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Researchers at University of Tsukuba have developed a theoretical framework to clarify the propagation of ultrasonic waves through complex materials such as rocks containing mixtures of oil, water, and gas bubbles. Understanding this process is essential for improving technologies such as enhanced oil recovery, which employs ultrasound to mobilize trapped heavy oil. This study reveals the coexistence of three types of compressional waves in these complex materials and emphasizes the role of heavy-oil viscoelasticity and bubble oscillations in controlling ultrasonic-energy dissipation.

Tsukuba, Japan—Ultrasound-based irradiation of rock formations has attracted considerable attention as a technique for enhancing heavy-oil (high-viscosity crude oil) recovery from deep underground reservoirs. However, a unified theoretical framework for wave propagation and energy dissipation in these formations remains lacking because water coexists with heavy oil within rock pores and gas bubbles in the water respond dynamically to ultrasonic excitation, thereby creating a complex system. Conventional theories typically treat oil as a purely viscous (Newtonian) fluid or assume frequency ranges markedly below the ultrasonic regime. Consequently, these theories inadequately capture oil viscoelasticity and the influence of bubble oscillations in the ultrasonic regime.

This study extends previous low-frequency models and constructs a theoretical framework applicable to ultrasonic frequencies by incorporating three notable elements into a unified system of equations: (i) heavy-oil viscoelasticity, (ii) dynamic capillary pressure at fluid-fluid interfaces, and (iii) oscillations of gas bubbles dispersed in water induced by ultrasonic pressure fluctuations. This framework analyzes wave velocities and frequency-dependent attenuation. The results reveal the coexistence of three longitudinal-wave types with distinct physical origins: a fast-propagating mode, strongly attenuated mode arising from relative motion between fluids and the rock matrix, and slow mode governed primarily by capillary dynamics at fluid interfaces.

These findings provide a theoretical basis for understanding and predicting ultrasonic wave behavior in multiphase porous media across a wide frequency range. The results are expected to support optimized strategy development depending on the intended application, such as selecting lower frequencies for wider spatial coverage and higher frequencies to exploit localized interfacial and viscoelastic effects.

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This study was partially supported by JSPS KAKENHI (No. 22K03898) from the Japan Society for the Promotion of Science, and JKA and its promotion funds from KEIRIN RACE. This work was also partially supported by the Top Runners in Strategy of Transborder Advanced Research (TRiSTAR) program conducted as part of the Strategic Professional Development Program for Young Researchers by MEXT.

Original Paper

Title of original paper:

Ultrasound propagation in multiphase porous media: A continuum-mechanical model for coupled effects of bubble dynamics and oil viscoelasticity

Journal:

Physics of Fluids

DOI:

10.1063/5.0323134

Correspondence

Associate Professor KANAGAWA Tetsuya
Institute of Systems and Information Engineering, University of Tsukuba

FUKUYA Tomohiro
Graduate School of Systems and Information Engineering, University of Tsukuba

Related Link

Institute of Systems and Information Engineering