NO20190559A1 - Downhole flow controller - Google Patents

Description

SYSTEM FOR REPLACING A DEVICE IN A TUBE IN A WELLBORE

The invention relates to a system for replacing a device in a wellbore.

Injection systems for injecting additives into the production tubing of wells near the bottom of the tubing are installed in substantially all offshore wells. To comply with common terminology, the additives are herein referred to as chemicals, though they may be any additive, and the injection system is referred to as a chemical injection system (CIS). The chemical is typically a liquid or dispersed within a liquid, e.g. as a solution, colloid, or suspension, and functions to improve fluid flow in the well and to delay corrosion problems, ultimately delaying the periods between well workovers. Applications for CIS are for example related to asphaltenes, scale, hydrates, emulsions, defoaming, paraffins, scavengers, corrosion inhibitors, and demulsifiers.

CIS are expensive and are known to have varying performance, and most of the CIS are not suited for automation or remote control. Generally, a CIS consists of a chemical injection pump and a flow control valve (FCV) positioned above the wellhead (e.g. subsea or at the surface), and a downhole injection tool (DIT) positioned near the bottom of a well, typically in the production tubing. The DIT is connected to the FCV and the pump through a supply line (see figure 1). The distance between the FCV and the DIT is thus at least in the order of the well depth, which can be several kilometres. A CIS typically have one FCV for each injection point.

Unfortunately, any known CIS with a FCV above the wellhead is subjected to difficulties related to large hydrostatic pressure differences between the supply line and the well at the injection point in the DIT. The supply line typically contains chemicals of fluid density while the well tubing, for example the production tubing, on the other side of the DIT may contain gas or oil of considerably lower density. For example, if the specific gravities of the chemical in the supply line and the fluid in the production tubing are 1.1 and 0.5, respectively, the pressure difference across the DIT in a 2000 m deep well will be about 120 bar. This large pressure difference across the DIT may cause the injection chemical to fall into the well through the DIT, which may leave the outlet of the FCV with substantially no outlet pressure. This phenomenon is known as the U-tube effect. The U-tube effect may therefore result in an evacuation of the supply line just downstream of the FCV, resulting in evaporation of the injection fluid and formation of a gas-liquid interface. Additionally, the well pressure in the production tubing varies due to slugs and other dynamic flow phenomena, whereby the pressure difference fluctuates and causes problems with stable injection. The pressure fluctuations may vary from very small to tens of bars.

There have been considerable operational problems caused by the U-tube effect. Guerra and Oliveira (Guerra, L.A. and Oliveira, G.H., Designing Downhole Chemical Injection Systems, Paper SPE 170772 presented at the SPE Annual Technical Conference and Exhibition, Amsterdam 27 – 29 Oct. 2014) discuss the problem with the U-tube effect at length, arguing that: “Tubing pressure oscillations (caused by well production instability or natural production fluctuations) are the cause of variability in the chemical flow rate delivered downhole. On top of that, column separation, which is liquid-vapor interface established inside the chemical injection line due to very low pressures, cannot always be avoided. This phenomenon intensifies flow rate oscillations and requires additional precautions during the design phase.”

Equinor (previously Statoi)l has also experienced problems associated with the U-tube effect. Olsen (Olsen, J.H., Statoil Experiences and Consequences to Continuous Chemical Injection, Paper SPE 146625 presented at the SPE Annual Technical Conference and Exhibition, Denver, 30 Oct. – 2 Nov.2011) discusses at length the problem with phase change in the supply line, leading to vacuum conditions and pressure variations. In attempts to avoid phase changes in the supply line, this is generally kept thin to increase the pressure drop along the line. However, a thin supply line has been known to cause plugging within said line, and Olsen postulates that many supply lines currently in use in wells are plugged.

Another way to avoid problems with the U-tube effect is to have a strong relief valve in the DIT, however, this requires a system with high operational pressure and difficulties maintaining steady flow due to fluctuations in the well pressure.

The invention disclosed herein has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art.

The object is achieved through features, which are specified in the description below and in the claims that follow.

The invention is defined by the independent patent claims. The dependent claims define advantageous embodiments of the invention.

A chemical injection system (CIS) for injecting a chemical into fluids in wells may comprise:

a downhole injection tool (DIT) positioned in a well;

a pump;

a supply line connecting the pump and the DIT; and

a flow control valve (FCV) positioned between the pump and the DIT along the supply line, said FCV controlling the flow of the chemical in said supply line,

wherein at least a portion of the flow control valve is placed in the well in the proximity of the DIT (as shown in figure 2). The chemical may typically be a liquid or be dispersed within a liquid, e.g. as a solution, colloid, or suspension, and the well fluids to inject the chemical into may typically, but not exclusively, be production fluids. Thus, by placing the FCV in the well close to the DIT, there will be no significant pressure difference between said DIT and the outlet of said FCV, whereby problems related to the U-tube effect may be avoided. The FCV may be installed in a retrievable mandrel for simple replacement. The FCV and the DIT may be placed anywhere in the well, even before the oil enters an inflow control device, which reduces scaling and corrosion.

The FCV may be a constant-flow FCV which has two functionalities: to set the desired flow rate of the chemical, and provide a system to keep the flow rate substantially constant regardless of variations in the inlet or outlet pressure. The constant-flow FCV may e.g. be of the type disclosed within patent document NO319627 and illustrated schematically in figure 3. The constant-flow FCV may e.g. comprise a flow control choke (FCC), which sets a desired flow rate, and a pressure compensation system, which functions to maintain said flow rate substantially constant. The pressure compensation system may e.g. be a device which controls the flow area of the FCV output, such that a change in FCV inlet or outlet pressure results in a corresponding change in outlet flow area. Thus, the pressure compensation system may be calibrated such that the FCV output flow is essentially constant.

The FCC may be e.g. a nozzle, a ball valve, a needle valve, or an orifice. Preferably, the FCC may be replaceable, whereby the chemical flow rate into the well may be changed during the life time of the CIS. For example, in one embodiment, the FCC may be retrievable or adjustable from the wellhead or the surface, e.g. through a wireline, whereby said FCC may be easily exchanged or adjusted.

The FCC may be made of or connected to a buoyant material. Thereby, the nozzle will float to the surface when there is no flow in the supply line, while drag will bring the nozzle downhole to an internal obstacle (e.g. a landing ring) at a predetermined position when flow resumes. Thus, the nozzle may be easily replaced at the surface.

The FCC may comprise an orifice plate with two or more openings which control the flow. If a lower flow rate is preferred, one or more correspondingly shaped restrictions, for example balls, may be dropped into the supply line to close one or more of the holes. The restrictions may have a high density for a permanent lowering of the flow rate, or it may be buoyant for a temporary lowering of the flow rate. If the restrictions are buoyant, they will be dragged downstream during flow, but float upstream if the flow ceases.

The FCC may comprise a seat to accept a ball restriction, where the seat is shaped such that the available flow area depends on the size of the ball. If the ball is buoyant, the flow area and thus flow rate may be easily changed by changing the ball to one with a different size. The seat may be shaped such that a specific ball size may completely block the flow area.

In an embodiment of the invention, the FCC may be contained within a disposable plug, and the supply line may comprise a plug-stopping system, which keeps the plug in the right position, as well as a disposal chamber for used plugs. The disposal chamber may e.g. be a section of a pipe. The plug-stopping system may also contain an automatic plug-releasing system, so that the used plug will automatically be released into the disposal chamber when a new plug is inserted into the supply line. Such a plug-stopping system, possibly also comprising an automatic plug-releasing system, may be used for any downhole application where flow through a tubing needs to be controlled and varied, also without the pressure compensation system. The plug-stopping system may e.g. comprise spring-loaded levers comprising lower lever members, which keep the plugs from falling further, connected to upper lever members, which may be shaped such that they will be expanded by an approaching plug. The levers may be displaced e.g. laterally or by rotation, and the upper and lower lever members may be connected such that displacement of the upper lever members results in displacement of the lower lever members. Thus, when a new plug approaches, it will expand the upper lever members, whereby the lower lever members will be displaced, allowing the old plug to continue down into the disposal chamber. As the plug is free of the upper lever members, the spring ensures that both the upper and lower spring-loaded levers will return to their original position, whereby the lower lever members will keep the new plug from continuing further down. This embodiment is versatile and robust, and it relies on mechanical or hydraulic control systems only. It may be installed in existing supply lines without the need for additional wires or pipes. The plug-stopping system may also comprise other obstacle means than levers, as longs as said obstacle means may be controlled to temporarily allow the plugs to continue their movement past said obstacle means. The obstacle means may be controlled mechanically by means of the force exerted by the incoming plug, possibly using hydraulics or pneumatics to regulate said force, or they may be controlled by applicable sensors and actuators in the supply line. The flow rate may be changed almost unlimited as the disposal chamber may be any size or length. Additionally, the plugs may be replaced during well operations so no well intervention or workover is required. A pressure relief valve may be installed near the FCC or the disposal chamber to facilitate new plug installation in case the FCC in the plug is blocked. The system may also easily be temporarily shut down by inserting a fully flow-blocking plug.

As noted above, the plug-stopping system, possibly comprising also an automatic plug-releasing system, can be used for other applications as well. Such applications may for example be replacing nozzles in inflow control devices, downhole gas lift valves, water injection valves, or any application where flow in tubes / pipes need to be regulated. Gas lift valves are commonly used to improve hydrocarbon production. If gas is injected through a dedicated tubular, a setup similar to the CIS can be used. In this situation, plugs can be used to easily change or stop the gas flow. One may either use a constant-flow FCV or a downhole choke to control the gas flow. Injection of a plug may also be used to control the flow rate of the reservoir into the well, or to stop gravel backflow by pumping down a plug after a gravel pack has been placed. The plugs may also contain other devices than nozzles. For example, the plug-stopping system may be used to replace sensors at specific positions in the well. The sensors may e.g. be temperature or pressure sensors, which are placed inside plugs.

The FCC may be situated above or in the proximity of the wellhead for easy access. In this particular embodiment, two different supply lines may extend from the FCC to the pressure compensation system in order to communicate with different portions of said pressure compensation system.

The FCC may be remote-controlled using e.g. electrical or hydraulic control solutions, whereby the flow rate may be changed at any instant. For example, a cable or small tubing may be clamped to the supply line or the well casing to provide the required control action, whereby the flow rate may be set from the surface. Another way to adjust the flow setting may be to have a FCC which may be changed by pressure pulses. Pulse level or duration may trigger a mechanism which sets the FCC to a specific flow rate.

The FCC may be an autonomous FCC, which may function by varying in a predetermined way its outlet flow area according to variations in inlet pressure, whereby the pressure drop across the autonomous FCC may be set as a function of inlet flow. The autonomous FCC may itself be an independent invention, which may be used to vary flow rates, also without the pressure compensation system. The autonomous FCC may e.g. comprise a spring-loaded piston with an orifice plate at the inlet end of the piston rod and a solid plate at the outlet end, said plates covering a crosssectional flow area of a flow path, and said spring-loaded piston arranged such that an increase in inlet pressure will displace the piston in the flow direction. In addition to the spring-loaded piston, the autonomous FCC may be formed with a widening of the available flow area just downstream of the solid plate (e.g. through the shape of a housing or a cavity carved into the housing wall), said widening formed such that a shift in the position of the spring-loaded piston in the flow direction will result in an increased flow path area. Thus, an increase in inlet pressure leads to increased flow across the autonomous FCC, which again leads to a decrease in pressure drop across said autonomous FCC. Therefore, correctly designed and calibrated, this autonomous FCC may provide a decrease in pressure drop upon increased flow rate. This relationship corresponds well with the requirements for the constant-flow FCV, whereby the flow rate of said constant-flow FCV may be easily controlled from above the wellhead by varying the inlet pressure.

The autonomous FCC may comprise an orifice plate and a spring-loaded restriction shaped complementary to the orifice plate opening. The spring-loaded restriction may be arranged such that the restriction will block the orifice plate opening at low inlet pressures, while an increase in inlet pressure will cause a downstream displacement of the restriction and a larger flow area through the orifice plate opening. The larger flow area results in a smaller pressure drop across the autonomous FCC. Thus, this embodiment of an autonomous FCC may also be designed and calibrated to provide a decreasing pressure drop upon increasing flow rate.

More than one CIS may be connected above the wellhead by a distribution system, e.g. a manifold. Said distribution system may be significantly simpler, and thus smaller, than known distribution systems, since the FCV of the CIS will be placed downhole. As the surface area on an oil platform is highly limited, any possible decrease in an instrument’s footprint (i.e. the surface area occupied by said instrument) will be of great value.

In the following is described examples of preferred embodiments illustrated in the accompanying drawings, wherein:

Fig.1 illustrates chemical injection system according to prior art;

Fig.2 illustrates a chemical injection system;

Fig.3 illustrates the functionality of the constant-flow flow control valve from patent document NO319627;

Fig.4 illustrates a chemical injection system, wherein the flow control valve is a constantflow flow control valve and the flow control choke is a retrievable orifice;

Fig.5 illustrates a flow control choke wherein the nozzle is buoyant;

Fig.6 illustrates a flow control choke comprising an orifice plate with two openings and a ball restriction closing one of the openings;

Fig.7 illustrates a flow control choke comprising a seat shaped to allow different flow areas depending on the size of a ball restriction, shown with a small ball in a view perpendicular to the axial direction (a), along the axial direction (b), and with a large ball in a view perpendicular to the axial direction (c);

Fig.8 illustrates a chemical injection system, wherein the flow control choke is contained within a disposable plug within the supply line, and where the supply line comprises a disposal pipe;

Fig.9 illustrates in a different scale an automatic plug-stopping and plug-releasing system for use with disposable plugs as in figure 8 in closed conformation;

Fig.10 illustrates the automatic plug-stopping and plug-releasing system for use with disposable plugs as in figure 9, where a new plug is approaching and thus opening said system;

Fig.11 illustrates another embodiment of an automatic plug-stopping and plug-releasing system for use with disposable plugs in closed conformation;

Fig.12 illustrates a chemical injection system wherein the flow control choke is situated above the wellhead;

Fig.13 illustrates an autonomous flow control choke; and

Fig.14 illustrates another autonomous flow control choke;

In the drawings, the reference numeral 1 indicates a chemical injection system (CIS). Identical reference numerals indicate identical or similar features in the drawings. The drawings are presented in a simplified and schematic manner, and the features therein are not necessarily drawn to scale.

Figure 1 demonstrates a well 3 with a CIS 1 according to prior art. The CIS 1 comprises a pump 5 and a flow control valve 7 (FCV) above the wellhead 9 and a downhole injection tool 11 (DIT) in the well 3. A supply line 13 connects the pump 5, the FCV 7, and the DIT 11. The shown CIS 1 will be subjected to problems related to the U-tube effect as described above.

Figure 2 shows a well 3 with a CIS 1. The CIS 1 comprises a pump 5 above the wellhead 9, and a FCV 7 and DIT 11 in the well 3. A supply line 13 connects the pump 5, the FCV 7, and the DIT 11. The shown CIS 1 will not experience any problems related to the U-tube effect.

Figure 3 demonstrates the most important features of a constant-flow FCV 7 as disclosed in patent document NO319627, to which we refer for further details. Briefly, the constant-flow FCV 7 comprises a pressure compensation system 15 and a flow control choke 17 (FCC) in fluid communication with the supply line 13. The FCC 17 sets the flow rate and may be e.g. a nozzle or an orifice. The pressure compensation system 15 includes a piston chamber 19 comprising a first inlet 21 communicating with the inlet 23 of the FCV 7, a second inlet 25 communicating with the outlet 27 of the FCC 17, an outlet orifice 29, and a spring-loaded piston 31. The piston 31 comprises a piston head 33 which is affected by the inlet pressure, and a piston rod 35 that further comprises a needle 37 at its tip. The needle 37 is shaped complementary to the outlet orifice 29 of the chamber 19, and said needle 37 is arranged such that it determines the size of said outlet orifice 29. The piston 31 further comprises a spring 39 with a spring force working against the inlet pressure, thereby pushing the piston 31 away from the outlet orifice 29. If the inlet pressure increases, the piston 31 will be displaced against the spring force towards the outlet orifice 29 of the chamber 19, thereby decreasing the flow area of said outlet orifice 29. As the pressure in the piston chamber 19 then increases due to the greater flow across the FCC 17 than through the outlet orifice 29, the piston 31 will slowly be displaced back to allow increased flow through said outlet orifice 29, until a new equilibrium is reached where the flow rate is substantially the same as before the increase in inlet pressure. Similarly, if the FCV 7 outlet pressure (i.e. the well pressure) decreases, the flow through the outlet orifice 29 will increase, whereby the piston chamber 19 pressure will decrease, and the piston 31 will be pushed down to decrease the outlet orifice 29 flow area, which again decreases the outlet flow rate to substantially the same flow rate as before the decrease in outlet pressure. Therefore, if the FCV 7 is properly calibrated, it will provide a substantially constant flow, controlled by the FCC 17, under variable pressure conditions.

Figure 4 shows a well 3 with a CIS 1, said CIS 1 comprising a constant-flow FCV 7, where the FCC 17 is a replaceable orifice plate 41, which is easily retrievable from above the wellhead 9 using a connected wireline 43. The position of the pressure-compensating system 15 of the constant-flow FCV 7 is also shown.

Figure 5 shows a FCC 17 comprising a landing ring 45 and a nozzle 47 connected to a buoyant material 48. The nozzle 47 and the buoyant material 48 constitute in Fig.5 a hollow cylinder where only a section cut of the cylinder is shown. The nozzle 47 will float to the surface when there is no flow in the supply line 13, while drag will bring the nozzle 47 downhole to the landing ring 45 when flow resumes. Thus, the nozzle 47 may be easily replaced at the surface to set the flow rate.

Figure 6 shows a FCC 17 comprising an orifice plate 49 with two openings 51, 53 which control the flow, and a ball 55 which has been dropped to stop the flow through the opening 51 for a lower total flow. The ball 55 can either be dense for a permanent lowering of the flow rate, or it can be buoyant for a temporary lowering of the flow rate. If the ball 55 is buoyant, it will have been dragged downstream during flow and will thus float upstream if the flow ceases.

Figure 7a-c shows a FCC 17 comprising a seat 57 to accept a ball 55. The seat 57 has four flow paths 59, most clearly shown in figure 7b, which are broader further away from the supply line centre. Figure 7a shows how a small ball 55 blocks a region towards the bottom of the seat 57, resulting in small flow paths 59, while figure 7c shows how a large ball 55 blocks a region toward the top of the seat 57 where the flow paths 59 are large. Thus, the available flow area depends on the size of the ball 55. If the ball 55 is buoyant, it will float to the surface when the flow ceases, whereby the flow may be easily changed by replacing the ball 55 with one of different size. The seat 57 may be shaped such that a specific ball 55 size may completely block the flow area.

Figure 8 shows a CIS 1 according to an embodiment of the invention, wherein the FCC of the constant-flow FCV 7 is contained within a disposable plug 61. In this embodiment, the supply line 13 comprises an injector head 63 for injecting new plugs 61, a downhole plug-stopping and automatic plug-releasing system 69, which keeps the plug 61 in the right position between the two sides of the pressure compensation function 15, and a pipe section 67 for used plugs 61. Examples of possible embodiments of the plug-stopping and automatic plug-releasing system 69 is shown schematically in greater scale in figures 9-11.

Figure 9 shows a plug-stopping and automatic plug-releasing system 69 in closed conformation. The system comprises spring-loaded levers 71 parallel to the axis of the supply line 13 and attached to said supply line 13 through attaching members 76. The attachment between the levers 71 and the attaching members 76 is constructed such as to allow some tilting and moving of said levers 71. Additionally, the levers 71 have hinges 78 at their centre positions so they are able to bend. The levers further comprise upper lever members 75 and lower lever members 73. The lower lever members 73 keep the plug 61 from falling by blocking the passage below, while the upper lever members 75 are shaped such that they will be displaced by an approaching plug 61. The levers 71 are constructed such that they can rotate around the rotation axis R (which is perpendicular to the plane of the paper and therefore only a point in the figure). Thus, as illustrated in figure 10, when a new plug 61 approaches, said plug 61 will displace the upper lever members 75, whereby the levers will tilt around the rotation axes R and bend around the hinges 78 so that the lower lever members 73 will also be displaced. Thus, the old plug 61 will be allowed to pass the lower lever members 71 and fall down into the pipe section 67 for used plugs 61. As the plug 61 is free of the upper lever members 75, the springs 77 will cause both said upper lever members 75 and the lower lever members 73 to return to their original position, whereby said lower lever members 73 will keep the new plug 61 from falling further down. Figure 11 shows another example of a plug-stopping and automatic plug-releasing system 69 in closed conformation, where the springloaded levers 71 are allowed to rotate around the shown dash-dotted rotation axis R. Thus, rotation of the levers 71 due to displacement of the upper lever members 75 as a new plug 61 is approaching will result in equal rotation of the lower lever members 73, whereby the old plug 61 will be able to fall further down. Note that the figures are only schematic and that real systems will include for example measures to ensure that the fluid does not escape the tubing, while at the same time allowing the levers 73, 75 to interact with the plug 61. A plug-stopping and automatic plug-releasing system 69 as those illustrated here can be applied to any downhole function that may need downhole adjustable flow control as described in the general description.

Figure 12 shows a well 3 with a CIS 1, said CIS 1 comprising a constant-flow FCV 7 where the FCC 17 is situated above the wellhead 9 for easy setting of the flow rate. In this embodiment, two supply lines 13, 79 extend from above the wellhead 9 to the two sides of the pressure compensation system 15 in the portion of the constant-flow FCV 7 in the well 3, as one supply line 13 passes the FCC while the other supply line 79 does not.

Figure 13 shows an autonomous FCC 17 for use as the FCC 17 in a downhole constant-flow FCV 7. The autonomous FCC 17 comprises a housing 81, which has an inlet side 83 with an inlet pressure and an outlet side 85 with an outlet pressure. The autonomous FCC 17 further comprises an axially displaceable spring-loaded piston 87, which at a first end 89 of the piston rod 91 towards the inlet side 83 of the housing 81 has an orifice plate 93 which allows fluid to flow through, while a second end 95 of the piston rod 91 towards the outlet side 85 of the housing 81 has a solid plate 97 which blocks the flow area when said autonomous FCC 17 is fully closed. Immediately downstream of the solid plate 97 is carved a small cavity 99 into the housing 81, said cavity 99 shaped to provide an increasing flow area of the housing 81 as a function of axial distance from the solid piston plate 97 in the downstream direction. The spring force of the piston spring 101 is opposite the flow direction, such that said piston spring 101 functions to press the orifice plate 93 against internal obstacles 103 when the inlet pressure is small, corresponding to a closed conformation. When the inlet pressure increases, the piston 87 is displaced towards the downstream direction, whereby a small flow path is created around the solid plate 97. This flow path functions as a choke which restricts flow, resulting in a large pressure decrease across the autonomous FCC 17. A further increase in inlet pressure will cause the piston 87 to be displaced further along the downstream direction, which will create a larger flow path around the solid plate 97. This larger flow path in turn causes an increase in flow rate and a corresponding decrease in pressure drop across the FCC 17. Thus, if properly designed and calibrated, the autonomous FCC 17 will provide a decreasing pressure drop upon increasing flow rate, which may be adjusted by adjusting the inlet pressure.

Figure 14 shows another embodiment of an autonomous FCC 17 for use in a downhole constantflow FCV. The autonomous FCC 17 comprises an orifice plate 105 and a spring-loaded restriction 107 shaped complementary to the opening 109 in the orifice plate 105. The spring-loaded restriction 107 is arranged such that said restriction 107 blocks the opening 109 of the orifice plate 105 at low inlet pressures, while an increase in inlet pressure causes a downstream displacement of the restriction 107 and a resulting larger flow area through said opening 109 of said orifice plate 105. This larger flow area results in a smaller pressure drop across the autonomous FCC 17. Thus, this embodiment of an autonomous FCC 17 may also be designed and calibrated to provide a decreasing pressure drop upon increasing flow rate.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims (8)

C l a i m s

1. A system for replacing a device in a wellbore (3), c h a r a c t e r i s e d i n that the system comprises:

− a pipe (13) extending downhole from the surface, the pipe (13) being configured to let a plug (61) comprising the device move through said pipe (13); and

− a plug-stopping system (69) for stopping the movement of the plug (61) through the pipe (13) at a specific position in said pipe (13).

2. A system according to claim 1, wherein the system further comprises an automatic plugreleasing system (69) for automatically releasing the plug (61) from the plug-stopping system (69) when another plug (61) is approaching.

3. The system according to claim 2, wherein the plug-stopping system (69) comprises springloaded levers (71) comprising lower lever members (73) configured to keep the plug (61) from moving further down, and wherein the automatic plug-releasing system (69) comprises upper lever members (75) which are shaped so that they will be displaced by an approaching plug (61), and wherein the upper (75) and lower (73) lever members are connected so that displacement of the upper lever members (75) results in a similar displacement of the lower lever members (73).

4. The system according to claim 2, wherein the automatic plug-releasing system (69) comprises a sensor for detecting an approaching plug (61) and an actuator for releasing the plug (61) from the plug-stopping system (69).

5. The system according to any of the preceding claims, wherein the system comprises a plug (61) comprising a device.

6. The system according to claim 5, wherein the device is a sensor.

7. The system according to claim 5, wherein the device is a flow control choke (17).

8. The system according to claim 7, wherein the flow control choke (17) is an autonomous flow control choke (17).