Effects of multiscale porosity system of compacted clays on water flow and chemical transport

Abstract

Understanding of the transport properties of compacted smectite-rich clays plays a key role in various geo-environmental engineering applications. Developing a sound theoretical and numerical description of the water flow and chemical transport processes in compacted highly swelling clays, such as bentonite, has remained a challenge. This challenge arises from the complex multiscale pore structure of bentonite which changes with varying water content, i.e. the number and volume of pores at different lengths scales depend on the saturation. In addition to this complexity, a large fraction of bentonite porosity, the micro- (≤ 2nm) and meso- (2~50nm) pores where ionic transport is impacted by naturally negative charged bentonite surface, is not detectable and quantifiable by direct imaging techniques. This work aims to advance the understanding of flow and ions transport in compacted bentonite clays taking into consideration multiscale pore evolution. This is achieved by a combination of theoretical developments, new pore network construction approaches and numerical simulations, all of which are validated with experimental data available in the literature. This thesis starts by proposing a predictive model for the hydraulic conductivity under confined wetting. A theoretical description of pore system evolution with changing relative humidity is presented based on a solid-solution geochemical modelling approach. Further, the Kozeny-Carman (KC) relationship for hydraulic conductivity is revisited as a basis to derive a new model for compacted bentonite, which incorporates the effects of key properties, such as porosity, specific surface area and tortuosity. The predictive results of the revised KC equation for saturated and unsaturated hydraulic conductivity of compacted bentonite are in close agreement with the experimental data. A novel algorithm is proposed for constructing 2D irregular pore network models (PNMs) at micro- and meso- scale without imaging data, but which represent statistically the size distributions of pores and solid grains obtainable with indirect methods (e.g. mercury intrusion porosimetry). Analysis of gas flow through constructed PNMs is performed by numerical solution of Stokes equation, and the results are used in two ways: for calculating tortuosity by a new numerical approach based on flow path lines and for calculating (intrinsic) permeability using Darcy’s law. The numerical approach of 2D PNMs shows that the majority of previous quantitative methods overestimate the tortuosity of systems with meso-pores (inter-particle pores). However, the permeability calculation with the improved tortuosity estimation is in close agreement with the revised Kozeny-Carman equation, particularly for bentonite samples with large fractions of inter-particle pores. A new approach is developed for constructing realistic 3D PNMs for multi-species diffusion in changed multiscale pore systems. The realism of the constructed 3D heterogeneous and anisotropic multiscale fabrics has been further justified by the experimental data of tritiated water (HTO) diffusion. Anion exclusion (repulsion) effect is simulated by developing a mathematical formulation based on Donnan layer theory. The 3D simulation of PNMs presents the effective diffusivities of bentonite samples agree well with measured results, especially for higher dry density one. The proposed transport theories and PNMs construction method in this thesis can be applied to a wide range of charged porous materials with known pore size distribution and fitted for linking to the flow and ions transport from micro-to-macro (> 50nm) scale.

Date of Award	1 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Andrey Jivkov (Supervisor) & Majid Sedighi (Supervisor)