Article pubs.acs.org/EF
Deep-Sea Field Test of the CH4 Hydrate to CO2 Hydrate Spontaneous Conversion Hypothesis Peter G. Brewer,*,† Edward T. Peltzer,† Peter M. Walz,† Elizabeth K. Coward,†,‡ Laura A. Stern,§ Stephen H. Kirby,§ and John Pinkston§ †
Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039, United States United States Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, United States
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ABSTRACT: We have carried out a small-scale deep-sea field test of the hypothesis that CH4 gas can be spontaneously produced from CH4 hydrate by injection of a CO2/N2 gas mixture, thereby inducing release of the encaged molecules with sequestration of the injected gas. Pressure cell studies have shown that, under some pressure and temperature conditions, this gas mixture can induce formation of a solid N2/CO2 hydrate with no associated liquid water production. We transported a cylinder of pure CH4 hydrate, contained within a pressure vessel, to the sea floor at 690 m depth off shore Monterey, CA, using the remotely operated vehicle (ROV) Ventana. Upon opening the pressure vessel with the vehicle robotic arm, we emplaced the hydrate specimen on a metal stand and covered this with a glass cylinder full of a 25% CO2/75% N2 gas mixture, thereby fully displacing the surrounding seawater (T = 4.92 °C). We observed complete and rapid dissociation of the CH4 hydrate with release of liquid water and creation of a mixed gas phase. This gas composition will undergo transition over time because of the high solubility of CO2 in the displaced water phase. We show that the experimental outcome is critically controlled by the injected gas/hydrate/water ratio.
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in the technical reports10 of a project executed in the Arctic Ignik Sikumi well 1 in early 2012 (http://www.netl.doe.gov/ research/oil-and-gas/methane-hydrates/co2_ch4exchange). This case of a more open system is quite different from the suite of contained laboratory pressure vessel studies that have thus far been undertaken. We note that the primary aim of such efforts is the safe and efficient recovery of CH4 as a fuel from hydrate deposits while simultaneously sequestering CO2 as a climate amelioration strategy. The sequestered CO2 can be disposed of safely in a geologic formation in any form (gaseous, dissolved, or solid hydrate phase), although Park et al.9 note that, for recovery of CH4 from oceanic hydrates, the formation of a CO2−N2 hydrate system might enable the sea floor to remain stabilized even after CH4 recovery because, with complete cage swapping, the same crystal structure would be maintained. However, the permanence of any CO2 hydrate within an oceanic system is uncertain because of the ∼10× greater solubility of CO2 relative to CH4 in the formation waters, and CO2 hydrates have been shown to undergo rapid dissolution in seawater.11 We have conducted two remotely operated vehicle (ROV)enabled deep-sea field experiments to test the exchange process on a liter scale that is large enough for fluid flow within the apparatus to occur and that allows for visual inspection of the course of events. We followed the evolving chemical signals in real time by in situ Raman spectroscopy.12,13 We did not include sedimentary material or other inorganic solid phases in our experiments as has been performed in many laboratory pressure vessel studies, thus allowing for better visualization of
INTRODUCTION Displacement of CH4 by CO2 in methane hydrates has been proposed as a mechanism for sequestering CO 2 and simultaneously accessing the methane reserve stored within the large hydrate deposits occurring along continental margins and permafrost areas. The proposal is based on the greater thermodynamic stability of CO2 hydrate over CH4 hydrate that potentially allows for an energetically favorable path. The injection of liquid CO2 into CH4 hydrate with conversion to CO2 hydrate has been reported from laboratory experiments in high-pressure cells with both synthetic and recovered hydrate samples.1−8 The CO2 conversion hypothesis was extended by Park et al.,9 who used a CO2−N2 gas mixture for the displacement−conversion reaction, noting that this provided a more favorable pathway by incorporation of CO2 into the large cages and N2 dominantly into the small cages of the hydrate structure. Moreover, this represents a more typical industrial combustion gas mixture and, thus, could eliminate the requirement for a costly CO2 separation procedure. They have proposed a mechanism for this, suggesting that it is a simple one-for-one substitution driven by the CO 2−N2 displacement gas mixture, in which the gas molecules in the cage structure shift and exchange, without the formation of liquid water, and CH4 is released to allow for the formation of a CO2−N2 hydrate structure, which is thermodynamically favored under specific pressure and temperature conditions.9 How this exchange would occur with gas substitution into the water cages without first cage breaking has not yet been fully described. Equally unclear is how such a process would occur on a large scale in situ in an unstirred geologic environment, where industry-scale gas injection would displace formation waters and both long-term CH4 gas extraction and CO2 sequestration is desired. One example of this is described © 2014 American Chemical Society
Received: June 27, 2014 Revised: September 30, 2014 Published: October 6, 2014 7061
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Figure 1. Image of the first field test to investigate CH4 displacement from a natural hydrate by the introduction of a separate gas phase, in this case N2. The dimensions of the experiment were approximately gas phase volume ≈ 1.7 L, sea water volume ≈ 1.7 L, and solid hydrate ≈ 200 mL. The ocean conditions are T = 4.22 °C, P = 8.0 MPa, and S = 34.29%.
Figure 2. Image of the in situ Raman probe being inserted upward into the experimental chamber. The sintered metal filter below the probe tip is shown here just below the gas−water interface.
the course of the experiment and permitting the spectroscopic analysis on the sea floor. This is not expected to influence the nature of the chemical reactions. We were not able to access field pressure and temperature properties that exactly
reproduced the pressure cell conditions by Park et al. (1200 dbar and 1 °C) because this represents a quite restricted ocean region. The depth and temperature regimes that we investigated are typical for the majority of oceanic hydrate 7062
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deposits.14 Here, we detail the execution of these experiments and the present results.
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the limit for displacement is about 53% CH4 in the gas space, at which time a new hydrate phase would form. Second Field Test. The success of the first experimental approach provided essential guidance for the time scale required and hints for an improved experimental technique. Therefore, a second, more controlled, experiment was carried out in the deep waters of Monterey Bay, CA, by adapting techniques pioneered by Rehder et al.11 In that study, samples of pure CH4 hydrate and CO2 hydrate were transported to a seafloor test site at 1000 m depth in Monterey Bay and monitored for 27 h while exposed to the ambient seawater. A major portion of the CH4 hydrate remained intact at the end of that experiment, while the CO2 hydrate rapidly dissolved into the surrounding seawater. For both the Rehder et al.11 experiment and the current work, pure, polycrystalline, sI methane hydrate was synthesized by previously established methods17 involving the warming and static conversion of a measured mass of granular water ice ( 5), then the percentage loss of CO2 from the gas phase is relatively small and the impact on the CO2/N2 hydrate stability boundary is greatly reduced. However, because methane is released from the gas hydrate as a result of the CO2−CH4 exchange reaction, CH4 is greatly diluted by the larger volume of injected gas. This reduces the upper limit of the methane concentration in the pore space, and from this, its recovery is made all the more difficult. The impact of the volume of injected gas to the volume of hydrate or residual pore water on the CO2/N2 hydrate stability boundary and the mixed hydrate (CH4/CO2/N2) stability boundaries can be seen in Figure 12. In Figure 12A, the stability boundary is calculated for a mixed gas of 20% CO2 and 80% N2 diluted with 0−100% CH4 with a low gas/liquid volume ratio (for 0% methane, the gas is just 20% CO2 and 80% N2; for 50% methane, it is 10% CO2, 40% N2, and 50% methane; and 100% CH4 is simply methane without CO2 or N2). In this case, much of the CO2 in the gas mixture is dissolved in the water phase and, thus, unavailable to make hydrate. In Figure 12B, these same stability boundaries are recalculated for the same gas mixtures but with a large gas/ liquid water ratio. In this case, a much greater fraction of CO2 is available to make hydrate, and the stability boundaries are shifted to the right (higher temperature at constant pressure) and upward (lower pressure at constant temperature). Because of the limited solubility of methane in water, its stability boundary remains relatively unshifted. From these observations and calculations, it would appear to be very difficult to execute a meaningful controlled displacement of CH4 gas from a subsea geologic hydrate formation. Simple injection of N2 gas can result in displacement of the formation waters, at which point rapid dissociation of CH4 can occur, with cage breakdown, cooling, and pooling of the water liberated from the hydrate. In the case of a CO2-enriched gas injection, much the same process would occur but with the added complexity of the greater tendency for the CO2 component to dissolve in both the original formation waters and the fresh water liberated from hydrate cage breakdown. This would result in a continued drift in gas-phase composition, making control difficult. The gas phase liberated would appear to be a CH4-enriched gas but by no means pure CH4. From Figure 12, it appears that under the pressure and temperature conditions of Park et al.,9 a limit of only 5−10% CH4 in the produced gas is possible, whereas perhaps 50% CH4 could be achieved under the higher temperature and lower pressure conditions of the field work at Hydrate Ridge. In no case were we able to observe CO2 hydrate formation, and in no case were we able to observe production of CH4 without direct cage breakdown and liberation of the clathrate water phase. A comparison of these results to largescale field tests in geologic formations may be difficult in the details, but overall, the result is not dissimilar to that from the technical reports of a project at the Ignik Sikumi well 1 in early 2012.10
gas−formation water interface around the reaction zone. Here, the greater solubility of CO2 compared to either CH4 or N2 must come into play, and it appears likely that CO2 depletion in the gas phase will occur over time and very possibly at a faster rate than any significant accumulation of CO2 hydrate can form. If a skin of CO2 hydrate did form, then this would also slow any further exchange with the gas medium because the diffusion rates of CH4 and CO2 in the solid hydrate phase are much slower than in the gas or liquid phases. It is possible to examine what steady-state conditions might be achieved if the gas conditions can be held constant. In Figure 11, we show the phase behavior of a CH4 hydrate−N2 gas
Figure 11. Example calculation of the phase behavior of CH4 displacement by injection of N2 gas for comparison to our pilot experiment at Hydrate Ridge, OR.15 The red + symbol indicates the site conditions and shows that an equilibrium point of about 50% N2/ 50% CH4 gas would be achieved.
system that covers the phase space included in our initial Hydrate Ridge natural hydrate displacement experiment (as shown in Figure 1). In Figure 12, we show a similar calculation for the phase behavior of a mixed gas (20% CO2 and 80% N2) similar to the gas experiment executed in Monterey Bay, CA. In this figure, we show the pressure and temperature conditions at our two field sites and for the Park et al.9 laboratory pressure vessel experiment. While a wide range of final CH4 gas-phase concentrations will result depending upon the initial conditions of the gas/hydrate/water ratio, for simplicity, we show only a limited set of final gas mixes than might be achieved when the new hydrate stability equilibrium condition is met. Our interpretation of the results is that the experimental outcome is strongly dependent upon the initial gas/hydrate/ water ratios. For example, when the volume of mixed gas (20% CO2/80% N2) added to a hydrate-bearing sediment is small relative to the volume of hydrate or residual pore water (moles of CO2/moles of liquid water < 0.5), an appreciable fraction of CO2 becomes dissolved in the liquid water, reducing the volume of the gas phase, which becomes enriched in nitrogen. This has the dual consequence of decreasing the concentration of CO2 available for the methane-swapping reaction and leading to the decomposition of the methane hydrate because the 7067
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Figure 12. Phase behavior of displacement of CH4 from a SI CH4 hydrate with a 20% CO2 plus 80% N2 gas showing the pressure and temperature conditions at (×) the “Hydrate Hill” OR 690 m site15 and (+) the “Hydrate Hotel” 1000 m site11 in Monterey Bay and (○) the laboratory pressure cell conditions reported by Park et al.9 In panel A, the stability boundaries are calculated for a mixed gas of 20% CO2 and 80% N2 diluted with 0− 100% CH4 with a low gas/liquid volume ratio. In this case, much of the CO2 in the gas mixture is dissolved in the water phase and, thus, unavailable to make hydrate. In panel B, these same stability boundaries are recalculated for the same gas mixtures but with a large gas/liquid water ratio. In this second case, a much greater fraction of CO2 is available to make hydrate because it has not dissolved in the water phase. As a result, the stability boundaries are shifted to the right (higher temperature at constant pressure) and upward (lower pressure at constant temperature). Because of the limited solubility of methane in water, its stability boundary remains relatively unshifted between the two examples.
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International Conference on Gas Hydrates; Vancouver, British Columbia, Canada, July 6−10, 2008; Paper 5635. (3) Zhou, X.; Fan, S.; Liang, D.; Du, J. Replacement of methane from quartz-bearing hydrate with carbon dioxide-in-water emulsion. Energy Fuels 2008, 22 (3), 1759−1764. (4) Espinoza, D. N.; Santamarina, J. C. P-wave monitoring of hydrate-bearing sand during CH4−CO2 replacement. Int. J. Greenhouse Gas Control 2011, 5, 1031−1038. (5) Schicks, J. M.; Luzi, M.; Beeskow-Strauch, B. The conversion of hydrocarbon hydrates into CO2 hydrates and vice versa: Thermodynamic considerations. J. Phys. Chem. A 2011, 115, 13324−13331. (6) Lee, H.; Yongwon, S.; Sea, Y.-T.; Moudrakovski, I. L.; Ripmeester, J. A. Recovering methane from solid methane hydrate with carbon dioxide. Angew. Chem. 2003, 115, 5202−5205. (7) Husebø, J.; Stevens, J. C.; Graue, A.; Kvamme, B.; Baldwin, B. A.; Howard, J. J. Experimental investigation of methane release from hydrate formation in sandstone through both hydrate dissociation and CO2 sequestration. Proceedings of the 6th International Conference on Gas Hydrates; Vancouver, British Columbia, Canada, July 6−10, 2008; Paper 5636. (8) Graue, A.; Kvamme, B.; Baldwin, B. A.; Stevens, J.; Howard, J. Magnetic resonance imaging of methane−carbon dioxide hydrate reactions in sandstone pores. Proceedings of the SPE Annual Technical Conference and Exhibition; San Antonio, TX, Sept 24−27, 2006; SPE Paper 102915. (9) Park, Y.; Kim, D. Y.; Lee, J. W.; Huh, D. G.; Park, K. P.; Lee, J.; Lee, H. Sequestering carbon dioxide into complex structures of naturally occurring gas hydrates. Proc. Natl. Acad. Sci. U. S. A. 2006, 103 (34), 12690−12694. (10) http://www.netl.doe.gov/research/oil-and-gas/methanehydrates/co2_ch4exchange. (11) Rehder, G.; Kirby, S. H.; Durham, W. B.; Stern, L. A.; Peltzer, E. T.; Pinkston, J.; Brewer, P. G. Dissolution rates of pure methane hydrate and carbon dioxide hydrate in under-saturated seawater at 1000 m depth. Geochim. Cosmochim. Acta 2004, 68 (2), 285−292. (12) Brewer, P. G.; Malby, G.; Pasteris, J. D.; White, S. N.; Peltzer, E. T.; Wopenka, B.; Freeman, J.; Brown, M. O. Development of a laser Raman spectrometer for deep-ocean science. Deep Sea Res., Part I 2004, 51, 739−753.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address ‡
Elizabeth K. Coward: Earth and Environmental Science Department, University of Pennsylvania, 240 South 33rd Street, Philadelphia, Pennsylvania 19104-6316, United States. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by a grant to the Monterey Bay Aquarium Research Institute from the David and Lucile Packard Foundation. The authors thank the captains and crew of the R/V Point Lobos and R/V Western Flyer and the skilled pilots of the ROVs Ventana and Doc Ricketts, who made deployment and operation of the various experiments possible. Support for Laura A. Stern and John Pinkston was provided in part by Interagency Agreement DE-FE0002911 between the United States Geological Survey Gas Hydrate Project and the United States Department of Energy (DOE)’s Methane Hydrate Research and Development Program. The authors thank T. Lorenson and P. Hart (both at the United States Geological Survey) for helpful reviews of this manuscript and two anonymous reviewers for helpful comments that much improved the manuscript.
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