Summary Several downhole measurement-while-drilling (MWD) systems are currently being developed and some are already in use on a commercial basis. These devices all have sensors located immediately above, or near, the drill bit to monitor drilling variables and, in some cases, formation properties. Information is collected downhole, and sent (telemetered) to the surface through either the drilling fluid, electrical conductors, the drill pipe, or the earth. The systems provide essentially real-time information for monitoring and controlling the drilling operation. Applications of MWD systems include monitoring and controlling the directional-drilling operation, assistance in detection of abnormal pressure zones, correlation logging, preliminary evaluation of some possible producing zones, and monitoring of weight on bit (WOB) and drilling torque at the bit. Operating experience indicates that the MWD systems are being accepted in the industry and are providing useful information. Several examples of information obtained are shown. Introduction MWD systems contain three primary subsystems:a downhole sensor package,a method to send (telemeter) information from the sensor package to the surface while drilling proceeds, andsurface equipment to receive the information and put it into a useful format. Fig. 1 is a block diagram of a typical MWD system. The MWD systems available now are beginning to satisfy a long-standing industry need. As early as the 1930's, attempts were being made to solve the many problems involved. In the last 45 years, more than 40 problems involved. In the last 45 years, more than 40 companies have been involved in MWD development. This paper describes systems that are now, or are soon expected to be, commercial. Information from downhole during drilling generally can be placed in two categories:drilling variables and hole information, andformation characteristics. Drilling variables and hole information might include hole direction and inclination, tool-face heading, WOB and torque, downhole temperature and pressure, mud properties, bit vibration and acceleration, and others. properties, bit vibration and acceleration, and others. Formation logs might include resistivity, natural gamma ray, and others. Many of these variables can be monitored with available systems. Additional sensors will no doubt be added as experience is gained. Successful implementation of MWD is the result of development of practical telemetry systems. All of the systems now available transmit through the drilling fluid inside the drill pipe. Downhole sensors and surface equipment technology were much more advanced and only required adaptation to MWD. One of the first MWD telemetry systems was field tested successfully by Mobil R and D Corp. during 1971. This telemetry method has been integrated into a complete MWD system by The Analysts/Schlumberger and was offered commercially beginning in early 1980. This system currently has the highest data rate and offers more sensors than any other commercial system. The first commercially available system, and the one with the most field experience, was put into commercial service by Teleco in Sept. 1978. This system supplies only directional information but is expected to offer other parameters soon. Christensen Diamond Products. Exploration Logging Inc., and Gearhart Industries Inc. MWD systems are also available commercially, with still others reported to be nearing commercial operation. MWD systems have had only about 4 to 5 years of commercial use. In this short time, they are beginning to establish a reputation for being reliable, useful, and cost effective additions to drilling technology. JPT P. 1439