CHANGE OF THERMAL CONDUCTIVITY OF GAS-SATURATED SEDIMENTS DURING HYDRATE FORMATION AND FREEZING

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1 Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom, July 17-21, CHANGE OF THERMAL CONDUCTIVITY OF GAS-SATURATED SEDIMENTS DURING HYDRATE FORMATION AND FREEZING E.M. Chuvilin and B.A. Buhanov Department of Geology Moscow State University Leninskie Gory, Moscow, RUSSIA ABSTRACT In this paper, the authors present their results on experimental estimations of variations of thermal conductivity in sediments during hydrate accumulation and freezing. These experiments were performed on a special gas hydrate cell, which has a cylindrical probe for measuring thermal conductivity of samples under gas pressure. The results show that the gas hydrate component has a significant effect on thermal conductivity, both for unfrozen and frozen sediments. This effect increases with increasing of hydrate saturation. Keywords: porous gas hydrates, thermal conductivity, frozen sediments. NOMENCLATURE S h hydrate saturation [%] W in initial water content [%] λ thermal conductivity [W/mK] INTRODUCTION In recent years, much attention is paid in the world to prospecting and exploring of gas hydrates, including within the Permafrost area. One of the main subjects is making and development of methods and technologies of gas production from natural gas hydrate accumulations. However, solutions of these issues are not possible without detail study of hydrate-bearing sediments properties, in particular thermal properties. These parameters are also important for traditional gas extraction in the permafrost area, where productive gashorizons are close to the base of frozen sediments and hydrate stability zone and they have low temperature. Gas production from these horizons is often accompanied by lowering the temperatures in well bottom zone to temperatures favorable for hydrate formation and sometimes ice-formation. Thereupon, researches of variations of thermal conductivity in gas saturated sediments during hydrate accumulation under low positive and negative temperatures are rather important. First abnormal low values of thermal conductivity for methane hydrate were found in 1979 by R.D. Stoll and G.B. Bryan [1].Later their data have been confirmed, specified and added by many other researchers [2-7]. As a whole, it is possible to say, that thermal conductivity of pure monolithic gas hydrates is studied well. Experimental data show, that thermal conductivity of gas hydrates (~0,6 W/mK) and water (~0,6 W/mK) are very similar, and almost in 4-times below that of ice (~3 W/mK). So, according to R.P. Warzinski with colleagues, value of thermal conductivity of dense methane hydrate is from 0,56 to 0,65 W/mK, and thermal conductivity of porous gas hydrate is 0,33-0,38 W/mK [7]. Also it was revealed, that temperature increase courses an abnormal increase of thermal conductivity of methane hydrates (from 0,56 W/mK at-10 o C to 0,62 W/mK at +8 o C and pressure 31,5 MPa) [4]. Thermal conductivity of hydrate bearing sediments is investigated poorly unlike pure gas hydrates. The first experimental study of thermal properties was carried out by A. Groysman [2]. He studied thermal properties of sandstones, saturated by gas hydrates. He compared thermal properties of sandstone in frozen and unfrozen states. Thus, thermal conductivity coefficient of frozen sandstone saturated by gas hydrates was

2 70 % less that of frozen sample without hydrates. G. B. Asher experimentally showed that the thermal conductivity coefficient of frozen quartz send is 80% higher than that of frozen hydrate saturated sand [8]. However, he does not give any quantitative data about hydrate saturation in porous medium of investigated sediments. Researches made by S. Fan and L. Huang [4, 9] on sand samples showed that thermal conductivity of methane hydrate bearing sand increased with the rise of temperature. Wright et al. [10] using needle probe technique measured thermal conductivities of laboratory specimens and recovered core from gas hydrate bearing reservoir at the Mallik gas hydrate production research site. They carried out the determination of thermal conductivity in a pressure chamber under gas pressure under equilibrium conditions. They also showed that the thermal conductivity of frozen samples is considerably higher then of hydrate-containing. Besides they give some data on thermal conductivity of frozen hydrate containing samples which is 20-25% higher then thermal conductivity of unfrozen hydrate containing samples. Authors noted that thermal conductivity of gas-hydrate-bearing sediments in frozen/unfrozen state should depend on the relative proportions of gas hydrate/ice and gas hydrate/water. Other experimental data by F. Waite showed, that thermal conductivity of hydrate saturated sand at positive temperature practically does not depend on ratio of hydrate and water in porous media. Authors explained it by similarity in thermal conductivity of hydrate and water [5]. Thus, formation of thermal conductivity of hydrate saturated sediments especially in frozen state is poorly studied. METHODS All our researches of thermal properties were carried out on a specially created experimental cell. This device allows us to investigate thermal conductivity of gas saturated sediments in pressures chamber under gas pressure during hydrate accumulation in porous media, and also during freezing of hydrate saturated sediment. The cell consists of a refrigerator to set necessary temperature, a pressure chamber with total volume of 200 cm 3, a gas balloon (volume 300 cm 3 ), connecting gas pipes and measuring system of thermal conductivity of the investigated samples. The pressure chamber, in which sediment sample is located, consists of two metal cylinders inserted one inside the other. External diameter of the internal cylinder is 23 mm, internal diameter of the external cylinder is 51 mm, and height of the working chamber is 100 mm. Thus the internal cylinder is a cylindrical probe for thermal conductivity definition. Heater and thermocouples are mounted in the cylinder in close thermal contact to its surface. Measuring system of thermal conductivity is executed as separate module, and consists of preamplifier, ADC (analog-to-digital converter). This system is connected to PC through LPT-port. The error of measurements of thermal conductivity does not exceed 5 %. To minimize errors and maintain set accuracy we used etalon materials, witch have thermal conductivity in a range from 0,3 to 3,0 W/mK for calibration. To study thermal conductivity of hydrate containing sediments we prepared artificial hydrate saturated samples of sand, sand-clay mixes, that consisted of sand and clay material (sand + 14% of kaolin, sand + 14% of bentonite) and loamy sand, which was selected from permafrost region (not far from city Vorkuta). Their characteristics are shown in tables 1 and 2. In our experiments we use methane (99,98%) and carbon dioxide (99,99 %) as hydrate forming gases. Type of sediment Particle size distribution/% mm mm <0.001 mm Sand 94,8 3,1 2,1 Kaolin 4,5 70,9 24,6 Bentonite 0,3 46,2 53,5 Sandy loam 4 53,7 4,5 Table.1. Grain size of sediments Type of Mineral composition, S/% sediment % Sand quartz> 90 0,012 Kaolin kaolinite clay-92 0,043 Bentonite montmorillonite > 93 1,988 Sandy loam quartz- 38 microcline + albite -55 0,075 Table 2. Mineral composition and salinity of sediments

3 Methods on experimental investigation of thermal conductivity of gas saturated sediments during hydrate accumulation and freezing include following actions: preparation of sediment sample with set water content and its placement into the pressure chamber, sealing and pumping out of chamber with the sample, filling of pressure chamber with hydrate forming gas (CH 4 or CO 2 ) and creation of conditions for hydrate and ice formation in porous media of sediment sample [11]. Measurements of thermal conductivity of the sediment sample, and also registration of temperature and pressure in the cell was carried out at each stage of cooling and heating. There were several cycles of cooling and heating for each sediment. In addition, for comparison sediment sample in the pressure chamber was exposed to freezing and melting, both at atmospheric and excessive pressure (up to 3-4 МPа).This pressure was created by nitrogen (N 2 ) which in conditions of experiment did not form porous hydrate. By changes of thermo-baric conditions during experiments with use PVT method we defined following parameters of samples: hydrate saturation, ice saturation, volumetric hydrate content and hydrate coefficient (share of pore water which has transformed into hydrate) [11]. RESULTS AND DISCUSSION Experimental researches showed, that thermal conductivity of investigated sediments varied slightly, no more than 1-2 % at small hydrate saturation in porous media (S h up to 30-35) (fig. 1). We did not trace influence of type of hydrate forming gas on thermal conductivity of hydrate saturated sediment samples. Significant change of thermal conductivity in gas saturated sediments is observed at big values of hydrate saturation (S h more than 35-40%) (fig. 1). Thus, in the sample of sandy loam (W in =18 %) with the increase of S h from 0 to 38%, thermal conductivity raised from 1,77 W/mK to 2,01 W/mK, which is about 14 % (fig. 1b). When S h increased to 38% the thermal conductivity coefficient of sand (W in =16 %) raised from 5 W/mK almost to 2,0 W/mK, which makes 8 % (fig. 1а). Thermal conductivity, W/mK Thermal conductivity, W/mK а) Sh, % b) Sh, % Figure 1. Influence of hydrate saturation on thermal conductivity coefficient in sediment samples at t = +2 0 С. 1-sand, W in =16%; 2-sand, W in =10%; 3- sand+14% kaolin, W in =15%; 4- sand+14% bentonite, W in =15%; 5- Sandy loam, W in =16%. Such considerable change of thermal conductivity of investigated sediments with big hydrate saturation (S h more than 35%), apparently, is connected with local redistribution of water in porous media during gas hydrate accumulation, that influences on thermal contacts in sediment. Comparison of thermal conductivity of frozen sediments containing ice and hydrates in porous media and containing only porous ice showed, that thermal conductivity in hydrate-containing samples is less than that in samples which have only porous ice (fig. 2).

4 Thermal conductivity, W/mK 2,5 2,0 1,5 0,5 0,0 with porous ice with porous hygrate and ice Figure 2: Comparison of thermal conductivity of frozen sediments, with and without porous hydrates, at t= -6 0 С (Win=15-16%). 1-sand (S h =51%); 2- sand with 14% of kaoline (S h =50%); 3- sand with 14% of bentonite (S h =43%); 4-sandy loam (S h =54%). Such variations of thermal conductivity of frozen samples containing and not containing porous gas hydrates formations is connected, on one hand, with difference between thermal conductivity of porous ice and porous hydrate, and on the other hand, with micro-structural transformations which, apparently, occur at freezing of residual water in hydrate containing sample, during its cooling to negative temperatures. It can cause upheaval of the sediment sample, formation of micro-cracks and additional hydrate accumulation on contacts of particles and aggregates. It is probable, that the observed differences in thermal conductivity of frozen hydrate containing samples of sandy loam and those without porous hydrates is connected with the considerable structural transformations, occurring in hydrate-bearing sediments during freezing of residual porous water. Similar sharp decrease of thermal conductivity of frozen hydrate saturated sediments was also observed under non-equilibrium conditions [13]. In [13] authors explain such low values of thermal conductivity of sediment samples to formation of numerous micro-cracks and emptiness in crystals of gas hydrates, caused by freezing and partial dissociation of porous hydrates under conditions of self-preservation effect. As to influence of hydrate saturation on thermal conductivity coefficient in frozen sediment samples (fig. 3) we established, that the increase of hydrate saturation in frozen sediment samples causes decrease of their thermal conductivity Thermal conductivity, W/mK Sh, %. Figure 3: Influence of hydrate saturation on thermal conductivity coefficient in frozen sediment samples (t= -4 0 С, Wi n =22-24%). 1- sand; 2- sandy loam. In the sand sample (W in =22 %) at rise of S h from 0 to 38% its thermal conductivity decrease from 2,00 W/mK to 1,73 W/mK, which is 14 %. Such dynamic was also observed in sandy loam (W in =24%): increase of S h to 40% caused λ decrease from 2,07 W/mK to 5 W/mK, which was about 11 %. Such decrease in thermal conductivity of frozen sediments during hydrate accumulation is, most likely, connected to ice/hydrate ratio change in porous media. I.e. increase of hydrate saturation of sediments leads to decrease of ice and increase of hydrate components in porous media. CONCLUSIONS In terms of analysis of the obtained experimental data, it is possible to make following conclusions: At low hydrate saturation (to 30-35%) thermal conductivity of investigated unfrozen gas saturated sediments varied slightly and did not exceed 1-2 %. Significant changes in thermal conductivity of gas saturated sediments were observed at high hydrate saturation (above 35-40%). Thus, in the sand sample with the increase of S h from 0 to 38% thermal conductivity raised from 5 W/mK almost to 2,0 W/mK, that makes about 8 %.

5 At freezing of hydrate saturated sediments, unlike samples without gas hydrates, abnormal fall of thermal conductivity was observed. Thus the difference in values of thermal conductivity of frozen hydrate containing sediments and frozen samples without hydrates can reach 10 and more percent. So in sand sample this distinction was 20%, and in samples of sandy loam sand more than 100%. Such behavior of thermal conductivity of hydrate saturated sediments is caused not only by difference in values of thermal conductivity of porous ice and porous hydrates, but also by structurally-textural changes in hydrate saturated sediments during freezing. Decrease of thermal conductivity was observed during hydrate accumulation in frozen sediments Thus, in sand sample (W in =22%) at S h =0% thermal conductivity was 2,00 W/mK, and at Sh = 38% thermal conductivity was 1,73 W/mK. In this case decrease of thermal-physic parameter was 14 %. ACKNOWLEDGMENTS The research has been made with a partial financial support of Schlumberger Moscow Research Center. REFERENCES [1] Stoll RD., Bryan GM. Physical properties of sediments containing gas hydrates. Journal of geophysical research, 1979; 84: [2] Groysman, AG. Thermophysical properties of gas hydrates. Novosibirsk, 1985 [3] Sloan ED. Clathrate hydrates of natural gases. Second edition. New York [4] Fan S., Huang D. Measuring and modeling thermal conductivity of gas hydrate-bearing sand. Journal of Chemical and Eng. Data 2004; 49(5): [5] Waite FW. Stern LA., Kirby SH. et al. Simultaneous determination of thermal conductivity, thermal diffusivity and specific heat in si methane hydrate. Geophys J. Int [6] Rosenbaum E. J., English N. J., Johnson J.K., Shaw D.W., Warzinski R. P. Thermal conductivity of methane hydrate from experiment and molecular simulation. J. Phys. Chem. B [7] Warzinski RP., Gamwo IK., Rosenbaum EJ., Myshakin EM. et al. Thermal properties of methane hydrate by experiment and modeling and impacts upon technology. Proceedings of the 6th International Conference on gas hydrates. Vancouver [8] Asher GB. Development of computerized thermal conductivity measurement system utilizing the transient needle probe technique. Dissertation T Colorado,1987. [9] Fan S., Huang D., Liang D. et al. Thermal conductivity of combination gas hydrate and hydrate-sand mixtures. Proc. 5th Int. Conf. on Gas Hydrates, 2005, V.2: [10] Wright JF., Nixon, SR., Dallimore SR., Henninges J., Cote MM., Thermal conductivity of sediments within the gas-hydrate-bearing interval at the JAPEX/JNOC/GSC et al. Mallik 5L-38 gas hydrate production research well. Scientific results from the Mallik 2002 gas hydrate production research well program, Mackenzie delta, Northwest Territories, Canada, Bulletin 585, [11] Chuvilin EM., Kozlova EV Experimental estimation of hydrate-containing sediments stability. Proccedings of the Fifth International Conference on Gas Hydrate. Thermodynamic Aspects. V.5. Trondheim, Norway: [12] Bukhanov BA., Chuvilin EM., Guryeva OM., Kotov PI. Experimental Study of the Thermal Conductivity of the Frozen Sediments containing Gas Hydrate. Proceedings of the 9 th International Conference on Permafrost. June 29-July 3. Fairbanks, Alaska: