RIASSUNTO
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Abstract
A magnetic survey correction method (MSCM) has been developed and used to process MWD surveys. The local magnetic process MWD surveys. The local magnetic field strength and dip angle required for the method were obtained from a commercially available computer model of the Earth's magnetic field. The method corrects for both radial and axial magnetic drillstring interference and has led to improved quality control for magnetic surveys. The only nonmagnetic tubular incorporated in the drillstring is the MWD's own nonmagnetic housing. Comparison of the MWD results with north-seeking gyro surveys confirmed that the theoretically predicted accuracy of the MWD surveys processed with the correction method is generally better than conventional surveys taken in a full length of nonmagnetic drillcollars. No additional rig time was incurred and operational benefits include reduced rental, inspection and risk of lost-in-hole costs.
Magnetic survey accuracy can be enhanced by using locally measured geomagnetic parameters for the correction method. parameters for the correction method. Field examples of enhanced magnetic surveying with electronic magnetic multishot tools are given. The achievable accuracy of magnetic surveying onshore in the Netherlands is comparable to the accuracy of current north-seeking gyro tools and a reduction in the number of gyro surveys is therefore planned.
Introduction
Magnetic surveying techniques are used extensively for borehole trajectory control in drilling operations. Photo-mechanical and recently developed Photo-mechanical and recently developed solid-state electronic instruments are used for this purpose. These instruments have to be run in a nonmagnetic section of the bottomhole assembly (BHA) in order to limit the azimuth error due to magnetic interference from the drillstring (Ref. 1). In Shell the length of the non-magnetic drillcollar (NMDC) section is determined such that this error does not exceed 0.25 degrees, based on typical measured pole strength values of BHA's (Ref. 2). However, during drilling operations NMDC's may pick up magnetic hot spots which deteriorate survey quality. Regular inspection of the NMDC's is therefore required. Furthermore, NMDC's are relatively weak components which are prone to failure, e.g. due to fractures, prone to failure, e.g. due to fractures, galling and over torqueing. Substantial inspection, maintenance and possible lost-in-hole costs are involved. Sometimes two survey instruments are run in tandem to improve reliability and to cross-check tool performance (Ref. 3). However quality control (QC) of conventional magnetic surveys is very limited and excessive drillstring interference, when present, cannot be detected.
This paper describes a magnetic survey correction method (MSCM) (Ref. 4), developed in Shell, to correct magnetic survey data of electronic instruments for interference from the drillstring. This allows accurate surveys to be obtained in one joint of NMDC's. In addition the method provides a set of QC-parameters which enable a quality assessment to be made of each individual survey. The achievable survey accuracy using the MSCM is described in terms of instrument characteristics and magnetic field parameters.
Finally field experience gained with the use of the method in the Nederlandse Aardolie Maatschappy B.V., Shell's Dutch operations, and possible future developments are reported.
2. THE EARTH'S MAGNETIC FIELD
The Earth's magnetic field at any location, and instant in time, is fully defined by the total field strength, B [ T], the dip angle, I [deg] and the declination, D [deg].