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Carbonate formation in Wyoming montmorillonite under high pressure carbon dioxide
International Journal of Greenhouse Gas Control 13 (2013) 149–155
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International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
Carbonate formation in Wyoming montmorillonite under high pressure carbon dioxide Tae-Bong Hur a,b , John P. Baltrus a , Bret H. Howard a , William P. Harbert a,b , Vyacheslav N. Romanov a,∗ a b
US DOE-National Energy Technology Laboratory, PO Box 10940, Pittsburgh, PA 15236, USA Department of Geology and Planetary Science, University of Pittsburgh, 4107 O’Hara Street, SRCC, Pittsburgh, PA 15260, USA
a r t i c l e
i n f o
Article history: Received 11 June 2012 Received in revised form 21 November 2012 Accepted 2 December 2012 Available online 16 January 2013 Keywords: Montmorillonite Carbonation reaction Structural deformation CO2 storage
a b s t r a c t Carbonation reaction with silicate minerals that are common components of the host rock and cap rock within geological storage reservoirs and the associated structural deformation were investigated for better understanding of the geochemical reactions associated with geologic CO2 storage. Exposure of a model expanding clay, Wyoming montmorillonite, SWy-2, to high-pressure CO2 resulted in the formation of a mineral carbonate phase via dry CO2 –clay mineral interactions at two different temperatures. The experimental evidence suggests that the properties of CO2 fluid at 70 ◦ C provide more favorable conditions for carbonate formation at the clay surface less accessible to CO2 at 22 ◦ C. The carbonation reaction occurred predominantly within the first couple of days of exposure to the fluid and then proceeded slower with continuing exposure. As compared to the as-received clay under the same ambient conditions, the (0 0 1) basal spacing of the clay bearing carbonates (after the CO2 exposure) was slightly expanded at a relative humidity (RH) level of 12% but it was slightly collapsed at the RH level of 40%. Experimental observations suggest that the carbonation reaction occurs at the external surface as well as internal surface (interlayer) of the clay particles. Published by Elsevier Ltd.
1. Introduction Carbon dioxide storage in geological formations, such as sedimentary rocks, saline aquifers, or unmineable coal seams, has been recognized as a mitigation option for the negative impact of greenhouse gas emissions. Understanding the physical processes and chemical reactions taking place in the reservoir’s host rock and cap rock, induced by injection of supercritical CO2 , is important for safe, long-term storage of anthropogenic carbon dioxide. The fluid injection into deep geological formations can shift the geochemical equilibrium, which may cause changes in porosity, permeability, wettability, and morphology of pre-existing rocks (Shao et al., 2010; Gaus et al., 2005; Luquot and Gouze, 2009). The buoyant plume of supercritical CO2 will eventually reach the surface of the reservoir seal and permeate through its native pores as well as fractures generated by the mechanical stress, thus enhancing the CO2 impact on the reservoir seal integrity. Mineral carbonation is often considered the most promising mechanism for safe and near-permanent CO2 storage underground. In nature, abundant silicate minerals that are rich in metallic divalent cations, such as Ca2+ , Mg2+ , and Fe2+ , are potential candidates for CO2 capture via mineral carbonation. Dissolution of
∗ Corresponding author. Tel.: +1 412 386 5476; fax: +1 412 386 4604. E-mail address: [email protected] (V.N. Romanov). 1750-5836/$ – see front matter. Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.ijggc.2012.12.001
metal-silicate minerals by carbon dioxide at CO2 –water–rock interfaces promotes the formation of carbonate precipitates (Luquot and Gouze, 2009; Oelkers et al., 2008; Loring et al., 2011; Huijgen et al., 2006). For example, using in situ mid-infrared spectroscopy, the mineral dissolution of forsterite (Mg2 SiO4 ) and subsequent carbonation reaction were observed under supercritical CO2 -containing dissolved water, but no detectable carbonation reaction with pure supercritical CO2 was observed under the same conditions (Loring et al., 2011). Similarly, primary clay mineral (phlogopite, KMg3 Si3 AlO10 (F,OH)2 ) dissolution and secondary mineral precipitation were investigated through the CO2 –H2 O–phlogopite reaction under geological CO2 sequestration conditions (Shao et al., 2010, 2011). Experimental observations (Shao et al., 2010, 2011; Loring et al., 2011) suggest that the presence of a thin water film adsorbed on the surface of the minerals plays a key role in their chemical reaction with CO2 . In another study (McGrail et al., 2009), experiments and molecular dynamic simulations showed that molecular water dissolved in CO2 fluid can be quite reactive toward silicate mineral surfaces. According to multiple prior publications (Luquot and Gouze, 2009; Matter and Kelemen, 2009; Scherer, 1999; Liteanu and Spiers, 2009), mineral carbonation can lead to various outcomes with regard to physical changes in rock formations. It may cause a decrease in porosity and permeability by increasing the solid volume and by filling the existing voids. On the other hand, dissolution of the primary minerals at the interface with CO2 fluid
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and the mechanical stress generated by formation of carbonate precipitate in the pore space may promote increased porosity and permeability. Numerous studies (Oelkers et al., 2008; Loring et al., 2011; Huijgen et al., 2006; Matter and Kelemen, 2009; Liteanu and Spiers, 2009) have been focused on mineral carbonation as well as structural deformation of cap rock in contact with CO2 fluid. It was suggested (Rochelle et al., 2004) that CO2 fluid may induce the process of water extraction from the cap rock, causing the increase of permeability by shrinkage of the rock. On the other hand, in situ X-ray diffraction experiments using a high-pressure environmental chamber showed that the degree of swelling of montmorillonite depends on its initial H2 O content (Giesting et al., 2012). However, there is a lack of detailed reports on the temperature effects of CO2 fluid and structural deformation of the minerals. There is a knowledge gap in understanding the kinetics of mineral carbonation as related to long-term, safe CO2 storage, especially with regard to carbonate precipitate formation and the associated structural deformations. Particularly interesting are carbonation reactions involving expanding clays such as smectite, which is a hydrated phyllosilicate mineral containing intrinsic water in the nano-gallery (interlayer) region (Gaus, 2010; Solin, 1997). Smectite is a common component of a typical host rock and cap rock within a potential CO2 storage reservoir. In the present study, we investigate the process of carbonation in Na-rich Wyoming montmorillonite (SWy-2) exposed to high-pressure CO2 fluids at different temperatures. The carbonation reaction between the hydrous clay and CO2 fluids is analyzed by Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and powder X-ray diffraction (XRD). The goal of this work is to understand the chemical reactions between high-pressure CO2 fluids and phyllosilicate minerals with hydrated cations in the interlayer gallery, with an additional objective of accurately characterizing the structural deformations induced by this reaction. The structural deformation of the clay induced by the reaction may affect changes in mechanical properties of reservoir rocks, which might lead to fracturing and changes in porosity. The overarching goal is to achieve better quantification and accountability of the geochemical processes associated with geologic sequestration of carbon dioxide.
high-purity CO2 (99.995%) at 70 ◦ C and 22 ◦ C, to create supercritical and liquid phases (scCO2 and liqCO2 ), respectively. For the carbon dioxide exposure, approximately 0.75 g of the as-received SWy-2 powder was placed in a 2 mL glass vial and then the vial was placed in a 1.3 L high pressure–temperature reactor vessel for each experiment. The vessel temperature was controlled using a Thar-CN6 controller (fluctuation < ±0.5 ◦ C) without agitation. Each clay sample was exposed to either supercritical or liquid CO2 for up to two weeks. To avoid solid CO2 formation, the pressure vessel with liquid CO2 was gradually heated up to ∼40 ◦ C during the slow depressurization following the completion of the test, after the pressure had been reduced to just below the critical point. The clay samples exposed to CO2 fluids were carefully mixed to increase their uniformity before characterization. The chemical composition of SWy-2 provided by the Clay Minerals Society is (weight percent): SiO2 (62.9), Al2 O3 (19.6), TiO2 (0.090), Fe2 O3 (3.35), FeO (0.32), MnO (0.006), MgO (3.05), CaO (1.68), Na2 O (1.53), K2 O (0.53), F (0.111), P2 O5 (0.049), and S (0.05). The chemical formula of the SWy-2 is reported as (Van Olphen and Fripiat, 1979) (Ca0.12 Na0.32 K0.05 )[Al3.01 Mg0.54 Fe(III)0.41 Mn0.01 Ti0.02 ][Si7.98 Al0.02 ] O20 (OH)4 ·nH2 O, with principal exchange cations of Na and Ca. 2.2. Sample characterization Molecular vibrational modes of the clay samples were observed by FTIR (Nicolet 8700, Thermo Scientific) spectroscopy. FTIR experiments were conducted in transmittance mode using a pelleted sample as well as in diffuse-reflectance mode using an environmental chamber with clay powder. The spectral range was 650–4000 cm−1 with a resolution of 4 cm−1 . The spectra of the as-received clay were used for background subtraction. All spectra were processed and converted into infrared absorbance using the manufacturer’s OMNIC 8 Research software. The initial infrared spectra, shown in Fig. 1, were collected from the pelleted samples (diameter 10 mm) prepared by pressurizing the mixed powder (5 mg clay and 200 mg KBr) to 8 metric tonnes. All other infrared spectra were collected from the powder samples in a high-purity He (99.999%) atmosphere by using an environmental chamber with a ZnSe window for in situ observations. For in situ observations of the temperature-dependent behavior of the carbonate phase, the clay sample was heated stepwise from room temperature, 22 ◦ C
2. Experimental 2.1. Materials As-received Na-rich SWy-2 montmorillonite clay (Wyoming), which had been acquired from the Source Clays Repository, the Clay Minerals Society, Department of Geology, University of Missouri, Columbia, Missouri, was used to investigate carbonation reactions under high-pressure CO2 conditions. Clay minerals are typically ultrafine-grained, which is smaller than 2 m in size. For the present study, the as-received clay samples were used without any size fractionation and purification processes. The “as received” clay sample consists of about 75% smectite, 8% quartz, 16% feldspar, and 1% gypsum plus smaller amounts of other impurities, with a broad size distribution, as previously reported (Chipera and Bish, 2001). The majority of the impurities were found to settle quickly from a water dispersion indicating a large particle size (Chipera and Bish, 2001) and subsequently an expected low surface area. Additionally, under the experimental conditions, especially the relatively short exposure times, the impurities were expected to be essentially “inert” in view of the characterization methods, so no attempt was made to remove them for this study. The clay samples were loaded into a high-pressure reactor vessel and the vessel was subsequently pressurized to 10.45 MPa (1500 psig) with
Fig. 1. Red and black spectra show the infrared bands of as-received and scCO2 exposed clays, respectively. The spectral range marked with open circle was magnified, as shown in inset. The spectra were collected from the KBr pellet samples mixed with clay. The exposure time for carbonation reaction was 14 days. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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to 450 ◦ C. At each temperature step, the sample was allowed to equilibrate for 10 min before the data was collected over 10 min (1000 scans). The deuterated triglycine sulfate (DTGS Tec) detector and the mercury cadmium telluride (MCT/A) detector were used for the transmittance and diffuse reflectance experiments, respectively. Quantitative measurements of the surface carbonate phase concentrations were done using XPS (PHI 5600ci, Physical Electronics). The XPS analysis was performed using non-monochromatic Al K␣ X-rays and an analyzer pass energy of 58.7 eV. Binding energies were referenced to the C 1s peak for adventitious carbon at 285.0 eV. Atomic concentrations were calculated based on peak areas using manufacturer-supplied sensitivity factors. Curve resolution of the C 1s spectra was performed using a pseudo-Voigt function containing both Gaussian and Lorentzian contributions; a Shirley background was used. Effects of the carbonation reaction on the clay structure were investigated by powder X-ray diffraction using a PANalytical X’Pert Pro MPD powder diffractometer having a theta-theta configuration, a Cu X-ray source operated at 45 kV and 40 mA and an X’Celerator detector equipped with a monochromator. Patterns were recorded over a 2 range of 2–30◦ . Since only a very small sample volume was available for analysis, samples were mounted as thin, essentially unoriented layers on zero background quartz slides. Samples were allowed to equilibrate with the atmosphere for a minimum of 15 min in the instrument prior to analysis. The (0 2 0) interlayer diffraction peak, which should be unaffected by the sample treatments, was used as an internal standard for comparison of 0 0 1 interlayer d-spacing changes. Diffraction patterns of the clay exposed to the CO2 phase were collected at approximately 12% and approximately 40% ambient relative humidity. Humidity was not controlled but was monitored using a Vaisala HUMICAP® Series HMT330 humidity and temperature transmitter.
3. Results and discussion 3.1. Infrared (IR) spectroscopy Fig. 1 shows the IR absorbance spectra of the as-received and CO2 -treated Wyoming montmorillonite. In both cases, the IR spectra are almost identical except for the carbonate bands that appear in the clay that had been exposed to supercritical CO2 at 70 ◦ C. The primary infrared absorption bands of the as-received SWy2 are observed in the OH-stretching region (3200–3800 cm−1 ), Si–O stretching regions (∼1040 cm−1 and 700–800 cm−1 ), OHcoordinated deformations region (800–950 cm−1 ), and the region of a combination bending and deformation band of adsorbed water (∼1634 cm−1 ) (Madejová and Komadel, 2001; Madejová, 2003). In addition, a trace-level absorption band may appear at ∼1425 cm−1 due to naturally occurring carbonate impurities (Van Olphen and Fripiat, 1979; Chipera and Bish, 2001; Arroyo et al., 2005); however, no clear evidence of it was observed in the present IR study. A broad CO3 2− asymmetric stretching (3 ) band that appeared in the spectral range of 1375–1550 cm−1 after the exposure to CO2 fluid is attributed to the carbonation reaction. The carbonate band that originated under the supercritical CO2 condition has a very broad band width, as shown in the inset, indicating that there is a large polarization-field splitting of the 3 mode that is usually observed (Farmer, 1974) in the infrared spectra of carbonate-mineral powders. The carbonate bands corresponding to 1 , 2 , and 4 in the lower-frequency region were not clearly observed due to their very weak intensities and/or their overlap with infrared bands of the host clay structure. Stability of the carbonate phase was examined by in situ testing of the clay exposed to supercritical CO2 for two weeks. The
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Fig. 2. In situ observation of the carbonate band in the SWy-2 exposed to scCO2 for 14 days. The infrared spectra of the clay were collected from powder samples as a function of temperature by using an environmental chamber.
clay sample containing the carbonate phase was heated from 22 ◦ C up to 450 ◦ C stepwise in a high-purity He atmosphere using an environmental chamber. The behavior of the carbonate band that has a very broad width in the spectral range of 1375–1550 cm−1 was monitored by collecting infrared spectra at each temperature step, as shown in Fig. 2. As the temperature increased to 450 ◦ C, the intensity of the absorption band at ∼1630 cm−1 owing to the combination of bending (2 ) and deformation vibrations of water adsorbed on the clay (Madejová and Komadel, 2001; Madejová, 2003) dramatically decreased, and finally the band almost disappeared at 450 ◦ C. As the clay sample temperature increased, the water band shifted to lower frequencies due to a decreasing amount of hydrogen-bonded H2 O molecules (Madejová, 2003). No significant change in the background-subtracted carbonate band was observed over the temperature range used in this test, although the band at ∼1420 cm−1 broadened with increasing temperature toward slightly lower wavenumbers. It is likely that there was no significant change in the carbonate band because CO2 molecules form a stable mineral carbonate phase in the SWy-2 clay structure by bonding with the metal cations within the structure. Silicate minerals that contain divalent cations such as Ca2+ , Mg2+ , and Fe2+ can allow the formation of carbonate phases to be energetically favorable (Lackner, 2002; Seifritz, 1990). In order to investigate the behavior of the carbonate phase formed under the supercritical CO2 conditions, the backgroundsubtracted carbonate band corresponding to the CO3 2− asymmetric stretching vibration (3 ) mode was monitored as a function of exposure time, as shown in Fig. 3. The broad band reflecting a large field splitting of the 3 mode can be separated into two bands. The first (primary) band assigned as ∼1420 cm−1 is relatively welldefined, but the second band assigned as ∼1476 cm−1 is irregular due to broadening corresponding to the field splitting of the mode (Farmer, 1974; White, 1971). The overall intensity of the carbonate band gradually increased with increasing CO2 exposure time, indicating that concentration of the carbonate phase depends on the length of exposure time. The IR absorbance bands of the carbonate phase formed under the supercritical and liquid CO2 conditions at 70 ◦ C and 22 ◦ C, respectively, were compared to each other, as shown in Fig. 4(a). Both carbonate bands formed under the different CO2 phases were normalized to the intensity of the primary band, which was centered at ∼1420 cm−1 after exposure to supercritical CO2 . After exposure to liquid CO2 , the primary CO3 2− asymmetric stretching
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Fig. 3. Carbonate bands appearing as a function of exposure time in the SWy-2 clay exposed to scCO2 . Background of the bands has been subtracted by using the IR spectrum of as-received clay as a baseline.
band (3 ) was observed at a relatively shorter wavenumber, ∼1417 cm−1 . The second band at ∼1476 cm−1 became significantly weaker (after the liqCO2 ) as part of a long tail extending to ∼1550 cm−1 . The corresponding difference between the two spectra, as shown in Fig. 4(b), reveals a distinct field-splitting band centered at ∼1476 cm−1 that becomes significant only at 70 ◦ C. Interestingly, it shows that the carbonation reaction occurs differently under the two different temperature conditions of CO2 fluid. It is well-known that supercritical CO2 fluid has lower viscosity, higher diffusivity, and near-zero surface tension (Horsch et al., 2006; Serhatkulu et al., 2006; Zhao and Samulski, 2003) compared to liquid CO2 . However it is not clear how the difference between the CO2 phases affects the carbonation reaction in this system. Therefore, the carbonation reaction should be further investigated under various temperature–pressure conditions in the future. It is
Fig. 5. XPS profiles of the clay samples exposed to scCO2 . The solid lines indicate the best fit of the raw data (♦) to a pseudo-Voigt function. (a) The carbon 1s spectral line for the carbonate, (b) the carbon 1s spectral line for adventitious carbon. The bottom profile shows the components of a representative best fit to the spectrum.
likely that CO2 fluid at 70 ◦ C provides more favorable conditions for carbonate formation at the interconnected external surface of clay particles as well as at the internal surfaces less accessible to the liquid CO2 phase formed at 22 ◦ C, such as interlayer galleries or isolated micro-pore voids between the crystals. The chemical composition of the mineral carbonate phases observed at the supercritical and liquid CO2 conditions cannot be well characterized due to the very weak intensities of the IR absorbance peaks, but the difference between the spectra of the carbonate phases formed under supercritical and liquid CO2 may be attributed to the different sites at which the carbonation reaction occurs at two different temperatures (White, 1971; Adler and Kerr, 1963; Zhang et al., 2006; Liu et al., 2005; Barriga et al., 2002). In addition, in order to investigate the effects of the cations intercalated into the interlayer gallery of the as-received clay, Na-SWy-2 and Ca-SWy-2 clays obtained by an ion-exchange process were exposed to the supercritical CO2 for three days. Interestingly, the supercritical carbon dioxide formed a carbonate phase in the CaSWy-2 clay (see supplementary data), but no carbonation reaction occurred in the Na-SWy-2 clay. It is likely that the carbonation reaction has a strong correlation with the type of cation in the structure of SWy-2 clay. 3.2. X-ray photoelectron spectroscopy
Fig. 4. (a) Infrared spectra of the clay exposed to liqCO2 (䊉) and scCO2 (); (b) baseline supplementary absorbance at the scCO2 vs. liqCO2 conditions (difference between Fig. 4(a) spectra).
In order to obtain additional evidence for the formation of the carbonate phase under high-pressure carbon dioxide conditions and to estimate the quantity of carbon present as carbonate, XPS measurements were conducted on the clay samples exposed to the supercritical and liquid CO2 phases. Fig. 5 shows representative C 1s XPS spectra obtained from the clay exposed to supercritical CO2 as a function of exposure time. The top spectrum was obtained from the as-received Wyoming SWy-2 clay. The lower intensity peak located at ∼289 eV is the carbon 1s spectral line for the carbonate phase formed in the clay sample by the exposure to the CO2 fluid. The XPS spectrum of the as-received clay includes a weak peak at 289.7 eV due to carbonate carbon indicating that the as-received clay contains surface carbonate impurities. As the total time of exposure to the CO2 phase increased, the relative intensity of the C 1s
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Fig. 6. Estimated concentration of carbon that consists of carbonate phase as a function of exposure time to scCO2 (䊉) and liqCO2 (). The solid lines are the best fits to the Elovich equation. Error bars correspond to the standard deviation from the average of duplicate samples.
spectral line for the carbonate phase gradually increased. The relative amount of carbonate in each sample was calculated by measuring the area of the C 1s peak due to carbonate and normalizing by the area of the Si 2p peak; the relative contribution to the XPS signal of Si from the clay was assumed to remain constant. The results are plotted in Fig. 6. The dependency of carbonate concentration on CO2 -exposure time illustrated in Fig. 6 is an indicator of the reaction between the clay structure and the CO2 fluids. Reactivity was measured under the supercritical and liquid CO2 conditions. The behavior of the carbonation reaction can be described by the Elovich equation (Bansal et al., 1970; Bulut et al., 2008; Polyzopoulos et al., 1986): dq = ˛ exp(−ˇq) dt
(1)
where q is the amount chemisorbed, t is the length of exposure time, and ˛ and ˇ are constants. The integrated form of the equation is q=
1 ˇ
ln(1 + ˛ˇt)
(2)
where the amount chemisorbed, q, used here is the carbon concentration in the C/Si molar ratio, which includes the initial quantity of the carbon present as carbonate in the as-received clay. The chemical reaction between the clay and the supercritical CO2 fluid was well described by the Elovich equation, but for the liquid fluid a serious deviation was observed between the best fit and the rate of carbonation. The carbonation reaction occurred rapidly within the first couple of days of exposure to the CO2 fluids. The reaction with the supercritical CO2 appears to continue at longer reaction times, while it begins to level off for the liquid CO2 . The measured net increase in carbonate concentration during the two weeks of CO2 -exposure under the supercritical conditions was about 0.033 when expressed as the C/Si molar ratio, indicating that 1 g of SWy2 clay may capture near-permanently about 0.35 mmol of carbon through the carbonation reaction. The results for chemical composition analysis of the clay were used to estimate the chemisorbed quantity in 1 g clay, on the basis of the molar ratio between the carbon and the silicon, under the assumption that the carbonate phase exists uniformly in the clay particles. 3.3. Powder X-ray diffraction Structural deformation of the clay resulting from the carbonation reaction was investigated by powder X-ray diffraction. The
Fig. 7. (a) Powder X-ray diffraction patterns at 12% relative humidity. Change in basal spacing of the clay exposed to (a) scCO2 and (b) liqCO2 .
XRD data was collected following equilibration of the sample with the ambient but known relative humidity. Two humidity levels were used, approximately 12% and approximately 40%. The procedure utilized for the XRD analysis resulted in two potentially useful pieces of information; the observation of new phase formation/alteration of the clay lattice and changes induced in the hydration behavior due to CO2 interaction with the clay interlayer. For this set of experiments, no detectable new phases were formed and changes in the montmorillonite lattice except for layer expansion/contraction were not observed. Fig. 7 shows the change in the basal spacing of the montmorillonite exposed to the supercritical and liquid CO2 as a function of exposure time. All patterns were collected at about 12% relative humidity. The interlayer spacing of a montmorillonite is very sensitive to ambient relative humidity; therefore, even though it was not possible to control the relative humidity for these XRD measurements, it was carefully monitored so that observed changes could potentially be correlated to the CO2 treatment. Under the XRD measurement conditions, the (0 0 1) reflection peak of the as-received Na-rich SWy-2 at about 12% relative humidity is located at 8.2◦ 2, corresponding to a basal spacing of 10.7 A˚ which falls between the zero water molecules layer and one water molecule layer hydration state (Morodome and Kawamura, 2009; Ferrage et al., 2005; Hensen and Smit, 2002). Following exposure to both supercritical and liquid CO2 , the (0 0 1) reflection peak was shifted to a slightly lower angle (higher d-spacing) when equilibrated at an ambient relative humidity level of 12% as compared to the as-received clay. There was no observed trend in d-spacing relative to exposure duration for either supercritical or liquid CO2 treatment. However, the shift of the (0 0 1) peak exposed to supercritical CO2 appears to be slightly larger than that of the clay exposed to liquid CO2 . Fig. 8 shows the basal spacing of several montmorillonite samples – each one having been previously exposed to supercritical CO2 – plotted as a function of the CO2 exposure time. All these XRD patterns were collected at about 40% relative humidity. Under the XRD measurement conditions, the (0 0 1) reflection peak of the asreceived Na-rich, SWy-2 at about 40% relative humidity indicated a basal spacing of 12.2 A˚ which falls near the one water molecule layer
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be important. For example supercritical CO2 fluid reaching the cap rock could alter the cap rock, either by extracting water from the cap rock, with clay minerals hence more likely to shrink, or by promoting expansion of the clay structure in cap rock. Our results indicate that the formation of carbonate may be an additional factor to alter the cap rock by the structural deformation of the clay, which in some scenarios could lead to change in porosity and permeability. Disclaimer
Fig. 8. Powder X-ray diffraction patterns of the clay exposed to scCO2 , as a function of exposure time. Relative humidity was approximately 40%.
hydration state (Morodome and Kawamura, 2009; Ferrage et al., 2005; Hensen and Smit, 2002). Following exposure to supercritical CO2 , the (0 0 1) reflection peak was shifted to a slightly higher angle (lower d-spacing) when equilibrated at an ambient relative humidity level of 40% as compared to the as-received clay. At 40% relative humidity, a slight trend in d-spacing to exposure duration was observed. Longer supercritical CO2 exposure times appeared to result in less clay expansion. The trends observed in the behaviors of the clay samples exposed to CO2 and then equilibrated under the two relative humidity levels used suggest that the CO2 exposure did alter the clay gallery conditions and, as a result, the interaction of the clay gallery species with water. This change was reflected in the alteration of the hydration behavior relative to the unexposed clay. At this time, the specific mechanism causing this change is not precisely known but likely involves CO2 interaction with the interlayer cations, and possibly, the formation of “carbonate” species in the clay gallery from the reaction of exchangeable cations with CO2 , which would alter the electrostatic interactions within the clay structure and subsequently the hydration behavior (Hensen and Smit, 2002; Botan et al., 2010; Segad et al., 2010; Laird, 2006; Hines and Solin, 2000). 4. Conclusions This study examined the CO2 -hydrous clay mineral interactions relevant to cap rock and leakage conduits from an experimental point of view. The experimental results suggest that high-pressure carbon dioxide forms a mineral carbonate phase by interaction with hydrous Na-rich SWy-2 clay under the conditions used in this study. The carbonation reaction shows a strong dependency on the physical properties of the CO2 fluid, which are defined by the temperature, and the length of exposure time. The carbonation reaction may be affected by the water molecules adsorbed on the surface as well as intercalated into the interlayer region of the hydrophilic clay mineral. Dissolution of the host material at CO2 –hydrous clay mineral interfaces in the presence of high-pressure CO2 fluid is another possible factor as suggested by Shao et al. (2010, 2011) and Loring et al. (2011). However, currently, the detailed mechanism of the carbonation is not clear; therefore, these chemical reactions should be investigated in more detail in the future. CO2 storage reservoirs are often sealed by caprock formations with significant clay fractions. In this study, the carbonation reaction in expanding clay minerals has been constrained by the conditions of our experiments; however, additional factors could
This project was funded by the Department of Energy, National Energy Technology Laboratory, an agency of the United States Government, through a support contract with URS Energy & Construction, Inc. Neither the United States Government nor any agency thereof, nor any of their employees, nor URS Energy & Construction, Inc., nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Acknowledgment This technical effort was performed in support of the National Energy Technology Laboratory’s ongoing research in CO2 Trapping Mechanisms in Clay Materials under the RES contract DEFE0004000. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ijggc. 2012.12.001. References Adler, H.H., Kerr, P.F., 1963. Infrared absorption frequency trends for anhydrous normal carbonates. American Mineralogist 48, 124–137. Arroyo, L.J., Li, H., Teppen, B.J., Johnston, C.T., Boyd, S.A., 2005. Oxidation of 1naphthol coupled to reduction of structural Fe3+ in smectite. Clays and Clay Minerals 53, 587–596. Bansal, R.C., Vastola, F.J., Walker Jr., P.L., 1970. Studies on ultraclean carbon surfaces: II. Kinetics of chemisorption of oxygen on graphon. Journal of Colloid and Interface Science 32, 187–194. Barriga, C., Gaitán, M., Pavlovic, I., Ulibarri, M.A., Hermos˜ın, M.C., Cornejo, J., 2002. Hydrotalcites as sorbent for 2,4,6-trinitrophenol: influence of the layer composition and interlayer anion. Journal of Materials Chemistry 12, 1027–1034. Botan, A., Rotenberg, B., Marry, V., Turq, P., Neotinger, B., 2010. Carbon dioxide in montmorillonite clay hydrates: thermodynamics, structure, and transport from molecular simulation. Journal of Physical Chemistry C 114, 14962–14969. Bulut, E., Özacar, M., S¸engil, I˙ .A., 2008. Equilibrium and kinetic data and process design for adsorption of Congo Red onto bentonite. Journal of Hazardous Materials 154, 613–622. Chipera, S.J., Bish, D.L., 2001. Baseline studies of the Clay Minerals Society source clays: powder X-ray diffraction analyses. Clays and Clay Minerals 49, 398–409. Farmer, V.C., 1974. The Infrared Spectra of Minerals. Mineralogical Society, London, 539 pp. Ferrage, E., Lanson, B., Sakharov, B.A., Drits, V.A., 2005. Investigation of smectite hydration properties by modeling experimental X-ray diffraction patterns: part I. Montmorillonite hydration properties. American Mineralogist 90, 1358–1374. Gaus, I., Azaroual, M., Czernichowski-Lauriol, I., 2005. Reactive transport modelling of the impact of CO2 injection on the clayey cap rock at Sleipner (North Sea). Chemical Geology 217, 319–337. Gaus, I., 2010. Role and impact of CO2 –rock interactions during CO2 storage in sedimentary rocks. International Journal of Greenhouse Gas Control 4, 73–89.
T.-B. Hur et al. / International Journal of Greenhouse Gas Control 13 (2013) 149–155 Giesting, P., Guggenheim, S., Koster van Groos, A.F., Busch, A., 2012. Interaction of carbon dioxide with Na-exchanged montmorillonite at pressures to 640 bars: implications for CO2 sequestration. International Journal of Greenhouse Gas Control 8, 73–81. Hensen, E.J.M., Smit, B., 2002. Why clays swell. Journal of Physical Chemistry B 106, 12664–12667. Hines, D.R., Solin, S.A., 2000. Physical properties of fixed-charge layer double hydroxides. Physical Review B 61, 11348–11358. Horsch, S., Serhatkulu, G., Gulari, E., Kannan, R.M., 2006. Supercritical CO2 dispersion of nano-clays and clay/polymer nanocomposites. Polymer 47, 7485–7496. Huijgen, W.J.J., Witkamp, G.-J., Comans, R.N.J., 2006. Mechanisms of aqueous wollastonite carbonation as a possible CO2 sequestration process. Chemical Engineering Science 61, 4242–4251. Lackner, K.S., 2002. Carbonate chemistry for sequestering fossil carbon. Annual Review of Energy and the Environment 27, 193–232. Laird, D.A., 2006. Influence of layer charge on swelling of smectites. Applied Clay Science 34, 74–87. Liteanu, E., Spiers, C.J., 2009. Influence of pore fluid salt content on compaction creep of calcite aggregates in the presence of supercritical CO2 . Chemical Geology 265, 134–147. Liu, Z., Ma, R., Osada, M., Takada, K., Sasaki, T., 2005. Selective and controlled synthesis of ␣- and -cobalt hydroxides in highly developed hexagonal platelets. Journal of the American Chemical Society 127, 13869–13874. Loring, J.S., Thompson, C.J., Wang, Z., Joly, A.G., Sklarew, D.S., Schaef, H.T., Ilton, E.S., Rosso, K.M., Felmy, A.R., 2011. In situ infrared spectroscopic study of forsterite carbonation in wet supercritical CO2 . Environmental Science & Technology 45, 6204–6210. Luquot, L., Gouze, P., 2009. Experimental determination of porosity and permeability changes induced by injection of CO2 into carbonate rocks. Chemical Geology 265, 148–159. Madejová, J., 2003. FTIR techniques in clay mineral studies. Vibrational Spectroscopy 31, 1–10. Madejová, J., Komadel, P., 2001. Baseline studies of the Clay Minerals Society source clays: infrared methods. Clays and Clay Minerals 49, 410–432. Matter, J.M., Kelemen, P.B., 2009. Permanent storage of carbon dioxide in geological reservoirs by mineral carbonation. Nature Geoscience 2, 837–841.
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McGrail, B.P., Schaef, H.T., Glezakou, V.-A., Dang, L.X., Owen, A.T., 2009. Water reactivity in the liquid and supercritical CO2 phase: Has half the story been negleted? Energy Procedia 1, 3415–3419. Morodome, S., Kawamura, K., 2009. Swelling behavior of Na- and Camontmorillonite up to 150 ◦ C by in situ X-ray diffraction experiments. Clays and Clay Minerals 57, 150–160. Oelkers, E.H., Gislason, S.R., Matter, J., 2008. Mineral carbonation of CO2 . Elements 4, 333–337. Polyzopoulos, N.A., Keramidas, V.Z., Pavlatou, A., 1986. On the limitations of the simplified Elovich equation in describing the kinetics of phosphate sorption and release from soils. Journal of Soil Science 37, 81–87. Rochelle, C.A., Czernichowski-Lauriol, I., Milodowski, A.E., 2004. The impact of chemical reactions on CO2 storage in geological formations: a brief review. Geological Society, London, Special Publications 233, 87–106. Scherer, G.W., 1999. Crystallization in pores. Cement and Concrete Research 29, 1347–1358. Segad, M., Jönsson, B., Åkesson, T., Cabane, B., 2010. Ca/Na montmorillonite: structure, forces and swelling properties. Langmuir 26, 5782–5790. Seifritz, W., 1990. CO2 disposal by means of silicates. Nature 345, 486. Serhatkulu, G.K., Dilek, C., Gulari, E., 2006. Supercritical CO2 intercalation of layered silicates. Journal of Supercritical Fluids 39, 264–270. Shao, H., Ray, J.R., Jun, Y.-S., 2010. Dissolution and precipitation of clay minerals under geologic CO2 sequestration conditions: CO2 –brine-phlogopite interactions. Environmental Science and Technology 44, 5999–6005. Shao, H., Ray, J.R., Jun, Y.-S., 2011. Effects of salinity and the extent of water on supercritical CO2 -induced phlogopite dissolution and secondary mineral formation. Environmental Science & Technology 45, 1737–1743. Solin, S.A., 1997. Clays and clay interaction compounds: properties and physical phenomena. Annual Review of Materials Science 27, 89–115. Van Olphen, H., Fripiat, J.J., 1979. Data Handbook for Clay Minerals and Other Nonmetallic Minerals. Pergamon Press. White, W.B., 1971. Infrared charaterization of water and hydroxyl ion in the basic magnesium carbonate minerals. American Mineralogist 56, 46–53. Zhang, Z., Zheng, Y., Ni, Y., Liu, Z., Chen, J., Liang, X., 2006. Temperature- and pHdependent morphology and FT-IR analysis of magnesium carbonate hydrates. Journal of Physical Chemistry B 110, 12969–12973. Zhao, Q., Samulski, E.T., 2003. Supercritical CO2 -mediated intercalation of PEO in clay. Macromolecules 36, 6967–6969.
Contents lists available at SciVerse ScienceDirect
International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
Carbonate formation in Wyoming montmorillonite under high pressure carbon dioxide Tae-Bong Hur a,b , John P. Baltrus a , Bret H. Howard a , William P. Harbert a,b , Vyacheslav N. Romanov a,∗ a b
US DOE-National Energy Technology Laboratory, PO Box 10940, Pittsburgh, PA 15236, USA Department of Geology and Planetary Science, University of Pittsburgh, 4107 O’Hara Street, SRCC, Pittsburgh, PA 15260, USA
a r t i c l e
i n f o
Article history: Received 11 June 2012 Received in revised form 21 November 2012 Accepted 2 December 2012 Available online 16 January 2013 Keywords: Montmorillonite Carbonation reaction Structural deformation CO2 storage
a b s t r a c t Carbonation reaction with silicate minerals that are common components of the host rock and cap rock within geological storage reservoirs and the associated structural deformation were investigated for better understanding of the geochemical reactions associated with geologic CO2 storage. Exposure of a model expanding clay, Wyoming montmorillonite, SWy-2, to high-pressure CO2 resulted in the formation of a mineral carbonate phase via dry CO2 –clay mineral interactions at two different temperatures. The experimental evidence suggests that the properties of CO2 fluid at 70 ◦ C provide more favorable conditions for carbonate formation at the clay surface less accessible to CO2 at 22 ◦ C. The carbonation reaction occurred predominantly within the first couple of days of exposure to the fluid and then proceeded slower with continuing exposure. As compared to the as-received clay under the same ambient conditions, the (0 0 1) basal spacing of the clay bearing carbonates (after the CO2 exposure) was slightly expanded at a relative humidity (RH) level of 12% but it was slightly collapsed at the RH level of 40%. Experimental observations suggest that the carbonation reaction occurs at the external surface as well as internal surface (interlayer) of the clay particles. Published by Elsevier Ltd.
1. Introduction Carbon dioxide storage in geological formations, such as sedimentary rocks, saline aquifers, or unmineable coal seams, has been recognized as a mitigation option for the negative impact of greenhouse gas emissions. Understanding the physical processes and chemical reactions taking place in the reservoir’s host rock and cap rock, induced by injection of supercritical CO2 , is important for safe, long-term storage of anthropogenic carbon dioxide. The fluid injection into deep geological formations can shift the geochemical equilibrium, which may cause changes in porosity, permeability, wettability, and morphology of pre-existing rocks (Shao et al., 2010; Gaus et al., 2005; Luquot and Gouze, 2009). The buoyant plume of supercritical CO2 will eventually reach the surface of the reservoir seal and permeate through its native pores as well as fractures generated by the mechanical stress, thus enhancing the CO2 impact on the reservoir seal integrity. Mineral carbonation is often considered the most promising mechanism for safe and near-permanent CO2 storage underground. In nature, abundant silicate minerals that are rich in metallic divalent cations, such as Ca2+ , Mg2+ , and Fe2+ , are potential candidates for CO2 capture via mineral carbonation. Dissolution of
∗ Corresponding author. Tel.: +1 412 386 5476; fax: +1 412 386 4604. E-mail address: [email protected] (V.N. Romanov). 1750-5836/$ – see front matter. Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.ijggc.2012.12.001
metal-silicate minerals by carbon dioxide at CO2 –water–rock interfaces promotes the formation of carbonate precipitates (Luquot and Gouze, 2009; Oelkers et al., 2008; Loring et al., 2011; Huijgen et al., 2006). For example, using in situ mid-infrared spectroscopy, the mineral dissolution of forsterite (Mg2 SiO4 ) and subsequent carbonation reaction were observed under supercritical CO2 -containing dissolved water, but no detectable carbonation reaction with pure supercritical CO2 was observed under the same conditions (Loring et al., 2011). Similarly, primary clay mineral (phlogopite, KMg3 Si3 AlO10 (F,OH)2 ) dissolution and secondary mineral precipitation were investigated through the CO2 –H2 O–phlogopite reaction under geological CO2 sequestration conditions (Shao et al., 2010, 2011). Experimental observations (Shao et al., 2010, 2011; Loring et al., 2011) suggest that the presence of a thin water film adsorbed on the surface of the minerals plays a key role in their chemical reaction with CO2 . In another study (McGrail et al., 2009), experiments and molecular dynamic simulations showed that molecular water dissolved in CO2 fluid can be quite reactive toward silicate mineral surfaces. According to multiple prior publications (Luquot and Gouze, 2009; Matter and Kelemen, 2009; Scherer, 1999; Liteanu and Spiers, 2009), mineral carbonation can lead to various outcomes with regard to physical changes in rock formations. It may cause a decrease in porosity and permeability by increasing the solid volume and by filling the existing voids. On the other hand, dissolution of the primary minerals at the interface with CO2 fluid
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and the mechanical stress generated by formation of carbonate precipitate in the pore space may promote increased porosity and permeability. Numerous studies (Oelkers et al., 2008; Loring et al., 2011; Huijgen et al., 2006; Matter and Kelemen, 2009; Liteanu and Spiers, 2009) have been focused on mineral carbonation as well as structural deformation of cap rock in contact with CO2 fluid. It was suggested (Rochelle et al., 2004) that CO2 fluid may induce the process of water extraction from the cap rock, causing the increase of permeability by shrinkage of the rock. On the other hand, in situ X-ray diffraction experiments using a high-pressure environmental chamber showed that the degree of swelling of montmorillonite depends on its initial H2 O content (Giesting et al., 2012). However, there is a lack of detailed reports on the temperature effects of CO2 fluid and structural deformation of the minerals. There is a knowledge gap in understanding the kinetics of mineral carbonation as related to long-term, safe CO2 storage, especially with regard to carbonate precipitate formation and the associated structural deformations. Particularly interesting are carbonation reactions involving expanding clays such as smectite, which is a hydrated phyllosilicate mineral containing intrinsic water in the nano-gallery (interlayer) region (Gaus, 2010; Solin, 1997). Smectite is a common component of a typical host rock and cap rock within a potential CO2 storage reservoir. In the present study, we investigate the process of carbonation in Na-rich Wyoming montmorillonite (SWy-2) exposed to high-pressure CO2 fluids at different temperatures. The carbonation reaction between the hydrous clay and CO2 fluids is analyzed by Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and powder X-ray diffraction (XRD). The goal of this work is to understand the chemical reactions between high-pressure CO2 fluids and phyllosilicate minerals with hydrated cations in the interlayer gallery, with an additional objective of accurately characterizing the structural deformations induced by this reaction. The structural deformation of the clay induced by the reaction may affect changes in mechanical properties of reservoir rocks, which might lead to fracturing and changes in porosity. The overarching goal is to achieve better quantification and accountability of the geochemical processes associated with geologic sequestration of carbon dioxide.
high-purity CO2 (99.995%) at 70 ◦ C and 22 ◦ C, to create supercritical and liquid phases (scCO2 and liqCO2 ), respectively. For the carbon dioxide exposure, approximately 0.75 g of the as-received SWy-2 powder was placed in a 2 mL glass vial and then the vial was placed in a 1.3 L high pressure–temperature reactor vessel for each experiment. The vessel temperature was controlled using a Thar-CN6 controller (fluctuation < ±0.5 ◦ C) without agitation. Each clay sample was exposed to either supercritical or liquid CO2 for up to two weeks. To avoid solid CO2 formation, the pressure vessel with liquid CO2 was gradually heated up to ∼40 ◦ C during the slow depressurization following the completion of the test, after the pressure had been reduced to just below the critical point. The clay samples exposed to CO2 fluids were carefully mixed to increase their uniformity before characterization. The chemical composition of SWy-2 provided by the Clay Minerals Society is (weight percent): SiO2 (62.9), Al2 O3 (19.6), TiO2 (0.090), Fe2 O3 (3.35), FeO (0.32), MnO (0.006), MgO (3.05), CaO (1.68), Na2 O (1.53), K2 O (0.53), F (0.111), P2 O5 (0.049), and S (0.05). The chemical formula of the SWy-2 is reported as (Van Olphen and Fripiat, 1979) (Ca0.12 Na0.32 K0.05 )[Al3.01 Mg0.54 Fe(III)0.41 Mn0.01 Ti0.02 ][Si7.98 Al0.02 ] O20 (OH)4 ·nH2 O, with principal exchange cations of Na and Ca. 2.2. Sample characterization Molecular vibrational modes of the clay samples were observed by FTIR (Nicolet 8700, Thermo Scientific) spectroscopy. FTIR experiments were conducted in transmittance mode using a pelleted sample as well as in diffuse-reflectance mode using an environmental chamber with clay powder. The spectral range was 650–4000 cm−1 with a resolution of 4 cm−1 . The spectra of the as-received clay were used for background subtraction. All spectra were processed and converted into infrared absorbance using the manufacturer’s OMNIC 8 Research software. The initial infrared spectra, shown in Fig. 1, were collected from the pelleted samples (diameter 10 mm) prepared by pressurizing the mixed powder (5 mg clay and 200 mg KBr) to 8 metric tonnes. All other infrared spectra were collected from the powder samples in a high-purity He (99.999%) atmosphere by using an environmental chamber with a ZnSe window for in situ observations. For in situ observations of the temperature-dependent behavior of the carbonate phase, the clay sample was heated stepwise from room temperature, 22 ◦ C
2. Experimental 2.1. Materials As-received Na-rich SWy-2 montmorillonite clay (Wyoming), which had been acquired from the Source Clays Repository, the Clay Minerals Society, Department of Geology, University of Missouri, Columbia, Missouri, was used to investigate carbonation reactions under high-pressure CO2 conditions. Clay minerals are typically ultrafine-grained, which is smaller than 2 m in size. For the present study, the as-received clay samples were used without any size fractionation and purification processes. The “as received” clay sample consists of about 75% smectite, 8% quartz, 16% feldspar, and 1% gypsum plus smaller amounts of other impurities, with a broad size distribution, as previously reported (Chipera and Bish, 2001). The majority of the impurities were found to settle quickly from a water dispersion indicating a large particle size (Chipera and Bish, 2001) and subsequently an expected low surface area. Additionally, under the experimental conditions, especially the relatively short exposure times, the impurities were expected to be essentially “inert” in view of the characterization methods, so no attempt was made to remove them for this study. The clay samples were loaded into a high-pressure reactor vessel and the vessel was subsequently pressurized to 10.45 MPa (1500 psig) with
Fig. 1. Red and black spectra show the infrared bands of as-received and scCO2 exposed clays, respectively. The spectral range marked with open circle was magnified, as shown in inset. The spectra were collected from the KBr pellet samples mixed with clay. The exposure time for carbonation reaction was 14 days. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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to 450 ◦ C. At each temperature step, the sample was allowed to equilibrate for 10 min before the data was collected over 10 min (1000 scans). The deuterated triglycine sulfate (DTGS Tec) detector and the mercury cadmium telluride (MCT/A) detector were used for the transmittance and diffuse reflectance experiments, respectively. Quantitative measurements of the surface carbonate phase concentrations were done using XPS (PHI 5600ci, Physical Electronics). The XPS analysis was performed using non-monochromatic Al K␣ X-rays and an analyzer pass energy of 58.7 eV. Binding energies were referenced to the C 1s peak for adventitious carbon at 285.0 eV. Atomic concentrations were calculated based on peak areas using manufacturer-supplied sensitivity factors. Curve resolution of the C 1s spectra was performed using a pseudo-Voigt function containing both Gaussian and Lorentzian contributions; a Shirley background was used. Effects of the carbonation reaction on the clay structure were investigated by powder X-ray diffraction using a PANalytical X’Pert Pro MPD powder diffractometer having a theta-theta configuration, a Cu X-ray source operated at 45 kV and 40 mA and an X’Celerator detector equipped with a monochromator. Patterns were recorded over a 2 range of 2–30◦ . Since only a very small sample volume was available for analysis, samples were mounted as thin, essentially unoriented layers on zero background quartz slides. Samples were allowed to equilibrate with the atmosphere for a minimum of 15 min in the instrument prior to analysis. The (0 2 0) interlayer diffraction peak, which should be unaffected by the sample treatments, was used as an internal standard for comparison of 0 0 1 interlayer d-spacing changes. Diffraction patterns of the clay exposed to the CO2 phase were collected at approximately 12% and approximately 40% ambient relative humidity. Humidity was not controlled but was monitored using a Vaisala HUMICAP® Series HMT330 humidity and temperature transmitter.
3. Results and discussion 3.1. Infrared (IR) spectroscopy Fig. 1 shows the IR absorbance spectra of the as-received and CO2 -treated Wyoming montmorillonite. In both cases, the IR spectra are almost identical except for the carbonate bands that appear in the clay that had been exposed to supercritical CO2 at 70 ◦ C. The primary infrared absorption bands of the as-received SWy2 are observed in the OH-stretching region (3200–3800 cm−1 ), Si–O stretching regions (∼1040 cm−1 and 700–800 cm−1 ), OHcoordinated deformations region (800–950 cm−1 ), and the region of a combination bending and deformation band of adsorbed water (∼1634 cm−1 ) (Madejová and Komadel, 2001; Madejová, 2003). In addition, a trace-level absorption band may appear at ∼1425 cm−1 due to naturally occurring carbonate impurities (Van Olphen and Fripiat, 1979; Chipera and Bish, 2001; Arroyo et al., 2005); however, no clear evidence of it was observed in the present IR study. A broad CO3 2− asymmetric stretching (3 ) band that appeared in the spectral range of 1375–1550 cm−1 after the exposure to CO2 fluid is attributed to the carbonation reaction. The carbonate band that originated under the supercritical CO2 condition has a very broad band width, as shown in the inset, indicating that there is a large polarization-field splitting of the 3 mode that is usually observed (Farmer, 1974) in the infrared spectra of carbonate-mineral powders. The carbonate bands corresponding to 1 , 2 , and 4 in the lower-frequency region were not clearly observed due to their very weak intensities and/or their overlap with infrared bands of the host clay structure. Stability of the carbonate phase was examined by in situ testing of the clay exposed to supercritical CO2 for two weeks. The
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Fig. 2. In situ observation of the carbonate band in the SWy-2 exposed to scCO2 for 14 days. The infrared spectra of the clay were collected from powder samples as a function of temperature by using an environmental chamber.
clay sample containing the carbonate phase was heated from 22 ◦ C up to 450 ◦ C stepwise in a high-purity He atmosphere using an environmental chamber. The behavior of the carbonate band that has a very broad width in the spectral range of 1375–1550 cm−1 was monitored by collecting infrared spectra at each temperature step, as shown in Fig. 2. As the temperature increased to 450 ◦ C, the intensity of the absorption band at ∼1630 cm−1 owing to the combination of bending (2 ) and deformation vibrations of water adsorbed on the clay (Madejová and Komadel, 2001; Madejová, 2003) dramatically decreased, and finally the band almost disappeared at 450 ◦ C. As the clay sample temperature increased, the water band shifted to lower frequencies due to a decreasing amount of hydrogen-bonded H2 O molecules (Madejová, 2003). No significant change in the background-subtracted carbonate band was observed over the temperature range used in this test, although the band at ∼1420 cm−1 broadened with increasing temperature toward slightly lower wavenumbers. It is likely that there was no significant change in the carbonate band because CO2 molecules form a stable mineral carbonate phase in the SWy-2 clay structure by bonding with the metal cations within the structure. Silicate minerals that contain divalent cations such as Ca2+ , Mg2+ , and Fe2+ can allow the formation of carbonate phases to be energetically favorable (Lackner, 2002; Seifritz, 1990). In order to investigate the behavior of the carbonate phase formed under the supercritical CO2 conditions, the backgroundsubtracted carbonate band corresponding to the CO3 2− asymmetric stretching vibration (3 ) mode was monitored as a function of exposure time, as shown in Fig. 3. The broad band reflecting a large field splitting of the 3 mode can be separated into two bands. The first (primary) band assigned as ∼1420 cm−1 is relatively welldefined, but the second band assigned as ∼1476 cm−1 is irregular due to broadening corresponding to the field splitting of the mode (Farmer, 1974; White, 1971). The overall intensity of the carbonate band gradually increased with increasing CO2 exposure time, indicating that concentration of the carbonate phase depends on the length of exposure time. The IR absorbance bands of the carbonate phase formed under the supercritical and liquid CO2 conditions at 70 ◦ C and 22 ◦ C, respectively, were compared to each other, as shown in Fig. 4(a). Both carbonate bands formed under the different CO2 phases were normalized to the intensity of the primary band, which was centered at ∼1420 cm−1 after exposure to supercritical CO2 . After exposure to liquid CO2 , the primary CO3 2− asymmetric stretching
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Fig. 3. Carbonate bands appearing as a function of exposure time in the SWy-2 clay exposed to scCO2 . Background of the bands has been subtracted by using the IR spectrum of as-received clay as a baseline.
band (3 ) was observed at a relatively shorter wavenumber, ∼1417 cm−1 . The second band at ∼1476 cm−1 became significantly weaker (after the liqCO2 ) as part of a long tail extending to ∼1550 cm−1 . The corresponding difference between the two spectra, as shown in Fig. 4(b), reveals a distinct field-splitting band centered at ∼1476 cm−1 that becomes significant only at 70 ◦ C. Interestingly, it shows that the carbonation reaction occurs differently under the two different temperature conditions of CO2 fluid. It is well-known that supercritical CO2 fluid has lower viscosity, higher diffusivity, and near-zero surface tension (Horsch et al., 2006; Serhatkulu et al., 2006; Zhao and Samulski, 2003) compared to liquid CO2 . However it is not clear how the difference between the CO2 phases affects the carbonation reaction in this system. Therefore, the carbonation reaction should be further investigated under various temperature–pressure conditions in the future. It is
Fig. 5. XPS profiles of the clay samples exposed to scCO2 . The solid lines indicate the best fit of the raw data (♦) to a pseudo-Voigt function. (a) The carbon 1s spectral line for the carbonate, (b) the carbon 1s spectral line for adventitious carbon. The bottom profile shows the components of a representative best fit to the spectrum.
likely that CO2 fluid at 70 ◦ C provides more favorable conditions for carbonate formation at the interconnected external surface of clay particles as well as at the internal surfaces less accessible to the liquid CO2 phase formed at 22 ◦ C, such as interlayer galleries or isolated micro-pore voids between the crystals. The chemical composition of the mineral carbonate phases observed at the supercritical and liquid CO2 conditions cannot be well characterized due to the very weak intensities of the IR absorbance peaks, but the difference between the spectra of the carbonate phases formed under supercritical and liquid CO2 may be attributed to the different sites at which the carbonation reaction occurs at two different temperatures (White, 1971; Adler and Kerr, 1963; Zhang et al., 2006; Liu et al., 2005; Barriga et al., 2002). In addition, in order to investigate the effects of the cations intercalated into the interlayer gallery of the as-received clay, Na-SWy-2 and Ca-SWy-2 clays obtained by an ion-exchange process were exposed to the supercritical CO2 for three days. Interestingly, the supercritical carbon dioxide formed a carbonate phase in the CaSWy-2 clay (see supplementary data), but no carbonation reaction occurred in the Na-SWy-2 clay. It is likely that the carbonation reaction has a strong correlation with the type of cation in the structure of SWy-2 clay. 3.2. X-ray photoelectron spectroscopy
Fig. 4. (a) Infrared spectra of the clay exposed to liqCO2 (䊉) and scCO2 (); (b) baseline supplementary absorbance at the scCO2 vs. liqCO2 conditions (difference between Fig. 4(a) spectra).
In order to obtain additional evidence for the formation of the carbonate phase under high-pressure carbon dioxide conditions and to estimate the quantity of carbon present as carbonate, XPS measurements were conducted on the clay samples exposed to the supercritical and liquid CO2 phases. Fig. 5 shows representative C 1s XPS spectra obtained from the clay exposed to supercritical CO2 as a function of exposure time. The top spectrum was obtained from the as-received Wyoming SWy-2 clay. The lower intensity peak located at ∼289 eV is the carbon 1s spectral line for the carbonate phase formed in the clay sample by the exposure to the CO2 fluid. The XPS spectrum of the as-received clay includes a weak peak at 289.7 eV due to carbonate carbon indicating that the as-received clay contains surface carbonate impurities. As the total time of exposure to the CO2 phase increased, the relative intensity of the C 1s
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Fig. 6. Estimated concentration of carbon that consists of carbonate phase as a function of exposure time to scCO2 (䊉) and liqCO2 (). The solid lines are the best fits to the Elovich equation. Error bars correspond to the standard deviation from the average of duplicate samples.
spectral line for the carbonate phase gradually increased. The relative amount of carbonate in each sample was calculated by measuring the area of the C 1s peak due to carbonate and normalizing by the area of the Si 2p peak; the relative contribution to the XPS signal of Si from the clay was assumed to remain constant. The results are plotted in Fig. 6. The dependency of carbonate concentration on CO2 -exposure time illustrated in Fig. 6 is an indicator of the reaction between the clay structure and the CO2 fluids. Reactivity was measured under the supercritical and liquid CO2 conditions. The behavior of the carbonation reaction can be described by the Elovich equation (Bansal et al., 1970; Bulut et al., 2008; Polyzopoulos et al., 1986): dq = ˛ exp(−ˇq) dt
(1)
where q is the amount chemisorbed, t is the length of exposure time, and ˛ and ˇ are constants. The integrated form of the equation is q=
1 ˇ
ln(1 + ˛ˇt)
(2)
where the amount chemisorbed, q, used here is the carbon concentration in the C/Si molar ratio, which includes the initial quantity of the carbon present as carbonate in the as-received clay. The chemical reaction between the clay and the supercritical CO2 fluid was well described by the Elovich equation, but for the liquid fluid a serious deviation was observed between the best fit and the rate of carbonation. The carbonation reaction occurred rapidly within the first couple of days of exposure to the CO2 fluids. The reaction with the supercritical CO2 appears to continue at longer reaction times, while it begins to level off for the liquid CO2 . The measured net increase in carbonate concentration during the two weeks of CO2 -exposure under the supercritical conditions was about 0.033 when expressed as the C/Si molar ratio, indicating that 1 g of SWy2 clay may capture near-permanently about 0.35 mmol of carbon through the carbonation reaction. The results for chemical composition analysis of the clay were used to estimate the chemisorbed quantity in 1 g clay, on the basis of the molar ratio between the carbon and the silicon, under the assumption that the carbonate phase exists uniformly in the clay particles. 3.3. Powder X-ray diffraction Structural deformation of the clay resulting from the carbonation reaction was investigated by powder X-ray diffraction. The
Fig. 7. (a) Powder X-ray diffraction patterns at 12% relative humidity. Change in basal spacing of the clay exposed to (a) scCO2 and (b) liqCO2 .
XRD data was collected following equilibration of the sample with the ambient but known relative humidity. Two humidity levels were used, approximately 12% and approximately 40%. The procedure utilized for the XRD analysis resulted in two potentially useful pieces of information; the observation of new phase formation/alteration of the clay lattice and changes induced in the hydration behavior due to CO2 interaction with the clay interlayer. For this set of experiments, no detectable new phases were formed and changes in the montmorillonite lattice except for layer expansion/contraction were not observed. Fig. 7 shows the change in the basal spacing of the montmorillonite exposed to the supercritical and liquid CO2 as a function of exposure time. All patterns were collected at about 12% relative humidity. The interlayer spacing of a montmorillonite is very sensitive to ambient relative humidity; therefore, even though it was not possible to control the relative humidity for these XRD measurements, it was carefully monitored so that observed changes could potentially be correlated to the CO2 treatment. Under the XRD measurement conditions, the (0 0 1) reflection peak of the as-received Na-rich SWy-2 at about 12% relative humidity is located at 8.2◦ 2, corresponding to a basal spacing of 10.7 A˚ which falls between the zero water molecules layer and one water molecule layer hydration state (Morodome and Kawamura, 2009; Ferrage et al., 2005; Hensen and Smit, 2002). Following exposure to both supercritical and liquid CO2 , the (0 0 1) reflection peak was shifted to a slightly lower angle (higher d-spacing) when equilibrated at an ambient relative humidity level of 12% as compared to the as-received clay. There was no observed trend in d-spacing relative to exposure duration for either supercritical or liquid CO2 treatment. However, the shift of the (0 0 1) peak exposed to supercritical CO2 appears to be slightly larger than that of the clay exposed to liquid CO2 . Fig. 8 shows the basal spacing of several montmorillonite samples – each one having been previously exposed to supercritical CO2 – plotted as a function of the CO2 exposure time. All these XRD patterns were collected at about 40% relative humidity. Under the XRD measurement conditions, the (0 0 1) reflection peak of the asreceived Na-rich, SWy-2 at about 40% relative humidity indicated a basal spacing of 12.2 A˚ which falls near the one water molecule layer
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be important. For example supercritical CO2 fluid reaching the cap rock could alter the cap rock, either by extracting water from the cap rock, with clay minerals hence more likely to shrink, or by promoting expansion of the clay structure in cap rock. Our results indicate that the formation of carbonate may be an additional factor to alter the cap rock by the structural deformation of the clay, which in some scenarios could lead to change in porosity and permeability. Disclaimer
Fig. 8. Powder X-ray diffraction patterns of the clay exposed to scCO2 , as a function of exposure time. Relative humidity was approximately 40%.
hydration state (Morodome and Kawamura, 2009; Ferrage et al., 2005; Hensen and Smit, 2002). Following exposure to supercritical CO2 , the (0 0 1) reflection peak was shifted to a slightly higher angle (lower d-spacing) when equilibrated at an ambient relative humidity level of 40% as compared to the as-received clay. At 40% relative humidity, a slight trend in d-spacing to exposure duration was observed. Longer supercritical CO2 exposure times appeared to result in less clay expansion. The trends observed in the behaviors of the clay samples exposed to CO2 and then equilibrated under the two relative humidity levels used suggest that the CO2 exposure did alter the clay gallery conditions and, as a result, the interaction of the clay gallery species with water. This change was reflected in the alteration of the hydration behavior relative to the unexposed clay. At this time, the specific mechanism causing this change is not precisely known but likely involves CO2 interaction with the interlayer cations, and possibly, the formation of “carbonate” species in the clay gallery from the reaction of exchangeable cations with CO2 , which would alter the electrostatic interactions within the clay structure and subsequently the hydration behavior (Hensen and Smit, 2002; Botan et al., 2010; Segad et al., 2010; Laird, 2006; Hines and Solin, 2000). 4. Conclusions This study examined the CO2 -hydrous clay mineral interactions relevant to cap rock and leakage conduits from an experimental point of view. The experimental results suggest that high-pressure carbon dioxide forms a mineral carbonate phase by interaction with hydrous Na-rich SWy-2 clay under the conditions used in this study. The carbonation reaction shows a strong dependency on the physical properties of the CO2 fluid, which are defined by the temperature, and the length of exposure time. The carbonation reaction may be affected by the water molecules adsorbed on the surface as well as intercalated into the interlayer region of the hydrophilic clay mineral. Dissolution of the host material at CO2 –hydrous clay mineral interfaces in the presence of high-pressure CO2 fluid is another possible factor as suggested by Shao et al. (2010, 2011) and Loring et al. (2011). However, currently, the detailed mechanism of the carbonation is not clear; therefore, these chemical reactions should be investigated in more detail in the future. CO2 storage reservoirs are often sealed by caprock formations with significant clay fractions. In this study, the carbonation reaction in expanding clay minerals has been constrained by the conditions of our experiments; however, additional factors could
This project was funded by the Department of Energy, National Energy Technology Laboratory, an agency of the United States Government, through a support contract with URS Energy & Construction, Inc. Neither the United States Government nor any agency thereof, nor any of their employees, nor URS Energy & Construction, Inc., nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Acknowledgment This technical effort was performed in support of the National Energy Technology Laboratory’s ongoing research in CO2 Trapping Mechanisms in Clay Materials under the RES contract DEFE0004000. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ijggc. 2012.12.001. References Adler, H.H., Kerr, P.F., 1963. Infrared absorption frequency trends for anhydrous normal carbonates. American Mineralogist 48, 124–137. Arroyo, L.J., Li, H., Teppen, B.J., Johnston, C.T., Boyd, S.A., 2005. Oxidation of 1naphthol coupled to reduction of structural Fe3+ in smectite. Clays and Clay Minerals 53, 587–596. Bansal, R.C., Vastola, F.J., Walker Jr., P.L., 1970. Studies on ultraclean carbon surfaces: II. Kinetics of chemisorption of oxygen on graphon. Journal of Colloid and Interface Science 32, 187–194. Barriga, C., Gaitán, M., Pavlovic, I., Ulibarri, M.A., Hermos˜ın, M.C., Cornejo, J., 2002. Hydrotalcites as sorbent for 2,4,6-trinitrophenol: influence of the layer composition and interlayer anion. Journal of Materials Chemistry 12, 1027–1034. Botan, A., Rotenberg, B., Marry, V., Turq, P., Neotinger, B., 2010. Carbon dioxide in montmorillonite clay hydrates: thermodynamics, structure, and transport from molecular simulation. Journal of Physical Chemistry C 114, 14962–14969. Bulut, E., Özacar, M., S¸engil, I˙ .A., 2008. Equilibrium and kinetic data and process design for adsorption of Congo Red onto bentonite. Journal of Hazardous Materials 154, 613–622. Chipera, S.J., Bish, D.L., 2001. Baseline studies of the Clay Minerals Society source clays: powder X-ray diffraction analyses. Clays and Clay Minerals 49, 398–409. Farmer, V.C., 1974. The Infrared Spectra of Minerals. Mineralogical Society, London, 539 pp. Ferrage, E., Lanson, B., Sakharov, B.A., Drits, V.A., 2005. Investigation of smectite hydration properties by modeling experimental X-ray diffraction patterns: part I. Montmorillonite hydration properties. American Mineralogist 90, 1358–1374. Gaus, I., Azaroual, M., Czernichowski-Lauriol, I., 2005. Reactive transport modelling of the impact of CO2 injection on the clayey cap rock at Sleipner (North Sea). Chemical Geology 217, 319–337. Gaus, I., 2010. Role and impact of CO2 –rock interactions during CO2 storage in sedimentary rocks. International Journal of Greenhouse Gas Control 4, 73–89.
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