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.
Introduction
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.
© 2008. Society of Petroleum Engineers
View full textPDF
(
1,112 KB
)
History
- Original manuscript received:
5 July 2005
- Meeting paper published:
9 October 2005
- Revised manuscript received:
25 April 2007
- Manuscript approved:
22 September 2007
- Version of record:
25 February 2008
View Cart
