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 and X-ray computer microtomography (µXRCT) measurements in combination with flow and transport experiments on the REV scale to experimentally elucidate differences in the evolution of porosity and permeability for two different fluid-solid reactions: salt precipitation during evaporation and (microbially-) induced calcite precipitation.

Salt precipitation during evaporation

Salt precipitation due to evaporation from porous media 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. Despite extensive research on evaporation of saline solutions from porous media, little is known about the effective transport properties of the developing salt crusts besides their importance for accurate modeling of evaporation.

In this project, the permeability of salt crusts that form on top (efflorescence) or inside (subflorescence) of the porous medium from evaporation of saline solution was determined with a gas permeameter set-up. The developed experimental approach with custom-made sample holders enabled the investigation of separated consolidated salt crusts. 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 revealed the deformation of the porous medium by salt precipitation. This in conflict with the common assumption that sublorescent precipitation occurs within a rigid porous structure. Overall, the experimental insights obtained in this project help to understand the impact of precipitation on evaporation on the pore scale. Additionally, 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 forms a basis for collaborations with SFB project areas A and C as well as associated projects.

Induced calcite precipitation

Microbially-induced calcite precipitation (MICP) is a promising technology for building underground barriers, soil consolidation, well bore sealing, and to support CO₂ sequestration. The changes in effective hydraulic properties such as porosity and permeability in response to biofilm growth and calcite precipitation are not very well understood. As a first step, flow experiments in sand columns with enzymatically-induced precipitation are being used to study the impact of calcite precipitation on the effective hydraulic properties in the absence of biofilm growth. At the same time, non-invasive imaging with MRI and µXRCT provide insights into the evolving pore space and into the development of preferential flow paths. The evolution of the permeability is determined by pressure measurements and correlated to the observed changes in pore space.

In the case of salt precipitation during evaporation, future experimental work in this project will focus on time-lapse imaging of evaporation processes and crust formation in homogeneous and heterogeneous porous media in order to obtain information on the development of the hydraulic properties of salt crusts, soil water content dynamics in salt crusts, as well as the deformation of unconsolidated porous media due to crust formation. In the case of induced calcite precipitation, the focus will be on non-invasive imaging of MICP using the developed experimental approach, which adds the complication of understanding how the presence of biofilms affect the porosity and permeability of porous media. The experimental data obtained in this project will provide valuable input for REV-scale modelling, which will be pursued in collaboration with partners from project areas A and C and associated partners.

The gas permeameter set-up was developed in collaboration with Uri Nachshon from the Volcani Institute, Beer’Sheva, Israel. The work on induced calcite precipitation benefits from the close connection of the SFB1313 with MSU Bozeman, USA.