6.2 Specific Recommendations Resulting from this Study
Standard uncertainty models published and reviewed by industry subject matter experts for In-Field Referencing (IFR) and multi-station analysis (MSA) should provide a sufficient framework for accurately locating wells and, if need be, drilling a relief well.
For these models to be applied, extra actions will be required above and beyond a typical drilling operation. Current data availability restricts the geographic area where these models can be soundly applied. The use of an IFR error model will require the use of aeromagnetic data to generate a local magnetic field model. This study was only able to locate data of sufficient quality for a portion of the Barents Sea.
The extent to which additional correction will be necessary will depend on how deep the well is to be drilled, the amount of step out in the well, and the expected drilling azimuth for the step out. The greater the extent of horizontal deviation in the wellbore and the closer the drilling azimuth is to horizontal East/West, the more likely it will be that advanced corrections are required to meet the appropriate levels of positional uncertainty. For wells drilled in a northerly or southerly direction, the application of IFR will provide the greatest amount of uncertainty reduction. Wells predominantly in an easterly or westerly direction will likely require both IFR and MSA as a minimum surveying standard.
The use of MSA in these applications will likely require that additional checkshots be taken for some sections of the well to ensure that adequate variation in survey orientation is achieved. Application of MSA will necessitate real-time compensation of the magnetic disturbance field in order to accurately model survey measurement deviation from reference values. The disturbance field error magnitudes observed in the Barents Sea can be as large or larger than those produced by MWD surveying errors and attempting to perform MSA on data that has not removed these errors may result in the application of an erroneous survey correction, potentially adding additional uncertainty to the bottom hole location. These issues may be exacerbated further in situations where there is limited orientation change across a drilling run.
There are many publicly available magnetic observatories in the vicinity of the Barents Sea, and use of any disturbance mitigation method (Nearest Observatory, IIFR, or DF) results in decreased remaining disturbance error. To meet OWSG IFR2 requirements as stated above, the Disturbance Function (DF) will need to be applied beyond ~50 km.
Anything beyond ~250 km from the nearest observatory will require a real-time local observatory. There was not sufficient data available to determine the effectiveness of disturbance mitigation methods at distances closer than 50 km.
In general, the errors are expected to be lower for all mitigation methods. This implies that the DF method will still be suitable, however it is possible that IIFR and nearest station methods may also have their errors drop to acceptable levels. For drilling activities expected to be within 50 km of a magnetic observatory additional study may be required if IIFR or nearest stations are to be used in order to determine if those mitigation methods will meet the requirements of the OWSG IFR2 error model.
Recommendations are as follows:
1. In the western part of the Barents Sea (West of 32° E longitude), IFR1 can be readily implemented with available data to meet the MWD+IFR1 tool code requirements. Results for this are in 5.3.1 Crustal Mitigation Results.
2. In the eastern part of the Barents Sea (East of 32° E longitude), higher resolution magnetic data (a maximum of 4 km line spacing, ideally 1-2 km) will be required to meet MWD+IFR1 requirements. This may already be available for discovery or purchase, or a new high-resolution aeromagnetic survey may need to be flown. If a new survey is flown, it should be acquired at 1km line spacing. Results in 5.3.1 Crustal Mitigation Results.
3. Seafloor magnetometers should be employed for any operation where cost is not a factor and the utmost accuracy is required. While expensive, they cut the disturbance uncertainty to essentially zero.
4. In regions of the Barents Sea within ~50 km of a magnetic observatory (near-shore, for example), the Disturbance Function method may be used to meet IFR2 tool code requirements. It is possible that either the Nearest Observatory method or IIFR may also be able to be used, however if they are to be employed a local study should be performed prior to drilling in order to confirm that uncertainties are within the IFR2 tool code. Of the latter two methods, IIFR will provide better results, as shown in Figure 4.19, Figure 4.20, and Figure 4.21, but is slightly more complicated to implement. Results in 5.3.2 Disturbance Mitigation Results.
5. In regions of the Barents Sea between ~50 and ~250 km from the nearest magnetic observatory, the Disturbance Function method must be used to meet IFR2 tool code requirements. Figure 4.21 provides a heat map of distances to the nearest observatory for locations in the Barents Sea. Results in 5.3.2 Disturbance Mitigation Results.
6. In remote regions of the Barents Sea beyond ~250km from the nearest magnetic observatory, a local magnetometer (seafloor or otherwise) with real-time data link must be deployed to meet IFR2 requirements. Results in 5.3.2 Disturbance Mitigation Results.