Method for inhibiting hydrate formation

A method for inhibiting the formation of gas hydrates in a petroleum fluid having hydrate-forming constituents is claimed. More specifically, the method can be used to treat a petroleum fluid, such as natural gas conveyed in a pipe, to inhibit the formation of a hydrate flow restriction in the pipe. The hydrate inhibitors used for practicing the method comprise substantially water soluble homopolymers and copolymers of surfactant monomers, wherein the surfactant monomer unit may be represented by the formula: ##STR1## where R.sub.1 and R.sub.2 independently are hydrogen or a methyl group, M is a metal cation, n is a number sufficient to produce a number average molecular weight between 1000 and 6,000,000, and o is a number from 1 to 5.

FIELD OF THE INVENTION
 The present invention relates to a method for inhibiting the formation of
 clathrate hydrates in a fluid. More specifically, the invention relates to
 a method for inhibiting the formation of gas hydrates in a pipe used to
 convey oil or gas.
 BACKGROUND OF THE INVENTION
 Petroleum fluids typically contain carbon dioxide and hydrogen sulfide, as
 well as various hydrocarbons, such as methane, ethane, propane, normal
 butane and isobutane. Water, present as a vapor and/or as a liquid phase,
 is also typically found mixed in varying amounts with such hydrocarbons.
 Under conditions of elevated pressure and reduced temperature, clathrate
 hydrates can form when such petroleum fluids contain water. Clathrate
 hydrates are water crystals which form a cage-like structure around
 "guest" molecules such as hydrate-forming hydrocarbons or other gases.
 Some hydrate-forming hydrocarbons include, but are not limited to,
 methane, ethane, propane, isobutane, butane, neopentane, ethylene,
 propylene, isobutylene, cyclopropane, cyclobutane, cyclopentane,
 cyclohexane, and benzene. Other gases which may form hydrates include, but
 are not limited to, oxygen, nitrogen, hydrogen sulfide, carbon dioxide,
 sulfur dioxide, and chlorine.
 Gas hydrate crystals or gas hydrates are a class of clathrate hydrates of
 particular interest to the petroleum industry because of the pipeline
 blockages that they can produce during the production and/or transport of
 natural gas and other petroleum fluids. For example, at a pressure of
 about 1000 kPa (145 psi), ethane can form gas hydrates at temperatures
 below 4.degree. C. (39.degree. F.), and at a pressure of 3000 kPa (435
 psi), ethane can form gas hydrates at temperatures below 14.degree. C.
 (57.degree. F.). Such temperatures and pressures are not uncommon for many
 operating environments where natural gas and other petroleum fluids are
 produced and transported.
 As gas hydrates agglomerate, they can produce hydrate blockages in the pipe
 or conduit used to produce and/or transport natural gas or other petroleum
 fluids. The formation of such hydrate blockages can lead to a shutdown in
 production and thus substantial financial losses. Furthermore, restarting
 a shutdown facility, particularly an offshore production or transport
 facility, can be difficult because significant amounts of time, energy,
 and materials, as well as various engineering adjustments, are often
 required to safely remove the hydrate blockage.
 A variety of measures have been used by the oil and gas industry to prevent
 the formation of hydrate blockages in oil or gas streams. Such measures
 include maintaining the temperature and/or pressure outside hydrate
 formation conditions and introducing an antifreeze such as methanol,
 ethanol, propanol, or ethylene glycol. From an engineering standpoint,
 maintaining temperature and/or pressure outside hydrate formation
 conditions often requires design and equipment modifications, such as
 insulated or jacketed piping. Such modifications are costly to implement
 and maintain. The amount of antifreeze required to prevent hydrate
 blockages is typically between 10% to 30% by weight of the water present
 in the oil or gas stream. Consequently, several thousand gallons per day
 of such antifreeze can be required. Such quantities present handling,
 storage, recovery, and potential toxicity issues. Moreover, these solvents
 are difficult to completely recover from the production or transportation
 stream.
 Consequently, there is a need for a gas hydrate inhibitor that can be
 conveniently mixed at low concentrations in the produced or transported
 petroleum fluids. Such an inhibitor should reduce the rate of nucleation,
 growth, and/or agglomeration of gas hydrate crystals in a petroleum fluid
 stream and thereby inhibit the formation of a hydrate blockage in the pipe
 conveying the petroleum fluid stream. As discussed more fully below, the
 inhibitors of this invention can effectively treat a petroleum fluid
 having a water phase, or a petroleum fluid containing water vapor that may
 condense to form a water phase, depending upon the operating environment.
 The use of polymeric inhibitors has been proposed, however, these materials
 have a tendency to precipitate out of solution at higher temperatures.
 This is an undesirable characteristic, since the inhibitor must stay in
 solution under a wide range of temperatures to be most effective. The
 surfactant monomers described herein yield homopolymers and copolymers
 with good inhibition properties as well as better solubility at higher
 temperatures.
 SUMMARY OF THE INVENTION
 According to the invention, there is provided a method for inhibiting the
 formation of clathrate hydrates in a fluid having hydrate-forming
 constituents. One embodiment of the method comprises contacting the fluid
 with an inhibitor comprising a polymer or copolymer which has been made
 from surfactant monomer(s). In an alternative embodiment, the fluid is
 treated with a copolymer of the surfactant monomer copolymerized with a
 comonomer that is known, when polymerized with itself, to exhibit hydrate
 inhibition.
 The polymers and copolymers of the invention can be classified as
 "polysurfactants" and are characterized by the general formula:
 ##STR2##
 where the sum of n and m is an average number sufficient to produce a
 number average molecular weight between about 1,000 to about 6,000,000,
 and A is the following surfactant "mer-unit":
 ##STR3##
 where both R.sub.1 and R.sub.2 are independently hydrogen or a methyl
 group, and o is a number from 1 to 5. Preferably, o is a number from 1 to
 3, and more preferably, o is a number from 1 to 2.
 The term "mer-unit" is used to describe both the monomers that are reacted
 to form polymers, and the polymer units that result from the conversion of
 one type of polymer units into another type of polymer units, by some
 reaction or conversion which occurs subsequent to the polymerization
 reaction.
 M is a metal cation selected from the group consisting of metals of Group
 IA of the Periodic Table, preferably sodium and potassium, and an ammonium
 cation. Sodium is most preferred. The corresponding anionic group is a
 sulfonate group, as shown above, but alternatively may be a sulfinate,
 sulfate, phosphonate, phosphinate, phosphate, or carboxylate group.
 Sulfonate and carboxylate groups are preferred, and sulfonate groups are
 most preferred. The two ionic groups are preferably associated as salts.
 B may be a surfactant mer-unit that is the same as, or a variant of, A.
 Alternatively, B is a monomer or mer-unit that is known, when polymerized
 with itself, to exhibit hydrate inhibition. For example, B may be an
 N-vinyl amide, an N-allyl amide, an acrylamide or methacrylamide, an
 N-vinyl lactam, a maleimide, or a vinyl oxazoline (a ring-closed cyclic
 imino ether).
 The A and B mer-unit proportions, or mole ratio of m to n, can vary. The
 mole ratio m:n may vary from about 5:95 to about 95:5, or from about 25:75
 to about 75:25, or from about 45:55 to about 55:45. Ratios which provide
 the most effective inhibitors for a given system are preferred.
 The polymers and copolymers consistent with the description above form a
 class of materials designated "polysurfactants." By polysurfactants, we
 mean that there are pendant groups on the polymer backbone that resemble
 surfactant-like materials, i.e., there is a hydrophilic portion and a
 hydrophobic portion. The polysurfactants of the invention fall within the
 generic class of amphoteric polymers, those which contain hydrophilic and
 hydrophobic groups on the same mer-unit. An example is shown below for the
 case of the C.sub.6 polysurfactant:
 ##STR4##
 The designation of "C.sub.6 polysurfactant" is based upon the number of
 carbon atoms below the nitrogen atom in the pendant group in the formula
 above, which results from the .alpha.-olefin used to produce the
 surfactant mer-unit. To obtain an effective hydrate inhibitor, it is
 important to properly balance the hydrophobic and hydrophilic nature of
 the mer-unit, such that the resulting polysurfactant is substantially
 water soluble.
 DETAILED DESCRIPTION OF THE INVENTION
 Inventive Method
 The inventive method of the invention inhibits the formation of clathrate
 hydrates in a fluid having hydrate-forming constituents. Formation of
 clathrate hydrates means the nucleation, growth, and/or agglomeration of
 clathrate hydrates. Such clathrate hydrates may be formed in a fluid
 whether it is flowing or substantially stationary, but are most
 problematic in flowing fluids conveyed in a pipe. For example, flow
 restrictions arising from partial or complete blockages in a fluid can
 arise as clathrate hydrates adhere to and accumulate along the inside wall
 of the pipe used to convey the fluid. In addition, the invention can be
 used for inhibiting formation of clathrate hydrates in substantially
 stationary fluids.
 In one embodiment of the invention, a concentrated solution or mixture of
 one or more of the inhibitors of the invention is introduced into a
 petroleum fluid. The term petroleum fluid includes fluids that are gases
 and/or liquids when under standard conditions, such as natural gas, crude
 oil, and various petroleum product streams. As the inhibitor solution of
 the invention is substantially dispersed in the fluid, it reduces the rate
 that clathrate hydrates are formed, and thereby reduces the tendency for a
 flow restriction to occur.
 In a preferred embodiment, the solid inhibitor is first dissolved into an
 appropriate carrier solvent or liquid to make a concentrated solution or
 mixture. Alternatively, the inhibitor may be provided in a solution where
 it is left as dissolved in its polymerization reaction solvent. The
 solvent will preferably dissolve the inhibitor and, for convenience, such
 liquids are referred to hereafter as solvents, whether they produce an
 inhibitor solution, emulsion, or other type of mixture. The carrier
 solvent's principal purpose is to act as a carrier for the inhibitor and
 to facilitate the inhibitor's dispersion into the petroleum fluid. Any
 solvent suitable for delivering the inhibitor to the petroleum fluid may
 be used. Such carrier solvents include, but are not limited to, water,
 brine, sea water, produced water, methanol, ethanol, propanol,
 isopropanol, glycol, or mixtures of such solvents. Other solvents familiar
 to those skilled in the art may also be used. Aqueous solvents (water,
 brine, sea water, produced water) are preferred.
 It should be understood that the use of a carrier solvent is not required
 to practice the invention, but it is a convenient method of introducing
 and dispersing the inhibitor into the fluid. In many applications, the use
 of a carrier solvent will facilitate treatment of the fluid stream. As
 noted above, water is frequently present along with hydrocarbons and other
 gases present in petroleum fluids. The presence of an aqueous phase in a
 petroleum fluid is not essential, but if present, will facilitate the
 dispersion of the inhibitor within the petroleum fluid. The presence of a
 significant aqueous phase in the petroleum fluid may reduce or eliminate
 the amount of carrier solvent required for dispersion of the inhibitor.
 Any convenient concentration of inhibitor in the carrier solvent can be
 used, so long as it results in the desired final concentration in the
 aqueous phase of the petroleum fluid. Higher concentrations are preferred,
 since they result in a reduced volume of concentrated solution to handle
 and introduce into the petroleum fluid. The actual concentration used in a
 specific application will vary depending upon the selection of carrier
 solvent, the chemical composition and molecular weight of the inhibitor,
 the system temperature, the inhibitor's solubility in the carrier solvent
 at application conditions, and the presence of an aqueous phase in the
 petroleum fluid. If there is no aqueous phase present in the petroleum
 fluid, a dilute solution may be preferred.
 The inhibitor mixture is introduced into the petroleum fluid using
 mechanical equipment, such as a chemical injection pump or other device
 which will be apparent to those skilled in the art. However, such
 equipment is not essential to practicing the invention. To ensure an
 efficient and effective treatment of the petroleum fluid with the
 inhibitor mixture, two factors should be considered.
 First, an aqueous phase is preferably present at the location the inhibitor
 solution is introduced into the fluid. In some petroleum fluid systems
 (particularly natural gas systems), an aqueous phase does not appear until
 the gas has cooled sufficiently for water to condense. If this is the
 case, the inhibitor solution is preferably introduced after the water has
 condensed. Alternatively, in the event that an aqueous phase is not
 available at the point the inhibitor solution is introduced, the inhibitor
 solution concentration should be selected to ensure that the inhibitor
 solution's viscosity is sufficiently low to facilitate its dispersion
 throughout the petroleum fluid.
 Second, because the inhibitor primarily serves to inhibit the formation of
 clathrate hydrates, rather than to reverse such formation, it is important
 to treat the fluid prior to substantial formation of clathrate hydrates.
 As a wet petroleum fluid cools, it will eventually reach a temperature,
 known as the hydrate equilibrium dissociation temperature, or T.sub.eq,
 below which hydrate formation is thermodynamically favored. A petroleum
 fluid's T.sub.eq will shift as the pressure applied to the fluid, and its
 composition, change. Various methods for determining a fluid's T.sub.eq at
 various fluid compositions and pressures are well known to those skilled
 in the art. Preferably, the fluid should be treated with the inhibitor
 when the fluid is at a temperature greater than its T.sub.eq. It is
 possible, but not preferable, to introduce the inhibitor while the
 temperature is at or slightly below the fluid's T.sub.eq, preferably
 before clathrate hydrates have begun to form.
 The inhibitor's solubility over a wide range of temperatures is important
 for ensuring that the polymer can be effectively injected under typical
 field conditions. Most polymeric inhibitors exhibit lower critical
 solution temperature, or LCST behavior when dissolved in water or brine.
 As the temperature of such solutions is increased, the polymer reaches a
 temperature where it will precipitate out of solution. The temperature
 above which the polymer will precipitate out of its solution is known as
 the polymer's cloud point, or T.sub.cp. Various methods for determining a
 polymer's T.sub.cp at various compositions and pressures are well known to
 those skilled in the art. When the inhibitor solution temperature exceeds
 the cloud point for a particular polymer, the polymer will precipitate out
 of solution.
 It is important to convey the inhibitor solution to the petroleum fluid at
 a temperature lower than its cloud point. The cloud point for a given
 polymer solution is dependent upon several factors, including the polymer
 concentration, other components present in the solution (such as dissolved
 salts), and the ambient temperature and pressure of the solution. In many
 oil and gas production situations, the inhibitor is injected under
 conditions where the temperature of the petroleum fluid to which the
 inhibitor is added can range as high as 100.degree. C.-150.degree. C.
 (212.degree. F.-302.degree. F.) or more. Consequently, it is desirable to
 select a polymer that exhibits a cloud point greater than the anticipated
 temperature of the petroleum fluid. Alternatively, the inhibitor could be
 injected at some point in the production system where the temperature of
 the petroleum fluid is below the polymer solution's cloud point.
 Sub-cooling is a measure of the effectiveness of a hydrate inhibitor. When
 a petroleum fluid contains hydrate-forming constituents, clathrate
 hydrates will begin to form rapidly at a given temperature. As the
 hydrate-forming constituents (typically gases) are consumed in forming
 clathrate hydrates, there is an abrupt and corresponding decrease in the
 volume of gas in the petroleum fluid as hydrates are formed. The
 temperature at which this abrupt decrease in the volume of gas is observed
 is known as the temperature of onset for hydrate formation, or T.sub.os.
 Various methods known to those skilled in the art, such as the mini-loop
 procedure described below, may be used to determine a fluid's T.sub.os. As
 noted above, the hydrate equilibrium dissociation temperature, or
 T.sub.eq, is the temperature below which hydrate formation is
 thermodynamically favored in an aqueous/gas solution without an inhibitor
 present. A hydrate inhibitor's sub-cooling, or T.sub.sub, is the
 difference between the T.sub.eq and the T.sub.os. (Note the subcooling is
 not actually a temperature, but a difference, measured in degrees, between
 two temperatures.) Therefore, for a given pressure, the greater the
 sub-cooling temperature, the more effective the inhibitor. Typically, an
 aqueous sea salt/gas solution with no inhibitor present produces a
 T.sub.sub of about 3 to 4.degree. C. (5 to 7.degree. F.).
 The surfactant polymers of the invention offer the unique advantage of
 effective hydrate inhibition, as measured by sub-cooling, combined with
 high cloud point temperatures.
 The concentration of inhibitor present in the aqueous phase of a petroleum
 fluid will typically vary from about 0.01 percent by weight (wt %) to
 about 5 wt %, based upon the aqueous phase present in the fluid.
 Preferably, the inhibitor will be present at a concentration of from about
 0.01 wt % to about 0.5 wt %. Most preferably, the inhibitor will be
 present in an aqueous phase at a concentration of from about 0. 1 wt % to
 about 0.5 wt %. The effective amount of an inhibitor for a particular
 application can be determined by those skilled in the art, by considering
 the inhibitor's performance factors, the degree of inhibition required for
 the petroleum fluid, and the inhibitor's cost. A higher inhibitor
 concentration can be used to lower the temperature at which a hydrate
 blockage would occur.
 Novel Inhibitors
 The inhibitors of the invention may be represented by the following general
 formula:
 ##STR5##
 where the sum of n and m is an average number sufficient to produce a
 number average molecular weight between about 1,000 to about 6,000,000,
 and where A is the following surfactant mer-unit:
 ##STR6##
 where R.sub.1 and R.sub.2 are independently hydrogen or a methyl group, and
 o is a number from 1 to 5. Preferably, o is a number from 1 to 3, and more
 preferably, o is a number from 1 to 2.
 M is a metal cation selected from the group consisting of metals of Group
 IA of the Periodic Table, preferably sodium and potassium, and an ammonium
 cation. Sodium is most preferred. The corresponding anionic group is a
 sulfonate group, as shown above, but alternatively may be a sulfinate,
 sulfate, phosphonate, phosphinate, phosphate, or carboxylate group.
 Sulfonate and carboxylate groups are preferred, and sulfonate groups are
 most preferred. The two ionic groups are preferably associated as salts.
 B may be a surfactant mer-unit that is the same as, or a variant of, A. If
 A and B are both surfactant mer-units, then one of A or B, for example, B,
 will have an o of from 1 to 3, while A may have an o of from 1 to 5. This
 will ensure that the resulting polysurfactant remains substantially
 water-soluble. The variation of the ratio of m to n will also impact
 polysurfactant solubility, and for the case described above, m:n would
 preferably range from 45:55 to 5:95, or from 45:55 to 25:75.
 Alternatively, B is a monomer or mer-unit that is known, when polymerized
 with itself, to exhibit hydrate inhibition. For example, B may be an
 N-vinyl amide, an N-allyl amide, an acrylamide or methacrylamide, an
 N-vinyl lactam, a maleimide, or a vinyl oxazoline (a ring-closed cyclic
 imino ether).
 The A and B mer-unit proportions, or mole ratio of m to n can vary. The
 mole ratio of m:n may vary from about 5:95 to about 95:5, or from about
 25:75 to about 75:25, or from about 45:55 to about 55:45. Ratios which
 provide the most effective inhibitors for a given system are preferred.
 In one alternative, the B mer-unit is an N-vinyl amide of the formula:
 ##STR7##
 where R.sub.3 is a hydrogen or a hydrocarbon group having one to six carbon
 atoms, and zero to two heteroatoms selected from the group consisting of
 oxygen, nitrogen, and combinations thereof, R.sub.4 is a hydrocarbon group
 having one to six carbon atoms, and zero to two heteroatoms selected from
 the group consisting of oxygen and nitrogen and combinations thereof; and
 R.sub.3 and R.sub.4 have a sum total of carbon atoms greater than or equal
 to one, but less than eight. The R.sub.3 and R.sub.4 carbon atoms may be
 branched, normal, or cyclic; R.sub.3 may be hydrogen or an alkyl,
 cycloalkyl, or aryl group; and R.sub.4 is an alkyl, cycloalkyl, or an aryl
 group.
 Preferred N-vinyl amides include N-methyl N-vinyl acetamide, also known as
 N-vinyl N-methyl acetamide (VIMA).
 Alternatively, the B mer-unit is an N-allyl amide of the formula:
 ##STR8##
 where R.sub.5 is a hydrogen or hydrocarbon group having one to six carbon
 atoms, and zero to two heteroatoms selected from the group consisting of
 oxygen, nitrogen, and combinations thereof; R.sub.6 is a hydrocarbon group
 having one to six carbon atoms, and zero to two heteroatoms selected from
 the group consisting of oxygen and nitrogen and combinations thereof;
 R.sub.5 and R.sub.6 have a sum total of carbon atoms greater than or equal
 to one, but less than eight. The R.sub.5 and R.sub.6 carbon atoms may be
 branched, normal, or cyclic; R.sub.5 is either hydrogen or an alkyl,
 cycloalkyl, or an aryl group; and R.sub.6 is an alkyl, cycloalkyl, or an
 aryl group.
 Alternatively, the B mer-unit is an acrylamide or methacrylamide of the
 formula:
 ##STR9##
 where R.sub.7 is hydrogen or a methyl group; R.sub.8 is a hydrocarbon group
 having one to ten carbon atoms, and zero to four heteroatoms selected from
 the group consisting of nitrogen, oxygen, sulfur, and combinations
 thereof; and R.sub.9 is a hydrogen atom or a hydrocarbon group having one
 to ten carbon atoms, and zero to four heteroatoms selected from the group
 consisting of nitrogen, oxygen, sulfur, and combinations thereof; R.sub.8
 and R.sub.9 have a sum total of carbon atoms greater than or equal to one,
 but less than eight. The R.sub.8 and R.sub.9 carbon atoms may be branched,
 normal, or cyclic; R.sub.8 is an alkyl, cycloalkyl, or an aryl group; and
 R.sub.9 is either hydrogen or an alkyl, cycloalkyl, or an aryl group.
 Preferred acrylamides and methacrylamides are N-substituted acrylamides and
 N-substituted methacrylamides, such as isopropylacrylamide (IPA),
 methacryloylpyrrolidine (MAPYD) and N-isopropyl methacrylamide (IPMA).
 Alternatively, the B mer-unit is an N-vinyl lactam of the formula:
 ##STR10##
 where p ranges from one to three, such as N-vinyl caprolactam (VCap), and
 N-vinyl pyrrolidone (VP), and N-vinyl piperidone (VPip). Preferred N-vinyl
 lactams include N-vinyl caprolactam (VCap), and N-vinyl pyrrolidone (VP),
 and VCap is particularly preferred.
 The polymers and copolymers consistent with the description above form a
 class of materials designated "polysurfactants." By polysurfactants, we
 mean that there are pendant groups on the polymer backbone that resemble
 surfactant-like materials, i.e., there is a hydrophilic portion and a
 hydrophobic portion. The polysurfactants of the invention fall within the
 generic class of amphoteric polymers, those which contain hydrophilic and
 hydrophobic groups on the same mer-unit. An example is shown below for the
 case of the C.sub.6 polysurfactant:
 ##STR11##
 The designation of "C.sub.6 polysurfactant" is based upon the number of
 carbon atoms below the nitrogen atom in the pendant group in the formula
 above, which results from the .alpha.-olefin used to produce the
 surfactant mer-unit.
 To obtain an effective hydrate inhibitor, it is important to properly
 balance the hydrophobic and hydrophilic nature of the surfactant mer-unit.
 If the inhibitor is too hydrophobic, or has a hydrophobic chain that is
 too long, it will exhibit an undesirably low cloud point, and could become
 insoluble in water. If the inhibitor is too hydrophilic, due to a
 hydrophobic chain that is too short, the inhibitor will exhibit a
 subcooling that is too low for the material to be a good inhibitor, or may
 even promote hydrate formation.
 Examples of polysurfactant copolymers of the type described above include
 materials such as:
 ##STR12##
 where M is sodium.
 As mentioned above, B can comprise one or more mer-units known to inhibit
 hydrate formation. Thus, a further example of this invention is
 illustrated by the following terpolymer:
 ##STR13##
 where M is sodium.
 Due to the ionic nature of the polysurfactants of the invention, direct
 measurement of molecular weight by, for example, Gel Permeation
 Chromatography (GPC) is difficult. However, molecular weights of the
 polymers described above are expected to fall within the 1,000 to
 6,000,000 number average molecular weight specified, based upon the
 structures identified and other polymerizations of this type.
 The generic structures above as well as the examples given are intended to
 cover any substantially water soluble polymers including, but not limited
 to, copolymers, terpolymers, other complex polymers, and blends and
 mixtures thereof, having the structural units described, whether such
 structural units or their related monomers were used to synthesize the
 polymer or not. The monomers disclosed below for synthesizing the polymers
 containing the preferred mer-units are not intended to limit the scope of
 the claims. Other starting materials and synthesis techniques, which are
 currently known or may become known, will be apparent to those skilled in
 the art as alternatives to synthesizing the polymers of the claimed
 invention. Accordingly, all polymers having at least the structural unit
 identified in the claims below, even though such polymers may be produced
 from starting materials and/or by means not explicitly referenced herein,
 are intended to fall within the scope of the claimed invention. Other
 polymers not specifically identified in the examples below will become
 apparent to those skilled in the art in light of the detailed discussion
 below. Such polymers are intended to fall within the scope of the claimed
 invention.
 The above-described polymers and copolymers can be used in a mixture with
 other polymers or additives useful for enhancing inhibitor performance, or
 operating parameters other than those specified here.
 Experimental Results
 Inhibitor Synthesis
 Standard laboratory procedures familiar to those skilled in the art were
 used to synthesize the polymers and copolymers identified below. Benzene
 or low molecular weight alcohols were used as reaction solvents. Many
 common azo free radical initiators, such as
 2,2'-azobis(2-methylpropionitrile), also known as AIBN, can be used for
 synthesizing copolymers. The polymers were isolated and characterized
 using techniques well-known to those skilled in the art, such as carbon-13
 (.sup.13 C) and proton (.sup.1 H) nuclear magnetic resonance spectroscopy
 (NMR).
 The surfactant monomers are prepared generally according to U.S. Pat. No.
 5,036,136. The following is an illustrative procedure for the C.sub.6
 surfactant monomer:
 A mixture of 21.0 g (31.2 mls) of 1-hexene and 55 g (68.2 mls) of
 acrylonitrile was cooled in an ice water bath. Subsequently, 28 g (14.4
 ml) of fuming sulfuric acid (30 wt % sulfur trioxide dissolved in sulfuric
 acid) was added drop-wise under vigorous agitation. The procedure took
 approximately 25 minutes. The temperature of the mixture was allowed to
 warm to room temperature over a period of several hours. The agitated
 solution was stirred overnight. The product was filtered as a solid
 powder, and rinsed several times with acrylonitrile. The powder was dried
 in a vacuum oven at room temperature for 24 hours. NMR and elemental
 analysis were used to determine the molecular structure as well as the
 monomer purity. Using this same procedure, C.sub.4 -C.sub.8 surfactant
 monomers were prepared. A methyl-substituted analogue, Me--C.sub.x
 surfactant monomer was also synthesized by treating methacrylonitrile with
 the appropriate .alpha.-olefin under acidic sulfonation conditions as
 described above.

EXAMPLE 1
 Illustrative--C.sub.5 Polysurfactant
 The monomers prepared in Examples 1 and 2 were prepared by treating
 acrylonitrile with the appropriate .alpha.-olefin under acidic sulfonation
 conditions as described above. After the preparation of the monomers,
 homopolymerizations were run using the acidic form of the monomers (unless
 otherwise specified) in water as the reaction solvent and potassium
 persulfate as the free radical initiator. The reactions were run at
 60.degree. C. for a 16 hour period.
 A 250 ml flask equipped with magnetic stir bar, thermometer, and a
 condenser with a nitrogen inlet/outlet was purged with nitrogen. 100 g of
 water, which had been degassed with nitrogen while cooled in an ice bath
 (1.degree. C.), was loaded into the flask. Potassium persulfate (0.2 g;
 7.4.times.10.sup.-4 moles) was then added and stirred until dissolved
 while keeping the temperature at 1.degree. C. Then C.sub.5 surfactant (10
 g, 0.045 moles) was added to the initiator solution and stirred for
 several minutes until dissolved. Once the monomer was dissolved, the
 reaction solution was polymerized by heating to 60.degree. C. After 16
 hours reaction time, the reaction was cooled and the polymer was
 neutralized with a slight excess of NaOH. The solution of the Na-salt of
 the polysurfactant was freeze-dried for 24 hours. The .sup.1 H NMR and
 .sup.13 C NMR of the polymer were consistent with the following structure:
 ##STR14##
 where M=Na, a poly(2-acrylamido-1-pentanesulfonic acid, sodium salt)
 polymer. When dissolved at 0.5 wt % in synthetic sea water, the polymer
 did not precipitate when heated to 100.degree. C.; consequently, its
 T.sub.cp (at atmospheric pressure) was above 100.degree. C.
 EXAMPLE 2
 Illustrative--C.sub.6 Polysurfactant
 The same procedure described in Example 1 above was used to make the
 C.sub.6 polysurfactant, except that 10 g (0.042 mol) of the C.sub.6
 surfactant monomer was employed instead of the C.sub.5 surfactant monomer.
 The .sup.1 H and .sup.13 C NMRs of the polymer product were consistent
 with the following structure:
 ##STR15##
 where M=Na, a poly(2-acrylamido-1-hexanesulfonic acid, sodium salt)
 polymer. This example was repeated using the same materials to make a
 second sample of the same polymer. When dissolved at 0.5 wt % in synthetic
 sea water, neither sample precipitated when heated to 100.degree. C.;
 consequently, the T.sub.cp for these polymers (at atmospheric pressure)
 was above 100.degree. C.
 EXAMPLE 3
 Illustrative--C.sub.7 Polysurfactant
 The same procedure described in Example 1 was used to make the C.sub.5
 polysurfactant, except that 10 g (0.040 mol) of the C.sub.7 surfactant
 monomer was employed instead of the C.sub.5 surfactant monomer. The .sup.1
 H and .sup.13 C NMRs of the polymer product were consistent with the
 following structure:
 ##STR16##
 where M=Na, a poly(2-acrylamido-1-heptanesulfonic acid, sodium salt)
 polymer. The polymer had a cloud point (when dissolved in 0.5% synthetic
 sea water at atmospheric pressure) of 39.degree. C.
 EXAMPLE 4
 Illustrative--Me--C.sub.6 Polysurfactant Homopolymer
 For this reaction, the Na salt of the monomer was used. The neutralization
 was carried out according to the following procedure: NaOH (4.01 g, 0.1003
 moles) was dissolved in 583 g of deionized water and cooled to -3.degree.
 C. by means of an icebath. Me--C.sub.6 surfactant monomer (25 g, 0.1003
 moles) was added slowly, making certain to maintain the temperature below
 0.degree. C. On completing the addition of the surfactant monomer, the pH
 of the solution was 2.5. The final pH was adjusted to 7.0 with a few drops
 of diluted NaOH, and the Na Me--C.sub.6 surfactant monomer isolated by
 freeze drying.
 To make the polymer, Na Me--C.sub.6 surfactant monomer and 38 g water were
 charged to a 250 ml flask equipped with magnetic stir bar, thermometer,
 and a condenser with a nitrogen inlet/outlet. The solution was flushed for
 1 hour with nitrogen. Then, the reaction was brought to 60.degree. C. and
 ammonium persulfate (0.150 g, 6.6.times.10.sup.-4 moles dissolved in 2 g
 water) was added. The reaction was heated at 60.degree. C. and stirred
 under nitrogen overnight. The next day, the reaction solution was freeze
 dried for 24 hours. The resulting polymer was redissolved in MeOH and
 reprecipitated in diethyl ether. The .sup.1 H and .sup.13 C NMRs were
 consistent with the following polymer structure:
 ##STR17##
 where M=Na, a poly(2-methacrylamido-1-hexanesulfonic acid, soldium salt)
 polymer. When dissolved at 0.5 wt % in synthetic sea water, the polymer
 did not precipitate when heated to 100.degree. C.; consequently, its
 T.sub.cp (at atmospheric pressure) was above 100.degree. C.
 EXAMPLE 5
 Illustrative--Copolymerization of C.sub.5 Surfactant with C.sub.7
 Surfactant
 Using the same general procedure described in Example 1 above, copolymers
 of the surfactant monomers can be made. Thus, a 250 ml flask equipped with
 magnetic stir bar, thermometer, and a condenser with a nitrogen
 inlet/outlet was purged with nitrogen. 100 g of water, which had been
 degassed with nitrogen while cooled in an ice bath (1.degree. C.), was
 loaded into the flask. Potassium persulfate (0.2 g 7.4.times.10.sup.-4
 moles) was then added and stirred until dissolved while keeping the
 temperature at 1.degree. C. Then 4.7 g (0.021 moles) C.sub.7 surfactant
 and 5.3 g (0.021 moles) C.sub.7 surfactant were added to the initiator
 solution and stirred for several minutes until dissolved. Once the
 monomers were dissolved, the reaction solution was polymerized by heating
 to 60.degree. C. After 16 hours reaction time, the reaction was cooled and
 the copolymer was neutralized with a slight excess of NaOH. The solution
 of the Na-salt of the polysurfactant was freeze-dried for 24 hours. The
 .sup.1 H and .sup.13 C NMRs of the copolymer were consistent with the
 following structure:
 ##STR18##
 where M=Na, a poly(2 acrylamido-1-pentanesulfonic
 acid-co-2-acrylamido-1-heptanesulfonic acid), sodium salt copolymer. When
 dissolved at 0.5 wt % in synthetic sea water, the polymer did not
 precipitate when heated to 100.degree. C.; consequently, its T.sub.cp (at
 atmospheric pressure) was above 100.degree. C.
 EXAMPLE 6
 Illustrative--Copolymerization of C.sub.5 Surfactant and C.sub.7 Surfactant
 The procedure outlined in Example 5 above was used to prepare a 50/50
 copolymer of the monomers of Examples 1 and 2, a
 poly(2-acrylamido-1-pentanesulfonic acid-co-2-acrylamido-1-octanesulfonic
 acid), sodium salt copolymer. When dissolved at 0.5 wt % in synthetic sea
 water, the polymer did not precipitate when heated to 100.degree. C.;
 consequently, its T.sub.cp (at atmospheric pressure) was above 100.degree.
 C.
 EXAMPLE 7
 Illustrative--Copolymerization of C.sub.6 Surfactant with IPMA
 N-isopropylmethacrylamide (IPMA) was purchased from Aldrich Chemical
 Company and recrystallized twice from hexane. Anhydrous methanol was
 purchased from Aldrich Chemical Company. Deionized water was degassed by
 sparging with nitrogen. The initiator, 2,2'-Azobis (2-amidinopropane)
 hydrochloride (V50) was obtained from WAKO Pure Chemical Industries and
 used as received. The C.sub.6 surfactant, prepared as described in Example
 2, was neutralized with NaOH as described above in Example 4.
 Na--C.sub.6 surfactant (13.37 g, 0.052 moles) and IPMA (6.62 g, 0.052
 moles) were charged to a 3-necked flask equipped with a condenser,
 stirrer, nitrogen inlet/outlet and thermometer. The powders were purged
 with nitrogen for about 1 hour. Then, 90 g MeOH and 90 g water, which had
 been purged separately, were added together to the reaction flask. The
 reaction mixture was brought to 60.degree. C., then initiated with 0.6 g
 (2.2.times.10.sup.-3 moles) V50, which was dissolved in a H.sub.2 O/MeOH
 mixture. The reaction was maintained at a constant temperature of
 60.degree. C. while it was stirred overnight. The next day, MeOH was
 removed from the reaction mixture on a rotary evaporator. The remaining
 water/polymer mixture was freeze-dried for 24 hours. This remaining
 product had .sup.1 H and .sup.13 C NMRs consistent with the following
 structure:
 ##STR19##
 a poly(2-acrylamido-1-hexanesulfonic acid, sodium
 salt-co-N-isopropyl-methacrylamide) copolymer. When dissolved at 0.5 wt %
 in synthetic sea water, the polymer did not precipitate when heated to
 100.degree. C.; consequently, its T.sub.cp (at atmospheric pressure) was
 above 100.degree. C.
 EXAMPLE 8
 Illustrative--Copolymerizations of Me--C.sub.6 Surfactant with IPMA
 Using a procedure similar to that outlined in the Example 7, the sodium
 salt of the Me--C.sub.6 surfactant monomer was copolymerized with IPMA,
 using MeOH/water mixtures as the solvent. The initiator, 2,2'-azobis
 methyl butyronitrile (V67) was obtained from DuPont Chemicals, and used as
 received. A table summarizing the recipes used follows:

Copolymer
 Reaction
 Composition
 Example IPMA NaMeC.sub.6 V67 H.sub.2 O/MeOH Temp
 (IPMA/NaMeC.sub.6)
 8A/8B 3.38 g 6.62 g 0.3 g 10 g/20 g 64.degree. C.
 48.4/51.6
 (0.027 mol) (0.027 mol) (0.0016 mol)
 8C/8D 5.43 g 4.57 g 0.3 g 10 g/20 g 64.degree. C.
 72.3/27.7
 (0.043 mol) (0.018 mol) (0.0016 mol)
 The copolymers were isolated by precipitating the reaction mixture into
 diethylether, followed by filtration. The products were then redissolved
 in MeOH and reprecipitated in diethylether. The .sup.1 H and .sup.13 C
 NMRs were consistent with the formation of
 poly(2-methacrylamido-1-hexanesulfonic acid, sodium
 salt-co-N-isopropyl-methacrylamide) copolymers. When dissolved at 0.5 wt %
 in synthetic sea water, the Example 8A polymer did not precipitate when
 heated to 100.degree. C., consequently its T.sub.cp (at atmospheric
 pressure) was above 100.degree. C. The Example 8C copolymer had a cloud
 point (when dissolved in 0.5 wt % synthetic sea water at atmospheric
 pressure) of 80-82.degree. C.
 EXAMPLE 9
 Illustrative--Copolymerizations of Me--C.sub.6 Surfactant with VCap
 A 100 ml 4-necked round bottom flask loaded with a mechanical stirrer,
 condenser, thermometer, and N.sub.2 inlet/outlet, was charged with
 N-vinylcaprolactam (VCap) (1.79 g, 0.0129 moles), which was used as
 received from Scientific Polymer Products. The flask was flushed with
 N.sub.2 for over 1 hour, then 6 ml of anhydrous MeOH was added. The
 solution was heated to 64.degree. C. Azobisisobutyronitrile (AIBN) (0.0150
 g in 4 ml MeOH) was then added to the VCap solution. NaMeC.sub.6
 surfactant (3.21 g, 0.0129 mole) was dissolved in 9.6 g of deionized
 H.sub.2 O, flushed with N.sub.2, and loaded in a 25 cc syringe. This
 solution was then pumped into the VCap solution at a rate of 0.064 g
 NaMeC.sub.6 per minute for about 50 minutes. A pumping procedure was used
 to achieve a random copolymer, due to the unequal reactivities of the two
 monomers employed here. The reaction was stirred and heated overnight. The
 following day, the reaction mixture was precipitated into acetone and
 filtered. The polymer was further purified by redissolving in acetone and
 reprecipitating into diethyl ether. The .sup.1 H and .sup.13 C NMRs were
 consistent with the following structure:
 ##STR20##
 a poly(N-vinyl caprolactam-co-1-methacrylamido-1-hexanesulfonic acid,
 sodium salt) copolymer. The copolymer had a cloud point (when dissolved in
 0.5% synthetic sea water at atmospheric pressure) of 35.degree. C.
 EXAMPLE 10
 Comparative--Poly (Na-AMPS), Poly (IPMA), and Poly (VCap)
 A sample of poly (2-arylamido-2-methyl-1-propanesulfonic acid sodium salt)
 or Poly (AMPS) was obtained from Polysciences Inc. (Warrington, Pa.). This
 polymer may be represented by the following structure:
 ##STR21##
 a poly(2-acrylamido-2-methyl-1-propanesulfonic acid, sodium salt) polymer.
 This material was evaluated as a comparative example, and was not expected
 to be an effective inhibitor. Although it falls within the class of
 amphoteric polymers, it is not considered a polysurfactant, since it has
 no hydrophobic tail, and consequently it is too hydrophilic.
 Samples of poly(N-isopropyl methacrylamide) or poly(IPMA), poly(N-vinyl
 caprolactam) or poly(VCap), and a copolymer of N-vinyl pyrroliddone (VP)
 and VCap, or poly VP/VCap, were obtained from previous work and used for
 comparison with the inhibitors of the invention. In addition, copolymers
 of AMPS with VCap, VP, and a combination of VP and VCap, were also
 prepared and used for comparison with the inhibitors of the invention.
 These polymers and copolymers may be synthesized by procedures known to
 those skilled in the art.
 Mini-loop Testing Procedure
 One method for evaluating an inhibitor's effectiveness uses a bench-scale
 high pressure apparatus referred to as a mini-loop apparatus. A mini-loop
 apparatus consists of a loop of stainless steel tubing with about a
 one-half inch inside diameter and about ten feet in length. The loop also
 has a transparent section for observing the fluid flow in the loop and the
 onset of hydrate formation in the loop. Fluid comprising about 40% by
 volume (vol %) synthetic sea water solution having about 3.5% total
 ionized salts, 40 vol % hydrocarbon condensate (i.e., C.sub.6 +), and 20
 vol % hydrocarbon gas mixture is circulated around the loop at constant
 pressure. The hydrocarbon gas mixture is comprised of approximately 85
 mole % methane, 5 mole % ethane, 5 mole % propane, and 5 mole % of C.sub.4
 +. The inhibitor is typically injected into the loop as an aqueous
 solution to produce the desired weight percent (wt %) concentration of
 inhibitor in the aqueous sea salt/gas solution. Generally, hydrate
 inhibitors are evaluated at about 0.5 wt % of the aqueous sea salt/gas
 solution.
 The fluid is circulated at a constant velocity of about 0.76 m/second (2.5
 feet/second). The loop and its pump are operated in a controlled
 temperature water bath to control the temperature of the fluid circulating
 in the loop. The bath water is circulated to ensure uniform temperature
 throughout the bath and rapid heat transfer between the bath water and the
 loop. As the loop temperature changes or as hydrates form, the gas volume
 in the loop will change accordingly. Therefore, to maintain constant
 pressure in the loop, a pressure-compensating device is required. Such a
 device can be comprised of a gas cell and a hydraulic oil cell separated
 by a floating piston. As the gas volume in the loop changes, oil may be
 added or removed from the oil cell to produce a commensurate addition or
 removal of gas to the loop. Mini-loop tests are typically run at a
 pressure of about 6996 KPa absolute (1,000 pounds per square inch gauge
 (p.s.i.g.)). However, any pressure between 101 to 20,786 KPa absolute (0
 to 3,000 p.s.i.g.) could be selected for evaluating an inhibitor's
 performance.
 The temperature of the water bath is reduced at a constant rate, preferably
 about 6.degree. F. (3.3.degree. C.) per hour, from an initial temperature
 of about 70.degree. F. (21.degree. C.). At some temperature, clathrate
 hydrates begin to rapidly form. As gas is consumed in forming clathrate
 hydrates, there is an abrupt and corresponding decrease in the volume of
 gas in the fluid as hydrates are formed. The temperature at which the
 abrupt decrease in the volume of gas is observed is measured as the
 temperature of onset for hydrate formation, or T.sub.os, and compared to
 the hydrate equilibrium dissociation temperature, or T.sub.eq, to
 determine the inhibitor's subcooling. A hydrate inhibitor's subcooling, or
 T.sub.sub, is the difference between the T.sub.eg and the T.sub.os. (Note
 the subcooling is not actually a temperature, but a difference, measured
 in degrees, between two temperatures.) For a given pressure, the greater
 the subcooling, the more effective the inhibitor.
 Mini-loop Test Results
 For the purpose of illustrating the invention, the various polymeric
 inhibitors described above were evaluated using the mini-loop testing
 procedure described above. The results of these evaluations are provided
 in the Table below.
 TABLE 1
 Polymer
 Cloud
 Mole Conc. Subcooling
 Subcooling Point
 Example Inhibitor Ratio (wt %) (.degree. F.)
 (.degree. C.) (.degree. C.)
 ILLUSTRATIVE
 Example 1 C.sub.5 polysurfactant NA 0.5 17
 9.4 &gt;100.degree.
 Example 2A C.sub.6 polysurfactant NA 0.6 19.5
 10.8 &gt;100.degree.
 Example 2B C.sub.6 polysurfactant NA 0.5 20.5
 11.4 &gt;100.degree.
 Example 3 C.sub.7 polysurfactant NA 0.5 14
 7.8 39.degree.
 Example 4 MeC.sub.6 polysurfactant NA 0.5 19
 10.6 &gt;100.degree.
 Example 5 C.sub.5 /C.sub.7 copolysurfactant 50:50 0.5 20
 11.1 &gt;100.degree.
 Example 6 C.sub.5 /C.sub.8 copolysurfactant 50:50 0.5 19
 10.6 &gt;100.degree.
 Example 7 C.sub.6 surfactant/IPMA Copolymer 52:48 0.5 24.5
 13.6 &gt;100.degree.
 Example 8A MeC.sub.6 surfactant/IPMA Copolymer 48:52 0.5 25
 13.9 &gt;100.degree.
 Example 8B MeC.sub.6 surfactant/IPMA Copolymer 48:52 0.5 26
 14.4 --
 Example 8C MeC.sub.6 surfactant/IPMA Copolymer 28:72 0.5 27
 15.0 80-82.degree.
 Example 8D MeC.sub.6 surfactant/IPMA Copolymer 28:72 0.5 27
 15.0 --
 Example 9 C.sub.6 surfactant/VCap Copolymer 56:44 0.5 31
 17.2 35.degree.
 COMATIVE
 Example 10A Poly (AMPS) NA 0.5 14.4
 8.0 &gt;100.degree.
 Example 10B Poly (IPMA) NA 0.5 24.0
 13.3 38.degree.
 Example 10C Poly (VCap) NA 0.5 22.5
 12.5 27.degree.
 Example 10D Poly (AMPSNCap) 50:50 0.5 17
 9.4 &gt;100.degree.
 Example 10E Poly (AMPSNP) 50:50 0.5 12
 6.7 &gt;100.degree.
 Example 10F Poly (VP/VCap/AMPS) 29:22:49 0.5 22
 12.2 67.degree.
 Example 10G Poly (VP/VCap) 50:50 0.5 21
 11.7 55.degree.
 The data show that the polysurfactant homopolymers and copolymers of the
 invention are effective inhibitors with a good balance of properties. The
 polysurfactant inhibitors of the invention are generally more effective
 (as measured by subcooling) than other known amphoteric polymers, such as
 poly(AMPS). Furthermore, copolymers of surfactant monomers copolymerized
 with monomers of effective hydrate inhibitors, such as poly(VCap) and
 poly(IPMA), are better inhibitors than the corresponding VCap and IPMA
 homopolymers. This is an unexpected result. Typically, properties and
 performance of copolymers are the additive of the two homopolymers. These
 systems exhibit an unexpected synergy.
 The polysurfactant homopolymers and copolymers of the invention are also
 advantageous because they generally have higher cloud points than other
 known inhibitors. Polymeric inhibitors other than poly (AMPS) typically
 exhibit cloud point temperatures (under atmospheric conditions) of less
 than 100.degree. C., and typically in the range of about 20.degree. C. to
 about 40.degree. C., for previously-known polymeric inhibitors which
 exhibit good subcooling properties. The surfactant monomers of the
 invention, when polymerized with monomers whose homopolymers have low
 cloud points, such as poly(VCap) and poly(IPMA), serve to raise the cloud
 point of the resulting inhibitors. Most of the polysurfactant copolymers
 of the invention have cloud points above 100.degree. C., while exhibiting
 subcoolings that indicate they are effective hydrate inhibitors. Thus,
 most of these inhibitors can be applied near the wellhead, where the
 hottest temperatures would be experienced, without precipitation.
 The means and method for practicing the invention and the best mode
 contemplated for practicing the invention have been described. It is to be
 understood that the foregoing is illustrative only and that other means
 and techniques can be employed without departing from the true scope of
 the invention as claimed herein.