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

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).