Seismic and electrical properties of sandstones at low saturations

Abstract

Laboratory measurements of elastic wave velocities, attenuation, and electrical resistivity of sandstones all show distinct changes in the measured properties at very low levels of water saturation. Velocity decreases, attenuation reaches a maximum, and electrical resistivity decreases over a limited range in saturation. We assume that all the water at low saturations is present in the rock as a layer of uniform thickness covering the entire internal surface area of the sample. With this model, we calculate the thickness of surface water that corresponds to the distinct low saturation region. In all cases, for both the seismic and the electrical data, the thickness is equal to one to four monolayers of water. This is interpreted as marking the transition in the behavior of water in a sandstone from that of a surface adsorbed phase to a bulk water phase. The changes in seismic properties can be modeled by considering the effect of adding surface and bulk water to grain contacts. In a similar treatment, the role of contacts is shown to be a dominant factor in the electrical resistivity of sandstones at low saturations, such that the addition of water to the contact area causes a dramatic decrease in the electrical resistivity.