Signal Processor Design and Additional Noise Models - Measurement While Drilling Signal Analysis, Optimization and Design - page 154

6.1 Desurger Distortion

The desurger, for example, as shown in Figure 6.1a, used at the rigsite
during drilling operations, suppresses strong fluid transients arising from
mudpump actions and is important to rig safety. Its role in MWD signal
processing, however, has been a source of confusion. Some engineers will
claim that it severely distorts the MWD signal, so much that signal detection is
sometimes impossible; others, however, never find problems with desurgers and
in fact embrace them. This chapter explains the discrepancies. Analytical
solutions for the desurger-distorted signal are derived which assist in developing
signal processing algorithms.

Figure 6.1a . Desurger.

The Lagrangian fluid displacement u(x,t) in a one-dimensional acoustic
field satisfies the classical wave equation w 2 u/wt 2 - c 2 w 2 u/wx 2 = 0 where c is the
speed of sound. When a signal (wave) travels uphole and reflects at the
mudpump piston (assuming that there is no desurger), the pressure doubles at
the piston face, then turns around and travels downward. The sign of the
pressure does not change at a solid reflector - this is evident from measurements
in very long wind tunnels, but this can also be proven mathematically from a
more complicated three-dimensional solution. Again, at the solid reflector, the
signal reflects with the same sign, but importantly, the shape of the signal does
not change and remains the same. The author has, in fact, instrumented pressure
transducers near to the mudpump to demonstrate improved signal recovery.
When a desurger is present, however, events can be unpredictable. A
desurger is typically used in the drilling setup at the rigsite to absorb dangerous
pressure transients from the mudpump. It is essentially a thick rubber
membrane within a metal housing that is “charged” with gas that is responsible
for energy absorption. However, the desurger can also affect the MWD signal.
When communicating with MWD engineers, one finds that different engineers
from different companies offer very different experiences that may be
contradictory and confusing.