• 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,and gamma-ray and resistivity may be added depending upon the specific tool type and/or application.
MWD tools consist of three basic sections;
- Power Supply - Generally 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 3 accelerometers and 3 magnetometers. The inclination and roll (gravity tool-face) of the tool is made by gravity based measurement devices called accelerometers. For simplicity's sake, they may be considered to be an electronic plumb-bob. Magnetometers measure the earths 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 day MWD tools transmit in one of two basic manners, by sending pressure waves through the drilling mud (mud pulse) or by transmitting electromagnetic "EM" signals through the earth to surface.
• Mud Pulse
Mud pulse tools operate by either opening or closing a valve in the tool that creates either a pressure surge (positive pulse) or drop (negative pulse). The data from the sensor section is encoded to one of a variety of formats (i.e. binary). Pressure signals in the range of 10-50 psi are detected at surface by a transducer, and the data decoded into useable information. Mud pulse tools have no practical depth limitation but are dependant upon the drilling fluid utilized and are relatively slow as compared to EM MWD systems. The tools do not function in under-balanced drilling where nitrogen or air is used as a drilling fluid. Both the negative and positive pulse systems add pressure increases to the circulating system, typically ranging from 150 up to 350 psi dependant upon the system type.
• EM MWD
Electro-Magnetic MWD systems use the same basic sensors and power supplies as the mud pulse systems. The main difference is in the transmission of data. Rather than use the drilling mud to send pressure waves, the tool sends either a magnetic pulse or electrical current through the ground to the surface. On surface the data is received through ground antennas and the data processed. EM systems are significantly faster (10x) than conventional mud pulse. In addition data can be sent at any time (not just when the rig pumps are circulating). The net result is faster overall drilling times. In addition the EM systems are the only practical method to drill under-balance wells involving the use of air, nitrogen, and foam. EM systems have no moving parts and do not create significant restrictions in the drill string. As a result the reliability is significantly higher, and damage from erosion caused by drill solids is minimal. EM tools do have depth limitations, which are a function of how much power can be supplied by batteries for the duration of the drilling interval, and at higher power settings the battery costs may be significant.
