Hydrate inhibition

Abstract

A method of inhibiting or retarding formation of hydrates from water and a lower alkane, such as methane or ethane, involves adding additives which comprise (i) a corrosion inhibitor and (ii) a salt which is of formula [R.sup.1 (R.sup.2)XR.sup.3 ].sup.+ Y.sup.-, wherein each of R.sup.1, R.sup.2 and R.sup.3 is bonded directly to X, each of R.sup.1 and R.sup.2, which may be the same or different is an alkyl group of at least 4 carbons, X is S, NR.sup.4 or PR.sup.4, wherein each of R.sup.3 and R.sup.4 which may be the same or different represents hydrogen or an organic group, with the proviso that at least one of R.sup.3 and R.sup.4 is an organic group of at least 4 carbons, and Y is an anion, the additives being added in an amount effective to inhibit or retard hydrate formation, to a medium susceptible to hydrate formation.

Description

BACKGROUND OF THE INVENTION

The present invention relates to hydrate inhibitors and a method for inhibiting the formation of hydrates in particular to a method for inhibiting the formation of hydrates in the petroleum and natural gas industries.

Hydrates are formed of two components, water and certain gas molecules, such as alkanes of 1-4 carbons, especially methane and ethane, such as those found in natural gas. These `gas` hydrates will form under certain conditions, i.e. when the water is in the presence of the gas and when the conditions of high pressure and low temperature reach respective threshold values. The gas may be in the free state or dissolved in a liquid state, for example, as a liquid hydrocarbon.

The formation of such hydrates can cause problems in the petroleum oil and natural gas industries.

Hydrate formation in the field may cause blocked pipelines, valves and other process equipment.

The problem is particularly of concern as natural gas and gas condensate resources are discovered where operating conditions surpass these threshold values, i.e. in deep cold water and on-shore in colder climates.

Hydrates can also form in association with the underground hydrocarbon reservoir thus impeding production by blockage of reservoir pores.

The problem of hydrate formation is however commonest during gas transportation and processing, the solid hydrate precipitating from moist gas mixtures. This is particularly true with natural gas which when extracted from the well is normally saturated with water. Often in such a case, in a cold climate, hydrates will form in downstream transportation networks and this can cause large pressure drops throughout the system and reduce or stop the flow of natural gas.

Hydrate formation may also occur during natural gas cryogenic liquefaction and separation.

A typical situation where hydrate formation can occur is in offs hore operations where produced fluids are transported in a long vertical pipeline, for example, a riser system. Such produced fluids normally include light gases known to form hydrates and water. In such a situation a temperature of 4.5.degree. C. and a pressure of 150 psi would be sufficient for hydrate formation.

Several methods are known to prevent hydrate formation and subsequent problems in pipelines, valves and other processing equipment.

Physical methods have been used, such as increasing gas temperature in the pipeline, drying the gas before introduction into the pipeline, or lowering the gas pressure in the system. However, these techniques are either expensive or are undesirable because of loss of efficiency and production.

Chemical procedures have also been used. Electrolytes, for example, ammonia, aqueous sodium chloride, brines and aqueous sugar solutions may be added to the system.

Alternatively, the addition of methanol or other polar organic substances, for example, ethylene glycol or other glycols may be used. Methanol injection has been widely used to inhibit hydrate formation. However, it is only effective if a sufficiently high concentration is present since at low concentrations there is the problem of facilitation of hydrate formation. Also for methanol to be used economically under cold environmental conditions there must be early separation and expulsion of free water from the well in order to minimise methanol losses in the water phase.

SUMMARY OF THE INVENTION

We have now found certain additives which may be used as effective hydrate inhibitors at low concentrations. Thus according to the present invention, there is provided a method of inhibiting or retarding hydrate formation, which method comprises adding additives (hereinafter called Additives) which comprise (i) a corrosion inhibitor and (ii) a salt which is of formula [R.sup.1 (R.sup.2)XR.sup.3 ].sup.+ Y.sup.-, wherein each of R.sup.1, R.sup.2 and R.sup.3 is bonded directly to X, each of R.sup.1 and R.sup.2, which may be the same or different is an alkyl group of at least 4 carbons, X is S, NR.sup.4 or PR.sup.4, wherein each of R.sup.3 and R.sup.4 which may be the same or different represents hydrogen or an organic group, with the proviso that at least one of R.sup.3 and R.sup.4 is an organic group of at least 4 carbons, especially at least 5 carbons, and Y is an anion, the additives being added in an amount effective to inhibit or retard hydrate formation, to a medium susceptible to hydrate formation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The Additive (i) is a corrosion inhibitor such as for steel and usually one suitable for use in anaerobic environments. It may be a film former, capable of being deposited as a film on a metal such as a steel surface such as a pipeline wall. It preferably has surfactant activity and especially surface wetting activity.

It is especially a nitrogenous compound with 1 or 2 nitrogen atoms. The corrosion inhibitor may be a primary, secondary or tertiary amine, or a quaternary ammonium salt, usually in all cases with at least one hydrophobic group, usually a benzene ring or a long chain alkyl group of, for example 8-24 carbons. It may be a quaternary ammonium salt, a long chain aliphatic hydrocarbyl N-heterocyclic compound or a long chain amine. The quaternary salt may be an (optionally alkyl substituted) benzyl trialkyl ammonium halide, in particular when at least 1 and especially 1 or 2 alkyl groups is of 1-20, in particular 8-20 carbons such as cetyl and the other alkyl groups are of 1-6 carbons such as methyl or ethyl; examples are benzyl alkyldimethyl ammonium chloride and Benzalkonium chlorides.

Other quaternary ammonium salts may be of formula [R.sup.5 R.sup.6 NR.sup.7 R.sup.8 ].sup.+ Z.sup.-, wherein Z is an anion such as a halide or sulphate, R.sup.5 is an alkyl or alkenyl group of at least 8 carbons, R.sup.6 is an alkyl or alkenyl group, each of at least 2 carbons or a N-heterocyclic group, and R.sup.7 and R.sup.8, which may be the same or different represents an alkyl group, with the proviso that at least one of R.sup.6 -R.sup.8 has less than 4 carbon atoms. R.sup.5 may be of 8-24 carbons, such as 10-18 carbons, especially, dodecyl, lauryl, cetyl, palmityl, stearyl or oleyl, while R6 may be selected from the same groups as R.sup.5, or may be ethyl, propyl, isopropyl, butyl or hexyl. R.sup.7 and R.sup.8 may be selected from the same groups as R.sup.6 but preferably represent methyl groups. Examples of these quaternary salts are cetyl trimethyl ammonium, dodecyl trimethylammonium and lauryl trimethylammonium halides, such as chlorides or bromides.

Other quaternary salt corrosion inhibitors are of formula [R.sup.9 NR.sup.10 R.sup.11 ].sup.+ Z- where Z is a anion such as those as defined above, R.sup.9 N or R.sup.9 NR.sup.10 forms a quaternizable N heterocyclic ring, and R.sup.11 represents an alkyl or alkenyl group each of at least 8 carbons such as as described for R.sup.5. The R.sup.9 N group may be N-heterocyclic group with 1 or 2 ring N atoms, especially with 1 or 2 heterocyclic rings, such as of 5 or particularly 6 ring atoms; examples of the rings are saturated ones such as piperidine. The group R.sup.9 NR.sup.10 may also be such an N heterocyclic group but with the R.sup.9 and R.sup.10 groups combined with the N atom to which they are bonded to form an unsaturated ring or fused N bridged ring system such as a pyridine ring. R.sup.10 if present may otherwise be an alkyl or alkenyl group described for R.sup.8. Examples of these quaternaries are cetyl pyridinium halides, such as the chloride.

The corrosion inhibitor may also be a long chain aliphatic hydrocarbyl N-heterocyclic compound, which is not quaternised. The aliphatic hydrocarbyl group in the heterocyclic compound usually has 8-24 carbons in the hydrocarbyl group, preferably a linear saturated or mono or diethylenically unsaturated hydrocarbyl group; cetyl-, stearyl and especially oleyl- groups are preferred. The N-heterocyclic compound usually has 1-3 ring N atoms, especially 1 or 2 which usually has 5-7 ring atoms in each of 1 or 2 rings; imidazole and imidazoline rings are preferred. The heterocyclic compound may have the aliphatic hydrocarbyl group on an N or preferably C atom in the ring; the ring may also have an amino-alkyl (such as 2-amino ethyl) or hydroxyalkyl (such as 2-hydroxyethyl) substituent, especially on an N atom. N-2-aminoethyl-2-oleyl-imidazoline is preferred. The long chain amine usually contains 8-24 carbons and preferably is an aliphatic primary amine, which is especially saturated or mono ethylenically unsaturated; an example is dodecylamine. Mixtures of any of the above corrosion inhibitors with each other may be used, such as a quaternary ammonium salt and a long chain aliphatic hydrocarbyl-N-heterocyclic compound (where each is preferably as described above), or mixtures with a tertiary aliphatic amine.

If desired the corrosion inhibitor such as a long chain amine may also comprise a phosphate ester salt, especially one with surface wetting activity. Such phosphate esters are anionic surfactants, which are salts of alkali metals such as sodium or a quaternary ammonium such as tetra methyl ammonium or tetrabutyl ammonium salts of acid phosphate esters, preferably with 1 or 2 organic groups and 2 or 1 hydrogen atoms; examples of the organic groups are allyl or alkenyl groups as described for R.sup.5 above. Examples of such phosphate ester salts are mono and dioctyl acid phosphate salts and mixtures thereof. A preferred blend comprises a long chain alkylamine and a phosphate ester salt for example as sold as NAL 1272 by Nalco.

The other essential Additive (ii) is a salt of formula I, [R.sup.1 R.sup.2 XR.sup.3 ].sup.+ Y.sup.-, in which each of R.sup.1 and R.sup.2 is an alkyl group of at least 4 carbons, which may be a linear alkyl or branched alkyl group, such as a secondary or tertiary alkyl group or especially an isoalkyl group. Each of R.sup.1 and R.sup.2 may be an alkyl group of 4-24 carbons, preferably 4-10 and especially 4-6 carbons, such as n-butyl, isobutyl, secbutyl, tertiary butyl, n-pentyl, sec pentyl, isopentyl or tertiary pentyl group, or hexyl group. R.sup.3 and R.sup.4 (if present as is preferred) are each hydrogen or an organic group of 1-24 carbons such as an alkyl or alkenyl group each preferably of 8-20 carbons, as is described for R.sup.5 above; however at least one or R.sup.3 and R.sup.4 contains at least 4 carbons preferably at least 5 carbons, especially in a group with a linear chain containing such numbers of carbon atoms. R.sup.3 may preferably be an alkyl group of 10-16 carbons, especially a mixture of 2 or more such alkyl groups. R.sup.4 may be hydrogen but is preferably alkyl of 1-10 carbons, such as methyl or ethyl but especially of 4-6 carbon atoms, such as is described for R.sup.1 or R.sup.2. Y is an anion which may be mono- or divalent and may be as defined for Z above. Preferred salts are tetra n-butyl, tetra n-pentyl, tetra-iso-pentyl ammonium (and phosphonium) halides such as chlorides or especially bromides, and C.sub.10-16 alkyl tri n-butyl ammonium (and phosphonium) halides, especially chlorides or bromides. Tri n-butyl, n pentyl or isopentyl sulphonium halides such as chlorides or bromides may be used.

The Additives (i) and (ii) may be present in weight ratios of 1-99:99:1 such as 10-90:90:10, but especially with a weight excess of Additive (ii) such as with a ratio of (i) to (ii) of 10-45:90-55. Mixtures of the Additives (i) and (ii) form another aspect of the invention, so there is also provided a blend which comprises a mixture of at least two Additives (i) and (ii).

The Additives (i) and (ii) for use as hydrate inhibitors are preferably water soluble, to at least 10 g/l in water at 20.degree. C. They may be used undiluted, but preferably are in solution such as aqueous solution, for example, as a solution in brine, or preferably an alcohol, for example, a water miscible one such as methanol or ethanol. Preferably are used Additives (i) and (ii), an aqueous solution of which has a pH 1.5-12, preferably 4-9, either naturally or after adjustment of the pH. Additives (i) and/or (ii) may be used in alcoholic solution.

Each Additive is suitably injected at concentrations in the range 10 to 20,000 ppm, preferably 30 to 10,000 ppm, especially 50-1200 ppm based on the total water volume in the medium, in which hydrate formation is to be inhibited, in particular at concentrations in the range 100-2000 preferably 200-1500 ppm for Additive (i) and 300-6000 preferably 500-5000 ppm for Additive (ii). The amount of methanol, ethanol, or mono, di or tri ethylene glycol added relative to the total water volume in the medium is usually less than 10%, preferably less than 5% or 2%, but especially less than 10,000 ppm, preferably 1000-8000 ppm.

The inhibitors may be injected at normal ambient conditions of temperature and pressure.

There may also be present with the Additive (i) at least one Additive (ix) which is at least one water soluble polymer of a polar ethylenically unsaturated compound and/or at least one Additive (x) which is a hydrophilic colloid. The Additive (ix) is usually water soluble to at least 10 g/l at 20.degree. C. and advantageously has a molecular weight of 1000-1500,000, preferably 5000-1,000,000, preferably 200,000-1,000,000 and especially 400,000-900,000. The ethylenically unsaturated compound is preferably a vinyl or methyl vinyl group, and the polar group may be an alcohol, carboxylic acid, sulphonic acid or N-heterocyclic group, especially pyrrolidone. Preferred polar compounds are thus vinyl sulphonic acid, acrylic and methacrylic acids and N-vinyl pyrrolidone and "vinyl alcohol". The polymers may be copolymers, but are preferably homopolymers of these polar compounds, especially polyvinyl alcohol (such as hydrolysed polyvinyl acetate), polyacrylates and polyvinyl pyrrolidone (PVP). The amount of said polymer is usually 10-1000%, such as 50-300% or 90-250% based on the weight of the total of Additive(s) (i) and (ii).

The hydrophilic colloid is an organic solid which is soluble in boiling water, preferably to at least 10 g/l or dispersible in boiling water and may be soluble (at least 10 g/l) or dispersible in water at 20.degree. C. It usually absorbs water strongly, preferably to at least three times such as 3-15 times its weight of water at 20.degree. C., and swells in water. It can form a colloidal solution or dispersion in water and may have an average molecular weight of at least 10,000, preferably 100,000-10,000,000. It may be a polysaccharide, preferably with at least 4 carbohydrate units, especially one with at least some galactose units, preferably 20-60% of such units, and may contain carboxylic acid residues, so that an aqueous solution or dispersion thereof can have an acidic reaction. The polysaccharide may be a natural gum, such as guar, agar, arabic, locust bean, karaya, carob or tragacanth gum, or a cellulosic material, such as starch, which may be unmodified or modified as an alkyl ether, such as methyl or ethyl cellulose or hydroxyalkyl ether, such as hydroxyethyl cellulose or carboxy alkylated starch, such as carboxy methyl cellulose (CMC). The polysaccharide may also be a synthetic, such as biosynthetic gum, the result of a microbiological process, such as fermentation; xanthan gum, which can be made by fermentation of dextrose with Xanthomonas campestris cultures, which is preferred, especially water soluble versions of xanthan gum. The colloid may also be proteinaceous, in particular gelatin or carrageenan (a seaweed extract), such as x-carrageenan. The colloid may also be a polyuronic acid or salt thereof, such as sodium or ammonium salt or ester thereof, such as a hydroxy alkyl ester (for example of propylene glycol), especially with beta-D-mannuronic acid residues; alginic acid and especially sodium alginate is preferred. The amount of Additive (x) may be 10-1000%, eg 50-300% or 90-250% by weight based on the total weight of Additives (i) and (ii).

Preferably there is present as Additive (xi) a Polymer of an ethylenically unsaturated N- heterocyclic carbonyl compound. The Polymer has a hydrocarbon chain with pendant N-lieterocyclic carbonyl groups, with the bonding to the chain via the heteroring -N- atom. The carbonyl group may be in any position in the N heteroring, but is especially alpha to the N hetero atom, so the N-heterocyclic rings are such as derived from lactams, especially of 4-8 preferably 5 or 6 ring atoms, such as those derived from butyric, pentenoic, pentanoic or hexanoic acid lactams (or 2-pyrrolidone, 2-pyridone, 2-piperidone or omega caprolactam). The polymer may have structural units from N- vinyl pyrrolidone and may be a homopolymer or copolymer, whether a graft copolymer or otherwise.

Preferably the Polymer as Additive (xii) has an aliphatic (N-heterocyclic carbonyl) polymer with a hydrocarbon backbone. It is water soluble or water dispersible, preferably to an extent of at least 0.01% by weight in water such as at least 0.05% but especially at least 0.5%, such as up to 10% by weight. Its molecular weight is usually 5000 to 1000000 preferably 10000 to 1000000 such as 1000 to 50000 and preferably has a K value of 10-150 especially 15-50, wherein the K value is obtained from the relative viscosity in aqueous solution via the FIKENTSCHER'S Formula, from which the average molecular weight is calculated as described in U.S. Pat. No. 2,811,499. The Polymer has a hydrocarbon chain with pendant N-heterocyclic carbonyl groups, with the bonding to the chain via the heteroring -N- atom and the N- heterocyclic carbonyl groups as described further above. The aliphatic group or groups in the polymer may be part of the hydrocarbon chain, or bonded to it or to the N-heterocyclic carbonyl ring; the aliphatic group may be linear or branched and maybe alkyl of 1-40 preferably 2-25 carbons or alkenyl preferably of 2-20 carbons, especially methyl, ethyl, butyl or octyl, tetradecyl, hexadecyl, octadecyl, eicosyl, tricosyl or ethylene, butylene or octylene. The molar ratio of aliphatic group to heterocyclic carbonyl group in the Polymer is usually 1:99 to 20:80 preferably 5-15:95-85.

The Polymer may be a copolymer having repeat units derived from at least one monomer which is an optionally alkyl substituted vinyl N-heterocyclic carbonyl compound) and at least one monomer which is an olefin; this copolymer may be simple copolymer formed by copolymerization of the monomers or a graft copolymer formed by grafting the olefin onto a polymer of the N-heterocyclic monomer. The Polymer may also be an alkylated derivative of a polymer of an optionally alkyl substituted (vinyl N-heterocyclic compound) especially a homopolymer of such a compound.

The optionally alkyl substituted vinyl N-heterocyclic carbonyl compound may be of general formula:

R.sup.10 R.sup.11 C.dbd.CR.sup.12 R.sup.13 wherein each of R.sup.11, R.sup.12 and R.sup.13, which may be the same or different, represents a hydrogen atom or an alkyl group of 1-20 carbons, such as methyl, ethyl, butyl, hexyl, decyl or hexadecyl, and R.sup.10 represents an N-heterocyclic carbonyl group with the free valency on the N atom; such as the N heterocyclic carbonyl group is as described above. The N-heterocyclic ring may contain 1-3 ring N atoms but especially 1 ring N atom and 0-2 other ring hetero atoms such as 0 or 5, but especially no ring hetero atom; the ring may contain in total 1 or 2 rings, which may be saturated or ethylenically unsaturated such as a pyrrolidine, piperidine, quinoline or pyridine ring. Preferably R.sup.11, R.sup.12 and R.sup.13 are hydrogen and R.sup.10 represents an N-(pyrrolidone), N-(2 pyrid-2-one) or N-(piperid-2-one) group.

The olefin is usually of 2-32 eg 4-18 carbon atoms and is generally a hydrocarbon. It is such as an alkene, especially a linear alkene and has in particular a terminal olefin group. It is such as a vinyl olefin eg of formula CH.sub.2 .dbd.CH--R.sup.14, where R.sup.14 is hydrogen or alkyl of 1-40 carbons, such as methyl, ethyl, propyl, butyl, hexyl or decyl, tetra decyl, octadecyl or octacosyl (so the olefin is tricosene). The olefin is such as butylene octene-1 or dodecene-1, hexa decene-1, octadecene-1, eicosene-1 or tricosene-1.

The Polymer may be made by free radical copolymerizing the N-heterocyclic carbonyl compound such as N-vinyl pyrrolidone with the olefin such as butylene in solution in the presence of a peroxide catalyst. The Polymer may also be made by free radical grafting of the olefin onto a polymer of the N-heterocyclic carbonyl compound such as poly vinyl pyrrolidone (PVP) with K value as described above. The copolymerizations may incorporate structural units from the olefin into the hydrocarbon polymer chain and/or insert such units into the N-heterocyclic rings.

The Polymer may also be made by direct alkylation of the polymer of the N-heterocyclic carbonyl compound such as PVP with an alkylating agent such as an alkyl halide such as butyl bromide or octyl bromide, optionally in the presence of a base, such as triethyl amine.

Finally the Polymer may be a homo or copolymer of an alkyl substituted N-(alkenyl) heterocyclic compound in which the alkyl substitutuent may be in the N-heteroring and/or present in the alkenyl side chain, the alkyl substituent may be as is preferred for the aliphatic group on the Polymer described above. The Polymer may have structural units from an N-vinyl-alkyl ring substituted heterocyclic carbonyl compound, such as N-vinyl-3-methyl pyrrolid-2-one and/or from an N-butenyl-heterocycle carbonyl compound, such as N-butenyl-pyrrolid-2-one. Such Polymers may be made by polymerization in solution in the presence of a free radical catalyst, in an analoguous way to polyvinyl pyrrolidine.

Thus the preferred Polymers are aliphatic (N-heterocyclic carbonyl) polymers with units derived from N-vinyl pyrrolid-2-one and butylene (sold as Antaron P 904), octylene, dodecylene, hexadecylene (sold as Antaron V216), eicosylene and tricosylene; the Antaron products are sold by International Speciality Products of Wayne, N.J., USA.

The Polymer may also be as Additive (xiii) a copolymer of (a) at least 1 ethylenically unsaturated N-heterocyclic ring compounds especially with at least one of (b) a different ethylenically unsaturated N-heterocyclic ring compound, (c) an ethylenically unsaturated carbocyclic carbonyl compound, and (d) a polar ethylenically unsaturated compound, different from said N-heterocyclic compounds. Polymer Additive xiii such as consists essentially of structural units derived from at least 2 of said (a)-(d) compounds, in particular (a), (b) and (d). Polymers (xiii) from heteroring ring compounds with different size rings for example with differences of 1-3 in the numbers of ring atoms are preferred, especially ones with 5 and 7 membered heterorings. Examples of the ethylenically unsaturated N-heterocyclic ring compounds are ones described above in relation to Polymers (xi) especially N-vinyl-pyrrolidone and N-vinyl omega caprolactam. The carbocyclic compound may be one with 4-8 preferably 6 or 7 ring atoms with the carbonyl group in the ring such as adjacent to a carbon atom carrying the ethylenically unsaturated group which is preferably an alkenyl group of 2-6 carbon atoms especially with a terminal CH.sub.2 .dbd.CH-group. The carbocyclic compounds are cyclic ketones with an unsaturated side chain. Examples of the carbocyclic compound are 2-vinyl-cyclohexanone and 2-vinyl-cycyloheptanone. The polar ethylenically unsaturated compound may be an ester of an alcohol containing more than 1 polar group and an unsaturated acid. The alcohol usually contains a hydroxyl group and at least one other hydroxyl group or aminogroup, which may be a primary secondary or especially tertiary amino group in particular in a non cyclic arrangement; thus the alcohol may be a diol or an amino alcohol, especially an aliphatic one, such as a dialkylaminoalkanol, with 1-4 carbons in each alkyl and 2-4 carbons in the alkanol. 2-Dimethylamino ethanol is preferred. The unsaturated acid is usually an aliphatic alkenoic acid with 3-10 carbons such as acrylic or methacrylic or crotonic acid. This unsaturated ester is especially a (meth)acrylate ester of a dialkylamino alkanol in particular dimethylaminoethyl acrylate or methacrylate. Polymer Additive xiii may be as described in WO94/12761, the disclosure of which is herein incorporated by reference.

The Polymer may contain structural units in molar % derived from 10-90% of the N-heterocyclic compound, 10-90% of the carbocyclic compound and 10-90% of the polar ethylenically unsaturated compound. A preferred compound is a copolymer of N-vinyl pyrolidone, N-vinyl-omega caprolactam and dimethylamino ethyl methacrylate, such as is sold as Antaron VC713 by International Speciality Products of Wayne, N.J.

The amount of said Polymer (xiii) is usually 10-1000% such as 50-300% or 90-250% based on the weight of the total of Additives (i) and (ii). Mixtures of the aliphatic (N-heterocyclic carbonyl) polymer (xii) and the copolymer (xiii) may be used especially in weight ratios of 10-90:90-10 in particular a majority of (xiii) in weight ratio of (xii) to (xiii) of 10-40:90-60.

The Formulations preferably contain at least one anti-foaming agent (xiv) especially when the Additive (i) or (ii) has foaming activity, such as when either contains at least one alkyl group of at least 8 carbon atoms. Examples of suitable anti-foaming agents are silicon containing compounds especially organosilicon oxygen or nitrogen compounds, such as polysiloxanes, including cyclic polysiloxanes, silicon polyethers, polysilazanes and fluorosiloxanes. Alkylpolysiloxanes are preferred especially dimethyl polysiloxanes such as 3556 from Th. Goldschmidt KG and AF 1520 from Dow Corning. The silicon anti-foaming agents are usually used as water dispersible emulsions. Silicon free anti-foaming agents, such as suds depressants used in detergents, may also be used. The amount of the anti-foaming agent is usually 10-70 ppm preferably about 40 ppm (based on the total water present), or 10-70 ppm preferably about 40 ppm (based on the total weight of Additives (i) and (ii)).

Formulations comprising Additive (i) and (ii) and preferably at least one of Additive (ix)-(xiii) may be used in total amount of 50-10,000 ppm, especially 150-2000 ppm, or 2000-8000 such as 4000-6000 ppm relative to the total water in the medium in which hydrates may form (including any water added in the formulation).

The Formulation may also contain another hydrate inhibitor and/or a water dispersant or surfactant, in particular an anionic one such as sodium dodecyl sulphonate or stearic acid and in amount of 1-10% of the Formulation weight and/or a biocide, such as formaldehyde, preferably in amount of 10-10,000 ppm and/or a metal complexant such as citric acid (for example in amount of 10-10,000 ppm) all amounts being in relation to the total weight of the Formulation.

The Formulations may be used to retard or inhibit hydrate formation in the same manner as the individual Additives, as described above.

The inhibitor Additives and Formulations of the present invention are suitable for use in media containing water and gas, in particular in the petroleum, natural gas and gas industries. The gas may be a hydrocarbon normally gaseous at 25.degree. C. and 100 KPa pressure, such as an alkane of 1-4 carbon atoms such as methane, ethane, propane n or isobutane, or an alkane of 2-4 carbon atoms such as ethylene, propylene, n- or isobutene; the gas preferably comprises by weight at least 80% and especially at least 90% of methane with 0.1-10% preferably 1-5% C.sub.2 hydrocarbon and 0.01-10% preferably 0.05-5% C.sub.3 hydrocarbon. A natural gas, which may or may not have been purified or processed is preferred. The gas may also contain nitrogen for example in amount of 0.01-3% by weight and/or carbon dioxide preferably in amount of 0.1-5% such as 0.5-2% by weight. The Additive (i) and (ii) can be more effective than poly vinyl pyrrolidone in the inhibition of gas hydrate formation, especially when the gas comprises carbon dioxide.

In particular, they may be suitable for use during the transportation of fluids comprising gas and water such as from oil or gas wells. They may also be suitable for use in oil based drilling muds to inhibit hydrate formation during drilling operations.

In another aspect therefore the invention provides an oil based drilling mud, which comprises as hydrate inhibitor at least one Additive, as such or in a Formulation.

When used during the transportation of fluids, such as gases with water and optionally oil such as condensate in conduits such as pipelines the inhibitors may be injected continuously or batchwise into the conduit upstream of conditions wherein hydrate formation may occur. Conditions under which gas hydrates may form are usually at greater than -5.degree. C. preferably greater than 0.degree. C. such as 0 to 15.degree. C. preferably 1-10.degree. C. and pressures such as of 0.1-30 MPa preferably 1-15 MPa the temperature of onset of gas hydrate formation depending on the pressure, and the presence of, and concentration of, salt in the water. As the temperature decreases and the pressure increases and the concentration of salt decreases the greater is the likelihood for hydrate formation to happen in the absence of the Additives and Formulations of the invention. Thus the conditions of use of the Additive are ones such that in its absence a gas hydrate may form. In the absence of the Additive ice may also form in addition to the gas hydrate especially at temperatures of -5.degree. C. to 5.degree. C. The pH of the water in the pipeline after addition of the Additives or Formulation is usually 3-9, especially 3.3-5 or 5-7.5.

In drilling operations the inhibitors may be added to the drilling muds in the mud tank at the wellhead.

The invention is illustrated in the following Examples.

EXAMPLES

To assess the efficiency of hydrate inhibitors suitable for use in the method of the present invention, tests were carried out using the following procedure:

The hydrate inhibitor test apparatus consisted of a simple 316 stainless steel pressure cell, with a usable internal volume of 1000 cm.sup.3 with a thermostated cooling jacket, a sapphire window, an inlet and outlet and a platinum resistance thermometer. The cell contained water which was stirred by a magnetic pellet. Temperature and pressure were monitored and the results provided on a computer data logger; gas hydrates were also detected visibly using a time lapse video recording system. Before each test the cell was cleaned thoroughly by soaking successively in 10% aqueous hydrochloric acid for 1 hour, 10% aqueous sodium hydroxide solution for 1 hour and then double distilled water.

Into the cell was placed 200 cm.sup.3 of pre-chilled double distilled water with or without the chemical to be tested. A PTFE stirrer pellet was then placed in the cell and the pH of the solution measured with subsequent adjustment if desired by the addition of small but concentrated amounts of hydrochloric acid or sodium hydroxide. After sealing the cell the water was then stirred at 500 rpm and allowed to cool to the operational temperature of 4.degree. C. When this temperature was reached the stirrer was stopped and the video recorder started. Methane was then admitted to the cell until the pressure reached 70 bar (7 MPa) and the temperature, pressure and time were noted. The stirrer was restarted to run at 500 rpm and the time noted. Hydrates were observed to form in the vessel when the solution in the vessel turned opaque, coincident with which was a sharp temperature increase of about 0.2.degree. C. and a gradual pressure reduction. The time from first contact of water and gas to formation of hydrate was read from the logger.

The experimental conditions are a very severe and accelerated test of gas hydrate formation and inhibition. The amounts of the Additive are expressed in ppm based on the volume of water. The inhibition time results given are an average of several results.

In the Examples, the following Additives were used:

Additive (i)(a) Benzalkonium chloride, which was benzyl hexyidimethylammonium chloride

Additive (i)(b) was a corrosion inhibitor blend sold by Nalco as "NAL 1272" believed to contain a tertiary amine and/or quaternary ammonium salt and/or aliphatic imidazoline, and possibly also a phosphate ester.

Additive (i)(c) was a corrosion inhibitor which was N-2-aminoethyl-2-oleylimidazoline, sold by BP Chemicals as IC 100.

Additive (i)(d) was a corrosion inhibitor, which was an alkyl benzyl diniethylammonium chloride sold by Hoechst under the Trade Mark DODIGEN 5462.

Additive (i)(e) was a corrosion inhibitor (sold under the Trade Mark "Champion" RU189 by Champion Technologies Inc., Texas, USA), which is believed to be a 1:1 mixture of quaternary ammonium salt and an aliphatic imidazoline.

Additive (ii)(a) was tetra-n-butylammonium bromide.

Additive (ii)(b) was tetra-n-pentylammonium bromide.

Additive (xii) was a "butylated polyvinyl pyrrolidone" sold under the Trade Mark "Antaron" P 904 by International Speciality Products of Wayne, N.J., USA, which is a water soluble copolymer powder with structural units derived from 90% N-vinyl pyrrolidone and 10% butylene and prepared by polymerization with t-butylperoxide; it is believed to be a graft copolymer of poly vinyl pyrrolidone (K30) and butylene, with the butylene structural units in the pyrrolidone ring and/or part of the hydrocarbon backbone and/or pendant thereon. The chemical has a weight average molecular weight of about 16000.

Additive (xiii) was a vinyl pyrrolidone copolymer sold under the Trade Mark "Antaron" VC 713 by International Speciality Products of Wayne, N.J., USA, which is a powder of a copolymer which is terpolymer with structural units from N-vinyl pyrrolidone, N-vinyl-omega caprolactam and dimethylaminoethyl methacrylate.

Additive (xiv) was an anti-foaming agent, which was a polydimethylsiloxane sold by Dow Corning under the Trade Mark AF1520.

Examples 1-5

The formulations were made up and added to the water so it contained the Additives below in amounts (in ppm relating to total water present). The results (averaged over several repeat experiments) were as follows:

______________________________________Additive, Amount (ppm) Inhibition Time/ EXP. (i) (ii) (xii) (xiii) (xiv) Total pH Mins______________________________________0 -- -- -- -- -- -- 6 1 (c) 500 (b) 1000 1000 2500 569 2 (d) 500 (b) 1000 1000 2500 6.2 2039 3 (a) 1000 (b) 2000 2000 5000 294 4 (e) 1000 (b) 2000 2000 5000 1201.sup.x 5 (e) 1000 (b) 1500 2000 3100 8.3 1360______________________________________ .sup.x denotes single result.

Examples 6-14

In these Examples, the process of Examples 1-5 were repeated with the methane replaced by a synthetic gas mixture (simulating natural gas) comprising by weight 95.3% methane, 3% ethane, 0.5% propane, 1% carbon dioxide and 0.1% each of iso-butane and n-butane. The pH of the medium was 4.0. The test was performed as described in Examples 1-5 apart from use of 5.degree. C. as the operational temperature.

The results (averaged over several repeat experiments) were as follows:

______________________________________Additive, Amount (ppm) Inhibi- tion Time/ EXP. (i) (ii) (xii) (xiii) (xiv) Total pH Mins______________________________________ 6 (b) 400 (a) 1500 500 1500 40 3940 504 7 (a) 400 (a) 1500 1500 40 3440 6.6 285 8 (a) 400 (a) 1500 500 1500 40 3940 456 9 (d) 400 (b) 1500 500 1500 40 3940 1543.sup.x 10 (a) 1000 (b) 2000 2000 5000 323 11 (b) 400 (b) 1000 1000 2400 6 159 12 (b) 400 (b) 1500 500 1500 40 3940 4500.sup.x 13 (b)* 400 (b) 1500 500 1500 40 3940 1186 14 (e) 500 (b) 2000 2000 4500 1290______________________________________ .sup.x Denotes a single result

Example 15

An inhibition blend was prepared with the proportions as in Example 6 but with a total of 4995 ppm (i.e. 510 ppm of (i)(b), 1900 ppm (ii), 635 ppm (xii), 1900 ppm (xiii) and 50 ppm (xiv) as well as the monobutyl glycol ether.

This inhibitor blend was tested in a field trial in the North Sea. Wet gas from a well was received at a first platform and was then passed subsea where it met gas from a second platform and the combined gas passed in a subsea line to a third platform. The wet gas arriving at the third platform comprised natural gas, which was in a mole % carbon dioxide 1.1%, nitrogen 1.6%, methane 92.95%, ethane 3.22%, propane 0.57% and C.sub.4+ hydrocarbons 0.47%, as well as water which was in amount of about 1.4 barrels per day per million standard cu. ft. of gas (bbld/mmscfd) and condensate in amount of about 0.1 bbld/mmscfd. The gas at the first platform was at 30.degree. C. and 7.5 MPa (75 bar) but cooled subsea. The gas on the second platform was under comparable conditions but was passed through a valve on the second platform, which could be partly closed to a fixed position, resulting in extra cooling to 8-9.degree. C. at the valve, which gave temperatures of about 10.degree. C. in the eventual subsea line just before the third platform and resulted in solid gas hydrates in the absence of added inhibitors. Formation of hydrates was shown by an increase in the back pressure in the line upstream of the valve, or by a reduction in the amount of water exiting the gas line at the third platform.

Methanol had been used as inhibitor in a blank trial, methanol addition was stopped and no other inhibitor added; 30 hrs later the amount of exit water at the third platform had reduced to zero and after 46 hrs the back pressure started to increase denoting significant solid gas hydrate formation. Methanol was then added and the valve reopened to raise the temperature and to remove the hydrates from the line.

Then into the gas on the first and second platforms with the valve open was added continuously the inhibitor blend for 4 hrs until the concentration of ingredients in the exit gas line to the third platform had become steady. The valve was partly closed to the fixed position; the blend was added continuously into each gas at 4995 ppm total level (relative to the water content of each gas) for 100 hr while monitoring the exit water flow rate, which went from an initial 0.7 bbls/mmscfd quickly to 1.4 bbls/mmscfd, where it remained steady denoting absence of formation of hydrates. The back pressure did not change over this period. The inhibitor blend addition rate was then reduced to a total of 3300 ppm and over the next 30 hrs again the water flow rate in bbl/mmscfd and the pressure remained substantially constant denoting absence of hydrates. The flow of inhibitor blend was then stopped resulting in a slow reduction in the water flow to zero and a subsequent increase in the back pressure, denoting reformation of hydrates. The valve was then opened and methanol was then added to clean the line of hydrates.

Example 16-18 and Comp. A and B

In these Examples the process of Example 6 was used with a temperature of 4.5.degree. C., a gas pressure of 37 bar the Additive mixture that of Example 6 (but used at varying levels), use of 200 ml double distilled water in the cell or 100 ml double distilled water and 100 ml hexane, and a gas, whose molar composition was 87.57% methane, 7.8% ethane, 2.69% propane, 0.26% isobutane, 0.44% n butane, 0.15% pentanes, and 1.09% carbon dioxide.

______________________________________Additive (ppm on total water) Hex- Inhibition EX. (i) (ii) (xii) (xiii) (xiv) Total ane Time______________________________________16 (b) 400.sup.x (a) 1500 500 1500 40 3940 Yes 48 hr+ 17 (b) 200.sup.x (a) 750 250 750 20 1970 Yes 48 hr+ A -- -- -- -- -- -- Yes 3.5 mins 18 (b) 400.sup.x (a) 750 250 750 20 2170 No 58 hr.sup.xx B -- -- -- -- -- -- No 11 mins______________________________________ x Denotes addition of (b) in monobutyl ehtylene glycol ether (10% on tota water) xx Denotes a small amount of hydrate present in vessel

Example 19-21 and Comp. C and D

The process of Example 16 was repeated with a gas pressure of 28 bar and a gas of molar composition 88.12% methane, 8.04% ethane, 2.18% propane, 0.23% isobutane, 0.37% n butane, 0.13% pentanes and 0.93% carbon dioxide, Additives of Example 6 or Example 12, (but at different levels) and instead of the hexane, 100 ml of a crude reservoir oil from the North Sea. The results were as follows, averaged over several experiments.

______________________________________Additive (ppm on total water) Inhibition EX. (i) (ii) (xii) (xiii) (xiv) Total Oil Time______________________________________19 (b) 400.sup.x (a) 1500 500 1500 40 3940 Yes 48 hr+ 20 (a) 400.sup.x (a) 1500 500 1500 40 3940 Yes 48 hr +.sup.xx C -- -- -- -- -- -- Yes 6 min 21 (b) 400.sup.x (a) 750 250 750 20 2170 No 68 hr+ D -- -- -- -- -- -- No 14 min______________________________________ x Denotes addition of (b) in monobutyl ethylene glycol ether (10% on tota water) xx Denotes the presence of hydrate lumps in the gas space over the liquid but none in the liquid.

Example 22-5 and Comp. Examples E and F

The process of Example 16 was repeated with operating temperatures of 4.degree. C. or 8.degree. C. and gas pressures of 70 bar and a gas molar composition as in Example 6-14. The results were as follows (averaged over several experiments).

______________________________________Additive (ppm on total water) Inhibition EX. (i) (ii) (xii) (xiii) (xiv) Total Oil Time______________________________________22 (b) 400.sup.x (a) 1500 500 1500 40 3940 Yes 480 mins (4.degree. C.) E -- -- -- -- -- -- No 5.2 min (4.degree. C.) 23 (b) 400.sup.x (a) 750 250 750 20 2170 No 192 hr + (8.degree. C.) 24 (b) 400.sup.x (a) 400 100 400 40 1340 No 71 hr (8.degree. C.) 25 (b) 200.sup.x (a) 750 250 750 20 1970 Yes 168 hr + (8.degree. C.) F -- -- -- -- -- -- No 153 min (8.degree. C.)______________________________________ x Denotes addition of (b) in monobutyl ethylene glycol ether (10% on tota water)

Examples 26-8 and Comp. G

The process of Example 16 was repeated with a temperature of 5.0.degree. C., a gas pressure of 70 bar and a molar composition of gas of 84.36% methane, 6.79% ethane, 2.54% propane, 0.43% isobutane, 0.77% n butane, 0.49% pentanes, 3.36% carbon dioxide and 0.81% nitrogen and in the cell 100 ml double distilled water and 100 ml crude reservoir oil from a North Sea field different from that in Examples 19-21. The results were as follows (averaged over several experiments).

______________________________________Additive (ppm on total water) EX. (i) (ii) (xii) (xiii) (xiv) Total Inhibition Time______________________________________26 (a) 1000 (b) 2500 -- 2500 -- 6000 14 hr 43 mins 27 (b) 400.sup.x (b) 1500 500 1500 40 3940 13 hr 26 mins 28 (a) 1000 (b) 1500 500 1500 -- 4500 8 hr 18 mins G -- -- -- -- -- -- 0.5 min______________________________________ x Denotes addition of (b) in monobutyl ethylene glycol ether (10% on tota water)

Claims

1. A method of inhibiting or retarding hydrate formation comprising adding to a medium susceptible to hydrate formation an effective inhibiting or retarding amount of (a) a corrosion inhibitor (hereinafter called Additive (i) and (b) a salt which is of formula [R.sup.1 (R.sup.2)XR.sup.3 ].sup.+ Y.sup.-, wherein each of R.sup.1, R.sup.2 and R.sup.3 is bonded directly to X, each of R.sup.1 and R.sup.2, which may be the same or different is an alkyl group of at least 4 carbons, X is S, NR.sup.4 or PR.sup.4, wherein each of R.sup.3 and R.sup.4 which may be the same or different represents hydrogen or an organic group, with the proviso that at least one of R.sup.3 and R.sup.4 is an organic group of at least 4 carbons, and Y is an anion (hereinafter called Additive (ii)).

2. A method according to claim 1 wherein in the weight proportion of Additive (i) to Additive (ii) is 10-45:90-55.

3. A method according to claim 1 wherein Additive (ii) each of R.sup.1 and R.sup.2 is an alkyl group of the 4-6 carbons, X is NR.sup.4 or PR.sup.4, R.sup.3 is an alkyl group of 1-24 carbons and R.sup.4 is an alkyl group of at least 4 carbons.

4. A method according to claim 3 wherein Additive (ii) is a tetra n butyl or tetra n-pentyl ammonium halide.

5. A method according to claim 1 wherein Additive (i) comprises a benzyl alkyl dimethyl ammonium chloride.

6. A method according to claim 1, wherein Additive (i) comprises a tertiary amine or imidazoline.

7. A method according to claim 1 further comprising adding at least one of (a) an Additive (ix), which is at least one water soluble polymer of a polar ethylenically unsaturated compound, and (b) an Additive (x) which is an hydrophilic colloid.

8. A method according to claim 1 further comprising adding an Additive (xii) which is an aliphatic (N heterocyclic carbonyl) polymer with a hydrocarbon backbone.

9. A method according to claim 8 wherein said Additive (xii) is a polymer with units from N-vinyl pyrrolidone and an alkene of 4-18 carbon atoms.

10. A method according to claim 1 further comprising adding an Additive (xiii) which is a copolymer of (a) at least 1 ethylenically unsaturated N-heterocyclic ring compound with at least one of (b) a different ethylenically unsaturated N-heterocyclic ring compound, and (c) a polar ethylenically unsaturated compound, different from said N-heterocyclic compounds.

11. A method according to claim 10 wherein Additive (xiii) is a copolymer of N-vinylpyrrolidone, N-vinylomegacaprolactam and dimethyl amino ethyl methacrylate.

12. A method according to claim 11 which comprises adding to said medium 200-1500 parts of Additive (i), 500-5000 parts of Additive (ii), with Additive (xii) and Additive (xiii) and a total of 2000-8000 parts of Additives, based on the total water volume of the medium, and a weight ratio of Additive (xii) to (xiii) of 10-40:90-60 and a percentage of Additive (xiii) to the total of Additives (i) and (ii) of 50-300%.

13. Method of transporting through a pipeline a medium susceptible to hydrate formation which comprises treating said medium with Additive (i) and Additive (ii) as defined in claim 1, under conditions where a temperature is in a range of -5 to 15.degree. C. and a pressure in a range of 0.01 to 30 mPa.