Photographic Surveying Tools
The oldest surveying instrument was known as an acid bottle. When taking a survey the tool aligned itself with the axis of the hole 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 have been used in directional wells since the 1930’s. The most simple tools consist of an instrument that measures the inclination and N-S-E-W direction of the well. A photographic disc contained within the instrument is used to produce an image of the surveying instrument. When the instrument is brought back to surface the disc is developed and the survey results recorded.
There are 3 methods of running and retrieving the photographic instrument:-
- It may be run and retrieved on wireline (sandline).
- It may be dropped down the drillpipe, then retrieved by running an overshot on wireline.
- It may be dropped free down the drillpipe and retrieved when a trip (check also tripping pipe procedures) is made (e.g. to change the bit). When the instrument reaches bottom it sits inside a baffle plate called a Totco ring which holds the instrument in position.
1- Magnetic Single shot Survey
2- Magnetic Multi shot Survey
3- Gyro Survey Tool – Single Shot Survey
Accuracy of Photographic Survey Results
There are two particular sources of error to be recognised when using photographic instruments:
- Instrument error – due to the inaccuracy of the device itself, infrequent calibration and damage caused to the instrument.
- Reader error – the developed film is easily mis-read. Some discs may have to be magnified to be read properly. Readings should be verified by another person (although this is seldom the case on the rig). Under ideal conditions (i.e. selecting correct angle unit, non-magnetic collars, centralization of tool etc.) inclination is accurate to +/-0.25 degrees, and direction to +/-2 degrees.