Note: Descriptions are shown in the official language in which they were submitted.
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SHEAR WAVE ACOUSTIC LOGGING SYSTEM
.
This invention relates to a shear wave acoustic well logging
system.
It is a well known to survey wells by acoustic loggirg
techniques. One known acoustic logging technique involves the
generation and reception of acoustic pulses and the determination of
the travel time of the pulse signals between a transmitter and
receiver or between spaced receivers. By this technique the velocity
of sound through a subterranean formation may be determined in order
to characterize the formation. Another acoustic logging technique
involves amplitude logging in which the loss of amplitude of an
acoustic signal as it travels between a transmitter and receiver, or
between spaced receivers, is measured. Velocity and amplitude logging
may be carried out separately or in combination, that is the logging
tool may be equipped with appropriate circuitry to detect both the
travel time of the acoustic signal and the loss in amplitude.
An acoustic signal may be transmitted through a subterranean
formation in both compressional and shear (transverse~ modes. Since a
shear wave cannot be transmitted along the borehole through liquid
therein9 it has been proposed to transmit and receive shear waves by
transducers positioned in contact with the borehole wall. For
example, U.S. Patent No. 3,949,352 discloses a shear wave acoustic
logging system employing transmitting and receiving transducers spaced
in close proximity with one another and loca~ed within a transducer
mounting pad which is pressed against the wall o~ the borehole. While
~his procedure requires a direct coupling of the transmitting and
receiving transducers to the borehole wall, shear wave logging
employing so-called "indirect" excitation of the borehole wall is
disclosed in Kitsunezakip "A New Method for Shear WavP Logging", OYO
Technical Note9 Urawa Research Institute, October7 1~7~. In this
prccedure an electromagnetic transducer is '1suspended" in water (the
borehole fluid) and employed to generate an asymlnetric shear wave
pulse through the borehole fluid into the formation. The transduc~er
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is oriented such that an excitation bobbin vibrates along an axis
normal to the axis of the well. The resulting shear wave is detected
at a plurality of receivers spaced longitudinally from the
transmitter. The receivers take the form of geophone type detectors
in a detector body suspended in the borehole fluid and having an
apparent density adjusted to be the same as the density of the
borehole fluid.
In one aspect, the present invention resides in an acoustic
well logging system; comprising:
(a) an elongated logging tool adapted for insertion into a
borehole,
(b) means forming a compartment in said tool containing a
coupling liquid therein,
(c) an acoustic transmitter mounted in said compartment and
comprising a bender~type transducer having opposed unrestricted
piezoelectric planar sur~aces oriented along the longitudinal axis of
said tool and exposed to said coupling liquid~ the dimensions of the
transducer in the direction parallel with the longitudinal axis of the
tool exceeding the dimensions of the transducer in the transverse
direction,
~ d) means for exciting said transmitter to simultaneously
flex said opposed piezoelectric surfaces in a conforming manner and
generate a positive pressure wave in one direction while
simultaneously generating a negative pressure wave in the opposite
direction, and
(e) an acoustic receiver mounted in said tool and spaced
longitudinally from said transmitter.
Preferably the receiver is also a bender-type transducer
mounted in a liquid ~illed compartment and having opposed unrestricted
planar sur~aces exposed to the liquid within the compartment and
oriented in substantially the sa~e direction as the transmitter.
In ~he accompanying drawings,-
Figure 1 is a schematic illustration of an acoustic loggingsystem according to one example oF the present invention,
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Figure 2 is an illustration partly in section showing details
of a portion of the tool shown in Figure 1, and
Eigure 3 is a sectional view taken along line 3-3 of Figure 2.
Referring to Figure 1, the logging system includes an
elongated logging tool 10 which is suspended from a cable 11 within a
borehole 12 which traverses a subterranean formation of interest
indicated by reference character 14. Formation 14 may be a suspected
oil or gas bearing formation which is to be characterized in regard to
its porosity, fluid saturation, or such other information as may be
desired. The well 12 is filled with a liquid such as drilling mud
indicated by reference numeral 16. The logging tool 10 comprises an
acoustic transmitter 17 and acoustic receivers 19 and 20. Transmitter
17 and preferably also receivers 19 and 20 take the form o~
ben~er-type transducers as described in greater detail hereinafter.
Signals from the logging tool 10 are transmitted uphole by
the conductors in cable 11 to any suitable utilization system at the
surface. For examplet th~ utilization system is illustrated as
comprising an uphole analysis and control circuit 22 and recorder 24.
A depth indicator produces a depth signal which is applied to the
recorder 24 in order that the output from circuit 22 may be correlated
with depth.
The logging system may be operated in a manner to measure one
or more parameters ascertainable with acoustic well logging systemsr
For example, the system may be operated in a velocity and/or amplitude
logging mode as described previously. The transmitter and receivers
are controlled through suitable timing circuitry located either
uphole, or in the logging tool itsel~. Typically, the control
circuitry will comprise a time base generator which operates to
produce pulses to excite transmitter 17 and which gates receivers 19
and 2Q. The elec~rical pulses produced by the time base generator are
preferably voltage spikès, that is, voltage pulses o~ relatively high
amplitude and short duration whose frequency spectrum is extremely
broad. Energizing acoustic transmmitter 17 by voltage spikes cause
the piezoelectric crystals (see Figure 2) to resonate at a frequency
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determined by the dimensions of the crystal as will be explained
infra. For example, the time base generator may generate a train of
triggering pulses which produce a pulse repetition rate from the
transmitter 17 of 15 acoustic pulses per second. Receivers 19 and 20
may be gated alternatively in order to prevent cross feed within the
cable 11 as will be readily recognized by those skilled in the arts.
For example, receiver 19 may be gated on during an interval of from
0.5 to 30 milliseconds subsequent to a first acoustic pulse from
transmitter 17. Receiver 19 is then gated off and after the next
succeeding pulse from transmitter 17, receiver 20 gated on. For
example, receiver 20 may be gated on during a similar interval from
0.5 to 30 milliseconds subsequent to the transmitter output pulse.
The logging tool may be moved through the well at any suitble rate
while it is operated to generate and receive the acoustic pulses.
Typically the tool will be lowered to the bottom of the interval to be
logged and then pulled upwardly during the logging measurements at a
speed of at least 20 feet per minute. Somewhat greater logging
speeds9 e.g. 60 feet per minute, normally can be usedO
At the surfaceJ the uphole circuitry operates on the sisnals
from receive~s 19 and 20 to produce signals representative of the
travel time between receivers 19 and 20 and the difference in
amplitude between the acoustic signals detected by receivers 19 and
20. The circuitry employed for determining the time interval between
the acoustic signal arrival at receivers at 19 and 20 may be of any
suitable type. For example~ the pulses emplnyed to trigger the
transmitter may also be applied to a ramp function generator to
initiate a signal which increases monotonically with time. For
example, the ramp function generator may respond to a triggering pulse
to generate a voltage which increases linearly with time. Thus, tne
amplitude o~ the voltage is directly proportional to the time
~ollowing generation o~ the acoustic signal by transmitter 17. The
output from the ramp ~unction generator is applied through gates
controlled by the outputs ~rom receivers 19 and 20 to respective
voltage storage means. Thus, when an acoustic signal is received at
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receiver 19, the resulting transducer voltage is applied to open one
gate to pass the voltage from the ramp function generator to a first
storage means. When the next signal is received by receiver 20, the
transducer signal is applied to open another gate to pass the output
from the ramp function generator to a second storage means. The two
voltage signals are then applied to a difference circuit, the output
o-F which is recorded in correlation with depth to provide a travel
time log. The amplitude parameter may similarly be determined through
the use of any suitable circuitry. For example~ the peak voltage
outputs from receivers 19 and 20 may be applied to a difference
circuit ~hich produces a voltage which is representative of the
difference in the maximum amplitudes of the acoustic signals received
by receivers 19 and 20. The output from this difference circuit is
then recorded to provide a log of attenuation within the formation.
Such analysis and control circuitry is well known to those skilled in
the art and for a further description thereof reference is made to
U.S. Patent No. 3,191,145. Also, while two receivers are shown it
will also be recognized that the logging tool may be equipped with
only one receiver in which case a measured parameter may be the travel
time bet~een transmitter 17 and the receiver. Preferably however, t~o
receivers as shown will be employed in order to avoid distortion of
the measured values due to borehole effects such as changes in the
borehole diameter. Typically, the first receiver 19 is spaced 5 to 15
feet from the transmitter with a spacing between adjacent receivers 19
to 20 of 2 to 5 feet.
As noted previously, shear wave acoustic pulses are produced
by means of a bender type transducer. Bender-type transducers are in
themselves well known and take the form ~f an ele~Ent which responds
to an applied electrical field such that its opposed surfaces bend in
the same direction in a conforming mannerO Thus, the transducer acts
as a point source for an acoustic shear wave signal which may be
characterized as comprising a positive pressure wave generated in one
direction from one surface and a simultaneous negative pressure wave
generated in the opposite direction from the other surface. For
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example, as described by Sheridan, C.A., et al, "Bender Bar
Transducers For Low-Frequency Underwater Sound Sources", presented at
the 97th Meeting of the Acoustical Society of America, Cambridge,
Massachusetts, June 15, 1979, ~loneywell Defense Electronics Division,
Seattle, Washington, August 20, 1979, a suitable bender-type
transducer may take the form of piezoelectric elements bonded together
in a manner such that one side o~ the transducer is driven in
extension while the other side is driven in contraction or is not
driven. The result is that both sides of the transducer then bend in
a conforming manner in response to an applied voltage. A suitable
bender-type transducer for use in the present invention is
commercially available and is comprised of two piezoelectric discs
which are bonded together and encased in a plastic t'potting"
compound. The two ceramic discs are reversed in polarity such that
one element responds to an applied voltage to expand while the other
contracts. The result is that the element ~lexes in response to each
voltage pulse such that one surface is concave and the other is
convex. The frequency of the acoustic signal produced by this
transducer ranges from about 1 to 6 KHz with a predominant ~requency
of about 3KHz.
The bender-tyhpe transducer is mounted such that the opposed
flexing surfaces are unrestricted and both are acoustically coupled to
the liquid within the wellbore.
Figures 2 and 3 illustrate an enlarged view of the
transmitter assembly 17 illustrating in detail the bender-type
transducer and the manner in which it is supported within the logging
tool. As shown in Figure 2, upper and lower panels 36 and 37,
respectively, de~ine a transducer compartment within the logging
tool. Extending between the panels 36 and 37 is a transducer mounting
bracket 40 which has an aperture therein which receives the
bender-type transducer 42. The transducer compartment is provided
with a peripheral window 43 which is relatively transparent to
acoustic energy. The window 43 may be formed of any suitable m~terial
which has an acoustic impedance close to that of the well liquid in
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order to minimize reflections from the window. The transducer
compartment is filled with a suitable coupling liguid as indicated by
reference numeral 45 in Figure 3 which also has an acoustic impedance
close to that of the liquid within the wellbore. For example, the
window 43 may be formed of neoprene rubber and the coupling liquid 45
within the compartment may be a light motor oil.
The mounting of the bender-type transducer 42 is shown in
greater detail in Figure 3. As shown in Figure 3, the transducer 42
comprises generally rectangular piezoelectric elements 42A and L2B
which are bonded together and encased in a plastic potting compound
42C. In accordance with the preferred embodiment, the length, L9 of
the piezoelectric planar surface or dimension parallel to the center
axis o~ the logginy tool is approximately three times greater than its
width, W, or dimension perpendicular to the center axis of the logging
tool. The transducer 42 is mounted in support 40 by means of a rubber
mounting ring 48 in order to reduce the transmission of acoustic
energy directly from the transducer to the structural components of
the well logging tool. Electrical leads 50 and 52 are bonded to the
outer surfaces af elements 42A and 428, respectively. Leads 50 and 52`
extend through panel 36 to a suitable source ~or a voltage pulse such
as a capacitor and inductor circuit which is periodially charged and
then discharged in response to a suitable triggering pulse as
described above.
The use of piezoelectric elements 42A, 42B ~or the
transmitter transducer 4~ which are elongated in the direction of the
axis of the tool enables lower resonant ~requencies to be generated
than is possible with circular piezoelectric elements. Thus
gen0ration of a resonant ~requency of lKHz would require circular
piezoelectric elements having a diameter o~ approximately six inches9
which is too large to be mounted within a logging tool for use in a
well borehole having a radial distance o~ approximately three inches.
The bender-type transducer described in the pre~erred
embodiment acts as a single point acoustic source and ideally produces
a shear wave displacement and radiation pattern of the type disclosed
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in the aforementioned article by Kitsunezaki. The shear wave
amplitude is at a maximum in the plane of the operating faces of the
bender-type transducer and falls off as a cosine function until it
reaches a minimum after displacement through an angle of 90. The
compressional wave pattern is 90 out of phase with respect to the
shear wave. Thus the compressional wave amplitude is at its maximum
along an axis normal to the operating surfaces of the bender-type
transducer.
The receiving transducer or transducers may, in accordance
with the broadest aspect of the invention, be of any suitable type.
For example, they may take the form of a suspension-type geophone
detector of the type disclosed in the paper by Kitsunezaki. It is
preferred, however, to employ a bender-type transducer of the same
shape and configuration as the transducer 42 for the reception of the
acoustic signal and to orient the receiving transducer such that its
opposed operating surfaces are oriented in substantially the same
direction as the transmitting transducer. Stated otherwise9 the
operating surfaces o~ the transmitting and receiving transducers are
located in substantially parallel planes~ Since the displacement
pattern of the shear wave is a cosine function, some deviation from
this standard can be tolerated and still provide a signal response
well over 90% of the maximum shear wave amplitude. ~here two
receivlng transducers are employed, both of the receivers should be
oriented in the same direction, particularly ~here the system is
operated in an amplitude logging mode, i~e. where attenuation of the
signal bet~een the receivers is arrived at by comparing the amplitu~es
o~ the signals received at the receivers.
