Note: Descriptions are shown in the official language in which they were submitted.
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APPARATUS FOR AND A METHOD ûF ACOUSTIC WELE-LOGGIN~
This invention relates to apparatus for and a method of
acoustic well logging, particularly in poorly consolidated formations,
such as unconsolidated sands or overburdens.
It has long been known acoustically to log open wellbores to
determine the velocities of compressional ("P") waves and shear ("S")
waves traveling through rock formations located in the wellbore region.
Logging devices have been used for this purpose which normally comprise
a sound source (transmitter) and one or more receivers disposed at
pre-selected distances from the sound source. For example, Kitsunezaki
has suggested one such device for use in shear wave logging wherein the
transmitter is located 3.2 meters from the first of 5 receivers, which
are spaced 1 meter apart down an elastic rubber tube which is intended
acoustically to isolate these elements from each other (see "A New
Method for Shear Wave Logging", by Choro Kitsunezaki, ~yo Technical Note
RP-4101, Oyo Corporation, Urawa Saitama 336 Japan, October, 1978).
8y timing the travel of compressional waves, shear waves,
and/or tube waves between the transmitter and each receiver, it is
normally possible to determine the nature of surrounding rock
formations. In logging loosely consolidated formations, however, it is
often difficult to distinguish between compressional, shear, tube and
secondary waves which may comprise portions of a wave train arriving at
a given receiver. The use of remotely spaced, multiple receivers is
thus intended to aid in distinguishing between arriving wave fronts and
from noise in the system. Multiple receivers permit the recognition of
similar wave patterns and wave fronts which are received at each
successive receiver. Since travel time differentials increase with
increasing distance from the transmitter source, wave fronts and
patterns which are closely spaced at proximate receiver locations will
separate by the time of their receipt at remote receiver locations.
Various signal timing and wave front analysis methods have also
been suggested for distinguishing between wave fronts received at a
given receiver. Most of these methods involve timing circuits which
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anticipate the receipt of, and facilitate the collection of, such wave
front information. For descriptions of various logging techniques for
collecting and analyzing compressional wave, shear wave, tube wave, and
secondary wave data, please refer to U.S. Patents 3,333,238 (Caldwell),
3,362,011 (Zemanek, Jr.), and U.S. Reissue 24,446 (Summers).
In the design of logging tools, various types of transmitters,
such as, piezoelectric or magnetostrictive transmitters, have been
suggested for creating acoustic logging signals. For conventional
logging operations, most such transmitters have been centrally located
in the borehole, and have been adapted to generate sound which is
radiated in a multidirectional (}60) pattern from the transmitter to
ad~acent wellbore surfaces. Such transmitters are well suited for
creating compressional waves in surrounding rock and sand formations.
Since compressional waves travel faster than those shear, tube
or secondary waves which may also be produced by a multidirectional
transmitter, calculation of compressional wave velocity is accomplished
by presuming that the first arriving wave front or wave pattern is that
o~ a compressional wave. In loosely consolidated formations, subsequent
arrivals of shear waves, tube waves and/or secondary waves are difficult
to distinguish. In such formations, multidirectional transmitters tend
to generate compressional waves of much greater amplitude than any shear
waves also produced thereby. Recognition of shear wave arrivals, is
thus particularly difficult.
Recently, attention has been directed to developing
transmitters which are particularly suited to shear wave logging. Such
transmitters generally attempt to achieve a single point source
application of sound energy to the borehole wall. The theory behind
point source transmitters, as generally outlined in the above-mentioned
Kitsunezaki paper, is that they are capable of directly generating S
- waves. Conventional multidirectional transmitters are said to becapable only of indirectly creating shear waves. Accordingly, point
source type transmitters produce shear waves of substantially higher
amplitudes than heretofore possible with conventional multidirectional P
wave transmitters. Accordingly, formations, such as loosely
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consolidated or unconsolidated sand, which do not propagate shear waves
in sufficient amplitudes to permit definitive detection using
conventional P wave receivers, may now be shear wave logged with these S
wave logging systems. Oyo Technical Notes RP-4105, entitled
"Development of a Suspension Type S-Wave Log System," by Kimio Ogura
(November 1979) and RP-4125, entitled "Development of the Suspension
S-Wave Logging System (Report No. 2)", by Kimio Ogura, et al (November
1980) provide additional information relating to S-wave logging systems.
In spite of the above described developmen~.s in logging
techniques and apparatus, difficulty is nonetheless encountered in
logging open boreholes, particularly those boreholes disposed through
unconsolidated or loosely consolidated formations, such as sandstone or
sand. In order to obtain the best possible logging data, it is
important to log boreholes as soon as drilling is completed. Dril~ing
mud has a tendency to damage the logging characteristics of an open
borehole, and may interfere with the gathering of reliable logging data
if time is permitted to elapse between the completion of drilling and
the logging operation. Not only is rig time extremely costly (currently
on the order of $50,000 to $100,000 per day) but there are inherent
dangers in prolonging any logging operation. For example, many
boreholes will not stay open for extended periods of time. It is thus
important to expedite the logging operation to obviate any necessity to
reinsert pipe into the borehole to flush with mud. Accordingly, a need
exists with loosely consolidated formations to obtain all data which may
be collected through the use of both conventional and shear wave logging
apparatus, and to do so in a manner which permits the efficient,
reliable collection of such data from an open, recently drilled borehole.
In one aspect, the invention resides in well logging apparatus
for continuously logging the shear and compressional wave transmission
characteristics of materials located adjacent to a borehole; comprising:
(a) a plurality of selectively activatable transmitters, said
transmitters comprising at least a first, multidirectional acoustic
transmitter and a second, point source acoustic transmitter;
;
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(b) a plurality of receivers, at least one for each
transducer, mounted in spaced relation from said transmitters, said
receivers comprising at least a first, compressional wave receiver
exhibiting a relatively greater sensitivity to acoustic waves produced
by said first transmitter, and at least a second, shear wave receiver
exhibiting relatively greater sensitivity to acoustic waves produced by
said second transmitter; and,
(c) sequencing means for selectively energizing each of said
transmitters in response to the detection of at least one wave by at
least one of said receivers.
In a further aspect, the invention resides in a method for
: acoustically logging an open borehole disposed-through poorly
consolidated materials, comprising the steps of:
(a) providing logging apparatus comprising at least one point
source type transmitter, at least one multidirectional compression wave
type transmitter, and a plurality of receivers disposed in spaced apart
relation to said transmitters, said receivers comprising at least one
shear wave type receiver showing increased sensitivity to waves produced
in a borehole by said point source type transmitter, and a compressional
wave type receiver showing relatively greater sensitivity to
compressional waves produced by said compressional wave type receiver;
- (b) placing said logging apparatus within said borehole to be
logged;
(c) sequentially and periodically activating said transmitters
; to produce periodic shear and compressional waves within materials
adjacent to said borehole;
(d) sequentially permitting selected ones of said receivers to
transmit information for collection to cause said shear wave receiver to
receive and transmit shear waves produced by said shear wave
transmitter, and to cause said compressional wave receiver to receive
and transmit information relating to said compressional waves produced
by said compressional wave transmitter;
. (e) recording said shear wave and compressional wave
information transmitted from said receivers; and
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(f) moving said apparatus through said borehole while
periodically repeating steps (a)-(e).
In a preferred embodiment, a plurality of receivers are
provided which are spaced apart from each of the transmitters at
selected intervals along`an acoustically isolating logging conduit. At
least one, and preferably two, receivers which are relatively more
sensitive to shear waves produced by the point source type transmitter,
are located closest to this transmitter. This juxtaposition of
transmitters and receivers takes advantage of the differing travel times
of shear and compressional waves to permit rapid cycling of the device
to collect optimal amounts of logging information. Preferably, cycle
speed is increased by alternatively energizing each transmitter in
response to the receipt of a suitable signal at the alternate wave-type
receiver.
In an alternative embodiment, which is intended for use under
conditions when increased acoustic isolation within the logging
apparatus is desired, the multidirectional compressional wave
transmitter is disposed between the compressional wave receivers and the
shear wave receivers. In this embodiment, all the shear wave receivers
are disposed closer to the compressional wave transmitter than is the
closest compressional wave receiver to that transmitter. In this
manner, receipt of a compressional wave by the closest compressional
wave receiver will ensure that the compressional waves generated by the
compressional wave transmitter have passed by the shear wave receivers
prior to onset of their duty cycles.
The present invention will now be more particularly described
with reference to the accompanying drawings, in which:
Figure 1 is a diagrammatic illustration of well logging
apparatus according to one example of the invention disposed within a
borehole in an unconsolidated sand,
Figure 2 is a diagrammatic illustration similar to Figure 1 of
well logging apparatus according to a second example of the invention,
and
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Figure 3 is a diagram which illustrates the preferred
sequencing of the logging device illustrated in Figure 1 and which
shows, starting from the top of the diagram, the energization of the
compressional transmitter (PT) and the point source transmitter (ST),
representative signals received by each of the compressional wave
receivers (Pl, P2 and P3), representative signals received by the
shear wave receivers (51 and S2), and the preferred gating (G) for
each of the aforementioned receivers, alternate gating modes being shown
in dotted outline.
Referring to Figure 1, the apparatus of the first example is
shown disposed in an open borehole 14 which, although foreshortened for
purposes of illustration, may be seen to extend from the ground surface
10 through Dverburden 18 eventually to and through a sand layer 20.
Although not ill~strated in Figure 1, borehole 14 would normally be
filled with a fluid, such as drilling mud, in which logging is
conducted. The logging apparatus of the first example is shown disposed
within the borehole 14 adjacent the sand layer 20 and connected to a
surface control panel 9 and recorder 8 by a conventional cable 12.
The logging apparatus shown in Figure 1 comprises an upper body
portion having an electronics section 100 and a multidirectional,
compressional (P) transmitter 102 and a point source (uni-directional),
shear wave transmitter 104 disposed in the order stated below the
electronics section 100. The multidirectional transmitter 102 may be a
piezoelectric or magnetostrictive transmitter of the type known in the
art for producinq suitably recordable compressional waves. The
preferred shear transmitter 104 is a bender type transmitter
which is described in Canadian Patent No. 1,152,201, Angona
et al, issued August 16, 1983. Suspended below transmitters
102 and 104 are a series of spaced apart receivers 106-114.
Each of these receivers is suspended on an acoustically
isolating conduit 118, which may be of the rubber hose type
described in the aforementioned Kitsunezaki paper.
Receivers 106 and 108 are preferably shear type
receivers also of the bender type, as described in the
aforementioned Canadian Patent No. 1,152,201 of Frank A.
Angona et al. These receivers are characterized by
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their high sensitivity to shear (s) waves and their relatively low
sensitivity to compressional and tube waves. In the preferred
embodiment, shear wave receiver 106 is located about 4-6 feet,
preferably about 5 feet, below shear wave transmitter 104, and shear
wave receiver 108 is located approximately 2-5 feet below shear wave
receiver 106. In this configuration, the distance between shear wave
transmitter 104 and remote shear wave receiver 108 ranges from a minimum
of about 6 to a maximum of about 11 feet.
Suspended below shear wave receivers 106 and 108 are a
plurality of compressional wave receivers 110, 112 and 114. These
compressional wave receivers may be of the conventional magnetostrictive
or piezoelectric type, which are characterized by relatively high
sensitivities to compressional and tube waves, and relatively lower
sensitivities to shear waves. In the preferred embodiment,
compressional wave receiver 110 is disposed at a distance of about 13-17
preferably 15 feet, from compressional transmitter 102. Second and
third compressional receivers 112 and 114 are suspended below receiver
110 at spacings of about 4-6, preferably about 5 feet.
Figure 2 is a diagrammatic illustration similar to Figure 1,
showing apparatus according to a second example having a different
juxtaposition of transmitters and receivers. Similar components in
Figure 2 have been numbered to correspond with the components described
in Figure 1, except "200" series numbers are used for the components in
Figure 2 instead of the "100" series numbers utilized in connection with
Figure 1. In the alternate logging apparatus shown in Figure 2, the
compressional wave transmitter 202 has been separated from the shear
wave transmitter 204 for the purpose of increasing acoustic isolation.
For the purpose of sequencing, shear wave receivers 206 and 208 are
still juxtaposed closer to compressional wave transmitter 202 than is
any compressional wave receiver. Otherwise, the spacings between each
transmitter and its respective receivers are as described in connection
with the device of Figure 1.
The preferred sequencing for the apparatus illustrated in
Figure 1 is set forth in Figure 3, which illustrates slightly more than
one duty cycle of this preferred embodiment apparatus. The signals
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indicated in each of the receiver traces of Figure 3 are based upon
experiments conducted in a loosely consolidated formation using shear
and compressional waves generated within holes 10 feet apart at a depth
of 20 feet. These signals have not been appropriately expanded or
contracted as they would have been if generated by receivers located at
distances of less than or greater than 10 feet. The signals are
nonetheless generally representative of the signals which would be
generated by the various receivers of the preferred embodiment apparatus
of Figure 1. Similarly, the time scale of Figure 3 is generally
representative of the actual velocities of the waves recorded in the
above mentioned experiments, but should not be interpreted as limiting
the present invention to any particular fixed time sequence. As
described hereinafter, the cycling frequency of the preferred embodiment
logging tool preferably depends upon the velocities of the waves
traveling through surrounding borehole formations.
The top trace (PT) is a trace showing the sequence of
activation of the compressional wave transmitter 102. The next lower
trace, ST, shows the sequence of activation of the shear wave
transmitter 104. In reference to both Figures 1 and 3, upon
energization of P wave transmitter 102 at time 0, as indicated by the
spike in Figure 3, a compressional wave will be generated which will
travel through the surrounding sand formation past the shear wave
transmitter 104 and shear wave receivers 106 and 108. This
compressional wave is then received by compressional wave receivers 110,
112 and 114, which produce the receiver traces indicated in Figure 3 as
traces Pl, P2 and P3 respectively. In the preferred embodiment,
each of these receivers is gated so that it will transmit its signals
beginning from the time that the compressional wave transmitter is
energized. The opening of this gating is indicated by the "+" condition
of gating traces Gpl, Gp2 and Gp3.
Conventional signal analysis means is preferably utilized to
ensure the receipt of the desired information relating to the
compressional wave, as reflected in signal traces Pl, P2 and P3.
Such signal analysis means may, for example "look" for a minimum
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amplitude of an arriving wave which will be presumed to be the
compressional wave front. The time of receipt of this amplitude may
then be used to calculate compressional wave velocity.
In the preferred embodiment, the shear wave transmitter 104 is
activated in response to the receipt of a compressional wave signal by
proximate compressional wave receiver 110, which generates trace Pl.
In alternate embodiments, the receipt of other compressional wave
signals, represented by traces P2 or P3, may be utilized to energize
the shear wave transmitter 104. Less preferably, the shear wave
transmitter 104 may be activated in time delayed response to the receipt
of one of the aforementioned compressional waves, particularly in
instances where high levels of acoustic noise have been encountered.
The particular location of the shear wave receivers 106 and 108
adjacent to the compressional wave transmitter 102 ensures that each
compressional wave will have passed by receivers 106 and 108 prior to
the activation of the shear wave transmitter. The gating for shear wave
receivers 106 and 108 may permit the collection of data beginning
immediately at the time of energization of the shear wave transmitter
104, as indicated by the dotted lines in Figure 3, or may be delayed for
a preselected interval to permit any minor compressional wave which may
be generated by the shear transmitter to pass by each shear wave
receiver prior to the beginning of shear wave data collection. In this
latter instance, the solid lined gating arrangement illustrated in
Figure 3 for shear wave receiver 106 (Gsl) and 108 (Gs2) would be
preferred. Such a time interval is easily calculated using recently
collected information relating to compressional wave velocity. Such
calculations should account for differential receiver spacings within
the tool. Thus, the shear wave receiver gating delay time is
simultaneously determined to account for compressional wave velocity
variations.
Analysis of shear wave information (Traces S, and S2) is
conducted in accordance with conventional prior art procedures, which
may be similar to the above referenced compressional wave analysis
procedures. Once the receipt of a shear wave front is detected at
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compressional wave receivers 112 and 114, the compressional wave
transmitter 102 may again be energized to restart the duty cycle. As
shown by the gating traces in Figure 3, the compressional wave receivers
110-114 will transmit information to the surface control panel while the
shear wave receivers are deactivated, and vice versa. In practice, the
apparatus is gradually moved through the borehole as the duty cycles are
repeated.
Although the timings vary somewhat for the alternate embodiment
device of Figure 2, it should be appreciated that the positioning of the
receivers and transmitters in the embodiment of Figure 2 permits a
cycling of this device in a signal receipt triggered manner similar to
that described in connection with the embodiment of Figure 1.
As seen from the above, the present apparatus and method for
logging open boreholes in unconsolidated formations facilitate the rapid
collection of high quality shear wave and compressional wave data
relating to those formations.
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