Measurement-while-drilling (MWD) systems, using mud pulse telemetry, are now available to the industry. The most popular sensor package is for directional drilling measurements. Accuracy of such measurements has been proved under field conditions and has resulted in considerable savings in rig time. This acceptance and usage has increased the demand for other sensors for improved drilling efficiency, kick detection, and formation evaluation.
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. Introduction 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". P. 515
American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Abstract The Direct Digital Logging System, first introduced in 1975 as a new concept in well logging, has proven the feasibility of digital systems for recording logs. The surface equipment necessary to produce logs includes a mini-computer and laser recorder, along with other input/output devices and is well sected for wellsite formation analysis. Log signals are sampled four times per foot and are recorded in engineering units on per foot and are recorded in engineering units on magnetic tape cartridges. Depth shifting required to match subsequent logs to the base log is performed through a calculator type keyboard, with the data interpolated between the depth shift points. Interpolation is possible because of the three inch depth increments. The advantage of interpolation in depth shifting is that no data is lost or created at the depth shift points. Environmental corrections are applied to the log trace data to remove borehole effects. Computer performed corrections such as borehole correction to the performed corrections such as borehole correction to the Induction Log and mud cake correction to the Sidewall Neutron Porosity Log removes the need for departure charts and hand calculations. With the digital system, wellsite quick look calculations are performed after the logs are recorded and environmentally corrected, all the data is studied, appropriate parameters are selected, and the program is executed. The digital system is an improvement over existing analog systems because the digital system does not require parameter selection before the logs are run, and the computing is not performed as the well is logged. Digital quick look techniques presently available are logarithmic F overlays and R with porosity. More detailed evaluation can be performed at wellsite, going beyond the quick look methods. For example, the Induction Electrical Log (IEL) and the Compensated Density Log (CDL) can be combined and solve for volume of shale, shale corrected porosity, and water saturation. Also the Sidewall Neutron Porosity Log (SNP) and the Compensated Density Log (CDL) combination will resolve cross-plot porosity and apparent matrix density, which combined with resistivity often provide good results in complex lithology. provide good results in complex lithology. The new concept of wellsite Direct Digital Logging and computed formation analysis opens a new era in log evaluation. The opportunity to obtain computed logs has been moved to the wellsite, and the applications are just being realized.
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