Modeling Drillstring Magnetic Interference of Wellbore Surveys Using Damagnetizing Factors

Abstract

Accurate directional borehole guidance requires Bottom Hole Assembly (BHA) non-magnetic spacing of Measurement While Drilling (MWD) systems from surrounding steel components. This is required to limit the magnetic interference and associated azimuth measurement error to acceptable levels.The current industry method of calculating the Estimated Drillstring Interference (EDI) on MWD magnetometers and the resulting azimuth error produced, are largely based on published material from the late 1970's to early 1980's. The current method models components as magnetic dipoles requiring the knowledge or "guess" of the pole strength of the BHA component and combination of components.Technological advances in drillstring design and complexity have rendered these methods inadequate. This inadequacy is due to the lack of knowledge of the pole strength information for modern components and/or the unlimited configurations of components, and the limited and simplified BHA configurations normally available in supporting software tools where the EDI calculation is performed. This paper presents the theory and method of computing the EDI using demagnetizing factors of a prolate spheroid. A worked example of this method is shown to illustrate its implementation with a comparison of the results to published dipole method examples as a reference.The operational advantages of the demagnetizing factor method are presented showing more consistency in calculation, elimination of BHA complexity limits, ease of computer implementation and system integration, and elimination of subjective and inconsistent selection of component pole strengths used in the dipole method, this providing a means of EDI calculation standardization across the industry.