Method and system for predicting performance of a drilling system

1. A method comprising:

measuring in real time a weight on bit, a torque on bit, and a bit revolutions per minute over a drilling interval in a wellbore;

inputting a modeled bit signature;

calculating, in real time, using a torque versus weight on bit model, an updated bit friction slope and an updated worn bit friction slope for the drilling interval using the measured weight on bit, torque on bit, and bit revolutions per minute and modeled bit signature; and

calculating at least one of an updated operating weight on bit and an updated operating bit revolutions per minute using the updated bit friction slope and the updated worn bit friction slope for drilling a next drilling interval of the wellbore;

wherein the updated bit friction slope and the updated worn bit friction slope are each slopes determined by a ratio of a change in the torque on bit to a change in the weight on bit.

2. The method of claim 1 further comprising measuring at least one of rate of penetration, and interval drilled.

3. The method of claim 2 wherein at least one of rate of penetration, weight on bit, torque on bit, bit revolutions per minute, and interval drilled is measured by a downhole sensor.

4. The method of claim 1 further comprising filtering the updated bit friction slope and the updated worn bit friction slope.

5. The method of claim 4 wherein the filtering comprises calculating a rolling average over a number of intervals.

6. The method of claim 1 further comprising storing at least the updated weight on bit and bit revolutions per minute, the updated bit friction slope and the updated worn bit friction slope in a database.

7. A system for drilling a well comprising:

a drill string in a wellbore having a bit at a distal end thereof;

at least one sensor to measure at least one of a weight on bit, a torque on bit, and a bit revolutions per minute over a drilling interval in a wellbore; and

a computer controller having a set of instructions stored therein to process the measured sensor measurements over the drilled interval to calculate, in real time, an updated bit friction slope and an updated worn bit friction slope and to calculate at least one of an updated weight on bit and a bit revolutions per minute for drilling the next drilling interval of the wellbore based on the updated bit friction slope and the updated worn bit friction slope;

wherein the updated bit friction slope and the updated worn bit friction slope are each slopes determined by a ratio of a change in the torque on bit to a change in the weight on bit.

8. The system of claim 7 wherein the at least one sensor comprises a sensor chosen from the group of: a weight on bit sensor, a torque sensor, and a bit revolutions per minute sensor.

9. The system of claim 8 wherein at least one of the at least one sensor is a downhole sensor disposed in the drill string.

10. The system of claim 7 wherein the computer controller further comprises instruction to filter the updated bit friction slope and an updated worn bit friction slope before calculating the updated weight on bit and a bit revolutions per minute.

11. The system of claim 7 further comprising a logging tool disposed in the drill string.

12. The system of claim 11 wherein the logging tool is chosen from the group consisting of a measurement while drilling tool and a logging while drilling tool.

13. A computer readable medium having a set of instructions stored thereon such that when executed by a computer perform a set of operations comprising:

calculating, in real time, using a torque versus weight on bit model, an updated bit friction slope and an updated worn bit slope for the drilling interval using a weight on bit, a torque on bit, and a bit revolutions per minute measured over a drilling interval in a wellbore; and

calculating an updated weight on bit and a bit revolutions per minute for drilling the next drilling interval of the wellbore based on the updated bit friction slope and the updated worn bit slope;

wherein the updated bit friction slope and the updated worn bit friction slope are each slopes determined by a ratio of a change in the torque on bit to a change in the weight on bit.

14. The computer readable medium of claim 13 wherein the operations further comprise filtering the updated bit friction slope and the updated worn bit slope.

15. The computer readable medium of claim 13 wherein the operations further comprise calculating a rolling average over a number of intervals.