11th International GeoRaman Conference (2014)
5040.pdf
Experimental determination of CO2 diffusion coefficient in aqueous solutions under pressure via Raman
spectroscopy at room temperature: impact of salinity (NaCl) on dissolved CO 2 diffusivity.
C. Belgodere1,2* ,J. Dubessy1**, J. Sterpenich1, J. Pironon1, D. Vautrin3, M.C. Caumon1, P. Robert1, A. Randi1 and
J.P. Birat4, 1 Universite de Lorraine, CNRS, Georessources Laboratory, BP 70239, F-54506, Vandoeuvre-LesNancy, France, 2CREGU, BP 70023, 54501, BP 30320, F-57283, Vandoeuvre-Les-Nancy, France, 3 IRCCyN, Ecole
Centrale de Nantes, 1 rue de la Noë, 44321, Nantes, France, 4 formerly : ArcelorMittal, now : ESTEP, 172 avenue de
Cotenbergh, Bruxelles, Belgique. * :[email protected], ** : [email protected]
Introduction:
The diffusivity rate of dissolved CO2 in aqueous solutions is a key parameter to predict CO2 migration in
reservoirs and leakage through caprock in the context
of greenhouse gas geological storage in saline aquifers.
[1] [2]. Among the different experimental techniques,
in situ Raman Spectroscopy in horizontal Fused Silica
Capillary (FSC) is a powerful technique to determine
CO2 diffusion coefficients at different pressures and
temperatures in the absence of advection contribution
[3]. Most of the previous experimental investigations
were dedicated to the study of the influence of temperature on CO2 diffusivity, at atmospheric pressure and
in pure water. While influence of pressure remains
unclear, collection of available data [4] shows unambiguous relationship between temperature and CO 2
diffusivity even at temperatures as high as 100°C [3].
The impact of salinity is poorly documented([5];[6];[7]) but show a decrease of the diffusion
coefficient with increasing salinity. Aqueous solution
of saline aquifers may display a wide range of salinities from 0 to 7 molNaCl.kg-1 H2O [8]. Considering the
scattering of experimental data of CO2 diffusion coefficient versus brine salinity, this study was focused on
the salinity (NaCl) dependence of CO2 diffusion over
from 0 to 6 molNaCl.kg-1 H2O, using Raman spectra of
an aqueous solution loaded in a High-Pressure Optical
Cell (HPOC, ([3] [9],[10],) at controlled CO2 pressure.
Materials and Methods
Pressurization device and Fused Silica Capsules
Seven experiments, on the range of salinity from 0 to 6
molNaCl.kg-1 H2O, were performed at 21±1°C and 40
bar, in capillary with infiling length higher than 17
mm [11,12,13]. The capillary was fixed to the x-y
micrometric moving stage and horizontally placed
under the optical microscope of the spectrometer to
record Raman spectra in the liquid phase at different
distances to the liquid/vapor interface.
Raman spectra acquisition
Collection conditions of Raman spectra of the CO2
Fermi resonance dyad and the bending mode of H 2O in
the 1200-1900 cm-1 spectral range were the following:
Labram-HR spectrometer (® Horiba Jobin-Yvon),
exciting radiation (0 = 514.532 nm; W0 = 60 mW),
exposition time of 30 s, 4 accumulations. Raman spectra were acquired sequentially over 12 hours. The
Raman peak areas of the 2ν2(CO2) and of the ν2(H2O)
bands (ACO2/AH2O) are considered to be proportional
to dissolved CO2 concentration in the molality scale.
Numerical determination of diffusion coefficient
CO2 diffusion from vapor / liquid interface to the
aqueous phase was treated as a one-dimensional diffusion process and modelled using the Fick’s second law.
For convenience, (ACO2/AH2O) are normalized with
(ACO2/AH2O)° value at 0.03 mm from the interface
and at equilibrium to define the variable Rnorm and
results in second Fick’s law:
(1)
CO2 diffusion coefficients in the aqueous solution were
determined fitting experimental data with a numerical
solution of the second Fick’s law using the least square
method and a finite difference numerical method modified after Lu et al., 2006, 2013([10],[3]).
Results
Figure 1 shows the evolution of the Raman spectra
with time and distance from vapor CO2/ aqueous solution interface in the pure water case.
Figure 1: a) Raman spectra collected at different times
after silica capillary pressurization by gaseous CO2 (40
bar, 21°C, pure water) at 11.025 ± 0.001 mm from the
CO2/H2O interface at t = 0. b) Raman spectra collected
at different distances from the CO2/H2O interface at t =
375 min after silica capillary pressurization (pure water).
Figure 2 shows the evolution of Rnorm in time and
space for the 3 molNaCl.kg-1 H2O solution. The asymptotic shape versus time for a given distance is clearly
evidenced even for short diffusion distances.
Abstract for 11th GeoRaman International Conference, June 15-19, 2014, St. Louis, Missourri, USA
11th International GeoRaman Conference (2014)
5040.pdf
diffusive transport of CO2 through cover rock and its
consequence on CO2 storage integrity.
Figure 2: Evolution of the normalized ratio Rnorm versus
time at different distances from the CO2/H2O interface as
- (PCO2=40 bar and T =21±1°C) for the 3 molNaCl.kg-1
H2O solution.
Discussion
The diffusion coefficient of CO2 in pure water at
21±1°C and 40 bar calculated from the present experimental study is 1.71×10-9m2.s-1, a value in good
agreement with previous studies (D°CO2 (20°C, 1 bar) =
1.66 ×10-9m2.s-1 ). The diffusion coefficient linearly
decreased with increasing salinity between 0.0 and 6.0
molNaCl.kg-1 H2O (Figure 3) and is in good agreement
with the results of name-here[5] and name-here[6]
determined at 1 bar. The diffusion coefficient determined in these two studies are slightly higher than in
the present work, probably because of a working temperature of 25°C. Moreover, our results agree with the
Wilke-Chang ([14]) theoretical model calculated at
temperature of 21°C and a pressure of 40 bar.
Acknowledgements:
This work was carried out as part of Clement Belgodere PhD Thesis supported by ArcelorMittal, CREGU,
CNRS, Universite de Lorraine, Nancy, France, and
CGSµLab ANR-12-SEED-0001. This work has been
performed while the first and fifth author were PhD
students at Georessources laboratory/ CREGU/ CNRS/
Universite de Lorraine, Nancy, France and IRccyN,
Ecole Centrale de Nantes, Nantes, France, respectively.
References:
[1] Lindenberg, E. B. and Wessel-Berg, B. (1997)
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et al., (1979). Chem. Eng. Journ., 17, 77–80. [7] Sell,
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et al., (2006). Appl. Specrt., 60, 122–129. [11] Chou, I.
M. et al., (2008). Geochim. Cosmochim. Acta, 72, 5217
-5231. [12] Ong, A. et al., (2013). Geofluids Journ.,
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Figure 3: Comparison of the diffusion coefficient of dissolved CO2 determined in this study with previous literature data.
Conclusion
The experimental device in this study was adapted
from Lu et al., 2006, 2013([10],[3]).and successfully
applied to high NaCl concentrated aqueous solutions.
It confirms that microfluidic techniques coupled with
Raman spectrometry may be a route for studying physical-chemical parameters of fluids. Further analytical
calculations will demonstrate the impact of salinity on
Abstract for 11th GeoRaman International Conference, June 15-19, 2014, St. Louis, Missourri, USA