The correction programme corrects the inclination and azimuth values for cross-axial and axial magnetic interference of the drillstring (including magnetic hot-spots in NMDCs), bias uncertainties of the accelerometers and magnetometers of the survey tool and toolface dependent misalignment.
The method does not correct for external disturbing magnetic fields such as from nearby casings or magnetic formations. The correction method assumes a constant magnetic field strength and dip angle during a complete survey. Surveys taken during magnetically unstable periods, e.g. magnetic storms, are unreliable, as discussed previously.
Magnetic interference correction as implemented in a correction programme consists of three basic elements:
1.axial correction (along the borehole axis);
2.cross-axial correction (in the cross-borehole plane);
3.quality control (accept/reject survey stations).
The axial correction is the most significant correction.
Axial correction
The axial magnetic interference correction minimises the distance and angle between measured and actual magnetic field vectors. This correction requires accurate knowledge of the local Earth's magnetic field. Axial correction can be applied at each individual survey station.
Cross-axial correction
The cross-axial correction compensates for imperfections of the cross-axial measuring system (i.e. cross-axial magnetic interference from the BHA and bias of cross-axial magnetometers and accelerometers).
The corrections are computed from a 'rotational shot', which has at least four surveys taken at constant depth and at various toolface angles. Experience has shown that 6 surveys taken at random toolface angles provide adequate input data for calculation of the cross-axial corrections.
Rotational shots are accepted if:
·the inclination is higher than a specified minimum (usually 2º);
·they consist of at least 4 accepted surveys;
·the azimuth spread in rotational shot surveys after application of the cross-axial corrections is less than before the corrections were applied;
·the inclination spread after application to the cross-axial corrections to accelerometer readings is less than 1.5 times the inclination spread before the corrections were applied.
It is recommended that at least one rotational shot for each BHA run is taken. The rotational shot should be taken in a magnetically undisturbed part of the well, i.e. at least 30 m away from the last casing shoe.
Quality control
correction programme accepts and rejects survey stations on the basis of comparing corrected magnetic and gravity field data with the local expected values. Corrected magnetic and gravity field data is computed from the axial and/or cross-axial corrected survey data. Survey stations are accepted if the following values are within specified tolerances.
·difference between local and calculated gravity field strength: ± 0.005 g
·difference between local and calculated dip angle: ± 0.5º
·difference between local and calculated magnetic field strength: ± 0.3 T
If a large number of survey stations are rejected, there are four potential sources of error:
1.tool movement during survey taking;
2.large variations in the Earth's magnetic field during surveying;
3.sensor performance is outside its specifications;
4.magnetic anomalies e.g. nearby casing, magnetic formation.
Large differences in gravity field strength values indicates tool movement. Earth's magnetic field variations can be checked with the Survey Focal Point. Incorrect sensor performance will be shown by large inclination and azimuth spreads of a rotational shot. Magnetic anomalies are shown because of their local anomalous nature.
Uncertainties
Uncertainties in local dip angle and total magnetic field strength contribute significantly to uncertainties in axial corrected azimuth values. The actual magnitude of the uncertainties will depend on the tool attitude and hence well path and the sensor calibration uncertainties. The resulting azimuth uncertainties can be computed with special software.