Method for preventing or retarding the formulation of gas hydrates

A composition for preventing or retarding the formation of gas hydrates during the transport of a fluid comprising water and a hydrocarbon through a conduit. The composition is a homopolymer of vinyl caprolactam, or copolymers thereof, having a low molecular weight in the range of 500 to 4000, which is made by polymerization of the monomer or monomers in a polymerization solvent which is a mixture of a glycol ether and water, preferably 2-butoxyethanol and water, at a pH of about 8-12.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates to a method for preventing or retarding the formation
 of gas hydrates, or for reducing the tendency of such hydrates to
 agglomerate, during the transport of a fluid comprising water and a
 hydrocarbon through a conduit, and, more particularly, to the addition to
 the fluid of a composition of a low molecular weight vinyl caprolactam
 polymer, or copolymers thereof, which is made in a defined polymerization
 solvent, to inhibit such gas hydrate formation.
 2. Description of the Prior Art
 It is well known in the art that the formation of gas hydrates in a
 conduit, e.g. in a pipeline, where an aqueous phase is inherently present,
 during the transport of liquids such as oil, and of gases, particularly
 lower hydrocarbons, e.g. methane, ethane, propane, butane, isobutane and
 natural gas, is a serious problem, especially in areas with a low
 temperature in the winter season or in the sea. Generally, the ambient
 temperatures in such areas are so low that gas hydrates are formed in the
 gas transportation pipeline, due to the inevitable presence of co-produced
 water therein. Insulation of the pipelines may decrease the opportunity
 for gas hydrate formation; however, if the field is relatively small and
 some distance from the production facilities, the cost of providing
 suitable insulation is too high to make such a field economically
 attractive. It is also known to add anti-freeze compounds, for example,
 ethylene glycol or methanol, during transport of such liquids and gases to
 minimize gas hydrate formation; however, large quantities of these highly
 flammable compounds are required to be effective which is expensive and
 unsafe.
 Further representative of the prior art in this field are U.S. Pat. Nos.
 4,915,176;5,420,370;5,432,292; and 5,723,524; EPO 0323774A1; EPA
 0457375A1; EPA 0526929A1; Can. Pat. Appln. 2,073,577; "Gas Hydrates and
 Hydrate Prevention", 73 GPA Annual Convention, pages 85-93; WO 96/08456;
 WO 96/08636; WO 93125798; WO 94/12761; WO 95/17579; and WO 95/32356.
 Representative of such art is the disclosure in U.S. Pat. No. 5,723,524
 that poly(vinyl caprolactam) (PVCL), synthesized in isopropanol, and
 having an average molecular weight of 36,000 amu, as determined by gel
 permeation chromatography (GPC), using polyethylene glycol as standard, is
 a gas hydrate inhibitor.
 Accordingly, it is an object of this invention to provide an improved
 composition and method for retarding the formation of gas hydrate in a
 hydrocarbon gas pipeline.
 SUMMARY OF THE INVENTION
 What is described herein is a composition for effectively preventing or
 retarding the formation of gas hydrates, or for reducing the tendency of
 gas hydrates to agglomerate, during the transport of a fluid comprising
 water and a hydrocarbon, through a conduit. The composition comprises a
 vinyl caprolactam homopolymer (PVCL), or copolymers thereof, for example,
 copolymers with vinyl pyrrolidone (VP), having a low molecular weight of
 about 500 to about 4000, which are made by polymerizing the monomer or
 monomers in a polymerization solvent which is a mixture of a glycol ether
 containing an alkoxy group having at least 3 carbon atoms, and preferably,
 2-butoxyethanol (BGE), and water, preferably in a predetermined weight
 ratio, at a pH of about 8-12.

DETAILED DESCRIPTION OF THE INVENTION
 The polymer which exhibits advantageous inhibitory characteristics in the
 composition of the invention is a homopolymer of vinyl caprolactam, or
 copolymer thereof, having a molecular weight of about 500 to about 4000,
 as determined by GPC using polyethylene glycol as the standard.
 The composition of the invention may also include low molecular weight
 copolymers of vinyl caprolactam with one or more monomers selected from
 N-vinylpyrrolidone; acrylamide; N-alkyl acrylamides, e.g. N,N-dimethyl
 acrylamide; N-[1-(2-pyrrolidonylethyl)] acrylamide; N,N-dialkyl aminoalkyl
 methacrylamide, e.g. N,N-dimethylamino propyl methacrylamide; N,N-dialkyl
 aminoalkyl (meth)acrylates; e.g. N,N-dimethylaminoethyl (meth)acrylate and
 quaternized salts thereof, including N-alkyl halides and the like;
 4-vinylpyridine; N-methyl-N-vinylacetamide; N-vinylacetamide; and
 N-vinylformamide. The homopolymer of vinylcaprolactam is preferred.
 Generally, the polymer solution used in the composition of the invention is
 present in an amount of about 30 to 60%, preferably 45 to 55%, by weight
 in admixture with the solvent. The polymer inhibition concentration in the
 pipeline, i.e. in the aqueous phase, water being inherently present
 therein, is about 0.1 to 3%, preferably 0.5-1%, by weight. The solvent*
 inhibition concentration, accordingly, is about 1 to 9% by weight of the
 aqueous phase.

FNT * total of all solvents present in the composition
 The polymer is synthesized from its monomer, or monomers, in a
 polymerization solvent which is a predetermined mixture of a glycol ether
 containing an alkoxy group having at least 3 carbon atoms, and water, at a
 pH of about 8-12. Representative of such suitable glycol ethers are
 2-butoxy-ethanol (ethylene glycol monobutyl ether); iso-butoxyethanol;
 propylene glycol butyl ether; (diethylene glycol) monobutyl ether; and
 2-isopropoxy-ethanol. 2-Butoxyethanol (BGE) is the most preferred glycol
 ether. Ammonium hydroxide is the preferred neutralizing agent to adjust
 the pH of the reacton mixture to the desired range.
 Suitable glycol ether:water weight ratios range from about 50-90 wt.%
 BGE:10-50 wt. % water. A 80:20 wt. ratio is preferred. The product of the
 polymerization is a composition of the polymer, e.g. poly(vinyl
 caprolactam) (PVCAP), in the polymerization solvent, e.g. BGE, water and
 the neutralizing agent. Generally, the solids content of the composition
 is about 30-90 wt. %, i.e. the weight of the polymer to the weight of the
 composition.
 The composition of the invention may be provided with a suitable carrier
 solvent such as monoethylene glycol (MEG), methanol, ethanol, propanol,
 1,4-butanediol, butanol, pentanol, hexanol, cyclohexyl pyrrolidone,
 propargyl alcohol, N-methylpyrrolidone and the like, preferably MEG.
 Preferably the concentration of MEG in the aqueous phase, i.e. under
 pipeline inhibition conditions, is about 2.5-10 wt. %, most preferably 4-5
 wt. %.
 The following examples are provided to illustrate the invention.
 A. GENERAL METHOD
 The gas hydrate inhibition tests were conducted in a 500 ml, 316 stainless
 steel autoclave vessel having a usable volume of 200 ml, equipped with a
 thermostated cooling jacket, sapphire window, inlet and outlet, platinum
 resistance thermometer (PRT) and magnetic stirring pellet. The rig is
 rated up to 400.degree. C. and down to -25.degree. C. Temperature and
 pressure are data logged, while the cell content is visually monitored by
 a boroscope video camera connected to a time lapsed video recorder.
 Hydrate formation in the rig is detected using a combination of three
 methods: visual detection of hydrate crystals, decrease in vessel pressure
 due to gas uptake and by the temperature exotherm created by heat released
 during hydrate formation.
 The rig was cleaned prior to running a blank and/or test solutions. An air
 drill with wet and dry emery paper was used to remove traces of any
 adsorbed chemicals therein with a small amount of water being added to the
 rig. The vessel was then rinsed several times with double distilled water.
 A blank solution of 200 ml of double distilled water was run to confirm
 the reproducibility of the test. Formation of hydrates within 4-10 minutes
 was taken as a standard time for a given set of testing conditions, e.g.
 synthetic gas, 60 bar and T =4.degree. C.
 A synthetic gas mixture having the following composition was used for
 hydrate testing: