Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-02-14
2001-11-20
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
active
06319971
ABSTRACT:
This application is a 371 application of PCT/NO98/00152 filed May 20, 1998.
This invention relates to a composition and a method for inhibiting the formation, agglomeration and plugging by gas hydrates of pipes, wells and equipment containing oil or gas. This is relevant for both drilling and production of oil and gas.
BACKGROUND OF THE INVENTION
Gas hydrates are clathrates (inclusion compounds) of small molecules in a lattice of water molecules. In the petroleum industry natural gas and petroleum fluids contain a variety of these small molecules which can form gas hydrates. They include hydrocarbons such as methane, ethane, propane, iso-butane as well as nitrogen, carbon dioxide and hydrogen sulfide. Larger hydrocarbons such as n-butane, neopentane, ethylene, cyclopentane, cyclohexane, and benzene are also hydrate forming components. When these hydrate forming components are present with water at elevated pressures and reduced temperatures the mixture tends to form gas hydrate crystals. For example, ethane at a pressure of IMPa forms hydrates only below 4° C. whereas at 3 MPa gas hydrates can only form below 14° C. These temperatures and pressures are typical operating environments where petroleum fluids are produced and transported. If gas hydrates are allowed to form inside a pipe or well used to transport natural gas and/or other petroleum fluids they can eventually block the pipe or well. The hydrate blockage can lead to a shutdown in production and significant financial loss. The oil and gas industry uses various means to prevent the formation of hydrate blockages in pipelines and wells. These include heating the pipe, reducing the pressure, removing the water and adding anti-freezes such as methanol and ethylene glycols which act as melting point depressants. Each of these methods is costly to implement and maintain. The most common method used today is adding anti-freezes. However, these anti-freezes have to be added at high concentrations, typically 10-40% by weight of the water present, in order to be effective. Recovery of the anti-freezes is also usually required and a costly procedure. Consequently, there is a need for alternate cheap methods for preventing hydrate blockages in oil and gas drilling and production. An alternative to the above methods is to control the gas hydrate formation process using nucleation and crystal growth inhibitors and/or other chemicals that prevent the formed hydrate crystals from agglomerating. These types of chemicals are widely known and used in other industrial processes. The advantage of using these chemicals to control gas hydrate formation is that they can be used at concentrations of 0.01 to 2% which is much lower than for anti-freezes.
It is an object of this invention to provide a method and composition for controlling gas hydrate formation using additives added at low concentrations to a gas hydrate forming fluid. To prevent gas hydrates from nucleating and growing into crystals, it is possible to design molecules which bind to hydrate surfaces in preference to the bulk water. To do this we need to understand the nature of hydrate surfaces formed during nucleation and crystal growth. It is assumed that Structure II hydrate will preferentially form in pipelines. Methane and ethane on their own form Structure I, but when small amounts of propane or iso-butane hydrocarbons are also present Structure II is the more thermodynamically preferred hydrate. Since C
3-4
hydrocarbons are always present in natural gas Structure II will always form in preference to Structure I in the field. Further, the concentration of C
3-4
components is in practice almost always high enough that formation of Structure II is thermodynamically preferred over Structure H in oil and gas pipelines. Structure II hydrate is made up of 5
12
6
4
and 5
12
cages. Hydrocarbons which fit the larger Structure II hydrate cages (5
12
6
4
) include propane, iso-butane and cyclopentane. These molecules do not need any help gas to form stable Structure II hydrates. Cyclohexane, benzene, n-butane, butadiene, cyclopentene, isobutylene and neopentane represent the larger hydrocarbons that can occupy the large Structure II cavities but they need the use of a help gas in the 5
12
cages, such as methane, to stabilize the structure.
The Structure II hydrate surface is very much dynamic but can be thought of as being made up of open 5
12
6
4
and 5
12
cages that would normally be filled with hydrate formers. One way to design a hydrate nucleation or growth inhibitor is to find certain groups which will interact strongly with these open cavities on the surface of the hydrate. Due to the low symmetry of the hydrate structure, the hydrate surface will change as it grows so that at different times the surface will have varying size and shape cavities. Therefore different groups in the same inhibitor molecule or a mixture of different inhibitors with different groups may be more favorable than using just one inhibitor with one interacting group.
It is known that polymers with monomers such as N-vinyl pyrrolidone (Int. Patent Appl. Publ. WO 93/25798), N-vinyl caprolactam (Int. Patent Appl. Publ. WO 94/12761), N-isopropylmethacrylamide (Int. Patent Appl. Publ. WO 96/41 834), and acryloylpyrrolidine (Int. Patent Appl. Publ. WO 96/08672) as well as small molecules such as some alkyl ammonium salts (Int. Patent Appl. Publ. WO 95/17579) are able to slow down the nucleation and growth of Structure II hydrates. Mixtures of a N-vinyl caprolactam polymer and a second N-vinyl pyrrolidone polymer are claimed in Patent Application, Publication WO 96/04462. All the above “known” technologies are not good enough for most field applications, i.e. their performance is not good enough for most applications.
In addition to their insufficient performance as a clathrate hydrate inhibitor, low deposition points of these polymers are also problematic for field applications. This is because the inhibitor must often be injected into a hot well stream e.g. at the well head, where it is required to dissolve in the produced aqueous phase. If the well head temperature is above the deposition temperature of the polymer, the polymer may precipitate out of the hot aqueous solution leading to a restriction or plugging of the injection or production line. Similarly, these clathrate hydrate inhibitor polymers may precipitate out from drilling fluids to which they have been added, especially when the drilling fluid has a high concentration of salts in the aqueous phase and the drilling fluid becomes hot in the well. The deposition point temperature of a water-soluble polymer useful as a clathrate hydrate inhibitor is often hard to measure but often lies 0-20° C. above the cloud point temperature. Thus, a useful indication of the deposition point temperature can be found from the more easily measured cloud point. For example, the cloud point of Gaffix VC-713, terpolymer of vinyl caprolactam, vinyl pyrrolidone, and dimethylaminoethylmethacrylate, is as low as 31° C. in distilled water. Polymers with such low cloud points are rarely applicable for real fields since the deposition point is often lower than the temperature of the produced fluids at the inhibitor injection point, or of the drilling fluids.
It is also claimed in Int. Patent Appl. Publ. WO 96/08672 that alkyl(meth)acrylamides can be used in combination with substantially water-soluble polymers from the group consisting of poly(N-vinyl caprolactam), poly(N-vinyl pyrrolidone) and N-acyl substituted polyalkeneimines. However, the technology is still not good enough since polymers disclosed in the application were prepared by conventional preparation methods so that their molecular weights would be too high to expect the sufficient performance and their deposition points should be too low to be practically used.
It is preferable that a polymer or polymer mixture prevents hydrate nucleation more than crystal growth as this will ensure no build up of hydrate crystals in the hydrate forming system. This is particularly advantageous in gas and oil pipelines as
Kelland Malcolm Andrew
Namba Takashi
Rodger Mark
Cain Edward J.
RF-Procom A/S
Wenderoth , Lind & Ponack, L.L.P.
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