GB2066363A - Deep-well and pipeline pumps - Google Patents

Abstract

A motor-pump unit, particularly but not exclusively for location in the downbore of a deep offshore oilwell, comprises a small diameter multi- stage fluid turbine, e.g. having at least thirty axial-flow stages, coupled to a multi-stage pump, e.g. of mixed-flow type. A portion of the oil pumped by the pump is cleaned, pressurized and returned by a conduit to drive the turbine, and on discharge from the turbine the portion mixes with freshly pumped oil being delivered to the head of the well. In contrast to previous units which employed an electric motor located at the bottom of the well, the present pump unit can be advantageously located at any suitable intermediate position in the bore and thereby operation and maintenance of the unit is considerably convenienced. Additionally, the present unit provides much greater pumping power than previous electrical units so enabling usage in very deep wells and the provision of greater capacity.

Description

SPECIFICATION Improved pumping system The present invention relates to a fluid pumping system for use in a pipeline or conduit, particularly but not exclusively the downbore of a well such as an oil well.

In the oil industry it is established practice in many oilfields to install a pump down the oil well to assist the production of oil from the well. In the early days of oil exploration and well development these pumps were of the reciprocating type, driven from the surface by means of rods. Such pumps were limited to comparatively shallow wells and comparatively small oil flows from the wells.

With the development of submersible electric motors during the last thirty years, and the drilling of deeper and often larger oil wells, it has become more economic to install a multistage centrifugal borehole pump within the well, driven by a submersible electric motor. The power to the motor is taken down the well from the surface by an insulated electric cable. Because of the general limitation of electrical supply frequency to 50 Hz or 60 Hz, such motors generally run at synchronous speeds -- with 3000 and 3600 RPM being the ones most commonly selected. At the same time, a fairly high generated pressure across the pump is frequently required for optimum oil production -- over 2000 ft. in many instances.

The limitations in diameter, imposed on both pump and motor design by the restrictions of the bore of the casing in the hole, therefore generally results in a pump with a very large number of stages (over 100 is not uncommon) and therefore a unit with a very high length to diameter ratio (sometimes several hundred).

Similarly, the submersible motor L/D ratio is also frequently a very large figure. Indeed downhole pumps and their associated submersible motor drivers are characterised by their extreme length and small diameter.

Recent increases in the price of oil have stimulated the exploitation of more marginal oilfields, where it has become necessary or desirable to install a larger horsepower into the oil well to generate sufficient boost head to achieve economic production. In various parts of the world ,there has also been rapid development of offshore oilfieids. One of the characteristics of these developments has been the drilling of comparatively large bore deviated wells from each offshore platform, with a resulting large flow of oil per well. Pressure boost for these wells necessarily involves comparatively large horsepowers (several hundred horsepower per well in many instances), installed in the downhole submersible-motor-driven pumps.

These recent developments have led to the progressive development of larger and much longer submersible motors. The extreme length and high power of these motors has led to a progressive increase in unit unreliability. Such motors, and the pumps they drive, must be installed at the base of the well, below the perforations in the casing which allow oil to flow from the rock strata into the well. The oil discharge tube, which is installed within the well casing to contain the oil flow as it passes up to the surface, must all be removed from within the casing to permit removal of the downhole pump and motor in the event of breakdown or to permit maintenance, repair or replacement.Thus the situation has developed where unit reliability has worsened, due to the design concepts being stretched to the absolute limit, and at the same time the cost associated with maintenance and replacement has escalated, particularly for the larger wells and in offshore situations.

According to the present invention a fluid pumping system for use in a pipeline or conduit comprises a rotodynamic fluid motor drivingly coupled to a pump, both the pump and motor being located in the pipeline or conduit.

The motor is preferably arranged for operation with an operating fluid of very high pressure.

Preferably the operating fluid for the motor comprises fluid pumped by the pump. In particular, a portion of the pumped fluid can be supplied to a separate relatively high pressure pump delivering to the motor.

In a preferred embodiment, the fluid discharged from the motor mixes with the discharge from the pipeline pump.

A novel form of machine is preferably provided to constitute the above rotodynamic motor and is in the form of a turbine of small diameter comprising a substantial number of stages and adapted to receive a very high pressure operating fluid. In particular, the turbine can have at least 30 rotor (and stator) stages, with operating fluid supplied at a pressure of greater than say 2000 p.s.i., for example circa 5000 p.s.i. The turbine is preferably an axial flow turbine provided with reaction blading.

Preferably the pump comprises a multi-stage pump with mixed-flow blading. In an alternative arrangement, the turbine discharge is returned to the pressurising pump by a separate conduit.

According to another aspect of the present invention there is provided in a well, especially but not exclusively an oil well, having a downbore with a production tube therein, a pumping system for production fluid in the well comprising a pumping unit located within the production tube, said pumping unit including a rotodynamic fluid motor.

Preferably the motor operating fluid is comprised by production fluid. Operating fluid to the motor can be delivered from a separate high pressure pump via a supply line located in the production tube, and the motor can conveniently be located above the pump. Preferably the fluid discharge from the motor is carried in the production tube by the production fluid from the production pump. The pumping unit is preferably located intermediate the ends of the down bore, and can be located by releasable fastening or support means to facilitate withdrawal of the unit.

The motor preferably comprises a multi-stage high pressure turbine, preferably of the reaction type, and many hundreds of horsepower can be developed e.g. 300 or more, to provide increased head and production in the well.

Preferably the pump comprises a multi-stage pump with not more than 1 5 stages, and preferably with between 6 to 10 stages.

In an alternative embodiment the motor could be located below the pump.

According to yet a further aspect of the present invention a pump unit for use in an oil well comprises an elongate multi-stage fluid turbine drivingly coupled to a co-axial multi-stage rotodynamic pump, the co-axial turbine and pump being arranged for location in a production oil delivery tube, the turbine having a fluid inlet connectable to a fluid supply line.

Preferably the fluid outlets of the pump and turbine are arranged to permit mixing of the respective fluid discharges in the delivery tube, production oil serving to drive the turbine.

In a preferred embodiment, the inlets to the pump and turbine are located at respective ends of the unit while the fluid discharges are positioned in juxtaposed relationship intermediate the ends of the unit.

The present invention is also a method of operation of a motor/pump unit in a fluid pipeline system, wherein fluid pumped by the pump in the pipeline serves for operation of the motor of the unit. Preferably the pumped fluid is cleaned prior to pressurisation and delivery to the motor.

In this invention the established practice of driving the downhole pump by a submersible electric motor is replaced by the concept in which the pump is driven by a rotodynamic fluid motor, particularly a multistage liquid turbine. This turbine is preferably axial flow, mounted immediately above the pump, and the liquid is preferably supplied to it at very high pressure (typically 4000 to 6000 psi) from a pump at or near the well head, by means of a comparatively small bore supply pipe which preferably passes down the inside of the well discharge tube.

After passing through the turbine, the driving liquid is then allowed to flow back up to the well head. There are several alternative systems incorporated in this invention for this return flow as will be described later.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 shows a partially sectioned side elevation of an oil well with a production oil pumping unit according to the present invention installed; Fig. 2 shows a partially sectioned side elevation of the downhole turbine of the pumping unit shown in Fig. 1; Fig. 3 shows a rotor/stator detail of the turbine of Fig. 2 to a larger scale, and as encircled in Fig. 2; Fig. 4 shows a cross-section of the turbine of Fig. 2; Fig. 5 shows a partially sectioned side elevation of the multi-stage pump of the pumping unit of Fig. 1; and Fig. 6 shows a side elevation of the coupled motor and pump of Figs. 2 and 5.

Referring to Fig. 1 a typical oil well contains a casing 1, and a production tube 2. Both the casing 1 and the tube 2 are assembled from a series of screwed tubes by methods well established in the oil industry. For the larger oil wells for which larger horsepowers (typically between 50 HP and 1000 HP) are required in the downhole pump, the casing may be 9-" o.d. and the production tube 685" o.d. (typically). Oil enters the production tube at the base of the well via perforations 3 in the casing 1. A downhole pumping unit comprising a pump 4 and a fluid turbine 5 is mounted within the production tube 2. Oil passing through the pump 4 is permitted to flow upwards through the annulus 6 formed between the outside diameter of turbine casing 5 and the bore of the production tube 2.In the present arrangement, a portion of the production oil at the well head after passing through gas and water separators is pumped at very high pressure circa 5000 p.s.i. back down the well through a supply pipe 8 inside the tube 2 to the turbine. This high pressure driving oil, after passing through the turbine, flows into the production tube 2 and back up to the well head. In other words, the turbine driving fluid is taken down the bore of the well by a high pressure supply pipe, but returns as part of the well production flow.

The casing of the pump 4 is provided with a location and sealing arrangement 7 which both locates the pump within the production tube 2 at the chosen depth below the well head, and provides a seal, preventing leakage of pressurised oil flowing from the pump discharge back to pump suction between the outside of the pump casing and the bore of the production tube 2.

The turbine driving oil is taken from the well head discharge, downstream of the gas and water separators, and is cleaned by subjection to solids separation to remove any suspended solids such as sand and grit, by filtration or centrifuging equipment. The driving oil is pressurised by a pump (not shown) at the well head to a very high pressure. This clean high pressure crude oil is then passed down the well by the high pressure supply tube 8, to the turbine. By adopting this very high supply pressure (e.g. 4000 to 6000 psi.) large horsepowers (typically 100 to 500 HP) can be obtained from the liquid turbine with a comparatively small diameter supply tube. For example it is possible to obtain 300 HP from a downhole liquid turbine according to this invention in excess of 5000 ft. below the well head, with a supply tube of only 2" bore, without introducing unacceptably high transmission losses in the supply pipe.

To utilise the hydraulic energy in the high pressure supply oil efficiently, the turbine is of multistage axial flow construction. Typically the turbine will have between 40 and 60 stages 30.

The turbine rotational speed, blade diameter D (Fig. 4) and number of stages 30 are inter-related and are affected by the supply pressure. A higher supply pressure will in general require a larger number of stages 30 within the geometrical limitations, imposed by the well and the stresses in the turbine components.

In general all the stages will be geometrically identical. Either impulse or reaction turbine blading profiles may be adopted. However, in general, reaction blading is to be preferred, since it tends to lead to a larger diameter and a less slender rotor, and results in better capability to resist erosion since blade inlet profiles are all rounded and maximum relative velocities are lower.

The high pressure oil passes alternately through fixed reaction blade rows or stators 9 and the moving reaction blade rows or rotors 10. The blade rows 9 and 10 which are of aerofoil section, are preferably made as precision castings, with integral shroud rings. Each of the fixed blade rows 9 are cast integral with an outer shroud ring 11, which acts as a linear inside the barrel casing 12.

Also cast integral with the fixed blade row 9 is an inner shroud ring 13, the bore of which provides a bearing surface for shaft sleeves 14 to run on at each stage 30. The moving blade rows 10 are cast integral with their inner shroud half sleeves 14.

These integrally bladed rings are mounted on the turbine shaft 15, such that torque is transmitted from the moving blade rows to the turbine shaft.

Fine clearances are maintained radially between fixed and moving components to minimise leakage losses. The integrally bladed rings are made from a highly erosion resistant material such as Stellite (T.M.), while the shaft 1 5 and the barrel are made of stainless steel.

The barrel includes radial strengthening webs 31. To assemble the turbine, the bladed rings 9, 10 are simply fitted in the barrel 12 and on the shaft 1 5 respectively. The turbine rotor is supported on the inter-stage seal rings and requires no external bearings, apart from example tilting pad axial thrust bearings running in the well oil at the non-drive end. There are no mechanical seals. The important feature of this turbine is that it is especially designed to run at very high pressure, so reducing the volume of driving oil required to a minimum thereby reducing the transmission losses of fluid passing down the hole, and provides a comparatively high speed machine with a very small barrel diameter.

As the oil supplied to the turbine 5 flows through each row of blades, its pressure is progressively reduced, until at the outlet 1 6 of the turbine this pressure is the same as that of the production oil flowing out of the pump 18 which the turbine drives. The oil flowing from the turbine 5 then mixes with the oil discharged from the pump and flows upwards around the outside of the turbine barrel casing 1 2.

Since the turbine as described above is of the 50% reaction type, a balance drum 1 7 is mounted on the shaft at the inlet end of the turbine, leaving only a small residual thrust from the turbine (and from pump 1 8 mounted beneath the turbine) to be carried by the thrust bearing 1 9. This thrust bearing is of the self regulating hydrostatic type as shown, with an axial clearance 'C' being maintained between collar 20 and stationary casing ring 21, by the pressure between these two components, as the driving oil flows between them. Alternative types of thrust bearing are also possible, e.g. tilting pad hydrodynamic arrangements.As shown, there are no mechanical seals on the turbine, and the driving liquid (crude oil) is allowed to flow through the fine clearances at the ends of the machine, and through the discharge branch, to the surrounding oil flowing upwards in the well discharge tube 2. Additional standstill or mechanical seals (not shown) may, however, be fitted with the object of preventing ingress of dirty oil to the inside of the turbine 5 when it is stationary. Similarly, spring loaded nonreturn valves may be fitted over all discharge openings from the barrel (e.g. at outlet 16), with the object of sealing the interior of the turbine from the environment when the machine is not running. A balance leak-off aperture 32 is provided in the turbine.

The turbine as described above is directly coupled by flanged couplings 33 to a pump 18 mounted below it, to form an integral unit.

Alternatively, the pump could be located above the turbine. Pump 18 is generally of the multistage bowl type. Thus the pump 18 comprises a plurality of stages 1 8A, 18B... each comprising a casing 34 housing a mixed-flow impeller 35 mounted on pump shaft 36, with diffuser passage 37 receiving the discharge from the impeller blading. 38 is the suction inlet and 39 the fluid discharge. A sealing arrangement 7 (Fig. 1) powered by high pressure hydraulic oil for removal, locks and seals the pump within the bore of the well discharge tube 2.

By having a rotational speed of between 5000 and 8000 RPM the number of stages in the downhole pump 18 can be reduced from upwards of thirty to between six and twelve. This higher rotational speed than existed previously in practice is acceptable due to the small size of the impeller diameter i.e. less than 5" and at this size the impellers could be manufactured from Stellite, or other erosion resistant materials.

Due to the reduction of the motor diameter from 8" to less than 5" the motor and pump can be placed at any intermediate point in the discharge tube 2 of the well, sealing the pump into the well tube by a hydraulically operated arrangement for which the technology already exists in the oil industry.

The position of the pump in the well is determined by that depth at which the gas is dissolved in the oil plus a marginal additional depth of NPSH. In the case of a typical North Sea field that will be about 4000 ft. Typical well depth is over 7000 ft deep and a submersible electric motor driven 300 HP pump has to be placed at the bottom of the well as the motor is too big in diameter to permit ol to flow past it in the 6" bore discharge tube - nor indeed can it even be fitted into the discharge tube. The latter must be inserted into the well after the electric motor has been installed at the base of the well. The present arrangement overcomes this problem.

Overall length of a 300 HP example with the concepts now proposed (pump plus turbine) is around 8 ft. The overall length of a typical 300 HP submersible electric motor driven pump is approximately ten times this figure.

To obtain reliability from the turbine, which will necessarily have quite thin section blades, erosion must be avoided. This is partly achieved by the very low relative velocities, inherent in the turbine design, but in addition it will be desirable to install hydrocyclines or filters in the suction of the pressurising pumps for the downhole turbines.

The high pressure pumps at the well heads may be centrifugal barrel case pumps bearing in mind that offshore oil wells are grouped together and these groups may all be powered from one production platform. Thus, this barrel case pump could have a duty of around 2000 GPM with a pumping head of approximately 8000 ft.

It is possible to have the downhole pressure supply pipe 8 (2" bore) either as a series of 30 ft sections with a reliable high pressure screwed joint, or alternatively as a continuous all-welded line on, say, a 1 5 ft. drum on the offshore platform, accepting some slight yielding as the tube is wound on the drum. This arrangement leads to a very fast installation and retrieval capability for maintenance and inspection purposes.

In an alternative arrangement (not shown) the driving fluid for the turbine is discharged from the turbine to a driving fluid discharge tube and passes up through this tube to the well head. In this arrangement the fluid power circuit for the turbine is kept separate from the crude oil flow in the well by the supply and discharge tubes and by mechanical or other seals fitted to the ends of the turbine. In this arrangement it is not necessary to use crude oil as the power fluid for the turbine, and hydraulic oil, mud or water, may therefore be used.

Generally at the start of a well's life the pump has to handle a comparatively large flow of comparatively small developed head, while as the well's production declines it is necessary to produce a higher head for a smaller well flow.

Thus the duty of the pump and/or turbine requires to be changed, during the well life, and the ease of withdrawal of the present pumping unit makes such a change comparatively inexpensive, in comparison with previous units utilising submersible motors.

As described above, the invention serves to boost the flow of crude oil from an oil well.

However, it is also applicable to wells, supplying water (e.g. from a subterranean aquafer) from below ground to the surface. In this case it is possible (and preferable) to use water as the driving fluid for the turbine.

The machine described in this invention need not be mounted vertically, and indeed will operate satisfactorily in any position from horizontal to vertical. For this reason it is particularly suited to deviated oil wells.

The machine may also be mounted within any pipeline (not necessarily within an oil well) as a pressure booster for the liquid in the pipeline, where it is difficult or impossible to fit a conventional electric motor or other primer mover directly to the pump. For example, the machine would be ideally suited for the boosting of the flow of water, oil, or condensed gas, in a pipeline on the sea-bed, or buried beneath the ground.

It is possible to mount two or more turbo-driven pumps according to this invention in series within the well production tube. With this arrangement the high pressure fluid supplied to the turbines may either pass sequentially through the turbines (i:e. turbines in series), or the flow may be split such that the turbines run with a parallel fluid supply, each turbine discharging into the production tube. By this means it is possible to generate comparatively high pressures in the production flow of oil at the well head, and to increase substantially the amount of pumping power which is available downhole. This can be of major importance for offshore oil wells, since by staging the downhole pumping units in this way, it is possible to keep the pressure of the production oil at the well head above the "bubble point", i.e.

above the pressure at which gas comes out of solution. This in turn permits the pumping of oil directly from the well head to the shore through a pipeline entirely on the sea bed, and obviates or minimises the need for gas and water separation equipment on the offshore platform, thereby reducing the size and cost of platform required.

Claims (26)

1. A pumping system for use in a pipeline or conduit comprising a rotodynamic fluid motor drivingly coupled to a pump, both the pump and motor being adapted for location in the pipeline or conduit.

2. A pumping system as claimed in claim 1, wherein the motor is arranged for operation with an operating fluid of very high pressure.

3. A pumping system as claimed in claim 1 or 2, wherein the operating fluid for the motor comprises fluid pumped by the pump.

4. A pumping system as claimed in claim 3, wherein a portion of the pumped fluid is supplied to a separate relatively high pressure pump delivering to the motor.

5. A pumping system as claimed in claim 3 or 4, wherein the fluid discharged from the motor mixes with the discharge from the pipeline pump.

6. A pumping system as claimed in any one of the preceding claims, wherein the motor is in the form of a turbine of small diameter comprising a substantial number of stages and adapted to receive a high pressure operating fluid.

7. A pumping system as claimed in claim 6, wherein the motor has at least 30 stages and receives operating fluid having a pressure greater than 2000 p.s.i.

8. A pumping system as claimed in claims 6 or 7, wherein the turbine is an axial flow turbine provided with reaction blading.

9. A pumping system as claimed in any one of the preceding claims, wherein the pump comprises a multi-stage pump with mixed-flow blading.

10. A pumping system as claimed in claim 4, wherein the motor discharge is returned to the pressurising pump by a separate conduit.

11. A well production system having a downbore with a production tube therein, and a pumping unit located within the production tube, said pumping unit including a rotodynamic fluid motor.

12. A well production system as claimed in claim 11, wherein the motor operating fluid is comprised by production fluid.

1 3. A well production system as claimed in claim 12, wherein a separate high pressure supply pump is provided supplying operating fluid to the motor via a supply line in the production tube.

14. A well production system as claimed in any one of claims 11 to 13, wherein the motor is located above the production pump.

1 5. A well production system as claimed in any one of claims 11 to 14, wherein the fluid discharge from the motor is carried in the production tube by the production fluid from the production pump.

1 6. A well production system as claimed in any one of claims 11 to 15, wherein the pumping unit is located intermediate the ends of the downbore, and is located by releasable fastening or support means to facilitate withdrawal of the unit.

17. A well production system as claimed in any one of claims 11 to 1 6 wherein the motor is a multi-stage high pressure turbine.

1 8. A well production system as claimed in any one of claims 11 to 17, wherein the pump is a multi-stage pump.

19. A production pump unit for use in an oil well, comprising an elongate multi-stage fluid turbine drivingly coupled to a co-axial multi-stage rotodynamic pump, the co-axial turbine and pump being arranged for location in a production oil delivery tube, the turbine having a fluid inlet connectible to a fluid supply line.

20. A pump unit as claimed in claim 19, wherein the fluid outlets of the pump and turbine are arranged to permit mixing of the respective fluid discharges in the delivery tube, production oil serving to drive the turbine.

21. A pump unit as claimed in claim 19 or 20, wherein the inlets to the pump and turbine are located at respective ends of the unit while the fluid discharges are positioned in juxtaposed relationship intermediate the ends of the unit.

22. A method of operation of a motor/pump unit in a fluid pipeline system, wherein fluid pumped by the pump in the pipeline serves for operation of the motor of the unit.

23. A method as claimed in claim 22, wherein the pumped fluid is cleaned prior to pressurisation and delivery to the motor.

24. A pumping unit substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

25. A well production system substantially as herein before described with reference to and as illustrated in the accompanying drawings.

26. A method of operation of a motor/pump unit substantially as hereinbefore described.