Geology Reference
In-Depth Information
2
Harmonic Analysis:
Six-Segment Downhole Acoustic Waveguide
High-data-rate pulsers in continuous wave MWD telemetry not only create
acoustic signals that propagate uphole, but in addition, down-going signals that
“reflect at the drillbit,” reverse direction and combine with the former to create
waves that may constructively or destructively interfere. The ultimate signal
that travels up the drillpipe depends on mud sound speed, the position of the
pulser in the drill collar and its operating frequency, and importantly, the details
of the bottomhole assembly forming the host waveguide. The implications are
both good and bad. Destructive interference in signal generation can severely
limit data rate and transmission distance, but constructive wave interference,
properly applied, can enhance MWD signal strength without the usual power
and erosion penalties incurred by purely mechanical methods.
“Reflection at the bit,” a phenomenon noted above, actually is more
complicated than simple acoustic “open” or “closed” models would have us
believe. In reality, signals
do
propagate through nozzles that may be small, and
signals
are
detectable in the annulus. Thus, a telemetry model used to study
fundamental physics and potential technical capabilities must not disallow
transmissions into the annulus; moreover, as explained in Chapter 1, it is
additionally important to model the antisymmetric disturbance pressure field
about the source, so that downhole constructive and destructive interference
processes and reverberant fields within the MWD collar can be properly studied.
Exact solutions for a basic six-segment waveguide model representing the
complete acoustic transmission channel are obtained for siren and poppet-type
sources and their implications in high-data-rate tool design are discussed.
Detailed solutions for a representative bottomhole assembly are provided in
which signal generation efficiency is evaluated as a function of source position
and frequency. These results suggest the use of new telemetry schemes that
specifically focus on the positive aspects of wave propagation in signal creation
and also the possibility that transmission rates and distances greatly exceeding
the values conventionally accepted can be developed. In Chapter 9, we find that
the model is more accurate than long flow loop analysis, since many industry
loops are built satisfying practical boundary conditions that are not consistent
with actual “radiation conditions” found in the drillpipe and annulus.
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