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Energies 2012, 5(2), 399-419; https://doi.org/10.3390/en5020399

Review
A Review on Research on Replacement of CH4 in Natural Gas Hydrates by Use of CO2
1
Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, Dalian 116024, China
2
Department of Civil Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia
3
CNOOC Research Center, Beijing 100027, China
*
Authors to whom correspondence should be addressed.
Received: 28 December 2011; in revised form: 6 February 2012 / Accepted: 8 February 2012 / Published: 22 February 2012

Abstract

:
This paper introduces the research advances on replacement of CH4 in Natural Gas Hydrates (NGHs) by use of CO2 and discusses the advantages and disadvantages of the method on the natural gas production from such hydrates. Firstly, the feasibility of replacement is proven from the points of view of kinetics and thermodynamics, and confirmed by experiments. Then, the latest progress in CH4 replacement experiments with gaseous CO2, liquid CO2 and CO2 emulsion are presented Moreover, the superiority of CO2 emulsion for replacement of CH4 is emphasized. The latest experiment progress on preparation of CO2 emulsions are introduced. Finally, the advancements in simulation research on replacement is introduced, and the deficiencies of the simulations are pointed. The factors influencing on the replacement with different forms of CO2 are analyzed and the optimum conditions for the replacement of CH4 in hydrated with different forms of CO2 is suggested.
Keywords:
gas hydrate; replacement; carbon dioxide; feasibility; emulsion; simulation

1. Introduction

Natural gas hydrates (NGHs) are non-stoichiometric crystalline compounds formed by natural gas and water molecules under low temperature and high pressure [1]. NGH widely exists below the ocean floor and in permafrost zones [2], and there is a large amount of NGH in Nature [3,4], so it is regarded as a potential future energy resource with great commercial exploitation potential. At present, the main mining methods for natural gas production from hydrates include depressurization, thermal stimulation and addition of inhibitors [5,6,7]. The methods promote hydrate decomposition to produce natural gas by destroying hydrate’s phase equilibrium under initial conditions. However, as various mining methods’ technical and economical features are taken into account, it has to be considered that NGH plays an important role in stabilizing the stratum where it exists [8]. During the process of NGH mining by use of the traditional methods, the decomposition of NGH may make the stratum where it exists become unstable and lead to geological disasters such as earthquakes, submarine landslides, etc. [9,10,11]. On the other hand, as the rapid development of human society produces massive discharges of greenhouse gases this has become a big environmental issue which humans have to face, and CO2 storage is an effective measure to reduce CO2 emissions and reduce the greenhouse effect [12,13].
Based on the two points on the above, Ebinuma [14] and Ohgaki et al. [15] suggested a new method for natural gas recovery from hydrates in sediments by use of CO2. As a method which combines CO2 long-term storage and NGH mining, this method can not only avoid some traditional mining methods’ disadvantages, such as a lack of heat source by depressurization method, the low efficiency of heat by thermal stimulation method, the high cost and damage to the stratum by the inhibitor addition method, but also provide a way to stabilize the stratum below the ocean floor and store CO2 for a long time [15,16].
The replacement method by use of CO2 is regarded as a promising NGH mining method, and has attracted great attention from researchers all over the world, but the slow replacement rate and low replacement efficiency restrict the method’s role in commercial applications [2,15]. According to the experimental data reported by Ota et al. [17] and Li et al. [18,19], during the replacement of CH4 by use of gaseous CO2 at temperatures ranging from 271.2 K to 276 K and at an initial pressure of 3.25 MPa, the replacement reaction rate became extremely slow after the early stages (about 10 h), besides, the ratio of CH4 released from hydrate was no more than 15% within 100 h. If the replacement rate and efficiency cannot be improved, the replacement method should not have any actual practical value.

2. Feasibility Study of Replacement Reaction

2.1. The Thermodynamic Feasibility of Replacement

Figure 1 is the equilibrium diagram of CH4-CO2-H2O system drawn based on the data from Sloan et al. [20]. In the diagram, areas A and B are above the equilibrium curve of H2O-hydrate-CO2 and below that of H2O-hydrate-CH4, so in theory, gaseous CH4 and CO2 hydrate can coexist in these areas. Thus, it can be concluded that CO2 hydrate is more stable than CH4 hydrate under certain condition.
Figure 1. Diagram of CH4-CO2-H2O phase equilibrium.
Figure 1. Diagram of CH4-CO2-H2O phase equilibrium.
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Experimental measurements and theoretical calculations both prove the possibility for CH4–CO2 replacement in the hydrate thermodynamically. Uchida et al. [21] used gas chromatography and Raman spectroscopy to analyze several formation and decomposition processes of CH4–CO2 mixed gas hydrates. The experiment and theoretical calculation demonstrated that, when the temperature is below 283 K, the equilibrium pressures of CH4 hydrate are higher than those of CO2 hydrate at the same temperature. Anderson et al. [22] studied the phase equilibrium conditions of CH4-CO2-H2O system in porous media. Their experimental data supported Uchida’s opinion.
Geng et al. [23] studied the stabilities of CH4 hydrate, CO2 hydrate, and CH4–CO2 mixed hydrate by molecular dynamics (MD) simulations; in these simulations, the temperature ranged from 260 to 280 K and a pressure of 5 MPa was chosen. The simulation results indicate that the CH4–CO2 mixed hydrate is the most stable among the three hydrates discussed, so in theory, after CO2 is injected into CH4 hydrate under suitable conditions, CH4 hydrate will transformed into the more stable CH4–CO2 mixed hydrate, and CH4 gas is produced at the same time.
Based on chemical thermodynamic basic theory, chemical reactions occur spontaneously if the Gibbs free energy for the reaction is negative. Yezdimer et al. [24] analyzed the thermodynamic feasibility of replacing CH4 gas from the hydrate with CO2 by MD simulation. Their simulation result showed the residual Gibbs free energy in the mutation process of CH4 into Ar was negative while that in the mutation process of CO2 into Ar was positive, this suggested the Gibbs free energy during the CH4–CO2 replacement in the hydrate is negative. Thus, the replacement of CH4 with CO2 is thermodynamically feasible.

2.2. The Kinetic Feasibility of Replacement

The distribution coefficients of CH4 and CO2 between the gas and the hydrate phase were calculated by Ohgaki et al. [15]. The results showed that compared with CH4, CO2 tended to distribute in the hydrate phase, so the result preliminarily proved the feasibility of replacement. Uchida [25] was the first researcher to prove that the replacement reaction occurs on the contact surface between CH4 hydrate and CO2 gas by use of Raman spectroscopy. Besides, his research group suggested that the replacement rate is very slow and the induction time is up to several days. Uchida et al. [21] measured the composition ratio of CH4 and CO2 in the vapor phase XCH4/XCO2 at various times during the formation process of CH4–CO2 mixed gas hydrates, they found that the ratio varied with the formation reaction progressing. The following logarithmic form was used to fit the data:
X CH 4 / X CO 2 = ( X CH 4 / X CO 2 ) 0 + α lg t
where t is the time, (XCH4/XCO2)0 is the initial composition ratio, and α is a fitting parameter related to the condensation rate of CH4 molecules from the vapor phase. From Equation (1), it can be concluded that the ratio of CH4 in the vapor phase increases with time, this result confirms the conclusion Ohgaki et al. drawn [15]. In the experiment, Uchida et al. found although the hydrate formation consumed more CO2 than CH4 as a whole, more CH4 was consumed in the early stages. This phenomenon perhaps results from the structure of hydrate and the sizes of CH4 and CO2 molecules. On a microscopic scale, crystal structure types of CO2 hydrate and CH4 hydrate are both type SI [1,2,26], SI hydrate’s unit cell consists of six medium cages (M-cage) and two small cages (S-cage). The CO2 molecular dimension is a little larger than the CH4 molecular dimension, and the molecular size of CO2 is between that of the M-cage and S-cage of SI hydrate [1,2,27], so CO2 molecules occupy the M-cages mainly. In the early stages, CH4 molecules were able to occupy both S-cages and M-cages while CO2 molecules only occupied M-cages, as a result, more CH4 gas was consumed. In the later stages, a replacement reaction occurred, and CO2 molecules occupied the M-cages which held CH4 molecules, CH4 gas was released, so the ratio of CH4 in the vapor phase increased. Uchida et al. divided the replacement reaction into two phases: (1) some CH4 hydrate decomposes and the generated gaseous CH4 transfers into the vapor phase; (2) CO2 molecules occupy the M-cages and CH4 molecules re-occupy the S-cages because of the memory effect. Figure 2 [28] is the schematic diagram redrawn from Ota et al. [28], it describes the guest molecule exchange process in the M-cages and the CH4 re-occupation in the S-cages.
Figure 2. Schematic diagram of the guest molecule replacement in the M-cage and the CH4 re-occupation in the S-cage.
Figure 2. Schematic diagram of the guest molecule replacement in the M-cage and the CH4 re-occupation in the S-cage.
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Hirohama et al. [29] suggested the driving force for CH4–CO2 replacement in the hydrate seemed to be the fugacity difference between different phases. Ota et al. [17,28] built the models for decomposition of CH4 hydrate and formation of CO2 hydrate during the replacement process, they assumed the driving force for CH4–CO2 replacement in the hydrate was proportional to the fugacity difference between the gas and the hydrate phase. The model of the CH4 hydrate decomposition is written as follows:
d n CH 4 . H d t = k Dec A ( f CH 4 . H f CH 4 . G )
1 k Dec = 1 k Dec . R + 1 k Dec . D
where nCH4.H is the remaining amount of CH4 in the hydrate phase, t is the reaction time, f is the fugacity, and kDec is the overall rate constant of the decomposition. kDec includes kDec.R, which is the reaction rate constant of decomposition and kDec, which is the rate constant of mass transfer in the hydrate phase. In Equation (2), A is the surface area between the gas and the hydrate phase, and H and G refer to the hydrate phase and the gas phase, respectively. The model of CO2 hydrate formation during the replacement can similarly be written as follows:
d n CO 2 . H d t = k Form A ( f CO 2 . G f CO 2 . H )
1 k Form = 1 k Dec . R + 1 k Dec . D
where nCO2.H is the amount of CO2 in the hydrate phase and kForm is the overall rate constant of the formation, kForm includes kForm.R and kForm.D rate constants.
Ota et al. calculated the fugacity from the model including the van der Waals–Platteeuw theory [20,30] and the Soave–Redlich–Kwong equation of state (SRK-EOS) [31]. The experimental conditions (T, P) were fixed in the SRK-EOS equation to calculate fugacity and the resulting compositions were measured by Raman spectroscopy. The surface area between the gas and the hydrate phase was treated as a constant. The slope of the Arrhenius plot was used to calculate activation energies for CH4 hydrate decomposition and CO2 hydrate formation. Ota et al. pointed that the rate constant of decomposition seemed to dominate the CH4 hydrate decomposition, while the mass transfer likely dominated the CO2 hydrate formation during the replacement process.
According to the experimental data for CO2 and CH4 hydrate formation respectively reported by Nagayev et al. [32] and Rueff et al. [33], the activation energy for CO2 hydrate formation is 57.98 kJ/mol and that for CH4 hydrate decomposition is 54.49 KJ/mol. While Ota et al. [17] calculated that the activation energy for CO2 hydrate formation is 73.3 KJ/mol and that for CH4 hydrate decomposition is 14.5 KJ/mol from these models. The data of Li’s experiment [19] shows the activation energies are 68.4 KJ/mol and 28.8 KJ/mol respectively from these models. These differences of activation energy may result from the interaction between formation of CO2 hydrate and decomposition of CH4 hydrate during the replacement process. By means of analysis and comparison of the activation energy during the replacement process, it can be concluded the formation of CO2 hydrate can supply enough heat for the decomposition of CH4 hydrate, and the extra heat can prevent CH4 hydrate’s self-protection and promote decomposition of CH4 hydrate further.

3. Experiment Study of Replacement Reaction

3.1. Replacement of CH4 Hydrate by Use of Gaseous CO2

Researchers have done the replacement experiment by use of gaseous CO2 under different conditions [16,17,18,19]. The adapted schematic diagram of the experimental apparatus for replacement with gaseous CO2 is illustrated in Figure 3 [17]. The apparatus mainly consists of a high-pressure cell which has a magnetic agitator inside it for hydrate formation and replacement reaction, a cooling system for keeping the temperature of the cell constant, a data acquisition system for data collection, and a laser Raman spectrometer or a gas chromatograph for gas phase composition analysis. The experimental procedure is as follows: (1) the desired amount of distilled water is firstly introduced into the cell, When the pressure is controlled at the set value, the magnetic agitator is started to promote the CH4 hydrate formation; (2) CH4 hydrate formation is considered to be completed when the experimental measurements remain unchanged. Then the cell is purged by high pressure CO2 gas, which is confirmed by the laser Raman spectrometer or the gas chromatograph; (3) the replacement reaction starts when the system is controlled at the required pressure and temperature. After the reaction starts, a little gas sample is taken out to be analyzed at required intervals; (4) after a given elapsed time, the hydrate mixtures are decomposed by heating, and the compositions are quantified.
Figure 3. Schematic diagram of the experimental apparatus. 1: CO2 cylinder; 2: Cooler; 3: CO2 buffer; 4: CH4 buffer; 5: Cell with cooling jacket; 6: Windows; 7: Stirrer; 8: Pressure reducer; 9: CH4 cylinder; 10: Thermocouples; 11: Pressure gauge; 12: Back pressure regulator; 13: Laser Raman spectrometer or a gas chromatograph; 14: Tank.
Figure 3. Schematic diagram of the experimental apparatus. 1: CO2 cylinder; 2: Cooler; 3: CO2 buffer; 4: CH4 buffer; 5: Cell with cooling jacket; 6: Windows; 7: Stirrer; 8: Pressure reducer; 9: CH4 cylinder; 10: Thermocouples; 11: Pressure gauge; 12: Back pressure regulator; 13: Laser Raman spectrometer or a gas chromatograph; 14: Tank.
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Figure 4 drawn based on the data from Ota et al. [17] shows how the amount of the decomposed CH4 hydrate (QCH4,Dec) and formed CO2 hydrate (QCO2,Form) changes with time respectively in the experiment.
Figure 4. Amount of the decomposed CH4 hydrate (QCH4,Dec) (a) and the formed CO2 hydrate (QCO2,Form); (b) against time.
Figure 4. Amount of the decomposed CH4 hydrate (QCH4,Dec) (a) and the formed CO2 hydrate (QCO2,Form); (b) against time.
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Based on the experimental data, we can reach three main conclusions: (1) it promotes CH4 hydrate decomposition and CO2 hydrate formation to increase the temperature appropriately at the same pressure; (2) the amount of decomposed CH4 hydrate is nearly consistent with that of formed CO2 hydrate. The phenomenon proves the replacement reaction’s essence is the process of CO2 molecules occupying CH4 molecules’ cages; (3) the reaction rate is rapid in the early stages (about 10 h), but it becomes slow after that.
The experimental data reported by Wang et al. [16] and Li et al. [18,19] show the variation tendency of the decomposed CH4 hydrate and formed CO2 hydrate during the replacement process is the same as that of Ota’s experiment. In Wang’s experiment, after the first 2 h, the rate of CH4 hydrate decomposition and CO2 hydrate formation slows, and in Li’s experiment, the high replacement rate is sustained for about 10 h, but in their experiments, the amount of CO2 hydrate formed is much more than that of decomposed CH4 hydrate. The highest ratio of formed CO2 hydrate and decomposed CH4 hydrate can be up to 5.6 and 6.0, respectively. The difference is mainly caused by the free water. In Ota’s experiment, there is no free water at the beginning of the replacement reaction, so the amount of decomposed CH4 hydrate is consistent with that of formed CO2 hydrate, while in the experiment of Wang and Li, free water exists in the hydrate stratum, and the amount of CO2 gas dissolved in the free water and formed CO2 hydrate with the free water is much more than that used for replacing CH4 from the hydrate. Thus, the factor of the free water in the NGH stratum should be taken into consideration in the actual exploitation of natural gas with CO2.
Based on the experimental results reported by different researchers, it is proven feasible to replace CH4 from the hydrate by use of gaseous CO2, but the replacement rate becomes extremely slow after the early stages of reaction, and the replacement efficiency can’t satisfy the requirements of commercial production of NGH. Thus more efficient methods of exploiting natural gas from the hydrate with CO2 should be developed.

3.2. Replacement of CH4 Hydrate by Use of Liquid CO2

Ota et al. [28] and Zhou et al. [34] have done the replacement experiment in the pure hydrate using liquid CO2. The initial temperature is 273.2 K and the initial pressure is chosen at 3.25 MPa in their experiments. Compared to the experimental apparatus used for replacement by use of gaseous CO2, a liquefying apparatus is added to be used for liquefying CO2 in this experiment. The procedure is as follows: (1) the desired amount of distilled water and CH4 gas is first introduced into the cell. When the system is pressurized to the required value, the agitation in the cell is started to promote the CH4 hydrate formation; (2) when the CH4 hydrate formation is completed, the cell is purged by high pressure CO2. Then the system is pressurized to the required value by keeping introducing CO2; (3) when the temperature of the system is controlled at the required value, saturated liquid CO2 is then introduced into the cell to replace CH4 from the hydrate; (4) the replacement process is observed and analyzed with in situ Raman spectroscopy. After a given amount of time, the CO2 remaining in the cell is released and the remaining hydrate mixture is resolved and analyzed.
Figure 5 drawn according to the data from Ota et al. [28] shows the time profile of the mole fractions of CH4 and CO2 in the hydrate phase in experiment. It can be seen the mole fraction of CH4 hydrate decomposition is consistent with that of CO2 hydrate formation, the phenomenon confirms the feasibility of replacement of CH4 in the hydrate by use of liquid CO2, and proves the replacement reaction’s essence is the process of CO2 molecules occupying CH4 molecules’ cages further.
Figure 5. Mole fraction of CH4 and CO2 in the hydrate phase as a function of time.
Figure 5. Mole fraction of CH4 and CO2 in the hydrate phase as a function of time.
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Zhou et al. [34] studied the replacement of CH4 in the hydrate by use of liquid CO2 under the same conditions as in Ota’s experiment [28]. Figure 6 redrawn from Zhou et al. [34] shows the CH4 ratios replaced from the hydrate of the two experiments, from the figure we can see that the results are equivalent approximately in less than 100 h. Li et al. [35] changed the experimental conditions in the replacement reaction by use of liquid CO2, the initial temperature and pressure were 282.2 K and 6 MPa respectively, and the experiment was conducted in porous sediment. His research showed the recovery ratio of CH4 can reach approximately 45% after 288 h, while in Ota’s experiment, the recovery ratio is about 37% after 307 h and the recovery ratio is 18.6% after 96 h in Zhou’s experiment.
Figure 6. CH4 ratios replaced from the hydrate of the two different experiments.
Figure 6. CH4 ratios replaced from the hydrate of the two different experiments.
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Ota et al. [28] observed the guest molecules’ transformation in the different cages and hydrate during the replacement process with Raman spectroscopy. Figure 7 redrawn from Ota et al. [28] shows the CH4 remaining in each cage and hydrate with time.
Figure 7. Time evolution of CH4 in the M-cages, S-cages and hydrate.
Figure 7. Time evolution of CH4 in the M-cages, S-cages and hydrate.
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It can be seen that the CH4 remaining in both the M-cages and S-cages decreased with time, however, the decay in the S-cages is much slower than that in the M-cages, and the ratio of CH4 in the M-cages is practically consistent with that of CH4 in the hydrate, which proves the replacement reaction mainly proceeds in the M-cages.
Researchers have applied experimental methods to prove the feasibility of replacing CH4 from the hydrate with liquid CO2. Compared with the replacement experiments with gaseous CO2, the replacement rate, efficiency and reaction time in the experiments with liquid CO2 are all improved, this shows that liquid CO2 is more suitable for the replacement than gaseous CO2.

3.3. Replacement of CH4 Hydrate by Use of CO2 Emulsion

Researchers have paid attention to finding out the reason why the replacement rate becomes slow after the early stages of reaction. According to an experimental study, Yoon et al. [36] pointed that in the early stages, the contact area between CO2 molecules and CH4 hydrate is large, so the reaction rate is fast. As the reaction progresses, the crust layer of CH4 hydrate is covered with CO2 hydrate, the CO2 hydrate layer provides a shielding effect, hinders the CH4 hydrate decomposition, and leads to the rate’s decrease and the reaction finally ceases.
In order to improve the replacement reaction rate, McGrail et al. [37] put forward the method of enhanced gas hydrate recovery (EGHR). The key of the method is to prepare an emulsion in which water is the continuous phase and CO2 is the dispersed phase, and the emulsion is substituted for gaseous and liquid CO2 to replace CH4 gas from the hydrate. In McGrail’s opinion, this method combines the advantages of controlled multiphase flow, heat, and mass transport processes in hydrate-bearing porous media, makes full use of the physical and thermodynamic properties of mixtures in the H2O-CO2 system, thus it can increase the contact area between CO2 molecules and CH4 hydrate and enhance the replacement reaction. White et al. [38] used numerical simulations to analyze the replacement process of CH4 from the hydrate stratum by use gaseous CO2, liquid CO2 and CO2 emulsion, and his simulation results show the replacement rate with CO2 emulsion is the highest among the three methods.
Replacement by CO2 emulsion is regarded as the best one among the three replacement methods by use of CO2, but the technique for preparing CO2 emulsions is still immature. DhanuKa et al. [39] used a new kind of emulsifier, TMN-6 (octa(ethylene glycol)-2,6,8-trimethyl-4-nonyl ether), for CO2 emulsion preparation. According to DhanuKa’s experiment, at a temperature below 318 K, the stability of the emulsion increases with pressure and the mass ratio of CO2 in the emulsion, in some cases the stable time of the emulsion with 90% CO2 in the emulsion can exceed 24 h.
A schematic illustration of the experimental apparatus for the CO2 emulsion formation redrawn form Zhou et al. [34] is shown in Figure 8. The apparatus mainly includes a CO2 liquefier used for liquefying CO2, a high-pressure cell for emulsion formation, a cooling system for controlling temperature and a high-speed magnetic agitator for promoting the CO2 emulsion formation. The emulsion was prepared as follows: (1) known amounts of water and TMN-6 are placed into the high-pressure cell; (2) CO2 gas is introduced into the cell to pressurize the system, after the pressure in the cell achieves the required value, liquid CO2 is injected into the cell, and the temperature is controlled to the required value by running the cooling system; (3) the magnetic agitator is started to promote the CO2 emulsion formation.
Figure 8. Experimental apparatus for CO2 emulsion formation. 1: CO2 cylinder; 2: plunger pump; 3: CO2 liquefier; 4: liquid distributing cell; 5: high-pressure cell; 6: pressure gauge; 7: PC; 8: data collector.
Figure 8. Experimental apparatus for CO2 emulsion formation. 1: CO2 cylinder; 2: plunger pump; 3: CO2 liquefier; 4: liquid distributing cell; 5: high-pressure cell; 6: pressure gauge; 7: PC; 8: data collector.
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Zhou et al. [34] used the prepared CO2 emulsion to replace CH4 in the hydrate in porous media. The procedures and apparatus of the experiment are similar to the experiment by use of liquid CO2, the only difference is that CO2 emulsion is substituted for liquid CO2. Zhou did three groups of replacement experiments using CO2 emulsion, the experimental initial pressure and temperature were the same, 90:10, 70:30, and 50:50 (WCO2:WH2O) CO2-in-water (C/W) emulsions were used to replace CH4 from its hydrate. The CH4 ratios replaced from the hydrate of the three groups of experiments by use of CO2 emulsion and one group of experiment by use of liquid CO2 at set intervals are calculated and shown in Figure 9 redrawn from Zhou et al. [34].
Figure 9. CH4 ratios replaced from the hydrate with different forms of CO2.
Figure 9. CH4 ratios replaced from the hydrate with different forms of CO2.
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Figure 10 redrawn from Zhou et al. [34] shows the replacement rates of CH4 versus time in different experiments. From the figures we can see that the replacement ratios of CH4 with the above emulsions are higher than that with liquid CO2, the former are about the latter 1.5 times, and the replacement efficiency increases with the mass ratio of liquid CO2 in the emulsion increasing. In addition, the replacement rate with emulsions reaches 5–7 times of the rate with liquid CO2. Thus, it can be concluded that CO2 emulsion is more efficient than liquid CO2 in replacing CH4 from the hydrate.
Figure 10. Replacement rates of CH4 with different forms of CO2.
Figure 10. Replacement rates of CH4 with different forms of CO2.
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In addition, in order to prove the superiority of CO2 emulsion in replacing CH4 from the hydrate, Zhou et al. [40] compared the replacement ratio of CH4 by use of CO2 emulsion and gaseous CO2. Zhou did three groups of contrast experiments under different conditions. The different molar quantities of CH4 gas replaced by gaseous CO2 and CO2 emulsion is shown in Figure 11 which is drawn based on the data from Zhou et al. [40].
Figure 11. Amount of the replaced CH4 gas by use of gaseous CO2 and CO2 emulsion against time.
Figure 11. Amount of the replaced CH4 gas by use of gaseous CO2 and CO2 emulsion against time.
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The graph (a) shows the molar quantity of free CH4 gas in the replacement process with gaseous CO2 and the graph (b) shows that in the replacement process with CO2 emulsion. From the figure, we can reach two conclusions: (1) CO2 emulsion is more effective than gaseous CO2 in replacing CH4 (2) the replacement rate with gaseous CO2 becomes slow after reaction for 10 h, and the reaction stops in about 50 h, the rate with CO2 emulsion is higher, and the reaction time can last over 100 h. Zhou pointed out that the result may due to the higher reaction temperature and the better conductivity and diffusibility of the CO2 emulsion. The heat given by emulsion promotes CH4 hydrate decomposition and increases the replacement rate, the better diffusibility of the CO2 emulsion leads to larger reaction area between CO2 molecules and CH4 hydrate, thus the reaction time extends.
Zhang et al. [41] pointed that the kinetics of CH4 replacement by CO2 emulsion, the varieties and contents of emulsifier, the ratio of water and liquid CO2, the influence on replacement rate of dispersed phase in the emulsion have to be further researched. In the actual exploitation, the existence of porous media, the statistical circumstances of CH4 hydrate presence in the stratum and the pumping of CO2 emulsion need to be considered.
Compared with gaseous CO2 and liquid CO2, CO2 emulsion is proved to be more efficient in replacing CH4 from the hydrate by experimental methods, but the preparation technique of CO2 emulsion is still immature, and the experimental research on the replacement with CO2 emulsion is scarce, so the influencing factors of reaction are not quite clear. Thus, the technique of CO2 emulsion preparation and the optimum conditions for replacement with CO2 emulsion should be studied further in the future work.

4. Advances in Simulation Research on Replacement

The Phase Field Theory (PFT) has been proved to be one of the most effective methods to model solidification in binary, ternary and multi-component melts over the past decade. PFT is applied to describe complex solidification morphologies, including thermal and solutal dendrites and eutectic/peritectic fronts [42]. The technique of Magnetic Resonance Imaging (MRI) is supposed to be a useful tool to visualize the process of hydrate formation and decomposition [43]. Baldwin et al. [44] and Ersland et al. [45] used MRI to observe CH4 hydrate formation and spontaneous conversion of CH4 to CO2 hydrate in porous media, and confirmed the feasibility of visualizing the replacement process by use of MRI. In order to investigate the micro-mechanism of replacement, researchers combine the PFT and the observation approach of MRI, and build corresponding models to simulate the replacement process.
Kvamme et al. [42,43] applied MRI to observe the replacement process by use of liquid CO2 and developed a corresponding model based on PFT. The model was used for describing the nucleation of hydrate in aqueous solution and the transformation of CH4 hydrate to CO2 hydrate. According to their reported results, the PFT-based model is more accurate than the classical nucleation theory in describing the nucleation process of hydrate. The theory simulations of Kvamme’s model indicate that the reformation kinetic rate is directly proportional to the kinetic rate characteristic for CO2 transport through an aqueous solution. The conclusion agrees with Yoon’s suggestion [36], but in Kvamme’s model, the effects of many other factors are ignored, and the model doesn’t give a good explanation of the replacement mechanism, so multiscale simulations are required for describing the replacement mechanism further.
Tegze et al. [46] built a multiscale model based on PFT to calculate the nucleation and growth rates of CO2 hydrate in aqueous solutions in which the parameters of the model were deduced from experiment and molecular dynamics simulation. In the paper, in order to determine the thickness of the CO2-hydrate–aqueous-solution interface, Tegze et al. performed molecular dynamics simulations taking realistic interaction potentials. The interface thickness from the MD simulations and the experimental interfacial free energy were used to fix the model parameters of multiscale model based on the PFT. Tegze et al. applied phase field calculations to determine the rate of the homogeneous nucleation and the velocity of the growth of CO2 hydrate in the aqueous solutions. The simulation result shows a homogeneous nucleation can be ruled out as a possible mechanism for initiating a hydrate formation, and the most probable obstruction factor of hydrate growth seems to be kinetic barriers caused by complex molecular motions.
In order to investigate the replacement microscopic mechanism further, Tegze et al. [47] built a multiscale model combining the phase field approach and the purely diffusive model, the model was used to describe the CH4 replacement process with liquid CO2 under conditions characteristic of underwater hydrate reservoirs. Data from experiments and atomistic simulations was used to fix the modes’ parameters, while the diffusion coefficient of CO2 in the hydrate phase was a set value, which was adjustable. The simulation result shows the replacement rates predicted by both the PFT and purely diffusive models are consistent with experimental results from MRI measurements. The agreement between the predicted and experimental rates supports the assumption that the hydrate conversion process is controlled by solid state diffusion. The PFT simulations indicate that hydrate conversion starts with the formation of a mixed SI hydrate and the simple purely diffusive model is proved to be useful to estimate transition kinetics with confidence, so it is concluded that multiscale simulation is an convenient tool to describe the replacement process microscopically. Besides, Tegze et al. pointed out that in further work, dedicated atomistic simulations should be added to the multiscale model to identify the micro-mechanism for the solid state transformation and to extend the model to the conversion of natural gas hydrates.
Multiscale simulation combining PFT and molecular dynamics provides a theoretical research method for analyzing the replacement microscopic mechanism, but current research on simulation of replacement focuses on the replacement with liquid CO2, the combination of simulations and advanced micro detection techniques are few, and the comparison between simulation results and experimental data is lacking, so in future work, researchers should pay attention to combining simulations and advanced detection techniques, develop simulations based on PFT and molecular dynamics, study the interfacial effect in the porous media and the effect of CO2 emulsion during the replacement process further, and build models which are suitable for explaining the replacement with CO2 emulsions.

5. Research on Factors Influencing the Replacement Reaction

Although the replacement method is regarded as a promising NGH exploitation method, the low replacement rate and efficiency are important obstacles to its application in commercial production. In order to improve the replacement rate and efficiency, researchers have utilized experimental and simulative methods to search for the optimal conditions for replacing CH4 from the hydrate with CO2.
The effects of the initial temperature and pressure during the replacement by use of gaseous CO2 have been studied with experimental methods [9,16,17,18,19,48,49]. According to the experimental data reported by the literature, higher initial temperature and pressure are both beneficial to improve the replacement rate and efficiency. Graph (a) in Figure 4 shows the effect of the initial temperature on the replacement efficiency with CO2 gas, and Zhou’s [48] experimental data show the ratio of released CH4 gas from the hydrate increases from 20.0% to 44.9% with the initial pressure of CO2 gas ranging from 3.97MPa to 4.84 MPa. Wang et al. [16] and Qi et al. [49] found the effect of temperature is more obvious than that of pressure. In addition, Li et al. [18] studied the effect of sodium dodecyl sulfate (SDS) on the replacement reaction, and the results show that SDS is beneficial to improving the replacement rate. Adding SDS may become a method for enhancing the replacement reaction, but the feasibility has to be studied further.
Ota et al. [50] investigated the effects of pressure and fugacity on the CH4–CO2 replacement in CH4 hydrate using quantitative analysis with in-situ laser Raman spectroscopy. It was found that the CH4–CO2 replacement at the boundary of liquid and hydrate phase (273.2 K and above 3.60 MPa) proceeds faster than that at the boundary of gaseous and hydrate phase (273.2 K and 3.26 MPa). The result shows the liquid CO2 is more effective in replacing CH4 from the hydrate than gaseous CO2. Zhou et al. reached the same conclusion with experimental method [40]. Ota et al. also found that in the replacement process by use of liquid CO2, pressure dependence was hardly observed under the conditions studied (3.60–6.00 MPa), but according to the experimental data reported by Xiong et al. [35], the recovery ratio of CH4 can reach approximately 45% after 288 h, which is higher than the results from obtained by Ota et al. [28], as in Ota’s experiment, the recovery ratio is about 37% after 307 h. The initial conditions in Xiong’s experiment are 282.2 K and 6.00 MPa, so through comparison of the two experiments, it can be concluded the initial temperature has an important influence on the replacement efficiency by use of liquid CO2, and a higher initial temperature favors of improved replacement efficiency.
White et al. [38] applied a simulation method to simulate the replacement process with gaseous CO2, liquid CO2 and CO2 emulsion, respectively, and proved the superiority of CO2 emulsion. Zhou et al. [34,40] used experimental methods to compare the replacement rate and efficiency using different forms of CO2, and the experimental results are shown in Figure 9, Figure 10 and Figure 11. From the figures, it can be concluded that the replacement rate and efficiency with CO2 emulsion are both higher than that with gaseous CO2 and liquid CO2. Furthermore, Zhou et al. proved that the higher quality fraction of CO2 in water results in the higher replacement efficiency, and Dhanuka et al. [39] pointed the varieties and contents of emulsifier have an important influence on the stability of CO2 emulsions and suggested that a higher pressure and quality fraction of CO2 in water can improve the stability of CO2 emulsions. To sum up, CO2 emulsion with higher quality fraction of CO2 and opportune emulsifier should be selected for the replacement reaction, and the initial pressure should be appropriately high. Literature about replacing Ch4 from the hydrate with CO2 are few at present, and the micro-mechanism of the replacement process with CO2 emulsion is still an issue, so the factors influencing preparation of CO2 emulsions should be studied in the future. In addition, in the actual exploitation, the influences of porous media, statistics circumstance of CH4 hydrate in the stratum should be also taken into account.

6. Conclusions

This paper introduces the research advances on replacement of CH4 from NGHs by use of CO2 at home and abroad, proves the feasibility of replacement from the points of view of kinetics and thermodynamics, summarizes the progress of experiments and replacement simulations, and analyzes the factors influencing the replacement with different forms of CO2. The following conclusions are drawn according to this paper:
(1) The feasibility of replacing CH4 from the hydrate by use of CO2 has been proven from the points of view of kinetics and thermodynamics, and experiments on replacement confirm the conclusions. Besides, by means of experimental observation, the replacement reaction mainly occurs in the hydrate phase and its essence is the process of CO2 molecules occupying CH4 molecules’ cages, so the replacement method is beneficial to stabilize the NGH stratum during natural gas recovery.
(2) Replacement of CH4 from the hydrate by combining the techniques of CO2 liquefaction and CO2 emulsification are new methods considered to be able to enhance the replacement reaction. According to the experiments on replacement with different forms of CO2, it is concluded that the replacement reaction with CO2 emulsion has faster reaction rates, higher replacement efficiencies and longer reaction times compared to that with gaseous and liquid CO2, so the replacement method with CO2 emulsion is considered as a new approach for commercial production of CH4 from the hydrates.
(3) The multiscale simulation combined the PFT and molecular dynamics provides a theoretical research method for analyzing the replacement microscopic mechanism. The simulation results prove the superiority of CO2 emulsion in replacement and indicate the diffusivity of CO2 in the hydrate phase is the key to the replacement efficiency, but the research on simulation of replacement is still in its infancy, and the descriptions of the micro-mechanism of replacement are still immature, so the simulation of replacement should be studied further, and multiscale models for replacement, especially for replacement with CO2 emulsion, should be built in the future work.
(4) The factors influencing the replacement reaction with different forms of CO2 are discussed. The factors mainly include the phases of CO2, the initial pressure and temperature, and the effect of additives. The ultimate purpose of the analysis of different factors is to find the optimal conditions for enhancing the diffusivity of CO2 in the hydrate reservoir and improve the replacement efficiency and rate. Besides, in actual exploitation, the sizes of the porous media, the distribution of NGH in the actual stratum, and the manners of CO2 injection to the NGH reservoir should be also taken into account.

Acknowledgment

This study has been supported by State Key Development Program for Basic Research of China (Grant No. 2009CB219507), Major National S&T Program of China (Grant No. 2011ZX05026-004-07) and Natural Science Foundation of China (Grant No. 51006017 and No. 50736001).

Nomenclature

t
time, s
XCH4/XCO2
ratio of CH4 and CO2 in the vapor phase
(XCH4/XCO2)0
initial ratio of CH4 and CO2 in the vapor phase
α
fitting parameter related to the a condensation rate of CH4 molecules from the vapor phase
nCH4.H
remaining amount of CH4 in the hydrate phase, mol
nCO2.H
amount of CO2 in the hydrate phase, mol
f
fugacity, MPa
kDec
overall rate constant of the decomposition, mol/s·m·MPa
kDec.R
reaction rate constant of decomposition, mol/s·m·MPa
kDec.D
decomposition rate constant of mass transfer in the hydrate phase, mol/s·m·MPa
kForm
overall rate constant of the formation, mol/s·m·MPa
kForm.R
reaction rate constant of formation, mol/s·m·MPa
kForm.D
formation rate constant of mass transfer in the hydrate phase, mol/s·m·MPa
A
surface area between the gas and the hydrate phase, m2
H
hydrate phase
G
gas phase

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