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Laboratory study of methane hydrate formation kinetics and structural stability in sediments
Marine and Petroleum Geology 58 (2014) 199e205
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Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Laboratory study of methane hydrate formation kinetics and structural stability in sediments Ch. V.V. Eswari a, B. Raju a, b, V. Dhanunjana Chari a, c, P.S.R. Prasad a, *, Kalachland Sain a a
Gas Hydrates Division, National Geophysical Research Institute, Council for Scientific and Industrial Research, Hyderabad 500007, India School of Physics, University of Hyderabad, Hyderabad 500046, India c Department of Physics, Osmania University, Hyderabad 500007, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 February 2014 Received in revised form 12 August 2014 Accepted 13 August 2014 Available online 28 August 2014
The natural gas hydrate deposits in the offshore of India are embedded in different sediments, namely clay rich silts and sands, coarse grain sand and volcanic fly ash. The variations in gas hydrate concentrations at different geological locations shows dependency on sediment mineralogy. It is also known that the particle size of the sediments plays an important role in hydrate formation and gas hydrate concentrations in sediments. We carried out systematic studies on the methane hydrate formation kinetics and methane hydrate volumetric yields, in stirred reactor experiments, using suspensions of synthetic silica and natural sediment from KrishnaeGodavari (KG) Basin. The hydrate formation behavior in silica and KG basin sediment is also compared with the formation of methane hydrates in a “pure system” without sediment or added silica grains. Our results show that the hydrate formation kinetics is faster in 50 mm silica system followed by that in natural marine sediment. Observed methane hydrate yield in the laboratory is higher (~39%) in both the pure (no sediment) and 1 mm silica suspensions. The gas intake is much quicker (~375 min) in the suspension of 50 mm silica system, while the hydrate yield is noticeably less (~29.38%). The methane hydrates are characterized by Raman spectroscopy and they show characteristic structure I (sI) methane hydrate signatures, with a hydration number in the range 5.93e6.12. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Methane hydrate Particle size Sediment Kinetics Hydrate yield Raman spectroscopy
1. Introduction Gas hydrates (GH) are ice-like crystalline compounds, comprising of water cages forming through hydrogen bonding, and these cages are occupied by the suitably sized gas molecules. The encaged gas molecules are often referred as guest molecules, and usual guest molecules in natural gas hydrates are methane (CH4), ethane, propane or carbon dioxide (Englezos, 1993). Much weaker Van der Waals interactions between the guest molecules and the surrounding water cage provide structural stability. Despite the fact that the gas hydrates look like ice, their physical properties are quite different; they can exist at temperatures well above the ice melting point at moderate pressures (Sloan and Koh, 2008). The four essential conditions for GH formation are (i) availability of host water molecules, (ii) suitable guest molecules with molecular diameter matching with available space within the host cages, (iii)
* Corresponding author. E-mail addresses: (P.S.R. Prasad).
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moderately high pressure and (iv) lower temperature typically in the vicinity of freezing point of ice. Methane (CH4) is the most common and predominant guest molecule (95e99%) in natural gas hydrates (NGH), and therefore we investigate the behavior of methane hydrates (MH) using silica/clay suspensions in this study. The stability of submarine gas hydrates is largely dictated by the pressure and the temperature, gas composition and pore water salinity etc. However, the physical properties and surface chemistry of marine sediments may also affect the thermodynamic state, growth kinetics, spatial distributions of gas hydrates (Clennell et al., 1999). Gas hydrates have been inferred to exists in the Saurashtra and KeralaeKonkan Basins along the east coast of India. Seismic evidence for the presence of gas hydrate also exists in the KrishnaeGodavari (KG), Mahanadi and Andaman sea regions of eastern India (Sain and Gupta, 2012). The NGHP-Expedition 01 (JODIES Resolution) confirmed the presence of hydrates in the KG Basin, Mahanadi Basin and Andaman region (Collett et al., 2008). The sedimentary property at these locations reveals that they are different from each other, e.g., clay rich marine sediment in KG Basin, coarse clay-rich silts and sands in Mahanadi and volcanic fly ash in Andaman (Sain and Gupta, 2012). Ginsburg et al., 2000
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demonstrated the role of sediment grain size on controlling the presence of hydrate at various sites on the Blake Ridge. Experimental results of Kawasaki et al. (2009) on sediments from the eastern Nankai Trough, indicate that particle size and clay contents are the two key factors determining the saturation level of gas hydrate in sediments; the finer the particle size and/or the higher the clay content, the lower the hydrate saturation. Therefore it is necessary to investigate the hydrate formation mechanism using host sediments to understand the role of mineralogy and grain size effects on the formation and structural stability of gas hydrate deposits. In the geological environment the hydrate formation can occur very quickly or may take a long time and previous laboratory reports show that the time required in converting the pore water to hydrate, using methane gas saturated water at about 17.4 MPa, was around 50 days (Spangenberg et al., 2005). However, in a stirred reactor system, gas hydrate form much faster and can be used to critically examine controls on gas hydrate formation. In a stirred system the water and gas interfacial area changes regularly, therefore, improves mass and heat transfer to increase the methane hydrate formation process (Hao et al., 2007). It is also well documented in the literature that the nucleation and growth of hydrates is considerably faster in stirred reactors (Hao et al., 2007; Ke and Svartaas, 2011; Prasad et al., 2012). Effect of the stirring speed on the kinetics of methane hydrate formation was investigated earlier and there is no simple correlation between stirring rate and nucleation rate (Hao et al., 2007). The higher stirring speed causes a reduction in hydrate induction time and an increase in hydrate growth rate (Hao et al., 2007). But beyond some critical speed (typically 1000 rpm for a laboratory reactor) the hydrate yield may decrease because of increase in the fluid temperature. On the other hand, at lower stirring speeds the hydrate induction time increases and the hydrate crystals formed tend to remain on the surface. Vysniauskas and Bishnoi (1983) have suggested an optimal stirring speed of 400 rpm or more to improve the efficiency of the process. Mesoscopic tools including X-ray computed tomography (XTG) (Kneafsey et al., 2007) and Scanning Electron Microscopy (SEM) (Kuhs et al., 2006) are being used to investigate the morphology
and spatial distribution of hydrate at the state of sediment pores. At molecular level, tools such as Raman (Uchida et al., 1999; Chari et al., 2014); Nuclear Magnetic Resonance (NMR) (Susilo et al., 2007) spectroscopy; X-ray and Neutron diffraction (Murshed and Kuhs, 2009) are used to analyze the molecular properties and structure of the hydrate phase. The features of micro-Raman spectroscopy can unambiguously provide information about the structure, the hydration number and the cage occupancy of encaged molecules in hydrate phase (Prasad et al., 2009; Sum et al., 1997; Chari et al., 2014). Additionally, chemical identification of the guest molecules is also possible using micro-Raman probe (Prasad et al., 2007). Further, we have used Raman spectroscopy to elucidate the structural information and hydration number of the methane hydrates synthesized within this study. 2. Experimental design 2.1. Materials and characterization The silica particles of diameter 1 mm and 50 mm were synthe€ bers method (Nozawa et al., 2005; sized using a modified Sto Sreenivasa Rao et al., 2005). The natural sediment was collected from KG Basin at a depth of ~40 m below sea floor (mbsf) in our previous cruise (Ramprasad, 2007) at latitude and longitude of 150.45.81N and 810.47.03E respectively. The silica/natural sediment with de-ionized ultra-pure water and a methane gas with a purity of 99.95% was used to synthesize the methane hydrates. In Figure 1, we show the morphology and elemental composition of the samples analyzed by a Field Emission Scanning Electron Microscopy (FESEM) and Energy-dispersive X-ray spectroscopy (EDX). SEM images show that the silica particles are mostly spherical in shape. However, the KG Basin sediment appears mostly as flakes. The bulk density of silica and KG basin sample is measured as 1.5 and 1.27 g cm3. Elemental analysis with an EDX system shows that the synthesized silica particles confirm the presence of Si and O2 with a ratio of close to 1:2. Whereas in KG basin sediment the elemental composition consists of mainly SiO2, K, Ca, Fe, MgAl etc., are corroborated with earlier report of KG basin mineralogy (Pattan, 2002).
Figure 1. FESEM images and elemental compositions from EDAX spectrum of (a) 1 mm silica (b) 50 mm silica and (c) KG basin sediment used in the present study.
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Molecular spectroscopic techniques are also used to identify the mineral composition and the purity of the sample. We have used Fourier Transform Infrared Spectroscopy (FTIR) to identify the clay and silica compositions. The sediment sample was dried in vacuum oven at a temperature of 150 C for 10e12 h to remove the pore waters trapped in the sample. The chemical purity of the samples were characterized by the FTIR are shown in Figure 2, and it was found to be 95%. The FTIR spectrum of sediment sample consists of several absorbance peaks at 490 cm1 (SieO), 540 cm1 (SieOeAl), 1090 cm1 (SieOeSi), 1650 cm1 stretching and other AleOeH modes. The best match from the mineral library for this data is for bentonite clay having structural composition as (Na, Ca) 0.3 (Al, Mg)2 Si4O10 (OH)2 nH2O (Preeti and Singh, 2007). The FTIR spectrum of silica samples with peaks at 490 cm1 (SieO), 800 cm1 and 1090 cm1 (SieOeSi) and 3600 cm1 eOH modes and the closest match is for silica of Nicolet library.
2.2. Laboratory apparatus and procedure A schematic experimental layout is shown in Figure 3. The main part of the apparatus is a cylindrical (SS-316) vessel that 100 ml can withstand the pressures up to 10 MPa. A stirrer with variable speed was installed in the vessel to agitate the fluids (Amar Equipments Pvt. Ltd., Mumbai, India). The required amount (3.15 g) of silica/ sediment and 27 g of degassed ultra-pure water (30 g in case of pure methane system) were measured by using Metler Toledo (AB104-S) accurate analytical balance. All the experiments were conducted with a fixed speed of 500 rpm. The reactor vessel, containing an aqueous solution (approximately 30% by volume of the vessel) was immersed into the temperature controlled bath and the cold fluid (water þ glycol mixture) was circulated around the vessel with the help of a circulator to maintain the temperature inside cell at a desired level. A platinum resistance thermometer (Pt100) inserted into the vessel to measure temperatures with an accuracy of ±0.2 K. The pressure in the vessel was measured with a pressure transducer (WIKA, type A-10 for pressure range 0e10 MPa with ±0.5 accuracy). The methane gas was supplied from a cylinder to desired level (7.0e7.5 MPa) using Teledyne ISCO Syringe pump. After obtaining the temperature and pressure stability far above from the hydrate formation region, the inline valve connecting the vessel and the ISCO pump/cylinder was closed. Subsequently, the cold fluid from the chiller was circulated around the reactor vessel
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to bring the system into the hydrate stability zone. The hydrate formation in the vessel was detected by sharp pressure drop at temperatures around 273 K-Pure, 274 K-Sediment, 276 K-50 mm, 277 K-1 mm. The insignificant head-pressure drop in the reactor over longer duration indicates the hydrate conversion ceased. Later on the temperature is increased stepwise to bring the system out of the hydrate-forming region, hydrate crystals partially dissociate, thereby substantially increasing the pressure. The temperature and pressure were logged at 60 s time intervals using the data acquisition system. A pressure temperature diagram (cooling and warming) was obtained for each experimental run. The molar concentration of methane gas (DnH, t) in the hydrate phase during the experiment is defined by the following equation (1):
DnH; t ¼ ng; 0 ng; t ¼ ðP0 V=Z0 RT0 Þ ðPt V=Zt RTt Þ
(1)
where Z is the compressibility factor calculated by the PengeRobinson equation of state. The gas volume (V) was assumed 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 taken at zero time and in the gas phase at time t, respectively (n represent the number of moles of feed gas). The hydrate yield was computed from the observed methane gas consumption to the expected values with ideal stoichiometry compositions (8CH4$46H2O) (Chari et al., 2012, 2013) and with the equation (2).
Conversion of water to hydrates ¼
DnH ; t hydration number nH2 O
mol % (2)
where DnH ; t is the number of moles of methane gas consumed for hydrate formation at the end of the experiment determined by equation (1) and nH2 O is the total number of moles of the water used in the system. 3. Results and discussion To study the effect of sediment grain size and composition on methane hydrate formation we conducted experiments in a stirred reactor with lower weight % of natural marine sediment samples and synthetic pure laboratory silica. The use of a stirred reactor cell
Figure 2. FTIR spectrum of silica samples and the silica spectrum from NICOLET-NEXUS library for comparison (left). FTIR spectrum of natural sediment from KG basin and the bentonite spectrum from NICOLET-NEXUS library for comparison (right).
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Figure 3. Schematic diagram of the experimental setup. 1. CH4 Gas cylinder, 2. ISCO Pump, 3. Inlet port for Gas, 4. Outlet/Vacuum port, 5. Magnetic Stirrer assembly, 6. Temperature sensor, 7. Pressure gauge and transducer, 8. Data acquisition and control, 9. Inlet for cold fluid, 10. Outlet for cold fluid, 11. Closed cycle refrigerant fluid circulator (LAB CAMPANION) and 12. Computer.
yielded faster nucleation and growth of methane hydrates by renewing the contact surface between the liquid and gas phases. Therefore a constant stirring (with a speed of 400 rpm) was applied throughout each experimental run (~20 h). In Figure 4, we show the pressureetemperature (peT) trajectory of CH4 þ water, CH4 þ water þ SiO2 (1 & 50 mm) and CH4 þ water þ Bentonite-clay suspensions. It is observed from Figure 4, that all systems require some amount of sub-cooling (5 e8 ) for hydrate nucleation,
growth and the sub cooling temperature found to be minimum for silica system than pure and natural sediment. As the hydrate formation is an exothermic process, associated with liberation of heat (Prasad et al., 2012), during further hydrate growth, the temperature increases to a maximum and then it slowly equilibrates with the outer wall temperature. After the nucleation, the hydrate growth is associated with a large pressure drop. The steady state temperature inside the reactor
Figure 4. The pressureetemperature trajectories showing the formation and dissociation of methane hydrate (MH) synthesized in stirred reactor in all the systems. The phase boundary curve for sI methane hydrate computed using CSMGem is shown by the solid line.
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Figure 5. Consumption of methane (CH4) gas in the reactor vessel as a function of time in pure and different sediment suspensions.
vessel has a small but noticeable dependence on the nature of the sediment, it is about 275 K for the natural sediment, 277 K for the pure hydrate system, and 279 K for those containing silica. This could be because of slower/faster heat transfer between the hydrates and the silica/sediment particles. The observed pressure drop was the greatest (2.77 MPa) for the pure gas hydrate system and the lowest (1.98 MPa) for the 50 mm silica system. The hydrate formation could be divided into three regions as shown in Figures 4 and 5. In region 1, a monotonic temperature and pressure decrease, was mainly due to the non compressive nature of the methane molecules. The second stage (region 2) where the temperature remained constant under steady state condition for about 9e10 h after hydrate nucleation for its maximum conversion/ growth. We also observed a significant pressure drop in the reactor vessel during this period. Thereafter, in region 3, the temperature and pressure monotonically decreased similar to region 1. The plot between gas consumed vs. time in these three regions provides necessary data on the rate of hydrate formation and the total gas consumed during hydrate formation (Abay and Svartaas, 2010; Prasad et al., 2012; Chari et al., 2013). The plot between the methane gas consumed (m$mol/mol$H2O) vs. time is shown in Figure 5. The methane gas content remained almost constant in the initial stage (region 1), indicating that there is no phase change for the water and methane gas present in reactor vessel. Very small change in the slope of the gas consumption curve with time during the hydrate formation (region 1) indicates the dissolution of methane gas in the water. In the second stage, there is a sharp rise in the slope of the gas consumption curve for the formation of hydrates and this continues for a while representing the hydrate growth (region 2). Finally, the constant slope signifies that there is no further gas consumption (region 3) and the hydrate conversion is ceased under these experimental conditions. Experiments are continued for longer duration (nearly 12e15 h) upon reaching a steady state temperature for achieving the maximum hydrate conversion. Though hydrate yield is lower in the case of 50 mm silica, the hydrate formation kinetics is quicker. After the hydrate nucleation, almost 80% of hydrate conversion is completed in 100 min. The time required for gas hydrate growth in the pure and 1m systems are similar (~500 min) and for the sediment it is around 185 min. The studies of Zhang et al. (2013), show that the hydrate growth occurs more readily in coarse grain systems. The water transfer characteristics in coarse grain sediments are observed to be more effective than in fine-grained sediments. Significant differences in the reactions in the two types of media arose from
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differences in the water retention capacity and lithology of media due to the internal surface area and pore size distributions. Our results indicate that the suspension of silica or clay sediments, primarily, acts as kinetic promoters, even though their thermodynamic inhibition is significantly low. It is well documented in the literature that the porous silica and clay particles show thermodynamic inhibition for methane and natural gas hydrates systems to a maximum extent of 1.0e1.5 K and this reaction is dependent on pore size (Kang and Seo, 2008; Østergaard et al., 2001; Aladko et al., 2004). Usually the smaller the pore size the greater the inhibition effects. The experimental studies on methane hydrate dissociation with different pore sizes were reported by Uchida et al. (2002). According to them there was a pore size inhibition effect on hydrate stability with respect to the bulk hydrate. They observed largest shift towards inhibition in case of 4 nm pore size around 12.3 K ± 0.2 K where as it was reduced to 0.5 K with a pore size of 100 nm. The studies by Riestenberg et al. (2003) reported that the kinetics of methane hydrate formation has been significantly affected by the presence of small amount of solid (bentonite) particles (200 mg/l). The required driving force (overpressure) at around 277e279 K (4e6 C) was ~5.37e4.13 MPa before observing hydrate formation in pure water, and this was significantly decreased in colloidal suspension of bentonite to ~0.66 MPa at 4.5 C. Thus the presence of bentonite particles may help formation of methane hydrates. As shown in Figure 5, the methane hydrate formation is significantly faster in the suspensions with silica/bentonite. Some recent studies reported that the cooling rate also effects the rate of nucleation and gas conversion. Under initial conditions, faster the cooling rate, shorter the nucleation time and the lower the methane gas conversion rate (Guanli et al., 2010). Further, from Figure 3 it can be concluded that the methane intake is considerably faster in silica suspension compared to that in natural clay sediments or pure hydrates. Precise reason for this is not known at present, but we feel that the grain surface effects and mineralogy play significant role in hydrate formation/growth, but our results are in broad agreement with recent literature (Riestenberg et al., 2003, Guanli et al., 2010). Methane hydrate formation/dissociation mechanism was also studied by Kneafsey et al. (2007) and Linga et al. (2009) in water saturated silica sand. The main conclusions of those studies are that the hydrate formation is quicker in silica þ water system with multiple nucleation sites. The rate of hydrate conversion is also not always proportional to the driving force in porous medium. The hydrate conversion typically occurred in three stages, an initial slower rate was followed by much quicker conversion and finally slower rates in the third stage. Linga et al. (2009) also reported the size and shape of silica sand beds has an influence over the hydrate formation. In our experiments we also observed the hydrate conversion is in multi-stages in pure, 1 mm silica, and bentonite-clay systems. Further, the overall duration of our experiments is less because rapid stirring promoted hydrate conversion. Recently methane hydrate growth simulations in the presence of silica surfaces have shown that the silica play a significant role in hydrate growth (Liang et al., 2011). They proposed that the silanol groups at the silica surfaces have a lower polarity compared to water molecules and thus proved a favorable location for methane bubble formation. Upon growth of hydrate crystal at the methane bubble, the bubble was found to move away from the advancing crystal growth front, consistent with the water-wet nature of the hydrates (Liang et al., 2011). It was also reported that sediment properties also affect the formation process of methane hydrate within it. In fine grained, the gas conversion rate is lowest; in coarse sand, the gas conversion rate is the greatest; and in fine sand, it is more intermediate. According Guanli et al. (2010) smaller the particle size of the host sediment, harder it is to grow hydrate.
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Table 1 Experimentally observed methane gas consumption during hydrate formation in suspensions with different silica particles and natural sediment (bentonite) from KG basin. The measured hydration number of hydrates formed in different systems. S. no Particle size
Yield (%)
nCH4 Consumed/H2O Hydration number (n) (m mol/mol)
1 2 3 4
39.74 39.25 39.72 29.38
63.21 62.41 63.19 46.06
Pure Sediment 1 um 50 um
(±3.430) (±3.615) (±2.430) (±0.884)
(±5.760) (±6.081) (±4.078) (±1.442)
5.93 6.02 5.96 6.12
(±0.13) (±0.22) (±0.21) (±0.09)
As shown in Table 1, experimentally measured methane consumption per mole water during the hydrate formation in the present study differ with the reports of Guanli et al. (2010). The hydrate conversion is about 20e30% lower in silica (50 mm) suspensions compared to the pure methane hydrate system (without sediment). Interestingly the suspensions with 1 mm silica also resulted in comparable hydrate yield or methane consumption to that of the pure methane hydrate and natural sediment sample system, but the kinetics were slower. Table 1 provides average methane consumption (m mol/mol H2O) for each experimental run (3 cycles of forming and dissociation) with identical cooling/heating rates and estimated standard errors. From the previous studies, it is well established that the Raman spectroscopy has unambiguous spectral signatures for the methane molecules in free/dissolved or encased in clathrate cages (Prasad et al., 2009; Sum et al., 1997; Chari et al., 2014). The procedure we followed in collecting the Raman signal and profile fitting were well described elsewhere (Chari et al., 2014). In Figure 6, we show the Raman spectrum of pure methane hydrate and the methane hydrates synthesized in sediments. The gas hydrates have distinguishable Raman shifts for methane molecules in clathrate phase (2904 and 2915 cm1), whereas in gas phase it is at 2917 cm1 (not shown in the figure). Such strikingly different signatures are useful in establishing the ground truth occupancy in the hydrate lattice of methane molecules. In Figure 6, these specific Raman signatures for pure methane hydrates and those synthesized with silica and sediment indicating that the methane molecules exist in a clathrate phase. Observed doublet in CeH stretching mode region clearly indicate that the methane molecules are enclathrated in water cages and the relative Raman intensity of these modes are consistent with sI structure. The calculated values of hydration number for the hydrate grown in the
pure gas hydrate system (without sediment), the natural sediment system and the synthetic silica system (See Table 1), in the temperature range 153 Ke233 K are in good agreement with other values reported in the literature that range from about 5.8 to 6.3 (Sloan and Koh, 2008). 4. Conclusions Methane hydrate formation was studied in a stirred reactor of 100 ml capacity. We have compared and contrasted the growth of methane hydrate in three systems: (1) Pure water and methane without any sediment, (2) a suspension of water, methane, and synthetic silica grains (8 wt % mixture; and grain size of 1 and 50 mm), and (3) a suspension of water, methane, and naturally occurring clay-rich sediment (characterized as bentonite) from KrishnaeGodavari Basin. The results obtained from the experiments clearly indicate a particle size and clay dependent trend of hydrate formation kinetics. When we compared to the pure water and methane system (without sediment), the growth of gas hydrate is significantly faster in the system with the suspension of water, gas, and synthetic silica with grain size of 50 mm. However, the volume of methane hydrate that formed in the pure water and methane system (without sediment) was greater than the volume that formed in the suspension of water, methane, and synthetic silica with a grain size of 50 mm, while it is comparable to the volume of methane hydrate that formed in the natural-clay-rich sediment, 1 mm silica systems. The structure of the hydrate grown in each experiment was confirmed by Raman spectroscopy to be structure I (sI) with a hydration number of 5.93e6.12. Indicating that there was no effect of the sediment on the structure or cage occupancy of the laboratory grown methane hydrate. Acknowledgments The author(s) wish to thank those that contributed to the success of the National Gas Hydrate Program Expedition 01 (NGHP01). NGHP01 was planned and managed through collaboration between the Directorate General of Hydrocarbons (DGH) under the Ministry of Petroleum and Natural Gas (India), the U.S. Geological Survey (USGS), and the Consortium for Scientific Methane Hydrate Investigations (CSMHI) led by Overseas Drilling Limited (ODL) and FUGRO McClelland Marine Geosciences (FUGRO). The platform for the drilling operation was the research drill ship JOIDES Resolution, operated by ODL. Much of the drilling/coring equipment used was provided by the Integrated Ocean Drilling Program (IODP) through a loan agreement with the US National Science Foundation. Wireline pressure coring systems and supporting laboratories were provided by IODP/Texas A&M University (TAMU), FUGRO, USGS, U.S. Department of Energy (USDOE) and HYACINTH/GeoTek. Downhole logging operational and technical support was provided by Lamont-Doherty Earth Observatory (LDEO) of Columbia University. The financial support for the NGHP01, from the Oil Industry Development Board, Oil and Natural Gas Corporation Ltd., GAIL (India) Ltd. and Oil India Ltd. is gratefully acknowledged. We also acknowledge the support extended by all the participating organizations of the NGHP: MoP&NG, DGH, ONGC, GAIL, OIL, NIO, NIOT, and RIL. Authors sincerely thank the Director of CSIR National Geophysical Research Institute, Hyderabad, for permission to publish this paper. This is a contribution to GEOSCAPE Project of NGRI under the 12th Five Year Scientific Program of CSIR. References
Figure 6. Characteristic methane stretching modes of CH4 molecules encaged in 512and 51262 cages of methane hydrates synthesized in pure and different sediments (8 wt %). The Raman spectra were collected at 153 K and 0.1 MPa.
Abay, H.K., Svartaas, T.M., 2010. Effect of ultralow concentration of methanol on methane hydrate formation. Energy Fuels 2, 752e757.
Ch.V.V. Eswari et al. / Marine and Petroleum Geology 58 (2014) 199e205 Aladko, E.Y., Dyadin, Y.A., Fenelonov, V.B., Larionov, E.G., Mel’gunov, M.S., Manakov, A.Y., Nesterov, A.N., Zhurko, F.V., 2004. Dissociation conditions of methane hydrate in mesoporous silica gels in wide ranges of pressure and water content. J. Phys. Chem. B 108, 16540e16547. Chari, V.D., Sharma, D.V.S.G.K., Prasad, P.S.R., 2012. Methane hydrate phase stability with lower mole fractions of tetrahydrofuran (THF) and tert-butylamine (tBuNH2). Fluid Phase Equilib. 315, 126e130. Chari, V.D., Sharma, D.V.S.G.K., Prasad, P.S.R., Murthy, S.R., 2013. Methane hydrates formation and dissociation in nano silica suspension. J. Nat. Gas. Sci. Eng. 11, 7e11. Chari, V.D., Prasad, P.S.R., Murthy, S.R., 2014. Structural stability of methane hydrates in porous medium: Raman spectroscopic study. Spectrochim. Acta Part A 120, 636e641. Clennell, M.B., Hovland, M., Booth, J.S., Henry, P., Winter, W.J., 1999. Formation of natural gas hydrates in marine sediments. J. Geophys. Res. 104, 985e1023. Collett, T.S., Riedel, M., Cochran, J., Boswell, R., Presley, J., Kumar, P., Sathe, A.V., Sethi, A.K., Lall, M., Sibal, V.K., NGHP Expedition 01 Scientists, 2008. NGHP Expedition 01 (2006). Initial Reports. Directorate General of Hydrocarbons, Noida and Ministry of Petroleum & Natural Gas, India, p. 4. Englezos, P., 1993. Clathrate hydrates. Ind. Eng. Chem. Res. 32, 1251e1274. Ginsburg, G., Soloviev, V., Matveeva, T., Andreeva, I., 2000. Sediment grain-size control on gas hydrate presence, sites 994, 995, and 997. Proc. Ocean. Drill. Program, Sci. Results 164, 237e245. Guanli, J., Qingbai, W., Jing, Z., 2010. Effect of cooling rate on methane hydrate formation in media. Fluid Phase Equilib. 298, 225e230. Hao, W., Wang, J., Fan, S., Hao, W., 2007. Study on methane hydration process in a semi-continuous stirred tank reactor. Energy Convers. Manag. 48, 954e960. Kang, S.P., Seo, Y., 2008. Proc. Int. Gas Union Research (IGRC), Paris, pp. 1e10. Kawasaki, T., Hailong, L.U., Fujii, T., Ripmeester, J.A., 2009. Investigations of particle size and clay mineral effect on gas hydrate formation in natural sediment. J. Geogr. 118, 872e882. Ke, W., Svartaas, T.M., 2011. Effects of Stirring and Cooling on Methane Hydrate Formation in a High-pressure Isochoric Cell. Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom. Kneafsey, T.J., Tomutsa, L., Moridis, G.J., Seol, Y., Freifeld, B.M., Taylor, C.E., Gupta, A., 2007. Methane hydrate formation and dissociation in a partially saturated corescale sand sample. J. Pet. Sci. Eng. 56, 108e126. Kuhs, W.F., Staykova, D.K., Salamatin, A.N., 2006. Formation of methane hydrate from polydisperse ice powder. J. Phys. Chem. B 110, 13283e13295. Liang, S., Rozmanov, D., Kusalik, P.G., 2011. Crystal growth simulations of methane hydrates in the presence of silica surfaces. Phys. Chem. Chem. Phys. 13, 19856e19864. Linga, P., Haligva, C., Nam, S.C., Ripmeester, J.A., Englezos, P., 2009. Gas hydrate formation in a variable volume bed of silica sand particles. Energy fuels 23, 5496e5507. Murshed, M.M., Kuhs, W.F., 2009. Kinetic studies of methaneeethane mixed gas hydrates by neutron diffraction and Raman spectroscopy. J. Phys. Chem. B 113, 5172e5180.
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Nozawa, K., Gailhanou, H., Raison, L., Panizza, P., Ushiki, H., Sellier, E., Delville, J.P., € ber silica particles: efDelville, M.H., 2005. Smart control of monodisperse sto fect of reactant addition rate on growth process. Langmuir 21, 516e1523. Østergaard, K.H., Anderson, K., Llamedo, M., Tohidi, B., 2001. Terra Nova 14, 307e312. Pattan, J.N., 2002. Volcanic ash and its enigma: a case study from the central Indian ocean basin. J. Geol. Soc. India 60, 127e130. Prasad, P.S.R., Prasad, K.S., Thakur, N.K., 2007. Laser Raman spectroscopy of THF clathrate hydrate in the temperature range 90e300 K. Spectrochim. Acta Part A 68, 1096e1100. Prasad, P.S.R., Sowjanya, Y., Prasad, K.S., 2009. Micro-Raman investigations of mixed gas hydrates. Vib. Spectrosc. 50, 319e323. Prasad, P.S.R., Chari, V.D., Sharma, D.V.S.G.K., Murthy, S.R., 2012. Effect of silica particles on the stability of methane hydrates. Fluid Phase Equilib. 318, 110e114. Preeti, S.N., Singh, B.K., 2007. Instrumental characterization of clay by XRF, XRD and FTIR. Bull. Mater. Sci. 30 (3), 235e238. Ramprasad, T., 2007. Acquisition of Long Sediment Cores in KrishnaeGodavari and Mahanadi Offshore Basins in Bay of Bengal Onboard R/V Marion Dufresne. Cruise Report: MD161-GH. NIO, Goa, pp. 11e28. Riestenberg, D., West, O., Lee, S., McCallum, S., Phelps, T., 2003. Sediment surface effects on methane hydrate formation and dissociation. J. Mar. Geol. 198, 181e190. Sain, K., Gupta, H., 2012. Gas hydrates in India: potential and development. Gondwana Res. 22, 645e657. Sloan, E.D., Koh, C.A., 2008. Clathrate Hydrates of Natural Gases, third ed. CRC Press, Taylor & Francis Group, Boca Raton. Spangenberg, E., Kulenkampff, J., Naumann, R., Erzinger, J., 2005. Pore space hydrate formation in a glass bead sample from methane dissolved in water. Geo. Res. Lett. 32, 1e4. Sreenivasa Rao, K., Hami, K El, Kodaki, T., Matsushige, K.J., 2005. A novel method for synthesis of silica nanoparticles. Collid. Interface Sci. 289, 125e131. Sum, A.K., Burruss, R.C., Sloan, E.D., 1997. Measurement of clathrate hydrates via raman spectroscopy. J. Phys. Chem. B 10, 7371e7377. Susilo, R., Ripmeester, J.A., Englezos, P., 2007. Tuning methane content in gas hydrates via thermodynamic modeling and molecular dynamics simulation. Chem. Eng. Sci. 62, 3930e3939. Uchida, T., Hirano, T., Ebinuma, T., Narita, H., Gohara, K., Mae, S., Matsumoto, R., 1999. Raman spectroscopic determination of hydration number of methane hydrates. AIChE 45, 2641e2645. Uchida, T., Ebinuma, T., Takeya, S., Nagao, J., Narita, H., 2002. Effects of pore sizes on dissociation temperatures and pressures of methane, carbon dioxide and propane hydrate in porous media. J. Phys. Chem. B. 106, 820e826. Vysniauskas, A., Bishnoi, P.R., 1983. A kinetic study of methane hydrate formation. Chem. Eng. Sci. 38 (7), 1061e1972. Zhang, P., Wu, Q., Yang, Y., 2013. Characteristics of methane hydrate formation in artificial and natural media. Energies 6, 1233e1249.
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Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Laboratory study of methane hydrate formation kinetics and structural stability in sediments Ch. V.V. Eswari a, B. Raju a, b, V. Dhanunjana Chari a, c, P.S.R. Prasad a, *, Kalachland Sain a a
Gas Hydrates Division, National Geophysical Research Institute, Council for Scientific and Industrial Research, Hyderabad 500007, India School of Physics, University of Hyderabad, Hyderabad 500046, India c Department of Physics, Osmania University, Hyderabad 500007, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 February 2014 Received in revised form 12 August 2014 Accepted 13 August 2014 Available online 28 August 2014
The natural gas hydrate deposits in the offshore of India are embedded in different sediments, namely clay rich silts and sands, coarse grain sand and volcanic fly ash. The variations in gas hydrate concentrations at different geological locations shows dependency on sediment mineralogy. It is also known that the particle size of the sediments plays an important role in hydrate formation and gas hydrate concentrations in sediments. We carried out systematic studies on the methane hydrate formation kinetics and methane hydrate volumetric yields, in stirred reactor experiments, using suspensions of synthetic silica and natural sediment from KrishnaeGodavari (KG) Basin. The hydrate formation behavior in silica and KG basin sediment is also compared with the formation of methane hydrates in a “pure system” without sediment or added silica grains. Our results show that the hydrate formation kinetics is faster in 50 mm silica system followed by that in natural marine sediment. Observed methane hydrate yield in the laboratory is higher (~39%) in both the pure (no sediment) and 1 mm silica suspensions. The gas intake is much quicker (~375 min) in the suspension of 50 mm silica system, while the hydrate yield is noticeably less (~29.38%). The methane hydrates are characterized by Raman spectroscopy and they show characteristic structure I (sI) methane hydrate signatures, with a hydration number in the range 5.93e6.12. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Methane hydrate Particle size Sediment Kinetics Hydrate yield Raman spectroscopy
1. Introduction Gas hydrates (GH) are ice-like crystalline compounds, comprising of water cages forming through hydrogen bonding, and these cages are occupied by the suitably sized gas molecules. The encaged gas molecules are often referred as guest molecules, and usual guest molecules in natural gas hydrates are methane (CH4), ethane, propane or carbon dioxide (Englezos, 1993). Much weaker Van der Waals interactions between the guest molecules and the surrounding water cage provide structural stability. Despite the fact that the gas hydrates look like ice, their physical properties are quite different; they can exist at temperatures well above the ice melting point at moderate pressures (Sloan and Koh, 2008). The four essential conditions for GH formation are (i) availability of host water molecules, (ii) suitable guest molecules with molecular diameter matching with available space within the host cages, (iii)
* Corresponding author. E-mail addresses: (P.S.R. Prasad).
[email protected],
http://dx.doi.org/10.1016/j.marpetgeo.2014.08.010 0264-8172/© 2014 Elsevier Ltd. All rights reserved.
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moderately high pressure and (iv) lower temperature typically in the vicinity of freezing point of ice. Methane (CH4) is the most common and predominant guest molecule (95e99%) in natural gas hydrates (NGH), and therefore we investigate the behavior of methane hydrates (MH) using silica/clay suspensions in this study. The stability of submarine gas hydrates is largely dictated by the pressure and the temperature, gas composition and pore water salinity etc. However, the physical properties and surface chemistry of marine sediments may also affect the thermodynamic state, growth kinetics, spatial distributions of gas hydrates (Clennell et al., 1999). Gas hydrates have been inferred to exists in the Saurashtra and KeralaeKonkan Basins along the east coast of India. Seismic evidence for the presence of gas hydrate also exists in the KrishnaeGodavari (KG), Mahanadi and Andaman sea regions of eastern India (Sain and Gupta, 2012). The NGHP-Expedition 01 (JODIES Resolution) confirmed the presence of hydrates in the KG Basin, Mahanadi Basin and Andaman region (Collett et al., 2008). The sedimentary property at these locations reveals that they are different from each other, e.g., clay rich marine sediment in KG Basin, coarse clay-rich silts and sands in Mahanadi and volcanic fly ash in Andaman (Sain and Gupta, 2012). Ginsburg et al., 2000
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demonstrated the role of sediment grain size on controlling the presence of hydrate at various sites on the Blake Ridge. Experimental results of Kawasaki et al. (2009) on sediments from the eastern Nankai Trough, indicate that particle size and clay contents are the two key factors determining the saturation level of gas hydrate in sediments; the finer the particle size and/or the higher the clay content, the lower the hydrate saturation. Therefore it is necessary to investigate the hydrate formation mechanism using host sediments to understand the role of mineralogy and grain size effects on the formation and structural stability of gas hydrate deposits. In the geological environment the hydrate formation can occur very quickly or may take a long time and previous laboratory reports show that the time required in converting the pore water to hydrate, using methane gas saturated water at about 17.4 MPa, was around 50 days (Spangenberg et al., 2005). However, in a stirred reactor system, gas hydrate form much faster and can be used to critically examine controls on gas hydrate formation. In a stirred system the water and gas interfacial area changes regularly, therefore, improves mass and heat transfer to increase the methane hydrate formation process (Hao et al., 2007). It is also well documented in the literature that the nucleation and growth of hydrates is considerably faster in stirred reactors (Hao et al., 2007; Ke and Svartaas, 2011; Prasad et al., 2012). Effect of the stirring speed on the kinetics of methane hydrate formation was investigated earlier and there is no simple correlation between stirring rate and nucleation rate (Hao et al., 2007). The higher stirring speed causes a reduction in hydrate induction time and an increase in hydrate growth rate (Hao et al., 2007). But beyond some critical speed (typically 1000 rpm for a laboratory reactor) the hydrate yield may decrease because of increase in the fluid temperature. On the other hand, at lower stirring speeds the hydrate induction time increases and the hydrate crystals formed tend to remain on the surface. Vysniauskas and Bishnoi (1983) have suggested an optimal stirring speed of 400 rpm or more to improve the efficiency of the process. Mesoscopic tools including X-ray computed tomography (XTG) (Kneafsey et al., 2007) and Scanning Electron Microscopy (SEM) (Kuhs et al., 2006) are being used to investigate the morphology
and spatial distribution of hydrate at the state of sediment pores. At molecular level, tools such as Raman (Uchida et al., 1999; Chari et al., 2014); Nuclear Magnetic Resonance (NMR) (Susilo et al., 2007) spectroscopy; X-ray and Neutron diffraction (Murshed and Kuhs, 2009) are used to analyze the molecular properties and structure of the hydrate phase. The features of micro-Raman spectroscopy can unambiguously provide information about the structure, the hydration number and the cage occupancy of encaged molecules in hydrate phase (Prasad et al., 2009; Sum et al., 1997; Chari et al., 2014). Additionally, chemical identification of the guest molecules is also possible using micro-Raman probe (Prasad et al., 2007). Further, we have used Raman spectroscopy to elucidate the structural information and hydration number of the methane hydrates synthesized within this study. 2. Experimental design 2.1. Materials and characterization The silica particles of diameter 1 mm and 50 mm were synthe€ bers method (Nozawa et al., 2005; sized using a modified Sto Sreenivasa Rao et al., 2005). The natural sediment was collected from KG Basin at a depth of ~40 m below sea floor (mbsf) in our previous cruise (Ramprasad, 2007) at latitude and longitude of 150.45.81N and 810.47.03E respectively. The silica/natural sediment with de-ionized ultra-pure water and a methane gas with a purity of 99.95% was used to synthesize the methane hydrates. In Figure 1, we show the morphology and elemental composition of the samples analyzed by a Field Emission Scanning Electron Microscopy (FESEM) and Energy-dispersive X-ray spectroscopy (EDX). SEM images show that the silica particles are mostly spherical in shape. However, the KG Basin sediment appears mostly as flakes. The bulk density of silica and KG basin sample is measured as 1.5 and 1.27 g cm3. Elemental analysis with an EDX system shows that the synthesized silica particles confirm the presence of Si and O2 with a ratio of close to 1:2. Whereas in KG basin sediment the elemental composition consists of mainly SiO2, K, Ca, Fe, MgAl etc., are corroborated with earlier report of KG basin mineralogy (Pattan, 2002).
Figure 1. FESEM images and elemental compositions from EDAX spectrum of (a) 1 mm silica (b) 50 mm silica and (c) KG basin sediment used in the present study.
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Molecular spectroscopic techniques are also used to identify the mineral composition and the purity of the sample. We have used Fourier Transform Infrared Spectroscopy (FTIR) to identify the clay and silica compositions. The sediment sample was dried in vacuum oven at a temperature of 150 C for 10e12 h to remove the pore waters trapped in the sample. The chemical purity of the samples were characterized by the FTIR are shown in Figure 2, and it was found to be 95%. The FTIR spectrum of sediment sample consists of several absorbance peaks at 490 cm1 (SieO), 540 cm1 (SieOeAl), 1090 cm1 (SieOeSi), 1650 cm1 stretching and other AleOeH modes. The best match from the mineral library for this data is for bentonite clay having structural composition as (Na, Ca) 0.3 (Al, Mg)2 Si4O10 (OH)2 nH2O (Preeti and Singh, 2007). The FTIR spectrum of silica samples with peaks at 490 cm1 (SieO), 800 cm1 and 1090 cm1 (SieOeSi) and 3600 cm1 eOH modes and the closest match is for silica of Nicolet library.
2.2. Laboratory apparatus and procedure A schematic experimental layout is shown in Figure 3. The main part of the apparatus is a cylindrical (SS-316) vessel that 100 ml can withstand the pressures up to 10 MPa. A stirrer with variable speed was installed in the vessel to agitate the fluids (Amar Equipments Pvt. Ltd., Mumbai, India). The required amount (3.15 g) of silica/ sediment and 27 g of degassed ultra-pure water (30 g in case of pure methane system) were measured by using Metler Toledo (AB104-S) accurate analytical balance. All the experiments were conducted with a fixed speed of 500 rpm. The reactor vessel, containing an aqueous solution (approximately 30% by volume of the vessel) was immersed into the temperature controlled bath and the cold fluid (water þ glycol mixture) was circulated around the vessel with the help of a circulator to maintain the temperature inside cell at a desired level. A platinum resistance thermometer (Pt100) inserted into the vessel to measure temperatures with an accuracy of ±0.2 K. The pressure in the vessel was measured with a pressure transducer (WIKA, type A-10 for pressure range 0e10 MPa with ±0.5 accuracy). The methane gas was supplied from a cylinder to desired level (7.0e7.5 MPa) using Teledyne ISCO Syringe pump. After obtaining the temperature and pressure stability far above from the hydrate formation region, the inline valve connecting the vessel and the ISCO pump/cylinder was closed. Subsequently, the cold fluid from the chiller was circulated around the reactor vessel
201
to bring the system into the hydrate stability zone. The hydrate formation in the vessel was detected by sharp pressure drop at temperatures around 273 K-Pure, 274 K-Sediment, 276 K-50 mm, 277 K-1 mm. The insignificant head-pressure drop in the reactor over longer duration indicates the hydrate conversion ceased. Later on the temperature is increased stepwise to bring the system out of the hydrate-forming region, hydrate crystals partially dissociate, thereby substantially increasing the pressure. The temperature and pressure were logged at 60 s time intervals using the data acquisition system. A pressure temperature diagram (cooling and warming) was obtained for each experimental run. The molar concentration of methane gas (DnH, t) in the hydrate phase during the experiment is defined by the following equation (1):
DnH; t ¼ ng; 0 ng; t ¼ ðP0 V=Z0 RT0 Þ ðPt V=Zt RTt Þ
(1)
where Z is the compressibility factor calculated by the PengeRobinson equation of state. The gas volume (V) was assumed 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 taken at zero time and in the gas phase at time t, respectively (n represent the number of moles of feed gas). The hydrate yield was computed from the observed methane gas consumption to the expected values with ideal stoichiometry compositions (8CH4$46H2O) (Chari et al., 2012, 2013) and with the equation (2).
Conversion of water to hydrates ¼
DnH ; t hydration number nH2 O
mol % (2)
where DnH ; t is the number of moles of methane gas consumed for hydrate formation at the end of the experiment determined by equation (1) and nH2 O is the total number of moles of the water used in the system. 3. Results and discussion To study the effect of sediment grain size and composition on methane hydrate formation we conducted experiments in a stirred reactor with lower weight % of natural marine sediment samples and synthetic pure laboratory silica. The use of a stirred reactor cell
Figure 2. FTIR spectrum of silica samples and the silica spectrum from NICOLET-NEXUS library for comparison (left). FTIR spectrum of natural sediment from KG basin and the bentonite spectrum from NICOLET-NEXUS library for comparison (right).
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Figure 3. Schematic diagram of the experimental setup. 1. CH4 Gas cylinder, 2. ISCO Pump, 3. Inlet port for Gas, 4. Outlet/Vacuum port, 5. Magnetic Stirrer assembly, 6. Temperature sensor, 7. Pressure gauge and transducer, 8. Data acquisition and control, 9. Inlet for cold fluid, 10. Outlet for cold fluid, 11. Closed cycle refrigerant fluid circulator (LAB CAMPANION) and 12. Computer.
yielded faster nucleation and growth of methane hydrates by renewing the contact surface between the liquid and gas phases. Therefore a constant stirring (with a speed of 400 rpm) was applied throughout each experimental run (~20 h). In Figure 4, we show the pressureetemperature (peT) trajectory of CH4 þ water, CH4 þ water þ SiO2 (1 & 50 mm) and CH4 þ water þ Bentonite-clay suspensions. It is observed from Figure 4, that all systems require some amount of sub-cooling (5 e8 ) for hydrate nucleation,
growth and the sub cooling temperature found to be minimum for silica system than pure and natural sediment. As the hydrate formation is an exothermic process, associated with liberation of heat (Prasad et al., 2012), during further hydrate growth, the temperature increases to a maximum and then it slowly equilibrates with the outer wall temperature. After the nucleation, the hydrate growth is associated with a large pressure drop. The steady state temperature inside the reactor
Figure 4. The pressureetemperature trajectories showing the formation and dissociation of methane hydrate (MH) synthesized in stirred reactor in all the systems. The phase boundary curve for sI methane hydrate computed using CSMGem is shown by the solid line.
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Figure 5. Consumption of methane (CH4) gas in the reactor vessel as a function of time in pure and different sediment suspensions.
vessel has a small but noticeable dependence on the nature of the sediment, it is about 275 K for the natural sediment, 277 K for the pure hydrate system, and 279 K for those containing silica. This could be because of slower/faster heat transfer between the hydrates and the silica/sediment particles. The observed pressure drop was the greatest (2.77 MPa) for the pure gas hydrate system and the lowest (1.98 MPa) for the 50 mm silica system. The hydrate formation could be divided into three regions as shown in Figures 4 and 5. In region 1, a monotonic temperature and pressure decrease, was mainly due to the non compressive nature of the methane molecules. The second stage (region 2) where the temperature remained constant under steady state condition for about 9e10 h after hydrate nucleation for its maximum conversion/ growth. We also observed a significant pressure drop in the reactor vessel during this period. Thereafter, in region 3, the temperature and pressure monotonically decreased similar to region 1. The plot between gas consumed vs. time in these three regions provides necessary data on the rate of hydrate formation and the total gas consumed during hydrate formation (Abay and Svartaas, 2010; Prasad et al., 2012; Chari et al., 2013). The plot between the methane gas consumed (m$mol/mol$H2O) vs. time is shown in Figure 5. The methane gas content remained almost constant in the initial stage (region 1), indicating that there is no phase change for the water and methane gas present in reactor vessel. Very small change in the slope of the gas consumption curve with time during the hydrate formation (region 1) indicates the dissolution of methane gas in the water. In the second stage, there is a sharp rise in the slope of the gas consumption curve for the formation of hydrates and this continues for a while representing the hydrate growth (region 2). Finally, the constant slope signifies that there is no further gas consumption (region 3) and the hydrate conversion is ceased under these experimental conditions. Experiments are continued for longer duration (nearly 12e15 h) upon reaching a steady state temperature for achieving the maximum hydrate conversion. Though hydrate yield is lower in the case of 50 mm silica, the hydrate formation kinetics is quicker. After the hydrate nucleation, almost 80% of hydrate conversion is completed in 100 min. The time required for gas hydrate growth in the pure and 1m systems are similar (~500 min) and for the sediment it is around 185 min. The studies of Zhang et al. (2013), show that the hydrate growth occurs more readily in coarse grain systems. The water transfer characteristics in coarse grain sediments are observed to be more effective than in fine-grained sediments. Significant differences in the reactions in the two types of media arose from
203
differences in the water retention capacity and lithology of media due to the internal surface area and pore size distributions. Our results indicate that the suspension of silica or clay sediments, primarily, acts as kinetic promoters, even though their thermodynamic inhibition is significantly low. It is well documented in the literature that the porous silica and clay particles show thermodynamic inhibition for methane and natural gas hydrates systems to a maximum extent of 1.0e1.5 K and this reaction is dependent on pore size (Kang and Seo, 2008; Østergaard et al., 2001; Aladko et al., 2004). Usually the smaller the pore size the greater the inhibition effects. The experimental studies on methane hydrate dissociation with different pore sizes were reported by Uchida et al. (2002). According to them there was a pore size inhibition effect on hydrate stability with respect to the bulk hydrate. They observed largest shift towards inhibition in case of 4 nm pore size around 12.3 K ± 0.2 K where as it was reduced to 0.5 K with a pore size of 100 nm. The studies by Riestenberg et al. (2003) reported that the kinetics of methane hydrate formation has been significantly affected by the presence of small amount of solid (bentonite) particles (200 mg/l). The required driving force (overpressure) at around 277e279 K (4e6 C) was ~5.37e4.13 MPa before observing hydrate formation in pure water, and this was significantly decreased in colloidal suspension of bentonite to ~0.66 MPa at 4.5 C. Thus the presence of bentonite particles may help formation of methane hydrates. As shown in Figure 5, the methane hydrate formation is significantly faster in the suspensions with silica/bentonite. Some recent studies reported that the cooling rate also effects the rate of nucleation and gas conversion. Under initial conditions, faster the cooling rate, shorter the nucleation time and the lower the methane gas conversion rate (Guanli et al., 2010). Further, from Figure 3 it can be concluded that the methane intake is considerably faster in silica suspension compared to that in natural clay sediments or pure hydrates. Precise reason for this is not known at present, but we feel that the grain surface effects and mineralogy play significant role in hydrate formation/growth, but our results are in broad agreement with recent literature (Riestenberg et al., 2003, Guanli et al., 2010). Methane hydrate formation/dissociation mechanism was also studied by Kneafsey et al. (2007) and Linga et al. (2009) in water saturated silica sand. The main conclusions of those studies are that the hydrate formation is quicker in silica þ water system with multiple nucleation sites. The rate of hydrate conversion is also not always proportional to the driving force in porous medium. The hydrate conversion typically occurred in three stages, an initial slower rate was followed by much quicker conversion and finally slower rates in the third stage. Linga et al. (2009) also reported the size and shape of silica sand beds has an influence over the hydrate formation. In our experiments we also observed the hydrate conversion is in multi-stages in pure, 1 mm silica, and bentonite-clay systems. Further, the overall duration of our experiments is less because rapid stirring promoted hydrate conversion. Recently methane hydrate growth simulations in the presence of silica surfaces have shown that the silica play a significant role in hydrate growth (Liang et al., 2011). They proposed that the silanol groups at the silica surfaces have a lower polarity compared to water molecules and thus proved a favorable location for methane bubble formation. Upon growth of hydrate crystal at the methane bubble, the bubble was found to move away from the advancing crystal growth front, consistent with the water-wet nature of the hydrates (Liang et al., 2011). It was also reported that sediment properties also affect the formation process of methane hydrate within it. In fine grained, the gas conversion rate is lowest; in coarse sand, the gas conversion rate is the greatest; and in fine sand, it is more intermediate. According Guanli et al. (2010) smaller the particle size of the host sediment, harder it is to grow hydrate.
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Table 1 Experimentally observed methane gas consumption during hydrate formation in suspensions with different silica particles and natural sediment (bentonite) from KG basin. The measured hydration number of hydrates formed in different systems. S. no Particle size
Yield (%)
nCH4 Consumed/H2O Hydration number (n) (m mol/mol)
1 2 3 4
39.74 39.25 39.72 29.38
63.21 62.41 63.19 46.06
Pure Sediment 1 um 50 um
(±3.430) (±3.615) (±2.430) (±0.884)
(±5.760) (±6.081) (±4.078) (±1.442)
5.93 6.02 5.96 6.12
(±0.13) (±0.22) (±0.21) (±0.09)
As shown in Table 1, experimentally measured methane consumption per mole water during the hydrate formation in the present study differ with the reports of Guanli et al. (2010). The hydrate conversion is about 20e30% lower in silica (50 mm) suspensions compared to the pure methane hydrate system (without sediment). Interestingly the suspensions with 1 mm silica also resulted in comparable hydrate yield or methane consumption to that of the pure methane hydrate and natural sediment sample system, but the kinetics were slower. Table 1 provides average methane consumption (m mol/mol H2O) for each experimental run (3 cycles of forming and dissociation) with identical cooling/heating rates and estimated standard errors. From the previous studies, it is well established that the Raman spectroscopy has unambiguous spectral signatures for the methane molecules in free/dissolved or encased in clathrate cages (Prasad et al., 2009; Sum et al., 1997; Chari et al., 2014). The procedure we followed in collecting the Raman signal and profile fitting were well described elsewhere (Chari et al., 2014). In Figure 6, we show the Raman spectrum of pure methane hydrate and the methane hydrates synthesized in sediments. The gas hydrates have distinguishable Raman shifts for methane molecules in clathrate phase (2904 and 2915 cm1), whereas in gas phase it is at 2917 cm1 (not shown in the figure). Such strikingly different signatures are useful in establishing the ground truth occupancy in the hydrate lattice of methane molecules. In Figure 6, these specific Raman signatures for pure methane hydrates and those synthesized with silica and sediment indicating that the methane molecules exist in a clathrate phase. Observed doublet in CeH stretching mode region clearly indicate that the methane molecules are enclathrated in water cages and the relative Raman intensity of these modes are consistent with sI structure. The calculated values of hydration number for the hydrate grown in the
pure gas hydrate system (without sediment), the natural sediment system and the synthetic silica system (See Table 1), in the temperature range 153 Ke233 K are in good agreement with other values reported in the literature that range from about 5.8 to 6.3 (Sloan and Koh, 2008). 4. Conclusions Methane hydrate formation was studied in a stirred reactor of 100 ml capacity. We have compared and contrasted the growth of methane hydrate in three systems: (1) Pure water and methane without any sediment, (2) a suspension of water, methane, and synthetic silica grains (8 wt % mixture; and grain size of 1 and 50 mm), and (3) a suspension of water, methane, and naturally occurring clay-rich sediment (characterized as bentonite) from KrishnaeGodavari Basin. The results obtained from the experiments clearly indicate a particle size and clay dependent trend of hydrate formation kinetics. When we compared to the pure water and methane system (without sediment), the growth of gas hydrate is significantly faster in the system with the suspension of water, gas, and synthetic silica with grain size of 50 mm. However, the volume of methane hydrate that formed in the pure water and methane system (without sediment) was greater than the volume that formed in the suspension of water, methane, and synthetic silica with a grain size of 50 mm, while it is comparable to the volume of methane hydrate that formed in the natural-clay-rich sediment, 1 mm silica systems. The structure of the hydrate grown in each experiment was confirmed by Raman spectroscopy to be structure I (sI) with a hydration number of 5.93e6.12. Indicating that there was no effect of the sediment on the structure or cage occupancy of the laboratory grown methane hydrate. Acknowledgments The author(s) wish to thank those that contributed to the success of the National Gas Hydrate Program Expedition 01 (NGHP01). NGHP01 was planned and managed through collaboration between the Directorate General of Hydrocarbons (DGH) under the Ministry of Petroleum and Natural Gas (India), the U.S. Geological Survey (USGS), and the Consortium for Scientific Methane Hydrate Investigations (CSMHI) led by Overseas Drilling Limited (ODL) and FUGRO McClelland Marine Geosciences (FUGRO). The platform for the drilling operation was the research drill ship JOIDES Resolution, operated by ODL. Much of the drilling/coring equipment used was provided by the Integrated Ocean Drilling Program (IODP) through a loan agreement with the US National Science Foundation. Wireline pressure coring systems and supporting laboratories were provided by IODP/Texas A&M University (TAMU), FUGRO, USGS, U.S. Department of Energy (USDOE) and HYACINTH/GeoTek. Downhole logging operational and technical support was provided by Lamont-Doherty Earth Observatory (LDEO) of Columbia University. The financial support for the NGHP01, from the Oil Industry Development Board, Oil and Natural Gas Corporation Ltd., GAIL (India) Ltd. and Oil India Ltd. is gratefully acknowledged. We also acknowledge the support extended by all the participating organizations of the NGHP: MoP&NG, DGH, ONGC, GAIL, OIL, NIO, NIOT, and RIL. Authors sincerely thank the Director of CSIR National Geophysical Research Institute, Hyderabad, for permission to publish this paper. This is a contribution to GEOSCAPE Project of NGRI under the 12th Five Year Scientific Program of CSIR. References
Figure 6. Characteristic methane stretching modes of CH4 molecules encaged in 512and 51262 cages of methane hydrates synthesized in pure and different sediments (8 wt %). The Raman spectra were collected at 153 K and 0.1 MPa.
Abay, H.K., Svartaas, T.M., 2010. Effect of ultralow concentration of methanol on methane hydrate formation. Energy Fuels 2, 752e757.
Ch.V.V. Eswari et al. / Marine and Petroleum Geology 58 (2014) 199e205 Aladko, E.Y., Dyadin, Y.A., Fenelonov, V.B., Larionov, E.G., Mel’gunov, M.S., Manakov, A.Y., Nesterov, A.N., Zhurko, F.V., 2004. Dissociation conditions of methane hydrate in mesoporous silica gels in wide ranges of pressure and water content. J. Phys. Chem. B 108, 16540e16547. Chari, V.D., Sharma, D.V.S.G.K., Prasad, P.S.R., 2012. Methane hydrate phase stability with lower mole fractions of tetrahydrofuran (THF) and tert-butylamine (tBuNH2). Fluid Phase Equilib. 315, 126e130. Chari, V.D., Sharma, D.V.S.G.K., Prasad, P.S.R., Murthy, S.R., 2013. Methane hydrates formation and dissociation in nano silica suspension. J. Nat. Gas. Sci. Eng. 11, 7e11. Chari, V.D., Prasad, P.S.R., Murthy, S.R., 2014. Structural stability of methane hydrates in porous medium: Raman spectroscopic study. Spectrochim. Acta Part A 120, 636e641. Clennell, M.B., Hovland, M., Booth, J.S., Henry, P., Winter, W.J., 1999. Formation of natural gas hydrates in marine sediments. J. Geophys. Res. 104, 985e1023. Collett, T.S., Riedel, M., Cochran, J., Boswell, R., Presley, J., Kumar, P., Sathe, A.V., Sethi, A.K., Lall, M., Sibal, V.K., NGHP Expedition 01 Scientists, 2008. NGHP Expedition 01 (2006). Initial Reports. Directorate General of Hydrocarbons, Noida and Ministry of Petroleum & Natural Gas, India, p. 4. Englezos, P., 1993. Clathrate hydrates. Ind. Eng. Chem. Res. 32, 1251e1274. Ginsburg, G., Soloviev, V., Matveeva, T., Andreeva, I., 2000. Sediment grain-size control on gas hydrate presence, sites 994, 995, and 997. Proc. Ocean. Drill. Program, Sci. Results 164, 237e245. Guanli, J., Qingbai, W., Jing, Z., 2010. Effect of cooling rate on methane hydrate formation in media. Fluid Phase Equilib. 298, 225e230. Hao, W., Wang, J., Fan, S., Hao, W., 2007. Study on methane hydration process in a semi-continuous stirred tank reactor. Energy Convers. Manag. 48, 954e960. Kang, S.P., Seo, Y., 2008. Proc. Int. Gas Union Research (IGRC), Paris, pp. 1e10. Kawasaki, T., Hailong, L.U., Fujii, T., Ripmeester, J.A., 2009. Investigations of particle size and clay mineral effect on gas hydrate formation in natural sediment. J. Geogr. 118, 872e882. Ke, W., Svartaas, T.M., 2011. Effects of Stirring and Cooling on Methane Hydrate Formation in a High-pressure Isochoric Cell. Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom. Kneafsey, T.J., Tomutsa, L., Moridis, G.J., Seol, Y., Freifeld, B.M., Taylor, C.E., Gupta, A., 2007. Methane hydrate formation and dissociation in a partially saturated corescale sand sample. J. Pet. Sci. Eng. 56, 108e126. Kuhs, W.F., Staykova, D.K., Salamatin, A.N., 2006. Formation of methane hydrate from polydisperse ice powder. J. Phys. Chem. B 110, 13283e13295. Liang, S., Rozmanov, D., Kusalik, P.G., 2011. Crystal growth simulations of methane hydrates in the presence of silica surfaces. Phys. Chem. Chem. Phys. 13, 19856e19864. Linga, P., Haligva, C., Nam, S.C., Ripmeester, J.A., Englezos, P., 2009. Gas hydrate formation in a variable volume bed of silica sand particles. Energy fuels 23, 5496e5507. Murshed, M.M., Kuhs, W.F., 2009. Kinetic studies of methaneeethane mixed gas hydrates by neutron diffraction and Raman spectroscopy. J. Phys. Chem. B 113, 5172e5180.
205
Nozawa, K., Gailhanou, H., Raison, L., Panizza, P., Ushiki, H., Sellier, E., Delville, J.P., € ber silica particles: efDelville, M.H., 2005. Smart control of monodisperse sto fect of reactant addition rate on growth process. Langmuir 21, 516e1523. Østergaard, K.H., Anderson, K., Llamedo, M., Tohidi, B., 2001. Terra Nova 14, 307e312. Pattan, J.N., 2002. Volcanic ash and its enigma: a case study from the central Indian ocean basin. J. Geol. Soc. India 60, 127e130. Prasad, P.S.R., Prasad, K.S., Thakur, N.K., 2007. Laser Raman spectroscopy of THF clathrate hydrate in the temperature range 90e300 K. Spectrochim. Acta Part A 68, 1096e1100. Prasad, P.S.R., Sowjanya, Y., Prasad, K.S., 2009. Micro-Raman investigations of mixed gas hydrates. Vib. Spectrosc. 50, 319e323. Prasad, P.S.R., Chari, V.D., Sharma, D.V.S.G.K., Murthy, S.R., 2012. Effect of silica particles on the stability of methane hydrates. Fluid Phase Equilib. 318, 110e114. Preeti, S.N., Singh, B.K., 2007. Instrumental characterization of clay by XRF, XRD and FTIR. Bull. Mater. Sci. 30 (3), 235e238. Ramprasad, T., 2007. Acquisition of Long Sediment Cores in KrishnaeGodavari and Mahanadi Offshore Basins in Bay of Bengal Onboard R/V Marion Dufresne. Cruise Report: MD161-GH. NIO, Goa, pp. 11e28. Riestenberg, D., West, O., Lee, S., McCallum, S., Phelps, T., 2003. Sediment surface effects on methane hydrate formation and dissociation. J. Mar. Geol. 198, 181e190. Sain, K., Gupta, H., 2012. Gas hydrates in India: potential and development. Gondwana Res. 22, 645e657. Sloan, E.D., Koh, C.A., 2008. Clathrate Hydrates of Natural Gases, third ed. CRC Press, Taylor & Francis Group, Boca Raton. Spangenberg, E., Kulenkampff, J., Naumann, R., Erzinger, J., 2005. Pore space hydrate formation in a glass bead sample from methane dissolved in water. Geo. Res. Lett. 32, 1e4. Sreenivasa Rao, K., Hami, K El, Kodaki, T., Matsushige, K.J., 2005. A novel method for synthesis of silica nanoparticles. Collid. Interface Sci. 289, 125e131. Sum, A.K., Burruss, R.C., Sloan, E.D., 1997. Measurement of clathrate hydrates via raman spectroscopy. J. Phys. Chem. B 10, 7371e7377. Susilo, R., Ripmeester, J.A., Englezos, P., 2007. Tuning methane content in gas hydrates via thermodynamic modeling and molecular dynamics simulation. Chem. Eng. Sci. 62, 3930e3939. Uchida, T., Hirano, T., Ebinuma, T., Narita, H., Gohara, K., Mae, S., Matsumoto, R., 1999. Raman spectroscopic determination of hydration number of methane hydrates. AIChE 45, 2641e2645. Uchida, T., Ebinuma, T., Takeya, S., Nagao, J., Narita, H., 2002. Effects of pore sizes on dissociation temperatures and pressures of methane, carbon dioxide and propane hydrate in porous media. J. Phys. Chem. B. 106, 820e826. Vysniauskas, A., Bishnoi, P.R., 1983. A kinetic study of methane hydrate formation. Chem. Eng. Sci. 38 (7), 1061e1972. Zhang, P., Wu, Q., Yang, Y., 2013. Characteristics of methane hydrate formation in artificial and natural media. Energies 6, 1233e1249.