The Influence of Ultrasonic Energy on Capillary Fluid Displacement

Abstract

In recent years, field tests and laboratory investigations have demonstrated that high intensity acoustic stimulation may enhance oil recovery in rocks. This technology is not only technologically feasible, but also serves as an economical, environmentally friendly alternative to currently accepted EOR methods. It requires low capital expenditure, and yields almost immediate improvement without any additional EOR agents. Despite a vast body of empirical and theoretical support, this technology lacks sufficient understanding to make meaningful, consistent engineering predictions. This is in part due to the complex nature of the physical processes involved, as well as due to a shortage of fundamental/experimental research. Much of what we believe is happening within ultrasonically stimulated porous media is speculative and theoretical.This paper aims at identifying and analyzing the influence of high-frequency, high-intensity ultrasonic radiation on the miscible and immiscible displacement of oil in capillary media. We first investigate the effect of ultrasound on flow through a capillary via the pendant drop method. Water was injected into a capillary which is submersed into various processed mineral oils and kerosene. The average drop rate per minute was measured at several ultrasonic intensities, and we determined that there exists a peak drop rate at a characteristic intensity, which strongly depends on oil viscosity and the interfacial tension between water and the oil. The second set of experiments comprises of Hele-Shaw type experiments designed to study instabilities at liquid-liquid interface when ultrasound is applied. Fractal analysis techniques were used to quantify the degree of fingering and branching, allowing a rough assessment of the degree of perturbation generated at the interface. Miscible Hele-Shaw experiments are also presented to illustrate the effect of viscous forces alone. We found that ultrasound acts to stabilize the interfacial front, and that such effect is most pronounced at low viscosity ratios. The third series of experiments focuses on the effect of ultrasound on spontaneous (capillary) imbibition of an aqueous phase into oil-saturated Berea sandstone and Indiana limestone plugs. Both counter-current and co-current geometries were tested. Due to the intrinsically unforced, gentle nature of the process, and their strong dependence on wettability, interfacial tension, viscosity and density, such experiments provide valuable insight into some of the governing mechanisms behind ultrasonic stimulation.