Mud Acoustics in Modern Drilling - Wave Propagation in Drilling, Well Logging and Reservoir Applications - page 288

Power requirements. Depending on rate-of-penetration, kinds of data
required, and data rate, power requirements can be substantial. Thus, batteries
do not provide the best alternative for power usage, and downhole turbines are
desirable and more flexible. But regulating power production, in an engineering
sense, is challenging. Generated power varies as the cube of flow rate, and
slight increases or decreases in flow rate relative to design conditions can be
detrimental to the power regulation. Torques and axial loads, for example, vary
as the square of flow rate; thus, good feedback and control systems are needed
to regulate pulser movement in high-data-rate applications
High pressure and temperature. MWD tools must be built to withstand
hundreds of degrees Fahrenheit and tens of thousands of psi. This harsh
environment is detrimental to sensor survival, electronics, batteries, seals and
materials. Under severe conditions, heightened concern with respect to
corrosion exists.
Fluid mechanics problems. Fluid mechanics can be extremely
unforgiving to pulser design. Obvious problems arise from increased signal
strength requirements: sirens and poppet valves, for example, must operate at
tighter gaps, thus creating increased local flow speeds and consequent erosion.
Unusual problems exist as well. Consider, say, the mud siren. Thus far, we
have not stated which of the two siren plates shown in Figure 5.6 is the rotor or
stator. We have only noted that rotation of the rotor relative to the stator in
circulating mud flow produces a cyclic signal that travels up the mud column to
be detected at the drill site surface, and that information is encoded by
selectively varying rotor rotation. Actually, mud sirens can act in irrational
ways that nature condones: the drilling mud naturally tends to “blow” the rotor
to a stable closed position which in turn blocks the mud flow. This jams the
MWD tool, as well as the drillstring; high pressure builds up at the mud pumps,
and dangerous field situations are created at interrupt drilling and logging
operations. This “stable closed” behavior is well known to MWD designers, and
over the years, proposed fixes have be purely “brute force” and mechanical.
Chin and Trevino (1988) uncovered the basic principles for “stable open” mud
siren design, noting that rotor placement in the downstream position, plus
special aerodynamically tapered cuts will cause the rotor to bias in the open
position. At any rate, we wish only to emphasize the important role of fluid
mechanics in downhole hardware design. The magnitudes of hydraulic torques,
for example, affect power requirements, electric versus hydraulic motor
selection, and the quality of the signal spectrum in generated pressure. Similar
anomalous behavior is found for positive pressure poppet valves and negative
pressure pulsers. Finally, there are the issues of low manufacturing cost, ease of
maintenance and repair, long “mean time between failure” or servicing, and so
on, that can be crucial to the commercial viability of any MWD system.