The oldest surveying tool for directional drilling was known as an acid bottle. When taking a survey, the tool aligned itself with the hole’s axis, but the surface of the acid remained level. The instrument was left in this position for about 30 minutes, allowing the acid to etch a sharp line on the glass container, which indicated the hole angle.
This system did not, however, determine the direction of the wellbore. Surveying tools in directional drilling have been used in directional wells since the 1930s. The most straightforward tools consist of an instrument that measures the inclination and N-S-E-W direction of the well. A photographic disc within the instrument produces an image of the surveying instrument. The disc is developed when the instrument is returned to the surface, and the survey results are recorded.
There are three methods of running and retrieving the photographic instrument:-
- It may be run and retrieved on a wireline (landline).
- It may be dropped down the drill pipe, then retrieved by running an overshot on the wireline.
- It may be dropped free down the drill pipe and retrieved when a trip (check also tripping pipe) is made (e.g., to change the drilling bit). When the instrument reaches the bottom, it sits inside a baffle plate called a Totco ring which holds the instrument in position.
Now here we are going to talk about :
- Magnetic Single Shot Survey
- Magnetic Multi shot Survey.
- Gyro Survey Tool – Single Shot Survey
- Measurement While Drilling
Accuracy of Surveying Tool In Directional Drilling
There are two particular sources of error to be recognized when using photographic instruments:
- Instrument error – due to the inaccuracy of the device itself, periodic calibration, and damage caused to the instrument.
- Reader error – the developed film is easily misread. Some discs may have to be magnified to be read correctly. Another person should verify the reading (although this is seldom true on the rig). Under ideal conditions (i.e., selecting the correct angle unit, non-magnetic collars, tool centralization, etc.), the inclination is accurate to +/-0.25 degrees and direction to +/-2 degrees.
