U.S. patent number 3,818,227 [Application Number 05/281,869] was granted by the patent office on 1974-06-18 for radioactive tracer system to indicate drill bit wear or failure.
This patent grant is currently assigned to Chevron Research Company. Invention is credited to Bernard A. Fries.
United States Patent |
3,818,227 |
Fries |
June 18, 1974 |
RADIOACTIVE TRACER SYSTEM TO INDICATE DRILL BIT WEAR OR FAILURE
Abstract
A radioactive tracer system for indicating drill bit wear or
failure utilizing radioactive krypton 85 in clathrate form, in the
form of water-soluble kryptonates, or dissolved in grease.
Preferably the radioactive krypton is placed so that when drill bit
wear or failure occurs, the radioactive krypton 85 is released and
effectively becomes diffused in the circulating drilling fluid. At
the surface, the radioactive krypton 85 gas is separated from the
circulating drilling fluid by gas-mud separating means and is
transported as a gas to a counting chamber where an accurate
radioactivity count of beta rays released from the krypton is
obtained. The beta rays indicate drill bit wear or failure when
detected above the normal background radioactivity of the
recirculating drilling fluid. The radioactive krypton may also be
implanted in the teeth or other surfaces of the cones of drill bits
or implanted in the shirttail or other body parts of the bit.
Inventors: |
Fries; Bernard A. (Orinda,
CA) |
Assignee: |
Chevron Research Company (San
Francisco, CA)
|
Family
ID: |
26734452 |
Appl.
No.: |
05/281,869 |
Filed: |
August 18, 1972 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
55635 |
Jul 17, 1970 |
|
|
|
|
Current U.S.
Class: |
250/303;
250/304 |
Current CPC
Class: |
E21B
12/02 (20130101); G01V 5/00 (20130101) |
Current International
Class: |
B23Q
17/09 (20060101); G01V 5/00 (20060101); E21B
12/02 (20060101); E21B 12/00 (20060101); G21h
005/02 () |
Field of
Search: |
;250/303,304 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Borchelt; Archie R.
Attorney, Agent or Firm: Freeland, Jr.; R. L. Keeling; E. J.
Buchanan, Jr.; J. A.
Parent Case Text
This application is a continuation-in-part U.S. Pat. application of
Ser. No. 55,635, filed July 17, 1970, now abandoned.
Claims
I claim:
1. In a process for detecting wear and failure of a drill bit
during a drilling operation in a borehole in which drilling fluid
is continuously recirculated, and wherein a radioactive tracer is
placed within a drill bit so that when drill bit wear or failure
occurs such failure can be detected by measuring the radioactivity
of the recirculating drilling fluid carrying the released
radioactive tracer, with said radioactive tracer being detected by
beta and gamma ray counting means for indicating both the transient
level of radioactivity in said recirculating drilling fluid and the
background level of radioactivity in said recirculating drilling
fluid as a function of time, the improvement comprising placing
krypton 85 contained in a soluble solid form in said drill bit so
that upon wear or failure said krypton 85 solid is dissolved in
said drilling fluid, passing the recirculating drilling mud after
it flows from the borehole through gas-separating means to remove a
substantial portion of said krypton 85 as a gas from said drilling
fluid, conducting said separated gas from said recirculating
drilling fluid to said beta and gamma ray counting means, and
detecting an abrupt rise in the transient level of radioactivity
due to the presence of beta rays in said gas as a primary
indication of wear or failure, with the relatively low level of
background radioactivity in the recirculated drilling fluid being
indicated by beta and gamma rays from gases continuously extracted
from said recirculated drilling fluid.
2. In a process for detecting wear and failure of a drill bit
having a plurality of rotatable cutters during a drilling operation
in a borehole in which drilling fluid is continuously recirculated,
and wherein a radioactive tracer is placed within a drill bit so
that when drill bit wear or failure occurs such failure can be
detected by measuring the radioactivity of the recirculating
drilling fluid carrying the released radioactive tracer, with said
radioactive tracer being detected by beta and gamma ray counting
means for indicating both the transient level of radioactivity in
said recirculating drilling fluid and the background level of
radioactivity in said recirculating drilling fluid as a function of
time, the improvement comprising placing krypton 85 dissolved in a
carrier in a grease reservoir or a sealed bearing for at least one
of said plurality of rotatable cutters of said drill bit so that
upon wear or failure of said bearing said krypton 85 is dissolved
in said drilling fluid, passing the recirculating drilling mud
after it flows from the borehole through gas-separating means to
remove a substantial portion of said krypton 85 as a gas from said
drilling fluid, conducting said separated gas from said
recirculating drilling fluid to said beta and gamma ray counting
means, and detecting an abrupt rise in the transient level of
radioactivity due to the presence of beta rays in said gas as a
primary indication of bearing wear or failure, with the relatively
low level of background radioactivity in the recirculated drilling
fluid being indicated by gamma rays from gases continuously
extracted from said recirculated drilling fluid.
3. The method of claim 2 wherein said carrier is grease and the
radioactive krypton is physically dissolved in a portion of the
grease in said reservoir.
Description
My invention is related to the detection of drill bit wear and
failure in a downhole drilling environment. It specifically
encompasses the use of radioactive krypton 85 gas, packaged in a
manageable form, which is placed in selective component parts of a
drill bit in combination with a simple and sensitive gas detection
system at the surface capable of monitoring the presence of
radioactive gas in the drilling fluid flow.
Drillers and their backup researchers in the oil industry have long
been confronted with the problem of devising a precise means of
determining when a given drill bit fails to function properly or
efficiently. Gross drill bit failure is eventually detected by a
marked drop-off in the rate of drill bit penetration. Minor
decreases in the rate of penetration are generally not a positive
indicator of drill bit wear or failure because such decreases can
frequently be attributed to encountering hard rock formations. On
the other hand, a high rate of penetration is not necessarily a
positive indicator that a given drill bit is functioning properly
because softer formations are also encountered. Since there was no
accurate way of knowing what type of formation had been encountered
without taking core samples or employing well-logging techniques,
both of which are highly impracticable in the ordinary drilling
situation, the rate of penetration is usually not a positive
indication of the physical condition of the drill bit. The only
means, then, that field drilling engineers rely on to detect drill
bit failure are complete drill stoppage, the detection of drill bit
debris in the effluent mud flow, a knowledge of how long a given
drill bit had been in use as compared to the average expected
lifetime of the drill bit, and vibrations in the drill string which
sometimes occur if a drill bit is not functioning properly.
The concept of using a radioactive substance as a tracer in a
drilling fluid or mud environment is disclosed by J. B. Warren in
U.S. Pat. No. 2,468,905; it is also shown J. J. Arps in U.S. Pat.
Nos. 2,658,725 and 2,659,046. Warren proposes that a compressed gas
means to be used to facilitate the mixing of a radioactive tracer
with the drilling mud when a predetermined level of wear had been
reached. Arps discloses that the radioactive tracer should be
selectively released downhole in order to monitor formation
characteristics in lieu of pulling the drill string and employing
conventional logging techniques. J. A. Rickard, in U.S. Pat. No.
3,115,576, discloses the use of a radioactive tracer in a drilling
mud to determine whether a balanced circulation is maintained. T.
J. Nowak, in U.S. Pat. No. 2,692,755, proposes the continuous
logging of the natural radioactivity of drilling mud due to its
contact with the downhole formations and to the inclusion of
drilling debris in the mud. In U.S. Pat. No. 2,658,724, noted
above, J. J. Arps proposes that radioactive tracers be utilized to
indicate the existence of abnormally high temperatures downhole to
detect undesirable drilling conditions. He discloses the use of an
ionization chamber immersed in the mud flow to detect the presence
of radioactive tracers within the mud. He also discloses a number
of radioactive salts and the possible use of a radioactive gas
under pressure, but does not discuss the problems to which the
present invention is addressed. The combination of a radioactive
substance in a vial enclosed in a drill bit and a cutter which
ruptures the vial when the axis of the cone ceases to coincide with
the axis of the drill shaft was disclosed by J. W. Graham in U.S.
Pat. No. 3,011,566.
British Pat. No. 1,126,916 (Farbwerke Hoehst) discloses the use of
radioactive krypton 85 in a clathrate form that is sealed in a
capsule in a drill bit. An expellant or propellant, to force the
krypton 85 clathrate into the drilling fluid is specified to assure
inclusion in the drilling fluid returns when wear causes rupture of
the capsule. However, no provision is made for removing the tracer
from the drilling fluid. This has two primary drawbacks. One is
that the tracer recirculates with the drilling fluid so that simple
decay or evaporation from the drilling mud pit are the only ways of
reducing radioactive background in the drilling fluid. This makes
subsequent detection of other tracer peaks (from other drill bit
failures) more difficult. The other drawback is that the patentees
propose to detect the radioactive krypton by immersing a radiation
detector in the drilling fluid. Unfortunately, radiation emitted by
radioactive krypton 85 is 99.6 percent beta rays and only 0.4
percent gamma rays. Beta rays are readily absorbed by drilling
fluid so that unless the krypton 85 is removed from the drilling
fluid, the sensitivity of the detection is inordinately poor as
compared to measuring the radiation in gaseous form.
All such previous proposals failed commercially. In some, no
suitable radioactive isotope was used. The half-life was either too
short so that difficult problems of manufacture and logistics were
created, or the isotope was in a form that left a residue which
contaminated the environment after they were used. In some a solid
radioactive substance was carried to the surface in the drilling
mud flow but was not mixed with the drilling mud flow. In a
fast-flowing mud stream, it was possible that such a spurt of
radioactivity associated with the solid radioactive substance could
be missed by even a relatively sensitive counter. None of the
proposed systems had the feature of separating a beta ray emmitting
radioactive material as a gas from the mud flow prior to counting
the level of radioactivity.
It is one object of my invention to implement a detection system
which measures the activity level of the radioactive tracer gas
after the tracer gas has been removed from the circulating drilling
mud so that absorption of the tracer radiation by the drilling mud
is eliminated.
A further object of my invention is the utilization of a detection
system which counts radioactivity after most of the radioactive
tracer from a failed drill bit has been removed from the drilling
mud flow so that the background level of radioactivity in the
drilling mud is not significantly built up over a period of time by
subsequent periodic releases of tracer by other drill bit
failures.
A more complete understanding of the elements and operation of my
invention may be had by reference to the drawings which are hereby
incorporated in this specification and in which:
FIG. 1 is a schematic illustration of the overall operation of my
invention, illustrating an embodiment of the gas-mud separation
means.
FIG. 2 is a cross-sectional view of a typical sealed bearing drill
bit, illustrating the relation between the drill bit, the drill
cone, the grease reservoir, the ring bearing, the ball bearings,
the teeth, and the ball bearing race, and also illustrating the
locations at which radioactive krypton 85 can be placed whether the
krypton is a gas dissolved in the grease, in solid clathrate form,
or in water-soluble kryptonate form.
FIG. 3 is a strip chart record of the amount of radioactivity
measured at the surface when krypton 85 in dissolved form was
introduced into the drilling mud flowing back to the surface at
time zero. The initial peak is sharp and represents the detection
of the extracted krypton 85 in gaseous form upon completion of the
first cycle of the circulating drilling mud. By such physical
extraction, the second peak is one-tenth as intense and is spread
out over a longer period of time. The residual recycled radioactive
krypton 85 is more widely dispersed throughout the circulating
drilling mud and is of such a low intensity that it does not
interfere with subsequent runs.
In essence, my invention is based upon the sensitivity and accuracy
with which it is possible to measure the radioactivity of a
separated sample of a gas and upon the fact that krypton 85 gas has
the property that it can be packaged in a non-gaseous form which
readily dissolves or disperses upon release into the circulating
drilling mud. Krypton 85 gas is suitable because its half-life is
10 years and therefore it can be placed in drill bits at some
convenient time prior to use and does not necessarily have to be
implanted in the drill bit at the drilling site; any gas with a
half-life greater than about 5 days possesses this desirable
property. Once krypton 85 has been prepared in a clathrate from,
has been kryptonated onto a water-soluble salt or physically
dissolved in bearing grease, it can be easily placed in a drill
bit. When drill bit wear or failure occurs, such a dissolved form
of krypton 85 comes into contact with the circulating drilling mud
and, in each case, the krypton 85 quickly becomes diffused in the
drilling mud. The water-soluble salts dissolve in the drilling mud,
the clathrate decomposes upon contact with the mud and the
liquified grease flows with the mud. Thus, after the mud flows from
the borehole, a gas-mud separator and a gas detection system is
capable of accurately determining how much radioactive gas has been
released below. Krypton possesses the desirable property that it
will not react with the constituents of the circulating drilling
mud and, therefore, reaches the surface intact other noble gases
also posses this property.
Clathrates are a newly discovered group of inclusion-type compounds
in which molecules of a captive substance are enclosed within the
crystalline structure of a host substance. Clathrates of
radioactive krypton 85, as disclosed in Radio-Release in Review
with Special Emphasis on Krypton 85 Clathrates and Kryptonates,
Carden, Joan E., Oak Ridge National Laboratory Publication No.
ORNL-IIC-18, can be prepared from an aqueous solution or from a
melt. For example, a 0.5 to 1.0 gram sample of solid hydroquinone
is put in a pressure bomb, a vacuum is achieved to eliminate
atmosphereic gases and the bomb is pressurized with krypton gas
containing a small percentage, say 5 percent, of radioactive
krypton 85. The bomb is maintained at 185.degree. C. for
approximately 2 hours to allow the krypton to saturate the melt.
Then the system is cooled to room temperature over a period of
several days. Finally, the fused krypton 85 hydroquinone clathrate
is removed and utilized in a powdered, granulated or pelleted
form.
Clathrates of radioactive krypton 85 can be used in a variety of
ways to detect drill bit wear and failure. The clathrate can be
placed directly in the grease reservoir of a sealed bearing drill
bit. Or, the clathrate can be placed within or underneath the teeth
in the cones of a drill bit, within holes drilled in the surface of
cones of drill bits and in the sides of the shirttail of drills. It
has been found that only very small amounts of the clathrate need
to be used. For example, in one series of experiments a few
milligrams of a hydroquinone clathrate of krypton 85 was mixed with
several hundred milligrams of powdered sugar. A small amount of
stearic acid was added as lubricant. Then, the mixture was placed
in a pelletizer and tiny pellets were prepared. These pellets were
placed in the aforementioned locations in a drill bit and the drill
bit was used to drill an exploratory hole.
Kryptonates are prepared by either ionizing krypton atoms and
impelling them into a solid or by placing krypton gas and a solid
in a pressure bomb under high pressure so that the krypton atoms
diffuse into the solid. Over a hundred solids have been found
capable of accepting krypton atoms in that manner but it is found
that solids which contain regular structural voids on an atomic
scale are preferable. Boron nitride, red iron oxide and platinum
dioxide have been found to be suitable for oil field use.
Ordinarily, the krypton held captive in kryptonates is released
only by heating. However, water-soluble kryptonates release the
captive krypton when the solid comes in contact with water.
Therefore, water-soluble kryptonates can serve as model tracers
because they release their krypton as soon as they come in contact
with the drilling mud downhole.
Radioactive krypton may also be incorporated in grease without
first being compounded as a clathrate or as a kryptonate by the
following simple procedure. A conventional lubricating grease such
as that used in automobiles and trucks chassis or wheel bearing
lubrication is placed in a rubber bag, or balloon, and a bubble of
radioactive krypton 85 gas introduced. The grease is then kneaded
until the gas becomes dissolved. The gas is soluble in the grease
and effectively disperses without leaving residual bubbles. The
grease could also be prepared during mixing of the original grease
batch by enclosing the chamber under pressure and stirring the
grease during compounding with a quantity of the radioactive
krypton gas in the chamber. The material is stable even at
relatively high temperatures but upon exceeding a particular
temperature, say 250.degree. F., would liquify and escape into the
drilling fluid. This would happen, for example, when a bearing
fails in a drill bit so that the cones will not turn, direct
friction between the bearing supported cones and the rock being
drilled will cause the grease reservoir temperature to rise
sufficiently so that the grease is liquified and runs out of the
bearing support. Such failure is relatively rapid after the melting
point of the grease is reached so that the melted grease containing
the radioactive krypton is readily incorporated in the drilling
fluid and returns to the surface as a part of the drilling mud
returns. Another mechanism of grease discharge is also possible;
intact grease may spurt out of the bearing seals without bit
heating, upon mechanical failure of the bearing seals.
A particular advantage of using krypton 85 dissolved in grease is
the low cost of krypton tracer in this form. The cost comparison
being on the order of two cents per millicurie versus about ten
dollars per millicurie for krypton in clathrate or kryptonate
form.
In FIG. 1 drill bit 13 with drilling cones 14 is being worked to
increase the depth of wellbore 12. Drilling debris resulting from
the extension of wellbore 12 into formation 15 is carried to the
surface by the upward movement of drilling mud 11. Upward flowing
drilling mud 11 has been introduced at the bottom of wellbore 12 by
an opening in the bottom of drill bit 13 after flowing from the
surface to drill bit 13 within annular drill string 10. Clathrates
and kryptonates of radioactive krypton 85 or physically dissolved
krypton 85 released at the bottom of wellbore 12 due to wear,
bearing or tooth failure, is caught up by upward flowing
circulating drilling mud 11 and transported to the surface.
At the surface in this embodiment of my invention upward flowing
circulating drilling mud 11 is channelled through conduit 18 and
discharged as flow 19 into separation trough 20. The drilling mud
moves by gravity flow through separation trough 20 and is
discharged as flow 24 into automatic shaker screen 28. As it passes
through separation trough 20 the circulating drilling mud is
agitated by stirring means 21 comprising electric motor 22 and
stirring blade 23. The lower end of stirring means 21 is immersed
in the drilling mud flow so that stirring blade 23 is constantly
agitating the mud and so that outer skirts 9 maintain an air-tight
seal with the flowing drilling mud. Fresh air is drawn into
stirring means 21 at inlet 8 by means of the negative pressure
within stirring means 21 created by vacuum pump 31 and communicated
to stirring means 21 by conduit 30. As krypton gas containing
radioactive krypton 85 is separated from the flowing drilling mud
by the agitation of stirring blade 23 it is caught up in the flow
of fresh air from inlet 8 and carried through conduit 30 to the
counting means 36.
Beta and gramma ray counting means 36 contains a counting element
35 which can be a Geiger tube, a scintillation counter, an
ionization chamber, a proportional counting means or any other beta
and gamma ray counting means which is reasonably compact and
inexpensive and therefore suitable for field use. Counting element
35 is calibrated under laboratory conditions to determine its
response as a function of the amount of radioactivity, the rate of
air flow, and the size of the chamber component of counting means
36. The output of counting element 35 is monitored continuously by
rate meter 38 equipped with an instantaneous readout dial 7.
Instantaneous readout dial 7 may be equipped with a visual or
audible alarm means which is activated whenever the instantaneous
intensity of radioactivity exceeds a predetermined level. Finally,
a record 41 of the level of radioactivity as a function of time is
kept on continuous strip chart 39 by means of galvanometer pin 40
driven by count rate meter 38.
After passage through the chamber of counting means 36, air
containing radioactive krypton 85 and non-radioactive krypton is
transported by conduit 37 to vacuum pump 31 and thence into the
atmosphere. Both forms of krypton in the atmosphere, do not become
concentrated in the region in which drilling is conducted. Streams
or ground water are not in danger of being polluted. Animal life
such as cow 42 and plant life such as alfalfa 6 are also not in
danger of taking in concentrated doses of radioactive krypton.
Exposure of the drilling crew and engineers to direct radiation,
i.e., to beta and gamma rays, is minimal because of the low
activities of the samples used, the rapid diffusion into the
atmosphere and the absorption of the drilling mud as well as the
absorption of other components of the separating and counting
means.
After circulating drilling mud 24 flows from separating trough 20
into automatic shaker screen 28, a major portion of the radioactive
krypton 85 has been removed. The residual radioactivity is not
great enough to obscure any readings that are taken after a given
portion of the mud completes another cycle through drill string 10
and out conduit 18. Automatic shaker screen 28 serves to recover
formation cuttings in the drilling mud and separates some of the
drilling debris from the drilling mud. Cleaner and more uniform
drilling mud is then stored in open pit 29 to be recirculated
downhole. When drilling is underway, pump 49 draws drilling mud
from open pit 29 through conduit 50 and forces it thence through
flexible hose 47, swivel head 46, drill pipe 48, and drilling head
17 to drill string 10. A relatively uniform flow is maintained by
the pressure imparted by pump 49 to the drilling mud introduced to
drill string 10.
Referring now to FIG. 2, a cross-sectional view of a single cone of
a three-cone sealed bearing drill bit, it can be seen that teeth 87
are implanted in a predetermined pattern on the surface of cone 89.
Cone 89 is disposed to rotate about the lower end of shank 82 by
means of sealed bearing 78 and ball bearings 90, and thereby comes
in contact with the formation to be drilled. Ball bearings 90 and
sealed bearing 78 move freely in race 72 by means of grease which
is introduced to race 72 from grease reservoir 70 via channel 71.
Ring seals 80 are constructed of a suitable material such as spring
steel and serve to prevent grease from escaping from race 72.
Grease reservoir 70 is typically constructed as shown with
cylindrical body 83 having a rubber boot 76 at one end to maintain
the grease in the reservoir under slight differential pressure and
having holes 79 opposite race 95, said race 95 being in
communication with channel 71, and said holes 79 permitting grease
to be supplied to the ball and sealed bearing race 72 via channel
71 whenever necessary. Cylindrical shell 83 of grease reservoir 70
is maintained in tight communication with shank 82 by means of 9
rungs 92 and 93. Grease is loaded into reservoir 70 at the factory
through the threaded bore 100 opening filled by set screw 81. A
grease nipple (not shown) is usually threaded into bore 100 for
filling with a conventional grease gum. This can also be done at
the drill site by temporary removal of set screw 81 and
substitution of a grease nipple. The nipple is then replaced by set
screw 81. In this manner, small grease wads containing dissolved
radioactive krypton 85 can be injected into the bit's grease
reservoir 70 just before the bit is run into the well on the end of
drill string 10. Such a procedure gives indefinite shelf life to
the bit since the tracer is added at the time of use only.
Additionally, only minor quantities of radioactive grease is
required so that cost per bit is greatly reduced.
External pressure on rubber boot 76 is maintained by the
hydrostatic pressure of the drilling mud. This pressure is
communicated to rubber boot 76 by drilling mud 84 which has passed
through screens 77 and 91 and is contiguous with the external
drilling mud. There is equal pressure on both sides of rubber boot
76. Only a very slight differential pressure develops which forces
the grease out of the reservoir as needed.
As indicated above, a wad or small quantity of the radioactive
krypton 85 activated grease is desirably inserted near the top of
reservoir 70 test. Tests indicate that even under downhole
temperature conditions the radioactive grease does not diffuse into
the regular body of the grease until such time as some portion of
the drill bit fails, as for example by damage to the cones, the
bearing supports, or the bearings themselves. However, excessive
wear of the drill cone 89, sealed bearing 78 or other parts of the
rotating bit including the shirttail 97, raises the drilling
temperature due to increased transfer of heat of grinding rock so
that the grease melts. The direct transfer of heat between the
grinding elements such as teeth 87 or the body portion 89 will heat
reservoir 70 sufficiently so that the grease is liquified and a
plug of grease, such as that indicated by phantom location 75, will
run out of the resevoir and drill bit bearing. The liquified grease
containing the radioactive krypton is then picked up by the
drilling fluid returns and carried to the detecting system at the
earth's surface. Alternatively, mechanical failure of the bearings
can also release the grease without any significant temperature
rise of the bit.
Clathrates and kryptonates of radioactive krypton 85 may be placed
as shown at various locations throughout the drill bit. In one
embodiment of my invention, small pellets composed of the
hydroquinone clathrate of radioactive krypton 85 in a powdered
sugar base were placed at location 75 in grease reservoir 70 and at
location 74 in channel 71. The drill was then used to drill an
exploratory hole. In one test the drill lasted for 7 hours after
initial radioactivity was detected at the surface. In another test
the drill functioned for 5 hours after the first sign of
radioactivity at the surface. The tracer, then, provided drilling
personnel with an indication of impending bearing failure. In
another embodiment, small pellets of clathrates and kryptonates of
krypton 85 are placed at location 85 underneath set screw 85 in the
surface of cone 89 and at location 88 under a tooth implanted in
the surface of cone 89. The diffusion of the radioactivity in these
pellets throughout the dirlling mud as it flows to the surface,
coupled with efficient detection of the radioactivity at the
surface, gives the drilling crew a clear indication of excessive
wear or serious damage to the teeth or cone downhole.
In drilling wellbores, it is well known that the external surface
of the drill bit, i.e., the smooth, tapered surface of shank 82
denoted as surface 97 in FIG. 2 and commonly called the shirttail,
is subject to wear due to friction with the sides of the previously
drilled section of the wellbore. When this occurs bearing seals 80
become exposed and bearing failure is imminent. Also, cone 89 is
then subject to shock from the side of the wellbore and will likely
fail. Thus, in one embodiment of my invention, a small amount of
radioactive krypton 85 in clathrate or water-soluble kryptonate
form is placed in shirttail 97 at location 94 beneath set screw 96.
If radioactivity is released from this source the drillers will
know that drill bit failure is imminent.
While certain specific embodiments have been described herein and
depicted in the drawings my invention is intended to be limited
only by the scope and spirit of the appended claims.
* * * * *