Modelling single-phase fluid-fluid reactive transport at the pore-scale

• Main Author: Al Nahari Alhashmi, Zaki Mahmoud Sharif
• Other Authors: Bijeljic, Branko ; Blunt, Martin
• Published: Imperial College London 2015
• Subjects: 551
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.695538

Reactive transport is an important field of study in the earth sciences. It captures both natural phenomena, and industrial and environmental applications, including transport of pollutants in the subsurface, nuclear waste storage, and carbon storage. The aim of this thesis is to provide a better understanding of coupled physico-chemical processes governing these phenomena as well as to be used as tools for better understanding these environmental applications. We introduce from first principles a novel pore-scale modelling approach capable of simulating single-phase fluid-fluid reactive transport directly on voxels of 3D images of porous media constructed from X-ray tomography. We use a streamline-based particle tracking method for simulating flow and transport, while for reaction to occur, both reactants must be within a diffusive distance. We assign a probability of reaction, as a function of the reaction rate constant and the diffusion length. The model for reaction is validated against analytical solutions in a free fluid as well as against experimental data on reactive transport in porous media. It takes into account the degree of incomplete mixing present at the sub-pore level. We demonstrate the nature of dynamic changes in the reaction rate, which is related to the degree of pore-scale mixing. Our model does not use any calibrating parameters to fit empirical data unlike other models published in the literature. The model is then extended to investigate the impact of pore structure heterogeneity, transport, and reaction conditions on the overall reaction rate in porous media by studying different classes of porous media. The overall reaction rate varies significantly according to the degree of heterogeneity and transport conditions. It is found that the rate of reaction is a subtle combination of the amount of mixing and spreading that cannot be predicted from the dispersion coefficient alone. At low Péclet number, the effective reaction rate is principally controlled by the amount of mixing due to diffusion. On the other hand, at high Péclet number the reaction rate is controlled by a combination of pore-scale mixing due to spreading and the degree of heterogeneity of the pore structure.