Mineral oils: processes and products – Asphalts – tars – pitches and resins; making – treating and... – Chemical modification of asphalt – tar – pitch or resin
Reexamination Certificate
1999-09-29
2001-08-07
Myers, Helane E. (Department: 1764)
Mineral oils: processes and products
Asphalts, tars, pitches and resins; making, treating and...
Chemical modification of asphalt, tar, pitch or resin
C208S309000, C208S039000, C208S045000, C208S044000
Reexamination Certificate
active
06270653
ABSTRACT:
The present invention relates to a method of controlling asphaltene precipitation and the use of certain compounds in such a method.
Asphaltene fractions are defined operationally as that portion of a crude oil or bitumen which precipitates on addition of a low molecular weight paraffin (usually n-pentane or n-heptane) but which is soluble in toluene. Asphaltenes are brown to black amorphous solids with complex structures, involving carbon, hydrogen, nitrogen, oxygen and sulphur and are basically formed of condensed aromatic nuclei associated with alicyclic groups. The particles are often surrounded by resins which are considered to add to dispersion stability. The molecular weight of asphaltene ranges from one thousand to several hundred thousand with a particle density of approximately 1200 kg/m
3
and a spheroidal shape about 10 nm in diameter. Colloidal asphaltene precipitation from petroleum reservoir fluids is recognised to present serious problems in numerous crude oil systems world-wide. Asphaltene precipitation may occur in the reservoir formation and cause permeability reduction or contribute to serious plugging problems in oil well tubing and surface facilities. Whilst not prevalent in the North Sea deposits, this phenomenon has lead to significant excess costs in the production operations of the oil industry in North America and the Middle East. The main approach to dealing with asphaltene problems is associated with well maintenance by either improved technology in clean-up methods for unplugging lines or asphaltene dissolution with various solutions.
We now propose a different approach to the problem of asphaltene deposits, which involves the prevention of the precipitation of the asphaltenes in the first place, rather than the approach taken in the prior art which involves dispersion of the precipitation.
Thus according to the present invention there is provided a method of controlling asphaltene precipitation in a fluid comprising the addition of a precipitation inhibitor.
In practice, the fluid will commonly be crude oil, and the precipitation inhibitor can be added using techniques known to those skilled in the art.
The stability of a dispersion in colloid science terms refers to the resistance of the particles to aggregation. The degree of this resistance is a measure of stability. Asphaltene colloidal dispersions in petroleum reservoirs are usually stable if free from any changes in physical properties. The behaviour of asphaltene in oil depends on the attractive and repulsive forces between adjacent particles. The interactions involved include van der Waals forces, steric effects and possibly electric double layer forces arising from charge at the interfaces. It is generally accepted that asphaltene coagulation and deposition occurs as a result of changes in parameters such as reservoir pressure, reservoir fluid temperature and oil composition brought about by normal recovery operations.
Surfactants can have either a stabilising or flocculating effect on dispersions into which they are introduced. The flocculating effect occurs especially when the reagent adsorbs on the particle surfaces and reduces charge, induces “bridging” effects between particles or causes hydrophobic interaction effects. Surfactants can also induce dispersion or maintain suspension stability. Then the mechanism is either by increasing surface charge or by contributing to steric (entropic) effects.
For effective stabilisation of dispersion to occur:
The surfactant has to adsorb on the particles
The dispersion medium has to be a “good solvent” for all or part of the surfactant molecule so that any carbon chains are extended and freely moving.
These two conditions are to an extent contradictory, but we have now found that a useful surfactant for dispersion/stabilisation often has an adsorbing part and, separately, a dissolving part of the molecule e.g. a block co-polymer of the AB or ABA type. The freely dissolved chains then give a repulsion when similar particles approach. The overlap region has a higher concentration of these chains and so an osmotic repulsion is produced.
Thus according to one embodiment of the present invention the precipitation inhibitor used in the present invention is a compound which has an adsorbing part and a dissolving part. Preferably the dissolving part of the molecule is a hydrocarbon-based chain or polymer, i.e. substantially composed of carbon and hydrogen.
More particularly, we have found a range of compounds which are especially suitable for use in the present invention. These compounds may be represented by the general formula I:
X—(R)
n
I
where X is the adsorbing part of the compound and is preferably a carbocyclic ring containing 6 to 16 carbon atoms and which may be a mono- or bi-cyclic ring, such as benzene (C
6
), naphalene (C
10
) and anthracene (C
14
). Naphalene is especially preferred; it is more soluble than anthracene and is less volatile than benzene.
R is the dissolving part of the molecule and is preferably an alkyl group containing 10 to 25 carbon atoms. Preferably R contains 12 to 20 carbon atoms, more preferably 14 to 18 carbon atoms, with 16 carbon atoms being especially preferred. In a most preferred embodiment R is hexadecyl (C
16
H
33
). It will be appreciated that R may be straight chain or branched chain. Preferably R is branched chain.
n is at least 1 and may equal the number of positions available for substitution in X. Preferably n is 1, 2 or 3. More preferably n is 1. When n is 2 or more, the R groups may be the same or different. Preferably there are two hexadecyl groups.
The carbocyclic ring may be optionally substituted in positions not occupied by R. Such substitutions should either not interfere with, or enhance, the properties of the compound. Suitable substituents may include C
1-6
alkyl groups such as methyl, ethyl, propyl, iso-propyl, n-butyl, iso-butyl, sec-butyl or t-butyl; or C
1-6
haloalkene such as fluoromethyl, difluoromethyl, trifluoromethyl, chloromethyl, dichloromethyl, trichloromethyl, 2-fluoroethyl, 2,2,2-trifluoroethyl and pentafluoroethyl.
Preferably the total number of substituents on X is no more than about 5 or 6. Once the number of substituents is greater than about 5 or 6, the adsorption of the X group on the asphaltene particle's surface may become impaired.
Similarly, the R group may be optionally substituted. Such substituents should either not interfere with, or enhance, the properties of the compound. Suitable substituents may include halogen, nitro, cyano, COOR′ (where R′ is H or C
1-6
alkyl) or a salt thereof, hydroxy and C
1-6
alkoxy.
According to an especially preferred embodiment of the present invention the molecule used to stabilize the asphaltene dispersion in crude oil is 2-hexadecyl naphthalene and as shown below as formula II
The hexadecyl chain will be in a good solvent in the aliphatic oil and so will be in an extended state and most effective at promoting steric repulsion. The naphthalene group was chosen because it is flat and hence, other things equal, it will have the maximum adsorption on a flat surface. The p electrons of the naphthalene molecule will render it more polarisable (with a high Hamaker coefficient) and hence more strongly adsorbed.
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patent: 3172473 (1965-03-01), Crowley et al.
patent: 3276519 (1966-10-01), Knox et al.
patent: 4428818 (1984-01-01), Derbyshire et al.
patent: 4441890 (1984-04-01), Feldman
patent: 5021498 (1991-06-01), Stephenson et al.
patent: 5214224 (1993-05-01), Stephenson et al.
patent: 258179 (1987-07-01), None
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XP 010978325; WPI Derwent; AN 96-475274—Derwent week 199647; “Composition for preventing and removing asphaltene-resin-paraffin deposits—contains aromatic solvent and monalkyl ether of . . . ”.
XP 009965424; WPI Derwent; AN 94-233137—Derwent week 199428; “Composition preventing asphaltene-resin-paraffin deposits—in oil industry equipment, contains specified surfactant, aromatic solvent and . . . ”.
Gochin Rodney J.
Smith Alec
Imperial College of Science Technology & Medicine
Myers Helane E.
Renner , Otto, Boisselle & Sklar, LLP
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