Downhole jarring tool adjuster

1. A downhole jarring apparatus, comprising:

a first mandrel;

a shaft, wherein the shaft and the first mandrel are configured to releasably engage;

a second mandrel;

a collar operatively attached to the shaft, wherein an axial position of the collar relative to the shaft is adjustable by access through an opening in an outer surface of the second mandrel; and

an energy store configured to exert a force against the shaft, wherein the force is adjustable in response to the axial position of the collar relative to the shaft.

2. The downhole jarring apparatus of claim 1 wherein the collar is operatively attached to the shaft by threaded engagement such that the axial position of the collar relative to the shaft is adjustable in response to rotation of the collar relative to the shaft via access of the collar through the opening in the outer surface of the second mandrel.

3. The downhole jarring apparatus of claim 1 wherein the outer surface of the second mandrel is substantially cylindrical.

4. The downhole jarring apparatus of claim 1 wherein the axial position of the collar relative to the shaft is adjustable by access through the opening in the outer surface of the second mandrel without disassembling any of the downhole jarring apparatus.

5. The downhole jarring apparatus of claim 1 wherein the energy store comprises a compressible element selected from the group consisting of:

a plurality of compressible washers;

a plurality of Belleville washers; and

a spring.

6. The downhole jarring apparatus of claim 1 wherein a first end of the energy store is substantially fixed relative to the second mandrel and a second end of the energy store is substantially fixed relative to the collar.

7. The downhole jarring apparatus of claim 1 wherein the first mandrel includes a hammer mandrel and an extension extending from the hammer mandrel, wherein a first shape of a first end of the extension and a second shape of a second end of the shaft cooperate to releasably couple therewith.

8. The downhole jarring apparatus of claim 7 wherein the first shape is substantially conical.

9. The downhole jarring apparatus of claim 1 wherein the shaft includes a collet having a radially expandable end configured to cooperate with the first mandrel to releasably couple therewith.

10. The downhole jarring apparatus of claim 9 wherein at least a portion of an internal profile limits expansion of the radially expandable end of the collet, wherein the internal profile includes:

a first recess configured to allow sufficient expansion of the radially expandable end of the collet to allow the first mandrel and the shaft to at least temporarily detach; and

a second recess configured to allow sufficient expansion of the radially expandable end of the collet to allow the first mandrel and the shaft to at least temporarily attach.

11. The downhole jarring apparatus of claim 10 wherein the second mandrel includes a third mandrel having the internal profile and the first and second recesses.

12. The downhole jarring apparatus of claim 1 further comprising a setscrew configured to limit rotation of the collar relative to the shaft.

13. The downhole jarring apparatus of claim 1 further comprising a collar guard having a guard opening, wherein the collar guard is rotatable around an axis of the second mandrel to selectively conceal the opening in the outer surface of the second mandrel and reveal the second mandrel outer surface opening through the guard opening.

14. The downhole jarring apparatus of claim 13 further comprising a setscrew configured to limit rotation of the collar guard relative to the second mandrel.

15. The downhole jarring apparatus of claim 1 wherein an inner profile that is rotationally fixed relative to the second mandrel includes a first cross-sectional shape configured to cooperate with a second cross-sectional shape of at least a portion of the first mandrel to limit rotation of the first mandrel relative to the second mandrel.

16. The downhole jarring apparatus of claim 1 further comprising a work string coupled at least indirectly to at least one of the first and second mandrels.

17. A method, comprising:

attaching a downhole jarring apparatus to a work string, the downhole jarring apparatus including:

a shaft and a first mandrel configured to releasably engage;

a collar operatively attached to the shaft, wherein an axial position of the collar relative to the shaft is adjustable by access through an opening in an outer surface of a second mandrel; and

an energy store configured to exert a force against the shaft, wherein the force is adjustable in response to the axial position of the collar relative to the shaft;

engaging the shaft and the first mandrel if the shaft and the first mandrel are disengaged;

adjusting the force exerted by the energy store against the shaft by accessing the collar through the opening to adjust the axial position of the collar relative to the shaft; and

increasing a work string force applied by the work string to the downhole jarring apparatus until the shaft and the first mandrel disengage.

18. The method of claim 17 wherein adjusting the force exerted by the energy store includes rotating the collar to adjust the axial position of the collar relative to the shaft.

19. The method of claim 18 wherein accessing the collar through the opening includes temporarily loosening a setscrew configured to limit rotation of the collar relative to the shaft.

20. The method of claim 17 wherein accessing the collar through the opening includes rotating a collar guard having a guard opening to temporarily reveal the second mandrel outer surface opening through the guard opening.