Note: Descriptions are shown in the official language in which they were submitted.
9~7
This invention relates to inhibiting the formation
of immobile wax deposits on the walls of vessels or
conduits carrying waxy oil, particularly petroleum crude
oils~ containing waxes.
When an oil containing wax is cooled and/or the
pressure is reduced, some of the dissolved wax may become
supersaturated. This supersaturation will lead to the
precipitation of paraffin solids either as immobile depo-
sits, or as waxy particles or crystals suspended or carried
in the fluid. The formation of immobile deposits is highly
undesirable since this can restrict production and necessi-
tate expensive remedial treatments to remove the blockages.
For example, as crude oil is produced from an oil well, it
flows from the underground reservoir to the surface and
into surface facilities and pipelines. During this time,
the temperature of the oil decreases from the reservoir
temperature, which may be up to 120C, down to ambient or
close to ambient temperature. This decrease in temperature
is primarily responsible for a decrease in the solubility
of the wax in the oil, resulting in precipitation of dis-
solved wax both as immobile deposits on the walls of the
conduits or vessels containing the oil, and as small waxy
particles carried along suspended in the oil.
It has long been known that if the wax can be main-
tained in the oil in the form of suspension of crystals or
pàrticles, there is then no tendency for the wax to be
incorporated into immobile wax deposits. Thus, the for-
mation of suspended particles of wax in the oil fluid is
highly desirable sinc~ these particles will be carried
along with oil fluid and not cause any flow restrictions
or plugging.
It has been observed in laboratory experiments that
the proportion of wax which is precipitated as immobile
deposits and the proportion which is precipitated as
1~39~97
suspended wax particles can be caused to vary depending on
a number of experimental parameters, e.g. the rate and
degree of cooling. Comparisons have been made between
compositions of wax deposits obtained from a crude oil at
different temperatures. In deposits obtained from fresh
samples of crude oil, the amount of the paraffin C50H102
contained in the immobilized wax deposit obtained at 35C
was 7 times greater than in the deposit obtained at ~0C.
In other words, the proportion of C50H102 which preci-
pitated as an immobile deposit changed by a factor of 7
between the two experiments. It was observed that the
supersaturation of C50H102 was precipitated as wax par-
ticles which were held in suspension in the fluid and were
not incorporated into the immobile deposit.
- In most oil production systems, it is impractical or
uneconomic to control the rate or degree of cooling of the
crude oil as above. However, it is possible to encourage
the formation of waxy particles by providing seed nuclei
or particles. Thus, in Coberly, U.S. Patent No. 2,303,823
there is described a method of retarding wax deposition in
which there is added to a hot waxy oil small particles of
a substance not normally present in the oil, these parti-
cles being of a nature to promote crystallization of the
wax in the oil about the particles. In this way, the wax
in the oil was inhibited from depositing on the walls of
the conduit. The material` which was used was small parti-
cles of resin or other foreign matter.
It is also a common commercial procedure to use
paraffin inhibitors, a wide variety of which are available.
However, these inhibitors are prohibitively expensive.
It is the object of the present invention to provide
a simple and inexpensive technique for inhibiting wax depo-
sition utilizing readily available materials.
Thus, the present invention in its broadest aspect
relates to a process for inhibiting the formation of wax
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deposits on the walls of a conduit or vessel in which a waxy
oil is carried. It comprises adding to the waxy oil a small
amount of an oil containing a large number of small wax
crystals suspended therein. The suspended wax particles act
as nuclei or centers of precipitation, so any supersatura-
tion of wax is preferentially precipitated about these small
wax crystals, which remain in suspension in the oil, thereby
decreasing the tendency of wax in the oil to form immobile
deposits on the walls of the conduit or vessel.
The invention can be applied to almost any kind of
petroleum oil, including crude oils, condensates and heavy
oils containing wax. It also applies to a wide variety of
oil conduits or vessels, including flowlines, pipelines,
surface facilities, etc.
In experiments it has been found that as a waxy oil
is cooled, large numbers of waxy particles or crystals
develop at temperatures between about 30-50C ~freezing
point). The exact temperatures vary depending upon both the
nature of the oil and the cooling rate. In clear crude
oils, this temperature is sometimes referred to as the cloud
point temperature of the oil since the formation of sus-
pended wax particles will give the crude a cloudy or murky
appearance. If the same oil containing the wax in the form
of suspended par~icles is again heated, the waxy particles
or crystals may not melt until ~he temperature is as much as
30C or higher than the cloud point. The difference between
these temperatures is believed to be due to kinetic limita-
tions on the rate of formation and dissolution of the waxy
particles. This difference between the melting point and
freezing point may be described as a hysteresis type effect.
The difference between the cloud point and the tempe-
rature required to dissolve the wax crystals allows the
introduction of a small stream of crude oil below its cloud
point temperature (whereby it contains a large number of
small wax crystals) into crude oil conduits or vessels
carrying waxy crudes in the order of 30C above their
cloud point, since the wax particles or seed nuclei will
survive some degree of reheating, and also provide
substantial inhibition of the formation of immobile wax
deposits on the walls of a conduit or vessel. The small
stream of crude oil may be withdrawn from the main body of
crude oil being carried in the conduit or vessel, cooled
to form small waxy crystals in suspension and then fed
back into the main body of crude oil. For instance, a
small sidestream of crude oil may be withdrawn from a
pipeline, cooled to form waxy crystals and then fed back
into the pipeline at a point further downstream.
Alternatively, a smail stream of oil from some other
source, e.g. a condensate, may be utilized and this may be
cooled to form a large number of wax particles and then
added to the hot waxy oil in the conduit or vessel as an
inhibitor.
ExamE~_
Experiments were conducted using a crude oil from
Alberta. A portion of this crude oil was cooled to form
therein a lar~e number of small wax crystals. About S00
ppm of the above cooled crude oil portion was added to the
same crude oil having a temperature of 50C. This provided
a 60% reduction in the amount of immobile wax deposit
formed at 20C. This reduction is comparable to the reduc-
tion obtained from the best of 13 commercial paraffin
inhibitors tested.
Obviously, many modifications and variations of the
invention, as hereinbefore set forth, may be made without
departing from the spirit and scope thereof, and therefore
only such limitations should be imposed as are indicated
in the appended claims.
