GB2399411A - Cased borehole investigation apparatus deviates parasitic echoes away from receiver - Google Patents

Abstract

An apparatus for investigating the cement annulus between a steel casing 5 and a borehole wall comprises an acoustic (ultrasonic) transmitter T and receiver R spaced apart and aligned at an angle from the normal to the wall of the casing, in a pitch-catch arrangement. Only a small amount of the emitted acoustic energy is transmitted through the casing to the probe the annulus, so the shape of the tool structure 9 is designed so as to deviate any unwanted parasitic echoes (specular reflections, tool reflections) away from the receiver R. The edge surfaces of the supporting structure 9 are serrated 12 to deviate unwanted signals, and the structure may further be coated in an absorbing material which attenuates unwanted signals. A second receiver may be provided to allow for computation of casing flexural wave attenuation. Deflector plates may also be used to further reduce unwanted waves.

Description

239941 1 Apparatus for acoustically investigating a borehole

FIELD OF THE INVENTION

The invention relates generally to an apparatus for acoustically investigating a wall of a borehole penetrating an earth formation. More specifically, the invention allows acoustical measurement of set cement in a borehole.

BACKGROUND OF THE INVENTION

Acoustic pulse echo techniques for investigating a casing set in a borehole are known in the art. These techniques generally fall into two classes: sonic cement evaluation and ultrasonic cement evaluation.

One sonic evaluation technique described in US Pat. No. 3,401,773 uses a logging tool employing a conventional, longitudinally spaced sonic transmitter and receiver. The received signal is processed to extract the portion affected by the presence or absence of cement. The extracted portion is then analyzed to provide a measurement of its energy, as an indication of the presence or absence of cement outside the casing. This technique provides useful information about cement defects at the interface between the casing and the cement. However, sonic techniques have several limitations such as poor azimuthal and axial resolutions and strong sensitivity to the bond quality between the casing and the cement, thus requiring, in the cases of poor bond quality, internal pressurization of the casing, which itself can degrade cement integrity.

Ultrasonic cement evaluation tools concentrate on the interface between the casing and the cement in order to determine whether cement or mud is adjacent to the casing. US patent 4,382,290 discloses a pulse echo tool comprising an ultrasonic transducer that is introduced in the borehole. The decay rate of an acoustic wave resonating in a direction normal to the casing is then analyzed. The main limitation encountered with these normal incidence waves techniques is that only the interface between the casing and the cement can be scanned. These measurements do not probe the entire thickness of the cement annulus. Therefore, in case a water channel -or any unwanted discontinuity- exists in the cement annulus behind the casing/cement boundary, it won't be possible to detect it.

A specific type of measurement to be made in the borehole involves acoustic waves that propagate towards the casing walls in a direction deviating from a direction normal to the wall of the casing. This specific geometry has been described in the international patent application published under W099/35490, which is incorporated here by reference. The geometry, also known as pitch/catch geometry, uses a pair of transducers. One transducer is used as transmitter while the other is used as receiver. The transducers are mounted in appropriate angles along the axis of the tool with an offset. This technique launches a flexural wave in the casing with a vertical direction of propagation. This wave propagates in the casing and radiates in the casing fluid and in the medium outside the casing. The outward propagating wave is eventually reflected at the formation wall back inside the casing. Then, the receiver detects both the direct flexural wave and the possibly weak echo from the formation wall. Therefore, this technique allows the acoustic signal to go further than the casing/cement boundary and consequently to scan the thickness of the cement annulus. It is thus possible to detect any discontinuity in the thickness of the cement layer between the casing and the annulus. The limitation with this pitch/catch techniques is that little of the acoustic energy is transmitted through the casing to probe the annulus. It is thus important to avoid any spurious echo that could reach the receiver and mask the formation echo.

Such parasitic echoes come from either specular reflection or from reflection of acoustic energy on the structure supporting the transducers. Specular reflection propagates only in the casing fluid and is reflected at the casing inner surface. Tool reflection, i.e. reflection of the acoustic energy on the structure supporting the transducers, propagates partly in the casing as a flexural wave, is reemitted in the fluid, reflected on any structure present inside the casing and converted again into the casing flexural wave, before detection at the receiver.

SUMMARY OF THE INVENTION

The object of the invention is thus to provide an apparatus for acoustically investigating a cased borehole, said apparatus comprising a tool structure supporting at least a pair of transducers among which a transmitter generates an acoustic wave that propagates in the casing and is detected by a receiver, characterized in that said tool structure is designed such that it deviates parasitic echoes from said receiver.

The apparatus of the invention thus provide a simple and very efficient way of eliminating any spurious echo that could reach the receiver and mask the formation echo.

Preferably, the surfaces of the tool structure that face the casing and that may be impacted by waves generated by the transmitter are made non parallel to the casing inner surface and oriented outward so that any parasitic echo impacting said tool surfaces is deviated away from said receiver front face. Furthermore the edges of said surfaces that are parallel to the casing longitudinal axis are serrated such that any parasitic echo impacting said edges is deviated away from the receiver. These technical features aim to deviate any signal that may impact the structure so that the receiver won't detect the parasitic echo.

Preferably, angles of the facets of any serration are optimized in order to deviate any parasitic echo impacting said facets away from the receiver.

Preferably, the pitch between each serration is made as large as possible.

This feature allows minimizing the amplitude of the signal that reflects on the tips of each serration and could consequently constitute a spurious echo.

According to a preferred embodiment of the apparatus of the invention, the tool structure is made of two longitudinal beams that are parallel to each other and oriented in a direction parallel to the axis of the well, said pair of transducers being placed between said two beams. In addition to the technical features that permit to deviate most of the spurious echoes, this feature simply permits to reduce the amount of parasitic echoes that are reflected by the tool structure itself.

In an advantageous implementation of an apparatus according to the invention, the tool structure is coated with an acoustically absorbing material. This feature combines the reduction of spurious echoes that may impact the receiver and the damping of one of said echo if ever it is not deviated and detected by the receiver.

Preferably, the tool structure further comprises deflector means that intercept specular waves and deviate them away from the receiver. These deflector means may comprise plates that are located close to said transmitter and/or receiver and deviate said specular waves.

Preferably, the transmitter generates flexural waves in the casing with a vertical direction of propagation and the receiver in situated in the same vertical plane as the receiver. Advantageously, the angle between said transducers and the casing normal is comprised between 28 and 38 degrees.

In an advantageous embodiment of the apparatus according to the invention, the distance between the transmitter and the receiver varies according to the diameter of the casing and the nature of the formation. The distance between the transmitter and the receiver is substantially comprised between 20 and 30 cm.

Preferably, the apparatus according to the invention comprises a second receiver situated in the same vertical plane as the first receiver, said second receiver being situated under said first receiver. The distance between the first and the second receiver is comprised between 10 and 20 cm.

Furthermore, the apparatus according to the invention further comprises other sensor means that is located between said transmitter and said receiver. This design allows reducing significantly the size of the tool while keeping the same range of measurement possibilities.

Preferably, the surfaces of said sensor means that may be impacted by waves generated from said transmitter are made non parallel to the casing inner surface and oriented outward so that any reflected wave that may impact said sensor means surfaces is deviated away from said receiver. This feature impeaches the structure of the sensor means to reflect spurious signals that may be detected by the receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and characteristics of the invention appear from the following description given by way of example and made with reference to the accompanying drawings, in which: - Figure 1 shows a schematic diagram of a logging operation; s - Figure 2 schematically represents spurious echoes that may occur while propagating a flexural wave in a casing; Figure 3 schematically represents an upper view of an apparatus according to the invention; - Figure 4 and 5 schematically represent a lateral view of a part of an apparatus according to the invention; - Figures 6a and 6b schematically represent a 3D view of an example of apparatus according to the invention; - Figure 7 represents the positions of the transducers and sensor means in the apparatus according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT (S)

Referring first to figure 1, which shows a schematic diagram of a logging operation, a sonde I (which may be self-contained, or part of drill string or other apparatus) for acquiring acoustic data includes a pressure-resistant housing 2 suspended by armored multi-conductor cable 3 (or coiled tubing or other means of eonveyanee) in a borehole which walls 4 are covered by a easing 5, the annulus between the easing and the borehole wall being filled by a cement layer 6. Cable 3 comprises conductors that electrically connect equipment within housing 2 with a data processing system 7, preferably located at the surface. A winch (not shown) is located at the surface and uses the cable 3 to lower and raise sonde I in the borehole, thereby traversing the earth formation.

Sonde 1 acquires acoustic data by emitting an acoustic pulse into easing 5 and detecting a reflected waveform. An apparatus 8 according to this invention comprises at least a pair of transducers among which a transmitter generates an ultrasonic acoustic wave that propagates in the casing and is detected by a receiver.

The signals detected by the receiver are analyzed by the data processing system 7.

Such a design is already described in patent WO 99/35490 incorporated herein by reference.

As schematically represented in figure 2, the transmitter T and the receiver R of the apparatus according to the invention are spaced apart and aligned at an angle a (measured with respect to the casing normal) larger than the shear critical angle of fluid-steel interface. This angle a needs to be adjusted according to the velocity of the casing fluid. To launch a flexural wave in the casing, this angle is computed according to the Snell's law = Arcsin( it'd' ), where VFuid is the Bra/ speed of sound in the casing fluid and VFexU, the speed of sound for the casing flexural wave. Since the speed of sound in the casing fluid may change with composition and pressure/temperature, the angle has to be adjusted accordingly.

Incorrect angles decrease the flexural wave amplitude and increase other unwanted mode amplitudes, for example the extensional mode. Angles ranging from 28 to 38 degrees cover the expected variation of fluid velocity, since the flexural velocity is approximately constant.

The outward propagating wave is eventually reflected at the formation wall 4 back inside the casing 5 through the cemented annulus 6. The receiver R. located in the same vertical plane as the transmitter T will detect both the direct flexural wave (see arrow 20 on fig.2) and the possibly weak echo from the formation wall 4 (see arrow 21 fig.2). This weak echo is thus the most important one to determine the nature of the material sitting behind the casing and, consequently, the integrity of the cemented annulus. It its thus important to avoid any spurious echo that could reach the receiver R. and mask the formation echo.

As it is represented on figure 2, such parasitic echoes come either from specular reflection (arrow 22) or from reflection of acoustic energy on the tool structure 9 supporting the transducers (arrow 23). Specular reflection propagates only in the fluid filling the casing 5 and is reflected at the casing inner surface.

Reflection on the tool structure 9 supporting the transducers T and R propagates partly in the casing as a flexural wave, is reemitted in the fluid filling the casing, reflected on any structure inside said casing and converted again into the casing flexural wave, before detection at the receiver. It is thus very important, in order to improve the accuracy of the acoustic measurements, to minimize these spurious reflections.

An example of an apparatus according to the invention is represented on figure 3, which combines two advantages: this structure permits to deviate most of the parasitic echoes but also to reduce a significant number of them. Actually, in order to reduce the number of reflected signals on the tool structure, it is possible to remove unnecessary parts from the region where acoustic energy is going to be radiated. Because of the relatively high frequency (300 kHz) used, the ultrasonic beam remains more or less collimated in the azimuthal direction defined by the transmitter. This leads to an example of tool structure 9 made of 2 side beams 9a and 9b, the pair of transducers T and R being located between said beams.

Therefore, apart from the transducers, the tool structure Cleaves the central zone free of any reflector means. These longitudinal beams 9a and 9b are parallel to each other and oriented in a direction parallel to the axis of the well 4.

The apparatus according to the invention aims at deviating any spurious echo from the receiver. In order to achieve this goal, it is necessary to avoid that the surfaces of the tool structure that face the casing are parallel to the casing longitudinal axis. Therefore, as it can be seen in the example of the tool structure represented on figure 3, the surfaces 10 of the beams 9a and 9b, that face the casing and that may be impacted by waves generated by the transmitter, are made non parallel to the casing inner surface and oriented outward so that any reflected wave that may impact said beam surfaces is deviated away from the receiver R (see Fig.3 arrow 24). However, when making said surfaces of the tool non parallel to the longitudinal axis, the edges of said surfaces remain a potential source of spurious echoes. As it can be seen in the example of figure 3, the remaining edges 11 of the surfaces 10 are diffracting some energy back to the casing and the receiver (Fig.3 arrow 25). Although weak, this spurious echo will decrease the ability to detect t formation reflections. This diffraction is troublesome because edges 11 are parallel to the casing axis: after reflection along the edge, the wave front reaching the receiver R is parallel to this receiver front face and this coherency across the receiver aperture generates the maximum signal.

Therefore, the structure of the apparatus according to the invention has been further modified in order to break this continuous edge by designing it serrated: it thus comprises serrations that are not parallel to the casing axis as it can be seen on figures 4 and 5. These serrations 12 are characterized by the pitch dL and the angles and O of their facets 13, said angles can be optimized. As it can be seen on figure S. in case the facet 13 is normal to the direction of arrival of the incoming wave (radiated by the flexural wave in the fluid at an angle of approximately 33 da), then the wave will be reflected back on the transmitter T. which does not affect the measurement. The only waves that travel toward the receiver R are the ones diffracted by the tips Pl and P2. it is thus advantageous to make dL as large as possible in order to reduce the number of these diffracting points.

Due to these serrated edges, the surfaces of the apparatus combine two deviating features: they are oriented in order to deviate any unwanted signal from the receiver and their edges are serrated such that they also deviate unwanted signals from the receiver. Due to mechanical reasons, the structured is likely to be made of steel. However, this feature won't constitute a problem when aiming at deviating any unwanted signal. Actually, steel presents a high contrast of acoustic lS properties (density and velocity) with those of the casing fluid, therefore, most of the unwanted signals will be deviated instead of being propagated in the tool structure.

In an example of an apparatus according to the invention it is also possible to combine deviation and attenuation of the unwanted signals by coating the supporting structure with a material that has a low contrast of acoustic properties with those of the fluid and high intrinsic acoustic absorption. For example, this! material could be constructed as a compound of a bonding agent, an elastomer and a high specific gravity material. More preferably, the acoustic absorbing material is constructed as a metal-loaded organic material. This material is a suspension of suitable metallic particles in an organic base, usually a bonding or setting material.

The metallic particles are preferably among brass, copper, gold, iron, lead, molybdenum, tungsten or tungsten carbide. The bonding or setting material is preferably a thermoplastic, a thermosetting or an elastomeric. i As it can be seen on figure 6a, in a preferred example of an apparatus according to the invention, a second receiver R' is located at a certain distance from the first receiver R. in the same vertical plane as said first receiver. This allows for the computation of the casing flexural wave attenuation, which requires a minimum of two receivers. Actually, when comparing the two wave signals that are received at each of the receivers, it is possible to deduce therefrom the wave attenuation that is due to the nature of the material in the annulus (cement, water, oil or gas). It also enables to detect the variation in the wave signals depending on transmitter/receiver spacing, with different amplitudes on each casing and formation. In particular, it will increase the likelihood of detecting the formation echo when it is weak.

Actually, this distance has to be long enough to make sure that a measurable signal attenuation will occur. However, for the wave signal being still measurable, this distance also has to be reasonable. A typical inter receiver distance is comprised between 10 and 20 cm.

The transmitter T position can be changed in order to adapt the overall transmitter to receivers distances to the diameter of the casing and formation. For example, a large casing and formation hole diameter requires a longer IS transmitter/receiver distance in order to enhance the formation echo amplitude. A typical distance from transmitter T to first receiver R is 20 to 30 cm.

In a preferred example of an apparatus according to the invention, it is possible to enhance the reduction of the unwanted waves by providing deflector means to minimize the specular reflection (coming from acoustic waves propagating only in the fluid and being reflected at the casing inner surface). Therefore, plates 14 can be added to the tool structure to intercept the specular wave, without! affecting the flexural wave. As it can be seen on figure 6b, these plates are fixed on each of the two beams 9a and 9b. Preferably, such plates are located close to the transmitter T or to the receiver, or both as it can be seen on figure 6b. These plates can either deflect the unwanted signals or absorb them. To deflect the acoustic energy, the material of the plate should present a high contrast of acoustic properties with those of the casing fluid, for example a steel plate of sufficient thickness will reflect most of the energy. To absorb energy, the material in contact with the fluid i should have a low contrast of acoustic properties with those of the casing fluid but a high intrinsic acoustic absorption as it has already been explained before in reference to the possible coating of the structure. Advantageously the plates 14 can be made out of steel and be coated with such an absorbing material, in order to combine both absorption and reflection, thus enhancing the performances. The size of these plates depends on the diameter of the casing. They have to be long enough to intercept an acoustic wave reflecting on the casing inner surface.

In another example of an apparatus according to the invention, it is also possible to add other sensor means like temperature sensors or density sensors (or any other sensor means), between the two beams 9a and 9b. In order to reduce the overall length of the apparatus according to the invention, it is preferable to locate this sensor means between the transmitter T and the first receiver R. However, this configuration requires that said sensor means is designed and oriented in such a way lO that it can not cause any unwanted reflected signals that may be detected at one of the receivers R or R'. Therefore, as it can be seen on figure 6a and 7 this sensor means 15 will be designed such that the surfaces of said sensor means that may be impacted and reflect waves propagating in the casing towards the receivers R or R' will deviate said signals from said receivers (see arrow 26). For example, these surfaces may be designed in a prismatic way and the angles optimized in order to send acoustic energy away from the receivers, both in the vertical and in the azimuthal directions. Furthermore, said surfaces may also be coated with an absorbing material has it has been explained here above.

In an example of an apparatus according to the invention, the supporting structure 9 comprises between the beams 9a and 9b an array of transducers comprising several transmitters T and several pairs of receivers R and R', said! transducers being disposed in a sequence of one transmitter and two receivers in the same vertical plane. Furthermore, it is also possible in said array to orientate the faces of a first sequence of transducers on one side on the borehole and the other sequence of transducers on the diametrically opposite side of the borehole, which reduces the required speed to rotate the apparatus in said borehole. In other examples of apparatus according to the invention, it is also possible to dispose several sequences of one transmitter and two receivers that are angularly spaced i along the all circumference of the casing, further reducing the required speed for rotating the tool.

Claims (18)

1. An apparatus for acoustically investigating a cased borehole, said apparatus (8) comprising a tool structure (9) supporting at least a pair of transducers among which a transmitter (T) generates an acoustic wave that propagates in the casing (5) and is detected by a receiver (R), characterized in that said tool structure is designed such that it deviates parasitic echoes from said receiver.

2. An apparatus according to claim I, wherein the surfaces (10) of the tool structure (9) that face the casing and that may be impacted by waves generated by the transmitter are made non parallel to the casing inner surface and oriented outward so that any parasitic echo impacting said tool surfaces (10) is deviated away from said receiver front face.

3. An apparatus according to claim 2, wherein the edges (11) of the surfaces (10) of the tool structure that are parallel to the casing longitudinal axis are serrated such that any parasitic echo impacting said edges is deviated away from the receiver.

4. An apparatus according to claim 4, wherein angles (+, 0) of the facets (13) of any serration (12) are optimized in order to deviate any parasitic echo impacting said facets away from the receiver.

5. An apparatus according to claim 4 or 5, wherein the pitch (dL) between each serration is made as large as possible. !

6. An apparatus according to claim any one of claims I to 5, wherein the tool structure (9) is made of two longitudinal beams (9a, 9b) that are parallel to each other and oriented in a direction parallel to the axis of the well, said pair of transducers (R. T) being placed between said two beams.

7. An apparatus according to any one of the preceding claims, wherein the tool structure is coated with an acoustically absorbing material.

8. An apparatus according to any preceding claims, wherein the tool structure further comprises deflector means (14) that intercept specular waves and deviate them away from the receiver.

9. An apparatus according to claim 8, wherein said deflector means comprise plates that are located close to said transmitter and/or receiver and deviate said specular waves.

10. An apparatus according to any one of the preceding claims, wherein the 5transmitter generates flexural waves in the casing with a vertical direction of propagation.

l l. An apparatus according to claim 10, wherein the receiver in situated in the same vertical plane as the receiver.

12. An apparatus according to any of the preceding claims, wherein the angle (c) 10between said transducers and the casing normal is comprised between 28 and 38 degrees.

13. An apparatus according to any one of the preceding claims, wherein the distance between the transmitter and the receiver varies according to the diameter of the casing and the nature of the formation.

1514. An apparatus according to claim 13, wherein the distance between the transmitter and the receiver is substantially comprised between 20 and 30 cm.

15. An apparatus according to any of the preceding claim, further comprising a second receiver (R') situated in the same vertical plane as the first receiver (R), said second receiver being situated under said first receiver.

2016. An apparatus according to claim 15, wherein the distance between the first and the second receiver is comprised between 10 and 20 cm.

17. An apparatus according to any one of the preceding claims, further comprising other sensor means (15) that is located between said transmitter and said receiver.

18. A system for acoustically investigating a cased borehole, said system comprising an array of an apparatus according to any one of the preceding claims.

3019. A system according to claim 18, wherein the apparatus of the array are angularly offset.

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18. An apparatus according to claim 17, wherein the surfaces of said sensor means 25that may be impacted by waves generated from said transmitter are made non parallel to the casing inner surface and oriented outward so that any parasitic echo impacting said sensor means surfaces is deviated away from said receiver.

19. A system for acoustically investigating a cased borehole, said system comprising an array of an apparatus (8) according to any one of the preceding claims.

20. A system according to claim 19, wherein the apparatus of the array are angularly offset. 1 -

Amendments to the claims have been filed as follows 1. An apparatus for acoustically investigating a cased borehole, said apparatus comprising a tool structure supporting: (i) at least one transmitter transducer that generates an acoustic wave that propagated in the casing; (ii) at least one receiver transducer, that detects said acoustic wave; wherein said tool structure is designed such that the surfaces facing the casing zone impacted by said acoustic wave from the transmitter, are made non parallel to said impacted casing zone and oriented outward so that any parasitic echo impacting said tool surfaces is deviated away from receiver front face.

2. An apparatus according to claim 1, wherein the edges of the surfaces of the tool structure that are parallel to the casing longitudinal axis are serrated such that any parasitic echo impacting said edges is deviated away from the receiver.

3. An apparatus according to claim 2, wherein angles of the facets of any serration are optimized in order to deviate any parasitic echo impacting said facets away from the receiver.

4. An apparatus according to claim 2 or claim 3, wherein the pitch between each serration is made as large as possible.

5. An apparatus according to claim any one of claims I to 4, wherein the tool structure is made of two longitudinal beams that are parallel to each other and oriented in a direction parallel to the axis of the well, said pair of transducers being placed between said two beams.

6. An apparatus according to any one of the preceding claims, wherein the tool structure is coated with an acoustically absorbing material.

7. An apparatus according to any one of the preceding claims, wherein the tool structure further comprises deflector means that intercept specular waves and deviate them away from the receiver.

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8. An apparatus according to claim 7, wherein said deflector means comprise plates that are located close to said transmitter and/or receiver and deviate said specular waves.

9. An apparatus according to any one of the preceding claims, wherein the 5transmitter generates flexural waves in the casing with a vertical direction of propagation.

10. An apparatus according to claim 9, wherein the transmitter in situated in the same vertical plane as the receiver.

ll. An apparatus according to any of the preceding claims, wherein the angle 10between said transducers and the casing normal is comprised between 28 and 38 degrees.

12. An apparatus according to any one of the preceding claims, wherein the distance between the transmitter and the receiver varies according to the diameter of the casing and the nature of the formation.

1513. An apparatus according to claim 12, wherein the distance between the transmitter and the receiver is substantially comprised between 20 and 30 cm.

14. An apparatus according to any one of the preceding claims, further comprising a second receiver situated in the same vertical plane as the first receiver, said second receiver being situated under said first receiver.

201S. An apparatus according to claim 14, wherein the distance between the first and the second receiver is between 10 and 20 cm.

16. An apparatus according to any one of the preceding claims, further comprising other sensor means that is located between said transmitter and said receiver.

17. An apparatus according to claim 16, wherein the surfaces of said other sensor 25means that may be impacted by waves generated from said transmitter are made non parallel to the casing inner surface and oriented outward so that any parasitic echo impacting said sensor means surfaces is deviated away from said receiver.