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.
The acronym MWD currently is used by the industry to describe telecommunications between the bottom of the drillstring and the surface. MWD at the surface is not new. Since the beginning of the drilling industry in the mid-1800's it has been a standard practice to record the time required to drill each foot of hole. A correlationtype log may be made by comparing the drilling time with the cuttings returned from downhole to establish the tops and bottoms of different formations. This driller's log was the sole logging method used by the industry until 1927, when the first electrical logs were developed. As early as 1932, attempts were made to make these downhole electrical measurements during the drilling process. Such efforts used an insulated cable inside the process. Such efforts used an insulated cable inside the drillpipe and were termed logging-while-drilling. Intended to replace the conventional logs, these electrical alternatives were not very successful for both technical and economical reasons. After the sudden increase in oil prices during the 1970's, renewed efforts were made by prices during the 1970's, renewed efforts were made by many companies to refine such methods. During this period, there was no standard industry nomenclature for period, there was no standard industry nomenclature for this procedure. In various companies it was called MWD for measurement-while-drilling, LWD for logging- while-drilling, or DHLWD for downhole-logging-while- drilling. In most instances, the information obtained is not intended to replace conventional well logs and often is nothing more than a few measurements taken intermittently on hole angle and direction. More descriptive terms might be downhole-measurements-made-while-drilling (DHM-MWD) or downhole-mud-pulse-telemetry-measurements (DHMPTM). These terms might better differentiate measurements made at the surface while drilling from downhole measurements telemetered to the surface while drilling. The entire industry has adopted the term MWD, although this generally refers only to downhole measurements telemetered to the surface while drilling.
The need for intermittent directional measurements (unlike the need for continuous information, as obtained by conventional wireline logs) may be another reason that MWD seems to be an accepted term for these new procedures. As the market matures it may develop a new procedures. As the market matures it may develop a new terminology, but for now it seems to be MWD. Other sensors and devices for improving dulling efficiency and safety, as well as providing correlation-type logs, are being added by most companies. The MWD directional methods show a considerable advantage over older techniques, since lost rig time for wireline surveying or other means to determine hole angle and direction in deviated wells can be very costly. These immediate savings have caused MWD directional services to find a ready market.
Mud pulse telemetry types currently available include:1. Negative pressure pulses generated by a bypass valve from inside the drillstring to the annulus (Fig. 1a).