Harmonic Analysis: Six-Segment Downhole Acoustic Waveguide - Measurement While Drilling Signal Analysis, Optimization and Design - page 56

this chapter is to identify the conditions under which p pipe /'p is weak, or on a
positive note, the conditions under which it can be optimized, so that signal
enhancement without the usual anticipated penalties related to power and
erosion are realized. We also recognize, of course, that any increase in p pipe /'p,
if it is obtained at a higher frequency, is subject to higher frequency-dependent
attenuation. The hope is that increases in signal amplitude more than offset
decreases incurred along the acoustic path. A complete telemetry analysis will
consider both the downhole model addressed in this chapter, plus the subject of
irreversible thermodynamic losses over large distances as well as the frequency
dependence of 'p on mechanical design, both of which are treated later.
2.2.6 Flowloop and field test subtleties .
In deep wells containing attenuative drilling mud, the up and down-going
waves created by the source will interact to form standing waves within the drill
collars, the lower annulus and the bit box, while upgoing propagating waves
will be found in the drillpipe and in upper annulus (this statement strictly applies
to a system excited by a constant frequency source) provided the travel path is
long. This observation also applies if the downward waves reflected from
mudpumps and desurgers (and from the mud pit) have attenuated enough that
they can be ignored - in shallow wells, we emphasize, this outgoing wave
condition may be inapplicable and standing wave assumptions in both pipe and
annulus may be required. In this chapter, we assume that propagating waves do
in fact exist, and use such “radiation” or “outgoing wave” conditions as
boundary conditions for the acoustic math model - that is, the drillpipe and
upper annulus both extend to infinity and are each in themselves semi-infinite
waveguides - or possibly, that they are both finite in length, but that their
reflections toward the pulser attenuate significantly enough that they are not
dynamically relevant. We implicitly assume deep wells.
The same careful observations apply to flow loop testing and
interpretation. In several flow loops operated by petroleum organizations,
lengths of 5,000 to 15,000 feet are available and have been used for MWD
telemetry testing. Typically, water is used as the test fluid since it is both clean
and requires significantly less pump power. Positive displacement pumps act at
one end of the loop while open reservoir conditions apply at the other. These
boundary conditions are not the ones encountered operationally in real wells -
they support systems of standing waves as opposed to radiating waves. Often a
valve or downstream choke near the reservoir is adjusted to control mean
pressure levels in order to prevent cavitation. This valve, while nearly closed, is
never completely closed because pumped fluid must be allowed to pass. This
opening, per the drillbit-pulser discussion above, permits signals to propagate
through it, since long waves will invariably never “see” the valve.