Gyro Single Shot
The inclination of the well can be assessed in the same way as in the magnetic tools. The Magnetic effects can be completely eliminated by using a gyroscopic compass.
A Gyroscope is a wheel which spins around one axis, but is also free to rotate about one or both of the other axes, since it is mounted on gimbals. The inertia of the spinning wheel tends to keep its axis pointing in one direction.
In a gyro single shot tool, a gyroscope is rotated by an electric motor at approximately 40,000 rpm. On surface the gyro is lined up with a known direction (True North) and as the tool is run in hole the axis of the tool should continue to point in the direction of true North regardless of the forces which would tend to deflect the axis from a northerly direction.
A compass card is attached to, and aligned with, the axis of the gyroscope and this acts as the reference direction from which all directional surveys are taken. Once the tool has landed in the required position in the drill collars the procedure is very similar to that for the magnetic single shot. Since the compass card is linked to the axis of the gyroscope it records a True North bearing which does not require correction for magnetic declination.
- Gyroscopes are very sensitive to vibration so the gyro single shot must be run and retrieved on wireline.
- The gyroscope may also drift away from its set direction while it is being run in the hole. When the instrument is recovered therefore, its alignment must be checked, and a correction applied to the readings obtained from the survey. Gyro single shots are often used to orient deflecting tools near casing.
- The Gyro Multi-Shot is used in cased holes to obtain a series of surveys along the length of the wellbore. The magnetic multi-shot cannot be used because of the interefence to the earths magnetic field, caused by the magnetisation of the casing.
- The directional surveyor must keep track of the depth at which the pre-set timer takes a picture. Only those shots taken at known depth when the pipe stationary will be recorded.
- When the multi-shot is recovered, the film is developed and the survey results read. In the case of both single shot and multi-shot instruments adequate centralization must be provided so that the instrument is properly aligned with the wellbore.