thermodynamic inhibitors for hydrate plug melting

822

Thermodynamic Inhibitors for Hydrate Plug Melting

XIAOYUN LI,

a

LARS HENRIK GJERTSEN, AND TORSTEIN AUSTVIK

Statoil Research Centre, Postuttak, 7005 Trondheim, Norway

A

BSTRACT

: The hydrate melting efficiency of thermodynamic inhibitors hasbeen shown to depend on hydrate plug properties, inhibitor properties, and theturbulence of the liquid system in question. Inefficient mixing of inhibitor andwater, lack of contact between inhibitor and hydrate, and a solid or gel precip-itation in the melting region have been demonstrated to give low melting effi-ciency. MeOH seems to be the most efficient inhibitor for melting porous plugs,but may not melt a plug with low porosity. MEG has the ability to penetrateinto a compact plug and cause melting. However, when used at high concentra-tions, MEG may freeze out as a solid or gel, which can lead to reduced hydratemelting efficiency. At high pressures and in the presence of a hydrocarbon liq-uid, MEG solutions were shown to freeze out at temperatures more than 30

�

Chigher than values given in the literature. TEG seems to easily freeze andbecome inefficient for melting hydrate plugs.

INTRODUCTION

In the North Sea, hydrate plugs are frequently formed in wells, flow lines and top-side. To remove hydrate plugs, thermodynamic inhibitors, such as methanol and gly-cols (MEG and TEG), are often employed. However, an inhibitor that is efficient forone case may be useless for the removal of the next hydrate plug. When inhibitorsare successfully applied, the time between injection and response is usually short,normally a few hours. When failing, a plug may not be removed by the inhibitor forweeks or months. For example, it has been experienced several times that TEG didnot melt a hydrate plug for more than a month. There are also cases where MeOH orMEG did not melt hydrate plugs effectively.

Low melting efficiency has been experienced in different systems such as oil pro-ducers, gas condensate lines, and in WAG injectors. Lab experiments as well as fieldexperience have shown that hydrate plugs formed in different systems, and at differ-ent conditions may be very different concerning porosity and oil content.

1

The char-acteristics of plugs and inhibitor properties, in turn, influence the inhibitor efficiencyfor plug melting. The inhibitor distribution, effective contact between inhibitor andhydrate, and solid or gel precipitation during plug melting are believed to be crucialfactors for hydrate plug melting efficiency when using inhibitors.

In this work the mixing between inhibitors and water, with inhibitor properties likedensity and viscosity as important parameters, has been studied. The conditions underwhich glycols may freeze out have also been studied and the melting efficiency of

a

Telecommunication. Voice: (47) 73584453; fax: (47) [email protected]

823LI

et al.

: THERMODYNAMIC INHIBITORS

three inhibitors on various plugs has been evaluated. The inhibitors studied weremethanol (MeOH), monoethyleneglycol (MEG), and triethyleneglycol (TEG).Important, selected properties of the inhibitors,

2

at 25

°

C and 1 bar, are given inT

ABLE

1.

INHIBITOR DISTRIBUTION IN WATER

The thermodynamic hydrate inhibitors are miscible with water, but the rate atwhich inhibitors are completely mixed with water is not well defined. When aninhibitor fails to melt a hydrate plug, it is frequently asked whether the inhibitor isat all in contact with the hydrate plug. The inhibitor distribution in water was studiedin a phase distribution rig to provide visual information. The rig was two meters highwith a diameter of 0.23 m, a 550 ml glass cylinder, and small beakers. In the distri-bution rig, inhibitors were injected either from the top or bottom, the rig was alreadyfilled with water and condensate. Air could be bubbled in through the bottom of the

T

ABLE

1. Important selected properties of the studied inhibitors at 25

�

C and 1 bar

Fluid Water MeOH MEG TEG

Density (kg/L) 0.995 0.787 1.11 1.12

Viscosity (centipoise) 0.923 0.539 17.647 37.783

Freezing point (

°

C) 0 –98 –13 –5

T

ABLE

2. MeOH concentrations along the aqueous phase at different sampling timeswhen MeOH was added from the top to the condensate and water phases

Sampling point (number of liters from bottom)

MeOH concentration (wt%) in the aqueous phase at different times

at time 1

a

a

Just after the MeOH injection when MeOH and water were separated.

b

After moderate bubbling of air from the bottom.

c

After even longer and stronger bubbling of air from the bottom.

d

After intense bubbling of air from the bottom.

at time 2

b

at time 3

c

at time 4

d

47 60 wt%

41 41 wt%

35 33 wt%

30

<

0.5 wt%

28 4 wt% 19 wt% 30 wt%

18 4 wt% 18 wt% 28 wt%

14 4 wt% 18 wt% 27 wt%

4 27 wt%

824 ANNALS NEW YORK ACADEMY OF SCIENCES

rig to allow mixing of the fluids. Along the sides of the rig and the cylinder flask,sampling points were mounted. The inhibitor concentrations at different levels of theaqueous phase were sampled and analyzed by gas chromatography (GC). Ink wasadded to the inhibitors to provide visual information.

Experiments with Methanol

MeOH was injected from the top of the rig filled with condensate (heavier thanMeOH) and water; the condensate separated the MeOH and water phases and pre-vented contact. Natural diffusion/convection and moderate air bubbling gave too lowa driving force to obtain good mixing between MeOH and water. Even after intensebubbling, a higher concentration of MeOH at the top of the water phase was detect-ed. In T

ABLE

2, the MeOH concentrations as a function of the height of the aqueousphase are listed. F

IGURE

1 shows a picture of the phase distribution rig when air wasbubbled through the water, condensate, and MeOH phases from the bottom. In the

FIGURE 1. Phase distribution experi-ment with MeOH on the top of water. TheMeOH was dyed to a dark blue color duringthe experiment.

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et al.


picture, the MeOH and the condensate phases are mixed mechanically, whereas thewater phase remains almost undisturbed.

In another experiment, MeOH was injected from the top of a flask filled with con-densate, lighter than MeOH, and water. The MeOH flowed easily through the con-densate and accumulated on the top of the water phase. After three days, there wasno noticeable mixing between the MeOH and water. The MeOH concentration at theupper and lower part of the aqueous phase was 90 and 0 wt%, respectively. Onlypowerful mechanical disturbance at the MeOH/water interface caused mixing.

To simulate the mixing of MeOH with melting water at the top of a compact plug,water was injected at the bottom of a flask filled with MeOH. Some gas was also bub-bled through the bottom, simulating gas released during hydrate melting. Althoughsome turbulence was created during the water and gas injection, very little MeOHwas mixed with the water. The MeOH concentration at the bottom of the water phasewas only 5 wt%, although 44 wt% should result from complete mixing. MeOH wasalso injected from the bottom of a water phase in one experiment. Gravity forced analmost complete and immediate mixing of the two phases.

Experiments with Glycols

In an experiment in which MEG was injected from the bottom of the distributionrig, MEG displaced the water at the lower part without mixing with the water. In theexperiments where MEG was added from the top, the condensate did not hinder thedownward motion of MEG. However, the degree of mixing between water and MEGdepended very much on how MEG was injected and the amount of water employed.When MEG was carefully added to the top of a high water column, the MEG con-centration in the upper part of the water phase was low. When TEG was added fromthe top into a water phase, most of the TEG flowed directly through the water phaseand accumulated at the bottom. Powerful forced convection was necessary to obtainan even distribution of the inhibitors in most of the glycol experiments.

By using a simple numerical method to solve the Fick’s second law of diffusion,

3

and using the diffusion coefficient in dilute water solutions,

2

the time needed to getan even inhibitor concentration in water has been calculated for some of the inhibitordistribution experiments. The results show that the diffusion time is in the range ofyears, confirming that inhibitor diffusion into water is a very slow process.

FREEZING OF GLYCOLS

The freezing point of pure MEG and TEG is

−

13

°

C and

−

5

°

C, respectively. Fur-thermore, aqueous solutions of MEG and TEG have lower freezing points than dothe pure substances.

1

However, field experience shows that at high pressures MEGsolutions may freeze out at temperatures higher than those given in the literature.The freezing experiments with MEG solutions were performed in a high pressuresapphire PVT cell.

5

The freezing temperature of MEG solutions was observed todepend on both pressure and the composition of the HC liquid present. For an aque-ous 75 wt% MEG solution in contact with a HC liquid at elevated pressure, the freez-ing point was more than 30

°

C higher than that given by Campbell.

4

The freezingpoints of MEG solutions from the present study and data from Campbell are plotted


in F

IGURE

2. At an onshore facility, a 70 wt% MEG solution in contact with a con-densate has been observed to freeze in the range of

−

26

°

C to

−

28

°

C at 67 bar. Thisresult is in accord with the measured freezing point of

−

28

°

C in the present work.According to Campbell, the freezing temperature of a 70 wt% MEG solution isbelow

−

60

°

C. The difference in freezing temperatures of MEG solutions betweenour high pressure data with a HC liquid present and the data given by Campbellseems to increase with decreasing MEG concentrations.

Some qualitative experiments were performed where MEG and TEG were usedto melt propane hydrate at

−

45

°

C and 1 bar. The equilibrium temperature for pro-pane hydrate at 1 bar is

−

11

°

C. It was observed that the TEG added at 80, 90, and100 wt% froze to a gel like phase in all the experiments. The frozen TEG gel wasextremely viscous and very little hydrate was melted by TEG in these experiments.MEG did not freeze to a gel like phase, but a kind of half frozen white solid wasobserved in the MEG experiments. Furthermore, some hydrate was melted on con-tact with MEG.

MELTING OF HYDRATES AND ICE

PVTsim, by Calsep, was used for hydrate equilibrium calculations with MeOH,MEG, or TEG as the inhibitor. The depression of the hydrate equilibrium tempera-ture for a model fluid, the composition of which is given in T

ABLE

3, at 200 bar has

FIGURE 2. Freezing temperature of MEG solutions under various conditions.

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been calculated as a function of inhibitor concentration. The corresponding hydrateequilibrium temperature without inhibitor is 23

°

C. In F

IGURE

3, a comparison of theinhibitor efficiency for three inhibitors is shown, with the inhibitor concentration inwt%, mol%, and vol%. TEG is most efficient on a mole basis. However, due to thehigh molecular weight, TEG is the least efficient inhibitor on a weight or volumebasis except at very high concentrations. Up to 55 vol%, MeOH and MEG are almostequally good, but MEG is more efficient at higher concentrations according to theequilibrium predictions.

Four ethane hydrate plug melting experiments were performed in a speciallydesigned steel cell at SINTEF,

6

two with MeOH and two with MEG. Liquids couldbe injected from the top of the cell, and a condensate lighter than MeOH was injectedprior to inhibitor injection in all four experiments (compare with T

ABLE

4).

Experiments with MeOH.

Even with gas circulation melting ceased only a shorttime after MeOH injection in the first experiment, where some water was injectedprior to inhibitor injection. In the second experiment, where no water was injectedand gas was not circulated either, the hydrate melted gradually.

T

ABLE

3. Composition of the model oil used for hydrate equilibrium temperature predictions

Component Mol%

C1 55.155

C2 13.07

C3 6.535

C8 0.002

C9 0.036

C10 1.867

C11 5.766

C12 7.064

C13 7.261

C14 0.244

T

ABLE

4. Information for the hydrate plug melting experiments in hydrate pipe

Inhibitor and Number

Extra water before inhibitor injection

Ethane circulatedfrom bottom

Condensate

MeOH I Yes Yes Yes

MeOH II No No Yes

MEG I Yes Yes Yes

MEG II No No Yes


FIGURE 3. Hydrate equilibrium temperature depression as a function of inhibitorconcentration in (A) wt%, (B) mol%, and (C) vol%.

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Experiments with MEG.

In the first experiment, with gas circulation, the hydratemelted gradually. In the second experiment, with no gas circulation, melting ceasedsome time after MEG injection.

Since all the inhibitors studied can melt ice in the same manner as meltinghydrates, qualitative studies with pure MeOH, MEG, or TEG as the inhibitor wereconveniently performed on porous snow and solid ice. The experiments were con-ducted in small beakers at atmospheric pressure and ink was added to the inhibitors.

Porous Snow Plugs.

MeOH was clearly the most efficient inhibitor followed byMEG, although TEG was the least efficient inhibitor to melt porous snow on a vol-ume basis. The experiments also revealed that a significant temperature gradient canexist in a plug. In snow melted by MeOH, the temperature in the upper part was

−

30

°

C and only

−

10

°

C in the lower part, with large temperature gradients lasted fora long period. For a snow plug melted by MEG, the temperature in the upper partwas

−

10

°

C and in the lower part

−

24

°

C. The melting temperature gradients in thesnow plugs indicate inhibitor concentration gradients, with high MeOH concentra-tion on the top in the MeOH experiments and high MEG concentration at the bottomin the MEG experiments.

Solid Ice Plugs.

MeOH remained on the top of the ice plug after having meltedsome ice at the interface. MeOH was unable to penetrate into the ice plug and melt-ing stopped after a short period. In the experiments with MEG and TEG, the inhibi-tors melted the outer layer of the ice plugs first, then they penetrated into the ice plug.MEG was the most efficient inhibitor for melting solid ice plugs.

DISCUSSION

Mixing of Inhibitor and Water

In common practice, an immediate mixing of water and inhibitor is expectedwhen risers and wells are inhibited. However, our results demonstrated that twofactors were very important in determining if the inhibitor and water may miximmediately.

Density Difference.

When a lighter phase was carefully injected on the top of aheavier phase, no detectable mixing occurred unless a strong forced convection wasapplied. Some turbulence during the injection caused limited mixing, but the turbu-lence was normally not strong enough to overcome the density difference and pro-vide good mixing. A third immiscible phase, like oil on the top of a water phase, mayreduce the degree of turbulence during injection. Furthermore, a heavy oil phase mayhinder the physical contact between inhibitor and water.

Viscosity.

Viscous forces are frictional forces that attempt to make all parts of afluid move at the same velocity. When the viscosity of a fluid is high, its ability tomix into another medium is low. The low viscous MeOH and water mixed easilywhen water was added from the top to an MeOH phase. Due to the high viscosity ofglycols, it was more difficult to get a good mixing of glycols and water although theglycol was added from the top into the water phase. Furthermore, it was more diffi-cult to mix the more viscous TEG with water than to mix MEG and water. When aninhibitor and water did not mix naturally, strong forced convection was necessary toobtain good mixing.


Inhibitor and Plug Properties versus Melting Efficiency

Theoretically, MeOH is an efficient hydrate inhibitor over the entire concentra-tion range (vol or wt%). Even used at extremely high concentrations, MeOH will notfreeze out due to its low freezing point. However, experiments indicated that specialconditions may make MeOH an inefficient inhibitor for plug melting. An oil phaseheavier than MeOH could hinder the physical contact between MeOH and thehydrate plug. Even when MeOH was in direct contact with a hydrate plug, a lowmelting rate could still be obtained. During the hydrate plug melting experiment withMeOH, where melting stopped early, the free water layer added prior to MeOHinjection was believed to prevent contact between MeOH and the hydrate plug. Fora compact plug, a water film released from melting could prevent MeOH from melt-ing the plug beneath it, as was demonstrated in one ice plug melting experiment.MeOH is believed to be most efficient in melting rather porous plugs.

When TEG is used to melt a hydrate plug, its high viscosity makes it move veryslowly towards and in the plug. As soon as some TEG comes into contact with theplug, melting starts. At high concentrations, the temperature in the melting regioncan be very low, which may cause the freezing out of TEG. The freezing out and thehigh viscosity of TEG at low temperatures hinders more TEG from coming into con-tact with the hydrate plug and the melting may stop. Furthermore, for a plug with ahigh water content, or when some hydrate has melted, the melting efficiency will below since TEG is an especially poor inhibitor at low concentrations (vol or wt%).Experiments with TEG show that TEG froze out at low temperatures and was inef-ficient for melting both hydrate and ice/snow plugs.

MEG is believed to have a similar behavior to TEG with respect to hydrate plugmelting but with a higher efficiency. At low concentrations (vol or wt%) the meltingefficiency of MEG is considerably higher than that of TEG. Furthermore, experi-ments show that it was easier to get an even distribution of MEG in water than forTEG. The high density of MEG allowed it to penetrate to the bottom, indicating thata low permeable plug could be melted by MEG. However, the plug melting experi-ments with MEG revealed that high concentrations of MEG could accumulate at thebottom without melting the hydrate above it effectively. The viscosity and freezingpoint of MEG solutions are lower, which also makes MEG better than TEG for plugmelting. However, the freezing out of MEG solutions at low temperatures may stillbe a problem, especially at high pressures and in the presence of a HC liquid phase.

There may be many explanations for the lack of ability to melt hydrate plugsefficiently with thermodynamic hydrate inhibitors. The freezing and slow motion ofMEG and TEG at low temperatures is one. Although not studied in this work, localfreezing of wax or gelling of oil at low temperatures can be another reason for lowmelting efficiency. The distribution of inhibitors and effective contact between theinhibitor and the hydrate plug are also important issues. When MeOH or MEGbecomes inefficient due to lack of contact between the inhibitor and the hydrate plug,only very strong turbulence in the system may enhance the mixing and, hence,increase the melting rate. However, this is generally difficult to achieve in actual plugremoval situations.

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CONCLUSIONS

The hydrate melting efficiency of thermodynamic inhibitors depends on the plugproperties, inhibitor properties, and the turbulence of the system. Inefficient mixingof the inhibitor and water, lack of contact between the inhibitor and the hydrate plug,and solid or gel precipitation in the melting region could cause low melting efficien-cy. MeOH is expected to be efficient for permeable/porous hydrate plugs, or when ahigh degree of turbulence can be obtained in the liquid system. However, if appliedon a compact plug, MeOH may stay stagnant on the top of the hydrate plug withoutmelting the plug beneath it. On a volume basis, MEG is at least as good as MeOH inmelting hydrate according to equilibrium predictions. However, MEG is rather vis-cous, which may cause a low penetration rate into a plug. Applied at high concentra-tions, the low melting temperature may cause the freezing of MEG solutions andreduce the melting rate. At high pressures and in the presence of a HC liquid, MEGsolutions froze out at temperatures more than 30

°

C higher than those reported in theliterature. TEG may be inefficient for melting hydrate plugs. Due to the high viscos-ity and density of TEG, no other inhibitors can be squeezed into the plug after afailed attempt with TEG. TEG should not be used for hydrate plug melting.

REFERENCES

1. A

USTVIK

, T., X. L

I

& L.H. G

JERTSEN

. 1999. Hydrate plug properties—formation andremoval of plugs. To be published in NGH’99.

2. Y

AWS

, C.L. 1995. Handbook of Transport Property Data. Gulf Publishing Company.3. N

OGGLE

, J.H. 1996. Physical Chemistry. Harper Collins College Publishers.4. C

AMPBELL

, J.M. 1994. Gas Conditioning and Processing, 7th edit. Campbell Petro-leum Series.

5. G

JERTSEN

, L.H. & F.H. F

ADNES

. 1999. Measurements and predictions of hydrate equi-librium conditions. To be published in NGH’99.

6. B

ORTHNE

, G., L. B

ERGE

, T. A

USTVIK

& L.H. G

JERTSEN

. 1996. Gas flow cooling effectin hydrate plug experiments. Proceedings of the 2nd Int. Conf. on Natural GasHydrates. Toulouse, France.

thermodynamic inhibitors for hydrate plug melting

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