Article pubs.acs.org/est
CO2 Sorption to Subsingle Hydration Layer Montmorillonite Clay Studied by Excess Sorption and Neutron Diffraction Measurements Gernot Rother,*,† Eugene S. Ilton,‡ Dirk Wallacher,§ Thomas Hauβ,§ Herbert T. Schaef,‡ Odeta Qafoku,‡ Kevin M. Rosso,‡ Andrew R. Felmy,‡ Elizabeth G. Krukowski,⊥ Andrew G. Stack,† Nico Grimm,§ and Robert J. Bodnar⊥ †
Geochemistry and Interfacial Science Group, Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6110, United States ‡ Pacific Northwest National Laboratory, P.O. Box 999, MS K8-98, Richland, Washington 99352, United States § Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner Platz 1, D-14109 Berlin, Germany ⊥ Fluids Research Laboratory, Virginia Tech, Blacksburg, Virginia 24061, United States S Supporting Information *
ABSTRACT: Geologic storage of CO2 requires that the caprock sealing the storage rock is highly impermeable to CO2. Swelling clays, which are important components of caprocks, may interact with CO2 leading to volume change and potentially impacting the seal quality. The interactions of supercritical (sc) CO2 with Na saturated montmorillonite clay containing a subsingle layer of water in the interlayer region have been studied by sorption and neutron diffraction techniques. The excess sorption isotherms show maxima at bulk CO2 densities of ≈0.15 g/cm3, followed by an approximately linear decrease of excess sorption to zero and negative values with increasing CO2 bulk density. Neutron diffraction experiments on the same clay sample measured interlayer spacing and composition. The results show that limited amounts of CO2 are sorbed into the interlayer region, leading to depression of the interlayer peak intensity and an increase of the d(001) spacing by ca. 0.5 Å. The density of CO2 in the clay pores is relatively stable over a wide range of CO2 pressures at a given temperature, indicating the formation of a clay-CO2 phase. At the excess sorption maximum, increasing CO2 sorption with decreasing temperature is observed while the highpressure sorption properties exhibit weak temperature dependence.
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INTRODUCTION Rising levels of carbon dioxide in the atmosphere are seen as major drivers of global climate change and ocean acidification. In geologic carbon storage (GCS), CO2 is compressed and pumped into porous, permeable underground reservoir rocks.1,2 The majority of the deposited CO2 will, at least initially, reside in the pore spaces as a supercritical fluid (scCO2) with mass density of about 0.3−0.8 g/cm3 at the depths being considered for geological storage.3 Even a CO2rich phase saturated with brine is still significantly less dense than the coexisting brine. Because the CO2 plume formed at the injection point is likely to be buoyant, it needs to be sealed with an overlying caprock formation, which is impermeable and stable against chemical alteration by scCO2 and brine. Thick layers of clay-rich shales or mudstones with high stability and without fractures are studied for their suitability.4,5 Caprock formations can be considered, to a first approximation, as mixtures of quartz and clay.6 While quartz is highly inert, certain clays may expand when exposed to water and some other fluids, which could be advantageous for sealing purposes. Montmorillonite (MM) clays, which are comprised of sheets of phyllosilicates with intercalated cation layers with variable water © XXXX American Chemical Society
saturations, are an important class of clays and have been extensively studied for their impacts on geomechanical properties of soils and for applications in radioactive waste storage.7 The atomic structure of MM is shown in Figure 1.8 The generic sum formula for Na-MM is Na0.33(Al1.67Mg0.33)(Si4O10)(OH)2•(H2O)x. The unit cell contains 2 formula units and has in-plane dimensions of a0 = 5.17 Å and b0 = 8.94 Å. From these data, the surface area presented in the clay interlayers is calculated to 92.4 Å2/unit cell or 690 m2/g for x = 2. The layer periodicity d001 varies between ca. 10 Å for dehydrated MM (x = 0 or 0W), ca. 12.4 Å for single layer hydrated (1W) Na-MM (x = 2), and ca. 15.2 Å for bilayer hydrated (2W) clay (x = 4).9 The bulk density varies with hydration state. From the crystallographic data, respective mass Special Issue: Carbon Sequestration Received: April 11, 2012 Revised: August 17, 2012 Accepted: August 23, 2012
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density peak of the water layer(s).26 No swelling of the full monolayer or bilayer hydrated clays was observed upon addition of CO2, in agreement with Giesting et al.25 By combination of excess sorption with neutron diffraction (ND) measurements, we studied interactions of anhydrous scCO2 with Na-MM (STX-1) with the goal to link the CO2 sorption behavior to the nanoscale structural properties of the sample. Specific objectives were to (1) relate the basal expansion to a quantified amount of CO2 sorption and (2) seek direct evidence for CO2 intercalation.
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EXPERIMENTAL SECTION Naturally occurring low iron MM (STX-1), obtained from the Clay Mineral Repository (Columbia, MO), that is well characterized,27−34 was used as a starting material. The as received clay, which has virtually all the layer charge emanating from the octahedral sites, has the published structural formula (Ca0.27 Na0.04 K0.01)(Al2.41 Fe3+0.09 Mntr Mg0.71Ti0.03)2Si8O20(OH)4. The clay was first reacted with 1 M sodium acetate buffer at pH 5 for 2 weeks to remove traces of calcite.35 Then, the acetate was removed by dialyzing the clay in deionized water (DI) until the conductivity of the washes was 18 MΩ cm. Separation of the 0.2 g/cm3, the pore fluid density remains approximately constant at ρp ≈ 0.25−0.4 g/ cm3 with increasing bulk density, which is consistent with zero excess sorption at ρb ≈ 0.3−0.4 g/cm3. Pore fluid depletion, relative to the bulk fluid, is found at even higher values of ρb. The pore densities in the depletion region appear to be only weakly temperature dependent, which is in agreement with the recently studied sorption behavior of CO2 to porous silica.41 The obtained value of the CO2 pore density (i.e., ρp ≈ 0.25− 0.4 g/cm3) can be compared to the peak height of the interlayer Bragg peak. The scattering intensity scales with the square of the difference in SLD between the two phases. The pore CO2 phase has an approximate SLD of ρ*p ≈ 0.8 Å−2 (calculated for an averaged ρp = 0.32 g/cm3). The contrast term would change from 16 · 10−12 Å−4 to ca. 11.6 · 10−12 Å−4, a reduction by ca. 25%. Indeed, the actual reduction in peak intensity is on the order of 20%, a good agreement considering the number of uncertainties contained in the calculation. As discussed above, the initial sample (before CO2 saturation) showed scattering contributions from 0W hydration fractions. For the CO2 saturated samples, all scattering is centered around the peak characteristic for the 1W hydration state. The small difference between the two results is attributed to a number of experimental uncertainties. There is some CO2 sorption to outer clay grain surfaces that is not visible in the neutron data. Separation of outer grain from interlayer sorption is impossible from our data, but it can be expected that at low pressure sorption takes place preferentially in the pore spaces, where fluid molecules may interact with the two clay sheet surfaces forming the interlayer pore simultaneously. At higher fluid density, fluid saturation of the interlayer spaces takes place and multilayer sorption at the outer grain boundaries may become more prominent, leading to values of the excess E
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(4) Benson, S. M.; Cole, D. R. CO2 sequestration in deep sedimentary formations. Elements 2008, 4, 325−331. (5) 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. In situ infrared spectroscopic study of forsterite carbonation in wet supercritical CO2. Environ. Sci. Technol. 2011, 45, 6204−6210. (6) Cole, D. R.; Chialvo, A. A.; Rother, G.; Vlcek, L.; Cummings, P. T. Supercritical fluid behavior at nanoscale interfaces: Implications for CO2 sequestration in geologic formations. Philos. Mag. 2010, 90, 2339−2363. (7) Meunier, A.; Velde, B.; Griffault, L. The reactivity of bentonites: A review. An application to clay barrier stability for nuclear waste storage. Clay Miner. 1998, 33, 187−196. (8) Tsipursky, S. I.; Drits, V. A. The distribution of octahedral cations in the 2:1 layers of dioctahedral smectites studied by oblique-texture electron diffraction. Clay Miner. 1984, 19, 177−193. (9) Strunz, H.; Nickel, E. H. Strunz Mineralogical Tables, 9th ed.; E. Schweizerbart’sche Verlagsbuchhandlung: Stuttgart, Germany, 2001. (10) Sposito, G.; Prost, R.; Gaultier, J.-P. Infrared spectroscopic study of adsorbed water on reduced-charge Na/Li monts. Clays Clay Miner. 1983, 31, 9−16. (11) Segad, M.; Jönsson, B.; Åkesson, T.; Cabane, B. Ca/Na mont: Structure, forces and swelling properties. Langmuir 2010, 26, 5782− 5790. (12) Devineau, K.; Bihannic, I.; Michot, L.; Villiéras, F.; Masrouri, F.; Cuisinier, O.; Fragneto, G.; Michau, N. In situ neutron diffraction analysis of the influence of geometric confinement on crystalline swelling of mont. Appl. Clay Sci. 2006, 31, 76−84. (13) De Siqueira, A. V.; Lobban, C.; Skipper, N. T.; Williams, G. D.; Soper, A. K.; Done, R.; Dreyer, J. W.; Humphreys, R. J.; Bones, J. A. R. The structure of pore fluids in swelling clays at elevated pressures and temperatures. J. Phys.: Condens. Matter 1999, 11, 9179−9188. (14) Schoonheydt, R. A.; Johnston, C. T. Surface and Interface Chemistry of Clay Minerals. In Handbook of Clay Science; Bergaya, F., Theng, B. K. G., Lagaly, G., Eds.; Elsevier Ltd.: London, 2006; Vol. 1. (15) Bildstein, O.; Kervevan, C.; Lagneau, V.; Delaplace, P.; Credoz, A.; Audigane, P.; Perfetti, E.; Jacquemet, N.; Jullien, M. Integrative modeling of caprock integrity in the context of CO2 storage: Evolution of transport and geochemical properties and impact on performance and safety assessment. Oil Gas Sci. Technol.-Revue De L Institut Francais Du Petrole 2010, 65, 485−502. (16) Credoz, A.; Bildstein, O.; Jullien, M.; Raynal, J.; Petronin, J. C.; Lillo, M.; Pozo, C.; Geniaut, G. Experimental and modeling study of geochemical reactivity between clayey caprocks and CO2 in geological storage conditions. Greenhouse Gas Control Technologies (GHGT-9); 2009; GCCC Digital Publication Series #08-03d; pp 3445−3452. (17) Shao, H. B.; Ray, J. R.; Jun, Y. S. Effect of salinity and the extent of water on supercritical CO2-induced phlogopite dissolution and secondary mineral formation. Environ. Sci. Technol. 2011, 45, 1737− 1743. (18) Shukla, R.; Ranjith, P.; Haque, A.; Choi, X. A review of studies on CO2 sequestration and caprock integrity. Fuel 2010, 89, 2651− 2664. (19) Schaef, H. T.; Ilton, E. S.; Qafoku, O.; Martin, P. F.; Felmy, A. R.; Rosso, K. M. In situ XRD study of Ca2+ saturated MM (STX-1) exposed to anhydrous and wet supercritical carbon dioxide. Int. J. Greenhouse Gas Control 2012, 6, 220−229. (20) Gaus, I. Role and impact of CO2-rock interactions during CO2 storage in sedimentary rocks. Int. J. Greenhouse Gas Control 2010, 4, 73−89. (21) Busch, A.; Alles, S.; Gensterblum, Y.; Prinz, D.; Dewhurst, D. N.; Raven, M. D.; Stanjek, H.; Krooss, B. M. Carbon dioxide storage potential of shales. Int. J. Greenhouse Gas Control 2008, 2, 297−308. (22) Loring, J. S.; Schaef, H. T.; Turcu, R. V. F.; Thompson, C. J.; Miller, Q. R. S.; Martin, P. F.; Hu, J.; Hoyt, Qafoku, O.; Ilton, E. S.; Felmy, A. R.; Rosso, K. M. In situ molecular spectroscopic evidence for CO2 intercalation into MM in supercritical carbon dioxide. Langmuir 2012, 28, 7125−7128.
are the nature of caprocks overlying the storage reservoir and the behavior of the clay-rich caprock following scCO2 injection. Our study shows that the interlayer region of a sub-1W Na montmorillonite accommodates limited amounts of CO2, which induces swelling in the z-direction by 0.5 Å; i.e., the expansion in the z-direction is on the order of 4%. The expansion determined for Na montmorillonite clays containing a higher fractions of 0W has been reported to reach 9%.25 Our data and the study by Schaef et al.19 suggest that CO2 does not displace H2O when entering the sub 1W interlayer region but rather makes room by pushing the structural units apart. It appears that scCO2 interacts with Na montmorillonite such that it partially substitutes for missing water molecules. This allows the initially under saturated system to assume a more homogeneous and perhaps thermodynamically more stable configuration with a d(001) typical for 1W hydration. The observed swelling of sub-1W Na MM as a result of interaction with scCO2 may lead to sealing of small cracks in the caprocks, enhancing storage security and reducing the possibility of leakage. Moreover, changes in pressure and temperature do not change the clay nanoscale structure, indicating that seal quality should not degrade as pressure changes during and following CO2 injection. We also note that the intercalation of CO2 into swelling clay interlayers provides an additional trapping mechanism, which can increase by a small amount the storage capacity in clay rich formations without degradation of seal quality.
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ASSOCIATED CONTENT
S Supporting Information *
More information about the experimental methods and additional Na-MM characterization results. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 865 574 2741; fax: 865 574 4961; e-mail: rotherg@ ornl.gov. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. David J. Wesolowski, Dr. Andreas Busch, Yves Gensterblum, and Pieter Bertier for helpful discussions and Dr. Dzombak and anonymous reviewers for valuable comments. This work was supported by the U.S. Department of Energy’s Office of Basic Energy Sciences, Geosciences Program through a Single Investigator Small Group research grant awarded to PNNL. E.G.K. and R.J.B. were supported through the NETLRegional University Alliance (NETL-RUA) in support of the National Energy Technology Laboratory’s ongoing research in carbon sequestration under the RES Contract DE-FE0004000.
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