Pore-scale network modeling of microporosity in low-resistivity pay zones of carbonate reservoir

Abstract

Archie's law is commonly used for the estimation of brine saturation using electrical conductivity of sedimentary rock. For a formation with narrow pore size distribution (PSD), low resistivity could mean high water saturation and hence, high water production. This, however, is not the case for many formations with wide PSDs, for which Low Resistivity Pay (LRP) zones have been reported with very low or none water cut production. One of the main causes resulting in LRP zones, especially in carbonates, is the existence of wide PSDs. Due to their small sizes, micro-pores have much higher threshold capillary pressures during drainage in a water-wet sample. In these systems, oil as the non-wetting phase preferably occupies larger pores leaving micro-pores saturated with brine. 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 a short circuit and, ultimately, lowers rock resistivity measured from logs. Here in this work, we investigate the impact of microporosity on the electrical properties of porous media under multiphase flow conditions. We develop a pore-scale network model for the simulation of displacement sequences during drainage and imbibition in the presence of micro-pore networks. To do so, we first construct ‘macro’ networks on 2D rectangular lattices which are later altered to include micro-pores. A comprehensive set of sensitivity analyses is performed to assess the impact of various parameters including the tortuosity, geometry, and location of the microporosity (i.e., parallel or in series, continuous or non-continuous). Also, 3D networks for two different rock samples are used to investigate the effect of PSD on the electrical resistivity curves. This work helps to examine the use of Archie's equation for field applications, specifically, for evaluating formations with wide PSDs.