WO2010111226A2 - A composition and method for inhibiting agglomeration of hydrates in pipelines - Google Patents

Abstract

The invention is an anti-agglomeration composition for preventing hydrates from binding together and forming plugs in pipelines, particularly from off shore gas wells. The inventive composition contains a combination of a surfactant and an alcohol cosurfactant, both present in low amounts to limit environmental effects as well as to reduce the cost of separation in downstream operations. The inventive composition comprises a surfactant and an alcohol co-surfactant each provided in effective amounts to cause anti-agglomeration of hydrates at high subcooling temperatures and/or at high water-cuts. Preferably, the surfactant is a Rhamnolipid biosurfactant and the alcohol cosurfactant is methanol, the alcohol cosurfactant being present at concentrations low enough such that side effects such as salt deposition are avoided. Generally, the alcohol cosurfactant should be present at from 0.05-5% wt., with the surfactant present at from 0.01 to 10% wt, more preferably 0.01-5% wt.. Using the present invention, agglomeration of hydrates in gas and oil pipelines can be reduced.

Description

A Composition and Method For Inhibiting Agglomeration Of Hydrates In Pipelines

Cross Reference To Related Applications:

This application claims priority from U.S. Provisional Patent Application serial nos.

61/263,464 filed November 23, 2009 and 61/162,364 filed March 23, 2009, each of which applications is incorporated by reference herein in its entirety.

Field of Invention:

This application is related to compositions and methods for preventing hydrate masses from impeding the flow of fluids particularly in gas and oil pipelines.

Background:

Natural gas has a high hydrogen to carbon ratio compared to petroleum fluids and coal. Due to availability, as well as economical and environmental considerations, natural gas is projected to be the premium fuel of the 21st century. It is also a clean burning fuel, which results in low production of CO₂. A large portion of natural gas is produced from the deep sea where the temperatures are low.

Thermodynamic conditions existing in gas production lines often favor formation of crystalline inclusion compounds known as gas hydrates. Water, co-produced with natural gas, forms lattice structures by hydrogen bonding, the structures stabilized by guest molecules such as methane, propane, etc., under high pressures, and low temperatures in the range of a few degrees to 25 ⁰C. Formation of gas hydrates occurs rapidly, unlike corrosion, scale, or wax buildup. This rapidity has undesirable safety and environmental consequences. Plug formation from hydrate may lead to production shutdowns, necessitating costly downtime to remove the plug. Hydrate formation in pipelines is a problem in gas and oil production from offshore fields.

Traditional hydrate prevention methods include physical means, such as insulation and electrical heating. Industry practice has also been to use thermodynamic inhibitors such as alcohols which affect bulk phase properties and inhibit hydrate formation. Thermodynamic inhibitors such as methanol are effective but sometimes a large quantity of methanol is needed, at ratios as high as 1 to 1 volume of alcohol to water, often these are significant amounts which have undesirable environmental and safety impacts.

An alternative is to use surfactants to alter surface properties of the hydrates. Surface property changes lead to hydrate kinetic inhibition which delays nucleation or growth of hydrates. Because hydrates may form slowly, the fluid flow can continue. Kinetic inhibitors have been used, generally in ratio of 0.005-0.02 volume of surfactant to water, for either inhibiting hydrate formation or reducing the rate of accumulation. The limitation with kinetic inhibitors is the ability to work under low subcooling conditions. In some deep sea environments, the subcooling can be as high as 20-25 ⁰C, because the sea bed temperature is about 4 ⁰C. Kinetic inhibitors are not effective at such subcooling temperatures.

The present inventors have found that a composition containing a surfactant such as a Rhamnolipid biosurfactant or a quaternary ammonium surfactant compound, combined with an alcohol co-surfactant provide anti-agglomeration with both tetrahydrofuran (THF) hydrates and cyclopentane hydrates, which are close in properties to the gas hydrates which occur in the fluid pipelines, and consequently, that such a composition would be effective as an anti-agglomeration agent in oil and gas pipelines, even at high subcooled temperatures and at relatively high water cuts.

Bio-surfactants can be very effective at low concentration, and the present inventors have found that the presence of an alcohol co-surfactant such as methanol at low concentration serves to enhance the anti-agglomeration effect. THF hydrates were first used to determine the anti-agglomeration effects with a combined bio-surfactant and methanol, as THF forms structure II hydrates and is much more soluble in water than any species in natural gas.

There was a concern that the crystallization in THF hydrates may be different from methane hydrates, as THF hydrates may form in the bulk phase whereas methane and propane hydrates may from on an interface between water and oil phases. Formation of THF hydrates unlike methane hydrates occurs at atmospheric pressure, which while an advantage in conducting experiments, left open the question of whether the composition would actually be effective in the field. To confirm the benefits of the present invention, the inventors conducted tests with cyclopentane hydrate formation, as these have a low solubility in water, and are in some respects close to hydrates from natural gas species. Cyclopentane was also used as the oil phase to form a water-in-oil emulsion, to confirm the anti-agglomeration effectiveness of the inventive composition containing a low concentration of a surfactant and an alcohol cosurfactant.

Summary Of The Invention

It is an object of the present invention to provide an anti-agglomeration composition for gas hydrates, effective at high subcooling temperatures which contains a combination of a surfactant and an alcohol cosurfactant.

It is a further object to provide an anti-agglomeration composition which uses low amounts of both the surfactant and co-surfactant to limit environmental effects as well as to reduce the cost of separation in downstream operations.

It is yet another object to provide an anti-agglomeration composition which does not require a stable water in oil emulsion to provide the beneficial anti-agglomeration effects.

One or more of these and/or other objects of the present invention are achieved by a composition comprising a surfactant and an alcohol co-surfactant provided in effective amounts to cause anti-agglomeration of hydrates at high subcooling temperatures and/or at high water-cuts.

In one embodiment, the surfactant is a Rhamnolipid biosurfactant and preferably, the alcohol cosurfactant is methanol, the alcohol cosurfactant being present at concentrations low enough such that side effects such as salt deposition are avoided.

Generally, the alcohol cosurfactant should be present at from about 0.5-5% wt., with the surfactant present at from about 0.001 to 10% wt., more preferably 0.01-5% wt.

Using the present invention, agglomeration of hydrates in gas and oil pipelines can be reduced using low levels of the inventive composition, also being effective at relatively high water cuts, and at relatively high subcooling temperatures.

Brief Description Of The Drawings

Figure 1 - Multiple screening-tube rocking apparatus.

Figure 2 - Typical freeze-thaw cycle data for THF mixture of two parts isooctane and no MeOH; example for mixture of 1.5 wt. % rhamnolipid.

Figure 3 - Typical freeze-thaw cycle data for THF mixtures with two parts isooctane and 5 or 2 wt. % MeOH; example for mixture with 0.5% rhamnolipid and 5% MeOH.

Figure 4 - Typical freeze-thaw cycle data for THF mixtures with two parts isooctane and 0.5 wt. % MeOH; example for mixture with 0.05% quat.

Figure 5 - Agglomeration state results for THF mixtures with four parts isooctane and small amounts of rhamnolipid. In all cases, significant adhesion of hydrate upon vial walls occurs immediately at all minimum temperatures (represented by • symbol). Data represents behavior of a given composition across ail minimum temperatures.

Figure 6 - Significant adhesion observed in THF mixtures of four parts isooctane, very low concentrations of rhamnolipid, and 5 wt. % MeOH or less, data shown in Figure 5. Sample shown in image contains 0.01 wt. % rhamnolipid and 5 wt. % MeOH.

Figure 7 - Typical plug appearance when small amounts, i.e., zero to two parts by weight, of isooctane are used in THF mixture. In this image, vial is upside down with most of vial volume blocked by hydrate; mixture being tested in this image is one containing two parts isooctane. Calculations using measured THF hydrate density⁵⁵ show the volume fraction of hydrate in mixtures of two parts isooctane is roughly 0.25. The sample shown in this image is for a mixture of two parts isooctane and 1.5 wt. % quat. Small bubbles seen in this image, as well as in Figure 15, are present in the bath fluid due to bath operation.

Figure 8 - Agglomeration state results for THF mixtures with two parts isooctane and rhamnolipid with and without MeOH: (■) stable dispersion — i.e., effective anti- agglomeration, ( • ) immediate and significant adhesion upon vial walls, (x) plugging tendency. Plugging tendency means that either total plugs occurred, i.e., steel ball is unable to move, or partial plugs occurred, i.e., steel ball is unable to move through the entire length of vial. Data represents behavior of a given composition across all minimum temperatures.

Figure 9 - Image of hydrate slurry in THF mixtures with two parts isooctane. This image was taken with vial almost horizontal in agitator rack and thus prior to complete slurry settling; due to high hydrate volume present in vial, it was not possible to capture a clear image of the slurry separate from the oil phase. This mixture shown here is for 0.5 wt. % rhamnolipid and 2 wt. % MeOH co-surfactant.

Figure 10 - Agglomeration state results for THF mixtures with two parts isooctane and ARQUAD 2C-75 with and without MeOH: (■) stable dispersion — i.e., effective anti- agglomeration, ( • ) plugging tendency. Plugging tendency means that either total plugs occurred, i.e., steel ball is unable to move, or partial plugs occurred, i.e., steel ball is unable to move through the entire length of vial. Data represents behavior of a given composition across all minimum temperatures.

Figure 11 - Average of freeze-thaw cycle data for THF mixtures including two parts isooctane with or without rhamnolipid. Given are Tc (shown as a solid triangle), t_c,

(shown as columns), and Ta (shown as ■). Error bars are present for all points; some may not be clear due to magnitude.

Figure 12 - Average of freeze-thaw cycle data for THF mixtures including two parts isooctane, rhamnolipid, and low MeOH concentrations. Given are Tc (shown as a solid triangle), t_c, (shown as columns), and Td (shown as ■). Error bars are present for all points; some Td and T]. β rror bars overlap and may not be clear. Figure 13 - Average of freeze-thaw cycle data for THF mixtures including two parts isooctane with or without ARQUAD 2C-75. Given are Tc (shown as a solid triangle), t_c, (shown as columns), and Ta (shown as ■). Error bars are present for all points; some may not be clear due to magnitude.

Figure 14 - Average of freeze-thaw cycle data for THF mixtures including two parts isooctane, ARQUAD 2C-75, and low MeOH concentrations. Given are Tc (shown as a solid triangle), t_c, (shown as columns), and Ta (shown as ■). Error bars are present for all points; some Ta and Tc error bars overlap and may not be clear.

Figure 15 - Partial plug that appears more as a concentrated hydrate slurry in a THF mixture of zero parts isooctane, 1.5 wt. % rhamnolipid, and 10 wt. % MeOH. Steel ball is barely visible, but the air bubble shows the vial is not filled with a solid hydrate plug as would be expected. The vial in this image is tilted at a roughly 45° angle away from the borescope, as evidenced by bubble position.

Figure 16. Freeze-thaw cycle data for mixtures of CP/H₂O/THF of composition 0.4/1/x. The results are for 0.01, 0.03, 0.05 wt. ratio of THF to H₂O.

Figure 17. Freeze-thaw cycle data for a mixture of CP/H₂O/THF/RI1 of composition

0.4/1/0.03/0.01 (weight ratio to water).

Figure 18. Effect of Rh on crystallization temperature (T_c) and dissociation temperature (Ta) for mixtures of CP/H₂O/THF/RI1 of composition 0.4/l/0.03/x (weight ratio to water).

Figure 19. Effect of CP amount on dissociation temperature (Td) for mixtures CP/H₂O/THF/Rh of composition x/1/0.03/0.01 (weight ratio to water).

Figure 20. Effect of MeOH on dissociation temperature (T_d) for mixtures of

CP/H₂O/THF/Rh/MeOH of composition 1.5/1/0.02/x/y (weight ratio to water).

Figure 21. Agglomeration states for mixtures of CP/H₂O/THF/Rh of composition 4/1 /x/y (weight ratio to water), where THF amount x and Rh amount y are control variables: (+) stable dispersion; (O) dispersible hydrate, hydrates that looks like plug but can be dispersed by heavy shaking, (x) plugging tendency. Plugging tendency means that either total plugs occurred, i.e., steel ball is unable to move, or partial plugs occurred, i.e., steel ball is unable to move through the entire length of vial .

Figure 22. Effect of THF and Rh on dissociation temperature (Td) for mixtures of CP/H₂O/THF/Rh of composition 4/1 /x/y (weight ratio to water).

Figure 23. Agglomeration states for mixtures of CP/H₂O/THF/Rh/MeOH of composition 2/1 /x/y /z, (weight ratio to water) where THF amount x, Rh amount y, and MeOH amount z are control variables: (+) stable dispersion; (O) dispersible hydrate, hydrates that looks like plug but can be dispersed by heavy shaking, (x) plugging tendency. Plugging tendency means that either total plugs occurred, i.e., steel ball is unable to move, or partial plugs occurred, i.e., steel ball is unable to move through the entire length of vial.

Figure 24a. Effect of THF on dissociation temperature (Td) for mixtures of CP/H₂O/THF/Rh of composition 2/ 1/x/y (weight ratio to water).

Figure 24b. Effect of Rh and MeOH on dissociation temperature (TdJ for mixxures of CP/H₂O/THF/Rh/MeOH of composition 2/1/0.02/x/y (weight ratio to water).

Figure 25. Stable dispersion observed for mixtures of CP/H₂O/THF/Rh of composition 2/1/0.02/x (weight ratio to water), where Rh x is 0.003-O.Olfor the data shown in Figures 23 and 24. Sample shown in image contains 0.005 wt. Rh and 0 wt. MeOH. Vial is tilted roughly 60° from horizontal with the bottom side up.

Figure 26. Agglomeration states for mixtures of CP/H₂O/THF/Rh/MeOH of composition 1.5/l/0.02/x/y(weight ratio to water): stable dispersion, where Rh amount x and MeOH amount y are control variables : (+) stable dispersion; (Δ) hydrates attached to bottom or wall of the vial. (^x) plugging tendency. Plugging tendency means that either total plugs occurred, i.e., steel ball is unable to move, or partial plugs occurred, i.e., steel ball is unable to move through the entire length of vial.

Figure 27. Significant hydrates being adhered to the side walls and bottom observed in mixtures of CP/H₂O/THF/Rh/MeOH of composition 1.5/1/0.02/0.003/0.005 (weight ratio to water); data shown in Figure 26 and 20. Vial is tilted roughly 60° from horizontal with the bottom side up.

Detailed Description Of The Invention

Due to availability, as well as economical and environmental considerations, natural gas is projected to be the premium fuel of the 21st century. With natural gas production, there is a risk of shut down of onshore and offshore operations because of blockage from hydrates formed from co-produced water and hydrate-forming species in natural gas. Industry practice has been to use thermodynamic inhibitors such as alcohols often in significant amounts which have undesirable environmental and safety impacts. Thermodynamic inhibitors affect bulk phase properties and inhibit hydrate formation. An alternative is changing surface properties through usage of polymers and surfactants, generally effective at 0.5 to 3 weight % of co-produced water. One group of low dosage hydrate inhibitors (LDHI) are kinetic inhibitors, which affect nucleation rate and growth. A second group of LDHI are anti-agglomerants (AA), which prevent agglomeration of small hydrate crystallites. Essentially, AA facilitate hydrate crystallite slurries that can be transported as a typical fluid can be, without fear of foiiiiiπg hydiate blockages. Despite great potential, work on hydrate anti-agglomeration has been very limited.

This invention centers on the effect of combining small amounts of alcohol co- surfactant in hydrate-forming mixtures with different anti-agglomerants. We first tested using as a model, oil, water, and tetrahydrofuran (THF) as a hydrate-forming species. For the surfactant, we used dicetyl dimethyl quaternary ammonium chloride (i.e., a "quat") as well as a Rhamnolipid biosurfactant, both of which have been used as anti-agglomerants. The alcohol co-surfactant employed was methanol, though other alcohols could also be used.

Results show that combining with alcohol co-surfactants may provide anti- agglomeration when traditional anti-agglomerants alone are ineffective. One specific instance when traditional AA alone may be ineffective is when the water-cut (i.e., ratio of water volume to that of oil) is too high. Generally, AA's are thought to be ineffective if water-cut is 50 % or greater, but smaller water-cuts may also be detrimental to anti- agglomeration. We found that traditional AA's up to 1.5 weight % are ineffective when water cut in these model mixtures increases from roughly 17 % to 34 %. That is, traditional AA alone in our model mixtures are effective when water-cut is 17 % but become ineffective at a 34 % water-cut. Furthermore, our results show that alcohol co- surfactant may be an effective aid in anti-agglomeration when water-cuts are increased in this manner.

Specifically, as low as 0.5 weight % methanol co-surfactant is shown to be effective in anti-agglomeration when water-cut is increased from 17 % to 34 %. Without the co- surfactant there will be agglomeration independent of the AA concentration, up to 1.5 weight % AA. Other alcohols will likely provide similar benefit, with the benefit believed to be effective with water-cut increases up to roughly 69%.

The inventive method thus uses small amounts of alcohol co-surfactant in combination with AA's administered to hydrate-forming mixtures where the traditional AA alone is ineffective at facilitating hydrate slurries.

Uses The composition and method will be used when oil or natural gas is being produced iogeiher wixh co-produced waier from subsurface weiib, followed by transport of the fluid mixtures in pipelines to downstream processing equipment. Specifically, it can be used under conditions when water-cut increases to a certain point where traditional AA alone are ineffective at facilitating hydrate slurries or those conditions where high subcooling is likely to occur.

Advantages

Over the lifetime of an oil or gas well, the relative amounts of oil and water being produced (and thus flowing through pipelines and other process equipment) varies. As a gas well ages (i.e., as it is used and fluid is extracted from the reservoir over time), the amount of water being extracted generally increases and the condensate liquid (i.e., hydrocarbon phase) decreases. Similar changes occur in oil wells. As demand for energy increases, more producers are attempting to extract as much oil and gas as is possible from a reservoir. Thus, it is crucial to have methods and compositions which can be used to extend the lifetime of these wells.

One method is to use an alcohol co-surfactants in addition to a traditional AA to inhibit hydrate plug formation so gas and oil production can continue unimpeded. Alcohol co-surfactants may enable anti-agglomeration when traditional AA alone are ineffective, such as when water-cut becomes too large.

Also, traditional thermodynamic inhibitors such as methanol, one of the main thermodynamic inhibitors used to date, are required in such large amounts that their presence encourages the precipitation of dissolved salts in the water phase. This can lead to increased problems with flow and cause corrosion in production equipment. If small amounts of alcohol co-surfactants can be used with an AA in a mixed compositions, then the problem of salt deposition can be eliminated as well as the need for large amounts of thermodynamic inhibitors. Inhibition by AA is very attractive because small concentrations are effective even at very large hydrate subcoolings, which occur more as wells are being developed further offshore. Thus, the addition of alcohol co-surfactant is another way to ensure that AA may still be used for process flow assurance in situations where large amounts of thermodynamic inhibitors may be the only other hydrate inhibition option.

PART I-THF HYDRATE TESTING AND ANALYSIS

Despite great potential, work υπ hyuiate aiui-agglomeratiofl has been very limited. Our work centered on the effect of small amounts of alcohol co-surfactant in mixtures of two vastly different anti-agglomerants. In a model with oil, water, and tetrahydrofuran as a hydrate-forming species, results show that alcohol co-surfactants may help with anti- agglomeration when traditional anti-agglomerants alone are ineffective. Specifically, as low as 0.5 wt. % methanol co-surfactant used in this study is shown to be effective in anti-agglomeration. Without the co-surfactant there will be agglomeration independent of the AA concentration.

It is also shown that a Rhamnolipid biosurfactant is effective down to only 0.5 wt.

% in such mixtures, yet a quaternary ammonium chloride salt, i.e., a quat can be used to limit agglomeration in hydrate slurries at concentrations as low as 0.01 wt. %. However, biochemical surfactants are less toxic and biodegradable and thus their use may prove beneficial even if at concentrations higher than chemical surfactants. Thermodynamic inhibitors shift equilibrium conditions to lower temperature and higher pressure.² Although well-characterized, these inhibitors often require large concentrations, as high as 60 to 100 wt. % of co-produced water, which increase costs and have serious environmental impacts.³

An alternative to thermodynamic inhibitors is the use of low-dosage hydrate inhibitors (LDHI). LDHI mainly influence hydrate surface properties and are effective at concentrations of 0.5 to 3 wt. %; rather than affecting thermodynamic equilibrium, they act upon kinetics or agglomeration. Kinetic hydrate inhibitors (KHI) are generally polymeric compounds that delay nucleation and decrease growth rate.⁴' ⁵ KHI may result in complete inhibition of hydrates⁶ but do not perform well at pipeline/well shut-in conditions or at high operating subcoolings, i.e., NT_op, the difference between equilibrium temperature and operating temperature at a given pressure. In some flow conditions, AΥ_op may be as high as 20 ⁰C; therefore, one would require effective LDHI for ΔT_0/7 = 20 °C. Shut-in conditions, that is when pipeline flow is paused for a period of time, may occur when pipeline/well maintenance is necessary or when inclement weather occurs.

A second class of LDHI are anti-agglomerants (AA) which prevent agglomeration but not formation of hydrate crystals and enable hydrate transportation as slurries. AA are generally effective at high t\l_op or at shut-in conditions.^7"9 AA may also possess kinetic inhibition features.^10"12 They are generally surfactants but may be low molecular weight oligomeric species.⁹' ¹³ AA have not been studied as extensively as KHI. Insight into hydrate anti-agglomeration and mechanism are found in surfactant and colloidal science.^14"

16

AA structure is key to their effectiveness and mechanism.¹⁷ Effective AA contain the head group that can interact with a water lattice, such as amine or carbonyl groups, through hydrogen bonding or electrostatic attraction. AA compounds may also contain head groups that act as hydrate guest molecules. This feature combined with hydrogen bonding may incorporate the AA into crystals. Molecules in this case may adsorb too strongly and become engulfed in the growing crystal, requiring higher concentrations. The hydrophobic tail renders hydrate more oil-wet, thus dispersible in the oil phase, and prevents separate crystals from agglomerating.¹⁸ AA often produce water-in-oil (w/o) emulsions — thus limiting hydrate growth to water droplets dispersed in oil phase.⁹'¹⁷'^19'24 However, emulsion stability is generally undesired in gas and oil production.²⁵'²⁶

Once transportation of well fluids is complete, it is desired that phase separation be attainable so that product quality standards can be met. If these emulsions are too stable, then additional processing or additives may be required once hydrate formation is of no concern.

AA may become ineffective if water occupies one third or more of the total liquid volume of the process stream, i.e., limited to 50 % water-cut. This requirement may be related to w/o emulsion formation, but other reasons such as high slurry viscosity with high hydrate volume fraction is also cited in the literature.¹⁷' ¹⁹' ²⁷ In most gas production flow-lines, the amount of hydrocarbon liquid is more than the amount of co-produced water and, therefore, the generation of w/o emulsions may not be an issue. However, in some cases, water production may be high and therefore the study of varying fluid composition on anti-agglomerant performance becomes necessary. Most anti- agglomeration studies⁹' ²⁰' ^{21> 28} use a relatively low and constant water-cut. One study used variable water-cuts, as high as 80%, with some positive results. However, the crude oil used also contained the hydrate forming components so there was less hydrate formed at high water-cuts.²⁹

Alcohol co-surfactants are discussed in the literature. It is known that co-surfactants aid in micro-emulsion formation, by interacting with primary surfactant in the interfacial region and reducing oil/water interfacial tension.³⁰ There is also evidence of co-surfactant effects such as modifications in primary surfactant packing and head area, reduction in interfacial layer thickness, and variation in continuous-phase viscosity.³¹' ³² The effect of different alcohols has been studied and it is found that medium-chain alkanols may be the most efficient co-surfactants, yet the smaller chain species such as MeOH are also effective.³⁰' ³³

There are limited data showing that alcohol affects the performance of kinetic inhibitors. Two studies show contrasting effects of MeOH in mixtures with KHI: synergy can be concluded in one study¹³ while the other³⁴ shows a well-studied inhibitor, PVCap, is less effective in MeOH's presence. One theoretical study has examined the effect of MeOH as a co-solvent with very little effect on anti-agglomeration.³⁵ To the best of our knowledge, there has been no experimental examination of the effect of alcohol co- surfactants in hydrate anti-agglomeration.

There now is an interest in biosurfactants as anti-agglomerants for ice³⁶ and hydrates.³⁷' ³⁸ Biosurfactants are often superior to chemical surfactants because of: 1) higher biodegradability; 2) lower toxicity; and 3) safety.³⁹'⁴⁰ There are only two known reports of the use of biosurfactants in the hydrate literature.³⁷' ³⁸ In Ref. 37, 500 ppm of a Rhamnolipid surfactant exhibited AA ability, but only for one of the oils used in the study. Because of the presence of natural surfactants in the oil, results from Ref. 37 may not apply to hydrates in natural gas systems. In Ref. 38, Rhamnolipid at concentrations down to 0.05 wt. % was shown to be effective as AA in tetrahydrofuran (THF) mixtures with sufficient model oil, i.e., four parts by weight or higher.

Chemically, the major Rhamnolipid are glycosides of rhamnose (6-deoxymannose) and B-hydroxydecanoic acid. Rhamnolipids are known to reduce surface and interfacial tension⁴¹ and have been used to create stable micro-emulsions.⁴² Typical commercial products consist of both the mono-rhamnolipid and di-rhamnolipid forms and are generally more expensive than the chemical counterparts.

This work utilized a multiple screening-tube rocking apparatus to investigate; I) the effect of increasing the water-cut, and 2) the use of alcohol co- surfactants on hydrate anti- agglomeration. The influence of AA concentration, ΔT_op and residence time at ΔT_op are other variables of focus. Rhamnolipid biosurfactant and a quaternary ammonium salt (quat) are the AA, as used in our reference 38. Shut-in testing in which vials with hydrates are allowed to stand un-agitated for a given period, and emulsion stability tests are also included. Through these variables, model w/o emulsions are judged according to hydrate formation/dissociation temperatures and visual observations of agglomeration state after hydrate formation.

THF is used as the guest molecule, since it forms structure II hydrates at atmospheric pressure, the same type that forms in most pipelines.⁴³ There are differences between THF and real systems, but THF is still considered to be an adequate model system. THF may partition significantly between the aqueous and organic phases.²¹ Another major difference is THF is much more soluble in water than any species found in a typical natural gas mixture. THF and some gases, e.g., CO₂, may initiate hydrate in the bulk water phase.⁴⁴' ⁴⁶ However, some authors present data and show methane, methane-ethane, and methane-propane hydrates form at the water/oil interface.⁴⁶' ⁴⁷ Since surfactants will reside at or near the interface in any system, AA shown effective for THF systems may also be effective for systems where hydrate formation and growth occur at the interface.²¹

II. Experimental Methods

A. Apparatus The experimental setup, a multiple screening- tube rocking apparatus, is shown in

Figure 1. It consists of a motor-driven agitator, with a rack holding up to 20 separate borosilicate glass scintillation vials with dimensions of 17 (diameter) by 60 (height) mm, submerged in a temperature bath. Each vial holds roughly 7.4 mL of a test mixture and an approximately 8 mm diameter stainless steel 316 ball to aid agitation as well as for visual observations. A Teflon-lined plastic screw-cap is used along with Teflon tape around the threads to seal the vials. The rack rotates the vials 150° to either side of the vertical direction, completing a cycle every 5 seconds. The temperature bath used is a Huber CC2- 515 vpc filled with 10 cSt at 24 ⁰C with silicon oil from Clearco Products Co., Inc., Bensalem, PA. Thermocouples, with an accuracy of ± 0.2 ⁰C from 70⁰C down to -20⁰C, is attached to the outside of the vials when crystallization and melting data are desired.

An Agilent 34970A data acquisition unh, recording temperature every 20 seconds, and an ice bath as fixed junction reference temperature is used with all thermocouples.

Agglomeration state images are obtained with a - 169 mm rigid borescope, a Hawkeye Pro Hardy from Gradient Lens Corp., Rochester, NY, and a Nikon Coolpix 5400 digital camera with samples still in bath fluid.

B. Chemicals

In all test mixtures, deionized water, obtained from a Barnstead Nanopure Infinity system with quality of roughly 5.5 x 10² μs/cm, and 99.5%+ purity THF (from Acros) are used. The oil phase consists of 99% purity 2,2,4-trimethylpentane (i.e., isooc:ane, from Acros).

The following surfactants are used: rhamnolipid (product JBR 425) was obtained from Jeneil Biosurfactant Co., Madison, WI. ARQUAD 2C-75, dicetyl dimethyl ammonium chloride, was obtained from Akzo-Nobel. It consists of 75 wt.% surfactant in solvent consisting of water (at 5-10 wt. %) and isopropanol (at 15-20 wt.%). Both were used as supplied. All the above chemicals used are the same as discussed in our previous work.³⁸ As co- surfactant, 99.8 % anhydrous MeOH with less than 0.05 ppm water was obtained from Acros.

C. Procedure

The experimental procedures are the same as in our previous work.³⁸ A composition of mostly 1/1/2/x parts (x is for varying surfactant concentration in different tests) by weight of water/THF/isooctane/surfactant is employed. Stoichiometric ratio of 0.235/1 for THF/water may lead to ice formation from heterogeneous nucleation.⁴⁸ Higher concentration of THF avoids ice formation.⁴⁹

When MeOH used as a co-surfactant was effective in preventing agglomeration, a systematic series of tests were conducted to examine the limits of both MeOH and AA concentration required in anti-agglomeration. MeOH concentrations of 5, 2, 0.5, and 0.1 wt. % were employed in the study. Limited agglomeration state testing was conducted with zero or one part isooctane; in these cases, up to 10 wt. % MeOH and only 1.5 wt. % AA was employed to examine the effect on agglomeration. MeOH in the amount of 10 wt.% is not used extensively in this study because much lower concentrations prove effective. Also, the effect of methanol co-surfactant on mixtures of four parts isooctane and very low rhamnolipid concentration, concentrations that resulted in agglomeration in our previous work³⁸ were also tested for agglomeration state only.

Temperature data was acquired separate from visual observations, since half of the sample vial surface area is covered when thermocouples are attached. However, temperature control of the bath and agitation are the same for both types of experiments.

Kinetic/Thermodynamic Data Acquisition

Select mixtures with 1/1/2/x of water/THF/isooctane/surfactant parts by weight (some mixtures also include 5, 2, or 0.5 wt. % methanol) were tested in the following manner. A typical trend of measured temperature data, referred to as a freeze-thaw cycle, is shown in Figure 2. Mixtures are brought to 7 ⁰C, allowed to reach equilibrium, and then a 10 °C/hr cooling ramp is employed to -8 ⁰C. The temperature is then raised back to 7 ⁰C at 14°C/hr. 7 ⁰C is chosen because it was used previously.²¹ The hydrate equilibrium temperature for these mixtures is expected to be around 2-3 °C ²¹' ⁴³ and will likely be higher, i.e., around 4 ⁰C as shown below, after some THF partitions into the oil phase.

As a mixture is cooled below the equilibrium — or dissociation-temperature, an onset of hydrate crystallization occurs and an exothermic heat release begins. The temperature of this transition is the crystallization temperature, or T_c. After crystallization, the sample temperature rejoins that of the bath fluid. The time, t_c, that the mixture spends crystallizing is directly related to the growth rate. Dissociation of the mixture upon heating shows as an endotherm, the beginning of which is labeled the dissociation temperature, Td. The accuracies of individual T_c and T_d data are determined at ±0.2 and ±0.5 ⁰C, respectively. The accuracy of t_c data is ±1.0 minutes.

With 5 or 2 wt. % MeOH in solution, t_c and T_d data become less clear than in all other instances — as shown in Figure 3. In this case, t_c can still be discerned within ±1.0 minute accuracy even though the crystallization peak does not end as abruptly. However, the accuracy in determining Td suffers after addition of MeOH. In these cases, the accuracy is more like ±0.7 ⁰C, although the determination is easier when the plot is enlarged. When only 0.5% MeOH is added, the effect on accuracy of t_c and Td is less, yet more than when no MeOH is present, as shown in Figure 4. In these cases, the crystallization peak ends more gradually than is shown in Figure 2, but t_c may still be determined within ±1.0 minute accuracy. The determination of Td is also as accurate as when no MeOH is added, although it is clear by Figure 4 that there is still some effect of MeOH on dissociation data.

Each composition was prepared in triplicate and experiments were repeated five times per sample. Thus, each sample was reused for five consecutive experiments. Some tests were separated by periods of heating at 7 ⁰C for 20 minutes. In other cases, un-agitated samples were kept in the bath overnight as it gradually warmed to room temperature before proceeding to the next test. There is no difference between the results from the use of samples exposed to room temperature and to those limited to heating at 7 ⁰C. Data shown below is the average of fifteen separate experiments per composition.

Agglomeration State

Experiments for visual observations were performed similarly to crystallization/dissociation testing. Agitated mixtures are equilibrated at 7°C and then 10 °C/hr cooling is applied to bring the mixtures to a minimum temperature of -8, -12, -16, or -20 ⁰C. The procedure deviates here whereby the minimum temperature is held constant for 24 hours. Observations were obtained at 10 minutes, 1 hour, and 24 hours into this period. This was repeated twice for each of the triplicate mixtures for a given composition, separated by periods of heating at 25-30 ⁰C for 30 minutes. There is no dependence of our results on this heating temperature. Observations were made mainly with the naked eye but also with the borescope. These observations show whether a dispersion or slurry of hydrate crystals is created by the surfactant or agglomeration occurs.

Shut-in testing was also conducted. All mixtures were left un-agitated at the various ΔT_Or, for 60 minutes (this test was conducted at the termination of the cooling ramp when desired temperature was reached). Agitation was then resumed to assess whether hydrates are re-dispersed or irreversible agglomeration occurred.

Emulsion Stability

The procedure for emulsion stability testing is similar to that employed by Zanota, et al.²¹ A similar approach is used in other hydrate anti-agglomeration studies.²⁹ Select mixtures with 1/1/2/x of water/THF/isooctane/surfactant parts by weight (some mixtures also include 5, 2, or 0.5 wt. % methanol) were prepared without the stainless steel ball and homogenized by hand shaking for 1 minute; the rate and motion of this agitation was the same as would be imparted by the multiple screening- tube rocking apparatus. The fluid was transferred at room temperature to a graduated cylinder with a glass stopper and the time for separation of 60 volume % of the initial aqueous phase was measured and used as an indicator of emulsion stability. This procedure is referred to as "fresh" sample emulsion stability testing. Tests at each composition were repeated three times.

Alternative assessment of emulsion stability, referred to as "used" sample emulsion stability testing, was performed in the following manner. After a visual observation test was finished, mixtures were allowed to equilibrate at room temperature. Samples were removed from bath and agitator, after which hand-shaking was applied for 1 minute, using the same rate and motion as used with "fresh" samples. The mixtures were then monitored for phase separation, with an approximate indicator of the 60 volume % separation being marks made on the vial during mixing that also corresponds to the height of the steel ball. Tests at each composition were repeated three times. Note that one difference between the two types of tests is the presence of the steel ball in the latter. Also, 1 minute of agitation was enough time to fully emulsify in both types of tests.

III. Results

A. Agglomeration State

Preliminary agglomeration state testing began with a few mixtures of 1/1 /4/x parts by weight of water/THF/isooctane/rhamnolipid, as continuation of previous work.³⁸ As shown in that paper, these mixtures begin to show significant hydrate adhesion upon vial walls at rhamnolipid concentrations of 0.01 wt. % and below. It was considered that small amounts of MeOH co-surfactant might allow full hydrate slurries to persist at lower rhamnolipid concentrations. As shown in Figure 5, up to 5 wt. % of MeOH does not alleviate the adhesion problem. An example of such adhesion is shown in Figure 6. It appears MeOH does reduce the magnitude of adhesion, but it is still considered significant.

Data shown in Figure 5, and in all of the agglomeration state plots shown, represents the behavior of a given composition across the entire temperature range tested. In all cases, this behavior is consistent across all minimum temperatures, i.e., at -8, -12, - 16, and -20 ⁰C. Some differences do occur for rhamnolipid concentrations, in data for both four and two parts isooctane mixtures, exhibiting adhesion, where there is typically increased adhesion for lower temperatures tested.

Next the effect of increasing water-cut upon anti-agglomeration was studied. Mixtures exhibiting effective anti-agglomeration with four parts isooctane were used as a reference. Mixtures with zero isooctane and 1.5 wt. % of either AA formed plugs immediately at -8 ⁰C and below. The same mixtures, but with 5 and 10 wt. %, respectively, MeOH added, also result in plugging behavior at -8 ⁰C and below. The same outcome is found in mixtures of one part isooctane with 1.5 wt. % AA and 0, 5, or 10 wt.% MeOH. An example of plugs in these cases, where the plug occupies most or all of the vial volume, is shown in Figure 7. The 10 wt. % MeOH is not used extensively, but only as a probe to determine if higher concentrations of MeOH, beyond 5 wt. %, would affect anti-agglomeration. It was determined that higher concentrations may not provide additional benefit.

When mixtures of two parts isooctane were tested, the true effectiveness of MeOH co-surfactant was discovered. Figure 8 shows the results of such tests with Rhamnolipid as AA. Plugs, either fully blocked or partially blocked such that the steel ball was blocked from moving across the entire length of the vial, were still observed when up to 1.5 wt. % Rhamnolipid was added without MeOH. However, when just 0.5 wt. % MeOH is added to these mixtures, flowable hydrate slurries are formed. The same is seen with up to 5 wt. % MeOH. An example of such slurries is shown in Figure 9. A significant difference exists over the slurries seen in our previous work³⁸ due to increased hydrate volume present in these samples. However, there is some agglomeration when a very low concentration of 0.1 wt. % MeOH co-surfactant is used.

Similar results are observed in mixtures with the quat AA. Figure 10 shows the results in these mixtures: AA alone does not produce slurries, but as little as 0.5 wt. % MeOH co-surfactant does. However, a 0.1 wt. % MeOH concentration does not result in anti-agglomeration, and so at least 0.5 wt.% MeOH is believed to be necessary to obtain the results of the invention.

All mixtures exhibiting stable hydrate slurries passed shut-in testing. These results are for all minimum temperatures. That is, mixtures that initially contain hydrate slurries at any minimum temperature are able to have the hydrate re-dispersed after agitation is paused for 60 minutes and then resumed.

Kinetic/Thermodynamic Characteristics Figures 11-14 show results of freeze-thaw cycles for select mixtures of both rhamnolipid and quat with 5, 2, and 0.5 wt. % MeOH, as well as mixtures without AA and/or MeOH. For both surfactants, AA was used in the amounts of 1.5, 0.5, and 0.05 wt. %. The difference between the dissociation temperature Td and the crystallization temperature T_c is the onset subcooling denoted by ΔT_on,.

B. Emulsion Stability

Table 1 and Table 2 provide emulsion stability results for quat and rhamnolipid, respectively, along with standard deviations. Mixtures of select AA concentration with 5, 2, 0.5, and 0 wt. % MeOH were tested using both "fresh" and "used" samples. For both surfactants, AA was used in the amounts of 1.5, 0.5, and 0.05 wt. %. Average and standard deviations are given to the nearest 0.1 minute, due to the relative instability of most compositions tested.

Table 1 - Quat-Induced Emulsion Stability Results (values in minutes)

Table 2 - Rhamnolipid-Induced Emulsion Stability Results (values in minutes)

Rhamnolipid Concentration (wt. %)

1 .5 0 .5 0. 35

Methanol (wt. % ) Fresh³ Used* Fresh Used Fresh Used

0 0.4 ± 0.2 2.3 ± 0.6 0.5 ± 0.0 2.1 ± 0.8 0.2 ± 0 1 0.3 ± 0.3

0.5 0.4 ± 0.1 1.3 ± 0.6 0.5 ± 0.1 0.5 ± 0.0 0.5 ± 0 3 0.1 ± 0.1

2 0.8 ± 0.1 2.3 ± 0.6 0.6 ± 0.1 1.2 ± 0.3 0.2 ± 0.0 0.2 ± 0.1

5 0.5 ± 0.2 3.0 ± 0.9 0.5 ± 0.2 3.1 ± 1.6 0.4 ± 0 1 0.1 ± 0.0

a - Procedure used by Zanota, et al.²¹ A fresh sample-without stainless steel ball- is hand-agitated for 1 minute and transferred at room temperature to a graduated cylinder and the time for separation of 60 volume % of the initial aqueous phase is measured and used as indicator of emulsion stability.

b - Samples used for visual observation and freeze-thaw cycles with stainless steel included-are hand-agitated at room temperature for 1 minute and the time for separation of 60 volume % of the initial aqueous phase is measured and used as indicator of emulsion stability.

IV. Discussion

A. Agglomeration State

Regarding the effectiveness of MeOH as a co- surfactant, low concentration is highly desirable. This is analogous to the LDHI concept, in that inhibitors effective at low concentrations should be used to reduce costs and other impact. Thermodynamic inhibitors, especially MeOH, may give rise to salt precipitation in petroleum fluid mixtures⁵⁰, and so it is crucial in this respect as well to be able to identify low concentrations at which MeOH co-surfactant will be effective.

When mixtures of zero part isooctane were tested, one sample with 1.5 wt. % rhamnolipid and 10 wt. % MeOH exhibited slurry consistency for a few minutes at -8 ⁰C. In these samples, visual inspection was performed immediately at the minimum temperature in addition to other times mentioned in the procedure. It soon appeared that the steel ball was partially blocked in the vial, although the appearance of the dispersion remained the same, i.e., it did not look like a plug, but rather still appeared as a highly concentrated hydrate slurry. This is shown in Figure 15.

It was thought that the initial blockage in this case was due to the shear volume of hydrate packed to the extent that the steel ball can not pass. All other samples of this composition, even for 1.5 wt. % quat, never exhibited slurry behavior at any temperature but the blockages appeared the same; occasionally, the steel ball was unable to move so full plugging does occur. The highly viscous nature of the hydrate slurry in this case appears to play a major role in such blockages. These observations have been made elsewhere.²⁷ Surprisingly, mixtures of one part isooctane always appear clearly as a full hydrate plug, even when 10 wt. % MeOH is added. The same can be said for mixtures of two parts isooctane showing plugging tendency.

The results showing plugging tendency in mixtures of zero or one part isooctane are expected due to the process by which anti-agglomeration must likely proceed. If no emulsion is created in the mixture, i.e., in the case with no oil, then hydrate anti- agglomeration may not be sustained. Additionally, hydrate slurries may not be sustainable if insufficient oil is present, perhaps owing to oil-in-water emulsion formation or simply the inability for all hydrate formed to be dispersed within the oil phase. Work has been reported on anti-agglomeration in pure water with NaCl as well as low subcooling, i.e., a few degrees or less. Under such conditions dispersion of crystallites in water are observed.³⁶' ⁵¹' ⁵² These limitations are exacted in such studies to control crystal growth. The point of these studies is to create transportable ice slurries for latent heat storage. In a hydrate system, this is not an option simply due to the impossibility of controlling conditions in the vicinity of a pipeline or well. Care must be taken to limit hydrate slurry volume and viscosity.

Water-cuts well below 50 % may still cause agglomeration in these model mixtures. Mixtures with one part isooctane contain a water-cut of roughly 69 % and so it is truly expected for blockages to form in them. However, mixtures of two parts isooctane contain roughly 34 % water-cut and results still show plug formation unless 0.5 wt. % or more MeOH co-surfactant is added. Since water-cut limitations will be different in real fluids, all that can be concluded from these observations is that more than water-in-oil emulsification and a 50% water-cut limit may be required for effective anti- agglomeration.

Small amounts of MeOH co-surfactant enables anti-agglomeration to occur. In low amounts, MeOH does not lead to salt deposition.⁵⁰ It is also thought that MeOH will be present mostly in the bulk water phase and the aqueous-side of the interfacial region in these mixtures. Thus, it appears that MeOH co-surfactant will be effective above a specific minimum concentration, and higher MeOH concentration will not be required. This concentration is believed to be around 0.5 wt .% or slightly less. Other alcohols may also be used as co-surfactants.

According to Figure 8, the presence of MeOH co-surfactant does aid anti- agglomeration via Rhamnolipid, down to very low Rhamnolipid concentrations. At 0.1 wt.% Rhamnolipid, significant adhesion occurs no matter how much MeOH is added. At 0.5 wt.% Rhamnolipid, slurries exist in mixtures down to 0.5 wt. % MeOH. It was desired to determine if any Rhamnolipid concentrations between these two values would also facilitate slurries. Thus, 0.25 wt.% Rhamnolipid was also tested and it was found that slurries are facilitated by this amount of surfactant specifically when 2 to 5 wt. % MeOH co-surfactant is added. 0.5 wt. % MeOH mixtures with 0.25 wt. % Rhamnolipid show a tendency to allow significant hydrate adhesion upon vial walls, defining a lower limit for these concentrations.

On the other hand, the quat is effective at anti-agglomeration over all the concentrations studied, i.e., down to 0.01 wt. %. A similar behavior is reported in our previous work and seems to indicate that the quat is effective at inducing steric repulsion between hydrate crystallites as well as in hydrate-wall interactions, whereas Rhamnolipid may only be effective at both classes of repulsion when present in sufficient amount. The quat solution contains 15-20 wt. % isopropanol, but it is assumed this does not play a co- surfactant role at the lower concentrations. For example, when 0.5 wt.% quat is added in mixtures of two parts isooctane, only about 0.05 wt. %, with respect to the total water amount of isopropanol, is present. When 0.01 wt. % quat is added, this amounts to only about 2.3 xlO^"3 wt. % of isopropanol in the water phase.

B. Kinetic/Thermodynamic Characteristics

In Figure 11 and Figure 13, a slight trend downward in T₀ values, i.e., a slight kinetic inhibition, is seen when surfactant is added in various amounts. This also means that ΔT_0M increases when AA is added especially at lower concentrations. Figure 11 shows clearly this trend for Rhamnolipid mixtures; the same but less clear trend is seen in quat mixtures as given in Figure 13. The same trend has been observed in our previous work³⁸ yet the differences are not significant due to the scatter in the data. Another noticeable similarity between the current results, without MeOH, and our earlier work is that t_c values decrease when rhamnolipid is added, yet this parameter increases when the quat is used. For all mixtures tested in this work, t_c values are generally larger than the values in mixtures of four parts isooctane due to the increased amount of hydrate being formed when water-cut is larger. Figures 11 and 13 reveal a decrease in dissociation temperature with an increase in concentration of AA, as expected.

The effect of MeOH co-surfactant as shown in Figure 12 and Figure 14 is explained as follows. The effect of MeOH on ^appears in Figure 12 as a clear trend in mixtures with rhamnolipid. That is, MeOH as a thermodynamic inhibitor depresses the equilibrium temperature and T_c will also be reduced since it is generally less than T_d through hysteresis, and the effect should be greater at higher concentrations. Taken in light of the scatter in the data, this trend is seen in Figure 12. This trend is not as clear for quat mixtures shown in Figure 14, but in general Rvalues are lower when more MeOH is added. In both mixture types, values are in the general range expected by both knowledge of thermodynamic inhibitors as well as from the only known published data⁵ on the effect of MeOH on THF hydrate equilibrium temperatures.

The effect of MeOH co-surfactant on AT_0n is unclear in either mixture type, largely due to the amount of scatter seen in the data. In rhamnolipid (Rh) mixtures, the data show increased ΔT_OT values for the extremes in composition, i.e., for high Rh/high MeOH and low Rh/low MeOH concentration, where high MeOH here simply means the higher concentration of 5 wt. %. However, Figure 12 clearly shows that a relatively high ΔT_0« value also occurs for concentration of 0.5 % Rh/2% MeOH. Figure 14 shows clearly a relatively large ΔT_on for composition of high quat/high MeOH; yet, all other compositions exhibit a slim margin between Td and T_c values.

The MeOH co-surfactant does not appear to alter t_c values significantly. As seen in Figure 3 and Figure 4, the crystallization peaks in presence of MeOH are generally broader so this is likely offsetting the affect of increased driving force, i.e. supersaturation,⁵¹ at lower T₀. Only in about half the cases does the data show thai addition of MeOH co-surfactant increases t_c.

C. Emulsion Stability Emulsion stability results in Table 1 and Table 2 reveal the same effect in used samples, those that have undergone freeze-thaw cycling, as seen in our previous work.³⁸ However, these emulsions are mostly unstable, so the difference between the two test types is small. Stable emulsions in real pipeline fluids are undesirable²⁵' ²⁶ and therefore low stability values are acceptable. In general, the differences between the two tests are more significant for higher amounts of AA and MeOH co-surfactant. For fresh mixtures, there is little or no difference between stabilities of rhamnolipid or quat mixtures, with or without MeOH. There is some difference between rhamnolipid and quat mixtures for the used samples, but only at 1.5 and 0.5 wt. % AA. There appears to be no effect of MeOH co-surfactant on emulsion stability in fresh samples of either AA. The effect of MeOH is only noticeable in the used quat samples, with stability values jumping significantly upon addition of 5 wt. % MeOH for all concentrations of quat. For 1.5 and 0.5 wt. % quat, there is a downward trend in stability for lower MeOH concentrations; for 0.05 wt. % quat, stability values jump to roughly 7 minutes upon addition of 5 wt. % MeOH but drop to normally unstable values, on the order of 0.5 minutes or less, for smaller concentrations of MeOH. In used rhamnolipid samples, there are only slightly higher stabilities for higher MeOH amounts. For both AA mixtures with any amount of MeOH, it is clear from these results that stable emulsions are not necessary for effective anti-agglomeration to occur.

V. Conclusions

Small amounts of alcohol co-surfactant have a significant effect on hydrate anti- agglomeration . MeOH is an effective co-surfactant as established through visual observations with a multiple screening-tube rocking apparatus using high operating subcooling and residence time as indicators of performance. Shut-in and emulsion stability tests also lend supporting evidence.

Several important conclusions can be drawn. Rhamnolipid biosurfactants are attractive AA candidates and stable water-in-oil emulsions are not required for an effective AA. It is also shown that AA may be ettective at concentrations below what the conventional limit was thought to be, i.e., 0.5 wt. %. Perhaps the most significant conclusion is that alcohol co-surfactants may be effective at low enough . concentrations so that side-effects such as salt deposition can be avoided.

PART H-CYCLOPENTANE HYDRATE TESTING

Experimental Section

Apparatus. The experimental setup used in this work is similar to the one discussed above for THF hydrate anti-agglomeration, that is, the setup is a multiple screening-tube rocking apparatus which consists of a motor-driven agitator with rack holding up to 20 separate borosilicate glass scintillation vials, all as described above.

Thermocouples with accuracy of ± 0.2 ⁰C from 70 ⁰C down to -20 ⁰C are attached to the outside of the vials when crystallization and melting data are desired. The thermocouples are attached to the outside wall of the vials, which are ideal for sample preparation and containment. An Agilent 34970A data acquisition unit recording temperature every 20 seconds and ice bath as fixed junction reference temperature is used with all thermocouples. A sketch of the apparatus is shown in Figure 1.

Chemicals. In all test mixtures, deionized water obtained from a Barnstead Nanopure Infinity system with quality of roughly 5.5 x 10^~2 μS/cm and 99%+ purity Cyclopentane (from Acros) (CP) are used. CP serves both as the oil phase and the hydrate former. The solubility of CP in water is very low, about 160 ppm by weight at 25 ⁰C and is nearly constant in the temperature range of 0 - 25 ⁰C.

Rhamnolipid biosurfactant (product JBR 425) (Rh) was obtained from Jeneil Biosurfactant Co., Madison, WI. It is a mixture of two forms at 25 wt. % in water. Rh was used as supplied and is the same as discussed above. The cosurfactant is 99.8% anhydrous MeOH with less than 0.05 ppm water, obtained from Acros.

Procedure. The experimental procedures are the same as discussed above. A composition of mostly x/l/0.02/y parts by weight of CP/water/THF/surfactant is used in our tests, where x is a varying amount of CP and y is varying surfactant concentration in different tests. In the tests with the cosurfactant MeOH, the mixture composition is x/l/0.22/y/z where z is the weight ratio of MeOH to water. Each sample is prepared in duplicate. (Exceptions are those data points without error bar.)

Temperature data was acquired separately from visual observations, since half of the sample vial surface area is covered when thermocouples are attached. However, temperature control of the bath and agitation are the same for both types of experiments.

Kinetic/Thermodynamic Data Acquisition.

Mixtures are brought to 11°C. This temperature is higher than the reported equilibrium temperature of CP hydrates at one atmospheric pressure, of 7 °c.^{57> 58} We allow the liquid mixture to reach equilibrium, and then a 5 °C/h cooling ramp is employed to a desired temperature for a specific sample where hydrates form without ice. The mixture is then heated back to 15 ⁰C at 15 °C/h. As the mixture is cooled below the equilibrium temperature, it crystallizes at the temperature (T₀) and an exothermic heat release begins. Once crystallization has occurred, the sample temperature rejoins that of the bath fluid. Dissociation of the mixture during heating shows as an endotherm, the beginning of which is labeled as the dissociation temperature, Td. This is the same as the equilibrium hydrate temperature.

Agglomeration States.

Experiments for gathering visual observations are conducted similarly to crystallization/dissociation testing. Agitated mixtures are allowed to equilibrate at 11 ⁰C and then 5 °C/h cooling is applied to bring the mixtures to a temperature where CP hydrates are formed without the formation of ice. The procedure deviates from kinetic/thermodynamic test whereby the minimum temperature is held constant. (Some samples were then heated to 1.5 ⁰C for ease of observation.) Observations were made at 2 hours and at 24 hours into this period, mainly with the naked eye but also with the borescope. These observations show whether a dispersion of hydrate crystals is facilitated by the surfactant, surfactant/cosurfactant or agglomeration occurs.

Emulsion Stability.

Mixtures of x/l/0.02/y and x/l/0.02/y/z of CP/water/THF/surfactant and CP/water/THF/surfactant/cosurfactant, by weight are prepared and homogenized by shaking by hand for 1 minute. The time it takes for 60 vol % of the initial aqueous phase to separate is measured and used as an indicator of emulsion stability.

Results and discussion Formation of CP hydrates. Attempts were made to obtain CP hydrates in mixtures of CP and water. It was found that no crystallites (CP hydrates) are formed as there was no exothermic peak, when cooling the sample to a temperature above 0 ⁰C. When cooling the sample to -2⁰C or below, ice was formed, indicated by the endothermic peak starting at 0 ⁰C. Whitman et al. reported that for mixtures of cyclopentane and water either as water-in-oil or oil-in- water emulsion, hydrate and ice always form simultaneously with ice forming preferentially.⁵⁸ Some authors have shown that methane and other gases, which can be incorporated into the smaller cavities of the CP hydrate at modest pressures, can serve as a helper molecule in the formation of CP hydrates.⁵⁹' ⁶⁰ The effect of THF in CP hydrates formation.

Helper molecules such as methane are used only under high pressure because of the solubility of these gases in water. THF was employed as a helper molecule because of its high solubility in water at atmospheric pressure. As can be seen in Figure 16, CP hydrates are formed by cooling samples of CP/water/THF with composition of 0.4/1/x with x=0.01, 0.03, and 0.05 to a temperature of 0.5⁰C (where ice does not form). The results clearly indicate that the presence of THF as a helper molecule give rise to hydrate formation. The dissociation temperature is around 7.0 °C in agreement with data from Refs. 57 and 58. The results reveal that THF does not measurably affect the dissociation temperature of CP hydrates. At higher concentrations than used, THF concentration may affect dissociation temperature or possibly THF hydrates may form.

The effect of Rh in CP hydrates formation.

Rhamnolipid (Rh) was added to the above samples to study the anti- agglomerant(AA) effect in concentration range of 0.001 to 0.05. The addition of Rh suppressed the CP hydrate formation temperature. No hydrates formed above 0⁰C, with Rh presence even in small concentration (as small as 0.001). The samples were cooled below 0 ⁰C to form hydrates.

Figure 17 shows the freeze-thaw cycle data for a sample of CP/H₂0/THF/Rh with composition of 0.4/1/0.03/0.01. Due to 3% fHF and 1% Rh in the mixture, no ice forms to a temperature of -4 ⁰C. For a conclusive study of anti-agglomeration of hydrates, the formation of ice should be avoided. In this experiment the weight ratio of CP to water is 0.4 : 1, and the molar ratio is 1 :10 which is higher than the hydrate stoichiometric molar ratio of 1 :17.⁴⁸ Figure 17 shows that the hydrate formation with surfactant Rh is accompanied by a high growth rate as compared to Figure 16.

The addition of Rh suppresses both the crystallization temperature and dissociation temperature. Figure 18 shows that the crystallization and dissociation temperatures both decrease as the concentration of Rh increases in the mixture. The decrease in dissociation temperature can be explained from bulk-phase thermodynamics. The lower crystallization temperature is from the need for driving force in hydrate formation which is a function of the cooling rate. The effect of CP concentration on hydrates formation.

The amount of CP in the mixture affects the ratio of hydrates to the sample volume. The total volume of the mixture is fixed at 7 mL. For a CPZH₂O ratio of 0.4/1 more hydrates form followed by a ratio of 2/1 and then 4/1. However, dissociation temperature data in Figure 19 clearly show that CP amount does not affect the dissociation temperature as expected.

The effect of MeOH on CP hydrates formation.

The addition of MeOH suppresses the dissociation temperature of CP hydrates. The data in Figure 20 reveal a decrease in dissociation temperature with increasing concentration of MeOH. There is a synergistic effect of Rh and MeOH in reduction of Td- The increase of Rh from 0.001 to 0.005 suppresses T_d by 0.54 ⁰C; the addition of MeOH by 0.01 lowers Td by 1.85 ⁰C. The combined effect of concentration increase of Rh from 0.001 to 0.005, and the addition of MeOH by 0.01 lowers T_d by 3.45 ⁰C, which is greater than the sum of the contribution from the increase in Rh and in MeOH when added individually by about 1°C.

Agglomeration state.

The purpose here is to confirm the anti-agglomeration (AA) effectiveness of the inventive composition with cyclopentane hydrate particles. The agglomeration state was determined by testing mixtures Of CPm₂OZTHFZRh of compositions 4ZlZ0.02Zx and 4ZlZ0.03Zx by weight. As Figure 21 shows, dispersible hydrates are formed with a low concentration of Rh. The samples with 0.02 THF were cooled to -2 ⁰C then kept at 1.5 ⁰C for AA state observation, while the samples with 0.03 THF were cooled to -3 ⁰C then kept at 1.5 ⁰C for AA state observation. There was no ice formation in the tests. Stable dispersion is observed for samples with Rh concentrations of 0.003 to 0.03 during the observation period of to 24 hours. However, for lower and higher concentrations of Rh, there is change in performance. At -2 ⁰C, the mixtures with Rh concentrations of 0.001 and 0.002 exhibit agglomeration upon the vial before 24 hours. When the concentration of Rh is 0.05, there seemed to be a plug when the samples were kept at -3 ⁰C for 24 hours. The hydrate plug would immediately disperse into stable dispersion when heavy shaking was applied. The "fake plug" may be due to the high viscosity of the solution for Rh concentration of 0.05. By shaking, i.e., by increasing the shear force, the high viscosity is overcome and there is a stable dispersion. By comparing the temperature at which the mixture was kept for 24 hours and the dissociations temperature shown in Figure 22, it can be seem that the anti-agglomeration effect of some these samples was tested at a subcooling of about 8 to 10 ⁰C.

In the above experiments related to hydrate anti-agglomeration in THF hydrates and the from the literature, it is known that the anti-agglomeration process becomes ineffective at a high water cut, and the following examines the effect of water cut on anti- agglomeration.

For samples with 2 parts of CP, stable dispersion forms at -3 ⁰C (for samples with

0.01-0.05 part of Rh) and -2 ⁰C (for samples with 0.003-0.005) when the concentration of Rh is 0.003 to 0.01 without MeOH. When the Rh concentration is high, in the range of 0.03 to 0.05, the AA effectiveness decreases probably due to the high viscosity effect. Plugs, either full or partial, appear when the Rh concentration is lower than 0.003.

As discussed above, the addition of small quantities of MeOH as a cosurfactant may prevent agglomeration of THF hydrates. We confirm here that small quantities of MeOH can be effective when added to samples with 2 parts of CP. Figure 23 demonstrates that MeOH at a concentration of 0.005 is very effective as a cosurfactant in anti-agglomeration where hydrate slurries are formed at -2 C. The slurries are also formed with 0.01 MeOH. Note that there is agglomeration of hydrates at a low MeOH concentration of 0.002. The dissociation data in Figure 24a and 24b and the temperature of -2 ⁰C where the hydrates are kept for a period of 24 hours show anti-agglomeration for a subcooling of about 6 to 9 ⁰C.

In Figure 25, it is shown that the vial wall and bottom are transparent with no crystallites adhered to the walls. The air bubble can travel freely in the vial, indicating there is no agglomeration.

The effectiveness of MeOH cosurfactant is further confirmed when MeOH is added to samples with 1.5 parts of CP, i.e. even higher water cut. Figure 26 shows the results. Plugs are formed to a Rh concentration of 0.01 when there is no MeOH. When the concentration of MeOH is only 0.005, hydrate slurries are formed at Rh concentrations of 0.005 and 0.01. When the concentration of Rh is 0.003, a MeOH concentration of 0.01 is required to form stable dispersion. In mixtures of 2 parts and 1.5 parts CP, a MeOH concentration of 0.002 MeOH is not effective in anti-agglomeration. The results in Fig 26 correspond to a subcooling of about of about 6 to 10 ⁰C. Experiments were conducted for mixtures in which the concentrations of CP are 1 and 0.4 (CP weight to water weight, water cut more than 50%), hydrate agglomeration appeared in all samples regardless of the concentration of Rh and MeOH. Figure 27 shows an example where there is hydrate agglomeration for a mixture containing 1.5 part CP and 1 part water.

Emulsion Stability.

In the past, emulsion stability in hydrate anti-agglomeration has been suggested to be very important.²¹ As discussed above, emulsion stability may not be critical when using the inventive composition. Using the methodology discussed above to measure emulsion stability, the time it takes to form 60% of water to separate in the mixture was determined. Table 3 gives average emulsion stability results with two duplicate tests. The weight ratio of CP to water in the mixtures was 1, 1.5 and 4 parts, with Rh at 0.001, 0.002, 0.003 0.005 and 0.01. For samples with 1.5 CP ratio to water, methanol concentration was 0, 0.002, 0.005. As can be seen from the Table, the Rh concentration increases emulsion stability in all mixtures. The CP concentration also increases emulsion stability. Addition of methanol generally increases emulsion stability, but the effect is not significant.

A fresh sample is hand-agitated for 1 min and the time for separation of 60 vol % of the initial aqueous phase is measured and used as an indicator of emulsion stability. Conclusions

The above confirms that agglomeration in cyclopentane hydrates is similar to THF hydrates, and that small amount of MeOH cosurfactant in a mixture with Rh has a significant effect on anti- agglomeration between hydrate particles. MeOH can lower the concentration of anti-agglomerant Rh in some of the mixtures. In other mixtures, MeOH results in anti-agglomeration when the concentration increase of biosurfactant does not lead to anti-agglomeration. A small amount of MeOH is sufficient to prevent agglomeration because of the cosurfactant nature of the alcohol. Also, the addition of THF as a helper molecule in the formation of cyclopentane hydrates does not affect the equilibrium hydrate temperature within the range of 1 to 5 wt % of water. In working with THF, a large subcooling (as high as 25⁰C) could be imposed without ice formation. With cyclopentane, the formation of ice allows a subcooling of about 10°C. However, the fact that within a period of 24 hours, a hydrate slurry can be maintained is believed to imply effectiveness at high subcooling.

The present invention thus provides an anti-agglomeration composition for gas hydrates, effective at high subcooling temperatures which contains a combination of a surfactant and an alcohol cosurfactant. Moreover, the anti-agglomeration composition is effective using relatively low amounts of both the surfactant and co-surfactant to limit environmental effects as well as to reduce the cost of separation in downstream operations. The anti-agglomeration composition of the invention also does not require a stable water in oil emulsion to provide the beneficial anti-agglomeration effects. Preferable, the composition comprises a surfactant and an alcohol co-surfactant provided in effective amounts sufficient to cause anti-agglomeration of hydrates, which is particularly useful where high subcooling temperatures and/or at high water-cuts occur.

Preferably, the surfactant is a Rhamnolipid biosurfactant and the alcohol cosurfactant is methanol, the alcohol cosurfactant being present at concentrations low enough such that side effects such as salt deposition are avoided. Generally, the alcohol cosurfactant should be present at from 0.05-5% wt., with the surfactant present at from 0.001 to 10% wt., more preferably, 0.01 to 5% wt. Using the present invention, agglomeration of hydrates in gas and oil pipelines can be reduced using low levels of the inventive composition, which is also effective at relatively high water cuts, and at relatively high subcooling temperatures. REFERENCES

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Claims

1. An anti-agglomeration composition comprising a surfactant and an alcohol co- surfactant provided in effective amounts to cause anti-agglomeration of hydrates formed in a fluid comprising water and hydrocarbons under conditions in which hydrates ordinarily form.

2. The anti-agglomeration composition of claim 1 wherein the surfactant is a Rhamnolipid biosurfactant.

3. The anti-agglomeration composition of claim 1 wherein the alcohol cosurfactant is methanol.

4. The anti-agglomeration composition of claim 1 wherein the alcohol cosurfactant is present at a concentration low enough such that salt deposition does not occur.

5. The anti-agglomeration composition of claim 1 wherein the alcohol cosurfactant is present at from 0.05-5% wt, and the surfactant is present at from 0.01 to 10 % wt, more preferably 0.01-5% wt.

6. The anti-agglomeration composition of claim 1 wherein Using the present invention, agglomeration of hydrates in gas and oil pipelines can be reduced using low levels of the inventive composition, also being effective at relatively high water cuts, and at relatively high subcooling temperatures.

7. The anti-agglomeration composition of claim 1 wherein the surfactant is a

Rhamnolipid biosurfactant present at 0.5 %wt, and the alcohol cosurfactant is MeOH present at 0.5 wt. % MeOH.

8. The anti-agglomeration composition of claim 1 wherein the surfactant is a Rhamnolipid biosurfactant present at 0.25 wt.% Rhamnolipid and the alcohol cosurfactant is MeOH present at from 2 to 5 wt. % MeOH is added.

9. A method for inhibiting agglomeration of hydrates in a fluid comprising water and hydrocarbons under conditions at which hydrates ordinarily form from water and hydrocarbons, the method comprising: adding to the fluid an anti-agglomeration composition comprising a surfactant and an alcohol co-surfactant each provided in effective amounts to cause anti-agglomeration of the hydrates formed in the fluid comprising water and hydrocarbons under conditions in which hydrates ordinarily form to prevent a plug from forming.

10. The method of claim 9 wherein the surfactant is a Rhamnolipid biosurfactant.

11. The method of claim 9 the alcohol cosurfactant is methanol.

12. The method of claim 9 wherein the alcohol cosurfactant is present at a concentration low enough such that salt deposition does not occur.

13. The method of claim 9 wherein the alcohol cosurfactant is present at from 0.05- 5% wt., and the surfactant is present at from 0.01 to 10% wt, more preferably 0.01-5% wt.

14. The method of claim 9 wherein the fluid is gas or oil and further comprising transporting the gas or oil through a pipeline.

15. The method of claim 9 wherein the composition is effective at a relatively high water cut, and at a relatively high subcooiing temperature.

16. The method of claim 9 wherein the surfactant is a Rhamnolipid biosurfactant present at 0.5 %wt, and the alcohol cosurfactant is MeOH present at 0.5 wt. % MeOH.

17. The method of claim 9 wherein the surfactant is a Rhamnolipid biosurfactant present at 0.25 wt.% Rhamnolipid and the alcohol cosurfactant is MeOH present at from 2 to 5 wt. % MeOH is added.