Investigations on Capillary and Viscous Displacement Under Ultrasonic Waves
Authors: Hamida T. , Babadagli T. 
Published: March 2010

Abstract
In this paper, the influence of ultrasonic waves on capillary and viscous displacement of oil in porous media was investigated. Capillary (spontaneous) imbibition experiments were conducted using various fluid pairs such as air-water, mineral oil-brine, mineral oil-surfactant solution, kerosene-surfactant solution, and mineral oil-polymer solution. Berea sandstone and Indiana limestone cores were used as the matrix. Oil saturated cores were immersed into the aqueous phase and subjected to high intensity ultrasound from a specially designed ultrasonic chamber. The resulting recovery was recorded against time, and compared to a control experiment without ultrasound. A substantial increase in ultimate recovery was observed for most of the fluid pairs, with some improvements in recovery rate.
To further investigate whether ultrasound induces a perturbation at the liquid-liquid interface of immiscible fluids, a series of Hele-Shaw type experiments were run. The resulting fingering pattern was strongly dependent on the interfacial tension of the fluid pair, and the injection rate. Ultrasound stabilized the fluid-fluid front of high interfacial tension fluid pairs, but generated larger instabilities when interfacial tension was low.

Introduction
For the past four decades, researchers have explored the use of acoustic energy to improve oil recovery that could potentially be an economic and environmentally-friendly alternative to current IOR methods. Beresnev and Johnson(1) provide an extensive review of the major developments of acoustic stimulation and its limitations. Guo et al.(2) discuss recent field results of seismic stimulation in China.
Despite numerous promising field trials and patents, the exact mechanism behind ultrasonic stimulations is poorly understood. Most of the findings have been speculative with little experimental verification. The reason is two-fold. Firstly, the problem is very complex, involving a superposition of several different mechanisms. Secondly, it is not certain how far an acoustic wave propagates into the reservoir, or how such propagation occurs. Ultrasonic applications are limited to the near-wellbore area due to the high attenuation through the rock or fluids. As a consequence, most research in recent years has shifted to low energy, low frequency waves that can propagate several kilometres into the reservoir. However, because low frequency waves disperse into high-frequency harmonics as they travel through a porous medium, one would still expect ultrasonic waves to be present in the reservoir. An analytical treatment of compression and shear wave propagation in saturated porous media was first developed in two classic papers by Biot.(3, 4)
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Acoustic waves are usually applied in three ways: pressure pulsing, down-hole vibration, and surface vibration. Applications range from enhanced oil recovery (EOR) to well stimulation, in situ upgrading, wellbore cleaning, and soil remediation.
Acoustic stimulation is believed to positively contribute to the flow of oil in porous media by:Increasing the relative permeability of the phases;Reducing the adherence of wetting films onto the rock matrix, due to non-linear acoustic effects such as in-pore turbulence, acoustic streaming, cavitation, and perturbation in local pressures(5);Reducing surface tension, density, and viscosity as a consequence of heating by ultrasonic radiation(6);Altering of rheological properties of non-Newtonian fluids;