Multistation Analysis of MWD Magnetic Directional Surveys

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 125677, ’Minimum Requirements for Multi-Station Analysis of MWD Magnetic Directional Surveys,’ by Erik Nyrnes, SPE, StatoilHydro ASA; Torgeir Torkildsen, SPE, SINTEF Petroleum Research; and Harry Wilson, Baker Hughes, originally prepared for the 2009 SPE/IADC Middle East Drilling Technology Conference and Exhibition, Manama, Bahrain, 26-28 October. The paper has not been peer reviewed. Multistation analysis (MSA) is a technique widely used in measurement-while-drilling (MWD) directional surveying to provide additional quality control and to correct for systematic errors. Although the potential of the method to enhance survey quality has been demonstrated, experience has shown that MSA can produce unstable solutions and poorly interpretable results. The full-length paper presents a set of fundamental requirements that have to be fulfilled to ensure the correct application of MSA. The requirements consist of a set of mathematical rules and corresponding acceptance limits. Introduction MSA is a more-powerful survey-quality-evaluation method than conventional single-station calculations. It makes it possible to identify and quantify different types of systematic errors, providing greater proof of whether or not surveys meet specification. Where systematic errors of significant magnitude are identified, they can be corrected for. However, the ability of MSA to correct failed surveys and provide increased confidence that surveys meet their stated specification is limited by several factors. For example, only a limited number of systematic-error terms can be estimated accurately for a given set of survey data, this being dependent on factors such as the geometry of the wellbore, the number of survey stations, and the number of estimated error terms. In current common practice, it is not always clear how the application of MSA to a given survey log affects its status with respect to its own accuracy specification. Improved accuracy may be claimed, but the link between MSA data manipulation and the error model that quantifies accuracy usually is not explicit. In the full-length paper, a basic MSA methodology suitable for adoption across the industry is proposed. The methodology consists of a set of requirements and acceptance limits that have to be fulfilled when applying MSA. The requirements can be applied to any survey log to determine which error terms can be estimated validly. In addition, mechanisms are included to ensure that the random-noise level in the survey data is of tolerable magnitude. The requirements are easy to apply and straightforward to implement and, therefore, should be easily communicated throughout the survey industry.