Mud Acoustics in Modern Drilling - Wave Propagation in Drilling, Well Logging and Reservoir Applications - page 279

components. Position sensors measure depth, direction and inclination.
Lithology sensors measure properties associated with electrical resistivity,
gamma ray, neutron porosity, acoustic impedance and so on. The telemetry
package encodes sensor information into signals and waveforms that are
transmitted to the surface. This process includes downhole processing to
compress data, e.g., calculating representative root-mean-square vibrations data
so that complete data streams need not be transmitted, plus data compression
from the communications engineer' s point of view. Different MWD systems
will use different telemetry and signal processing schemes, but the basic
requirement is a transmission mode that “minimizes bit error” and “maximizes
data rate” in the presence of sub-optimal “signal-to-noise ratios.” The
“modulator” is the hardware that performs the actual transmissions, which are
decoded at the surface by complementary “surface receivers” and “signal
processors.” Finally, the “power system” may require no more than batteries
for shallow depth and low temperature applications, but more than likely,
“downhole turbines” are used which draw energy from the mud stream,
converting it into electrical and hydraulic power which drive the MWD
modulator and all sensor electronics.
5.5.2 Candidate transmission technologies -
with brief survey of early work.
So far, we have refrained from discussing specific implementations for
Measurement-While-Drilling. Over the years, advances in microelectronics,
mechanical and electrical packaging, hydraulics, and materials technology, have
continually improved such that the meaning of MWD has likewise evolved. In
the late 1960s, the pioneering downhole recorder developed for real-time
measurements at the Esso Production Research Company (now, ExxonMobil )
may be viewed as a forerunner to modern MWD systems (e.g., see Koch (1967),
and Deily, Dareing, Paff, Ortloff and Lynn (1967)). Denison (1976a,b)
described a high-data-rate system where modified drillpipe containing special
electrical circuitry allows instantaneous transmission from the holes while
drilling (the system, capable of high data rates, transmitted at least 600 data
samples/sec). Electric circuits from the surface to the bottom of the borehole
allowed continuous transmission of downhole data whether or not drilling was
in progress, and did not require the use of downhole power sources. Denison
(1979) later reported a significant increase in data rate to more than 36,000
samples per minute. In the 1980s, one oil service company offered both mud
pulse telemetry and recorded downhole data , and in research-oriented efforts,
developed high-data-rate wireline transmission methods for use while drilling
(e.g., see Wolf, Zacksenhouse and Arian (1985)).