Note: Descriptions are shown in the official language in which they were submitted.
SYSTEM FOR DETERMINING THE FREE
POINT OF PIPE STUCK IN A BOREHOLE
BACRGROUND OF T~E INYENTION
1. Field of the Invention
The invention relates to a method and apparatus for
determining the point at which a pipe is stuck in a bore-
hole and, more particularly, ~o a system for magnetically
determining a pipe's ree point location without the
necessity of attaching apparatus ~o the pipe wall.
2. ~istory of the Prior Art
In the drilling of oil and gas wells through earth for-
mations it often occurs that the drilling pipe will become
stuck in the borehole being formed. This may happen because
of a collapse or cave-in of the subterranean formation
surrounding the borehole. It may also occur as a result of
fluid absorption and swelling of certain downhole formations
which re~trict the mo~ement of the drilling pipe within the
borehole, as well as for many other reasons. When this phe-
nomenon does occur, the drilling pipe becomes jammed, opera-
tion ceases and no further progress can be made in deepening
of the borehole until the stuck pipe is removed.
The first step in clearing a jammed pipe in a borehole
q~
~ ~3~ 7
--2~
is locating the point along the borehole, often several
thousand fee~ beneath the surface, at which the pipe is
stuck. Numerous techniques have been developed over the
years for locating the free point of the pipe in the bore-
hole so that the pipe portion above the stuck region can beremoved. ~he mosk popular technique involves the lowering
of a tool down the central passageway of the drilling pipe
and the attachment of a pair of relatively movable sensor
members to the inside pipe wall. '~he drill pipe is then
stretched either longi~udinally or in torsion so that any
relative movement between the two fixed members indicates
that the members are fixed to the pipe wall at a location
above the stuck point~ Of course, stresses in the drill
string which are induced from the surface are only reflected
in that portion of the drill string which is aboYe the stuck
point. As soon as the sensor pair is affixed to the walls
of the pipe below the stuck point and the drill string is
stressed, there will be no r~lative movement between the two
members. Thus, by sequential measurement and movement of
the sensors along the inside of the drill pipe, the stuck
point is located. Systems of this type are, however, rela-
tively slow in that the sequential attachment and detachment
of the sensor mem~ers requires time and time in the opera-
tion of a drilling rig is very expensive. In addition, the
~23~3~7
-3-
contacting type of stuck pipe detectors also require ela-
borate mechanical or magnetic means for attaching the sensor
members to the wall of the pipe.
A known characteristic of ferromagnetic pipe is that the
magnetic permeability of the material changes as a function
of stresses in the material. Another prior art stuck point
detector system has utilized this principle rather than ~he
mechanical elonga~ion of the pipe. Employment of this tecn-
nique allows the co~struction and use of a non-contacting
stuck poin~ detector which does not need to engage the
sidewalls of the pipe. As shown in U.S. Patent No.
2,686,039 to Bender, a high frequency oscillator 10 is tuned
to a frequency on the order of 20 to 50 RHz by a coil 17 and
lowered into the axial bore of a stuck drill pipe. The coil
is inductively coupled to ttle wall of the steel pipe which
loads the coil and is thus a part of the tuned tank circuit
of the oscillator 10. The magnetic permeability of the pipe
determines the degree of loading of the coil 12, therefore,
the inductance of the tank circui~ and the frequency of the
oscillator. As the coil passes the stuck point of a drill
pipe under stress, the oscillator will shift in frequency
due to the fact tha~ the magnetic permeability of the
unstressed pipe below the point is different from that of
the stressed pipe above the stuck point. While the Bender
system is capable of detecting the stuck point withouL
physical attachment of sensors to the pipe walls such
a system includes a number of inherent disadvantages.
Perhaps the greatest of these is that the inductive
coupling of the pipe into an oscillator tank circuit
requires the use of relatively high frequencies. The
depth of penetration of high frequency electromagnetic
waves are limited by skin effect and thus, the overall
accuracy and reliability of the technique is limited.
The sensitivity of the Bender system is also restricted
by the teaching of a single logging run to detect stuck
point which does not allow sufficient tolerance for
magnetic permeability variance between different pipe
materials and sizes.
While certain other prior art tools have included
means for measuring the permeability of pipe or tubing,
these are generally utilized only in caliper tools for
determining thicknesses and inside diameter of unstressed
pipe. For example, U.X. Patent Application No. 2,037,439
published July 9, 1980 by Schlumberger Limited, and U.S.
Patent No. 2,992,390 issued to DeWitte both utilize
various aspects of magnetic permeability for pipe
measurements.
For example, in the Schlumberger U.K. application,
there is described a tool for measuring the wall thickness
of a well casing by means of magnetic flux. Three pairs of
transmitter and receiver coils are employed, one for
measuring inside diameter, one for measuring casing thickness
and one for measuring casing wall permeability. Variations
in each of these parameters affect one another so that
measurements of all three simultaneously can be used to
correct one another and produce a highly accurate thickness
measurement. While the Schlumberger U.R. application
discloses a two coil, two log approach to magnetic per-
meability measurement, it is only disclosed in connection
with a caliper tool and none of these proposals have culmi-
nated in a commercially satisfactory stuck point detector.
Although the prior art is replete with both method and
apparatus for downhole measurement of pipe permeability,
the problem of accurately locating stuck pipe in a borehole
has still existed. The system of the present invention has
overcome the disadvan~ages of the prior art to produce a
highly successful tool by providing a non-contacting magne-
tic stuck point detector which uses rela~ively low frequency
to detect changes in permeability occurring in s~ressed pipe
within a borehole. In ~his manner, an effective system andmethod is provided for locating the point along the borehole
at which a drill pipe section is lodged.
~30~
--6--
Summary of the Invention
The invention comprises a system for determining the
stuck point of a pipe within a borehole by providing a pair
of coils located on a common a~is and spaced apart a
S prescribed distance. The exciter coil is energized at a
relatively low preselected frequency while the ~oils are
lowered into the drilling pipe when the pipe is in a
generally unstressed condition. A log of the output of the
receiver coil is taken. Thereafter, the sidewalls of the
pipe are placed in stress and the process repeated to take a
second log. A comparison of ~he two logs is made to give an
indicatio~ of the location of the stuck point within the
borehole due to the change in signal received by the coil.
The signal change i6 a result of magnetic permeability shift
between the stressed and unstressed condi~ions of the drill
pipe above and below the stuck point.
In another aspect, the invention includes an improved
method for detecting the stuck point location of drilling
pipe lodged within a borehole of the type wherein a tool is
lowered through ferromagnetic pipe sections for detecting
permeability changes therein. The improvement comprises the
steps of providing a wire line tool having a pair of spaced
apart coils adapted for descent within the drilling pipe to
sense the permeability of the pipe. An alternating fre-
~L~3~3~7
--7--
quency primary magnetic flux is generated with one of thecoils of the tool and induced into the walls of the drill
pipe, The secondary flux signal generated by eddy currents
induced in the drill pipe is detected by the receiving coil
of the tool as an indicia of permeability, The tool is
moved along the drill pipe within the borehole, with the
pipe in an unstressed condition, for generating a first log
of pipe permeability, The tool is then moved along th~
drill pipe within the borehole, with the pipe in a stressed
condition, for generating a second log of pipe permeability,
The first and second logs are then compared to locate the
variation in permeability indicative of the stuck point of
the pipe within the borehole.
In yet another aspect, the aforesaid method of
lS generating of the first log includes the step of estimating
the depth within the borehole of the stuck point, calcu-
lating the approximate weight of drill pipe above the stuck
point, and applying an upward force to the drill string
within the borehole to substantially remove compression
forces from the drill pipe in the region of the stuck point.
The step of generating the second log then includes the step
of applying a compression force to the drill string within
the borehole to increase the stress within the drill pipe
section in the region of the stuck point; The step of
7~
--8--
generating the second log may also include the step of
applying a torsional load to the drill string within the
borehole for imparting a high torsional stress to the drill
pipe section in the region of the stuck point. The method
may also include the step of separating the first and second
coils within the tool a distance from one another by a
prescribed distance on the order of six inches and exciting
the first coil at a frequency on the order of 130 Hz.
Brief Description of the Drawin~
For a more complete understanding of the present inven-
tion and for further objects and advantayes thereof,
reference may now be had to the following description taken
: in conjunction wi~h the accompanying drawing, in which:
Figure 1 is a side elevational partially cross-sectional
view of a drilling rig forming a borehole;
Figures 2A-2E are sequential, enlarged, partially side
elevational and partially longitudinal cross-sectional views
of a stuck point detection tool constructed in accordance
with principles of the present invention;
Figure 3 is block diagram of the system of the present
invention;
Figure 4 is a series of graphs of receiver coil output
voltages as a function of spacing between the coils within
the tool of FIG. 2 and the excitation frequency; and
2~
~3~
g
Figure 5A and 5B are schematic diagrams o~ one embodi-
ment of a circuit used in conjunction with the system of the
present invention.
Detailed Descri tion of the Preferred Embodiment
p
Referring first to Figure 1, there is shown a drilling
rig 11 disposed atop a borehole 12. The rig 11 includes
draw works having a crown block 13 mounted atop the rig and
a traveling block 14 which is hooked to the upper end of a
drill string 18. The drill string 18 consists of a plura-
lity of series connected sections of drilling pipe 15 which
are threaded end to end in a conventional fashion. A
drilling bit 22 is located at the lower end of the drilling
string 18 by a drill collar 19. The drilling bit 22 serves
to carve the borehole 12 through the earth formations 24~
~rilling mud 26 is pumped from a storage reservoir pit 27
near the wellhead 28 down an axial passageway through the
center of each of the drill pipes 15 comprising the drill
string 18, out of apertures in the bit 22 and back to the
surface through the annular region 16. Metal casing 29 is
shown positioned in the borehole 12 near the surface or
maintaining the integrity of the upper portion of the bore-
hole 12.
Still referring to Figure 1, the annulus 16 between the
drill stem 18 and ~he side walls 20 of the borehole 12 form
--10--
the return flow path for the drilling mud. Mud is pumped
from the storage pit ~6 near the wellhead 28 by pumping
system 30. The mud travels through a mud supply line 31
which is coupled to the central passageway extending through
the length of the drilling string 18. Drilling mud is, in
this manner, forced down through the string 18 and exits
into the barehole through apertures in the drill bit 22 for
cooling and lubricating the drill bit and carrying the for-
mation cuttings produced during the drilling operation back
to ~he surface. A fluid exhaust conduit 32 is connectedfrom the annular passageway 16 at the wellhead for con-
ductiny ~he return mud flow from the borehole 12 to the mud
pit 26.
As is also illustrated in Figure 1, a cave-in of the
walls of th~ borehole can occur around the drilling stem 18
so that the pipe section 15a is stuck in the hole as
illustrated at point S. The system of the invention func-
tions to locate this point S along the length of the bore-
hole 12 and drilling stem 18 at a measured distance from the
wellhead so that all of the free sections of drill pipe 15a~ove pipe joint l5a, which is immovably jammed in the bore-
hole 12, can be removed. Once all of the pipe above the
freepoint S is removed equipment can be brought into the
borehole 12 to unstick joint 15a and thereafter resume
~Z~ 77
the drilling operation.
It should be understood that the system of the present
invention includes a wireline tool which is lowered down
through the central bore formed in each of the sections of
drill pipe by means not shown. The necessary wireline
trucks, guide pulleys and the like are positioned over the
borehole at the well head in a conventional fashion to
operate the tool while still controlling the weight on the
bit 22 and drill string 18 by means of the crown and tra-
velling blocks 13 and 14.
Still referring to FIG. 1, a tool 10 constructed inaccordance with the principles of the present invention, is
lowered into the borehole 12 through the central passageway
in the drill string 18 by means of a wireline (not shown).
The w;reline is conventional and consists of an armored
coaxial tws conductor cable which provides both a mechanical
and electrical connection between ~he tool 10 and the wire-
line control and monitoring equipment at the surface. The
tool 10 descends down through the central aperture in the
drilling string 18 from the wellhead in order to locate the
stuck point S through ~easurable chanqes in the physical
characteristics of the pipe related thereto~
It is well known that when a ferromagnetic member such
as a drill pipe is stretched J compressed or torqued the
3~
-12-
maynetic permeability of the material changes. Further, if
a magnetic field is induced into the walls of drill pipe,
eddy currents will be qenerated in the drill pipe wall. The
pattern and strength of the eddy currents will be related to
the permeability of the matexial comprising the pipe. The
preferred way to measure the permeability related eddy
currents in a drill pipe is by using a receiving coil to
detect the electroma~netic fields produced by those eddy
currents in the pipe material. In general, the measurement
parameters are defined by the classical eddy current
eguation:
J~ o~
B = ~e s~ o3-J
The above equation defines equates magnetic flux den-
sity B, at a depth d, within the material wheno
Bo = magnetic flux density of the surface;
d = depth in centimeters;
f = frequency in Hz;
= magnetic Dermeability;
= resistivi~y in micro-ohm centimeters; and
t = time in seconds.
The amplitude variation of magnetic flux density with
depth into the material is:
A ~ Pl I T.~ ,!3 = Bo e
-13-
Phase shift with depth d is indicated by the following
equation:
Magnetic flux induced into the drill pipe by an input
signal will thus produce eddy currents which will in turn
create an electromagnetic field. This secondary magnetic
field produced by eddy current flow in the pipe may be
detected by a receiving coil. If the input signal as well
as all other variables are held constant then the signal on
the receiving coil will vary in amplitude and phase as a
function of the magnetic permeability of the pipe.
Referring still to FIG. 1, the method of the present
invention incorpora~es several steps for enhancing accuracy
and reliability of ~he critical downhole measurements. A
driller faced with a stuck pipe will utilize the system 10
by approximating the depth within the borehole 12 at which
the pipe is stuck. This may be accomplished by puliinq the
drill string upwardly hy means of the crown block and tra-
velling blocks 13 and 14 with a preselected quantity of
force. For example, 20,000 pounds upward force will produce
a measurable degree of elongation in the drill string lS.
Knowing the degree to which steel drill pipe of a known type
77
elongates under a preselected force, the operator can then
estimate the length from the surface to the stuck point S
over which the stretching of the pipe is occurring. In this
manner the approximate location of the stuck point S of the
drill pipe 15 may be es~imated within an accuracy of a few
hundred feet. The approximate length of pipe between the
well head and the downhole stuck point S permits calculation
of the weight of that pipe down ~o that depth and the amount
of upward force necessary to substantially remove the weight
of the pipe from the section l5a lodged at the stuck point.
This creates a generally zero stress condition within the
pipe section l5a in the region of the stuck point S.
When the pipe section 15a in the region of the stuck
point S is supported in a generally z~ro stress condition, a
first log of drill pipe permeability is tak~n. This log
records the permeability of the drill string along the
approximate region where the pipe is believed to be stuck.
Thereafter, the driller places a preselected degree of
stress on the pipe in the r~gion of tbe stuck point. Stress
in the drill string 18 in the region of the section 15a may
be created by either placing the pipe in a high degree of
tension through pulling on the string, by placing the pipe
in compression by releasing the weight of the drill string
onto the region or by applying torsion to the drill string
through twisting.
7~
-15-
When the drill string 18 in the region ~f section 15a
and stuck point S is in a mechanically stressed condition, a
second drill pipe permeability log is run by means of the
system 10. The stressed log is then compared to the
unstressed log of the same region. The comparison clearly
ind~cates the downhole point at which the stress on the
drill string is suddenly xelieved, tha~ is, the point below
the stuck point SO The tool 10 used in conjunction with the
system o the present invention may also incorporate means
near its lower end for mounting a string shot, chemical
cutter or the like for loosening or severing of the drill
pipe immediately above section 15a and the stuck point S so
that the drill string in the upper portion of the borehole
can be removed.
Referring now to.Figures 2A-2E, there are shown a series
of longitudinal cross sectional views of a tool 10
constructed in accordance with the principles of ~he present
invention. Referring first to F~GS. 2C-2E there is shown a
portion o the instrument housing portion of the tool 10
which comprises an outer cylindrical housing or shell 41
formed of non-magnetic material such as non-magnetic
stainless steel alloy. ~he outer housing walls are relati-
vely thick so as to protect the internal coils and el~ctro-
nics of the tool 10. The housing 41 is also constructed to
resist the shocks produced by explosive charges of the type
used to uncouple drill pipe joints within the borehole 12
once the stuck point S has been located. As shown in FIG.
2B, the upper end of the cylindrical housing 41 is coupled
to a cylindrical shaft 42 having a central aperture 43
formed therethrough. The central shaft 42 is threadedly
received into a socket 44 in the upper end of tool housing
41. As shown in FIG. 2A, the upper end of the shaft 42
includes a mechanical and electrical connec~ing socket por-
tion 45 for receiving and coupling to the lower end of acoaxial wireline ~not shown) used to lower the tool 10 into
the central aperture of the drill pipe and provide com-
munication between the tool and the re~uisite power supply
and control equipment at the surface. Adjacent the socket
portion 45 is an upper spring guide portion 46 having plura-
lity of azimuthally spaced guide slots 47 formed therein
which receive one end of a centralizing spring 48. As shown
in FIG. 2B, the other end of the centralizing spring 48 is
mounted in a lower guide slot 49 of a lower bushing 51.
There are, preferably, three centralizing springs 48 spaced
at 120 degrees around the axis of the tool. A helical
spring assembly 52 insures that the three centralizing
springs 48 center the axis of the tool 10 within the central
axis of the drill pipe aperture.
3~
-17-
Referring to the portion of the tool 10 shown in FIGS.
2A and 2B the central aperture 43 carries a coaxial conduc-
tor 50 which is electrically insulated from the sidewalls of
the central aperture 43 and carries electrical power and
signals from the central conductor of the coaxial wireline
to the instrument portion of the tool through a connector
assembly 53. The conductor 50 is connected between the
wireline and electronic circuitry within housing 54 (FIG.
2D) where DC power from the surface is delivered to the
electronics of the tool 10 and from which an AC voltage data
signal is passed back up the wireline to the surface.
As shown in FIGS. 2C and 2D the non-magnetic QUter shell
41 of the tool lO houses an exciter coil 61 comprising
multiple turns of wire wound about a core 62 formed of a
magnetic material. A receiver coil 63 is spaced a prese-
lected distance "d~ from the exciter coil 61 and also
compri~es a plurality of turns of wire wound circumferen-
tially about an insulative coil core 64. ~he two coils 61
and 63 are spaced from one another the preselected distance
by a coil spacer 65 which is affixed to the opposing flanged
ends of the respective coil cores 62 and 64. The electro-
nics positioned within the chamber 54 comprises circuitry
wnich will be described below for use in connection with
generating the excitation signal and measuring a received
~3~37~
-18-
signal in accordance with the teachings of the present
invention.
The lower portion of the tool 71 sho~n in FIG. 2~ inclu-
des means 72 for attaching tbe lower end to an explosive
charge or a chemical cutter as is required for the par-
ticular downhole condition. Additionally, the lower end 71
provides a connector 73 for coupling a signal from the wire-
line to the surface to detonate the explosive charge or to
activate the chemical cutter. This action i~ necessary to
separate the lodged drill pipe 15a in the vicinity of the
stuck points from ~he rest of the drilling string above it
so that the string can be removed. Thus t the stuck portion
of the string may be properly handled for removal or by-
pass in accordance with known tec~niques.
Referring now to the block diagram of FI~. 3 there is
shown the manner of operation of the overall system. Asillustrated~ the exciter coil 61 is driven by an oscillator
81 through a drive amplifier 82 to generate an AC variation
in magnetic flux. This flux ~ariation is used to produce
eddy currents in the wall of the drilling pipe schematically
and illustratively shown as 15. The receiver coil 63 has a
voltage induced therein by the magnetic flux in the pipe
wall to which it is exposed because of the flowing eddy
currents. The output from the receiver coil 63 is connected
-19~ 30~
through a receiver amplifier 83 to a peak detector 84 which
measures the peak-to-peak voltage of the output of the
amplifier 83. The output of the peak detector 84 is coupled
through a voltage to frequency converter 85 which produces a
series of output pulses. The frequency of the pulses from
the voltage controlled oscillator contained ~ithin the
voltage-to-frequency converter 85 is controlled by the value
of the signal from the peak detector 84. The output signal
from the converter 85 is passed back up the wireline 86 to
the surface where it is fed into a rate meter 87. A signal
indicative of the downhole frequency is generated by rate
meter 87 and logged as a function of tool position by
recorder 880 A DC power supply 89 feeds a DC voltage down
the wireline 86 to power the electronics and drive the
exciter coil Çl and receive the signal from the pick-up coil
63~ The recorder 88 may be of the convention~l strip chart
recorder type for generating logs of pipe magne~ic per-
meability as a function of position of the wireline tool
along the borehole. Thus, mechanical graphs may be produced
for comparison. Alternatively, recorder 88 may include data
storage and processing means which records, analyzes and
compares sequential logging runs to give a direct output of
variations therebetween.
In th~ method and apparatus of the present invention it
3~7
-20-
has been found that there are several significant parameters
which must be met with regard to the successful operation of
the system. For example, the frequency with which the
exciter coil 61 is driven is important for maximum sen-
sitivity and accurate measurement of magnetic permeabilitydownhole. It has also been found that the spacinq d between
the exciter and xeceiver coils, 61 and 63, is particularly
significant and is also related to the excitation frequency
at which maximum sensitivity to permeability changes in the
steel drill pipe is present in the system.
~ eferring now to Figure 4, there is shown a series of
thr~e superimposed graphs of output voltage for a constant
input as a function o frequency of excitation for each of
three different distances between the exciter and receiver
coils 61 and 63, respectively. The lower curve 91 shows
normalized received voltage values for a spacing of about 5
inches between the opposing ends of the exciting and
receiving coils 61 and 63. The peak sensitivity for this
spacing occurs at a frequency on the order of-130 Hz.
Similarly, curve 92 shows receiver coil voltage for a
spacing of about 7 inches between the coils with a similar
peak sensitivity occurring in the range of 130-150 Hz. The
upper curve 93 shows tha~ maximum receiver voltage sen-
sitivity is obtained at a spacing of about 6 inches between
-21- ~ 7
the excitation and receiver coils and at frequency on the
order of 130 Hz. Thus, it can be seen that an operating
excita~ion frequency on the order of 130 Hz and a spacing of
approximately 6 inches between the excitation and receiving
coils yields optimum results with respect to obtaining the
maximum sensitivity for the detection of a change in magne-
tic permeability of a ferromagnetic pipe as a function of
stress therein.
As was generally discussed above, the system of the pre-
sent invention, as illustrated in the circuitry shown in FIG.
3, could also be provided with a phase detector on the out-
put o~ the amplifier 83 rather than the amplitude detector
84. A phase detector would, of course, require connection
to the output of amplifier 82 as a reference phase in order
to detect the phase shift of the signal on the receiver coil
63 with respect ~o the driving signal on the exciter coil
61. Phase shift could be used to detec~ the magnetic per-
mea~ility change in a stressed pipe across the region of a
stuck point.
Referring now to Pigures 5A and 5B, there is shown a
schematic diagram of the circuitry shown in Figure 3.
Specifically, the exciter coil 61 is driven by means of an
oscillator circuit 81 which comprises a crystal oscillator
101 connected through a dividing coun~er 102. The crystal
~5
-22- ~3~37~
101 operates at a frequency on the order of 1 MHz and is
divided down through counter 102 to output leads 1~3, 104,
and 105, to an AND/OR SELECT gating circuit 106. The
AND/OR SELECT gate 106 is of a type such as a CD4019B which
provides a suitable drive for a bridge ~ype coil driver cir-
~uit 107.
Driver circui~ 107 consists of four field effect tran-
sistors (FETS) 108, 109, 110, and 111. The FETS 108 and 109
are connected in tandem while FETS 110 and 111 also woxk in
tandem. The AND/OR SELECT gate circuit 106 operates so that
FE~S 108 and 109 are turned on for a preselected perisd of
time and then off for a preselected finite period of time
prior to the turning on of FETS 110 and 111. In this
manner, the sensitive transistors 108-110 are protected from
the possibility of overloading and damage. The square-wave
switching by the FETS is converted to a smoo~h sinusoidal
excitation signal by means of induc~ance coils 112 and 113
operating ~hrough capacitors 114 and 115. The exciter coil
61 is thus driven at a preselected AC frequency by a sinu-
soidal signal.
Still referring to FIGS. 5A and 58, the receiver coil 63
is connected to the input of amplifier 83 and by means of
capaci~ors 121 coupled into a first stage amplifier 122 the
output of which is connected to a second stage of amplifica-
3~7
-23-
tion 123. The output of the second amplifier 123 is coupled
into a pair of series connected amplifiers 124 and 125 con-
nected in a peak-to-peak detector configuration. The output
of detector 84 is connected through couplinq resistor 126
into the voltage to frequency converter 85. The converter
85 comprises an integrator amplifier 127 and a comparator
amplifier 128 connected to control the frequency of opera-
tion of a pulse generator 131 through a switch 132. The
output of the voltage to frequency converter 85 is coupled
through an operational amplifier driver 133 a~d to a line
driver 134 which places a series of line voltage pulses onto
the wireline 9, fcr ~ransmission to the surface equipment.
The wireline 9 also carries, between the armor 9b and the
center core conductor 9a, a DC voltage which is coupled into
a power supply 89 comprising a first voltage regulator 141,
which drops the 30 volt input to 15 volt. A second voltage
regulator 142 is coupled to regulator 141 for producing a
lower power supply voltage of 7.5 volts suitable for driving
the operational ampliiers of the present circuitry.
As discussed above, the oscillator 81 serves to drive
the exciter coil 61 by means of the bridge driving circuit
; 107. This produce an ~C variation in magnetic flux in
exciter coil 63 at a frequency on the order of 128-130 Hz.
The signal which is induced into the receiving coil 63 is
~3~
-24-
amplified through amplifier 83 and then measured in the
peak-to-peak detector 84. The output of the peak-to-peak
detector 84 is connected to voltage-to-frequency converter
85 which produces a series of output pulses, the frequency
of which is indicative of the input voltage. The output
pulses are passed through line driver 134 and back up the
wireline 9 to the surface where they are received by the
rate meter and recorded.
In operational summary, a first wireline log of the
magnetic permeability of the steel walls of the sections of
drill pipe 15 is run in the region of the stuck point with
all the stress remo~ed from the drill string as described
above. Thereafter, ~he drill string is stressed by the
application of force to the drill pipe at the surface by
means of longitudinal tension, compression, or rotational
torque, and a second log run along the same region of pipe.
A comparison of the two logs reveals a sharp variation in
magnetic permeability value at the stuck point S due to the
differences in s~ress above an~ below the stuck point. This
Yariation in permeability shown by comparison of the two
logs precisely locates the joint of drill pipe lSA which is
stuck in the borehole. An explosive charge carried at the
lower end of the tool 10 may then be detonated immediately
from the surface while torque is applied ~o the string and
~3~
-25-
the upper por~ion of the drill string loosened at that
jointO Alternatively, the chemical cutter carried by the
lower end of the tool may also be activated to cut the drill
string section so that the upper portion th.ereof can be
S removed from the borehole.
It is ~hus believed that the operation and construction
of th~ present invention will be apparent from the foregoing
description. While the method and appara~us shown and
described has been characterized as being preferred, it will
be obvious that various changes and modifications may be
made therein without departing from ~he spirit and scope of
the invention as defined in the following claims:
