Pore-scale electrical numerical simulation and new saturation model of fractured tight sandstone

Fractured tight sandstone reservoirs are characterized by low matrix porosity, low matrix permeability, and strong heterogeneity. Fractures developed in rocks improve reservoir permeability, but they bring new challenges to formation evaluation from log interpretation. These challenges lead to the inaccurate calculation results of formation water saturation (Sw) by using Archie's equation. In addition, the difficulty in coring and preserving fractured core samples limits the application of conventional petrophysical experiments. Recently, the development of digital rock technology provides a new means for studying the physical properties of complex reservoir rocks such as fractured rocks. In this work, a set of fractured digital rocks with different fracture apertures and dips is constructed to study the electrical properties of rocks. The formation factor is calculated using the resistivity simulation results by the finite element method (FEM) and a correlation among cementation exponent, fracture aperture, and fracture dip is established. In addition, the resistivities of partially saturated fractured rocks are calculated by the FEM and an exponential correlation between resistivity index (RI) and Sw is fitted by the RI-Sw crossplot. Combined with the basic principles of Archie's equation, a new saturation model for fractured tight rock reservoirs is proposed. Considering the deep Cretaceous fractured tight sandstone gas reservoirs from Kuqa as an example, the model is observed to be an effective and reasonable representation of the subsurface properties of interest. We believe that this study provides a novel and feasible method of determining formation properties, especially saturation, which are close to the ones at reservoir conditions.