International Conference on Gas Hydrates (ICGH) (6th : 2008)

CARBON DIOXIDE GAS HYDRATES ACCUMULATION IN FREEZING AND FROZEN SEDIMENTS Chuvilin, Evgeny; Guryeva, Olga 2008-07-31

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   CARBON DIOXIDE GAS HYDRATES ACCUMULATION IN FREEZING AND FROZEN SEDIMENTS   Evgeny Chuvilin  Department of Geology Moscow State University Vorob’evy Gory, Moscow, 119899 RUSSIA  Olga Guryeva  Department of Geology Moscow State University Vorob’evy Gory, Moscow, 119899 RUSSIA   ABSTRACT The paper presents results of the experimental research on the process of CO2 gas hydrates formation in the porous media of sediments under positive and negative temperatures. The subject of research were sediment samples of various compositions including those selected in the permafrost area. The research was conducted in a special pressure chamber, which allowed to monitor pressure and temperature. Using the monitoring results it was possible to make quantitative estimation of the kinetics of CO2 hydrates accumulation in the model sediments. In the course of the research it was demonstrated, that active hydrates accumulation occurred in frozen sediments under negative temperatures (about -4 оС). At the same time a comparative analysis of СО2 and СН4 hydrates accumulation was made in the porous media of the sediment under negative temperatures. The performed experiments enabled to estimate an influence of temperature, sediment composition and water content on kinetics of CO2 hydrates accumulation in porous media. Besides, we made an estimation of the amount of hydrates, which could be formed in hydrates containing sediments at freezing of the remaining pore water.  Keywords: CO2 gas hydrates, sediment, gas hydrate formation, kinetic, ice, freezing.                                                         Corresponding author: Phone/Fax: +7 495 939 19 27 E-mail: chuvilin@geol.msu.ru NOMENCLATURE Fh share of hydrate which forms at freezing [%] Hv volumetric hydrate content [%] Kh hydrate coefficient [u.f.] Sh hydrate saturation [%] Win initial water content [%]  INTRODUCTION It is known, that CO2 hydrates as well as methane hydrates can be formed, and exist under natural conditions [1]. Besides, formation of СО2 hydrates is possible at the CO2 disposal as a greenhouse gas in the marine sediments [2,3]. Keen interest in the process of СО2 hydrates formation is also caused by possibility of carbon dioxide usage for methane extraction out of hydrate deposit [4-6].  In recent years permafrost area has been considered to be the environment for possible disposal of CO2. The favorable factors for preserving CO2 in liquid and gas hydrate states in frozen sediments and underpermafrost horizons are as follows: great thickness of frozen sediments, Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  low permeability in comparison with thawed sediments, and favourable conditions for hydrates formation.  In this connection the experimental research of formation and existence conditions of СО2 gas hydrates in permafrost and underpermafrost sediments are of great importance for estimation of CO2 disposal conditions in permafrost, and for working out specific sequestration schemes.  First works on CO2 gas hydrates formation, conducted by Z. Wroblewski and P.Villard who made first estimation of its composition and environment, date back to the end of the 19th century. Later on many scientists performed experimental studies on СО2 bulk hydrates formation. One can single out several directions in studying СО2 hydrates formation: hydrate formation in the system of liquid СО2-water [7-10 et al], hydrate formation in the system of gaseous СО2 - water or ice [11-15], and hydrates formation out of dissolved СО2 [16]. Thus, T. Komai et al (2002) figured out temperature dependence of СО2 hydrates formation on the surface of ice particles. They demonstrated that the maximum rate of hydrates formation is characteristic of the temperature range of –1÷–2 oС [12]. Kuhs et al (2000) showed that СО2 porous hydrates form on the surface of ice [15]. Kuhs et al research revealed, that the rate of СО2 hydrates formation is 2-3 times higher than that of methane [17].  Significantly less experimental researches devoted to СО2 hydrates formation process in porous media were carried out [6, 18-25]. The main matter of concern in those works was nucleation, observation of the kinetics of hydrate formation, and thermobaric conditions of СО2 hydrates accumulation in the porous media. Individual experimental researches were devoted to СО2 hydrates accumulation in freezing sediments [26]. The possibility of СО2 hydrates accumulation in frozen sediments had been poorly investigated.  METHODS Experimental modeling of СО2 gas hydrates formation in freezing and frozen sediments has been carried out on experimental installation consisting of the following basic elements: a pressure chamber, a thermostate to provide temperature conditions of pressure chamber, a converter of temperature and pressure gauge electrical signals into digital data, a computer [27].  As a subject of research we used model sediments with different dispersity and mineral composition. They were chosen to estimate influence of soil constitution on hydrates accumulation kinetics in porous media of sediments. Soils of disturbed structure were used in the course of experiments. The following soils were used as model ones: samples of silty sand selected from Yamburg gas field (which is located in the North of West Siberia), fine-grained quartz sand and sandy-clay mixes presented by fine-grained sand with kaolinite or montmorillonite clays particles. The weight content of clay particles were 7% and 14%. Their characteristic is shown in Table 1. As a rule, water content of soils was set as 10%, ensuring developed gas-water interfacial area in porous media. Moreover, we used sediment samples with water content 17% to estimate influence of initial water content on kinetics of gas hydrates accumulation. Sample’s height was about 10cm, its diameter – 4,6 cm.  Type of sediment Particle size distribution/% 1-0.05 mm 0.05-0.001mm <0.001 mm Silty sand 84 14 2 Fine-grained sand 94.8 3.1 2.1 Sand with 7% montmorillonite 88.2 2.9 8.9 Sand with 7% kaolinite 88.9 7.5 3.6 Sand with 14% kaolinite 83.7 11.4 4.9  Table 1: Grain size of investigated sediments  Experiments on СО2 gas hydrates accumulation in porous media of sediment samples were carried out at positive and negative temperatures. Individual experiments were carried out on the methane-saturated sediments in order to estimate the influence of gas hydrate-former on kinetics of hydrates accumulation in porous media.  In the first case prepared for experiment pressure chamber with soil was pressurized at room temperature (T≈+20 ÷ +22 оС) up to 3.7 - 4MPa. Then the temperature in the chamber was lowered from room temperature to low positive values (about 2оС). The sample in the chamber was kept at this temperature till the end of hydrates formation process, detected by sharp pressure drop in pressure chamber. Than the pressure chamber with the sample was cooled to negative temperature (–7 ÷ –8oС). As a result remaining pore water which had not transformed to hydrates froze-out and frozen artificially hydrates saturated sample formed.  In the second case we cooled the prepared for experiment chamber with wet soil to –7 ÷–8oC. Then it was pressurized up to 3.0MPa (for methane about 6.0MPa) and temperature was set to the experimental value of –3.8 oC. The chamber was kept at this negative temperature till the end of hydrates formation process. In contrast to the first case, hydrates were formed out of pore ice. As a result we got frozen hydrate-saturated samples under CO2 gas pressure. Analysis of pressure chamber thermo-baric conditions changes in the process of hydrate and ice formation allows to detect parameters of phase transition in soil samples. The hydrate saturation, volumetric hydrate content and hydrate coefficient (share of water transforming in hydrate) were determined from changes in thermobaric conditions incorporating compressibility according to Mendeleyev-Clapeyron equation with account of CO2 solubility in pore water.  EXPERIMENTAL RESULTS Kinetics of CO2 hydrates accumulation in the porous media at positive and negative temperatures. The undertaken analysis of thermobariс conditions change in time in pressure chamber shows that CO2 hydrates accumulation in porous media actively occurred under positive as well as negative temperatures. However intensity of hydrates formation and amount of hydrates accumulation under positive temperatures, when pore water was in liquid state were higher than those under negative temperatures. Calculated data on kinetics of CO2 hydrates saturation of fine-grained sand showed, that the accumulation intensity was considerably higher initially under positive temperatures (about +2 oC), later on though this difference decreased, because of significant acceleration of dying out of hydrate accumulation process under positive temperatures in comparison with the negative temperature (–3.8 oC) (Figure 1).  As a result, the total hydrates accumulation under negative temperatures did not differ greatly from that under positive temperatures (Table 2). At that Kh, which characterized the share of pore water turning into hydrates, reached the point of 0.79 under negative temperatures at the end of experiment, which is to a little degree lesser than that under positive temperatures, where Kh was 0.88. 02040600 50 100 150T i m e ,  hSh, %t= -3,8 ºCt= + 2 º C Figure 1  Kinetics of CO2 gas hydrate accumulation in fine-grained sand (Win=10%) at different temperatures.   Parameters Silty sand  Win=17% Fine-grained sand  Win=10% –3.8 oC +2 oC –3.8 oC +2 oC Kh 0.66 0.76 0.79 0.88 Hv 22 25 14 15 Sh 49 56 34 40  Table 2. Characteristics of sediment samples saturated by hydrates under various temperatures.  Similar results were obtained for silty sand with initial water content of 17%. In the experiment carried out under positive temperature hydrate saturation of porous media was only 7% higher, than that under negative temperature –3.8оС. In contrast to fine-grained sand, 76% of pore water turned into hydrate in silty sand under positive temperature, and only 66% under negative temperature. It is connected to greater water content and dispersity of silty sand in comparison with fine-grained sand.   Kinetics of CO2 hydrates accumulation in various porous media  Studies destined to determine influence of sediment composition and its water content on the intensity of hydrates accumulation in porous media was carried out on sand-clay mixes samples under positive temperatures. It revealed that the share of pore water turning into hydrate decreases with addition of clay particles (Figure 2).  00 , 2 50 , 50 , 7 510 25 50 75T i m e ,  hKh, u.f.s a nds a nd+ 7% ka ols a nd+ 7%m ont  Figure 2  Hydrate coefficient during the hydrate accumulation in different model sediments (Win=10%).  The greatest decrease in CO2 hydrate growth rate occurred when added montmorillonite particles, which induced significant change in energy of pore water, preventing its transition in hydrates. However it should be pointed out, that at the initial stage the intensity of hydrates formation in sand samples with kaolinite particles was higher than that even in sand samples without clay particles. Such behavior as this is apparently caused by occurrence of more enhanced gas-water contact of sand with kaolinite particles in porous media. It is possible to trace on Figure 3 the influence of clay particles content on kinetics of hydrates accumulation in porous media.  00,250,50,7510 25 50 75T i m e ,  hKh, u.f0%7%14% Figure 3  Hydrate coefficient during the hydrate accumulation at different content of kaolinite clay particles (Win=10%).  As it is known increase in clay particles content in sediments at fixed water content stimulates increase in quantity of bound water, transmission into hydrate state of which is highly restricted. This results in decrease in hydrates accumulation rate and logical decrease in hydrate coefficient with increase in content of clay particles. Thus by the end of the experiment Kh in sediment samples with initial water content of 10% changed from 0.88 in fine-grained sand to 0.58 in sand with 14% of kaolinite particles.  Kinetics of hydrates formation in porous media and porous hydrates accumulation were significantly influenced by initial water content (Figure 4).  00,20,40,60,80 25 50 75 100T i m e ,  hKh, %17%10% Figure 4  Hydrate coefficient during the hydrate accumulation at different water content of sediment samples (sand with 7% of montmorillonite clay).  At low water content in sediment, there existed an enhanced gas-water contact, which stimulated increase in hydrates accumulation and water share turning into hydrate. The area of gas-water contact decreases as saturation in porous media increases. This result in decrease in Kh though the total hydrates accumulation can be higher than that in more saturated sample.  Gas hydrate-former influence on kinetics of hydrates accumulation in frozen sediments  One can trace the influence of gas hydrate-former on kinetics of hydrates accumulation in frozen sediments by the example of silty sand sediment with the initial water content of 17% (Figure 5).  02040600 100 200 300T i m e ,  hSh, %CH4C O 2 Figure 5. Kinetics of СО2 hydrates and methane accumulation in frozen silty sand sediments (Win=17%) under negative temperature (-3.8оС) According to the experimental data the СО2 hydrates accumulation rate and quantity of accumulated hydrate under negative temperature in silty sand was higher than that of methane. The reasons for this were: gas reactivity, which was higher for СО2 than for methane [13], phase water composition of the frozen sediment under gas pressure. There would occur an active interaction of СО2 with unfrozen water in СО2 gas-saturated sediment, and subsequently its dissolution in unfrozen water and increase in its content, which results in acceleration of hydrates formation. Besides, microstructure of the forming hydrate will apparently play a special role in it. СО2 hydrates forming on the surface of the porous ice have more friable and porous structure in comparison with methane hydrate, this feature stimulates acceleration in hydrate forming rate. However it requires special microstructure researches on.   Influence of freezing on СО2 hydrates accumulation in porous media Not all the pore water turned into hydrate in the course of hydrates formation under positive temperatures. In the course of our experiments from 38% to 88% of pore water turned into СО2 hydrate under +2°С (Table 3).  Type of sediment Fh / % Kh before freezing after  freezing Silty sand, Win=17% 8 0.76 0.86 Fine-grained sand  Win=10% 6 0.88 0.93 Sand with 14% of  kaolinite Win=10% 26 0.58 0.78 Sand with 7%  kaolinite, Win=10% 18 0.70 0.86 Sand with 7% of  mont. Win=10% 17 0.69 0.84 Sand with 7% of  mont. Win=17% 28 0.38 0.53  Table 3. Characteristics of СО2 hydrates formation at the stage of freezing of remaining pore water.  The remaining pore water, which had not been transformed into hydrate, froze out after dying out of hydrates formation process in porous media of wet sediment and subsequent cooling below 0оС. In the course of the remaining pore water freezing, there released dissolved gas, and might occur deformation of sediment skeleton. As a result there appeared new gas-water contacts, which stimulated hydrate forming process and increase in hydrate-saturation and Kh. The amount of pore water turning into hydrate phase, depended on several factors: mineral content of the sediment, content of clay particles, and initial water content of the sediment. According to the experimental data, up to 28% of the total quantity of the СО2 hydrates were formed at freezing of the hydrate-saturated sediments (Table 3). The lowest amount of hydrate was formed at the stage of freezing in sand sediments. The amount of hydrates forming at freezing increased with the increase in clay particles content. Thus, the amount of hydrates formating in sand samples (Win=10%) at freezing increased from 6% to 17% at addition of kaolinite particles (up to 14%) (Table 3). This was connected to growth of remaining water content with increase in clay particles content.   Conclusion The performed experimental research on СО2 hydrates formation process in freezing and frozen sediments enables to make the following conclusion. СО2 hydrate accumulation takes an active place in porous media not only under positive, but also under high negative temperatures (–3 ÷ –4оС), when the water is mainly in the form of ice in porous media.  The comparative analysis of hydrates accumulation demonstrates, that СО2 hydrate is characterized by more intense accumulation in frozen sediments under negative temperatures then CH4 hydrate. Study of kinetics of СО2 hydrates accumulation in porous media shows, that dispersity, mineral composition and sediment water content make an influence on intensity of hydrates accumulation. The share of water turning into hydrate decreases at the increase in content of clay particles, especially those of montmorillonite composition. An increase in extent of porous media saturation with water, decreases hydrates accumulation intensity and hydrate coefficient, though the total hydrates accumulation might rise. 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