Fluid-actuated hammer bit

1. A drilling assembly, comprising:

a string of downhole tools connected to a drill bit with a bit body intermediate a shank and a working face;

the drill bit being connected to the string of tools at the shank;

a continuous fluid passageway being formed within the bit body and the string of tools;

a valve mechanism disposed within the fluid passageway adapted to substantially cyclically build-up and release pressure within the fluid passageway such that a pressure build-up results in radial expansion of at least a portion of the fluid passageway and wherein a pressure release results in a contraction of the portion of the fluid passageway;

wherein the expansion and contraction of the portion of the fluid passageway varies a weight loaded to the drill bit.

2. The assembly of claim 1 , wherein the valve mechanism comprises a rotary valve or a relief valve.

3. A method for forming a wellbore, comprising the steps of:

providing a string of downhole tools connected to a drill bit with a bit body intermediate a shank and a working face, the drill bit being connected to the string of tools at the shank;

providing a continuous fluid passageway being formed within the bit body and the string of tools;

deploying the bit when connected to the string of tools into a wellbore;

continuously passing fluid through the fluid passageway;

loading at least a portion of the weight of the string of downhole tools to the drill bit;

substantially cyclically building up and releasing pressure within the fluid passageway such that a pressure build-up results in radial expansion of at least a portion of the fluid passageway and wherein a pressure release results in a contraction of the portion of the fluid passageway;

substantially cyclically varying the weight loaded to the drill bit by expanding and contracting the portion of the fluid passageway.

4. The method of claim 3 , wherein the step of substantially cyclically varying the weight loaded to the drill bit vibrates the drill bit.

5. The method of claim 4 , wherein a magnitude of the vibrations varies according to the physical properties of a formation being drilled.

6. The method of claim 4 , wherein the vibrations of the tool string produce acoustic signals; the signals being received by acoustic receivers located at the tool bit, tool string, or earth surface.

7. The method of claim 3 , wherein the drill bit is a shear bit or a rollercone bit.

8. The method of claim 3 , wherein the drill bit is rigidly connected to the string of tools at the shank.

9. The method of claim 3 , wherein the step of expanding and contracting the inner wall of the tool string is continuous.

10. The method of claim 3 , wherein the step of building up and releasing pressure within the fluid passageway is controlled by a valve mechanism disposed within the fluid passageway.

11. The method of claim 10 , wherein the valve mechanism comprises a rotary valve or a relief valve.

12. The method of claim 10 , wherein the valve mechanism is adapted to restrict all fluid flow within the fluid passageway.

13. The method of claim 10 , wherein the valve mechanism is adapted to restrict a portion of fluid flow within the fluid passageway.

14. The method of claim 10 , wherein at least a portion of a jack element being disposed within the body and comprising an end forming at least a portion of the valve mechanism in the fluid passageway and a distal end substantially protruding from the working face.

15. The method of claim 14 , wherein the jack element is rotationally isolated from the string of downhole tools.

16. The method of claim 10 , wherein a portion of the valve mechanism is adapted for attachment to a driving mechanism.

17. The method of claim 16 , wherein the driving mechanism is a motor, turbine, electric generator, or a combination thereof.

18. The method of claim 16 , wherein the driving mechanism is controlled by a closed loop system.

19. The method of claim 3 , wherein the substantially cyclical building-up and releasing of pressure comprises a rate of 0.1 to 500 cycles per second.

20. The method of claim 3 , wherein the step of substantially cyclically varying the weight loaded to the drill bit induces a resonant frequency of the formation being drilled.