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Description  |
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FIELD OF THE INVENTION
This invention relates to an apparatus and method for detecting borehole
wall discontinuities using an acoustic pulse echo technique. More
particularly, this invention relates to an apparatus and method for
detecting vertically oriented borehole discontinuities such as fractures.
BACKGROUND OF THE INVENTION
Techniques for the acoustic detection of fractures have been described in
the art. These techniques involve the generation of an acoustic wave in
the earth formation surrounding the borehole and detecting the degree of
attenuation of an acoustic wave as it is strongly influenced by fractures
in the path of the acoustic wave. Typically, the shear wave is recognized
as not being transmitted through an open or fluid filled fracture. Hence,
any crack or fissure in the earth formation in the path of a shear wave
will strongly attenuate it. Known techniques for fracture detection thus
involve transmitting an acoustic pulse into the formation and detecting
the acoustic attenuation of the received waveform portion where the shear
wave ought to be. A strong attenuation indicates the presence of a
fracture and the orientation of the acoustic transmitter-receiver system
relative to the borehole indicates the orientation of the fracture. Prior
art patents which describe such transmissive attenuation type fracture
detection techniques are the U.S. Pat. No. 2,943,694 to Goodman; U.S. Pat.
No. 3,406,776 to Henry; U.S. Pat. No. 3,474,878 to Loren; U.S. Pat. No.
3,775,739 to Vogel and U.S. Pat. No. 3,794,976 to Mickler.
These prior art detection techniques involve reliance upon the transmissive
influence by fractures, whose presence are deduced either from the absence
of a shear signal or its very small amplitude when in view of the
knowledge of the lithology of the formation, greater shear amplitudes
would be expected. Since the receiver waveform from such fracture
detection does not provide a positive indication of a signal
representative of a fracture, its detection is more difficult.
Acoustic pulse-echo techniques have been described in the art to
investigate boreholes; see, for example, U.S. Pat. No. 3,883,841 to Norel
et al and U.S. Pat. No. 4,255,798 to Havira. These latter techniques
involve the generation of an acoustic pulse to cause reflections from
material interfaces in the path of the pulse. The reflections are then
processed to evaluate the cement bond.
In the U.S. Pat. No. 3,502,169 to Chapman, a sonic borehole televiewer
device is described to obtain a visual presentation of the wall of the
borehole. An acoustic transmitter is used, operating in a frequency range
of the order of about 2 MHz, to direct acoustic pulses at the borehole
wall. The acoustic reflections from the wall are plotted as a function of
azimuth, or circular scan to present a visual indication of wall
fractures, cracks, as well as distinctions between hard and soft
formations.
The U.S. Pat. No. 3,474,879 to Adair describes an acoustic pulse echo
technique for scanning surface characteristics of a borehole with a
rotationally mounted receiver-transmitter acoustic transducer. An acoustic
beam generated by this transducer is directed at an angle relative to the
borehole wall. The beam glances off with relatively little reflections in
case of a smooth borehole wall, but when the beam is incident upon a wall
discontinuity such as a cavern fracture or a rock interface, a detectable
acoustic reflection is generated. The U.S. Pat. No. 3,464,513 to Roever
teaches use of a similar system as in the Adair patent except that a
plurality of stationary transducers are used to scan the periphery of the
borehole wall.
The scanning of acoustic beams may be done mechanically as taught in the
Adair patent or electronically as shown in the Roever patent or in U.S.
Pat. No. 3,693,415 to Richard. Various techniques have been proposed to
electronically steer an acoustic beam, see for example the U.S. Pat. No.
3,732,945 to Lavigne. An acoustic transceiver employing a flat array of
transducers to enable the retrieval of a fish lost within a borehole is
described in U.S. Pat. No. 3,935,338 to Aldrich et al.
SUMMARY OF THE INVENTION
In a borehole investigation technique in accordance with the invention,
acoustic pulses are introduced by a tool mounted transmitter towards the
borehole wall at a beam forming frequency and in such direction so as to
promote the excitation of transverse acoustic waves in the borehole wall.
The transverse acoustic waves do not traverse a fluid filled fracture,
which, in response to the preferentially excited transverse waves causes a
reflection towards an acoustic receiver on the tool. The acoustic
receiver, which is located in the vicinity of the transmitter, or may be
the same transducer as the transmitter, detects the acoustic reflections
and produces a waveform signal representative thereof. The waveforms may
then be recorded or further processed as indicative of reflected
transverse waves to identify the presence of the fractures.
As described with respect to one form of the invention, acoustic pulses are
introduced in the borehole medium by an acoustic transmitter operating at
such frequency as to produce an acoustic beam whose principal direction
lies in a reference plane. The reference plane for the purpose of
detecting vertical fractures is generally transverse to the longitudinal
axis of the borehole. The acoustic beam is further so oriented within the
reference plane to direct the beam with a predetermined angle relative to
the normal to the surface of the borehole wall region upon which the beam
is incident so as to promote the generation of transverse waves such as
shear or pseudo Rayleigh waves. The transverse waves travel away from the
incidence region generally in a direction dictated by the incident
acoustic beam. A highly angled fluid filled borehole wall discontinuity in
the path of such transverse waves causes a substantial acoustic reflection
which may then be detected by a sonic receiver.
Since the fractures of interest may occur over a range of inclination
angles relative to the longitudinal borehole axis, the acoustic
reflections produced by transverse waves travel away from the fractures in
different directions. These directions are determined by the angle of
incidence of the transverse waves with a normal to the fracture. Thus, for
inclined fractures of interest, the primary or maximum amplitude
reflections are not likely to be returned to the sonic receiver. In
another form of the invention, therefore, the transmitter acoustic beam is
further scanned so as to enhance detection of highly angled fractures over
a desired range of inclination angles.
With the acoustic investigation technique in accordance with the invention,
highly angled fractures are positively identified with a strong signal
thus also enabling a precise determination of the location of such
fractures in the borehole wall.
It is, therefore, an objection of the invention to provide a method and
apparatus for the detection of borehole wall discontinuities such as
highly angled fractures, cracks and edges of voids in a positive manner.
It is still further an object of the invention to detect borehole wall
discontinuities and precisely determine their positions on the borehole
wall.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other advantages and objects of the invention can be understood
from the following description of several embodiments described in
conjunction with the following drawings.
FIG. 1 is a horizontal section view of a tool located in a borehole for the
detection of borehole wall discontinuities in accordance with the
invention;
FIG. 2 is a perspective view of a portion of an apparatus in accordance
with the invention for the detection of borehole wall discontinuities;
FIG. 3 is a top plan view partially broken away of the apparatus shown in
FIG. 2;
FIG. 4 is a horizontal section of tool pads employed on a borehole wall
with an apparatus in accordance with the invention;
FIG. 5 is a simplified vertical schematic representation of a fracture
detection technique with an apparatus in accordance with the invention;
FIG. 6 is a front schematic view of a transducer array employed in
accordance with the invention;
FIG. 7 is a perspective schematic view of a transducer and portion of a
borehole wall having a highly angled borehole wall discontinuity;
FIG. 8 is a schematic view of a fracture detection technique taken in a
plane which is transverse to the axis of the borehole;
FIG. 9 is a schematic block diagram of a signal processing network used to
form a log of reflections attributable to high angled fractures detected
in accordance with the invention; and
FIG. 10 is a block diagram of a technique for controlling and operating an
acoustic transducer in a beam scanning mode in accordance with the
invention.
DETAILED DESCRIPTION OF EMBODIMENTS
With reference to FIGS. 1-3, a borehole 10 is shown formed in an earth
formation 12. The borehole 10 may be a perfectly round hole, but in
practice it is likely to have discontinuities in the form of short
fissures such as 14 or a cavity such as 16. The borehole 10 also
intersects discontinuities in the form of long fractures, which may be
generally horizontally inclined such as when different earth layers are
traversed by the borehole 10 or vertically inclined.
Of particular interest in the detection of natural gas or petroleum are
borehole wall discontinuities in the form of vertically oriented fractures
such as shown at 18. Such vertical fractures may be aligned with a radial
of the borehole axis 20 such as fractures 18.1, 18.2 and 18.3, while other
vertically oriented fractures such as 18.5, 18.6 and 18.7 intersect the
borehole wall 22 chordally or tangentially.
According to one technique of this invention, an acoustic beam of energy is
directed from a tool 24 carrying acoustic transducers 26.1-26.4. The tool
is suspended from a cable 28 in borehole 10. Cable 28 is connected to
surface located equipment 29 from which electrical power is obtained and
to which measurements made with tool 24 are transmitted.
The acoustic transducers 26.1-26.4 are formed so as to be able to emit
acoustic energy with a directivity as suggested by the beam 30.1 emanating
from face 32.1 of transducer 26.1 in FIG. 1. The transducers 26.1-26.4
further are selected to be able to produce such beam-shaped acoustic
energy in the form of short pulses. In this manner reflections produced by
borehole wall discontinuities can be detected and waveform signals
indicative thereof produced from the transducers during intervals between
acoustic pulses. The formation of an acoustic beam is well known and is
among other factors a function of the surface area of the emitting face 32
of transducers and frequency.
The operating frequency is selected sufficiently high so that the acoustic
wavelengths employed enable formation of a beam, yet the frequency is
sufficiently low to reduce the attenuation from borehole fluids such as
drilling mud. An operating center frequency of the order of about 500 KHz
may be used with a rectangular transducer face 32 surface area of about an
inch wide by one and a half inch high. The transducers 26 are mounted on
tool 24 so that their acoustic beams 30 are directed at a predetermined
angle .theta. relative to the normal 34 to the borehole region upon which
beams 30 are incident. The angle .theta. is so selected that the acoustic
beams 30 promote generation of transverse waves in the borehole wall in a
manner as taught, for example, by the previously identified U.S. Pat. No.
3,775,739 to Vogel. The transverse wave may be a shear wave or a
pseudo-Rayleigh wave, both of which travel along the surface of a borehole
wall 22.
In order to preferentially excite transverse waves in the borehole wall,
the transducers 26 are so oriented that the sine of angle .theta. is
approximately equal to the ratio of the velocity of sound in the borehole
liquid to the velocity of the transverse wave to be execited. This angle
may thus vary, but when it is about 40.degree. transverse waves are
normally enhanced over a wide range of earth formation conditions. With an
angle .theta. of about 40.degree. the transducers 26 are located near the
periphery of tool 24 and, if necessary, are mounted on pads 36.1, 36.2 as
shown in FIG. 4.
In the operation of tool 24, short duration acoustic pulses are regularly
generated by transducers 26, for example in the manner and of the type as
shown in the U.S. Pat. No. 4,255,798 to Havira. Each acoustic pulse
directs acoustic energy at the borehole wall 22 so as to promote the
generation of a transverse wave. The transverse waves travel along the
borehole wall away from the region such as at 38 upon which the acoustic
beam 30 is incident.
When a fracture, such as 18.2 is in the path of the transverse wave, the
liquid at the fracture interface is unable to pass the transverse wave,
which is, therefore, reflected. The acoustic reflection, in turn,
introduces acoustic compressional waves at the boundary with the borehole
fluid. Since the transducer 26.1 is oriented to optimize sensitivity to
transverse waves traveling along the borehole wall, a waveform signal
representative of the transverse wave is obtained at the output of
transducer 26.1. The reflections represent positive indications of the
presence of fractures. FIG. 5 shows a simplified planar view of a path 40
followed by a transverse wave generated by a transducer 26.1. When a
fracture such as 18.1 parallel to the borehole axis is encountered and is
perpendicular to path 40, a reflection travels along the path 40 but in
the direction indicated by arrow head 42 and is incident upon the face
32.1 of transducer 26.1.
The borehole may be inclined relative to the vertical of the earth and the
fractures of interest may have inclinations relative to the earth vertical
while still being of sufficient interest. The fractures of interest also
may not all lie in a plane parallel to the borehole axis and may be in
fact inclined with an inclination angle .alpha. with respect to the
borehole axis 20 such as fracture 18.2. The transverse wave incident upon
detection of such fracture 18.2 with a transverse wave traveling along
path 40 is difficult because a reflection returns along a path such as 44,
which depending upon the size of the inclination angle .alpha. may result
in avoiding incidence upon transducer 26.1 and thus detection.
Detection of inclined fractures thus depends upon the operating beam width
of transducer 26.1. This tends to be a function of frequency and at a
center frequency of about 500 KHz is quite narrow, of the order of about
seven degrees between the half power points. At such beam width, fracture
inclination angles of only several degrees are detected since the
reflections are reflected at twice the angle of inclination.
Although a plurality of transducers 26 could be employed with differently
inclined fractures, a preferred technique in accordance with the invention
employs a transducer 26.1 with an electronically steerable beam. This is
obtained by using an array 48 of acoustically energizable and sensitive
strip-shaped elements 50 with the array distributed along the direction in
which the acoustic beam is to be scanned.
FIGS. 2 and 6 illustrate a transducer 26.1 on which the piezoelectric
elements in the form of parallel rectangular shaped strips 50 are used.
The number of elements 50 may vary, though five may be sufficient to yield
an ability to scan over an angular range of .+-.20.degree. relative to a
plane which is substantially perpendicular to borehole axis 20. Such scan
range is deemed sufficient to detect mot fractures of interest. The
elements 50 are shown as exposed, though in practice they are protected by
an appropriate acoustic coupling layer, which is deleted for clarity
though shown in the view of FIG. 4. Techniques for forming such array of
acoustic materials are known in the art; see, for example, the previously
mentioned U.S. patents to Roever, Aldrich et al and others. As an example,
the elements 50 may have a width, W, of 0.1 inches, a length, 1, of about
one inch and separated from each other with a spacing, s, of about 0.010
inch. The transducers 26 are mounted adjacent the peripheral surface of
the tool 24 so that the desired angle of incidence of the acoustic beam on
the borehole surface 22 can be obtained. Since the acoustic beam is
scanned, the portion of tool 24 in front of the arrays 48 is flared upward
at 51 above the arrays 48 and downwardly at 53 below the arrays. The flare
angles are selected sufficiently large to avoid interference with the
steered acoustic beams and reflections caused thereby.
In practice the acoustic path followed by the acoustic pulses and
reflections is not the simplified planar representation as shown in FIG.
5, but a more complex path 52 as illustrated in Fig. 7. There, acoustic
pulses are launched at the borehole wall 22 along an initial path 54
through the borehole with a tool 24 as shown in FIG. 2 or through an
acoustic coupling layer 56 as illustrated in FIG. 4 until the acoustic
energy is coupled into the borehole wall 22 at a region 38. The acoustic
coupling layer may be formed of an appropriate impedance matching material
as is well known in the art.
Acoustic waves then propagate away from region 38, generally along a path
58.1 and in a direction determined by the angle of incidence of the
acoustic pulse at region 38. The path 58 lies along a circular portion of
the normally cylindrical borehole wall 22 in the case of a direction for
path 54 which lies in a plane which is perpendicular to the borehole axis
20.
When, however, the beam direction 54 is scanned by the array 48, such as
along a plane parallel with the borehole axis, the resulting travel paths
58.2 and 58.3 are non-circular so that at least one set of preferentially
enhanced transverse waves traveling along 58.2 may intersect a fracture
such as 18.1 along a perpendicular direction thereto.
With the use of a scannable acoustic beam, fractures may be detected over a
range of inclination angles depending upon the orientation of the
transducers 26. In the embodiment of tool 24 in FIG. 2, the transducers
are oriented to detect vertical fractures having high inclination angles
as measured relative to a plane which is perpendicular to the borehole
axis. The tool 24 is held within the center of the borehole 10 with
centralizers (not shown) which are well known in the art. In FIG. 4 a tool
24' is used employing wall engaging pads and in which transducers such as
26 are mounted to scan for and detect vertical fractures.
Scanning of the acoustic beam of transducer 26 is obtained by controlling
the time when each element 50 in the array 48 is activated during a
transmit mode, or sampled during a receive mode.
With reference to FIGS. 8 and 9, a network 70 is shown for generating a log
72 to indicate borehole wall discontinuities as a function of depth. The
log 72 may be made by storing signals on a magnetic medium or in the
memory of a data processor or a visible record as shown in FIG. 9 may be
formed. A pulse network 74 provides transducer 26 with electrical drive
pulses on line 76 while disabling a receiver amplifier 78 with a signal,
T, on line 80. At the end of a transmitter pulse, the disabling signal on
line 80 is removed and reflection signals representative of acoustic
reflections on line 76 from transducer 26 are amplified by amplifier 78.
The amplified reflection signals are applied to a fullwave rectifier 82
whose output 84 is compared by a comparator 86 with a threshold value on
line 88 from a threshold network 90. In the event the threshold value is
exceeded, a reflection signal appears on output line 92 which is applied
to recorder 94 with which log 72 is formed. The reflection signal on line
92 has an amplitude representative of the amplitude of the acoustic
reflections. In this manner the magnitude and time of arrival of
reflections as recorded on log 72 are indicative of the distance of
transducer 26 from a borehole wall discontinuity such as fracture 18.1.
Log 72 is formed of a time log 96 on which reflections 98 from fractures
are recorded. In addition, a scan angle log 100 is employed to indicate
the inclination angle of the fracture. For the time-log 96 the recorder 94
is provided with a time sweep signal representative of the time following
a common event such as a transducer acoustic pulse, t.sub.o, to thus
indicate the time interval when a reflection from a borehole wall
discontinuity is detected relative to this common event. The time sweep
signal commences its sweep signal with the actuation of the transducer 26
and terminates a predetermined maximum time, t.sub.m, thereafter. The time
sweep signal may be obtained with a generator 101 or a signal processor
102. The duration of the time-sweep signal is selected sufficiently long
to enable the recording of reflections which exceed the threshold level
from threshold network 90 and may occur over the operating range of the
transducers. The successive arrivals of the reflections or the intervals
of time relative to the common event may thus be used to indicate the
relative inclination of a discontinuity such as a fracture with respect to
the direction of logging by tool 24 in the borehole.
The signal processor 102 is preferably employed for controlling scan angles
as well as generate signals indicative of the scan angle at which recorded
reflections 100 were detected when a beam scanning mode is employed.
Alternatively to the use of a sweep signal from the sweep generator 101,
the signal processor 102 can produce a sweep signal on line 104 as a
function of the acoustic wave velocity in the earth formation for the
depth at which the transducers 26 are located for a more accurate
indication on log 96 as to the angular location and inclination of
fractures on borehole wall 22.
FIG. 8 illustrates how the path length 54 through the borehole medium in
case of a tool as in FIG. 2 or through the coupling layer 56 in case of a
pad arrangement as in FIG. 4 can be estimated. The two-way travel time of
the acoustic pulse along path 54 can be calculated using the scan angle
.phi. and the known velocity of an acoustic pulse through the borehole
medium. The velocity of transverse waves in the earth formation along path
58 may be generally known from acoustic velocity or travel time interval
(.DELTA.T) measurements in units such as microseconds per foot as a
function of depth for borehole 10. Hence, either signals representative of
.DELTA.T and depth measurements are applied to a signal processor 102 on
lines 106, 108 or stored in memory associated with the signal processor.
One technique for determining the azimuth of a fracture includes a
measurement of the time lapsed between the detection of a reflection and
the generation of the acoustic pulse which produced it. This measurement
may be made inside signal processor 102 with a clock 110 applied to drive
a register 112 which is reset to a particular value each time the
transducer 26 is activated with the signal on line 80.
Hence, when an acoustic reflection is detected and a reflection signal
indicative thereof occurs on line 92, an AND gate 114 is enabled to
transfer the count of register 112 via transfer logic network 116 to a
control and processing unit 118 in signal processor 102. While this
transfer occurs, an inhibit network 120 is activated to inhibit further
transfers for a time estimated sufficient for the reflection signal to
pass.
A signal indicative of the length of path 58 to a borehole discontinuity
may then be derived by subtracting the two way travel time of the acoustic
energy along path 54 and dividing the remainder by the interval travel
time of the acoustic transverse waves for the particular earth formation
depth.
The signal processor 102 may transform the length of path 58 to an azimuth
position for the fracture 18.1 based upon the known placement of
transducer 26 on the tool 24 and the known path length 54 by using known
geometric relationships. An azimuth signal may then be produced on a line
122 and applied to recorder 94 to record the angular position of the
detected borehole wall discontinuity.
The network 70 has been described for use with a single transducer 26. In
such case a single fracture 18.1 along the path 58 can be detected,
fractures such as 18.2 lying behind the first fracture are less likely to
be detected. Accordingly, as shown in the embodiment for tools 24, 24' in
FIGS. 2 and 4, a plurality of transducers 26 are employed to each
investigate a portion of the borehole wall for discontinuities. Network 70
is correspondingly expanded to accommodate the signals to and from
additional transducers.
FIG. 10 illustrates one technique 130 for controlling the beam scanning
mode for a transducer 26 having an array 48 of separately energized strip
elements 50. The elements 50 are sequentially energized with slight delay
intervals depending upon the desired direction of the acoustic beam.
Following activation of the elements 50, they are used in a receive mode
and combined in accordance with direction determining delays. The receive
mode is active for a time period sufficient to detect the presence of
borehole wall discontinuities within a predetermined distance from the
transducer 26. This distance is strongly affected by the amount of
attenuation. Normally, a two way travel distance limit of about six inches
may be imposed so that the number of transducers 26 employed to
investigate the entire borehole wall may be correspondingly increased
depending on the perimeter length of the borehole.
In the technique 130 of FIG. 10, a signal processor, such as 102 of FIG. 9,
is employed to control and set the delays needed to scan the acoustic beam
during the transmit mode and combine the receiver outputs during the
receive mode. Each element 50 in the transducer array 48 is connected to a
separately energized drive circuit 132 and separate amplifier such as 78
in FIG. 9.
The drive circuits 132 in turn are energized by pulses from individual
registers 134, each of which produce an output pulse for one acoustic beam
pulse at the appropriate delay time as a function of preset delay counts
from counters 136. The preset delay counts are selected to, for example,
step the acoustic beam through fixed scan positions, e.g. eight for a
maximum scan range of about forty degrees (.+-.20.degree. relative to a
plane such as which is perpendicular to the borehole axis 20).
At 138 the respective sets of delays needed to scan the acoustic beam are
stored for various array beam angles and at 140 the discrete scan angles
for the acoustic beam are selected so that the corresponding delays can be
output at 142 in the proper sequence to the delay preset counters 136.
The registers 134 are then activated at 144 so that each produces an output
pulse to a drive circuit 132 at the proper delayed instant while the
receiver gates are inhibited. A short delay at 146 is provided following
the generation of an acoustic pulse. The receiver gates 78 are then
enabled at 148 and the receiver outputs are passed through variable delay
lines 150 having delays set at 149 in correspondence with the delays
previously employed in counters 136. The outputs from the delay lines 150
are combined in a summing network 152 to provide a single receive waveform
signal from transducer 26 for the particular array scan angle. The
combined waveform may then be converted to digital form with an analog to
digital converter 154 and the waveform stored in memory at 156.
At 158 the stored waveform is scanned for a peak value indicative of the
presence of a fracture. At 160, the time position of the detected peak in
the waveform is transformed to a distance to the fracture from the
receiver and this measurement, together with the peak amplitude and scan
angle are recorded at 162.
Following the receive mode, a slight delay is implemented at 164 followed
by a return at 166 to step 140 for an acoustic investigation for the next
successive beam scan angle position.
The receiving mode may be implemented by sampling each of the waveform
signals obtained from a strip element 50 for the interval of interest and
then storing the samples in a memory of signal processor 100. The stored
samples of different waveforms may then be combined with time shifts which
correspond with the delays set for the waveforms in counters 136. This
technique may be performed within a signal processor located either
downhole in tool 20 or above ground.
Having thus described an apparatus and method for the detection of
fractures, the advantages of the invention can be understood. Variations
from the described embodiments can be made without departing from the
scope of the invention.
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