Apparatus and methods for pressure compensated contact with the borehole wall

1. A well logging tool, comprising:

a housing;

at least one piston moveably disposed on the housing;

at least one sensor disposed on the at least one piston;

a pressure compensation system operatively coupled to the at least one piston such that a pressure at a back side of the at least one piston is substantially identical to a pressure outside the well logging tool; and

a piston deployment mechanism for deploying the at least one piston.

2. The well logging tool of claim 1 , wherein the piston deployment mechanism comprises a piston spring disposed at the back side of the at least one piston.

3. The well logging tool of claim 1 , wherein the pressure compensation system comprises a pressure-compensating chamber having an opening to an exterior of the housing, a fluid-filled reservoir, and a compensating piston moveably disposed between the pressure-compensating chamber and the fluid-filled reservoir such that a pressure inside the pressure-compensating chamber and a pressure inside the fluid-filled reservoir are substantially identical.

4. The well logging tool of claim 3 , further comprising a compensating spring connected to the compensating piston for exerting a force on the compensating piston.

5. The well logging tool of claim 4 , wherein the piston deployment mechanism comprises a piston spring disposed at the back side of the at least one piston.

6. The well logging tool of claim 1 , further comprising at least one piston liner moveably disposed between the housing and the at least one piston for extending a reach of the at least one piston.

7. The well logging tool of claim 4 , further comprising a controller connected to the compensating spring for regulating the force exerted by the compensating spring.

8. The well logging tool of claim 7 , wherein the controller is operated by a motor.

9. The well logging tool of claim 7 , wherein the controller is operated by hydraulic pressure of drilling fluids.

10. A well logging tool, comprising:

a housing;

at least one piston moveably disposed on the housing;

at least one sensor disposed on the at least one piston;

means for keeping a pressure at a back side of the at least one piston substantially identical to a pressure outside the well logging tool; and

means for deploying the at least one piston.

11. A method for well logging, the method comprising:

disposing a well logging tool in a borehole, the well logging tool having at least one piston carrying a sensor;

maintaining a pressure at a backside of the at least one piston substantially equal to a pressure outside of tie well logging tool;

deploying the at least one piston to establish contact between the ax least one piston and a wall of the borehole, and

measuring a formation property using the at least one sensor.

12. The method of claim 11 , wherein the deploying comprises exerting a force on the back side of the at least one piston by a spring.

13. The method of claim 11 , wherein the deploying comprises using a hydraulic pressure of a drilling fluid.

14. The method of claim 11 , the measuring was performed while drilling.

15. The method of claim 11 , wherein the at least one sensor is selected from a temperature sensor, a pressure sensor, a resistivity sensor, an inductive imager, a density sensor, a neutron sensor, a sonic sensor, a nuclear magnetic resonance sensor, a dipmeter, and a seismic sensor.