U.S. patent number 3,703,096 [Application Number 05/101,739] was granted by the patent office on 1972-11-21 for method of determining downhole occurrences in well drilling using rotary torque oscillation measurements.
This patent grant is currently assigned to Chevron Research Company. Invention is credited to Hugh G. McDonald, Albert L. Vitter, Jr..
United States Patent |
3,703,096 |
Vitter, Jr. , et
al. |
November 21, 1972 |
METHOD OF DETERMINING DOWNHOLE OCCURRENCES IN WELL DRILLING USING
ROTARY TORQUE OSCILLATION MEASUREMENTS
Abstract
Method and apparatus are used to detect and measure oscillations
of the rotary torque generated while well drilling, particularly
oscillations of a frequency near the expected frequency of
torsional oscillation of the drill string. The method and apparatus
are particularly useful in indicating bit damage.
Inventors: |
Vitter, Jr.; Albert L. (New
Orleans, LA), McDonald; Hugh G. (Monroe, LA) |
Assignee: |
Chevron Research Company (San
Francisco, CA)
|
Family
ID: |
22286153 |
Appl.
No.: |
05/101,739 |
Filed: |
December 28, 1970 |
Current U.S.
Class: |
73/152.47;
73/152.58; 175/39 |
Current CPC
Class: |
E21B
44/00 (20130101); E21B 47/00 (20130101); E21B
12/02 (20130101) |
Current International
Class: |
E21B
47/00 (20060101); E21B 44/00 (20060101); E21B
12/02 (20060101); E21B 12/00 (20060101); E21b
047/00 () |
Field of
Search: |
;73/151,137,136R
;175/39 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Myracle; Jerry W.
Claims
We claim:
1. The method of determining bit damage in rotary drilling
comprising rotating a drill string having a bit attached thereto in
a hole and monitoring the rotary torque for characteristic
oscillations, said characteristic oscillations having a frequency
not more than twice and not less than half the expected frequency
of torsional oscillation of the drill string, and said
characteristic oscillations having an amplitude substantially in
excess of the oscillations produced while rotating an undamaged
bit.
2. The method of claim 1 further characterized by activating an
alarm signal in response to the occurrence of said characteristic
oscillations.
3. The method of determining bit damage in rotary drilling
comprising driving an electric motor to rotate a drill string
having a bit attached thereto in a hole, producing a signal
proportional to the current to the electric motor as indicative of
rotary torque, passing said signal through a filter to suppress
undesired frequencies more than twice and less than half the
expected frequency of torsional oscillation of the drill string to
produce an information signal, recording the said information
signal passed through said filter and monitoring said information
signal for oscillations characteristic of bit damage.
4. The method of claim 3 further characterized in that the expected
period of torsional oscillation of the drill string is between 3
and 12 seconds.
5. The method of claim 3 further characterized in that the
information signal is recorded on a strip chart.
6. The method of claim 5 further characterized in that the
information signal is recorded and monitored only at predetermined
sample time intervals.
7. Apparatus for use in well drilling comprising a drill string,
drive means for rotating said drill string, torque measuring means
connected to said drive means for producing a signal proportional
to the torque produced by said drive means, filter means connected
to said torque measuring means, said filter means selected to
suppress relatively high frequency signals having a period near to
the rotary table period and to suppress relatively low frequency
signals of less than one twenty-fourth cycle per second and
recording means connected to said torque measuring means through
said filter means for recording the signal passed by said filter
means.
8. Apparatus for use in well drilling comprising a drill string,
electric drive means for rotating said drill string, shunt means
connected to said electric drive means for producing s signal
proportional to the current utilized by said electric drive means,
band pass filter means connected to said shunt means, said band
pass filter selected to suppress relatively high frequency signals
having a period near to the rotary table period and to suppress
relatively low frequency signals of less than one twenty-fourth
cycle per second, and recording means connected to said shunt means
through said band pass filter for recording the signal passed by
said band pass filter.
9. The apparatus of claim 8 further characterized in that said
recording means includes strip chart means and means for
periodically actuating said strip chart means for a limited sample
period.
10. The apparatus of claim 8 further characterized in alarm means
responsive to actuation by oscillations having a period near the
expected period of the drill string and having an amplitude
substantially in excess of the amplitude of oscillations produced
while rotating an undamaged bit.
Description
FIELD OF THE INVENTION
The present invention is directed to rotary well drilling and, more
specifically, the present invention is directed to obtaining, at
the surface, indications of drill bit damage utilizing rotary
torque oscillation measurements.
BACKGROUND OF THE INVENTION
Wells are drilled for oil utilizing a drill bit connected to the
surface by a string of drill pipe. The drill pipe is rotated to
cause the bit to rotate and cuttings are circulated up the well
annulus by means of a circulating fluid. As the drill string is
rotated, the drill bit tends to wear down and eventually fail. In
deep hole drilling e.g., 10,000 feet) balance must be made between
operating the drill bit until a failure occurs which necessitates a
fishing job, and pulling the bit prematurely from the well, i.e.,
while there is still useful life in the bit, because in deep hole
drilling, half a day, or more, may be required to pull the bit from
the hole, place a new bit on the drill string, and run it back into
the hole. It is, therefore, desirable to get maximum use out of
each drill bit. Heretofore no satisfactory method has been found to
determine when the drill bit is damaged, and should be pulled.
One form of rotary drilling utilizes roller cones which are
connected to the bit body and which are bearing mounted so as to
roll on the drilled rock face when the drill bit is rotated.
Usually a roller cone bit has three or four roller cones. As noted
above, it is desirable to operate these bits to a point just short
of complete failure, i.e., it is generally desirable to operate the
bit just short of the point where the roller cones become detached
from the bit body. In operation, the bearings of the roller cones
tend to freeze after a period of use. Even then, the bits with the
frozen roller cones may still be operated for a short additional
period. However, if the frozen bits are operated for too long a
time, the bushings are likely to be destroyed and the cones will
fall free of the bit body when the drill pipe is pulled. If this
occurs, a "fishing job" must be performed to remove the roller
cones from the hole before further drilling may proceed. The
fishing job requires a complete roundtrip with a drill string
having special fishing apparatus connected to its lower end. This
operation is time-consuming and expensive and is to be avoided, if
at all possible.
Several types of methods have been tried in the past to determine
when it is desirable to pull a bit in order to place a new bit on
the drill string. One of the most promising methods involved
embedding radioactive tracers in the grease in the roller cones of
a bit. The tracers are released into the drilling fluid stream when
the roller cones fail. The tracers are then detected by monitoring
apparatus at the surface. This method, however, is somewhat
inadequate in that the travel time of the mud up the annulus may be
as much as an hour and, therefore, much additional damage can be
done to the bit before anything is detected at the surface. The
nearest approach to the method of the present invention has perhaps
been the observation of drill pipe torque changes at the surface,
but these were changes in the average value of the torque as
observed on a meter in front of the driller or on a drilling
recorder. (See e.g., the advertisement of Technical Oil Tool
Corporation, p. 4,571 of the Composite Catalog of Oil Field
Equipment and Services. 1970-71 Edition. Published by World Oil,
Houston, Tex. Also see p. 3,237 of same publication for an
advertisement ad of Martin-Decker Corp. on the same subject.)
Changes in the average torque can result from many causes, the
commonest being a change in the formation being drilled, and so
they are not a reliable indication of bit damage. There is still
need, therefore, for a reliable method of detecting bit damage.
BRIEF DESCRIPTION OF THE INVENTION
The present invention provides a method of detecting bit damage
during rotary drilling by utilizing, as an indication of such
damage, very particular oscillatory changes in rotary torque
produced while drilling. In accordance with the invention, the
rotary torque produced during rotary drilling is monitored for
characteristic oscillations of a frequency in the neighborhood of
the expected frequency of torsional oscillation of the drill
string. These oscillations usually have a frequency significantly
different from the fundamental frequency of rotation of the rotary
drill string and the harmonics of that frequency, so that they may
be unambiguously detected. The expected period of torsional
oscillation of the drill string in most drilling operations of
interest is of the order of 3 to 12 seconds. In accordance with the
present invention the method of determining bit damage in rotary
drilling includes rotating a drill string having a bit attached
thereto in a hole and monitoring the rotary torque for
characteristic oscillations, said oscillations having a frequency
not more than twice and not less than half the expected frequency
of torsional oscillation of the drill string and said oscillations
having an amplitude substantially in excess of the amplitude of any
oscillations produced by rotation of an undamaged bit.
The preferred apparatus of the present invention includes a drill
string and electric drive means for rotating the drill string. A
shunt is connected to the electric drive means for producing a
signal proportional to the current utilized by the electric drive
means during rotation of the drill string. A filter means is
connected to the shunt. The filter is selected to suppress
relatively high frequency signals having a period near to the
rotary table period and to suppress relatively low frequency
signals of less than one twenty-fourth cycles per second. A
recording means is connected to the shut through the filter for
recording the signal passed by the filter.
In a preferred embodiment of the present invention rotary torque is
measured by determining the current required to operate the
electric motor which rotates the drill string. A shunt is provided
between the generator and the rotary drive electric motor from
which a continuous measurement of current is made. The signal
obtained is filtered through an appropriate band pass filter
selected to pass frequencies depending, as detailed later, on the
dimensions and masses of the various portions of the drill string.
The signal is recorded on a suitable strip chart recorder. It has
been found that significant oscillations in the passed signal are
indicative of bit damage. These oscillations may be observed on the
strip chart.
OBJECTS OF THE INVENTION
A particular object of the present invention is to provide a
driller with a method of determining bit damage utilizing
oscillatory changes in rotary torque which characteristically occur
as a result of such damage. Further objects and advantages of the
present invention will become apparent from the following detailed
description read in view of the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic view and illustrates a preferred embodiment
of apparatus assembled in accordance with the present
invention.
FIG. 2 is a facsimile of a portion of a strip chart recording
revised for clarity and illustrates oscillations in rotary torque
recorded against time and is typical of records produced in
accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Refer now to FIG. 1 where a schematic illustration of the preferred
embodiment of apparatus assembled in accordance with the present
invention is shown. A well 10 is being drilled by means of rotating
bit 12 with drill string 14. Bit 12 has rotary cones 16 which
rotate against the drilled rock face 18 as the bit is rotated. The
rotary cones 16 have teeth which engage the rock face 18 and break
it into chips. These chips are carried to the surface by means of
drilling mud which comes down through drill string 14 and
circulates back up annulus 11 and out the overflow pipe 13. The
drill pipe is rotated at the surface by means of rotary table 20.
The topmost portion of the pipe, the so-called kelly, is square in
external cross section so that it may be gripped for rotation, but
may still move freely through the rotary table vertically. The
rotary table 20 is rotated by electric motor 24 through appropriate
linkage 26. Power for the electric motor is supplied through
suitable circuitry by generator 28.
In accordance with the present invention it has been found that as
bearings of the drill bit cones 16 begin to fail, characteristic
oscillations are produced in the rotary torque of drill string 14.
These oscillations have a period longer than the fundamental period
of rotation of the rotary table 20 and approximately equal to the
expected period of torsional oscillation of the drill string. In
order to more fully understand the present invention a description
is now given of a method of calculating the expected period of
torsional oscillation of a drill string. The description is based
upon readily available material in a well-known textbook,
"Vibration Problems in Engineering" by S. Timoshenko, 3rd edition,
Van Nostrand Company, 1955.
As discussed by Timoshenko on pages 9 and 10, the periods of
torsional oscillation in bodies such as a drill string are given by
equations of the type of Equation (1) (Timoshenko's Equation (9),
page 10):
.tau. = 2.pi. .sqroot.I/K (1)
.tau. = the period of torsional oscillation
I = an effective moment of inertia
k = an effective spring constant (torque per radian of twist)
For a simple rigid disk, Timoshenko calculates the moment of
inertia from Equation (2) (an unnumbered equation on Timoshenko's
page 11):
I = WD.sup.2 /8g (2)
W = weight of the disk
D = diameter of the disk
g = gravitational acceleration
For the effective spring constant of a simple solid shaft
Timoshenko gives Equation (3) (Timoshenko's Equation (a), page
10):
K = (.pi.d.sup.4 G/32L) (3)
d = diameter of shaft
L = length of shaft
G = the shear modulus
The drill string of interest here comprises a long, relatively
flexible drill pipe, and a set of relatively stiff drill collars
attached to this drill pipe above the bit. Thus two complications
must be introduced into the simple Timoshenko formulas. These
complications are introduced by methods which Timoshenko has
outlined or indicated in his text. Equation (2) is readily
converted to an equation for a hollow cylinder instead of a solid
disk in the following manner: First, Equation (2) is written in an
expanded form in terms of a specific weight, w, and disk height,
h:
I = (wh.pi.D.sup.2 /4) (D.sup.2 /8g) (4)
Then for a hollow cylinder of two different diameters, D and d:
I = (1/8 g) (wh.pi. /4) (D.sup.4 - d.sup.4) (5) But D.sup.4 -
d.sup.4 = (D.sup.2 - d.sup.2) (D.sup.2 = d.sup.2)
So I = (1/8 g) (wh.pi./4) (D.sup.2 - d.sup.2)
or I = (W(D.sup.2 + d.sup.2)/8g (6)
where in Equation (6), W is now the weight of the hollow
cylinder.
Equation (6) is the basic formula for computing the moments of
inertia of the parts of the drill string in the problem of
interest, but for the upper part of the string, the drill pipe
itself, the effective moment of inertia is less than that indicated
by Equation (6) because the upper part of the drill pipe is
relatively stationary during the oscillation. The drill pipe itself
participates actively in the oscillation only toward its bottom.
Timoshenko shows in his pages 29 and 30 how, with the use of
Rayleigh's method, one may calculate the effective moment of
inertia of a simple shaft undergoing this type of motion. His
analysis shows that, in such a circumstance, an expression of the
type of Equation (6) should be multiplied by a factor of
one-third.
For the system concerned here it is convenient then to write down
an equation of the type of Equation (6) involving three terms, one
for the drill pipe, and one for each of two sections of drill
collar, denoted respectively by the subscripts 1, 2 and 3:
The remaining necessity at this point is to formulate an expression
for the effective spring constant to insert in Equation (1).
Equation (3) may be readily converted to an equation for a hollow
cylinder of two different diameters, D and d:
but the system concerned has three different lengths, of different
elastic properties. Timoshenko show on page 11 how a composite
solid shaft, composed of two lengths of different diameters, acts
as if it were a shaft of the single smaller diameter but of an
"equivalent" length less than the actual composite length. The
stiffer part of larger diameter acts as if it were shorter than its
actual length. By reasoning identical to that of Timoshenko,
Equation (8) may be converted to Equation (9): ##SPC1##
in which the subscripts have the same meanings as in Equation (7)
and the denominator represents an effective length composed of the
actual length of the drill pipe and two "equivalent" lengths of
drill collar shorter than their actual length by factors which
account for their relative stiffnesses.
The two Equations (7) and (9) give the quantities that are to be
inserted in Equation (1) which, in turn, will give the period of
torsional oscillation that is desired to be calculated.
EXAMPLE I
Set out below is an example calculation of the natural torsional
period of a typical drill string which comprises 15,600 feet of
4-1/2inch drill pipe, 1,200 feet of 4-1/2inch drill collars and 200
feet of 6-1/2inch drill collars:
1. 4-1/2 inch Drill Pipe (I.D. = 3.826 inch) weighing 16.6
lb./ft.
Length of 15,600 feet .times. 16.6 lb./ft. = 259,000 lb.
D.sub.1 = 4.5" D.sub.1.sup.2 = 20.25 D.sub.1.sup.4 = 410 d.sub.1 =
3.826" d.sub.1.sup.2 = 14.60 d.sub.1.sup.4 = 213 W.sub.1 =
259,000.pi. D.sub.1.sup.2 +d.sub.1.sup.2 = 34.85 in..sup. 2
D.sub.1.sup.4 -d.sub.1.sup.4 = 197 in..sup.4
2. 4-1/2 inch Flexweight Drill Collars (I.D. = 2.76 inch) weighing
32 lb./ft.
Length of 1,200 feet .times. 32 lb./ft. = 38,500 lb.
D.sub.2 = 4.5" D.sub.2.sup.2 = 20.25 D.sub.2.sup.4 = 410 d.sub.2 =
2.76" d.sub.2.sup.2 = 7.60 d.sub.2.sup.4 = 57.8 W.sub.2 =
38,500.pi. D.sub.2.sup.2 -d.sub.2.sup.2 = 27.85 in..sup.2
D.sub.2.sup.4 -d.sub.2.sup.4 = 352.2 in..sup.4
3. 6-1/2 inch Drill Collars (I.D. = 3.0 inch) weighing 80
lb./ft.
Length of 200 feet .times. 80 lb./ft. = 16,000 lb.
D.sub.3 = 6.5" D.sub.3.sup.2 = 42.25 D.sub.3.sup.4 = 1780 d.sub.3 =
3.0" d.sub.3.sup.2 = 9.00 d.sub.3.sup.4 = 81 W.sub.3 = 16,000.pi.
D.sub.3.sup.2 -d.sub.3.sup.2 = 51.25 in..sup.2 D.sub.3.sup.4
-d.sub.3.sup.4 = 1699 in..sup.4 ##SPC2##
The discussion thus far considers the drill string in effect
rotating in a vacuum or a fluid which presents an insignificant
amount of drag on the torsional oscillation of the drill string. In
practice the drill string is immersed in a heavy drilling mud and
will in fact also rub against the sides of the hole it has drilled.
The effect of drag or friction is to increase the period of
oscillation. Therefore, as noted herein it is desirable to look at
a range of frequencies near the calculated frequency.
It has been calculated, in a similar manner to that shown above,
that the expected torsional period of oscillation of most drill
strings is between 3 and 12 seconds when drilling is occurring at
the depths where the present invention finds most utility. In most
well drilling at such depths, the drill bit is rotated at speeds in
the range of from 50 to 200 revolutions per minute. This means that
the period of the rotation of the drill string imposed by the
rotary table is in the range from 0.3 to 1.2 seconds. The expected
torsional period of the drill string, therefore, is on the order of
10 times as great as the rotational period of the rotary table.
Therefore, the expected torsional oscillations are relatively easy
to distinguish from oscillations occurring with an undamaged bit.
Such background oscillations have the constantly imposed rotational
period of the rotary table, or are formed from oscillations that
are harmonics of those imposed oscillations.
It is important to note that the relatively low frequency of the
characteristic torsional oscillations not only makes then
distinguishable from oscillations at the constantly imposed
rotational frequency; it also makes the characteristic torsional
oscillations not susceptible in the first place to excitation by
impulses occurring at repetition rates corresponding to those
higher frequencies. As is well known in the physics of vibrations,
neither regular nor random impulses tend to excite oscillations of
a system whose natural frequency is lower than their average
repetition rate. On the other hand, either random or regular
impulses tend to excite oscillations in a system whose natural
frequency is higher than their average repetition rate (except for
certain regular impulse sequences at exactly submultiple
frequencies, which tend to cancel their own excitations). So, it is
not only possible to distinguish the characteristic torsional
oscillations from oscillations at the higher frequencies
corresponding to the imposed rotational frequency and its
harmonics; it is also reasonable to ascribe those oscillations to
impulsive causes at significantly lower repetition rates.
The exact mechanism by which the torsional oscillations used in the
present invention are set up is not known; and it would be
extremely difficult to investigate. However, the mechanism is
believed to be a type of "stick-slip" phenomenon in which the bit,
say, with an impaired cone, tends to stick in position, and the
lower end of the drill pipe actually stops rotating for a fraction
of a second, until the torsional strain induced in the drill pipe
builds up such a large forward torque that the sticking is
overcome, and the bit rotates impulsively through a relatively
large angle. This sends a torsional pulse up the drill pipe which
must be reflected at its upper end. Then, if the pulse is strong
enough, or if it is followed by a sufficient number of similar
pulses, at random intervals, torsional resonance is set up in the
drill pipe.
Refer now to FIG. 1 for a detailed description of detection
apparatus useful to detect the torsional oscillations of interest.
In the electrical path from the power source 28 to the motor 24, a
shunt 30 is inserted so that the power current or a known fraction
of that current may be continuously measured. A signal proportional
to the instantaneous current strength is transmitted from shunt 30
to filter 34. This filter is preferably a band pass filter whose
upper and lower cutoff frequencies may be varied. Generally then,
when the expected period of torsional oscillation of the drill pipe
is expected to be between 3 and 12 seconds the upper pass frequency
may be set to suppress relatively high frequency signals having a
period near the rotary table period and the lower pass frequency
may be set to suppress relatively low frequency signals of less
than one twenty-fourth cycle per second. In a more specific
instance, if the expected period of torsional oscillation of the
drill pipe is expected to be about 8 seconds (a frequency of
one-eighth second), the upper pass frequency may be set at, say,
0.25 cycles per second and the lower frequency cutoff at, say, 0.06
cycles per second. The signal passed by the filter 34 is used to
activate the strip chart recorder 38, which marks on the chart 39.
The stylus 40 of the strip chart recorder produces a record 42 of
the current variations which are indicative of the torsional
oscillations of drill string 14. If desired, an alarm device such
as light 44 and/or bell 43 may be activated by a threshold filter
responsive to the signal passed through band pass filter 34.
FIG. 2 represents a typical section of strip chart which was
produced utilizing the method of the present invention. In FIG. 2
the oscillatory component of the current for the rotary drive motor
is plotted as a function of time. Clock times may be marked on the
chart by the recorder itself. Corresponding drilling depths may
also sometimes be advantageously entered on the chart. The recorded
current represents the variation of the torque on the drill string.
The normal torque variation on the drill string is illustrated in
the lower portion of the chart below the vicinity of the chart
where the time indication is 4:16 P.M. There, a substantially
constant torque is being required by, and imposed upon, the
drilling apparatus. In the portion of the chart beginning above the
4:30 P.M. marking, relatively large characteristic torsional
oscillations begin, indicating that the drill bit is now damaged.
These characteristic oscillations are substantially greater in
amplitude than the minor oscillations which occurred when drilling
with an undamaged bit, i.e., before 4:16 P.M. In the particular
strip chart in question each vertical chart division represents 1
recording minute. Thus the period of the oscillations is about 10
seconds. It has also been found that it is sufficient in field
practice to record the torque for a 1 minute interval every 15
minutes. Thus as shown in FIG. 2, after one minute of recording
between 4:00 P.M. and 4:01 P.M. a fourteen minute break is taken
before actuating the torque recorder for another 1 minute interval.
As illustrated in FIG. 2 evidently the oscillations began sometime
after 4:16 P.M. since characteristic oscillations are shown
beginning with the 4:30 P.M. recording. These characteristic
oscillations have an amplitude substantially in excess of the minor
oscillations occurring before 4:16 P.M. as a result of rotating an
undamaged bit in the well.
The proper time to replace a bit is a function of many variables
including the hole depth, the formation being drilled, and the
probable cost of a complete failure, which would require a
subsequent fishing job. The start of oscillations which are
recorded in accordance with the method of the present invention,
however, informs the driller that the bit is damaged and immediate
attention must be given to whatever considerations should precede
actual pulling of the pipe. In some instances, the driller will
want to pull and replace the bit within two hours after he
determines that the oscillations are continuing.
The following Field Examples illustrate the effectiveness of the
present invention in indicating bit damage.
As a result of these Examples it was decided that in future
operations, in the drilling of similar wells, a bit should be
pulled after 2 drilling hours beyond the initial appearance of
oscillations in torque.
FIELD EXAMPLE I (BIT PULLED BEFORE FAILURE)
A straight hole was drilled in the South Timbilier Block 86 Field.
The hole was 8-1/2 inches in diameter and was being drilled below
9-5/8 inch protective pipe set at 14,953 feet. At this depth,
roundtrip time averaged 9-10 hours. The rotating roller bits used
in this operation had long tooth tungsten carbide inserts.
Generally, their life on the bottom is approximately 40 to 60
rotating hours. The practice was to use bit weights of 40 to 45
thousand pounds and a rotary speed of 45 to 50 rpm. Due to the long
length of protective pipe in this hole, oscillations in torque on
the surface were quite apparent as the bit bearings became damaged.
After 43 hours of drilling, characteristic oscillations of the type
illustrated in FIG. 2 appeared on the torque record. The bit was
run 3 more hours in the manner specified above, i.e., with 40 to 45
thousand pounds bit weight and at 45 to 50 rpm rotary speed. The
bit was then pulled from the hole and no cones were lost from the
bit in the hole. However, the lower bearings from two of the cones
were left in the hole and one bit cone was cracked. It is believed
that if additional drilling had been done for any extent of time,
one or more of the roller cones would have been left in the hole
necessitating an undesirable fishing job.
FIELD EXAMPLE II (BIT PULLED AFTER FAILURE)
A hole was being drilled below 16,000 feet. The weight on the bit
was 20 thousand pounds and the rotational speed was 87 rpm. The
hole proceeded from 16,194 feet to 16,280 feet where the weight on
the bit was 42 thousand pounds and the rotational speed was 78 rpm.
At this depth, characteristic oscillation of the rotary torque
appeared on the strip chart. Drilling continued for 13 hours during
which time the oscillation also continued on the strip chart. At
the end of this time the bit was pulled and three cones were left
in the hole. An expensive fishing job was required to remove the
cones from the hole.
Although only certain preferred embodiments of the present
invention have been described in detail, the invention is not meant
to be limited to these embodiments only, but rather to the scope of
the appended claims.
* * * * *