Page | 41
6.2 Interpolated In-field Referencing
One solution to diurnal variations is to use a reference station on surface. In this way, the observed variations observed at surface can be applied to the Downhole Data which will experience similar variation. This is not always practical and requires a magnetically clean site with power supply nearby and some method of transmitting the data in real time from the temporary observatory. The other issue is establishing the baseline from which these variations are occurring in order to correct to the right background field values. In a combined research project between Sperry Sun and the British Geological Survey, it was discovered that the diurnal and other time variant disturbances experienced by observatories, even a long way apart follow similar trends. The researchers compared observations made at a fixed observatory with derived observations interpolated from those taken at other observatories some distance away. The match was very encouraging and a new technique for diurnal correction was established called Interpolated In-Field Referencing or IIFR (not be confused with IFR discussed below). This technique is a patented method of correcting for time variant disturbances in the E h’s m gn c f ld but is widely used under licence from the inventors. The readings observed at the nearby stations are effectively weighted by the proximity to the drill site and the time stamped combined corrections applied to the Downhole observations either in real time or retrospectively.
LINK
BGS - GeoMagnetic
CONTENTS
Points to Ponder
IFR has been one of the greatest developments for improving well placement in recent years, but it is still not widely used in some regions. Generally. money spent on surveying will be more than compensated for by improved production value. As advantageous as it is, below are caveats when using IFR and IIFR. 1. All IFR corrections are published from a global prediction model. If you use a different global prediction model later in the life of the field, the corrections will be less valid. 2. IFR corrections should be applied to the background field magnetic data and not to the azimuth directly. That is because it is easy to forget to apply corrections from survey to survey, but if the reference is corrected once it will apply throughout. The exception, of course, s d ‘cub s’ wh ch m n ob n pol dfo ch survey, but it is still good discipline to report the corrected magnetic data at each station rather than just the new azimuth. 3. Be very careful using a local MWD sensor as a local reference. This is unlikely to be as good as interpolating between highly calibrated and isolated observatories. The following should be borne in mind: a. An MWD reference station has similar error sources as the MWD downhole. The scale and bias on all the sensors are similar. b. It is possible to determine dip angle and field strength from a sensor pack but not declination. Therefore it is common practice to assume that if the sensor is averaged over a few hours, that can be your base value for azimuth from which any instantaneous azimuth is assumed to be a diurnal
Made with FlippingBook flipbook maker