Stories from the Field,Fundamental Questions and Solutions - Measurement While Drilling Signal Analysis, Optimization and Design - page 41

Figure 1.5x. CNPC Headquarters, Beijing.
1.2.5.5 Example test results.
Here we highlight some interesting test results. The first pertains to signal
strength as a function of rotation rate with flow speed fixed. Early
Schlumberger papers claim that 'p's obtained at high frequencies are
independent of frequency, i.e., the siren behaves as an orifice. We believed
otherwise. As the rotor turns, it brings oncoming mud to a halt whatever the
frequency. However, the water hammer signal must weaken as rotation rate
increases because less time is available for fluid stoppage and rebound. Our test
setup is summarized in Figure 1.6a.1. The expected monotonic decrease of 'p
with increasing frequency is seen, for instance, in Figure 1.6a.2, where we
typically test up to 60 Hz as suggested by calculations in Figures 1.2b,c,d. The
low 'p's associated with existing “siren alone” approaches reinforced our
efforts to seek more innovative signal enhancement methods utilizing ideas from
constructive wave interference. Good signal strength alone does not imply a
usable siren - a workable design must be low in resistive torque to enable rapid
rotational speed changes. Figure 1.6a.3 provides a spreadsheet example of
torque data collected as functions of flow and rotation rate. Signal strength,
torque and erosion pattern (obtained by flow visualization) are catalogued for
each siren prototype.
Figure 1.6a.1. Simultaneous signal strength and torque measurements.