Geology Reference
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demonstrates, following Chapter 1, that linear modeling sufficed. This early
paper importantly showed that the wave form of the pulse changes very little
with distance, that is, that the wave propagation was nondispersive; again, the
linear theory afforded by the classical wave equation appeared to be valid.
Finally, the author suggested drilling methods driven by surface vibrators, which
took advantage of controlled resonant interactions to increase penetration rate.
The design of resonant vibrators, incidentally, is given in the summary
paper of Eisner (1964). An important suite of vibration data, collected by
research workers at (then) Esso Production Research Company over a two year
span, was published three years later in the late 1960s (e.g., see Koch (1967),
and Deily, Dareing, Paff, Ortloff and Lynn (1967)). “Downhole Measurements
of Drill String Forces and Motions,” by Deily, Dareing, Paff, Ortloff and Lynn
(1968), provided the first detailed description of the real environment downhole,
pointing to the significance of dynamic behavior in drilling operations. The
paper described a self-contained, specially-designed downhole recording
instrument that was used to measure and capture drillstring forces, moments and
motions at the source. Eight signals were recorded by “pulse-width modulation”
on magnetic tape, namely, axial, torsional and bending loads, axial, angular and
lateral accelerations; and internal pipe and external annular pressure.
The hardened mechanical device was used over several years to collect
downhole data, under a wide range of field drilling conditions, for numerous
wells. A complete dataset would collect approximately ten minutes of
cumulative recordings, at which point the tool would be retrieved and returned
to the surface. There, the data was converted from magnetic tape format to one
suitable for oscilloscope display (selected portions of the dataset were digitized
for analysis purposes, and studied in the cited paper). The authors noted that
variations in measured downhole bit load ranged from 25 to 50 percent of the
mean value, indicating that weight-on-bit was by no means unchanging.
Maximum bit loads reached 3.5 times their mean values at times, accompanied
by large annular pressure variations. Frequencies of weight, torque and bending
traces showed evidences of rock bit tooth action, of cone action, of rotation and
pump pulsations. The paper also documented cases of “bit bounce,” provided
exact measured magnitudes for a suite of forces and accelerations, and
demonstrated the coupling between bit weight and torque. This dynamic
information, plus details on the mechanical construction of the downhole
recorder, would no doubt be of value to MWD designers still a decade away.
Cunningham (1968), studying the Esso data further, noted irregular bit
rotations, torque oscillations, periodic beating, bit bouncing, bit weight
fluctuations, mud pump pressure fluctuations and large losses of rotary
horsepower along the drill stem. The model of Bailey and Finnie (1960), which
assumed an undamped mass-spring surface system and an undamped
longitudinally vibrating rod, and Angona (1965), who experimentally
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