Geology Reference
In-Depth Information
10
Advanced System Summary and
Modern MWD Developments
In this final chapter on Measurement-While-Drilling analysis and design,
we discuss the basic elements of a hypothetical high-data-rate MWD system
which embodies the ideas in this topic and the authors' field and modeling
experiences. Our exposition applies key telemetry and acoustics concepts which
are also useful in other possible prototypes and highlights the pitfalls and
fallacies behind commonly accepted engineering design rules-of-thumb.
At the present time, several MWD service companies provide mud pulse
logging worldwide. Three largely operate in the 1 bit/sec (or less) range, one
operates siren tools at 3-6 bits/sec (bps), possibly faster, and the last, in recent
product literature related to siren-like pulsers, claims extraordinarily data rates.
The exact specifications in all cases are not well documented, that is, technical
details related to mud type, borehole depth and environment are not available,
but the high data rate at least points to mechanical capabilities which we believe,
importantly, are nowhere near present technology limits.
Can we do better? Yes. And just what
is
required to send and receive
signals over large distances successfully at high rates? Three key factors apply:
strong amplitudes, high frequencies and low torque. First, the acoustic signal
should be large and ideally created “intelligently” with minimal impact on
erosion and power demand. Second, the telemetry sequence should be simple to
operate and decode, with very small possibilities for error. Third, surface signal
processing should be robust and based on well-defined acoustics principles,
allowing sensitive piezoelectric transducers to “see” beneath high levels of
noise. Fourth, the pulser should require minimal mechanical power for
operation. And finally, as we will explain, drilling mud should be selected for
minimal attenuation. We will discuss each of these ideas in detail.
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