Seismoelectric Effect of Oil-Wetted Porous Media Containing Two-Phase Flow

Abstract

In oil reservoirs, if oil mainly has wettability in the solid phase, such as in carbonate reservoirs, the medium is oil-wetted. For oil-wetted porous media containing an oil and water two-phase flow, there are electric double layers at both the oil–solid interface and the oil–water interface, which can stimulate the seismoelectric effect. To date, most of the studies on the seismoelectric effects of porous media have mainly focused on water-wetted porous media, however, there are few reported studies on cases of oil-wetted porous media, especially on oil-wetted porous media containing an oil–water two-phase flow. In this paper, we adopted the oil-wetted pore model, in which oil and water are assumed to be immiscible, and each phase is continuous and distributed in parallel. We also considered the influence of the electric double layer at both the oil–solid interface and the oil–water interface on the seismoelectric effect. It was concluded that the seismoelectric effect of oil-wetted porous media containing a two-phase flow is mainly caused by the electric double layer at the oil–water interface, while the effect of the electric double layer at the oil–solid interface can be ignored. We regarded the two-phase flow as an equivalent fluid, and then we derived a governing equation of the seismoelectric effect and proposed the flux-averaging method to derive the electrokinetic coupling coefficients under the excitation of a steady acoustic field and a time-harmonic acoustic field. We also investigated the effects of formation parameters, namely, water saturation, pore size, water viscosity and porosity, on the seismoelectric effect, which can provide a theoretical reference for the study of seismoelectric logging in oil-wetted porous formations containing a two-phase flow.

Seismoelectric Coupling Equations of Oil-Wetted Porous Medium Containing Oil and Water

“…where J ws is the microscopic streaming current in pores, and J wc denotes the microscopic conduction current in pores. According to the research of Ref [23], the streaming current in the oil-wetted pores with dual phase fluid is mainly caused by the electric double layer (EDL) at the fluid interface; therefore, J ws can be expressed as follows:…”

“…E is the macroscopic electric field. According to Ref [23], by defining the flux averaging of Q wo relative to v wo , the macroscopic streaming current of oil-wetted porous medium with dual phase fluid can be expressed as follows:…”