Reduction of Well-Bore Positional Uncertainty Through Application of a New Geomagnetic In-Field Referencing Technique (1995)

Abstract. A new In-Field Referencing (IFR) technique, for measuring local geomagnetic-field parameters at, or very close, to the well site is described. IFR is shown to enable reduction in the uncertainty associated with the estimates of geomagnetic field values, normally obtained from main-field geomagnetic models. This significantly reduces the magnitude of certain critical directional uncertainty error terms which are inputs to survey tool accuracy performance models, reducing positional uncertainty for well-planning purposes. Logistical problems and high unit-cost have precluded IFR development to date, but recent innovative thinking which has evolved from dialogue between oil and drilling-service companies and the scientific/academic community has refined the concept and greatly improved the feasibility. Introduction. In-field-referencing (IFR) is the name given to the practice of measuring the geomagnetic field at, or close to, a drilling site. Magnetic survey tools rely on the principle of measuring the direction of the well bore relative to the direction of the local geomagnetic field. The hole direction can then be referenced to a geographic coordinate system from a knowledge of the direction of the geomagnetic field, relative to true north. This true north reference direction is normally obtained from charts or from a computerised model of the geomagnetic main-field. Both short term magnetic variations and errors in the modeled local magnetic field direction are ignored in this surveying technique. Survey errors are potentially large, giving rise to considerable directional uncertainty. Also the accuracy performance of some of the widely used drillstring interference correction methods can be sensitive to errors in certain magnetic parameters which are obtained from the main-field model and used as input values. Well bore positional uncertainty tends to be greater for magnetic surveys than for high accuracy gyro systems. This is due, to a considerable extent, to uncertainty in declination and the influence of drill string interference. Measurement of the local geomagnetic conditions during surveys can significantly reduce survey errors and directional uncertainty. IFR therefore, has the potential to improve the accuracy of magnetic surveys. Previous authors have indicated that local measurement of the geomagnetic field can result in lateral position uncertainties for modern magnetic survey tools approaching those normally reserved for high-accuracy gyros. IFR has been actively discussed for several years by several major oil operators and service-contractor companies, but without significant progress. The practice of IFR is not entirely untried, at least on land locations, but there is a paucity of public domain information describing practical applications of the technique. There are no reports in the literature of IFR being applied to offshore drilling activity. It is the aim of this paper to redress this situation and also to describe a recent refinement which could lead to wider usage and application of IFR, including offshore drilling projects. This in turn could lead to significant drilling cost reductions in high-cost drilling areas such as the Gulf of Mexico and the North Sea. This would be achieved via optimisation of directional survey programs, and also reduction in the quantity of non-magnetic hardware in BHAs.