Halliburton Energy Services, Inc.

Baroid Technology, Inc.

Anne Holmes

Paul F Rodney

Gordon M Shiells

This invention relates to surveying wellbores. In particular this invention provides methods of correcting magnetic surveys for the effects introduced by magnetic mud. The method of the present invention comprises, in broad terms, measuring gravitational and magnetic fields at least one position in the wellbore; comparing the measured fields with theoretical values, and introducing scale factors to adapt the measured values to equal the theoretical values thus making it possible to cope with the effects of magnetic mud.

Wellbore surveying

A method is disclosed for correcting for the distorting effect of cross-axial magnetic interference on the readings of a well survey tool using a reduced number of measurements and measuring locations. A well survey tool connected to a drill string assembly measures the gravitational and distorted terrestrial magnetic fields at two or more axially displaced locations in an axially extending well bore. The measurements are made from two or more different orientations (tool faces) of the tool about the well axis. Initial values for the tool inclination, azimuth and tool face angle are first determined without correction for the cross-axial magnetic interference introduced by the drill string assembly and the survey tool. Measurements made at each location and tool face are referred to the toots coordinate system and variations introduced by the different tool face angles are used to estimate the cross-axial interference. The estimated interference is used to improve the estimate of the azimuth. Where the azimuth and inclination vary between measurement, the process is iterated until a desired degree of convergence is reached or until no further improvement in the azimuth is possible. Where azimuth and inclination remain substantially constant between readings, the azimuth correction is directly calculated without iteration. Where the cross-axial interference varies slowly and systematically between measurements, a variational trend in the cross-axial interference is calculated with trend analysis techniques. Where rapid variations between cross-axial measurements are detected such that the only information obtained is that the cross interference exists, the method is used to determine that the related azimuth calculations may be identified as being faulty. After correction for the cross-axial interference, the existence of an external magnetic field acting on the measurements is also determined from residual error that varies in magnitude and direction along the well bore trajectory.

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Cheltenham, United Kingdom

Anne Holmes