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Available online at www.sciencedirect.com ScienceDirect Energy Procedia 00 (2017) 000–000 www.elsevier.com/locate/procedia 13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland Experimental study of CO2-saturated water ‒ illite/kaolinite/montmorillonite system at 70-80 °C, 100-105 bar Eszter Sendulaa*, György Falusb, Mariann Pálesa, Barbara Péter Szabóc, Beatrix Udvardib, István Kovácsb, Péter Kónyab, Ágnes Freilerb, Anikó Besnyib, Csilla Királya, Edit Székelyc, Csaba Szabóa Lithosphere Fluid Research Lab, Eötvös University, Pázmány Péter sétány 1/C, Budapest, H-1117, Hungary b Geological and Geophysical Institute of Hungary, Stefánia str. 14, Budapest, H-1143, Hungary C Department of Chemical and Environmental Process Engineering, Budapest University of Technology and Economics, Budafoki út 8., Budapest, H-1111, Hungary a Abstract Low permeability caprock prevents migration of injected CO2 towards the surface and potable water reservoirs. As a result of geochemical reactions among caprock minerals and CO2-saturated pore water, physical properties such as porosity, permeability and tortuosity may change, which could affect the sealing capability of caprocks. Due to the common high clay mineral content of caprocks, the reactivity of these minerals should be well-studied at CO2-saturated reservoir conditions. The main aim of this study is to understand better the geochemical behavior of CO2–brine–clay mineral system by using laboratory experiments at reservoir conditions on reference clays. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of GHGT-13. Keywords: CO2-clay interaction, carbon sequestration, lab experiment 1. Introduction CO2 injection into the subsurface is considered to be an effective way to reduce emission of anthropogenic CO 2 into the atmosphere [e.g. 1-4], besides, CO2 is also used in the oil and gas industry as a working fluid for enhanced oil and gas recovery (CO2-EOR and CO2-EGR) [e.g. 5, 6] and ultimately proposed as a possible fracturing fluid for shale gas production [7]. Since the injected supercritical CO2 (scCO2) is less dense than the formation water, buoyancy forces push it to migrate upwards, therefore, caprocks are important physical barriers to hinder the vertical migration of CO2 towards the surface. For this reason, the sealing capacity of the caprock is crucial in the 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of GHGT-13. 2 Author name / Energy Procedia 00 (2017) 000–000 evaluation and planning of a potential CO2 storage reservoir or CO2-EOR project, to ensure that CO2 does not escape into overlying shallow rock units, especially drinking water aquifers [8]. The overlying caprock likely will be exposed to two phase (aqueous brine + variously hydrated scCO 2) or one phase fluid (aqueous brine containing dissolved CO2). Consequently, modeling and experimental studies should focus on the effects of mineral reactivity in both environments. Caprocks are typically fine-textured shales, mudstones, or carbonate rocks with low permeability for any fluid, therefore, flow rates across them can be very slow even at large pressure gradients. More importantly, the small pore spaces have very high capillary entry pressures, which cause the caprock to act as a membrane that allows water to pass but blocks scCO2 unless its pressure exceeds the capillary entry pressure [4]. In a properly-designed CO2 storage site, the supercritical CO2 should not overcome this barrier, therefore, CO 2 percolation into the originally water-wet caprock is relatively easier with molecular diffusion, or as a dissolved constituent in the brine [8]. The dissolution of CO2 into the porewater will decrease its pH, thereby enhancing the dissolution of the porous matrix. It has been described from the study of natural CO 2 occurrences that CO2-saturated brine could react with the caprock in geological timescale causing mineral dissolution and precipitation within the caprock [9, 10], even though CO2 leakage is not observed in a system currently. Considering that various types of clay minerals (e.g. illite, montmorillonite, kaolinite, chlorite and glauconite) are common constituents of caprocks, their reaction behavior with variously hydrated scCO2 and CO2-saturated porewater should be well studied. Over the last few years increasing number of experimental studies has been published regarding the reaction of clay minerals (mostly montmorillonite) and variously hydrated scCO 2. Several papers reported that scCO2 can be intercalated in the montmorillonite’s interlayer space [e.g. 11-16] resulting in expansion of the mineral structure, and therefore, promoting the self-sealing of caprock fractures. However, it has been also shown that depending on the hydration state of scCO2, it can cause shrinkage of the montmorillonite [12, 13], which can lead to the reduction of sealing integrity of the caprock. The volume change of montmorillonite modifies the pore structure of the caprock and affects its porosity and permeability. As mentioned above, variously hydrated scCO 2 should not reach the capillary entry pressure of the caprock, therefore, these experimental studies are mostly relevant only for minor volume of the caprock (i.e. at the interface of the reservoir and the caprock, or along the walls of fractures and faults within the caprock, where scCO2 can move by advection). The major part of the caprock is likely filled with CO2saturated or undersaturated porewater [8]. Some of the papers provide examples for modeling studies for caprockCO2 interactions [17-22] and some experimental works exist on the behavior of bulk caprock samples from different locations with CO2-saturated porewater [e.g. 21, 23-27]. However, at least to our best knowledge, only few articles have been published investigating the reaction between clay minerals and CO2-saturated water [e.g. 23, 28]. Hence, the main goal of this study is filling an existing gap in experimental studies in order to understand better the geochemical behavior of CO2-saturated water–clay mineral system. For this reason, approximately 100 hours long laboratory experiments were carried out under reservoir conditions relevant to geological CO 2 storage (70-80 °C, 100-105 bar) using mixtures of illite (IMt-1), kaolinite (KGa-1b) and Na-montmorillonite (SWy-2) reference clays. The resulting solid phases were examined by Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR) and X-ray powder diffraction (XRD), whereas the fluid compositions were measured by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). For comparison, ‘as-shipped’ samples and control samples exposed to chemically inert argon (Ar) at the same pressure-temperature conditions were also studied the same way as the CO2-exposed samples. 2. Materials and methods 2.1. Clay samples In this study we used illite (IMt-1, Silver Hill, Montana, USA), kaolinite (KGa-1b, Coss-Hodges pit, Washington County, Georgia, USA) and Na-montmorillonite (SWy-2, Crook County, Wyoming, USA) reference clays from the Source Clays Repository of The Clay Minerals Society. These reference clays have been previously well characterized by Van Olphen and Fripiat [29], Chipera and Bish [30], Madejová and Komadel [31], Mermut and Cano [32], Kogel and Lewis [33] and Mermut and Lagaly [34]. Since each of these clay minerals have different crystal structure and chemistry, they are expected to react differently with CO2-saturated porewater. The mineral Author name / Energy Procedia 00 (2017) 000–000 3 composition of the reference clays was reported by Chipera and Bish [30] for KGa-1b [~96 % kaolinite and trace dickite, 3% anatase, 1% crandallite + quartz(?)] and for SWy-2 [~75% montmorillonite, 8% quartz, 16% feldspar, 1% gypsum + mica and/or illite + kaolinite(?) and/or chlorite(?)] and by Köster [35] for IMt-1 [mostly illite, trace of quartz/rutile/anatase, 0.98% calcite]. In the experiments we used 1:1 weight ratio mixtures of the above mentioned reference clays (IMt-1 + KGa-1b, SWy-2 + IMt-1, SWy-2 + KGa-1b). 2.2. Experimental setup The experiments were carried out under reservoir conditions typical for the Pannonian Basin (70-80 °C, 100-105 bar) at the Department of Chemical and Environmental Process Engineering, Budapest University of Technology and Economics, Hungary. The mixture of ultra-pure distilled-deionized water and powdered-mixed (1:1 mass ratio mixtures) reference clays (illite-montmorillonite, kaolinite-montmorillonite, illite-kaolinite) was treated by CO2. Control experiments were run simultaneously at the same temperature and pressure using Ar, which is considered to be a non-reactive gas. Two stainless steel autoclaves with 37 ml inner volume were used in parallel, i.e. one for CO 2 experiments and one for reference CO2-free (Ar-treated) experiments (Fig. 1). Table 1 summarizes the water-rock ratios, the experimental conditions (p, T, duration) and the used gases (CO2 or Ar) in the experiments. The duration of each experiment was approximately 100 hours, therefore, to speed up the chemical reactions in this limited time interval, we artificially increased the surface area by powdering the clay samples. High water-rock ratio was applied (14-19) to facilitate the brine sampling and analyses. The clay particles were maintained in suspension by continuous stirring which provided a higher reactive surface area to the clay minerals, since it prevented the deposition of the particles. Moreover, stirring is likely to have reduced the diffusion control of the fastest reaction in the solutions surrounding the mineral grains. Minor changes in temperature and pressure were observed during the experiments, the latter likely due to minor loss of CO 2 and Ar from batch reactor and/or the uptake of CO 2 in reactions. The reacted solid phases were examined by XRD and ATR-FTIR and the fluid samples collected during the experiments were measured by ICP-OES. Ar-treated control samples and ‘as shipped’ samples were examined the same way as the CO2-exposed samples. 2.3. X-ray powder diffraction (XRD) Semi-quantitative XRD analyses were conducted at the Geological and Geophysical Institute of Hungary with a Phillips® PW 1730 diffractometer (Cu cathode, 40 kV accelerating voltage and 30 mA tube-current, graphite monochromator, goniometer speed 2°2θ/minute) to determine the mineral abundances in the ‘as shipped’, the CO2treated and the Ar-treated samples. All samples were analyzed in random orientation. The semi-quantitative assessment of the relative concentrations of phases was performed by relative intensity ratios and half width of specific reflections of minerals by using XDB Powder Diffraction Phase Analytical software 2.7. 4 Author name / Energy Procedia 00 (2017) 000–000 Fig. 1. Experimental setup used in the laboratory experiments. The numbers are: 1) ISCO pump, 2) control valve, 3) reactor inner space, 4) manometer, 5) thermometer, 6a) thermostating fluid inlet, 6b) thermostating fluid output, 7) magnetic stirrer, 8) stirring core, 9) filter, 10) control valve, 11) sampling vial. Table 1. Summary of the experimental conditions (p, T, duration), weight of clay mixtures and pure water, water-rock ratios and the used gas (CO2 or Ar) in each experiment. Mixed reference clays Sample name Gas used Weight of clay mixtures (g) Weight of pure water (g) Water-rock ratio Temperature (°C) Pressure (bar) SWy-2 + KGa-1b SK8K CO2 0.8165 14.9964 18.4 79.1 ± 1.2 102.5 ± 1.2 SWy-2 + KGa-1b SK12K Ar 0.8357 14.1752 17.0 81.4 ± 0.7 105.0 ± 1.6 IMt-1 + SWy-2 IS10K CO2 0.8595 13.4515 15.7 81.4 ± 0.9 104.7 ± 1.0 IMt-1 + SWy-2 IS11K Ar 0.9466 13.7936 14.6 70.4 ± 7.1 103.2 ± 2.8 IMt-1 + KGa-1b IK9K CO2 0.8023 15.1472 18.9 72.4 ± 8.8 101.9 ± 7.3 IMt-1 + KGa-1b IK13K Ar 0.8140 15.1261 18.6 82.4 ± 1.6 100.2 ± 8.9 2.4. Attenuated Total Reflectance Fourier Infrared Spectroscopy (ATR-FTIR) The powdered and mixed clay samples were measured before and after CO2/Ar treatment by ATR-FTIR at the Geological and Geophysical Institute of Hungary. This method is highly sensitive to the OH --bearing phases, therefore beneficial for the study of clay minerals. The instrument is a Bruker Vertex 70 spectrometer with a single reflection ATR cell (Bruker Platinum diamond ATR) and an MCT detector (mercury-cadmium-telluride). This instrument was utilized to acquire ATR spectra in the mid–infrared spectral range (400–4000 cm–1) with 64 scans and a resolution of 4 cm-1. The samples were dried at least for 30 minutes at 80°C in an oven right before the measurements in order to remove the majority of adsorbed water, which significantly facilitates the interpretation of the spectra. Tóth et al. [36] showed that the relatively low temperature of the heat treatment does not cause any first order alterations in the clay mineral structures. Furthermore, since the experiments themselves were carried out at 70-80 °C, we do not expect significant alterations due to our sample preparation process. The mixed and powdered clay sample was placed on the diamond ATR crystal and pressed with a compression clamp and was analyzed. The analysis was repeated 3 to 5 times and the measured IR spectra were averaged after atmospheric compensation and ATR correction. In some cases, quick re-adsorption of water from the atmosphere was experienced on montmorillonite-bearing mixtures. In these cases, we used the average spectrum of the first two spectra only where the effect of re-adsorption was not significant. ATR correction was necessary since the light penetration depth into the sample is a function of the IR radiation’s wavelength, refractive indices of the sample and the ATR crystal [37]. The ATR correction was carried out taking into account 1.56 mean reflection index of the samples, single reflection in the ATR crystal and 45° incidence angle. The baseline was corrected by the automatic algorithm of OPUS 7.2 software (concave rubber band correction, 1 or 2 iterations, 64 numbers of baseline points). For the spectral region of 1730-1330 cm-1 baseline correction was carried out separately and then the spectra were smoothed (15 smoothing points). As all the spectra were recorded under identical experimental conditions, the intensity and wavenumber of the characteristic infrared bands depend only on the particle size, degree of crystallinity, crystallographic anisotropy and the chemical composition of the samples. 2.5. Inductively coupled plasma optical emission spectroscopy (ICP-OES) Fluid samples collected at several time steps during and right after both CO2 and CO2-free control (Ar) experiments were analyzed by ICP-OES, HORIBA Jobin Yvon® ULTIMA 2C at the Geological and Geophysical Institute of Hungary to determine the Na +, K+, Ca2+, Mg2+, Mn2+, Al3+, H2SiO3(aq) and Fe(total) concentrations. It should be noted here that the results of ICP-OES should be treated with caution, since only small volume of fluid samples could be taken during the experiments (0.1-0.8 ml at each sampling period) despite of the high water/solid ratio (14-19, Table 1). These amounts were not enough for the measurements that required 5 ml fluid sample, Author name / Energy Procedia 00 (2017) 000–000 5 therefore the samples were diluted by the factor of 6-33 (Appendix A), resulting in an increased analytical error. Noted however, that any appearance of the above mentioned elements in the fluid samples suggest chemical reaction in the clay-water-CO2/Ar system, since distilled and deionized water was used as initial fluid. In some cases, where it was possible, duplicate samples were taken and analyzed to ensure the quality of the data. 3. Results 3.1. XRD Mineral composition of the mixtures of the ‘as shipped’ reference clays and the CO2/Ar-treated samples were characterized by bulk XRD analysis (Table 2) to detect the changes in the mineralogy as a result of the experiments. All original, non-treated clay mixtures show the presence of quartz, K-feldspar and some amorphous phases. Kaolinite and minor amount of anatase detected in the mixtures containing KGa-1b reference clay. The SWy-2 bearing mixtures contain montmorillonite, 2-3% calcite and minor amount of mica and/or illite. Additionally, illite detected in the IMt-1 bearing mixtures (Table 2). The resulting solid phases from the experiments were measured after drying the samples at ambient conditions. In the CO2-exposed illite-kaolinite mixture (IK9K), slight decrease of the illite peak was detected, whereas, no significant changes were observed after Ar-treatment (IK13K) (Table 2, Fig. 2). In the montmorillonite-bearing samples the lack of the initially present calcite was observed as a result of CO 2-treatment, while in the Ar-treated montmorillonite-illite mixture (IS11K) trace amount of calcite was still recognized even after the experiment (Table 2, Fig. 2). Furthermore, shift of the initial d001 spacing of the montmorillonite peak were experienced by about 2 Å (from ~13 Å to ~15 Å) in the case of both CO 2 and Ar-treated samples, together with the increase of the peak intensity (Table 2, Fig. 2). 3.2. ATR-FTIR Infrared spectroscopy has been used to characterize clay minerals both in the ‘as shipped’ and CO 2/Ar-treated samples, since the specific absorption bands of structural OH - and Si–O groups play an important role in the differentiation of clay minerals from each other [38]. All bands presented here were characterized according to data from Madejová and Komadel [31], Madejová [38], Tóth et al. [36], Krukowski et al. [15] and Kovács et al. [39]. Kaolinite was identified by the four absorption bands in the OH - stretching region (strong bands around 3620 and 3695 cm-1 and weak absorptions around 3669 and 3653 cm-1) [38]. Illite and montmorillonite show a single band at 3620 cm-1 corresponding to the hydroxyl group at the aluminum-enriched octahedral positions. Furthermore, related to montmorillonites, a broad band around 3400 cm-1 is usually observed corresponding to the H-O-H vibration of mainly interlayer water with a decreasing shoulder near 3240 cm-1 representing an overtone of bending mode of molecular water. A band at ~1630 cm-1 also appears related to the OH- bending mode of molecular water. d001 reflection of montmorillonite Amorphous Anatase Calcite Plagioclase K-feldspar Quartz Kaolinite Mica Gas used Illite Sample name Montmorillonite Table 2. Mineralogical composition of the samples and position of the d001 reflection of montmorillonite before and after experiments by semiquantitative XRD analyses. Å % SWy-2 + KGa-1b - 35 - 3 50 6 1 - 2 1 2 13.049 SK8K CO2 39 - 2 49 2 1 1 - 1 5 14.546 SK12K Ar 32 - 2 57 4 2 1 - 1 1 15.137 IMt-1 + SWy-2 - 33 44 - - 11 5 - 3 - 4 12.807 6 Author name / Energy Procedia 00 (2017) 000–000 IS10K CO2 65 26 - - 4 2 - - - 3 14.998 IS11K Ar 41 46 - - 6 3 - 1 - 3 14.437 IMt-1 + KGa-1b - - 30 - 62 3 2 - - 1 2 - IK9K CO2 - 21 - 56 11 2 - - 1 9 - IK13K Ar - 27 - 66 3 1 - - 1 2 - Fig. 2. The figure shows XRD patterns obtained from non-treated (blue), CO2-treated (green) and Ar-treated (purple) mixtures of montmorillonite-illite, illite-kaolinite and montmorillonite-kaolinite after drying the samples at ambient conditions. The position of d001 reflection of montmorillonite (m, shaded) is increased by ~2Å as a result of the experiments. Abbreviations: m-montmorillonite, i-illite, kkaolinite, q-quartz, f-feldspar, c-calcite. The 1300-400 cm-1 range is characteristic for the stretching and bending vibrations of Si-O and Al-O and bending of OH-. The strong band near 1000 cm-1 is due to Si–O–Si stretching vibrations, while the band at 1116 cm-1 refers to the Si-O stretching (longitudinal mode). The Al-Al-OH bending bands of kaolinite appear at 914 and 936 cm-1. The bands at 795 cm-1 and 778 cm-1 are corresponding to quartz and the band at 751 cm-1 is related to the perpendicular stretching of Si-O. The ATR-FTIR spectrum of the CO2-treated illite-kaolinite mixture (IK9K) shows relative decrease of band intensities compared to the non-treated clay mixtures at 3695 cm-1 and around 1430 cm-1 corresponding to OHstretching in kaolinite structure and vibration of CO32- in carbonates, respectively (Fig. 3). The intensity of bands increased at 1630 cm-1 related to OH- bending and at 1001 cm-1 and 1026 cm-1 in the Si–O–Si stretching vibration region. In the Ar-treated sample (IK13K) no remarkable changes were observed in the 1700-1300 cm-1 region where the bands of carbonates and molecular water appear. However, the intensity of bands decreased both in the OH- stretching region related to kaolinite and at 1001 cm-1 and 1026 cm-1 assigned to Si–O–Si stretching. No significant shift was observed in the peak positions, except in the band around 1630 cm-1 related to the bending mode of molecular water. The position of this band shifted from the original 1643 cm-1 to 1639 cm-1 in the case of the Ar-treated sample (IK13K) and to 1635 cm-1 in the CO2-treated sample (IK9K). In comparison, in the CO2-treated montmorillonite-kaolinite mixture (SK8K) relative increase of the bands around 3400 cm-1, 1630 cm-1, 1430 cm-1 and decrease at 1001 cm-1, 1026 cm-1 and between 3620 cm-1 and 3659 cm-1 were observed (Fig. 3). Moreover, the absorbance at 3620 cm-1 associated with both kaolinite and montmorillonite Author name / Energy Procedia 00 (2017) 000–000 7 relatively increased compared to the band intensity at 3659 cm-1 related to kaolinite only. Contrary, in the Artreated sample (SK12K) the band intensities in the OH- stretching region did not change significantly, while in the Si–O–Si region increased. No significant changes were observed in the 1700-1300 cm-1 region and around 3400 cm1 . In the CO2-treated montmorillonite-illite mixture (IS10K), the relative intensity of the bands increased in both the OH- stretching and Si–O–Si stretching regions (Fig. 3). The only band that decreased around 1430 cm-1 is related to the vibration of CO32-. In the Ar-treated montmorillonite-illite mixture (IS11K) the band related to structural OHvibration at 3620 cm-1 decreased. Bands associated with molecular water around 3400 cm-1 and at 1630 cm-1, and the peak intensities in the Si–O–Si stretching vibrations band around 990-996 cm-1 did not change significantly. The CO32- peak at 1430 cm-1 slightly increased. 3.3. Fluid composition Fluid samples were taken at regular time intervals (1, 20-27, 43-48 and 94-98 hours after the start and right after the end of the experiments) to investigate fluid chemistry development as a function of time of exposure to CO 2saturated water. The results of fluid sample analysis taken during and after CO 2-free (Ar) and CO2 experiments are summarized on Fig. 4 and in Appendix A. It must be noted that in the case of the last sampling (after 95, 97, 98 hours), mineral precipitation due to the reactor depressurization and cooling could potentially modify the solution compositions compared to original experimental conditions. Since the starting solution was ultra-pure water (distilled and deionized), presence of any measured cations (Na +, K+, Ca2+, Mg2+, Mn2+, Al3+, H2SiO3(aq) and Fe(total)) in the resulting solution is due to geochemical reactions in the fluid-clay mineral-CO2/Ar system. Note that the concentration of Al3+ and Fe(total) were below the detection limit and K+ concentration was measured with relatively high uncertainty (±20%) in most cases (Appendix A). Argon gas is assumed to be inert, therefore, changes in the solution composition in the case of Ar-treated samples is expected to be driven by the pressure and temperature of the system and initial disequilibrium between the solid sample and pure water. Compared to the reference data of the chemically inert Ar, the differences in the CO2-exposed cases assumed to be the effect of CO2. 8 Author name / Energy Procedia 00 (2017) 000–000 Fig. 3. Average ATR-FTIR spectra of the mixed clay samples are shown before (blue) and after treatment with CO2 (green) and Ar (purple). Standard deviance of the spectra indicated with dashed lines. In the 3800-3000 cm-1 region, the labelled bands correspond to: O-H stretching of inner surface hydroxyl group in kaolinite (3690 cm-1), O-H stretching of inner hydroxyl group in kaolinite (3621 cm-1), O-H stretching of hydroxyl group at the aluminum-enriched octahedral positions in mixtures containing montmorillonite and illite (3623 cm-1), H-O-H asymmetric and symmetric stretching associated with interlayer water related mostly to montmorillonite (around 3400 cm-1), H-O-H 2ν2 bending overtone mode (3240 cm-1). In the 1730-1330 cm-1 region the bands are related to: H-O-H bending mode of molecular water (1640-1630 cm-1) and vibration of CO32- (1430 cm-1). In the 1300-500 cm-1 region the bands correspond to: longitudinal Si-O stretching (1116 cm-1), in-plane Si-O stretching (1026 cm-1, 1001 cm-1 and 996-990 cm-1), O-H stretching of inner surface hydroxyl group in kaolinite (940 cm-1), Al-Al-OH deformation (914-908 cm-1), quartz (795 cm-1 and 778 cm-1) and perpendicular stretching of Si-O (751 cm-1) [15, 31, 36, 38, 39]. Fig. 4. ICP-OES results of the fluid samples were collected at several time steps during the experiments with CO2 and Ar. Squares show the results of montmorillonite-kaolinite mixtures, circles represents the illite-kaolinite mixtures and hexagons indicates the mixture of montmorillonite-illite. Error bars indicate uncertainties of the measurements. The last sampling can potentially represents modified solution composition due to mineral precipitation because of the reactor depressurization and cooling. Cations (Ca2+, Mg2+, Mn2+, K+, Na+) and H2SiO3(aq) previously absent in the starting solution, appeared in all CO2 experiments already 1 hour after the start of the experiment indicating fast dissolution of reacting solids (Fig. 4). Further increase of the cation concentrations during the CO2 experiments was not clear. In the CO2-free (Ar) experiments Ca2+ and Mn2+ concentration of the resulting fluid samples was around the detection limit of ICP-OES. Furthermore, the concentration of Mn2+, K+, Na+ and H2SiO3(aq) were significantly lower than in the experiments with CO2. The only exception is the illite-kaolinite mixture, where the Na+ concentration was higher in the Ar-treated sample (IK13K). Slight increase in the concentration of Mg2+, K+, Na+ Author name / Energy Procedia 00 (2017) 000–000 9 and H2SiO3(aq) was detected during the Ar-treated control experiments (Fig. 4), which suggests a slower reaction rate at the same pressure-temperature conditions than in the CO2-exposed case. 4. Discussion 4.1. The relevance of CO2-staurated water‒clay system in CO2 geological storage Following the early study of Fripiat et al. [40] on the interlamellar adsorption of CO2 by smectites at low temperature, in the last few years several experimental works have been published on the behavior of montmorillonites with variably hydrated (from dry to H2O-saturated) scCO2 fluids under the conditions relevant to CO2 geological storage [e.g. 12-16, 41-44]. The focus of these studies was on the intercalation of CO 2 into the interlayer region of montmorillonite and volume change due to swelling or contraction of the clay mineral structure as a function of the initial hydration state of montmorillonite and water saturation of scCO 2. Corresponding to the intercalation of 0, 1, 2 or 3 planes of H2O molecules in the interlayer region, the hydration state of montmorillonite typically referred to as 0W (dehydrated), 1W (monohydrated), 2W (bihydrated) or 3W (trihydrated), however, these hydration states usually coexist due to structural heterogeneities [45]. The CO2 intercalation into montmorillonite under geological storage conditions was suggested based on the expansion of CO2 exposed montmorillonite measured by XRD [11-13]. However, the first direct evidence of CO2 intercalation was provided by Loring et al. [41] using Ca-STx-1 montmorillonite with ~1W initial hydration state and the combination of in-situ XRD, MAS-NMR and ATR-IR techniques. For Na-montmorillonite, Loring et al. [14] showed that CO2 intercalation is maximal from ~0W to 1W initial hydration states, and decreases with further hydration of the montmorillonite. Under geological storage conditions (up to a few km depth) montmorillonites are believed to be stable mostly between 0W and 1W hydration state, while at shallower reservoir environments 3W hydration state could be also in equilibrium [46]. It was shown that uncompacted Na-montmorillonite can swell up by 9% when exposed to high purity CO2 [11], however Jong et al. [44] measured only ≤3% swelling strain on compacted samples at similar experimental conditions. Schaef et al. [16] extended the experiments to Ca- and Mgsaturated montmorillonites, giving an upper limit on the capacity of clays to sequester CO 2. Ilton et al. [13] and Schaef et al. [12] found evidence for partial dehydration of Na + and Ca2+ exchanged montmorillonite with ≥2W initial hydration states exposed to wet scCO2, however Ilton et al. [13] showed that above ~65% H2O saturation in scCO2 the sub-3W montmorillonite swelled up to 3W hydration state. Both hydration and dehydration processes can be rapid and take place in few seconds-few hours [13, 15, 44]. Only few papers studied illite and kaolinite behavior in scCO2 environment. Kaolinite is used often in several experiments as a control sample, since it is typically regarded as non-expandable clay mineral [e.g. 14] which is also inert to chemical degradation by carbonic acid [e.g. 43]. All the above mentioned articles studied the clay-scCO2 system with variable H2O saturation, however, to our best knowledge, only very few experimental studies focus on the clay mineral‒CO2-saturated water system [e.g. 28, 43], which could be relevant at longer time scales and further away from the injection well. Depending on the CO 2 injection rate, the reservoir permeability and thickness, the distance from the injection point of CO2 to the caprock and the plume velocity, the time required to the CO 2 plume to reach the base of the caprock can vary. Since the water uptake of scCO2 can be very fast, scCO2 can reach saturation in H2O if enough water is accessible in the pore space. If the scCO2 reaches 100% water saturation at the given temperature and pressure conditions, montmorillonite could only swell based on the work of Ilton et al. [13], however, in case of lower H2O saturation of scCO2 shrinkage of montmorillonites can occur at the bottom of the caprock. Volume change of the montmorillonites in the caprock can cause porosity and permeability change and therefore can affect the capillary entry pressure of CO2 into the caprock during the injection period. Since in a properly-designed injection site, injected CO2 pressure should not reach the capillary entry pressure of the caprock to avoid leakage of the storage system, CO2-saturated water is more likely to enter into the caprock through diffusion [8]. Therefore, reaction between clay minerals and CO2-saturated porewater is more likely in the caprock, whereas variously hydrated scCO2 possibly can react with the caprock only on the reservoir-caprock interface, or in the fractures where the scCO 2 can move by advection. 10 Author name / Energy Procedia 00 (2017) 000–000 4.2. Discussion of experimental results 4.2.1. Illite-kaolinite mixture In the case of illite-kaolinite mixture, XRD measurements suggest the dissolution of illite in the CO 2 experiment relative to the other phases (IK9K), and no significant changes in the Ar-treated sample (IK13K). The higher extent of the dissolution of illite-kaolinite mixture in the CO2 exposed experiment confirmed by the ICP-OES results. The concentration of dissolved Ca 2+, Mg2+, Mn2+, K+, Na+ and H2SiO3(aq) increased in the fluid samples compared to the initial pure water. Minor dissolution in Ar-treated samples was also observed, however the concentration of K + and H2SiO3(aq) were 2-3 times lower than in the CO2-treated case, and Ca2+, Mg2+ and Mn2+ concentration was under the limit of detection. The increased Ca2+, Mg2+, Mn2+ content of the CO2-treated fluid sample is indicating the dissolution of carbonates initially present in the IMt-1 reference clay [35]. Carbonate dissolution is supported by ATR-FTIR spectrum, where decrease of the peak intensity around 1430 cm-1 was observed associated to CO32- in carbonates (IK9K). The higher K+ and H2SiO3(aq) concentration of the fluid samples in the CO2-treated case suggests that presence of CO2 enhanced the decomposition of clay minerals in the mixture. This result is in agreement with the experiment of Galán and Aparicio [43], who reported minor decomposition of illite indicated by the increased concentration of soluble elements (Si, Al, Na and K) after CO2 treatment with excess water. Dissolution of K-feldspar could also contribute to the increase of K + and H2SiO3(aq) concentrations of the fluid samples, however, only small amount was initially present in the samples. Interestingly, the Na+ concentration of the fluid samples in the Ar-treated sample was four times higher than in the CO 2 exposed case, which might be the result of artificial contamination of the sample. 4.2.2. Montmorillonite-bearing mixtures In the case of montmorillonite-bearing samples, the shift of the initial d001 basal reflection by ~2 Å (from ~13 Å to ~15 Å) on the XRD diffractograms suggests the expansion of the crystal structure. The expansion could be the result of cation-exchange (e.g. replacement of monovalent Na+ by divalent Ca2+) and/or CO2 incorporation into the interlayer or simply the changes in the hydration state of the montmorillonite (from 1W to 2W hydration state). In order to understand the reason of d001 shift, we re-measured the illite-montmorillonite samples (IS10K and IS11K) after the same preparation procedure that was applied before the ATR-FTIR measurements (drying it for 30 minutes at 80°C in an oven). In this case, the previously experienced shift of the d001 montmorillonite peak was no longer detected in the samples. At the same time, the increased intensity of the montmorillonite peak relative to the illite peak remained recognizable in the CO2 exposed samples (IS10K), while disappeared in the Ar-treated samples (IS11K) (Fig. 5). The result clearly shows that the relative increase in the amount of montmorillonite was the result of reaction between the clay sample and CO2-saturated water. ATR-FTIR spectra can be used to confirm if exchange of the interlayer cation occurred or not during the experiments. According to Madejová [38] the position of the broad band of H-O-H stretching vibration on ATRFTIR spectra near 3400 cm-1 decreases in order K-Na-Ca-Mg interlayer cation, because of the increasing polarization power of the interlayer cation due to the charge/radius relation. Since no shift was observed in the position of this peak, cation exchange during the experiments is unlikely. It is unclear, whether CO2 incorporation into the interlayer occurred or not. Michels et al. [47] showed on a dehydrated LiF-hectorite sample that intercalated dry CO2 can desorb from the interlayer space upon heating and LiF-hectorite is able to retain the CO2 up to 35°C. However, this threshold temperature is highly dependent on the type of interlayer cation and to our best knowledge, there is no similar experiment on Na-or Ca-montmorillonite, therefore the threshold temperature for CO 2 desorption is not known for these clay minerals. Nevertheless, it is unlikely that significant CO2 intercalation into the interlayer could occur with the high water concentration used in our experiments (CO2-saturated water), since several previous experimental and modeling studies showed even in the CO2-rich environments (variously hydrated scCO2) that CO2 intercalation decreases with increasing hydration of the montmorillonite [e.g. 14]. Therefore, the most likely explanation is that the detected d001 shift on the XRD diffractograms is a result of the increased hydration state of montmorillonite from 1W to 2W because of the experiments with high water content. The measured d001 values for montmorillonite are in agreement with the values reported as a function of integral hydration states of 0W (9.7– 10.2 Å), 1W (11.6–12.9 Å), 2W (14.9–15.7 Å) and 3W (18.0–19.0 Å) at ambient conditions [45]. Author name / Energy Procedia 00 (2017) 000–000 11 In agreement with the XRD results, the relative increase of the bands around 3400 cm -1 and 1640-1630 cm-1 associated with interlayer water on ATR-FTIR spectra could indicate the increase of the relative amount of montmorillonite in the samples, assuming that the hydration state of the montmorillonite was the same in all samples after the heat-treatment in the oven. As both the XRD and FTIR spectra show proportional amount of the minerals in the sample, this relative increase could be slightly influenced by the dissolution of other minerals in the mixture (e.g. calcite or K-feldspar), however, this effect likely to be minor given the small initial amount of these minerals. In addition, absorption strength on ATR-FTIR spectra is sensitive to the physical size of the particles [48-50], therefore, increased absorbance of the CO2-treated samples may be the result of aggregation of montmorillonite particles during the experiments. Fig. 5. The figure shows XRD patterns obtained from CO2-treated and Ar-treated illite-montmorillonite samples (IS10K and IS11K) after drying the samples at ambient conditions and at 80°C in oven for at least 30 minutes. Diffractogram of the ‘as shipped’ sample (blue) provided for comparison. The position of d001 montmorillonite peak (m, shaded) is increased by ~2Å in the samples dried at ambient conditions after the experiments compared to the ‘as shipped’ sample. However, no significant change was observed in the peak position after drying the same samples in oven. Relative increase in the intensity of montmorillonite peak compared to the illite peak disappeared in the Ar-treated sample (IS11K) after oven drying, but remained in the CO2-treated sample (IS10K) suggesting that relative amount of montmorillonite increased as a result of CO2 treatment. Abbreviations: m-montmorillonite, i-illite, q-quartz, c-calcite. Based on the results of ICP-OES (increased Ca2+, Mg2+ and Mn2+ concentration of the fluid samples) and XRD measurements, dissolution of minor amount of calcite initially present in the samples is likely in the CO 2 exposed cases, while there was no or only minor dissolution observed in Ar-treated samples. Since the cation concentration of the CO2 exposed fluid samples was already increased at the very first fluid sampling after one hour and further clearly increasing trend was not detected, rapid chemical reactions could be assumed. This is in agreement with previous studies [e.g. 15] reporting fast equilibration of the system. Control experiments with Ar can support this argument, since increasing concentration of cations could be detected until the end of the experiments, indicating that the fast reaction kinetics was not caused by pressure and temperature but most likely related to CO 2. 12 Author name / Energy Procedia 00 (2017) 000–000 5. Conclusion In this study, we conducted batch experiments at reservoir conditions suitable for CO2 sequestration to understand the reaction of montmorillonite, kaolinite and illite with CO 2-saturated water. Control samples exposed to chemically inert Ar were also studied as comparison. The experimental results (XRD, ICP-OES) indicate rapid and total dissolution of originally present carbonates in the CO2-treated samples. In the Ar-exposed control samples, carbonate dissolution was negligible at the same pressure-temperature conditions. The clay mixtures, initially containing montmorillonite, were more reactive than the sample of illite-kaolinite mixture. In these reactive samples, both XRD and ATR-FTIR measurements suggest slight increase in the relative amount of montmorillonite and/or aggregation as a result of CO2-treatment, while no remarkable changes were observed in the Ar-treated samples compared to the ‘as shipped’ ones. Expansion of the crystal structure of montmorillonite was observed on the XRD diffractograms after both CO 2 and Ar experiments. The reason is most likely the high water concentration we used in the experiments, which caused change in hydration state of the montmorillonite from 1W to 2W. The effect of CO 2 intercalation or cation exchange is very likely negligible. In the fluid samples primary cation (Ca2+, Mg2+, Mn2+, K+, Na+) and H2SiO3(aq) concentration increased in the CO2-exposed samples, however, no significant changes in Ca2+ and Mg2+ concentration were observed in Ar-treated samples. Based on the fluid chemistry, fast dissolution of the CO2-treated samples and slower dissolution of Artreated samples were detected. Acknowledgements This research was carried out in agreement between Eötvös Loránd University and Hungarian Geological and Geophysical Institute (TTK 2461/1/2013 and MFGI 206-114/2013) with partial financing by the Hungarian National Research Fund (K 115927 to Gy. Falus). The project was partially supported through the New National Excellence Program of the Ministry of Human Capacities, Hungary (ÚNKP-16 to Cs. Király). Work of E. Székely was supported by the Hungarian Academy of Science by the János Bolyai Research Grant. This is the 84 publication of Lithosphere Fluid Research Lab at Eötvös University. Appendix A. Appendix A. ICP-OES results of the fluid samples are collected at several time steps during the experiments. The results presented here are corrected with the dilution factor indicated in the table, as well. The uncertainties are demonstrated by the type of numbers (bold numbers indicate ±2%, regular numbers represent ±5% and italic numbers show ±20% analytical error, <LOD indicate concentrations below the limit of detection). It must be noted that in the case of the last sampling (after 95, 97, 98 hours), mineral precipitation due to the reactor depressurization and cooling can potentially change the solution compositions compared to original experimental conditions. Sample name Gas used SK8K CO2 SK12K Ar IS10K CO2 IS11K Ar Duration hours 1 20 43 95 2 27 48 1 26 48 97 1 Dilution factor for the measurements 15 24.9 13.1 10 14 11.8 16 14.1 6.35 19.95 6 12.3 Concentration (corrected with dilution) 2+ Mg K+ Na+ 42.2 37.8 24.0 34.8 <LOD <LOD <LOD 11.1 10.8 7.7 10.4 2.0 0.9 0.79 34.0 39.6 60.2 25.2 0.39 14.9 28.5 15.2 16.0 8.6 10.2 9.1 32.8 34.6 65.3 25.0 12.4 207.5 244.7 197.0 222.0 119.6 187.0 198.6 296.6 264.4 398.8 172.3 56.7 Ca 137.7 147.6 196.2 84.1 <LOD 2+ Fe(total) mg/l 1.3 <LOD <LOD <LOD 3.9 1.2 <LOD Mn2+ H2SiO3(aq) Al3+ 0.45 0.51 0.42 0.6 0.015 0.015 <LOD <LOD <LOD <LOD <LOD 0.042 0.04 0.12 <LOD <LOD 1.3 1.8 2.2 1.3 <LOD 89.5 95.4 82.6 77.6 78.4 72.8 88.1 119.3 180.4 316.1 93.7 27.0 20.8 <LOD <LOD <LOD <LOD <LOD <LOD <LOD Author name / Energy Procedia 00 (2017) 000–000 IK9K CO2 IK13K Ar 26 97 1 20 43 94 95 2 26 98 31.25 10 12.375 14 12.5 12.1 10 12.5 11.7 10 <LOD 0.46 80.6 86.0 82.3 80.1 71.7 <LOD <LOD 2.8 1.14 1.1 24.2 27.8 27.7 25.5 24.1 2.6 3.1 2.2 29.8 8.7 23.3 22.9 26.6 26.8 26.7 6.1 6.6 16.9 138.7 155.7 14.8 12.0 13.4 12.2 12.2 51.9 49.0 44.4 1.22 0.8 0.9 <LOD <LOD <LOD <LOD 0.10 <LOD <LOD 13 0.018 0.005 1.1 1.3 1.6 1.9 1.7 <LOD <LOD <LOD 64.8 113.4 57.0 89.2 93.4 91.2 75.9 15.6 40.0 49.8 <LOD <LOD <LOD <LOD <LOD <LOD <LOD <LOD <LOD <LOD References [1] Bachu S, Gunter WD, Perkins EH. 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