Micro-computed tomography pore-scale study of flow in porous media: Effect of voxel resolution

Abstract

A fundamental understanding of flow in porous media at the pore-scale is necessary to be able to upscale average displacement processes from core to reservoir scale. The study of fluid flow in porous media at the pore-scale consists of two key procedures: Imagingreconstruction of three-dimensional (3D) pore space images; and modelling such as with single and two-phase flow simulations with Lattice-Boltzmann (LB) or Pore-Network (PN) Modelling. Here we analyse pore-scale results to predict petrophysical properties such as porosity, single-phase permeability and multi-phase properties at different length scales. The fundamental issue is to understand the image resolution dependency of transport properties, in order to up-scale the flow physics from pore to core scale. In this work, we use a high resolution micro-computed tomography (micro-CT) scanner to image and reconstruct three dimensional pore-scale images of five sandstones (Bentheimer, Berea, Clashach, Doddington and Stainton) and five complex carbonates (Ketton, Estaillades, Middle Eastern sample 3, Middle Eastern sample 5 and Indiana Limestone 1) at four different voxel resolutions (4.4 µm, 6.2 µm, 8.3 µm and 10.2 µm), scanning the same physical field of view. Implementing three phase segmentation (macro-pore phase, intermediate phase and grain phase) on porescale images helps to understand the importance of connected macro-porosity in the fluid flow for the samples studied. We then compute the petrophysical properties for all the samples using PN and LB simulations in order to study the influence of voxel resolution on petrophysical properties. We then introduce a numerical coarsening scheme which is used to coarsen a high voxel resolution image (4.4 µm) to lower resolutions (6.2 µm, 8.3 µm and 10.2 µm) and study the impact of coarsening data on macroscopic and multi-phase properties. Numerical coarsening of high resolution data is found to be superior to using a lower resolution scan because it avoids the problem of partial volume effects and reduces the scaling effect by preserving the pore-space properties influencing the transport properties. This is evidently compared in this study by predicting several pore network properties such as number of pores and throats, average pore and throat radius and coordination number for both scan based analysis and numerical coarsened data.

Critical Resolution and Sample Size of Digital Rock Analysis for Unconventional Reservoirs

“…Alyafei et al [34] applied numerical approach to change the image resolution and no clear trend of permeability dependence on resolution was observed. Shah et al [35] studied function between permeability and voxel size and the result showed that permeability continued changing when approaching the smallest voxel size. All these results fail to answer the question whether a high enough resolution exist to obtain a stable permeability.…”

“…Works have been done on the effect of sample size and resolution, however almost all of them are done separately [31][32][33][34][35][36][37]. The effect of sample size is well studied since Bear [38] brought up the idea of representative element volume (REV), which is the smallest volume over which the measurement result is representative of the macroscopic property.…”

Enhancing Computational Precision for Lattice Boltzmann Schemes in Porous Media Flows

“…By tuning the unphysical relaxation parameters, viscosity-independence can be achieved with the bounce-back boundary conditions [15]. Although boundary interpolation schemes demonstrate slightly more consistent behaviour [16,17], we have found that the standard bounce-back method gives accurate results for flow in complex porous media [18] and remain with this approach here for its computational efficiency. The fluid is driven by a body-force term, for which a precise treatment is needed to eliminate error terms in velocity gradients [19] and is incorporated into the MRT framework [20,21].…”

High-Resolution 3D FIB-SEM Image Analysis and Validation of Numerical Simulations of Nanometre-Scale Porous Ceramic with Comparisons to Experimental Results

“…In this model, a discrete form of the Boltzmann scheme is solved on a Cartesian grid and can be shown to reduce to the incompressible Navier-Stokes equations (He and Luo 1997). The LB method has been applied extensively to problems in porous media flows (Sukop and Thorne 2007;Boek and Venturoli 2010;Shah et al 2015;Yoon et al 2015) due to its computational efficiency and simplicity. The central variable is the distribution function f i (x, t) which represents the number of fluid particles at a grid position x, at time t having a velocity which is one of a discrete set indexed by i.…”