Katharina Heck

Abstract

The exchange of gas components across the subsurface/atmosphere interface influences multiphase flow and reactive transport in the subsurface and is crucial for many biogeochemical processes, for the emission of greenhouse gases, and for the fate of volatile contaminants. In this study, we present a modeling approach to simulate non-isothermal multiphase flow and multicomponent reactive transport in coupled subsurface/atmosphere compartments. The model is based on a coupled porous medium/free flow domain in which the Navier-Stokes equation is used to describe single-phase (gaseous) flow in the free-flow subdomain and Darcy's law is applied for two-phase flow in the porous medium subdomain (i.e., two-domain approach). The implementation is performed by coupling COMSOL Multiphysics and PhreeqcRM, which enables the investigation of the interplay between multi-physical processes (i.e., flow, mass and heat transport) in the coupled compartments and geochemical reactions in the porous medium. We first present a set of benchmark examples in which key features of the proposed model are tested against other numerical simulators and an analytical solution. Successively, we take advantage of the unique capabilities of the proposed approach to explore conservative and reactive transport of gas components in a coupled porous medium/free flow domain. The results show that the exchange processes between the compartments control the location of reactive zones and the extent of geochemical reactions (i.e., mineral dissolution) by changing the spatiotemporal distribution of fluid phases and enhancing the interphase mass transfer of key gas components such as oxygen and carbon dioxide.

Abstract

We present version 3 of the open-source simulator for flow and transport processes in porous media DuMux. DuMux is based on the modular C++ framework Dune (Distributed and Unified Numerics Environment) and is developed as a research code with a focus on modularity and reusability. We describe recent efforts in improving the transparency and efficiency of the development process and community-building, as well as efforts towards quality assurance and reproducible research. In addition to a major redesign of many simulation components in order to facilitate setting up complex simulations in DuMux, version 3 introduces a more consistent abstraction of finite volume schemes. Finally, the new framework for multi-domain simulations is described, and three numerical examples demonstrate its flexibility.

Abstract

Multicomponent gas transport in porous media and at the interface between porous media and free flow occurs in a wide range of technical and environmental systems. Modeling tools are required for the quantitative description of such coupled environmental compartments. We present a benchmark study to compare different diffusion models as well as different coupling concepts employed for the description of multicomponent gas flow and transport at the porous medium/free-flow interface. The benchmark problems allowed us to compare two diffusion models (Fickian and Maxwell-Stefan formulations) for predicting the transport behavior of multicomponent mixtures in porous media and in presence of a free flow. The examples highlighted the importance of interaction between different diffusing components by applying the Maxwell-Stefan formulation, as well as the impact of the free flow velocity on gas migration at the interface under diffusion- and advection-dominated conditions. The problems were solved using two simulation tools, incorporating different coupling concepts to describe the physics of flow and transport processes. In COMSOL Multiphysics, we implemented a single-domain Brinkman approach, whereas in the DuMuX simulator we used a two-domain approach with the Navier-Stokes equation for the free flow and Darcy’s law for the porous medium. Also, the formulation of the Maxwell-Stefan equation for multicomponent transport was different between the two codes. Despite the different modeling concepts, mathematical descriptions and numerical schemes implemented in the two simulators, their outcomes demonstrate excellent agreement for all benchmark problems, as well as the capability to reproduce the results of previous modeling and experimental studies.