Geographical Reference Systems
The position of a point in the well can only be defined in three dimensions when, in addition to the TVD of the point, its lateral displacement and the direction of that displacement is known. The lateral displacement is expressed in terms of feet (or meters) from the wellhead in a Northerly and Easterly direction or in degrees of latitude and longitude. All displacements are referenced to the wellhead position. The position of the wellhead is determined by land or satellite surveying techniques and quoted in latitude and longitude or an international grid co-ordinate system (e. g. Universal Transverse Mercator UTM system). Due to the large number
of digits in some grid co-ordinate systems, a local origin is generally chosen and given the co-ordinates zero, zero (0,0). This can be the location of the well being drilled, or the centre of an offshore platform. When comparing the position of points in a well, and in particular for anti-collision monitoring, it is important that all coordinate data are ultimately referenced to a single system.