The present invention relates to a fluid lifting system to be placed in a fluid production well, comprising:

- a fluid pump, having a pump stator and a pump rotor defining an intermediate fluid pumping space, the pump rotor having an impeller received in the intermediate fluid pumping space, the pump rotor being rotatable around a longitudinal rotation axis,

- a gas turbine comprising a turbine rotor having blades and a turbine stator defining a gas expansion chamber with the turbine rotor,

- a gas injection duct, configured to receive a gas flow from an external gas source and to introduce the gas flow in the gas expansion chamber to drive the turbine rotor in rotation around the longitudinal rotation axis, the turbine rotor being rotatable around the longitudinal rotation axis, the turbine rotor and the pump rotor being mechanically coupled such that the rotation of the turbine rotor produced by gas injection from the gas injection duct drives in rotation the pump rotor.

The fluid is for example a multiphase fluid comprising various phases including liquid and gaseous hydrocarbons and/or water. The multiphase fluid may comprise a gas content which varies along time, for example a gas content which progressively decreases with time.

During production, a change of phase composition of the produced fluid, in particular a decrease of the gas content may lead in some instances to an increase of pressure of the fluid column at the bottom of the well.

Indeed, the liquids conveyed in the fluid column in the well have a higher weight contribution when the gas flow rate is smaller due to higher liquid hold-up in the fluid column.

The productivity of the installation can thus be significantly affected. In some instances, below a critical gas flow rate, the liquid cannot any more be lifted by the gas. It then continuously accumulates in the fluid column, until the production eventually stops by installation self-killing.

If such an accumulation of liquid in the fluid column occurs, a potential stop of production can be avoided by creating an artificial lift of the produced liquid is needed. The artificial lift can be carried out through gas lift or/and through pumping. Such solutions increase the production costs.

Gas lift is usually carried out by injecting gas from the annulus to the production tubing via a gas lift orifice. The injection of gas into the fluid column lightens the fluid column and enhances its circulation towards the surface of the well. Similarly, a fluid pump at the bottom of the well is able to pump fluid to the surface. As indicated above, these two solutions are able to provide lift, but increase production costs.

In order to alleviate this problem, WO 2015/065574 discloses a fluid lifting system in which the gas injected for gas lift is injected in a gas turbine mounted in series with the pump. The expansion of the gas in the gas turbine drives a rotor of the pump in rotation, which pumps the liquid upwards. At the same time, the gas expanded in the turbine is injected in the liquid column above the fluid lifting system to lighten the fluid column.

Such a solution is not entirely satisfactory. Indeed, the combined length of the gas turbine and of the pump in the fluid lifting system may be detrimental to the insertion of the fluid lifting system in the well, in particular if the well comprises bent regions. In critical cases, bending of the fluid lifting system may occur, which can lead to jamming the fluid lifting system in the well.

In addition, the conveying of the gas to the gas turbine must be isolated from the passage of the pumped fluid through the pump. This implies providing specific pipes through the fluid passage within the fluid lifting system, which makes manufacturing more complex and leads to an increased pressure losses.

One aim of the invention is to provide a fluid lifting system in a fluid production well which enhances production of fluid, when the fluid column becomes more heavy, the fluid lifting system being easy to put in place and very efficient to operate.

To this aim, the subject matter of the invention is characterized in that the turbine rotor and the pump rotor are at least partly in longitudinal overlap in projection on the rotation axis.

The lifting system according to the invention may comprise one or more of the following features, taken solely or according to any technical feasible combination:

- the fluid pump is motorless, the rotation of the pump rotor being driven exclusively by rotation of the turbine rotor;

- the turbine rotor and the pump rotor are directly mechanically coupled such that rotation of the turbine rotor at a given angular rotation speed around the longitudinal rotation axis drives the pump rotor in rotation at the same angular rotation speed around the rotation axis;

- the fluid pump comprises a mechanical decoupler, inserted between the pump rotor and the turbine rotor, the rotation of the turbine rotor at a first given angular rotation speed driving in rotation the pump rotor at a second given angular rotation speed, different from the first angular rotation speed, advantageously smaller than the first angular rotation speed; - the mechanical decoupler comprises a non-Newtonian fluid, an epicyclical gear or/and a magnetic coupler inserted between the turbine rotor and the pump rotor;

- the fluid comprises a radial gas passage fluidly connecting the expansion chamber to the intermediate fluid pumping space;

- the impeller of the pump rotor defines gas mixing through holes;

- the turbine rotor and the pump rotor have a full longitudinal overlap in projection on the rotation axis;

- the pump rotor is received within the turbine rotor ;

- the turbine stator receives the turbine rotor, the turbine rotor receiving the pump rotor and being mechanically coupled to the pump rotor, the pump rotor receiving the pump stator;

- the impeller of the pump rotor has vanes protruding radially towards the longitudinal rotation axis, the blades of the pump rotor protruding radially apart from the rotation axis.

The invention also concerns a fluid production installation comprising a fluid production well having:

- a production tubing inwardly defining an inner production canal and outwardly defining an outer annular space being connected to a gas source at the surface of the well, the production tubing delimiting a gas injection inlet extending between the outer annular space and the production canal,

- a fluid lifting system according as defined above, the gas injection duct being connected to the gas injection inlet.

The invention also concerns a fluid lifting process comprising the following steps:

- providing a fluid lifting system as defined above in a fluid production well;

- providing a gas flow from an external gas source to the gas injection duct, and introducing the gas flow in the gas expansion chamber to drive the turbine rotor in rotation around the longitudinal rotation axis,

- jointly driving the pump rotor with the turbine rotor to pump fluid through the intermediate fluid pumping space.

The fluid lifting process according to the invention may comprise one or more of the following features, taken solely or according to any technical feasible combination:

- it comprises transferring expanded gas from the expansion chamber to the intermediate fluid pumping space to mix the expanded gas with the pumped liquid;

- it comprises producing a first negative thermal power in the gas turbine by expansion of gas in the gas chamber and producing a second positive thermal power in the fluid pump by pumping fluid through the intermediate pumping space, and providing heat exchange between the gas turbine and the fluid pump to at least partly compensate the second thermal power with the first thermal power.

The invention will be better understood, based on the following description, given solely as an example, and made in reference to the following appended drawings in which:

- [Fig 1] Figure 1 is a schematic cross-section of the bottom part of a well equipped with a fluid lifting system according to the invention;

- [Fig 2] Figure 2 is a schematic functional view of the fluid lifting system according to the invention;

- [Fig 3] Figure 3 is a structural view of the fluid lifting system of figure 2.

In the following description, the terms “upper”, “upwards”, “upwardly”, “uphole”, “lower”, “downwards”, “downwardly”, “downhole” are given relative to their orientation from the top of the installation to the bottom of the installation.

The terms “inward”, “inside”, “outward”, “outside” are given relative to a local central axis of the installation. The term “inside” generally means closer or oriented towards the central axis, whereas the term ’’outside” generally means further away or oriented away from the central axis.

A first fluid production installation 10 according to the invention is shown partially in figure 1.

The fluid production installation 10 comprises at least one well 12 bored in a subsoil 14, an outer casing 19, and a production tubing 18 inserted in the casing 19.

The fluid production installation 10 further comprises a downhole fluid lifting system 20 according to the invention. It comprises a surface gas source 22 connected to an annular space 24 between the casing 19 and the production tubing 18 to feed gas to the fluid lifting system 20 through the annular space 24.

In the region shown in figure 1 , the well 12 extends along a local central axis A-A’ which is here shown vertical. In a variant, the well 12 has inclined regions, and/or horizontal regions.

The well 12 extends from the surface of the subsoil 14 (not shown), which can be located on the ground or at the bottom of a body of water, to a lower formation including a fluid reservoir 28. As known in the art, the well 12 is closed by a wellhead (not shown) located at the top of the well 12.

The casing 19 lines the well 12. In this example, the casing 19 comprises an assembly of cylindrical metal strings, which are advantageously held in place with cement.

The casing 19 or an alternative lower liner (not shown) is in communication with the reservoir geological formation 28 containing hydrocarbons and/or water. Thus, a multiphase fluid stream, which can comprise hydrocarbons, is produced through perforations 27 emerging in the well 12, downhole of the lower end of the production tubing 18.

In this example, the reservoir geological formation 28 produces gas in particular, hydrocarbons such as methane. It also produces liquids, for example water and/or liquid hydrocarbons called oil or condensates.

The multiphase fluid stream produced from the reservoir 28 is under pressure, for example at a pressure greater than 10 bars. Preferentially, the liquid is dispersed in the gas in the form of droplets (“mist flow”), or at intervals in the form of liquid pockets, (“slug flow”). In variant, the gas is dispersed in the liquid, in the form of droplets or in the form of gas pockets.

The production tubing 18 is surrounded at its lower end with an annular outer packer 30, interposed between the casing 19 and the production tubing 18.

The packer 30 seals the annular space 24 at the bottom of the production tubing 18. Hence, the fluid produced from the reservoir 18 necessarily flows to the production tubing 18 without entering the annular space 24 uphole of the outer packer 30.

The production tubing 18 defines an internal production canal 32, which extends from the bottom of the production tubing 18, in the vicinity of the reservoir 28, to the wellhead at the surface. The annular space 24 is radially delimited towards the axis A-A' by the production tubing 28.

The production tubing 18 delimits at least a radial gas injection inlet 34 which extends through the production tubing 18 to connect the annular space 24 to the internal production canal 32. The gas injection inlet 34 allows feeding gas received from the gas source 22 through the annular space 24 to the fluid lifting system 20 located in the internal production canal 32, as will be described below.

As shown in figure 2, the fluid lifting system 20 is anchored in the production canal 32 at a longitudinal position facing the gas injection inlet 34.

The fluid lifting system 20 comprises an inner fluid pump 40, to pump the multiphase fluid flowing upwardly in the internal production canal 32, and an outer gas turbine 42 coaxially receiving the inner fluid pump 40, and driving the fluid pump 40 in rotation.

The fluid lifting system 20 further comprises an anchoring system 44 to longitudinally anchor the outer gas turbine 42 in the production tubing 18, and a sealing system 46 to guide the gas received from the gas injection inlet 34 exclusively to the gas turbine 42.

As shown in figures 2 and 3, according to the invention, the outer gas turbine 42 and the inner fluid pump 40 longitudinally overlap over the whole length of the outer gas turbine 42, in projection on the axis A-A’. In this example, the inner fluid pump 40 is totally received at the center of the outer gas turbine 42. In reference to figure 3, the outer gas turbine 42 comprises, in a radial direction towards the axis A-A’, an outer turbine stator 50, fixed to the production tubing 18 via the anchoring system 44, and a turbine rotor 52 rotatably mounted into the turbine stator 50 about a rotation axis A-A’ which corresponds to the center longitudinal axis A-A’ of the production tubing 18.

The outer gas turbine 42 further comprises a gas injection duct 54, to feed the turbine rotor 52 with gas from the gas injection inlet 34.

The turbine stator 50 comprises a sleeve 56 defining an outer wall of the gas turbine. The sleeve 56 inwardly delimits, with the turbine rotor 52, a gas expansion chamber 58 receiving gas from the gas injection duct 54 to drive the turbine rotor 52 in rotation.

The turbine rotor 52 comprises for example a rotating hub 60, and rotating blades 62 protruding from the hub 60 towards the sleeve 56 in the expansion chamber 58. The expansion of gas received from the gas injection inlet 34 in the expansion chamber 58 acts on the rotating blades 62 to jointly drive the blades 62 and the hub 60 of the turbine rotor 52 in rotation around the axis A-A’ with regards to the turbine stator 50.

The pump 40 comprises an outer pump rotor 70, mechanically connected to the inner turbine rotor 52 to be driven in rotation by the turbine rotor 52, and an inner pump stator 72, placed at the center of the pump rotor 70 to guide in rotation the pump rotor 70 about the axis A-A’.

The pump rotor 70 comprises an inner impeller 74 having vanes protruding towards the pump stator 72 in the intermediate fluid pumping space 76 defined between the pump rotor 70 and the pump stator 72.

The intermediate fluid pumping space 76 opens downwardly and upwardly in the internal production canal 32 to allow intake of fluid from the internal production canal 32, downhole of the fluid lifting system 20, and ejection of pumped fluid uphole of the fluid lifting system 20, after having being pumped by the outer pump rotor 70.

The outer pump rotor 70 is mounted rotatable about the axis A-A’ with regards to the inner pump stator 72.

In the example shown in figure 3, the pump rotor 70 is directly mechanically connected to the turbine rotor 52, such that a rotation of the turbine rotor 52 at a given angular rotation speed around the axis A-A’ provides a joint rotation of the pump rotor 70 at the same angular rotation speed around the rotation axis A-A’.

In a variant, shown in dotted line in figure 3, a mechanical decoupler 68 is provided between the turbine rotor 52 and the pump rotor 70.

The mechanical decoupler 68 for example comprises a non-Newtonian fluid to adjust the decoupling based on the angular rotation speed of the pump rotor 70 as a function of the turbine rotor 52 speed. Alternatively, the mechanical decoupler 68 comprises an epicyclical gear or a magnetic coupler having magnets. Preferentially, the mechanical decoupler 68 comprises at least a bearing 79 to maintain the turbine rotor 52 and the pump rotor 70 well centered and to support rotation.

Generally, the mechanical decoupler 68 allows the pump rotor 70 to be driven in rotation at a second given angular rotation speed, when the turbine rotor 52 is driven in rotation at a first given angular rotation speed. The second given angular rotation speed is for example greater or lower than the first given angular rotation speed. Advantageously, the mechanical decoupler 68 comprises a gearbox (not shown) able to modify the ratio of the second given angular speed to the first given angular speed.

In any case, the pump rotor 70 is driven in rotation around axis A-A’ exclusively by the turbine rotor 52, without provision of rotative drive by a hydraulic or electric motor.

The pump stator 72 extends in the middle of the outer pump rotor 70, along the axis A-A’. It comprises a cylindrical base, and arms (not shown) connecting to the turbine stator 50. The arms define between them through openings for fluid to flow through.

The anchoring system 44 comprises at least an anchor 80 protruding radially from the sleeve 56 of the turbine stator 50 to attach to the production tubing 18. The production tubing 18 preferably comprises a corresponding housing receiving the anchor 80.

The anchor 80 radially spaces the turbine stator 50 from the production tubing 18 such that a peripheral space 82 is defined around the turbine stator 50, between the turbine stator 50 and the production tubing 18.

The sealing system 46 comprises at least an upper seal 84 to tightly close the peripheral space 82 in the uphole direction and at least a lower seal 86 to tightly close the peripheral space 82 in a downhole direction.

Hence, the peripheral space 82 exclusively communicates with the gas injection inlet 34 on the one side and with the gas injection duct 54 on the other side. Gas from the source flowing from the annular space 24 through the gas injection inlet 34 directly and exclusively flows to the expansion chamber 58 through the gas injection duct 54.

In a preferred embodiment, the hub 60 of the turbine rotor 52, the mechanical decoupler 68 if present and the pump rotor 70 define at least a radial gas through passage connecting the expansion chamber 58 to the intermediate fluid pumping space 76.

Preferably, the impeller 74 vanes define through longitudinal through holes 75, through which gas flowing from the radial passages is evacuated and mixed.

Thus, the gas expanded in the intermediate chamber 58 is evacuated through the radial through passages to the intermediate fluid pumping space 76 and mixes with the fluid pumped in the intermediate fluid pumping space 76. Further mixing and gas lift effect is advantageously provided by the gas passage through the holes 75 made in the vanes of the impeller 74.

The operation of the fluid lifting system 20 to enhance fluid production in the installation 10 will now be described.

In production, fluid from the reservoir 28 enters the well 12 and flows up in the production tubing 18, downhole of the fluid lifting system 20.

At the same time, gas from the gas source 22 is injected in the well annular space 24 and flows downhole to the gas injection inlet 34. The gas enters the peripheral space 82 around the turbine stator 50 and is confined longitudinally in the peripheral space 82 by the seals 84, 86 of the sealing system 46.

The gas thus flows to the gas injection duct 54, and enters the expansion chamber 58. In the expansion chamber 58, the gas expands and interacts with the blades 62 to drive the turbine rotor 52 in rotation around the rotation axis A-A’.

The rotation of the turbine rotor 52 jointly drives in rotation the pump rotor 70 around the pump stator 72.

Fluid flowing downhole of the fluid lifting system 20 thus enters the intermediate fluid pumping space 76 and is pumped uphole, to be ejected from the fluid lifting system 20 towards the surface.

Preferably, the gas which has expanded in the chamber 58 flows into the intermediate fluid pumping space 76 through the or each radial passage and mixes with the pumped fluid. A jet effect is advantageously generated, as the gas circulates into the through holes 75 made in the vanes of the impeller 74.

Thanks to the arrangement of the fluid lifting system 20, with the inner pump 40 being received within the outer gas turbine 42 and driven in rotation by the outer gas turbine 42, the fluid can be lifted with low energy consumption. The fluid lifting system 20 is also very compact which facilitates its introduction in the well 12.

The gas provided to the outer gas turbine 42 is totally separated from the fluid which is pumped in the inner pump 40, which simplifies the structure and tubing within the fluid lifting system 20. Nevertheless, a further mixing of the gas expanded in the outer gas turbine 42 with the pumped fluid can be carried out within the uphole stages of the inner pump 40. In some instances, a jet pump effect can be obtained, in particular when through holes are provided in the vanes of the pump impeller 74.

Thus, fluid management within the fluid lifting system 20 is simplified and a lesser pressure drop occurs.

Another advantage of the arrangement of the fluid lifting system 20 according to the invention is that the thermal effects occurring in the fluid lifting system 20 are compensated. Indeed, the expansion of the gas within the expansion chamber 58 of the outer gas turbine 42 generates negative thermal power. On the contrary, the compression of the fluid within the inner pump 40 provides positive thermal power. Since the inner pump 40 has at least a longitudinal overlap with the outer gas turbine 42, thermal power can be exchanged between the inner pump 40 and the outer gas turbine 42 to compensate the above- mentioned thermal effects.