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placed in a uniform flow loop and not a replica of the bottomhole assembly.
Likewise, “real world” field test results are likely to lead to confusing
conclusions if not interpreted properly - and similarly, successful high-data-rate
MWD telemetry will not be optimal unless preliminary job planning is
performed to determine the range of desirable frequencies and also the suite of
useful telemetry schemes for any particular rigsite operating scenario.
2.2.7 Wind tunnel testing comments .
Analogous test interpretation problems may arise with wind tunnel testing,
that is, the use of hundred-feet versus thousand-feet long wind tunnels for MWD
telemetry acoustic testing. For instance, different pressure patterns are obtained
accordingly as the fluid source is positive displacement piston driven (that is, a
solid reflector) or acts as a centrifugal pump (an open-end reflector). While
such wind tunnels are ideal for 'p source strength testing and preliminary signal
processing instrumentation and software design, particularly because such tests
are convenient and inexpensive, care must be taken to understand the physical
phenomenon actually modeled and how it relates to telemetry in a deep well.
For example, if outgoing wave conditions are to be modeled at both inlet and
outlet, enough damping along all propagation paths (say, using soft acoustic
liners) and sufficient length are required to eliminate end reflections. In the next
section, we develop a detailed waveguide model for the complete MWD
telemetry channel. In later chapters, however, models for finite loops are
designed for laboratory application.
2.3 Downhole Wave Propagation Subtleties
We have discussed the complexities related to source signal generation in a
continuous wave environment from a downhole perspective. In Figure 2.4,
where constant frequencies are assumed, single arrows denote pure propagating
wave motions while double-arrows indicate the existence of standing waves.
Diagram (a) illustrates the situation encountered downhole for deep wells,
while Diagrams (b), (c) and (d) illustrate other situations that may be appropriate
to shallow wells or wind tunnel tests (the darkened cross-sections indicate that
alternative boundary conditions might apply, e.g., shortened wind tunnel
lengths, shallow wells, open reservoirs or orifice plates). In this chapter, we
develop and discuss model results for (a) only, although similar results for the
other configurations are easily obtained by modifying one or two end boundary
conditions. In addition, our model applies to sirens and poppet dipole valves
and not negative pulsers or monopoles.
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