Note: Descriptions are shown in the official language in which they were submitted.
CA 02261415 1999-02-09
657P01CA
-1-
METHOD AND APPARATUS FOR EXTRACTING OIL
The present invention relates to a method and apparatus for
extracting oil, and more particularly to apparatus
comprising a downhole pump, and a method of operating the
same.
One source of oil is from wells which have a depth of less
than 3000m, and which are not artesian, i.e. in which the
oil will not reach the ground surface unaided. Oil from
such sources may be extracted by raising it from a static
level (i.e. the level to which oil will rise in the casing)
to the surface by means of a pump sent down a borehole at,
or close to, the depth of the oil-bearing stratum.
At present, mechanically driven pumps, such as rotodynamic
(axial flow turbine) and positive displacement (travelling
cavity) are used to raise the oil. Of these, the former
type is quickly destroyed by grit present in the oil and
latter is similarly damaged, although perhaps less quickly.
Hydraulically driven pumps, such as double-acting piston or
jet pumps, are also used. Of these, the former type tend to
become blocked, are quickly worn out by grit, and are
suited only to low production rates.
In a jet pump, pressure energy in a motive fluid is
converted to kinetic energy by passing it through a nozzle.
The resulting jet of motive fluid leaves the nozzle at high
velocity, and crosses an entrainment zone, where an induced
flow (including the matter to be extracted) is drawn into
the jet. The two flows are mixed in a mixing chamber, and
the pressure of the resultant fluid is increased in a
diffuser, in order to drive the pipeline, and carry the
CA 02261415 1999-02-09
-2-
fluid to the surface.
In current arrangements for downhole pumps, a motive fluid
is arranged to form a "central jet", so defined because of
the axial solid-prismatic form of the jet. The pump
configuration is such that the induced fluid enters
laterally or circumferentially and is entrained by the
outer surface of the motive jet.
As a jet pump includes no moving parts, it might appear to
be particularly suitable for transporting abrasive matter,
such as oil from a borehole.
However, conventional jet pump designs incorporate small
passage diameters through which the induced fluid passes,
so that it may be introduced to the motive jet laterally.
As the extracted fluid is a mixture of, inter alia, oil and
grit, these passages have a tendency to become blocked. In
these circumstances, functioning of the pump ceases, and
the pump must be brought back to the surface for
maintenance.
Furthermore, because of the small passage diameters,
existing designs of jet pump are unsuitable for the heavier
grades of oil which have higher viscosities.
Also, efficient functioning of existing jet pumps for oil
extraction rely upon high fluid velocities, which results
in a high rate of wear, particularly in the mixing chamber,
due to the abrasive nature of the mixture of grit and fluid
being extracted.
It has been established that the viability of many holes
and fields is marginal, but that the viability would be
disproportionately improved if a reduction in operating
CA 02261415 1999-02-09
-3-
costs could be achieved, and/or if downtime could be
reduced by increasing the reliability of the operation of
the extraction pump. This would result in the viability of
oil production of a particular field being less sensitive
to variations in the well-head price of oil.
It is therefore an object of the present invention to
provide a jet pump arrangement better suited to downhole
oil production; and/or to provide improvements in relation
to one or more matters discussed above; and/or to provide
improvements generally.
According to an aspect of the present invention there is
provided a downhole pump for the extraction of
oil/petroleum comprising a motive fluid conduit for the
conveyance of a motive flow, and a transport conduit
connected to the motive fluid conduit for the conveyance of
the induced flow and motive flow to the surface; the
transport pipe comprising a suction inlet for intake of the
induced fluid, and housing a jet pump comprising a nozzle,
a mixing chamber and a diffuser; the transport pipe and/or
motive fluid pipe being configured to produce an annular or
cylindrical motive jet through the nozzle, whereby an axial
induced flow is produced through the suction inlet. The
downhole pump therefore comprises an annular jet pump.
By providing an annular or hollow jet of motive fluid, and
ensuring that the induced flow passes through the centre of
the jet, the induced flow passes through larger passage
clearances than is the case in prior art jet pumps.
Therefore, the pump arrangement may be configured to
eliminate the thin annuli, which are required in prior art
pumps to produce an annular flow of the induced fluid. By
eliminating the thin annuli, the likelihood of internal
blockage by induced grit, which is common in central-type
CA 02261415 1999-02-09
-4-
jet pumps at present in use, is greatly reduced.
The configuration of the present invention also allows
larger passage clearances for the oil in the induced flow,
than is the case with existing designs of central jet pump.
The apparatus is therefore intrinsically more able to
handle the heavier oils, i.e. those of greater viscosity,
which are unable to flow freely through very narrow
passages.
Preferably, the transport fluid conduit comprises a tube
located inside the motive fluid conduit, which also
comprises a tube, thereby defining an annulus which forms
a path for the motive fluid.
It is preferred that the motive fluid pipe and the
transport fluid pipe be concentric. In this way, the
configuration of the pump apparatus takes maximum advantage
of a given borehole (casing) diameter, than in some prior
art systems in which the motive flow conduit and the
transport conduit are located adjacent one another.
Perforations are provided to allow oil to enter the casing.
This embodiment also has the advantage that the pump
apparatus may readily be installed down a cased borehole
for the production of oil, the casing being the cylindrical
metal conduit comprising the borehole. In an embodiment,
the outer tube forming the motive fluid conduit comprises
the casing. In an alternative embodiment, a separate casing
is provided.
In an embodiment, the motive fluid, or power fluid,
comprises either a light oil or water. In an embodiment the
motive fluid comprises produced water, i.e. the water
recovered with the oil product from the borehole system in
CA 02261415 1999-02-09
-5-
the induced flow. The induced mixture will generally
comprise oil, produced water and grit ("grit" refers to all
solid matter, from clays to gravel).
The produced water is a convenient source of fluid which
may be readily pressurised to form the motive fluid, and
which has a low viscosity, so that it may readily pass
through the nozzle of the jet pump to form the annular jet.
In an embodiment, the jet pump is of the free type, i.e. it
is not attached to the casing of the borehole and is
dropped into position under its own weight. Such an
embodiment has the advantage that the pump may be recovered
e.g. for maintenance purposes, without the need to remove
tubing or casing.
It is preferred that an extraction sleeve may be fitted to
the pump body, for example to the diffuser, which is
adapted to co-operate with a retrieval tool. This enables
the apparatus to be withdrawn readily for inspection and
maintenance, minimising operational downtime and increasing
the financial viability of a site.
In an embodiment, the spacing between the nozzle and the
mixing chamber is adjustable, as is the annular nozzle gap.
This enables the pump to be adjusted to suit the required
conditions of lift and flow.
In an embodiment there is provided anti-arching means to
prevent external blockage of the suction inlet by solid
material at the point of entering the pump. Arching refers
to the arch formed by inter-particle contact in a non-
linear flow of liquid containing solid particles. Either
differential acceleration of the phases causes the
volumetric concentration to rise until contact occurs or
CA 02261415 2006-11-16
- 6 -
differential velocity of fine and coarse particle sizes
causes a concentration of the fine within the coarse until
the space occupied becomes effectively solid.
It is preferred that the anti-arching means comprises anti-
arching jets.. High velocity jets blast are used to disrupt
establishment of a incipient arches. In an embodiment,
anti-arching jets are supplied from the motive fluid supply.
It has been found that anti-archi.ng jets may be supplied
from the main motive fluid., without significantly effecting
the performance of the main pump. In this way, the need to
provide a separate fluid supply in the borehole to power the
anti-aching jets is avoided..
The. anti-arching means may comprise additionally or
alte.rnatively a re'-entrant or Borda design of suction inlet.,
which also serves to deny the arch-a springing, to reduce or
prevent arching.
In accordance with a first aspect of the present invention-,
there is provided a downhole pump for use in association
with a borehole casing for the extraction of oil from a
borehole., the downhole pump comprising a motive fluid
conduit constructed and arranged to be located inside and
spaced apart from an inner perimetric surface of the
borehole casing, and a transport conduit located inside and
connected to the motive fluid conduit, and comprising a
suction inlet, and housing a jet pump comprising a nozzle, a
mixing chamber and a diffuser, wherein
CA 02261415 2006-11-16
-6a-
(i) an inner surface of the motive fluid conduit and an
outer surface of the transport conduit together define a
motive flow input path;
(ii) at least one of the motive fluid conduit and the
transport conduit are configured to produce a motive jet of
the motive flow through the nozzle;
(iii) the suction inlet is constructed and arranged to
convey an induced flow of oil therethrough in response to
the motive flow; and
(iv) the transport conduit is constructed and arranged to
convey the motive flow and the induced flow to a discharge
outlet.
1.5
An embodiment of the invention will now be described by way
of example only with reference to the following illustrative
drawing in which:
20. Figure 1 is a section through a down hole jet pump
arrangement according to the present invention..
As can be seen in Figure 1, downhole pump apparatus 10 for
the extraction of oil/petroleum according to the present
25 invention is provided. ."Downhole" refers to the location of
the extraction pump.. It need not be at or near the bottom
of the hole since there may be more than one oil-bearing
stratum, and the borehole may also penetrate aquifers. A
borehole is formed by a casing 20. Perforations allow oil
30 to enter the casing 20. Packers (not shown) are used to
CA 02261415 2006-11-16
-7-
isolate sections of the casing. They fill and seal the
annulus between casing and tubing inside the casing which
forms the conduits to convey materials up or down the
borehole.
The downhole pump apparatus 10 comprising a motive fluid
conduit in the form of a cylindrical outer tube 22 for the
conveyance of a motive flow A, and a concentric transport
conduit in the form of cylindrical inner tube 24, of
smaller diameter than the outer tube 22, for the conveyance
of the induced flow and motive flow B to the surface.
The inner tube 24 comprising a suction inlet 26 for the
intake of the induced fluid from the casing 20. The inlet
26 is in the form of a re-entrant or Borda inlet, that is
an inlet in which the inlet conduit projects into the space
containing the fluid to be induced. Such an inlet has been
found to minimised arching around the inlet tube 26, as
discussed above.
Both inner tube 24 and outer tube 22 are connected through foot 28
which may also incorporate arch-breaker nozzles 30. The
inner tube 24 houses a jet pump 32 comprising a nozzle 34,
a mixing chamber 36 and a diffuser 38. The nozzle 34 is an
extension of the inlet 26, and serves to convert the
pressure energy in the motive fluid A into kinetic energy,
such that the fluid leaves the nozzle in a high energy
stream. The mixing chamber 36 refers to the conduit in
which energy is shared between the motive and induced
flows. The annular gap between nozzle 34 and the mixing
chamber 36 forms the motive jet. The diffuser 38 is the
divergent taper employed in hydraulic devices to recover
pressure energy from velocity energy.
The spacing of the nozzle 34 and mixing chamber 36 is
CA 02261415 1999-02-09
-8-
maintained by split lantern 40 integrated by garter springs
42. "Lantern" is a metaphoric term applied to cylindrical
components having lateral holes for access or flow. Split
shims 44 allow adjustment of the annular nozzle gap to suit
the required conditions of lift and flow.
The inner tube 24 and outer tube 22 are configured to
produce an annular or cylindrical motive jet through the
nozzle 34, whereby an axial, or central, induced flow is
produced through the suction inlet 26 which is entrained by
the inner surface of the motive jet. The jet pump 32 is
therefore termed an annular jet pump. The pressure in the
motive fluid A is maintained between inlet seal 46 and
discharge seal 48.
In use, the pump apparatus 10 is inserted into the borehole
by gravity and retrieved by a retrieval device inserted
down the inner tubing 24. An extraction sleeve 50 may be
fitted to the diffuser 38 to assist in attachment of the
retrieval tool.
Motive (power) fluid A is passed down to the pumping
apparatus 10 under pressure. The motive fluid comprises
produced water, i.e. the water recovered with the oil
product from the borehole system. The fluid A flows inside
the outer tubing 22 but outside the inner tubing 24. Oil
within the casing 20 is induced by the pump 32 to form
induced flow B. This flow B will generally contain also
produced water and grit.
Flow A and B together form the discharged flow C which is
conveyed to the surface inside inner tubing 24.
Some of Flow A will be emitted from the arch-breaker
nozzles 30 formed in foot 28. If induced flow is gritty,
CA 02261415 1999-02-09
-9-
then arching around the suction inlet 26 can be a problem.
An arch may be prevented from forming by using the motive
flow A to produce additional jets.
It has been found that a downhole pumping apparatus
according to the present invention, unlike all central-type
jet pumps, has characteristics that accurately match the
pumping requirements of downhole pumping.
In addition, as the pump can operate at a lower motive
pressure, when compared with conventional central jet pump
apparatus, to provide an equal discharge pressure. The
lower operating pressure leads to reduced wear rates and
greater in-situ life than the central-type jet pumps or
travelling cavity pumps at present in use.
Apparatus according to the present invention has also been
found to induce product at a higher head ratio than
central-type pumps and also at a higher flow ratio at high
head ratios, the flow ratio being defined as the ratio of
the flow rate of the induced mixture to the flow rate of
the motive fluid and the head ratio as the ratio of the
head developed by the motive fluid.
