Drainage mechanisms in porous media: From piston‐like invasion to formation of corner flow networks

Abstract

Water drainage from porous media is a highly dynamic process often marked by rapid piston‐like air invasion events at the front and other rapid interfacial reconfigurations. Liquid phase entrapped behind the moving front drains at significantly slower rates often via gravity driven flow through corners and crevices. This distribution of slowly draining residual water phase determines the plant available water and biological functioning of soils. The study aims to determine the conditions for the flow regime transition from piston‐like invasion at a drainage front to slower corner dominated flow at the pore and sample scale. This transition was observed experimentally for sand and glass beads with fast X‐ray tomography, revealing water fragmentation into clusters of full pores interconnected by water corner films. The observed liquid morphology at the transition from piston to corner flow was reproduced by a quasi‐static pore network model and predicted by percolation theory. The amount of capillary‐retained water at flow transition controlling the subsequent drainage dynamics could be reproduced by an idealized star shaped pore whose geometry is deduced from macroscopic properties of the porous medium. Predictions of water content thresholds at flow transitions were in agreement with other critical saturation values associated with cessation of solute diffusion and of internal drainage (at field capacity) highlighting the criticality of water phase continuity disruption for formation of relatively stable unsaturated conditions controlled by slow corner flow that support life in soil.

Scale Issues in Soil Hydrology

“…Second, new tomographic techniques led to considerable new insights into multiphase dynamics. It is now possible to directly visualize local water saturation within the pore space and even the movement of individual water–gas interfaces during changing water saturation (Schlüter et al, 2017; Hoogland et al, 2016). Based on continuously improving tomographic techniques, this is now possible for three‐dimensional natural porous media and not just two‐dimensional artificial micromodels.…”

Numerical Simulation of Multiphase Flow in Nanoporous Organic Matter With Application to Coal and Gas Shale Systems

“…In nanoporous organic matter, fluid flow property is highly dependent on pressure, temperature, and pore size. In Figures , the corresponding water saturation for the relative permeabilities crosspoint can shift to the value larger than 0.5 when pore pressure and average pore radii are lower than 10 MPa and 9.36 nm, respectively, and temperature is larger than 550 K. The fluid distribution in a single pore is influenced by the pore geometry (Arns et al, ; Blunt et al, ) and numerous pore network models predict multiphase flow pattern based on the corner geometry (Afsharpoor et al, ; Hoogland et al, ; Joekar‐Niasar et al, ; Ryazanov et al, ; Valvatne & Blunt, ). However, the aim of this study is to understand the major effect of the multiphase flow pattern change in various condition.…”

Drainage dynamics controlled by corner flow: Application of the foam drainage equation

“…Note that in the present study the onset of a corner flow dominated drainage process depends on the applied flow rate defining the critical water content. An alternative and simplified approach for transition to corner flow could be based on morphological arguments [Hoogland et al, 2016] defining a threshold water content when the network of water filled pores becomes fragmented and clusters of full pores are interconnected by corner flow. This critical water content can be related to concepts of percolation theory and a critical percolation threshold as was done in Hunt and Gee [2002] and Hunt and Ewing [2003].…”

“…At air invasion in angular pores an interfacial jump takes place to water-air interfaces pinned in pore corners with drainage radius of curvature r d and corresponding drainage saturation S d as defined by minimization of interfacial energy (so called MS-P method based on the work of Stowe [1965, 1966] and Princen [1969aPrincen [ , 1969bPrincen [ , 1970). Expressions relating r ic to r d , and computing saturation S d based on the star geometry and the MS-P method are given in Hoogland et al [2016]. The model was related to macroscopic medium properties of porosity /, air entry value h b and grain specific surface area SSA in Hoogland et al [2016] by the following equations:…”