Well string assembly

1. A well string assembly comprising:

an upper tubular well string part having upper and lower ends between which there is formed a passageway;

a lower well string part having upper and lower ends, which lower end is spaced apart from the lower end of the upper well string part and which lower end is connectable to or includes a drill bit;

a releasable well string interconnection means for selectively interconnecting the lower and upper well string parts above the lower end of the lower well string part; and

an auxiliary tool arranged such that it can be passed along the passageway of the upper well string part, wherein the auxiliary tool comprises an operating means for manipulating the well string interconnection means so as to disconnect the lower well string part from the upper well string part, and wherein the auxiliary tool is arranged such that it can be passed, after disconnecting the upper and lower well string parts, through an opening at the lower end of the upper well string part while suspending the lower well string part, so as to reach a working position in which at least part of the auxiliary tool is not radially surrounded by a part of the well string.

2. The well string assembly according to claim 1 , wherein the auxiliary tool further comprises a lower well string connection means for connecting the auxiliary tool to the lower well string part simultaneously with or before disconnecting the well string interconnection means.

3. The well string assembly according to claim 2 , wherein the interconnection means, auxiliary tool and the lower well string connection means are arranged such that the lower and upper well string parts can be reconnected again after disconnecting.

4. The well string assembly according to claim 1 , wherein the auxiliary tool comprises a hang-off device co-operating with the lower end of the upper well string part.

5. The well string assembly according to claim 1 , further comprising a means for performing an operation in relation to the borehole or formation surrounding the auxiliary tool, when the auxiliary tool is in the working position.

6. The well string assembly according to claim 5 , wherein the means for performing an operation forms part of the auxiliary tool.

7. The well string assembly according to claim 5 , wherein the means for performing an operation is selected from the group consisting of a logging tool, a sampling tool, a fluid injection tool, a placement tool.

8. A method of performing an operation in a borehole, external of the well string, comprising using the well string assembly of claim 1 .

9. The well string assembly according to claim 1 , wherein the interconnection means and auxiliary tool are arranged such that the lower and upper well string parts can be reconnected again after disconnecting.

10. A well string assembly comprising:

an upper tubular well string part having upper and lower ends between which there is formed a passageway;

a lower well string part having upper and lower ends, which lower end is spaced apart from the lower end of the upper well string part and which lower end is connectable to or includes a drill bit;

a releasable well string interconnection means for selectively interconnecting the lower and upper well string parts above the lower end of the lower well string part; and

an auxiliary tool arranged such that it can be passed along the passageway of the upper well string part, wherein the auxiliary tool comprises an operating means for manipulating the well string interconnection means so as to disconnect the lower well string part from the upper well string part, and wherein the auxiliary tool is arranged such that it can be passed, after disconnecting the upper and lower well string parts, through an opening at the lower end of the upper well string part, so as to reach a working position in which at least part of the auxiliary tool is not radially surrounded by a part of the well string, further comprising a means for performing an operation in relation to the borehole or formation surrounding the auxiliary tool, when the auxiliary tool is in the working position, wherein the means for performing an operation is selected from the group consisting of a logging tool, a sampling tool, a fluid injection tool, a placement tool, and wherein the auxiliary tool comprises a first member which includes the lower well string connection means and a second member which includes the operating means, which second member is arranged movably so that it can assume a first and a second position relative to the first member, wherein in the first position the lower well string connection means is connectable, at least when upper and lower well string parts are interconnected, to the lower well string part without operating the well string interconnection means, and wherein after connecting the auxiliary tool to the lower well string part the well string interconnection means can be operated by moving the second member including the operating means between the first and the second position.

11. The well string assembly according to claim 10 , wherein the lower well string connection means is arranged near the lower end of the first member, wherein the operating means is arranged near the lower end of the second member, and wherein the second member is arranged longitudinally slideably along the passageway with respect to the first member, so that the first relative position is an upper position of the second member, and wherein the second member is moved relative to the first member in downward direction when moving it towards the second relative position.

12. The well string assembly according to claim 11 , wherein the first member of the auxiliary tool comprises a substantially tubular body in which the second member is coaxially slideably arranged, wherein the lower well string part comprises at its upper end an outer sleeve and a coaxial inner sleeve, wherein the upper end of the outer sleeve is arranged to cooperate with the lower well string connection means so as to lock the auxiliary tool to the outer sleeve, wherein the upper end of the inner sleeve is arranged to cooperate with the operating means of the auxiliary tool so that the interconnecting means is operated by longitudinally sliding the inner sleeve with respect to the outer sleeve.

13. The well string assembly according to claim 12 , wherein the interconnection means, auxiliary tool and optionally lower well string connection means are arranged such that the lower and upper well string part can be reconnected again after disconnecting.

14. A method of performing an operation in a borehole, external of the well string, comprising using the well string assembly of claim 13 .

15. A method of performing an operation in a borehole, comprising the steps of:

providing a well string assembly in the borehole, the well string assembly comprising an upper tubular well string part having upper and lower ends between which there is formed a passageway and a lower well string part having upper and lower ends, which lower end is spaced apart from the lower end of the upper well string part and which lower end is connectable to or includes a drill bit;

passing an auxiliary tool along the passageway of the upper well string part, thereby manipulating a well string interconnection means and thereby disconnecting the lower well string part from the upper well string part;

subsequently passing the auxiliary tool through an opening at the lower end of the upper well string part while suspending the lower well string part by the auxiliary tool;

bringing the auxiliary tool in a working position in which at least part of the auxiliary tool is not radially surrounded by a part of the well string;

performing a selected operation in a borehole when the auxiliary tool is in the working position.

16. The method of claim 15 , wherein the opening at the lower end of the upper well string part is formed after disconnecting the lower well string part from the upper well string part.

17. The method of claim 15 , further comprising suspendably connecting the auxiliary tool to the lower well string part simultaneously with or before disconnecting the lower well string part from the upper well string part.

18. A well string assembly comprising:

an upper tubular well string part having upper and lower ends between which there is formed a passageway;

a lower well string part having upper and lower ends, which lower end is spaced apart from the lower end of the upper well string part and which lower end is connectable to or includes a drill bit;

a releasable well string interconnection means for selectively interconnecting the lower and upper well string parts above the lower end of the lower well string part; and

an auxiliary tool arranged such that it can be passed along the passageway of the upper well string part, wherein the auxiliary tool comprises an operating means for manipulating the well string interconnection means so as to disconnect the lower well string part from the upper well string part, and wherein the auxiliary tool is arranged such that it can be passed, after disconnecting the upper and lower well string parts, through an opening at the lower end of the upper well string part, so as to reach a working position in which at least part of the auxiliary tool is not radially surrounded by a part of the well string, wherein the auxiliary tool further comprises

a lower well string connection means for connecting the auxiliary tool to the lower well string part simultaneously with or before disconnecting the well string interconnection means;

a first member which includes the lower well string connection means

a second member which includes the operating means, which second member is arranged movably so that it can assume a first and a second position relative to the first member, wherein in the first position the lower well string connection means is connectable, at least when upper and lower well string parts are interconnected, to the lower well string part without operating the well string interconnection means, and wherein after connecting the auxiliary tool to the lower well string part the well string interconnection means can be operated by moving the second member including the operating means between the first and the second position.

19. The well string assembly according to claim 18 , wherein the lower well string connection means is arranged near the lower end of the first member, wherein the operating means is arranged near the lower end of the second member, and wherein the second member is arranged longitudinally slideably along the passageway with respect to the first member, so that the first relative position is an upper position of the second member, and wherein the second member is moved relative to the first member in downward direction when moving it towards the second relative position.

20. The well string assembly according to claim 19 , wherein the first member of the auxiliary tool comprises a substantially tubular body in which the second member is coaxially slideably arranged, wherein the lower well string part comprises at its upper end an outer sleeve and a coaxial inner sleeve, wherein the upper end of the outer sleeve is arranged to cooperate with the lower well string connection means so as to lock the auxiliary tool to the outer sleeve, wherein the upper end of the inner sleeve is arranged to cooperate with the operating means of the auxiliary tool so that the interconnecting means is operated by longitudinally sliding the inner sleeve with respect to the outer sleeve.

21. The well string assembly according to claim 20 , wherein interconnection means, auxiliary tool and the lower well string connection means are arranged such that the lower and upper well string parts can be reconnected again after disconnecting.

22. A method of performing an operation in a borehole, comprising the steps of:

providing a well string assembly in the borehole, the well string assembly comprising an upper tubular well string part having upper and lower ends between which there is formed a passageway and a lower well string part having upper and lower ends, which lower end is spaced apart from the lower end of the upper well string part and which lower end is connectable to or includes a drill bit;

passing an auxiliary tool along the passageway of the upper well string part, thereby manipulating a well string interconnection means and thereby disconnecting the lower well string part from the upper well string part;

positioning the lower one of the lower end of the of the lower well string part or the drill bit at a position in the borehole at a distance above the bottom of the borehole;

subsequently passing the auxiliary tool through an opening at the lower end of the upper well string part;

bringing the auxiliary tool in a working position in which at least part of the auxiliary tool is not radially surrounded by a part of the well string;

performing a selected operation in a borehole when the auxiliary tool is in the working position.

23. The method of claim 22 , wherein the opening at the lower end of the upper well string part is formed after disconnecting the lower well string part from the upper well string part.

24. The method of claim 22 , further comprising suspendably connecting the auxiliary tool to the lower well string part simultaneously with or before disconnecting the lower well string part from the upper well string part.