Geology Reference
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in nondestructive testing. X-ray and Catscan methods play similar roles in
medicine and engineering.
4.2.11.1 Drillstring imaging.
Why look ahead of the bit? One application for vibration modeling is the
detection of mechanical drillstring dysfunctions while drilling. In this chapter,
the detection of large downhole lateral vibrations using axial surface
measurements is discussed. Here we describe some diagnostic tools that have
been field-tested with optimistic results. Henneuse (1992) cites case histories
using Elf Aquitaine's “Drilling Dynamics Control Unit,” a vibration monitoring
sub located at the top of the drillstring dedicated to data acquisition and signal
analysis. Dynamic surface measurements were useful in detecting downhole
problems such as bit wear, bit bouncing, blocked cones, shock sub performance,
stick-slip, stabilizer hangup, backwhirl, and bit and stabilizer balling. Rappold
(1993a,b) noted that additional Elf objectives included optimization of the rock
destruction process, elimination of premature pullout of the bit, reduction in the
number of drillstring fatigue failures, tracking of the rock formations drilled
without time lag, and optimization of mud circulation, cleaning, and lubrication.
When stick-slip torsional oscillations are progressing, the bit stalls and
accelerates cyclically as the surface rotary speed remains constant (a full
discussion is provided later). During stick-slip drilling, penetration rates are low
and the bit wears faster. According to Rappold, stick-slip oscillations can cause
downhole rotational speeds up to ten times the specified surface speed; such
fluctuations last from 1 sec to 40 sec, depending on the BHA. Momentarily high
torques can develop, exceeding the elastic limit of drillpipe. This can cause
premature aging of the drillstring, and damage MWD equipment, downhole
motors and shock subs.
Dufeyte and Henneuse (1991), delving deeper into the problem, analyze
simultaneous downhole and surface dynamic measurements to characterize
stick-slip accurately. Surface measurements of static and dynamic axial and
torsional forces provide useful indicators of existing mechanical conditions far
downhole. Combined with real-time MWD data, e.g., root-mean-square “rms”
information lateral shock load and frequency, a more complete and accurate
interpretation of real-time environment is possible.
4.2.11.2 Seeing ahead of the bit: MWD-VSP and vibration logging.
The ability to detect potential downhole dysfunctions along the drillstring
before they become dangerous is an important objective of vibrations modeling.
Another objective that has intrigued industry professionals is “formation
imaging,” or put simply, “seeing ahead of the bit.” Different approaches are
possible and we discuss some of these.
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