Pore-scale dynamics of salt precipitation in
drying porous media
Nima Shokri
School of Chemical Engineering and Analytical Science, University of
Manchester, Manchester, UK
Active projects in our group
1.
Dynamic of evaporation from porous media
2.
Salt transport and precipitation in porous media
3.
Fracture patterns and scaling in desiccating clay
4.
Foam in porous media for enhanced oil recovery
5.
Microfluidic analysis of two-phase flow in porous media
6.
Building energy efficiency
7.
Entry capillary pressure in angular pores
8.
Drying of nano-suspensions
9.
Bioclogging in porous media
Building energy efficiency
Crack dynamics
Foam for EOR
Entry capillary pressure
Salt precipitation
Drying of nanosuspension
200 µm
 We apply various experimental and
theoretical tools to extend physical
understanding of multiphase processes
occurring in porous media
Bioclogging
Polymer flooding
Why is it important?
 Evaporation from saline porous media and
salt precipitation is of major concern in
many processes such as soil salinity,
terrestrial ecosystem functioning,
durability of building materials,
preservation of historical monuments, and
CO2 sequestration
 Soil salinization is a serious issue with many
adverse environmental and hydrological
consequences. For example, in EU, ~1 million
hectares of land is affected by soil salinity.
In Spain 3% of the 3.5 million hectares of
irrigated land is affected by soil
salinization; another 15 % is under risk
Scherer (2004)
Prediction remains a challenge
 As evaporation proceeds, salt concentration
increases which modifies the surface tension,
vapor pressure, fluid density among other
factors modifying the evaporation process
 During evaporation when salt concentration
exceeds its solubility limit, salt precipitates
which modifies the pore structure of porous
media adding complexity to the problem
 How parameters such as type of salt,
pore size distribution, wettability,
heterogeneity or atmospheric
conditions, influence the drying of
saline porous media and the associated
precipitation patterns is not wellunderstood
Zhang et al. (2007)
Before evaporation
After evaporation
Shokri et al., TiPM, 2015
Objectives
•
To describe mechanisms controlling salt precipitation in drying porous
media
•
To delineate the effects of grain and pore size on salt precipitation
patterns and dynamics
•
To investigate of the effects of grain angularity on salt precipitation
Salt affected land, Dasht-e-Kavir, Iran
Salt transport in drying porous media
Ions are transported toward surface via
upward capillary flow while diffusion tends to
distribute salt homogenously in space
•
As a result of water evaporation, salt
concentration continuously increases in
evaporation sites at surface (assuming Pe>1)
and it precipitates when its concentration
exceeds the solubility limit
•
Preferential evaporation at the
surface of porous media strongly
influences the dynamics and
precipitation patterns
5 mm
•
Shokri et al., PRE, 2010
Shokri and Sahimi, PRE, 2012
•
Looking into solute transport and distribution
A cylindrical column packed with sand saturated with Cal2 solutions
(5% by weight) was used to study 3D dynamics of solute transport at
a spatial and temporal resolution of 0.012 mm and 30 min using
synchrotron X-rays
Advanced Photon Source - Argonne National Laboratory, IL, The USA
Solute concentration at pore-scale
•
Every time the sand column was set to be imaged, the column was
scanned two times with different X-ray energies which were just
below (33.0690 keV) and above (33.2690 keV) the absorption edge
(K-edge value) of the Iodide (33.1694 keV)
•
Taking the difference between the pixel gray values enabled us to
delineate the salt concentration at pore scale
Concentration is not uniform even within one single pore!
Shokri, Phys. Fluids, 2014
4D investigation of salt precipitation
•
Preferential salt deposition at the surface significantly influences
the precipitation dynamics
•
X-ray tomography was used to investigate the dynamics of
precipitation in drying porous media. Cylindrical columns packed with
sand saturated with NaCl solutions was used to investigate the
dynamics of salt precipitation with high temporal and spatial
resolution
Norouzirad, Shokri and Sahimi, Phys. Rev. E, 2013
4D investigation of salt precipitation
•
We were able to quantify 4D dynamics (3D space
+ time) of salt precipitation in porous media
during evaporation
•
The figure below illustrates a typical vertical
close-up of the surface at the end of the
experiment. The spectrum of orange to yellow
indicates the addition of precipitated salt in
each scan. Brighter colors indicate longer times
(later scan).
Norouzirad, Shokri and Sahimi, Phys. Rev. E, 2013
Norouzirad et al., TiPM, 2015
Effects of grain size on precipitation
•
We quantified the
effects of grain size on
salt precipitation
patterns using six
quartz sand differing in
particle size
distribution
•
Two round of
experiments were
conducted. In each
round, the sand columns
were taped together
and scanned at the
same time as a bundle
to ensure consistency in
the atmospheric
conditions.
Norouzi Rad, Shokri, Keshmiri, Withers, TiPM, 2015
Evaporation curves
•
The same methodology was used to quantify the
evaporative water losses as well as salt
precipitation dynamics
•
The constant slope of the computed mass loss
curves indicates that in all cases the evaporation
process was in stage-1
Norouzi Rad, Shokri, Keshmiri, Withers, TiPM, 2015
Pore scale precipitation patterns
•
•
Pore-scale heterogeneity induces preferential salt deposition
Initially the number of precipitation sites increases as evaporation
proceeds resulting in increasing precipitation rate at the early stage
Norouzi Rad et al., TiPM, 2015
Norouzirad, Shokri and Sahimi, Phys. Rev. E, 2013
Patchy vs crusty precipitation & thickness
•
Since there are less fine active
pores on the surface of coarse
sand, the salt crust will be thicker
but more discrete
•
The patterns of precipitation are
significantly different in different
medium under same cumulative
amount of precipitated salt!
Norouzi Rad, Shokri, Keshmiri, Withers, TiPM, 2015
Estimation of salt crust thickness
•
During stage-1, immediately after onset of evaporation, the surface
saturation decreases to a nearly constant value representing the
active pores contributing to the evaporation as well as precipitation
whose area can be estimated as 𝑆𝜀𝐴
•
Dividing the “active area” by the average area of a pore provides us
with an estimation of the number of the pores participating in the
precipitation.
•
From conservation of mass, one can obtain the following equation for
mass of precipitated salt per unit area 𝑀𝑠
𝑀𝑠 𝑑
𝜔~
𝑆𝜌𝑠 𝜀𝑙(𝑡)
Norouzi Rad, Shokri, Keshmiri, Withers, TiPM, 2015
•
Estimation of salt crust thickness
The width of the crust
on each grain may
increase from zero to
the diameter of the grain
𝐷. We propose the
following equation to
describe 𝑙(𝑡)
𝑙 𝑡 = D 1 − 𝑒 −𝑡/𝜏
where t indicates the time of
precipitation and 𝜏 is a factor
affecting the lateral spreading
of the precipitated salt
𝑀𝑠 𝑑
𝜔~
𝑆𝜌𝑠 𝜀𝑙(𝑡)
Norouzi Rad, Shokri, Keshmiri, Withers, TiPM, 2015
•
Effects of grain angularity
Evaporation experiments
were conducted using sand
and glass beads with similar
average particle size and
porosity, but different pore
size distribution
Norouzi Rad and Shokri, WRR, 2014
Effects of grain angularity
•
The total area covered by salt is larger
but thinner in the case of sand
compared to glass beads due to the
presence of more evaporation sites at
the surface
•
More precipitation in the case of glass
beads compared to sand under a same
cumulative mass loss!
Norouzi Rad and Shokri, WRR, 2014
•
A puzzling question!
Why during stage-1 evaporation, the drying rate remains nearly
constant while the surface is covered by the precipitated salt?
18 hours
168 hours
Norouzirad, Shokri and Sahimi, Phys. Rev. E, 2013
Summary and conclusions

Salt precipitation rate initially increases followed by a relatively constant
precipitation rate during stage-1 evaporation

The occurrence of crusty vs. patchy efflorescence at the surface was
described

The porous structure of precipitated salt is probably the reason for
maintaining evaporative fluxes despite of the presence of salt at the
surface

Larger particles resulted in more discrete but thicker crust formation
Acknowledgments

Thanks to Mansoureh Norouzi Rad for her contributions

This project is funded by ACS-Petroleum Research Fund
(PRF No. 52054-DNI6) and The Leverhulme Trust to
support this research (RPG-2014-331)
Thank you for your attention; Question?
References for more details
1. Jambhekar, V.A., R. Helmig, Natalie Schroder, N. Shokri (2015), Free-flow-porous-media coupling for evaporation-driven
transport and precipitation of salt, Trans. Porous. Med., 110(2), 251-280.
2. Norouzi Rad, M., N. Shokri, A. Keshmiri, P. Withers (2015), Effects of grain and pore size on salt precipitation during
evaporation from porous media: A pore-scale investigation, Trans. Porous. Med., 110(2), 281-2954.
3. Shokri, N., P. Zhou, A. Keshmiri (2015), Patterns of Desiccation Cracks in Saline Bentonite Layers, Trans. Porous. Med.,
110(2), 333-344.
4. DeCarlo, K.F., N. Shokri (2014), Effects of substrate on cracking patterns and dynamics in desiccating clay layers, Water
Resour. Res., 50, 3039-3051.
5. DeCarlo, K.F., N. Shokri (2014), Salinity effects on cracking morphology and dynamics in 3-D desiccating clays, Water
Resour. Res., 50, 3052-3072.
6. Norouzi Rad, M., N. Shokri (2014), Effects of grain angularity on NaCl precipitation in porous media during evaporation,
Water Resour. Res., 50, 9020-9030.
7. Shokri, N. (2014), Pore-scale dynamics of salt transport and distribution in drying porous media, Phys. Fluids, 26, 012106.
8. Norouzi Rad, M., N. Shokri, M. Sahimi (2013), Pore-Scale Dynamics of Salt Precipitation in Drying Porous Media, Phys. Rev.
E, 88, 032404.
9. Norouzi Rad, M., N. Shokri (2012), Nonlinear effects of salt concentrations on evaporation from porous media, Geophys.
Res. Lett., 39, L04403.
10. Shokri, N., M. Sahimi (2012), The structure of drying fronts in three-dimensional porous media, Phys. Rev. E 85, 066312.
11. Shokri, N., P. Lehmann, D. Or (2010), Liquid phase continuity and solute concentration dynamics during evaporation from
porous media- pore scale processes near vaporization surface, Phys. Rev. E, 81, 046308.
Drying curve
•
This talk is focused on salt transport and precipitation during
stage-1 evaporation during which liquid vaporization is limited by
atmospheric conditions and not by the vapor transport through the
unsaturated zone
Shokri and Or, Water Resour. Res., 2011
Stage 1
Stage 2