Soaking Process for the Enhanced Methane Recovery of Gas

Nov 16, 2015 - Replacement technique is known as a promising method for potential energy extraction from gas hydrate deposits and greenhouse gas seque...
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Soaking Process for the Enhanced Methane Recovery of Gas Hydrates via CO2/N2 Gas Injection Young-ju Seo,†,‡ Daeok Kim,† Dong-Yeun Koh,† Joo Yong Lee,‡ Taewoong Ahn,‡ Se-Joon Kim,‡ Jaehyoung Lee,*,‡ and Huen Lee*,† †

Department of Chemical and Biomolecular Engineering (BK21+ Program), Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea ‡ Petroleum and Marine Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), 124 Gwahang-no, Yuseong-gu, Daejeon 305-350, Korea S Supporting Information *

ABSTRACT: Replacement technique is known as a promising method for potential energy extraction from gas hydrate deposits and greenhouse gas sequestration into deep ocean sediments. When it comes to the utilization of the replacement method in field applications, the soaking duration or the frequency of CO2/N2 gas injection becomes an important process variable. In this study, the influence of soaking duration and frequency on the CH4 replacement efficiency was quantitatively investigated for the first time by imitating the gas hydrate-bearing sediments. The CH4 replacement process with CO2/N2 was performed through two consecutive stages: dynamic replacement and soaking process. While any additional soaking process after the dynamic process enhanced the CH4 replacement efficiency from 35−36 to 52−60%, several replenishments of a fresh CO2/N2 gas mixture into the vapor phase were considered more effective than solely increasing the soaking time. The present study will help in establishing the basic process variables for obtaining an enhanced CH4 replacement efficiency during the well design and operation of the replacement technique in field production tests.

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INTRODUCTION

Recently, Lee et al. investigated the kinetic mechanism of CH4 replacement by supplying liquid CO2 and confirmed that the replacement rate gradually decreased as time progresses as a result of the low diffusivity of CH4 gas through the preformed hydrate layer.14 Koh et al. also performed a quantitative process assessment of CH4 replacement with CO2/N2 gas injection upon demonstrating an 8 m-scale experiment.15 The authors verified the replacement efficiency dependence upon the injection rate under a continuous CO2/N2 gas flow. Meanwhile, a gas hydrate field test by means of the replacement technique was successfully exploited in Prudhoe Bay, Alaska, in early 2012. According to the technical report, a binary CO2/N2 gas mixture (22.5 mol % CO2 and 77.5 mol % N2) was charged into the hydrate-bearing sandstones for 14 days and, after a few days in well work, CH4 production was initiated.16,17 During the time of injection and well work, gas hydrates were soaked in a CO2/N2 gas mixture for several days, namely, indicating that the soaking process was applied to this field production test. Despite the significant procedure of soaking in the field applications, soaking process variables and those effects on CH4 replacement efficiency are still unproven within the gas hydrate-bearing sediments. In this study, we initially imitated the natural geologic occurrence of gas hydrates using analogous sands with the field sample. The field sands were acquired by the second drilling expedition of Ulleung Basin (UBGH 2-6 site) located in East Sea, Korea. Then, the CH4 replacement reaction with CO2/N2

Natural gas hydrates have received considerable attention as future energy resources because of the large amounts of massive hydrate reservoirs and global estimates of geologic accumulations in the permafrost environments.1,2 Conventional methods for gas hydrate production have been historically divided into three distinct approaches: depressurization, thermal stimulation, and inhibitor injection.3,4 In addition to the previous techniques, the replacement method has been widely studied over the past decade as a prominent method to achieve both CH4 recovery from natural methane hydrates and the simultaneous reduction of anthropogenic emissions of CO2 gas.5−8 When the CH4 replacement process occurs with CO2, the CH4 molecules of the gas hydrate phase are substituted by the CO2 molecules of the vapor phase as a result of the preferential guest occupations of CO2 molecules in the CH4−CO2 hydrate equilibrium system.5−11 The most important feature of this technique shows that the structure of gas hydrates is sustained without large-scale melting or thermal issuing during the methane exploitation.12,13 This guarantees secure production of CH4 gas from natural gas hydrate reservoirs and avoids the catastrophic emissions of greenhouse gas into the atmosphere. For those reasons, the replacement technique is known as a non-destructive and natural production method. Furthermore, the use of a binary CO2/N2 gas mixture instead of CO2 gas alone has been reported as greatly enhancing the replacement efficiency because the residual CH4 molecules entrapped in the small cages of the CH4 hydrates are substituted by N2 gas.8,9 To apply the replacement method in field production, quantitative evaluation of the process variables is required. © 2015 American Chemical Society

Received: September 18, 2015 Revised: November 12, 2015 Published: November 16, 2015 8143

DOI: 10.1021/acs.energyfuels.5b02128 Energy Fuels 2015, 29, 8143−8150

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Energy & Fuels

in Figure 1b.19 Figure 1c shows the experimental apparatus for this study. The inner space of the high-pressure reactor for the CH4 hydrate formation/replacement had a dimension of 55 mm in diameter and 460 mm in height. The temperature of the cell was controlled by the thermal jacket, which was connected to the cooling chiller, and thermocouples were installed on every top and bottom place of the reactor. The inner pressure of the reactor was detected by a pressure transducer (HEISE, error range of 99.9 mol %) and CO2/ N2 (20:80 molar ratio) gases were supplied by Jeil Gas (Korea). Deionized water with an ultrahigh purity was supplied by Millipore (Billerica, USA). Experimental Descriptions. Throughout the second drilling expedition of Ulleung Basin (UBGH 2-6 site), the target site for CH4 production was comprised of alternating layers of sand and clay, suggesting that enriched gas hydrates existed within the voids of the sand layer.18,19 Thus, we mainly focused on the replacement reaction that occurred in the gas hydrate-bearing sands layer, as shown in Figure 1a, which shows a schematic illustration of the replacement reactor containing the artificial CH4 hydrates. The natural gas hydrate samples consisted of various sand contents, and our concern was only on the occurrence of the disseminated type of gas hydrates, as shown

Figure 2. XRD patterns of field (UBGH 2-6) and artificial (HAMA-8) samples. Asterisks indicate diffractions from crystalline SiO2. sample were due to the presence of impurities in natural sand sediments. These peaks were identified as the feldspar albite (NaAlSi3O8) using Jade software (version 9.1.4, MaterialsData Inc.). To compare each particle size distribution, sieve analysis was performed for these two samples. As shown in Figure 3a, both the UBGH 2-6 and HAMA-8 samples represented similar patterns of percent retained (%), which had a large portion of particle sizes ranging from 75 to 300 μm. In other words, those samples passed through the 0.3 mm sieve perfectly (Figure 3b), and their particle sizes were less than 300 μm, respectively. The real digital images of the field (UBGH 2-6) and artificial (HAMA-8) samples are shown in panels c and d of Figure 3. The XRD and particle size distribution indicated

Figure 1. (a) Schematic illustration of the replacement reactor containing the gas hydrate-bearing sediments, (b) disseminated types of gas hydrates within the natural sediments from UBGH 2-6 holes, and (c) schematic diagram of the experimental apparatus for CH4 replacement with the CO2/N2 gas mixture. 8144

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Figure 3. Sieve analysis results of field (UBGH 2-6) and artificial (HAMA-8) samples: (a) percent retained and (b) percent finer. Digital images of (c) field and (d) artificial samples. that the HAMA-8 sample was adequate for use as sand sediment instead of a field sediment. To investigate the soaking influence on CH4 replacement efficiency in the artificial gas hydrate-bearing sand system, the CH4 hydrates were prepared with the following procedure. First, the reactor was packed with water-saturated HAMA-8 sands. Then, we measured the porosity and absolute permeability with reference to the general equation of linear steady flow.21,22 More detailed equations of material property were included in eqs 1 and 2 of the Supporting Information. Next, the reactor containing water-saturated sands was flushed with nitrogen gas several times to make sands with irreducible water saturation. Thus, the pores of sands were filled with an irreducible water phase, which would turn into partial CH4 hydrates. The irreducible water saturation was also calculated from the equation of refs 21 and 22, which is also represented in eq 3 of the Supporting Information. All of the physical properties of sand sediments are shown in Table 1. Finally, the high-pressure reactor was pressurized up

Table 1. Physical Properties of the CH4 Hydrate-Bearing Sediment

a

physical properties (each case)

porosity (%)

absolute permeability (darcy)a

irreducible water saturation (%)

hydrate saturation (%)

6 h case 24 h case

36.2 36.2

0.88 0.86

42.4 43.7

20.6 20.1

1 darcy = 0.987 × 10−12 m2.

to 8.96 MPa upon supplying CH4 gas, and then the reactor was closed. The reactor was cooled to 274 K, and the pressure reduction occurred. Exothermic peaks were detected as a result of the CH4 hydrate formation, as shown in Figure 4a. Our previous phase equilibria results23 indicated that these pressure and temperature conditions, which were within the hydrate stability region for pure CH4 hydrates, were adequate for their formation. By observing the stabilization of pressure and temperature along with time, we concluded that the CH4 hydrate formation completely finished in the reactor. On the basis of the previous pressure reduction, the gas hydrate saturation was calculated through the equation proposed by Sakamoto et al.15,24 The equation was indicated in eq 4 of the Supporting Information. The saturation value was based on the CH4 hydrate formation within the pore volume of sand sediments, and it represented about 20−21%. Each experiment was performed after calculating the value of hydrate saturation to ensure the experimental reliability before the replacement reaction. Replacement Procedures. After confirmation of CH4 hydrate formation within the pores of sand sediments, the reactor was refilled by CH4 gas until it reached operating pressure. When the total

Figure 4. Pressure and temperature change as a function of time: (a) during the hydrate formation and (b) during the entire replacement process. pressure indicated a stable value, the first step of the replacement process, called dynamic replacement, was conducted. The CO2/N2 gas mixture was continuously injected at a constant flow rate of 300 standard cubic centimeters per minute (sccm) during the entire process of dynamic replacement. However, the pressure of the experimental system was maintained at 8.96 MPa (shown in Figure 4b) because the produced gas was simultaneously discharged through the outlet connected to the back pressure regulator (BPR). The realtime composition of the produced gas was directly measured by gas chromatography (GC). The injection of the CO2/N2 gas mixture into a reactor containing the hydrate-bearing sediments was stopped when the CH 4 composition of the outlet gas appeared to be less than 1 mol %. After ceasing the first step of CH4 production, we closed the inlet and 8145

DOI: 10.1021/acs.energyfuels.5b02128 Energy Fuels 2015, 29, 8143−8150

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Energy & Fuels outlet valves of the reactor and waited for a targeted soaking time. We expected that, during the soaking time, the previously injected CO2/ N2 gas may diffuse into an unproduced CH4 hydrate phase confined within the pores of sands. Then, the CH4 hydrates would be replaced with CO2/N2 molecules at a static CO2/N2 pressure. In reality, we observed no distinct change in pressure during the sequential soaking steps, regardless of the initial fluctuation, as shown in Figure 4b. We may think that the amount of CH4 released from the hydrate phase corresponds to the CO2/N2 consumption of the gas phase during the soaking steps. When the first soaking time expired, we replenished the CO2/N2 gas mixture to sweep the additionally produced CH4 gas from the CH4 hydrates. Similar to the dynamic replacement, the replenishment of CO2/N2 gas was terminated when the CH4 composition appeared to be less than 1 mol %, so that a fresh CO2/N2 gas mixture remained in the gas phase. Each soaking step was followed by these procedures and repeated 6 times, sequentially. In addition, we confirmed that the hydrate stability was maintained during the entire replacement process because the equilibrium pressure of mixed gas molecules mainly existed below 9 MPa at a given temperature, with the exception of the N2-rich region. The ternary diagram of the CH4/CO2/N2/H2O system was generated using CSMGem software (version 1.10, CSM), and more detailed information was included in Figure S1 of the Supporting Information. The reported replacement efficiency was derived under various conditions, and different methods, such as mass balance, Raman, and nuclear magnetic resonance (NMR) spectroscopy, were used.25 In this study, we used the equation below with reference to the previous investigation.15 total replacement efficiency (%) = volume of produced (CH4 gas + CH4 gas after each soaking step) /volume of CH4 in the hydrate phase × 100 The volume of produced CH4 gas was calculated by subtracting the initially supplied CH4 gas from the total amount of released CH4 gas that was detected by GC analysis. The volume of CH4 in the hydrate phase is the amount of methane consumed during the hydrate formation, which is proportional to the injected CH4 volume in the refill. An increase in replacement efficiency through the soaking process was calculated by adding the volume of produced CH4 gas after each soaking step was terminated.

Figure 5. (a) Composition change of produced gas as a function of time. (b) N2/CO2 ratio change as a function of time during the dynamic replacement process.

Initially, the N2/CO2 ratio represented a value smaller than 4 and increased over time until 200 min, as shown in Figure 5b. The reason may be attributed to the guest-specific preference behavior of the replacement reaction. In structure I of the CH4 hydrates, there were two small cages and six large cages in a unit cell [2(512), 6(51262), 46H2O]. In general, the CO2 molecules dominantly occupied in the large cages, and the N2 molecules were more preferentially occupied in the small cages.8,9 The CH4 cage occupancy (the ratio of small cage/large cage) was maintained at a constant value of 1:3 during the CH4 replacement with the CO2/N2 gas mixture.9,15,26 This means that the CO2 gas is 3 times more likely to be replaced than the N2 gas at the same time. Thus, the N2/CO2 ratio in the vapor phase increased up to 200 min. After that time, the N2/CO2 value gradually decreased up to an ideal composition of N2/ CO2, 4, as the replacement reaction was terminated. Both soaking cases represented similar patterns of N2/CO2 change over time, and there only existed a little time difference, as shown in Figure 5b. From the experimental results of the dynamic replacement, as shown in panels a and b of Figure 5, we suggest that both cases have similar fluid patterns of produced gas over injection time. The CH4 replacement efficiencies were calculated to be around 35−36%, respectively (as shown in Figure 10). This guarantees that each case has the experimental reliability before a different time duration is set up for soaking. To summarize, dynamic replacement was carried

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RESULTS AND DISCUSSION As previously reported, the replacement reaction is quantitatively estimated through dynamic replacement and soaking process. During the dynamic replacement, which is the first step of CH4 production, released CH4 gas was mainly attributed to the fast replacement reaction with the CO2/N2 gas mixture on the surface of gas hydrates.14,15 The vapor composition within the pore volume of sands was continuously measured from GC analysis. Thus, the composition change of produced gas as a function of time was observed (Figure 5a) before each soaking step progressed. As shown in Figure 5a, the CH4 gas was emitted right after the CO2/N2 gas mixture was injected into the CH4 hydrates but its amount included both the initially charged CH4 gas and recovered CH4 gas as a result of the dynamic replacement. Each of the CO2 and N2 gases came out with a little time delay. The composition of CH4 in the vapor phase rapidly decreased up to 200 min, indicating that the replacement reaction occurred drastically within that time. Moreover, the N2/CO2 ratio in the vapor phase was a precise indicator of the progression of the replacement reaction.15 Ideally, the N2/CO2 ratio should be 4 because the injected gas contains 80 mol % N2 and 20 mol % CO2, but the value was transformed with time as the replacement reaction occurred. 8146

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the composition change of the released CH4 gas was easily compared, as shown in panels a and b of Figure 7. As previously mentioned, the initial CH4 composition behind each soaking step gradually declined as a result of the sequentially released CH4 gas, which was confined in the hydrate phase. Thus, each progress time of CH4 recovery, where the CH4 composition approached less than 1 mol %, decreased as soaking steps progressed (panels a and b of Figure 7). We also confirmed the cumulated amount of produced CH4 gas after each soaking step gradually increased through the sequential soaking steps, but the amount of produced CH4 gas within each step became lower, as shown in panels c and d of Figure 7. In comparison of the total amount of produced CH4 gas, the 24 h soaking case represented a high value but not a 4 times higher value than that of the 6 h soaking case. This indicates that the produced CH4 gas amount is not in direct proportion with the contact time of soaking between the CH4 hydrate phase and the injected CO2/N2 gas phase. In the closed system of the soaking process, the real time monitoring of the produced CH4 gas and the quantitative analysis of gas consumption are required for an accurate understanding of the soaking mechanism. However, this analysis is tough because the replacement reaction occurs within the pore volume of sand sediments, which shows a large difference from the previous results of the pure CH4 hydrate system.8,9,14,27 Thus, we could indirectly quantify the amount of produced CH4 gas after each soaking step by injecting the CO2/N2 gas mixture into the reactor. We also determined the replacement patterns through measuring the N2/CO2 ratio. The N2/CO2 ratio was applied again to the soaking process and shown in panels a and b of Figure 8. Both cases had similar patterns of the N2/CO2 ratio; the initial N2/CO2 ratio slightly decreased and started to recover, which is similar to the dynamic replacement result. We think that the initial decrease is due to the deviation between the composition of the previously saturated vapor and the injected composition of fresh CO2/N2 gas depending upon the reactor length. Namely, it means that the previously produced CH4 gas and newly injected CO2/N2 gas came out simultaneously for the time of each CH4 recovery. However, from the following patterns of the N2/CO2 ratio, the vapor composition partially contained the new CH4 recovery from the dynamic replacement because the N2/CO2 ratio increased as a result of the fast CO2 consumption rather than N2. As the sequential soaking steps progressed, the effect of dynamic replacement diminished (panels a and b of Figure 8). To summarize, after each soaking step, produced CH4 came out mainly from the soaking effect and partially contained the new CH4 recovery by injecting the fresh CO2/N2 gas mixture. Fortunately, the clear separation of each mass balance is not required in the field application because pursuing an enhanced method of the CH4 recovery is considered urgent and significant in the replacement technique. In addition, we confirmed the cumulated amount of produced gas during each step of the entire replacement process. As the amount of produced CH4 gas went down through each soaking step, the total amount of injected CO2 and N2 gas also decreased (panels a and b of Figure 9). In both cases, they appeared as identical features at the dynamic replacement step but underwent variant gradients of decrease in the cumulated amount through the sequential soaking steps on account of the different soaking durations. To compare the final amount of produced gas, the CH4 gas amount was relatively smaller than that of the injected CO2/N2 gas at each step. In

out 3 times prior to each soaking case. Also, no significant difference was observed at this step. The soaking process, which is the second step of CH4 production, was carried out at the static pressure of the CO2/N2 gas mixture in the closed system. Also, the soaking duration was determined to be 6 and 24 h, and each soaking step occurred 6 times, successively. The CH4 replacement slowly occurs because the replaced CH4 molecules are becoming enriched in the gas phase of sand pores, which decrease the driving force of the replacement reaction and the limited diffusion-controlled reaction through the preformed mixed hydrate layer.14,15,27,28 To verify the influence of the soaking process on the CH4 replacement efficiency, we compared the composition of the produced CH4 gas after each soaking step was terminated. Panels a and b of Figure 6

Figure 6. Composition change of produced CH4 gas as a function of time during the entire replacement process: (a) 6 h soaking case and (b) 24 h soaking case.

show the produced CH4 concentration during the entire replacement process and indicate that CH4 gas is produced whenever the soaking process is completed. However, the initial compositions of the produced CH4 gas after the first step of 6 and 24 h soaking cases represented similar values around 6 mol %, even though the 24 h soaking duration was 4 times as high as that of 6 h. As the sequential soaking steps progressed, the initial composition of each soaking case gradually decreased as a result of the diminished value of hydrate saturation in the hydrate phase. We rearranged all of the soaking results shown in panels a and b of Figure 6 into the same time scale, and then 8147

DOI: 10.1021/acs.energyfuels.5b02128 Energy Fuels 2015, 29, 8143−8150

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Figure 7. Composition change of produced CH4 gas as a function of time after each soaking step: (a) 6 h soaking case and (b) 24 h soaking case. Cumulated amount of produced CH4 gas as a function of time after each soaking step: (c) 6 h soaking case and (d) 24 h soaking case.

Figure 8. N2/CO2 ratio change as a function of time after each soaking step: (a) 6 h soaking case and (b) 24 h soaking case.

particular, we could estimate the cost of CO2/N2 injection by comparing the relative proportion between the entirely injected CO2/N2 amount and produced CH4 amount. Thus, the economic evaluation on the total cost of CO2/N2 injection

Figure 9. Cumulated amount of produced gas during the entire replacement process: (a) 6 h soaking case and (b) 24 h soaking case.

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CONCLUSION We aimed at quantitatively evaluating the soaking process for the enhanced CH4 recovery within the gas hydrate-bearing sediments. The reason is that soaking variables are basically required for the well design and operation of CH4 replacement applications with the CO2/N2 gas mixture. In our experiments, the artificial CH4 hydrates within the pore voids of sands were initially prepared to closely approach the natural occurrence of gas hydrates and physical characteristics of the field sample. Next, the CH4 replacement process with CO2/N2 was carried out through dynamic replacement and the sequential soaking steps. From the results of the produced CH4 amount after each soaking step, significant findings are as follows: (1) Soaking steps are certainly beneficial to recover additional CH4 gas and enhance the CH4 replacement efficiency. We identified that CH4 gas was released whenever each soaking step was terminated, and the total CH4 replacement efficiency increased from 35 to 60% through the sequential soaking steps of the 24 h case. (2) CH4 replacement efficiency was not in direct proportion with the contact time of soaking between the CH4 hydrates and the injected CO2/N2 gas mixture. Therefore, adequate soaking time that depends upon the various scales of the replacement system should be determined for field test production. (3) Several replenishments of a fresh CO2/N2 gas mixture into the vapor phase were considered as a more efficient method than solely increasing the soaking time. This is because the short-term soaking frequency of CO2/N2 gas injection enables it to sustain a driving force of the CH4 replacement reaction in the vapor phase. Therefore, we believe that the soaking process has a significant potential to enhance the CH4 recovery from the natural gas hydrate reservoir. Also, this result will contribute in setting up the process variables of soaking for future on-/ offshore gas hydrate production based on the replacement technique. However, economic evaluations should be undertaken in advance because a huge amount of CO2/N2 gas mixture is injected into the CH4 hydrates to recover a relatively smaller amount of CH4 gas during the sequential soaking steps. Thus, we should estimate the probable cost and benefit of the soaking process prior to extending our bench-scale concept into field-scale applications.

and mixed gas separation from the production well should be performed before we extend our replacement technique into field applications. Using the amount of cumulated CH 4 gas, the CH 4 replacement efficiency at each step was calculated, as shown in Figure 10. Each calculation was performed 3 times and

Figure 10. CH4 replacement efficiency during the entire replacement process for both the 6 h soaking case and the 24 h soaking case. Yellow square marks indicate each replacement efficiency after the first step of the 24 h soaking case and the fourth step of the 6 h soaking case.

represented the mean value of the replacement efficiency with error bars. More detailed data were included in Table S2 of the Supporting Information. Runs 1 and 4 were stopped in the sixth and fifth steps of soaking, respectively, as a result of the sudden plugging of gas flows, but we could identify a similar tendency of the replacement efficiency with the small values of standard errors. In the case of 24 h soaking, the final outcome of the CH4 replacement efficiency resulted in about 60%, which was a little higher than 52% of the 6 h case. In the beginning, the CH4 replacement efficiency remained at 35−36%, but increased up to 52−60% through the sequential soaking steps (Figure 10). Thus, the addition of the soaking process is certainly useful for enhancing the CH4 replacement efficiency. Moreover, we could derive precise information through the results of CH4 replacement efficiency after using different soaking frequencies. In the 6 h case, the CH4 replacement efficiency after the fourth step represented a more enhanced efficiency than that of the first step of the 24 h case. This indicates that the short-term soaking frequency is more efficient than that of long-term frequency. We marked the points in yellow squares and compared the efficiencies to each other, as shown in Figure 10. For an intuitive understanding, we derived Figure S3 in the Supporting Information and compared the replacement efficiency over the cumulated soaking time of each case. Consequently, several replenishments of the vapor phase through injecting a fresh CO2/N2 gas mixture could have more enhanced CH4 replacement efficiency than solely increasing the soaking time between the CH4 hydrates and the injected CO2/ N2 gas mixture in the batch system. This is because, to sustain the driving force for the CH4 replacement reaction, the vapor phase composition needs to be replenished with a fresh CO2/ N2 gas mixture, which enables the gradient of chemical potential to increase between the hydrate phase and the surroundings.26,27

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b02128. Calculation of sand porosity (eq 1), calculation of absolute permeability (eq 2), calculation of irreducible water saturation (eq 3), calculation of hydrate saturation (eq 4), ternary diagram of the CH4/CO2/N2/H2O system at 274 K (Figure S1), CH4 replacement efficiency during the entire experiment (Table S2), and CH4 replacement efficiency over the cumulated soaking time (Figure S3) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 8149

DOI: 10.1021/acs.energyfuels.5b02128 Energy Fuels 2015, 29, 8143−8150

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Energy & Fuels Notes

(21) Sung W. Petroleum Natural Gas Engineering, 1st ed.; Goomibook: Seoul, Korea, 2009. (22) Dake, L. P. Fundamentals of Reservoir Engineering, 17th ed.; Elsevier: Amsterdam, Netherlands, 1998. (23) Seo, Y-j.; Seol, J.; Yeon, S.-H.; Koh, D. Y.; Cha, M.; Kang, S. P.; Seo, Y. T.; Bahk, J. J.; Lee, J.; Lee, H. J. Chem. Eng. Data 2009, 54, 1284−1291. (24) Sakamoto, Y.; Komai, T.; Haneda, H.; Kawamura, T.; Tenma, N.; Yamaguchi, T. Proceedings of the 5th International Conference on Gas Hydrates (ICGH); Trondheim, Norway, June 13−16, 2005; pp 866− 874. (25) Junfeng, Q.; Werner, F. K. Proceedings of the 8th International Conference on Gas Hydrates (ICGH); Beijing, China, July 28−Aug 1, 2014. (26) Koh, D. Y.; Kang, H.; Kim, D. O.; Park, J.; Cha, M.; Lee, H. ChemSusChem 2012, 5, 1443−1448. (27) Cha, M.; Shin, K.; Lee, H.; Moudrakovski, I. L.; Ripmeester, J. A.; Seo, Y. Environ. Sci. Technol. 2015, 49, 1964−1971. (28) Schicks, J. M.; Luzi, M.; Beeskow-Strauch, B. J. Phys. Chem. A 2011, 115, 13324−13331.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Knowledge Economy (MKE) through the Project “Studies on Gas Hydrate Development & Production Technology” under the management of the Gas Hydrate Research and Development Organization (GHDO) of Korea and the Korea Institute of Geoscience and Mineral Resources (KIGAM). It was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1A6A3A01060008).

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DOI: 10.1021/acs.energyfuels.5b02128 Energy Fuels 2015, 29, 8143−8150