Geology Reference
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Figure 2.10h
. Drillpipe p/'p to 1,000 Hz.
The implications for MWD telemetry are clear. Phase-shift-keying (PSK)
not only confuses surface detectors with “ghost reflections” created at the
drillbit: they typically reduce the wave amplitudes needed for transmission over
large distances. On the other hand, frequency-shift-keying (FSK) schemes in
which 0's and 1's are transmitted by alternating between frequencies with high
and low transmission efficiency offer significant advantages. First, constructive
wave interference is used to enhance signal strength without incurring power or
erosion penalties as would be the case in mechanical methods, e.g., mud sirens
now increase signal only by reducing rotor-stator gap.
Second, the multiplicity of available frequencies allows significantly
higher transmission data rates, say, by switching rapidly between 100 Hz, 110
Hz and 120 Hz. Third, the closeness of these frequencies implies that the
mechanical power requirements needed to change from one rotation rate to
nearby close ones are small, and this is particularly so with low-torque, rotor-
downstream sirens such as those described in Chin and Trevino (1988) and Chin
(2004). Finally, the large number of available optimal frequencies suggests that
next-generation tools with multiple sirens should be investigated. Numerous
“conversations” can be multiplexed along the channel using methods standard in
communications theory.
Over the years, petroleum industry practitioners have accepted a maximum
threshold of about 25 Hz for MWD transmissions based on flawed assumptions.
The results of flow loop tests, for one, have not been interpreted properly, in the
sense that commingled destructive wave interference and true attenuation effects
have never been separately analyzed. Adding to the confusion, long flow loops,
as will be explained in Chapter 9, offer boundary conditions that differ from
those in real wells - they tend to support irrelevant standing wave systems rather
than provide test platforms for evaluating novel telemetry concepts.
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