Mud Acoustics in Modern Drilling - Wave Propagation in Drilling, Well Logging and Reservoir Applications - page 278

Other research issues. Once the reader familiarizes himself with the
wave equation algorithm presented in this topic, the details highlighted in
various of its applications can be implemented straightforwardly. A swab-surge
program constructed in this manner might serve as a test platform for numerical
experimentation. Useful computer codes can be devised to solve important field
problems, e.g., swab-surge effects induced by bit-bounce, or the effect of more
realistic attenuation models on dynamic pressure fields. We now address
another important application of borehole mud acoustics, namely, that
encountered in MWD mud pulse telemetry. The exposition below provides
elementary introductions only. For a complete theory, developed with greater
detail and rigor, the reader should refer to the author' s recent topic Measurement
While Drilling Signal Analysis, Optimization and Design (Chin et al , 2014).
5.5 MWD Mud Pulse Telemetry
What is “Measurement-While-Drilling” or “MWD”? In conventional
rotary drilling, rig operations are periodically interrupted in order to make
directional and wireline logging measurements. This can mean removing mile-
long, million-pound drillstrings from the hole, section-by-section, an extremely
tedious, time-consuming process. By contrast, MWD provides surface
personnel with real-time information while drilling about instantaneous drill bit
position and lithological properties of newly penetrated formation. Real-time
directional information is useful because drillers can react more rapidly to
unanticipated changes in planned wellbore trajectory; two-way communications,
for example, allows steerable bottomhole assemblies. MWD also provides
useful safety information related to overpressured zones, formation and
hydrocarbon properties, lost circulation, gas influx and blowout prevention.
Formation information obtained from MWD is more reliable than traditional
wireline data. The formation is interrogated by downhole sensors before the
penetrated rock is altered or damaged due to drilling fluid invasion, mechanical
damage during tripping or wellbore stress relief. MWD is rapidly gaining in
popularity for several reasons. First, modern wells, which tend to be highly
deviated or horizontal, cannot be logged by wireline. Second, the fact that
modern wells tend to be thousands of feet and even miles long makes real-time
MWD logging attractive, since the costs and down times associated with making
and unmaking million-pound drillstrings are prohibitive.
5.5.1 Basic MWD system components.
Conceptually, a modern MWD tool can be viewed as a stand-alone
downhole information system consisting of sensor, telemetry, and power
packages that reside within the drill-collar, uncomplicated by wired connections
to the surface, situated near the drillbit. We briefly summarize the basic system