservoir characterization and oil quantification accuracy. By utilizing digital core technology and numerical multiphase drainage simulation, the research aims to investigate the complex relationships between micro-scale rock properties and resistivity response behavior which cannot be captured by the conventional logging tools and other in-site measurements. In this study, a novel methodology was implemented involving a 3D digital rock construction simulation. The simulation was utilized to fabricate three distinct rock samples, each exhibiting the same average porosity but differing in structural pore systems. Specifically, one sample represented a single porosity system at the micro-scale, while the remaining two samples were designed with laminations, incorporating variations in porosity at the micro-scale to maintain comparable average porosity levels. Subsequently, a multiphase drainage simulation model was employed to generate different water saturation profiles based on pore entry radius representing the capillary pressure. This simulation can perform the calculation of resistivity in three orthogonal directions (x, y, and z), based on a solution of the Laplace equation with charge conservation boundary conditions using a conjugate gradient technique, enabling a comprehensive analysis of the impacts of structural heterogeneity on resistivity responses. The study's findings reveal that the resistivity values obtained when current flow direction is parallel to the bedding planes in laminated samples are consistently lower—by approximately half—compared to those of non-laminated samples across all current flow directions. Conversely, resistivity measurements taken perpendicular to the bedding in laminated samples exhibit significantly higher values. Additionally, a trend of decreasing resistivity with increasing porosity was observed. These phenomena can be attributed to the enhanced connectivity of the pore structure and the distribution of water saturation aligned with the current flow direction, leading to reduced resistivity in the examined scenarios. This highlights the importance of understanding the influence of micro-scale structural variations on the resistivity response within rock formations. This paper presents an extended horizon in advancing the accuracy of reservoir characterization and reserve estimations. The findings provide valuable insights into understanding the challenges of detecting low resistivity pay zones, which are often overlooked yet contain substantial hydrocarbon reserves.