Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Disclosure
Recently acquired Dodge U.S. Patent 4,328,567
assigned to the cor~on assignee hereo~ sets forth an acoustic
well logging system. This disclosure is directed to an
improvement over the structure set forth therein, and in
particular, to an acoustic caliper tool for measuring the
diameter of a well borehole.
During the drilling of an oil or gas well, a
drill bit attached to a drill stem is advanced into the
earth. The drill bit cuts what is ideally a circular hole.
The shape of the borehole is less than ideal in ordinary
circumstances. Because of the manner in which the drill
bit penetrates various geologic,l strata, the hole may be
-precisely the diameter cut by the drill bit or may be
lS somewhat larger. The hole may be irregular in shape. The
size and shape of the hole varies widely dependent on a
number of factors. Whatever the case, it is highly
desirable to measure the diameter of the borehole.
Mechanical caliper tools are readily available. They
depend typically on protruding fingers which touch or
contact the wall of the borehole. This apparatus does not
have protruding fingers which run the risk of breakage.
Moreover, this tool is quite ac~urate, capable of more
accuracy than mechanical caliper tools. This tool is not
required to move along the hole in jumps; rather, the tool
moves smoothly in continuous motion.
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The device of this tool is particularly able to
be run in open hole, that is a well borehole filled with
some type of fluid. The borehole may be filled with
drilling mud, water or oil, or a mixture. It may also be
run in a dry borehole with no fluid.
The acoustic caliper tool of this disclosure is
particularly immune to randomly occurring noise in the
environment~ As the tool is pulled through the borehole,
the tool or the cable on which it is suspended may bang
against the borehole wall and create incoherent, randomly
occurring noise. There are other sources of randomly
occurring noise which may be in the vicinity, and all of
this noise impinges on the tool, obscuring the quality of
data obtained from operation of the device. The tool is
also imune to coherent noise sources such as motor
vibrations, etc.
In general, the use of a caliper tool is
important at different stages in drilling. For instance, a
caliper tool should be used to measure the diameter oE a
borehole in advance of cementing casing in the borehole.
It is also important to caliper a hole to determine if
there are lateral voids encountered by ~he borehole. Such
lateral voids may give rise to substantial loss of drilling
mud or cement. It is also important to caliper a hole to
assure that there is sufficient spacing between casing in
advance of placing the casing in the hole to determine
whether or not the spacing will permit proper injection of
cement behind the casing. This type of caliper can be used
to measure the hole diameter during drilling since it is
insensitive to both incoherent and coherent noises.
This apparatus discloses a linearly swept
frequency acoustic signal. The frequency is swept in
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linear fashion between two frequencies. It may sweep from
a lower frequency limit upwards or from a higher frequency
downward. For instance, it may be swept from lOO khz to
one mhz. The sweep rate is specified, for instance a sweep
rate of four k!hz per millisecond. This scale factor is
applied to the linear sweep freguency oscillator. The
signal which i$ transmitted is also applied to a mixer. A
second input to the mixer is the receiver signal. The
receiver signal has an acoustic wave travel time delay compared
with the transmitted signal, the two signals being mixed
and the output is a difference freq~ency signal. The mixer
has other output frequencies, as will be discussed, but
they are filtered out so that only the difference signal is
observed. The difference signal frequency is proportional
to acoustic wave travel time, this referring to the elapsed
time of signal travel between transmission and return of
the reflected acoustic wave front. The elapsed time is
proportional to distance. Accordingly~ the difference
freq~ency signal measures the distance. A constant of
`2~ proportionality applied to the difference frequency
provides a representation of the distance of interest.
This apparatus particularly utilizes a pair of
diametrically positioned transducer transmitters and
receivers facing opposite directions along the same
diameter. The sum of three values represents the borehole
diameter. The first and third values are the me~sured
distances between the transducer pairs and the facing wall;
the other measure i5 a fixed value which approximately
represents the diameter of the acoustic caliper tool and
particularly the distance between the two transmitters.
These three values are added, multiplied by a constant of
~2~
proportionality, and the value so obtained is the diameter
of the borehole.
The equipment is particularly adapted to make
multiple measurements. Several sets of transducers can be
spaced around the tool so that difference measurements
radially of -the tool can be obtained.
Brief Description of the Drawings
So that the manner in which the above recited
features, advantages and objects of the present invention
are attained and can be understood in detail, more
particular description of the invention, briefly summarized
above, may be had by reference to the embodiments thereof
which are illustrated in the appended drawings.
It is to be noted, however, that the appended
drawings illustrate only typical embodiments of this
invention and are threfore not to be considered limiting of
its scope, for the invention may admit to other equally
effective embodiments.
Fig. 1 shows an acoustic caliper tool in
accordance with the teachings of this disclosure in a well
borehole measuring diameter of the borehole;
Fig. 2 is a section~l view taken along the line
2-2 of Fig. 1 of the drawings; and
Fig. 3 is a view taken along the line 3-3 of Fig.
2 showing details of construction oE a transducer pair
including acoustic transmitter and receiver.
Detailed Description of the Preferred Embodiment
The present apparatus is directed to an acoustic
transducer system for use in a borehole. This apparatus
utilizes a linearly swept frequency oscillator coupled with
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a transmitting transducer. A receiving transducer is
spaced immediately adjacent to the transmitting transducer.
Acoustic energy is transmitted, and a reflected wave front
is received after traversing the space between the acoustic
caliper tool and the wall of the borehole immediately
adjacent to the toolO Acoustic energy is propagated
radially outwardly from a transducer. While there are
other modes of propagation, the primary or compression wave
transmitted carries the energy through the medium. The
medium which is immediately adjacent to the tool is well
fluids in the well bore and includes drilling mud.
Alternatively, the Nell fluid may also include water or oil
from the adjacent formation. Alternately it may be air or
gas and no fluid. The compression wave front which is
transmitted impinges on the facing wall of the borehole.
In typical circumstances, this tool can be used in an open
hole, that is, a well which has not been cased or lined
with casing. In theory, the borehole may be a smooth
walled cylindrical passage drilled by the rotary bit. In
practice, this is not ordinarily the case, and indeed, the
borehole is ususally non-round and irregular in shape.
Depending on the geological strata penetrated by the
borehole, the formation may break into varied Eashion, the
pieces shattering and thereby leaving an oversized non-
round borehole. It is important to measure the diameter ofthe borehole as a preliminary to conducting subsequent well
completion steps, and this apparatus is an acoustic device
which enables such measurements to be made.
It has been discovered that the velocity of
propagation of acoustic wave fronts in a borehole filled
with drilling mud, water, air or oil or any mix thereof is
reasonably constant for a selected set of circumstances.
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Moreover, the interface at the wall of the borehole
reflects the wave front of acoustic energy transmitted in
the borehole. The wave front is reflected by the borehole
wall and returned to the receiving transducer, all with a
view of determining the time interval of transmission and
hence the distance of the propagation path.
The procedure set forth below utilizes a linearly
swept frequency oscillator. The tool forms two output
signals, one being a difference signal having a frequency
which is the difference between the instantaneous
transmitted and received signals. Output amplitude is of
importance only insofar as it is large enough to be
observed. Since the data of interest is encoded in a
difference signal having a duration of a few milliseconds,
the system is relatively immune to white noise or other
incoherent noise sources. This method of coherent
detection is very effective in its ability to discriminate
against both incoherent noise and even coherent noise, so
long as the coherent noise does not change frequency at the
same rate as the transmitter. For instance, the cable and
the acoustic tool which is supported thereby may bang
against the borehole creating shock acoustic energy. The
shock may have a relatively high amplitude, but even in
that case, it poses no problems because this type of noise
does not obscure the data obtained from the acoustic
caliper tool of this disclosure. Such noise is fairly well
screened by the detection procedure set forth below and the
output is therefore relatively immune or non-sensitive to
background noise.
Considering first of all the structure shown in
Fig. 1 of the drawings, the acoustic caliper tool 10 is
suspended in a well borehole 11 which penetrates several
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earth formations 13 and in the case discussed herein is
filled with well fl~id 12 The well fluid is either
drilling mud, oil or water or a mix of these. The acoustic
caliper tool 10 is enclosed in a sonde 14 which is lowered
on a well logging cable 15. The well logging cable 15
passes over a sheave 16 that directs the cable to a spool
(not shown) where the cable is stored and paid o~t over the
sheave to enable the acoustic logging tool 10 to be lowered
to great depths in the well borehole.
The equipment located in the sonde 14 includes a
transmitter 17. It is a linear amplifier capable of
driving an acoustic transducer with a constant voltage over
a broad frequency range. It is adapted to transmit in a
frequency range up to about 2 mhz sweeping across a
specified range. The transmitter 17 faithfully amplifies
the output of a sweep frequency oscillator 18. The sweep
frequency oscillator 18 is driven between frequency limits
in a specified manner. A small portion of the transmitter
output is applied to a mixer circuit 19. The mixer circuit
is provided with two inputs, one being an attenuated
version of the transmitted signal, and the other signal
being the received signal from the receiver 20. The
transmitter circuit forms a swept frequency output signal.
The received signal changes frequency at the same rate as
the transmitted signal, but its instantaneous frequency and
phase are different from the transmitted signal. The
mixer 19 o-ltput lS the product of the instantaneous
transmitted and received signals. That product includes
sum and difference frequencies of the instantaneous
transmitted and received signals. One of the frequencies
is the sum of the two inputs. The sum frequency signal is
of no interest; it is rejected at the output of the mixer
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circuit 19 by a band pass filter 21. The band pass filter
rejects such high frequency signal inputs; lower
frequencies are passed, and it is therefore selected to
pass frequencies representing the frequency difference
between the transmitted and received signals. In other
words, the band pass filter 21 has a relatively low pass
band. A telemetry circuit 22 is provided with inputs from
the oscillator 18 and the output filter 21.
The transmitter 17 is connected to a multiplex
circuit 24~ In turn, that is connected to transmitting
transducers 25 at spaced locations. There are several
transmitting transducers 25 spaced around the sonde 14.
The receiver 20 has inputs from a multiplexer 26. The
multiplexer 26 is provided with input from several
receiving transducers 27. They are also spaced around the
sonde. More will be noted regarding ~he number and
location of the transmitting and receiving transducers.
Attention is next directed to Fig. 2 of the
drawings. Fig. 2 is a sectional view taken through the
sonde 14 and shows several sets of transmit~ing and
receiving transducers. They are better shown in Fig. 3
wherein the transmitting transducer 25 is a relatively
small circular transducer surrounded by a receiving
transducer 27. Alternately, the outer ring transducer may
be used as a transmitter and the inner small circular
transducer as the receiver. This concentric construction
enables the two transducers to face along a common radial
line to the tool, the tool being circular in cross-section.
Fig. 3 is the preferred arrangement so that lateral offset
is held to a minimum between the transmitter and receiver
transducer pair. The schematic of Fig. 1 shows them in
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near proximity, but the preferred embodiment is the
arrangement shown in Fig. 3.
Typically, a magnetostrictive or piezoelectric
transducer is acceptable. Various types of transducers
will suffice in ordinary circumstances. It is desirable
that the transmitting transâucers radiate the same amount
of acoustic energy for the same voltage input over the
range of frequencies swept by the swept frequency
oscillator 18. Similarly, it is desira~le that the
receiving transducers give the same voltage output for a
given pressure difference input over the range swept by the
swept frequency oscillator. Thus, low Q transducers
mounted in damped configurations with resonances out of the
operation range improve the system operation.
The arrangement shown in Fig. 3 is thus
duplicated at multiple locations around the tool as shown
in Fig. 2. Here, there are four sets of transn~itter and
receiver transducers. At one location identified by the
numeral 40, there is a first set. There is a similar set
located at 41. Similar sets are also included at 42 and
43. The several similar sets function in like manner, and
they are triggered i~ operation by the multiplexer which
is connected to them. The first set of transducers is
radially spaced inwardly from the borehole wall by a
distance dl. The other transducers are spaced from the
borehole wall by distances which differ depending on the
shape of the borehole and the position of the sonde in the
borehole. Fig. 2 shows an irregular borehole~ This is
more probable; in fact, the manner in which the formation
breaks, fractures or otherwise yields to the drill bit
varies widely and forms an irregular drilled hole of the
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fashion shown in Fig. 2. This shape is merely
representative, and the shape does vary widely.
The four transducer sets shown in Fig. 2 measure
four different distances radially outwardly to the facing
wall. The four sets of transducers cooperate in the
fashion described to measure the distances along the radial
directions from the transducers in the radial direction as
illustrated. The tool has a diameter D which is a fixed
dimension. The acoustic caliper tool 10 measures the
diameter of the borehole. This measurement is indicated by
the symbol BHD and is given an Equation (1):
(1) BHD - dl + D ~ d3 ,
As can be seen, one of the three values on the
right hand side of the equation is a fixed value
representing the distance between transducers 1 and 3.
Assume that the swept frequency oscillator 18 is swept
between 200 khz and 1200 khz during an interval of 20
milliseconds. In this instance, the sweep rate is 50 khz
per millisecond. If the well fluid is assumed to be water
and has a primary wave velocity of 5~000 f-t./second or
60,000 inches/second, then the difference signal is
approximately 830 hertz per inch. Thus, a difference
signal of 830 hertz would imply a spacing of ~ inch between
reflector and transducers (one inch total wave travel
length). Assuming that the sweep does last 20
milliseconds, the center of the recorded frequency
difference spectrum is 830 hertz per inch. Since the
difference frequency is only present Eor at most 20
milliseconds its spectrum is a band of frequencies (an
envelope) centered around the desired difference frequency,
the band having components at most 50 hertz each side of
the desired difference frequency. The percentage error
658~
decreases as the difference frequency and ~he measured
distance increases.
As the distance increases, the difference
frequency is greater. Equation (2~ may be used to relate
borehole diameter to the measured difference frequencies.
(2) BHD = K( fl ~ FD + f3)
In the foregoing equation, the borehole diameter BHD is
thus dependent on two variables and a constant descriptive
oE the diametrical distance between the two transmitters.
The other two values are measured, and they are the
measurements of frequency difference obtained from
diametrically opposite transducer pairs. This refers to
the opposite pairs 40 and 42 in Fig. 2; an alternate is
transducer pairs 41 and 43.
The symbol FD is the frequency equivalent of the
diameter of the caliper tool or more specifically the
diametrical distance between transducer sets on the tool
body. The tool can be equated to a fixed frequency
difference. There is no need to measure the diameter.
Rather, a fixed value can be calculated for the diameter in
equivalent frequency difference and this value can be
easily known. Equation (2) thus requires the measurement
and summation of three frequencies, one being fixed and the
other two being variable. The constant given in Equation
(2) is readily obtained for a given velocity of wave
propagation in a medium. The constant K is exactly equal
to the acoustic velocity of propagation in the well bore
medium divided by the transmitter sweep rate. Accordin~ly,
the constant K is stored for selected well fluids and
conditions/ and the use of this constant therefore readily
follows in the reduction of data from frequency difference
measurements so that Equation (2) can be evaluated.
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In some instances, it may be desirable to measure
the average borehole diameter. In this instance, the
acoustic caliper tool 10 is equipped with N acoustic
transducer pairs where N is an integer. Equation (3) is
thereEore similar to Equation (2) except that the data from
all the transducer pairs is used, and such data is
thereafter used to obtain an average of the diameter of the
borehole. As will be understood, the average has more
significance by increasing N to some larger number. In any
event, the borehole diameter BHD on an average basis is
yielded by Equation (3~:
(3) BHD = K FD ~ fl . . . + fN
,
where N = an integer
fN = measured frequency di~ference at
transducer set N
FD = frequency equivalent of tool
K = a constant equal to the acoustic
velocity of propagation in the
well bore medium divided by the
transmitter sweep rate
It will be recalled that the received signal and
a small fraction of the transmitted signal are both input
to the mixer. The received signal is typically ~ few
orders of magnitude smaller than the transmitted signal.
The transmitted signal is partially attenuated, and the
received signal is amplified, at least to some extent, in
the receiver to thereby provide two signals input to ~he
mixer. The mixer is operated in the customary manner to
mix or multiply the two signals which are input to it.
Alternately,this product can be regarded as the sum of two
~;~6~65~2
signals of different frequencies. One signal has a
frequency which is the difference between the two input
signals; the other signal has a frequency which is the sum
of the two input signal frequencies. The difference
frequency is small if the wall of the borehole is quite
close to the pair of transducers. On the other hand, even
a widely spaced wall still provides a relatively small
difference frequency signal when compared to the sum
frequency signal from the mixer circuit. Through the use
l~ of a band pass filter set at a relatively low pass band,
the mixer forms an output of the frequency difference of
the two input signals~ and that in turn is proportional to
spacing of the borehole wall which reflects signals back
toward the transducers. Accordingly, the maximum measure
of the device can be adjusted by setting the maximum
frequency permitted through the filter.
The present apparatus can be operated to obtain
multiple readings at a given elevation in a well. If the
sonde 14 is stopped, and data is obtained over a 20
millisecond sweep, and this is followed by an interval in
which the next transducer is operated, each transducer can
be operated to obtain several data points. For instance,
during a one second interval,a 20 millisecond sweep can be
formed 50 times per second. With four separate
transducers, this enables the equipment to obtain twelve
readings of the distances from each of the four sets of
transducers shown in Fig. 2. The frequency difference
signals are furnished to the surface through the logging
cable 15 as transmitted by the telemetry equipment. At the
surface, a power supply system 28 is utilized to furnish
power for operation of the equipment in the sonde. A
signal detector 29 is provided with the output signal from
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the telemetry circuit 22~ That is connected to a diameter
computer 30 which functions in the fashion set forth for
Equations (2) or (3). This provides a measure of diameter
which is output to the recorder 31 and it is recorded on a
suitable medium. The data is recorded in correlation with
depth of the sonde 14 in the well bore, the sheave 16 being
connected by a depth indicator 32 of either a mechanical or
electronic construction which inputs the depth o-f the sonde
14 in the borehole 11. The depth is input to the recorder
31.
Several advantages of the device will be noted.
First of all, it does not have to be centralized in the
well borehole. If the caliper lO hangs to one side, one of
the two measurements will be relatively small but the other
will be relatively large. Further, it can be used in
practically any fluid or in the absense of fluid. The rate
of propagation of acoustic energy in well Eluids including
drilling mud, water and oil is well known. Further, the
device is reasonably immume to white noise, shock impulses,
acoustic noise and the like. This is true whether the
noise is coherent or not~ Through the use of conventional
amplifier stages in the receiver and with the use of
suitable attenuators to reduce the amplitude of the
transmitted pulse applied to the mixer, the two signals can
be fairly well controlled in amplitude; amplitude
variations are meaningless as long as there is sufficient
amplitude to derive a difference frequency output from the
mixer. Moreover, the device can also be used ~ith sets of
diametrically opposed pairs of transducers so that
different radial measures relative to the tool can be
obtained in the well bore. ~n situations where knowledge
of an average borehole diameter is desirable statistically,
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this improves the quality of the data. For instance,
Equation (3~ specifies that up to N sets of transducers can
be used. If N is increased to about 10 or more, the number
of measurements yield an average diameter; or the data can
be broken out utilizinq opposing pairs so that difference
measurements of the diameter along the different diametric
lines can be obtained. As will be observed in Equations
(1)-(3), the tool diameter is a fixed value which can be
converted into frequency difference.
While the foregoing is directed to the preferred
embodiment, the scope is determined by the claims which
follow~
