Research Project C05

Fluid-solid reactions pose a challenge for modelling flow and transport in porous media due to the dynamic changes in the pore space and the associated alteration of the permeability. This project aims to use Magnetic Resonance Imaging (WRI) and X-ray computer microtomography (µXRCT) measurements in combination with flow and transport experiments on the REV scale to improve understanding of how reactions at the fluid-solid interface affect flow and transport in porous media.

In the first funding period, we focused on salt precipitation due to evaporation from porous media as an exemplary solid-fluid reactions. This process leads to the formation of a salt crust that affects the flow and transport of water and solutes in the top layer of a porous media, and causes amongst others soil salinization, erosion and land loss, and damage to building materials.

A key result was the investigation of the permeability of salt crusts that form on top (efflorescence) or inside (subflorescence) of the porous medium for which we developed a gas permeameter set-up. Although efflorescent and subflorescent salt crusts developed differently, the gas permeability of the dried crusts were similar and within one order of magnitude. µXRCT scans of subflorescent MgSO₄ crusts (Figure 1) revealed the deformation of the porous medium by salt precipitation. This in conflict with the common assumption that subflorescent precipitation occurs within a rigid porous structure. Overall, the experimental insights helped to understand the impact of precipitation on evaporation on the pore scale. The information and the evolution of porosity and REV-scale permeability can be used to improve Darcy-scale models that describe the evaporation of saline solution from porous media. This formed the basis for collaborations with SFB project areas A and C as well as associated projects.

In the current (second) funding phase, we continued to investigate solute dynamics and salt precipitation associated with evaporation. A key results was obtained together with project A02 and associated researcher Stefanie Kiemle (CX5). We investigated density-driven downward redistribution of solutes during evaporation for soils with a supply of water from below using MRI (Figure 2). Strong differences in solute concentration distributions were observed for samples with different intrinsic permeability, and the implications for evaporation rates and time to the start of crust formation were confirmed in a wind tunnel experiment performed together with external partner Nima Shokri from the Technical University Hamburg. Numerical simulations with DuMux were able to reproduce both evaporation experiments.

We also investigated a second solid-fluid reaction: induced calcite precipitation. In collaboration with project C04, we investigated how different injection strategies for enzyme-induced calcite precipitation (EICP) affected the intrinsic permeability as well as the distribution of calcite precipitation as determined both MRI and XRCT. The results showed that constant pressure injection led to a faster reduction of permeability and a more homogeneous precipitation distribution compared to constant flow injection (Figure 3). This confirms previous findings of project C04 obtained with microfluidic experiments.

For the third funding period, we propose to extend the aim of the project and also focus on how interactions between fluid-solid reactions and deformation affect flow and transport in porous media. The work will be focused on:

• Development of an experimental benchmark for evaporation-induced salt precipitation in highly controlled experimental conditions using the advanced experimental facilities of the CRC 1313 (with project A06 and Z02). This unique experimental benchmark with quantified uncertainty will be used to organize a community-wide benchmark study to evaluate the performance of REV-scale models together with project D03.
• Deformation during subflorescent salt precipitation for different salt types and environmental conditions using a novel set-up to determine strain of granular samples. Selected samples will also be investigated using time-lapse XRCT imaging to obtain detailed 3D information on deformation, and to assess the importance of this deformation for evaporation processes.
• Desiccation cracking of evaporating porous media through time-lapse imaging with a digital camera. Thin samples of bentonite clay will be investigated to obtain information on how salt type, solute concentration, and salt precipitation affect desiccation cracking. This will be followed by experiments with thicker soil samples, where more intricate hydromechanical interactions are anticipated. This work will be jointly done with B02, where a model for desiccation cracking will be developed.

The REV-scale experiments on evaporation-induced salt precipitation are a central component of the “salt vision” of the overall CRC. The acquired data will be important for the further development and validation of both pore-scale and REV-scale modelling approaches for coupled free-flow porous-media flow in the benchmark study “evaporation-induced salt precipitation” that will be developed together with A06 and D03.

Together with external partner Nima Shokri, we performed wind tunnel experiments to verify that density-driven redistribution of solutes lead to a delayed start of salt crust formation.

The work on induced calcite precipitation benefits from the close connection of the CRC 1313 with the Montana State University in Bozeman, USA.

We joined forces in the first funding phase with Uri Nachshon from the Volcani Institute. Together, we developed the gas permeameter set-up and jointly analyzed the permeability and XRCT measurements.