Acoustic Telemetry: The New MWD System
Authors: Tochikawa T. , Sakai T. , Taniguchi R. , Shimada T. 
Published: December 2007

Abstract
A new MWD system utilizing acoustic telemetry system has been researched, based on the principle of elastic wave propagation and magnetostrictive technology. The system is intended for data transmission through jointed drill strings with much higher reliability and transmitting efficiency.
In order to achieve a good acoustical propagation characteristics through the drillstring the elastic wave signal requirements have had to be less than 1 kHz. The development of an optimum oscillator that generates such a low elastic wave frequency under harsh drilling conditions has been one of the technical breakthroughs.
In this regard, one of the features of the system is the use of a magnetostrictive material as an oscillator in the transmitter.
Utilizing the phenomena in which a magnetostrictive material drastically distorts by applying magnetic field to it, the oscillator generates and elastic wave for propagation and transmission through the drill strings.
Several field tests have proven that the system could transmit canned data from the depth of 1914 meters and data demodulation at the surface free from troubles while drilling forty nine (49) degrees deviated well.
These results provide a comprehensive understanding of wave propagation characteristics which the authors are assured that practical MWD equipment with higher than normal transmission rates can be developed and commercialized. Since the system can offer several unique features especially in terms of miniaturization and non-dependence on drilling fluids it can be applied to the emerging technologies such as slim-hole drilling, coiled tubing drilling, local communication on the bottom hole assembly. Furthermore it can offer wider application for the drilling environment such as underbalanced drilling.

Introduction
MWD technology which obtains real-time measurement of various data at the bottom of the well during drilling process is one of the most common technologies of today. Mud pulse propagation has been the normal method used for MWD technology, however, one of the limitations of this method is the low transmission rate due to the attenuation mechanisms in the drilling mud. The use of telemetry by acoustic wave through the drill string has been suggested since 1940's due to steel tubulars such as drill pipe having good and effective sound propagation.
A field test was performed in 1948 and a signal loss of about 12 dB/1000ft was reported. As a result of this test, the technology of telemetry by acoustic wave has been thought to be extremely difficult to be developed.
In 1975, Sun Oil Company conducted tests using ball-pin hammer and reported that attenuation in the actual field situation was very low, less than 4 dB/1000ft. The former report was investigated the adequacy of the theory developed by the laboratory experiment to the actual field environment and concluded the following results in his paper (Ref. 1).
* It is important to use pass band propagating acoustic waves through the drill strings. * Using pass band, it will expect a signal loss of 4 dB/1000ft. * An acoustic telemetry device is needed to apply these characteristic.
Nevertheless, since then, there have been no reports of any other successful results. It is thought that the key for the practical application of this technology is the reliable acoustic telemetry device.
With this background, a new project on a MWD system that has focused on the development of a acoustic wave source was commenced in 1994.
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