GB2482672A - Reciprocating jack pump driver - Google Patents

Abstract

A drive mechanism for a reciprocating jack pump comprises a pump drive string polished rod 1 connected via a bridle 3 to first ends of pairs of cables or belts 4 at least partially wrapped around a pulley 5 or drum and attached at their other ends to a counterweight 8 having a central cavity and mounted coaxially of the mechanism. The cables attaching to the bridle piece pass through the cavity. The pulley is driven by a reversible electric motor. e.g. a permanent magnet brushless DC or multiphase servomotor, and the cable is guided through the cavity by rollers 11, 12. The coaxial counterweight counter balances drive string loads, allows a sinusoidal drive and its position eliminates structural side loads. It is protected by a fibreglass cover 19. A shaft encoder and current and voltage demand sensing enable measuring of out of balance forces and other information.

Description

Improvements in artificial lift mechanisms The present invention relates to the design of mechanisms to lift liquids from deep wells or boreholes by means of a reciprocating pump. In such a "jack pump" mechanism, a piston and non-return-valve unit at the base of the well or borehole (which may be several thousand metres deep) is generally connected to the drive mechanism at the surface by means of a long steel rod that, being assembled in sections and screwed together, is known as a string. The topmost section of that string -the section that emerges from the well through a pressure seal -necessarily has a higher surface finish and is known as the polished (or polish) rod. The polished rod is connected to the reciprocating mechanism that forms the invention described herein.

For several hundred years it has been known to construct such pump drive mechanisms in the form of an oscillating horizontal beam having a hammer-shaped end, over which is wrapped a chain or cable from which the pumping string is suspended. Alternative designs (by James Watt) have used a four-bar linkage to convert the swinging arc of the beam into a clean vertical motion of the pump rod. Other, mechanisms exist in which, for example, the polished rod has to have a long stroke (a long vertical travel distance) and forms part of an elevator mechanism. In such mechanisms the polished rod is suspended from one or more steel ropes or belts, which are raised and lowered by the rotation and contra-rotation of a rotary drum or pulley over which the cable is passed. Nevertheless, the reciprocating hammer-head beam or "nodding donkey" is still the most popular device for moving a rod string.

The powered end of the reciprocating beam unit has either been driven by a linear actuator, such as the very earliest kind of steam engine (the atmospheric or Newcomen engine) or by a crank mechanism, driven in turn by a rotary engine, such as a petrol, gas or diesel engine. Such a rotary engine usually has a low output torque and so a heavy duty gearbox must be interposed to reduce the rotary speed and to increase the torque of the crank that moves the beam.

It will also be understood that the long steel string that connects the drive mechanism at the top of the well with the pump itself at the base of the well has a deadload mass of several tonnes, which must also be supported by the beam. To increase the efficiency of the mechanism, the deadload has to be counterbalanced. Although some recent mechanisms have used a gas spring (in the form of a pneumatic cylinder as a "prop" beneath the loaded end of the beam) it is more common for the counterbalance to be in the form of eccentric weights, attached to the shaft of the crank mechanism that drives the oscifiating beam.

From the earliest times the pumping stroke has traditionally been about 100 inches (two or three metres) and the pumping frequency has been around 5 (between I and 10) strokes a minute. It will be understood that the traditional choices have been determined by the large masses involved, the great length of the rod string (which stretches under load) and by the asymmetric action of the device, which places high stress on the parts and causes significant wear on the bearings of the "nodding donkey" mechanism.

The traditional machine has many moving parts and it is required to operate for 24 hrs a day, 365 days a year for several years, so it wifi be understood that it needs regular inspection, lubrication, maintenance and repair.

Because the machines are placed in remote locations, the regular maintenance is expensive.

It will be further understood that pumping mechanisms in the past have been designed with regard to their mechanical function alone -that is to say, the process of their design has been entirely focussed on providing a simple method of raising and lowering a long pumping string within a shaft, without any consideration for the automatic and continuous monitoring of the pumping conditions at the base of the well. For example, the mechanisms of the prior art do not generally incorporate within themselves the ability to sense and to react appropriately to conditions such as a dry well, a broken rod string or a stuck valve. Such conditions could only be discovered or diagnosed as a result of routine inspection and maintenance -and before that discovery the untreated condition will have lost output and may have been the cause of damage to the pumping mechanism.

In recent years a variety of alternative systems have been devised, some using a hydraulic ram to raise and lower the polished rod directly. Other alternatives have proposed the use of linear electric motors acting vertically and directly connected to the polished rod where it emerges from the top of the well. None of those designs has been commercially successful. We now consider a number of proposals that pre-date our invention and which may be taken together as representative of the prior art.

US Patent 5,960,875 (1997) describes a mechanism by which the piston of an oil pump (placed deep within the borehole and close to the base of the well) is so constructed as to be combined with the cylindrical armature of a linear electric induction motor. It wifi be understood that, to be lowered into the bore of the well, the outer diameter of that armature has to be restricted to just a few inches. Since the continuous rated thrust of an electromagnetic actuator is proportional to the mass of the armature, a small diameter demands that the length shall be substantial -which is often impractical. US Patent 5,960,875 proposes that the combined piston and armature shall be driven vertically upwards and downwards by forces induced in the said armature by a surrounding set of cylindrical coils -and since they must also be lowered into the borehole, the said cylindrical powered coils must also have an outer diameter not significantly greater than that of the piston. That reduces the available thrust per unit length of the armature, making design yet more difficult.

Unfortunately, therefore, the design of electric linear motors for reciprocating pumpjacks proved to be beyond the known technology of that time. The weight of oil that must be raised by every upward stroke of the pump is a few tonnes, so that the force that must be produced by the small diameter electromagnetic piston has to be several tens of thousands of Newtons. But no such small-diameter linear motor of reasonable length, efficiency and cost could previously have been constructed to meet the requirements of the artificial lift mechanism and to satisfy the safety requirements of the oil business. The concept of using a linear electric actuator of conventional form was therefore impractical of commercial realisation.

US Patent 5,196,770 (1989) also describes a cylindrical linear electric actuator. In this case the actuator is placed at the head of a well and drives the heavy rod string to move the submerged pump. The proposed linear electric motor is much larger, heavier, more complex and more expensive than that proposed in US 5,960,875. It is described as being of inductive design, or in the alternative as being of synchronous, asynchronous or variable reluctance design. Unfortunately, all of those machine types are known to be inefficient at the velocities and reciprocation rates that are typical of jack pumping operations. It is also costly to make such linear electric motors to a standard that would allow them to pass the safety regulations for electrical power devices in a flammable gas environment.

There is the further disadvantage that, to produce a strong magnetic field and an adequate electromagnetic force, both the moving part or armature and the fixed part or stator must be continuously supplied with many kilowatts of electrical power. Thus there are cooling difficulties that result from the motor inefficiencies and the need to conduct heat away from a slowly moving body. Other practical problems relate to the incessant flexing of the power cable to the moving part. No reliable electric linear actuator can therefore be constructed to meet the demands of the invention whilst having a reasonable price, size and weight.

Canadian Patent CA 2,250,739 (1997) describes a machine in which the prime mover is a simple linear motor that is conceived to have a short rectangular armature that runs between two rectangular stators. It is very difficult -if not impossible -to design such a motor that is capable of producing the large forces required to drive the machine whilst remaining efficient at the slow speeds demanded by the application and meeting the statutory requirements for safety in an oil field environment.

Further, the said Patent Application teaches that the form of the preferred counterbalance is that of a very large steel spring, compressed between the base of the machine and the electrical armature, so as to support the armature against the deadload of the rod string.

It should be noted that the weight of the rod string may approach ten tonnes and that the force exerted by a spring is directly proportional to its compression or extension. Hooke's famous anagram of the Law of elastic stress and strain (which he used to hide his discovery from Newton) decodes as "ut tensio sic vis" A common requirement for such a counterbalance is that the force shall remain within ten percent of its set value whilst the armature travels through a stroke distance of plus or minus 1.25 metres (total displacement 2.5 metres or 100 inches). That means that the length of the spring when compressed to support the ten tonne rod string must be about 12.5 metres or 40 feet -and that its uncompressed length has to be about 50 feet. Such a massive spring would probably have an outer diameter of several metres. Although alternative counterbalancing systems using pneumatic or hydraulic cylinders are mentioned, they are not described and no related invention is claimed. A further alternative type of counterbalance is mentioned, being a mechanical counterweight connected via a cable and sheave, which at that time would had have the same disadvantages that we have described in relation to US Patent 5,196,770 earlier in this document.

The type of linear motor described in CA 2,250,739 would appear to be a switched variable-reluctance machine with individual parts of the stator being commutated under microprocessor control. The design of such a motor would be very complex and we know of no such motor that would be capable of meeting the exacting demands of a viable pumpjack design.

Further, although CA 2,250,739 teaches the advantages of an electric linear motor with respect to its ability to change its stroke and speed under remote control and in automatic response to local emergency conditions, it does not describe any method by which those conditions might be detected -and the ability of the machine to detect and to respond to emergency conditions is not therefore claimed.

The prior art also includes a number of inventions that are based on the use of a cylindrical linear electric motor that is direcdy connected to the rod string and whose deadload weight is counterbalanced by an auto-tuned gas spring. That is to say the actuator in each case is designed to be a dual-action linear electromagnetic ram. PCT/CA2005/001271, for example, suggests the use of such a machine to sense and respond to pumping conditions.

PCT/CA2007/001714 teaches the use of an adaptive gas spring to minimise power demand and PCT/CA2009/000194 describes the key features of particular type of dual-action linear electric machine that uses planar wireless technology in its design.

However, all artificial lift mechanisms that use linear electric motors have the following principal disadvantages: -A linear electric motor always costs more than its rotary equivalent. That is because the stator of a linear machine is always bigger than its armature -sometimes much bigger. So it costs money to build and operate parts of the linear motor that are never in continuous use. The same principle applies to the bearings, which have to be there for use at those times when they are needed; but that is never continuously.

The force produced by a linear motor is always less than its rotary equivalent. That is because the armature of a linear machine has to travel back and forth within the space of the stator -the armature only lies against a fractional part of the stator at any one time. In contrast, the armature of a rotarymachinealways fltscloselyinto theairgapofthestator-allofthe armature is always acting with all of the stator together, so as to produce the required force.

There are strong limits to the physical proportions of a simple artificial machine that relies on a simple linear electric motor at its core. For erample, if the machine stroke is increased from 2.5 m to 6m or lOm (which is sometimes desirable for modern conditions of oil extraction) the tolerances on the straightness of the machine axis become very difficult to maintain, especially in the morning and the evening, when different ends -and different sides -of the machine can quickly adopt different temperatures. To allow for this, and perhaps to accommodate more guide bearings, the proportions of the magnetic circuit have to be changed. There is an inevitable loss of magnetic flux density -and therefore of machine efficiency.

A]! such dual-action linear actuator machines using a gas spring and sliding seal have the further disadvantages that: -The entire operation of the machine is critically dependent on the satisfactory performance of the gas spring that is at the heart of the machine. Whilst the machine is relatively tolerant of imperfections in the linear electric motor, it demands a very high degree of perfection from the gas spring.

There are many desirable properties of a gas spring, amongst which the most attractive is perhaps its very low mass. However, the gas spring has to be contained by a sliding seal within a volume that is penetrated by at least one moving shaft. The travel distance of the sliding seal between service inspections or replacements has to exceed hundreds of kilometres -which is just a year's working life for an oil pump. To the best of our knowledge no sliding seal has ever been designed to meet such a tough specification and it may cost a great deal of money and many years of testing to develop one.

It should be noted especially that the gas spring seal friction must be reduced to a negligible value to prevent it from swamping the other forces experienced by the actuator, from which the down-hole parameters of the pumping system may otherwise be deduced. Such very small values of seal friction can be achieved in high precision hydraulic systems of our experience, but such seals are very expensive and quite unsuitable for use in

oil field pumping applications.

As the armature moves back and forth (actually up and down) during its normal operation, the pressurised gas that forms the gas spring has to get out of the way of the moving armature and then recover its original state when the armature has passed by. Since the armature must (for magnetic reasons) fit snugly into the tube formed by the stator -and because the cross-sectional area of the "piston" has to be large so that it can support the weight of the rod string at an acceptable pressure -the enforced gas flow takes place under difficult conditions twice every cycle. The pressurised gas has to be rapidly accelerated to squeeze past (and even through) the moving armature, which heats the gas and wastes a significant amount of energy.

Thus the most severe technical difficulties in the development of an artificial lift machine based on a linear electric actuator and a gas spring are associated with the gas seals, the internal paths for the gas flow and the bearing alignment -all of which are associated with the gas spring counterbalance and not with the electric linear actuator.

The principal objectives of this invention are to avoid the use of a dual-action linear electromagnetic ram and a self-tuning gas spring whilst retaining the following powerful advantages: - * Mechanical simplicity, with the minimum number of moving parts; * Inherent reliability resulting from the above; * Reduced physical volume and relatively light weight; * Complete self-containment giving protection from adverse environmental conditions; * In-line counterbalancing with no external forces; * Silence, safety and protection for humans and animals in close proximity; * Direct sensing of rod-string forces, allowing accurate calculation of down-hole conditions; * Automatic optimisation of pumping conditions whilst protecting the mechanism from damage "Elevator mechanisms" As mentioned in earlier paragraphs of this document, it is also known to construct artificial lift mechanisms based upon the principles of a conventional elevator -a passenger elevator for a tall building, for example.

In the simplest of terms, and without mention of the various redundancies and extra features that are commonly installed for personnel safety, the rod string and pump assembly (which in this simile are represented by the passenger cage of an elevator) is mounted in an enclosed shaft, at the top of which is mounted a drum over which is passed the cable or cables that support the load.

An appropriately-sized counterweight is affixed the other end of the cable that supports the load, so that the counterweight descends to balance the rise of the load and vice versa, by which means much of the energy in the moving parts of the machine may be conserved and recycled.

Several previous attempts have been made to produce artificial lift mechanisms of this "elevator mechanism" design, but they have experienced a number of problems; including those resulting in a dangerous instability of the supporting structure, motor unreliability and high surge current demands.

The chief cause of those problems has been that the weights and masses common to artificial lift mechanisms are generally an order of magnitude (ten times) as great as those common to passenger or good lifts in occupied buildings. The resulting peak electrical power demands and the forces required to accelerate and decelerate the loads -and the corresponding structural stresses -are therefore much greater. The electrical and mechanical forces within the mechanism are significantly reduced by our invention.

For example, our invention discloses means by which the peak forces required to accelerate and to decelerate the mechanism at each end of its working stroke are significantly reduced. Using the method of control described herein, both the mechanical and the electrical stresses required to operate the pumpjack are lessened; extending the working life of the machine and increasing its safety during regular inspection and maintenance visits.

It is therefore a further objective of this invention to eliminate the problems of the rotary elevator mechanism of the prior art, whilst retaining many of the most attractive features thereof.

It is another object of this invention to teach the design of a machine that is very flexible in its conformation, so that it can be made suitable for a range of different applications and can be adapted to a very wide range of markets with different pumping conditions such as thick and thin oils, short and long stroke pumps, hot and cold environments, etc. It is a further object of this invention that off-the-shelf components shall be used as much as possible, so as to permit a demonstration prototype machine to be built quickly and to minimise production costs.

It is therefore a further objective of this invention that the novel artificial lift mechanism, in combination with an electronic drive unit of conventional design and standard specification, shall be capable of sensing a variety of pumping conditions, of reporting them to a distant monitoring centre, of responding to remote commands and/or of reacting autonomously to several emergency conditions.

It is therefore a further objective of this invention that the novel artificial lift mechanism shall be compact, fully enclosed, relatively light in weight and lower in cost than mechanisms of the prior art, whilst meeting all statutory regulations for such equipment in oilfield conditions.

It is a further objective of this invention that the machine shall be so constructed that each standard marc or model can be so controlled as to suit a wide range of wells, thus reducing the number of different models that are necessary to satisfy the market, minimising stock inventory, manufacturing

cost and training for field work.

In this invention the machine may be conceived to be analogous to that of a conventional passenger elevator in non-stop reciprocating operation, comprised a high-torque rotary electric motor mounted on a platform at the top of the structure, the motor driving a drum or pulley that has an appropriate cable or belt connection to the load; there being also a counterweight of predetermined mass, moving within the same shaft in vertical opposition to that of the load. So as to reduce the horizontal forces on the structure, the primary load-carrying forces and the counterbalancing forces are designed to be closely aligned with the central axis of the drum and with the axis of the polished rod, for which reason the mass of the counterweight is symmetrically disposed around the same axis.

The traveffing pump chamber deep in the well is connected to the drive belt or cables via the rod string and a bridle, so that by the controlled reciprocating motion of the motor, the correct pumping action is produced.

In this invention the physical shape of the counterweight surrounds and is symmetrical about the axis of motion of the rod string. It may, for example, be cylindrical, constructed from a number of ring-shaped elements and the central axis of the counterweight (which passes through the centre of mass of the counterweight) is arranged to be closely coincident with the axis of motion of the polished rod that supports the rod string.

So that the forces (and the effects of their inequalities) remain closely balanced at all times, there are two or another even number of cables or belts linking the rod string bridle and its cable connections to the surrounding mass of the physical counterweight. This pair (or these pairs) of cables or belts are wrapped around the same winch drum or sheave.

The connection of the said cables to the polished rod may be by means of a bridle that allows the frtment of a rod rotator independently of the drive mechanism. The rod rotator itself need not be of any special kind, is not part of this invention and is not shown in any of the diagrams The whole assembly is supported by an inner sub-frame or tube that supports the axially-aligned loads of the pumping system and its counter weight. The inner frame is designed so that the total load of the rod string and the counterweight does not exceed the Rayleigh bending criterion and the central positioning of the vertical force prevents any instability of the sub-frame, which is contained within in a single vertical housing of (e.g.) fibreglass that does not carry any vertical load but which protects the machine from all weathers and also prevents accidental contact with the dangerous moving parts of the mechanism by animals or by untrained and unauthorised persons. It will be understood that the outer protective casing is not placed in position until after the counterweight segments have been adjusted to the correct value and the satisfactory operation of the machine has been tested and carefully inspected.

There is also fitted a small a top cover made from fibreglass or other tough weather-resistant plastic that protects the mechanism from contaminants such as rain, snow, sand, ferrous dust and concentrations of explosive or corrosive gases. The lightweight top cover and certain panels of the cylindrical casing may be removed completely for access and inspection at any time.

Whilst the invention is not inherently dependent on the use of high-torque (gearless) planar wireless motor technology for the construction of the driver motor, the use of gearless planar wireless motor construction it likely to be a desirable improvement that wifi substantially reduce the mass of the motor, which must be mounted (and regularly inspected) at the top of a structure that may be more than 10 metres high.

It will be noted that the centre of the drive motor assembly has to lie over the axis of the hollow mounting pillar in which the mechanism is contained It is a feature of the machine that the vertical position of the down-hole pump shall be accurately known by means of a position transducer. For example a rotary transducer may be affixed the armature of the drive motor so that it may measure accurately the number of revolutions of the pulley or drum over which the drive belt or cables are passed. Because the relative position of the armature and stator is accurately known, the drive motor may be of the type known as a brusNess DC or multiphase servomotor and use an electronic drive unit so as to allow efficient and precise control of the pump motion at all times.

The brushless DC servomotor also has the useful property of sensing accurately and at all times the forces (torques) acting upon it. It may be obvious that the recorded data produced by such a DC servomotor can easily be processed off-line so as to deduce the down-hole pumping conditions by the solution of several simultaneous linear equations over the complete pumping cycle.

A further useful property of the servomotor is that the current demands over a complete pumping cycle (or an exact number of such cycles) can be processed to show the optimum value of the counterweight for any chosen pumping condition. That is to say, it is possible to provide an on-site indication of the correctness of the counterbalance weight then in use.

Our new invention has the following innovative and commercially- useful features: - 1. Uses a standard rotary motor -there is no need for a linear machine of any kind to be specially developed for the pumpjack 2. Does not require a gas spring counterbalance to be developed -and so does not need a traveffing gas seal to be developed either 3. Has one highly-developed low-cost rotary bearing of a type that is in common use world-wide and is easily capable of supporting a 20 tonnes load with low friction 4. The drive assembly is built and tested as a single module -which is centrally positioned at the top of the tower 5. The rod string loads and the counterweight forces are coaxial and thus allow the pumpjack to be positioned directly above the well head 6. The simple mechanism within the strong support tower is silent, unseen and inaccessible to children, unauthorised adults or graying animals 7. A fibreglass outer casing protects against all weather conditions and so reduces the need for inspection and maintenance whilst prolonging the life of the moving parts 8. The first prototypes (and perhaps even the production machines) do not need to use a planar wireless motor -so there is no need to design and develop a special drive unit before the rest of the machine can be designed and built around it 9. In external shape the mechanism looks similar to an Inteffimax and provides a simple, compact and lightweight low-footprint drive mechanism 10. It uses an artificial lift mechanism of a type similar to one that is already in common use world-wide. There is very little technical risk 11. Nevertheless, it has a number of characteristic features by which it may be clearly differentiated from all known machines of the prior art 12. The parts do not need to be finely -toleranced and a demonstration prototype may be quickly constructed at low cost by using off-the-shelf components 13. There do not appear to be any strong upper or lower limits to the flexibility of the design in respect of stroke length, operating speed or well depth.

In order that the invention may be more readily understood, we provide the accompanying drawings by way of an example of the embodiment of the principal elements of the machine: -Figure 1 illustrates the basic principles of the prior art using a counterbalance weight and it is not intended to represent any existing artificial lift mechanism. The numbers used to refer to the various components of the machine are used for the equivalent components in other drawings of this Patent Application.

Figure 2 is a diagrammatic side view of a structure that illustrates the principles of our invention, showing especially that the centre of mass of the hollow counterbalance weight is coaxial (or closely coaxial) with the axis of motion of the rod string.

Figure 3 is a diagrammatic front view of a structure that illustrates the principles of our invention, showing especially that the cables or belts that support the rod string pass through the centre of mass of the hollow counterweight. The diagram also shows the monolithic (or otherwise rigid) structure of the motor drive, guide roller and drum assembly.

Figure 4A shows the waveform of a typical pumping mechanism of the prior art and Figure 4B shows the corresponding force demand at the surface of the drive drum Figure 5A shows the preferred sinusoidal drive waveform for our invention Figure 5B shows the corresponding force demand at the surface of the drive drum for the same load as Figure 4B. It should be noted that the peak force values (and therefore the values of electric current demand) are very much less than those of the equivalent prior art. It should also be noted that, whilst it is highly recommended, it is not mandatory that a sinusoidal drive waveform be used in our invention.

In more detailed description of our invention we refer again to the following diagrams: -Figure 1 (showing the prior art) is particularly clear in showing the mechanical disadvantages of earlier designs. It will be recalled from earlier paragraphs of this document that the force on the polished rod -that is, the tension in the supporting belt or cable 4 -varies violendy twice in every pumping cycle. When descending, the effective weight hanging from the belt or cable 4 might be 9 tonnes but when rising it may be 12 tonnes and vice-versa; changing back and forth in only a fraction of a second.

Meanwhile, the counterweight value might be set to the arithmetic median value of 10.5 tonnes. It is reasonable to assume that the effective diameter of the drum or pulley 5 (perhaps extended by rollers 11 and 12, which carry the supporting cables or belts well clear of the supporting structure 9) is

about two metres in a machine of the prior art..

Using those typical values, there is a net anticlockwise torque of 1,500 Newton metres bending the tower to the left on the down stroke and a clockwise torque of 1,500 Newton metres bending it to the right on the up stroke. The forces change direction very quickly and the resulting sudden change in the position of the tower as it sways back and forth is known to alarm those persons who may then be at the top of the tower. What is more, very much larger forces are necessary to accelerate and to decelerate the masses of 22.5 tonnes upwards and of 19.5 tonnes downwards, which forces produce yet larger and more alarming movements of the tower.

The first aspect of our invention is therefore the removal of all overturning moments and the provision of a safe and stable working platform at the top of the tower 9.

Figure 2 shows that the polished rod 1, supported by the cable or belt 4 via the bridle piece 3, is raised and lowered through the well-head seal 2 by rotation and contra-rotation of the pulley or belt drum 5, which is driven by the rotary motor 17. The rotary motor may have any convenient form but it is preferably of the type known as a brushless three-phase permanent magnet machine, driven by a standard off-the-shelf electronic drive unit.

The motor is fitted with a rotary shaft position encoder (not shown), by which the position of the load and the counterweight 8 can be readily deduced and by which the precise angular relationship between the motor armature and its stator is also known. That feature allows the motor to be most efficiently controlled -and the reciprocation of the pump rod to be made to follow a smooth sinusoidal motion by which the mechanical stresses on the mechanism are minimised. The motor assembly may incorporate a gearing unit (not shown) so as to increase the torque applied to the drum 5 by the motor 17.

It is to be noted that the counterweight 8 is hollow. In particular, the central cavity is so arranged that the polished rod support cable(s) or belt(s) 4 (from which the polished rod 1 is hung) pass vertically through its central axis and the belts 4 are guided to that position by rollers or pulleys 12. In a similar manner the cable(s) or belt(s) 20 that support the counterweight 8 are guided to lie close to but not touching the cables 4 by means of rollers or pulleys 11.

It will be understood, therefore, that there is no differential torque resulting from the changes in tension in the belt or cable 4 as the force on the polished rod 1 alters rapidly at each extreme of the pumping stroke. Neither is there any side load on the guide bearings 9 by which the counterweight 8 is lightly constrained to move in the vertical axis. There is no vertical friction in the mechanism except that due to the roller bearings 11, 12, 6 and 17 -and for that reason it is possible to use the known current demands of the motor 17 as valid parameters in a set of simultaneous linear equations relating to the full motion of the rod string. Solutions of those equations yield useful knowledge concerning the pumping conditions of the well, including the weight of the oil column, the correct operation of the valves (or otherwise) the rate of oil flow towards the pump, and so on.

The gross vertical loading of the drive shaft (which is usually in the order of tonnes) is designed to be carried mainly by the centre bearing 6, which may perhaps be chosen from a range of well-proven long-life bearings originally designed for use in rail transport systems. That bearing, the motor(s) and their end bearings 17 and the transverse guide rollers or pulleys 11 and 12 are mounted as a rigid assembly and contain all the forces produced by the motor(s) 17 and the drum or pulley 5. The net physical effect of the machine operation is therefore reduced to a tendency for the complete assembly (17, 5, 11, 12) to rotate about an axis close to that of the top mounting plate 21. The vertical force is carried to the base plate 10 via the structure 9, which does not need to carry any side forces.

Full protection against the effects of all wind and weather conditions is provided by the external casing 19 and the top cap 18, which may consist of fibreglass, for example.

Figure 3 shows a diagrammatic side view of the same structure, in which it can be seen that at least two belts or sets of cables 20 pass over the drive drum or pulley 5 to support the counterweight 8. It will also be understood that the other ends of those belts of cables 4 pass freely through the central cavity of the counterweight 8 and are connected to the polished rod 1 via the bridle piece 3 The pulleys or rollers 11 act to bring the belts or cables 20 into the central axis region of the counterweight 8 and the corresponding rollers or pulleys 12 (not shown in this figure) act to guide the belts or cables 4 through the central cavity of the counterweight 8 so as to connect with the bridle 3 and the polished rod 1.

By the use of our invention, therefore, we have simplified the design, minimised the mechanical stresses and improved the sensitivity of an artificial lift mechanism that is designed to sense "down-hole" conditions by monitoring the current of an electric drive motor. Whilst some motors may be lighter in weight or lower in cost than others, and may therefore be preferred in a particular application, it is to be noted that the choice of motor is not a critical feature of our invention. It is, however, important that the motor is constructed as a permanent magnet servomotor, so that it may be controlled by a precision drive unit that produces a stream of measurements of the drive current, instant by instant throughout the pumping cycle. Such measurements can then be used to calculate a number of important parameters concerning the safe and efficient operation of the well itself.

We now describe other features of our invention by which the performance of the artificial lift mechanism may be further improved by choice of motor control means and by selection of the actual waveform of the pumping cycle.

Figure 4A shows a typical pumping waveform that is in common use at this time. The motor is switched on at the low point of the pump travel and the polished rod (and therefore the oil column which surrounds it is raised at a constant velocity of (say) one metre per second, so that a well having a stroke of ten metres will take ten seconds for the bridle to reach the uppermost extremity of it motion and to discharge the oil from the top of the column. When the top of the travel is reached a relay or contactor is tripped, so that the direction of motion is immediately reversed and the travelling valve opens. The rod string is brought to a halt and then accelerated downwards to lower the rod string through the oil column to the bottom of the well at a similar constant speed. In this case we have assumed for convenience of illustration that the downwards velocity is the same as the upwards velocity. Whether or not that is actually the case, the waveform is termed as "triangular" and is especially simple to contrive.

Unfortunately, the triangular waveform places heavy demands on the motor and on the electrical system to which the motor is connected, often resulting in considerable damage to the motor and increased costs of the electrical supply. That is because the mechanism has to deal with a large inertial load; the mass of the rod string, oil column and counterweight together often exceeds twenty tonnes. When the pump reaches the lower end of its stroke the twenty tonnes of mechanism is traveffing at a metre a second -sometimes more -and it has to be brought to a halt as quickly as possible by driving the motor in reverse. The massive load must then be accelerated in the opposite direction -and the pump traveffing valve closes at that same instant, adding a few more tonnes to the mass that the motor has to handle. Similar problems occur at the top of the pump stroke, of course.

Figure 4B shows how such crude motor control results in very high peak current demands twice every cycle at the extremes of the pumping motion.

The resulting stresses within the motor can cause displacement of the windings and even demagnetisation of the permanent magnets.

Figure 5A shows a sinusoidal pumping waveform. The use of this waveform is highly recommended because it minimises the mechanical stresses in the structure, whilst increasing the efficiency of the machine.

The sinusoidal waveform is the smoothest reciprocating waveform that can be produced; it chief feature being that there are only two points in the cycle in which there is no acceleration -which both occur at the times when the velocity of the rod string reaches its maximum and begins to decelerate and reverse direction thereafter.

As a result all associated accelerations blend smoothly one into another and the resulting inertial forces on the structure (and on the drive motor) are minimised. The waveform is also well known for its simple relationships between the instantaneous position of the load, the velocity of the load (and any viscous friction), and the acceleration of the load (plus any resulting inertial forces). It will be understood that all sudden peak accelerations are eliminated by the use of this pumping profile.

Figure 5B shows the motor torque demands that result from the use of this waveform in a machine that is otherwise identical to that whose behaviour is illustrated in Figure 4A. It should also be noted that the peak torque demands shown in Figure 4B are four times as great as those arising in Figure 5B; which clearly demonstrate the advantages of the sinusoidal waveform.

Figure 5B also shows how the motor demand current varies over a complete cycle and from which a set of linear differential equations may be chosen so that, for example, the correct counterbalance weight may be calculated and displayed at any time.

Claims (10)

1. Claims 1. A mechanism suitable for driving a reciprocating pump or "artificial lift apparatus", the pump being connected to the surface equipment by means of a rod string; the drive mechanism being placed directly over the well-head and connected to the pump rod string by at least one pair of cables or belts that are wrapped or partially-wrapped around a pulley or drum, the pulley or drum being driven by a reversible rotary electric motor; the other end of the drive belts or cables supporting at least one counterbalance weight that is symmetrical about the axis of the machine and has at least one continuous hollow central space, the said counterweight being driven in the opposite direction to the rod string by the action of the drive drum, characterised in that the lines of action of the load and the counterbalance forces are arranged to be closely coaxial with the centre line of the mechanism and to pass through the central region of the counterweight.
2. 2. A mechanism constructed in accordance with Claim 1, the position of the polished rod being known precisely (by the drive motor having a shaft position encoder, for example) and being controlled by an electronic drive unit in such a way that the instantaneous current demand is an accurate measure of the difference between the load and counterbalance forces.
3. 3. A mechanism constructed in accordance with Claim 1, in which the bearing friction and the resistance to flexing of the drive belts or cables are minimised, so that the peripheral force or motor torque demand is not dominated by that friction
4. 4. A mechanism constructed in accordance with any previous claim in which the electric motor is a permanent-magnet brushless DC or multiphase servomotor and is driven by an electronic drive unit that is capable of providing continuous measurements of the current and voltage demands of the machine
5. 5. A mechanism constructed in accordance with any previous claim, in which the measured values of the instantaneous current and voltage demands of the rotary electric motor are also processed to calculate the machine efficiency at any point in the pumping cycle, to extract the parameters of the pumping operation, to detect faults and anomalies, to initiate appropriate automatic responses and/or to prepare information for transmission to a remote site.
6. 6. A mechanism constructed in accordance with any previous claim, in which the measured values of the instantaneous current and voltage demands of the motor are used to calculate and display the optimum value of the counterweight at any time or its difference from the value then in use.
7. 7. A mechanism constructed in accordance with any previous Claim, in which the power drive assembly, including the motor and drive drum or cable sheave and its associated guide drums or pulleys, is built and tested as a separate module.
8. 8. A mechanism constructed in accordance with any previous claim in which the whole of the mechanism, including the moving counterweight, is encased in a tough weatherproof housing such as fibreglass, having a removable top for inspection and servicing of the motor and drive drum and having side access covers for similar purposes
9. 9. A mechanism constructed in accordance with any previous claim, in which the drive mechanism is so powered by the electronic drive unit as to cause the waveform to be sinusoidal or a close approximation thereto, so as to further minimise the mechanical and electrical stresses on the mechanism.
10. 10. A mechanism constructed in accordance with any previous claim in which the electric motor uses planar wireless motors technology as described in our co-pending Applications such as PCT/GB2007/003482, PCT/GB2009/000201/, PCT/2009/000199, PCT/001815 and their corresponding National Applications.