Stories from the Field,Fundamental Questions and Solutions - Measurement While Drilling Signal Analysis, Optimization and Design - page 44

Figure 1.6e. High-frequency (45 Hz) signal recovery.
Our experiences with constructive wave interference “at the drillbit” are
also worth noting. In U.S. Patent 5,583,827 or Chin (1996), where the use of
downhole constructive interference for signal enhancement was first suggested,
the published analytical model mistakenly assumed the bit as a solid reflector.
In fact, it is now known that MWD signals are detectable in the annulus, where
their absence is used as an indicator of gas influx. The six-segment waveguide
model presented in Chapter 2 is used to study typical MWD collars, e.g., see
Figure 1.2a. It is more general and does not assume any particular reflection
mechanism on an a priori basis. Detailed calculations show that, more often
than not, the drillbit acts as an open reflector - attesting to the dangers of
“common sense” and visual inspection. This model creates plots similar to
Figures 1.2b,c,d. The wave characteristics of siren and positive pulsers from
present MWD vendors are consistent with those in Figure 1.2b.
1.2.6 Conclusions.
We have summarized our strategy for high-data-rate mud pulse telemetry
and means for developing the technology. Our target objective of 10 bits/sec at
30,000 feet appears to be doable. The signal amplification approach used,
together with new surface signal processing techniques, plus the use specially
designed tools that are integrated with mud and drillpipe properties, provide a
systems oriented process that optimizes data transmission. Needless to say, we
have acquired much in our testing program, and we are continually learning
from our mistakes and developing new methods to improve the technology.
Prototype (metal) tools have been built, using one or more sirens, and are
presently being tested for mechanical integrity and telemetry performance; an
example is given in Figures 1.1a,b. The top photograph in Figure 1.5b.1 shows
the “long wind tunnel” described in this paper, however, it also operates with
mud or water using a mud pump and redesigned pulser section (not shown) that
is controllable from a test shed at the center of the loop. Real mud laboratory
and field tests are in progress and results will be presented at a later date.