Pore‐scale mechanisms for the enhancement of mixing in unsaturated porous media and implications for chemical reactions

Abstract

Porous media in which different fluid phases coexist are common in nature (e.g., vadose zone and gas‐oil reservoirs). In partially saturated porous media, the intricate spatial distributions of the wetting and nonwetting phases causes their flow to be focused onto preferential paths. Using a novel 2‐D experimental setup allowing pore‐scale measurement of concentration fields in a controlled unsaturated flow, we highlight mechanisms by which mixing of an invading fluid with the resident fluid is significantly enhanced when decreasing saturation. The mean scalar dissipation rate is observed to decrease slowly in time, while under saturated conditions it decays rapidly. This slow decrease is due to sustained longitudinal solute fingering, which causes concentration gradients to remain predominantly transverse to the average flow. Consequently, the effective reactivity is found to be much larger than under saturated conditions. These results provide new insights into the role that multiphase flows play on mixing/reaction in porous media.

Mixing in a three‐phase system: Enhanced production of oil‐wet reservoirs by CO2 injection

“…The degree of mixing is defined as [e.g., Jha et al, 2011] The degree of mixing is defined as [e.g., Jha et al, 2011] …”

Identification of Channeling in Pore‐Scale Flows

“…Notably, flow and transport phenomena are affected not only by the degree of heterogeneity of the medium but also by the spatial arrangement of its hydraulic properties, a prominent role being played by connectivity (Knudby & Carrera, 2005). Understanding the mechanisms driving flow to concentrate in high-velocity channels is key for proper prediction of first arrival times of dissolved chemicals at critical targets (Nissan & Berkowitz, 2018;Tartakovsky & Neuman, 2008;Zinn & Harvey, 2003) and the characterization of multiphase flow processes (Dai & Santamarina, 2013;Jiménez-Martínez et al, 2015), with direct implications in several settings, including, for example, environmental risk assessment or enhanced oil recovery. It can be regarded as a measure of the presence of preferential flow paths (or fast channels) across which flow tends to focus and be associated with high velocity values.…”

Hydrothermodynamic mixing of fluids across phases in porous media

“…These studies, however, are mainly limited to fully miscible single‐phase flow of two‐component incompressible fluids with usually constant diffusion coefficients. Only recently, Jiménez‐Martínez et al [, ] conducted pore‐scale experiments for multiphase (mostly immiscible) systems.…”