Geology Reference
In-Depth Information
Because flowloops are finite as opposed to infinite despite their mile-long
length, pulsers positioned within almost all flowloops that the author is familiar
with will create systems of standing waves. These waves - and not propagating
waves that attenuate with distance - are the ones measured experimentally and
whose data must be reinterpreted in the context of wave motions encountered in
the well. As pulser frequency changes within the flowloop, the nodes and anti-
nodes of the standing wave move. A pressure transducer located at a fixed
position will measure amplitude changes related both to attenuation and node
movement that are difficult to distinguish if a single transducer is used - or if
multiple transducer data is not properly interpreted. In several publications,
measured pressure changes that occurred with frequency increases have been
incorrectly identified with attenuation - a term that, in the scientific literature, is
usually reserved for irreversible thermodynamic losses. In fact, for a given 'p,
the measured pressure at a given location versus frequency increase does not
always decrease - it may increase, or otherwise vary non-monotonically with
periodic character, but importantly, always in a manner dependent on the sound
speed of the fluid, the length of the flow loop and its end boundary conditions.
We emphasize that the measurement of 'p across a positive pulser or mud
siren, say using differential pressure transducers, is correct and perfectly
legitimate whether the flowloop is finite or infinite because the continuous
background wave field (containing even the most complicated reflections) is
subtracted out at both sides. However, the measurement of pressure itself needs
interpretation because it is subject to the vagaries of reflection - care must be
taken to determine if it is in fact a propagating wave result, a standing wave
anomaly, or a wave that has been contaminated by reflections.
Many field reports obtained at different service companies have provided
confusing and contradictory reports related to high-data-rate signal generation.
Invariably, workers failed to distinguish between positive versus negative pulser
sources and failed to note the type of drillbit used or the length of the hole. As
we have discussed, downhole reflection patterns depend on source type, the
manner of reflection at the drillbit, not to mention mud sound speed, pulser
frequency and the details of the bottomhole assembly. The parameters discussed
thus far explain the sources of confusion encountered by workers in MWD
telemetry. Measured standpipe signals have often varied significantly from one
rig to another without explanation. Sometimes, extremely weak signals are
found with tools that are functioning perfectly mechanically. However, in all
cases, differences can be reconciled when bottomhole assembly geometries, the
lengths of the boreholes, and other acoustic parameters, are considered within
the framework of a comprehensive and rigorous mathematical model.
Again, flowloop testing can be dangerous when pressures (not to be
confused with 'p's) are to be measured, and particularly so, when the pulser is
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