Method of qualifying a borehole survey

• US 5,787,997 A
• Filed: 11/21/1996
• Issued: 08/04/1998
• Est. Priority Date: 11/21/1995
• Status: Expired due to Term

• Current Assignee: Shell Oil Company (Shell Plc)
• Original Assignee: Shell Oil Company (Shell Plc)
• Inventors: Hartmann, Robin Adrianus
• Primary Examiner(s): Tsay, Frank
• Application Number:
  US08/752,988
• Time in Patent Office: 621 Days
• Field of Search: 175/45, 175/50, 175/61, 175/40, 33/302, 33/304
• US Class Current: 175/45
• CPC Class Codes: E21B 47/022 of the borehole, e.g. using geomagnetism

Abstract

A method of qualifying a survey of a borehole formed in an earth formation is provided. The method includes the steps of a) selecting a sensor for measuring an earth field parameter and a borehole position parameter in said borehole, b) determining theoretical measurement uncertainties of said parameters when measured with the sensor, c) operating said sensor so as to measure the position parameter and the earth field parameter at a selected position in the borehole, d) determining the difference between the measured earth field parameter and a known magnitude of said earth field parameter at said position, and determining the ratio of said difference and the theoretical measurement uncertainty of the earth field parameter, and e) determining the uncertainty of the measured position parameter from the product of said ratio and the theoretical measurement uncertainty of the position parameter.

Claims

1. A method of qualifying a survey of a borehole formed in an earth formation, the method comprising:

a) selecting a sensor for measuring an earth field parameter and a borehole position parameter in said borehole;

b) determining theoretical measurement uncertainties of said parameters when measured with the sensor;

c) operating said sensor so as to measure the position parameter and the earth field parameter at a selected position in the borehole;

d) determining the difference between the measured earth field parameter and a known magnitude of said earth field parameter at said position, and determining the ratio of said difference and the theoretical measurement uncertainty of the earth field parameter; and

e) determining the uncertainty of the measured position parameter from the product of said ratio and the theoretical measurement uncertainty of the position parameter.

2. The method of claim 1, wherein said sensor comprises a solid state magnetic survey tool including at least one magnetometer and at least one accelerometer.

3. The method of claim 2, wherein the solid state magnetic survey tool comprises three magnetometers and three accelerometers.

4. The method of claim 1 wherein the step of determining theoretical measurement uncertainties of said parameters comprises determining the theoretical measurement uncertainties of a group of sensors to which the selected sensor pertains.

5. The method of claim 1 wherein said theoretical measurement uncertainties are based on at least one of the sensor uncertainty and an uncertainty of the earth field parameter.

6. The method of claim 1 further comprising the step of disqualifying the measurements if said ratio exceeds 1.

7. The method of claim 1 wherein said position parameter is selected from the borehole inclination and the borehole azimuth.

8. The method of claim 7, wherein in a first mode of operation the position parameter forms the borehole inclination, the earth field parameter forms the earth gravity field, and the theoretical uncertainties of the position parameter and the earth field parameter are based on the sensor uncertainty.

9. The method of claim 7 wherein in a second mode of operation the position parameter forms the borehole azimuth, the earth field parameter forms the earth magnetic field strength, and the theoretical uncertainties of the position parameter and the earth field parameter are based on the sensor uncertainty.

10. The method of claim 7 wherein in a third mode of operation the position parameter forms the borehole azimuth, the earth field parameter forms the earth magnetic field strength, and the theoretical uncertainties of the position parameter and the earth field parameter are based on the uncertainty of the earth magnetic field.

11. The method of claim 7 wherein in a fourth mode of operation the position parameter forms the borehole azimuth, the earth field parameter forms the dip-angle of the earth magnetic field, and the theoretical uncertainties of the position parameter and the earth field parameter are based on the sensor uncertainty.

12. The method of claim 7 wherein in a fifth mode of operation the position parameter forms the borehole azimuth, the earth field parameter forms the dip angle of the earth magnetic field, and the theoretical uncertainties of the position parameter and the earth field parameter are based on the uncertainty of the earth field parameter.

13. The method of claim 9 wherein the step of determining the uncertainty of the measured position parameter comprises determining the maximum absolute value of the uncertainties of the measured position parameters determined in the second, third, fourth and fifth mode of operation.