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Magnetic Resonance Imagi

ng 29 (2011) 281–288

In situ observations by magnetic resonance imaging for formation anddissociation of tetrahydrofuran hydrate in porous media☆

Jiafei Zhao, Lei Yao, Yongchen Song⁎, Kaihua Xue, Chuanxiao Cheng, Yu Liu, Yi ZhangKey Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, Dalian, 116024, P.R. China

Received 24 April 2010; revised 7 August 2010; accepted 27 August 2010

Abstract

Tetrahydrofuran (THF) hydrate has long been used as a substitute for methane hydrate in laboratory studies. This article investigated theformation and dissociation characteristics of THF hydrate in porous media simulated by various-sized quartz glass beads. The formation anddissociation processes of THF hydrate are observed using magnetic resonance imaging (MRI) technology. The hydrate saturation during theformation is obtained based on the MRI data. The experimental result suggests that the third surface has an effect on hydrate formation. THFhydrate crystals lean to form on the glass beads and in their adjacent area as well as from the wall of the sample container firstly. Furthermore,as the pore size diminishes, or as the formation temperature decreases, the nucleation gets easier and the formation processes faster. However,the dissociation rate is mostly dependent on the dissociation temperature rather than on the pore size.© 2011 Elsevier Inc. All rights reserved.

Keywords: Tetrahydrofuran hydrate; Porous media; Formation; Dissociation; MRI

1. Introduction

Clathrate hydrates are equilibrium crystal structures thatare formed when certain components of natural gas (e.g.,methane, carbon dioxide, propane) and volatile liquids (e.g.,certain alcohols, tetrahydrofuran) in molecules becomepermanently encaged by water molecules [1]. Watermolecules form a framework containing relatively largecavities through hydrogen bonding. Gas molecules, whosediameter is less than the size of the cavity, can occupy thecavities. Because of the nonbonded interactions between theencaged gas molecules and water lattice, the hydratestructure is thermodynamically stabilized. There are threeprominent structures that hydrates crystallize in— structuresI, II and H — depending on the nature and size of the guestmolecules [2].

☆ This study has been supported by the Key Program of the NationalNatural Science Foundation of China (Grant no. 50736001), the NationalNatural Science Foundation of China (Grant no. 51006017), the NationalHigh Technology Research andDevelopment of China (863) Program (Grantno. 2006AA09A209-5) and the National Basic Research Program of China(973 Program) (2009CB219507).

⁎ Corresponding author.E-mail addresses: jfzhao@dlut.edu, powere@dlut.edu.cn (Y. Song).

0730-725X/$ – see front matter © 2011 Elsevier Inc. All rights reserved.doi:10.1016/j.mri.2010.08.012

Hydrates are of interest for a number of differentapplications including as a potential source of energy, as apotential greenhouse gas storage medium, for gas transpor-tation, desalination, refrigeration and for flow assurance inoil and gas pipelines where they may form solid plugs andlead to less production [3]. Although the physical propertiesand thermodynamics of hydrates are known to an acceptabledegree for most engineering applications, the kinetics stillremains a scientific curiosity. Most hydrates form at highpressure and low temperature, so specialist equipment isgenerally needed to form and study them. Among numerouscompounds known as hydrate formers, tetrahydrofuran(C4H8O, hereafter THF) has long been used as a proxy ofmethane in laboratory studies [4–6]. The main advantages ofTHF are as follows: (I) ability to remain in the liquid stateunder atmospheric pressure; (II) complete miscibility inliquid water, which enables relatively rapid, homogeneousformation of THF hydrate and close control on hydratesaturations; and (III) ability to form a structure II hydrate atatmospheric pressure and easily achieved temperatures,making the experiments easier [7–10]. No highly mobileand compressible gases are released during the dissociationof THF hydrate, so the poroelastic complication can also beavoided [11].

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The THF/water solution allows hydrate crystals to formand grow anywhere within it. If it is composed of THFand water at a molar ratio of 1:17 (or 19% by weight),THF forms a structure II hydrate with water and themelting point of the solution is raised to approximately4.4°C [4]. The mass transfer process can be eliminatedfrom the process of crystal growth, thereby offering amore simplified system for studying hydrate growth. Insuch a system, the rate of crystal growth is considered tobe determined by the transfer of heat generated at thesurfaces of growing crystals to the surrounding solutionand to the reaction at the surfaces [12]. Pinder [13] foundthat hydrate formation rate was controlled by the diffusionrate of the hydrate film, not by the heat transfer. Larsen[14] suggested that the crystal growth is mainlydetermined by the heat transfer unless the heat transferis much increased by some means. Bollavaram et al. [15]suggested that the surface-reactive restraints could not beoverlooked even if the heat transfer is much increased.The third surface also has an effect on hydrate nucleation.Large areas of interactive surface and possible surfaceordering have an impact on the thermodynamic andkinetic characteristics of hydrate formation [16,17]. Sincethe porous media have a large specific surface area, astrong surface tension and capillary condensation phe-nomenon, they also have an impact on hydrate phaseequilibrium [18]. Even the 3-Å molecular sieve powdercan shorten THF hydrate nucleation time and promotehydrate growth [19].

Several experimental techniques have been used tomeasure hydrate formation and dissociation processes,including ultrasonic detection, electrical resistance mea-surement, laser interferometry and visual microscopy[20,21]. Magnetic resonance imaging (MRI) has beenshown to be an effective tool for observing the formationand dissociation of THF hydrate [9]. The MRI signal isstrong for THF/water solution, but not detected above thebackground noise for solid hydrate. So it can be used todetermine the spatial distribution of hydrate and nonhydratephases, hydrate saturation and the rate of hydrate formationand dissociation.

In this study, in situ observation by MRI of theformation and dissociation of THF hydrate in various-sized quartz glass beads is presented. The saturationsduring the process were measured. The work also providesan improved basic understanding of the processes andcorresponding dynamics.

Fig. 1. Schematic diagram of the experimental system. (1) Thermostaticbath, (2) sample, (3) thermocouple, (4) recovery bottle, (5) output system,(6) MRI system, (7) manual pump.

2. Experiments

2.1. Apparatus and materials

Five different diameters of quartz glass beads (AS-ONE,Co., Ltd, Japan) — BZ-4 (3.962–4.699 mm, average4.5 mm), BZ-3 (2.500–3.500 mm, average 3.0 mm), BZ-2(1.500–2.500 mm, average 2.0 mm), BZ-1 (0.991–1.397

mm, average 1.2 mm) and BZ-04 (0.350–0.500 mm,average 0.4 mm) — were used to simulate the porousmedia. The porous media exhibited almost the sameporosity of around 35%. THF (Tianjin Damao ChemicalReagent Factory, Tianjin, China) with a minimum purityof 99.0% and deionized water were mixed at a mass ratioof 19:81 which corresponds to 1:17 on a molar basis inour experiments.

Hydrate formation and dissociation were achieved bycontrolling the temperature of the coolant continuouslycirculating through the MRI cell around the exterior of thesample. The experimental system is illustrated in Fig. 1.Fluorinert FC-40 (3M, St. Paul, MN, USA) was used as thecoolant because it contains no imageable hydrogen and haslow dielectric properties to minimize RF losses. Thetemperature controller was an F25-ME-type circulator(Julabo, Inc., Germany) with a temperature control rangefrom −28°C to 200°C and a precision of ±0.01°C. THFsolution was pumped into the sample container by a JB-80-type manual pump (Haian Oil Scientific Research Appara-tus, Co., Ltd, Jiangsu, China).

A cross-sectional diagram of the temperature-controlledcell used inside the MRI is shown in Fig. 2. The partimaged by MRI was made up of polyimide. The other partswere made of titanium and the two were joined by atapered thread. The glass bead samples were sealed in theinner sample container and the coolant, FC-40, waspumped from the bath through the sleeve around thesample container and back to the bath, giving a goodcontrol of the sample temperature and a quick response totemperature changes.

A Varian 400-MHz NMR (Varian, Inc., Palo Alto, CA,US) was used to monitor the formation and dissociation ofTHF hydrate. It operated at a resonance of 400 MHz, 9.4

Fig. 2. Cross-sectional diagram of the MRI cell. (1) Coolant inlet or outlet, one for the thermocouple; (2) end cap; (3) sealing o-ring; (4) high-pressure joint; (5)inner sample container; (6) sleeve; (7) sieve; (8) pressure pad.

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T, to measure hydrogen. A spin-echo multislice (FSEMS)pulse sequence was used with a repetition time of 500 msand an echo time of 10.14 ms. Images were all collectedwith 256 points for read out and phase encoding. Field ofview was set to 40×40 mm, giving a voxel size of0.156×0.156 mm. Furthermore, the thickness of the MRIslices is 100 μm.

2.2. Procedure

The quartz glass beads were saturated by the stoichio-metric THF/deionized water solution. Temperature washeld at −4.0°C, −3.0°C, −2.0°C and −1.0°C, respectively,by circulating the coolant in the THF hydrate forma-tion cycle.

In the formation process, the crystal nucleation is difficultsince the nucleation in the liquid phase interior needs a greatdeal of energy input to overcome the surface barrier.Therefore, the initiation of the nucleation is important. Inthis work, all of the samples were gently shaken in the sameway after cooling for 1 min to promote nucleation, but therewas no shaking thereafter.

A strategy was used to ensure the formation of THFhydrate instead of ice under our experimental condition. Themethod used in this research was to raise the temperature to3°C (still under the THF solution equilibrium temperatureof 4.4°C) after the solution was completely converted tosolid and to keep the temperature for 5 h. If the crystalsmelt, the resultant is considered to be ice; otherwise, theproduct is THF hydrate. In our experiments, the crystals didnot show any obvious change, which confirmed theformation of THF hydrate.

For the dissociation cycle, temperature was raisedto 5.5°C in BZ-3 and BZ-04 or to 8°C in BZ-4, BZ-2and BZ-1.

Since the hydrate crystal growth spot is in randomnessand the sample container is larger than the field of view,each experiment was repeated four times and monitoredby MRI.

3. Results and discussion

3.1. THF hydrate formation

The processes of THF hydrate formation at −4.0°C,−3.0°C, −2.0°C and −1.0°C, respectively, were monitoredby MRI. Images of one of the axial imaging planes selectedto monitor the process at −2.0°C in quartz glass beads BZ-4, BZ-3, BZ-2, BZ-1 and BZ-04 are shown in Fig. 3. Theblack area in the images represents the glass beads andhydrate phase, while the white area is the nonhydrate phase(aqueous THF solution). When the only signal observed inthe images was background noise, indicting that the liquidphase was all converted to solid, THF hydrate formationwas completed.

The nucleation process of THF hydrate is different fromthat of the common gas hydrate. Since THF can becompletely soluble in water, nucleation of THF hydratewith no external medium added is a homogeneous processand anywhere in the solution could be a nucleation spot. Butporous media have a large specific surface area and a strongsurface tension, so they have an influence on hydrateformation. In our experiments, the hydrate started formingfrom the wall of the sample. This might be due to the fact thatthe wall of the sample container reached the requiredformation temperature first. It was also found that the THFhydrate crystals formed on the glass beads and in theiradjacent area first. It seemed that hydrate growth wasinfluenced by porous media and the crystals leaned to formon them.

Furthermore, the mean MRI signal intensities of thesamples during the formation processes are shown inFig. 4. With the growth of THF hydrate, the signalintensity decreases.

The hydrate saturation can be calculated from the MRIdata. Two methods are commonly used to determine theporosity of porous media [16,17]. First, the ‘bulk method’is based on the mean signal related to that of the entiresample volume. For this method, it is assumed that the

Fig. 3. MR images of THF hydrate formation processes at −2.0°C in various-sized quartz glass beads.

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response of the MRI signal has a linear relation to thewater saturation. Second, the ‘distribution shape method’is based on the shape of the probability distribution

obtained from the signal for each mapped voxel. For thismethod, the threshold needs to be found from theprobability distribution. In this work, the bulk method

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0 20 40 60 80

MR

I In

tens

ity BZ-4

BZ-3

BZ-2

BZ-1

BZ-04

Time,t(mins)

Fig. 4. The mean MRI signal intensities of the samples during theformation processes.

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was used to avoid any subjectivity. Thus Eq. (1)was used to calculate the hydrate saturation in theformation process.

Si =I0 − IiI0

× 100k ð1Þ

Where Si is the hydrate saturation at t=i minutes, I0 isthe mean MRI intensity of the imaging plane at t=0 minand Ii is the mean MRI intensity of the imaging plane att=i minutes. In our experiments, the hydrate saturation inBZ-2 reached about 24% in 7 min, about 54% in 10 min,about 72% in 33 min and 87% in 50 min.

The time when hydrates could be observed in MRimages with the set resolution (hereafter, hydrate appear-ance time) is shown in Fig. 5. The smaller the size of

10

20

30

40

50

60

-5 -4 -3 -2 -1 0

Tim

e,t (

min

utes

)

BZ-4

BZ-3

BZ-2

BZ-1

BZ-04

Temperature,T1(ºC)

Fig. 5. Hydrate appearance time in various-sized quartz glass beads as afunction of temperature.

the glass bead is, or the lower the formation temperatureis, the shorter the time to start hydrate formation. Fig. 5also suggests that, as the formation temperature getshigher, the differences between the nucleation rates ofhydrate forming in different pore sizes became moreobvious. The formation of THF hydrate in pure THF/watersolution with no glass beads was also measured in thepresent study. It took about 30 min at −4.0°C and about 70min at −2.0°C to start the formation. It confirms thesignificance of the existence of the third surface to thecrystal nucleation. Porous media increase the contactsurface and reduce the surface energy and the chemistrypotential barrier that the nucleation must overcome. Thus,the nucleation becomes easier.

The entire formation time (hydrate appearance time notincluded) in different quartz glass beads is shown in Fig. 6. Itsuggests that THF hydrate formation rate is also faster inporous media with a smaller pore size at a lower temperature.For the pure THF/water solution, it took about 75 min at−4.0°C and about 85 min at −2.0°C to complete the hydrateformation of the same amount. This result does not agreewith our previous work. The previous work was notundertaken with an in situ MRI monitoring. The formationprocess was observed through the visual window first, andthen the sample was imaged using MRI for further study. Forthe small glass beads, the hydrate crystal was not easy toobserve unless it grew larger. This might lead to the mistakenconclusion that a larger pore size shortens the formation timein the previous work.

The pure THF/water hydrate formation in the test tubewas also tried. However, no obvious phenomenon wasobserved within 2 days. It is not quite known why pureTHF hydrate could be formed in the MRI cell but couldnot be formed in the test tube. It appears that the shapeand the smoothness of the sample container causedthe difference.

20

40

60

80

100

-5 -4 -3 -2 -1 0

BZ-4

BZ-3

BZ-2

BZ-1

BZ-04

Tim

e,t (

min

utes

)

Temperature,T1(ºC)

Fig. 6. The entire formation time in various-sized quartz glass beads as afunction of temperature.

Fig. 7. MR images of THF hydrate dissociation processes in various-sized quartz glass beads at 5.5°C in BZ-3 and BZ-04 and at 8°C in BZ-4, BZ-2 and BZ-1

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Moreover, experiments on THF hydrate formationabove 0°C (under an equilibrium temperature of4.4°C) were also carried out. It took a longer time

.

to start and complete the entire formation and theexperimental result also showed the tendency sug-gested above.

10

15

20

25

30

35

0 1 2 3 4 5

TTim

e,t (

min

s)

T1=-1.5ºC, T2=8ºC

T1=-2.5ºC, T2=8ºC

T1=-3.5ºC, T2=8ºC

T1=-4.5ºC, T2=8ºC

Diameter, d (mm)

Fig. 9. The entire hydrate dissociation time in BZ-4, BZ-2 and BZ-1 at 8°CT1 is the formation temperature; T2 is the dissociation temperature. Thevalue of the error bar is ±5%.

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3.2. THF hydrate dissociation

The bath temperature was raised to 3°C after the entireformation process was completed, and it was held there for5 h. No obvious change was observed. Then the temperaturewas increased to 5.5°C in BZ-3 and BZ-04 and to 8°C in BZ-4, BZ-2 and BZ-1, and the hydrate dissociation processbegan. Images were also collected by MRI during thedissociation cycle, as shown in Fig. 7. The images slowlygained in intensity with time.

Baldwin et al. [9] found that the MRI intensity across thestone plug uniformly increased with the time duringdissociation. They thought that the unexpected uniformitycould be the result of the slow warming process and of theheat transfer properties of the rock matrix. They suggestedthat the dissociated portion would be observed as a ringbeginning from the outer edge of the sample at a fasterwarming rate. The same phenomenon was also found in ourexperiment. But a rind of stronger intensity at the outsideedge of the sample could also be observed.

The entire dissociation time in BZ-3 and BZ-04 at5.5°C is shown in Fig. 8. It took about 46 min in BZ-3and 43.5 min in BZ-04 to complete the dissociation at5.5°C. The entire dissociation time in BZ-4, BZ-2 and BZ-1 at 8°C is shown in Fig. 9. It took about 24 min in BZ-4,25.5 min in BZ-2 and 24 min in BZ-1 to dissociateentirely. It seems that the dissociation time is mainlydependent on the dissociation temperature rather than onthe sizes of the glass beads under our experimentalsituation. No obvious dissociation time difference wasobserved for different formation temperatures and differentdiameters. This result does not quite agree with the factthat smaller glass beads have larger contact areas andtherefore cause a faster heat transfer to the hydrate. Itmight be caused by the present high dissociationtemperature, so the difference was not obvious. For the

30

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50

55

0 1 2 3 4 5

Tim

e,t (

min

s)

T1=-1.5ºC, T2=5.5ºC

T1=-2.5ºC, T2=5.5ºC

T1=-3.5ºC, T2=5.5ºC

T1=-4.5ºC, T2=5.5ºC

Diameter, d (mm)

Fig. 8. The entire hydrate dissociation time in BZ-3 and BZ-04 at 5.5°C. T1is the formation temperature; T2 is the dissociation temperature. The valueof the error bar is ±5%.

.

pure THF hydrate, it took about 110 min at 5.5°C and 65min at 8°C to complete the total dissociation of the sameamount of hydrate. It suggests that even at the presentdissociation temperature the glass bead size does have animpact if large, to a certain extent.

4. Conclusions

An experimental investigation on THF hydrate formationand dissociation in porous media was carried out using MRI.Based on the experimental results, the following conclusionscan be drawn:

(1) Since porous media provide a large specific surfacearea and contact area, they can promote THF hydratenucleation and formation compared with the situationwithout porous media.

(2) THF hydrate crystals start to form on porous mediaand in their adjacent area as well as from the wall ofthe sample container. The smaller the pore size is, orthe lower the formation temperature is, the faster thenucleation and the shorter the entire formation time.As the formation temperature gets higher, the effect ofthe porous media became more obvious.

(3) For the dissociation cycle, the dissociation time ismostly dependent on the dissociation temperaturerather than on the size of the glass beads. The impactof the glass bead size on the dissociation might beobvious at a lower temperature.

Further studies should consider THF hydrate dissociationin various-sized glass beads at different temperatures toinvestigate the dissociation characteristics and the effect ofglass bead size. The structure of the porous media shouldalso be explored in detail as it may affect the THF hydrateformation and dissociation.

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References

[1] Sloan ED. Clathrate Hydrates of Natural Gases. New York: MarcelDekker; 1990.

[2] Je!rey GA. Inclusion Compounds I. London: Academic Press; 1984.[3] Sloan ED. Fundamental principles and applications of natural gas

hydrates. Nature 2003;426:353–63.[4] Leaist DG, Murray JJ, Post ML Davidson DW. Enthalpies of

decomposition and heat-capacities of ethylene-oxide and tetrahydro-furan hydrates. J Chem Phys 1982;86:4175–8.

[5] Rueff RM, Sloan ED. Effect of granular sediment on some thermalproperties of tetrahydrofuran hydrate. Ind Eng Chem 1985;24:882–5.

[6] Lu XB, Wang L, Wang SY, Li QP. Study on the mechanical propertiesof the tetrahydrofuran hydrate deposit. Proceedings of the 8thInternational Offshore and Polar Engineering Conference, Vancouver,Canada; 2008.

[7] Handa YP. Enthalpies of fusion and heat capacities for H218O ice and

H218O tetrahydrofuran clathrate hydrate in the range 100–270 K. Can J

Chem 1984;62:1659–61.[8] Bondarev EA, Groisman AG, Savvin AZ. Porous medium effect

on phase equilibrium of tetrahydrofuran hydrate. Proceedings of the2nd International Conference on Natural Gas Hydrates, Toulouse,France; 1996.

[9] Baldwin BA, Moradi-Araghi A, Stevens JC. Monitoringhydrate formation and dissociation in sandstone and bulkwith magnetic resonance imaging. Magn Reson Imaging 2003;21:1061–9.

[10] Yun TS, Santamarina JC, Ruppel C. Mechanical properties of sand,silt, and clay containing tetrahydrofuran hydrate. J Geophys Res2007;112:B04106.

[11] Lee JY, Yun TS, Santamarina JC. Observations related to tetrahydro-furan and methane hydrates for laboratory studies of hydrate-bearingsediments. Geohim Mineral Petrol 2008;8:1525–2027.

[12] Tomoyuki I, Hideaki M, Takaaki M, Yasuhiko HM. Formation anddissociation of clathrate hydrate in stoichiometric tetrahydrofuran–water mixture subjected to one-dimensional cooling or heating.Geohim Mineral Petrol 2001;56:4747–58.

[13] Pinder KL. Time-dependent rheology of the tetrahydrofuran-hydrogensulphide gas hydrate slurry. Can J Chem Eng 1964;426:132–8.

[14] Larsen R. Clathrate hydrate single crystals: growth and inhibition. Ph.D. thesis, Norwegian University of Science and Technology 1997,Trondheim, Norway.

[15] Bollavaram P, Devarakonda S, Selim MS, Sloan ED. Growthkinetics of single crystal s II hydrates, elimination of mass and heattransfer effects. Annals of New York Academy of Sciences 2000;912:533–43.

[16] Cha SB, Ouar H, Wildeman TR, Sloan ED. A third-surface effect onhydrate formation. J Chem Phys 1988;92:6492–4.

[17] Kleinhans MG, Jeukens CRLPN, Bakker CJG, Frings RM. Magneticresonance imaging of coarse sediment. Sediment Geol 2008;208:69–78.

[18] Riestenberg D, West O, Lee SG, McCollum S, Phelps TJ. Sedimentsurface effects on methane hydrate formation and dissociation. MarGeol 2003;198:181–90.

[19] Zang XY, Fan SS, Liang DQ, Li DL, Chen GJ. Influence of 3Amolecular sieve on tetrahydrofuran (THF) hydrate formation. SciChina Ser B Chem 2008;51:893–900.

[20] Song YC, Yang MJ, Liu Y. Progress on the experimental detectiontechnology of gas hydrate formation and decomposition. Nat Gas Ind2008;28(8):111–5.

[21] Zhou XT, Fan SS, Liang DQ, Wang DL, Huang NS. Use of electricalresistance to detect the formation and decomposition of methanehydrate. J Nat Gas Chem 2009;18:337–40.