Drillstring Vibrations: Classic Ideas and Modern Approaches - Wave Propagation in Drilling, Well Logging and Reservoir Applications - page 170

MWD-VSP. A mature technology known as “vertical seismic profiling”
(or, “VSP”) has been used over several decades to provide seismic earth images
near a wellbore. Researchers have speculated about the application of signal
sources other than those traditional to the seismic industry. For example, Rector
and Marion (1989a,b) propose an MWD - VSP , where checkshot surveys use the
drillbit as a downhole source of seismic energy. In this approach, the vibrations
induced by a drillbit while drilling are used to produce VSP data. But unlike
MWD methods based on mud pulse telemetry, which are relatively expensive,
low in data rate and prone to mechanical failure, downhole instrumentation is
not required to acquire data. Also, the data acquisition process does not
interfere with drilling operations. The continuous signals generated by the
drillbit are monitored with a reference sensor attached to the top of the
drillstring. These are processed together with signals from receivers placed at or
near the earth' s surface to produce VSP data; in a marine environment, the same
sensors would be deployed on the ocean floor. This idea deserves further
development, as problematic issues remain. A quick scan through Geophysics ,
or Geophysical Prospecting , and other seismological journals, reveals many
papers addressing subtleties due to acoustic source characterization, modeling
and interpretation. Commonly used sources range from “impulsive” dynamite
blasts, to “2D” seismic lines towed by geophysical ships, to Vibroseis land
signatures generated by controlled thump trucks. The acoustic source signature
generated by the drillbit, here complicated by coupled mode drillstring
vibrations as well as changing drillstring length, is not likely to be any cleaner.
Vibration logging of the formation. The application of drillstring
vibrations to formation evaluation is as old as rotary drilling itself. Drillers will
reminisce with stories about “rough running” and “twist-offs,” which indicate
hard rock formations. On the other hand, high penetration rates with minimal
bit wobbling are suggestive of soft, unconsolidated sands. Drilling engineers
today monitor the bottomhole environment by studying downhole and surface
measured changes to weight-on-bit and torque-on-bit. Changes in such
measurements yield, in addition, information about dogleg severity, drillbit
wear, lost cones and borehole friction. Researchers dealing with detailed
downhole vibrations data have noted strong, positive correlation between
vibration logs and resistivity logs as the drill bit penetrates alternating sand and
shale sequences.
The use of vibration data for detailed logging is not new but appears to
have been neglected in recent years. A successful early effort dealing with
dynamical effects is found in the very interesting paper of Lutz, Raynaud,
Gstalder, Quichaud, Raynal and Muckleroy (1972) entitled “Instantaneous
Logging Based on a Dynamic Theory of Drilling.” With a simplified analysis
involving only longitudinal acceleration measurements taken at the top of the
string, the authors were able to produce usable logs which gave information on