Field of the invention

The invention relates to a pumping device, in particular to a sucker rod pump, a pumping arrangement, a method for pumping fluid, and a use of a pumping device.

Background of the invention

Sucker rod pumps are used throughout the entire industry and are adapted for pumping various kinds of liquids. These pumps are very popular in oil well production also and cover more than 80 % of oil wells on artificial lift worldwide. There are three basic types of constructions like insert pumps, tubing pumps and casing pumps.

However, the pump principle is similar for the three different pump types. In such pump devices, in particular in sucker rod pumps, a pump is actuated in a defined depth by the sucker rod string which is hence reciprocated by a beam pumping unit at the surface. The pump is basically composed of a pump plunger that is movably supported in the bore hole and that comprise a so called travelling valve. At the bottom of the bore hole a pump barrel is installed that includes a standing valve.

During an upstroke of the pump, the plunger is lifted together with the entire column of fluid (such as liquid) that is gathered in the bore hole above the plunger. The travelling valve at the pump plunger is therefore closed and hinders the liquid column to a flow back at the bottom of the bore hole. At the same time the standing valve of the pump barrel is open and allows fluid to fill the pump.

During a downstroke the static standing valve of the pump is closed while at the same time the travelling valve opens in order to make all the fluid in the pump flowing through the plunger above into a tubing string .

WO 2009/146713 discloses a pumping device for pumping fluids. The pumping device comprises a force transmitting element, a tension unit coupled to the force transmitting element and a seal. The force transmitting element is adapted for transmitting an upstroke force and a downstroke force to a pump plunger for pumping fluid . The tension unit is adapted for applying a tension

AD:mp force to the force transmitting element for keeping the force transmitting element under tension during the upstroke and the downstroke. The seal is adapted for sealingly preventing pumping fluids during the downstroke and for enabling pumping fluid during the upstroke. A part of the seal is coupled with the force transmitting element.

Although the pump according to WO 2009/146713 is very powerful, there is further room for improving the efficiency of pumping devices.

Object and summary of the invention

It is an object of the invention to provide a pumping device having a high efficiency in pumping fluid .

In order to achieve the object defined above, a pump device, a pumping arrangement, a method for pumping fluid, and a use of the device for pumping fluid according to the independent claims are provided.

According to an exemplary embodiment of the invention, a pumping device for pumping fluid is provided, wherein the pumping device comprises a force transmitting element, a pump plunger, and a seal, wherein the force transmitting element is adapted for transmitting an upstroke force and a downstroke force to the pump plunger for pumping fluid, wherein the seal is adapted for sealingly preventing pumping fluid during the downstroke and for enabling pumping fluid during the upstroke, wherein a movable part of the seal is coupled with the force transmitting element, and wherein the force transmitting element has a free unbiased end extending through and beyond a static part of the seal even when the pump plunger is located at an upper reversal position.

According to another exemplary embodiment, a pumping arrangement is provided which comprises a ground with a bore hole, the ground comprising fluid to be pumped, and a pumping device having the above-mentioned features for pumping the fluid out of the ground . In particular, the pumping device may be positioned in the bore hole so that, at the beginning of the pumping, a fluid level in the bore hole circumferentially surrounding the pump plunger is equal or substantially equal to (for instance slightly above) an upper reversal position of the pump plunger, in particular an upper reversal position of a travelling valve of the pump plunger.

According to a further exemplary embodiment, a method for pumping fluid by a pumping device (in particular by a pumping device having the above- mentioned features) is provided, wherein the method comprises transmitting an upstroke force and a downstroke force by a force transmitting element to a pump plunger for pumping fluid, sealingly preventing pumping fluid during the downstroke and enabling pumping fluid during the upstroke by a seal, and coupling a movable part of the seal with the force transmitting element.

Optionally, the pumping device may be positioned in a bore hole so that, at the beginning of the pumping, a fluid level in the bore hole circumferentially surrounding the pump plunger is equal or substantially equal to an upper reversal position of the pump plunger, in particular an upper reversal position of a travelling valve of the pump plunger.

According to a further exemplary embodiment, an above-described pumping device is used for drainage purposes, oil production, water catchment, dewatering in particular of gas wells, or geothermic systems.

In the context of the present application, the term "force transmitting element has a free unbiased end extending through and beyond a static part of the seal even when the pump plunger is located at an upper reversal position" may particularly denote that no biasing force (for instance caused by a spring mechanism) acts on the pump plunger, nor any separate member (such as a compensation weight) is attached to the free end of the force transmitting element. In particular, such a force transmitting element may be a structure (such as a rod) which simply ends at a position lower than the static part of the seal during any operation state of the pumping device, i.e. when the pump plunger is at an upper reversal point but also when the pump plunger is at a lower reversal point or in between the upper reversal point and the lower reversal point.

According to an exemplary embodiment, a pumping device is provided which experiences only very small changes of dynamic load during a working cycle due to the strong suppression or even cancellation of differences of the buoyancy force acting on the pumping device during an upstroke (i.e. an upwards motion) and a downstroke (i.e. a downward motion). Therefore, substantially only static forces (buoyancy forces are such static forces, however gravitational forces, etc. may be present as well) act on the pumping device which can be easily compensated by certain measures, for instance

compensation weights. However, the generation of dynamic forces can be essentially decreased, thereby increasing the lifetime of components of the pumping device so that the efforts for repairing and maintenance are significantly reduced. In addition, the energy efficiency of the pumping device can be significantly improved. While in conventional approaches, the buoyancy effect is remarkably different during an upstroke and a downstroke (during an upstroke, buoyancy force is caused by the dynamic fluid level in the annulus, while during the downstroke buoyancy force is caused by fluid in the tubing string), these differences of the buoyancy force during upstroke and downstroke are

significantly suppressed or even fully eliminated according to exemplary embodiments. Since for oil production and other applications with sucker rod pumps, pump exchange and repair and the corresponding time delay of production are the main cost factors, the increase of the lifetime of the pumping device achievable with the load equilibration or load reduction of the

embodiments of the present invention significantly increases the efficiency of the oil production or other application. This also holds for the production of other fluids such as water or gas. Also emulsions and fluids with solid particles may be pumped by the pumping device. Exemplary embodiments of the invention also have the advantage that the inventive concept can be easily retrofitted to a conventional pump by removing components such as tension weight and/or biasing spring from a lower end of the force transmitting element or by

exchanging a lower sealing arrangement and plunger.

When, as accomplished by an exemplary embodiment of the invention, the bottom end of the force transmitting element (such as a force transmitting rod) remains located below the static part of the seal in each operation state of the pumping device (in particular also at an upper reversal position of the pump plunger), the buoyancy force and corresponding pressure exerted onto the bottom of the force transmitting element by the surrounding level of the fluid remains always the same (or at least substantially the same) in each of the different operation states of the pumping device, whereby dynamic changes of the mechanical load exerted onto the force transmitting element are prevented or at least reduced.

In the following, further exemplary embodiments of the pumping device, the pumping arrangement, the method and the use will be described .

In an embodiment, the pumping device has no tension unit adapted for applying a tension force to the force transmitting element for keeping the force transmitting element under tension during the upstroke and the downstroke. In contrast to this, the force transmitting element may remain free of externally exerted tension forces. Thus, in the described embodiment, the pumping device has no such tension unit, neither embodied as tension mass nor embodied as spring unit.

In an embodiment, the pumping device comprises a control unit adapted for controlling the pumping of fluid from a bore hole so that, at the beginning of the pumping, a fluid level in the bore hole circumferentially surrounding the pump plunger is substantially equal to an upper reversal position of the pump plunger, in particular an upper reversal position of a travelling valve of the pump plunger. When this condition is fulfilled, the pressure exerted by the fluid column is minimum (and also disturbing effects of buoyancy force are minimum) so that the pumping device may operate substantially free of losses, which results in a long lifetime of the pumping components and low effort for repair and

maintenance of the pumping device. In the context of the present application, the term "substantially equal" may particularly denote that the fluid level is either exactly equal to or slightly higher (in particular not more than 100 m higher, more particularly not more than 50 m higher) than the upper reversal position. Even when the fluid level is slightly higher than the upper reversal position, the impact of the fluid pressure on the components of the pumping device are still acceptably low.

In an embodiment, the force transmitting element is configured to, when the pump plunger is located at the upper reversal position, extend through the static part of the seal by a length in a range between 5 cm and 2 m, in particular in a range between 10 cm and 1 m. Thus, the extension of the force transmitting element (in particular a rod) through the static seal at the lowest position is relatively low but never zero, and the given values have turned out to provide proper results in terms of wear reduction.

In an embodiment, the force transmitting element is configured to extend, when the pump plunger is located at the upper reversal position, with a part or portion through the static part of the seal which part or portion has a weight in a range between 100 g and 10 kg, in particular in a range between 500 g and 3 kg . These relatively small weights resulting from the own weight of the portion of the force transmitting element extending beyond the seal in the upper reversal point operation state are significantly lower than compensation weights or a compensating biasing force applied to force transmitting elements of conventional pumping devices. This reduces the load acting on the pumping device and its components.

In an embodiment, a stroke length of the pump plunger is in a range between 0,5 m and 10 m, in particular in a range between 2 m and 4 m. In particular, the stroke length may be larger than the extension of the force transmitting element beyond the static part of the seal at the upper reversal position.

In an embodiment, a length of the force transmitting element is in a range between 20 m and 4000 m, in particular in a range between 500 m and 2000 m. Hence, the entire length of the tubing may be significantly larger than the extension of the pumping rod beyond the static part of the seal at the upper reversal point.

In an embodiment, the pumping device, in particular the force

transmission element of the pumping device, is adapted so that a buoyancy force (in particular dominated by surrounding fluid) acting on the pumping device is the same during the upstroke and during the downstroke. By configuring the pump so that the buoyancy force of the dynamic fluid level during the upstroke and the buoyancy force of pump components during the downstroke are rendered substantially identical, the dynamical load acting on the pumping device is reduced which protects the pump components from wear and damage. Thus, the pump design may be configured particularly such that the values of the buoyancy forces in the upstroke and in the downstroke conditions are the same.

In an embodiment, the pumping device comprises a compensation unit, in particular a compensation weight or a hydraulic load unit, configured for at least partially compensating static loads acting on the pumping device during operation. While dynamic loads cannot be compensated or can at least not be easily compensated, compensation of static forces acting on the pumping device are easily compensable by measures such as the provision of a compensation weight providing a counterforce to the static load, or a hydraulic drive being operated to compensate the static load.

In an embodiment, the pumping device is, in particular the force

transmission element in combination with the compensation unit are, adapted so that the buoyancy force acting on the pumping device is extremely small and very close to zero (in particular substantially zero, preferably less than 100 N) during the upstroke and is extremely small and very close to zero (in particular substantially zero, preferably less than 100 N) during the downstroke. While in conventional approaches, the buoyancy force is usually larger during the upstroke as compared to the downstroke due to the effects of the dynamic fluid level, exemplary embodiments of the invention provide the possibility to substantially lower or even eliminate the buoyancy force even during the upstroke. This has a significantly positive impact on the reduction of the wear acting on the pumping device during operation.

In an embodiment, the force transmitting element is adapted for moving along a vertical direction or slanted with regard to a vertical direction during the upstroke and the downstroke. In one embodiment, the motion of the pump plunger together with the force transmitting element may be parallel to the force of gravity or perpendicular to the ground plane. However, due to the reduced load acting on the components of the pumping device thanks to the configuration of the free unbiased end of the pump plunger, the range of inclinations (i.e.

angular deviations from a perpendicular orientation) can be significantly increased as compared to conventional approaches. Therefore, the range of possible applications of the pumping device is extended, since it allows to comply with different angular requirements. In view of the lower mechanical loads acting on the components of the pumping device, and in view of the omission of any tension weights and/or tension spring at the free unbiased end of the force transmitting element, an increased inclination becomes possible, since no undesired interaction of such compensation weights or the like with the wall of a borehole occur. For instance, inclinations of up to 45° to 70°, or (at least section- wise, for instance at an end portion) even up to 90°, are possible in

embodiments of the invention.

In an embodiment, the pumping device comprises a tubular container within which the pump plunger and part of the force transmission element are located and being filled at least partially with the fluid to be pumped during operation of the pumping device. By allowing the force transmitting element together with the pump plunger to reciprocate in an interior of the tubular container located in a borehole, the fluid level within the container and outside of the container may become different. In particular, an interior of the container may have a higher fluid level as a result of the pumping performance.

In an embodiment, the pumping device comprises a pump barrel with a duct, wherein the duct is adapted for connecting a fluid reservoir with the pump barrel, wherein the movable part of the seal is adapted for opening the duct during the upstroke and for sealing the duct during the downstroke, and wherein the pump barrel is adapted for being filled with the fluid during the upstroke and for being discharged during the downstroke. Hence, the duct may cooperate with the movable part of the seal so as to be closed during at least part of the downstroke and opened during at least part of the upstroke.

In an embodiment, the seal is adapted for guiding the reciprocating force transmitting element during its motion. In other words, a duct of the seal is also capable to function as a guiding element for guiding the force transmitting element (such as a rod) to reciprocate along a direction defined by the duct of the seal . Therefore, loads acting on the pumping device and resulting from a misalignment of the force transmitting element can be suppressed . By guiding the force transmitting element through the static part of the seal (which may also be denoted as a seal valve) even at the upper reversal point of the pump plunger, only the external fluid in the borehole applies pressure to the pumping device, wherein an increased pressure difference between an interior and an exterior of the pumping device provides energy and a force which reduces the risk of buckling .

In an embodiment, the movable part of the seal comprises one of the group consisting a cylinder, a ball (or a sphere), and a cone. While the exact geometry of the movable part of the seal does not matter, it should be

configured so as to provide a form closure with the duct of the static part of the seal in the closed state of the seal so as to provide for a fluid-tight sealing when the pump plunger moves downwardly. The geometry of the duct of the seal and of the movable part of the seal may therefore be adapted to one another to provide for such a sealing effect. Particularly during the downstroke, the movable part of the seal may move relatively to the force transmitting element so as to provide a continuous sealing effect. The force transmitting element then moves relatively to the static part of the seal while the movable part of the seal rests sealingly on the duct. In contrast to this, during an upstroke, the movable part of the seal may move together with the force transmitting element and relative to the duct so that fluid may pass through the duct. The movable part of the seal may be a recessed sphere, a recessed cylinder, a recessed cone, etc., wherein the (for instance cylindrical) force transmitting element is guidable through the (for instance hollow cylindrical) recess. The sealing effect of the movable part of the seal results from its mass and the pressure of the pumped fluid both providing for a sealing during the downstroke of the pumping device.

In an embodiment, the fluid (which, in the context of this application, may denote a liquid and/or a gas, optionally comprising solid particles) is selected from one of the group consisting of crude oil, gas and water. Hence, the pumping device may be capable of pumping any desired fluids, such as liquids or liquid solid mixtures, for instance crude oil or ground water.

In an embodiment, the force transmitting element is one of the group consisting of a rod string, in particular one of a hollow rod string, a full material rod string, and a combination of a hollow rod string and a full material rod string, a pull rope, and a combination of a rod string and a pull rope. Hence, in one embodiment, the force transmitting element is a rod string having a hollow interior so as to provide a relatively low weight while being simultaneously capable to apply a downwardly (and upwardly) oriented force (in view of hydrostatic forces and friction forces) during the downstroke. In an alternative embodiment, the rod string may be made of a full solid material if a particularly high stability and robustness is desired. It is also possible to assemble the rod string from a first portion being made of a hollow rod and a second portion being made of a solid rod . This may have particular advantages in terms of load reduction of the pumping device. Also a pull rope may be used as force

transmitting element since it is capable of applying a tension force to the pump plunger during the upstroke. It is also possible that an upper portion of the force transmitting element is realized as a pull rope while the lower portion is realized as a (for instance hollow or solid) rod string. Since the liquid in the hollow body in which the force transmitting element is located (such as a tubing or a pipe) provides a buckling preventing force, it is possible that the force transmitting element (for instance made of a material such as a metal like steel or plastic) is at least in a lower portion thereof embodied as a hollow tube or even a rope.

In an embodiment, the pumping device is configured as a sucker rod pump. However, other pump types are possible as well.

In an embodiment, pumping the fluid is terminated when a level of the fluid to be pumped reaches the lower reversal position of the pump plunger.

As a drive unit for driving the force transmitting element and the pump plunger, an electric motor, a combustion engine, or a hydraulic drive may be used, for instance.

Preferably, the force transmitting element has a constant diameter and/or shape around its external surface over the entire extension thereof in the borehole. This allows proper cooperation with the movable part of the seal.

Brief description of the drawings

The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited :

Figure 1 to Figure 4 illustrate schematic views of a pumping device according to an exemplary embodiment of the present invention in different operation modes.

Figure 5 to Figure 8 illustrate schematic views of a pumping device according to another exemplary embodiment of the present invention in different operation modes.

Figure 9 illustrates a part, which is located above ground level, of a pumping device according to an exemplary embodiment of the present invention.

Figure 10 is a diagram showing a dependency between the time and a polished rod load of a pumping device according to an exemplary embodiment compared to a conventional pumping device.

Figure 11 is a diagram showing a dependency between a displacement and a load of a pumping device according to an exemplary embodiment compared to a conventional pumping device.

Figure 12 schematically compares a conventional pumping device with a pumping device according to an exemplary embodiment.

Figure 13 is a diagram which corresponds to Figure 11 and relates to conditions of a pumping device according to an exemplary embodiment compared to a conventional pumping device in which the fluid level in the bore hole circumferentially surrounding the pump plunger is substantially equal to an upper reversal position of the pump plunger.

Figure 14 is a diagram which corresponds to Figure 10 and relates to conditions of a pumping device according to an exemplary embodiment compared to a conventional pumping device in which the fluid level in the bore hole circumferentially surrounding the pump plunger is substantially equal to an upper reversal position of the pump plunger. Description of the embodiments

The illustrations in the drawings are schematically. In different drawings similar or identical elements are provided with the same reference signs.

Before exemplary embodiments of the invention will be described in more detail referring to the figures, some basic consideration underlying the present invention will be described based on which exemplary embodiments of the invention have been developed.

Sucker rod pumps are some kind of artificial lift systems that are used in the oil industry for producing crude oil from wells that are not naturally flowing. A sucker rod pump may also be implemented in a gas well. A corresponding pumping system may comprise or consist of three parts:

- A pump jack at the surface, which drives the pump and supports the load of the system.

- A sucker rod string, which connects the pump jack with a subsurface pump.

- The subsurface pump which is submerged in oil and transfers the required energy for the lifting process.

The working principle of the subsurface pump is the following : Two check valves are alternately opened and closed by fluid pressure. At the beginning of the pumping cycle a pump plunger is at a lowest position. As the plunger starts moving upwards, a travelling valve installed at the bottom of the pump plunger is closed and a standing valve is opened by the inflow of the reservoir fluid . The barrel is filled with reservoir fluids. As the plunger starts moving in the opposite direction, the standing valve closes, the fluid in the barrel is trapped, the travelling valve is opened and allows inflow into the production tubing. At the end of the cycle the pump plunger is back at its initial position and the whole procedure starts again.

A shortcoming of conventional sucker rod pumps is that its working principle causes cyclic production and thus cyclic loads in the system, especially in the sucker rods and the pump jack. Only static loads, that are loads which stay constant during upstroke and downstroke, can be balanced properly at the pump jack using counter weights. Dynamic loads, for instance caused by the fluid production, cannot be balanced properly, thus causing undesirable stress vibrations in the system.

For a conventional pump design the surface loads during upstroke and downstroke are the following :

- During the upstroke, the total load is composed of the weight of the equipment (such as rods, plunger), the load of the produced fluid and its inertia effects, the viscous and coulomb friction forces and the buoyancy effect, caused predominantly by the dynamic fluid level.

- During the downstroke, the total load is composed of the weight of the equipment (such as rods, plunger), minor effects from fluid production, the viscous and coulomb friction forces and the buoyancy effect, caused

predominantly by the fluid in the tubing string .

Forces caused by fluid production and friction cannot be balanced for upstroke and downstroke anyway. But it can be seen that a commonly static force, the buoyancy force, also changes during upstroke and downstroke. During the upstroke, it has a very small value, because the dynamic fluid level above the pump is normally very low, thus the provided assistance for lifting is small.

During the downstroke the buoyancy force is big because the

corresponding fluid level is at the surface. In combination with friction forces, a section directly above the plunger is under compression and buckling of the rod string may occur. Buckling is not caused by buoyance because the rod

surrounding fluid pressure stabilizes the rod string, but only small additional forces (for example friction forces) can cause buckling . Buckling is a pressure area force.

When changing the design of the pumping device according to an exemplary embodiment of the invention in way that only the pressure of the dynamic fluid level will cause buoyance, there are some huge advantages, as can be understood based on the following example:

In this example, 7/8" sucker rods are assumed, as well as a TVD (true vertical depth) of 865m, and a fluid density of 1000 kg/m ³ . A dynamic fluid level at pump is assumed as well.

A first advantage of an exemplary embodiment of the invention is that the buoyancy force is constant during upstroke and downstroke. Thus, it can be balanced perfectly at the surface with counterweights. As a consequence, less stress vibrations occur.

The differences between the buoyancy forces in a conventional pumping device and a pumping device according to an exemplary embodiment of the invention can be seen in the following table: Conventional pumping device Pumping Device according to an

embodiment of the invention

Fbuoyance_upstroke ⁼ 3300 N Fbuoyance ~ 0 N

Fbuoyance_downstroke ~ 0 N

A further advantage of an exemplary embodiment of the invention is that the stabilizing effect of the rod surrounding fluid still remains high. A 7/8" sucker rod in air can withstand a compressive load of about 250 N. Submerged in fluid it can withstand the compressive load caused by the surrounding fluid pressure and a small additional compressive load caused by the rod stiffness. When removing the buoyant compressive load, the rod string can support much higher

compressive loads (for instance from friction) without buckling .

The critical compressive load for conventional pumps under some conditions is:

FcNticai = 3300 N + 250 N =3550 N

Without buoyancy, the rod sting still can support 3550 N compressive load (by friction, etc.). Higher inclinations (in particular of a pump plunger ending section) are possible (up to 90°).

A further significant advantage of pumping devices according to exemplary embodiments of the invention is a significant energy saving. During the downstroke there is less resistance (the rod string is easier falling), therefore more energy can be recovered during the downstroke and the system is much more efficient. Using the example well, with the conventional design an average power of about 2330 W (under the assumption that the energy of the

downstroke can be recovered) are required for lifting. Using the design of a pumping device according to an exemplary embodiment of the invention, the average power is reduced to 2060 W, which is an energy saving of about 12 percent (see also Figure 10 and Figure 11).

A further significant advantage of a pumping device according to an exemplary embodiment of the invention is a stress reduction in the rod . The lifetime of the sucker rods is increased because the magnitude of the load change during upstroke and downstroke can be reduced . This can be derived from the following table:

Figure 1 to Figure 4 illustrate schematic views of a pumping device 100 configured as a sucker rod pump according to an exemplary embodiment of the present invention in different operation modes. Figure 1 to Figure 4 also show a ground 130 with a bore hole 118, wherein the ground 130 comprising fluid 108 (such as crude oil) to be pumped.

The pumping device 100 comprises a reciprocating force transmitting element 102 which is here embodied as a rigid force transmitting rod . In the shown embodiment, the force transmitting element 102 is adapted for

reciprocating along a vertical direction during the upstroke and the downstroke. The force transmitting element 102 cooperates with a pump plunger 104.

Moreover, a seal 106 is arranged in a deeper position within the borehole. A movable part 110 of the seal 106 comprises a ball with a central recess through which the force transmitting element 102 extends. The seal 106 is hence adapted for guiding the force transmitting element 102.

The force transmitting element 102 is adapted for transmitting an upstroke force and a downstroke force to the pump plunger 104 for pumping fluid 108, here crude oil. The seal 106 is adapted for sealingly preventing pumping fluid 108 during the downstroke (see operation mode of Figure 3 and operation mode of Figure 4, in which a corresponding motion direction of the force transmitting element 102 is indicated with an downward oriented arrow 197) and for enabling pumping fluid 108 during the upstroke (see operation mode of Figure 1 and operation mode of Figure 2, in which a corresponding motion direction of the force transmitting element 102 is indicated with an upward oriented arrow 199). A movable part 110 of the seal 106, i.e. a part of the seal 106 which is capable of moving together with the force transmitting element 102, is mechanically coupled with the force transmitting element 102. As can be taken best from a detail 300 shown in Figure 3, the force transmitting element 102 has a free unbiased end 112 extending through and beyond a static part 114 of the seal 106 delimiting a duct 128 even when the pump plunger 104 is located at an upper reversal position 116 (which may also be denoted as top dead center, as in Figure 3), i.e. at an uppermost position during the entire pumping cycle illustrated in Figure 1 to Figure 4. The force transmission element 102 is adapted so that a buoyancy force acting on the pumping device 100 is substantially the same during the upstroke and during the downstroke. As can be taken from Figure 3 as well, the force transmitting element 102 is configured to, when the pump plunger 104 is located at the upper reversal position 116, extend through the static part 114 of the seal 106 by a length D in a range between 10 cm and 1 m (wherein the corresponding weight of the portion of the force transmitting element 102 with the length D is not more than few kilograms). In all other operation modes (see Figure 1, Figure 2 or Figure 4), the portion of the force transmitting element 102 extending through the static part 114 of the seal 106 is even larger than D.

A stroke length L, i.e. a distance between the upper reversal position 116 and a lower reversal position 186 (which may also be denoted as bottom dead center) of the pump plunger 104 is in a range between 2 m and 4 m. For comparison, a length of the force transmitting element 102 may be significantly larger, for instance several hundred meters

The pumping device 100 comprises a tubular container 124 within which the pump plunger 104 and part of the force transmission element 102 are located and being filled at least partially with the fluid 108 to be pumped during operation of the pumping device 100. The pumping device 100 comprises a pump barrel 126 with a duct 128, wherein the duct 128 is adapted for connecting the fluid 108 from a fluid reservoir with the pump barrel 126. The movable part 110 of the seal 106 is adapted for opening the duct 128 during the upstroke and for sealing the duct 128 during the downstroke. The pump barrel 126 is adapted for being filled with the fluid 108 during the upstroke and for being discharged during the downstroke.

Figure 1 shows the pumping device 100 during an upstroke (as indicated by arrow 199) in an operation state at which the pump plunger 104 is located at a relatively low position within the borehole, i.e. at or close to a lower reversal point 186. Figure 2 shows the pumping device 100 in another operation mode still within the upstroke, wherein however now the pump plunger 104 is close to the upper reversal point 116.

Figure 3 shows the pumping device 100 during a downstroke (see arrow 197), wherein the pump plunger 104 is now directly at the upper reversal point 116. As can be taken from detail 300 of Figure 3, the movable part 110 of the seal 106 through which the force transmitting element 102 is guided already seals with the free unbiased end 112 of the force transmitting element 102 extending even in this uppermost position of the force transmitting element 102 by a dimension D through the static part 114 of the seal 106.

Figure 4 shows the pumping device 100 in yet another operation mode still during the downstroke, wherein the pump plunger 104 is now even deeper inside the borehole than in the operation mode according to Figure 3.

Due to the configuration and operation of the pumping device 100 shown in Figure 1 to Figure 4, the high pressure forces acting particularly on the force transmitting element 102 during the downward stroke (see arrow 197 in Figure 3 and Figure 4) are significantly suppressed and prevented . Buckling in the force transmitting element 102 is suppressed or eliminated as well. During an ordinary cycle, the pumping device 100 is operated first according to Figure 1, then according to Figure 2, then according to Figure 3, and then according to Figure 4, followed by a further pumping cycle starting again with Figure 1.

Figure 5 (corresponding to Figure 1), Figure 6 (corresponding to Figure 2), Figure 7 (corresponding to Figure 3) and Figure 8 (corresponding to Figure 4) illustrate schematic views of a pumping device 100 according to another exemplary embodiment of the present invention in different operation modes.

While a spherical body with a central cylindrical recess for accommodating the force transmitting element 102 has been implemented as movable part 110 of the seal 106 according to Figure 1 to Figure 4, Figure 5 to Figure 8 implement a cylindrical body with a cylindrical recess as movable part 110 of the seal 106 of the pumping device 100. Apart from this difference, operation of Figure 5 to Figure 8 corresponds to operation according to Figure 1 to Figure 4.

Figure 9 illustrates a part, which is located above ground level, of a pumping device 100 according to an exemplary embodiment of the present invention. As can be taken from Figure 9, the pumping device 100 comprises a control unit 900 adapted for controlling the pumping of fluid 108 from bore hole 118 so that, at the beginning of the pumping, fluid level 120 in the bore hole 118 circumferentially surrounding the pump plunger 104 is preferably equal to an upper reversal position 116 of travelling valve 122 of the pump plunger 104. The control unit 900 may cooperate with a drive unit such as an electric motor or a combustion engine for moving the components of the pumping device 100, particularly for moving the force transmitting element 102 and the pump plunger 104 so as to carry out a reciprocating motion. Moreover, the pumping device 100 comprises a compensation unit 902 embodied as a compensation weight and configured for partially or fully compensating static loads acting on the pumping device 100 during operation. The force transmission element 102 in combination with the compensation unit 902 are adapted so that a buoyancy force acting on the pumping device 100 is substantially zero during the upstroke and during the downstroke. By the compensation weight 902 outside of the bore hole 118, the static loads acting on the components of the pumping device 100 during the upstroke and during the downstroke may be equilibrated .

Figure 10 is a diagram 1000 showing a dependency between the time and a polished rod load of a pumping device 100 according to an exemplary embodiment compared to a conventional pumping device. The diagram 1000 has an abscissa 1002 along which the time during an operation cycle of the pumping device 100 is plotted and comprises an ordinate 1004 over which the mechanical loads acting on the force transmitting element is plotted . A first curve 1020 illustrates the performance of a pumping device 100 according to an exemplary embodiment of the invention, while a second curve 1010 shows, for comparison purposes, the performance of a conventional pump. One whole cycle according to Figure 1 to Figure 4 is plotted along the abscissa 1002 of the diagram 1000. As can be seen in a downstroke range 1030, the behaviour of the pumping device 100 differs from that of the conventional pumping device in the downstroke range 1050 in which the compared pumping devices move downwardly with different loads. Figure 10 shows that the whole cycle time takes about 20 seconds, wherein more generally it can be in a range between typically 4 seconds and 30 seconds.

Figure 11 is a diagram 1100 showing a dependency between a

displacement and a load of the pumping device 100 according to an exemplary embodiment compared to the conventional pumping device. The diagram 1100 has an abscissa 1102 along which a displacement of the respective pumping device is plotted . Along an ordinate 1104, the load over the entire stroke is plotted . Again, a first curve 1120 shows the performance of the pumping device 100 according to an exemplary embodiment of the invention, whereas a second curve 1110 shows the performance of a corresponding conventional pumping device. The area included within the hysteresis- 1 ike curves 1110, 1120 is a measure for the required energy for operating the respective pumping device. As can be taken from a comparison of curves 1110, 1120, the energy sufficient for operating the pumping device 100 according to an exemplary embodiment of the invention is significantly lower than the energy for operating the conventional pumping device. The energy reduction is about 12% in the shown example (see hashed area 1130). Apart from the reduced energy required for operating the pumping device 100, it is also possible to increase the frequency of the pumping device 100 as compared to an operation frequency of the conventional pumping device, since no tension weight or the like needs to be provided with the pumping device 100.

Figure 12 schematically compares a conventional pumping device 1200 with a pumping device 100 according to an exemplary embodiment.

The operation mode of the pumping device 100 shown in Figure 12 corresponds to the operation mode according to Figure 4 or Figure 8,

respectively. With the pumping device 100, no or no significant buoyancy force acts on the pumping device 100 during the upstroke and the downstroke. In particular, there is no pronounced difference between the buoyancy force between the upstroke and the downstroke. The compensation of static loads can be performed by compensation rates 902 located outside of the bore hole 118. The force transmitting element 102 embodied as a force transmitting rod is stabilized by the fluid 108 which results in a protection against buckling . The production of fluid 108 occurs, according to the pumping device 100, mainly or exclusively during the upstroke. As can be taken from Figure 12, the entire tubular container 124 may be filled with fluid 108 up to the ground or almost up to the ground.

An exterior diameter S of the bore hole 118 can be in an order of magnitude between 5 inch and 7 inch. The inner diameter B of the tubular container 124 can be in a range between 1.5 inch and 3 1/2 inch. The diameter F of the force transmitting element 102 may be approximately 5/8 inch to 1 1/4 inch.

Figure 13 is a diagram 1300 which corresponds to Figure 11 and relates to conditions of a pumping device according to an exemplary embodiment compared to a conventional pumping device in which the fluid level in the bore hole circumferentially surrounding the pump plunger is substantially equal to an upper reversal position of the pump plunger. Figure 14 is a diagram 1400 which corresponds to Figure 10 and relates to conditions of a pumping device according to an exemplary embodiment compared to a conventional pumping device in which the fluid level in the bore hole circumferentially surrounding the pump plunger is substantially equal to an upper reversal position of the pump plunger.

When comparing Figure 13 and Figure 14 on the one hand with Figure 11 and Figure 10 on the other hand, it can be seen that with the fluid level being equal to the upper reversal position (as in Figure 13 and Figure 14), the advantages derivable from Figure 11 and Figure 10 are even more pronounced.

Finally, it should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The words "comprising" and "comprises", and the like, do not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice versa. In a device claim

enumerating several means, several of these means may be embodied by one and the same item of software or hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.