GB2528326A - Method of determining a condition of a borehole and apparatus - Google Patents

Abstract

A method of determining a condition of a borehole 100 which is cased or lined by first 102 and second tubulars 104 includes providing data associated with at least one return wave detected by at least one detector 107 in the borehole 100, the return wave being returned away from the wall of the borehole in response to transmission of at least one ultrasonic wave toward the wall from at least one transmitter disposed within the first casing in the borehole. The data may then be processed to determine at least one property of the second casing 104 or the medium adjacent 105 to an outer surface of the second casing 104, so as to determine the condition of the borehole 100. The method may be used to determine the quality and distribution of cement in a cementing operation.

Description

Method of determining a condition of a borehole and apparatus

Technical field

The present invention relates to the surveying of a borehole, and in particular, to a method of determining a condition of a borehole, and associated apparatus. In particular variants, the invention relates to determining the quality of the cement bond around an outer casing in the wall structure of the borehole.

Background

Boreholes are formed in the subsurface of the Earth in many contexts. They provide access to the interior of the Earth's crust, as may be desirable for example to construct a well to extract fluids from geological formations in the Earth, or perhaps to explore or make measurements of the subsurface. The borehole is drilled using drilling equipment and is typically cased or lined with tubular sections of casing or lining. The casing or lining can help to support and stabilise the geological formation into which the borehole is drilled in order to prevent collapse of the formation. It may also help to prevent fluid pressure loss or build up in the borehole, which can be important for safely performing further borehole operations such as drilling.

In order to case the borehole, an initial casing is inserted at a desired location in a drilled section of the borehole. Cement is pumped and injected into the borehole to enter the space surrounding the inserted casing. The cement circulates up along the outside of the casing in the surrounding space between the casing and the formation, and is left to set and harden to secure the casing in place.

At more advanced stages, a further casing may be installed. The further casing has a smaller internal diameter and is inserted radially within the initial casing, approximately concentrically therewith forming an annular space between the inner surface of the initial casing and an outer surface of the further casing. The further casing is installed in the same way as the initial casing, with cement pumped into the borehole and forced up through the annular space between the initial and further casing.

In this way, a multi-cased region can be defined in the borehole where the borehole has a wall structure including multiple layers of casing spaced apart from each other in a radial direction with respect to the borehole long axis, toward the formation.

A difficulty with the casing process in practice is that cement may not completely or perfectly fill the annular spaces surrounding the casings. Accordingly, there may be gaps where cement has not reached, and potential pathways for fluid from the formation to leak into the borehole, or vice versa, which can create problems for pressure control in the borehole. In addition, if a borehole or well is to be abandoned the borehole is required to be plugged to prevent leakage of fluid from the formation to the surface. Cement plugs can be acceptable for this purpose, but must comply with stringent leakage and pressure containment requirements.

It is known to assess the quality of the cementation and whether the cement is adhered solidly to the surfaces of the casing in a logging operation. Sonic logging tools have been used for this purpose. A more recent technique is to obtain cement evaluation logs, which give detailed 360-degree representations of the integrity of the cementation.

In some techniques, variations in amplitude of an acoustic signal travelling in the casing wall between a transmitter and a receiver are detected and used to determine the quality of the cement bond on the exterior casing wall. The fundamental principle of this determination is that the acoustic signal is more attenuated in the presence of cement than if the casing were not cemented. This technique has limitations in that the measurement is largely qualitative, as there is no indication of azimuthal cement variations such as channelling and as it is sensitive to the effect of a microannulus.

Pulse echo techniques have been developed where an ultrasonic transducer, in transmit mode, emits a high-frequency acoustic pulse towards the borehole wall, where it is reflected back to the same transducer operating in receive mode. The measurement consists of the amplitude of the received signal, the time between emission and reception, and sometimes the full waveform received. Tools that use this technique either have multiple transducers, facing in different directions, or rotate the transducer while making measurements, thereby obtaining a full image of the borehole wall. Pulse-echo techniques are used in the borehole televiewer. In cased hole, the waveform is analyzed to give indications of cement-bond quality and casing corrosion.

In addition, it is known to excite flexural waves in the casing, obtain an amplitude signal and use the attenuation of the signal to determine properties of the material, whether that be a solid, e.g. cement, liquid or gas, adjacent to the casing.

It is known to generate flexural waves of this type using an ultrasonic pulse-echo tool with a transmitter arranged to transmit a pulse obliquely incident with respect to the casing and a receiver arranged to receive reflections or echoes of the pulse from interfaces in the borehole wall.

Summary of the invention

The oblique transmitters used in current pulse-echo tools for generating flexural waves are arranged to transmit pulses with an incident angle of around 30 to 60 degrees with respect to the casing. This is done in order to maximise flexural wave production. This also ensures generation of significant S-wave energy in the material adjacent to the exterior casing wall, which is useful to help distinguish between the presence of a solid material, such as cement, which can support S-wave propagation and the presence of a fluid, such as liquid or gas, which does not support S-wave propagation well.

Such previous techniques have focused on analysing the cement bond associated with the innermost casing in the borehole. In cases where multiple casings are installed and overlap to provide sections one wthin another, evaluation of the quality of the cement bond of the outer casing sections normally does not take place. It has typically been thought that the conventional cement bond logging techniques are suited only to the innermost casing, due to concerns about noise and penetration depth. For example, it can be noted that a current borehole recording device for oblique ultrasound incidence, an Isolation Scanner (IBC tool) by the Schlumberger Wireline division, is known, but that in the theory and published literature for the tool the evaluation of the cement bond to outer casings in a double-layered casing is not defined.

The present inventors have gone against this thinking.

Various aspects of the invention are set out in the claims as appended hereto.

By way of the invention, the quality of the cement bond of an outer casing can be determined which is advantageous for the assessment of well integrity. Both the quality of the cement bond of the first and second casings can be obtained in one logging run. In turn, this can reduce costs in plug and abandonment operations and reduce casing recovery costs.

Further advantages of the various features and embodiments the invention will be apparent from the description, drawings and claims.

Each of the aspects may have further features as described in any other aspect, or as described elsewhere in the description, drawings and claims. Features described in relation to one embodiment or aspect may be included in other embodiments or aspects, as an additional feature or in exchange for another like feature.

Drawings and descrirnion There will now be described by way of example only embodiments of the invention with reference to the accompanying drawings, in which: Figure 1 is a representation of a borehole tool for use in an embodiment of the invention; Figure 2 is a representation of propagation geometry and signals recorded at receivers in the borehole in response to transmission of ultrasonic pulses into the borehole wall using a tool as in Figure 1; Figure 3 is a representation of reflection and transmission geometry and conversion of P-waves across a thin metal layer borehole casing interface; Figure 4 is a representation of energy contained in different wave modes generated upon transmitting P-waves incident upon a thin metal layer interface with different angles of incidence; Figure 5 is a representation of wave mode propagation behaviour in a thin metal layer

casing example;

Figure 6 is an overview representation of a borehole using the tool of Figure 1 to survey the quality of the cement bond against an outer casing in a multi-cased region where an inner casing is arranged within the outer casing; Figure 7 is a close up cross-sectional representation of the borehole wal in the multi-cased region of Figure 6; Figure 8 is a time-series of amplitudes of energy returned from the borehoe wall (echo) in response to the transmission of an ultrasonic pulse, using a tool as in Figure 1; Figure 9 is a flow chart of a method for processing recorded amplitude and phase data associated with energy returned from the borehole in response to a pulse transmitted by the tool of Figure 1, and obtaining impedance properties of the material surrounding the outer casing of Figure 6 from that data, to evaluate the quality of the cement bond; Figure 10 is a schematic representation of a computer device for use in determining the property of the material adjacent to the casing, in an embodiment of the invention; and Figure 11 is a graph of a group delay spectrum illustrating the parameters by which the group delay can be characterised, in performing the method of Figure 9.

Turning firstly to Figure 1, there is shown a pulse-echo borehole tool 1 for use in the invention, in determining a condition of a wall of a borehole. In use, the tool is provided in the borehole, for example run on a wireline, to perform a survey of the borehole wall.

The tool 1 has two transmitters 2a, 2b in this case, each arranged to transmit ultrasonic pulses into the borehole, radially toward the wall of the borehole. The transmitter 2a directs energy at an oblique angle with respect to the borehole wall, whilst the source 2b directs energy more or less at an angle perpendicular to the borehole wall. The transmitters typically take the form of transducers.

The tool has receivers 3a-3c to detect waves returned from the borehole in response to the transmitted pulse. They are arranged to detect waves, which propagate back to the tool due to the transmission of ultrasonic pulses by the source. They are spaced apart from at least one of the sources 2a, 2b along the axis of the borehole, and tool body, in an arrangement favourable for detecting waves that have returned from interfaces in the wall structure of the borehole after transmission of the pulse. For example, the sources 2a and 2b are arranged to direct energy at an angle of incidence with respect to an object interface, e.g. borehole wall, and the receivers 3a-3c are arranged to detect waves that have reflected back from the interface at angle of reflection with respect to the object interface, which corresponds with the angle of incidence. The use of multiple receivers allows energy to be detected over a range of angles of reflection, and or reflections from interfaces of the borehole wall at different distances away from the tool.

The receivers are used to record amplitudes of waves returned from the borehole wall as time-series data. Figure 2 shows time-series amplitude traces 4a, 4b recorded at the receivers Sc, Sb in an example borehole section cased with a single casing. The recorded time series are associated with waves returned from interfaces in the borehole wall in response to the transmitted ultrasonic pulse.

The time series traces 4a, 4b both have amplitude events 5a, 5b associated with the casing layer, whilst trace 4a additionally shows a significant amplitude event 6a associated with the interface between the cement annulus and the formation. The amplitude event 6a is referred to as a third interface echo (TIE).

As demonstrated by the time series traces 4a, 4b, by transmitting obliquey, the casing becomes "transparent". In other words, the normal incidence configuration sees only the casing (the only significant event relates to the casing as shown in trace 4b) whereas the oblique configuration, which produces flexural waves, addtionally sees interfaces behind the casing (significant events associated with the casing and with the third interface are shown in trace 4a).

The TIE event is of particular interest in the present technique, in a double casing scenario, where it is recognised that a TIE can derive from the outer casing (i.e. when compared with Figure 2, a second casing is present in place of the interface between the cement and the formation).

The underlying wave field and wave mode behaviour is explained with reference to Figures 3, 4, and 5. A transmitter 10 transmits longitudinal P-waves 11 toward a thin metal layer 12 (e.g. a casing between materials), along a path forming an oblique angle of incidence 13 with respect to the layer. At the surface of the layer, refracted P-waves are produced which propagate into the medium beyond the surface. In addition, refracted S-waves are produced which propagate in the medium. The refracted P- waves and S-waves propagate with different refraction angles 14 and 15. The P-waves are refracted to a higher refraction angle than the S-waves. As the angle of the incident P-waves in the path 13 is increased, the P-wave critical angle is reached before the S-wave critical angle.

When an oblique ultrasonic beam from transmitter lOis incident upon a thin metal layer 12 various Lamb wave modes are generated inside the layer. The dominant modes generated inside the layer depends on the angle of incidence 13. When the incident angle is between 30 and 67 degrees, the predominant Lamb mode is flexural (A0).

This mode will travel inside the body of the layer while a "leaking P" mode propagates in-between an inner tubular (on which the transmitter may be mounted) and an outer tubular the casing (see Reference [1] in the "References" section below). For all incidence angles, as Lamb waves propagate in the inner tubular they radiate (or leak) energy into the surrounding fluid medium. In the present technique, we analyse the TIE generated as a result of interaction between the "Leaky P" mode and the outer tubular (see Reference [1] below). This is the wave B in Figure 5. The Leaky P (planar and compact) mode is transferred to the media between the inner and outer tubulars as a result of the propagation of the zero-th anti-symmetric Lamb mode A0 (flexural mode) inside and along the inner tubular. In some embodiments, the tool is employed inside production tubing (constituting the inner tubular), and the Leaky P mode s transferred in the same way to the medium between the production tubing and an outer tubular constituted by casing outside of the production tubing.

The double casing scenario is shown in Figure 6. In Figure 6, a borehole 100 is shown as extending into the geological subsurface of the Earth. The borehole is cased with two casings 102, 104 constituting inner and outer tubulars having different diameters, and overlapping with each other in the region 130 of the borehole. The respective casings 102, 104 have surrounding annular spaces 105, 106 adjacent to their outer surfaces. The space 105 is defined between the casing and formation 101, whilst the space 106 is defined between the inner surface of the casing 102 and the outer surface of the casing 104. During casing of the borehole, cement is introduced into the borehole and directed into these annular spaces 105, 106 to install the casings, with the intention that the cement seals the borehole from the formation and helps with containing fluid and controlling pressure of fluid in the borehole. In Figure 6, cement is present in the annular spaces 105, 106. As will be appreciated however, the cement bond may not be perfect in all places.

In order to survey or evaluate the quality of the bond of the cement, in particular the bond of the cement against the outer surfaces of the casings 102, 104, an ultrasonic bond logging tool 107 is run in the borehole at the region 103. The tool 107 is located in the borehole space 108 and is configured in the same manner as the tool of Figure 1 described above. The tool is used to transmit ultrasonic pulses in the frequency range of 100-700 kHz from a source into the wall of the borehole in the region 103, so as to generate Lamb wave modes as described above. Receivers arranged on the tool are used to detect return energy from the medium interfaces in response to the transmissions, including returns due to the leaky P wave interacting with the second casing or outer tubular. The corresponding TIE event is recorded.

Figure 7 shows in close up a cross-secton of the borehole wall structure in the region 103. The wall structure has a first casing section 104s, of the casing 104. The first casing section has an inner surface 104 defining the borehole space 108 in which the tool is located. This is the first section of the casing that the transmitted pulse encounters as it travels from the source. The wall structure further has a second casing section 102s, of the casing 102, arranged outside of the first casing section 104s (with respect to the borehole space 108 and bore longitudinal axis 108a). The second casing section has an inner surface 102i. The wall structure includes the annular space 106 which is present on the outside of the first casing section 104s, between the outer surface 104e and the inner surface 102i of the second casing section 102s. Similarly, the annular space 105 is defined on the outside of the second casing section 102s, between the outer surface 102e and the formation wall surface 101w. The annular space 106 is arranged to contain cement as described above to form a sealed contact and bond with the casing surfaces 104e, 102i facing the annulus 106. Likewise, the annular space 105 is arranged to contain cement and bond against the casing surface 102e and the formation wall surface 101w. Thus, the wall structure of the borehole has a layered nature with alternating casing and casing annulus layers against the formation wall 101w. The borehole space 108s extends within the innermost casing 104 for accessing the bottom of the borehole and performing operations therein. The casings are typically formed of metal, such as steel, providing an acoustic impedance contrast with the material in the annular spaces 105, 106.

In order to evaluate the quality of the bond of cement in the space 105 against the surface 102e of casing 102s, the ultrasonic P-wave pulses are directed from the tool at an oblique angle of incidence 109 with respect to the casing 104. The angle is selected to be in the range of 5 to 38 degrees. For example, an angle cf 19 degrees could be selected to provide particularly good transparency through the first casing 104. This oblique angle helps to retain and maximise P-waves propagating across the casing 104 and into the material in the annular space 104. Flexural waves and Lamb modes are generated as described above. Moreover, the pulse can put the casing into resonance. The flexure of the casing from the flexural waves produces P-waves in the adjacent media. In this way, a good TIE response from the second casing can be obtained.

The P-wave energy propagates through the medium, preferably cement, of the annular space 106, and into the casing 102. P-waves are directed typically with an oblique incident angle to the inner surface 102i of the casing 102s are converted similarly to produce further refracted P-waves or S-waves or both, and flexural waves. The transmission may take place so that the incident angle of P-waves at the second casing is in the range of 5 to 38 degrees, or greater. Flexural waves and Lamb modes are produced as described above, to produce Leaky-P wave return energy at the second (outer) casing. The second casing, like the first casing, is put into resonance.

It will be appreciated that waves incoming toward the borehole wall interact, e.g. reflect and/or refract, at the interfaces between the casing and adjacent materials generating return waves which are returned back away from the borehole wall and can be detected by the receivers. In this way, at least some energy from the transmitted pulse into the borehole wall is returned, as an echo. The returned energy is measured at the detectors, for example in the form of amplitudes of detected pressure charges.

Figure 8 shows a typical amplitude time series associated with energy returned from the borehole wall and detected at a detector. Various amplitude events 110, 112 and 114 are seen at progressively later recordal times. These are respectively associated with reflection events at the casings 104, 102 and at the formation 101. Prominent events are produced by the interface between the casing 104 and the medium in the annular space 106, the interface between the casing 102 and the medium of the annular space 105, and the interface between the medium in the space 105 and the formation 101. The waves returned from layers farthest from the source are seen later in the time series, and will in general have lower amplitudes, as progressively less energy reaches the far interfaces. The event 112 is associated with TIE produced at the second casing 102 as described above.

The recorded data associated with each interface are analysed to determine properties associated with the medium in the annular space adjacent to the outer surface of each casing, to determine whether cement has bonded effectively to the casing.

In particular, in the technique of the present invention, the recorded data associated with the TIE event 112 is processed to determine the acoustic impedance associated with the interface between the (outer) casing 102 and the annular space 105.

Referring now to Figure 9, the processing of the TIE event associated with the outer tubular (e.g. second casing) is described. The processing is performed to determine the acoustic impedance using the so-called "TA3" processing technique. The "TA3' technique is described in the Reference [2]. In this case, the T'i technique is applied on TIE data as input, which in turn is obtained by applying the technique described in the Reference [3] to a raw recorded waveform data. The processing includes the following process steps Si to SB, (numbered correspondingly in the Figure 9): Si. At this step, the recorded TIE time series data associated with the outer tubular (e.g. second casing) interface are identified. The first arrival from the second casing may be located. Process and normalisation windows may be defined.

S2. The time-series data within the process window are transformed to the frequency domain, for example by a Fast Fourier Transform (FFT).

S3. The frequency spectrum is analysed. Resonant waves in the casing are damped at different rates according to the medium adjacent to the casing. Resonant frequencies can be identified from the amplitude spectrum as local minima or, preferably, the phase spectrum as changes in slope. By analysing these, the resonant frequency tO and fractional bandwidth delta_fJfO can be determined. A plot of phase against frequency may be made (i.e. the phase spectrum).

S4a. From the frequency spectrum, the group delay is determined and characterised.

The group delay is obtained by taking the derivative of phase with respect to the frequency, i.e. the derivative of the phase spectrum. The group delay spectrum can then be characterised by determining the parameters tau_min, delta_tau, fO and delta_f, as shown in Figure 11. The parameter tau_min is the minimum value of the group delay, delta tau is the value of the group delay at 40% above the minimum, fO is the resonant frequency, and delta_f is the bandwidth of the group delay at delta_tau.

The casing thickness is proportional to 1/f 0, and the acoustic impedence is proportional to the group delay bandwidth, tau_min and thickness.

S4b. From the parameters of the group delay, the acoustic impedance and second casing thickness are estimated.

S5. A planar interface model waveform is calculated from the estimated acoustic impedance and second casing thickness estimate. Parameters of this calculated model waveform is compared with that actually measured, e.g. parameters of the group delay of the calculated model estimate and the actual data may be compared.

S6. If the comparison shows poor agreement, then the casing thickness and impedance estimated in step S4b is adjusted, and step S5 is repeated until upon comparison there is satisfactory agreement.

S7 & S8. When there is satisfactory agreement, the estimated thickness and impedance (which is a plane model approximation) is corrected for the non-planar geometry of a borehole, and from this, the impedance of the annulus material is output.

Based on the impedance output, it is readily determined whether gas, liquid or solid, well-bonded cement can be found adjacent to the outer surface of the second casing 102. Accordingly, the quality of the cement bond of the second casing 102 can be determined.

Referring to Figure 10, there is shown a computer device 200. The computer device has an In/Out device 201 used for communicating with the detector and/or transmitter through the In/Out device. The computer device has a microprocessor 202 arranged to process or execute instructions as for example defined by a computer program. The processor 202 may also process data received from the detector and process data or instructions for sending to the detector and/or transmitter. The device further comprises a memory device 203, which may be used for storing data, such as may be obtained from the detector, for example in one or more databases. The memory may also contain a computer program with instructions for processing the response data and signals associated with the waves returned from the borehole wall in response to the ultrasonic wave transmission. The computer device 202 may be a distributed arrangement for example with wireless communication between components or with communication across a network. In addition, the computer device may be located at a surface location and may communicate with the borehole tool transmitter and/or receiver whilst the borehole tool is in use in the borehole.

By way of the method and apparatus described, it is possible to quantify the cement bond of an interval behind second (outer) tubular or casing with a tool deployed inside a first (inner) tubular or casing. In some embodiments the invention may be employed where the inner tubular is production tubing surrounded by fluid and a cemented outer tubular in the form of casing. In other embodiments, the invention may be employed where the inner tubular and outer tubular are both cemented casings. In fact, the inner and outer tubulars may be constituted by any generally cylindrical barriers and the medium surrounding each tubular may be solid (e.g. cement or formation), liquid (e.g. water) or gas.

The technique takes account of the scattering and absorptions of the signal along the path in the analysed layer by taking into account both types of attenuation: shear wave attenuation and flexural wave attenuation. Calibration is provided in terms of scattering and absorptions. The TIE is associated with the outer tubular and can be put to use to evaluate a cement bond, whereas signals of this type have in the prior art not been used or have been used merely for tool centralisation when logging the first, inner tubular.

The invention provides several advantages, such as the following: * Improved data for post-cement job qualification/disqualification.

* Improved data utilisation.

* Existing logging technology is limited to one layer of pipe. In order to log behind the second layer of pipe the inner pipe needs to be removed which sometimes is impossible.

* Enables cement logging in old wells and defining minimum criteria for plug and abandonment (P&A) operations.

* Reduced cost for casing recovery.

* Reduced cost for P&A strategy.

* Reduced risk for wellhead fatigue investigation.

* Increased safety of well by recognizing outer casing fatigue.

* Information about formation mechanical properties.

* Information about the cement mechanical properties.

Various modifications and improvements may be made without departing from the scope of the invention herein described. In particular, although the description has been made referring to a region in which there are two tubulars (e.g. casings), it will be appreciated that further tubulars may be used in other variants of the invention, where for example the first and second casings described above are arranged within another casing which is adjacent to the formation wall, and the invention may be applied to determine the quality of the cement bond on the exterior of the casing nearest to the formation. It can also be noted that production tubing may take the place of the first casing, with the transmitter and receiver located inside the production tubing. The method can then be performed in the same way, except with the transmission of energy through the production tubing, to allow the cement bond evaluation of the second casing (which may be the only tubular or the second tubular outside the production tubing), in the manner described above.

REFERENCES: [1] Smaine Zeroug and Benoit Froelich, Ultrasonic Leaky-Lamb Wave Imaging Through A Highly Contrasting Layer, 2003, PROCEEDINGS IEEE ULTRASONICS SYMPOSIUM, vol. 1, p. 794-798.

[2] Method and apparatus for the acoustic investigation of a casing cemented in a borehole, United Stales Patent 5216638, Filed 04/20/1990, Published 06/01/1 993.

[3] Method of analyzing waveforms, United States Patent 5859811, Filed 02/29/1996, Published 01/1 2/1 999.

Claims (26)

1. CLAIMS: 1. A method of determining a condition of a borehole which is cased or lined by first and second tubulars, the first tubular being arranged within the second tubular, the method comprising the stops of: (a) providing data associated with at least one return wave detected by at least one detector in the borehole, the return wave being returned away from the wall of the borehole in response to transmission of at least one ultrasonic wave toward the wall from at least one transmitter disposed within the first tubular in the borehole; and (b) processing the data to determine at least one property of the second tubular or the medium adjacent to an cuter surface of the second tubular, so as to determine the condition of the borehole.
2. 2. A method as claimed in claim 1, which further comprises transmitting the ultrasonic wave toward the wall of the borehole using the transmitter.
3. 3. A method as claimed in claim 1 or 2, which further comprises detecting the return wave using the detector, in order to provide the data.
4. 4. A method as claimed in any preceding claim wherein the return wave comprises a third interface echo from the second tubular.
5. 5. A method as claimed in claim 4 wherein the processing comprises processing data associated with the third interface echo.
6. 6. A method as claimed in claim 4 or 5, wherein the third interface echo is generated as a result of a Leaky P wave interacting with the second tubular.
7. 7. A method as claimed in any of claims 4 to 6, wherein the processing is performed by using the TA3 processing method applied to the data associated with the third interface echo.
8. 8. A method as claimed in any preceding claim, wherein the method further comprises identifying the data associated with the third interface echo.
9. 9. A method as claimed in any preceding claim, wherein the data comprises time-domain data, and the processing step comprises converting the time-domain data to frequency-domain data.
10. 10. A method as claimed in claim 11, wherein the processing further comprises determining a group delay from the frequency-domain data.
11. 11. A method as claimed in any preceding claim, wherein the property of the medium adjacent to the outer surface of the second tubular is acoustic impedance.
12. 12. A method as claimed in claim 11, when dependent upon claim 10, comprising determining said acoustic impedance based on the group delay or parameters thereof.
13. 13. A method as claimed in any preceding claim, wherein the return wave comprises at least one flexural wave generated from either or both of the first and second tubulars.
14. 14. A method as claimed in any preceding claim, wherein the transmission is performed so as to generate resonance in either or both of the first and second tubulars.
15. 15. A method as claimed in any preceding claim, wherein the ultrasonic wave has a frequency in the range of 100 to 700 kHz.
16. 16. A method as claimed in any preceding claim, wherein the transmission is performed to direct the ultrasonic wave, or at least one P-wave being derived from the ultrasonic wave, to have an oblique angle of incidence with respect to either or both of the first tubular and the second tubular.
17. 17. A method as claimed in claim 16, wherein the angle of incidence is in the range of 5 to 33°.
18. 18. A method as claimed in claim 16 or 17, wherein the angle of incidence is less than or equal to 20°
19. 19. A method as claimed in any of claims 16 to 18, wherein the angle of incidence is selected such that at least one P-wave propagates in the material adjacent to an outer surface of either or both of the first and second tubulars, and the P-wave energy in said material is greater than any S-wave energy.
20. 20. A method as claimed in any preceding claim, which further comprises identifying a signal associated with waves returned from the second tubular or medium outside the second tubular.
21. 21. A method as claimed in any preceding claim performed to determine the presence of cement adjacent to the outer surface of the second tubular and/or the quality of bonding of cement against said outer surface.
22. 22. A method as claimed in any preceding claim, wherein the detector comprises at least one transducer arranged to sense pressure.
23. 23. A method of determining a condition of a borehole which has a production tubing installed and is cased or lined by a tubular of casing or lining, the production tubing being arranged within the tubular, the method comprising the steps of: (a) providing data associated with at least one return wave detected by at least one detector in the borehole, the return wave being returned away from the wall of the borehole in response to transmission of at least one ultrasonic wave toward the wall from at least one transmitter disposed within the production tubing in the borehole; and (b) processing the data to determine at least one property of the tubular or the medium adjacent to an outer surface of the tubular, so as to determine the condition of the borehole.
24. 24. Apparatus for performing the method of any preceding claim.
25. 25. A computer program for use in performing the method at any preceding claim.
26. 26. A computer arranged to execute the computer program of claim 25, to perform the method of any of claims ito 23.