New Gyro Drilling Technology Delivers Accurate Azimuth and Real-Time Quality Control for All Well Trajectories
In response to the increasing demand for a reduction in the uncertainty of wellbore placement to minimize drilling risks and potential liabilities, a new gyro survey tool capable of operating at any attitude during the drilling process has been developed; an all attitude gyro while drilling tool.
Gyro survey systems determine the direction of the survey tool in the wellbore, the tool azimuth, using measurements of the horizontal components of Earth’s rotation in a process known as gyro compassing or north finding. Current gyro while drilling systems are based on angular rate measurements taken about two axes only. These measurement axes are both perpendicular to the direction of the wellbore and perpendicular to each. Whilst two-axis systems provide accurate estimates of azimuth near vertical, this accuracy degrades as inclination increases, with the azimuth becoming indeterminate due to a mathematical singularity at 90 degrees inclination. To overcome this, an additional rate measurement about the along-hole axis of the tool needs to be performed. Only gyro instruments with 3 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 advances that have allowed all attitude gyro while drilling (GWD) to become a reality, including:
- a brand new gyroscope designed specifically for the down hole drilling environment
- a new system mechanization to control gyro biases
- a signal processing technique known as Continuous Adaptive Processing that estimates the g-sensitive bias errors at each gyrocompass station
- 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 kick-offs, in-fill 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 MWD data. Further, with the memory multi-shot capability that is available, reliable high accuracy surveys from section TD to surface are generated verifying both MWD and GWD performance on the trip out 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 gyro system over a range of well trajectories and latitudes.
The all attitude gyro while drilling system not only allows the survey reliability goals to become a reality in a cost effective manner – as set out in the earlier QC papers published in conjunction with the SPE Well Positioning Technical Section – but also does this real time, while drilling.