Chapter 4 - Formation Resistivity and Water Saturation

Abstract

The resistivity of reservoir rocks is a function of salinity of formation water, effective porosity, and quantity of hydrocarbons trapped in the pore space. Relationships among these quantities indicate that the resistivity decreases with increasing porosity and increases with increasing petroleum content. Resistivity measurements are also dependent upon pore geometry, formation stress, composition of rock, interstitial fluids, and temperature. Resistivity is, therefore, a valuable tool for evaluating the producibility of a formation. A rock that contains oil and/or gas will have a higher resistivity than the same rock completely saturated with formation water. This chapter provides an introduction to spontaneous potential (SP) curve. It is a recording of naturally occurring physical phenomena in in situ rocks. The SP curve records the electrical potential or voltage produced by the interaction of formation connate water, conductive drilling mud, and shale. The SP curve can be recorded simply by suspending a single electrode in the borehole and measuring the voltage difference between this electrode and a ground electrode, making electrical contact with the earth at the surface.