A new approach to acoustic communication along the drill string employing zero-order torsional acoustic waves is presented. This new approach is introduced because the presented. This new approach is introduced because the extensional acoustic waves presently used by the petroleum industry in drill string acoustic telemetry are fundamentally limited to short communication distances along the string, except for extremely low data rates. This limitation occurs because extensional waves are inherently severely attenuated in propagating along the string. The physical mechanism propagating along the string. The physical mechanism responsible for this attenuation is described. Analysis is presented that provides estimates of this attenuation and presented that provides estimates of this attenuation and indicates a very large value for extensional waves in typical drill-string acoustic telemetry applications. Conversely, the analysis indicates a very small attenuation for zero-order torsional acoustic waves. Therefore, the communication distance along the string should be significantly increased by he use of zero-order torsional waves.
Two new types of drill string acoustic communication system that are made feasible by the low attenuation of zero-order torsional waves are mentioned. Both of these systems have major advantages relative to conventional drill string acoustic telemetry systems.
A series of field experiments measuring zero-order torsional acoustic wave propagation on drill strings in wells has been completed and another series is in progress, to verify the analytical predictions for these waves. These experiments are described and preliminary results are presented.
In drilling a well, the usual method employed by the petroleum industry to obtain down-hole drilling data consists of petroleum industry to obtain down-hole drilling data consists of lowering an instrument package down the inside of the drill string on a wire line. As presently used, this method requires the interruption of the drilling process and the opening of the drill string in order to lower the instrument package into the well. In a deep well, the time required for this operation can amount to an hour or more. Moreover, in directional drilling applications the interruption of drilling to obtain steering data often introduces errors, due to the absence of torque on the drill string during the measurements. A similar situation exists in open-hole logging, where the drill string must be removed from the bore hole before the open-hole logging instrument package can be lowered into the well. In a deep well, the time consumed during this logging operation may be several hours.
A wireless system for telemetering data from down hole to the surface would not require opening the drill string and interrupting the drilling process for long periods of time and would have immense operational and economic advantages over the methods presently used for obtaining down-hole drilling and logging data. Such a system could be used to provide virtually continuous information on drilling parameters, such as bore-hole direction and pressure, and on formation parameters, such as conductivity and density. In addition, a system that could function while directional drilling is actually in progress would eliminate the errors in directional steering data caused in present systems by the drill-string torque differences between drilling and data measurement conditions.
Because of the potential advantages of a wireless telemetry system, during the past thirty or more years there have been many attempts to communicate down-hole data to the surface by means of acoustic waves propagating along the drill string. The success of these attempts has been severely limited, since communication has been achieved only over short lengths of drill string, except when the data rate has been restricted to extremely low values. This limitation in performance is a direct consequence of the very large acoustic attenuation associated with the type of acoustic wave - the extensional wave - that has been used for previous attempts at drill string communications. previous attempts at drill string communications. An extensional wave on a drill string consists of propagating regions of longitudinal compressive and extensive strain propagating regions of longitudinal compressive and extensive strain in the drill pipe and tool joints. Because these waves consist of regions of longitudinal strain, they have sometimes been called longitudinal waves. Strictly speaking, however, the term "longitudinal" is applied only to waves in extended solids, such as seismic "P" waves in the earth. In an extensional wave, at each region of compression the pipe is bulged and at each region of extension the pipe is necked down, the magnitude of bulging or necking being related by Poisson's ratio to the magnitude of the strain in the longitudinal direction. Because of this interaction between longitudinal and radial strain, as extensional waves propagate along the drill string they are accompanied by radial propagate along the drill string they are accompanied by radial waves that radiate acoustic power into the surrounding drilling fluid and formation. It is this radiation loss that accounts for the very large attenuation associated with these waves.
