Characteristics of acoustic emissions induced by fluid front displacement in porous media

Abstract

The dynamics of fluid displacement in porous media often affect phase entrapment and shape macroscopic transport properties and thus are of considerable interest for a range of natural and engineering applications. The macroscopic motion of a displacement front is composed of numerous abrupt pore‐scale invasion events that involve rapid interfacial jumps and reconfigurations with associated mechanical and interfacial energy release detectable as acoustic emissions (AE). We conducted systematic experiments of fluid displacement and measured associated AE during passage of fluid fronts (primarily drainage) within assemblies of glass beads of different sizes. Results indicated distinct acoustic signatures associated with different displacement processes, reflecting dependency on porous media pore size, displacement flow rate, and liquid properties. The rich AE signals associated with front dynamics exhibited power law relationships between the number of AE events and their amplitudes, reminiscent of avalanche‐like invasion processes. In addition to AE signals emanating from rapid emptying or filling of pores (Haines jumps), other processes such as redistribution and interfacial reconfigurations behind a drainage front and grain rearrangement may generate AE. Characteristic AE signatures generated by displacement processes in different media and under various boundary conditions offer a promise for remote detection of pore‐scale fluid interfacial dynamics in porous media that may shape macroscopic transport properties (e.g., linked with phase entrapment).

Pore-scale modeling of phase change in porous media

“…volume quantities. In this section we show that the proposed diffuse-interface model is able to capture pressure fluctuations generated as liquid-vapor interfaces advance, in accordance with experimental observations [77][78][79][80][81][82] (Figs. 20-23).…”

“…In particular, we show that the diffuse-interface model is able to capture pressure fluctuations 084302-2 generated as liquid-vapor interfaces advance, in accordance with experimental observations [77][78][79][80][81][82]. We also conclude that phase-change fronts are controlled by heterogeneity, a feature that is typical of evaporation fronts, interpreted as modified invasion percolation processes [83][84][85][86][87].…”

“…24). The structure of acoustic emissions during two-phase displacements has been proposed as a proxy to understand the flow processes involved [77]. fluctuations with other flow mechanisms.…”

A new general 1‐D vadose zone flow solution method

“…The capillary-weighted redistribution employed in T- O [2008] moved water vertically in the profile and was incorrect because our intention in simulating this process is that it is a free-energy minimization process that involves changes in interfacial energy, not potential energy. In the improved method, we call this process capillary relaxation after Moebius et al [2012], as it moves water from regions of low capillarity to high capillarity at the pore scale and at the same elevation, producing no advection at or beyond the REV scale.…”

“…In unsaturated soils in the presence of gravity, water infiltrating into a partially saturated porous medium flows faster through larger pores because they are the most efficient conductors [Gilding 1990]. Capillarity then acts to move this advected water from these larger pore spaces into smaller pore spaces at the same elevation in a process we call ''capillary relaxation'' after Moebius et al [2012] who have observed this phenomenon using acoustic emissions.…”