Limits on Data Communication Along the Drillstring Using Acoustic Waves
- Li Gao (Halliburton) | Wallace R. Gardner (Halliburton Energy Services Group) | Carl Robbins (Halliburton Energy Services Group) | Don H. Johnson (Rice University) | Mahsa Memarzadeh (Rice University)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- February 2008
- Document Type
- Journal Paper
- 141 - 146
- 2008. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 1.12.2 Logging While Drilling, 5.6.4 Drillstem/Well Testing, 1.5 Drill Bits, 1.6.7 Geosteering / Reservoir Navigation, 5.1 Reservoir Characterisation, 1.6 Drilling Operations, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.10 Drilling Equipment, 4.1.5 Processing Equipment, 1.6.1 Drilling Operation Management
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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.
LWD plays crucial roles in the exploration of hydrocarbons. LWD is used to gain understanding of the Earth formations in real time. This knowledge is not only useful for hydrocarbon-reservoir characterization, it is also important in geosteering. To transmit LWD data to the surface, the current standard technology in the industry is mud-pulse telemetry, in which pressure pulses are generated downhole and propagate to the surface where they are detected and decoded (Arps and Arps 1964). Unfortunately, the throughput of the mud-pulse system is only a few bits per second. On the other hand, with the ever -ncreasing complexity of LWD sensors, more data are required to be transmitted to the surface than ever before. Clearly, faster-data-rate wireless-telemetry systems are needed. In fact, the industry has been searching for such systems for a long time. As early as 1948 (Cox and Chaney 1981), the transmission of data by means of acoustic stress waves propagating along the drillpipe was identified as a potential method for high-speed communication. Theoretical studies were carried out by Barnes and Kirkwood (1972) as well as by Drumheller (1989) in an attempt to analyze acoustic-wave propagation in drillstrings. Work by Lee (1991) and Ramarao (1996) on wave propagation in fluid-laden drillstrings further advanced understanding of the attenuation processes of acoustic attenuation.
In the 1990s, Halliburton launched an effort to develop a wireless telemetry system for nondrilling applications. As a result, Acoustic Telemetry System (ATS) (Halliburton; 2000; Houston) was commercialized in 2000, demonstrating the value of acoustic telemetry in nondrilling applications such as drillstem testing. Since then, effort has been extended into research of an LWD acoustic-telemetry system that is operable under drilling conditions (Shah et al. 2004).
To the authors' knowledge, our work represents the first systematic study on the communication-channel capacity of drillstring under drilling conditions. After a brief discussion of the motivation, the channel characteristics of the drillstring are examined. This is followed by an introduction to the general channel-capacity theory. The theory is then applied to 3,000-, 5,000-, and 6,000-ft-long drillstrings to derive their respective capacities. Implications of the capacity result and potential means to achieve maximum data rate are investigated.
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