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Title:
POLYMERS FOR USE AS HYDRATE INHIBITORS
Document Type and Number:
WIPO Patent Application WO/1996/041834
Kind Code:
A1
Abstract:
The instant invention relates to novel copolymers of N-vinyl amides and acrylamides and N-allyl amides and acrylamides. The polymers are used for inhibiting the formation of clathrate hydrates in a fluid having hydrate forming constituents. The polymers can also be used in treating a petroleum fluid stream such as natural gas conveyed in a pipe to inhibit the formation of a hydrate restriction in the pipe.

Inventors:
Colle
Karla
Schall, Costello
Christine
Ann, Berluche
Enock, Oelfke
Russell
Harlan, Talley
Larry
Dalton
Application Number:
PCT/US1996/009725
Publication Date:
December 27, 1996
Filing Date:
June 07, 1996
Export Citation:
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Assignee:
EXXON PRODUCTION RESEARCH COMPANY COLLE
Karla
Schall, Costello
Christine
Ann, Berluche
Enock, Oelfke
Russell
Harlan, Talley
Larry
Dalton
International Classes:
F17D1/00; C07C7/20; C08F26/02; C08F220/10; C08F220/56; C08F222/40; C09K8/52; C09K8/524; C10L3/10; E21B37/06; F17D1/05; (IPC1-7): C08L39/02; C08F226/02
Foreign References:
US4921621A
US4533708A
US4973775A
US5420370A
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Claims:
We claim :
1. A composition comprising a polymer having the following units: where, Rx is a hydrogen or hydrocarbon group having 1 to 6 carbon atoms, R2 is a hydrocarbon group having 1 to 6 carbon atoms and RL and R2 have a sum total of carbon atoms greater than or equal to 1, but less than 8; R3 is a hydrogen or hydrocarbon group having 1 to 3 carbon atoms; R4 is a hydrogen or hydrocarbon group having 1 to 6 carbon atoms and R5 is a hydrocarbon group having 1 to 6 carbon atoms; x+y is an average number of units for producing an average molecular weight for said polymer between about 1,000 and about 6,000,000; and z is either 0 or 1.
2. The polymer of claim 1 wherein R4 and R5 may be linked to form a cyclic group with the nitrogen atom to which they are attached, said cyclic group having a sum total of at least 2 but less than 8 carbon atoms.
3. The polymer of claim 2 wherein R3 is a hydrogen or a methyl group.
4. The polymer of claim 2 wherein said sum total of carbon atoms is greater than or equal to 4, but less than 8.
5. The polymer of claims 2, 3 or 4 wherein said cyclic group has 1 to 2 heteroatoms selected from the group consisting of oxygen and nitrogen and combinations thereof.
6. The polymer of claim 1 wherein Rx is said hydrogen or an alkyl, cycloalkyl, or aryl group having 0 to 2 heteroatoms selected from the group consisting of oxygen and nitrogen and combinations thereof, and R2 is an alkyl, cycloalkyl, or aryl group having 0 to 2 heteroatoms selected from the group consisting of oxygen and nitrogen and combinations thereof.
7. The polymer of claim 1 wherein R4 is said hydrogen or an alkyl, cycloalkyl, or aryl group having 0 to 2 heteroatoms selected from the group consisting of oxygen and nitrogen and combinations thereof, and R5 is an alkyl, cycloalkyl, or aryl group having 0 to 2 heteroatoms selected from the group consisting of oxygen and nitrogen and combinations thereof.
8. The polymer of claim 1, 6 or 7 wherein R3 is a hydrogen or a methyl group.
9. The composition of any one of claims 18 inclusive further comprising an additive, said additive comprising a surfactant, at least a second polymer, or a solvent and combinations thereof.
10. The polymer composition of claim 9 wherein the polymer additive is selected from the group consisting of poly(vinylpyrrolidone) , poly(vinylcaprolactam) , poly(NvinylNmethylacetamide) , a copolymer of Nvinyl Nmethylacetamide and isopropylmethacrylamide, a copolymer of NvinylNmethylacetamide and acryloylpiperidine, a copolymer of NmethylN vinylacetamide and methacryloylpyrrolidine, a copolymer of NmethylNvinylacetamide and acryloylpiperidine, and combinations thereof and said solvent additive is selected from the group consisting of water, brine, sea water, produced water, methanol, ethanol, propanol, isopropanol, glycol, and mixtures thereof.
11. A composition comprising a polymer having the following units : where, R is a hydrogen or hydrocarbon group having 1 to 6 carbon atoms, R2 is a hydrocarbon group having 1 to 6 carbon atoms and R^ and R2 have a sum total of carbon atoms greater than or equal to 1, but less than 8; R3 is a hydrogen or hydrocarbon group having 1 to 3 carbon atoms; R4 is a hydrogen or hydrocarbon group having 1 to 6 carbon atoms and R5 is a hydrocarbon group having 1 to 6 carbon atoms; x+y is an average number of units for producing an average molecular weight for said polymer between about 1,000 and about 6,000,000; and z is either 0 or 1; whereby an alkyl R3 group having 1 to 3 carbon atoms enhances the clathrate hydrate inhibition activity of a counterpart polymer having said Rlf R2, R4 and R5 groups and only hydrogen as a R3 group.
12. The polymer according to claim 11 wherein said alkyl R3 group increases the subcooling temperature of said counterpart polymer.
13. The polymer according to claim 12 wherein said alkyl R3 group increases the subcooling temperature of said counterpart polymer by at least about 2°F.
14. The polymer according to claim 13 wherein said subcooling increase is the difference in the subcooling temperatures for said polymer having an alkyl R3 group and said counterpart polymer as measured by a pressurized test conducted at a 1,000 psig.
15. The polymer according to claim 14 wherein said test is conducted with an apparatus selected from the group consisting of an autoclave, a sapphire cell, and a miniloop apparatus.
16. The polymer according to claim 11 wherein said R3 group is a methyl group.
17. The polymer according to claim 11 wherein said R3 group is an ethyl group.
18. The polymer according to claim 11 wherein said R3 group is a propyl group.
Description:
POLYMERS FOR USE AS HYDRATE INHIBITORS

This application is based on U.S. provisional patent application Serial No. 60/000,065, filed on June 8, 1995, and is related to copending U.S. Serial No.

08/624,277 filed March 28, 1996, which is also based on U.S. provisional application Serial No. 60/000,065.

FIELD OF THE INVENTION

This invention relates to polymers useful in inhibiting formation of gas hydrates .in a pipe used to convey oil or gas.

BACKGROUND OF THE INVENTION

Carbon dioxide, hydrogen sulfide, and various hydrocarbons, such as methane, ethane, propane, normal butane and isobutane, are present in natural gas and other petroleum fluids. However, water is typically found mixed in varying amounts with such petroleum fluid constituents. Under conditions of elevated pressure and reduced temperature clathrate hydrates can form when such petroleum fluid constituents or other hydrate formers are mixed with water. Clathrate hydrates are water crystals which form a cage-like structure around guest molecules such as hydrate forming hydrocarbons or gases. Some hydrate forming hydrocarbons include, but are not limited to, methane, ethane, propane, isobutane, butane, neopentane, ethylene, propylene, isobutylene,

cyclopropane, cyclobutane, cyclopentane, cyclohexane, and benzene. Some hydrate forming gases include, but are not limited to, oxygen, nitrogen, hydrogen sulfide, carbon dioxide, sulfur dioxide, and chlorine. Gas hydrate crystals or gas hydrates are a class of clathrate hydrates of particular interest to the petroleum industry because of the pipeline blockages that they can produce during the production and/or transport of the natural gas and other petroleum fluids. For example, at a pressure of about IMPa ethane can form gas hydrates at temperatures below 4 °C, and at a pressure of 3MPa ethane can form gas hydrates at temperatures below 14 °C. Such temperatures and pressures are not uncommon for many operating environments where natural gas and other petroleum fluids are produced and transported.

As gas hydrates agglomerate they can produce hydrate blockages in the pipe or conduit used to produce and/or transport natural gas or other petroleum fluid. The formation of such hydrate blockages can lead to a shutdown in production and thus substantial financial losses. Furthermore, restarting a shutdown facility, particularly an offshore production or transport facility, can be difficult because significant amounts of time, energy, and materials, as well as various engineering adjustments, are often required to safely remove the hydrate blockage.

A variety of measures have been used by the oil and gas industry to prevent the formation of hydrate blockages in oil or gas streams. Such measures include

maintaining the temperature and/or pressure outside hydrate formation conditions and introducing an antifreeze such as methanol, ethanol, propanol, or ethylene glycol. From an engineering standpoint, maintaining temperature and/or pressure outside hydrate formation conditions requires design and equipment modifications, such as insulated or jacketed piping. Such modifications are costly to implement and maintain. The amount of antifreeze required to prevent hydrate blockages is typically between 10% to 30% by weight of the water present in the oil or gas stream. Consequently, several thousand gallons per day of such solvents can be required. Such quantities present handling, storage, recovery, and potential toxicity issues to deal with. Moreover, these solvents are difficult to completely recover from the production or transportation stream.

Consequently, there is a need for a gas hydrate inhibitor that can be conveniently mixed at low concentrations in the produced or transported petroleum fluids. Such an inhibitor should reduce the rate of nucleation, growth, and/or agglomeration of gas hydrate crystals in a petroleum fluid stream and thereby inhibit the formation of a hydrate blockage in the pipe conveying the petroleum fluid stream.

One method of practicing the present invention uses gas hydrate inhibitors which can be used in the concentration range of about 0.01% to about 5% by weight of the water present in the oil or gas stream. As discussed more fully below, the inhibitors of this

invention can effectively treat a petroleum fluid having a water phase.

SUMMARY OF THE INVENTION

According to the invention there is provided a composition comprising a polymer having the following units:

where,

R x is a hydrogen or hydrocarbon group having 1 to 6 carbon atoms, R 2 is a hydrocarbon group having 1 to 6 carbon atoms and R- L and R 2 have a sum total of carbon atoms greater than or equal to 1, but less than 8;

R 3 is a hydrogen or hydrocarbon group having 1 to 3 carbon atoms;

R 4 is a hydrogen or hydrocarbon group having 1 to 6 carbon atoms and R 5 is a hydrocarbon group having 1 to 6 carbon atoms;

x+y is an average number of units for producing an average molecular weight for said polymer between about 1,000 and about 6,000,000; and

z is either 0 or 1.

According to a second embodiment of the invention R 4 and R 5 may be linked to form a cyclic group with the nitrogen atom to which they are attached, said cyclic group having a sum total of at least 2 but less than 8 carbon atoms.

DETAILED DESCRIPTION OF THE INVENTION

The polymers of this invention inhibit the formation of clathrate hydrates in a fluid having hydrate forming constituents. Formation of clathrate hydrates means the nucleation, growth, and/or agglomeration of clathrate hydrates. Such clathrate hydrates may be formed in a fluid whether it is flowing or substantially stationary, but are often most problematic in flowing fluid streams conveyed in a pipe. For example, flow restrictions arising from partial or complete blockages in a fluid stream can arise as clathrate hydrates adhere to and accumulate along the inside wall of the pipe used to convey the fluid.

Nonetheless, the polymers can be used for inhibiting formation of clathrate hydrates in substantially stationary fluids.

In one embodiment, a concentrated solution or mixture of one or more of the inhibitors of the type

described below is introduced into a petroleum fluid stream having an aqueous phase. As the inhibitor solution or mixture of this invention is substantially dissolved in the aqueous phase or dispersed in the fluid stream it reduces the rate that clathrate hydrates are formed, and thereby reduces the tendency for a flow restriction to occur.

In a preferred embodiment, the solid polymer is first dissolved into an appropriate carrier solvent or liquid to make a concentrated solution or mixture. It should be understood that many liquids may effectively facilitate treatment of the fluid stream without dissolving the inhibitor. Many liquids, however, will preferably dissolve the inhibitor and, for convenience, are referred to hereafter as solvents whether they produce an inhibitor solution, emulsion, or other type of mixture. The solvent's principal purpose is to act as a carrier for the inhibitor and to facilitate the inhibitor's absorption into the aqueous phase of the petroleum fluid. Any solvent suitable for delivering the inhibitor to the fluid's aqueous phase may be used. Such carrier solvents include, but are not limited to, water, brine, sea water, produced water, methanol, ethanol, propanol, isopropanol, glycol, or mixtures of such solvents. Other solvents familiar to those skilled in the art may also be used.

It should be understood that the use of a carrier solvent is not required to use the inhibitors, but it is a convenient method of introducing the inhibitors into

the fluid. In many applications the use of a carrier solvent will facilitate treatment of the fluid stream. Any convenient concentration of inhibitor in the carrier solvent can be used, so long as it results in the desired final concentration in the aqueous phase of the petroleum fluid. Higher concentrations are preferred, since they result in a reduced volume of concentrated solution to handle and introduce into the petroleum fluid. The actual concentration used in a specific application will vary depending upon the selection of carrier solvent, the chemical composition of the inhibitor, the system temperature, and the inhibitor's solubility in the carrier solvent at application conditions. The inhibitor mixture is introduced into the aqueous phase of the petroleum fluid using mechanical equipment, such as, chemical injection pumps, piping tees, injection fittings, and other devices which will be apparent to those skilled in the art. However, such equipment is not essential to using the inhibitors of this invention. To ensure an efficient and effective treatment of the petroleum fluid with the inhibitor mixture two points should be considered.

First, an aqueous phase is preferably present at the location the inhibitor solution is introduced into the fluid. In some petroleum fluid systems (particularly natural gas systems) , an aqueous phase does not appear until the gas has cooled sufficiently for water to condense. If this is the case, the inhibitor solution is preferably introduced after the

water has condensed. Alternatively, in the event that an aqueous phase is not available at the point the inhibitor solution is introduced, the inhibitor solution concentration should be selected to ensure that the inhibitor solution's viscosity is sufficiently low to facilitate its dispersion through the fluid and permit it to reach the aqueous phase.

Second, because the inhibitor primarily serves to inhibit the formation of clathrate hydrates, rather than reverse such formation, it is important to treat the fluid prior to substantial formation of clathrate hydrates. As a wet petroleum fluid cools it will eventually reach a temperature, known as the hydrate equilibrium dissociation temperature or T e q, below which hydrate formation is thermodynamically favored. A petroleum fluid' s T e q will shift as the pressure applied to the fluid and its composition change. Various methods of determining a fluid's T e q at various fluid compositions and pressures are well known to those skilled in the art. Preferably, the fluid should be treated with the inhibitor when the fluid is at a temperature greater than its T e q- It is possible, but not preferable, to introduce the inhibitor while the temperature is at or slightly below the fluid's T e q, preferably before clathrate hydrates have begun to form. The quantity of inhibitor introduced into a petroleum fluid with an aqueous phase solvent will typically vary between about 0.01 wt% to about 5 wt% by weight of the water present in the fluid. Preferably, the inhibitor concentration will be about 0.5 wt%. For

example, a laboratory study has shown that adding 0.5 wt% of a copolymer of N-methyl-N-vinylacetamide and acryloylpyrrolidine (VIMA/APYD) to a petroleum fluid allowed the fluid to cool to a temperature which was about 15.5 °C below its T e q without rapid formation of a hydrate blockage. A higher inhibitor concentration can be used to lower the temperature at which a hydrate blockage is obtained. A suitable concentration for a particular application, however, can be determined by those skilled in the art by taking into account the inhibitor's performance under such application, the degree of inhibition required for the petroleum fluid, and the inhibitor's cost.

POLYMER DESCRIPTION

The term "homopolymer" as used herein includes a polymer having the same monomeric repeating unit, while the term "copolymer" as used herein includes polymers having two or more different monomeric repeating units. Many of the copolymers of N-vinyl amides are considered novel compounds, nonetheless many N-vinyl amide homopolymers will be discussed below for completeness. Accordingly, a generic structure for such classes of polymers having a N-vinyl amide unit is depicted as follows:

where ,

R is a hydrogen or hydrocarbon group having one to six carbon atoms, R 2 is a hydrocarbon group having one to six carbon atoms, wherein R λ and R 2 have a sum total of carbon atoms greater than or equal to one, but less than eight, and x is an average number of units for producing an average molecular weight for said polymer between about 1,000 and about 6,000,000. The R 2 and R 2 carbon atoms may be branched, normal, or cyclic wherein R may be either a hydrogen or an alkyl, cycloalkyl, or an aryl group having one to six carbon atoms and zero to two heteroatoms selected from the group consisting of oxygen and nitrogen and combinations thereof, and R 2 is an alkyl, cycloalkyl, or an aryl group having one to six carbon atoms and zero to two heteroatoms selected from the group consisting of oxygen and nitrogen and combinations thereof.

Other polymers of N-vinyl amides useful for inhibiting formation of hydrates include copolymers and terpolymers of N-vinyl amides with other N-vinyl amide monomers, acrylamides, maleimides, acrylates, vinyl carboxylates, and alkenyl cyclic imino ethers.

Also, many of the copolymers of N-allyl amides are considered novel compounds, nonetheless N-allyl amide homopolymers will be discussed below for completeness. A generic structure for these classes of N-allyl amide polymers is depicted as follows:

where, R x is a hydrogen or hydrocarbon group having one to six carbon atoms, R 2 is a hydrocarbon group having one to six carbon atoms, wherein R and R 2 have a sum total of carbon atoms greater than or equal to one, but less than eight, and x is an average number of units for producing an average molecular weight for said polymer between about 1,000 and about 6,000,000. The R a and R 2 carbon atoms may be branched, normal, or cyclic wherein R-_ may be either a hydrogen or an alkyl, cycloalkyl, or an aryl group having one to six carbon atoms and zero to two heteroatoms selected from the group consisting of oxygen and nitrogen and combinations thereof, and R 2 is an alkyl, cycloalkyl, or an aryl group having one to six carbon atoms and zero to

two heteroatoms selected from the group consisting of oxygen and nitrogen and combinations thereof.

Copolymers of N-allyl amides preferably used for hydrate inhibition include polymerizing a first N-allyl amide with a second N-allyl amide or polymerizing a first N-allyl amide with a second and third N-allyl amide, respectively.

It should be understood that the generic names used to identify the class of polymers, such as polymers of N-vinyl amides or poly(N-vinyl amides) and polymers of N-allyl amides or poly(N-allyl amides), are selected for convenience of discussion. These generic names are intended to cover any substantially water soluble polymers including, but not limited to, homopolymers, and copolymers and blends or mixtures thereof, having the structural units described above, whether such structural units or their related monomers were used to synthesize the polymer or not. The monomers disclosed below for synthesizing the polymers described herein are preferred examples of the starting materials and are not intended to limit the scope of the claims. Other starting materials and synthesis techniques, which are currently known or may become known, will be apparent to those skilled in the art as alternatives to synthesizing the polymers of the claimed invention. Accordingly, all polymers having at least the structural units identified in the claims below, even though such polymers may be produced from starting materials and/or by means not explicitly referenced herein, are intended to fall

within the scope of the polymers described and claimed herein.

Copolymers of N-vinyl amides comprise an N-vinyl amide monomer copolymerized with another vinyl monomer. The other vinyl monomer can be chosen from a class of monomers including, but not limited to, other N-vinyl amides, acrylamides, maleimides, acrylates, vinyl carboxylates and alkenylcyclic imino ethers, such that the resultant copolymer is substantially water soluble. For example, N-vinyl-N-methylacetamide (VIMA) is copolymerized preferably with various N-substituted acrylamides, such as acryloylpyrrolidine (APYD) or acryloylpiperidine (APID) , and more preferably, is copolymerized with N-substituted methacrylamides, such as N-isopropylmethacrylamide (iPMAM) and methacryloylpyrrolidine (MAPYD) to produce copolymers in the class described above.

The above-described N-vinyl amide homopolymers and copolymers could be used in a mixture with other polymers or additives useful for enhancing inhibitor performance, including but not limited to, various polymers, such as poly(vinylpyrrolidone) (PVP) , poly(vinylcaprolactam) (PVCap) , copolymers of PVP and PVCap, other homopolymers and copolymers of N- vinylamides, homopolymers and copolymers of N- allylamides; various surfactants, such as sodium n-butyl sulfate, sodium n-butyl sulfonate, sodium n-octyl sulfonate, sodium n-pentanoate; various alcohols, such as methanol, ethanol, propanols, and butanols; and

various alkyl zwitterions having one to ten carbon atoms, such as butyl zwitterion, as identified below,

Copolymers of N-allyl amides preferably used as hydrate inhibitors would comprise a first N-allyl amide of the class described above copolymerized with a second N-allyl amide of the class described above. Also, the above-described N-allyl amide homopolymers and copolymers could be used in a mixture with other polymers or additives useful for enhancing inhibitor performance, including but not limited to, various polymers, such as poly(vinylpyrrolidone) (PVP) , poly(vinylcaprolactam) (PVCap) , copolymers of PVP and PVCap, homopolymers and copolymers of N-vinylamides, and other homopolymers and copolymers of N-allylamides; various surfactants, such as sodium n-butyl sulfate, sodium n-butyl sulfonate, sodium n-octyl sulfonate, sodium n-pentanoate; various alcohols, such as methanol, ethanols, propanols, and butanols; and various alkyl zwitterions having one to ten carbon atoms, such as butyl zwitterion, as identified below,

H 3 C-(CH 2 ) 3 N + (CH 2 ) 4 S0 3 "

CH 3

Poly(N-vinvl amide) Examples

Without limiting the scope of the invention, and for the purpose of illustrating the invention, various N-vinyl amide polymers and copolymers were evaluated, including poly(N-methyl-N-vinylacetamide) (PVIMA) , copolymers of N-methyl-N-vinylacetamide and diethylacrylamide (VIMA/DEAM) , copolymers of N-methyl-N- vinylacetamide and iso-propylmethacrylamide (VIMA/iPMAM) , copolymers of N-methyl-N-vinylacetamide and acryloylpyrrolidine (VIMA/APYD) , copolymers of N- methyl-N-vinylacetamide and methacryloylpyrrolidine (VIMA/MAPYD) , copolymers of N-methyl-N-vinylacetamide and acryloylhexamethyleneimine (AHMI) , copolymers of N- methyl-N-vinylacetamide and acryloylpiperidine (VIMA/APID) , copolymers of N-methyl-N-vinylacetamide and vinyl butyrate (VIMA/VBu) , copolymers of N-methyl-N- vinylacetamide and ethyl maleimide (VIMA/EME) and terpolymers of N-methyl-N-vinylacetamide, N,N- dimethylacrylamide and ethyl maleimide (VIMA/DMAM/EME) . The above-identified N-vinyl amide homopolymers and copolymer structures are depicted below:

PVIMA

VIMA/DEΞAM

VIMA/iPMAM

VIMA/MAPYD

VIMA/APID

VIMA/AHMI

VIMA/EME

VIMA/DMAM/EME

VIMAΛ/Bu

Other N-vinyl amide polymers of interest include copolymers of N-methyl-N-vinylacetamide and isopropylacrylamide (VIMA/iPAM) , N-methyl-N- vinylacetamide and cyclohexylacrylamide (VIMA/CHAM) , N- methyl-N-vinylacetamide and acryloylmorpholine

(VIMA/AMorph) , poly(N-vinylpropionamide) (PVPA) , poly(N- n-propyl-N-vinylpropionamide) (PVPPA) , copolymers of N- vinylpropionamide and N-methyl-N-vinylacetamide

(VPA/VIMA) , copolymers of VPA and isopropylmethacrylamide (VPA/iPMAM) , copolymers of N-n- propyl-N-vinylpropionamide and N-methyl-N-vinylacetamide (VPPA/VIMA) :

VIMA/iPAM

VIMA/CHAM

VIMA/AMorph

PVPA

PVPPA

VPA/iPMAM

VPA/VIMA

VPPA/VIMA

Poly(N-allyl a ide) Examples Without limiting the scope of the invention, and for the purpose of illustrating selected N-allyl amide polymers for practicing the invention, various homopolymers and copolymers in the class that are preferred candidates for inhibitor evaluation include the polymers described immediately below.

A preferred class of N-allyl amide homopolymer candidates for evaluation include:

where R x = H or a hydrocarbon group having one to three carbon atoms and R 2 = a hydrocarbon group having one to three carbon atoms, wherein R x and R 2 have a sum total of carbon atoms greater than or equal to one, but less than eight, and x is an average number of monomeric units for producing an average molecular weight of between about 1,000 and about 6,000,000 for the homopolymer; more preferably, however, R 1 = H and R 2 = a hydrocarbon group having one to three carbon atoms; and most preferably, R x = H and R 2 = a hydrocarbon group having three carbon atoms. A preferred class of N-allyl amide coopolymer candidates for evaluation include:

where R x = H or a hydrocarbon group having one to three carbon atoms and R 2 = a hydrocarbon group having one to three carbon atoms, wherein R x ≠ R 2 and R x and R 2 have a sum total of carbon atoms greater than or equal to one, but less than eight, and the sum of x and y yields an average number of units for producing an average molecular weight of between about 1,000 and about 6,000,000 for the copolymer.

POLYMER SYNTHESIS

General Synthesis Procedure for Poly(N-vinyl amides)

N-methyl-N-vinylacetamide (VIMA . ) is commercially available or may be synthesized according to published procedures (see e.g., A.I. Askenov, et al , Zhurnal Obschei Khimii , 57 (2), pp. 1634-1637 (1987)). Many of the vinyl monomers used in synthesizing various VIMA copolymers described below are commercially available. Acrylamide monomers which were not available were synthesized from the appropriate amine and acryloyl chloride according to published procedures (see e.g.,

S. Ito, Kobunshi Ronbunshu, 46 (7), pp. 437-443 (1989)). Alternatively, poly(N-vinyl amide) homopolymers can be synthesized by functionalization of poly(vinyl amine) . This technique is described by Akashi et al. in Japanese Patent Kokai 7 [1995] and also by T. Fischer and W. Heitz in Macromol. Chem. Phys. , 195(2), 1994, 679- 687. The technique consists of acylating the amine group of poly(vinyl amine) by reaction with an acyl chloride or a carboxylic acid.

Higher analogs of N-vinyl amides, such as N-methyl- N-vinylpropionamide and N-methyl-N-vinylbutyramide, which are useful in preparing polymers of this invention, are preferably synthesized by alkylation enamide enolates. This synthesis process is described by Kannappan Chockalingam, Bijan Harirchian, and Nathan L. Bauld in Synthetic Communications, 20(2), pages 189- 202, 1990.

Standard laboratory procedures familiar to those skilled in the art were used to synthesize the evaluated polymers and copolymers identified above. Benzene or low molecular weight alcohols were used as reaction solvents. Many common azo free radical initiators, such as 2,2' -Azobis (2-methylpropionitrile) , also known as AIBN, can be used for synthesizing poly(N-vinyl amides) . The polymers were isolated and characterized using techniques well-known to those skilled in the art, such as carbon-13 ( C) and proton ( 1 H) nuclear magnetic resonance spectroscopy (NMR) and gel permeation chromatography (GPC) , to confirm their structures. Some examples of synthesis procedures that may be adapted to synthesizing other polymers and copolymers similar to those identified above are provided below.

Polymerization of N-methyl-N-vinylacetamide (VIMA)

t-Butanol was dried overnight over activated molecular sieves and then purged for about 4 hours with a stream of dry nitrogen gas. A 250 mL flask equipped with an overhead stirrer, condenser with drying tube,

thermometer and nitrogen inlet was purged with nitrogen. 25.0 g (0.25 moles) N-methyl-N-vinylacetamide (Aldrich) was loaded into the flask with about 100 mL t-butanol. 0.46 g (0.0028 moles) AIBN (Eastman) was added and the reaction heated at 65 C for about 8 hours. The reaction was cooled and the product isolated by vacuum evaporation of the solvent. The product was characterized by C NMR and GPC.

Polymerization of Other N-Vinyl Amide Monomers

Other N-vinyl amide monomers may be used to produce higher analogs of N-vinyl amide homopolymers, such as poly(N-vinylpropionamide) and poly(N-n-propyl-N- vinylpropionamide) , and other copolymers according to published procedures known to those skilled in the art of polymer synthesis, see e.g., A.I. Askenov, et al , Zhurnal Obschei Khimii , 57 (2), pp. 1634-1637 (1987), Hartwimmer et al, U.S. Patent 3,531,471, and Lederer et al., U.S. Patent 3,696,085.

Copolymerization of N-methyl-N-vinylacetamide and Substituted Acrylamides

In general, the copolymerization of VIMA with acrylamides, specifically N-substituted (or N,N disubstituted) acrylamides is slow. Thus, if the two monomers are charged together in a batch copolymerization, the N-substituted acrylamide will polymerize to some extent, as a homopolymer, or at least

a copolymer that is very rich in the acrylamide. To alleviate this problem, a pumping process is used to ensure that the VIMA is substantially incorporated into the copolymer. In general, the VIMA is charged to a reactor with initiator. The "faster" reacting monomer, the N-substituted acrylamide, is pumped into the VIMA solution. The optimum addition time is based on the relative reactivity of the comonomers. For our purposes, a pumping time of two to four hours was adequate. An example of this procedure is given below for the copolymerization of N-methyl-N-vinylacetamide (VIMA) and N-isopropylmethacrylamide . (iPMAM) to produce a VIMA/iPMAM copolymer.

Copolymerization of N-methyl-N-vinylacetamide (VIMA) and N-isopropylmethacrylamide (iPMAM)

N-isopropylmethacrylamide was purchased from Aldrich and recrystallized twice from hexane. Anhydrous benzene was purchased from Aldrich and used without further purification. All transfers were done under an inert atmosphere. N-vinyl-N-methylacetamide was purchased from Aldrich and purified by fractional distillation. AIBN was recrystallized from methanol.

VIMA (8.43g (0.066 mol) ) was passed through a commercially available inhibitor removal column and dissolved in benzene (45ml) . This solution was charged to a 3-necked flask equipped with a condenser, stirrer, nitrogen inlet/outlet, and a port through which the

iPMAM was pumped. The VIMA/benzene solution was purged further with N2 for 1/2 hour. A solution of iPMAM (6.57g (0.663 mol) in 90ml benzene) was purged for 1/2 hour, then charged to a syringe pump. After purging the reaction kettle containing the VIMA solution, it was brought to 60°C. The reaction was initiated by injecting an AIBN solution (0.115g in 2 cc benzene) into the VIMA solution. Then the NiPMAM solution was pumped in over the next 2 and 1/2 hours. After pumping was completed, the reaction mixture was kept at 60°C under nitrogen with stirring. The next day, it was precipitated into an excess of hexane, redissolved in a minimum amount of THF, then reprecipitated into hexane.

The sample was then dried under vacuum at 60°C at 10~ 3 torr. The final yield of purified copolymer was 6g

(40%) . 13 C NMR indicated that the VIMA/iPMAM ratio was 13/87.

Copolymerization of N-methyl-N-vinylacetamide (VIMA) and Ethylmaleimide (EME)

VIMA monomer was purchased from Aldrich and purified by distillation. EME monomer was used as received from Aldrich. Anhydrous benzene was purchased from Aldrich and transferred under an inert atmosphere. AIBN was recrystallized from methanol.

11.4 g (0.115 moles) VIMA monomer and 3.6 g (0.029 moles) EME monomer were dissolved in 154 mL benzene, loaded into a three-necked flask fitted with a

condensor, a thermometer, and a nitrogen inlet/outlet, and purged with nitrogen for one hour. The solution was then heated to 60 °C. The reaction was initiated by injecting an AIBN solution (0.115 g in 2 cc benzene) into the mixture. The reaction mixture was maintained at 60 °C overnight, which resulted in a slightly viscous solution. The next day, the polymer was precipitated into hexane, dissolved into acetone, then reprecipitated into hexane. The reaction product was then dried at 40 °C and 10 "3 torr overnight. The reaction product was characterized by X H and 13 C NMR and GPC. The VIMA/EME ratio produced in the copolymer was about 62/38.

Copolymerization of VIMA and Other N-Vinyl Amides with Acrylamides. Maleimides. Carboxylates. and Cyclic Imino Ethers

It will be apparent to those skilled in the art of polymer synthesis, that VIMA or higher N-vinyl amide analogs, such as N-n-propyl-N-vinylpropionamide and N- vinyl propionamide, will produce copolymers with various carboxylates, cyclic imino ethers, and various other acrylamides and maleimides using procedures substantially similar to those described above for VIMA/iPMAM and VIMA/EME.

General Synthesis Procedure for Poly(N-allyl amides)

The synthesis of poly(N-allyl amides) is well known to those skilled in the art. The methods for

controlling the rate of polymerization and the molecular weights for such polymers by using complexing agents, such as Lewis acids, and the appropriate initiator are disclosed in two patents, Canadian No. 1,251,892 and U.S. No. 4,644,042, and one patent application, European Application No. 84307460.0. Also, F. M. Menger et al. , in J. Orσ. Chem.. 1995, 60, 6666-6667, disclose a technique for functionalizing a poly(allylamine) to produce a poly(N-allyl amide) . Generally, the technique consists of acylating the amine group of a poly(allylamine) . This reaction is conducted with the appropriate mole ratio of the poly(allylamine) and either an acyl chloride, R j COCl, or a carboxylic acid, RiCOOH. A generic reaction sequence for producing a poly(N-allyl amide) is provided below:

where R = a hydrocarbon group having one to six carbon atoms and zero to two heteroatoms selected from the group consisting of oxygen and nitrogen, and combinations thereof and x is an average number of units for producing an average molecular weight of between about 1,000 and about 6,000,000 for the final homopolymer product.

POLYMER EVALUATION AS A HYDRATE INHIBITOR

Mini-lppp Testing Procedure

One method for evaluating a polymer's effectiveness as an inhibitor uses a bench-scale high pressure apparatus referred to as a mini-loop apparatus. A mini- loop apparatus consists of a loop of stainless steel tubing with about a one-half inch inside diameter and about ten feet in length. The loop also has a transparent section for observing the fluid flow in the loop and the onset of hydrate formation in the loop.

Fluid comprising about 40% by volume SSW solution having about 3.5% total ionized salts, 40% by volume hydrocarbon condensate (i.e., C e +) , and 20% by volume hydrocarbon gas mixture is circulated around the loop at constant pressure. The hydrocarbon gas mixture is comprised of about 76 mole% methane, 9 mole% ethane, 7 mole% propane, 5 mole% n-butane, 2 mole% iso-butane, and 1 mole% of C 5 +. The inhibitor is typically injected into the loop as an aqueous solution to produce the desired weight percent concentration of inhibitor in the aqueous

sea salt/gas solution. Generally, many hydrate inhibitors are evaluated at about 0.5 wt% of the aqueous sea salt/gas solution.

The fluid is circulated at a constant velocity of about 2.5 feet/second. The loop and its pump lay in a controlled temperature water bath for controlling the temperature of the fluid circulating in the loop. The bath's water is circulated to ensure uniform temperature throughout the bath and rapid heat transfer between the bath water and the loop. As the loop temperature changes or as hydrates form, the gas volume in the loop will change accordingly. Therefore, . to maintain constant pressure in the loop a pressure compensating device is required. Such a device can be comprised of a gas cell and a hydraulic oil cell separated by a floating piston. So as the gas volume in the loop changes, oil may be added or removed from the oil cell to produce a commensurate addition or removal of gas to the loop. Mini-loop tests are typically run at a pressure of about 1,000 pounds per square inch gauge (p.s.i.g.). However, any pressure between 0 to 3,000 p.s.i.g. could be selected for evaluating an inhibitor's performance.

The temperature of the water bath is reduced at a constant rate, preferably about 6 °F or 3.3 °C per hour, from an initial temperature of about 70 °F or 21 °C. At some temperature, clathrate hydrates begin to rapidly form. As the dissolved gas is used to form clathrate hydrates there is an abrupt and corresponding decrease in the volume of dissolved gas in the aqueous sea

salt/gas solution. The temperature at which this abrupt decrease in the volume of dissolved gas is observed is known as the temperature of onset for hydrate formation (T os ) . Recalling from the discussion above, the hydrate equilibrium dissociation temperature or T eq is the temperature below which hydrate formation is thermodynamically favored in an aqueous sea salt/gas solution without an inhibitor present. Therefore, another measure of an inhibitor's effectiveness is the difference between T eq and T os which is known as the inhibitor's subcooling, T gub . Therefore, for a given pressure, the greater the subcooling-the more effective the inhibitor. Typically, an aqueous sea salt/gas solution with no inhibitor present produces a T sub of about 6-7 °F or 3.3-3.9 °C.

Mini-loop Test Results

Without limiting the scope of the invention, and for the purpose of illustrating the invention, various polymeric N-vinyl amide and acrylamide inhibitors were evaluated using the mini-loop testing procedure described above. The results of these evaluations, where available, are provided below.

TABLE 1

Generally, copolymerizing N-vinyl amides, such as VIMA, with other water soluble polymers, such as various acrylamides (e.g., AHMI, iPMAM, APYD, MAPYD, and APID) , carboxylates (e.g., VBu) , and maleimides (e.g., EME), produced an unexpected improvement over the corresponding homopolymer's inhibitor performance. As indicated above, the VIMA homopolymer' s subcooling was either about equal to or significantly less than the subcooling for other known homopolymer inhibitors, such as PDEAM, PAPYD, PAPID, or PiPMAM. Also, a simple mixture of a poly(N-vinyl amide) , such as PVIMA, with a second water soluble polymer, such as PVCap, slightly diminishes the inhibition activity of the second polymer, when comparing systems having the same total inhibitor concentrations. For example, compare the results for the PVIMA, PVCap mixture having equal weight percentages of each homopolymer, 0.25 wt% for a total 0.5 wt% concentration, to the corresponding copolymer having a 50:50 mole ratio of VIMA/VCap at a total 0.5wt%. The 50-50 PVIMA-VCap mixture has a slightly lower subcooling, 22.3 °F (12.4 °C) , compared to the PVCap homopolymer' s subcooling, 22.5 °F (12.5 °C) . However, the copolymer with a 50:50 mole ratio of VIMA/VCap produced a substantially higher subcooling, 29.0 °F (16.1 °C) , versus that of the PVCap homopolymer. Consequently, it was unexpected that copolymerizing an N-vinyl amide, such as VIMA, with the monomers used for producing other known homopolymer inhibitors would enhance, rather than diminish, the corresponding homopolymer' s inhibition activity. PDEAM is the only

anomaly among this otherwise consistent and unexpected influence that an N-vinyl amide, such as VIMA, can have when used to produce a copolymer, terpolymer, or other complex polymer. With the exception of VIMA/DEAM, three other VIMA copolymers, VIMA/APYD, VIMA/APID, and

VIMA/iPMAM, demonstrated an improvement in subcooling by about 10% to 100% over the corresponding acrylamide homopolymer.

We believe that VIMA is similarly enhancing the inhibition activity of other acrylamides (e.g, AHMI), carboxylates (e.g., VBu), and maleimides (e.g., EME) . Also, other members of the N-vinyl amide family, such as poly(N-vinyl propionamide) and poly(N-vinyl-N-n- propylpropionamide) , may also demonstrate a similar synergism when polymerized with one or more other monomers selected from the class of acrylamides, carboxylates, or maleimides.

The results in Table 1 also indicate that water soluble polymers with alkylated backbones are effective as clathrate hydrate inhibitors. Alkylating the backbone appears to enhance the hydrate inhibition activity of various water soluble polymers. Comparing the subcooling results of comparable polymers with nonalkylated versus alkylated backbones, we see that PiPAM (nonalkylated) has a 20°F (11.1°C) subcooling versus the 24°F (13.3°C) subcooling for PiPMAM (methylated) and VIMA/iPAM (nonalkylated) , with a 42:58 ratio, has a 24°F (13.3°C ) versus the 29.0°F (16.1°C) subcooling for VIMA/iPMAM (methylated) with about the same ratio, 40:60. The subcooling for VIMA/iPAM was

also significantly lower versus the average subcooling obtained for all VIMA/iPMAM samples evaluated, 24.0°F (13.3°C) versus 28.2°F (15.7°C) ,respectively. Also, VIMA/APYD (nonalkylated) , in a ratio of about 30:70, has an average subcooling of 26°F (14.4°C) and, in a ratio of 49:51, has a subcooling of 25°F (13.9°C) versus the 29.5°F (16.4°C) subcooling for VIMA/MAPYD (methylated) . Consequently, alkylating the backbone surprisingly and unexpectedly enhances the polymer's hydrate inhibition activity. As indicated by the results in Table 1, alkylating the backbone of a polymer with at least one methyl group will produce a subcooling for the alkylated polymer that is at least about 2°F (about 1.1°C) greater than its nonalkylated counterpart. It is also believed that alkylating the backbone of a polymer with either an ethyl or propyl group could similarly improve the polymer's subcooling performance.

Mini-loop test results are not yet available for other members of the N-vinyl amide family, such as poly(N-vinyl propionamide) and poly(N-vinyl-N-n- propylpropionamide) , or for the preferred poly(N-allyl amide) candidates specified above for inhibitor evaluation. Nonetheless, based on an inhibitor design method disclosed in copending U.S. Patent application, Serial No. 08/600,386, these polymers are expected to have a T sub of at least 12.5 °F or 6.9 °C.

The means and method for practicing the invention and the best mode contemplated for practicing the invention have been described. It is to be understood that the foregoing is illustrative only and that other

means and techniques can be employed without departing from the true scope of the invention as claimed herein.