Combined Gyroscopic and Magnetic Surveys Provide Improved Magnetic Survey Data and Enhanced Survey Quality Control

Abstract

The use of advanced solid-state gyroscopic sensors has now become both a viable and practical option for high accuracy wellbore placement, with the potential to out-perform traditional mechanical gyroscopic systems. This paper describes how the contributions of the new gyroscope technology are causing service providers to reconsider current survey practices, and to examine how the new gyroscopic survey tools can be best used for wellbore surveying and real-time wellbore placement. The simultaneous application of multiple survey tools, largely made possible as a result of the unique attributes of solid-state gyroscopic sensors (including small size and significant power reduction), has clear benefits in terms of enhanced well placement, reliability and the detection of gross errors in the survey process. Further benefits accrue through the combination of different, but complimentary survey methods. This paper focuses mainly on the benefits of combining gyroscopic and magnetic measurements to reduce or remove the known errors related to the Earth's magnetic field to which magnetic survey systems are susceptible; errors in total magnetic field, declination and dip angle. In this context, the use of statistical estimation techniques based on performance models of the survey systems used is described. For post-drilling surveys (using drop survey tools or wireline-conveyed tools for example), post-run analysis of the data using least-squares estimation techniques is appropriate. Alternative methods capable of achieving real-time data correction during drilling are also described and results are presented to demonstrate the potential for enhanced magnetic survey performance. The principles described may be used when running basic magnetic measurement while drilling (MWD) systems, and for systems that employ field correction methods, such as the various in-field referencing (IFR) techniques, that are frequently used. The proposed methodology is of particular benefit in the former case, allowing enhanced magnetic surveying to be achieved without the need for expensive and complex magnetic field correction procedures. The potential also exists either to identify or to correct possible errors in the IFR data when such methods are used. This information may be of great value for the safe drilling of additional wells in the same region.