Apparatus and method for determining corrected weight-on-bit

1. A method of determining a corrected weight on a drill bit (weight-on-bit) during drilling of a wellbore, comprising:

measuring a first weight-on-bit while a fluid flows through the drill and no applied weight-on-bit using a sensor on a sensor package in the drill bit, wherein the sensor package includes a member bound by end sections that have a larger transverse dimension than the member, wherein the member is located within a passage that is separate from a flow path for drilling fluid;

measuring a second weight-on-bit with the sensor in the drill bit while drilling the wellbore using the drill bit; and

determining the corrected weight-on-bit from the first weight-on-bit and the second weight-on bit.

2. The method of claim 1 , wherein the corrected weight-on-bit is determined by subtracting the first weight-on-bit from the second weight-on-bit.

3. The method of claim 1 , wherein the corrected weight-on-bit is determined by one of: processing signals from the sensor downhole or on the surface.

4. The method of claim 1 , wherein measuring the first weight-on-bit comprises:

determining a temperature of the fluid flowing through drill bit;

determining acceleration of the drill bit; and

processing signals from the sensor in the drill to measure the first weight-on-bit when the determined temperature meets a selected criterion and the determined acceleration meets a selected criterion.

5. The method of claim 4 further comprising:

determining the temperature using a temperature sensor in the drill bit; and

determining the acceleration using an accelerometer in the drill bit.

6. The method of claim 1 further comprising determining a pressure differential across an effective area of the drill bit while drilling the wellbore, wherein determining the corrected weight-on-bit comprises determining the corrected weight-on-bit from the first weight-on-bit, the second weight-on bit and the determined pressure differential.

7. The method of claim 6 , wherein determining the pressure differential comprises determining pressure differential between a pressure inside the drill bit and a pressure outside the drill bit.

8. The method of claim 6 , wherein determining the pressure differential comprises using a sensor having a first sensing element sensing pressure at the inside of the drill bit and a second sensing element sensing pressure at the outside the drill bit.

9. A drill bit comprising:

a first sensor in the drill bit for measuring a weight-on-bit, wherein the sensor is positioned on a sensor package that includes a member bound by end sections that have a larger transverse dimension than the member, wherein the member is located within a passage that is separate from a flow path for drilling fluid; and

a processor configured to:

measure a first weight-on-bit using the measurements made by the first sensor with a fluid flowing through the drill bit and no weight applied to the drill bit;

measure a second weight-on-bit using measurements from the first sensor while drilling the wellbore using the drill bit; and

determine a corrected weight-on-bit from the first weight-on-bit and the second weight-on bit.

10. The drill bit of claim 9 , wherein the first sensor is disposed in a shank of the drill bit configured to measure weight-on-bit.

11. The drill bit of claim 10 , wherein the processor is configured to determine the corrected weight-on-bit by subtracting the first weight on-bit from the second weight-on-bit.

12. The drill bit of claim 11 , wherein the processor is enclosed in a module in the drill bit.

13. The drill bit of claim 12 further comprising a data communication device coupled to the processor and configured to transmit data from the drill bit to a location outside the drill bit.

14. The drill bit of claim 9 , further comprising a second sensor in the drill bit configured to measure pressure differential across an effective area of the drill bit; and

a processor configured to:

determine a third weight-on-bit from the measurements of the pressure differential; and

determine corrected weight-on-bit using the determined first weight-on-bit, the second weight-on-bit and the third weight-on-bit.

15. The apparatus of claim 14 , wherein the second sensor comprises a first sensing element configured to measure pressure inside the drill bit and a second sensing element configured to measure pressure outside the drill bit.