Abstract for 11 th GeoRaman International Conference, June 15-19, 2014, St. Louis, Missourri, USA Experimental determination of CO 2 diffusion coefficient in aqueous solutions under pressure via Raman spectroscopy at room temperature: impact of salinity (NaCl) on dissolved CO 2 diffusivity. C. Belgodere 1,2* ,J. Dubessy 1** , J. Sterpenich 1 , J. Pironon 1 , D. Vautrin 3 , M.C. Caumon 1 , P. Robert 1 , A. Randi 1 and J.P. Birat 4 , 1 Universite de Lorraine, CNRS, Georessources Laboratory, BP 70239, F-54506, Vandoeuvre-Les- Nancy, France, 2 CREGU, 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. * :clement.belgodere@gmail.com, ** : jean.dubessy@univ-lorraine.fr Introduction: The diffusivity rate of dissolved CO 2 in aqueous solu- tions is a key parameter to predict CO 2 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 CO 2 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 temper- ature on CO 2 diffusivity, at atmospheric pressure and in pure water. While influence of pressure remains unclear, collection of available data [4] shows unam- biguous relationship between temperature and CO 2 diffusivity even at temperatures as high as 100°C [3]. The impact of salinity is poorly document- ed([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 salini- ties from 0 to 7 mol NaCl .kg -1 H 2 O [8]. Considering the scattering of experimental data of CO 2 diffusion coef- ficient versus brine salinity, this study was focused on the salinity (NaCl) dependence of CO 2 diffusion over from 0 to 6 mol NaCl .kg -1 H 2 O, using Raman spectra of an aqueous solution loaded in a High-Pressure Optical Cell (HPOC, ([3] [9],[10],) at controlled CO 2 pressure. Materials and Methods Pressurization device and Fused Silica Capsules Seven experiments, on the range of salinity from 0 to 6 mol NaCl .kg -1 H 2 O, 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 CO 2 Fermi resonance dyad and the bending mode of H 2 O 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 spec- tra were acquired sequentially over 12 hours. The Raman peak areas of the 2ν 2 (CO 2 ) and of the ν 2 (H 2 O) bands (ACO 2 /AH 2 O) are considered to be proportional to dissolved CO 2 concentration in the molality scale. Numerical determination of diffusion coefficient CO 2 diffusion from vapor / liquid interface to the aqueous phase was treated as a one-dimensional diffu- sion process and modelled using the Fick’s second law. For convenience, (ACO 2 /AH 2 O) are normalized with (ACO 2 /AH 2 O)° value at 0.03 mm from the interface and at equilibrium to define the variable R norm and results in second Fick’s law:           (1) CO 2 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 modi- fied after Lu et al., 2006, 2013([10],[3]). Results Figure 1 shows the evolution of the Raman spectra with time and distance from vapor CO 2 / aqueous solu- tion interface in the pure water case. Figure 1: a) Raman spectra collected at different times after silica capillary pressurization by gaseous CO 2 (40 bar, 21°C, pure water) at 11.025 ± 0.001 mm from the CO 2 /H 2 O interface at t = 0. b) Raman spectra collected at different distances from the CO 2 /H 2 O interface at t = 375 min after silica capillary pressurization (pure water). Figure 2 shows the evolution of R norm in time and space for the 3 mol NaCl .kg -1 H 2 O solution. The asymp- totic shape versus time for a given distance is clearly evidenced even for short diffusion distances. 5040.pdf 11th International GeoRaman Conference (2014)