Advanced System Summary and Modern MWD Developments - Measurement While Drilling Signal Analysis, Optimization and Design - page 339

The fast-charging, deep discharge and high-cycling capability of the Li-ion
electrochemistry will enable the MWD tool to remain in continuous downhole
operation. The C-size cell functions as an energy buffer and is commonly used
in oil & gas drilling applications, while the higher power D-size cell was
designed for oil exploration operations. The Li-ion cells are integrated into
customized, rugged, cylindrical MWD battery staves. A key part of the stave
design is to provide complete mechanical integrity, even under extreme
temperatures, vibration and pressure. This involves careful selection of the
construction materials and specialized manufacturing techniques, such as the
cross-ply, tape-wrapping process. The staves also incorporate electronic
controls, such as diodes to protect primary cells or balancing circuits to manage
rechargeable cells.”
10.4.4 Tool requirements.
We have addressed the issues encountered in designing high-data-rate
MWD systems with 10 bps capability or more. Several actions are required. P 0
must be increased by constructive interference using “smart FSK” telemetry
without complete rotor stoppage or multiple sirens, or both, additionally
employing “turbine on top of siren” designs, and optimizing sirens for high 'p
and low torque. P xdcr should be minimized by using sensitive piezoelectric
transducers and advanced multiple transducer noise removal methods. Low
rotor torques (which allow rapid changes in frequency) with low erosion should
be designed in the wind tunnel which are also consistent with high signals.
Larger diameter drillpipe should be used if possible, and optimum low-
attenuation muds should be employed in field operations with proper values of
U, P and c selected with the model in Figure 10.7. A well-designed, integrated
system should embody all of the design principles in this topic, which we
emphasize are based on rigorous acoustics and fluid-dynamics principles.
Signal processing efforts deserve special mention. While excellent
conventional treatises exist on digital signal processing methods, e.g., the now
classic topics by Oppenheim and Schafer (1975, 1989), the MWD environment
poses extremely difficult challenges. The telemetry channel is essentially one-
dimensional and supports strong plane wave noise emanating from both
downhole and uphole ends, i.e., mud motor “thumping” and mudpump
“banging.” In between, shape-distorting reflections are found at desurgers while
reverberations are induced at collar-pipe impedance mismatches. These effects
are not addressed by existing methods and provide a fertile area for continuing
work for all researchers. Because “high data rates” for mud pulse telemetry are
still slow by modern standards, e.g., cell phone or Internet connections, real-time
processing for relatively complicated algorithms, e.g., not unlike those disclosed
in this topic, is possible, since time steps for digitization finer than 0.001 sec are
likely not required.