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OPEN
Received: 5 September 2018
Accepted: 29 March 2019
Published: xx xx xxxx
Self-preservation and Stability of
Methane Hydrates in the Presence
of NaCl
Pinnelli S. R. Prasad1 & Burla Sai Kiran1,2
Gas hydrate, a solid transformed from an ensemble of water and gaseous molecules under suitable
thermodynamic conditions, is present in marine and permafrost strata. The ability of methane hydrates
to exist outside of its standard stability zone is vital in many aspects, such as its utility in gas storage
and transportation, hydrate-related climate changes and gas reservoirs on the planet. A systematic
study on the stability of methane hydrates divulges that the gas uptake decreased by about 10% by
increasing the NaCl content to 5.0 wt%. The hydrate formation kinetic is relatively slower in a system
with higher NaCl. The self-preservation temperature window for hydrate systems with NaCl 1.5, 3.0 and
5.0 wt% dramatically shifted to a lower temperature (252 K), while it remained around 270 K for NaCl 0.0
and 0.5 wt%. Based on powder x-ray diffraction and micro-Raman spectroscopic studies, the presence
of hydrohalite (NaCl·2H2O) phase was identified along with the usual hydrate and ice phases. The
eutectic melting of this mixture is responsible for shifting the hydrate stability to 252 K. A systematic
lattice expansion of cubic phase infers the interaction between NaCl and water molecules of hydrate
cages.
The gas hydrates, a class of clathrate compounds, composed of several polyhedral cages made up of
hydrogen-bonded water molecules. Those cages are filled with suitable guest molecules, such as methane, ethane,
carbon dioxide, nitrogen, hydrogen sulfide. The gas hydrates look similar to ice, and the natural gas hydrates are
also popularly known as “burning ice”. These materials are attractive for several reasons1–6. The volume of gas
(guest) in a unit volume of hydrate is at least two orders of magnitude higher compared to the standard pressure
(p) and temperature (T) conditions. The stability conditions (p and T) of hydrate phase are milder (compared to
compressed form of natural gas -CNG or liquefied form of natural gas -LNG) and thus these solid gas hydrates
(SGH), i.e., hydrates formed with natural gas as guests, are being considered as an economically feasible form
for fuel gas storage and transportation. However, slow formation kinetics (several hours to days) and inefficient
hydrate conversion are pertinent constraints for its large-scale applications. Usage of some porous materials as
confining matrix for water molecules, and the addition of some surfactants/additives even in a smaller dosage
(<1 wt%) were proven to be active promoters of gas hydrates, in particular, for methane hydrates7–10. Apart from
gas storage applications, gas hydrate-based methodology has established its presence in the gas separation, water
desalination, refrigeration technology and so on11,12.
The bulk of the experimental work on the phase and structural stability of methane hydrates focused on
the binary system, i.e., H2O-CH4. Several experiments have also been conducted in this system with the aid
of additives and porous sediments to understand the formation and dissociation kinetics7–10. The pioneering
phase behaviour study of the ternary system, i.e., NaCl-H2O-CH4, is described by de Roo et al.13. Possible solid
phases in this ternary system are NaCl·2H2O (hydrohalite), ice and hydrate, and these phases are in addition to
the brine-liquid and gas phases. Investigations on these binary and ternary systems revealed that the melting
of hydrohalite phase is independent of initial NaCl concentration in the solution and the gas pressure, whereas
the ice melting is independent of gas pressure but strongly depend on NaCl concentration (vary along the
brine-liquidus curve)14–16.
On the other hand, the hydrate phase melting depends strongly on both the gas pressure and NaCl content.
In all these studies the NaCl concentration was rather high (>10 wt%). Earlier studies with higher salt content
were exploited extensively to find the means of preventing hydrate formation in the process. Cha et al.17 have
1
Gas Hydrate Division, CSIR–National Geophysical Research Institute (CSIR–NGRI), Hyderabad, 500 007, India.
Academy of Scientific and Innovative Research (AcSIR), CSIR–NGRI Campus, Hyderabad, 500 007, India.
Correspondence and requests for materials should be addressed to P.S.R.P. (email: psrprasad@ngri.res.in)
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compared the phase equilibria of methane hydrates in the presence of NaCl, KCl and NH4Cl solutions, using the
isochoric method and high-pressure differential scanning calorimetry (DSC) and inferred that the detected phase
stability points from both the methods are consistent, and the hydrate inhibition is more for NaCl. More practical
usage of salts in the gas hydrate related research is to use them as hydrate inhibitors in deep and ultradeep oil/gas
transmission pipelines. Therefore, earlier gas hydrate studies were conducted at higher pressures (up to 200 MPa)
and with higher salt concentration (up to 30 wt%)18. The results can be summarised as the following: (i) The phase
equilibrium boundary moves towards lower temperature and higher pressure as the salt concentration increased
to its saturation limit, and it remains unaltered even at higher concentrations. (ii) Hydrate formation is also possible with high salinity (above saturation limit) coupled with salt precipitation, and both of them are competing
effects. (iii) Supersaturation of the solutions potentially self-inhibits hydrate formation due to strong electrostatic
forces between ions and water, causing retardation in hydrate formation.
The stability of natural gas hydrate-bearing sediments is susceptible to fluctuations in the pressure/temperatures in their locations. Additionally, percolation of salt into the deposits also destabilises the gas hydrates by
reducing the extension and the thickness of their thermodynamic stability zone. Recent studies by Riboulot et al.19
inferred that the fluctuations in the salinity also is a critical parameter controlling the stability of gas hydrates of
the Black Sea, particularly at the near surface. You et al.20 studied the methane hydrate formation and dissociation
with salinity-buffered solutions in the gas-rich region; and concluded that the salt is excluded during hydrate
formation. Thus, changing the salinity of the solution causing a shift in the thermodynamic boundary.
Another unique property, associated with some gas hydrates, like CH4 and CO2, is commonly acknowledged
as self- (or anomalous) preservation effect and which is useful in gas storage and transportation applications at
ambient pressure21,22. According to this effect, the gas hydrates show abnormal stability even outside its thermodynamic stability limits and the gas encased in hydrates can be preserved for a longer duration. The rate of gas
release (or hydrate dissociation) at atmospheric pressure exponentially increase with temperature, and the gas
releasing rate significantly decreases at around 240 K and a similar trend will persist until 272 K. This temperature
zone, i.e., 240 to 272 K is known as self-preservation zone. Although the precise mechanism for this unusual
property is unknown, the formation of some ice layer around hydrate grains, an additional kinetic barrier for gas
diffusion, is responsible. The self-preservation effect depends on several factors, such as guest gas composition,
hydrate grain size, the presence of other additives and co-guest molecules, and so on23.
On the other hand, the role of electrolytes in hydrates, particularly in the vicinity self-preservation region is
less known. Sato et al.24,25, have examined the decomposition rates of methane hydrate in the presence of dilute
electrolyte solutions (≤34 mol/m3), in the temperature range 233 to 273 K, and reported that the decomposition
is remarkably suppressed (slower than hydrates with pure water) immediately below the eutectic temperature.
Authors have also observed a brief upsurge in the decomposition rate at the eutectic temperature. Mimachi et
al.26, also have studied the dissociation behaviour of methane hydrates, prepared from 3.0 and 10 wt% NaCl solutions, and have reported a faster decomposition for hydrates synthesised with 10 wt% NaCl solutions at 253 K.
While the decomposition behaviour for hydrate with 3.0 wt% NaCl solution is comparable to the pure water
system.
The present study is aimed at revisiting the thermodynamic inhibiting nature of methane hydrates in the
presence of NaCl, particularly in under-saturated condition. We also examined the self-preservation behaviour of
methane hydrates synthesised with pure water (0.0 wt% NaCl), mild salt solutions (0.5 & 1.5 wt% NaCl) and salt
content comparable with sea-water (3.0 & 5.0 wt%). Such studies will provide a detailed insight into the hydrate
dissociation behaviour.
Results and Discussion
Hydrate formation in electrolyte solution. The methane hydrates were synthesised using an aqueous
solution prepared from 0.0, 0.5, 1.5, 3.0 and 5.0 wt% NaCl. Additionally, 0.5 wt% l-methionine (l-met) was also
added. Addition of l-met to the aqueous solution helps in efficient and rapid methane hydrate conversion even in
non-stirred configuration9. We conducted all the experiments in constant volume mode by charging the reactor
vessel with 29 mL of stock solution and pressurising it with ~7.5 to 8.0 MPa methane gas at ambient temperature
(298 K). The schematic experimental set-up is shown in Supplementary Information Figure (SI-1).
An illustrative pressure-temperature (p-T) trajectory for each system is shown in Supplementary Figure
(SI-2), wherein the black and red coloured dots denote recorded behaviour during cooling and thawing cycles
respectively. The phase boundary curve is generated, using CSMGem model1 and is shown in the blue line. A
sharp decrease in the methane gas pressure, coupled with a small temperature rise, indicative of state change to
hydrate phase, is observed in all the systems. The completion/saturation of the phase changes are denoted by an
insignificant pressure decrease over a longer time span at some lower temperature. The hydrates are dissociated
in a thawing cycle by, increasing the temperature at a rate of 4–6 K/h. The fast heating can induce measurable drift
from the computed phase boundary curve. However, it is desirable to conduct the dissociation at much slower
viz 0.5 to 1.0 K/h ramping rate, to find the correct phase boundary point. However, the primary objective of the
present work is not on the phase boundary points but to verify the gas uptake, kinetics and stability of methane
hydrate system in the presence of NaCl. Thus, the hydrates were dissociated at a faster rate. We calculated the gas
uptake/release during the hydrate formation/dissociation using Eq. 1, and the data from at least three repeated
measurements were considered in all interpretations.
Figure 1 shows the average of gas uptake kinetics during the hydrate conversion process (first 600 min from
the nucleation) in the presence of NaCl. The onset for hydrate nucleation is identified by an abrupt change in the
pressure drop, and at the same instant, an exothermic temperature peak also appeared. The inset graphs show the
temporal variations in the measured temperatures (hydrate/aqueous phase) for two systems (NaCl 0.0 & 5.0 wt%).
The insets also depict the gas uptake kinetics for these two representative systems. As shown in the inset figures
it is not always possible to have a stronger exothermic temperature peak. Several factors such as heat transfer
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Figure 1. The methane gas uptake during hydrate formation in CH4-H2O-NaCl system in the first 600 min
from the nucleation event. The dominant gas uptake for hydrate forming systems with 0.0, 0.5 and 1.5 wt%
NaCl, while it occurred in two stages for 3.0 and 5.0 wt% NaCl. The inset shows recorded temporal variations in
temperature (orange) and gas uptake for 0.0 and 5.0 wt% NaCl.
Figure 2. The gas uptake in the process of hydrate conversion in CH4-H2O-NaCl system.
efficiency, placement of sensor etc., will influence the detectability of the exothermic peak. Thus, the combination
of an exothermic peak coupled with a change in the slope of vapour phase pressure is considered as a triggering
point for the hydrate phase27.
Presence of electrolytes such as NaCl is a well-known thermodynamic inhibitor and also retards the gas uptake
during the hydrate conversion. However, the time taken for 90% of hydrate conversion in no or less saline systems
is really fast (i.e., 15 and 25 min for 0.0 and 0.5 wt% NaCl). On the other hand, the hydrate conversion process
occurred in longer time spans for 1.5 (80 min), 3.0 (200 min) and 5.0 (260 min) wt% NaCl. The hydrate nucleation
also occurred in multi-stages in higher (3.0 and 5.0 wt%) saline system. As said the electrolytes are thermodynamic inhibitors, while amino acid (l-met) is a good promotor for methane hydrates9. These contrary effects are
responsible for the dual stage gas uptake, which has been observed predominantly in high saline systems. These
results are in tandem with earlier reports. The overall gas consumption in hydrate formation, computed from p-T
trajectories, (Fig. 2) shows a progressive decrement by about 10% at higher salinity. It is interesting to note that
Chong et al.28, have reported similar observations in methane hydrates formed in the presence of NaCl (1.5 and
3.0 wt%) in a silica bed reactor. Those authors stated that the presence of NaCl induces a delay (1.5 times to pure
water) in hydrate formation and also about 30% decrease in the hydrate conversion between NaCl solutions with
0.0 wt% and 3.0 wt%. Our results indicate that the addition of the amino acid is helpful for equivalent gas uptake
in the pure and saline systems, leading to similar hydrate conversion.
Self-preservation effect. Some gas hydrates are abnormally stable outside their general thermodynamic
stability regions, and this property is valid only to some specific systems. The hydrate systems with guest molecules such as methane or carbon dioxide can be classified into this category. The mechanism for such a peculiar
property is still not understood, but the hydrates are stable for more prolonged periods even at atmospheric pressures while preserving them at sub-zero temperatures. This property, readily identified as “self- or anomalous-“
preservation effect, has exciting applications in the gas storage & transportation sector7,8. In Fig. 3, we plot the
dissociation behaviour of CH4 - H2O hydrate system in the presence of 0.0, 0.5, 1.5, 3.0 and 5.0 wt% NaCl, along
with computed phase boundary curves of the end-membered (0.0 and 5.0 wt% NaCl) systems using CSMGem1.
The reactor vessel was equilibrated to the atmospheric pressure by removing the residual methane gas at a lower
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Figure 3. Observed methane pressure built-up during hydrate dissociation in CH4-H2O-NaCl system. The full
and dash (red colour) lines indicate the computed phase boundary using CSMGem for 0.0 and 5.0 wt% NaCl
systems.
temperature (~250 K), and the hydrate system firmly is in its metastable state. A small portion of hydrate sample
is transferred to 8 mL pressure vessel (pre-cooled to 150 K) and subjected to dissociation by increasing the temperature (by placing it in a cotton filled glove box). The pressure build-up is remarkably sluggish in CH4 - H2O
(0.0 wt% NaCl) system when the temperature is below 268 K. The gas increases rapidly in the temperature window
of 268 to 271 K. At about 270.8 K the total pressure of the reactor reached to 2350 kPa, which is a phase boundary
point for pure methane hydrates. After that, the gas release by hydrate dissociation is primarily driven by the
thermodynamic conditions.
Similarly, the dissociation of hydrates, formed with 0.5 wt% NaCl, activated at a somewhat lower temperature.
Further, increase in the salinity such as 1.5, 3.0 and 5.0 wt% brought some remarkable changes in the dissociation
process (see Fig. 3), namely the quick release of methane gas observed in the temperature close to 252 K, and also
the phase-boundary curve shifted to the inhibition (left) side. It is interesting to note that the rapid gas release
occurred in two distinct temperature windows, such as 268 to 271 K and 249 to 253 K. The former temperature
window is in the vicinity of ice melting point, while the latter corresponds to the eutectic temperature for hydrohalite (sodium chloride dihydrate: NaCl·2H2O) phase. A decrease in the freezing point of water with increasing
NaCl content is a well-known phenomenon, and the growth of hydrohalite phase is probable in H2O-NaCl system at higher NaCl (~23 wt%) weight, otherwise brine ice will form at lower temperatures. However, according
to Mimachi et al.26 the hydrate grains in the ternary system with 10 wt% NaCl, were surrounded by the rims of
NaCl·2H2O and as such no such lamellar objects were reported at lower NaCl content.
Ex-Situ Characterisation of Gas Hydrate. Powder xrd study. Experimentally synthesised solid phases
were characterised by the analytical techniques such as powder x-ray diffraction (PXRD) and laser Raman spectroscopy under ambient pressure and cryo-temperature conditions. The pressure vessel containing the hydrates
were quenched to 150 K, by placing them in liquid nitrogen. The residual gas was completely removed intermittently, and the hydrate samples were preserved at low temperatures. Figure 4 shows the XRD pattern of hydrate
samples recorded from fine powders (collected from different spatial locations of the hydrate sample) at 150 K
with NaCl content 0.0, 3.0 and 15.0 wt%. Three solid phases possibly existed in H2O-NaCl-CH4 system, namely,
methane hydrates (cubic), ice (hexagonal) and to some lesser extent hydrohalite (monoclinic). The recorded
PXRD pattern was indexed by the CheckCell29 programme using space groups Pm3n, P63/mmc and P21/c respectively. The red, blue and green coloured bars indicate the computed positions for the cubic hydrate, hexagonal ice
and hydrohalite phase respectively.
Observed PXRD pattern for the hydrate-forming system without NaCl (see Fig. 4A) is dominant of cubic
hydrate phase features, and the unit cell length is estimated as 11.8806 ± 0.002 Å. In particular, the observed pattern around low-angles (2θ) consisting of 10.522 (0 1 1); 14.902 (0 0 2); 16.676 (1 0 2) and 18.282 (1 1 2) indicate
the presence of hydrate phase (Pm3n). The hexagonal ice phase (P63/mmc) has distinguishable diffraction peaks
at 22.796 (0 1 0); 24.265 (0 0 2); 25.954 (0 1 1) and 33.531 (0 1 2). On the other hand, the hydrate-forming system
with NaCl as 15 wt% (synthesised to collect the specific signatures of the monoclinic phase) show a distinctly
different pattern (inset of Fig. 4C). Three low-angle peaks are observed at 15.335 (1 0 0), 17.506 (0 2 0) and 17.673
(−1 1 0). These characteristic features of hydrohalite phase are distinctly different from the cubic hydrate phase,
and they are absent. However, as shown in Fig. 5, the characteristic Raman features for CH4 molecules encased
in hydrate cages are visible. Thus, the hydrate phase and the hexagonal ice also coexisted along with hydrohalite.
As shown in Fig. 4C the PXRD lines for hydrohalite and hydrate peaks are weaker than the ice phase. The lattice
parameter of the hydrate phase is estimated as 11.9062 ± 0.0036 Å. The hydrate-forming system with 3.0 wt%
NaCl (Fig. 4B) shows mixed features, namely, dominant hydrate and weaker ice and hydrohalite signatures.
Presence of hydrate and hydrohalite phases are observed from the characteristic low-angle peaks (see inset). The
lattice parameter for this system is estimated as 11.8975 ± 0.0034 Å. Similar XRD pattern namely the co-existence
of hydrate and hydrohalite phases are observed in hydrate forming systems with NaCl. The PXRD patterns for
other NaCl (0.5, 1.5 and 6.0 wt%) concentrations are shown in the Supplementary Information (SI-3).
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Figure 4. Recorded powdered X-ray diffraction patterns for the solid phases in the CH4-H2O-NaCl system.
PXRD was recorded at 150 K. Top (A), and bottom (C) segments correspond to hydrate systems with 0.0, and
15.0 wt% NaCl and the middle (B) trace is with 3.0 wt% NaCl. Computed diffraction peak positions for cubic
hydrate (Pm3n), hexagonal ice (P63/mmc) and monoclinic hydrohalite (P21/c) are represented by red, blue and
green coloured bars respectively. Blown-up portions of the low-angle diffraction peaks are shown as insets for
clarity.
Figure 5. Recorded Raman spectra of hydrates in the wavenumber window 2800–3650 cm−1 for CH4-H2ONaCl system. All the spectra were recorded at 150 K.
Micro–raman spectroscopic study. A small portion of hydrate sample (selected from different spatial locations)
is placed on the LINKAM FTIR-600 stage, which was pre-cooled to 150 K. Spectral window in the range 2800 to
3700 cm−1 will have the characteristic signatures for methane molecules encased in the hydrate phase and also
the OH stretching mode of network water molecules. Further, the typical modes for the hydrohalite phase are
also seen prominently in this spectral window. In Fig. 5, we show the Raman spectrum for methane hydrates prepared in the presence of NaCl. The characteristic vibrational bands of methane molecules encased in 51262, and
512 cages of sI are observed at 2905 and 2915 cm−1 9,30. Also, a broader band around 3106 cm−1 is a signature band
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Figure 6. The Raman spectrum of hydrates samples synthesised with 0.0 wt% (A) and 3.0 wt% (B) NaCl at
different temperatures.
for OH stretching mode for sI31. It is interesting to note some additional bands in this window, such as a triplet at
3406, 3420 and 3436 cm−1 and another band at 3537 cm−1 32,33, particularly in the hydrate-forming system with
10 and 15 wt% NaCl. These bands also observed with lesser intensity for the systems with NaCl 1.5, 3.0, 5.0 wt%.
Interestingly the hydrohalite signatures always appeared along with gas hydrate signatures and thus inferred that
the hydrate grains are enclosed by a layer of hydrohalite. Further, such characteristic modes of hydrohalite phase
are invisible in the hydrates synthesised with 0.0 and 0.5 wt% NaCl. Thus, the additional features appearing in
the spectral window, particularly at higher NaCl content, indicates the presence of hydrohalite phase along with
methane hydrates and ice phases. These observed features are grossly independent of the spatial location of the
test specimen indicating the steady growth of hydrate. Our PXRD and Raman spectroscopic analysis explicitly
indicate the presence of three solid phases namely cubic hydrate, hexagonal ice and monoclinic hydrohalite. The
hydrohalite phase is particularly abundant at higher NaCl content.
Temperature dependent micro–raman spectroscopic study. To elucidate the thermal stability of CH4 molecules
entrapped in the hydrate cages at ambient pressure, we carried-out detailed Raman spectroscopic studies in the
range 150–273 K. The Raman spectrum of methane hydrates with 0.0 and 3.0 NaCl wt% at different temperatures
is shown in Fig. 6. The samples are exposed to ambient pressure conditions. Similarly, the thermal evolution for
hydrate systems with NaCl wt% (0.5, 1.5, 5.0) is shown in the Supplementary Information (as SI-4). The Raman
signatures of methane molecules in hydrates without NaCl, namely, the bands at 2905 and 2915 cm−1 diminish
as the temperature approaching ice melting point, while those with 3.0 wt% NaCl decrease close to 252 K. This
behaviour is similar to the degassing observed in the pressure vessels (see Fig. 3). Variation in the amount of
methane encased in the hydrate cages (measured as the ratio for the characteristic CH4 band to the H2O band
at 3106 cm−1) by increasing the temperature is shown in Fig. 7. Interestingly, the ratio rapidly decreased around
273 K (ice melting temperature) for hydrated synthesised with 0.0 and 0.5 wt% NaCl, while the ratio dramatically
reduced around 252 K for all other systems with higher NaCl content. Therefore, the hydrates with a certain
amount of NaCl are less stable.
Mechanism of self-preservation. Although the self- (or anomalous) preservation effect of methane hydrate is a
well-known phenomenon, the exact molecular mechanism is still puzzling. Nevertheless, this property is very
much useful for gas transportation applications. The requirement of high-pressure vessels could be avoided as the
hydrates show more extended stability in its self-preservation window21–23,34. However, previous studies suggest
the following: the gas from gas hydrates diffuses rapidly in the temperature less than 240 K, and a thin water layer
(product in the dissociation process) transforms into ice particulates and forms layer around hydrate grains. This
layer of ice acts as an additional barrier for gas diffusion and prevent hydrate dissociation. Thus, the hydrates
continue to stay in the meta-stable conditions until the temperature reaches the ice melting point.
As seen from the data plotted in Figs 3 and 7 the degassing of methane hydrates formed in the presence of
NaCl occurred in two different temperature windows. The meta-stability of hydrates with pure and mild (0.5 wt%)
NaCl solutions persists until ice melting temperature. The hydrates formed using a higher amount of NaCl have
shown rapid gas release around 252 K. Interestingly, this is the eutectic temperature for the hydrohalite phase
formed in NaCl-H2O system. The NaCl·2H2O usually develop from the solution at higher NaCl wt%. However,
the present PXRD and Raman spectroscopic results (Figs 4 and 5) indicate the existence of this hydrohalite phase
even with lower NaCl wt%. Further, a significant downshift of the meta-stable region to the eutectic temperature
suggests that the hydrohalite may exist as defects in the protective ice layer around the hydrate grains. Thus, rapid
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Figure 7. The variation in the amount of trapped CH4, measured from the intensity ratio the characteristic
stretching modes of encased CH4 and cage-network H2O molecules, at different temperatures. The shaded
vertical bars (guide to the eye only) show significant gas release in two different temperature windows. All the
experiments were conducted at ambient pressure (@ 101 kPa).
Figure 8. Estimated lattice parameter of the hydrate phase with different NaCl content. The red coloured line is
a guide to the eye.
gas diffusion is possible around the eutectic temperature. As said, the formation of hydrohalite requires saturated
NaCl solutions, and this could have been facilitated by the accumulation of excess NaCl during the hydrate formation. Significant rejection of dissolved salts during the hydrate growth process is a well-known phenomenon,
and as such, the gas-hydrate based methodology is useful in the desalination process.
The lattice parameter of the cubic hydrate phase also increases by increasing the concentration of NaCl.
These variations are systematically plotted in Fig. 8. The line is a guide to the eye. Estimated lattice parameter for
the pure system is 11.8806 Å, and it increases to 11.906 Å for hydrates with NaCl 6.0 wt%, and it remained the
same for 15 wt% NaCl system. The extent of change is small (0.22%) but systematic. This indicates an interaction
between Na+ with the H2O molecules of hydrate cages. Cha et al.17, have shown that the hydrates with NaCl are
better inhibitors than KCl and NH4Cl and this is because of their ability to interact more with the water molecules
in the hydration layer. Further, Sa et al.35, have also shown that the amino acid molecules are incorporated into the
hydrate cages leading to expansion of the lattice as a function of amino acid content.
An interesting point to note is the dissociation trend at somewhat high pressures (>2000 kPa) for hydrates
(see Fig. 3). The dissociation trend strictly follows the thermodynamic phase boundary curve for pure hydrates.
On the other hand, the dissociation pattern for the hydrates prepared with NaCl 1.5, 3.0 and 5.0 wt% is shifted to
inhibition side and follows the thermodynamic phase boundary for NaCl system. It is worthwhile to recall that
the stability of methane hydrates in the self-preservation window depends on several factors and granular size is
one among them. Nakoryakov and Misyura34 have shown that the gas diffusion in natural hydrates (0.7 mm) is
slower than the synthetic hydrates (2.4 mm), although the average granular size of natural hydrates is considerably
less. On the other hand, Falenty et al.23, have reported that the gas diffusion from unconsolidated hydrate grains,
synthesised from 0.3 mm or less, ice particles is significantly faster compared to highly consolidated hydrate
grains. Thus, the stability of gas hydrates in its meta-stable window is more complicated and the properties of the
solid ice layer, which is acting as an additional barrier for gas diffusion, also attains considerable significance. It is
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essential to have perfect packing, and thicker ice crust around hydrate grains, and also lesser defects will ensure
greater hydrate stability even in the meta-stable region.
As said, the phase boundary points for the ternary system such as CH4-H2O-NaCl has been experimentally
verified at higher pressures, and it is not possible to compute the same from CSMGem1 also at low pressures
and temperature conditions. Extrapolation of the phase boundary using some proposed empirical relations13,16
from the literature causes significant deviations from the CSMGem model. It is difficult to understand the
self-preservation effect as these empirical relations predicts the hydrate stability at significantly lesser pressures
than CSMGem model (see SI-5). Nevertheless, the salt-solutions are well-known thermodynamic inhibitors for
methane hydrates, and thus it is fair to assume that the stability pressure is more than that of pure hydrates at a
given temperature. In other words, we consider the phase boundary for hydrates synthesised with and without
NaCl to be identical at lower temperatures. As shown in Fig. 3 (Fig. also SI-5) the dissociation of hydrates, prepared with the lesser amount of NaCl, is predominately around ice/brine -melting temperature. There is a significant reduction in the gas release rate upon cumulative pressure approaching equilibrium pressure (at a given
temperature). Plausibly of a sudden upsurge in the gas release around 252 K, in the hydrates with a higher amount
of NaCl, is because of the eutectic melting of hydrohalite defects in the brine-ice layer surrounding hydrate grains.
Both the PXRD and Raman spectroscopic investigations show the presence of hydrohalite and also its abundance
increases with NaCl content.
Conclusions. In summary, we systematically examined methane hydrate formation behaviour of the aqueous
solution with (0.0 to 5.0 wt%) weight fraction of NaCl, under isochoric and non-stirred configuration. The methodology could be easily adaptable for large-scale applications. Overall water to hydrate conversion is 88.68% for
0.0 wt% NaCl system, while it decreased progressively to 79.24% in 5.0 wt% NaCl system. Transformation into
the hydrate phase is rapid and occurred in a single step in lower NaCl (1.5 wt%) contents, while it is considerably
slower and happened in two stages in hydrate forming systems with higher NaCl. By PXRD and micro-Raman
investigations, co-existence of hydrohalite phase, particularly in the systems with higher NaCl contents, is established. The significantly rapid gas release is detected around 252 K, which is a eutectic temperature for brine-ice
and hydrohalite, for the methane hydrates formed with 1.5, 3.0 and 5.0 wt% NaCl. Whereas, the methane hydrates
from 0.0 and 0.5 wt% NaCl are stable up to ice-melting temperature. Thus, self-preservation of the methane
hydrates is strongly influenced by the presence of NaCl.
Experimental Section
The de-ionized ultra-pure water (Millipore–type 1) was used, and dissolved gases were removed by evacuation.
High purity (99.95%) methane was acquired from M/S Bhuruka Gas Company.
Apparatus. The hydrate crystalliser, the central part of the experimental setup, is made up of a solid SS-316
rod, which can withstand gas pressures up to 20 MPa, and volume of the vessel was 250 mL. The inside temperature of the hydrate crystalliser was maintained at a desired level by immersing it in a cold fluid (water + glycol
mixture) tank, coupled with a closed loop chiller. A platinum resistance thermometer (Pt100) was inserted into
the vessel to measure the temperature of aqueous/hydrate, with an accuracy of ±0.5 K. The pressure in the vessel
was measured with the pressure transducer (WIKA, type A-10 for pressure range 0–25 MPa with ±0.5% accuracy). The small vessel (8.0 mL), used for gas release measurements, has a similar arrangement for p &T logging.
Procedure. The experiments were conducted in constant volume mode. Initially required amount (29 g)
aqueous solution was added to the hydrate crystalliser and was pressurised with experimental values using
Teledyne ISCO syringe pump. The atmospheric gas was removed by purging three/four times with methane gas
(1.0 MPa). The reactor vessel was completely isolated from the gas tank assembly and was immersed in the tank
with a cold fluid to lower the temperature. At some suitable temperature, a sharp decrease in the gas pressure
indicates the state change. The irrelevant head-pressure drop in the crystalliser over a longer duration specifies
the saturation in hydrate conversion. The temperature and pressure were logged for every 30 seconds interval. The
molar concentration of methane gas (∆nH, t) in the hydrate phase at time t, is defined by the following equation:
∆nH, t = ng , 0 − ng , t = (P0V /Z 0RT0) − (PV
t / Z t RTt )
(1)
The compressibility factor (Z) is calculated by the Peng-Robinson equation of state. The gas volume (V) was
presumed as constant during the experiments, i.e., the volume changes due to phase transitions were neglected.
ng, 0 and ng, t represent the number of moles of feed (methane) gas at zero time and in the gas phase at time t,
respectively.
Raman measurements.
The Raman spectroscopic measurements were conducted on Horiba-T64000
system, coupled with an air-cooled argon ion laser emitting 514.5 nm which is the excitation source. We used
LINKAM FTIR 600 cryo-stage is used to collect the Raman spectra at different temperatures in the range 153–
300 K. The samples were exposed to laser radiation, using a 50X long distance focusing lens, only during the data
accumulation. The Raman data was processed using GRAMS/3 software, and overlapped Raman bands were
fitted into several Lorentzian components. The peak position, width and intensity for all the constituents were
allowed to vary as free parameters for a convergent fitting.
Powder x-ray diffraction measurements. The crystalline phases were analysed on a Bruker (Advance
D8) diffractometer. The diffractometer was operational in the θ/2θ scan mode and the x-rays were having the
wavelength of 1.5406 Å (Cu- radiation). An Anton-Paar (TTK-450) non-ambient stage was fixed in the sample
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chamber, and all the measurements were conducted at 150 K. The PXRD pattern was recorded in the range 2θ = 8
to 60°, in step scan mode. The dwell time and step size respectively are 0.5 sec and 0.02°.
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Acknowledgements
Authors sincerely thank the Director of the National Geophysical Research Institute, Hyderabad, for his
encouragement, and permission to publish this paper (NGRI/Lib/2018/112). Partial financial support from MoES
(India) and DGH-NGHP (India) are acknowledged. A generous grant from ExxonMobil under the knowledgebuild programme is acknowledged. Constructive comments/suggestions of anonymous reviewers helped in
improving the manuscript significantly, and we sincerely thank them.
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Author Contributions
P.S.R.P. conceived and designed all the experiments and B.S.K. has helped in conducting the experiments and data
analysis. Both the authors contributed to the scientific discussions and reviewed the manuscript.
Additional Information
Supplementary information accompanies this paper at https://doi.org/10.1038/s41598-019-42336-1.
Competing Interests: The authors declare no competing interests.
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