Simulating dispersion in porous media and the influence of segmentation on stagnancy in carbonates |
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| Institution: | 1. Qatar Carbonates and Carbon Storage Research Centre (QCCSRC), Department of Chemical Engineering, South Kensington Campus, Imperial College London, London SW7 2AZ, UK;2. Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK;1. School of Petroleum Engineering, UNSW Australia, Sydney, Australia;2. Department of Applied Mathematics, Research School of Physics and Engineering, Australian National University, Canberra, Australia;3. NCI VizLab, ANU Supercomputer Facility and NCI National Facility, Australian National University, Canberra, Australia;1. Department of Mathematics and Informatics, University Politehnica of Bucharest, Bucharest, Romania;2. Department of Mathematical Sciences, University of Nevada Las Vegas, Las Vegas, NV 89154, USA;1. Department of Geodynamics, Sciences Faculty, University of Granada, Campus Fuentenueva, E-18071, Granada, Spain;2. Department of Geological Sciences, University of Delaware, Penny Hall, 255 Academy Street, 19716 Delaware, USA;1. The University of Texas Health Science Center at Houston, Department of Diagnostic & Interventional Imaging, 6431 Fannin Street, Houston, Texas 77030;2. The University of Texas Health Science Center at Houston, Department of Neurology, 6431 Fannin Street, Houston, Texas 77030 |
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| Abstract: | Understanding the transport of chemical components in porous media is fundamentally important to many reservoir processes such as contaminant transport and reactive flows involved in CO2 sequestration. Carbonate rocks in particular present difficulties for pore-scale simulations because they contain large amounts of sub-micron porosity. In this work, we introduce a new hybrid simulation model to calculate hydrodynamic dispersion in pore-scale images of real porous media and use this to elucidate the origins and behaviour of stagnant zones arising in transport simulations using micro-CT images of carbonates. For this purpose a stochastic particle model for simulating the transport of a solute is coupled to a Lattice-Boltzmann algorithm to calculate the flow field. The particle method incorporates second order spatial and temporal resolution to resolve finer features of the domain. We demonstrate how dispersion coefficients can be accurately obtained in capillaries, where corresponding analytical solutions are available, even when these are resolved to just a few lattice units. Then we compute molecular displacement distributions for pore-spaces of varying complexity: a pack of beads; a Bentheimer sandstone; and a Portland carbonate. Our calculated propagator distributions are compared directly with recent experimental PFG-NMR propagator distributions (Scheven et al., 2005; Mitchell et al., 2008), the latter excluding spin relaxation mechanisms. We observe that the calculated transport propagators can be quantitatively compared with the experimental distribution, provided that spin relaxations in the experiment are excluded, and good agreement is found for both the sandstone and the carbonate. However, due to the absence of explicit micro-porosity from the carbonate pore space image used for flow field simulations we note that there are fundamental differences in the physical origins of the stagnant zones for micro-porous rocks between simulation and experiment. We show that for a given micro-CT image of a carbonate, small variations in the parameters chosen for the segmentation process lead to different amounts of stagnancy which diffuse away at different rates. Finally, we use a filtering method to show that this is due to the presence of spurious isolated pores which arise from the segmentation process and suggest an approach to overcome this limitation. |
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