WO2013053770A1

WO2013053770A1 - Method of inhibiting the formation of gas hydrates using amine oxides

Info

Publication number: WO2013053770A1
Application number: PCT/EP2012/070077
Authority: WO
Inventors: Malcolm Andrew Kelland
Original assignee: Eco Inhibitors As
Priority date: 2011-10-11
Filing date: 2012-10-10
Publication date: 2013-04-18
Also published as: GB201117476D0

Abstract

The present invention relates to methods of inhibiting the formation or agglomeration of gas hydrates using amine oxides. The invention further relates to compositions comprising amine oxides that find use as gas hydrate inhibitors, as well as processes for preparing alkylated amine oxides.

Description

Method of Inhibiting the Formation of Gas Hydrates using Amine Oxides

The present invention relates to clathrate hydrate inhibitors and methods of inhibiting the nucleation, formation, agglomeration, and deposition of clathrate hydrates. Methods for preparing hydrate control compounds and hydrate inhibitor compositions are also provided. The invention is especially useful in inhibiting blockages due to clathrate hydrates in pipelines for production and transport of oil and natural gas, in drilling operations, completion, stimulation and fracturing operations, and in injection and re-injection operations. Gas hydrates are clathrates (inclusion compounds) of small molecules in a lattice of water molecules. In the petroleum industry, natural gas and petroleum fluids contain a variety of these small molecules, which can form gas hydrates. They include hydrocarbons such as methane, ethane, propane, isobutane as well as nitrogen, carbon dioxide and hydrogen sulphide. Larger hydrocarbons such as n-butane, neopentane, ethylene, cyclopentane, cyclohexane and benzene are also hydrate-forming components. When these hydrate- forming components are present with water at elevated pressures and reduced

temperatures, the mixture tends to form gas hydrate crystals. For example, ethane at a pressure of 1 MPa forms hydrates only below 4 °C, whereas at 3 MPa gas hydrates can only form below 14 °C. These temperatures and pressures suited to hydrate formation are typical operating environments where petroleum fluids are produced and transported and in drilling, completion or fracturing operations in the oil and gas industry.

If gas hydrates are allowed to form inside a pipe containing natural gas and/or other petroleum fluids, they can eventually block the pipe. The hydrate blockage can lead to a shutdown in production and significant financial loss. The oil and gas industry therefore uses various means to prevent the formation of hydrate blockages in pipelines. These include heating the pipe, reducing the pressure, removing the water and adding thermodynamic inhibitors (antifreezes) such as methanol and ethylene glycols, which act as melting point depressants. Each of these methods is costly to implement and maintain. The most common method used today is the addition of antifreezes. However, these antifreezes have to be added at high concentrations, typically 10-60% by weight of the water present, in order to be effective. Recovery of the antifreeze is also often required and is a costly procedure.

An alternative to the above methods is to control the gas hydrate formation process using nucleation and crystal growth inhibitors. These types of chemicals are widely known and used in other industrial processes. The advantage of using these chemicals to control gas hydrate formation is that they can be used at concentrations of 0.01 to 3%, i.e. much lower than concentrations typically used for antifreezes. Thus, these chemicals are often called low dosage hydrate inhibitors (LDHIs).

Gas hydrate nucleation inhibitors are called kinetic hydrate inhibitors (KHIs). Examples of KHIs include polyvinylpyrrolidone, copolymers of vinyl pyrrolidinone (e.g. with alpha-olefins, vinyl caprolactam or dimethylaminoethyl methacrylate), polymers containing

pyrrolidinocarbonyl aspartate groups, polyesteramides and polyvinyllactams. KHI polymers are often expensive, therefore a lower concentration of KHI polymer (perhaps 40-60% as much) is often used with the addition of a cheaper synergist to improve the performance and lower the overall cost. A commonly used KHI synergist is the quaternary ammonium salt, tetrabutylammonium bromide (TBAB). US Patent No. 6,102,986 mentions the use of amine oxides of formula R¹,R²,R³N-0 or the acid addition salts thereof as hydrate inhibitors. The compounds suggested contain a single amine oxide moiety and have been found to act, not as kinetic hydrate inhibitors alone, but merely to have some effect as synergists for KHI polymers such as polyvinyllactam.

Some kinetic hydrate inhibitor polymers cannot be used on some oil/gas fields because they have a cloud point (or lower critical solution temperature) in the produced aqueous fluid below the temperature where the polymer would be injected, e.g. at the wellhead. This would cause the polymer to deposit near the injection point rendering it ineffective for the job for which it was designed. It could also cause a restriction in the conduit near the injection point. It would therefore be advantageous if alternative additives could be found.

Besides KHIs, there is another class of LDHIs called anti-agglomerants (AAs). AAs do not inhibit the formation of gas hydrates to the same level as KHIs, rather their primary activity is in preventing the agglomeration and deposition of hydrate crystals. A hydrocarbon phase provides a transport medium for the hydrates which are referred to as hydrate slurries so that the overall viscosity of the medium is kept low and can be transported along the pipeline. As such, the hydrate crystals formed in the water-droplets are prevented from agglomerating into a larger crystalline mass. Chemicals acting as anti-agglomerate hydrate inhibitors are typically quaternary ammonium or phosphonium salts, such as

tributylhexadecylphosphonium bromide and tributylhexadecylammonium bromide.

Unfortunately, such compounds have undesirable levels of toxicity, are poorly biodegradable and don't function well in water with relatively low salt concentrations (such as some areas of the North Sea). Due to the above-mentioned problems relating to cost, performance and environmental impact, a need exists for alternative compounds for inhibiting and controlling the formation of gas hydrates in connection with hydrocarbon production, storage and transportation including production, drilling, completion, fracturing, stimulation and injection and re-injection operations.

It is therefore an object of the present invention to find novel and effective compounds which retard the formation of gas hydrates (kinetic inhibitors) or keep the gas hydrate crystals small and pumpable (anti-agglomerants). Amine oxides are compounds that contain the functional group R₃N⁺-0^~, i.e. an N-0 bond with three additional hydrogen and/or organic groups attached to the nitrogen atom.

Alternative notations are R₃N→0 and R₃N=0.

It has been surprisingly found that compounds containing more than one, i.e. two or more, amine oxide groups give good results as low dosage hydrate inhibitors, that is as kinetic hydrate inhibitors, synergists for kinetic hydrate inhibitors and/or anti-agglomerants.

Polymers containing more than one amine oxide group have been found to be particularly effective as KHIs and excellent results have been found for bis-amine oxides as anti- agglomerants. As all current commercial KHI polymers contain amide groups, the poly(amine oxides) of the present invention represent a completely different approach to designing KHIs as they do not necessarily contain amide groups. They have also been found to be excellent synergists for various KHI polymers such as vinylcaprolactam polymers. Excellent results have also been found for other compounds containing more than one amine oxide group, such as bis-amine oxide surfactants which show markedly improved effects as anti-agglomerants when compared to mono amine oxides. Bis-amine oxides have also been found to have a very strong kinetic inhibition effect which lends them to use as synergists with KHI polymers to improve the polymer performance, or as kinetic hydrate inhibitors themselves - believed to be the first example of a non-polymeric KHI.

Thus, the present invention provides alternative compounds for inhibiting and controlling the formation of gas hydrates in connection with hydrocarbon production, storage and

transportation including production, drilling, completion, fracturing, stimulation and injection and re-injection operations. The compounds can act as synergists for new or existing KHI polymers, as anti-agglomerants and as kinetic hydrate inhibitors themselves.

Thus, viewed from a first aspect, the present invention provides a method of inhibiting the formation or agglomeration of gas hydrates in a system, said method comprising adding to the system a compound comprising two or more amine oxide groups (i.e. groups of general formula R₃N⁺-0^~), or a salt thereof.

Viewed from a further aspect, the invention provides the use of a compound as herein defined for inhibiting the formation or agglomeration of hydrates in a system, preferably a system for hydrocarbon drilling, production, storage and/or transportation, including production, drilling, completion, fracturing, stimulation and injection and re-injection operations. Compositions comprising the compounds described herein form a further embodiment of the invention, as to do certain new compounds containing two or more amine oxide groups.

Bis-amine oxides and poly(amine oxides), particularly alkylated versions, are preferred compounds according to the present invention.

In a preferred aspect, the compoun is of general Formula (I)

(I) or a salt thereof, where

n is an integer from 1 onwards,

R¹, R², R³ and R⁴ are independently H or an organic group comprising 1 -20 carbon atoms and optionally one or more heteroatoms and

X is a bond or linker moiety.

The compound comprising two or more amine oxide moieties of the present invention may be an oligomer or a polymer. The polymers may be branched or linear. Especially preferably each R group of the amine oxide groups in the compounds described herein, e.g. the organic group comprising 1 -20 carbon atoms and optionally one or more heteroatoms of the above formula, is a C2-2₀ organic group (e.g. an optionally substituted, cyclic, linear or branched saturated or unsaturated hydrocarbon). The R groups may be the same or different.

Especially preferably each R group is a C_2-16 alkyl group, especially preferably a C_2-6 alkyl group, particularly propyl, n-butyl, n-pentyl, iso-pentyl. Especially preferably all of the R groups in the amine oxide groups, e.g. those of formula (I), are independently selected from butyl, e.g. tert-butyl, n-butyl, sec-butyl or iso-butyl. Especially preferably one or more (e.g. all) of the R groups are n-butyl.

In a further aspect, two or more of the R groups in R₃N⁺-0^~ or the groups R¹, R², R³ and R⁴ in Formula (I) are independently chosen from n-butyl, n-pentyl and iso-pentyl groups.

As used herein, references to "R" generally means any of R¹-R⁴ in Formula (I).

Preferably, in Formula (I), none of R¹-R⁴ contains heteroatoms, and more preferably none of R¹-R⁴ or X contains heteroatoms.

Preferably, each R group in Formula (I) is independently H or a Ci_-6 alkyl, wherein at least one of the R groups is a C_3-6 alkyl, more preferably at least two of the R groups are C_3-6 alkyl. Preferably, each R group in Formula (I) is independently H or a Ci_-6 alkyl, wherein at least one of the R groups is a C₄-₆ alkyl, more preferably at least two of the R groups are C_4-6 alkyl.

Preferably, each R group in Formula (I) is independently H or a Ci_-6 alkyl, wherein at least one of the R groups is a C_3-5 alkyl, more preferably at least two of the R groups are C_3-5 alkyl.

Preferably, each R group in Formula (I) is independently H or a Ci_-6 alkyl, wherein at least one of the R groups is a C_4-5 alkyl, more preferably at least two of the R groups are C_4-5 alkyl.

Preferably, each R group in Formula (I) is independently a C_3-6 alkyl, more preferably C_3-5 alkyl, with C_4-5 alkyl being the most preferred. X is preferably a linker group, i.e. a group providing a backbone chain of up to 20 atoms, such as -(CH₂)_y- optionally containing a heteroatom such as oxygen or nitrogen (e.g. an ether (-0-), an ester (-C(=0)0-) or an amide (-C(=0)NH-)), where y is an integer from 1 upwards, preferably 2 to 20, especially preferably 2 to 10, particularly preferably 2 to 6, most preferably 3 to 5, e.g. 4.

Thus, X denoting -(CH₂)4- optionally containing an ether could represent -CH₂0(CH₂)3- or - (CH₂)₂0(CH₂)₂- or the like, while -(CH₂)₄- optionally containing an ester could represent - CH₂COO(CH₂)₂-, -(CH₂)₂COOCH₂- or the like. Preferably, the heteroatom (i.e. ether or the non-carboxyl part of the ester or amide) is not the group immediately adjacent and bonded to the amine oxide moiety. Including such groups improves the biodegradability of the compounds.

As used herein, by the term "alkyl" is meant linear or branched, unsubstituted, acyclic alkyl group containing the recited number of carbon atoms.

Especially preferably none of the R groups are H.

Salts of compounds containing two or more amine oxide groups are also suitable for use in the method, uses and compositions of the invention. Examples of salts are protonated amine oxides (R₃N=0⁺H) with various ions such as bicarbonate, halides, sulphate, sulphonates, carboxylates, phosphonates etc.

In the compounds of the invention, one or more of the R groups of an amine oxide moiety (e.g. one or more of R¹-R⁴ of Formula (I)) may be linker groups which are attached to a further moiety, e.g. to a further amine oxide group or to a polymer. In this way amine oxide groups, such as the structural units described above may be connected through one of the R groups to become a pendant group of many oxygen-containing or nitrogen-containing polymers. Such polymers include, but not limited to polyacrylate, polymethacrylate, copolymers of acrylate and methacrylate, polyacrylamide, polymethacrylamide, copolymers of acrylamide and methacrylamide, and polymers and copolymers of N-vinylcaprolactam. Such nitrogen containing polymers and copolymers can be obtained by the Michael addition reaction between polyethylenimine and acrylic or methacrylic acids. The copolymers may also include N-vinylcaprolactam, N, N-dimethylacrylamide, N- ethylacrylamide, N- isopropylacrylamide, N-butylacrylamide, or N-tert. butylacrylamide. Amine oxide derivatives of linear and branched polyethyleneimines, polyvinylamines, polyallylamines, polymers from dialkylaminoethyl(meth)acrylates and

polydialkylaminopropyl(meth)acrylates. i.e. methacryl or acryl, polylysine are also preferred. Alternatively, the polymers of the invention may be polymers of amine oxides, i.e. where the amine oxide groups form the polymer backbone, as in Formula (I) when n is a suitably high number.

The compounds of the methods, uses and compositions of the invention therefore include polymers comprising two or more amine oxide moieties. Preferably the polymeric compounds have the structure of Formula (I) as described herein, in which case n is preferably an integer from 20-2000, more preferably 50-1000, e.g. 250-750. Such polymers preferably have a molecular weight of 200 to 10,000,000 Daltons, preferably 500-5,000,000 Daltons, more especially 1 -100 kDa, e.g. 2-25 KDa. Polymers in which the repeat unit consists essentially of units of Formula (I) as well as polymers in which two or more amine oxide groups are present in the polymer backbone or in one or more side chains are encompassed. Thus the nitrogen atom of the amine oxide group can be part of the backbone, or the nitrogen atoms can be in a side chain. Units of formula (I) may make up the whole or the majority or a minority of the overall polymer and may be positioned randomly or in blocks throughout the overall polymer. The overall polymer may be linear, branched or cross-linked. An example of a preferred polymer for the uses, methods and compositions of the invention is shown below.

In a further aspect, the compounds for the uses, methods and compositions of the invention may be amphiphiles or surfactants, particularly amphiphiles or surfactants with a molecular weight of less than 1000 Daltons. Where the compound is an amphiphile or surfactant, typically one of the R groups of the amine oxide groups, preferably one of R¹-R⁴, is or comprises a long chain hydrocarbon group, e.g. a C_8-2o alkyl group, preferably a C12-18 alkyl group such as a hexadecyl group. It is also preferred that the surfactant compound is a bis or tris amine oxide, i.e. that of formula (I) when n is 1 or 2.

Further preferred compounds are bis-amine oxides, such as compounds of Formula (I) when n= 1 and X is a linker as described above. Higher analogues, such as tris and tetrakis amine oxides are also preferred, although bis-amine oxides are particularly preferred.

In a preferred aspect, the compounds of the invention contain one or more biodegradable linkages such as esters, amides, ethers, or C=C double bonds.

More than one compound as described herein may be added to the system in the method and uses of the invention. For example, mixtures of two or more of the compounds as herein described may be used. Especially preferred compounds for the methods, uses and compositions of the invention are; bis-amine oxides, such as C1 1 amido-bis amine oxide, C13 amido-bis amine oxide, C15 amido-bis amine oxide, C17 amido-bis amine oxide, N,N,N'N'-tetra-n-butyl-1 ,6- hexanediamine bis-oxide, 1 ,1 ,6,6-tetrabutylhexanediamine bis-oxide,

tris(ethylenedibutylamine oxide)amine tetraoxide and polyamine deriviates such as n-Pr- polyamine oxide (MW = 10kDa), n-Butyl-Polyamine oxide (MW = 10kDa), Butyl-polyamine oxide (MW = 25kDa), Butyl-polyamine oxide (MW = 10kDa), Butyl-polyamine oxide (MW = 2kDa), Propyl-polyamine oxide (MW = 10kDa), Pentyl-polyamine oxide (MW = 10kDa), polybutylated polyethylenediamine. Butylated polyethyleneimine oxide is particularly preferred.

The compounds as described herein can be used as kinetic hydrate inhibitors themselves or as synergists (performance enhancing chemicals) for new and existing kinetic hydrate inhibitors, i.e. KHI polymers. In a preferred aspect the method of the invention further comprises adding a kinetic hydrate inhibitor to the system. Use of the compounds herein described as KHI synergists forms a further embodiment of the invention.

Examples of KHIs include oligomers, polymers, homopolymers, graft-polymers and copolymers of N-vinyllactam, N-vinylcaprolactam, N-vinyl-pyrrolidone and alkylated vinylpyrrolidones, alkyl- and dialkylacrylamide polymers and copolymers, and terpolymers of vinylpyrrolidone, vinylcaprolactam and further anionic, cationic and neutral comonomers having a vinylic double bond such as 1 -olefin, N-alkylacrylamides, N-vinylacetamide, acrylamide, sodium 2-acrylamido-2-methyl-1 -propanesulfonate (AMPS) or acrylic acid, hyperbranched polymers or dendrimers including polyesteramides, polymers and copolymers of maleic anhydride, which have been reacted with alkylamines to form imide or amide groups, polysaccharides and derivatives of such including sugars and starch,

polyoxyalkylenediamines, small alcohols, small glycol ethers or ketones, proteins, peptides polyaminoacids, and amphiphilic molecules with molecular weight of less than 1000 Daltons. Preferably, the kinetic hydrate inhibitor polymer is a polymer, copolymer or graft polymer prepared from or one or more N-vinyl lactams, N-alkylacrylamides, N,N-dialkylacrylamide, N- alkylacrylamides, N,N-dialkylacrylamide, N-vinyl-N-alkyl alkanamides, or a hyperbranched poly(esteramide), or a peptide or protein including polyaspartamides or a polymer or copolymer containing pyroglutamate groups. Also suitable are mixtures of the compounds as herein described (containing two or more amine oxide groups) with homo- and copolymers of N,N-dialkylacrylamides such as N-acryloylpyrrolidone, N-acryloylmorpholine and N- acryloylpiperidine. Likewise suitable are mixtures with alkylpolyglycosides,

hydroxylethycellulose, carboxymethylcellulose and other ionic or nonionic surfactant molecules.

In particular, polymers containing two or more amine oxide groups and compounds according to Formula (I), particularly those where n=1 , i.e. bis-amine oxides, have been found to be very effective as KHI synergists.

The invention also provides use of the compounds as herein described as kinetic hydrate inhibitors. In particular polymers containing two or more amine oxide groups and compounds according to Formula (I), particularly those where n=1 , i.e. bis-amine oxides, have been found to be very effective as kinetic hydrate inhibitors.

Thus in a further embodiment, the method of the invention is a method is for inhibiting the formation of gas hydrates and said compounds are polymers containing two or more amine oxide groups or compounds according to Formula (I), particularly those where n=1 , i.e. bis- amine oxides.

Thus viewed from a further aspect, the present invention provides the use of a polymer containing two or more amine oxide groups or a compound according to Formula (I), particularly those where n=1 , i.e. bis-amine oxides as herein described as a kinetic hydrate inhibitor.

In a further aspect, the method of the present invention is a method for inhibiting

agglomeration of gas hydrates. Preferred compounds for this aspect are surfactants comprising two or more amine oxide groups, e.g. compounds of formula (I) in which n=1 , as herein described.

Thus viewed from a further aspect, the present invention provides the use of a compound comprising two or more amine oxide groups as herein described as a hydrate anti- agglomerant.

The compositions, methods and uses of the invention are applicable to any system or situation in which gas hydrate formation is desired to be controlled. In particular, they are applicable to systems for hydrocarbon drilling, production, storage and/or transportation, including production, drilling, completion, fracturing, stimulation and injection and re-injection operations. Typically, the "system" referred to herein is a fluid and/or a conduit.

Addition of the compounds to the system may be achieved through any known means and in amounts typical in the art. However, due to the surprising efficacy of the compounds of the invention, lower amounts may be required than of conventional hydrate inhibitor or anti- agglomerant compounds. Typical use concentrations, calculated as 100% of active substance, are 0.005 to 8%, preferably 0.0075 to 5%, more especially 0.01 to 3% especially concentrations of from 0.02 to 1 wt % (100-10,000 ppm) by weight based on the water present in the system.

The present invention is useful for inhibiting hydrate formation or inhibiting agglomeration of hydrates for many hydrocarbons and hydrocarbon mixtures, e.g. those which include methane, ethane, propane, n-butane, isobutane, isopentane and mixtures thereof. Other examples include various natural gas mixtures that are present in many gas and/or oil formations and natural gas liquids (NGL). The hydrates of all of these low-boiling hydrocarbons are also referred to as gas hydrates. The hydrocarbons may also comprise other compounds including, but not limited to C0₂, hydrogen sulphide, and other compounds commonly found in gas/oil formations or processing plants, either naturally occurring or used in recovering/processing hydrocarbons from the formation or both, and mixtures thereof. The methods and uses of the present invention involve contacting a hydrocarbon and water mixture with a compound or composition as described herein. When an effective amount of the compound/composition is used, hydrate blockage is inhibited. The contacting may be achieved by means of standard equipment such as injection pumps or the like, the good water solubility of the amine oxides resulting in rapid and uniform distribution of the inhibitor in the aqueous phase which has a tendency to form hydrates. It is generally sufficient for the aqueous amine oxide solution to be added to ensure uniform distribution.

The contacting can be made in-line or offline or both. When the compounds are added in a composition, the various components of the composition may be mixed prior to or during contact, or both. If needed or desired, the composition or some of its components may be optionally removed or separated mechanically, chemically, or by other methods known to one skilled in the art, or by a combination of these methods after the hydrate formation conditions are no longer present. The pressure at which the compounds/compositions are contacted with the

hydrocarbon/water mixture is usually at or greater than atmospheric pressure, (i.e. about 101 kPa), preferably greater than about 1 MPa, and more preferably greater than about 5 MPa. The pressure in certain formation or processing plants or units could be much higher, for example greater than about 20 MPa. There is no specific high-pressure limit. The present invention can be used at any pressure that allows formation of hydrocarbon gas hydrates.

Since the inhibitor primarily retards or prevents the formation of gas hydrates, the addition of the inhibitor should ideally take place before gas hydrates are formed, i.e. at above the equilibrium temperature of hydrate formation. The temperature for contacting is usually below, the same as, or not much higher than the ambient or room temperature. Lower temperatures tend to favour hydrate formation, thus requiring the treatment with the compositions/compounds of the present invention. For anti-agglomerant applications, the compound or composition may be added before or after hydrate formation, preferably before. In the methods and uses of the present invention, the compounds and compositions herein described may be added to the system at any stage or location suitable to inhibit formation or agglomeration of hydrates. The conduits into which the compounds/composition of the invention are added are typically hydrocarbon conduits extending for at least part of the length from the site within a hydrocarbon well at which hydrocarbon enters the borehole to the facility remote from the well at which hydrocarbon compositions are processed.

Typically, the compounds/compositions are added to a process stream containing

hydrocarbons and water by injection via a single port or multiple ports. In one aspect, the compound may be injected into the reservoir matrix surrounding a hydrocarbon production well. In a further aspect, the compound may be injected into a hydrocarbon production well. Preferably, the compound is injected at the well head. The compounds of the invention may be used alone or together with a further component, such as a hydrate inhibitor, a liquid solvent, a solid carrier and/or an excipient.

A further embodiment of the invention is the provision of hydrate inhibitor or anti-agglomerant compositions. Thus from a further aspect, the present invention provides a hydrate inhibitor or anti-agglomerant composition comprising a compound as herein described and a kinetic hydrate inhibitor, a solvent (e.g. a liquid solvent), a carrier (e.g. a solid carrier) and/or an excipient. In a particularly preferred aspect, the composition of the invention is a hydrate inhibitor composition comprising a kinetic hydrate inhibitor together with a compound as herein described. The compositions may be used in the methods and uses described herein.

The ratio of kinetic hydrate inhibitor to compound of the invention is preferably from 99:1 to 1 :99 by weight. Further preferred additives for use together with the compounds of the invention, in the methods, uses and compositions of the invention include polymers, amphiphiles and surfactants. These may be non-ionic or anionic. Examples are alkylpolyglycosides, hydroxylethycellulose, carboxymethylcellulose and other ionic or nonionic surfactant molecules. Especially preferred are anionic surfactants. Other suitable additives are corrosion inhibitors and scale inhibitors.

Suitable solvents, carriers and excipients are known in the art and include oxygenated solvents such as water, alcohols, ether solvents and mixtures thereof. Solvents, carriers or excipients are typically present in the inhibitor compositions in the range from 0 wt% to 95 wt%, e.g. 20 wt% to 95 wt%, preferably 50 wt% to 95 wt% of the total composition.

Preferably, the kinetic hydrate inhibitor polymer is a polymer, copolymer or graft polymer prepared from or one or more N-vinyl lactams, N-alkylacrylamides, N,N-dialkylacrylamide, N- alkylacrylamides, N,N-dialkylacrylamide, N-vinyl-N-alkyl alkanamides, or a hyperbranched poly(esteramide), or a peptide or protein including polyaspartamides or a polymer or copolymer containing pyroglutamate groups. Especially preferably the KHI is a

polyvinyllactam.

The present invention also provides novel methods for preparing compounds used in the hydrate inhibition methods, uses and compositions herein described. The synthesis of the amine oxides may be carried out according to known methods, preferentially by oxidation of the corresponding tertiary amine with peroxides or peracids, preferably by oxidation with hydrogen peroxide in aqueous or aqueous/alcoholic solution as shown e.g. in J. Am. Chem. Soc. 1957, 79, 964. Under these conditions a low-viscosity solution of the desired amine oxides in water or alcohol/water mixtures is produced directly. In principle, the products may also be employed as an anhydrous pure substance, but advantageously they are generally used in the form of an aqueous solution, to ensure convenient proportioning at low viscosity. However, it has now been found that alkylated (e.g. butylated) poly(amine oxides) can be produced in two easy steps from commercial hyperbranched polyethylene imines (HPEI).

Step 1 requires the use of an alkylating agent, e.g. an alkyl halide, e.g. butyl bromide (BuBr), a base and a solvent which is recovered after filtration. Step 2 requires hydrogen peroxide and an alcohol or glycol solvent which is left in the final product. The HPEI should ideally be water-free in Step 1 . Furthermore, bis-amine oxide surfactants can be produced by adding an esterification with a fatty acid (this is cheap and has only water as side product) between steps 1 and 2. Other types of poly- and bis-amine oxides can be produced using different starting materials, e.g. polyvinylamine, polylysine, linear polyethyleneimines etc.

These new preparation methods enable the hydrate inhibitors compounds according to the invention to be obtained significantly more cheaply than current commercial additives. Thus, viewed from a further aspect, the present invention provides a process for the preparation of an alkylated amine oxide said process comprising the steps of:

(i) reacting an amine, e.g. an ethyleneamine, with an alkylating agent, e.g. an alkyl halide and a base in the presence of a solvent and

(ii) reacting the product of (i) with hydrogen peroxide and an alcohol or glycol solvent.

In order to form a poly(amine oxide), a polyethyleneimine is used as the ethyleneamine. In order to form a non polymeric amine oxide product, e.g. a bis amine oxide, the amine is non polymeric. Preferred polymeric ethyleneamines are hyperbranched polyethylene imines. Preferred non-polymeric amines include ethylenediamine, diethylenetriamine (DETA) 1 ,6- hexanediamine and triethylenetetramine.

In a further aspect the present invention provides a process for the preparation of an alkylated amine oxide surfactant comprising two or more amine oxide groups, said process process comprising the steps of: (i) reacting an amine, e.g. an ethyleneamine, with an alkylating agent, e.g. an alkyl halide and a base in the presence of a solvent,

(ii) esterifying the product of (i) with a fatty acid (e.g. a C1₀-2₀ fatty acid or derivative thereof, e.g. a fatty acid chloride) and

(iii) reacting the product of (ii) with hydrogen peroxide and an alcohol or glycol solvent.

In the above processes, step 1 typically involves refluxing the reactant in the solvent (e.g. tetrahydrofuran) for 10-24 hours, e.g. around 16 hours. The mixture may then be filtered (solvent optionally removed in vacuo) and the residue dissolved in a solvent (e.g. isopropyl alcohol) before treating with hydrogen peroxide for around 16 hours with stirring. The resulting solution may be warmed to destroy excess hydrogen peroxide, leaving the product. Solvent residue may be removed in vacuo if desired. Examples of suitable bases are K₂C0₃, NaC0₃, NaOH, KOH etc. THF is a suitable solvent.

Preferably, the alkyl group of the alkyl halide is selected from the group consisting of

alkyl, preferably Ci_-6 alkyl, e.g. butyl. Preferred alkyl halides are n-butyl chloride and n-butyl bromide.

Compounds produced by these processes form a further aspect of the invention. They are also applicable to the uses, methods and compositions of the invention.

Until now, fully alkylated amine oxide derivatives of polyamines and non-polymeric ethylene amines have been difficult to prepare, however, the new preparation process herein described has made these compounds obtainable for the first time. Moreover, amide derivatives may be produced by reacting the products with organic carboxylic acids. Amine oxide derivatives may be produced according to the processes described above.

For example, polybutylating an ethyleneamine and leaving one N-H group unbutylated gives a mixture of isomers e.g. two isomers of tetrabutyl-DETA. This product is new as are its amide derivatives made by reacting tetrabutyl-DETA with organic carboxylic acids. The amine oxide derivatives, prepared, for example, as set out above are also new. These compounds and the products of these processes form further aspects of the present invention. Thus, from a further aspect, the present invention provides an alkylated amine oxide compound comprising two or more amine oxide groups, said compound being selected from the group consisting of

(i) alkylated polyethyleneimines and (ii) alkylated amine oxide derivatives of non-polymeric ethyleneamines such as

ethylenediamine, diethylenetriamine (DETA) and triethylenetetramine, wherein said alkyl groups are the same or different and are selected from the group consisting of C1-12 alkyl, preferably Ci_-6 alkyl, e.g. pentyl or butyl.

Preferred compounds are butylated polyethyleneimine oxide and tetrabutyl-DETA and the amide and amine oxide derivatives thereof.

These new compounds are also applicable to the methods, uses and compositions herein described.

The compounds described herein, particularly the polymers, may also be used to protect against corrosion, i.e. in some cases it may be unnecessary to use another molecule as a specific corrosion inhibitor if the compounds of this invention can do the job. Alternatively, less corrosion inhibitor may be necessary due to the partial protection provided by the compounds of the invention. The compounds described herein may also have biocidal or scale inhibition properties. Thus, from a further aspect, the present invention provides the use of a compound as herein defined as a corrosion inhibitor, a biocide or a scale inhibitor.

The invention will now be further described with reference to the following non-limiting examples:

Example 1 : Synthesis of polybutylated polvamine oxide based on polyethylene

Hyperbranched polyethyleneimine (MW 2000) was refluxed in tetrahydrofuran (THF) for 16 hours with 1.2 molar equivalents of potassium carbonate and butyl bromide per mol of nitrogen atoms. The mixture was filtered, solvent removed in vacuo from the filtrate, the residue dissolved in isopropyl alcohol (I PA) and treated with 1.1 molar equivalents of 30% hydrogen peroxide (per mol of nitrogen atoms) for 16 hours with stirring. The solution was warmed to 70 °C to destroy excess hydrogen peroxide to leave the polyamine oxide of polybutylated polyethylenediamine as a solution in IPA. The IPA can be removed in vacuo if desired.

Example 2: Synthesis of a bis-amine oxide (bisAO)

1 ,6-hexanediamine and 4 molar equivalents of potassium carbonate and n-butyl bromide in THF were refluxed for 16 hours. After filtration the solvent and volatiles were removed from the filtrate. The residue was dissolved in isopropyl alcohol (IPA) and treated with 2.1 molar equivalents of 30% hydrogen peroxide for 16 hours with stirring. The solution was warmed to 70 °C to destroy excess hydrogen peroxide to leave the 1 ,1 ,6,6-tetrabutylhexanediamine bis- oxide. The IPA can be removed in vacuo if desired.

Example 3: Synthesis of a bis-amine oxide surfactant

Diethylenetriamine (DETA) was butylated by refluxing with 4 molar equivalents of n-butyl bromide and 4 equivalents of potassium carbonate in THF for 16 hours. After filtration volatiles were removed from the filtrate. The residue, which is a mixture of butylated DETA products was dissolved in diethyl ether. One molar equivalent of triethylamine was added and the solution stirred. Lauroyl chloride (C11 H2₃COCI) was slowly added dropwise. The solution was stirred for a further 1 hour and then filtered. Volatiles were removed from the filtrate and the residue dissolved in isopropyl alcohol (IPA). This solution was treated with 1 .05 molar equivalents of 30% hydrogen peroxide for 16 hours with stirring. The solution was warmed to 70 °C to destroy excess hydrogen peroxide to leave C1 1 amido-bisAO which is a mixture of products. Other surfactant mixtures with C13-C17 tails were prepared similarly.

Example 4: Tetrahydrofuran hydrate crystal growth tests

Tetrahydrofuran (THF) forms Structure II hydrate crystals at about 4.4 °C under atmospheric pressure. NaCI (26.28 g) and THF (99.9%, 170 g) was mixed and distilled water was added to give a final volume of 900 ml_. This gives a stoichiometrically correct molar composition for making Structure II THF hydrate, THF.17H₂0. With this added salt the equilibrium temperature for THF hydrate formation is approximately 3.3 °C. The test procedure was as follows (M.A. Kelland and L. Del Villano, Chem. Eng. Sci., 2009, 64, 3197):

1 . 80 mL of the aqueous THF/NaCI solution was placed in a 100 mL glass beaker.

2. The test chemical was dissolved in this solution to give the desired concentration, for example 0.32g of polymer in 80 ml of solution gives a 0.4 wt.% (4000 ppm) solution of the polymer. 3. The beaker was placed in a stirred cooling bath pre-set to a set temperature, e.g. -0. °C (±0.05 °C) which represents about 3.8 °C subcooling.

4. The solution was briefly stirred manually with a glass rod every 5 minutes, without touching the glass beaker walls, whilst being cooled for 20 minutes.

5. A hollow glass tube with inner diameter 3 mm was filled at the end with ice crystals kept at -10 °C. The ice crystals are used to initiate THF hydrate formation.

6. The glass tube was placed almost halfway down in the cooled polymer/THF/NaCI solution after the solution had been cooled for 20 minutes.

7. THF hydrate crystals were allowed to grow at the end of the glass tube for 60

minutes.

8. After this time, the glass tube was removed and the amount of THF hydrate crystal formed at the tip was weighed.

Table 1 lists the results from the THF hydrate crystal growth tests. Table 1. THF hydrate crystal growth in gram/hr after 1 hr growth.

The results show that particularly, bis- or polyamine oxides with butyl groups are good at inhibiting the growth of THF hydrates.

Example 5: High Pressure Gas Hydrate Kinetic Hydrate Inhibitor Tests

To evaluate the performance of the hydrate inhibitors of this invention, the examples given herein use high pressure 40 ml stainless steel rocking cells placed in a cooling bath (RC5 equipment designed by PSL Systemtechnik, Germany) and a stainless steel jacketed 200 ml stirred cell, previously described (L. Del Villano and M.A. Kelland, Chem. Eng. Sci., 201 1 , 66, 1973). All tests were performed using distilled water and synthetic natural gas (SNG) forming a Structure II hydrate (Table 2).

Table 2. Composition of synthetic natural gas (SNG).

Component mole %

Methane 80.67

Ethane 10.20

Propane 4.90

/so-butane 1.53

n-butane 0.76

N₂ 0.10

C0₂ 1.84

5a. Rocking Cell Experiments

A description of the general test procedure in the rocking cells is given here:

1 ) The additive to be tested was dissolved or dispersed in distilled water to a specified active concentration.

2) The rocking cells were filled with 20 ml of the aqueous solution containing the additive to be tested. A steel ball was placed in each of the 5 cells and the cells sealed and placed in a cooling bath.

3) The temperature of the cooling bath was adjusted to 19.5 °C, just outside the hydrate region at the pressure conditions to be used in the experiment.

4) The cell was purged twice with the SNG with rocking at 30 bar.

5) The data logging was started, and the cell was loaded with the SNG to 76 bar pressure while rocking at 20 rocks/min.

6) When the temperature and pressure in the cell had stabilised the cell was cooled from 19.5 °C and 76 bar to 1 °C over 18.5 hours with 20 rocks/min at a 40° angle.

The onset temperature (To) for hydrate formation was recorded as the first drop in pressure not due to the temperature drop in a closed system. The temperature at which fast hydrate formation occurred, Ta, was also recorded. The results are given in Table 3.

Table 3. Steel Rocker Rig KHI Constant Cooling Test Results. (Average of minimum 5 results unless otherwise stated) Entry Additive Concentration To Ta

(ppm)

7 No additive (15 tests) 18.1 17.2

8 PVP Mw = 4k 5000 1 1.1 10.7

9 VCap:VP copolymer Mw = 6k 2500 8.1 5.9

10 VCap:VP copolymer Mw = 6k 5000 6.9 4.2

1 1 Tributylamine oxide 2500 12.5 1 1.1

12 Tributylamine oxide 5000 14.5 1 1.7

13 DETA Imine 2500 9.6 9.2

14 DETA Imine 5000 8.3 7.9

15 C1 1 amide-tetrabutylDETA-AO 2500 9.3 9.0

single isomer

16 C1 1 amide-tetrabutylDETA-AO 5000 9.1 8.1

single isomer

17 Bu-PAO 25k 5000 8.4 7.2

18 Bu-PAO 10k 5000 8.4 7.2

19 Bu-PAO 2k 5000 7.7 6.8

20 Pr-PAO 10k 5000 13.1 12.5

21 Pe-PAO 10k Not soluble

Clearly, bis or polyamine oxides particularly with n-butyl groups perform well as KHIs lowering the onset temperature compared to tests with no additives. 5b. Steel Autoclave KHI Experiments

Stirred Cell Experiments

A description of the general test procedure in the 200ml stirred steel cell is given here:

2) The cell was filled with 50ml of the aqueous solution containing the additive to be tested.

3) 8ml of the aqueous solution containing dissolved inhibitor was placed in the cell (above the cell bottom) as well as the cell housing using a pipette, and the top end piece was fitted.

4) The temperature of the cooling bath was adjusted to 19.5°C, just outside the hydrate region at the pressure conditions to be used in the experiment.

5) The cell was purged twice with the SNG with stirring at 30 bar.

6) The data logging was started, and the cell was loaded with the SNG to 76 bar pressure while stirring at 600 rpm. 7) When the temperature and pressure in the cell had stabilised the cell was cooled from 19.5 °C and 76 bar to 1 °C over 18.5 hours at 600 rpm stirring.

The onset temperature (To) for hydrate formation was recorded as the first drop in pressure not due to the temperature drop in a closed system. The temperature at which fast hydrate formation occurred, Ta, was also recorded. The results are given in Table 4.

Table 4. Steel Autoclave KHI Rig results: Constant cooling, 3 results unless otherwise stated

Example 6: High Pressure Anti-agglomerant (AA) Rocker Rig Experiments

The equipment used was a RCS20 sapphire cell rocker rig purchased from PSL

Systemtechnikk, Germany. 6 sapphire cells each with volume 20 ml were used. They were mounted in a 400 litre bath the temperature of which was controlled by a cooler/heater to an accuracy of about 0.1 °C. Each cell contained a steel ball which rolled back and forth in the cell when the cells were rocked. The ball run-times, as well as the pressure and temperature in each cell were recorded. A video recording was taken of all 6 cells in each experiment.

A description of the general AA test procedure is given here:

1 . The additive to be tested was dissolved or dispersed in 1.5 wt% aqueous NaCI solution or European white spirit to a specified active concentration based on the aqueous phase.

2. The 20ml sapphire cell was filled with 3 ml of the 1 .5 wt% saline solution and 6 ml of European white spirit to give a water cut of about 33%. The cell was then mounted in the rocker rig.

3. The temperature of the cooling bath was adjusted to 20.5°C, just outside the hydrate region at the pressure conditions to be used in the experiment. 4. Prior to loading the cell with recombined SNG the air was briefly pumped out and SNG reloaded to ca. 2 bar.

5. The data acquisition and video recording were started, and the cell was loaded with the SNG to the 76 bar pressure while rocking at 2 rocks/min and at a 40° angle.

For the constant cooling tests, the cells were cooled to 2 °C over 18.5 hrs with rocking.

Rocking was continued at 2 °C for a minimum of 14 hours or until no significant further pressure drop occurred. The ranking of the experimental results was made using ball run time, temperature and pressure data and visual observations.

For the start-up tests, gas hydrates were formed at 2 °C until no further pressure drop occurred. Then the bath was warmed and the hydrates were melted at 20 °C for 2 hours with rocking. Rocking was stopped and the bath cooled to 6 °C over ca. 10 hours. After the temperature stabilised at 6 °C rocking was started at 20 rocks/min. Rocking was continued at 6 °C for 14 hrs or until no significant further pressure drop occurred.

The ranking of the AA tests was as follows:

A - fine dispersed hydrates, no deposits, no significant change in run time throughout test

B - No plugging is observed in video, but run time a little increased (No deposits but coarse hydrate particles)

C - Significant deposits at end of cell, steel ball still moving slowly

D - plug of hydrate, run time decreases as the ball breaks up hydrate lumps after some hours E - plug of hydrate, ball stops moving

The results are given in Tables 5 and 6.

Table 5. Sapphire Rocker Rig AA Constant Cooling Tests Results:

Entry Additive Concentration To (°C) AA

based on water ranking phase (ppm)

28 No additive - 12.0-13.6 (6 tests) E

29 C13amido-Bis AO 15000 9-1 1 hrs after reaching A

2.0 (2 tests)

30 C13amido-Bis AO 10000 9-1 1 hrs after reaching E

2.0 (2 tests)

31 C13amido-Bis AO 5000 9-1 1 hrs after reaching E

2.0 (2 tests) 32 C13amido-Bis AO 10000 14 hrs after reaching A in 3.6 wt % NaCI 2.0 (2 tests)

33 C15amido-Bis AO 10000 14 hrs after reaching A

in 7.0 wt % NaCI 2.0 (2 tests)

34 C17amido-Bis AO 15000 2.1 and 2.7 C

35 C17amido-Bis AO 10000 2.1 and 2.9 C

Table 6: RCS20 Sapphire Rocker Rig AA Results: Memory effect start-up test results

Entry Additive Concentration To AA ranking

based on water (°C)

phase (ppm)

36 No additive - 6.0 E

37 C13amido-Bis AO 15000 6.0 A

38 C13amido-Bis AO 10000 6.0 A

39 C13amido-Bis AO 5000 6.0 E

40 C15amido-Bis AO 10000 6.0 A

41 (3.6% NaCI)

42 C15amido-Bis AO 5000 6.0 A

(3.6% NaCI)

43 C17amido-Bis AO 15000 6.0 A

44 C17amido-Bis AO 10000 6.0 A

Claims

1 . A method of inhibiting the formation or agglomeration of gas hydrates in a system, said method comprising adding to the system a compound comprising two or more amine oxide groups (i.e. groups of general formula R₃N⁺-0^~), or a salt thereof.

2. The method as claimed in claim 1 wherein the compound is of general formula (I)

(I) or a salt thereof, where

n is an integer from 1 onwards,

X is a bond or linker moiety.

3. The method as claimed in claim 1 or claim 2 wherein each R group, which may be the same or different, is a C_2-16 alkyl group

4. The method as claimed in claim 2 or claim 3 wherein X is -(CH₂)_y- optionally containing an ether (-0-), an ester (-C(=0)0-) or an amide (-C(=0)NH-), where y is an integer from 2 to 6.

5. The method as claimed in any one of the preceding claims wherein each R is

independently H or a Ci_-6 alkyl, wherein at least two of the R groups are C₃-₆ alkyl.

6. The method as claimed in any one of the preceding claims, wherein each R group independently denotes C_3-6 alkyl.

7. The method as claimed in any one of the preceding claims, wherein each R group independently denotes C_3-5 alkyl.

8. The method as claimed in any one of the preceding claims in which two or more of the R groups are independently chosen from n-butyl, n-pentyl and iso-pentyl groups.

9. The method as claimed in any one of the preceding claims wherein the compound is an oligomer or a polymer.

10. The method as claimed in any one of the preceding claims wherein said system is one for hydrocarbon drilling, production, storage and/or transportation, including production, drilling, completion, fracturing, stimulation and injection and re-injection operations.

1 1 . The method as claimed in any one of the preceding claims wherein said method is for inhibiting the formation of gas hydrates and said compound is a polymer.

12. The method as claimed in any one of the preceding claims wherein said method is for inhibiting the formation of gas hydrates and said compounds are polymers containing two or more amine oxide groups or compounds according to Formula (I) where n=1 .

13. The method as claimed in any one of the preceding claims wherein said method is for inhibiting the agglomeration of gas hydrates and said compound is a surfactant comprising two or more amine oxide groups.

14. The method as claimed in any one of the preceding claims further comprising adding a kinetic hydrate inhibitor to said system.

15. Use of a compound as defined in any one of the preceding claims for inhibiting the formation or agglomeration of hydrates in a system, preferably a system for hydrocarbon drilling, production, storage and/or transportation, including production, drilling, completion, fracturing, stimulation and injection and re-injection operations.

16. A hydrate formation inhibitor or anti-agglomerant composition comprising a compound as defined in any one of the preceding claims and kinetic hydrate inhibitor, a solvent, a carrier and/or an excipient.

17. A process for the preparation of an alkylated amine oxide said process comprising the steps of:

(i) reacting an amine, e.g. an ethyleneamine with an alkylating agent e.g. an alkyl halide and a base in the presence of a solvent and (ii) reacting the product of (i) with hydrogen peroxide and an alcohol or glycol solvent.

18. An alkylated amine oxide produced by the process of claim 17.

19. An alkylated amine oxide compound comprising two or more amine oxide groups, said compound being selected from the group consisting of

(i) alkylated polyethyleneimines and

(ii) alkylated amine oxide derivatives of non-polymeric ethyleneamines such as