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Points to Ponder
The error models we use to predict the uncertainty in the wellbore position use certain assumed values for magnetcic interference. It may be obvious, but if the magnetic interference from our BHA components exceeds these assumptions we will underestimate the positional uncertainty. BHAs and BHA components that sit for a m n h h’s m gn c f ld w ll n bly pol s w h h oppos polarity. The longer they sit, the closer to North/South orientation, and the closer to the equator (zero dip angle), the stronger that natural magnetisation. This chapter only suggests approximate pole strengths but it would be useful to measure these occasionally rather than rely on estimates. The main reason we like to use shorter collars is to keep the survey sensors as close to the bit as possible, which allows the directional driller to see the effect of any changes made as soon as possible. However, this needs to be balanced with how accurate these sensors are likely to be if they are too close to steel. 6. nF ld f ncng
6.1 Measuring Crustal Anomalies using In-field Referencing
The biggest source of error in MWD is usually the crustal variation. The global models such as the BGGM and HDGM can only take into account the longer wave length variations in the Earth Field and cannot be expected to allow for the localised effects of magnetic rock in the basement formations. In order to correct for these effects, the magnetic field has to be measured on site. From these local measurements, a series of corrections from a global model can be mapped out for the field so that in future years, the more permanent effect of local geology can be added to the secular effects for an up-to-date local field model.
IFR is a technique that measures the strength (Field Strength), direction (Declination) and vertical angle (Dip Angle) in the vicinity of the drilling activity to give the MWD contractor a more accurate reference to work to.
To accurately measure the magnetic field locally we can take direct measurements from the land, the sea or the air. On land, a non-magnetic theodolite with a fluxgate magnetometer aligned on its viewing axis, is used to measure the orientation of the magnetic field against a true north, horizontal reference from which accurate maps can be made. A proton or Caesium magnetometer is used to accurately the local field strength. In the air, only the field strength variations can be measured but if a wide enough
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