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
A large gap exists between the type of equipment in general field use for mineral exploration and the latest state of the art type digitizing equipment. Economic considerations and technical capability of the average logging operator are the two main reasons for this gap. A new generation of digitizing equipment used for digitizing depth and gamma count rate in uranium exploration holes presents the data on printed tape. This is one of the simplest systems available but still requires key punching before the log data is computer compatible. Such systems are used primarily in the development drilling and economic evaluation of ore bodies. Exploratory logging units are seldom equipped with even this simple type of digitizing equipment since the rate of uranium discoveries is quite low and actual usage would be minimal if on all logging units. Highly sophisticated digitizing systems of a laboratory type are being field tested for exploratory logging units engaged in basic mineral prospecting. Such systems analyze and process well log data on-site and identify basic elements. The evaluation of californium-252 as a neutron source for mineral exploration by neutron activation analysis is assisted by the use of such a computer system. Introduction Digitizing of well logs at the well site seems to be lagging behind the previous expectations of people in the industry. There are specific areas where well site digitizing has been widely accepted, such as in uranium logging. Digital techniques and automatic data processing systems offer the only practical solution for handling large volumes of data accumulated. In the evaluation of uranium deposits by the use of well logs, the logs are accumulated rapidly and final results from the processed data must be obtained quickly. Therefore, beginning with simple digital tape printouts and in some cases advancing to punched tapes, printouts and in some cases advancing to punched tapes, uranium logging field units are currently producing most of their data in both analog and digitized form. Field examples are used to demonstrate typical systems in current use. Such systems are relatively inexpensive and require a minimum of skill to operate. Highly sophisticated well site digitizing systems are being used for research purposes and one such system is described. INDUSTRY PRACTICES Evaluation of the economics involved in all phases of the exploration and development of a mineral deposit have always been essential to the planning of a successful venture.
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.
One of the more extensive uses of directional drilling anywhere in the world has been in the development of the East Wilmington Oil Field in Long Beach, California. The average well is deviated from vertical in excess of 50° and wells with a maximum deviation in the 70° to 80° range are not uncommon before they are dropped off to 50° or less when penetrating the completion interval. Over 780 wells have been drilled in this field to date, requiring the highest degree of control and accuracy in order to avoid intersection of other wells and to obtain proper bottom hole spacing. The Measurement-While-Drilling (MWD) directional system has been tested on several wells and proven to provide the required accuracy, along with many advantages over past methods, used in the field development.Accurate transmission by MWD of bottom-hole measurements to the surface is provided by mud pressure pulses generated in the drill pipe downhole and detected by a pressure transducer includes the means for detecting, recording and processing these pressure pulses, to translate the information from the pressure pulses to rig floor displays usable by the drilling crew.
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