The following survey tools and techniques are recommended/ available:
- Solid state magnetic survey tools
- Conventional magnetic survey tools
- Conventional gyroscopic survey tools
- North seeking gyro survey tools
- Inertial navigation survey systems
Approval of tools
Inclination-only tools are not regarded as survey tools but as inclination indicator tools. They can at best only be used to obtain provisional, intermediate inclination data in between or prior to full surveys.
Techniques recommended during drilling:
- single and multi-shot surveying;
- wireline steering;
- MWD.
The survey interval for magnetic surveys for isolated vertical wells (inclination < 5º) is 100 m. For other wells it is 10-30 m depending on directional drilling requirements.
In addition to or to replace the magnetic surveys, gyroscopic surveys shall be taken in the following cases:
- a single-shot at the bottom of the conductor; ·to check magnetic surveys while nudging away from adjacent wells;
- to check magnetic surveys in case of collision risk;
- to reduce the position uncertainty over the open hole section if required.
Survey intervals for these surveys depend on directional drilling requirements.
Survey tools and techniques for verification surveys
An independent verification survey should be taken after a hole section is drilled to verify previous survey uncertainties.
Systematic uncertainties such as depth, misalignment and declination which can not be determined from a single survey can be determined from a verification survey. In addition, verification surveys should also be taken while drilling if accuracy requirements are most likely not to be met. Verification surveys should be obtained by gyroscopic (conventional, North Seeking or Inertial Navigation) tools. The survey data frequency for gyroscopic (conventional and North Seeking) tool is 25 m. This frequency is chosen to ensure that large uncertainties introduced by interpolation and minimum curvature calculations will not occur. For Inertial Navigation surveys the survey data frequency is 10 m.
- Isolated vertical wells (inclination < 5º): verification surveys may be omitted if the well will not penetrate a potential blow-out zone in the next open hole section provided:
- there are gyroscopic or solid state magnetic surveys in the previous section, and
- the open hole of that section is 'acceptable' or 'good' if a conventional magnetic survey tool is used
- Other wells: For wells which are non-isolated or non-vertical, a verification survey shall be run. For the last hole section of the well the gyroscopic verification survey may be replaced by a solid state magnetic survey.
Survey standards
Wellpath planning and anti-collision monitoring (COMPASS)
Magnetic interference correction
Magnetic interference correction by means of a correction program is preferred over magnetic interference reduction by means of a full NMDC length. However, for highly inclined wells above 70º, East/West ± 20º a full NMDC length should be used.
NMDCL
In the event that correction programme is not used for magnetic interference correction, a full NMDC length should be used. Softwares are available to compute the required length of NMDCs and the optimum compass spacing for each location.
BGGM
The British geological survey Global Geomagnetic Model (BGGM) offered by the British Geological Survey (BGS) is a standard for the computation of the Earth's magnetic field data. The BGGM is a module in COMPASS.