Communicating uncertainty in geomagnetic field estimates provided by the BGS to aid directional drilling

Navigating underground when drilling for oil and gas has become more challenging as companies try to hit smaller targets in reservoirs already congested with existing wells. One widely used method is Measurement While Drilling (MWD) using magnetic survey tools to direct the drill head. The provision of accurate geomagnetic field values with verifiable estimates of uncertainty is of utmost importance as the estimates help mitigate the risk of collision or missing a target. The BGS offers three options to the oil industry depending on accuracy required: the basic option is to use estimates of the geomagnetic field from the annually updated BGS Global Geomagnetic Model (BGGM); the second and more accurate option, In-Field Referencing (IFR), includes estimates of the local crustal magnetic field; the third and most accurate option, Interpolation In-Field Referencing (IIFR), includes estimates of the rapidly time-varying magnetic field at the oil field. The estimates of uncertainty in the geomagnetic field values supplied under each of these options are almost as mportant as the values themselves because they are incorporated into company software which derives positional error ellipsoids along the well-path. We describe work done over several years on the derivation and communication of geomagnetic field uncertainty for the oil industry.