Determination of the water saturation is the crucial point in formation evaluation. For this purpose the well log data are widely used. Traditional estimation of the water saturation from the resistivity logs uses models described by Archie?s law. These models consider different pore systems in rocks as a single one. However, carbonate formations have a complex pore space that consists of a primary (matrix porosity and secondary pores (vugs, channels, and cracks. The effective conductivity of a double-porosity formation depends on each pore-type component and the traditional approach cannot be applied for this medium (non Archie?s rocks. The radial distribution of resistivity tensor around a borehole relates to the saturation of both the primary and secondary pores. The saturation of each pore type can be different in the transition zone because of different permeabilities of the matrix- and secondary-pore systems. These circumstances require development of adequate models for the double porosity carbonate formations which allow us to assess the influence of a multicomponent fluid distribution in complex pore structures.
In this paper we propose a technique for determining the water saturation in double-porosity carbonate formations. The technique uses the unified pore microstructure model where the matrix and secondary pores have different saturations. We have presented saturated carbonates as a material composed by solid grains and pores of ellipsoidal shapes with different aspect ratios for the primary and secondary pores. To calculate the formation resistivity we have applied the effective medium approximation method. The following models were considered:
the mixture of pores completely saturated with water or non-conductive (oil, gas fluids;
pores approximated by layered ellipsoids where the wet-fluid corresponds to the outer ellipsoid layer; and
saturated pores represented by the combination of two previous models.
Calculation of the electrical resistivity consisted of two steps. First, we calculated the resistivity of the matrix with primary small-scale pores (matrix homogenization. Then the porous matrix was treated as homogeneous isotropic host where the large-scale secondary pores were embedded. The approach presented allows us to predict the water saturation in carbonate formations using the resistivity logs with different depth of investigation. Applying the joint inversion of micro-electrical (MSFL and acoustic data, the pore microstructure (type and concentration of secondary pores is estimated. Then, based on the LLS and LLD data the water saturation can be estimated. The examples of the water saturation determination are presented.