The uncertainties associated with current wellbore surveying techniques can generate time consuming challenges when sidetracking around a fish at high inclinations. If the sidetrack is not planned with a significant safety margin and/or the bottomhole assembly does not include the proper amount of non-magnetic spacing then the MWD surveys of the sidetrack wellbore can be compromised significantly by external magnetic interference leaving the well to be drilled blind. One means of monitoring the approach between a drilling well and a secondary wellbore has been to use the MWD sensors to monitor for external magnetic interference. When monitoring the convergence of two wellbores in a close approach situation, the relative positions between the wells must be calculated. However, calculating the relative position between wells is a challenge when the inclination of the two wellbores exceeds 80 degrees.

This method increases safety while drilling. Using GWD eliminates the need to run wireline conveyed gyros, saving considerable expense. GWD allows data to be collected frequently while the drilling progresses, minimizing the risk of intercepting the fish without slowing the drilling process. Equally it is possible to be flexible in sidetrack trajectories as any attitude is available. There is no longer a need to steer by inclination only, improving efficiency in drilling operations.

This paper looks at a case study where a passive MWD ranging method uses both the output of the magnetic sensors from the MWD and the directional information calculated from an all-inclination GWD. This allows the calculation of the spatial relationship (distance and direction), between the drilling well and the fish, at 90 degrees of inclination. As the drilling sidetrack and the fish are at 90 degrees inclination and the two wellbores are not parallel, a high inclination gyro must be used to calculate the azimuth required for passive ranging calculations. Since an electromagnetic target cannot be placed in the original hole, only a single entry ranging technique can be used. This tool combination allows the positional relationship between the drilling well and the fish to be continuously monitored until the collision risk has passed.