System and method for EM ranging in oil-based mud

摘要

Nearby conductors such as pipes, well casing, etc., are detectable from within a borehole filled with an oil-based fluid. At least some method embodiments provide a current flow between axially-spaced conductive bridges on a drillstring. The current flow disperses into the surrounding formation and causes a secondary current flow in the nearby conductor. The magnetic field from the secondary current flow can be detected using one or more azimuthally-sensitive antennas. Direction and distance estimates are obtainable from the azimuthally-sensitive measurements, and can be used as the basis for steering the drillstring relative to the distant conductor. Possible techniques for providing current flow in the drillstring include imposing a voltage across an insulated gap or using a toroid around the drillstring to induce the current flow.

主权项

1. A method for detecting a conductive feature from a borehole filled with an oil-based fluid, the method comprising:
providing current flow between two axially-spaced conductive bridges on a conductive tubular in the borehole, said current flow dispersing into a surrounding formation and causing a secondary current flow in the conductive feature; detecting a magnetic field from the secondary current flow with at least one azimuthally-sensitive antenna in the borehole; and obtaining magnetic field measurements at multiple azimuths from the borehole and, based at least in part on said measurements, determining a direction of the conductive feature from the borehole.