First Successful Field Test of Continuous Surveying While Drilling in Slim Hole

Abstract Definitive directional surveys are typically acquired off-bottom in a static environment using a measurement-while-drilling (MWD) tool embedded in the bottom hole assembly (BHA). The flow-off survey process can take up to 15 minutes. With continuous surveying it is possible to shrink the flow-off time to the drill pipe connection time, reducing the risk of stuck pipe events. Hence, it is desirable to take survey data continuously during drilling – covering a fine point grid along the borehole. Although the concept of rotational continuous surveying (RCS) was introduced previously, the lack of proper digital signal processing within the surveying tool has made it impossible to cope with shocks and vibrations caused by the harsh drilling environment. Furthermore, the rotation of the drill string causes eddy currents to contribute an additional cross-axial offset and deteriorate results, if not properly compensated. Recent advancements in digital signal processing, algorithmic developments, and error modeling enable high-accuracy RCS. As the drill string is rotating, the raw sensor measurements from accelerometers and magnetometers are oscillating and cannot be averaged. Therefore, sensor measurements need to be transformed to allow filtering. Such a transformed survey set is created by calculating a complex envelope of the sensor signals. This approach opens the opportunity for rotational continuous surveying also for existing MWD survey tools. As a quality assessment, RCS data from two wellbore profiles are compared statistically to static surveys by means of Chi-Square tests on direction (inclination and azimuth) as well as position, utilizing appropriate error models for static and continuous operational modes and correlation assumptions. The results verify that special attention should be paid to precise cross-axial sensor phase delay compensation prior to deriving a definitive continuous survey at surface. Such phase delays are caused by different filters in the magnetometer and accelerometer signal paths and the magnetic field induced by the eddy currents. Static-survey-like quality can only be achieved when phase shifts are correctly compensated. The paper presents results and case studies of continuous surveying with small diameter rotary steerable BHAs - an industry first. Fundamental principles, challenges, and relevant system components are outlined and a method for conducting downhole quality control is discussed. RCS has the potential to reduce costs and safety risks, and improve battery utilization, wellbore positioning, micro-dogleg detection, stuck-pipe prevention, and eliminate static survey time.