Pore-Scale Network Modeling of Petrophysical Properties in Samples with Wide Pore Size Distributions

Abstract

AbstractArchie's law is commonly used for the estimation of petrophysical properties of porous media linking electrical resistivity to water saturation. Therefore, low resistivity formation is expected to have high water saturation and hence, high water production. This, however, is not the case for many reservoirs around the world, for which low resistivity pay zones have been reported with very low or none water cut.One of the main causes of Low Resistivity Pay (LRP) phenomena, especially in carbonates, is microporosity. Due to their small pore sizes, micro-pores have much higher threshold capillary pressures than macro-pores during drainage in the water-wet samples which resembles the original state of reservoirs before oil migrations. Because of that, we often find formations in which micro pores and macro pores are saturated with brine and oil, respectively. This indicates that there is a correlation between pore fluid occupancies and pore size distributions. The existence of connected pathways through micro-pores that are fully saturated with a conductive phase (i.e., brine) creates ‘shortcuts’ for the electrical current which causes short circuit and, ultimately, lowers rock resistivity measured from log analysis.The purpose of this work is to investigate the impact of microporosity on the electrical properties of porous media through pore-scale network modeling techniques. To achieve this, a tortuous pore network is constructed on 2D rectangular regular lattice to represent macro pores and throats in the network. Next, the macro network is modified to include micro-pores. This has been done by adding a small rectangular lattice network of micro pores and throats. Radii and lengths of each element are chosen from the pre-specified ranges. This is done carefully to ensure that all networks of different scales fit geometrically within the lattice of a given size. We are specifically interested to investigate flow and electrical properties as a function of the locations, dimensions, and orientations of the micro-porosity regions. To achieve this goal, a comprehensive set of sensitivity analyses are done to assess the impact of various parameters including number of pores, tortuosity, geometry and location of microporosity (i.e., parallel or in series, continuous or non-continuous). The results are compared against Archie's equation, which is commonly used in the industry for log interpretation. This work helps to further expand the use of this equation for field applications, specifically, for formations containing rocks with wide pore size distribution.