Effects of Ultrasonic Waves on Immiscible and Miscible Displacement in Porous Media

Abstract

Abstract Insight into the physical principles governing the mobilization of oil by ultrasonic waves is vital for developing novel techniques of oil extraction.Various mechanisms such as peristaltic transport due to mechanical deformation of the pore walls, reduction of capillary forces due to the destruction of surface films, coalescence of oil drops due to the Bjerknes forces, oscillation and excitation of capillary trapped oil drops, forces generated by cavitating bubbles, and sonocapillary effects may be responsible for enhancing the flow of oil through porous media in the presence of an acoustic field. This paper aims at analyzing the influence of high-frequency, high-intensity ultrasonic radiation on the immiscible and miscible displacement in porous media.Capillary imbibition (co- and counter current) and viscous displacement (Hele-Shaw) experiments were performed with and without ultrasonic waves. We observed that the increase in ultimate recovery is more pronounced than the increase in recovery rate when ultrasonic waves were applied in capillary imbibition experiments.The cases yielding enhancements on displacement efficiency (higher recovery rate and lower residual oil saturation) due to ultrasonic effects were identified. To analyze the viscous displacement under ultrasonic waves, experiments were performed on Hele-Shaw models.Immiscible (for mineral oil-brine and mineral oil-surfactant pairs) and miscible (for mineral oil-pentane, mineral oil-kerosene and mineral oil - 2-Propanol pairs) displacement experiments were performed.Changes on the displacement fronts and patterns were observed under ultrasound.Fractal techniques were used to analyze the changes and correlate them to the intensity of ultrasonic waves. Introduction The use of sonic energy to stimulate and enhance oil recovery has been investigated in the past four decades. It provides an economically feasible and environmentally friendly alternative to established secondary and tertiary recovery methods. The interest in vibroseismic stimulation dates back to the early 50s when increased oil recovery was observed as a consequence to cultural noise and earthquakes. Beresnev and Johnson1 provided an excellent review of the major developments in seismic stimulation in the USA and Russia. Guo et al.[2] discussed various field tests of seismic application in China. When a seismic (high wavelength) wave passes through fractured porous media, it will disperse into higher harmonics thereby generating waves at ultrasonic frequencies within the reservoir.[3] Therefore, knowledge of the effect of ultrasound on flow through porous media is relevant to technologies such as pressure pulsing, downhole acoustic stimulation and the use of explosives as form of seismic stimulation. The potential of ultrasound as a viable method to enhance oil recovery in mature fields was first investigated by Duhon and Campbell.[4] They performed a series of brine flood experiments on oil saturated sandstone samples under ultrasound at a frequency range of 1 - 5.5 MHz. Their observations showed that ultrasonic energy did have a considerable effect on displacement efficiency by creating a more uniform displacement front. They postulated that ultrasound generates localized pressure surges during cavitation, which may force trapped oil into adjacent pores. Later, Fairbanks and Chen[5] performed a series of experiments testing the effect of heat transfer on the percolation of fluids through porous media under ultrasound. Increase in percolation rate was observed for all cases investigated. Since the temperature strongly affects viscosity, interfacial tension and density of the phases, it was not clear whether the observed improvements were due to temperature or due to the vibrations by ultrasound. Percolation of oil in different porous rock types were also investigated by Mikhailov et al.[6], Dyblenko et al.[7], Nereting et al.[8]. Gadiev9 introduced ultrasound to oil saturated unconsolidated sand packs and observed considerable increase in oil production rate and cumulative oil production. He postulated that this effect was due to a phenomenon called "sono-capillary effect", by which the liquid level within a capillary is raised due to an additional pressure generated from collapsing bubbles during cavitation. This effect has been studied by numerous authors[10–13].