Magnetic Referencing and Real-Time Survey Processing Enable Tighter Spacing of Wells
Authors: Wilson Adam 
Published: January 2017

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 175539, “Magnetic Referencing and Real-Time Survey Processing Enable Tighter Spacing of Long-Reach Wells,” by Stefan Maus, SPE, Magnetic Variation Services, and Jarod Shawn Deverse, SPE, Surcon, prepared for the 2015 SPE Liquids- Rich Basins Conference—North America, Midland, Texas, USA, 2–3 September. The paper has not been peer reviewed.
Wellbore position is computed from survey measurements taken by a measurement-while-drilling (MWD) tool in the bottomhole assembly (BHA). The MWD tool uses accelerometers and magnetometers to measure the Earth’s gravitational and geomagnetic fields. Knowing the direction of these two fields and using them as a frame of reference enable drillers to calculate inclination and direction (azimuth) of the wellbore. Local high-resolution in-field referencing (IFR) models have been produced for the Permian Basin that significantly improve the accuracy of MWD azimuth measurements.
Ellipse of Uncertainty (EOU)
Numerous error sources are associated with MWD-survey measurements, and each error source contributes in some form to the magnitude of uncertainty that propagates along the computed wellbore trajectory. The Industry Steering Committee for Wellbore Survey Accuracy (ISCWSA) developed a framework for quantifying the magnitude of uncertainty. The Operator’s Wellbore Survey Group (OWSG), an ISCWSA subcommittee, continued development on the original error model and published a set of instrument-performance models that enables the computation of EOU for specific surveying methods. This consolidated set is referred to as the OWSG set of tool codes. Fig. 1 illustrates the difference between EOUs for standard MWD vs. advanced corrections using MWD with IFR and corrections for sag (MWD+IFR1+SAG).
Generally, one can see that the large uncertainty of standard MWD can be reduced by 11 to 38% by use of IFR, while further applying multistation analysis (MSA) can reduce the uncertainty by 50 to 61%. As shown in Fig. 1, vertical uncertainty can also be reduced by advanced survey-correction methods. Bending of the BHA brings the accelerometer out of alignment with the inclination of the wellbore trajectory. This effect can be corrected for by applying corrections for sag.
IFR
The objective of IFR is to use local-magnetic- field measurements to produce an accurate 3D magnetic reference model for the drilled volume. The most cost-effective method is to use aeromagnetic surveys, which have been used for more than 50 years to map geology and tectonic features. Earlier IFR methods used fast Fourier transforms or equivalent-source techniques on planar 2D grids. A recent analysis showed that these grid methods can lead to significant errors in the declination and dip reference values because they assume that the crustal magnetic anomalies are entirely contained within the grid. A superior approach is to tie the grid into the global crustal field inferred from satellite measurements and represent the solution in terms of global high-degree ellipsoidal harmonic functions.
The IFR method used by the authors first produces an accurate baseline magnetic field for a given reference date. This baseline for the reference date is supplied as an IFR model together with an IFR calculator to the drilling engineer. The baseline can then be extrapolated to any desired drilling date using the main field model embedded in the IFR calculator.