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Classic soil physics relies heavily on the concept of representative elementary volume (REV), which is necessary to perform upscaling from the studied soil samples and parameterize continuum scale hydrological models (e.g., based on Richards or Darcy equations). In this paper, we explore the boundaries of the classic REV concept and conventional representativity studies that claim REV for any given physical property, and that its values converge to a steady value with increasing sample volume. We chose two undisturbed soil samples of standard size from Ah and B horizons, subcropped two subvolumes within each of them and performed pore‐scale flow simulations using binarized X‐ray microtomography scans as input data. The volume of the simulation domains was 9003 voxels, with a physical volume within two orders of magnitude of the whole soil core. Based on the 3D pore geometry images and the resulting flow velocity and pressure fields, we performed REV analysis for the saturated hydraulic conductivity and porosity. Although in general subvolumes showed classical REV behaviour (convergence of porosity and saturated hydraulic conductance (Ksat)to plateau‐like behaviour), their flow properties converged to different REV values. We also evaluated the stationarity of pore structures by computing the directional correlation functions to explain the observed non‐unique behaviour. We concluded that neither of the studied samples can be considered to be representative due to their structural non‐stationarity. We extensively discussed the implications of these results for the upscaling and parameterization of continuum‐scale flow models. We argued that the plateau in Ksat (or any other physical property) is a necessary, but insufficient, condition for the REV, which requires (at least) pore structure stationarity as an additional criterion. REV as a concept is much broader than previously anticipated and the possibility of establishing REVs in structured soils will require an analysis of tensorial flow properties with correct boundary conditions, multiscale soil structure imaging with stationarity analysis, and pore‐scale simulations on fused multiscale images. Highlights: Ksat analysis within subvolumes showed classical REV behaviour but resulted in non‐unique REVs for the soil samples studiedBased on the stationarity analysis we argued that the conventional soil core samples studied were not REVsPlateau‐like behaviour in physical properties is a necessary but insufficient condition for REVSoil structure information is necessary to properly establish REV or upscale flow properties [ABSTRACT FROM AUTHOR]

Permeability is one of the fundamental properties of porous media and is required for large-scale Darcian fluid flow and mass transport models. Whilst permeability can be measured directly at a range of scales, there are increasing opportunities to evaluate permeability from pore-scale fluid flow simulations. We introduce the free software Finite-Difference Method Stokes Solver (FDMSS) that solves Stokes equation using a finite-difference method (FDM) directly on voxelized 3D pore geometries (i.e. without meshing). Based on explicit convergence studies, validation on sphere packings with analytically known permeabilities, and comparison against lattice-Boltzmann and other published FDM studies, we conclude that FDMSS provides a computationally efficient and accurate basis for single-phase pore-scale flow simulations. By implementing an efficient parallelization and code optimization scheme, permeability inferences can now be made from 3D images of up to 10 9 voxels using modern desktop computers. Case studies demonstrate the broad applicability of the FDMSS software for both natural and artificial porous media. [ABSTRACT FROM AUTHOR]

Pore-network models were found useful in describing important flow and transport mechanisms and in predicting flow properties of different porous media relevant to numerous fundamental and industrial applications. Pore-networks provide very fast computational framework and permit simulations on large volumes of pores. This is possible due to significant pore space simplifications and linear/exponential relationships between effective properties and geometrical characteristics of the pore elements. To make such relationships work, pore-network elements are usually simplified by circular, triangular, square and other basic shapes. However, such assumptions result in inaccurate prediction of transport properties. In this paper, we propose that pore-networks can be constructed without pore shape simplifications. To test this hypothesize we extracted 3292 2D pore element cross-sections from 3D X-ray microtomography images of sandstone and carbonate rock samples. Based on the circularity, convexity and elongation of each pore element we trained neural networks to predict the dimensionless hydraulic conductance. The optimal neural network provides 90% of predictions lying within the 20% error bounds compared against direct numerical simulation results. Our novel approach opens a new way to parameterize pore-networks and we outlined future improvements to create a new class of pore-network models without pore shape simplifications. [ABSTRACT FROM AUTHOR]

• Biopore stiffness and permeability correlate positively with coating cohesion. • Coating cohesion affects crack nucleation and propagation due to biopore compaction. • Biopore stability is higher in the axial direction than in the diametral direction. Exudates and finer particles often coat the surface of biopores, increasing the mechanical stability and altering the physicochemical properties (e.g. wettability or sorption) of the surrounding. Properties of the biopore surface govern the macropore-matrix mass exchange processes during preferential flow in the soil macropores. However, the relationships between mechanical and hydraulic properties of coated biopore regions are not fully understood nor expressed by numerical models. Correlations between soil hydraulic and mechanical properties could perhaps be established by quantifying the water flow in defined pore structures of the biopore. The objective was to develop a model-based approach for studying the effects of clay-organic coatings on hydro-mechanical properties of biopore walls. The technical challenge was to develop a one-way directed solution (i.e., structural impact on fluid flow) between discrete element method (DEM) and a finite difference Stokes solver (FDMSS) to perform hydro-mechanical simulations of a coated biopore structure. Increasing the coating cohesion by DEM simulation to account for concentrating organo-mineral cementing agents, increased the overall stiffness as well as crack propagation through the soil matrix. As a result, biopore structure displayed higher yield strength and higher permeability through the interface between coating and matrix. Coating cohesion was positively correlated with Young's modulus and permeability displaying non-linear relationships. The one-way directed solution between DEM and FDMSS Stokes solver could be further extended to describe dynamic hydro-mechanical properties of more complex soil structures. [ABSTRACT FROM AUTHOR]

While it is well known that permeability is a tensorial property, it is usually reported as a scalar property or only diagonal values are reported. However, experimental evaluation of tensorial flow properties is problematic. Pore-scale modeling using three-dimensional (3D) images of porous media with subsequent upscaling to a continuum scale (homogenization) is a valuable alternative. In this study, we explore the influence of different types of boundary conditions on the external walls of the representative modeling domain along the applied pressure gradient on the magnitude and orientation of the computed permeability tensor. To implement periodic flow boundary conditions, we utilized stochastic reconstruction methodology to create statistically similar (to real porous media structures) geometrically periodic 3D structures. Stochastic reconstructions are similar to encapsulation of the porous media into statistically similar geometrically periodic one with the same permeability tensor. Seven main boundary conditions (BC) were implemented: closed walls, periodic flow, slip on the walls, linear pressure, translation, symmetry, and immersion. The different combinations of BCs amounted to a total number of 15 BC variations. All these BCs significantly influenced the resulting tensorial permeabilities, including both magnitude and orientation. Periodic boundary conditions produced the most physical flow patterns, while other classical BCs either suppressed crucial transversal flows or resulted in unphysical currents. Our results are crucial to performing flow properties upscaling and will be relevant to computing not only single-phase but also multiphase flow properties. Moreover, other calculation of physical properties such as some mechanical, transport, or heat conduction properties may benefit from the technique described in this study. [ABSTRACT FROM AUTHOR]

The paper is focused on high efficiency Stokes solver that is applied to the incompressible flow in porous media. Computational domains are represented by binarized 3D computed tomography voxel models. The solution procedure is constructed around a fast algebraic multigrid solver that utilizes the power of graphics processing units (GPUs). In order to minimize memory footprint and accelerate the solver, a simple MAC-type staggered finite difference discretization is used and a coupled Stokes saddle-point type system is solved directly on a GPU. The MAC discretization is discussed, taking particular care of internal solid boundaries around pores. The method includes topological domain analysis on a GPU, which removes isolated volumes with no flow in the domain (based on the connected component labeling), depending on the boundary conditions. We consider various types of boundary conditions and efficient parallel strategies for the GPUs, including fast matrix assembly and residual regularization. Our method is extensively benchmarked both against analytical solutions and applied problems from digital rock physics in terms of computational wall time, precision, approximation order, and convergence. We demonstrate that it takes up to 5–23 s on a modern GPU card to obtain a solution with 1 ⋅ 1 0 − 6 error residuals for 3D geometries with 300–450 3 voxels and a porosity range of 5–37%. [Display omitted] • Coupled second order Stokes solver for pore-scale modeling is presented. • The solution for 4503 voxels is obtained within 20 s on a modern GPU card. • The solver uses approximate commutator, multigrid preconditioners, Krylov methods. • The topological domain analysis removes null subspaces in linear operators. • The software is implemented on modern GPU architecture; it is extensively tested. [ABSTRACT FROM AUTHOR]