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Direct observations of three dimensional growth of hydrates hosted in porous media
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Prasad Kerkar1, Keith W. Jones2, Robert Kleinberg3, W. Brent Lindquist4, Stan
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Tomov5, Feng Huan6, Devinder Mahajan1, 2*
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We present the first visualization of time-resolved 3-D growth of tetrahydrofuran
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(THF) hydrates in porous media using X-ray computed microtomography (CMT).
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The 1119μm x 1630μm x 1443μm volume rendered from a stack of images shows
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patchy hydrates formed from excess THF in aqueous solution. Hydrate growth is
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found convex away from the grains, showing that liquid, not hydrate, is the wetting
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phase. This is similar to ice growth in porous media in which mineral grains are
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coated with unfrozen water films1, 2. Hydrates formed at several locations in the
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system before the first images were taken, 28 hours after the system was cooled to
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hydrate-forming temperature. The size and shape of hydrate patches varied within
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a sample of glass spheres of uniform size, consistent with the random nature of
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nucleation and growth. Tracking individual grains in tomoscans taken over a three
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day period indicated grain movement during the hydrate growth. No container-wall
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effect was observed. The extension of the observed growth behavior to methane
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hydrate could have implication in understanding the role of hydrate in seafloor
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stability3 and climate change4.
Material Science Department, Stony Brook University, Stony Brook, NY 11794, USA
Brookhaven National Laboratory, Upton, NY 11793-5000, USA
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Schlumberger Doll Research, Cambridge, MA 02139, USA
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Department of Applied Mathematics and Statistics, Stony Brook University, NY 11794
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Computer Science Department, University of Tennessee, Knoxville, TN 37996
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Department of Earth and Environmental Studies, Montclair State University, Montclair,
NJ 07043
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Gas hydrates are known to occur worldwide in locations such as the permafrost
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regions of Siberia5, the Mackenzie Delta6, and the Prudhoe Bay and Kuparuk River oil
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fields7 on the North Slope of Alaska. Gas hydrates have also been found beneath the
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seafloor offshore Vancouver and Oregon8, South Carolina9, Costa Rica10, Japan, and
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E-mail: dmahajan@bnl.gov
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India. Methane hydrates are prevalent where high pressure and low temperature
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conditions naturally coexist11, 12. Oceanic hydrates are found up to a few hundred meters
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below seafloor. Though earlier methane hydrate research was solely driven by the need to
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avoid natural gas pipeline plugging13, the quest to develop this potentially vast but
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unconventional energy resource is now fueling global research on the subject. Moreover,
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since CH4 is about 17 times more potent as a greenhouse gas than CO2, the possibility of
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rapid release through methane hydrate dissociation is of concern14. If hydrates are
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encountered during drilling to deeper hydrocarbon targets, their unintentional dissociation
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could lead to a blowout, loss of support for pipelines, and sea-floor failure resulting in
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underwater landslides. Therefore, a thorough understanding of geochemical and
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geotechnical aspects of hydrate and its surroundings, including the structure of hydrate in
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sediments during growth and dissociation is of interest.
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cementing at grain contacts, grain coating, grain supporting, pore filling, or massive15.
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The microstructural model of sediment-hydrate interaction governs the mechanical
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strength of the formation or wellbores3 that may have consequence in the event of release
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of methane. Yun et al.16 conducted a study on the THF hydrates in sediment system and
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found the greatest impact of hydrate on the skeletal stiffness of the sediments at a hydrate
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concentration of >40%.
Hydrates are classified as
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X-ray computed microtomography (CMT) holds considerable potential for
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revealing the pore scale interaction between hydrate and the mineral grains of rocks,
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soils, and sediments. In a study of hydrate sample cores recovered from the seafloor of
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the Amazon Fan, Soh17 reported images of a plume-shaped, fluidized structure containing
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gas bubbles that indicated dissociation of a relatively large nodule of gas hydrate. Mikami
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et al.18 produced CT images of samples collected from the JAPEX/JNOC/GSC Mallik
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2L-38 exploratory well to demonstrate that gas hydrates dissociated simultaneously on
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exposed surfaces and within the pore spaces of granular sand cores during
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depressurization. Jin et al.15 characterized the porosity from the 2-D images of 10-mm
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thick cylinders cut from artificial methane hydrate sediments produced at -30oC and 10
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MPa. Freifeld and Kneafsey19 prepared a synthetic methane hydrate sample in a 28.6-mm
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diameter and 32-mm long pressure vessel using packed 12/20 mesh Ottawa sand (30%
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porosity). A cross-section of the prepared sample was imaged with 200 m resolution in
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a portable CT scanner. The 2-D images showed hydrate dissociation initiating at the walls
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of the vessel and progressing inwards. Follow-up 2-D imaging by Kneafsey20 presented
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local temperature and density changes during methane hydrate formation and dissociation
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in a partially saturated sand sample formed in an X-ray transparent aluminum vessel.
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Sato21 investigated the density and hydrate saturation distribution of methane hydrates in
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Toyoura sand (average grain size 0.2 mm) at ~10 MPa and -30oC with CMT.
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Though most natural deposits contain methane as the primary guest gas molecule
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which forms Structure I hydrates, sites such as the Mississippi Canyon 852/853 (at
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~1050-1060 m water depth) in the Gulf of Mexico contain Structure II hydrates of mixed
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(C1-C5) hydrocarbons22. The production of methane hydrates in the laboratory requires
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high pressures (~10 MPa) and low temperatures (~ 0°C). However a 19 wt% solution of
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tetrahydrofuran and water (stoichiometric mole ratio of THF/H2O = 1/17) forms Structure
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II hydrate at 4.4oC and atmospheric pressure (Figure 1), making THF a convenient
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surrogate to understand hydrate growth. Mork et al.23 performed NMR imaging of THF
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hydrate in Quartz sand. However the attempts to acquire CT images were unsuccessful
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due to the negligible density difference between the stoichiometric THF-water mixture
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(0.978 g/cm3) and THF hydrate (0.971 g/cm3). Tohidi et al.24 visually observed THF
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hydrate in glass micromodels. In coarse grained micromodels (0.313 mm feature size)
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THF hydrate grew in the centers of pores, leaving liquid water at the solid surfaces. In
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finer grained micromodels (0.070 mm feature size) hydrates encapsulated the grains.
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Recently, Takeya et al.25 performed experiments with 19 wt% THF solution at a 35 keV
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monochromatic synchrotron x-ray beam line to reveal the density difference of THF
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hydrate in a 3-D image. However, utilization of the time-resolved CMT technique to
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monitor the growth of methane or THF hydrate has not been reported.
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In the present CMT study, we focused on visualization of hydrate growth
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phenomena at a micro scale (total volume ~1 mm3) in a THF/H2O/BaCl2/glass bead
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system. The use of THF as a surrogate for methane allowed convenient operation at
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ambient pressure in the beamline. The particle size distribution of natural sediments
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typically span a broad range, so a uniform packing of 500 μm-sized glass beads was used
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to remove uncertainty related to this heterogeneity. BaCl2 was used to enhance the
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density contrast between aqueous THF solution and THF-hydrate; it also helpfully
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lowered the freezing point of the solution to -6.85oC. To initiate THF-hydrate formation,
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sample was cooled with a circulating fluid at -3°C. Hydrate formation was monitored
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over three days and the resulting data was processed using a multi-step data
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reconstruction procedure that produced 2-D and 3-D images.
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The hydrate formation appears to start at a few locations in the system before the
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first images were taken at 28 hours. Figure 2 shows the growth pattern of THF hydrate
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and its interaction with glass beads. Time lapse bead-to-bead matching indicates that the
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growth of hydrates displaces beads within the unconsolidated pack. Further, the 2-D
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images from the stack show that the hydrate size and shape is independent of container-
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walls. These observations are consistent with previous NMR23 and visual observations24
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and the random nature of the nucleation process. A magnified image of one of the
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growing hydrates from Figure 2 is shown in Figure 3. Clearly, the hydrates grow in pores,
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similar to the pore-filling model described by Dvorkin et al26. This implies progressive
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but significant reduction of mechanical strength of the sediment upon dissociation of
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hydrates from pore walls by retraction from the pore wall followed by shrinkage in the
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pore space3. The hydrate dissociation from large pores may trap gas within pores until
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hydrate saturation reaches low values, permitting the flow of gas. The 2-D hydrate
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growth is found convex away from the grains, showing clearly in Figure 3 that THF, not
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hydrate, is the wetting phase in the form of thin film with thickness less than 37 µm.
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This is analogous to ice growth in porous media in which a water film remains unfrozen1
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and consistent with the contact angle arguments of Miller27 and Clennell28.
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Figure 4a shows a 1119μm x 1630μm x 1443μm volume after 29 hours of
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cooling. The density based transfer function was selected to show only the growth of
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hydrates. Figures 4b and 4c show images of continuing hydrate growth of the same
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volume after 54 hours and 78.5 hours, respectively. Hydrate saturation values were 7.19,
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8.09, and 8.79% respectively.
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THF/H2O/glass bead system is homogeneous. The patchy growth is consistent with the
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weak dependence of sound speed on natural hydrate saturation at low saturation values29.
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Note that the 60-40 wt% ratio THF-H2O solution leaves excess THF compared to the 19-
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81 wt% THF-H2O stoichiometric solution (1:17 molar ratio).
The hydrate distribution is patchy even though the
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The sample porosity was directly determined by segmenting the tomographic data
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into solid (glass beads) and pore (water) spaces based on the differences in their x-ray
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attenuation coefficients.
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somewhat less than the theoretical porosity of a random dense pack of uniform spheres,
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38%. The mean pore diameter for the 500 µm glass bead sample is calculated using
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Kozeny’s equation. The experimental porosity value of 34.6% leads to a pore diameter of
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177 µm. The contact angle between hydrate and glass bead is also obtainable directly
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from analysis of sections through the volume tomographic data. Typical results are
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shown in Figures 5b and Figures 5c. The value for the contact angle measured in this way
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was 140.7° averaged over five measurements.
The measured porosity value was 34.7%.
This value is
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Figure 5d is adapted from Clennell et al28 to describe hydrate formation within
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pores. Upon sufficient temperature depression, a convex hydrate front moves from larger
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pore (rb) into a smaller cylindrical pore (re) with a nonfreezing layer of water layer at the
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pore wall. The Gibbs-Thomson equation30 can be written for the hydrate-water system
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within pores as follows:
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T pore  Tbulk
N o
. K
2 hwTbulk cos  hw

 m
kg J
 h H f re
. .m
m 3 kg
(1)
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where γhw is surface energy between hydrate and water, θhw and ρh are contact angle and
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density of hydrate respectively, and ΔHf is enthalpy of melting. The depression of
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freezing temperature in pores (Tpore) below the bulk freezing temperature (Tbulk) depends
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on the pore radii (re) and the contact angle for the hydrate. The growth habit of ice and
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hydrate is argued to be similar1, 30. The surface free energy of water-ice interface can be
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approximated for the water-hydrate interface (γhw = γiw = 0.032 J/m2)1. We calculated the
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dependence of the equilibrium temperature shift on the pore radius for THF hydrates and
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methane hydrates using measured contact angle from the reconstructed 2D images
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(Figure 5) and substituting it in Equation 1. These data were used to plot the temperature
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depression ratio (Tpore/Tbulk) versus capillary radius in Figure 6. For reference, a plot (line
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a) is included for the ice-water case in which water is assumed to be the wetting phase
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(contact angle = 180o). The specific enthalpy of dissociation and the density of THF
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hydrate values of 263.15 kJ/kg31 and 971 kg/m3, respectively were used to construct a
6
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plot for THF hydrate (line c, Figure 6) based on equation 1. The summary of parameters
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used is included in Table 1 as an addendum.
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It is apparent from Figure 6 that for capillary radius greater than 1000 Å, the Tpore/Tbulk
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value is ~1 suggesting negligible effect of the pore radii and contact angle terms on
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temperature depression. For the present THF hydrate system with glass beads of 500 μm
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uniform diameter and ~34.68% porosity, the pore radii was calculated to be 177 μm (1.77
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x 106 Å) that lies on the far right of the plot. However, Figure 6 shows a plot for the
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methane hydrate system from data by Turner et al.32 where a 180o contact angle (hydrate
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as the wetting phase) was assumed. It should be noted that the angle θ, from Turner et al,
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has been modified to fit in equation 1 with a non-negative right term. When these data
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were modified to include the contact angle term (cosθ ≠ 1), the line d shifted to line b,
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showing a significant effect of the contact angle term. For fine host sediments such as
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those found at hydrate sites like the Gulf of Mexico, the smaller effective pores exhibit
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higher capillary pressure and increased specific surface energy between solid-liquid
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interfaces, that result in the liquid phase thermodynamically favored down to lower
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temperatures than that at bulk conditions. Bob: Please add the two summary sentences
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from your notes.
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METHODS
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The CMT study was carried out at Beamline X2B at the National Synchrotron Light
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Source (NSLS) at Brookhaven National Laboratory (BNL). The intense X-ray beam (5-
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mm horizontal and 1-mm vertical) passed through the sample and impinged on a thin
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yttrium-aluminum-garnet (YAG) scintillation X-ray detector. The light from the
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scintillator was imaged on a CCD camera after 90 o reflection from a mirror placed at 45o
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to the beam and passing through a focusing lens. The images were recorded using a CCD
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camera (pixel size = 0.00373 mm, area = 1317x1035 pixels) with 3500-5000 msec
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exposure in the 24-26 keV X-ray beam at a 0.15o angular increment. A total of 1200
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views were collected in an assembled file (.prj) for a selected region of interest (ROI)
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keeping the sample container within the ROI from angle 0 to 180o.
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The sample for the CMT study was prepared as follows. An aqueous solution
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containing 25 wt% BaCl2 (saturation limit of BaCl2 in H2O is 30 g/mL at -3oC) was
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prepared and then mixed with THF in the 40/60 wt% ratio to yield a colorless
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homogeneous solution. A 1 mL polypropylene syringe was fitted with a cooling jacket
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and filled with about 0.6 mL of the prepared THF-H2O-BaCl2 solution. Glass beads of
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uniform 500 μm size that served as a surrogate for host sediments were then added to the
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syringe until the total volume was about 1 mL. The syringe was then cooled to -3oC by
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circulating ethylene glycol through the cooling jacket. A total of 10 tomograms were
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scanned during the 79-hour time-resolved study of THF-hydrate formation under
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isothermal conditions. Of the 10, three were selected for detailed image reconstruction. A
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typical reconstruction involved selecting 300 slices from the assembled files in a
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tomogram and converting them into a stack of jpegs using IDL tomography software. All
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1200 angular images were reconstructed to get horizontal cross-section. The vertical axis
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was optimized for each reconstruction to reduce artifacts in the images. The 3-D volume
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from the stack of images could be created using a number of softwares such as Drishti33,
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Cmtvis34, or with commercially available plug-ins for ImageJ35 developed at the
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Australian National University, University of Tennessee, Knoxville, and National
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Institute of Health respectively. The conversion of each stack of images in this analysis
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involved cmtvis and volume rendering software, Drishti. The final processing yielded
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contrasting images in which THF-Water, THF-hydrate, and glass beads could be
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differentiated based on their attenuation coefficients.
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Acknowledgements
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This work was supported by the Office of Fossil Energy, US Department of Energy under
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contract No. DE-AC02-98CH10886 and Brookhaven National Laboratory under the
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Laboratory Directed Research and Development (BNL LDRD) program.
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Figure 1: THF-Water Phase Diagram at 1 Atmosphere
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a
b
c
Figure 2: Observation of random THF hydrate (black) growth hosted in glass beads
(white spheres) is representative of 2-D cross sections (7 mm diameter). The images
are recorded at (a) 54:06 h (b) 70:30 h (c) 74:07 h.
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Figure 3: A 3-D images of THF-hydrate in glass beads. The image was reconstructed
from 300 slices such as those shown in Figure 2. The embedded bar and accompanying
graph relates to absorption coefficients that clearly differentiate hydrate (1), liquid THF
and water (2), and glass beads seen as faded spheres (3).
Figure 4: Time resolved THF hydrate growth in glass beads serving as host. The 3-D
structures are rendered from tomography scans at cooling times (a) 28:53 h, (b) 54:06 h
and (c) 78:39 h. The glass beads are not shown to allow enhancement of the contrast for
distinct observation of THF-hydrate growth (shown in grey scale).
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Glass Bead
θhw rhw
Pore
Hydrate
141.07
140.35
40.94
4
338
339
340
341
342
343
344
345
346
THF-Water
Solution
141.14
THF Hydrate
b
a
c
d
Figure 5: Contact angle measurement and capillary model of hydrate in pores
(d)(adapted from Clennell et al., 200028). A convex contact angle (b,c) is analyzed with
‘angle tool’ in ImageJ after processing a 2-D image (a) from a vertical stack with tool
such as ‘find edges’ followed by ‘sharpen’ in ImageJ.
1.00
a
b
c
0.95
T pore /T bulk
Ice-Water
Our study applied to CH4 hydrate
Our study for THF hydrate
d
CH4 hydrate (from Turner et al.)
0.90
0.85
10
100
1000
10000
Capillary Radius (Å)
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350
351
352
353
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Figure 6: Effect of pore radii and contact angle on equilibrium temperature shift for
present THF hydrate analysis, its extension for Methane Hydrate. In reference 31,
the contact angle for methane hydrate-grain system is assumed to be zero (perfectly
wetting).
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Addendum - Table 1: Summary of Parameters used to plot the Temperature
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depression ratio (Tpore/Tbulk) versus Capillary radius
357
Turner et al.32
Our Study
Our Study
Ice-Water
CH4HYD
THFHYD
CH4HYD
----
γ, N/m
2.67E-0228
2.67E-0228
2.67E-0228
3.20E-02
θ, deg
0
140
140
180
ρ, kg/m3
91032
971
91032
916
ΔHf, J/kg
4.36E+0536
2.63E+0531
4.36E+0536
6.01E+06