AU612261B2 - Method and system for measuring azimuthal anisotropy effects using acoustic multipole transducers - Google Patents

Description

COMMONWEALTH OF AUSTRALIAI 26~ PATENTS ACT 1952-69 COMPLETE SPECIFICATION

(ORIGINAL)

Class Ilot. Class Application Number: Lodged: 35033/89 19th May 1989 Complete Specification Lodged: Accepted: Published: Priority: Art: Nam~e of Applicant: 0000 Address of Applicant1 00 0 00 0 0 0 0Q000 04006 Actual Inventor: 0 00 00 0 000 0 00 00 0 Address for Service: EXXON PRODUCTION RESEARCH CCMPANY 3120 Buffalo Speedway, Houston, Texas 77001, United States of America DAVID F. BECKER, SEN-TSUEN CHEN, AZII( GERAH11 A. WItNBOW I. PERELBERO and MMMAUMAMMRWatermark Patent Trademark Attorneys 50 QUEEN STREEHT, MELBOURNE, AUSTRALIA, 3000.

Complete -Specification for the Invention entitled; 0040 0 00 4 0 06 0 0 44 ME~THOD AND SYSTEMv FOR MEASURING AZIMUTHAL ANISOTROPY EFFECTS USING ACOUSTIC MULTIPOLE TRANSDUCERS The following statement Is a full description of this Invention, Including the best method of performing It known to us p -1- METHOD AND SYSTEM FOR MEASURING AZIMUTHAL ANISOTROPY EFFECTS USING ACOUSTIC MULTIPOLE TRANSDUCERS FIELD OF THE INVENTION The invention is a method and system for acoustic well logging. More particularly, the invention is a well logging method and system using at least one muitipole transducer for measuring the velocity of acoustic waves in a subterranean formation traversed by a well.

BACKGROUND OF THE INVENTION In conventional acoustic well logging operations, the velocity of an acoustic wave propagating through an earth formation traversed by a well may be determined. Throughout this specification, the synonymous terms "well" and "borehole" 0. 40 will be used interchangeably, and the phrases "acoustic wave" and "acoustic signai" will be used in their general sense to denote any compressional wave, shear wave, guided acoustic wave, •or any other elastic wave. A conventional acoustic well logging system includes: a logging sonde that may be suspended downhole a.o in the borehole fluid, a source contained within the sonde for generating compressional waves in the borehole fluid, and one or a more detectors within the sonde and spaced apart from the compressional wave source for detecting compressional waves in -2the borehole fluid. Compressional-wave energy in the borehole fluid generated by the source is refracted into the earth formation surrounding the borehole.

Some of the energy in the compressional waves in the fluid is refracted into the formation surrounding the borehole. Some of the refracted energy then propagates in the formation as a refracted compressional wave, and some propagates in the formation as a refracted shear wave. Another portion of the energy radiated by the compressional-wave source is converted into the form of guided waves that travel in the borehole fluid and the part of the formation adjacent to the borehole. A portion of the energy in each refracted compressional wave and shear wave is refracted back into the borehole fluid in the form c 00 90 o 15 of compressional waves and reaches the detector in the logging 0 0 oo 0o 0 sonde. The guided waves are also detected by such detector.

000o o 0 0* 0 o Any wave that is one of these three types of waves detected by o o the detector may be called an arrival. The compressional-waves a0 0 0 0 0 in the borehole fluid caused by refraction of compressional 20 waves in the formation are called the compressional wave arrivals; those caused by refraction of shear waves in the Sformation are called the shear-wave arrivals; and those caused by guided waves are called the guided-wave arrivals. Thus, if the signal generated by the source is an impulsive signal, the signal detected by the detector is a composite signal which includes a number of impulsive components including the compressional-wave arrival, the shear-wave arrival and the guided-wave arrivals. In earth formations compressional waves I i -3travel faster than shear waves, and shear waves in the formation usually travel faster than the guided waves. Therefore, in the composite signal detected by the detector, the compressionalwave arrival is the first arrival, the shear-wave arrival is the second arrival, and the guided-wave arrivals are the last arrivals. In measuring the compressional-wave velocity of the formation, the time interval between generation of compressional waves and detection of the first arrival by the detector gives the approximate travel time of the refracted compressional wave in the formation. Hence the later shear-wave and guided-wave arrivals do not affect measurement of the compressional-wave velocity of the formation. The ratio of the distance between the source and detector to the time between generation and detection of the energy in the compressional-wave arrival yields 15 the velocity of compressional waves in the formation. The 0 distance between source and detector is usually fixed and known oooo s.6o so that measurement of the time between compressional-wave generation and detection of the compressional- 0 0 Swave arrival i. sufficient to determine the velocity of compressional waves in the formation. For better accuracy, such distance is usually much greater than the dimensions of the source or detector. Alternatively, measurement of the time a 90 interval between the detections of a compressional-wave arrival, at two detectors separated by a known distance, can be used to measure the velocity of compressional-waves in the formation.

S S, S 5 -4- Information important for production of oil and gas from subterranean earth formations may be derived from the compressional-wave velocities of such formations. It is also known that determination of the velocity of shear waves may yield information important for production of oil and gas from the formations. The ratio between the shear-wave velocity and compressional-wave velocity may reveal the lithology of the subterranean earth formations. The shear-wave velocity log may also enable seismic shear-wave time sections to be converted into depth sections. The shear-wave log is also useful in determining other important characteristics of earth formations such as porosity, fluid saturation and the presence of fractures.

Conventional compressional-wave logging sources of the on "a 15 monopole type generate compressional waves that are symmetrical Do 00 O about the axis of the logging sonde. When such monopole 0000 °ooo compressional waves are refracted into the surrounding earth oa o 0 0 00 0 00o a formation and detected with conventional receivers of the 9o 0 o monopole type, the relative amplitudes of the refracted monopole shear and compressional waves are such that it is difficult to 0400 distinguish the later shear-wave arrival from the earlier o 4 compressional-wave arrival and its reverberations in the borehole.

However, it has been proposed that a multipole transmitter-detector pair a dipole-source/dipole-receiver pair, a quadrupole-source/quadrupole-receiver pair, or a higherorder-multipole-source/receiver pair where the multipole order i- of the source matches that of the receiver) be used in a well logging system to facilitate direct shear-wave velocity logging. Such a multipole well logging system, operated at the proper frequency, will produce arrivals at the detector such that the amplitude of the detected shear-wave arrival is significantly higher than that of the compressional-wave arrival. By adjusting the triggering level of the detector (and the system for recording the detected signal) to discriminate against the compressional-wave arrival, the shear-wave arrival is detected as the first arrival. Dipole acoustic wave well logging systems of this type are disclosed in U.S. Patent 4,606,014 issued August 12, 1986 to Winbow, et al.; European Patent Application 031,989 by Angona, et al. (published July 1981); and U.S. Patent 3,593,255 issued July 13, 1971 to White.

0o ,a 15 However, the prior art (including the cited references) teaches 0 0° that the source and receiver (or receivers) of a dipole (or 0 0 0400 00soO" higher order multipole) system should be aligned so that each 00 0o0* source and receiver is associated with substantially the same 04 0G S0 azimuthal angle relative to the borehole's longitudinal axis the azimuthal angle between the source and receiver is so as to maximize the sensitivity of the system4.

OG 0 Similarly, the references teach that if the azimuthal angle between a dipole source and a dipole receiver is 90', the r* *receiver will be insensitive to dipole wave energy produced by the source, and that if the angle between a quadrupole source and a quadrupole receiver is 45°, the receiver will not detect quadrupole radiation produced by the source.

ri 00 00 S0 0 0 00 S46 S0 0 0 0 4 a 9 a04 0a 9o 000 0 a 0O 0 04 -6- Multipole transducers of the quadrupole, octopole, and higher-order multipole types are described in the following Applications and Patent, all assigned to the same assignee as is the present Application: U.S. Patent Application Serial Number 379,684, filed May 19, 1982; U.S. Patent Application Serial Number 440,140, filed November 8, 1982; and U.S. Patent 4,649,526, issued March 10, 1987 to Winbow, et al.

It has for some time been known that thinly bedded horizontal formations and horizontally fractured rocks exhibit transverse isotropy. In this situation, the velocity of compressional and shear waves generally depends on their direction of propagation with respect to the vertical. However, their velocity is independent of the azimuthal direction in which they propagate. Alternatively, the formation may exhibit azimuthal anisotropy, in which compressional waves may travel at different speeds in different azimuthal directions away from (or toward) a vertical borehole. Similarly, the speeds of shear waves depend on the azimuthal direction (relative to the borehole axis) in which they propagate. Azimuthal anisotropy may be caused by vertical fractures, among other geologic factors.

In a azimuthally anisotropic medium, the velocity of a shear wave also depends on the polarization direction the plane containing the particle motion). For example, the velocity of a vsrtically propagating shear wave whose polarization is North-South will in general differ from the velocity of a vertically propagating shear wave whose polarization is East-West.

In their simplest form, azimuthally anisotropic media have five independent elastic constants associated with them as compared with two independent elastic constants for fully isotropic media. In principle, more complex types of anisotropy are also possible, and describing those forms of anisotropy may require as many as 21 independent elastic constants.

Azimuthal anisotropy has been reported to be a widespread phenomenon. Even a small amount of azimuthal anisotropy (for example, where the amount of azimuthal anisotropy is 0 15 defined to be c=I(Vl *100% where 0 0 L I 00' 0 Vi=velocity of wave propagating with polarization parallel 0 0 to a selected direcion, and V =velocity of wave having the same frequency content but having polarization perpendicular to 0 0 the selected direction (for example, parallel and perpendicular to fracture orientation V 1 and V 1 will be slow and fast velocities) has a significant effect on correlating shear-wave 0o seismic data.

a *4 o If azimuthal anisotropy could be detected and quantified during a wkll logging operation, the information could be used to help interpret direct hydrocarbon indicators, locate and define fractured reservoirs, and deduce lithologic information from seismic data. Azimuthal anisotropy data gathered during -8such a well logging operation, might also be used in performing production studies of fractured reservoirs since the information regarding azimuthal anisotropy is indicative of the existence and orientation of vertical fractures provide high-permeability pathways for hydrocarbons and reservoir quality.

Until the present invention, it has not been recognized how the effects of azimuthal anisotropy may be measured using welllogging tools.

SUMMARY OF THE INVENTION The inventive method uses at least one multipole transducer to measure the azimuthal anisotropy of a formation traversed by 15 a borehole. The phrase "multipole transducer" is used throughout this Specification, including the claims, to denote ooaa aoeo 0transducers having multipole order, n, greater than zero (i.e.

0a O OoD c*o a the phrase "multipole transducer" denotes a dipole transducer, a *0 0* Sa0 quadrupole transducer, an octopole transducer, or a higher-order multipole transducer, but does not denote a monopole transducer). For a monopole transducer, n=0, for a dipole transducer, n=l, for a quadrupole transducer, n=2, and so on.

In general, for a 2n-pole transducer, the multipole order of the transducer is equal to n. The multipole order, n, is always a non-negative integer. The term "transducer" will be used throughout this specification, including the claims, to denote either a transmitter or a receiver. The synonymous terms "detector" and "receiver" will be used interchangeably. The waves produced by a multipole transducer will be referred to as "multipole waves." In the inventive method, an acoustic signal is generated at a transducer disposed in the borehole, the signal propagates into the formation, and an arrival of the signal is detected at another transducer disposed in the borehole. At least one of the transducers is a multipole transducer oriented at a first azimuthal angle relative to the borehole's longitudinal axis.

Another acoustic signal arrival, associated with a second azimuthal angle relative to the borehole longitudinal axis, is also detected at a transducer disposed in the borehole. This latter arrival either includes wave energy generated at a 15 multipole transmitter oriented at the second azimuthal angle, or S* it is detected at a multipole detector oriented at such angle, or is generated by such transmitter and detected by such *4 o detector. If the formation is azimuthally anisotropic, 4 4 different acoustic wave velocities will be associated with the two arrivals. The anisotropy may be studied by analyzing the detected arrivals.

4 In a preferred embodiment of the inventive method, a single 4"4 dipole transmitter is operated to generate the acoustic signal (or signals) employed, and the arrivals are detected at two S* 4* dipole receivers oriented at different azimuthal angles. In an alternative preferred embodiment, a single monopole transmitter is operated to generate the acoustic signal (or signals), and 4 the arrivals are detected at two quadrupole receivers oriented at different azimuthal angles. More generally, when studying formations exhibiting azimuthal anisotropy, it is preferred that for each transmitter- receiver pair used in performing the inventive method (or included in the inventive system), if the multipole order of one transducer in the pair is n, the multipole order of the corresponding transducer should be n+2m, where n+2m>O, and m is an integer (positive, negative, or zero).

There are numerous variations on the preferred embodiments.

For example, a single acoustic signal may be generated, and arrivals of this wave then detected at two detectors.

Alternatively, a first signal may be generated and detected and thereafter, a second signal generated and detected. The two i

B

15 signals may be generated by the same transmitter, and detected *by the same detector, if either the source or detector is a rotated azimuthally relative to the other between the two signal Sditction events. Alternatively, two distinct source-detector pairs may be employed in performing the method, The inventive borehole logging system will preferably include at least one dipole transmitter and at least one dipole k*I, 4detector, or one monopole transmitter and at least one quadrupole detector, or at least one quadrupole transmitter and 4 25 one monopole detector. Preferably, two multipole transmitters (or two multipole detectors) are included, where the two transmitters (or detectors) are positioned at distinct azimuthal -11orientations. Alt ernatively, only one multipol- transducer is included, and a means for rotating the multipole transducer between acoustic signal generation (or detection) events is also included.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a schematic view of a well logging system that may embody the invention.

FIGURE 2 is a cross-sectional view of a formation traversed by a borehole and having fractures oriented along the x-axis.

V~ FIGURE 3 is a perspective view of a portion of a borehole 15 logging tool disposed in a borehole, showing the paths of two 0 4 4 acoustic waves propagating between a pair of transducers in the tool.

4, FIGURE 4 is a graphical representatlon of three acoustic wave arrivals typical of those detected in logging an azimuthally anisotropic formation in accord with the inventive to A, method.

i- Ai 4 FIGURE 5 is a simplified, perspective view of a preferred 25 embodiment of the inventive acoustic well logging system* FIGURE 6 is a simplified, perspective view of an alternative embodiment of the inventive acoustic well logging system, -12- DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGURE 1 is a schematic view of an acuustic well logging system that may embody the invention. Logging sonde 10 is adapted to be raised and lowered into well 20 surrounded by earth formation 22. Sonde 10 houses transducer 12 for generating an acoustic wave and two transducers (14 and 16) for detecting arrivals of the acoustic wave(s). To initiate logging, sonde 10 is suspended in fluid 18 contained in borehole 20. Detectors 14 and 16 are mounted in sonde 10 in positions spaced along the longitudinal axis of borehole 20 from each other and from source 12. Source 12 is connected to firing and recording control unit 24. Although the firing and recording control unit is shown in FIGURE 1 as a separate unit S" 15 from the logging sonde, the part of unit 24 that powers the 4o 6C S° acoustic wave source may, for convenience in operation, be b housed by the logging sonde. Signals recorded by detectors 14 o0 0 and 16 are fed to band-pass filter 26, amplifier 28 and timeo t interval unit 30. After amplification by amplifier 28 tte signal may be digitized and recorded in unit 32, or may be ,g displayed in unit 33, or may be both recorded and displayed il t* units 32 and 33.

O t 000 Firing and recording control unit 24 is operated to fire source 12 which produces an acoustic wave (which may include both shear and compressional wave components) in formation 22.

Acoustic wave arrivals are detected by detectors 14 and 16.

-13- Sonde 10 will typically contain a pre-amplifier (not shown in FIGURE 1) which amplifies the acoustic wave arrivals detected by detectors 14 and 16. The amplified signals are then filtered by filter 26 and amplified again by amplifier 28. The time interval between the arrival at detector 14 and the corresponding arrival at detector 16 is then measured by timeinterval unit 30. The measured time interval may be stored or displayed as desired.

Alternatively, the detected signals may be displayed (such as by display unit 33) before, instead of, or in addition to being processed in time-interval unit 30. In another alternative embodiment, a signal representing the time at which source 12 transmits an acoustic wave may be supplied to S* 15 filter 26, amplifier 28, and time-interval unit 30. In this 9 4 4 ,o alternative embodiment, unit 30 will measure the time interval between transmission of an acoustic signal from source 12 and 4 *,wCa the arrival of acoustic energy in the signal at a single detector either detector 14 or 16).

In accordance with the invention, either detectors 14 and 16 or source 12, or all three of these transducers must be of the multipole type. The invention relies on the property that a multipole transducer may be oriented in a particular azimuthal direction relative to the longitudinal axis of a borehole.

FIGURE 2 is a cross-sectional view of borehole 37 having longitudinal axis perpendicular to the plane of FIGURE 2. A multipole acoustic source (not shown) in borehole 37 will have 2n vibrating elements (where n is the multipole order of the source, an integer greater than zero) arranged around the tool's longitudinal axis. The transducers of the inventive logging system (when operated in a borehole traversed by an azimuthally anisotropic formation such as formation 38 of FIGURE 2) may be oriented to produce multipole acoustic waves that have polarization (identified by arrow 34 in FIGURE 2) in the azimuthal direction 9. Formation 38 traversed by borehole 37 is azimuthally anisotropic since it is permeated with vertical fractures 39 that are oriented parallel to the x-axis. Wave S (the acoustic wave initially propagating with polarization in the direction of arrow 34) will have component waves S and S (with relative amplitudes determined by 8, the sc azimuthal orientation of the initial stress creating the wave) 4 15 whose polarizations, 35 and 36 respectively, are different as shown in FIGURE 2. Since the speeds along the borehole's 4 longitudinal acis of the component waves S and S are S' different, if the wave S has a velocity component perpendicular to the plane of FIGURE 2 along the longitudinal axis of the borehole) one will observe a splitting of the energy into two component waves SI and S between whose times of arrival the difference increases the further down the borehole Saxis one masures the response. This phenomenon permits one to detect azimuthal anisotropy and measure its magnitude.

4 t, FIGURE 3 and 4 illustrate the energy splitting of two components of a nIultipole acoustic wave produced by source 44 in tool 45 disposed in fluid-filled borehole 40. The z-axis is parallel to the longitudinal axis of the borehole. Formation 42 is permeated by vertical fractures 43. Fractures 43 are parallel to the x-z plane. Component W of the wave has polarization in the y-z plane (perpendicular to fractures 43); component W 1 of the wave has polarization in the x-z plane (parallel to fractures 43). Since formation 42 is azimuthally anisotropic, in a time interval t, cox- nent W will in general propagate to depth dl while component W1 will propagate to depth d2, where d2-dl=Ad, and Ad>0. In other words, the times of arrival of W 11 and W at a given depth will be different. If detector 46 is a multipole detector oriented so as to be sensitive only to component WI, then the sigal received at detector 46 representing the 9 9 arrival of the wave will appear as in FIGURE 4A, with a peak amplitude at time t. If detector 46 is a multipole detector oriented so as to be sensitive only to component Wn, then *4 *II .o the signal received at detector 46 representing the arrival of the wave will appear as in FIGURE 4C, with a peak at time t 2 If detector 46 is a multipole detector oriented (rotated into a *0o 20 desired azimuthal angular position) so as to be sensitive to both components W 1 and then the signal received at the detector representing the arrival of the wave will appear as in FIGURE 4B, with two peaks, separated by time At=t2-t i 4. where At corresponds to Ad in FIGURE 3. In contrast, if 4 00 formation 42 were isotropic (so that all components of an acoustic wave having a given frequency content will have the same speed, vf), then the signal received at detector 46 representing the arrival of the wave would not exhibit splitting

I

r[ -16as the detector is azimuthally rotated. Rather the detected arrival would have a peak at a time tf determined by the distance the wave propagates between transmission and detection and the fixed velocity vf of the isotropic medium.

In the inventive method, a first acoustic wave is generated at a transducer in a borehole so as to propagate through an earth formation surrounding the borehole. An arrival of the first wave is detected at another transducer in the borehole.

At least one of the transducers is of the multipole type, and has a first azimuthal orientation. Thus, the wave arrival will have appearance similar to either FIGURE 4A, 4B, or 4C, depending on whether the first azimuthal orientation happens to a a be parallel (or perpendicular) to any axis of the formation's azimuthal anisotropy (in either of which cases, the arrival will not be split, as in FIGURES 4A and 4C) or happens to be neither a Oo 0 parallel nor perpendicular to such an axis of azimuthal o 0 anisotropy (in which case, the arrival may be split as in FIGURE 4B).

0.c° *a a o.a. Also, in performing the inventive method, a second acoustic wave arrival is detected at a transducer disposed in the a borehole. This second arrival may be associated with the first wave described in the preceding paragraph may contain a S4 25 portion of the energy of such first wave), or may be associated with a second acoustic wave that has been generated by the same or a different transducer than the one that generated the first wave. The second arrival must be associated with a multipole -17transducer (either with a multipole source or detector, or with both), and the multipole transducer must have a second azimuthal orientation different from the first orientation. Comparison of the first and second arrivals will reveal information about the existence and type of azimuthal anisotropy of the formation.

For example, if splitting is observed in one or both of the arrivals, then it follows that the formation is azimuthally anisotropic. If both arrivals exhibit splitting, the relative amplitudes of the double peaks of the arrivals reveal information about the direction of azimuthal anisotropy.

Two preferred embodiments of the inventive method will next be described. The first preferred embodiment employs at least 0 o one source of dipole acoustic waves ("dipole source") and at os oo 4 15 least one detector sensitive to dipole acoustic waves ("dipole 0 sdetector"). The second preferred embodiment employs at least o oe 0 0 0 0 0 0 to a one detector sensitive to acoustic quadrupole waves ("quadrupole *m 0 I detector"), and either at least one source of monopole acoustic waves ("monopole source") or at least one source of quadrupole 20 acoustic waves ("quadrupole source"), or both a quadrupole source and a monopole source.

Applicants have recognized that the polarization of dipole waves generated by a dipole source is predominantly linear, so 25 that a "dipole-source/dipole-detector" system is adequate for measuring the splitting of dipole acoustic waves in an azimuthally anisotropic formation. In one variation of the inventive method, a single dipole acoustic wave is generated, S-18and arrivals of the wave are detected at two different dipole detectors, where the detectors have different (and preferably, orthogonal) azimuthal orientations. In another variation, two dipole acoustic waves are generated and each is detected at a different dipole detector, or each is detected at the same dipole detector, where the detector is rotated azimuthally between the two wave-detection events.

The inventors have recognized that in an azimuthally anisotropic formation, some of the acoustic wave energy produced by a monopole source in a borehole will couple to a quadrupole mode and hence propagate as a quadruple wave in the formation.

Thus the arrival detected at a detector in the borehole will be 0 o e o a mixture of monopole and quadrupole signals. Accordingly, 0 od 15 either a monopole source, or a quadrupole source, with two S.o* quadrupole detectors having different azimuthal orientations (or t a a qradrupole detector that may be rotated azimuthally) may be used to measure anisotropy effects in a preferred embodiment of the inventive method. Similarly, two quadrupole sources with 20 different azimuthal orientations (or one azimuthally rotatable 00: quadrupole source) may be used in an alternative preferred embodiment.

to* In order to describe a preferred embodiment of the invention, the phrase "simple" azimuthal anisotropy shall be used herein to describe the symmetry of formations that are isotropic in one direction but anisotropic in the azimuthal directions orthogonal thereto and having only two orthogonal r -19azimuthal directions of symmetry. Examples of formations exhibiting simple azimuthal anisotropy include formations that have parallel, vertically oriented cracks, but are otherwise homogeneous. For any cource-receiver pair employed in performing this preferred embodiment of the inventive method, the multipole order of one transducer in the pair is a non-negative integer n (such as n=0, corresponding to a monopole transducer; n=l, corresponding to a dipole transducer; n=2, corresponding to a quadrupole transducer; n=3, corresponding to an octopole transducer; and so on), and the multipole order of the other transducer in the pair is n+2m, where n+2m is greater than or equal to zero, and m is an integer (m may be a positive integer, a negative integer, or may be equal to zero). However, one of the transducers in the pair must be a multipole 15 transducer, so that one of the following conditions must be maintained: n>O, m>0, or both n>0 and m>O. For example, the following source-receiver pairs may be employed in performing this embodiment of the inventive method: dipole-source/dipolereceiver, quadrupole-source/quadrupole-receiver, octopole- 20 source/octopole-receiver, modopole-source/quadrupole-receiver, quadrupole-source/monopole-receiver, monopole-source/16-polereceiver, 16-pole-source/monopole-receiver, dipolesource/octopole-receiver, of octopole-source/dipole-receiver.

t There may be more than one transducer having different multipole orders in a particular system, one transducer having a multipole order of n+2m, and a second transducer having a multipole order of n+2p. The selection of p would require the same general considerations as the selection of m.

i ,I The preferred embodiment described in the previous paragraph is particularly effective for surveying formations exhibiting simple azimuthal anisotropy. In order to study other types of formations formations exhibiting complex azimuthal anisotropy), it may be preferable that the limitations in the embodiment of the preceding paragraph be relaxed to permit the integer to be selected from the set of numbers including half-integer numbers as well as integer numbers. For example, to study such complex azimuthally anisotropic formations, it may be desirable to employ a system including a dipole transmitter and a quadrupole detector A preferred design for the inventive well logging tool will next be described with reference to FIGURE 5. The transducers 4 15 of the FIGURE 5 system are mounted in sonde 50 which may be suspended in a borehole from cable 51. Monopole source M, which 44 may be selected from those conventional monopole sources that 4 4 are commercially available, is used to generate monopole signals as in a conventional sonic tool. Not shown are the electrical *io, 20 connections between the acoustic sources of FIGURE 5 and a power 04 source and a firing and control unit, since such connections may be accomplished in a well known manner, and are schematically shown in FIGURE 1. Dipole acoustic wave transmitter unit D S* includes eight equal segments of a piezoelectric cylinder that are electrically driven so that each pair oi oppositely facing segments can be driven as a dipole source. The particular design of unit D shown in FIGURE 5 thus comprises four distinct dipole sources each azimuthally oriented at 450 with respect to the adjacent dipole source pairs 1-5, 2-6, 3-7, and 4-8 are 4 stinct dipole souztes).

-21- Receiver unit R l includes two acoustic quadrupole wave receivers. Receiver unit RD1 includes four acoustic dipole wave receivers. R and RD together will be collectively referred to as a receiver station. Although a single receiver station is shown in FIGURE 5, preferably there will be at least two receiver stations in housing 50, each containing both dipole receivers (as in RDl) and quadrupole receivers (as in RQ 1 Each receiver unit shown in FIGURE 5 is electrically divided into eight equal segments (similarly to multi-dipole source D described above). Each of the four dipole receivers of RDl is aligned, respectively, with one of the four dipole sources of transmitter unit D.

4e t4 S* The segments of receiver unit RD are electrically 0 l15 connected to form four dipole receivers separated at 450 azimuthal angles, and the segments of unit R are 4 QQ electrically connected to form two quadrupole receivers (1-3-5-7 and When one dipole source is fired source Sii, any receiver in each receiver station is activated to 20 record an arrival of the generated signal. The same source can 4c 4 be fired four times and consequently record the signal at four 44 4 differently oriented receivers, or one can fire one dipole source and simultaneously record the signals received at all four dipole receivers. As a result of sequentially firing the four dipole sources and detecting the associated arrivals at one receiver station, one detects sixteen dipole signals associated with four different azimuthal angles with four duplications which may be analyzed to distinguish and measure the degree of -22formation azimuthal anisotropy. When the monopole source M is fired, one or two quadrupole receivers (receiver 1-3-5-7, or receiver 2-4-6-8, or both of these receivers) will be operated to detect.arrivals of the signal produced by source M. By analyzing the two detected quadrupole wave arrivals in the maanner described above, one detects the existence and magnitude of formation anisotropy. Other receiver stations positioned at larger separations from the source will provide redundancy which will improve the accuracy of the system.

An alternative embodiment of the inventive well logging tool is shown in simplified form in FIGURE 6. Source 62 and detector 64 are housed in sonde 60, which may be suspended in a borehole at the end of cable 61 so that the sonde's longitudinal 15 axis is substantially parallel to the borehole's longitudinal 4 as 4 0 axis. Transducers 62 and 64 may be selected using the criteria described above so that the FIGURE 6 system is capable of performing the inventive method. Preferably, transducers 62 and 64 will be of the dipole type, or source 62 will be a monopole source and detector 64 will be a quadrupole detector.

S*4 Stepping motors 66 and 72 are mounted in sonde 60 for rotating 4 44 transducers 62 and 64 relative to each other about the longitudinal axis of sonde 60. Transducers 62 and 64 are S mounted, respectively, on hollow shafts 68 and 70 which are aligned with the longitudinal axis of sonde 60. Motors 66 and 72 may be operated independently in response to electrical signals supplied via electrical lines 74, 76. Line 74 extends through shafts 68 and 70 and is connected to motor 72. Line 76 extends through shaft 68 and is connected to motor 66.

Motors 66 and 72 may be selected from commercially available models.

-23- In all embodiments of the invention, the frequency content of the acoustic wave generated by the transducer (or transducers) employed will determine the relative portions of energy in the wave that will propagate through the formation in the shear-wave mode and in the compressional-wave mode. The preferred operating frequencies broducing shear waves or compressional waves of various multipole orders have been described in U.S. Patent 4,606,014, issued August 12, 1986 to Winbow, et al., U.S. Patent Application Serial Number 379,684 (filed May 19, 1982); U.S. Patent Application Serial No. 440,140 (filed November 8, 1982); and U.S. Patent 4,649,526, issued March 10, 1987 to Winbow, et al., all assigned to the assignee 6* to of the present application.

4 4* o o 4 4 ve* As an example of the inventive method's resolution, consider Gas the dipole-transmitter/dipole-detector embodiment, where the S*transmitter is operated at a frequency such that the emitted dipole wave has predominant frequency of about 4kHz. In a typical formation having a shear-wave slowness of (150 Usec/ft) with an azimuthal anisotropy of 10, we expect to detect a shear-wave slowness variation of 4, (15 ps/ft) between the two principal polarization axes.

For a logging tool with a separation of ten feet between dipole source and dipole detector, the time difference between the two polarized shear arrivals would be approximately 150 ps, which is about 60% of a complete cycle for a 4kHz shear-wave signal.

The highest resolution one can achieve in this embodiment using a standard pulsing technique wherein the dipole transmitter -24emits a dipole pulse is approximately 4 ps. For the situation described above, using an inter-receiver separation of 1 foot, 4 ps of time resolution corresponds to detecting an anisotropy of 2.67%. To increase the resolution a larger inter-transducer separation can be used.

Driving the dipole transmitter with a sine-wave power generator instead of the more usual pulsing technique would further enhance the system's resolution because the phase shifts between monochromatic sine waves can be determined more precisely than can the time interval between the arrival of two impulses. The technique of measuring phase shifts between the sine waves detected at separated locations in a well logging system is well known. An example of the phase precision achievable using sinusoidal signals is which is equivalent oc to about 1.4 ps for a 4kHz sine wave. As above, for a P ga° shear-wave slowness of (150 ps/ft)' 1.4 ps time resolution allows 1% of anisotropv to be detected using a one-foot inter-receiver separatie If the number of dipole transducers (each associated with a pair of vibrating elements) in each receiver unit of the inventive tool (ie. receiver unit i0. RDl of FIGURE 5) is increased to 8 or 16 if the number SI**rl of vibrating elements in each unit is increased to 16 or 32), an increase in the azimuthal resolution is obtained, A 32-element receiver unit would form 16 pairs of dipole receivers with a relative angular orientatior of 11i25".

If greater resolution is needed, an additional dipole receiver unit may be added, each transducer in this additional, unit pair being oriented at 5.6250 relative to the corresponding transducer in the unit pair described in the preceding paragraph. This latter, more complex tool, would be suitable for investigating more complex forms of anisotropy created, for example, by the presence of more than one set of fractures in the formation.

In embodiments employing sine wave acoustic signals, it may be desirable to employ a swept-frequency sine wave signal, or a pseudo-random sweep signal, analogous to those commonly produced by seismic land-vibrators during seismic surveying operations, but for this purpose operating at logging frequencies (kHz).

4 0 iO o. In all embodiments of the invention, the absolute orientation of the tool may be determined using a gyro-compass, or any other conventional means suitable for accomplishing this purpose, Various modifications and alterations in the inventive *4 method and system will be apparent to those skilled in the art 4 *4 4'44* without departing from the scope of this invention. Although 4 4 the invention has been described in connection with specific preferred embodiments, it should be understood that the S. 2,3 invention as claimed should not be unduly limited to such Sembodiments specific embodiments.

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Claims (26)

1. A method, usinLg at least one multipole transducer, for. measuring azimuthal anisotropy of a formation traversed by a borehole having a longitudinal axis, including the steps of: operating a first transducer disposed in the borehole to generate a first acoustic signal that will propagate into the formation from within the borehole; detecting an arrival of the first acoustic signal at a second transducer disposed in the borehole, where at least one of the first and second transducers is a multipole transducer oriented at a first azimuthal angle relative to the longitudinal axis of the borehole; and at a location within the borehole, detecting 15 another acoustic arrival hep&^bhe a de ia-t a -t*p 4is associated with a second azimuthal angle relative to the longitudinal axis of the borehole. 4 4

2. The method of claim 1, wherein the first acoustic signal is impulsive.

3. The method of claim 1, wherein the first acoustic signal is sinusoidal.

4. The method of claim 3S wherein the first acoustic signal is a swept-frequency sine wave. t i 4 4 -27- The method of claim 3, wherein the first acoustic signal is a pseudo-random sweep signal.

6. The method of claim 1, wherein the first transducer has mu!.tipole order n, and is oriented at said first azimuthal angle relative to the borehole's longitudinal axis, and the second transducer has multipole order n+2m, where n+2n>0, and m is an integer.

7. The method of claim 6, wherein the first transducer is multipole transducer and the arrival detected in step is associated with a second acoustic signal azimuthally distinct from the first acoustic signal, and also including the step of: operating a third transducer disposed in the 15 borehole, where the third transducer is a multipole transducer having multipole order n and is oriented at said S* second azimuthal angle relative to the borehole's longitudinal axis, to generate the second acoustic signal in such a manner that the second acoustic signal will propagate into the formation from within the borehole. 44 I 4

8. The method of claim 7, wherein the arrival detected in S* step is detected at a fourth transducer, having multipole order n+2m and being disposed in the borehole.

9. The method of claim 7, wherein the first and third transducers are located at substantially the same location along the borehole's longitudinal axis. -28- The method of claim 7, wherein the first and third transducers are quadrupole transmitters and the second transducer is a monopole detector.

11. The method of claim 7, wherein the first and third transducers are octopole transmitters, and the second transducer is a dipole detector.

12. The method of claim 6, wherein the second transducer is a multipole detector oriented at said first azimuthal angle relative to the borehole's longitudinal axis, and the arrival detected in step is detected by a third transducer, having multipole order n+2p and disposed in the borehole, where p is an integer and n+2p>Q, where the third transducer is oriented at S. 15 said second azimuthal angle relative to the borehole's o 9 longitudinal axis, and wherein the acoustic arrivals detected in S• steps and are both arrivals of the first acoustic signal. 9

13. The method of claim 12, wherein the first transducer is a monopole transducer, and the second and third transducers are multipole transducers. 4r 14. The method of claim 12, wherein the second and third transducers are located at substantially the same location along .25 the borehole's longitudinal axis, but are oriented at different azimuthal angles relative to the borehole's longitudinal axis. -29- The method of claim 1, wherein the arrivals detected in steps and are both detected at the second transducer.

16. A method, using at least one multipole transducer, for measuring azimuthal anisotropy of a formation traversed by a borehole having a longitudinal axis, including the steps of: operating a first transducer disposed in the borehole to generate a first acoustic signal that will propagate into the formation from within the borehole; detecting an arrival of the first acoustic signal at a second transducer disposed in the borehole, where at least one of the first and second transducers is a multipole transducer oriented at a first azimuthal angle relative to •the longitudinal axis of the borehole; S 15 rotating said second transducer to orient it at a as second azimuthal angle relative to the longitudinal axis of the borehole; operating said first transducer to generate a second acoustic signal azimuthally distinct from said first acoustic signal; and detecting the arrival of ',he second acoustic P signal while the second transducer is oriented at said second azimuthal angle. S" w

17. A method for measuring azimuthal anisotropy of a formation traversed by a borehole, said borehole having a longitudinal axis, including the steps of: operating a first transducer having multipole order n at a first location along the longitudinal axis to generate an acoustic signal that will propagate into the formation from within the borehole; detecting the arrival of the acoustic signal at a second transducer disposed in the borehole, where the second transducer is a multipole receiver oriented at a first azimuthal angle relative to the longitudinal axis and having multipole order n+2m, where m is a member of the set of integers and half-integers; and ap at a location within the borehole, detecting vn t. 15 another acoustic arrival, \whe=t=a-l- t- d fig-- 4 0 -&up-4 is associated with a second azimuthal angle relative to the longitudinal axis of the borehole. S*

18. A method for measuring azimuthal anisotropy of a formation traversed by a borehole, said borehole having a longitudinal axis, including the steps of: operating a first transducer having multipole order n at a first location along the longitudinal axis to generate an acoustic signal that will propagate into the 25 formation from within the borehole; -31- detecting an arrival of the acoustic signal- at a second transducer disposed in the borehole, where the second transducer is a multipole receiver oriented at a first azimuthal angle relative to the longitudinal axis and having multipole order n+2m, where m is a member of the set of integers and half-integer numbers; and detecting an arrival of the acoustic signal at a third transducer disposed in the borehole, where the third transducer is a multipole receiver oriented at a second azimuthal angle relative to the longitudinal axis and having multipole order n+2m.

19. The method of claim 18, wherein the first transducer is a monopole transmitter and the second and third transducers are 4. 4 15 quadrupole receivers. 4 4 o*°o 20. The method of claim 18, wherein the second and third SOotransducers occupy azimuthally distinct portions of a single receiver unit, and receiver unit being located at a second location spaced along the longitudinal axis from the first So i location. .4

21. The method of claim 18, wherein the acoustic signal is an impulsive signal, and also including the step of: measuring the time interval between the arrival times detected in steps and i. -32-

22. The method of claim 18, where the acoustic signal.is. sinusoidal, and also including the step of: measuring the relative phase of the arrivals detected in steps and

23. The method of claim 20, wherein the acoustic signal is an impulsive signal, and also including the steps of: detecting the arrival of the acoustic signal at a fourth transducer disposed in the borehole at a third location spaced along the longitudinal axis from the second location, where the fourth transducer is a multipole receiver oriented at the second azimuthal angle relative to the longitudinal axis and having multipole order n+2m; and measuring the time interval between the arrival 4o *4 4 times detected in steps and 4 4 4 4

24. A method for measuring azimuthal anisotropy of a formation traversed by a borehole, said borehole having a longitudinal axis, including the steps of: operating a first transducer having multipole order n in the borehole to generate a fire acoustic signal that will propagate into the formation from within the borehole; 4 t C -33- detecting an arrival of the first acoustic signal at a second transducer disposed in the borehole, where at least one of the first and second transducers is a multipole transducer, and the multipole order of the second transducer is n+2m, where m is an integer and n+2m10, and the detected arrival of the first acoustic signal is associated with a first azimuthal angle relative to the longitudinal axis; operating a third transducer having multipole order n+2p, where p is an integer and n+2p>0, to generate a second acoustic signal that will propagate into the formation from within the borehole; detecting an arrival of the second acoustic signal at the second transducer, where at least one of the second S, and third transducers is a multipole transducer, and the .4 15 detected arrival of the second acoustic signal is associated *r 4 with a second azimuthal angle relative to the longitudinal ae axis. The method of claim 24, wherein the first and third transducers occupy azimuthally distinct portions of a transmitter unit positioned at a single location spaced along 4 the longitudinal axis from the second transducer. 4 2 t «t I -34-

26. A method for measuring azimuthal anisotropy of a formation traversed by a borehole, said borehole having a longitudinal axis, including the steps of: operating a rotatable multipole transmitter having multipole order n, where n>0, and oriented at a first azimuthal angle relative to the longitudinal axis to generate a first acoustic signal that will propagate into the formation from within the borehole; detecting an arrival of the first acoustic signal at a receiver disposed in the borehole, said receiver having multipole order n+2m, where m is an integer and rotating the multipole transmitter about the longitudinal axis to reorient said transmitter at a second azimuthal angle relative to the longitudinal axis; Sa 15 after step operating the multipole 0 transmitter to generate a second acoustic signal that will oo*, propagate into the formation from within the borehole; and *4 detecting an arrival of the second acoustic signal 4 1 at a receiver disposed in the borehole, said receiver having multipole order n+2p, where p is an integer and S* 04 0a

27. The method of claim 26, wherein the transmitter is a quadrupole transmitter, and the receiver recited in step is a monopole receiver and is the same receiver as is recited in 25 step 0 *4

28. A method for measuring azimuthal anisotropy of a formation traversed by a borehole,, said borehole having a longitudinal axis, including the steps of: operating an acoustic transmitter having multipole order n to generate a first acoustic signal that will propagate into the formation from within the borehole; detecting an arrival of the first acoustic signal at a rotatable multipole receiver having multipole order n+2m, where m is an integer and n+2m>0, said receiver being disposed in the borehole and being oriented at a first azimuthal angle relative to the longitudinal axis; rotating the receiver to cause it to be oriented at a second azimuthal angle relative to the longitudinal *axis; after step operating the transmitter to "a" generate a second &coustic signal that will propagate into the formation from within the borehole; and 4 a detecting an arrival of the second acoustic signal at the receiver. 449*

29-. The method of claim 28, wherein the transmitter is a monopole transmitter. A system for measuring azimuthal anisotropy of a 25 formation traversed by a borehole, including: a housing adapted to be suspended in the borehole and having a longitudinal axis; -36- a first transducer having multipole order n mounted in the housing and capable of generating a first acoustic signal that will propagate from within the borehole into the formation; a first multipole receiver, having multipole order n+2m, where n is a non-negative integer, m is a member of the set of integers and half-integer numbers, and and capable of detecting an arrival of the first acoustic signal, said first multipole receiver being mounted in the housing at a first azimuthal angle relative to the longitudinal axis, and a second multipole receiver, having multipole order n+2p, where n is a non-negative integer, p is a member of the set of integers and half-integer numbers, and 15 and capable of detecting an arrival of the first signal, .t.o said second multipole receiver being mounted in the housing S* 1 0440 ,4 at a second azimuthal angle relative to the longitudinal e0 0 0.0 4axis, 31, The system of claim 30, including a receiver unit «4 having longitddinal dimension not greater than the approximate longitudinal dimension of the first transducer, and wherein the I first and second receivers comprise azimuthally distinct portions of the receiver unit. S* I :1 3 -37-

32. A system for measuring azimuthal anisotropy of a formation traversed by a borehole, including: a housing adapted to be suspended in the borehole and having a longitudinal axis; a first multipole transmitter having multipole order n+2m, where n is a non-negative integer, m is a member of the set of integers and half-integer numbers, and said first multipole transmitter being mounted in the housing at a first azimuthal angle relative to the longitudinal axis, and being capable of generating a first acoustic signal that will propagate from within the borehole into the formation; a second multipole transmitter, having multipole order ni2p, where p is a member of the set of integers and 15 half-integer numbers, and n+Rp>0, said second multipole transmitter being mounted in th? housing at a second azimuthal angle relative to the longitudinal axis, and being capable of generating a second acoustic signal that will propagate from within the borehole into the formation; and 20 a receiver, having multipole order n, mounted in the housing at a position spaced along the longitudinal axis from the first transmitter and the second transmitter, and capable of detecting arrivals of the first and second acoustic signals. 4r 4 4a 4 94 4 4 40 9r 9 L4 44 4 '444 j -38-

33. The system of claim 32, including a transmitter unit having longitudinal dimension not greater than the approximate longitudinal dimension of the receiver, wherein the first and second multipole transmitters comprise azimiuthally distinct portions of the transmitter unit.,

34. A system for measuring azimuthal anisotropy of a formation traversed by a borehole, including: a housing adapted to be suspended in the borehole and having a longitudinal axis; a first transducer, having multipole order n, mounted in the housing and capable of generating an acoustic signal that will propagate from within the borehole into the formation; 4* 4 S, 15 a second transducer having multipole order n+2m, a where m is a member of the set of integers and half-integer S numbers, and n+2m)0, said second transduczer being mounted in 044 4 o the housing and capable of detecting an arrival of the acoustic signal, where at least one of the first and second ?0 transducers is a multipole transducer;and 4*4 means for rotating at least one of the first and 4. 44S 4, second transducers azimuthally with respect to the other. 4 4 The system of claim 34, wherein the rotating means 25 includes a stepping motor. 4 DATOW this 19th clay of May 1989. EXXON PRODUI'ION RESEARCH COMPANY WATERMaRK PATNt TRADAK ATORNEYS 50 QUL]IN STREEiT MELBOURNE, VIC., 3000, I 0o,0it t t 0'I and the preoding pages aw a