Geology Reference
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it is difficult to ascertain true system performance. However, we can estimate
the conditions under which these claims are realistic. If we assume that its
rotary shear valve possesses characteristics similar to that of the mud siren, not
an unreasonable assumption, a simple reduction of the viscosity from 50 cp to
20 cp would allow transmission depths to 35,000 ft, as the results for Run C
show - the calculated value of 17.6 Hz would easily support 3 bps but no more.
In Run D, we increase our viscosity to 50 cp but limit travel distance to 3,600 ft.
The critical frequency increases to 665 Hz, demonstrating at 40 bps is not
unexpected. Cumulative results are given in Figure 10.8.
SG CP C DIA L P0 P Fcrit
A 1.7 50 3000 4 25000 145 0.4 13.8
B 1.7 50 3000 4 35000 145 0.4 7.0
C 1.7 20 3000 4 35000 145 0.4 17.6
D 1.7 50 3000 4 3600 145 0.4 664.9
Figure 10.8. Cumulative results, critical carrier frequencies.
Finally, we ask, “What data rates are possible using all the technology
elements developed in this topic for mud pulse telemetry?” Again, we
emphasize that maximizing the critical carrier frequency requires us to optimize
the ratio P 0 /P xdcr following the strategies indicated earlier. Some experimental
evidence suggests that delta-p source strengths are independent of siren
frequency at higher frequencies exceeding 10 Hz. Now let us assume the same
baseline numbers used in Run A for Schlumberger's mud siren. For Run E,
instead of PSK, we operate FSK with constructive interference with the siren
optimally positioned and assume a 1.7 factor increase in signal output as might
be suggested by Figure 10.2. Thus, the P 0 = 145 psi used previously is replaced
by P 0 = 1.7 u 145 or 246.5 psi, increasing the critical frequency to 16.4 Hz - not
enough to increase data rates substantially.
In Run F, we additionally apply the “sirens in series” design suggested in
Figure 10.4, which would double the 246.5 psi to 493 psi. This only increases
the critical frequency to 20.1 Hz. However, if we increase the drillpipe diameter
to 5 in as in Run G, f crit increases to a remarkable 31.4 Hz. In Run H, we
decrease the mud viscosity to 20 cp, showing in increase in critical frequency to
78.5 Hz for the assumed 25,000 ft transmission - or, per a prior analysis, at least
10 bps. In Run I, depth is increased to 35,000 ft, and our 78.5 Hz decreases to
40.1 Hz, which should allow 6-7 bps.
The software model of Figure 10.7 can be used to select drilling muds that
facilitate high-data-rate transmissions too. In Run J, let us formulate a mud with
a specific gravity of 2, a plastic viscosity of 40 and a sound speed of 4,000 ft/s.
Then, employing a single siren, but with the use of FSK and constructive
interference, we have a high value of 34.1 Hz at 35,000 ft. In our final Run K,
we reduce P xdcr to 0.2 psi and demonstrate that the critical frequency increases to
41.9, for efficient 6-7 bps operation (piezoelectric transducers with such
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