Experimental Investigation into the Production Behavior of Methane

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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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Energy & Fuels

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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

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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

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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

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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

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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.

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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

2

(-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

5

"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

9

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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1

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.

9

3. RESULTS AND DISCUSSION

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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

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experimental runs were carried out to study the dissociation characteristics of methane

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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

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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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1

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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1

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


Energy & Fuels

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

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


Page 12 of 23

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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


Energy & Fuels

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

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

28

Hydrates in Nature-Current Knowledge and Challenges. J. Chem. Eng. Data 2015, 60 (2),

29

319-329.

30

(3) Klauda, J. B.; Sandler, S. I. Global distribution of methane hydrate in ocean sediment.

31

Energy Fuels 2005, 19 (2), 459-470.

32

(4) Makogon, Y. F.; Holditch, S. A.; Makogon, T. Y. Natural gas-hydrates—A potential

33

energy source for the 21st Century. J. Petrol. Sci. Eng. 2007, 56 (1), 14-31.

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(6) Ji, C.; Ahmadi, G.; Smith, D. H. Natural gas production from hydrate decomposition by

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accumulations in oceanic sediments. Energy Convers. Manage. 2007, 48 (6), 1834-1849.

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(9) Heeschen, K. U.; Abendroth, S.; Priegnitz, M.; Spangenberg, E.; Thaler, J.; Schicks, J. M.

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depressurization test in Mallik, Northwest Territories, Canada. Energy Fuels 2016, 30 (8),

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investigation into the production behavior of methane hydrate in porous sediment with hot

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brine stimulation. Ind. Eng. Chem. Res. 2008, 47 (23), 9696-9702.

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porous media. AIChE J. 1989, 35 (6), 1049-1052.

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(22) Li, X. Y.; Gjertsen, L. H.; Austvik, T. Thermodynamic inhibitors for hydrate plug

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melting. Ann. N.Y. Acad. Sci. 2000, 912 (1), 822-831.

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Mcmullen, N.; Palermo, T.; Shoup, G.; Talley, L. Natural Gas Hydrates in Flow Assurance,

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(24) Kinnari, K.; Hundseid, J.; Li, X. Y.; Askvik, K. M. Hydrate management in practice. J.

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Chem. Eng. Data 2015, 60 (2): 437-446.

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32 (8), 1321-1333.

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(26) Yousif, M. H. Effect of underinhibition with methanol and ethylene glycol on the

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hydrate-control process. SPE Prod. Facil. 1998, 13 (3), 184-189.

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(27) Ke, W.; Svartaas, T. M.; Abay, H. K. An experimental study on SI hydrate formation in

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presence of methanol, PVP and PVCap in an isochoric cell. In the 7th International

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Conference on Gas Hydrate, Edinburgh, Scotland, U.K. 2011.

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(28) Sira, J. H.; Patil, S. L.; Kamath, V. A. Study of hydrate dissociation by methanol and

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glycol injection. In Proceedings of the SPE Annual Technical Conference and Exhibition.

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New Orleans, LA, USA, 23-26 September 1990.

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(29) Yuan, Q.; Sun, C. Y.; Yang, X.; Ma, P. C.; Ma, Z. W. Gas production from

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methane-hydrate-bearing sands by ethylene glycol injection using a three-dimensional

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(30) Dong, F. H.; Zhang, X. Y.; Li, D. L.; Fan, S. S.; Liang, D. Q. Experimental investigation

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on propane hydrate dissociation by high concentration methanol and ethylene glycol solution

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injection. Energy Fuels 2009, 23 (3), 1563-1567.

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(31) Qureshi, M. F.; Atilhan, M.; Altamash, T.; Tariq, M.; Khraisheh, M.; Aparicio, S.;

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Tohidi, B. Gas hydrate prevention and flow assurance by using mixtures of ionic liquids and

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synergent compounds: combined kinetics and thermodynamic approach. Energy Fuels 2016,

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measurement and thermodynamic modeling of methane hydrate dissociation conditions in the

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presence of aqueous solution of ionic liquid. Fluid Phase Equilib. 2013, 354, 312-318.

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(33) Vojta, D.; Vazdar, M. The study of hydrogen bonding and pi...pi interactions in

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phenol...ethynylbenzene complex by IR spectroscopy. Spectrochim. Acta, Part A 2014, 132,

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6-14.

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(34) Tohidi, B.; Anderson, R.; Chapoy, A.; Yang, J. H.; Burgass, R. W. Do we have new

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solutions to the old problem of gas hydrates? Energy Fuels 2012, 26 (7), 4053-4058.

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(35) Kelland, M. A. History of the development of low dosage hydrate inhibitors. Energy

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Fuels 2006, 20 (3), 825-847.

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on the stability of clathrate hydrates. Can. J. Chem. 1981, 59 (17), 2587-2590.

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growing shell model in water-in-oil dispersions. Chem. Eng. Sci. 2009, 64 (18), 3996-4004.

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16

Gupta, A. Methane hydrate formation and dissociation in a partially saturated core-scale sand

17

sample. J. Petrol. Sci. Eng. 2007, 56 (1-3), 108-126.

18

(39) Fitzgerald, G. C.; Castaldi, M. J.; Schicks, J. M. Methane hydrate formation and thermal

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based dissociation behavior in silica glass bead porous media. Ind. Eng. Chem. Res. 2014, 53

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(40) Li, G.; Li, X. S.; Li, B.; Wang, Y. Methane hydrate dissociation using inverted five-spot

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water flooding method in cubic hydrate simulator. Energy 2014, 64, 298-306.

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(41) Li, B.; Li, X. S.; Li, G.; Feng, J. C.; Wang, Y. Depressurization induced gas production

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from hydrate deposits with low gas saturation in a pilot-scale hydrate simulator. Appl. Energy

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2014, 129 (6), 274-286.

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dissociation conditions in porous media using two thermodynamic approaches. J. Chem.

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Thermodyn. 2008, 40, 1464-1474.

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(43) Fan, S. S.; Zhang, Y. Z.; Tian, G. L.; Liang, D. Q.; Li, D. L. Natural gas hydrate

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Energy & Fuels

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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


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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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

1

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


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


Temperature (oC)

2 3

Pressure (MPa)

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

Page 20 of 23

CH4 Gas Cylinder

Energy & Fuels

Page 21 of 23

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


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


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

Page 22 of 23

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


7

Runs 1-3 (original data and data adjusted by Run 0).

8


Page 23 of 23

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


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


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.

8


Experimental Investigation into the Production Behavior of Methane

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