Research Project CX3

Investigation of salt-precipitation processes in coupled porous-media free-flow systems – a detailed consideration at the pore-scale

Publications

Publications in scientific journals

  1. (Journal-) Articles

    1. Shokri-Kuehni, S. M. S., Raaijmakers, B., Kurz, T., Or, D., Helmig, R., & Shokri, N. (2020). Water Table Depth and Soil Salinization: From Pore-Scale Processes to Field-Scale Responses. Water Resources Research, 56(2), Article 2. https://doi.org/10.1029/2019wr026707

Research

About this project

The fundamental understanding of salt precipitation in porous media due to evaporation processes is important in different environmental and technical applications. For example, salt precipitation in building materials generate stresses, which can lead to weathering and damage of constructions and cultural monuments. The salinization of soil in arid and semi-arid zones is a further challenge. This is often induced by use of saline water for irrigation, due to the limited availability of irrigation water of high quality. In combination with high evaporation rates and poor drainage this can lead to salt precipitation in the root zone or at the soil surface and a resulting reduction of crop yield.
During this processes water evaporates at the evaporation front at the brine-air interface. Due to the loss of water the salt concentration in the brine increases at the evaporation front and salt precipitates when the solubility limit is exceeded. The evaporation process is influenced by the fluid properties like the salt concentration, by the surrounding atmospheric conditions like temperature, humidity of the air or wind speed and the porous medium properties like permeability.

During the evaporation of a saline solution, the salt concentration increases at the evaporation front and solid precipitates in the porous medium when the solubility limit is exceeded. At the evaporation front a layer of higher densitiy develops due to the accumulation of salt in the liquid phase. This can lead to density-driven instabilities which prevent a further increase of the salt concentration and the precipitation of salt. Many process-controlling properties of salt precipitation depend on interface-driven processes on the pore-scale like the spatially varying salt concentration. To investigate this interface-driven processes experiments and simulations are done on the pore scale, where the single pores and interfaces can be resolved. The formation of density-driven instabilities is considered on the REV-scale, where the porous medium is averaged over a representive elementary volume (REV).

 

The project investigates salt precipitation on the pore scale using pore-network modelling and micro-CT experiments. Also the influence of density-driven instabilities on salt precipitation is investigated on the REV-scale.

In this project a so-called pore-network model is used, that efficiently resolves pore-scale phenomena using a network of pore bodies and pore throats, representing the larger pore spaces and the connections in-between. The aim is to developed a reactive transport model for the pore-network model including the precipitation reaction and the resulting change in pore-space geometry. To investigate the influence of the free-flow on the evaporation processes in detail the pore network must be coupled to a free-flow domain.

Further, experiments are conducted at the microCT-facility of the Oregon State University to improve and validate the model. A column filled with a porous medium and salt solution which is open to the atmosphere is scanned after several periods of evaporation using X-ray tomography. From these scans three-dimensional information about the distribution of the liquid and gaseous phase can be gained as well as knowledge about the position and amount of precipitated salt. From this experiments an extensive dataset is obtained which is of great value for the further development and improvement of the numerical model.

Density-driven instabilities due to evaporation.

The results of these detailed pore-scale considerations will be used to identify relevant processes and find relations between pore-scale and macro-scale parameters. The overall goal is to improve macro-scale models, which are able to represent and investigate field-scale problems.

The investigations of density-driven instabilities on the REV-scale are done in collaboration with Hans van Dujin (TU Eindhoven) and Carina Bringedal (A05).  It is of importance for the investigation of salt precipitation to understand the parameters influencing the occurence and development of instabilities. So the influence of parameters like permeability and the evaporation rate on the development of instabilities will be investigated for saturated and unsaturated porous medium.

Results

Segmented micro-CT scan showing the increased salt concentration at the top due to evaporation.

To investigate salt-precipitation processes in porous media different aspects were investigated. To get a better understanding of the interface-driven processes first investigations were done on the pore-scale. A pore-network model was developed for one-phase flow which is able to simulate the change in pore space due to precipitation.

Further, pore-scale, two-phase flow experiments were conducted in collaboration with Dorthe Wildenschild (Oregon State University). The scans were segmented and the distribution of the different fluid phases as well as the solid salt were investigated in detail.

These investigations were supplemented by REV-scale investigations of density-driven instabilities of saturated porous media in collaboration with Hans van Dujin (TU Eindhoven) and Carina Bringedal (A05). Here the influence of different permeabilities on the onset time of the instabilities was investigated and compared to results of a linear stability analysis.

Salt concentration in the pore network. Increased at the top due to evaporation.
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