WO2016182451A1

WO2016182451A1 - Back pressure valve for a completion string comprising sand screens, completion string comprising said back pressure valve, and method for lowering said completion string

Info

Publication number: WO2016182451A1
Application number: PCT/NO2016/050086
Authority: WO
Inventors: Michael Kennedy WILLIAMSON; Christian Berven; Jon Eirik Haugneland KVITHYLL
Original assignee: Toolserv As
Priority date: 2015-05-13
Filing date: 2016-05-11
Publication date: 2016-11-17
Also published as: GB2555257A; NO340579B1; GB201718013D0; GB2555257B; NO20150590A1

Abstract

Herein is described a back pressure valve ( 1) for a completion string (10) comprising sand screens (101), the back pressure valve (1) comprising: - a valve body (2); - a valve seat (7); - a valve head (8) for sealing against the valve seat (7); - a valve stem (4) connected to the valve head (8) for guiding the valve head (8); and - a resilient member (9) arranged around a portion of the valve stem (4); wherein the valve stem (4) is guided by at least two supports (13, 15) which are spaced apart in the longitudinal direction of the valve stem (4), the at least two supports (13, 15) being configured for allowing translation of the valve stem (4) in its longitudinal direction only. Furthermore, a method for using said back pressure valve (1) is described, wherein the method comprises the step of providing the back pressure valve (1) in a completion string.

Description

BACK PRESSURE VALVE FOR A COMPLETION STRING COMPRISING SAND SCREENS, COMPLETION STRING COMPRISING SAID BACK PRESSURE VALVE, AND METHOD FOR LOWERING SAID COMPLETION STRING

The present invention relates to a back pressure valve for a completion string comprising sand screens, the back pressure valve comprising a valve body, a valve seat, a valve head for sealing against the valve seat, a valve stem connected to the valve head for guiding the valve head and a resilient member arranged around a portion of the valve stem. The invention also relates to a completion string comprising said back pressure valve. Furthermore, the invention relates to a method for lowering said completion string comprising said back pressure valve.

A back pressure valve is a type of check valve designed to hold pressure from below, or downstream of the valve, at the same time as it enables fluids to be pumped from above, or upstream of, the valve. There are many fields in which back pressure valves are used. The back pressure valve of the present invention, however, is designed for use in the production of petroleum from a petroleum well, more specifically in a completion string comprising sand screens.

Wherever used herein, the words upstream/above and downstream/below refer to the relative positions of parts of the valve and/or completion string when in use in a well.

Completion of a petroleum well is the process of making the well ready for production. In that process the maximizing of recovery from the petroleum reserves is a main issue which requires management of fluid flow through the reservoir. Inflow control devices, for instance in relation with sand screens, like the Schlumberger ResFlow inflow control device are increasingly popular for this use. In order to install such an inflow control device successfully, a greater pressure must be maintained within the completion string, than in the annulus, as this will keep the inflow control device in a closed position thus preventing any fluids or debris from entering inside of the completion string during the completion phase. One way of maintaining such higher pressure inside the completion string, is to use a heavy weight fluid, for instance a heavy weight low solids oil based mud inside of the string and a light weight fluid in the annuius, creating a hydrostatic imbalance and keeping the inflow control device valves closed.

After lowering and installation of the completion string, the heavy weight fluid is flushed through the back pressure valve into the annuius and returned to the surface.

One problem connected to lowering and installing a completion string with an inflow control device as mentioned above, is that prior art float valves or back pressure valves at the end of the completion string, below the sand screens, have shown not to be able to prevent the heavy weight fluid, for instance low solids oil based mud, from entering the annuius through the back pressure valve. When the heavy weight mud enters the annuius the pressures inside and outside of the completion string will be equalized and the valves of the sand screens will open at an undesirable time.

Calculations show that the back pressure valve should have an opening pressure of about 200 psi, i .e. the back pressure valve should be able to withstand a pressure of about 200 psi from the heavy weight fluid inside of the string, in order to be suitable for use in a completion string comprising sand screens.

In order to overcome this problem, it is known from prior art to use an inner string which is removed after successfully lowering the completion string. However, the use of an inner string is time consuming.

There are many known back pressure valves, for instance the poppet valve of patent publication EP 0 255 269 Bl, which is a fluid activated poppet valve installed in the casing to prevent back flow of cement into the casing from the annuius if the pressure in the casing is reduced.

Another example of a prior art back pressure valve, is the plunger-type valve described on the web-site of America West Drilling Supply.

Patent publication US 5413180 A discloses method and apparatus for completing wells, especially for isolating a production interval during backwashing and sand control operations performed prior to placing the well in production. A check valve is provided wherein the check valve has an upper conical valve portion with cylindrical valve stem depending therefrom. The valve stem is slidably mounted within a valve stem guide. A spring is wound around the valve stem, said spring biasing the conical valve to a closed position. WO 2007/014009 Al discloses a downhole non-return valve for injection of fluid, for instance water or gas, into a pressurized downhole environment. The non-return valve comprises a housing, a plug movable between an open position and a fully sealed position, and a biasing member urging the plug towards the fully sealed position but where the biasing member is selected to be insufficient to move the plug to the fully sealed position.

EP 2636840 Al discloses a bottomhole assembly for a capillary injection system comprising a plurality of injection valves connected in series. Each injection valve includes a tubular housing having a valve seat, a valve member, and a biasing member pushing the valve member towards engagement with the valve seat.

Conventional back pressure valves for cemented liners are designed primarily to hold pressure from below, hence the testing requirements do not involve verifying the valve opening pressure. For use in a completion string comprising sand screens, this is however, as mentioned above, critical. The theoretical opening pressure of a certain conventional poppet valve was calculated to be 30 psi, whereas tests surprisingly showed that the valve in fact opened at only 4 psi. The intuitive solution to this would be to simply replace the valve spring with a stronger spring, but even when replacing the spring with a spring six times stronger, the opening pressure was still found to be 4 psi . Thus a standard poppet valve is not suitable for providing the necessary opening pressure of 200 psi for running of the completion string with sand screens without an inner string.

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least to 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.

In a first aspect the invention relates more particularly to a back pressure valve for a completion string comprising sand screens, the back pressure valve comprising :

- a valve body;

- a valve seat;

- a valve head for sealing against the valve seat;

- a valve stem connected to the valve head for guiding the valve head; and

- a resilient member arranged around a portion of the val e stem; wherein the valve stem is guided by at least two supports which are spaced apart in the longitudinal direction of the valve stem, the at least two supports being configured for allowing translation of the valve stem in its longitudinal direction only.

By valve seat is meant herein a valve member which either forms part of the inner surface of the valve body or which is a separate ring or element connected to the inner surface of the valve body, where the valve seat is arranged to be able to receive the valve head such that the valve head cannot pass through the valve seat. The valve seat comprises a sealing surface arranged to seal against a corresponding sealing surface of the valve head.

The valve body may be a tubular valve body.

By "supports" is meant herein any kind of element which is suitable for guiding the valve stem and making sure to prevent "wobbling" or any sideways movement of the valve stem. The supports may therefore be for instance pins or other elements extending from the inner surface of the valve body in such a way that they support the valve stem from different sides. Further, the supports may be plates or discs partly or completely enclosing the valve stem either directly or indirectly, by supporting another element which supports the valve stem directly, for instance a housing enclosing the valve stem.

When the valve stem is only allowed to move in a longitudinal direction, the pressure required to open the valve, is more predictable. The prior art back pressure valves have that in common that the valve stem is only guided by one support, either in the end near the valve head or in the opposite end. By guiding the valve stem with at least two spaced apart supports, such that the valve stem can only move in its longitudinal direction, the valve stem will not give way sideways when a force is applied to the valve head. The at least two supports thus prevent wobbling or swinging of the valve stem, which otherwise would cause it to allow fluid to pass with much lower pressures than the calculated pressure based on the strength of the resilient member.

The sealing between the valve head and the valve seat may be a metal-to-metal seal. By providing a metal-to-metal seal, the back pressure valve can hold the well pressure for the entire lifetime of the well, as opposed to a valve in which the sealing comprises polymer or another material which will only be able to hold the pressure for much shorter time periods, for example for hours or days, but not over several years. A metal-to-metal seal will provide less flexibility of the material of the valve and therefore further add to the stability and high opening pressure of the valve. The valve may also comprise a double seal, whereof one of the seals is the above- mentioned metal-to-metal seal and the other seal may be a polymer seal, for instance a rubber seal .

The rubber seal may for instance be provided in a seal channel which is machined out in the valve seat. The rubber used may be a vulcanized rubber. The effect of this additional seal is that it contributes to a very accurate opening pressure of the valve. For this purpose it is of less importance if the rubber deteriorates over time after the installation of the completion string in the well, as the metal-to-metal seal will make sure the valve stays closed.

The valve head may have a planar upstream surface which will make the pressure applied to the valve head more evenly distributed over the surface than with another shape of the surface. Furthermore, the planar upstream surface will aid in giving consistent opening pressures and reduce the chance of trapping debris in the seal area.

The back pressure valve may comprise a tensioning means or a compression means, for example a nut, for adjusting the tension of the resilient member which makes it possible to use the valve for different pressures without necessarily having to change the resilient member. Said nut may be formed such that it allows for the movement of the valve stem in the longitudinal direction of the valve stem .

Furthermore, the tensioning means, such as the nut, may constitute one of the supports for the valve stem . It is an advantage to use the same element, in this case the nut, for providing more than one function in order to make the construction of the valve simpler. The nut itself may be supported either directly or indirectly by the valve body.

In one possible embodiment of the invention, the resilient member may be a spring. The resilient member may, however, be of any known type, for instance in one embodiment of the invention the resilient member may be a gas cylinder filled with nitrogen .

The back pressure valve may further comprise a housing enclosing the resilient member. This has the effect that the resilient member, for example the spring, is protected from the fluid which is pumped through the valve when it is open. Furthermore, the housing may be the indirect support of the valve stem, as mentioned above, by, in one embodiment, constituting the connection between the nut and the valve body. The nut may be threaded on the outside and fastened to the housing, while being hollow on the inside allowing for the valve stem to move in the longitudinal direction of the valve stem.

The at least two supports may further be configured for keeping the housing centred in the valve body. This gives strength to the valve assembly and makes the fluid flow more even_*

One of the supports may be arranged in the annulus formed between the housing and an inner wall of the valve body, encircling the housing and making contact with the inner wall of the valve body in at least two points. One possible shape of such support may be a circular body with two or more radially extending legs.

In one embodiment one of the supports may be a disc shaped unit comprising at least one aperture, and more preferably multiple apertures. It is important for the intended use of this valve that the apertures have a size large enough to allow a heavy weight fluid, like low soiids/high solids, drilling fluid like oil based mud, to pass through in large volumes. The specific gravity of the fluid may, as an example, be about 1.4, typically in the range from 0.84-2.4. Said fluid will be pumped through the valve at a rate of for example 6 barrels per minute («0.95 m³/minute), and up to 12 barrels per minute, which is a considerable amount and which requires the valve opening and the apertures of the disc shaped unit to be large enough to prevent clogging of the valve.

Another embodiment, similar to the above, is an embodiment wherein one of the supports is disc shaped and is in contact with the inner wall of the valve body in the entire periphery of the support.

One of the supports may be positioned in a portion upstream of the resilient member and downstream of the valve head.

In still another embodiment, the valve head may be provided with at least one flow port. Said flow port is arranged to be able to divert the flow of fluid, for example mud, which flows through the valve in its open position, away from the sealing surfaces of the valve seat and valve head. In this way abrasion of the sealing surfaces is prevented. This is particularly of importance when the valve is used together with highly abrasive fluid, like for instance cement, which is relevant for this particular embodiment of the invention.

In a second aspect the invention relates more particularly to a completion string comprising a back pressure valve according to the first aspect of the invention. Such completion string may comprise at least one sand screen. Furthermore, the invention concerns a method for lowering the completion string comprising the back pressure valve described herein. When using a back pressure valve as described above, able to withstand pressures up to 200 psi, the completion string may be lowered without having to use an inner string, which saves time. With the use of a prior art valve, opening at much lower pressure, an inner string has to be used in order to prevent the heavy weight fluid from entering into the annulus.

In a third aspect the invention relates more particularly to a method for use of the back pressure valve according to the first aspect of the invention, wherein the method comprises the step of providing the back pressure valve in a completion string.

In one particularly advantageous use of the back pressure valve, the method may further comprise the steps of:

- providing the completion string with at least one sand screen; and

- gradually lowering the completion string into the well, while at the same time:

- filling the completion string with a fluid of higher density than a fluid surrounding the completion string in the well, such that the pressure inside of the completion string at any time during the lowering of the completion string is higher than the pressure in the well acting on the sand screen, and such that the pressure inside of the completion string is lower than the opening pressure of the back pressure valve, for keeping the sand screen and the back pressure valve closed.

Since the back pressure valve according to the invention is able to withstand considerable pressures before it opens, no fluid will leak from inside of the completion string to the annulus outside of the completion string. The hydrostatic imbalance created due to the relatively heavier fluid inside of the completion string compared to the fluid outside of the completion string, will keep the sand screens closed. This means that an inner string, as used in the prior art, is not needed.

The pressure inside the completion string may be kept above 30 psi and below 200 psi. The pressure needed inside the completion string in order to keep the sand screens closed is dependent on the ambient pressure, i.e. the pressure acting on the sand screens from the annulus/the well. It is therefore the differential pressure, the pressure inside the completion string relative to the ambient pressure, which is of relevance. It takes only a very small pressure to keep the sand screens closed if no pressure is acting from outside. Therefore, theoretically, any pressure above 0 psi inside of the completion string may be sufficient to keep the sand screens closed. However, in operation, it is likely that the pressure inside of the completion string should be kept for instance above 30 psi in order to keep the sand screens closed. In addition to keeping the pressure high enough to make sure to keep the sand screens closed, it is also essential to keep the pressure below the opening pressure of the back pressure valve. The opening pressure of a back pressure valve according to the invention may be around 200 psi. It must be understood that adjustments may be made such that the opening pressure may vary. However, the opening pressure is significantly higher than the opening pressure of prior art back pressure valves for this use.

The method comprises to lower the completion string to a desired depth, and when such depth has been reached, the method further comprises the step of:

- increasing the pressure inside of the completion string, in order to overcome the opening pressure of the back pressure valve, to flush the fluid out through the back pressure valve for returning it to the surface via the annulus of the well .

During said last step, the pressure inside the completion string may be increased by applying pump pressure from surface. The fluid present in the completion string will then be flushed out of the valve and returned to the surface through the annulus of the well, in order to be re-used.

In a second particularly advantageous use of the back pressure valve, the method may also, when the completion string comprises a back pressure valve with at least one flow port provided in the valve head, further comprise the steps of:

- lowering the completion string to a desired depth;

- providing the completion string with a cement for cementing the string to the well; and

- flushing the cement through the back pressure valve.

The flow port or flow ports will, as earlier mentioned, divert the highly abrasive fluid, which the cement is, away from the sealing surfaces of the valve. This valve is therefore especially suitable for cementing purposes, as the back pressure valve will stay closed and intact for the entire lifetime of the well, once the cementing has been completed.

When using said valve with at least one flow port the method may further comprise keeping the pressure in the string using pumps on the rig, while running the completion assembly into the wellbore. This kind of valve is particularly useful in a completion string without sand screens; and in the case of running a completion string without sand screens, there is no need to use fluids of differing densities inside of the completion string compared to the fluids in the annular. The valve, however, works in the same manner in all methods described herein. In the following is described an example of a preferred embodiment illustrated in the accompanying drawings, wherein :

Fig. 1 is a planar end view of an embodiment of a back pressure valve according to the invention;

Fig. 2 is a plane view of a cross section of a back pressure valve, indicated as A- A in Figure 1 ;

Fig. 3 is a perspective view of the cross section shown in Figure 2;

Fig. 4 is a principle drawing of a completion string with a back pressure valve;

Fig, 5 is a plane view of a cross section of a second embodiment of the back pressure valve according to the invention;

Fig. 6 is a perspective view of the cross section shown in Figure 5;

Fig. 7 is a plane view of a cross section of a third embodiment of the back pressure valve according to the invention;

Fig. 8 is a perspective view of the cross section shown in Figure 7;

Fig. 9 is a plane view of a cross section of a fourth embodiment of the back pressure valve according to the invention, wherein the valve is shown in a closed position; and

Fig. 10 shows the back pressure valve of Figure 9, wherein the valve is in an open position.

Reference is first made to Figure 1 showing a back pressure valve 1 seen from below, or downstream, in working position. A valve body 2 is provided with an opening 3 through which it is possible to see a valve stem 4 enclosed by a tensioning means 5, here shown as a nut 5, which again is enclosed by a housing 6.

In Figure 2, the valve body 2 is shown to have a tubular body part 21 and a lid 22. The lid 22 is only required for testing purposes, and is not part of the valve in operational use. The inner surface of the tubular body part 21 forms a constriction or narrowing of the passage, the walls of said constriction being arranged to receive a valve seat 7 in an upstream portion of the tubular body part 21. The valve seat 7 has inclined surfaces forming sealing surfaces 71 against a valve head 8 when the back pressure valve 1 is in a closed position. The seal is a metal-to-metal seal 71, 82 formed by the sealing surface 71 of the valve seat and a sealing surface 82 of the valve head. The valve head 8 is fixedly connected to a first, upstream, end of the valve stem 4. The valve stem 4 is provided with a collar 41 for engaging with a resilient member 9, here shown as a spring 9, which encloses the valve stem 4 downstream of the collar 41. The resilient member 9 and the part of the valve stem 4 encircled by said resilient member 9 are held in the housing 6. In its second, downstream, end the valve stem 4 is connected to the nut 5, which is used for adjusting the tension of the spring 9. In this particular embodiment the upper part of the valve stem 4 is guided, via the housing 6, by a first support 13, here in the form of a disc 14, and by a second support 15, here represented by the nut 5, The two supports 13, 15 are arranged such that the valve stem 4 is only movable in its longitudinal direction.

When a force directed downstream is applied to an upstream surface 81 of the valve head 8, it will, once it overcomes the tension of the spring 9, force the back pressure valve 1 to open. The valve head 8 is pushed such that both the valve head 8 and the valve stem 4 are moved in a longitudinal direction causing the sealing surfaces of the valve seat 7 and of the valve head 8 to separate from each other. The compression of the spring 9 causes the valve head 8 to again form a sealing connection with the valve seat 7 as soon as the downstream directed force is removed or drops to a level where it can no longer overcome the spring force.

In the perspective view of Figure 3, it is more clearly shown how the housing 6 enclosing the spring 9 in one embodiment can be made in one piece with the circular disc 14 constituting the first support 13. In this particular embodiment the disc 14 is shown with circular apertures 16.

Figure 4 shows a principle drawing of a completion string 10 in a well 20. The completion string 10 comprises sand screens 101 with inflow control devices 102. Further, the completion string comprises a float valve or back pressure valve 1 below, i.e. downstream of, the screens 101. Heavy weight fluid, like low solids oil based mud, is kept inside the string 10 and light weight fluid is in an annulus 30 between the string 10 and the wall of the well 20. The resulting hydrostatic imbalance between the inside and the outside of the string 10, will keep the inflow control devices 102 in their closed position. The back pressure valve 1 will prevent the heavy weight fluid from escaping out into the annulus 30.

Figures 5 and 6 show, in a planar view and in a perspective view, respectively, an embodiment of the back pressure valve 1 wherein the valve seat 7 is formed as an integrated part of, i.e. in one piece with, the inner surface of the tubular body part 21 of the valve body 2. This is a practical embodiment with fewer parts, compared to the above-mentioned embodiment, needing to be formed to fit each other. The metal-to- metal seal 71, 82 of this embodiment is constituted by sealing surfaces 71, 82 of the valve head 8 and the integrated valve seat 7

Figures 7 and 8 show, in a planar view and in a perspective view, respectively, an embodiment of the back pressure valve 1, which comprises a double seal. A channel 72 is machined in the valve seat 7 such that the channel 72 faces the valve head 8. Said channel 72 may then be filled with a polymer, for instance rubber, preferably vulcanized rubber, such as to form a rubber seal against the valve head. In the figures, the channel 72 is shown before it is filled with the polymer.

The valve seat 7 may be threaded, which constitutes a reliable connection to the tubular body part 21 of the valve body 2, and which also makes it easier to connect two or more back pressure valves 1 together.

Figures 9 and 10 show, a planar view of the back pressure valve 1 in a closed position and in an open position, respectively, an embodiment of the back pressure valve 1 which is particularly suitable for use in completion strings where highly abrasive fluids are used. Also in this embodiment the valve seat 7 is shown formed as an integral part of the tubular part 21 of the valve body 2, but the valve seat 7 could as an alternative also in this embodiment be a separate part. In this embodiment the valve head 8 has been modified, compared to the above described embodiments of the back pressure valve 1, by extending it such that an extension 83, or sleeve 83, goes through the opening of the valve seat 7. The valve head 8 is provided with at least one flow port 84 and said flow port 84 is in the shown embodiment provided in the extension 83. When the valve is in its closed position, Figure 9, the flow port 84 is closed by the inner wall of the tubular body part 21 of the valve body 2. When a force exceeding the opening pressure of the back pressure valve 1 is applied to the planar upstream surface 81 by the fluid in the completion string 10, the valve opens and the fluid flows through the flow port 84 which is arranged to divert the fluid away from the sealing surface 82 of the valve head 8. The embodiment in the figures shows a plurality of flow ports 84.

Furthermore, to supplement/assist the flow ports 84, there may also be provided corresponding flow ports 171 in a housing 17 provided such that it encloses the area between the first support 13 and the valve seat 7. When the valve 1 opens, the flow ports 84 of the valve head 8 align with the flow ports 171 of the housing 17 such that the fluid, even more efficiently, is diverted away from the sealing surfaces 71, 82, and out into the apertures 16 of the disc 14.

The housing 17 couid be a separate part, or an extension of the first support 13, or alternatively it could be an extension of the housing 6 which also houses the resilient member 9, 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 ciaim. 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.

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.

Claims

C l a i m s

1. Back pressure valve (1) for a completion string (10) comprising sand screens (101), the back pressure vaive (1) comprising:

- a valve body (2);

- a valve seat (7);

- a valve head (8) for sealing against the valve seat (7);

- a valve stem (4) connected to the valve head (8) for guiding the valve head (8); and

- a resilient member (9) arranged around a portion of the valve stem (4); c h a r a c t e r i z e d i n that the valve stem (4) is guided by at least two supports (13, 15) which are spaced apart in the longitudinal direction of the valve stem (4), the at least two supports (13, 15) being configured for allowing translation of the valve stem (4) in its longitudinal direction only.

2. Back pressure valve (1) according to claim 1, wherein the valve head (8) forms a metal-to-metal seal against the valve seat (7).

3. Back pressure valve (1) according to claim 1 or 2, wherein the valve head (8) and the valve seat (7) form a double seal against each other,

4. Back pressure valve (1) according to claim 3, wherein one of the seals is a polymer seal.

5. Back pressure valve (1) according to any of the preceding claims, wherein the valve head (8) has a planar upstream surface (81).

6. Back pressure valve (1) according to any of the preceding claims, further comprising a nut (5) for adjusting the tension of the resilient member (9).

7. Back pressure vaive (1) according to claim 6, wherein the nut (5) constitutes one of the supports (13, 15) for the valve stem (4).

8. Back pressure valve (1) according to any of the preceding claims, wherein the resilient member (9) is a spring.

9. Back pressure valve (1) according to any of the preceding claims, further comprising a housing (6) enclosing the resilient member (9).

10. Back pressure valve ( 1) according to claim 9, wherein the at least two supports ( 13, 15) are further configured for keeping the housing (6) centred in the valve body (2).

11. Back pressure valve ( 1) according to claim 9 or 10, wherein the valve head is provided with at least one flow port.

12. Completion string (10) comprising a back pressure valve (1) according to any of the preceding claims.

13. Completion string (10) according to claim 12, wherein the completion string ( 10) is provided with at least one sand screen.

14. Method for using a back pressure valve ( 1) according to any of claims 1-11, wherein the method comprises the step of providing the back pressure valve (1) in a completion string.

15. Method according to claim 14, wherein the method further comprises the steps of:

- providing the completion string (10) with at least one sand screen; and

- gradually lowering the completion string (10) into the well, while at the same time:

- filling the completion string (10) with a fluid of higher density than a fluid surrounding the completion string (10) in the well, such that the pressure inside of the completion string ( 10) at any time during the lowering of the completion string (10) is higher than the pressure in the well acting on the sand screen, and such that the pressure inside of the completion string ( 10) is lower than the opening pressure of the back pressure valve, for keeping the sand screen and the back pressure valve ( 1) closed.

16. Method according to claim 15, wherein the pressure inside the completion string is kept above 30 psi and below 200 psi.

17. Method according to any of claims 14 to 16, wherein, when the completion string has reached a desired depth, the method further comprises the step of:

- increasing the pressure inside of the completion string in order to overcome the opening pressure of the back pressure valve ( 1), and to flush the fluid out through the back pressure valve ( 1) for returning it to the surface via the an- nulus of the well. Method according to ciaim 14 when in combination with the back pressure vaive according to c!aim 11, wherein the method further comprises the steps of;

- lowering the completion string to a desired depth;

- providing the completion string with a cement for cementing the string to the well ; and

- flushing the cement through the back pressure valve.