The azimuth of the borehole is determined from the Earth's magnetic field using a magnetic survey tool.

Two basic types of tools exist, i.e. conventional photo-mechanical and solid state tools. For conventional tools the length of NMDC should be such that the interference from the steel drillstring is acceptable. For solid state survey instruments, a shorter length of NMDC can be used and the magnetic interference from the drillstring measured and corrected for.

Magnetic interference from adjacent wells or objects cannot be corrected for and will produce erroneous azimuth readings.

In all situations, for accurate magnetic surveys, the Earth's magnetic field must be stable during the survey and appropriate values of magnetic field must be available.

The Earth's magnetic field

The Earth can be considered to be a magnet surrounded by its magnetic field. Close to the Equator the field lines are parallel to the Earth's surface and at the magnetic poles they are near vertical. The Earth's core magnetic field varies slowly with time. Its spatial and time dependency is modelled in the British Geological Survey Global Geomagnetism Model (BGGM).

Short-term disturbances are the result of magnetic fields external to the Earth caused by electric currents flowing in the ionised region of the atmosphere. One variation is regular and often referred to as the daily variation, the other is irregular and referred to as a magnetic storm. The short-term disturbances mean that the direction of the geomagnetic field vector can differ by several degrees from the estimated value given by the BGGM. If a magnetic survey is taken during a significant magnetic disturbance this will lead directly to an uncertainty in azimuth.

Dip angle: The angle between the Earth's magnetic field lines and the horizontal is called the dip angle The dip angle ranges, from -90º at the South pole, through zero at the Equator to 90º at the North pole.

Magnetic field strength: The magnetic field strength of the Earth is expressed in micro Tesla (mT) and varies from 30 mT at the magnetic equator to 60 mT at the magnetic poles.

Declination: Declination is the angle, expressed in degrees, between the horizontal component of the Earth's magnetic field lines and the lines of longitude. Its value is needed to convert azimuth values between the Magnetic North and True North reference systems.

Types of magnetic survey tools

There are different designs and implementations of magnetic survey instruments, but they can be split into two basic types:

• conventional magnetic survey tools, where a compass card aligns itself to the local magnetic field and is photographed;
• solid state magnetic tools where the magnetic field is measured electronically and the azimuth calculated.

Both these types can either be 'single-shot' where only one reading is recorded, or 'multi-shot' where more than one reading is recorded. In addition, solid state instruments are also applied in wireline steering tools and MWD systems.

Magnetic interference

There are two possible solutions to drillstring magnetisation effects on surveys:

1.use sufficient length of NMDC to reduce the azimuth errors to an acceptable level;

2.where applicable, use a shorter length of NMDC and magnetic azimuth correction techniques.

It must be realised, that in particular for long NMDC section requirements in case of option one, application of the second option is preferred, where possible, because:

• the distance between bit and sensors becomes unacceptably long, making directional drilling control more difficult;
• NMDC are less strong than steel drill collars;
• the use and potential loss of NMDC is costly;
• use of correction programme enables advanced quality control.

It is recommended to take the first magnetic survey measurement 15 m below the casing shoe.

Magnetic field of NMDCs

Non-magnetic drillstring components, however, may also introduce additional magnetic fields due to manufacturing deficiencies, mechanical damage or embedded magnetic particles. All NM DCs and stabilisers should be tested for magnetic hot-spots prior to being accepted for use when new. In addition to regular mechanical inspection they should be tested to ensure magnetic requirements: ·after a thread is re-cut; ·after suspected (or actual) mechanical damage; ·after prolonged jarring; ·after unexplained magnetic survey uncertainties.

NMDC charts can be used to estimate the possible azimuth uncertainty due to using a shorter NMDC length than that recommended.

Magnetic interference correction

Magnetic interference correction techniques correct the raw measured data for magnetic interference from the drillstring. These correction techniques can not correct for magnetic interference from non-drillstring components such as nearby casings or magnetic formation anomalies. Magnetic interference correction techniques enable the required length of NMDC to be reduced. Magnetic interference correction can only be applied where the individual components of the Earth's magnetic and gravity field strength are measured. This is only the case for solid state tools, in which separate magnetometers and accelerometers measure these field strength components. Magnetic interference correction techniques should not be used in high inclination wells, >70º, in East/West direction ± 20º, hence the intervals 70-110º and 250-290º. In these instances the full recommended length of NMDC should be used and the techniques not applied.

A number of Survey COmpanies offer different magnetic azimuth correction techniques and associated survey procedures.