Reactive/Adsorptive transport in (partially-) saturated porous media: from pore scale to core scale

Abstract

Pore-scale modeling provides opportunities to study transport phenomena in fundamental ways because detailed information is available at the microscopic pore scale. This offers the best hope for bridging the traditional gap that exists between pore scale and macro (lab) scale description of the process. As a result, consistent upscaling relations can be performed, based on physical processes defined at the appropriate scale. In the present study, we have used a Multi-Directional Pore Network (MDPN) for representing a porous medium. One of the main features of our network is that pore throats can be oriented not only in the three principal directions, but in 13 different directions, allowing a maximum coordination number of 26. Using MDPN, flow and transport processes were simulated at the pore scale in detail by explicitly modeling the interfaces and mass exchange at surfaces. The solution of pore network model provides local concentrations and it allows relating concentrations and reaction rates at the macro scale to concentrations and reaction rates at the scale of individual pores, a scale at which reaction processes are well defined. Then, comparing the result of pore-scale simulations with the model representing the macro-scale behavior, we could study the relation between these two scales. We have considered mass transfer of reactive/adsorptive solutes though interfaces, under both saturated and partiality saturated conditions. While under saturated conditions the interfaces are only those of solid-water interfaces, under saturated conditions in addition to solid-water interfaces there will be mass transfer though air-water interfaces as well. Macroscopic quantities were obtained through averaging over the pore network domain. To meet our objectives we focus on both physical heterogeneity and topology (differently sized pores and coordination number distribution) and chemical processes. There are many other novel and unique aspects to this book, through which, we develop more accurate and realistic schemes to study flow and transport processes. For this purpose we have developed an extensive FORTRAN 90 Modular Package which spans though generation of random structure networks, discretizing pore spaces on the basis of their saturation state, and solving flow and reactive transport under both saturated and unsaturated conditions using several complex algorithms. The governing equations are solved applying a fully implicit numerical scheme; however, efficient substitution methods have been applied which made the algorithm more computationally effective and appropriate for parallel computations. Through coupling MDPN with multi-component reactive simulator, BRNS, we could simulate transport of species which may cause changes in porosity and permeability due to reaction with the solid phase. By averaging over a representative MDPN, we calculated upscaled parameters such as permeability, dispersion coefficient and measure of plume spreading, and upscaled adsorption parameters under saturated conditions. For partially saturated conditions the results were capillary pressured-saturation relation, relative permeability, unsaturated dispersivity-saturation relation, and upscale adsorption parameters. Whenever possible, we compared our results with the results of experimental observations and analytical equations. In this way, we could evaluate the limitations and sufficiency of the available analytical equations and macro scale models for prediction of transport behavior of (reactive) solutes.