Phase estimation from rotating sensors to get a toolface

1. A method of estimating an orientation of a bottomhole assembly (BHA), the BHA including a drillbit and at least one measurement-while-drilling (MWD) sensor, in a borehole intersecting an earth formation, the method comprising:

obtaining a non-axial component of a magnetic field using a measurement from a magnetic sensor responsive to the magnetic field at a location on the BHA closer to the drillbit than the closest sensor to the drillbit of the at least one MWD sensor;

estimating an axial component of the magnetic field from the non-axial component of the magnetic field; and

estimating a near-bit azimuth of the BHA at the location using the axial component of the magnetic field.

2. The method of claim 1 further comprising obtaining the non-axial component using a measurement from an uncalibrated magnetic sensor on the BHA.

3. The method of claim 2 wherein the uncalibrated magnetic sensor is a near-bit magnetometer.

4. The method of claim 2 wherein the measurement comprises a cross-axial component of the magnetic field.

5. The method of claim 1 further comprising estimating the near-bit azimuth using an additional measurement of the magnetic field remote from the bit.

6. The method of claim 5 wherein estimating the near-bit azimuth using the additional measurement comprises applying to the non-axial component at least one of: i) a scale factor determined from a measurement of the magnetic field at a magnetometer remote from the bit while the BHA is at a borehole depth and a measurement of the magnetic field from a near-bit magnetometer while the BHA is at the borehole depth; ii) a scale factor determined from a measurement of the magnetic field at a magnetometer remote from the bit at a borehole depth and a measurement of the magnetic field from a near-bit magnetometer at the borehole depth; and iii) a scale factor determined from a measurement of the magnetic field from a near-bit magnetometer at a borehole depth and axial field strength at the steering unit estimated as a function of total magnetic field strength (HTA), a magnetic dip angle (DIPA), near-bit inclination (INCA), and an azimuth orientation of the steering unit.

7. The method of claim 1 further comprising:

detecting a trend in near-bit azimuth change over time using the estimated near-bit azimuth ; and

using the trend to identify azimuthal walk.

8. The method of claim 1 further comprising using the estimated near-bit azimuth of the BHA for controlling a direction of drilling.

9. The method of claim 1 further comprising estimating the axial component of the near-bit magnetic field using an additional measurement of the magnetic field remote from the bit.

10. The method of claim 1 further comprising estimating the component Hxy.

11. The method of claim 1 further comprising correcting the estimated axial component using an offset between a gravitational toolface and a magnetic toolface.

12. The method of claim 1 , further comprising transmitting information about the estimated orientation to a surface location.

13. The method of claim 12 , where the information is transmitted to the surface location by one of: mud pulse telemetry, electromagnetic telemetry, acoustic telemetry, wired drillpipe communication, the wired drill pipe comprising direct electrical transmission, inductive coupling, capacitive coupling or optical transmission.

14. The method of claim 12 , further comprising sending at least one command to the drilling BHA, in response to the received information about the BHA orientation.

15. The method of claim 12 , further comprising changing at least one drilling parameter at the surface location, in response to the received information about the BHA orientation, the parameter chosen from a group of: weight on bit, drilling fluid flow rate, and drill string rotational speed.

16. The method of claim 1 , wherein the estimating the near-bit azimuth of the BHA is conducted at a surface location.

17. An apparatus for conducting drilling operations, the apparatus comprising:

a bottomhole assembly (BHA) configured to be conveyed into a borehole, the BHA including a drillbit and at least one measurement-while-drilling (MWD) sensor;

at least one magnetic sensor at a location on the BHA closer to the drillbit than the closest sensor to the drillbit of the at least one MWD sensor; and

at least one processor configured to:

obtain a non-axial component of a magnetic field using a measurement from the magnetic sensor responsive to the magnetic field;

estimate an axial component of the magnetic field from the non-axial component of the magnetic field; and

estimate a near-bit azimuth of the BHA at the location using the axial component of the magnetic field.

18. The apparatus of claim 17 wherein the at least one near-bit magnetic sensor is uncalibrated.

19. The apparatus of claim 17 wherein the at least one near-bit magnetic sensor is configured to provide a signal responsive to a cross-axial component of the magnetic field.