Downhole production casing string

1. A downhole production casing string for insertion in a pre-drilled borehole in a reservoir, the downhole production casing string having a first end nearest a top of the borehole and a second end furthest away from the top, the downhole production casing string extending along a longitudinal axis and comprising:

at least one opening which during production allows hydrocarbon-containing fluid from the reservoir into the downhole production casing string,

a plurality of casing parts having end sections and a base section between the end sections, the base section having an outer diameter, and

at least one annular projecting element having an outer face and at least one helical groove arranged in or on the outer face and having an overall outer diameter which is larger than the outer diameter of the base section, wherein the at least one annular projecting element is fixed to the plurality of casing parts so as to rotate together during insertion of the string down the borehole, the helical groove being configured to act as a screw to rotatably engage one or more borehole projections remaining after drilling and tending to impede free passage of the string and to advance the string past the borehole projections,

wherein the downhole production casing further comprises at least one fluid-expandable annular barrier, the annular projecting element being positioned in front of the annular barrier closer to the second end of the casing string, and

wherein a diameter of the annular projecting element is larger than a diameter of the annular barrier before expansion of the annular barrier, so as to protect the annular barrier against damage when, following drilling, the annular projecting element and the annular barrier are together inserted into the borehole.

2. A downhole production casing string according to claim 1 , wherein the annular projecting element is a casing collar connecting the casing parts.

3. A downhole production casing string according to claim 1 , wherein the annular projecting element is arranged between two annular barriers.

4. A downhole production casing string according to claim 1 , wherein the annular projecting element at least partly secures the at least one expandable annular barrier to one of the casing parts.

5. A downhole production casing string according to claim 4 , wherein the annular barrier comprises a casing part , an expandable sleeve surrounding the casing part and having an inner sleeve face facing the casing part and an outer sleeve face facing the borehole, each end of the expandable sleeve being connected with the casing part in two connections, and an annular space between the inner sleeve face of the expandable sleeve and the casing part, and wherein the annular projecting element is arranged on an outer casing face adjacent at least the connection closest to the second end of the production casing string.

6. A downhole production casing string according to claim 4 , wherein the annular barrier comprises a casing part, an expandable sleeve surrounding the casing part and having an inner sleeve face facing the casing part and an outer sleeve face facing the borehole, each end of the expandable sleeve being connected with the casing part in two connections, and an annular space between the inner sleeve face of the expandable sleeve and the casing part, and wherein the annular projecting element at least constitutes the one connection arranged closest to the second end of the production casing string.

7. A downhole production casing string according to claim 1 , wherein the opening has an angle in relation to a radial direction transverse to the longitudinal axis so that the hydrocarbon-containing fluid is guided into the production casing string in the angle different from 90°.

8. A downhole production casing string according to claim 1 , wherein the helical groove has a cutting edge.

9. A downhole production casing string according to claim 1 , wherein the annular projecting element comprises several grooves forming a helix about the longitudinal axis.

10. A downhole production casing string according to claim 1 , wherein the annular projecting element tapers towards the second end of the production casing string.

11. A downhole production casing string according to claim 1 , wherein the outer diameter of the annular projecting element is the overall outer diameter of the production casing string.

12. A downhole production casing string according to claim 1 , wherein the opening is arranged in the groove for letting fluid from the reservoir into the production casing string.

13. A downhole production casing string according to claim 12 , wherein the production casing string has an inner face along which a sliding sleeve is slidably arranged for sliding between a closed position, in which the sliding sleeve blocks the opening, and an open position, in which the fluid is allowed to flow through the opening and into the production casing string.

14. A downhole production casing string according to claim 12 , wherein the opening is arranged closer to the first end of the production casing string than to the second end of the production casing string, or closer to the second end of the production casing string than to the first end of the production casing string.

15. A downhole production casing string system for completing a well downhole, comprising:

a production casing string according to claim 1 , and

a rotation equipment for rotating the production casing string about the axis as the production casing string is inserted into the borehole.

16. A method of implementing a production casing string according to claim 1 in a borehole downhole, comprising:

connecting casing parts and the at least one annular projecting element for forming the production casing string,

following drilling, entering the production casing string into the borehole as the casing parts are assembled, and

rotating the production casing string and the at least one annular projecting element about the axis as the production casing string enters the borehole, the helical groove acting as a screw to engage the borehole projections and advance the string past the borehole projections as a result of rotation of the helical groove.

17. A method according to claim 16 , further comprising detaching part of a wall of the borehole from the wall by cutting in or hitting against the borehole wall by means of the annular projecting element.

18. A method according to claim 16 , further comprising allowing fluid to flow from the borehole, in through an opening in the annular projecting element and into the downhole production casing string.