EXPERIMENTAL STUDY OF SELF-PRESERVATION MECHANISMS DURING GAS HYDRATE DECOMPOSITION IN FROZEN SEDIMENTS

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1 Proceedings of the 7th International Conference on Gas Hydrates (ICGH 11), Edinburgh, Scotland, United Kingdom, July 17-1, 11. EXPERIMENTAL STUDY OF SELF-PRESERVATION MECHANISMS DURING GAS HYDRATE DECOMPOSITION IN FROZEN SEDIMENTS Evgeny Chuvilin, Boris Buhanov, Olga Guryeva Department of Geology Moscow State University Leninskie Gory, Moscow, RUSSIA Vladimir Istomin Gazprom VNIIGAZ, Razvilka, Leninsky Rayon, Moskovskaya oblast, RUSSIA Satoshi Takeya National Institute of Advanced Industrial Science and Technology, Higashi, Tsukuba JAPAN Akihiro Hachikubo Kitami Institute of Technology 165 Koen-cho, Kitami, Hokkaido JAPAN ABSTRACT Gases released from shallow frozen sediments may include gaseous methane due to metastable hydrate's decomposition. Relict gas hydrates in permafrost may be in metastable state as a result of self-preservation effect. So a series of experiments with gas hydrates in porous sediments have been desired to confirm and to detail this possibility. The experimental technique included the following stages: i) saturation of sediments by methane or carbon dioxide hydrates, ii) freezing of the hydrate-containing samples, iii) studying of the hydrate decomposition process including self-preservation in frozen samples after the reducing gas pressure below line of three phase equilibrium (gaseous phase gas hydrate ice). Core samples of different composition and porous structure recovered from gas-showing horizons of permafrost sediments were used. Experimental data for hydrate's decomposition in frozen sediments when pressure is reduced below equilibrium line are presented. Some interesting features of the self-preservation effect in hydrate-containing sediments at negative (Celsius) temperatures were discussed. Keywords: gas hydrates, self-preservation; frozen sediment, methane, carbon dioxide, unfrozen water, nonclathrated water Corresponding author: Phone/Fax: chuvilin@geol.msu.ru

2 NOMENCLATURE H gravimetric hydrate content [%] H v volumetric hydrate content [%] K h hydrate coefficient [u.f.] m G weight of gas [g] M molar mass of gas [g/mol] n porosity of the sediment sample [u.f.] P i pressure at a point in time i [MPa] R universal gas constant [J/K mol] S h hydrate saturation [%] S i ice saturation [%] T i temperature at a point in time i [K] V free volume of the pressure chamber [cm 3 ] W gravimetric water content of sample [%] W Н amount of water that transformed into hydrate [% to weight of dry sample] W in initial water content [%] z gas compressibility [-] ρ density of the sediment sample [g/cm 3 ] ρ Н density of hydrate [g/cm 3 ] INTRODUCTION Permafrost sediments occupy significant part of the Arctic areas of continental Eurasia and America. Their thickness reaches 3-7 meters. Permafrost also exists within the framework of a continental shelf of the Arctic seas. Continuous frozen sediments on Arctic shelf can occur up to sea depth of 6 meters. As the Arctic seas are shallow, the shelf frozen ground occupy greater territories, thus their thickness reach -3 meters [1]. Processes of cooling and long-term frost penetration which periodically occurred in Arctic regions promoted formation of not only powerful thicknesses of frozen sediments, but also natural gas hydrates congestions, primary methane gas hydrates. The stability zone of natural gas hydrates formations in cryolitozone where methane gas hydrates can be formed and exist, begins with depths of -5 meters and extends in sub-permafrost layers up to depths of 8-15 meters [, 3]. Recently obtained data allow to consider methane gas hydrates existence in thicknesses of frozen sediments above zone of its thermodynamic stability (up to depths of -5 meters) This zone may be called the zone of gas hydrate s metastability or zone of relict hydrate existence [4, 5]. The relict natural gas hydrate accumulations in permafrost may have formed under favorable thermodynamic conditions during glacial periods. Subsequently, glacial retreat induced metastable hydrate preservation due to self-preservation effect [6-8]. Gas hydrates self-preservation phenomenon can be defined as a very slow decomposition of gas hydrates when the external pressure drops below a three-phase equilibrium pressure of the gas-icehydrate system at sub-zero temperature (below -3 - o C) as a result of thin ice film emergence on gas hydrate surface [8, 9]. This effect was initially discovered and described in detail over , by researches from Canada (Ottawa National R&D Center) and Russia (VNIIGAZ and Moscow State University) [6, 1-1]. The term gas hydrate self-preservation was introduced by Russian researches after laboratory experiments which showed that gas hydrate decomposition come to a virtually halt when hydrate particles cover by thin ice shell at the initial stage of decomposition. At present the existence of natural gas hydrates at shallow depth at nonequilibrium conditions leads to a serious geological hazard during exploration drilling. The dissociation of shallow metastable methane hydrate may contribute to global warming by adding significant amount of a greenhouse gas (methane) to the atmosphere. Up to now relict natural gas hydrates are poorly understood, so the experimental research of gas hydrates decomposition in frozen sediments under non-equilibrium conditions is of significant importance. EXPERIMENTAL METHODS Physical modeling of porous gas hydrate dissociation process when reducing gas pressure below the equilibrium is based on our original experimental equipment. It includes a high pressure chamber of a working volume about 4 cm 3 with a special container for artificial hydratesaturating of the sediment samples, cryothermostate for automatic control of temperature mode in the pressure chamber, the device for transformation of electric signals from gages of temperature and pressure in digital form and operating computer. Accuracy of temperature gages is.5 o C, and pressure sensors in the pressure chamber have accuracy.5 MPa [13]. As a subject of research we used natural quartz sand (sand 1) and sediments selected from gascontaining horizons in Permafrost areas in the North of West Siberia - sand (Yamburg gas field) and sandy loam (Zapolyarnoe gas field). Their characteristics are shown in tables 1and.

3 The initial sediment water contents ranged from 1 to %. Methane (99.98%) and carbon dioxide (99.99%) were used as hydrate-former gases. Type of sediment Particle size distribution/% 1-.5 mm.5-.1mm <.1 mm Sand Sand Sandy loam Table 1. Grain size of sediments. Type of Mineral S, % sediment composition, % Sand 1 quartz >9.1 Sand quartz 8.76 feldspar - 9 Sandy loam quartz - 64 microcline - 9 albite -5. Table. Mineral composition and salinity of sediments. As a rule, samples with incomplete degree of water saturation were used for providing good gaswater contact in porous medium of the sediments. Sample s height was about 1 cm, its diameter 4.6 cm. It was set into the pressure chamber, sealed, and then saturated by gaseous methane (pressure 6-8 MPa) or carbon dioxide (pressure MPa). We used two schemes of the sample saturation by hydrates and preparation of frozen hydratecontaining samples. In the first scheme formation of gas hydrates in sediment sample occurred during cooling of the chamber from room (+ +5 o C) to small positive temperature (+1 +3 o C). After attenuation of the hydrate-formation process the chamber with the sample was frozen, so the residual porous water partially froze. During freezing additional hydrate-formation took place in the porous media. Therefore in several experiments for increasing of hydrate content in the sample the cyclic freezingthawing technology was used. In the second scheme the hydrate accumulation in the sample began at negative temperature (-4-6 o C). Then the temperature was gradually risen to small positive values (+1 +3 o C). Process of hydrate formation in pore space was intensified due to ice melting and creating some additional gas-water contacts. After attenuation of hydrate formation process in the sediment sample, the temperature was decreased to -6-8 o C. This approach developed by authors allowed receiving the soil samples with high hydrate saturation of pore spaces up to 6 % and more. Experiments on dissociation kinetics of pore hydrates in soils were carried out by two original methods. According to the first method, pressure in the chamber with frozen hydrate-bearing sample was decreased to atmospheric. Then the pressure chamber was opened, the sample was taken out and studied by different petrophysical methods. As a rule, it happened in 3 minutes after pressure reducing. Petrophysical methods included macroand micromorphological study, level-by-level determination of water content, density, gas- and hydrate-contents, porosity, hydrate coefficient (share of water which has transformed into hydrate), hydrate- and ice-saturation [13, 14]. For estimation of heat conductivity of frozen hydratecontaining samples under non-equilibrium conditions the analyzer KD by company Decagon Devices, Inc (USA) was used [15]. Determination of gas-content of the sample was carried out by measurement of gas volume allocated at thawing of the sample in saturated NaCl solution. Calculation of hydrate content was carried out using hydrate number 5.9 for methanebearing samples and 6.1 for the samples saturated by СО [14]. Part of the sample was placed into special containers for long storage at different negative temperatures from - o C up to- o C. To prevent the sublimation process the samples were covered by ice crumbs. In certain time intervals we selected samples to determine gas- and hydrate contents. These tests proceeded up to attenuation of pore hydrate decomposition process. Observation period reached one month or more. Using these data we obtained the dependences of hydrate content on time. So the hydrate decomposition kinetics and the self-preservation effect of pore gas hydrates in frozen sediments were studied. The second experimental method for studying of pore hydrate dissociation kinetics was based on the PVT-method. It allowed estimating hydrate dissociation rate directly in the pressure chamber using the changes in thermobaric conditions after

4 gas pressure decrease below the equilibrium. This approach gives possibilities to study dissociation kinetics in a wide range of temperatures and pressures, and to estimate more precisely the intensity of hydrate dissociation at the first stage of the process after gas pressure decrease. The hydrate accumulation parameters (hydrate saturation (S h ), volumetric hydrate content (H v ) and hydrate coefficient (K h ) were calculated on the basis of analysis of pressure chamber thermo-baric conditions changes using formula (1): Pi V M mg =, (1) R Ti z Volumetric hydrate content (H v ) was calculated by formula (): H ρ H v = ρ H () Hydrate saturation (S h ) was calculated by formula (3): H v S h = (3) n Hydrate coefficient (К h ) was calculated by formula (4): WH K h = (4) W EXPERIMENTAL RESULTS The frozen hydrate-containing samples have been received during the experiments on artificial hydrate accumulation (methane or СО hydrates in soil pore space) and their subsequent freezing to -8 o C. In general, the test samples were characterized by homogeneous sediment structure and uniform distribution of hydrate and ice in the pore space resulting from initial uniform water distribution. The examined samples were characterized by massive cryohydrates. Porosity of hydratecontaining samples varied in a range of Hydrate saturation under equilibrium conditions was from to 7 %, and the hydrate coefficient reached Our experimental data show that self-preservation reduces decomposition of pore-space hydrate in all samples. The self-preservation effect forms a thin ice film around particles of dissociating gas hydrate at negative temperatures. As a result of this effect hydrate can remain in a metastable condition for a long time. Self preservation of gas hydrate in frozen sediment depends on many factors such as thermobaric conditions, hydrate and ice saturation, gas permeability, structure of organ-mineral skeleton of sediment, pore water salinity and micromorphology of pore hydrate. Experiments revealed that high rate of dissociation takes place only at the first stage of the process. Then the rate slowly reduces up to the complete stop of the dissociation process (fig 1). Sh/Si,6,4, Figure 1: Change in time of hydrate (Sh) and ice saturation (Si) in the sample of gas-saturated sand (W in =18%, t= -6.5 о С) after pressure release to.1 MPa As one can see from the figure 1, two processes take place at the same time: decrease of hydratesaturation and increase of ice content which forms at the dissociation of gas hydrate (fig.1). Thus, in the beginning of the experiment hydrate saturation (S h ) was 46% and ice saturation S i was 8%. After 845 hours S h lowered to 11% and S i increased to 43%. Pore ice, forming from freezing of residual water, plays an important role in the self-preservation of gas hydrates. Appearance of this ice promotes the enhanced stability of gas hydrate and their initial preservation. Frozen hydrate-containing sediment samples with higher ice content are characterized by faster attenuation of the dissociation process after pressure release below equilibrium. Thus, with the increase in ice saturation before the pressure release (at -6-7 о С), the selfpreservation effect and stability of gas hydrates in sandy samples increases (fig.). 1

5 Hv, % 3 1 Si=% Si=38% Hv, % Figure : Influence of the ice saturation (S i ) on the kinetics of pore gas hydrates dissociation in the sample of sand (t= -6-7 o C; P=.1 MPa) Results of the experiments made on sand and loam samples show that intensity of pore gas hydrate dissociation sharply decreases at lower temperatures. In the silty sand sample (W in =14 %, S h =6% at all temperatures) hydrate-saturation in hours after pressure release fell to 5% at - C, to 8% at -4 C, and to 43% at -7 o C (Fig. 3). In 15 hours at - o C, a small amount of hydrate remained, however, at -4 o C and -7 o C decomposition rate was decreased significantly. Sh, % t=-7oc t=-4oc t=-oc Figure 3: Methane hydrate dissociation kinetics in artificially hydrate-saturated frozen sand- (W in =14 %) after pressure release to.1 MPa. In the course of experiments on samples selected from frozen gas-showing horizons within Yamburgskoe and Zapolyarnoe gas-condensate fields we showed the possibility of long-term preservation of gas hydrates at reservoir temperature (-6-7 o C) due to the self-preservation effect (fig. 4). Figure 4: Change of volumetric hydrate-content (H v ) in the samples of sand (1) and sandy loam () at pressure release below equilibrium (W in =19- %; t= -5-7 о C; Р=.1 МРа). In the course of the performed experiments we revealed the influence of mineral skeleton and structure of porous medium on the selfpreservation effect under negative temperatures. Addition of clay particles to fine-grained sand decreased preservation of pore hydrates under non-equilibrium conditions. Thus, in the sand sample with 7% of kaolin, the intensity of methane hydrate dissociation at -7 o C was higher then in sand sample without clay particles. Initial hydrate saturation in both samples was almost equal (3-33%). In 3 minutes after pressure release to the atmospheric hydrate-content decreased 4.5 times in the sample with clay particles and it was only 6% lower than initial in the sand sample. Further observation showed that methane hydrate dissociation rate was also significantly lower in the sand sample. Such behavior can be probably explained by difference in equilibrium water content (non-clathrated water content) in hydratecontaining sample during pressure change. Clay particles absorb liquid water which forms after pressure release and don t allow the protective ice layer formation around hydrate particles. The same dependence was obtained for higher content of clay particles in sand matrix. The study of thermal conductivity of hydratebearing sediment samples under non-equilibrium conditions allowed observing the kinetics of thermal conductivity change when pore hydrate decomposed, and to make a correlation between thermal conductivity and hydrate saturation of frozen samples. Thermal conductivity of hydratebearing samples increases with time after the pressure release. It is the result of slow dissociation of pore hydrate and structure-textural

6 transformations [15]. Thus, in the sample of hydrate-bearing silty sand thermal conductivity gradually increased from.5 Wm - 1 C -1 to 1.74 Wm - 1 C -1 during the testing period of 55 hours at t= -8 o C (volumetric hydrate content changed from 13 to 3%). Using PVT- analysis, the influence of various residual gas pressures on intensity of porous gas hydrates dissociation during the decrease of gas pressure in pressure chamber was experimentally shown. It was determined that the value of relative (in comparison with equilibrium) pressure reduction affects considerably the intensity of gas hydrates dissociation in porous media and the development of self-preservation effect. Usually more considerable pressure decrease leads to the increase of pore gas hydrate preservation. Thus in frozen hydrate saturated sandy loam sample during the lowering of pressure to1.6 MPa after 14 hours at -3. o C about 6% of porous hydrate preserved, while about 8% of methane hydrate preserved at the same temperature during pressure reduction to.1 MPa (fig.5). Relative change in Sh (% of Sh) ,6 МПа,1 МПа Figure 5: Relative change in hydrate saturation (S h ) in time at pressure decrease to.1 and to 1.6 MPa in the sample of sandy loam at t= -3. o C. According to the calculation of pore gas hydrate dissociation rate after the pressure release in the sample of sandy loam, the greatest difference in dissociation rates was observed during first hours of the experiment (fig. 6). Dissiciation intensity (dsh/t), % /h ,6 MPa,1MPa Figure 6: The intensity of methane pore hydrate dissociation at pressure release to.1 and to 1.6 MPa in the sample of sandy loam at t= -3. o C We obtained experimental data on the influence of initial ice (or water) saturation and temperature on the kinetics of hydrate dissociation in porous media at different gas pressures. The most significant difference in hydrate dissociation rates depending on ice saturation and temperature was observed during first hours after pressure decrease when ice layer forms on the surface of hydrate particles. Then the dissociation rate decreases. The degree of dissociation rate decrease depends on the ice layer around hydrate particles (fig. 7, 8, 9). % of Sh Sw =8% Sw =67% Figure 7: Relative change of hydrate saturation (S h ) in time at pressure decrease to 1.5 MPa in the sample of sandy loam with different initial ice saturation (S i ) at t= -3. o C.

7 Dissociation intensity (dsh/ dt), %/h 3 1 t= - 11,5oC t= - 6,5oC Figure 8: The intensity of pore hydrate dissociation at pressure release to.8 MPa in the sample of sand 1 at different negative temperatures. covering of partly-dissociated hydrate particles. Porous media essentially affects on mechanisms of ice covering of hydrate particles. Below we consider some important factors which influence the process of ice covering formation. The factor of initial pressure drop to different values up to atmospheric pressure (below pressure of three-phase equilibrium gas bulk ice gas hydrate and/or below metastable three-phase equilibrium gas-bulk supercooling water hydrate (see fig. 1 for methane hydrate). 1 Dissociation intensity, dsh/ dt, %/h t= - 6, oc t= - 3,3 oc Figure 9: The intensity of CO hydrate dissociation at pressure decrease to.5 MPa in the sample of sand 1 at different negative temperatures. According to experimental data the dissociation of CO hydrate in porous media (at pressure decrease below equilibrium) is more intensive to compare to CH 4 hydrates. Smaller stability of CO hydrate could be explained by several reasons. Firstly, the layer of supercooled water which forms at hydrate dissociation is acidated by dissolved CO, which complicates its freezing. Next, the ice layer which forms at freezing of supercooled water during methane hydrate dissociation probably has more monolithic and dense structure than the ice forming at CO hydrate dissociation. But this suggested mechanism requires a detailed microstructure researches. DISCUSSION The obtained experimental data showed that hydrate particles without porous media in many tested cases the first rapid stage of pore hydrate dissociation process transforms to the second more slowly dissociation stage (the stage of selfpreservation), which is explained by the ice shell Figure 1: The zones on phase diagram for possibility of methane hydrate decomposition to gaseous methane plus ice I h and to gaseous methane plus supercooled water [8]. As one can see from the phase diagram (fig. 1), methane gas hydrates can dissociate both to ice and gas, and to supercooled water and gas. In the last case the supercooled water quickly crystallizes, forming ice layer around incompletly decomposed hydrate particle. More significant pressure decrease leads to stronger driving forse of decomposition. It means that the self-cooling of hydrate particle is more significant which leads to the increase of the possibility of supercooled water crystallization into ice phase. During pressure reducing the important moment is a crossing of a metastable equilibrium line «gas supercooled water methane hydrate» [8]. Below this line the mechanism is opening to surface hydrate decomposition directly on supercooled

8 water and gaseous methane without any induction time (similar behavior of the simple substances melting process) and further start to freezing of the supercooled water to ice (which covering hydrate particles by a good ice film ). So in this case we have possibility for realizing of the selfpreservation effect. On the other hand if we reduce pressure above a line of metastable equilibrium gas supercooled water methane hydrate the hydrate dissociation process to ice phase is only possible. In such case the porous ice is formed during the decomposition of hydrate particle. It means that we don t have a good self-preservation effect. In general, the porous media (soils and sediments) affect in the direction of reducing of residual hydrate content in the sample on the second slowly stage of decomposition (the self-preservation stage). Microstructure and the average size of hydrate particles. The increase of hydrate particles average size the residual hydrate-content of the sample increases. It may be explained by heat exchange of hydrate particles with a porous matrix (the small particles have more chances to decompose fully before its covering by ice film). The result is consistent with earlier study on pure methane hydrate particles [16]. The presence of a residual liquid water phase in hydrate-containing porous media. At the stage of formation of the hydrate-containing sample the important factor is a proportion between nonclathrated water (equilibrium with hydrate phase) and the residual unfrozen water (which is not yet transformed to hydrate phase on kinetic reasons). Then at the stage of pressure reducing we have the increasing of the equilibrium water content at fixed negative temperature (on Celsius) which affects the decomposition kinetics. Porosity and grain size composition of sediments indirectly affect on hydrate decomposition process through the quantitative characteristics of liquid water content (unfrozen and nonclathrated water contents), and also through the influence on the of liquid water transfer in hydrate-containing samples. For example the addition of the kaolin particles to sand sample significantly decreased metastability of hydrate accumulation. CONCLUSIONS On the basis of suggested techniques we obtained the experimental data on the influence of temperature, pressure, hydrate- and ice-saturation on the kinetics of methane gas hydrates dissociation in porous media. It was shown that the dissociation of pore gas hydrates below melting point of ice (negative temperatures) leads to the increase of thermal conductivity of hydratebearing samples. It can be explained by the increase of amount of pore ice, which forms during hydrate decomposition. It has been noticed that there are two mechanisms of initial stage of hydrate decomposition depending on pressure reduction. It means that the initial stage essentially influences on further decomposition process (the self-preservation stage). Experimental researches on kinetics of gas hydrate dissociation in frozen sediments samples selected from gas showing horizons within Yamburg and Zapolyarnoe gas fields confirm an opportunity of long preservation of pore methane gas hydrate formations in frozen sediments at bedded negative temperatures and pressures below equilibrium due to self-preservation effect. It was shown that the possibility of gas hydrates self-preservation in frozen sediments depends on some factors, such as thermodynamic conditions, ice content, phase composition, gas permeability, composition of organic-mineral matrix, salinity, structure-textural peculiarities of hydrate-bearing sediments, including micro-morphology of hydrates. The residual pore water (which was not transformed to hydrate phase and partly frozen) plays an important role at hydrate metastability in porous media. On base of experimental data the theoretical models and mechanisms of gas hydrate self-preservation in frozen sediments may be further developed. ACKNOWLEDGMENTS This research was supported by JSPS and RFBR under the Japan - Russia Research Cooperative Program. REFERENCES [1] Yershov E. General Geocryology. Cambridge University Press., [] Istomin V., Yakushev V. Gas hydrates in natural conditions, 199. (in Russian). [3] Collett T.S, Dallimore S.R. Permafrost- Associated Gas Hydrate. In: Max M, editor. Natural Gas Hydrate in Oceanic and Permafrost

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