Measurement While Drilling ("MWD") systems are installed in the drill string to provide real time measurements of basic trajectory parameters such as inclination, direction, tool-face and temperature. Additional sensors such as pressure, gamma-ray and resistivity may be added depending upon the specific application.
MWD tools consist of three basic sections:
- Power Supply - Most tools are powered by lithium batteries or in some cases down-hole generators (turbines).
- Sensor Section - Hole trajectory is measured by a sensor stack that has three accelerometers and three magnetometers. The inclination and roll (gravity tool-face) of the tool is made by gravity based measurement devices called accelerometers. Simply stated, they can be considered to be an electronic plumb-bob. Magnetometers measure the earth’s local magnetic field. Combined with inclination sensors the tool can provide a reference direction to magnetic north. This is corrected for true north by adding the localized value for magnetic declination. Other sensors such as pressure, gamma-ray, and resistivity are typically housed in separate dedicated tool sections.
- Transmitter - Current MWD tools transmit in two basic manners, by sending pressure waves through the drilling mud (mud pulse) or by transmitting electromagnetic (EM) signals through the earth to surface.
The EM MWD tool is a wireless MWD System which utilizes low frequency current-loop signals to communicate data to the surface.
Mud Pulse Systems
The symmetrical, performance-enhancing design of the Resistivity system and the centrally located antennas provide real-time compensation eliminating invasion effects due to measurement delays, and improve accuracy through cancellation of variations in receiver channels. The Resistivity system is effective in all mud types and provides a range of transmission formats. High resolution data can be stored downhole, retrieved and processed as required.