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
2
-2
-6
0
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
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6
8
10
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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.
–
–
–
–
–
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