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Electrical Resistivity Measurements of Methane Hydrate during N/CO Gas Exchange 2

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Dongwook Lim, Hyeyoon Ro, Young-ju Seo, Joo Yong Lee, Jaehyoung Lee, Se-Joon Kim, Youngjune Park, and Huen Lee Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01920 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on December 1, 2016

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Electrical Resistivity Measurements of Methane Hydrate during N2/CO2 Gas Exchange Dongwook Lima, Hyeyoon Roa, Young-ju Seoa,b, Joo Yong Lee b, Jaehyoung Lee b, Se-Joon Kim b, Youngjune Parkc,* and Huen Leea,* a

Department of Chemical and Biomolecular Engineering (BK21+ Program), Korea Advanced

Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea b

Petroleum and Marine Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), 124 Gwahak-ro, Yuseong-gu, Daejeon 34132, Republic of Korea c

School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology (GIST), Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea

ACS Paragon Plus Environment

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ABSTRACT Natural gas hydrates, known to exist in both continental margins and permafrost regions, have received tremendous attention owing to their potential use as an unconventional natural gas resource. Among the options to develop natural gas hydrates, a gas exchange method utilizing an external CO2 or N2/CO2 mixture is considered one of the most promising technologies because (i) the process can prevent structural destruction of the gas hydrate deposits by swapping CO2 or N2/CO2 for CH4 molecules and (ii) the injected CO2, a global warming gas, can be sequestered and locked away through the formation of thermodynamically stable CO2 or N2/CO2 hydrate. During and after N2/CO2 injection, however, the progress of gas exchange and the stability of the mixed CH4/N2/CO2 hydrate must be monitored. In this study, the electrical resistivity of CH4 hydrate before, during, and after N2/CO2 swapping was investigated using a lab-constructed tube-type reactor system for in-situ electrical resistance measurement. The natural environment of a gas hydrate-bearing sediment was simulated by forming CH4 hydrate in the pore spaces of glass beads, and its electrical properties were examined. Finally, changes in electrical resistivity were used to interpret CH4 recovery yields, while the guest composition of the gas hydrate was simultaneously analyzed by gas chromatography.


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1. INTRODUCTION Clathrate hydrates, commonly called gas hydrates, are solid-crystalline inclusion compounds composed of host water and gaseous or liquid guest molecules. By forming three-dimensional hydrogen-bonded networks, water molecules create empty nanocavities where guest molecules can be trapped inside.1 After gas hydrates were created in the laboratory in the early 1800s, their significance as an industrial hazard was recognized as the formation of gas hydrate in the oil or natural gas transportation process usually leads to pipe blockage.2 For this reason, securing flow assurance by preventing gas hydrate formation has long been an important issue in the related industries. Recently, natural gas hydrates (NGHs) existing in deep ocean sediments or in permafrost regions have been studied as a potential energy resource due to their huge amounts of unconventional natural gas content, and the recovery of CH4 from NGH deposits accordingly has become an important issue.3,4 To develop NGH, several methods including thermal stimulation, depressurization, and inhibitor injection have been proposed or demonstrated.5,6 Those methods are basically based on the dissociation of the NGH deposit, so this may involve geological and environmental risks. On the other hand, a swapping approach, which utilizes molecular exchange phenomena between CH4 occupying the cages of NGH and external gas molecules (i.e. CO2 or N2/CO2), can produce natural gas while minimizing dissociation or observable destruction of the NGH deposit.7,8 Previous reports have revealed that the use of pure CO2 for CH4 hydrate replacement could recover ~ 64% of CH4, whereas a N2/CO2 binary mixture may produce 85% of CH4 or more due to the distinct roles of N2 and CO2; N2 and CO2 replace CH4 in small and large cages, respectively.9,10 Recently, NGH recovery with air or air/CO2 was also examined, and the critical CH4 concentration was observed for stable CH4 recovery by a replacement process.11


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A previous study reported that the extent of CH4 yield could be affected by various factors, such as pressure, temperature, viscosity, permeability, density, and solubility.12 To apply the gas exchange methods to actual hydrate production, however, the fundamental properties of NGH, particularly for monitoring guest molecule migration within the hydrate cages during and after CO2 or N2/CO2 injection, must be further examined. Naturally occurring gas hydrates are distributed in various types of geologic sediments, such as sand, silt, and clay. The porosity is relatively high in sand but low in silt and clay. Therefore, the understanding of sediments is very important for research and exploitation of deep-sea NGH deposits. In particular, we mainly focus on the sand layers in gas hydrate reservoirs, because most NGH sediments in UBGH sites have alternating layers consisting of two distinct parts of sand and clay layers. Though the clay layers are expected to contain a considerable amount of the intercalated methane hydrate, clay layers are considered to be practically impermeable due to compact packing between clay particles. On the other hand, sand layers are considered to be practically permeable.13,14 The hydrate distribution and pore-filling structure in actual gas hydrate fields in the ocean floor have been provided by seismic surveys. Seismic surveys are considered one of the most effective tools for the exploration of natural gas hydrate deposits so far. For example, compressional p-wave velocity has been utilized to survey a gas-bearing zone at the hydrate stability zone.15 However, the quantitative analysis of the natural gas hydrates using sonic logs has been difficult to achieve because of uncertainties in the mechanical properties of the sediment as a result of the inclusion of gas hydrate.16 Therefore, well logging or electromagnetic measurement for NGH systems could be a practical option to investigate the concentration or geometric distribution of NGH deposited in ocean sediments.

17-25

In particular, since gas

hydrates can be considered an electric insulator, understanding the electrical properties of


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complex NGH systems is an essential task. Moreover, it is known that electrical resistivity is strongly affected by various factors, such as the ionic concentration of the aqueous solution, fluid-filling porosity of the pores, and other conduction effects.26 Nonetheless, little effort has been directed toward investigating the electrical resistivity of simple CH4 hydrate or CH4 hydrate-bearing sediment systems.27-31 Archie’s equation has been used to determine the hydrate saturation (volume fraction of hydrate in pore space) in sediment by measuring electrical properties.32,33 Therefore, the electrical properties of clathrate hydrates have been investigated for application in exploration methods for hydrate deposits. In this study, we investigated the electrical resistivity of a CH4 hydrate-bearing sediment system during the N2/CO2 gas exchange process using a laboratory-constructed in-situ electrical resistance measurement system. Artificial glass beads and 3.35 wt% of NaCl solutions were used to synthesize CH4 hydrate aiming to explore the fundamental relationship between electrical resistivity and guest distributions within gas hydrate cages. Distinct patterns of change in electrical resistivity during the processes of CH4 hydrate formation, N2/CO2 replacement, and gas hydrate dissociation were observed. The electrical resistivity of CH4 hydrate before, during, and after N2/CO2 swapping was also examined accompanied by simultaneous gas chromatography (GC) analysis to estimate CH4 recovery yields.

2. EXPERIMENTAL SECTION 2.1. Materials


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Artificial glass beads (Potters- Ballotini AB, Co. Ltd., Japan) were used to simulate the natural environment of NGH. The average particle size and density of the glass beads were 100 µm and 2.5 g/cm3, respectively. CH4 gas with a purity of 99.95 mol% and N2/CO2 mixed gas (20 mol% CO2 with N2 balance, 99.995 mol% purity) were purchased from Special Gas Co. (Republic of Korea). CH4/N2/CO2 mixed gases with a minimum purity of 99.95 mol% were purchased from PS CHEM (Busan, Republic of Korea). NaCl aqueous solution (3.35 wt%) was used to synthesize CH4 hydrate aiming to mimic the natural environment. 2.2. Apparatus and Procedures Figure 1 presents a schematic diagram of the experimental apparatus used in this study. A tube-type reactor having an electrode part was constructed using 316 stainless steel tubes. The tube-type reactor was placed in a circulating water-ethanol bath (RW-2025G, JEIO TECH, Republic of Korea) to maintain a low temperature for gas hydrate formation. A mass flow controller (MFC, Brooks 5850E, Hatfield, PA) was used to control the gas injection. A pressure transducer (PMP4070, GE Sensing Co., Rochester, NY) was equipped to monitor the internal pressure change. A back pressure regulator (TESCOMTM 26-1765-24, Emerson Electric Co., Elk River, MN) was used to retain the internal maximum pressure during the continuous gas injection. A wet gas meter (W-NK-1, Shinagawa Co. Ltd., Japan) was also used to measure the volume of the produced gases. For a quantitative analysis of the guest composition of the synthesized gas hydrates, a gas chromatograph (YL-6100, Young Lin Instrument Co. Ltd., Republic of Korea) equipped with a TCD (Thermal Conductivity Detector) was used. Electrode parts were designed to measure the electrical resistance of the gas hydrate. It consists of two stainless steel-plate-electrodes. The cross sectional area (A) and the distance between the


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two electrodes (L) were 0.785 cm2 and 1.55 cm, respectively. The electrodes were inserted into a cross union component (SS-12-VCR-CS, Swagelok Co., Solon, OH), and a TeflonTM spacer with epoxy resin was used for electrical insulation. The electrical resistance of the gas hydrates was measured by an electrical interface device (SI 1287 & SI 1260, Solartron Analytical, UK). During the measurement, the imposed voltage was set to 0.1 V, which could minimize undesired effects (i.e. electrode polarization, electrochemical reactions on electrodes). The specific resistivity was then calculated from the measured electrical resistance. To synthesize CH4 hydrate for N2/CO2 replacement, artificial glass beads were first filled into the reactor. The initial porosity was calculated by injected volumes, and it was about 39%: ϕ % = where V indicates the volume, and

!"#$

=

, ,

and ρ

!"#$

,

are the weight and density of the glass

beads, respectively. A water-NaCl mixture was then injected into the reactor. The degree of water saturation in the specimen was calculated by the difference between the volumes of initially injected and drained aqueous solution. The irreducible water saturation indicates the inside condition of the reactor before hydrate formation. Detailed conditions for CH4 hydrate formation with irreducible water saturation (&'( ) and hydrate saturation (&)*# ) are provided in Table 1. &'( % =

&)*# % =

+,,'-.!/0!# − +,,#("'-!# × 100 +,,'-.!/0!#

+,# × ∆6 × 78 172 × 68 × 7; × < !"#$ × =

!"#$

×>

× 100


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Here, +,# is the volume of water displaced by the injected gas; ∆6 is the pressure drop by hydrate formation; 78 is the standard temperature; the ideal volume ratio of stored methane per hydrate is 172; 68 is the standard pressure; 7; is the temperature during the hydrate formation;
is the porosity of the porous media. CH4 or CH4/N2/CO2 mixtures were introduced into the reactor up to 90 bar, and the reactor temperature was maintained at 283.15 K. The temperature was then lowered to 274.15 K to form gas hydrates. For the synthesis of CH4/N2/CO2 mixed hydrates, a N2/CO2 mixture having a fixed mole ratio of 4:1 was used with variation of the CH4 composition from 50 to 90 mol%. The composition of the mixed feed gas and guest molecules in the mixed gas hydrates are summarized in the first and second columns of Table 2.

3. RESULTS AND DISCUSSION Prior to exploring the electrical resistivity of the gas hydrate system, the electrical resistance (R) of the irreducible water saturation-glass beads system was measured. The electrical resistivity (ρ) was calculated using the following equation, A

ρ = β ∙ B C, where β is the shape factor (β =1 for simple cylindrical shape), A is the cross-sectional area of the gas hydrate-bearing sediment in the tube-type reactor, and L is the distance between two electrodes. As shown in Figure 2, the irreducible water saturation-glass bead system exhibited a


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low electrical resistivity of ~ 0.4 Ω·m; the measured resistivity in this reactor system also showed similar results to previous measurements for brine.22 Moreover, no effect of CH4 pressure on the electrical resistivity was observed by measurement of the electrical resistivity in the out of the thermodynamic condition for CH4 hydrate formation. The electrical resistivity of CH4 hydrate during the formation process was explored and the results are shown in Figure 3. The initial pressure was about 90 bar by CH4 gas, and the pressure slightly decreased at the beginning stage of the experiment (about 0-300 minutes) due to isochoric cooling as the temperature was lowered from 283.15 to 274.15 K before hydrate nucleation. A pressure drop of ~15 bar was subsequently observed after 300 minutes because the CH4-water-NaCl mixture was converted to solid CH4 hydrate through nucleation and crystal growth process. Simultaneously, a dramatic increase in electrical resistivity was also observed during the same time interval. It was confirmed that the solid hydrate phase with CH4 guest molecules has much larger resistivity than the water phase. The electrical resistivity was measured twice, and the maximum electrical resistivities were compared (Table 1). In the case of the first run, the hydrate conversion was 82.2% and the corresponding maximum electrical resistivity was 31.34 Ω·m. However, the second run exhibited a higher electrical resistivity of 35.17 Ω·m with 92.5% hydrate conversion. The results imply that, as the hydrate conversion increases, the electrical resistivity could increase. To investigate the effect of CH4 concentration in the gas hydrate phase on electrical resistivity, CH4/N2/CO2 mixed hydrates were synthesized with varying CH4 guest composition, and their electrical resistivities were measured, as shown in Figure 4. As presented in Figure 4, the electrical resistivity of the CH4/N2/CO2 mixed hydrates increased as the CH4 concentration increased. According to Archie’s law, and as observed in previous studies, the electrical


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resistivity of hydrate-bearing-sediment is affected by hydrate saturation, and the electrical resistivity showed higher values with higher hydrate saturation.33-35 However, with mixed guests hydrate systems, it is very difficult to determine the hydrate saturation because each guest has a different hydrate composition with different participating hydrate formation due to the thermodynamic stability and the volume expansion from water to hydrate. In the case where only CO2 is injected, carbon dioxide could easily form CO2 hydrate thermodynamically; hence, the electrical resistivity would be increased with higher hydrate conversion.12 On the other hand, only N2/CO2 mixed injection gas has lower thermodynamic stability of the hydrate phase formed from it; hence, the electrical resistivity of mixed hydrates also could be lowered. Moreover, minor electrical conduction through the hydrate lattice could occur during the reorientation of the lattice water molecules to stabilize the lattice defects. The defect formations of hydrate lattice varied with different guests by volume expansion for hydrate formation and the L-defect formation from host-guest hydrogen bonding.36-42 CH4 hydrate formation has smaller expansivity (Vhyd ~ 1.234 Vw, Vw = volume of water) than CO2 hydrate formation (Vhyd ~ 1.279 Vw).43 Nitrogen guests form different types of hydrate structure (sII) with CH4 or CO2 (sI); therefore, a direct comparison of volume expansion with CH4 or CO2 is difficult. Because of the slight difference between the volume expansion during hydrate formation of CH4 and CO2, the thermodynamic stability might primarily affect the hydrate saturation. Therefore, with a larger composition of methane in mixed hydrate, which would lend higher thermodynamic stability, higher electrical resistivity would be obtained. The electrical resistivity change during the replacement process for CH4 hydrate by injection of the N2/CO2 mixture was measured, and the results are shown in Figure 5. First, CH4 gas was introduced into the reactor to form CH4 hydrate (Zone 1). After the CH4 hydrate was fully


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converted, CH4 gas was refilled to 90 bar and stabilized to prepare for the injection of the N2/CO2. Subsequently, an N2/CO2 mixture (80 mol% N2 balanced with CO2) was injected into the CH4 hydrate with a constant flowrate of 100 sccm (standard cubic centimeters per minute) for a continuous replacement process (Zone 2). At the beginning of N2/CO2 injection, the electrical resistivity dropped immediately. In this stage, this transient occurrence may have been due to momentary dissociation at the interface by contacting the injected gas or partial opening of the hydrate cage for replacement between guests.12 During the replacement process, the electrical resistivity stabilized at a lower value than that of only CH4 hydrate in Zone 1. The electrical resistivity after N2/CO2 replacement roughly stabilized at ~ 13 Ω·m in this system. According to the electrical resistivity for CH4/N2/CO2 mixed hydrate, as shown in Figure 4, it is speculated that the replaced CH4 hydrate in Figure 5 could contain ~ 60 mol% of CH4 as a guest molecule, which means that 40% of CH4 was recovered by the N2/CO2 mixture. This CH4 recovery yield was similar to the result obtained by GC analysis, namely, 39% CH4 recovery. Finally, the hydrate was dissociated by depressurization, and the electrical resistivity returned to the initial value of about 0.4 Ω·m before CH4 hydrate formation.


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4. CONCLUSION The electrical resistivity of CH4 hydrate during the N2/CO2 replacement process was experimentally investigated employing an in-situ high-pressure gas hydrate formation system equipped with laboratory-designed electrodes for electrical resistance measurement. The electrical resistivity for CH4 hydrate was measured up to ~ 35.18 Ω·m. In addition, the electrical resistivity for CH4/N2/CO2 mixed hydrates was measured with varying CH4 guest composition to reveal the relation between the electrical resistivity and CH4 concentration. As the CH4 concentration in the gas hydrate phase increases, the electrical resistivity also increased. The electrical resistivity change of CH4 hydrate during the replacement process for CH4 hydrate by injecting the N2/CO2 mixed gas was examined and the results imply that one may estimate the CH4 recovery yield from CH4 hydrate by injecting N2/CO2 by monitoring the changes of the gas hydrate’s electrical resistivity.


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AUTHOR INFORMATION Corresponding Author *Huen Lee: Tel.: +82-42-350-3917. Fax: +82-42-350-3910. E-mail: [email protected] *Youngjune Park: Tel.: +82-62-715-2836. Fax: +82-62-715-2434. E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was funded by the Ministry of Trade, Industry & Energy (MOTIE) through the “Field Applicability Study of Gas Hydrate Production Technique in the Ulleung Basin” project [KIGAM – Gas Hydrate R&D Organization]. It was also supported by Mid-career Researcher Program through National Research Foundation Korea (NRF) grant funded by the Ministry of Science, ICT and Future Planning (MSIP) (NRF Grant No. 2010-0029176).


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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 ACS Paragon Plus Environment

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Figure 1. Schematic diagram of the experimental apparatus for simultaneous gas hydrate formation and electrical resistivity measurement


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Figure 2. Electrical resistivity of irreducible water saturation-glass bead system as a function of CH4 pressure


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Figure 3. Electrical resistivity and pressure change during CH4 hydrate formation.


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Figure 4. Electrical resistivity of CH4/CO2/N2 mixed hydrates as a function of CH4 concentration in the mixed hydrate phase.


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Figure 5. Electrical resistivity change during replacement process for CH4 hydrate by injecting CO2/N2 mixture


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Table 1. Experimental details for CH4 hydrate formation Hydrate Irreducible Hydrate

conversion

saturation

(Hydrate/pore

(Sh, %)

filled water,

water Run

Porosity (%)

Electrical resistivity

saturation (Ω∙m)

(Sir, %) vol %) 1

39.2

54.0

17.4

82.2

31.34

2

39.4

57.6

21.0

92.5

35.17


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Table 2. Compositions of feed gas mixture, guests composition in the CH4/N2/CO2 hydrates, and their electrical resistivities

Feed gas composition

Mole fraction of CH4 among

(CH4/N2/CO2, mol%)

the guest molecules in the

Electrical resistivity (Ω·m)

mixed hydrate phase 100/0/0

1.00

35.17

90/8/2

0.91

29.81

80/16/4

0.82

22.92

70/24/6

0.74

17.96

60/32/8

0.66

14.81

50/40/10

0.53

10.51


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