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NOTICE CONCERNING COPYRIGHT RESTRICTIONS
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Developments at Schlumberger Cambridge Research
G. Cooper, Sedco Forex Schlumberger, Montrouge, France
SUMMARY
Research in Drilling Mechanics at Schlumberger
Cambridge Research is aimed at improving the
efficiency and safety of drilling in two ways. First, by
understanding the steady-state response of the drill bit
to weight on bit and rotary speed for any particular
combination of bit, rock and hydraulics, we hope to be
able to optimise the normal operating parameters and
also to obtain information about the rocks being
penetrated. Second, and, we believe, of equal value, we
have been studying the means of identifying a variety
of "Drilling Events". These are non-steady and
generally unwelcome occurrences such as bit bearing
failure, bit balling, influxes of various types etc. If
detected at an early stage, such events are relatively
straightforward to deal with, but if left undetected, they
can become costly to correct and very dangerous.
The main method of attack has been to study
torque and rate of penetration as a function of weight
on bit (WOB) and rotary speed, not only as steady-state
values, averaged over a period ranging from seconds to
hours, but in terms of their characteristic vibrations.
The latter have been shown to contain quantitative
information about the state of wear of the bit teeth and
bearings, and have been related to changes in rock
indentation resistance.
STEADY STATE MEASUREMENTS
Optimisation of operating parameters
The measurement and optimisation of the
steady-state drilling parameters for any combination of
bit, rock and operating conditions is of course not new.
It is, however, important for the driller in the field to
know what bit to select in any given conditions, and
although much information is available from the bit
manufacturers, it is not always complete. It is, for
example, difficult to obtain comparisons between bits
coming from different manufacturers, or to obtain
information about the performance of a bit in any wear
state other than new. In the latter context, for example,
it is known that the rate of penetration of a new
milled-tooth bit is superior to that of the nominally
equivalent tungsten carbide insert bit in spite of the fact
that the latter may cost perhaps double. It is also
known, however, that in most rocks, the wear of the
teeth of the insert bit will be negligible, and its
performance will be maintained, whereas the milled
tooth bit will wear progressively, and its rate of
penetration will fall.
gil.3
The question is thus posed'"After how long will
the overall performance of the insert bit overtake that
of the milled-tooth bit, and when will it become
economically advantageous ?" Clearly, the answer
depends upon many factors, including those related to
the rig type and operating environment. At SCR, we
have been using our full-scale drilling teit machine
(Cooper and Peltier, 1986) to measure the performance
of bits in different wear states in order to provide the
basic drilling information from which, when the
external factors are included, a rational choice can be
made.
Detection of Bit tooth wear.
Experiments on bits in different wear states also
serve another function, to support work on the
development of diagnostic methods for determining
the state of wear of the drill bit from the drilling
response. Analysis of the ratio of torque divided by
weight-on-bit to rate of penetration per bit rotation is
the basis of the Mechanical Efficiency Log (Burgess and
Lesso, 1985). This is a diagnostic technique which is
capable of determining tooth wear in milled tooth bits.
The method works well for shales, but is less accurate
when the lithology is changing rapidly, essentially
because there is difficulty in distinguishing increases in
rock hardness from additional wear of the teeth. Work
is under way at SCR to separate the two effects.
An alternative method for determining bit wear is
by the analysis of drilling vibrations. If a spectrum is
obtained of the vibrations occurring above the bit,
many peaks are seen, whether in torsion or vertical
movement force or acceleration. Many of these peaks
can be associated with the impact of successive teeth on
particular rows and on specific cones of the bit. Since
the number of teeth on any row is known a priori, the
measurement of the frequency of the relevant peak(s)
gives a measurement of the cone rotation speed. By
comparing this value with the bit rotary speed, one can
then infer the effective rolling radius of the cone. This,
of course, decreases progressively as the cone teeth
wear, and so a diagnostic for thi wear of the teeth may
be obtained. This method at present appears to give a
value which is independent of lithology, although in
some cases considerable signal processing is required to
obtain a reliable result. Figure 1 shows the result of
applying the technique in a favourable case to a milled
tooth bit for soft rock ( IADC code 1:3:6 ) (Cooper et
al., 1987). Here, the upward,shift in frequency of the
main peak can be clearly seen as the wear state changes
from T : l to T : 5.
1
Another method which we have attempted to use
to derive a diagnostic for tooth wear is the Drill-off Test.
(Lubinski, 1958). In this test, the • driller applies
weight-on-bit to some predetermined value, then sets
the brake. As the bit drills ahead, WOB decays at a rate
determined by the rate of penetration and the drill
string compliance. Meanwhile, the surface WOB is
recorded, and if ROP is proportional to WOB, it is
expected that WOB will decay to zero exponentially
with time (Bourdon et al.,1987). This is indeed the case
in many circumstances, but sometimes the weight does
not decay to zero (see Fig. 2 ). Under such conditions, it
was believed that the offset might be an indicator of
wear of the bit, the argument being that a certain
minimum weight would be necessary to cause the bit to
drill, and that this threshold would increase with the
size of the wear flat. To test this idea, careful
comparisons have been made with the same new and
worn bits tested in· the field and in the laboratory in the
drilling test machine. The latter tests have shown that
a threshold is never recorded in the laboratory, and so
we have been led to the conclusion that the observation
of the offset in field conditions is not an indicator of bit
wear, but is more likely to be associated with poor
weight transfer (eg stabiliser hanging or similar
problerns).
Detection of changes in lithology
While we are actively engaged in this area, there
is little which can be reported at present. Considerable
effort is being deployed to develop drilling models
which contain an explicit link between the observed
drilling parameters and the rock properties as they
would be measured in a conventional rock-mechanics
test. This is relatively straightforward for drag bits, but
roller-cone bits have a very complicated cutting action
which has to be accurately modelled.
At SCR, a combined theoretical and experimental
approach has been adopted. For the experiments, we
have built an "instrumented bit", which comprises the
three cones of a commercial roller cone bit which have
been removed from the bit body and reassembled on an
instrumented carrier. Two of the cones are attached to
shaft encoders, so that their angular positions may be
determined. The third cone has been cut so as to
separate the tooth rows, and each tooth row is carried
separately on strain-gauged supports. With this cone
also linked to a shaft encoder, we can obtain all the
information necessary to reconstitute the force-
indentation behaviour of each of the bit teeth as they
drill. Fig. 3 shows a record of the vertical forces on the
inner and outer tooth rows of one of the cones of an
IADC code 1:3:6 bit during one complete rotation of
the cone. The force peaks from each of the nineteen
teeth on the outer row and the nine teeth on the inner
row can be seen clearly. Knowing the angular relations
between the teeth, such data can then be used to
construct the force - indentation curves for the
individual teeth, and examples are shown in Fig. 4 for
four different rock types (Cooper et al. 1987). These
curves may now be compared with the equivalent data
obtained in single-point indentation experimer•ts, and
thence with the suite of more conventional rock
mechanics tests. The ultimate objective is, of course, to
infer the mechanical properties of the rock from its
drilling response.
EVENT DETECTION
As mentioned above, we believe that the timely
detection of abnormal behaviour during drilling is at
least as important as optimising the steady-state
operating parameters, and is probably more so
regarding questions of safety. Within the Drilling
Mechanics Group, we have been investigating various
ways in which some of these events may be detected.
One of the most annoying, and possibly the most
preventable events is the loss of one or more of the
cones of a roller cone bit following failure of the
bearings. If the cones are lost, then a fishing trip must
be undertaken, because drilling cannot, recommence
until the lost cones are recovered.
A technique which we have used to detect the
early stages of bit bearing failure (before the cones are
lost) is the analysis of torque and weight-on-bit in real
time using a moving window technique (Peltier et al.,
1987). The object is to detect the intermittent snatching
of the cone bearing which occurs during the early stages
of failure, before the cone locks or falls off. The method
consists of monitoring the ratio of torque to weight on
bit and looking for anomalies. in this ratio in a time
scale appropriate to the scale of the event which it is
hoped to detect (This is a few seconds in the case of
bearing failures). It should be realised that the ratio
may be affected by changes other than changes in
bearing condition - lithology, for example. In this case,
however, it is not expected that the rock quality will
change dramatically in a timescale of a few seconds, and
so the time constant of the diagnosis may be changed to
"tune out" such unwanted influences.
Fig. 5 shows an example of the application of the
method. It records a test in which an 8 1/2 " bit was run
at constant rotary speed, but with weight on bit varying
between approximately six and twenty tons. The bit had
been used in the field, but was believed to be still in
good condition. Before the test, holes were drilled in
each bit lug, and thermocouples were placed as close as
possible to the bearings to record bearing temperatures
during the test.
In the figure, the first two tracks show the changes
of WOB and torque during the test. Track three shows
one of the three thermocouple records, which was the
only one to show a large temperature rise during the
test (indicating failure in that one particular bearing
only). It will be seen that there were no problems in the
early part of the test, at low weight on bit, but that there
was a sharp rise in temperature after about 130 seconds,
2
:
and another at about 170 seconds. Clearly, these twOincreases in temperature indicate incipient bearingfailure at the higher loads. Track four shows the torquediagnosis signal derived from tracks one and two. Iteasily detects the two points of bearing failure, and withas much precision as the thermocouple. The advantageof the torque analysis, however, over the thermocouplemethod, is that with the former, it is not necessary tomodify the bit to obtain the measurement, as the inputvalues could be obtained by, for example, a MWD unitmounted above the bit.
This technique appears to be applicable to thedetection of a variety of events; so far we have been ableto use a very similar method in the detection of thebeginning of bit-balling, and other applications areunder investigation.
If the early stages of bearing failure have passedundetected, there is another diagnostic method whichwe believe may be of value, through the analysis of thetorque signal at a higher frequency (Cooper et al., 1987).Fig. 6 shows traces of torque and torsional accelerationtaken over about three seconds for a bit in goodcondition (right) and one with one of the cones locked(left). As might be expected, the bit with the locked coneshows a higher average level of torque (lower traces),but the signals are noisy, and not easy to distinguish.Compare, however, the records of torsional acceleration(above). Here it is immediately obvious that whereasthe bit which is in good condition gives a relativelyquiet signal, the bit with the locked cone shows a seriesof sharp one-sided spikes. These almost certainly comefrom the impact of the bit teeth of the locked coneagainst the roughnesses of the hole bottom. They are avery characteristic feature of the vibration signature of abit in this condition, and as such, form a very goodindicator of a locked cone.
Fig. 1. The upward shift in frequency of thecharacteristic tooth-generated vibration peak as anIADC code 1:3:6 bit wore from T: 1 (dotted) to T: 5(solid line).
Cooper, G. A., Lesage, M., Sheppard, M., and Wand, P.,1987. The interpretation of roller-cone drill bitvibrations for bit wear and rock type. Proceedings of1987 Rapid Excavation and Tunnelling Conference,New Orleans, LA., 14-18 June, 1987.
Lubinski, A., 1958. Proposal for future tests. ThePetroleum Engineer, B 50 - B 52, Jan 1858.
Peltier, B. P., Cooper, G. A., and Curry, D.A., 1987. Use oftorque analysis to determine roller-cone bit bearingfailure. SPE Paper 16698, presented at 62nd. SPEMeeting, Dallas, TX., 27-30 Sept., 1987.
0.5000E-01M/S-2
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REFERENCES
Bourdon, J-C., Cooper, G. A., Curry, D. A., and McCann,D. P., 1987, Comparison of field and laboratory 5simulated Drill-off tests. SPE Paper 16162, presented 5 150at the 1987 SPE/IADC Drilling Conference, New •Orleans, LA., 15-18 March, 1987. &!
m 100Burgess, T. M. and Lesso, W. G., 1985. Measuring the 5wear of milled tooth bits using MWD Torque and w
Weight-on-bit. SPE Paper 13475, presented at the1985 SPE/IADC Drilling Conference, New Orleans,LA., 6-8 March, 1985.
0 15 30 45 60 75 90Cooper, G.A., and Peltier, B., 1986. Advanced techniques 105 120 135 150 1for laboratory full-scale drilling tests. SPE Paper TIMEs14783, presented at the 1986 SPE/IADC Drilling Fig. 2. Record of a field drill-off test, in whichConference, Dallas TX., 10-12 Feb., 1986. weight-on-bit did not decay to zero.
3
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Temperature $*./tr'-1.C 100 -0.0000 Degrees 359.0 10AVERAGED VERTICAL FORCES Instrumented Bit Drilling Portland LS 500 4
Diagnosis -• 1Fig. 3. Vertical force on the outer (above) and inner N.mi(below) tooth rows as a function of rotation angle forone of the cones on an IADC code 1:3:6 bit. 1 1
0 25 50 75 100 125 150 175 200 225Time s
10.00 Fig. 5. Detection of the beginning of bit bearing failureas weight-onfbit was increased in five stages at constant
KN rotary speed. Tracks 1 and 2, weight-on-bit and torque.Track 3, record from a thermocouple in one of the bit
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0.0000 0 0008 ' 2.000COMPARISON BETWEEN NEW ANO USED J22 BITDRILLING MARBLE 2T W. 0. B. 31 RPMUSED BIT HAS ONE CONE STUCK
Fig. 6. Detection of locked cone from an analysis oftorsional vibrations. Note the sharp spikes in torsionalacceleration from the failed bit (top left). This ti,aceshows a much more striking difference from that •of thenew bit ( top right) than do the corresponding torquesignals (bottom left and right)
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