Limits on Data Communication Along the Drillstring Using Acoustic Waves

Summary The first commercial acoustic-telemetry system was introduced successfully in 2000 as part of a drillstem-testing system. However, duplicating the success of the acoustic telemetry to environmentally challenging logging-while-drilling (LWD) applications at high data throughput remains a formidable task. The primary limitation arises from normal drilling operations that produce inband acoustic noise at multiple sources at intensities comparable to the transmitter output. This noise, together with the signal attenuation along the drillstring, adversely affects the data throughput. To determine the communication capacity of the drillstring channel using acoustic waves, we examined the impact of channel characteristics, signal attenuation, and noise in detail. On the basis of a communication model that incorporates the effects of both drillstring acoustic channel and noise, we extensively studied the capacity of the system using the waterfilling method. For this analysis, realistic downhole transmitter power output, experimentally measured noise at the surface, and measured attenuation of acoustic waves in the drillstring channel were used as input parameters. The results show that a typical drillstring channel has a potential capacity of up to several hundred bits per second under noisy drilling conditions. Implications of the channel capacity on acoustic-telemetry-system designs are discussed. A communication technique that comes close to realizing a high-rate telemetry system is introduced. Methods to optimize various aspects of the system such that maximum drillstring-channel utilization can be realized under drilling conditions are also discussed. Potential enhancement to data rates through application of error control-coding is covered briefly.