WO2001081764A1

WO2001081764A1 - Method for lift pumping and lift pump

Info

Publication number: WO2001081764A1
Application number: PCT/NO2001/000167
Authority: WO
Inventors: Bjørn EILERTSEN
Original assignee: Engineering & Drilling Machinery As
Priority date: 2000-04-27
Filing date: 2001-04-19
Publication date: 2001-11-01
Also published as: AU5277601A; NO311906B1; NO20002177D0; NO20002177L

Abstract

A lift pump is designed so that the movement of the elongate lifting member (1) up and down in a wellbore causes a compression of an air mass in a tank (3, 9). The energy in this compressed air mass is used for carrying out the movement of the lifting member (1) upwards in the wellbore. The compression takes place by means of a piston (22) that is moved inside a vertical cylinder (3) under the influence of a train of rack segments (15) that are hinged to each other and are drive-actuated by a drive comprising frequency-controlled electromotors (30, 31) and pinions (35, 36) driven thereby which are in engagement with the teeth of the rack segments (15) in the rack segment train.

Description

METHOD TO LIFT PUMPING AND LIFT PUMP

The invention relates to a method of lift pumping, where an elongate, essentially vertical lifting member is moved up and down in a wellbore.

The invention also relates to a lift pump, comprising an elongate, essentially vertical lifting member designed for running into a wellbore, and a driving mechanism assigned to the lifting member for movement thereof up and down.

The invention has been especially developed in connection with the pumping of oil from an onshore deep low-pressure reservoir.

Traditionally, such lift pumping is done by oil pumps known as "nodding donkeys". A normal pump stroke for a pump of this kind is 30 feet, i.e., a little more than 9 metres.

The weight of the oil volume lifted with each pump stroke is estimated to be about 300 kg, but a much greater downhole mass, about 27 tonnes, must be accelerated and retarded during a pump stroke.

Usually, the pump performs four cycles per minute, i.e., four lifts and four lowerings - a total of eight strokes.

The object of the present invention is to provide a method and an apparatus which permit a continuous and reliable pumping operation over a long period of time. It is also an object of the invention to provide a lift pump that is maintenance-friendly and simple to repair, and that can also be transported by road, rail and sea.

The basic idea of the invention is to make use of the energy in the mass moved during the downward movement of the lifting member to compress a gas medium, preferably air, and to utilise the stored energy during the subsequent upward movement of the lifting member in the wellbore.

The lift pump is therefore provided with a pneumatic arrangement to balance the mass moved. In a practical embodiment, the pneumatic arrangement comprises a 10-metre long cylinder in which a piston is moved. The piston is attached to one end of a rack segment train, the other end of which is connected to the downhole pumping arrangement. When the mass moved is lowered, the energy is used to compress air inside a cylinder and a pressure tank. The inside pressure of the cylinder is nominal, i.e., it balances the downhole mass at about a half stroke. The piston will be in its outer position when the downward stroke commences, and therefore the cylinder pressure will then be lower than the nominal pressure.

During the first half of the downward stroke, the mass is accelerated. After the piston has passed a "balance point", the second half of the stroke is retarded as a result of an increase in the cylinder pressure. The downward movement is stopped by the piston entering a closed "high-pressure" cylinder at the end of the stroke. The volume in this "high-pressure" cylinder can be adjusted so as to compensate for different loads, retardation lengths, ambient temperature etc.

A large part of the energy is accumulated in the compressed air, and can be used during the lifting stroke.

The kinetic energy is used together with two frequency-controlled electromotors which drive the rack segment train.

According to the invention, a method of lift pumping is thus proposed, where an elongate, essentially vertical lifting member is moved up and down in a wellbore, and what characterises the method of the invention is that the downward movement of the lifting member in the wellbore is used to compress an air mass in a tank, and that the energy in the compressed air mass is used to carry out the upward movement of the lifting member in the wellbore.

Additional features of the method will be apparent from the dependent method claims.

According to the invention, there is also proposed a lift pump, comprising an elongate, essentially vertical lifting member designed to be run into a wellbore, and a driving mechanism assigned to the lifting member for movement thereof up and down, which lift pump according to the invention is characterised in that the driving mechanism includes a train of rack segments hinged to each other and movable in a vertical U- shaped path, and a drive comprising pinions in engagement with the rack segment train for movement of the train back and forth in the U-shaped path, a rack segment placed in a first U-leg being connected to the lifting member, whereby this is moved up and down in accordance with the movement of the train. Additional features of the lift pump according to the invention will be apparent from the dependent apparatus claims.

The invention will now be described in more detail with reference to the drawings, wherein

Fig. 1 shows a lift pump according to the invention, in an upright or raised operating state; Fig. 2 shows the lift pump in a lowered transport state;

Fig. 3 shows on a larger scale a section of the upper part of the lift pump;

Fig. 4 shows on a larger scale a schematic section taken along the line IV-IV in Fig. 1 ; and

Fig. 5 shows a section through the lift pump.

The lift pump shown in the drawings comprises an elongate lifting member 1 that is run into a non-illustrated well through a wellhead 2 located at the surface.

The lift pump further comprises two vertical hollow cylinders 3 and 4. The hollow cylinder 3 is closed at the top at 5, and is open at the bottom at 6.

The other vertical hollow cylinder 4 is open at the bottom at 7, and has a cylinder wall split 8 extending in the longitudinal direction, see Fig. 4 in particular.

Arranged between these two vertical cylinders 3 and 4 is an elongate cylindrical pressure tank 9. The three components 3, 4 and 9 are combined into one unit through the use of suitable stays/braces 10. At the bottom, the two vertical cylinders 3, 4 are connected together by a brace 11 and 12, and vertical supports or stays 13 are also provided for the pressure tank 9. At the bottom, protective plates 14 are also provided over a transition area between the two vertical cylinders 3, 4, which constitute vertical legs of a U-shaped path for a train of rack segments 15 that are hinged to each other, see in particular Fig. 3 and Fig. 5.

As shown in Fig. 5, the hinge-connected rack segments 15 run in the vertical hollow cylinder 4 and then over to the vertical hollow cylinder 3 via a transitional section 16, where the rack segments 15 are guided in a semicircular path, as shown. The uppermost rack segment 15 in the cylinder 4 is connected to a trolley 17, made having a projecting bracket 18 that is connected to the vertical lifting member 1. The trolley 17 is supported inside the cylinder 4 by means of support/guiding wheels 19. These support wheels 19 help to take the bending moment from the downhole load.

The individual rack segment 15 is guided towards the inside wall of the cylinder 4 by guiding wheels 20, see in particular Figs. 3 and 4. As shown in particular in Fig. 4, these guiding wheels 20 also act as guiding components inside the second vertical cylinder 3. The rack segments 15 are hinged to each other as shown at 21 in Figs. 3 and 4.

The rack segments 15 form a train of rack segments that are hinged to each other, as in shown in particular in Fig. 5. The end of the rack segment train in the cylinder 3 is connected to a piston 22 that is closely fitted in the cylinder 3, which thus forms a working cylinder where the piston 22 can move up and down under the influence of the rack segment train or the pressure in the cylinder 3. The working chamber in the cylinder 3 is flow-connected at 23 to the vertical, elongate pressure tank 9, see Figs. 3 and 5.

The vertical cylinder 3 is closed at the top as shown at 24 in Fig. 3. In the bottom end 24 there is a threaded bore 25 for a control rod 26, to the available end of which inside the cylinder 3 there is fastened a piston 27. The purpose of the adjustable piston 27 will be explained in more detail below.

The rack segments 15 are provided with two longitudinal toothed portions 28, 29, so that the tooth segments in the vertical state form two continuous toothed bars.

A drive is arranged immediately above the turning section 16, see in particular Fig. 4. The drive comprises two frequency-controlled electromotors 30, 31 which via couplings 32 and planetary gears 33, 34 drive two respective pinions 35, 36 that are in engagement with the teeth on the rack segments 15. In Fig. 4 just two pinions 35, 36 are shown . The motor 30 also drives two similar pinions that are inside the box 38. The pinions 35, 36, i.e., those that are driven by the motor 31, are arranged in a similar elongate box 37, which has been cut through in Fig. 4. Mounted on the outside of the cylinder 3 and the cylinder 4 are counter support wheels 39, 40. These are supported in a suitable manner in non-illustrated bearings connected to the individual cylinders 3, 4 and act as counter support wheels for the pinions.

As already mentioned, the whole of the lift pump, i.e. the vertical cylinders and the air tank, is assembled as one unit, and this unit is pivotally supported at 41 in a support frame 42, so that the whole unit can be swung down to a storage or transport position on the support frame 42, as shown in Fig. 2.

A ladder 43 is also provided in the unit to enable operating personnel to gain access to the control rod 26 on top of the cylinder 3.

The lift pump works in the following way:

Starting from the position shown in Figs. 1 and 5, the motors 30, 31 are started so that they drive the rack segment train downward in the cylinder 4 and upward in the cylinder 3. The lifting member 1 is then moved down into the wellbore through the wellhead 2. The piston 22 moves up in the cylinder 3 and compresses the air available there. The air is forced into the compressed air tank 9 through the connection 23.

To begin with, the lifting member is accelerated under the influence of the mass by which it is actuated in the wellbore. After the piston 22 has passed a balance point with regard to the pressure in the cylinder 3, there will be a retardation. This happens after about half a downward stroke length. The reason for the retardation is that the pressure in the cylinder 3 increases. The downward movement is stopped when the piston 22 enters the upper cylinder part 44, above the pipe 23 which provides a flow connection to the pressure tank 9. This situation is shown in more detail in Fig. 3. After the piston 22 has passed the pipe 23, the chamber 44 will constitute a closed high-pressure chamber, which offers great resistance and stops the downward movement. The power supply to the motors is cut when the piston 22 approaches the end of its upward stroke.

The volume of the chamber 44 can be adjusted by the piston 27 so as to compensate for different loads, retardation lengths, ambient temperature etc.

It will be understood that a large part of the lowering energy will now be accumulated in the compressed air and that this compressed air can be used during the subsequent lifting stroke, i.e., the upward movement of the lifting member 1 in the wellbore. At the same time the motors 30, 31 are reversed so that they also contribute to the upward movement of the lifting member 1. When the lift stroke is terminated, the motor activity is controlled so that it gradually decreases, thereby allowing the desired retardation to be obtained.

Of course, it is desirable that the nominal pressure in the pneumatic cylinder 3 should always be at the right level. To compensate for heat loss and leakage from the cylinder 3, a small air compressor 45 is provided on the support frame 42. This air compressor is connected to the cylinder 3 or the tank 9. The compressor 45 is also used during the start-up of the system.

In a practical exemplary embodiment, the two vertical cylinders 3, 4 have a length of about 10 metres and a diameter of about 60 cm. The rack segments 15 may be about 50 cm in length, and the toothed portions 28, 29 extend along the full length of each individual rack segment 15.

The use of two motors 30, 31 with associated pinions allows equal exertion of force on each side or in each leg of the U-shaped path for the rack segments to be obtained. This gives reduced wear on the rack segments and pinions.

A rack segment length of 50 cm means that 25 rack segments are used in the train.

The whole pump apparatus will have a transport length of a little more than 15 metres, and a length in the operational state of 12 metres. The width can easily be kept within 2.4 metres, with a transport height of 4.3 metres, so that the whole apparatus can relatively easily be transported by road, rail and sea.

The apparatus can easily be tilted about a pivot point 16, for example by using a mobile crane. Maintenance and repairs can be done when the apparatus is in the horizontal transport state.

Claims

P a t e n t c l a i m s

1.

A method of lift pumping, where an elongate, essentially vertical lifting member (1) is moved up and down in a wellbore (2), characterised in that the downward movement of the lifting member (1) in the wellbore is utilised to compress an air mass in a tank (3, 9), and that the energy in the compressed air mass is used for carrying out the upward movement of the lifting member (1) in the wellbore.

2.

A method according to claim 1 , characterised in that the compression is carried out in that a piston (22) actuated by the lifting member (1) is passed into a cylinder (3) which has flow communication (23) with an accumulator tank (9).

3.

A method according to claim 2, characterised in that in the cylinder (3) at the end of the downward movement of the lifting member (1) there is established an adjustable, closed damper chamber (44) in the cylinder (3).

4.

A method according to claim 2 or 3, characterised in that the movement of the lifting member (1) up and down in the wellbore is transmitted to the piston (22) via a train of rack segments (15) that are hinged to each other and that run in a vertical U-shaped path (3, 4).

5.

A method according to claim 4, characterised in that the rack segment train (15 ... 15) is drive-actuated by motor-driven pinions (35, 36) during the movement of the lifting member (1) up and down in the wellbore.

6.

A lift pump, comprising an elongate, essentially vertical lifting member (1) designed to be run into a wellbore (2), and a driving mechanism (30, 31, 35, 36, 15) assigned to the lifting member (1) for the movement thereof up and down, characterised in that the driving mechanism comprises a train of rack segments (15) that are hinged to each other and movable in a vertical U-shaped path and also a drive comprising pinions (35, 36) in engagement with the rack segment train (15 ... 15) for movement of the train back and forth in the U-shaped path (3, 16, 4), wherein a rack segment (15) located in a first U- leg (4) is connected to the lifting member (1), whereby this is moved up and down in accordance with the movement of the train.

7.

A lift pump according to claim 6, characterised in that the other U-leg (3) is made in the form of a vertical, hollow cylinder that is closed at the top, and that one end of the rack segment train (15 ... 15) is assigned to a piston (22) in the hollow cylinder (3).

8.

A lift pump according to claim 6 and/or claim 7, characterised in that the said first U-leg (4) is made in the form of a vertical hollow cylinder with a longitudinal cylinder wall split (8).

9.

A lift pump according to claim 7, characterised in that the piston stroke chamber in the vertical hollow cylinder (3) that is closed at the top is connected to an accumulator tank (9).

10.

A lift pump according to claim 9, characterised in that the accumulator tank (9) is in the form of a vertical cylindrical tank, arranged between the two U-legs (3, 4).

11. A lift pump according to one of claims 6-10, characterised in that the rack segments (15) are provided with a guide wheel (20) for guiding the rack segments (15) in the vertical U-shaped path (3, 16, 4).

12. A lift pump according to one of preceding claims 6-11, characterised in that there is provided a counter support wheel (39, 40) opposite each pinion (35, 36).

13.

A lift pump according to one of claims 7-12, characterised in that the drive comprises two drive motors (30, 31), each of which drives two pinions (35, 36) each assigned to its respective U-leg (3, 4).

14.

A lift pump according to one of preceding claims 6-13, characterised in that it is pivotally supported (41) as a unit on a support frame (42) so that it can be swung from a horizontal storage/transport position to a vertical operational position.

15.

A lift pump according to one of claim 7-14, characterised in that uppermost in the vertical hollow cylinder (3) that is closed at the top there is arranged a piston (27) adjustable in the stroke direction.