Position of the Reference Point
The depth referencing system used when surveying was discussed extensively in chapter 11.
The depth of the survey station is provided by the the driller and is calculated on the basis of the length of drillstring in the wellbore and the distance between the drillbit and the survey tool.
2.2 Azimuthal Direction of Wellbore
The direction in which the drillbit is pointing when a survey is taken is expressed in degrees azimuth. Azimuth is the angle in degrees (°) between the horizontal component of the wellbore direction, at a particular point, measured in a clockwise direction from the reference (generally North). Azimuth is generally expressed as a reading on a 0 — 360° (measured from North) scale.
For directional surveying, there are three azimuth reference systems :
• Magnetic north;
• True (Geographic) North;
• Grid North.
This is the direction of the horizontal component of the Earth’s magnetic field lines at a particular point on the Earth’s surface. A magnetic compass will align itself to these lines with the positive pole of the compass indicating North.
True (Geographic) North (TN)
This is the direction of the geographic North Pole. This lies on the axis of rotation of the Earth. The direction is shown on maps by the meridians of longitude.
Grid North (GN)
The meridians of longitude converge towards the North Pole and South Pole, and therefore do not produce a rectangular grid system. The grid lines on a map form a rectangular grid system, the Northerly direction of which is determined by one specified meridian of longitude. The direction of this meridian is called Grid North. For example, in the often used Universal Transverse Mercator (UTM) co-ordinate system the world is divided into 60 zones of 6 degrees of latitude, in which the central meridian defines Grid North. Grid North and True North are only identical for the central meridian. Comparison of co-ordinates is only valid if they are in the same grid system.
To be meaningful, all azimuths must be quoted in the same reference system. This is usually the Grid North system. In practice, azimuths are often measured in systems other than the grid North system. Two conversions normally have to be applied to the measured azimuths:
Grid Convergence
Grid convergence converts azimuth values between the Grid North and the specified True North system. The grid convergence angle is the angle between the meridians of longitude (TN) and the North of the particular grid system (GN) at a given point. By definition, the grid convergence is positive when moving clockwise from True North to Grid North, and negative when moving anti-clockwise from True North to Grid North. The value of grid convergence depends upon location. Close to the Equator the convergence is small and it increases with increasing latitude.
Declination
Declination converts azimuth values between the Magnetic North and True North systems. Declination is the angle between the horizontal component of the earth’s magnetic field lines and the lines of longitude. By definition, the declination is positive when moving clockwise from True North to Magnetic North, and negative when moving anti-clockwise from True North to Magnetic North. Values of declination change with time and location and those representative of the parameters at the time of drilling should be used.
2.3 Inclination of the wellbore
The inclination of the wellbore is the angle in degrees that the wellbore is deviated from the vertical.