Zone isolation method and annular barrier with an anti-collapsing unit

1. An annular barrier to be expanded in an annulus between a well tubular structure and a wall of a borehole downhole for providing zone isolation between a first zone having a first pressure and a second zone having a second pressure of the borehole, the annular barrier comprising:

a tubular metal part for mounting as part of the well tubular structure, the tubular metal part having an outer face,

an expandable sleeve surrounding the tubular metal part and having an inner face facing the tubular metal part and an outer face facing the wall of the borehole, each end of the expandable sleeve being connected with the tubular metal part, and

an annular space between the inner face of the expandable sleeve and the tubular metal part, the annular space having a space pressure,

wherein the annular barrier comprises an anti-collapsing unit comprising an element movable at least between a first position and a second position, the anti-collapsing unit having a first inlet which is in fluid communication with the first zone, and a second inlet which is in fluid communication with the second zone, and the anti-collapsing unit having an outlet which is in fluid communication with the annular space, and in the first position, the first inlet is in fluid communication with the outlet, equalising the first pressure of the first zone with the space pressure, and in the second position, the second inlet is in fluid communication with the outlet, equalising the second pressure of the second zone with the space pressure.

2. The annular barrier according to claim 1 , wherein the anti-collapsing unit comprises a shuttle valve and the element is comprised in the shuttle valve.

3. The annular barrier according to claim 1 , wherein the anti-collapsing unit is arranged on the outer face of the tubular metal part or on an outer face of the well tubular structure.

4. The annular barrier according to claim 1 , wherein the anti-collapsing unit is arranged adjacent to or in abutment with the expandable sleeve.

5. The annular barrier according to claim 1 , wherein one or both of the ends of the expandable sleeve is/are connected with the tubular part by means of connection parts, and the anti-collapsing unit is arranged outside the annular space adjacent to or in the connection part.

6. The annular barrier according to claim 1 , wherein the outlet of the anti-collapsing unit is fluidly connected to the annular space through a fluid channel.

7. The annular barrier according to claim 1 , wherein the anti-collapsing unit is arranged in the first or the second zone.

8. The annular barrier according to claim 1 , wherein a compound is enclosed in the annular space.

9. The annular barrier according to claim 8 , wherein the compound comprises nitrogen.

10. The annular barrier according to claim 8 , wherein the compound is thermally decomposable below a temperature of 400° C.

11. The annular barrier according to claim 1 , wherein the well tubular structure comprises a first opening opposite the first zone and a second opening opposite the second zone, the second opening being in fluid communication with the first inlet and the first opening through an inside of the well tubular structure.

12. The annular barrier according to claim 1 , wherein the first inlet is in fluid communication with the first zone through a conduit extending through the annular space.

13. The annular barrier according to claim 1 , wherein a screen is arranged on the outer face of the tubular part and upstream of the first inlet and/or the second inlet.

14. The annular barrier according to claim 1 , wherein the element is a piston movable in a piston housing between the first position and the second position, the piston housing comprising a spring being compressed when the piston moves in a first direction.

15. The annular barrier according to claim 2 , wherein the shuttle valve has a housing having an insert made of metal, ceramics, an elastomeric material or a polymeric material.

16. The annular barrier according to claim 1 , wherein the tubular part comprises an expansion opening being in fluid communication with the inside of the tubular part and the annular space.

17. The annular barrier according to claim 16 , further comprising a shear pin assembly fluidly connecting the expansion opening and the annular space in order to allow expansion fluid within the well tubular structure to expand the expandable sleeve.

18. The annular barrier according to claim 17 , wherein the shear pin assembly has a first position in which expansion fluid is allowed to flow into the annular space and a second position in which the expansion opening is blocked, preventing expansion fluid from entering the annular space.

19. A downhole system comprising:

a well tubular structure, and

the annular barrier according to claim 1 .

20. A zone isolation method for providing and maintaining zone isolation between a first zone having a first pressure and a second zone having a second pressure of the borehole, the method comprising the steps of:

positioning the annular barrier according to claim 1 in a well tubular structure,

expanding the expandable sleeve of the annular barrier to provide zone isolation between the first zone and the second zone of the borehole, and

maintaining zone isolation between the first zone and the second zone when the first pressure of the first zone is higher than the second pressure of the second zone by equalising the first pressure of the first zone with the space pressure by arranging the element in the first position, whereby the first inlet is in fluid communication with the outlet, and

maintaining zone isolation between the first zone and the second zone when the second pressure of the second zone is higher than the first pressure of the first zone by equalising the second pressure of the second zone with the space pressure by arranging the element in the second position, whereby the second inlet is in fluid communication with the outlet.