Teledrift is an inclination-only mud pulse MWD system that can measure inclinations up to 10.5º. The system is available from Scientific Drilling International (SDI).
The Teledrift is a purely mechanical system. The tool consists of a pendulum at the bottom that moves along a series of graduated stop shoulders, and a signalling plunger at the top that can traverse a series of seven annular restrictions to produce pressure pulses in the mud stream.
Teledrift - Inclination-only MWD
The plunger travel is controlled by the coding system so that as the inclination increases, the number of pulses also increases. The tool is adjusted before running in the hole so that it can measure any seven successive incremental angles of inclination between 3.5° (minimum step size 0.5°) up to a maximum of 10.5° (step size: 1.5°).
If the normal pump rate is substantially higher than the optimum signalling range, hold the pump speed within the signalling range for about 2 minutes before bringing it up to normal speed. Drilling may begin as soon as the pumps are started.
If the flow rates are below the minimum values, the signal amplitude may be too low for clear detection, and the spacing between signals may be excessively long. If the flow rate is too high, the signal amplitude will be too high and the signal spacing too short for clear distinction.
Pre-survey check list
- Check power supply and air for the surface recorder and purge system.
- Check for suitable connections for hoses and diaphragm.
- Check the expected inclination of the hole and the corresponding inclination range for the tool.
- Check required size of Teledrift sub and instrument. Determine optimum flow rate and maximum flow rate.
- Check BHA connections and hydraulics with the planned position of Teledrift sub.
- Check maximum expected temperature.
Making Teledrift readings
Teledrift readings can be made any time during drilling. When drilling is stopped during both the measuring and the signalling periods, the maximum downtime is generally 2 to 4 minutes. Downtime may be reduced to only 1 to 1-1/2minutes by resuming drilling during the signalling period. For maximum clarity of signals, take readings between, not during, connections, when the mud is very fluid, with no air in the pipe. Downtime may also be minimised by measuring the inclination just before making a connection and receiving the signals after completing the connection, when the pump is started.
The most reliable operation is obtained by making the inclination measurement and recording signals in one unbroken sequence between connections. This produces the maximum signal clarity with minimum possibility of uncertainty.
Uncertainties of Teledrift tool
The accuracy of the tool is at best equal to the angle step size of 0.5-1.5°, dependent on the selected range. Whether the system accuracy is actually achieved also depends on having the inclination range set correctly, receiving the full set of pressure pulses and having the tool operate correctly. The tool is of simple design, but a failure of the downhole measuring system may be difficult to detect, and consequently a single-shot inclination-only survey should be run occasionally as a check.