Chapter 4 - Formation Resistivity and Water Saturation

Abstract

Sedimentary formations are capable of transmitting an electric current only by means of their interstitial and adsorbed water content. They would be nonconductive if they were entirely dry. A rock that contains oil and/or gas will have a higher resistivity to electric current than the same rock when it is completely saturated with formation water. The greater the connate water saturation is, the lower the formation resistivity is. This relationship to saturation makes the formation resistivity factor an excellent parameter for the detection of hydrocarbon zones. Several correlations relating the formation resistivity factor to: salinity of formation water, effective porosity, pore geometry, formation stress, temperature, interstitial fluids, and composition of rock, are discussed in this chapter. Methods for determining the formation resistivity are discussed, including: Archie equation, chemical analysis, spontaneous potential (SP) curve, and shale properties. Statistical correlations of the formation resistivity factor to several petrophysical parameters, including porosity, tortuosity, cementation factor m, water saturation, and permeability, are discussed in this chapter. Several models to estimate water saturation and flow units in shaly reservoirs are presented. Most concepts are illustrated with numerical examples. Many exercises are included at the end of the chapter.