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Experimental Investigation into the Production Behavior of Methane Hydrate under Methanol Injection in Quartz Sand Gang Li, Danmei Wu, Xiao-Sen Li, Yu Zhang, Qiunan Lv, and Yi Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00464 • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on March 28, 2017
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Experimental Investigation into the Production Behavior of
2
Methane Hydrate under Methanol Injection in Quartz Sand
3 4
Gang Li†,‡,§,∥, Danmei Wu†,‡,§,∥,⊥, Xiaosen Li*,†,‡,§,∥, Yu Zhang†,‡,§,∥, Qiunan Lv†,‡,§,∥, Yi Wang†,‡,§,∥
5
†
Key Laboratory of Gas Hydrate, Chinese Academy of Sciences, Guangzhou 510640, P. R China
6
‡
Guangdong Provincial Key Laboratory of New and Renewable Energy Research and
7
Development, Chinese Academy of Sciences, Guangzhou 510640, P. R. China
8
§
9
P. R. China
10 11 12 13
Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640,
∥
Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou
510640, P. R. China ⊥
Nano Science and Technology Institute, University of Science and Technology of China,
Suzhou 215123, P. R. China
14 15
ABSTRACT: In this work, the dissociation behavior of methane hydrate in quartz sand
16
sediment by injecting a thermodynamic inhibitor (THI), methanol (MeOH) was investigated
17
using a one-dimensional experimental apparatus. The experimental results indicated that the
18
hydrate dissociation process included four stages: free gas production, methanol dilution,
19
major hydrate dissociation, and residual gas production. The overall liquid production rate
20
was smaller than the injection rate during the whole production process. The cumulative gas
21
produced from hydrate under methanol solution injection was adjusted with the reference
22
experiment. A new strategy of the adjustment of the experimental runs was introduced,
23
which was based on the ratio of the water and methanol solution injection rates. In general,
24
with the increase of the methanol injection rate and the methanol concentration, the
25
cumulative hydrate-originating gas produced increased. During the major hydrate
26
dissociation stage, the production efficiency was enhanced continuously with the increase of
27
the injection rate and concentration of the methanol solution, while the methanol efficiency
28
increased and reached a maximum value when the concentration was 60 wt% and then
29
gradually decreased.
30
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1. INTRODUCTION
2
Natural gas hydrates (NGHs) are non-stoichiometric crystalline solids that form from water
3
molecules and small gas molecules (e.g., methane, ethane, carbon dioxide and hydrothion,
4
etc.) at low temperatures and high pressure in the sediments of continental margins and
5
permafrost regions.1,2 The guest molecules are filled in interstices in the cage-like structure
6
formed by the host molecules that are held by hydrogen bonds. Methane is the most common
7
gas in the natural gas hydrate, and one volume of gas hydrate can contain as much as 164
8
volumes of natural gas.3 NGHs are considered to be one of the most potential future energy
9
resource and have attracted more and more attentions from researchers all over the world.4,5
10
To recover natural gas from hydrate reservoirs, four primary dissociation principles have
11
been proposed: 1) depressurization,6-9 to decrease the deposit pressure below the equilibrium
12
hydrate dissociation pressure at a specified temperature; 2) thermal stimulation,10-12 to heat the
13
reservoir above the hydrate dissociation temperature with hot water, steam, or hot brine; 3)
14
thermodynamic inhibitor (THI) injection,13-15 to infuse chemical additives, such as inorganic
15
salts (e.g., NaCl, NaOH, CaCl2) or alcohols/glycols (e.g., methanol (MeOH) or ethylene
16
glycol (EG)) to shift the thermodynamic equilibrium condition for hydrate formation to lower
17
temperatures and/or higher pressures; and 4) carbon dioxide replacement,16-18 which replaces
18
the methane molecules in hydrate by another gas such as carbon dioxide. Of the above
19
methods, the dissociation time of depressurization is the longest, and production rate is
20
relatively low. Furthermore, it is easy to cause secondary hydrate formation due to the
21
endothermic hydrate dissociation reaction.19 The injected energy of thermal stimulation
22
method can spread through not only hydrate-bearing layer but also the hydrate-free
23
surrounding layers, which causes low heat efficiency.20,21 The present replacement efficiency
24
of the carbon dioxide replacement method still requires improvement. Inhibitor injection
25
features include simple and easy operation, and has extensive application in reducing the risk
26
of hydrate blockage formation.22-24 In comparison, chemical inhibitor stimulation is an
27
efficient technique to produce gas from the hydrate-bearing sediment.25
28
Experimental investigations of hydrate dissociation behaviors under THI injection in the
29
hydrate reservoir have been reported. A laboratory study of Yousif et al.26 was conducted to
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determine the effects of methanol and EG on the hydrate formation process. The results
2
showed the promoting effect of methanol at low concentrations (1 to 5 wt%) to the water and
3
the inhibiting effect at high concentrations. Furthermore, the presence of these chemicals
4
seems to affect the size of the forming hydrate particles. Ke et al.27 studied the hydrate
5
formation in the presence of low concentration methanol, and the results suggested that
6
methanol at 100 - 3000 ppm had no significant effect on nucleation, while it showed a weak
7
promotion on that at the early stage of spontaneous hydrate growth. Li et al.13 developed an
8
one-dimensional apparatus to study the gas production behavior of methane hydrate in
9
unconsolidated sediment by injecting EG solution, and the results indicated that the
10
production efficiency was affected by both the concentration and injection rate, and it reached
11
a maximum with the EG concentration of 60 wt%. Sira et al.28 experimentally studied the
12
dissociation characteristics of methane hydrate by injecting methanol and EG (both with 30.0,
13
20.0 and 10.0 wt%), respectively. The results suggested that the gas-producing rate of hydrate
14
dissociation was a function of the methanol and EG concentrations, the temperature, the
15
pressure of the chemical solution, the chemical injection rate, and the hydrate/chemical
16
interfacial area. Yuan et al.29 used a reactor with an internal diameter of 300 mm and an
17
effective length of 100 mm to simulate the process of methane hydrate dissociation from
18
quartz sand samples. They found that the EG solution may accelerate the dissociation rate of
19
methane hydrate and the gas production efficiency increased with the decrease of the EG
20
quantity and the increase of the EG concentration. Dong et al.30 carried out a series of
21
experiments to study the dissociation characteristics of propane hydrate by injecting methanol
22
(30.0, 60.1, 80.2, and 99.5 wt%) and EG (30.0, 60.1, 69.8, 80.2, and 99.5 wt%), respectively.
23
It was found that the average dissociation rate increased with the mass increase of the alcohol
24
solutions and reached its maximum value when 99.5 wt% EG solution was injected. Qureshi
25
et al.31 tested and compared the thermodynamic inhibition performances of 10 wt% methanol
26
and 10 wt% ionic liquids (ILs), which was an aqueous solution also acting as THI and
27
prepared by mixing 5 wt% [PMPy][Cl] and 5 wt% [PMPy][triflate] in equal ratio. The results
28
showed that methanol had relatively weaker inhibition effect on the hydrate formation than
29
that of the above ILs, because of its shorter alkyl chains,32 strong interaction of the -OH group
30
with the hydrogen bonds in hydrates clusters and additional interaction of the -CH3 group
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with the C-C or hydrogen bonds within hydrate clusters.33 Methanol injection for hydrate
2
dissociation has already been employed for flow assurance in the petroleum industry.34 THIs
3
are normally used in high concentrations (usually 20-50 wt%)35 and large quantities. However,
4
few publications about gas production behavior of methane hydrate in porous media using
5
methanol with different concentrations and injection rates could be found. Furthermore, the
6
quantification of the efficiencies of the hydrate dissociation under THI injection and the usage
7
of the thermodynamic inhibitors is meaningful for the future application of THI.
8
In this work, a one-dimensional experimental apparatus was developed to simulate the
9
dissociation behavior of gas hydrate in porous media under methanol solution injection. In the
10
experiments, methanol with concentrations from 30 wt% to as high as 70 wt% and an average
11
temperature and pressure of 2.0 oC and 3.80 MPa, was injected into the hydrate vessel by the
12
injection rate of 4.9-8.9 mL/min. The production process for the hydrate-bearing sediment
13
with methanol injection was analyzed. Meanwhile, the effects of the concentration and
14
injection rate of methanol on the production characteristic of gas and liquid, and production
15
and methanol efficiencies during the hydrate dissociation period were acquired. Previous
16
studies indicated that methanol do not incorporate into methane hydrate,36 and this is not
17
considered in this work.
18
2. EXPERIMENTAL SECTION
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2.1. Materials. In this work, the methane gas used had a purity of 99.99% and was
20
supplied by the Foshan Kody Gas Chemical Industry, Co., Ltd., China. Ultrapure water
21
equipment was used to prepare deionized water with a resistivity of 18.25 mΩ/cm, and was
22
produced by the Nanjing Ultrapure Water Technology, Co., Ltd., China. Methanol with a
23
purity of 99.99% was supplied by the Guangzhou Chemical Reagent Factory, China.
24
2.2. Apparatus. Figure 1 is a schematic diagram of the one-dimensional experimental
25
apparatus used in this work. The experiment device mainly consisted of a high-pressure
26
hydrate vessel, a gas injection system, an aqueous solution injection system, a
27
thermostatically controlled air bath, a back-pressure regulator, some measurement units, and a
28
data acquisition system. A cylindrical hydrate vessel, with the internal diameter of 38 mm and
29
an effective length of 500 mm, was made of stainless-steel 316. The hydrate vessel could
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withstand pressures of up to 30 MPa was placed in an air bath. Four Pt100 thermal couples
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(-20-200 oC, ±0.1 oC), two pressure transducers (0-20 MPa, ±0.25%) and three differential
3
pressure transducers were uniformly inserted into the hydrate vessel to monitor the pressure,
4
temperature and differential pressure profiles during the experiments. A metering pump
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"Beijing Chuangxintongheng" 2PB00C with the range of 0-9.99 mL/min, which could
6
withstand pressures up to 20 MPa, was used to inject the aqueous solution into the hydrate
7
vessel. To protect the metering pump from corrosion by the methanol, middle containers were
8
used for the solution injection. Two D07-11A/ZM gas flow meters with the range of 0-1000
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mL/min produced by the Seven Star Company were used to measure the cumulative gas
10
injected into the hydrate vessel, the cumulative gas production and the gas production rate
11
produced from the hydrate vessel. A coiled pipeline in the air bath was used to precool the
12
injected solution. A back-pressure regulator connected to the outlet of the hydrate vessel was
13
used to control the pressure of the production well. The driving force of the back-pressure
14
regulator was provided by a nitrogen gas cylinder. Two electronic weighing balances,
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Sartorius BS 2202S, 0-2200 g, ±0.01 g, were used to monitor the flow rates of liquid. The
16
data acquisition system recorded the temperature, differential pressure and pressure in the
17
hydrate vessel, the amount of the cumulative gas production and injection, and the gas/liquid
18
production rate and injection rate.
19
2.3. Procedure. Raw, dry quartz sand with a 300-450 µm size range was tightly packed in
20
the hydrate vessel and then the vessel was placed in an air bath. The vessel was evacuated
21
three times to remove air in it with the vacuum pump. The quartz sand in the vessel was
22
completely filled with deionized water by the metering pump. The effective pore volume of
23
the porous media was 174.62 mL, which was measured by the volume difference between the
24
injected and produced water. Methane gas was then injected into the vessel from the gas
25
cylinder to increase the pressure to a much higher level than that of the hydrate equilibrium
26
pressure in the porous media at the working temperature. Subsequently, the inlet and outlet
27
valve of the hydrate vessel were closed and the temperature of the air bath was set at 2.0 oC.
28
Hydrate began to form in the hydrate-bearing sediment with the decrease of the pressure. The
29
hydrate formation process usually lasted for 2 to 4 days, and was completed when the system
30
pressure did not decrease obviously.
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After the end of the hydrate formation, the dissociation characteristics of methane hydrate
2
by methanol injection were examined using the following procedures. First, the methanol
3
solution of the required mass concentration (wt%) was prepared. Next, the back-pressure
4
regulator was set to 3.80 MPa and the outlet valve was opened. Then, the methanol solution
5
was injected into the hydrate vessel via the middle containers with constant injection rate (in
6
volume). The hydrate began to dissociate and both the gas and the water solution began to
7
flow out from the hydrate vessel. Finally, when the gas flow rate was very limited, the
8
dissociation experiment was assumed to be completed.
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3. RESULTS AND DISCUSSION
10
Table 1 provides the specific experimental conditions of methane hydrate formation for
11
each run. It can be seen that there were similar properties for the hydrate synthesized in all the
12
experiments. Table 2 provides the experimental conditions including the methanol
13
concentration, the density of the methanol solution, the injection rate, the average system
14
pressure and temperature during methanol solution injection. The density of methanol
15
solution varied with the methanol concentration, as shown in Table 2. In this work, eight
16
experimental runs were carried out to study the dissociation characteristics of methane
17
hydrate in unconsolidated sediment by injecting methanol of constant concentrations (0, 30,
18
40, 50, 60, and 70 wt%) with the injection rates of 4.9, 6.8, 8.9 mL/min, respectively. Run 0
19
was a reference experiment, in which deionized water instead of methanol solution was
20
injected into the hydrate vessel with the injection rate of 8.9 mL/min. The purpose of this
21
reference experiment was to eliminate the influence of gas produced from hydrate with the
22
injection of liquid.
23 24 25 26
3.1. Hydrate Formation. Figure 2 shows the evolution of temperature and pressure during methane hydrate formation. First, the pressure in the hydrate vessel from the beginning of the experiment to Point A, decreased from 5.43 to 5.09 MPa due to the gas dissolved in water.
27
Next, from Point A to Point B, although the pressure (approximately 5.09 MPa) was much
28
higher than the corresponding equilibrium pressure approximately 3.20 MPa at the working
29
temperature 2.0 oC, there was no hydrate synthesized in the hydrate vessel. The reduction of
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the system pressure was very limited, and this period was considered to be the induction time
2
of the hydrate formation process.
3
Then, the pressure decreased gradually from 5.09 to 3.59 MPa from Point B to Point C,
4
indicating a relatively high consumption rate of gas. This process could be mainly divided
5
into three stages: (1) Initial rapid formation, which was associated with hydrate film
6
formation at the gas-liquid interface; (2) hydrate crystal growth, which was likely controlled
7
by the slow diffusion of gas through the hydrate shell.37 The hydrate film barricaded
8
remaining free water from the gas phase; (3) Shell breakage or cracking, which was caused by
9
exposing the trapped free water to the gas phase, and allowing a more rapid hydrate formation
10
that was no longer diffusion limited.38,39
11
Finally, the descent speed of pressure slowed down due to the decrease of the driving force
12
for hydrate formation. When the pressure maintained stable, the hydrate formation process
13
could be considered to be completed.
14
The hydrate saturation SH, the gas saturation SG, and the water saturation SA at the stabilized
15
temperature and pressure, were calculated according to the consumed deionized water and
16
CH4 amount during the hydrate formation. The properties of SH, SG and SA from quartz sand
17
samples synthesized in our work for the eight experimental runs are listed in Table 1. Their
18
relationship was:
19 20
S H + SG + S A = 1
The phase saturation expressed as follows:40,41
21 22 23
(1)
SG =
SA =
vm n m , G V pore
mW 0 - N H ( nm 0 − nm,G − nm,W ) M W
ρW V pore SH =
( nm 0 − nm ,G − nm ,W ) M H
ρ H V pore
(2) (3) (4)
24
Where Vpore is the total pore space (mL) of the hydrate vessel. vm is the molar volume
25
(mL/mol) of the methane gas, which is determined using the fugacity model of Li et al.42 nm,G
26
and nm,W are the molar amounts (mol) of the methane remaining in the gas phase and
27
dissolved in aqueous phase in the hydrate vessel, respectively. mW0 is the total mass (g) of the
28
injected water, and nm0 is the total molar amount (mol) of the methane gas. NH=5.75 is the
29
hydration number of the hydrate. MW and MH represent the molar masses (g/mol) of water and
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hydrate, respectively. ρW and ρH are the densities (g/mL) of water and hydrate, respectively.
2
3.2 Driving Force.
3
Figure 3 shows the methane hydrate phase diagram with different methanol concentrations.
4
As mentioned earlier, THI shifted the hydrate stability zone to the left, and this effect grew
5
stronger with the increase of the MeOH concentration. In other words, the higher the MeOH
6
concentration, the stronger the inhibition effect on the hydrate existence. Point G in Figure 3
7
represents the thermodynamic conditions, 3.64 MPa and 2.0 oC for Run 5 (Table 2). Point H is
8
on the equilibrium line of methane hydrate with 50 wt% MeOH concentration, which
9
represents the hydrate dissociation pressure at the same temperature with Point G. The
10
pressure difference between Point H and Point G is the theoretical maximum value of the
11
driving force for hydrate dissociation. Considering the dilution process of the methanol
12
solution, as discussed above, the effective driving force during the hydrate dissociation
13
process was much smaller than the theoretical maximum value. Furthermore, there were
14
various other factors that can affect the effective driving force, which was corresponding to
15
the real time methanol concentration affected by both the hydrate and methanol distribution in
16
the hydrate vessel. Besides the injected methanol concentration and the injection rate, these
17
factors may also include the amount of the original water (methanol-free pure water after
18
hydrate formation) driven out by the injected solution, the distribution of the irreducible water
19
in the hydrate vessel, and the liquid water produced from hydrate dissociation.
20
3.3 Gas and Liquid Production. The variations of the gas and liquid production profiles
21
during the whole methanol injection process for all experimental runs were similar, and we
22
chose Run 5 as a typical one. Figure 4 shows the instantaneous gas production rate with time
23
for Run 5. Figure 5 shows the cumulative gas production and mass of liquid injection and
24
production with time for Run 5.
25
As shown in Figure 4 and Figure 5, the production process consisted of four stages as
26
follows.
27
Stage 1 - free gas production (0-5 min): The free gas remained in the hydrate vessel after
28
hydrate formation was released and produced. The gas production rate suddenly jumped up to
29
548 mL/min, with the average value of 169 mL/min in this stage. There was no liquid
30
produced during this stage, due to the significant density difference between the gas and
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liquid phases. Furthermore, the injected methanol solution also drove the gas phase towards
2
the outlet of the hydrate vessel.
3
Stage 2 - methanol dilution (5-9 min): Comparing with the free gas production rate in Stage
4
1, the average gas production rate was much lower (70 mL/min in Stage 2). The liquid began
5
to be produced in this stage. During Stage 1 and Stage 2, a total of 80.1 mL (8.9 ml/min × 9
6
min) methanol solution was injected in the hydrate vessel. Based on the effective pore volume
7
in the hydrate vessel (174.62 mL) and the three phase saturations before methanol injection
8
(Run 5 in Table 1), the initial gas, water and hydrate volumes in the vessel were calculated to
9
be 117.59 mL, 44.93 mL and 12.10 mL, respectively. It was obvious that the volume of the
10
injected methanol solution (79.2 mL) was much larger than that of the original deionized
11
water (44.93 mL). It should be noted that in each experimental runs in this work, both the
12
injection rate and the concentration of the methanol solution were constant. Some part of the
13
original water was driven and produced from the outlet of the hydrate vessel. The injected
14
methanol solution was also diluted by the remaining water in the vessel. The overall
15
concentration of the methanol solution in the hydrate vessel increased with time, and this
16
effect continued in the next stage (Stage 3).
17
Stage 3 – major hydrate dissociation (9-30 min): In the major hydrate dissociation stage,
18
the hydrate in the vessel began to dissociate into water and gas under the effect of the injected
19
methanol solution acting as a THI of gas hydrate. The average gas production rate was
20
approximately 69 mL/min in Stage 3, which seemed slightly lower than that in Stage 2.
21
However, the overall instantaneous gas production rate at the first half of Stage 3 was
22
obviously higher than that in Stage 2, as shown in Figure 4, which was caused by the
23
emergence of the hydrate-originating gas (gas from hydrate dissociation). Because of the
24
limited hydrate saturation (6.93% of Run 5 in Table 1), the amount of gas from hydrate
25
dissociation decreased on the second half of Stage 3, which caused the decline of the overall
26
gas production rate. Similar with that in Stage 2, as discussed above, the dilution process of
27
the methanol solution continued in Stage 3. The components of the liquid solution produced
28
from the hydrate vessel included the methanol solution, the remaining water in the hydrate
29
vessel after hydrate formation, and the hydrate-originating water. The slight fluctuation of the
30
liquid production trend was caused by the unsteady dilution process of the methanol solution
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and its stimulation effect on hydrate dissociation.
2
Stage 4 - residual gas production (30-39 min): Comparing with Stage 3, the hydrate
3
dissociation rate and the gas production rate decreased to a relatively low level and there was
4
still hydrate existed and remained undissociated in the vessel. And most of the produced gas
5
was the residual gas in the vessel. The overall liquid production rate was smaller than the
6
injection rate, because some of the injected solution maintained in the vessel and replaced the
7
original gas.
8
3.4 Effect of methanol injection rate. Figure 6 shows the variation of the cumulative gas
9
production with time during Stage 3 for Run 0 and Runs 1-3, including the original data and
10
the data that were adjusted by Run 0).
11
Table 3 provides the gas production results of hydrate dissociation by methanol injection,
12
including both the whole experiment (Stage 1-4) and the hydrate dissociation stage (Stage 3).
13
The solution injection time was the total duration of the experiment. The total mass of
14
methanol injected was the total amount of methanol in the solution injected into the vessel. It
15
was noted that the methanol injection rate was the value of the total mass of methanol injected
16
divided by the solution injection time, rather than the methanol solution injection rate. The
17
total gas produced represented the amount of gas flow out from the vessel during the whole
18
experimental runs. The average gas production rate during the whole experiment was also
19
shown in Table 3. In Stage 3, the duration of this hydrate dissociation stage, the cumulative
20
gas produced from hydrate (the hydrate-originating gas), the average rate of gas produced from
21
hydrate, and the percentage of hydrate dissociated were listed in Table 3. The percentage of
22
hydrate dissociated was calculated with the cumulative gas produced from hydrate and the
23
total methane gas trapped in the solid hydrate, which could be obtained from the hydrate
24
saturation in Table 1.
25
It should be noted that the experimental data of the cumulative gas produced from hydrate
26
were adjusted with the reference experiment Run 0, as mentioned above in Figure 6 and Table
27
3. The strategy of the adjustment of the experimental runs was based on the fluid (water and
28
methanol solution) injection rate. The deionized water injection rate of Run 0 and Runs 3-7
29
were all 8.9 mL/min, while the methanol injection rates of Run 1 and Run 2 were 4.9 and 6.8
30
mL/min (Table 2), respectively. To evaluate the effect of the fluid injection on gas production,
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the ratio of the methanol and deionized water injection rates was used. For Run 1 and Run 2,
2
the ratios were 4.9/8.9 and 6.8/8.9, respectively.
3
Take the end point of Stage 3 in Run 1 for example. The duration of Stage 3 in Run 1 was
4
34 min (Table 3 and Figure 6). The original data of the cumulative gas produced from hydrate
5
in Stage 3 in Run 1 was 767 mL (Point M in Figure 6), while the corresponding cumulative
6
gas at 34 min in Run 0 was 605 mL (Point P in Figure 6). Hence, the value of the cumulative
7
gas produced from hydrate after adjustment in Run 1 was equal to 767 - 4.9/8.9 × 605 = 434,
8
as shown in Table 3 and Point N in Figure 6.
9
As shown in Table 3, from Run 1 to Run 3, with the increase of the injection rate (4.9, 6.8
10
and 8.9 mL/min in Table 2), the duration of Stage 3 (the major hydrate dissociation stage)
11
decreased (34, 30, and 26 min), while the cumulative and the average rate of the gas produced
12
from hydrate enhanced significantly. Meanwhile, the percentage of hydrate dissociated
13
increased from 20.7% to 36.7%. This indicated the essential effect of the injection rate on
14
hydrate dissociation. Previous studies have suggested that (1) the gas-producing rate of
15
hydrate dissociation was a function of the chemical injection rate, the concentration of the
16
chemical solution, the pressure and the temperature of the chemical solution, and the
17
hydrate/chemical interfacial area.28 The hydrate dissociation rate with EG injection was
18
demonstrated to be nearly linear to the flow rate, and were 0.056, 0.075, and 0.085 mol/min at
19
the flow rate of 80, 100, and 120 mL/min, respectively.43 Therefore, the above phenomenon in
20
this study was due to the fact that an acceleration of injection rate can increase the contact
21
area for the inhibitor and hydrate, and raise the dissociation rates at the same time. In Figure 6,
22
for the data been adjusted by Run 0, the cumulative gas produced from hydrate increased and
23
then reached a certain level and maintained almost stable at the second half of Stage 3. This
24
confirmed that the division of Stage 3 and Stage 4 were appropriate and reasonable.
25
3.5 Effect of methanol concentration. Figure 7 shows the variation of the cumulative gas
26
production with time during Stage 3 for Run 0 and Runs 3-7 (data adjusted by Run 0). The
27
effects of the methanol concentration on the cumulative gas production from hydrate (hydrate
28
dissociation) are shown in Table 3 and Figure 7. The curves of Runs 3-7 in Figure 7 were all
29
based on experimental data adjusted by Run 0, with the same adjustment strategy discussed
30
above in Figure 6. For Run 0 and Runs 3-7, the methanol solution injection rates all kept the
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1
same at 8.9 mL/min. Thus, for Runs 3-7, the ratios of the methanol and deionized water
2
injection rates were all equal to 1.0 due to the same injection rates with Run 0.
3
It can be seen from Runs 3-7 in Table 3, the effect of methanol concentration (30-70 wt%)
4
on the duration of hydrate dissociation (26-19 min, Stage 3) were relatively limited. It was as
5
expected that both the cumulative and average value of the gas produced from hydrate
6
increased significantly with the concentration of the injected methanol solution. In Run 5,
7
Run 6 and Run 7, the percentage of hydrate dissociated at the end of Stage 3 increased
8
significantly to from 51.1% to over 70% with the methanol solution concentration as high as
9
50 - 70 wt%, as shown in Figure 7 and Table 3. With the similar original hydrate saturation
10
and methanol solution injection rate, this phenomenon strongly demonstrated and confirmed
11
the inhibition effect of methanol on hydrate. It was consistent with the results observed by
12
Dong et al.30 As a thermodynamic inhibitor, methanol changed the phase equilibrium of
13
methane hydrate and made the hydrate "solved" quickly.
14
On the other hand, in the experimental runs with low injection rate or methanol
15
concentration, such as Run 1, Run 2, Run 3 and even Run 4, the uptrend of the curves of the
16
cumulative gas production (after adjustment) weakened with time, and did not increase
17
obviously at the end of Stage 3, as shown in Figure 6 and Figure 7. Be accompanied by the
18
water dilution process, as mentioned above, the effect of methanol solution on hydrate
19
dissociation was limited for lack of enough driving force.
20
3.6 Production Efficiency and Methanol Efficiency. The production efficiency (ηp) was
21
defined as the ratio of the methane gas produced from hydrate to the mass of the methanol
22
solution injected in unit time. The methanol efficiency (ηm) was defined as the ratio of the
23
methane gas produced from hydrate to the mass of pure methanol injected in unit time. In this
24
study, the efficiencies during the major hydrate dissociation stage (Stage 3) were analyzed,
25
and they were calculated with the cumulative gas production, the mass of pure methanol and
26
methanol solution injected, and the duration of Stage 3 (see Table 3).
27
Figure 8 shows the effects of injection rate and concentration of methanol solution on
28
production and methanol efficiencies during Stage 3 for Runs 1-7. It can be seen from Figure
29
8 that both ηp and ηm were enhanced gradually with the increase of the methanol solution
30
injection rate. With higher injection rate, there was more original deionized water driven and
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Energy & Fuels
1
produced from the hydrate vessel. Meanwhile, there was more methanol injected into the
2
vessel in unit time, which weakened the methanol dilution effect with the same amount of
3
original water in the vessel.
4
For Runs 3, 4, 5, 6 and 7, with the increase of the methanol concentration from 30 wt% to
5
70 wt%, ηp increased continuously, which suggested that the efficiency of gas released from
6
hydrate and been produced from the vessel was enhanced when more methanol was injected
7
into the vessel. However, considering the efficiency of the usage of methanol in the injected
8
solution, ηm increased and reached a maximum value when the concentration was 60 wt%,
9
and then gradually decreased. As discussed above in Figure 7, the cumulative gas production
10
increased with the increase of the methanol concentration. Nevertheless, the effect of the
11
concentration of methanol on cumulative gas production was not obvious after the
12
concentration was raised to a certain level (50-60 wt%). In other words, with the increase of
13
the methanol concentration, the consumption of the mass of methanol increased significantly,
14
which resulted in a reverse effect on the efficiencies. The production and methanol
15
efficiencies could be used to evaluate the gas production efficiency and the economic
16
performance of the thermodynamic inhibitors.
17
4. CONCLUSION
18
In this work, eight experimental runs were carried out to investigate the gas production
19
behavior from hydrate reservoir by methanol injection using a one-dimensional experimental
20
apparatus. From the experimental results, we can draw the following conclusions:
21
(1) The whole production process by methanol injection in the hydrate vessel consisted of
22
four stages: free gas production, methanol dilution, major hydrate dissociation, and
23
residual gas production.
24
(2) The cumulative gas produced from hydrate under methanol solution injection was
25
adjusted with the reference experiment Run 0. A new strategy of the adjustment of the
26
experimental runs, which was introduced in this study, was based on the fluid (water and
27
methanol solution) injection rate.
28
(3) Some of the injected methanol solution maintained in the vessel and drove the original
29
gas out, and the overall liquid production rate was smaller than the injection rate during
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1
the whole production process. In general, the cumulative hydrate-originating gas
2
produced increased from the vessel with the increase of the methanol injection rate and
3
the methanol concentration.
4
(4) During the major hydrate dissociation stage, the production efficiency was enhanced
5
continuously with the increase of the injection rate and concentration of the methanol
6
solution. However, the methanol efficiency increased and reached a maximum value
7
when the concentration was 60 wt% and then gradually decreased.
8 9
AUTHOR INFORMATION
10
Corresponding Author
11
*Tel.: +86-20-87057037; Fax: 86-20-8703-4664. E-mail:
[email protected]. (X.-S. Li.)
12
ORCID
13
G. Li: 0000-0002-4549-293X
14
Notes
15
The authors declare no competing financial interest.
16 17
ACKNOWLEDGMENTS
18
This work is supported by National Natural Science Foundation of China (51376183,
19
51676196, 51506203), Frontier Sciences Key Research Program of the Chinese Academy of
20
Sciences (QYZDB-SSW-JSC028), and Natural Science Foundation of Guangdong Province
21
of China (2014A030313669), which are gratefully acknowledged.
22 23
24
(1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press (Taylor
25
and Francis Group): Boca Raton, FL, 2008.
26
(2) Collett, T.; Bahk, J. J.; Baker, R.; Boswell, R.; Divins, D.; Frye, M.; Goldberg, D.;
27
Husebo, J.; Koh, C. A.; Malone, M.; Morell, M.; Myers, G.; Shipp, C.; Torres, M. Methane
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Hydrates in Nature-Current Knowledge and Challenges. J. Chem. Eng. Data 2015, 60 (2),
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319-329.
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(3) Klauda, J. B.; Sandler, S. I. Global distribution of methane hydrate in ocean sediment.
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Energy Fuels 2005, 19 (2), 459-470.
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(4) Makogon, Y. F.; Holditch, S. A.; Makogon, T. Y. Natural gas-hydrates—A potential
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energy source for the 21st Century. J. Petrol. Sci. Eng. 2007, 56 (1), 14-31.
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1
TABLE Captions
2
Table 1. Formation conditions and results of methane hydrate.
3
Table 2. Experimental conditions after methanol solution injection.
4
Table 3. Gas production results of hydrate dissociation by methanol injection.
5 6 7
FIGURE Captions
8
Figure 1. Schematic of the experimental apparatus.
9
Figure 2. Evolution of temperature and pressure during methane hydrate formation.
10
Figure 3. Methane hydrate phase diagram with different methanol concentrations.
11
Figure 4. Evolution of gas production rate with time for Run 5.
12
Figure 5. Cumulative gas production and mass of liquid injection and production with time
13
for Run 5.
14
Figure 6. Variation of the cumulative gas production with time during Stage 3 for Run 0 and
15
Runs 1-3 (original data and data adjusted by Run 0).
16
Figure 7. Variation of the cumulative gas production with time during Stage 3 for Run 0 and
17
Runs 3-7 (data adjusted by Run 0).
18
Figure 8. Effects of injection rate and concentration of methanol solution on production and
19
methanol efficiencies during Stage 3 for Runs 1-7.
20 21 22 23 24
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Table 1. Formation conditions and results of methane hydrate. runs
0
1
2
3
4
5
6
7
P0 (MPa)
5.38
5.32
5.4
5.72
5.4
5.43
5.53
5.41
T0 (oC)
17.7
17.8
17.5
17.7
17.5
17.8
17.7
17.6
Pend (MPa)
3.5
3.65
3.65
3.75
3.65
3.64
3.59
3.72
Tend (oC)
1.8
1.8
1.7
1.8
1.7
1.7
1.8
1.5
SG (%)
67.79
67.8
68.54
67.69
68.54
67.34
67.48
67.32
SA (%)
24.79
25.38
24.92
25.88
24.92
25.73
25.38
26.08
SH (%)
7.42
6.82
6.54
6.43
6.54
6.93
7.14
6.6
2 3
Table 2. Experimental conditions after methanol solution injection. runs
0
1
2
3
4
5
6
7
Methanol concentration (wt%)
0
30
30
30
40
50
60
70
Methanol solution density (g/mL)
1
0.956
0.956
0.956
0.94
0.922
0.902
0.879
Solution injection rate (mL/min)
8.9
4.9
6.8
8.9
8.9
8.9
8.9
8.9
Average pressure (MPa)
3.56
3.72
3.82
3.71
3.64
3.64
3.8
3.87
Average temperature (oC)
1.6
1.7
2
2.1
2.3
2
1.8
2.1
6
7
4 5
Table 3. Gas production results of hydrate dissociation by methanol injection. runs
0
1
2
3
4
5
Stage 1-4 Solution injection time (min)
119
49
51
33
33
39
31
37
Total methanol injected (g)
0
69.1
99.8
84.9
110.9
158.7
148.6
203.2
Methanol injection rate (g/min)
0
1.4
2.0
2.5
3.4
4.1
4.8
5.5
Total gas produced (mL)
2799
1596
2227
2282
2006
2879
2040
2587
24
33
44
69
61
74
66
70
-
34
30
26
21
21
19
21
-
434
589
727
706
1090
1070
1435
-
13
20
28
34
52
56
68
-
20.7
29.3
36.7
35.1
51.1
48.7
70.7
Average gas production rate (mL/min) Stage 3 Duration (min) Cumulative gas produced from hydrate (adjusted by Run 0) (mL) Average rate of gas produced from hydrate (adjusted by Run 0) (mL/min) Percentage of hydrate dissociated (%)
6
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N2 Gas Cylinder
1
Figure 1. Schematic of the experimental apparatus. 6.0
5.5
Initial rapid formation
5.0
4.5
Shell breakage/cracking
4.0
3.5
A
3.0 0
200
B 400
C 600
800
1000
1200
1400
1600
1800
20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 2000
Time (min)
4 5
Figure 2. Evolution of temperature and pressure during methane hydrate formation.
6
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Temperature (oC)
2 3
Pressure (MPa)
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CH4 Gas Cylinder
Energy & Fuels
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1000
Lw-H or H-V (Hydrate exist) 50 wt% MeOH
Driving force (Run 5)
40 wt% MeOH
H
Pressure (MPa)
100
(Run 4)
(Run 3)
Lw-H-V (Equilibri
30 wt% MeO
H
20 wt% MeO
H
10 wt% MeOH 0 wt% MeOH
10
G
um)
Lw-V (Hydrate free) 1 0
1
2
3
4
5
6
7
8
o
Temperature ( C)
1 2
Figure 3. Methane hydrate phase diagram with different methanol concentrations.
3 600
Stage 1 Stage 2 550
Stage 4
Stage 3
500 450
Gas production rate (mL/min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
400 350 300 250 200 150 100 50 0 0
5
10
15
20
25
30
Time (min)
4 5
Figure 4. Evolution of gas production rate with time for Run 5.
6
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35
40
Energy & Fuels
450
3500
Stage 1 Stage 2
400
Stage 3
Stage 4 3000
2500 300 250
2000
200
1500
150 1000 100
gas production liquid injection liquid production
50
Cumulative gas production (mL)
Cumulative liquid mass (g)
350
500
0
0 0
5
10
15
20
25
30
35
40
Time (min)
1 2
Figure 5. Cumulative gas production and mass of liquid injection and production with time
3
for Run 5.
4 1400
Run 0 Run 1 Run 1 - 4.9/8.9 • Run 0 Run 2 Run 2 - 6.8/8.9 • Run 0 Run 3 Run 3 - Run 0
1300 1200 1100
Cumulative gas production (mL)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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1000 900
Point M
800 700
Point P
600 500 400 300
Point N
200 100 0 0
5
10
15
20
25
30
35
Time (min)
5 6
Figure 6. Variation of the cumulative gas production with time during Stage 3 for Run 0 and
7
Runs 1-3 (original data and data adjusted by Run 0).
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ACS Paragon Plus Environment
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1600
Run 0 Run 3 - Run 0 Run 4 - Run 0 Run 5 - Run 0 Run 6 - Run 0 Run 7 - Run 0
1500 1400
Cumulative gas production (mL)
1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0 0
5
10
15
20
25
30
Time (min)
1 2
Figure 7. Variation of the cumulative gas production with time during Stage 3 for Run 0 and
3
Runs 3-7 (data adjusted by Run 0).
4 0.7
Run 6
Run 5
Run 7
0.6
ηm
Run 4
0.5
Efficiency (mL/(g⋅min))
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Run 3 0.4
Run 2
Run 7
Run 6
0.3
ηp
Run 5
Run 1 Run 4
0.2
Run 3 Run 2 Run 1
0.1
0.0 25
30
35
40
45
50
55
60
65
70
75
Methanol concentration (wt%)
5 6
Figure 8. Effects of injection rate and concentration of methanol solution on production and
7
methanol efficiencies during Stage 3 for Runs 1-7.
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ACS Paragon Plus Environment
