ABSTRACT

It is well known that the electrical resistivity of a formation varies with the level of water saturation (Sw). In order to accurately determine Sw from borehole resistivity measurements, however, an understanding is needed of the basic mechanisms responsible for electrical conduction in partially saturated rocks. This makes it necessary to isolate and account for contributions to electrical conduction from parameters other than Sw. In a three-component system, composed of the solid rock, water, and hydrocarbon, the electrical resistivity can be related to the volume fractions and the geometries of the three phases. Conduction in this three component systems can occur as volume conduction through the volumes of the individual components and as surface conduction, at the interfaces between the components. While the concept of surface conduction at the rock/water interface is widely accepted, the concept of surface conduction at the hydrocarbon/water interface has been neglected. We suggest that this a key parameter in modeling conduction in partially saturated rocks that should be incorporated in existing theories, Laboratory data are presented for three sandstones with resistivity measured during imbibition/drainage cycles. The pore fluids are deionised water and air. Significant hysteresis is observed at high saturations with resistivity measured during imbibition consistently less than resistivity measured during drainage. With a simple model of fluid geometries, this variation in resistivity can be related to the changing size of the air/water interface, which acts as a source of conduction. In addition, it was found that the resistivity of a partially saturated sample was less than the resistivity of the fully saturated sample, a phenomenon that can again be attributed to the presence of surface conduction at the air/water interface in the partially saturated sandsone; an effect that will not be present in a fully saturated sandstone. A survey of the literature has confirmed the presence of an electrical double layer at an air/water and hydrocarbon/water interface. As with conduction at the rock/water interface, the magnitude of the effect depends on the size of the interface and the conductivity of the water. We conclude that conduction at fluid/fluid interfaces is an additional parameter that should be included in modeling conduction in partially saturated rocks.

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