OR/16/030 Oil industry developments

Thomson, Alan W P. 2016. Geomagnetism review 2015. British Geological Survey Internal Report, OR/16/030.

Oil services quality control activities: Brian Hamilton and Orsi Baillie.

Declination standard errors from the industry error model (left) and from a BGGM look-up table (right). Note that the industry model unnecessarily overestimates errors in the polar region.

Different methods of defining geomagnetic latitude: dipole (blue) and quasi-dipolar (red) contours at 2015.0.

Understanding the directional and field strength uncertainties in global models is important in oil industry error modelling. In addition, for navigational referencing, understanding the local geomagnetic latitude is also important, not least when using IIFR services. We therefore discuss some BGS service developments in this area made in 2015.

Accurate knowledge of the navigational error associated with global magnetic field models continues to be important for directional drilling. The industry typically uses a simple error model involving, at most, four parameters each of which is assumed constant in value. BGS has long been a proponent of a more comprehensive error model involving three parameters but whose values are drawn from look-up tables arranged by latitude and longitude and which are then usable with high (statistical) confidence levels. Look-up tables would allow errors arising from the auroral electrojets to be better modelled.

In 2010 BGS published a paper in an industry journal (Macmillan and Grindrod, 2010) detailing the work behind the derivation of look-up tables at different levels of confidence. However the industry needs standard errors so they can be combined with other sources of errors associated with wellbore position, and they are also keen to reduce errors. We will therefore be working on this with industry partners in 2016.

In 2015 a BGS online geomagnetic coordinate calculator was developed and made available. The new calculator uses coefficients for all spherical harmonic degrees (up to 13) of the 12th Generation International Geomagnetic Reference Field and, for comparison, a simple tilted dipole coordinate system is included, where we use only the degree one coefficients of IGRF revision 12. The calculator is available at www.geomag.bgs.ac.uk/data_service/models_compass/coord_calc.html and has proven popular.

Structural and dynamical features of the ionised and neutral upper atmosphere are strongly organised by the geomagnetic field, and several magnetic coordinate systems have been developed to describe this organisation and the impact it has on defining geomagnetic latitude. The significance of this is that locations which have similar geomagnetic latitudes are likely to experience similar levels of magnetic disturbance.

This is useful for directional drilling if sourcing magnetic data when a nearby observatory is not available. However it is acknowledged that the morphology of the internal field, local time and conductivity structures can also influence magnetic disturbance levels and hence also geomagnetic latitude.