An account for the geomagnetic disturbances due to space weather events
during offshore directional drilling. A proper use of the adjacent
land-based observatory magnetic field data
Abstract
Directional drilling in the oil fields relies particularly on the
“on-fly” measurements of the natural magnetic field (measurements
while drilling; MWD); the MWD are eventually used to construct the well
path. These measurements are the superposition of the signals from the
internal, core and crustal, and external, ionospheric and magnetospheric
sources and the noise from magnetic elements in the borehole assembly.
The internal signals are mostly constant in time and accounted for
through the Earth’s internal field models. The signals of external
origin give rise to diurnal and irregular spatio-temporal magnetic field
variations observable in the MWD. One of the common ways to mitigate the
effects of these variations in the MWD is to correct readings for the
data from an adjacent land-based magnetic observatory/site. This method
assumes that the land-based signals are similar to those at the seabed
drilling site. In this paper, we show that the sea level and seabed
horizontal magnetic fields differ significantly, reaching up to 30% of
sea level values in many oceanic regions. We made this inference from
the global forward modeling of the magnetic field using realistic models
of conducting Earth and time-varying sources. To perform such modeling,
we elaborated a numerical approach to efficiently calculate the
spatio-temporal evolution of the magnetic field. Finally, we propose and
validate a formalism allowing researchers to obtain trustworthy seabed
signals using measurements at the adjacent land-based site and
exploiting the modelling results - without needing additional
measurements at the seabed site.