Method for inhibiting hydrate formation

Chemistry of hydrocarbon compounds – Hydrate or production thereof

Reexamination Certificate

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C585S950000, C095S153000

Reexamination Certificate

active

06194622

ABSTRACT:

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° C. (39° F.), and at a pressure of 3000 kPa (435 psi), ethane can form gas hydrates at temperatures below 14° C. (57° 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:
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”:
where both R
1
and R
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
6
polysurfactant:
The designation of “C
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 &agr;-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 i

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