Geology Reference
In-Depth Information
10.1 Overall Telemetry Summary
For convenience, we discuss the foregoing points separately to emphasize
key ideas in depth. Doing so allows us to develop supporting arguments and
suggestions more clearly.
10.1.1 Optimal pulser placement for wave interference.
Conventional misunderstandings are common to hardware design. Perhaps
the most misunderstood is the requirement for downhole turbines to remain at
the bottom of the MWD collar, for fear that uphole turbines would block the
upgoing signal. But this is impossible because rotor-stator configurations are
always open and will pass both flow and signals. Thus, there are no acoustic
restrictions on turbine location. This conclusion is important for one crucial
reason: by placing say, a siren or positive pulser closer to the bottom and by
applying the correct phasing, the use of constructive wave interference for signal
enhancement is rendered faster and more practical.
The time required for downgoing waves to reflect upward and add to later
upgoing waves is significantly decreased and fewer wave cycles are required to
establish stronger signals that can be clearly seen. This implies higher data
rates. At the same time, the use of constructive interference for signal
enhancement means reduced erosion penalties at the pulser and decreased power
demands on the turbine or any batteries - all of which imply increased life span
and reliability. The increased signal strengths mean that attenuation, while
problematic at higher frequencies, will be less of a problem, thus permitting
greater travel than is otherwise possible. Of course, sometimes there are other
considerations, for instance, connections to rotary steerable systems may require
top-mounted sirens, that preclude this arrangement.
But how close to the drillbit can we position our siren? Certainly, from a
wave or signal strength perspective alone, the closer the better. This would,
however, defeat the purpose of Measurement-While-Drilling, which attempts to
use near-bit logging information quickly and effectively for geosteering. To
address this concern, and to determine practical numbers, we allow placement of
a resistivity-at-bit (or, “RAB”) tool between the mud siren collar and the drillbit
(the use of any other tool, e.g., a turbodrill, is acceptable for our analysis). From
the vendor advertisement for a RAB tool in Figure 10.1, a typical near-bit tool
length might be on the order of 10 ft. For our analysis, we assume that the
MWD collar is 20 ft long, which is consistent with several commercial designs.
We now apply the six-seqment acoustic waveguide model in Chapter 2,
which we emphasize includes the correct outgoing wave radiation conditions for
both drillpipe and borehole annulus, and hence, is representative of real drilling
scenarios. Different perspectives of our computed color results are shown in
Figures 10.2a,b,c. The parameters in Figure 10.3 are assumed for the
calculations (RAB geometry appears in “mud motor drill collar” input text
boxes).
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