Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Maintenance of Oit Production and Refining Equipment
The present invention relates to the removal of deposits in oil production and
refining equipment and in
particular to the removal of deposits of asphaltene and/or petroleum waxes
from such equipment.
Crude petroleum is a mixture of many components which varies depending on the
source. Typically, it
will include relatively low molecular weight components, mostly hydrocarbons
including aliphatic and
aromatic compounds. Relatively low molecular weight products such as naphtha,
gasoline, diesel or
light fuel oil, benzene and toluene are obtained mainly from these components.
Usually crude
petroleum also includes materials having much higher molecular weights
including those known as
asphaltenes and petroleum waxes. Asphaltenes vary depending on the source of
the oil, but are
typically polycyclic, usually aromatic or partly aromatic, compounds,
including heterocyclic especially N
and S atoms, and typically with multiple aliphatic substituent chains.
Asphaltenes are probably not
truly soluble in most crude oils, but are present as fine particles or
platelets. Their dispersion in the oil
is aided by the presence of resinous materials usually called maltenes and the
relatively high
temperatures of most oil producing formations. Petroleum waxes are long chain,
typically C15 to
0100, usually mainly open chain aliphatic compounds. They are often described
as paraffins and may
be straight or branched chain materials. Usually they are soluble in crude
oil, particularly at the
temperatures of most oil producing formations.
In refining, the normal fate of these materials is to be cracked into lower
molecular weight compounds
that then form part of useful fractions in the products e.g. petroleum waxes
can be converted into
useful shorter chain alkanes or alkenes, or to end up as still bottoms e.g. as
part of bitumenic products.
During production and processing, and in particular at temperatures below
those of the oil producing
formation, asphaltenes and petroleum waxes may separate from the bulk of the
oil and may become
solidified as deposits on surfaces with which they are in contact. These
deposits can block the well or
other pipes through which the oil passes, or be deposits at the bottom of
separation vessels or storage
tanks during production and the early stages of refinery operations. It is
important to remove such
deposits to avoid blockage of pipes and reduction in the capacity of vessels.
Conventionally, aromatic solvent, such as xylene, sometimes in combination
with dispersants are used
to remove both asphaltenes and petroleum waxes from pipes and vessels. Such
aromatic materials
are good solvents, but environmental considerations are leading to pressure to
reduce the proportion
of such volatile aromatic compounds used in industrial applications.
The present invention is based on the discovery that certain, particularly
alkyl, esters of aromatic
carboxylic acids, particularly benzoic acid, are very effective solvents for
materials deposited in oil
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recovery and refining equipment, particularly asphaltenes and petroleum waxes
and are
environmentally much less objectionable than the xylenic solvents currently
used for such purposes.
The present invention accordingly provides a method of removing from or
preventing or inhibiting the
deposition in a well pipe or vessel of asphaltenes and/or petroleum waxes
which comprises contacting
the asphaltene and/or petroleum wax or the well pipe or vessel adjacent a
location where deposition of
asphaltene and/or petroleum wax is expected, with a solvent including at least
one compound of the
formula (I):
R1 - (AO)n - OOC - (CH2)m - Ph - (R2)p (1)
where
R1 is a C1 to C20 hydrocarbyl group, particularly a C3 to C1g alkyl or alkenyl
group;
AO is an alkyleneoxy group, particularly an ethyleneoxy or a propyleneoxy
group, and may vary
along the (poly)alkyleneoxy chain;
n 0 or from 1 to 100, desirably 0;
m is 0, 1 or 2, desirably 0; and
Ph is a phenyl group, which may be substituted with groups (R2)p; where
each R2 is independently a C1 to Cq, alkyl or alkoxy group; and p is 0, 1 or
2, desirably 0;
and subsequently removing the solvent with softened, dissolved or dispersed
asphaltenes and/or
petroleum waxes from the well, pipe or vessel.
In the compound of the formula (I) used in the invention R1 can be an alkyl or
alkenyl group. Alkyl
groups have the advantage that they are more stable, particularly to oxidation
than alkenyl groups, but
alkenyl esters generally remain fluid at lower temperatures than alkyl esters,
especially for longer chain
materials. Desirably, an alkenyl group includes only a single double bond as
multiple unsaturation
generally gives poor stability. In removing asphaltenes, R1 is particularly a
relatively short chain, such
as a C2 to C10 chain, for example a C3 to Cg, alkyl group. Desirably R1 is
branched e.g. it is an iso-
propyl (prop-2-yl), sec-butyl (but-2-yl), iso-butyl (2-methyl-prop1-yl), terf
butyl and/or 2-ethyl hexyl
group, to reduce the ease with which the ester can be hydrolysed. Esters with
secondary alcohols are
particularly useful in this regard and R1 is thus especially a C3 to C5
secondary alkyl group and very
desirably an iso-propyl group. Other relatively short chain alkyl esters that
can be used include ethyl,
nonyl, and other straight chain alkyl benzoates such as propyl, butyl, pentyl
and hexyl benzoates. A
benefit of relatively short chain esters is that they have low viscosity. In
removing petroleum waxes,
longer chain esters may be desirable to improve the low temperature solubility
of the waxes in the
solvent. Thus, R1 can be a Cg to C20, particularly a Cg to C1 g alkyl or
alkenyl group which may be
straight chain e.g. as in mixed esters such as (mixed C12/C13 alkyl) benzoate,
or branched e.g. as in
2-ethylhexyl or iso-nonyl or branched chain C~ g alkyl as in so-called iso-
stearyl (actually a mixture of
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mainly branched C14 to C22 alkyl with an average chain length close to C1 g).
Unsaturated longer
chain groups include oleyl. Where longer chain length groups are used,
particularly longer than C12, it
is desirable that they are or include branching and/or unsaturation and/or
that mixtures of such esters
are used, as these promote liquid esters where straight chain saturated ester
compounds may be solid
and thus more difficult to use.
Although the carboxylic acid used in the ester can be a dihydrocinnamic acid
or a phenylacetic acid, it
is very desirably ~a benzoic acid i.e. desirably m is 0. Similarly,. although
the phenyl ring of the acid may
be substituted, it is desirable that it is unsubstituted i.e. desirably p is
0. The esters used in the
invention may include a (poly)alkyleneoxy chain, (AO)n in formula (1), between
the carboxyl group and
the group R1. When present the (poly)alkyleneoxy chain is desirably a
(poly)ethyleneoxy, a
(poly)propyleneoxy chain or a chain including both ethyleneoxy and
propyleneoxy residues. Generally,
it is desirable not to include such a chain in the ester i.e. desirably n is
0.
Particularly for dissolving asphaltenes, an especially useful ester is iso-
propyl benzoate and the
invention specifically includes a method of the invention in which the solvent
is or includes iso-propyl
benzoate. Iso-propyl benzoate has a combination of properties that make it
exceptionally useful in the
solvent role. As a pure material, it has a wide liquid range having a high
boiling point (BP ca 219°C)
and remaining fluid at temperatures below normally expected environmental
temperatures (pour point
< -60°C); it has a flash point (ca 99°C) so that it is
classified as non-flammable and under normal use
conditions it has a low vapour pressure; it lias a density similar to that of
water (1.008 kg.l-1 at 25°C);
and a low viscosity (2.32 cSt at 25°C; measured by the U tube method,
equivalent to 2.34 mPa.s).
By comparison other alkyl benzoates have viscosities (at 25°C) as
follows:
ethyl benzoate: 1.9 cSt; 2-ethyl hexyl benzoate: 6.1 cSt; nonyl benzoate:
7.5cSt; (mixed
C12/C13 alkyl) benzoate: l4cSt; and iso-stearyl benzoate: 30cSt.
To provide a balance of solubility for both asphaltene and petroleum wax,
mixed esters; having a
variety of groups R1, or blends of compounds of the formula (I), may be
advantageous by providing a
combination of solvency properties matching combinations of asphaltene and
waxes. Such mixed
esters of blends can have the additional benefit that they are more liquid
than pure, especially linear
saturated compounds of similar overall R1 carbon number.
The solvent used in the method of the invention can be wholly of one or more
compounds of the
formula (I), or it may contain other solvents in admixture. Although xylenes
can be included it is
unlikely that xylenes or other solvent including a substantial proportion of
aromatic hydrocarbons will
be used as a major component of any such mixed carrier fluid, because of its
adverse environmental
impact. Mixtures with paraffinic liquid solvents may improve the solubility of
petroleum waxes, but are
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likely to reduce the solubility of asphaltenes. Other solvents such as water
soluble alcohols may be
included either as carriers for additives (see below) or to improve
compatibility with aqueous materials
in use. In such mixed solvents, the proportions used will thus depend on the
particular nature of
material deposited in the pipe or vessel, and probably on the balance and
detailed composition of
asphaltenes and waxes. When mixtures are used, compounds of the formula (I)
will typically be
present in at least 25%, usually at least 40%, more usually at least 50%,
desirably at least 60%, and
particularly at least 75%, by weight of the total carrier fluid used. When
present, other solvent
components will desirably be used at level typically of from 1 to 75, usually
1 to 40%, more desirably 2
to 25, and particularly 5 to 15% by weight of the total carrier fluid used.
To aid in dispersing the asphaltene and/or petroleum wax, the solvent may
include dispersants,
particularly non-ionic surfactants and dispersants such as alcohol
alkoxylates; reaction products
between poly-iso-butylene succinic anhydrides (PIBSA's) and alcohol
alkoxylates, particularly C10 to
C1g e.g. C13 to C15 alcohol ethoxylates; reaction products between PIBSA's and
alkanolamines such
as di- and tri-ethanolamine; and sorbitan fatty acid esters, especially mono
esters and particularly
esters of unsaturated fatty acids e.g. sorbitan mono-oleate; sulphonic acid
dispersants such as alkyl
aryl sulphonic acids; or resinous dispersants such as phenol formaldehyde
resin dispersants and
ethylene vinyl acetate co-polymers. When used dispersants will typically be
included as from 1 to
40%, more usually 1 to 30% and desirably from 1 to 20%, by weight of the
solvent formulation.
Further additives such as fluid loss agents particularly such as synthetic
polymers such as
polyacrylamides, polyacrylates, polyamides and similar polymers (some of which
can also function as
viscosity improving agents); corrosion inhibitors; demulsifiers; scale
inhibitors; oxygen scavengers; and
other similar additive materials, can be included in the solvent formulation
used in the invention.
Particularly when such other additives are used, one or more co-solvents may
be used e.g. a water
soluble alcohol, such as propanol, particularly for use in systems operating
in the presence of water,
and/or a dispersant for the additive may be included.
Generally, in production wells, the temperatures of the oil bearing formation
and the crude oil are often
superambient typically within the range 50 to 150°C, particularly 60 to
120°C. Asphaltene and/or
waxes tend to be deposited at temperatures below those of the formation, but
generally within or
somewhat below the ranges given above, particularly in the range 40 to
110°C. The compounds of the
formula (I), particularly iso-propyl benzoate, are better solvents for such
materials at such moderately
elevated temperatures. Thus it may be advantageous to operate at superambient
temperature e.g. by
heating the solvent, either deliberately or e.g. in an oil well by contact
with rock formations at elevated
temperatures, to improve solvent performance. The temperature of pipes, tanks
and refinery
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equipment will generally be determined by the desired operating temperatures
(often linked to the feed
temperature).
The equipment treated can be pipes such as pipes in oil well structures,
including the interior bore of
well casings, pipelines, including well head pipes, undersea pipes and
pipework in refineries; or
vessels such as oil separators (to separate gas, oil water and resinous
phases); storage tanks,
particularly near the well head and at or near refineries; and refinery
equipment. The removal of
deposits of asphaltenes and/or waxes can be by way of introducing a body of
solvent into contact with
the deposit, if necessary providing circulation or agitation of the solvent,
and removal of the softened,
dissolved or dispersed deposit from the equipment. In storage tanks and other
refinery process
locations, the use of solvents to remove tank bottoms has the advantage of not
needing to open the
tank for mechanical cleaning. The tank contents, tank bottoms and/or solvent
can be heated to aid
solubilisation and the tarry solid tank bottoms dispersed into the solvent and
usually added to a crude
oil stream for further refinery processing.
The invention includes a method of treating an oil storage tank, vessel, or
oil refinery pipework to
remove deposits of asphaltenes and/or petroleum waxes, in which a treatment
material including a
compound of the formula (I) as defined above is introduced into the tank,
vessel or pipework at or
adjacent the location of a deposit of asphaltene andlor petroleum wax, and
subsequently softened
dispersed or dissolved asphaltene and/or petroleum wax is removed from the
tank, vessel or pipework.
Particularly in oil wells, conventional techniques include preventative
application of solvents optionally
including dispersants with the aim of preventing flocculation and deposition
of asphaltenes and/or
waxes. These methods generally involve continuous treatment by pumping the
treatment material
down the well e.g. using capillary tubing, or by a slip stream. This lays down
a thin layer of treatment
material in the area where deposition is considered likely and can effectively
prevent flocculation and
deposition in the tubing and flow lines. They are less effective in preventing
deposition in the near
wellbore area e.g. within the production formation itself. In such cases the
treatment material needs to
be placed in the formation where it can inhibit solid deposition e.g. by
squeezing the treatment material
into the formation.
The invention accordingly includes a method of treating an oil well to remove
a deposit of asphaltenes
and/or petroleum waxes, in which a solvent including a compound of the formula
(I) as defined above
is introduced into the oil well in or adjacent to a deposit on a surface or in
a rock formation.
The invention further includes a method of treating an oil well to inhibit or
prevent the deposition of
asphaltenes and/or petroleum waxes, in which a stream of a treatment material
including a compound
of the formula (I) as defined above is introduced into the oil well in a
location, particularly onto a
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surface or into a rock formation, where deposition is expected thereby
carrying away asphaltenes
and/or petroleum waxes before they are deposited on the surface or in the
formation.
The following Examples illustrate the invention. All parts and percentages are
by weight unless
otherwise specified.
Example 1
Scampton C4 crude oil was added to hexane to precipitate asphaltene (the
hexane dissolves
asphaltene stabilising resins), the upper solvent layer was removed and
residual hexane allowed to
evaporate to give asphaltene as a viscous liquid residue. 5.2 g iso-propyl
benzoate solvent was added
to 0.048 g of asphaltene and after a few minutes at ambient temperature
virtually all the asphaltene
had dissolved (a few very small particles of asphaltene remained visible in
the solution). The
estimated (minimum) solubility was calculated as ca 0.85% by weight.
Example 1 a
In a separate experiment, asphaltene was made as described in Example 1 and
its solubility was
assessed in the various solvents as decribed in Example 1. The solvents were
ethyl benzoate, iso-
propyl benzoate, 2-ethyl hexyl benzoate, nonyl benzoate, 3:1 by weight mixture
of iso-propyl benzoate
and nonyl benzoate, (mixed C12/C13 alkyl) benzoate and iso-stearyl benzoate.
In each case the
majority of the asphaltene dissolved but a few very small particles remained
visible in the solution.
Example 2
About 0.025 g asphaltene (obtained as described in Example 1 ) was smeared
onto a weighed 5 cm x
1 cm rectangular mild steel coupon and placed inside a weighed glass jar which
was then reweighed to
give the amount of asphaltene by difference. About 2 ml solvent (weighed
accurately) was added, the
jar sealed and the sealed jar placed on moving rollers, so that the metal
coupon was constantly
covered with solvent at ambient temperature, for about half an hour. The
asphaltene was removed
completely from the metal coupon, indicating an effective minimum solubility
of asphaltene in the
solvent of 1.3%. Similar results were obtained using xylene as the solvent.
Example 2a
In a separate experiment, Example 2 was repeated using the solvents listed in
Example 1a. In each
case, the solvent removed all of the asphaltene from the metal coupon.
Example 3
Example 2 was repeated using about 0.18 g asphaltene and about 0.5 ml iso-
propyl benzoate. Again
the solvent removed all the asphaltene from the metal coupon.
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Example 3a
In a separate experiment, Example 3 was repeated using different quantities:
0.4 g of asphaitene and
1 ml solvent, and the solvents listed in Example 1 a. In each case, at the end
of the rolling most of the
asphaltene was removed, but small traces were left adhering to the metal
surface. The residues could
not be measured quantitatively, but visual inspection of the metal coupons
suggested that the ranking
of the solvents in this test was (from best to worst):
ethyl benzoate > nonyl benzoate > 2-ethyl hexyl benzoate > 3:1 by weight
mixture of iso-
propyl benzoate and nonyl benzoate > iso-propyl benzoate > (mixed C12/C13
alkyl)
benzoate > iso-stearyl benzoate.
Example 4
The solubility of soft white paraffin wax [mp 49 to 59°C] in iso-propyl
benzoate was measured. At
ambient temperature, the wax solubility was low (less than 1 %), but on
warming to 50 to 60°C more
than 60% by weight (on solvent) of the wax could be dissolved.
Example 4a
In a separate experiment, Example 4 was repeated to assess the solubility of
soft white paraffin wax in
the solvents listed in Example 1 a. In each case the solubility appeared to be
low at ambient
temperature but at 60°C more than 60% by weight of wax cound be
dissolved in each of the solvents.
Example 5
Weighed metal coupons were coated with the wax described in Example 4 by
smearing the soft
paraffin onto the coupon surface and tested for wax removal by iso-propyl
benzoate using the method
described in Example 2. The amount of wax was about 10% by weight of the
solvent. At ambient
temperature, a small amount of wax was removed giving a cloudy solution, but
at 59°C all the wax was
readily removed from the coupon surface.
Example 5a
In a separate experiment, Example 5 was repeated using soft white paraffin wax
and the solvents
listed in Example 1 a. Each of the solvents removed all the wax at ambeint
temperature giving cloudy
solutions. Visual inspection of the extent of cloudiness suggested that the
ranking of the solvents in
this test was (from best to worst):
iso-stearyl benzoate - iso-stearyl benzoate > nonyl benzoate -- 2-ethyl hexyl
benzoate >
ethyl benzoate - iso-propyl benzoate -- 3:1 by weight mixture of iso-propyl
benzoate and
nonyl benzoate.
