Geology Reference
In-Depth Information
Transmission efficiency plays a strong role in later calculations, i.e., for a
given BHA and mud sound speed, the dependence of p/'p on position in the
drill collar and on frequency is of interest. In practice, mud properties and BHA
are not within the control of the MWD operator. However, the complete range
of frequencies can be “swept,” with signals evaluated at the surface, to
determine optimal values for phase or frequency shift keying.
2.5 An Example: Optimizing Pulser Signal Strength
2.5.1 Problem definition and results.
We now consider a practical example and examine signal strength
optimization by constructive wave interference, which is relevant to carrier
wave enhancement for phase-shift-keying (PSK) schemes or frequency selection
for frequency-shift-keying (FSK) methods. The reader new to acoustic methods
should bear in mind the two examples from classical physics illustrated in
Figure 2.6. At the top, an incident P 0 signal doubles at the reflector, whereas
for the telescoping waveguide, an incident signal quadruples (the area ratio is
two and the length of the extension is a quarter wavelength). The first
amplification method was in fact successfully tested on the standpipe by the
author using one hundred feet of hydraulic hose and subsequented patented (see
Chapter 6). Our point is this: signal amplification is as natural as signal loss.
Figure 2.6 . Classical wave amplification solutions.
What happens with the bottomhole assembly in Figure 2.7? For our input
variables, we assumed water as the drilling fluid, and neoprene rubber as the
mud motor stator medium. In calculating the cross-sectional area available to
wave propagation for the mud motor, we subtracted out that corresponding to
the metal rotor, whose acoustic impedance greatly exceeds that of water or
rubber. Also, the presence of the MWD tool body within the MWD collar was
ignored for simplicity; its effect is simply to reduce the collar area. In our
calculations, the position of the MWD pulser x s was incremented every foot,
starting with x s = 1 ft immediately above the motor, to x s = 22 ft just below the
drillpipe; also, transmission frequencies ranging from 1 Hz to 50 Hz were taken
in single Hertz increments. The 50 u 22 matrix of runs required approximately
five seconds of computing time on a typical personal computer, making the
algorithm practical for MWD telemetry job planning, say, in applications where
ideal frequencies are to be identified. Representative numerical results are given
in Figure 2.8a below, while the surface plots in Figures 2.8b and 2.8c are based
on the entire set of 1,100 data points.
Search WWH ::




Custom Search