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Detection of Occupancy Differences in Methane Gas
Hydrates by Raman Spectroscopy Susanne Brunsgaard
Hansen1,2, Rolf W. Berg1 and Erling H.
Stenby2 1Department of Chemistry, DTU, 2800 Kgs.
Lyngby, 2IVC-SEP, Department of Chemical
Engineering
Introduction Gas hydrates are crystalline
compounds which grow from micro crystals to bulk
masses. Since hydrates exist at elevated
pressures and temperatures above the ice point,
they can cause severe plugging problems during
production and transportation of reservoir
fluids. Methods to prevent hydrate formation are
in use (injection of inhibitors). From
environmental and security points of view an easy
way to detect the formation of gas hydrates is of
interest. We have tried to detect methane hydrate
formation by Raman spectroscopy. Gas hydrate
structure In ices water molecules form lattice
structures based on hydrogen bonding. The
structure may be visualised as composed of only
one kind of basic building block with 12 pentagon
faces given the abbreviation 512. By linking
such blocks, different gas hydrates form as
illustrated in Fig. 1. Structure sI is cubic BCC,
formed by linking the vertices of 512, leaving
another kind of cavities, 51262. Structure sII,
FCC, is obtained by linking the faces of 512. In
the even more complicated structure sH, a layer
of linked 512 connects two kind of holes, 51268
and 435663.
Raman equipment The Raman spectra were obtained
by use of a confocal DILOR XY instrument using
532 nm laser light as exciting source (180 o
scattering). The laser light was sent 1) through
the methane vapour phase, 2) through the liquid
phase and finally 3) through the formed gas
hydrate at different pressures and positions.
Raman spectra of gas hydrates The Raman spectrum
of a typical gas hydrate is shown in Fig. 7C
(data read from Ref. 3). It is seen that the ?1
vibration appears at two different wavenumbers,
2915 and 2905 cm-1. This splitting is in
accordance with the fact that methane occupies
both the small and larger cavities. The intensity
ratio of the band areas I(2915)/I(2905), is in
agreement with the ratio between the number of
small and larger cavities in sI (26), cf. Fig.
1. Some of our obtained spectra of methane gas
hydrates at two different pressures are shown in
Fig. 7A and 7B. We note two things i) the
intensity ratio between the two ?1 bands is not
constant and ii) the occupations of small and
larger cavities are the opposite of what has been
reported in the literature3,4. An explanation
could be one of two The structure of our hydrate
was sI with relatively low occupation of large
cavities or the structure was sII. Studies
presented in the literature have shown that upon
compression, sI methane hydrate transforms to sII
structure at pressures about 100 MPa5. In Fig. 8
are shown hydrate Raman spectra in the region of
lattice vibrations. The broad band at 217 cm-1
is the translational lattice mode. The features
of this band and the band 290 cm-1 are similar
to what is seen in hexagonal ice6,7, but with
wavenumbers 10 cm-1 lower8. Thus, we conclude
that the hydrates formed in our pressure cell
have the sI structure, which therefore must be
able to exist with different occupancies of the
cavities.

Fig. 4. Raman spectrum of methane in the vapour
phase, 13.0 MPa, 22 C.
Raman spectrum of the vapour phase The vapour
phase spectrum is shown in Fig. 4. The band at
2915 cm-1 is the methane ?1 vibration. A part of
the spectrum is enlarged in order to make other
bands more obvious. The band at 3020 cm-1 is the
methane ?3 band and the band at 3070 cm-1 is the
overtone 2?2 as concluded in Ref. 1. Recently we
have discovered that the intensity ratio between
these two bands depends on the pressure. The band
intensities, I(?3) and I(2?2), were measured at
varying pressures, for a pure methane sample, two
methane / ethane mixtures and a methane / N2
mixture. The intensity ratios I(?3) / I(2?2) were
plotted as a function of the total pressure, cf.
Fig. 5. As seen, the intensity ratio as a
function of pressure is independent on the
composition of the methane mixture. The intensity
of the two weak methane bands were measured as
indicated with lines in Fig. 4. The intensity
ratio was determined to be 0.60 and the pressure
in the cell was 13.0 MPa. It is seen that the
point (13, 0.60) is in good agreement with the
curve shown in Fig. 5. The ratio method has been
described in detail in Ref. 2.
Fig. 7. Raman spectra of the ?1 stretching mode
of methane incor-porated into hydrates. Curves A
and B this work and curve C data read from Ref.
1.
Fig. 1. The gas hydrate cell.
0.60
Methane gas hydrate formation A photo of the gas
hydrate cell is shown in Fig. 2. The cell
consists of a box of stainless steel with
stainless steel fittings and a Bourdon gauge. The
box is equipped with two quartz windows through
which the laser light was sent for Raman
measurements. The cell was loaded with water and
thereafter with methane (N45) to a pressure of
13.0 MPa, Fig. 3a. The cell was cooled in a
freezer to 2-4 oC. A thin layer of hydrate was
to be observed at the water surface. The cell was
shaken, resulting in rapid gas hydrate formation,
Fig. 3b.
13 MPa
Fig. 5. Intensity ratio, I(?3) / I(2?2), as a
function of total pressure.
Raman spectrum of the liquid phase The liquid
phase spectrum is presented in Fig. 6. The broad
water band is seen in the region 3700-2900 cm-1.
The weak methane ?1 band seen at 2910 cm-1
indicates that some of the gaseous methane has
been dissolved in the water.
Fig. 8. Raman spectra of the methane gas hydrate
in the region of the lattice vibrations.
Acknowledgement SBH acknowledge IVC-SEP, DONG A/S
and the Danish Research Agency for financial
support. STVF supplied the 532 nm Spectra-Physics
laser. References 1. S. Brunsgaard Hansen, R. W.
Berg and E. H. Stenby, Appl. Spectrosc. 55, 55-50
(2001). 2. S. Brunsgaard Hansen, R. W. Berg and
E. H. Stenby, J. Raman Spectrosc., 33, 160-164
(2002). 3. S. Nakano, M. Moritoki and K. Ohgaki,
J. Chem. Eng. Data 44, 254 (1999). 4. S.
Subramanian and E. D. Sloan, Jr., Fluid Phase
Equilibria 158(1), 813 (1999). 5. I-M. Chou et
al, Proc. National Acad. Sci. USA 97, 13485
(2000). 6. B. Minceva-Sukarova, W. F. Sherman and
G. R. Wilkinson, J. Phys. D Solid State Phys.
17, 5833 (1984). 7. I-M. Chou, J. G. Blank, A F
Goncharov, H-K Mao and R J Hemley, Science 281,
809 (1998). 8. P. Bosi, R. Tubino and G. Zerbi,
J. Chem. Phys. 59, 4578 (1973).
Fig. 3a. The cell filled with water and methane.
Room temperature, P13.0 MPa.
Fig. 3b. Hydrate formation after cooling and
shaking. Temperature 2-4 oC. P not determined.
Fig. 6. Raman spectrum of methane dissolved in
the aqueous liquid, 13.0 MPa and room temperature.
Poster presented at the 2002 ICORS, Budapest,
Aug. 25-30, 2002.