Increase of thermodynamic shift of gas hydrate formation/ decomposition con- ditions in ... 1998) that the transition of part of the soil water to hydrate is regis-.
Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7
Experimental investigation of gas hydrate and ice formation in methane-saturated sediments E.M. Chuvilin, E.V. Kozlova, N.A. Makhonina Faculty of Geology, Moscow State University, Russia
V.S. Yakushev Gazprom, VNIIGAZ, Russia
ABSTRACT: The presented experimental results allow us to characterise kinetics and thermodynamics of water phase transitions in sediments under methane pressure. It is found that only a part of the ground water transforms to hydrate and the rest of the water freezes in the negative (subzero) temperature spectrum. Thermodynamic parameters of gas hydrate and ice formation/decomposition in porous media are determined for the investigated sediments. Influence of soil grain size, mineral composition, water content and salinity on P/T conditions of gas hydrate and ice formation are quantitatively shown. Increase of thermodynamic shift of gas hydrate formation/ decomposition conditions in porous media with growth in soil dispersion, humidity decrease and salinity rise in comparison with free volume (water-gas) is documented. The data on ice crystallisation conditions in porous media under methane pressure were obtained during cooling of artificially hydrate-saturated soils of different composition. Experimental results provide a new perspective on gas hydrate and ice formation/existence conditions in nature. studied by a number of researchers (Makogon 1974, Handa & Stupin 1992, Melnikov & Nesterov 1996, Uchida et al. 1998, Wright et al. 1998, Clennell et al. 1999, Tohidi et al. 2000, Chuvilin et al. 2000, Chuvilin et al. 2001). The majority of these reported thermodynamic shift of hydrate formation/decomposition conditions to the region of lower temperatures and higher pressures compared to the system of “free gas-pure water”. When hydrates form in sediments, only a part of the water transforms to hydrate. Residual water transforms to ice forming a cryogashydrate structure if further freezing takes (Chuvilin et al. 2000). This paper presents an experimental study of gas hydrate and ice formation/decomposition in sediments of different composition.
1 INTRODUCTION Natural gas containing layers have often been documented for permafrost sediments in different regions of the world in the course of well drilling (Yakushev et al. 1992, 2000, Dallimore et al. 1998, Chuvilin et al. 1998). However origin of gas and the form of its existence within permafrost was not completely understood until now. Shallow large gas accumulations are of special interest because they are situated in sediments having high specific water (ice) content and are probably related to the gas hydrate form of gas existence. The cryolithozone is the thermodynamic region where hydrates could be stored under non-equilibrium conditions due to self-preservation phenomena. In this situation, gas hydrates are in a metastable state and they are called relict hydrates (being formed in the past under equilibrium conditions, they are now in non-equilibrium conditions). The self-preservation phenomenon extends the thermodynamic area of gas hydrate existence to the whole thickness of permafrost sediments and gas hydrates and can be discussed as one of the permafrost rock components such as ice and unfrozen water. Gas hydrates, like ice, can form different textures in permafrost sediments (cement, inclusions). Many physical properties of hydrate-containing sediments are very similar to the properties of frozen (ice-containing) sediments of the same composition. But the processes of hydrate formation and decomposition in sediments of different composition have been studied very poorly. This could be attributed to the complexity of experimental simulation of gas hydrate formation in sediments. Hydrate formation in porous media was
2 METHODS Physical modelling of hydrate and ice formation in dispersed sediments has been conducted in a special pressure chamber. Pressure and temperature sensors connected to a data acquisition system enable us to record incoming data to a computer at any step in time. Pressure inside the chamber was regulated via connection with a gas cylinder (methane 99.98%, P 15 MPa). Gas hydrate formation/decomposition took place under cyclical temperature alteration at certain initial pressure conditions. Each experiment included testing of 2 samples: one in the experimental chamber and a control sample of the same composition and structure outside the chamber, but at the same temperature conditions. Samples were prepared using the layer-by-layer consolidation method. 145
sand was used in some experiments for mixtures with montmorillonite clay. Clay content of the mixtures was 7% (from dry sand weight). Initial water content for these mixtures was the same in all experiments – 17%. Kaolinite clay of different salinity was used in the experiments. Salt content for these samples was 0.2%, 0.4%, 1% and 2% (from dry sand weight). Observation of temperature and pressure condition changes with time allowed us to investigate the kinetics of hydrate formation and decomposition in the soils. Using experimental data on temperature and pressure condition changes inside the chamber the quantity of gas participating in hydrate formation or decomposition was calculated. Hydrate formation or decomposition rate in soil was also calculated. Moreover quantitative estimation of the proportion of the water transferred into hydrate (hydrate ratio), the proportion transferred into ice and the proportion transferred into unfrozen water was made.
The procedure of hydrate-containing sample production included the following steps: 1) loading of the prepared sample into the chamber and assembly of the chamber; 2) soil saturation by methane and pressure elevation inside the chamber up to 8–9 MPa; 3) placing of the chamber and control sample into the refrigerator and temperature reduction to 0.5 to 4°C; 4) storage of the chamber and control sample at this temperature until pressure stabilization within chamber (primary hydrate formation); 5) temperature reduction in refrigerator down to 7 to 9°C and sample freezing; 6) temperature gradual elevation in refrigerator up to 17 to 20°C (ice melting, hydrate decomposition). The last step was the end of one complete cycle of hydrate and ice formation/decomposition. After hydrate decomposition the pressure was reduced to 1.5–2 MPa and the steps were repeated in subsequent cycles from step 4. When pressure inside the experimental chamber was dropped to atmospheric at temperatures below 0°C there was incomplete decomposition of methane hydrate due to the gas hydrate self-preservation phenomenon. This phenomenon made it possible to study hydrate-containing samples produced in a cold room at subzero temperature and atmospheric pressure and apply procedures developed for normal frozen sediments (Yershov et al. 1993). Sediments of various salinity, grain size and mineral composition were used in experiments. Their main parameters are shown in Table 1. Montmorillonite (bentonite) and kaolinite clays with water content close to the plastic limit values for these clays were used in the experiments. So, initial water content of montmorillonite was in the range of 70 and 130% (relatively to weight of dry sediment) and for kaolinite about 35%. Quartz fine-medium-grained
3 RESULTS AND DISCUSSION 3.1
Kinetics of the water phase transition in methane-saturated soils
Observation of temperature and pressure condition changes with time in wet methane-saturated soil samples under cyclic cooling and heating allows us to investigate the kinetics of water phase transitions in soil porous media. It has been shown (Wright J.F. et al. 1998) that the transition of part of the soil water to hydrate is registered at the cooling stage and takes place under methane pressure. The rest of the water transforms to ice during further subzero cooling. Thawing of the porous ice and further gas hydrate decomposition during heating is also registered. The analysis of the changes in pressure and temperature with time allows investigation of hydrateaccumulation intensity in various soil media and
Table 1. Grain size and micro-aggregate composition of the researched soils.
Grain size distribution, % Grain size, mm Sediment title
1–0.5
0.5–0.25
0.25–0.1
0.1–0.05
0.05–0.01
0.01–0.005
0.005–0.001
0.001
Quartz sand
1
53
45
1
–
–
–
–
Montmorillonite (bentonite) clay Kaolinite clay
0.0 (2.0) 0.7 (1.5)
0.0 (1.0) 0.5 (0.2)
0.2 (1.3) 0.4 (0.2)
0.1 (8.5) 2.9 (1.2)
18.8 (15.6) 19.5 (32.2)
7.3 (5.9) 11.2 (24.0)
20.1 (15.4) 40.2 (34.5)
53.5 (50.3) 24.6 (6.2)
Values: 0.1 – certain grain size particles content. (8.5) – micro-aggregates of certain diameter content. 146
Soil title according to grain size classifications of V.V. Okhotin for clay-contining sediments and E.M. Sergeev – for pure sand Fine/medium grain size sand Heavy Clay Silty clay
estimation of the proportion of water, which was transformed to hydrate under the experiment conditions. Based on P/T conditions data, the maximum quantity of water which was transformed to hydrate (about 80% of the total) was detected in sandy samples (Win 17%). In kaolinite clay (Win 35%) this value was about 45%. The minimum water quantity (about 15%) was transformed to hydrate in montmorillonite clay (Win 70%). Hydrate ratio naturally decreased in accordance with rise of bonding energy of pore water and mineral surface. Decrease in the quantity of hydrate-transformed water was also observed with sediment salinity increase. The portion of water, which was transformed into hydrate decreased up to 2% in kaolinite clay salted by a solution of NaCl (Dsol 2%). Data on the structure of frozen artificially hydrate-saturated soils show that sandy soils are characterised by pore ice-hydrate cement formation and also formation of hydrate on sample edges connected with water migration towards them. In more dispersed soils, formation of ice and hydrate occurs inside the samples only. Sands are characterized mainly by massive cryohydrate texture, sandy-argillaceous mixtures – by porphyry-massive cryogashydrate texture and clays by lenticularporphyraceous texture. The rates of methane absorption (hydrate formation) on cooling of gas-saturated dispersed sediments were calculated by experimental curves of change in P/T conditions with time (Fig. 1a). The diagram shows that the maximum rate of hydrate formation takes place in sandy samples – 1 * 103 mol/min. The hydrate formation rate is decreasing from sand to clay. The rate of gas absorption under hydrate formation also decreases with rising salinity. The hydrate formation rate is minimum in the saline kaolinite clay (Dsol 0.2%) and was about 6 * 105 mol/min in the period of time 300 min. Such change of hydrate formation rate with time is caused by the influence of chemical-mineral structure of mineral sediment skeleton and the energy condition of the ground water during gas hydrate accumulation in the pore space. The hydrate decomposition rates and gas release also decrease from sand to clay and when salinity rises (Fig. 1b). However, hydrate decomposition in a sediment is characterised by a special feature. It can be observed in an example of montmorillonite clay and sand with clay particles. Figure 1b shows that in the first period of time the rate of hydrate decomposition is very small (1 * 104 mol/min). Then the rate sharply increased and then the dissociation rate fades away. Such characteristic hydrate dissociation can be caused by the forms of hydrate existence in a sediment (B. Tohidi et al. 2000). In our case hydrate is present in the form of porous cement and various inclusions also. Apparently, the first step corresponds to dissociation of hydrate cement
a
0,12
a b c d e
methane consumption (mol)
0,1 0,08 0,06 0,04 0,02 0 0
100
200
300
400
500
600
time, min 0,12
b
methane release (mol)
0,1
a
0,08
b c
0,06 0,04 0,02 0 0
50
100
150
200
250
300
time, min
Figure 1. The rate of methane hydrate formation (a) and dissociation (b) in investigated sediments: a – sand (Win 17%); b – sand with 7% of montmorillonite clay (Win 17%); c – montmorillonite clay (Win 70%); d – kaolinite clay (Win 35%); e – kaolinite clay (Win 35%, Dsol 0.2%).
(porous hydrate) and the second step corresponds with dissociation of various gas hydrate inclusions (B. Tohidi et al. 2000). 3.2
P/T conditions of the hydrate formation/ dissociation in sediments
The temperature and pressure conditions of gas hydrate formation in dispersed sediments depend on the whole range of factors. These factors are grain size and mineral structure of the sediment medium, the water content, the salinity and kinetic factors. Often, the influence of these factors is complex. It can result in a considerable range of experimental data. The analysis of experimental data has shown obvious shifts of P/T equilibrium conditions of hydrate formation in sediment pore media to the region of higher pressures and lower temperatures in comparision of the system “water (ice) – free gas – gas hydrate”(P/Tg-w). This shift is not constant and it increases with rise of environment temperature. Apparently, it could be attributed to energy condition of the pore water of sediments and change of physical-chemical properties of the sediment at higher temperatures. It was established that the shift of P/T parameters to the field of higher pressures and lower temperatures increases with increase of soil grain size (under the 147
It can therefore strongly tie a great quantity of porous water. In case of kaolinite clay hydration occurs only on external surfaces of particles. The specific active surface of kaolinite does not exceed 30 m2/g. Therefore porous water is less subjected to energy influence of a mineral surface and conditions of its transition to hydrate are more favourable. Experimental data show that increase of dispersed sediment salinity also causes shift of temperature and pressure conditions of hydrate formation/dissociation to the field of higher pressures and low temperatures. So, in a sequence of pure kaolinite clay – kaolinite clay salted by NaCl solution with Dsol 0.2% – kaolinite clay salted by NaCl solution with Dsol 2%, the shift on the temperature scale changed from 2.5°C (for pure kaolinite clay) up to 5.5°C (kaolinite clay with Dsol 2%) and on the pressure scale – from 1.8 MPa to 4.5 MPa, respectively. Apparently, this could be attributed to the salt ions dissociated during interaction with water and connected to water polar molecules. Therefore, the energy condition of pore water varies. It becomes the osmotically connected water. Its transition to hydrate requires more energy expenditure in comparison with free water. Therefore, the more concentration of pore solution, the more water will be tied by salt ions and the conditions of hydrate formation will became more difficult. The experimental data on salinity influence on conditions of gas hydrate decomposition are presented in Figure 3. These data show that in salted kaolinite clay (Dsol 0.2%) the deviation in temperature and pressure changes from 1.3°C up to 2.5°C and from 1.4 MPa up to 2.2 MPa accordingly. At salinity 2% T reaches 3.6°C and P – 3 MPa (Fig. 3). Influence of salt content on P/T conditions of hydrate decomposition in free volume (solution) and pore water is not the same. In the pore solution the shift is larger. This fact indicates joint influence of salt and mineral skeleton on hydrate decomposition conditions in soil pore medium.
12 10
d
Pressure, MPa
c 8 6 e
b
4 a
2 0
-2
0
2
4
6
8
10
12
14
Temperature, ºC
Figure 2. Temperature and pressure conditions of methane hydrate decomposition in sand and sand with montmorillonite clay: a – P/T conditions in system “gas-water”; b – P/T conditions in sand (Win 17%); c – P/T conditions in sand with 7% montmorillonite clay (Win 17%), d – P/T conditions in kaolinite clay (Win 35%); e – P/T conditions in montmorillonite clay (Win 70%).
constant initial water content value). Shift of temperature (T) and pressure (P) is 1.0–1.5°C and 0.7 MPa, respectively in pure sand (Win 17%). These values increase in sand with montmorillonite particles (Win 17%). The shift reached 1.6–2°C in temperature and 0.7–1.5 MPa in pressure range. Shifts of temperature (T) and pressure (P) of hydrate dissociation in fine- and medium-grained sands are 0.6–1.2°C in temperatures and 0.4–0.6 MPa in pressure, in sandy-argillaceous mixtures T is 1.3°C and P is 1 MPa (Fig. 2). Such distinction is explained by increase of bonding energy of pore water with mineral surface with addition of montmorillonite particles with high superficial energy. The influence of mineral structure can be considered by reference to an example of clay sediments of montmorillonite and kaolinite structure. It was revealed that the shift of P/T conditions of formation and decomposition in montmorillonite clay is more than in kaolinite clay in spite of the large initial humidity of montmorillonite clay. So, the shift in temperature is 3–4.5°C for montmorillonite clay (Win 70%) and 2.5–3°C for kaolinite clay (Win 35%). The value of the pressure shift reaches 3.0 MPa and 1.8 MPa for montmorillonite and kaolinite clay, respectively. Under decomposition of hydrate in the pore space of these clays a similar tendency is observed. In the case of kaolinite clay (Win 35%) deviation on the pressure scale (P) does not exceed 2.1 MPa and on the temperature scale 2°C. In the case of montmorillonite clay this shift reaches 2.5°C in temperature and 2.8 MPa in pressure (Fig. 2). These shifts are much larger than in other soils studied. The similar behaviour of montmorillonite clay is caused by its crystal-chemical structure. It owes of active crystal lattice and a high specific active surface (up to 600 m2/g).
3.3
Ice formation in hydrate-saturated sediments
The experiments data on freezing of the porous water under pressure of nitrogen (impossibility of hydrate formation) and under pressure of methane (opportunity of hydrate formation) are presented in given paper. These data allow us to estimate the gas pressure influence on the freezing point in sediments of different composition. In sand samples under nitrogen pressure the coefficient of pore water freezing point reduction at increasing pressure is equal to 0.1°C/MPa, that is very close to the value for closed soil systems (Fig. 4). In hydrate-containing soils the coefficient varied (Fig. 4). In sands (Win 17%) the coefficient was equal to 0.4°C/MPa, in montmorillonite (bentonite) 148
12
a
a c
8
b d
0
2
4
6
8
10
8
10
-2
Tbf, oC
Pressure, MPa
10
Pressure, MPa 0
6
-4
4 -6
2 -8
Closed system z=0% z=0,2% z=0,4% z=2%
0 0
2
4
8 6 Temperature, ºC
10
12
14
b
Pressure, MPa 0
Figure 3. Temperature and pressure conditions of methane hydrate decomposition in saline kaolinite clay: a – P/T conditions in system “gas-water”; b – P/T conditions in unsaline kaolinite clay (Win 35%); c – P/T conditions in saline kaolinite clay (Win 35%, Dsol 0.2%); d – P/T conditions in saline kaolinite clay (Win 35%, Dsol 2%).
2
6
4
8
Tbf, ºC
6
Closed system N2 % z=0% z=0,4% z=2%
Tbf, oC
Figure 5. Freezing temperature reduction under methane pressure in different saline sediments: a – kaolinite clay (Win 35%), b – sand (Win 17%).
10
-2
-4
-6
4
-4
-8
Pressure, MPa 0
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-6
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0
3.4 Closed system
Peculiarities of gas hydrate formation/ decomposition in dispersed sediments
Open system Sand (W=17%) -8
-10
Recorded experimental data gave an opportunity to predict more precisely boundaries of the Hydrate Stability Zone (HSZ) in a section of sediments of different composition, especially in the cryolithozone depth interval. In particular, these boundary positions can be several tens of metres different from predicted using P/T hydrate formation condCns for the system “free gas-pure water” (Fig. 6). The most considerable shift should be observed at the lower boundary of HSZ in clay sediments of low water content and elevated salinity. Such sediments are wide spread in permafrost regions. On the other hand, ice formation in pore space when residual water freezing in hydrate-containing sediments stabilizes gas hydrates even when pressure reduction occurs. As a result, gas hydrate can exist in the pore space of frozen sediments for a long time at pressures considerably lower than equilibrium values. So relict hydrates can be encountered all over the cryolithozone.
Kaolinite clay (W=35%) Montmorillonite clay
(W=70%)
Montmorillonite clay
(W=110%)
Figure 4. Freezing temperature reduction under methane pressure in different sediments.
clay (Win 70%) it was 0.6°C/MPa, in kaolinite (Win 35%) the coefficient was about 0.1°C/MPa. So large difference in the values is probably attributed to the mineral surface influence on the thickness of bonded water and to the different volume of residual water in soils after hydrate formation. In soils with elevated salinity the freezing point is dependent not only on the mineral surface but also on salt composition and content. In samples of kaolinite clay (Win 35%) containing NaCl the freezing point reduced from 0.25°C (pure kaolinite) to 3.4°C (Dsol 2%) at atmospheric pressure. In hydratecontaining samples under methane pressure the coefficient for pure kaolinite was 0.1°C/MPa and for kaolinite with Dsol 2% it was 0.2°C/MPa (Fig. 5a). In salted sand the freezing point gradually reduced from 0°C to 3.98°C when salinity increased from 0% to 2% (Fig. 5b). The coefficient of freezing point reduction under pressure was 0.4°C/MPa.
4 CONCLUSION The following conclusions could be made on the basis of the experiments conducted: – when hydrate formation takes place in dispersed sediments in the course of continuous cooling, not 149
-10 -8
-6
-4
Temperature, ºC -2 -0 2 4 0
REFERENCES 6
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Chuvilin, E.M., Perlova, E.V., Makhonina, N.A. & Yakushev, V.S. 2000. Research of hydrate and ice formation in soils during cyclic fluctuations of temperature. In: Ground Freezing-2000 (J-F. Thimus, ed): 9–14. Rotterdam: Balkema. Chuvilin, E.M., Perlova, E.V., Makhonina, N.A., Kozlova, E.V. & Yakushev, V.S. 2001. Gas hydrate formation and dissociation conditions in clay sediments of different composition. Proc. of the 7th International Symposium on Thermal Engineering and Sciences for Cold Regions, Seoul, Korea: 231–235. Chuvilin, E.M., Yakushev, V.S. & Perlova, E.V. 1998. Gas and possible gas hydrates in the permafrost of Bovanenkovo gas field, Yamal Peninsula, West Siberia. Polarforshung 68 (erschienen 2000). Clennell, M.B., Hovland, M., Booth, J.S., Henry, P. & Winters, W.J.J. 1999. Formation of natural gas hydrates in marine sediments. Journal of Geophysical Research B, 104, 22985. Dallimore, S.R. & Collett, T.S. 1998. Gas hydrates associated with deep permafrost in the Mackenzie Delta, N.W.T., Canada: regional overview. Proc. of the 7th International Permafrost Conference, Yellowknife, Canada: 201–206. Handa, Y.P. & Stupin, D. 1992. Thermodynamic properties and dissociation characteristics of methane and propane hydrate in 70-A-radius silica gel pores. Journal of Physical Chemistry v. 96, 21: 8599–8603. Makogon, Yu.F. 1974. Hydrates of natural gases. M: Nedra, Russia: 208. (in Russian). Melnikov, V. & Nesterov, A. 1996. Modelling of gas hydrates formation in porous media. Proc. of the 2nd International Conference on Natural Gas Hydrates, Toulouza, France: 541–548. Tohidi, B., Ostergaard, K.K., Llamedo, M. & Burgass, R.W. 2000. Measuring hydrate phase in porous media. Proc. of the VI International Conference on Gas in Marine Sediments, St. Petersburg, Russia: 133–135. Uchida, T., Ebinoma, T. & Ishizaki, T. 1998. Dissociation Pressure of methane hydrates in porous media. Proc. of the International Symposium on Methane Hydrates: Resources in the Near Future?, Japan National Oil Corp., Chiba, Japan: 253–258. Wright, J.F., Chuvilin E.M., Dallimore S.R., Yakushev, V.S. & Nixon, F.M. 1998. Methane Hydrate Formation and Dissociation in Fine Sands at Temperature near 0°C. Proc. of the 7th International Permafrost Conference, Yellowknife, Canada: 1147–1153. Yakushev, V.S. & Chuvilin, E.M. 2000. Natural gas and gas hydrate accumulations within permafrost in Russia. In Cold Regions and Technology 31: 189–197 Yakushev, V.S. & Collett, T.S. 1992. Gas Hydrates in Arctic Regions: Risk to Drilling and Production. Proc. of the Second International Offshore and Polar Engineering Conference. San Francisco, USA, vol.1: 669–673. Yershov, E.D., Lebedenko,Yu.P., Chuvilin, E.M. & Yakushev, V.S. 1993. Peculiarities of gas hydrate formation in sands. Proc. of the 6th International Permafrost Conference, Beijing, China: 160–163.
1 100 2 200 HSZ* HSZ
3 300
4
4 400 1 5 500 3
6 600 7 700 2 8 800 Pressure, Depth, m 9 900 MPa 10 1000
1 - equilibrium curve of methane-hydrate existence in "water-gas" medium; 2 - equilibrium curve of methane-hydrate existence in porous medium; 3 - curve of temperature and pressure in section; 4 - permafrost boundary; HSZ - zone of hydrate existence in "water-gas" medium; HSZ* - zone of hydrate existence in porous medium.
Figure 6. Hydrate stability zone (HSZ) in the cryolithosphere under the influence of porous medium.
–
–
–
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all the pore water transforms to hydrate. Residual water forms ice when the freezing point is reached; decrease of sediment grain size and increase of sediment salinity results in reduction of total hydrate content; hydrate formation rate (velocity) in dispersed sediments reduced in the sequence sand-silt-loam-clay and with increasing salinity; thermodynamic shift of hydrate formation conditions increases with grain size reduction and greater salinity; the coefficient of freezing point reduction under pressure changes in hydrate-containing sediments in the range 0.1 to 0.6°C/MPa depending on grain size, water content and salinity; experimental data on hydrate formation/decomposition conditions shows considerable influence of sediment composition and structure on thermodynamic stability of gas hydrates in geologic section.
ACKNOWLEDGEMENTS The research presented in this paper has been partly funded by the programme Russian Foundation for Basic Research No. 01-05-64653. 150
