Today, Measurements While Drilling (MWD) is a widely available and accepted service industry in the international marketplace. As it has matured, the focus has shifted from the novelty of telemetry to the measurements themselves; what can be measured, and how can that information be of value?
The most widely used and best understood service is the directional survey. The promptness and accuracy of MWD surveys has proven to be of real value in the control of difficult directional wells. The MWD service of formation measurements is growing. It is being used, not only for marker selection, but as the sole logging service for some footage. In exploratory wells, the immediate nature of the logs takes some of the guesswork out of the real-time decisions and the quality of measurements made in an undamaged borehole improves interpretation. Evaluation also continues on a host of MWD sensors that bear on the efficiency, safety and control of drilling.
From the first commercial runs in the late 1970's, the Measurements While Drilling service industry has grown to approximately 300 field systems. Atleast five different companies offer services internationally and several others are commercial in a localized area or still in development and testing. All are competing for a market estimated to be around 200 million U.S. dollars in 1985 and growing annually at a 15 to 20 percent rate. percent rate. The earliest technical successes in MWD were actually research projects by some of the major operating companies. In a couple of these. an electrical conductor supplied power to the measuring apparatus downhole and carried the measurements back to the surface. This technique, now called "hardware", was historically important because of the information gained from those measurements and the impetus the success gave to the companies developing commercial MWD systems. The hardwire technique offers a higher transmission rate than any other known MWD telemetry technique and eliminates the need for a downhole power source. However, because of the expense and cumbersome nature of the equipment, the competitive service on the technique.
The MWD technique on which the presently commercial systems are based is called "mud pulse telemetry" because the pressure drop of the mud flowing through the bottom hole assembly is modulated in order to produce a signal. At the surface, a pressure transducer is mounted on the standpipe and a pressure transducer is mounted on the standpipe and a receiver searches for the fluctuations in pressure that were produced by the tool downhole. There are some systems, still under development, that use low frequency electromagnetic waves to transmit through the earth to an antenna on the surface; this technique is known as "EM telemetry". Since, in either case, there is no wire, the downhole tool must get its power from batteries or a mud flowdriven electrical generator.
There are various ways to produce the pressure modulation in mud telemetry. Some systems use a bypass valve that vents a portion of the mud directly through the collar wall into the annulus. This has been given the name"negative pulse" because a drop in pressure is produced at the standpipe. Other systems are based on a restriction that momentarily increases the pressure at the standpipe and are known as "positive pulse"systems. One special case of pressure restriction has a pair of toothed wheels, pressure restriction has a pair of toothed wheels, axially aligned with the drill string, one rotating and one stationary. As the teeth pass each other, a fixed frequency pressure wave is introduced into the pipe and information is encoded by phase shifting pipe and information is encoded by phase shifting that wave. This technique received the nickname, mud siren".