[0001] This invention relates to a method for recovering oil from a submarine oil reservoir
and to a system for the injection of seawater into such a reservoir.
[0002] Present day techniques for the waterflooding of subsea petroleum reservoirs involve
lifting seawater to the surface production facility and then pumping that seawater
into the reservoir via a suitable injection system to maintain reservoir pressure
and displace oil. The seawater may be de-aerated before being pumped into the injection
system to minimize corrosion of wellhead and other equipment.
[0003] Quite clearly, the provision of such topside pumping facilities constitutes a very
considerable capital investment when the injectioniproduction well is situated at
other than a very short distance from the surface production facility. Not only must
the required flowlines for produced oil and gas be laid between the wellhead and the
surface facility, but also suitable flowlines must be provided for passing the pumped
seawater from the surface facility to the injection site. In addition, the vast quantity
of seawater that is carried by the surface facility at any one time, together with
the pumping and treating equipment for that seawater, add considerably to the weight
that must be borne by the surface facility.
[0004] The present invention now seeks to eliminate the necessity for remote surface pumping
facilities _ and interconnecting flowlines in subsea waterflooding, by providing a
method and system in which a pump located adjacent that subsea wellhead provides the
injection water from the seafloor environment.
[0005] According to the present invention, there is provided a method for recovering oil
from a submarine petroleum-containing formation penetrated by an injection well and
a production well in which seawater is injected under pressure into the formation
through the injection well and oil is recovered from the formation through the production
well, characterised in that the seawater is injected through a pump located adjacent
the seafloor injection wellhead.
[0006] The provision of the seawater injection pump on the seafloor adjacent the injection
wellhead, in contrast to its conventional situation on the production platform at
a considerable distance from the injection well, results in a number of important
advantages. Firstly, the cost of providing and maintaining a long run of piping connecting
the topside pump and the wellhead is elminiated. Secondly, the volume and weight of
the pump and its associated equipment and the weight of the water that is in it at
any one time is removed from the platform. Thirdly, de-aeration and fine filtering
of the water drawn from near the seafloor may not be essential. Other advantages resulting
from the method and system of the invention are discussed in detail below.
[0007] The invention resides in the use of a seawater injection pump that is located on
the seafloor adjacent the injection wellhead. That pump may be electrically or hydraulically
driven but is preferably powered by an electric motor which is itself preferably directly
coupled to the pump. Quite clearly the pump -and, when the pump is driven by an electric
motor, the motor also -must be capable of operating in the submarine environment and
are therefore constructed of materials that are highly corrosion resistant and capable
of operating for long periods of time without regular maintenance and repair.
[0008] The pump is most suitably a multistage centrifugal pump that is capable of delivering
up to 2500 m
3 per day (or about 100 m
3 per hour) of seawater at a wellhead pressure of up to 35,000 kPa, although routine
operations may require from 1500 to 2400 m
3 per day (65 to 100 m" per hour) at a wellhead pressure of 20.000 to 35.000 kPa.
[0009] The motor that is preferably used to drive the pump is suitably a high voltage electric
motor, for example a 750 kW or 1500 kW motor operating on a 3.3 kV or 6.6 kV, 50 Hz
or 60 Hz altemating current supply. The motor is preferably of the water flooded type
to eliminate potential sealing problems -especially at the motor-pump junction -and
the thrust and joumal bearings for the motor are therefore of materials that are suitable
for seawater lubrication and cooling, such as elastomeric compositions or ceramics
such as silicon carbide. The bearings in the pump are suitably of similar construction.
[0010] The electric power for the motor will, of course, be supplied by the surface facility
or platform via a suitable high voltage submarine power cable. That cable can be combined
with a control umbilical incorporating control lines for the water injection tree
and pump system, monitoring lines for pressure and flow readings and, if desired or
necessary, small bore tubing for chemical injection.
[0011] The pump and motor are suitably directly coupled and mounted on an appropriate base
and connected to power, control and monitoring lines and to the wellhead injection
system by remotely- operated hydraulic couplings to facilitate removal of the pump/motor
unit to the surface for maintenance and/or repair.
[0012] It is common practice to de-aearate seawater used for injection on the conventional
surface facility but, quite clearly, such a procedure is not possible in the method
of the present invention. It is therefore essential that the pump, pipework and well
tubulars are constructed of suitably selected materials based on raw seawater duty,
as outlined in NACE Recommendation MR-0175, for example. It is not expected that any
problem associated with seawater oxygen content will be encountered in the reservoir
since the reducing nature of the reservoir environment will rapidly consume dissolved
oxygen.
[0013] It is, of course, necessary to contain the pump/motor system within a secure enclosure
to prevent accidental damage from water-borne debris and to prevent such debris and
marine flora and fauna being drawn into the pump along with seawater. Normally, that
enclosure will include a coarse-mesh sieve to prevent such ingress although it may
be necessary to provide fine mesh filters where the amount of suspended matter exceeds
a certain value.
[0014] In the North Sea, for example, the total suspended matter (TSM) content of seawater
at about 10 m above the seafloor is of the order of 0.4 mg/I. This is somewhat higher
than that encountered at greater heights above the seafloor (for example. 0.04 mg/I
at 60 m above the seafloor at a location where water is drawn up to a platform for
injection pumping), but is still well below the "polished injection water" standard
of 2 mg/I TSM. As a result, fine filtration is not believed necessary and formation
plugging is not expected to be a problem. However, in locations where the seawater
is likely to have a high TSM content, especially shallower waters such as those in
the Gulf of Mexico, fine filtration to remove particles larger than 200 um, in diameter,
may be necessary to prevent formation plugging problems and to prevent damage to pump
and motor bearings.
[0015] Raw seawater contains also sulfate reducing bacteria (SRB). If unchecked, these organisms
give rise to increasing levels of H
zS in the reservoir. SRB require a reducing environment to multiply - hence injection
of non-deaerated seawater will inhibit their growth in the vicinity of the wellbore.
Further into the reservoir, however, the removal of free oxygen as described above
will lead to reducing conditions which suit the SRB. Some form of biocide injection
will therefore be necessary in order to inhibit SRB growth. This may be achieved by
providing small bore tubing, for example of stainless steel, within the system control
umbilical, to carry biocide to the wellhead. If desired, scale inhibitor can be combined
with the biocide.
[0016] It is, of course, essential to monitor the volume of seawater injected into the well
and this can be achieved by any one of a number of known flow measurement systems.
A preferred system utilizes a standard vortex meter mounted in the suction side of
the pump. Alternative systems include a magnetic flow meter in the discharge line
of the pump, an orifice plate or flow nozzle in the discharge line of the pump and
a differential pressure transmitter, and a turbine meter. The vortex meter, magnetic
flow meter and turbine meter all generate a frequency signal proportional to the flow,
and a generated signal of 4 to 20 mA DC can be transmitted up to about 12 km. Other
data transmission systems such as subsea telemetry and fiber optic data transmission
can also be used in this respect.
[0017] The invention will now be described in greater detail by way of example only with
reference to the accompanying drawings, in which
Figure 1 is a diagramatic representation in side elevation of a pump/motor unit in
a protective enclosure; and
Figure 2 is a diagramatic representation in side elevation of a system similar to
that shown in Figure 1 but including fine filters to remove all but the finest particles.
[0018] Referring first to Figure 1 of the drawings, a containment tank 1 is attached via
stud bolts (not shown) to a fabricated steel plinth 18 which is itself dowelled to
the seabed 20 by piles through holes 19 in the plinth. The conical roof 2 of the tank
houses a plurality of sieves of, for example, 6 mm mesh. The mesh should be of a copper
alloy to prevent growth of algae. Water flows (as indicated by the arrows) down through
the mesh to the bottom of the tank where an inner tank 4 directs the water up towards
the inlet 5 of a pump 6. The downward flow followed by the upward flow occurs at such
a velocity that unacceptable entrained solids gravitate to the bottom of the tank
1 whence they can be removed.
[0019] The elimination of the water contaminated by gravitated particles can be achieved
by means of an outlet pipe 8 in which there is an electrically or hydraulically operated
pump which is energised periodically, or an ejector continuously energised by high
pressure water obtained as a side stream from the discharge of the pump 6. The outlet
pipe 8 is flanged to a pipe 24 for conveying the discharged water to a dispersal zone.
[0020] The pump 6 is coaxially attached to an electric motor 7, of about 1500 kW running
at about 2950 rpm and fed by a 3.3. kV or 6.6 kV supply. The motor is of the water
flooded type and of the "solid" type, being either a one-piece forging or of laminar
construction, with a final coating of dielectrical material. The casing of the motor
is of the same material as the pump, namely, a duplex stainless steel defined generally
as of 50 percent austenitic and 50 percent ferritic steel. All other constructional
components such as the shaft, screws, keys, washers, bearings, housings, etc., are
made of duplex stainless steel with the exception of the rotor which may be constructed
of a modified duplex steel or other established magnetic materials used in the construction
of electrical machinery.
[0021] Stator laminations are of a duplex stainless steel and insulated in a manner similar
to that of the rotor. Electrical windings are protected by corrosion resisting materials.
Motor and pump may have individual shafts in which case the power is transmitted via
a suitable coupling.
[0022] The pump is of 10 stages with a co-axial back- to-back impeller array, mounted above
the motor. Delivery pressure is attained half way along the shaft and the pump outlet
9 is located in this position. Construction throughout (excepting the rubbing components
of bearings) is of duplex stainless steel. The inlet orifice 5 is typically 150 mm
in diameter and the delivery orifice 9- is typically i00 mm in diameter.
[0023] For support, the combined pump
/motor unit may be secured to the inner tank 4 by brackets 10, or suspended from the
top of inner tank 4. Outlet nozzle 9 is connected by means of a flange 12 to a delivery
pipe 11. Pipe 11 is guided through removable access plates 13 and 14 in the walls
of tanks 1 and 4 and sealed to prevent passage of water.
[0024] The conical roof 2 serves as a support framework for the sieves 3 and also as a lifting
hanger. Bolting unites the framework of roof 2 to tank 1, and after the delivery pipe
11, sludge outlet pipe 8, and any electrical junctions, have been disconnected the
whole assembly may be lifted.
[0025] Figure 2 illustrates an alternative system for use when filtration of the seawater
has to be augmented so that very fine particles, for example, as small as 5 um can
be removed. Power driven filters 25 and their driving motors 29 are housed in the
containment tank 1. Seawater enters the tank via filtration grids 3 supported on roof
framework 2, and enters the filters at inlet ports 26, the filtrate being conveyed
to the pump by pipework 27. It is arranged by control valves 28 that one filter at
a time can be backflushed by means of independently controlled valves in the auxiliary
piping. Filters 25 are disposed peripherally around the inside of tank 1. The remainder
of the device is essen- tally the same as that described above with reference to Figure
1.
[0026] The containment tanks 1 of Figures 1 and 2 will generally have a height of up to
12 m and a diameter of up to 4 m. The inner tank 4 shown in Figure 1 may have a diameter
of about 2.5 m and a height of about 6.5 m.
1. A method for recovering oil from a submarine petroleum-containing formation penetrated
by an injection well and a production well, in which seawater is injected under pressure
into the formation through the injection well and oil is recovered through the production
well, characterized in that the seawater is injected through a pump located adjacent
the seaffoor injection well.
2. A method according to claim 1, wherein the pump is a multistage centrifugal pump.
3. A method according to claim 1 or claim 2, wherein the pump is driven by a motor
directly coupled to the pump.
4. A method according to claim 3, wherein the motor is an electric motor.
5. A method according to claim 3 or claim 4, wherein the pump and the motor have bearings
that are lubricated by seawater.
6. A method according to any one of claims 1 to 5, wherein the pump and motor are
located within a protective enclosure which is attached to the seafloor, and which
incorporates a coarse fitter for water-bome solid particles.
7. A method according to claim 6, wherein particles passing through the coarse filter
are permitted to settle within the protective enclosure.
8. A method according to claim 6, wherein seawater enters the pump through one or
more fine filters for water-borne solid particles.
9. A method according to any one of claims 1 to 8, wherein the pump includes means
for monitoring the flow of water through the pump.