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properties, to some extent. It also appeared that whether or not the drillbit was
off-bottom mattered. Very often, common sense dictated that the drillbit acted
as a solid reflector, since nozzle cross-sectional areas were “pretty small”
compared to pipe dimensions. Yet, this line of reasoning was contradictory and
had its flaws; strong MWD signals by then had been routinely detected in the
borehole annulus, where their existence or lack of was used to infer gas influx.
It became clear that what the human eye visually perceived as small may not be
small from a propagating wave's perspective.
Lack of controlled experiments also pervaded the industry and still does.
Whenever any service company design team was lucky enough to find a test
well, courtesy of obliging operating company customers, engineering “control”
usually meant installing the same pressure transducer in the same position on the
standpipe. New tools that were tested in one field situation would perform
completely differently in others: standpipe measurements had lives of their own,
it seemed, except at very low data rates of 1 bit/sec or less, barring mechanical
tool failure, which was often. Details related to surface plumbing, bottomhole
assembly, bit-box geometry, drilling motor details and annular dimensions, were
not recorded and were routinely ignored. The simple “piston at the end of pipe
model” didn't care - and neither did most engineers and design teams.
By the mid-1990s, the fact that higher data rate signaling just might depend
on wave propagation dawned upon industry practitioners. This revelation arose
in part from wave-equation-based seismics - new then, not quite understood, but
successful. I began to view my confusion as a source of inspiration. The
changing patterns of crests and troughs I had measured had to represent waves -
waves whose properties had to depend on mud sound speed and flow loop
geometry. At Halliburton, I would obtain patents teaching how to optimize
signals by taking advantage of wave propagation, e.g., signal strength increase
by downhole constructive wave interference (without incurring erosion and
power penalties), multiple transducer array signal processing to filter unwanted
signals based on direction and not frequency, and others.
Still, the future of mud pulse telemetry was uncertain, confronting an
unknowing fate - a technology held hostage by still more uncontrolled
experiments and their dangerous implications. At the time, industry experts had
concluded that mud pulse telemetry's technology limits had been attained and
that no increases in data rate would be forthcoming. At Louisiana State
University's ten-thousand-feet flow loop, researchers had carefully increased
MWD signal frequencies from 1 to 25 Hz, and measured, to their dismay,
continually decreasing pressures at a second faraway receiver location. At
approximately 25 Hz, the signal disappeared. Completely. That result was
confirmed by yours truly, at the same facility, using a slightly different pulser
system. Enough said - the story was over. Our MWD research efforts were
terminated in 1995 and I resigned from the company in 1999.
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