Downhole disconnect tool

1. An apparatus comprising:

a downhole tool for connecting and selectively disconnecting within a wellbore first and second portions of a downhole tool string from each other, wherein the downhole tool comprises:

a first connector sub connectable with the first portion of the downhole tool string;

a second connector sub connectable with the second portion of the downhole tool string;

an internal chamber; and

a fastener connecting the first and second connector subs, wherein the fastener fluidly separates the internal chamber into a first chamber portion and a second chamber portion, wherein the first chamber portion is fluidly connected with a space external to the downhole tool, and wherein the downhole tool is selectively operable to disconnect the first and second connector subs from each other to disconnect the first and second portions of the downhole tool string from each other.

2. The apparatus of claim 1 wherein the fastener contains an explosive charge selectively operable to detonate to sever the fastener and thus disconnect the first and second connector subs from each other.

3. The apparatus of claim 1 wherein the fastener comprises:

a first fastener portion connected with the first connector sub; and

a second fastener portion connected with the second connector sub, wherein the downhole tool is selectively operable to disconnect the first and second fastener portions from each other to thereby disconnect the first and second connector subs from each other.

4. The apparatus of claim 3 wherein the fastener is or comprises a bolt, wherein the first fastener portion is or comprises a shank of the bolt, and wherein the second fastener portion is or comprises a head of the bolt.

5. The apparatus of claim 3 wherein the second fastener portion is latched against a shoulder of the second connector sub, and wherein the second fastener portion is movable within the internal chamber when the first and second fastener portions are disconnected from each other.

6. The apparatus of claim 3 wherein, while the downhole tool is conveyed within the wellbore, a port permits wellbore fluid to flow into the first chamber portion from the wellbore thereby forming a pressure differential between pressure within the first chamber portion and pressure within the second chamber portion, and wherein, after the first and second fastener portions are disconnected from each other, the pressure differential facilitates movement of the second fastener portion within the internal chamber to permit flow of the wellbore fluid into the second chamber portion.

7. The apparatus of claim 1 wherein, while the downhole tool is conveyed within the wellbore:

an opening in the downhole tool permits pressure within the first chamber portion to be maintained substantially equal to pressure within the wellbore external to the downhole tool; and

the fastener fluidly isolates the second chamber portion from the first chamber portion to maintain pressure within the second chamber portion appreciably lower than the pressure within the first chamber portion thereby forming a pressure differential between the pressure within the first chamber portion and the pressure within the second chamber portion.

8. The apparatus of claim 1 wherein, while the downhole tool is conveyed within the wellbore, the fastener blocks wellbore fluid from flowing into the second chamber portion.

9. An apparatus comprising:

a downhole tool for connecting and selectively disconnecting within a wellbore first and second portions of a downhole tool string from each other, wherein the downhole tool comprises:

a first connector sub connectable with the first portion of the downhole tool string;

a second connector sub connectable with the second portion of the downhole tool string, wherein the first and/or second connector subs at least partially define an internal chamber; and

a fastener connecting the first and second connector subs and fluidly isolating the internal chamber from external space, wherein the fastener is separable into first and second fastener portions to disconnect the first and second connector subs and thereby disconnect the first and second portions of the downhole tool string from each other.

10. The apparatus of claim 9 wherein, while the downhole tool is conveyed within the wellbore, the fastener blocks wellbore fluid from flowing into the internal chamber.

11. The apparatus of claim 9 wherein, while the downhole tool is conveyed within the wellbore, the fastener fluidly isolates the internal chamber from wellbore fluid to maintain pressure within the internal chamber appreciably lower than wellbore fluid pressure thereby forming a pressure differential across the fastener.

12. The apparatus of claim 11 wherein, while the downhole tool is conveyed within the wellbore:

the second fastener portion blocks the wellbore fluid from entering the internal chamber; and

after the first and second fastener portions are separated from each other, the pressure differential facilitates movement of the second fastener portion within the internal chamber to permit the wellbore fluid to flow into the internal chamber.

13. The apparatus of claim 9 wherein the fastener divides the internal chamber into first and second chamber portions, wherein the first chamber portion is fluidly connected with the external space, and wherein the fastener fluidly isolates the first chamber portion from the second chamber portion.

14. The apparatus of claim 9 wherein the fastener contains an explosive charge selectively operable to detonate to separate the fastener into the first and second fastener portions.

15. The apparatus of claim 9 wherein the second fastener portion is latched against a shoulder of the second connector sub, and wherein the second fastener portion is movable within the internal chamber when the first and second fastener portions are separated from each other.

16. A method comprising:

connecting a first connector sub of a downhole tool with a first portion of a downhole tool string and connecting a second connector sub of the downhole tool with a second portion of the downhole tool string to connect the first and second portions of the downhole tool string, wherein a fastener of the downhole tool connects the first and second connector subs;

conveying the downhole tool string within a wellbore while the fastener blocks wellbore fluid from flowing into an internal chamber of the downhole tool; and

operating the downhole tool to separate the fastener into first and second fastener portions thereby permitting the wellbore fluid to flow into the internal chamber to disconnect the first and second connector subs and thus disconnect the first and second portions of the downhole tool string from each other.

17. The method of claim 16 further comprising connecting the second fastener portion with the second connector sub by slidably inserting at least a portion of the fastener into the internal chamber such that the second fastener portion:

is disposed against a shoulder of the second connector sub; and

fluidly seals the internal chamber.

18. The method of claim 16 further comprising, while conveying the downhole tool within the wellbore, maintaining the internal chamber at a pressure that is appreciably lower than hydrostatic wellbore pressure.

19. The method of claim 16 wherein, after the first and second fastener portions are separated from each other, wellbore fluid pressure facilitates movement of the second fastener portion within the internal chamber to permit the wellbore fluid to flow into the internal chamber.

20. The method of claim 16 wherein operating the downhole tool comprises detonating an explosive charge disposed in association with the fastener to separate the fastener into the first and second fastener portions.