New Gyro While Drilling Technology Delivers Accurate Azimuth and Real-Time Quality Control for All Well Trajectories

Abstract

Summary In response to the increasing demand for a reduction in the uncertainty of wellbore placement to minimize drilling risks and potential liabilities, a new gyroscopic survey tool capable of operating at any attitude during the drilling process has been developed: an all-attitude gyro while drilling (GWD) tool. Gyro survey systems determine the direction of the survey tool in the wellbore, the tool azimuth, by use of measurements of the horizontal components of Earth's rotation in a process known as gyrocompassing or north finding. Current GWD systems are based on angular rate measurements taken about two axes only. These measurement axes are perpendicular to the direction of the wellbore and perpendicular to each other. Although two-axis systems provide accurate estimates of azimuth near vertical, this accuracy degrades as inclination increases, with the azimuth becoming indeterminate because of a mathematical singularity at 90° inclination. To overcome this, an additional rate measurement about the along-hole axis of the tool needs to be performed. Only gyroscopic instruments with three orthogonal sensitive axes, as adopted in the new tool, are capable of measuring the full horizontal Earth rate signal in all wellbore geometries. This paper describes a number of innovative technological developments that have allowed all-attitude GWD to become a reality, including a new gyroscope designed specifically for the downhole drilling environment, a new system mechanization to control gyroscope biases, a signal-processing technique known as continuous adaptive processing that estimates the g-sensitive bias errors at each gyrocompass station, and a more robust quality control scheme. The new high angle GWD system is well-suited to handle any wellbore-placement needs. These include high inclination kickoffs, infill drilling, relief wells, and the provision of improved accuracy in east/west wells. With the improved reliability of the data comes increased confidence that the resulting survey is representative of the true wellbore trajectory. The detection of gross errors can also be accomplished by comparing real-time GWD and measurement-while-drilling (MWD) data. Further, with the memory multishot capability that is available, reliable high accuracy surveys from section total depth (TD) to surface are generated, verifying both MWD and GWD performance on the trip out and eliminating the need for separate and rig-time-dependent gyroscopic surveys. Finally, a number of case studies are presented to illustrate the performance capabilities of the new gyroscopic system over a range of well trajectories and latitudes. The all-attitude GWD system not only allows the survey-reliability goals to become a reality in a cost-effective manner—as set out in the earlier quality control papers published in conjunction with the SPE Well Positioning Technical Section—but also does this in real time while drilling.