CA1206582A - Acoustic caliper tool - Google Patents

Abstract

ACOUSTIC CALIPER TOOL

Abstract of the Disclosure An acoustic caliper tool is set forth in this disclosure. In the preferred and illustrated embodiment, acoustic transducers propagating highly directional acoustic waves transmit signals which are reflected at the interface of the borehole. An acoustic return signal is received. An acoustic swept frequency having a specific linear rate of sweep is utilized. The transmitted signal is mixed with the received signal; the frequency difference between the two is proportional to the wave transit time between transmitter and receiver. Distance of wave propagation is inferred by the frequency difference.
Utilizing transducer transmitter and receiver pairs at locations around the caliper tool, the hole diameter can then be measured. Diametrically spaced pairs measure radial segments of distance to the borehole, and the transit times are converted into distance. The diameter of the tool is a fixed value. The measures are summed thereby yielding the diameter of the borehole.

Description

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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 :~2~65~3~

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 i58~

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.

~ZQ~B~2 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 ~2C~6S~;~

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 ,~ ~!,, i ' 5~

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 ~ 65~
--9_ 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 ~2~5~2 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, lZ~5~2 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~

Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An acoustic caliper tool for use in wireline well logging measurements of the diameter of a well borehole, comprising:
a fluid tight hollow body member sized and adapted for passage through a well borehole and housing therein, first, second, third and fourth acoustic energy transmitters, said transmitters being disposed in different respective circumferential quadrants of said body member and being acoustically coupled from said body member to the well bore fluid, first, second, third and fourth acoustic receiver means for receiving reflected acoustic energy, each of said receiver means being associated with one of said transmitter means and located in said body member immediately adjacent to its associated transmitter, thereby providing diametrically opposed transducer pairs in different respective circum-ferential quadrants of said body, means for energizing said transmitters with a continuous swept frequency modulated signal for causing said transmitters to emit acoustic energy at a frequency which is varied as a preselected known function of time, means for mixing said transmitted swept frequency signal with received signals from each of said transducer pairs to produce first, second, third and fourth difference signals whose frequency is related by said known function to the distance said reflected acoustic signals have traveled in the borehole fluid, and means for combining said first, second, third and fourth difference signals to provide an output signal indicative of borehole diameter.

2. The apparatus of claim 1 wherein said known function comprises a linear function of time.

3. The apparatus of claim 1 wherein said means for combining includes means for applying a constant proportional to the tool diameter to each of two diametrically opposed pairs of difference signals in providing said output signal.

4. The apparatus of claim 1 and further including means for moving said tool through a well borehole and recording said output signals as a function of borehole depth.

5. The apparatus of claim 1 wherein said transmitting and receiving means comprise piezoelectric transducers.

6. The apparatus of claim 1 wherein said transmitting and receiving means comprise magnetostrictive transducers.