Method of controlling asphaltene precipitation in a fluid

A method of controlling asphaltene precipitation in a fluid comprising the addition of a precipitation inhibitor to the fluid containing the asphaltene.

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.sup.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:
EQU X--(R).sub.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.sub.6), naphalene (C.sub.10) and
 anthracene (C.sub.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.sub.16 H.sub.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.sub.1-6 alkyl groups such as methyl, ethyl, propyl, iso-propyl, n-butyl,
 iso-butyl, sec-butyl or t-butyl; or C.sub.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.sub.1-6 alkyl) or a salt thereof, hydroxy and
 C.sub.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
 ##STR1##
 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.

The compounds for use in the present invention may be prepared by
 techniques known to those skilled in the art. An example of such a
 preparative method is given below in Example 1.
 EXAMPLE 1
 Preparation of 2-hexadecyl naphthalene
 2-hexadecyl naphthalene was prepared by the Wolff-Kishner reduction of
 2-hexadecanoyl naphthalene using the Huaing-Minlon modification (Anderson
 JACS)(1953) p449). 2-Hexadecanoyl naphthalene was prepared by the
 Friedel-Craft's reaction between 2-hexadecanoyl chloride (palmitoyl
 chloride) and naphthalene in the presence of anhydrous aluminium chloride
 and nitrobenzene using the method of Buu-Hoi & Cagnint (Bull. Soc. Chim.,
 12 (1945) p307). The presence of nitrobenzene seems to ensure the
 formation of the 2-isomer rather than a mixture of the 1- and 2-isomers.
 The compound obtained was then vacuum distilled and further purified by
 column chromatography on an alumina column in petroleum ether. Its purity
 was confirmed by melting point and infra-red spectroscopy.
 An alternative synthesis, which may be more appropriate for commercial
 exploitation, would be to react naphthalene directly with hexadecanol in
 the presence of an appropriate catalyst. This would give a mixture of
 isomers. The infra red spectroscopy pattern of 2-hexadecyl naphthalene is
 shown in FIG. 1.
 EXAMPLE 2
 Determination of Mode of Action of 2-hexadecyl naphthalene in Asphaltenic
 Crude Oils
 It is generally accepted that asphaltenes exist in petroleum oil as
 particles in a dispersed state, colloidally stabilised at least to some
 extent by the resins which act as peptizing agents. Resin molecules
 surround asphaltene particles and can form a layer giving a steric shield.
 If this protective shield is removed by for instance the dissolution of
 the resins into the fluid phase, the asphaltene particles start to
 aggregate into larger particles (i.e. coagulate) which can result in
 asphaltene deposition onto surfaces. The presence of 2-hexadecyl
 naphthalene will reduce the instability of asphaltene by mimicking the
 action of resins for maximum effectiveness in aliphatic solvents such as
 crude oil. The stabilisation of asphaltene particles by 2-hexadecyl
 naphthalene occurs when 2-hexadecyl naphthalene molecules are attached to
 the surfaces of asphaltene particles by the naphthalene heads and stretch
 the aliphatic chains out into the oil to form a steric stabilisation
 layer. Though this will only happen to maximum effect in an aliphatic
 liquid which is good solvent for the hexadecyl chain such as oil, it
 should however be partially effective in solvents such as toluene.
 The particle size distribution of a solid in crude oil can not be easily
 determined by conventional techniques. A laser back-scattering technique
 was developed to avoid the difficulty in strongly absorbing dispersion
 media such as crude oil. The technique uses photon correlation
 spectroscopy, also called quasi elastic light scattering, but in the
 back-scatter mode rather than the more conventional forward scattering.
 This is particularly useful for concentrated dispersions or for strongly
 absorbing solutions as in this work. The instrument used in this work was
 supplied by Brookhaven Instruments, New York. The particle detection range
 was from 2 nm up to 10 .mu.m. The instrument was supplied with a fibre
 optic probe to allow measurements in remote locations. Despite the
 relative sophistication of such an instrument, the measurement of particle
 size distribution remains difficult if the fluid has strongly absorbing
 characteristics like crude oil. This is because a considerable amount of
 laser light is absorbed and the intensity of back-scattered light can be
 very weak. In our version of the Brookhaven instrument, in order to go
 through a thick window in high pressure cells the optical probe was
 modified to give a focus point distance from the probe tip to the center
 of the scattering volume of about 4 mm. Two different cells were designed
 to carry out measurements of asphaltene particle sizes in oil. The first
 cell was designed in rectangular shape from black (carbon filled) PTFE
 with a 3.8 mm thick push fit quartz window. The other cell consists of a
 cylindrical housing made from thick plastic material with a window
 (quartz) held in one end of the cylindrical housing. With this technique
 the size distributions of asphaltene particles were monitored in the
 presence of the inhibitor 2-hexadecyl naphthalene.
 FIG. 2 shows a plot of average particle volume (expressed as a diameter)
 against time on the addition of 50% by volume of n-heptane to asphaltene
 solutions in toluene. The increase in size of asphaltene particles in the
 absence of 2-hexadecyl naphthalene was found to be 0.037 .mu.m/minute,
 whereas in the presence of 2-hexadecyl naphthalene the aggregation rate
 dropped to 0.0022 .mu.m/minute. This shows that 2-hexadecyl naphthalene
 slows the aggregation rate by 17 times even though the 2-hexadecyl
 naphthalene was not designed to be especially effective in toluene. The
 linearity of the volume plot (even though expressed as the equivalent
 diameter) indicates that the major process occurring has been aggregation
 (coagulation) rather than growth by deposition of molecules on to existing
 particles.
 FIG. 3 shows the effect of 2-hexadecyl naphthalene on the stabilisation of
 carbon particles dispersed separately in hexadecane and toluene. It
 clearly shows that 2-hexadecyl naphthalene is considerably more effective
 at stabilising carbon particles in the aliphatic hexadecane than in
 toluene. This is no doubt due to the greater extension of the hexadecyl
 chain in hexadecane than in toluene, hexadecane being a good solvent for
 2-hexadecyl naphthalene.