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MWD Geomagnetic Models
The ISCWSA MWD Model was originally developed using low resolution (maximum spherical harmonic degree 13) BGS Global Geomagnetic Model (BGGM) from the mid 1990's, i.e. before the current era of magnetic survey satellites. 
Although the BGGM has been widely used in the oil and gas industry, there are earlier and less accurate geomagnetic models in use [e.g. the International Geomagnetic Reference Field (IGRF) and World Magnetic Model (WMM)] as well as more recent developments such as the High Definition Geomagnetic Model (HDGM).
NOTE: Clicking the Hyperlink should open a new window and give additional information on the geomagnetic models.  
One suggested method for using alternative geomagnetic models in the MWD Tool Error Models is to scale the BGGM values using a multiplier (Reference: SPE 151436). The multiplier was derived by comparing measured Total Magnetic Field Strength values with those predicted by the geomagnetic models. The Declination and Magnetic Dip angular values were checked to confirm the same multiplier was valid. 
This multiplier could be applied to the BGGM Uncertainty Look up Tables (Reference: SPE 119851) based on low to medium resolution BGGMs (maximum spherical harmonic degrees 13-30) or to the BGGM Error Values published in the original paper (Reference: SPE 67616). These values are shown in the Table below and should be read in conjunction with the original papers:
 Total FieldDip AngleAzimuth (Constant)Azimuth (BH Dependant)
MFI (nT)  MDI (Deg) AZ (Deg) DBH (Deg.nT)
BGGM1300.20 0.36 5000 
HDGM1070.16 0.30 4118 
IGRF/WMM1570.24 0.43 6029 
WARNING: As with any error model term the applicability and validity of the values need to checked before use.
Geomagnetic Model Comparison
The effect on position uncertainty from these different models can be seen in the Lateral Uncertainty for the three ISCWSA well profiles. The results are summarised in the Tables below for 'Standard MWD' and 'MWD+Axial Correction'.   
Standard MWD (Rev.3 Toolface Independent) Semi-Major Axis Uncertainty at 1-Standard Deviation
At 8000 m MD At 12500 ft MD At 4030 m MD 
BGGM 84.36 m 32.01 ft 12.64 m
HDGM 79.60 m 31.04 ft 11.82 m
IGRF/WMM 90.62 m 33.32 ft 13.72 m
MWD+Axial Correction (Rev.3 Toolface Independent) Semi-Major Axis Uncertainty at 1-Standard Deviation
 At 8000 m MDAt 12500 ft MD At 3000 m MD* 
BGGM 201.46 m 28.31 ft 9.65 m
HDGM 172.69 m 25.96 ft 8.56 m
IGRF/WMM 232.99 m 31.12 ft 10.99 m
* Note: The comparison is at 3000 m MD because the Axial Correction fails at 3030 m MD in this well profile.
The Uncertainty at TD in profile ISCWSA#1 is much larger with Axial Correction than the Standard MWD and this indicates that the correction is not applicable for the whole of this well profile.

SPE 67616, "Accuracy Prediction for Directional Measurement While Drilling." Hugh S. Williamson, SPE Drilling & Completion Vol 15 No.4 December 2000 page 221-233 (Designated SPE 67616)
SPE 119851, "Confidence Limits Associated With Values of the Earth's Magnetic Field Used for Directional Drilling", Susan Macmillan and Steve Grindrod, SPE Drilling & Completion, 25(2), 230-238. DOI: 10.2118/119851-PA
IADC/SPE 151436 "High Definition Geomagnetic Models: A New Perspective for Improved Wellbore Positioning", Stefan Maus, Manoj C Nair (NOAA & CIRES, University of Colorado) Benny Poedjono (Schlumberger), Shola Okewunmi (Chevron Corporation), Derek Fairhead (GETECH), Udo Barckhausen (German Federal Institute for Geosciences and Natural Resources), Peter R. Milligan (Geoscience Australia) and Jurgen Matzka (Technical University of Denmark); 2012 IADC/SPE Drilling Conference and Exhibition, San Diego, USA, 6-8-Mar-2012.