Laboratory observations of particle release in a porous medium using increasing ultrasound stimulation

Abstract

Summary It is well established that seismic waves disperse into higher harmonics, thereby generating waves at ultrasonic frequencies within layers. Field tests and laboratory investigations have demonstrated that ultrasonic stimulation can enhance particle release in natural systems, but the intrinsic mechanisms of ultrasonic waves are not fully comprehended. We investigated the underlying mechanics of such waves by generating increasing ultrasound stimulation upon enhanced release of trapped particles from pores. Experiments were conducted using a novel soil column apparatus that is capable of applying ultrasonic waves, controlling the confining pressures, regulating the soil column height, and controlling the temperature of the soil column. Particles with median diameters of 9.63 μm in the range of 1–35 μm were evaluated in a series of tests under three flow rates. Stimulations were applied for 60 s at power levels of 600, 1000, 1400, and 1800 W. The particle concentration in the effluent during each test was recorded. The laboratory results showed that hydrodynamic drag was the dominant force in pre-stimulation particle release. However, the flow rate was found to be constant in pre- and post-stimulation phases, and cavitation caused by ultrasound stimulation was judged to be the most likely reason for post-stimulation particle release. Furthermore, the nature of the post-stimulation particle release was distinctly different from that of the pre-stimulation release, in that an abrupt concentration change was observed in each stimulation application. These abrupt concentration changes were attributed to ultrasound stimulation, but the release concentration and the duration of particle release were controlled by the power of the ultrasound stimulation and the quantity of particles deposited in the sand. Furthermore, the release rate coefficient first decreased and then increased as the number of ultrasound stimulations increased. Two mechanisms of particle release were identified that could explain the rate change: (1) only particles deposited in the porous medium by fouling mechanisms could be released by ultrasound stimulations at power levels below a certain critical value, or (2) ultrasound stimulations at power levels above a certain critical value are sufficient to change the structure of the porous medium, producing more dead-end pore openings that allow particle flow through the porous medium. The results presented are unique in indicating that successive particle release can be induced using increasing ultrasound stimulation.