GB2528888A

GB2528888A - Method, downhole tool and transducer for echo inspection of a well bore

Info

Publication number: GB2528888A
Application number: GB1413706.1A
Authority: GB
Inventors: Willem Hubertus Paulus Maria Heijnen; Thomas Hahn-Jose
Original assignee: Maersk Olie og Gas AS
Current assignee: Total E&P Danmark AS
Priority date: 2014-08-01
Filing date: 2014-08-01
Publication date: 2016-02-10
Also published as: WO2016016186A1; GB201413706D0

Abstract

A method, downhole tool (20) and transducer (30) for echo inspection of a well bore (2) equipped with a casing and/or liner (12) extending inside the well bore (2) using sound signals. A sound signal (24) is emitted into the fluid (18) within the casing or liner (12) and to the wall (14) of the casing or liner (12), an echo signal including superimposed reflections (38, 40, 36) of the emitted sound signal from the wall (14) of the casing or liner (12) any from portions of the well bore(2) outside the casing or liner (12) is received, and the echo signal is analysed and it is discriminated between signal reflections (38, 40) from the wall (14) of the casing or liner (12) and signal reflections (36) from said portions of the well bore (2) outside the casing or liner (12).

Description

Method, Downhole Tool and Transducer for Echo Inspection of a Well bore

Field of the Invention

The invention relates to a method, a downhole tool and a ultrasonic piezoeleotric transducer for inspecting a well bore equipped with a casing and/or liner extending inside the well bore using sound signals (echo inspection)

Background and Summary of the Invention

Well bores are used in the petroleum and natural gas industry to produce hydrocarbons (production well) or to inject fluids, for example water, 002 and/or Nitrogen (injection well) . Typically, such fluids are injected to stimulate, i.e. to enhance the hydrocarbon recovery.

Lately, 002 injection has been introduced to this to reduce the 002-concentration in the atmosphere in order to defeat global warming.

Typically, a well bore is lined with a steel pipe or steel tubing, generally referred to as casing or liner, and cemented in the overburden section to reduce the risk of unwanted evacuation of fluids from the overburden and/or the reservoir into the surface environment. In the reservoir there are several options typically used in the industry, namely open hole completion of the reservoir section or completion of the reservoir section with a liner with several formation packers for sealing off sections of the annulus around the steel liner, or completion of the reservoir section with a steel liner which is cemented in place and access to the reservoir is gained by perforating the liner and cement in a later stage of the completion, or completion of the well with a liner in open hole which has predrilled holes in the liner to gain access to the reservoir. It should be noted that the later holes can also be made in a later stage of the well life.

The invention described herein mainly relates to, but is not limited to those completions, where the liner is not cemented in place. The invention can also be used in non-reservoir sections.

During the production or injection of fluids from a well bore in an earth formation the well bore can enlarge due to chemical reactions and/or an instability of the borehole.

This may occur due to injection or production pressure changes and/or erosion which can take place e.g. in case of production from unstable geological formations such as turbidites known for their unpredictable sand face failure resulting in massive sand production leading to well failure. Furthermore, when injection processes are being used fractures can be generated resulting in undesired direct communication between the injection and production wells. On the other hand the well can collapse, for example caused by compaction, a process which happens when the pressure in the reservoir reduces, or by the use of chemicals used to improve injectivity or productivity. The latter can cause a collapse of the annulus and therewith possibly block the access to the reservoir and, therewith, preventing injection or production. Another important aspect is a phenomenon which is called cross flow in the annulus. Cross flow in the annulus is the result of pressure differences along the liner of the production or injection well in an un-cemented completion. The latter can lead to loss of production and/or loss of economic reserves.

It is particularly desirable to determine the afore-mentioned phenomena, which is, however, difficult because of the casing or liner, which is typically a steel pipe or steel tubing extending between the reservoir and the access. Therefore, if instruments should be placed in the well bore, they are usually placed in the casing or liner.

It has been proposed to use glass fibre optic lines put outside of the liner providing pressure and/or temperature data from which, using interpretation techniques, some of the above phenomena may be addressed (see "Optical Fiber Sensors for Permanent Downwell monitoring Applications in the Oil and Gas Industry" by Alan D. Kersey in IEICE Trans. Electron., Vol. E83-C, No.3, March 2000). However, not all desired parameters can be derived from the data obtained by the glass fibre optics. E.g. the hole size and the position of the liner in the well bore seem not to be derivable from the data obtained by the glass fibre optics technique.

It is known to use sonic signals for cementation quality assessments. There are two main areas, namely cement bond logging (CBL) and ultra sonic imaging (USI) . With cement bond logging it is determined, if there is any cement in the annular space around a steel pipe in the well bore.

With ultrasonic technology it is determined, if the cement is in contact with a steel pipe in the well bore. However, e.g. these technologies do not provide information in a non-cemented liner and other maybe desired information.

Therefore, it is an object of the invention to provide a method and tooling equipment for inspecting a well bore equipped with a casing and/or liner, in particular to provide tooling equipment which can be introduced within the casing and/or liner and which nevertheless, provides information about the well bore outside the casing and/or liner and to provide a corresponding inspection method.

A further aspect of the object of the invention is to provide a method and tooling equipment for inspecting a well bore equipped with a casing and/or liner which is able to provide information about the above-mentioned phenomena.

A further aspect of the object of the invention is to provide a method and tooling equipment for inspecting a well bore equipped with a casing and/or liner which provides desired information in a non-cemented casing and/or liner.

A more specific aspect of the object of the invention is to provide a method and tooling equipment for imaging and/or gauging the inside wall or boundary of the well bore through the wall of casing and/or liner.

The object of the invention is achieved by subject matter of the independent claims. Preferred embodiments of the invention are subject of the dependent claims.

An aspect of the invention is a method for inspecting a well bore equipped with a casing and/or liner extending inside the well bore in an earth formation, in particular an underground earth formation. The casing and/or liner may be a steel pipe or steel tubing having a circumferential wall with a thickness as usual in hydrocarbon drilling technology. However, the casing and/or liner could also be made of another metal, e.g. aluminium or could be non-metallic, e.g. made of glass-fibre reinforced plastic or carbon-fibre reinforced plastic. The well may be a production or injection well.

The method comprises transmitting or emitting a sound signal to the steel wall of the casing or liner from inside the casing or liner and to receive an echo signal including reflections of the transmitted or emitted sound signal from a different boundary, i.e. the method provides an echo inspection.

A downhole tool having a sonic transducer system comprising a sonic transmitter/receiver may be brought into the casing or liner. The transmitter directs a sound signal from inside the casing or liner to the steel wall of the casing or liner. A small part of the emitted sound signal passes through the steel wall of the casing or liner and a part of that is reflected by a boundary outside the casing or liner, e.g. by the inside wall or boundary of the well bore. An again small part of this echo or response signal passes again through the steel wall from outside to inside of the casing or liner and is received by the receiver. Tt may be understood that the transmitter and the receiver may be combined within the same device, e.g. an ultrasonic piezoelectric transducer used as a sonic transceiver. The sound signal may be a directional signal and at least a part of the directional sound signal is emitted in radial direction.

The received echo signal includes superimposed reflections of the emitted sound signal from the steel wall of the casing or liner and from portions of the well bore outside the casing or liner, in particular from the inside wall of the well bore acting as a boundary. As explained above, the sound signal being reflected from said portions of the well bore outside the casing or liner has to pass twice the steel wall of the casing or liner, thereby significantly attenuating the amplitude of this part of the echo signal.

On the other hand, the steel wall of the casing or liner itself creates a strong echo signal.

According to the invention, the echo signal received by the receiver inside the casing or liner is analysed and signal reflections from the steel wall of the casing or liner and signal reflections from said portions of the well bore outside the casing or liner are discriminated.

Analysing the received echo signal may include filtering and/or other signal processing, where e.g. the build-in firmware in the control system of the downhole tool provides processing and control of the emitted and received sound signals, improving the discrimination between signal reflections from the steel wall of the casing or liner and signal reflections from said portions of the well bore outside the casing or liner.

Summarising, the invention provides echo inspection data from behind the casing and/or liner, wherein the sound signal partially passes the steel wall of the casing or liner which allows the sound signal to reach areas behind the steel wall, to be reflected from there and to be received after a further passing of the steel wall of the casing or liner in opposite direction.

Advantageously, the method provides information about the well bore outside the casing and/or liner with a downhole tool which is positioned inside the casing or liner.

Further advantageously the invention is also applicable in a non-cemented casing or liner. Phenomena like well bore enlargement due to chemical reactions and/or instability of the borehole due to injection or production pressure changes and/or erosion e.g. in case of production from unstable geological formations such as turbidites with unpredictable sand face failure resulting in massive sand production leading to well failure and/or fractures resulting in undesired direct communication between the injection and production wells and/or collapse of the well, for example caused by compaction, e.g. due to pressure reduction in the reservoir, or by the use of chemicals used to improve injeotivity or productivity and/or collapse of the annulus and blockage of the access to the reservoir and/or cross flow in the annulus may be detected and/or examined by the invention.

The method preferably includes coupling of the sound signal by the transmitter directly into the fluid which is present within the casing or liner, i.e. between the sonic transducer system and the circumferential steel wall of the casing or liner by constructing or positioning the downhole tool within the casing or liner so that the sonic transducer system is held radially distant from the inside surface of the circumferential steel wall of the casing or liner. In other words the sonic transducer system is held radially distant from the inside surface of the

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circumferential steel wall of the casing or liner, so that the sound signal emitted by the sonic transducer system is coupled into the fluid which is present between the sonic transducer system and the circumferential steel wall of the casing or liner and is not coupled directly into the steel wall of the casing or liner. For this, a sound emitting surface of the transducer is in contact with the fluid in the casing or liner and couples the sound signal into the fluid. Thus, the emitted sound signal propagates at least some way through the fluid within the casing or liner before reaching the steel wall of the casing or liner. In other words the sonic transmitter(s)/receiver(s), or more precisely their sound emitting/receiving surfaces, are not in direct contact with the wall, but are sonioally coupled to the fluid in the casing or liner. This allows proper coupling of the signal in the well and smooth movement of the downhole tool inside the casing and/or liner.

Advantageously, inspection may be performed in a casing and/or liner having a varying diameter or shape.

Furthermore, inspection may be performed even when sediments, scale build up and/or corrosion occurs.

The sound signal which propagates through the fluid in the casing or liner and then through the steel wall of the casing or liner forth and back is received and filtered, in particular to filter out echoes or reflections from the inside and outside surface of the steel wall of the casing or liner and the echoes or reflections from the annular space caused for example by floating particles or the reflections of the inside wall of the bore hole are received and processed. From those echoes or reflections data may be derived e.g. such as hole sizes, flow velocities (if any) , wash outs (parts of the hole which are bigger than the nominal hole size caused during and/or after the drilling of the hole) , fractures and/or fracture zones in the earth formation (parts of the hole where natural and/or artificial fractures are present), fill in the annulus being particles dropping out from the production or injection fluid as well as reservoir particles breaking out of the inside wall of the bore hole and/or holes in the steel wall of the liner which can be artificially made or as a result from chemical reactions creating corrosion. Also corrosion and/or scale build up can be detected since this will result in different reflections of the inner and outside wall of the steel wall of the casing or liner in the well bore.

The distance between the emitting surface of the sonic transducer system to the casing or liner is preferably selected, depending on the propagation speed of the sound signal in the fluid inside and/or outside the casing or liner, so that the expected direct echo signal being reflected from the inside wall of the well bore is timely between expected reverberated reflections from the wall of the casing or liner. The fluid in the casing or liner and in the annulus around the casing or liner comprises particularly oil, natural gas and/or water of specific pressure and temperature, so that the propagation speed of the sound signals depends on these fluid properties.

Positioning the direct echo signal timely between said expected reverberated reflections facilitates the discrimination between the desired part of the echo signal from undesired parts, in particular the reverberated reflections from the steel wall of the casing or liner.

According to an exemplary embodiment of the invention, the downhole tool may comprise two or more transducer systems with different distance to the wall of the casing or liner.

Advantageously, it can be ensured that at any longitudinal position of the casing or liner at least one of the transducer systems provides an echo signal from the inner boundary of the well bore which is timely positioned between reverberated reflections from the wall of the casing or liner (maybe single reverberations and/or multiple reverberations) , despite a varying diameter and/or shape of the well bore and/or the casing or liner.

The method according to the Invention is advantageously used to determine the size of the well bore, the contour of the inside wall of the well bore, the position of the casing or liner in the well bore, the dimensions of the circumferential wall of the casing or liner, fabricated holes in the liner, and/or cracks in the well bore, in response to the signal analysis of the echo signal. E.g.

desired information on the annulus around the casing or liner can be gained and corrosion of the casing or liner, even corrosion of the outer surface of the casing or liner, can be detected.

According to a preferred embodiment of the method, a three-dimensional image of the well bore is generated in response to the signal analysis of the echo signal which represents a particularly advantageous information on the well bore.

Preferably, the spatial and time-based sound field of the emitted sound signal is set up so that the expected distortions of the sound signal at the circumferential wall of the casing or liner are compensated. That is, the distortions of the sound signal when passing the wall of the steel casing or liner can be included in simulation models.

Preferably, the emitted sound signal is a focused sound signal which is directed or non-isotropic. Such directed sound signal is preferably emitted in a polar angle of not larger than +/-15° and/or in an azimuthal angle of not larger than +/-45°.

Further preferably, the emitted sound signal includes directional components being directed at an angle other than 900 to the wall of the casing or liner, i.e. in forward or backward direction and Doppler-type measurements are performed. Preferably, the forward or backward angle of the transmitter(s)/receiver(s) relative to the steel wall of the casing or liner varies allowing Doppler-type of measurements resulting in velocity values in the casing or liner and around the casing or liner, i.e. in the annular space between the casing or liner and the inside surface of the well bore.

According to a preferred embodiment of the invention, the sonic transducer system rotates around a longitudinal axis of the casing or liner, thereby emits the sound signal and receives the echo signal over an extended range of polar angles during that rotation. Therewith the contour of the inside wall of the well bore, and/or the dimensions of the circumferential wall of the casing or liner, and/or fabricated holes in the liner, and/or cracks in the well bore can be determined over an extended range of polar angles around the longitudinal axis of the casing or liner.

Preferably, the sonic transduoer system oontinuously rotates within the casing or liner around the longitudinal axis over an angle of equal or larger than 3600 during the inspection for emitting the sound signal in radial directions covering 360° and receiving the echo signal and determining the contour of the inside wall of the well bore and/or corrosion of the steel wall of the casing or liner and/or holes in the casing or liner over an angle of 360°.

Further preferably, the downhole tool is continuously moved longitudinally within the casing or liner during the inspection for emitting the sound signal, receiving the echo signal along the longitudinal axis to determine the contour of the inside wall of the well bore and/!or corrosion of the wall material of the casing or liner and/or holes in the casing or liner over an extended length along the longitudinal axis of the casing or liner.

According to a preferred embodiment, the sonic transducer system is, during the inspection, rotated within the casing or liner around the longitudinal axis andmoved longitudinally within the casing or liner, preferably simultaneously for inspecting the wall of the casing and/or liner and/or the well bore outside the casing or liner along a helical curve of movement.

According to another preferred embodiment, angular coverage can also be achieved by using a sonic transducer system which includes an array of sonic transmitters and sonic receivers or an array of sonic transceivers essentially covering an extended polar angle around the longitudinal axis of the casing or liner up to 3600.

In other words, the transmitter(s) /receiver(s) preferably cover the full circumference of the well to obtain a complete contour of the well bore. The latter can be achieved by using an array of transmitters/receivers covering the full circumference or by making the transmitter(s) /receiver(s) rotating and therewith covering the full circumference.

Advantageously, the contour of the well bore can be scanned therewith, and a two-or three-dimensional image of the well bore can be generated.

In particular, the emitted sound signal is an ultrasonic signal, preferably in a frequency interval between 100 kHz and 10 MHz, more preferably in a freguency interval between 300 kHz and 4 MHz. It has turned out that frequencies in the order of 570 kHz are appropriate. The inventors have found that the mentioned frequency ranges provide a relatively strong echo signal from outside of the casing or liner, so that said discrimination may be improved.

According to a preferred embodiment, the emitted sound signal includes two or more different frequencies, or more precisely two or more different frequency peaks, which may be tailored to the propagation properties of the fluid inside and/or outside the casing or liner. The sonic energy coupled into the fluid varies with the type of fluid, because if the fluid is gaseous the speed of sound is very different from the speed of sound in liquids due to the mass differences. For that reason, the control device for the sonic transmitter(s)/receiver(s) allow for a variety of frequencies tailored to the fluid in the well bore. This may positively influence the signal quality of the signal coupling and, therewith, the intensity of the echo signal and the quality of the signal analysis.

Preferably, the emitted sound signal is a pulsed signal.

The emitted sound signal may include a single pulse or multiple pulses. The emitted sound signal may include multiple pulses having different frequencies peaks (multitone burst) , and/or the emitted sound signal may use pulse coding to calibrate the signal reflections. This allows for an improved analysis of the echo signal and may improve the discrimination between desired and undesired echoes.

If pulse coding is used to calibrate the reflections, the calibration is performed on the basis of model tests in the laboratory using the casing or liner material as installed in the well together with the well bore fluid. This calibration can be used by the firmware to enhance the control and processing of the emitted and received signals.

It is advantageous to measure the orientation and/or acceleration of the downhole tool within the casing or liner in coincidence with the sound signal for correlating the orientation and/or position of the downhole tool along the longitudinal axis of the casing or liner with the analysis of the echo signal. This may improve the provision of a three-dimensional image of the inside wall of the well bore, which may advantageously facilitate the detection of cracks and/or depositions in the well bore.

According to a preferred embodiment of the invention, the downhole tool includes a wiper system for wiping the sound emitting and/or receiving surface or surfaces for coupling the sound signal into the fluid of the transmitter and/or receiver of the scnic transducer system during the inspection. Any disturbance of the emitted or received sonic wave will influence the measurement negatively. In injection and production wells there will be typically some particles which attach itself to the downhole tool run into the casing or liner. If these particles cover the sonic transmitter(s)/receiver(s) this could influence the measurements negatively. The wiper system cleans the sound coupling surface of the sonic transmitter(s)/receiver(s) as and when required, advantageously preventing or at least mitigating such negative influence.

Preferably, the signal analysis of the echo signal includes a spectral analysis of the echo signal and the discrimination between signal reflections from the circumferential wall of the casing or liner and signal reflections from the inside wall of the well bore includes using spectral information of the echo signal obtained by said spectral analysis.

Ihe discrimination between signal reflections from the circumferential wall of the casing or liner and signal reflections from the inside wall of the well bore may include analysing a combination of the frequency spectrum of the emitted sound signal and the frequency spectrum the received echo signal. For this e.g. a two-dimensional plot of the received freguency in dependence from the emitted frequency may be created and examined. Ihis advantageously may improve the discrimination and signal-to-noise-ratio of the desired echo signal.

More generally, the discrimination between signal reflections from the wall of the casing or liner and signal reflections from the inside wall of the well bore may include filtering with at least one filter parameter or a combined filtering with at least two filter parameters, wherein the filter parameters are selected from a group of filter parameters including: i) timing of the received echo signal, ii) emit frequency of the emitted sound signal, iii) receive frequency of the received echo signal.

According to another aspect of the invention it is provided a downhole tool for inspecting a well bore equipped with a casing and/or liner extending inside the well bore in an earth formation, the downhole tool being adapted to be moved to and positioned in an underground portion of the casing or liner. The downhole tool includes equipment for performing the method described herein and may comprise: a sonic transducer system having a transmitter for emitting a sound signal and a receiver for receiving an echo signal, wherein the receiver receives an echo signal including superimposed reflections of the emitted sound signal from the circumferential wall of the casing or liner and from the inside wall of the well bore, a signal analyser for analysing the echo signal and comprising means for discriminating between signal reflections from the circumferential wall of the casing or liner and signal reflections from portions of the well bore outside the casing or liner, in particular from the inside wall or surface of the well bore.

The downhole tool may further comprise means for holding the sonic transducer system, i.e. the emitting and/or receiving surfaces of the transmitter and receiver, respectively, radially distant from the inside surface of the circumferential wall of the casing or liner, so that the sound signal is coupled into the fluid in the casing or liner and propagates through said fluid at least partially.

In other words, the sound coupling surfaces of the transmitter and/or receiver do not have direct contact with the wall of the casing or liner and the sound signal is not directly coupled into the wall of the casing or liner. This can be achieved by the way the transducer is built in the downhole tool, maybe by a radial spacer construction or the like.

The means for holding is adapted to hold the sonic transducer system or the emitting and/or receiving surfaces of the transmitter and receiver, respectively, at a predetermined distance from the inside surface of the circumferential wall of the casing or liner, wherein said predetermined distance is selected depending on the propagation speed of the sonic signal in the fluid inside and/or outside the casing or liner, so that the expected direct echo signal being reflected from the inside wall of the well bore is timely between expected reverberated reflections from the wall of the casing or liner.

Preferably, the downhole tool comprises moving means for moving the downhole tool with the sonic transducer system along a longitudinal axis within the casing or liner. The moving means may include a drive unit integral to the downhole tool, and/or a drive unit separately connected to the downhole tool, and/or a tube or pole to move the downhole tool, and/or a flexible cable to move the downhole tool within the casing or liner. The downhole tool may also be fluid flow driven inside the casing or liner.

The transmitter and the receiver may be combined in a sonic transceiver -Preferably, the signal analyser includes means for determining the size of the well bore and/or for determining the contour of the inside wall of the well bore, and/or for determining the radial position of the casing or liner in the well bore, and/or for determining the dimensions of the circumferential wall of the casing or liner, and/or for determining fabricated holes in the liner, and/or for determining cracks in the well bore, in response to the signal analysis of the echo signal.

Further preferably, means for generating a three-dimensional image of the well bore in response to the signal analysis of the echo signal are included.

According to a preferred embodiment of the invention, the transmitter is operable to emit a sound signal having a predetermined spatial and/or time-based sound field, which is selected so that the expected distortions of the sound signal at the circumferential wall of the casing or liner are compensated.

The transmitter may be operable to emit a sound signal which is focused in a polar angle (around the longitudinal axis of the casing or liner) of not larger than +/-15° and/or in an azimuthal angle (forward/backward angle with the longitudinal axis) of not larger than t/-45°.

The transmitter may be operable to emit a sound signal including directional components being directed at an angle other than 900 to the wall of the casing or liner in forward or backward direction, e.g. for providing Doppler- type measurements -The downhole tool preferably comprises a rotational drive for rotating the sonic transducer system around a longitudinal axis within the casing or liner during the inspection, so that the echo signal can be received and the contour of the inside wall of the well bore and/or corrosion of the wall material of the casing or liner and/or holes in the casing or liner can be determined over an extended range of polar angles (preferably 3600) around the longitudinal axis of the casing or liner.

According to a preferred embodiment the downhole tool comprises more than one sonic transducer system.

A further sonic transducer system may be rotated by a further rotational drive around a longitudinal axis within the casing or liner during the inspection, so that the echo signal of the two (or even more) sonic transducer systems can be received and the contour of the inside wall of the well bore and/or corrosion of the wall material of the casing or liner and/or holes in the casing or liner can be determined over an extended range of polar angles around the longitudinal axis of the casing or liner.

The sonic transducer system and the further sonic transducer system preferably rotate in opposite rotary directions.

According to a further preferred embodiment the sonic transducer system(s) may include an array of sonic transmitters and sonic receivers or an array of sonic transceivers essentially covering an extended range of polar angles (preferably 3600) around the longitudinal axis of the casing or liner.

The downhole tool may further comprise a longitudinal drive for moving the downhole tool along the longitudinal axis of the casing or liner during inspection, so that the echo signal can be received and the contour of the inside wall of the well bore and/or corrosion of the wall material of the casing or liner and/or holes in the casing or liner can be determined over an extended length along the longitudinal axis. The one or more sonic transduoer systems may follow a helical curve, i.e. perform a combined longitudinal and rotational movement.

In particular, the transmitter is an ultrasonic signal transmitter operating at a frequency in an interval between kHz and 10 MHz, preferably in a interval between 300 kHz and 4 MHz, most preferably in the order of 570 kHz.

Preferably, the transmitter is operable to emit a sound signal including two or more different frequencies, or more precisely frequency peaks, tailored to the propagation properties of the fluid inside and/or outside the casing or liner.

According to a preferred embodiment, the transmitter is operable to emit the sound signal in the form of a pulsed signal, and/or including a single pulse or multiple pulses, and/or including multiple pulses having different frequencies peaks (multitone burst) , and/cr using pulse coding to calibrate the signal reflections.

According to a preferred embodiment of the invention, the downhole tool comprises an accelerometer, e.g. a MEF4S element and/or a gyro and/or pattern recognition means for measuring the acceleration and/or orientation and/or velocity of the downhole tool within the casing or liner.

Therewith, the orientation and/or longitudinal position of the downhole tool within the casing or liner may be recorded. The acceleration and/or orientation and/or velocity of the downhole tool may be measured in coincidence with the echo signal, so that the recorded orientation and/or longitudinal position of the downhole tool can be correlated with the analysis of the echo signal, which provides a three-dimensional image of the inside wall of the well bore, in particular with the absolute orientation and/or longitudinal position information.

The downhole tool may comprise a wiper system for wiping the emitting and/or receiving surface of the transmitter and/or receiver of the sonic transducer system.

Preferably, the transmitter and/or receiver include at least one ultrasonic piezoelectric transducer.

Preferably, the at least one piezoelectric transducer includes at least one piezoelectrically active layer which includes a bulk matrix layer and a plurality of piezcelectric elements made of piezoelectric material, wherein the piezoelectric elements are embedded in the bulk matrix layer.

The embedded piezoelectric elements may be in the form of rods which are parallel arranged in an array being embedded in said bulk matrix layer.

According to a preferred embodiment the at least one piezoelectric transducer includes multiple piezoelectrically active layers each comprising a bulk matrix layer and a plurality of piezoelectric elements made of piezoelectrio material being embedded in the corresponding bulk matrix layer.

Preferably, the at least one piezoelectric transducer includes a damping layer and/or one or more matching layers operable to optimize the sonic transmission into the fluid in the casing or liner.

The piezoelectric material may be a ceramic piezoelectric material, e.g. lead zirconate titanate (PZT) . The bulk matrix material may be a polymer material.

The inventors have found that the afore-mentioned arrangements and materials provide a proper coupling of the sound signal into the fluid and may provide proper directional properties of the sound signal.

According to a further aspect of the invention an ultrasonic piezoelectric transducer is provided which is particularly suitable to be used for the above-mentioned method and in the downhole tool.

The ultrasonic piezoeleotric transduoer includes at least one piezoelectrically active layer which includes a bulk matrix layer and a plurality of piezoelectric elements made of piezoelectric material, wherein the piezoeleotric elements are embedded in the bulk matrix layer.

The piezoelectric elements may be in the form of rods which are parallel arranged in an array being embedded in said bulk matrix layer.

The ultrasonic piezoelectric transducer preferably comprises multiple piezoelectrically active layers each comprising a bulk matrix layer and a plurality of piezoelectric elements made of piezoelectric material being embedded in the corresponding bulk matrix layer.

The ultrasonic piezoelectric transducer may include a damping layer and/or one or more matching layers being operable to optimize the sonic transmission into a fluid.

The piezoelectric material may be a ceramic piezoelectric material, e.g. lead zirconate titanate (PZI) . The bulk matrix material may be a polymer material.

The invention is described in more detail and in view of preferred embodiments hereinafter. Reference is made to the attached drawings wherein like numerals have been applied to like or similar components.

Brief Description of the Figures

It is shown in Fig. 1 a schematic cross-sectional drawing of an earth formation with a downhole system in a casing/liner within a well bore; Fig. 2a a schematic top view drawing of a downhole tool according to an embodiment of the present invention; Fig. 2b a schematic cross sectional drawing along line A-A in Fig. 2a; Fig. 3 a schematic cross-sectional drawing of a well bore equipped with a liner and a transducer system as well as a schematic representation of echo signals; Fig. 4 a schematic representation of passage and reflection of sonic signals; Fig. 5 curves of the speed of sound over temperature in salt water at different pressures; Fig. 6 a frequency spectrum of a sonic pass signal through a steel wall in a test configuration; Fig. 7 a schematic representation of signal filtering using a split filter bank; Fig. 8 curves of the echo signal amplitude over propagation time of echo signals for different steel tubes in a test configuration; Fig. 9 curves of the echo signal amplitude over propagation time of echo signals for different steel tubes in a test configuration with an outside steel tube reflector; Fig. 10 test measurements of off-center frequency excitation and filtering; Fig. 11 a two-dimensional plot of the echo signal intensity as a function of the frequency of the emitted sound signal and the frequency of the received echo signal from the steel wall of a casing or liner in a test configuration; Fig. 12 a two-dimensional plot of the echo signal intensity as a function of the freguency of the emitted sound signal and the frequency of the received echo signal from behind the steel wall of a casing or liner in a test configuration; Fig. 13 a schematic exploded drawing of a sonic transducer according to an embodiment of the invention; Fig. 14a a schematic top view drawing of a downhole tool according to a further embodiment of the present invention; Fig. 14b a schematic cross sectional drawing along line A-A in Fig. Ha; Fig. 15 a schematic cross sectional drawing along the longitudinal axis of a downhole tool according to a further embodiment of the present invention; Fig. 16 a flow chart of a method according to an exemplary embodiment of the present invention.

Detailed Description of the Invention

Referring to Fig. 1 a well bore 2 is drilled in an earth formation 4 to exploit natural resources like oil, gas or the like. The well bore 2 continuously extends from the surface 6 to a terminating end 8 of the well bore 2 distal from the wellhead 10.

A casing/liner 12 in the form of an elongate steel pipe or steel tubing is located within the well bore 2 and extending from the wellhead 10 to an underground section of the well bore 2. The well bore 2 and the casing/liner 12 located within the well bore 2 define a flow channel 13 for fluid flow outside the liner 12. The annular flow channel 13 is defined by the outside surface 12b of the casing/liner 12 and the inside wall or surface 2a of the well bore 2. The longitudinal axis of the liner 12 is represented by line A. The flow channel 13 and the casing/liner 12 are typically filled with a fluid 16, 18, respectively. The fluids 16, 18 are e.g. oil or gas in case of a production well or water, 002 or nitrogen in case of an injection well.

A downhole tool 20 may be moved by moving means 21, generally known to the skilled person, within the casing or liner 12 to any desired position in the casing or liner 12.

The downhole tool 20 may be controlled from the surface 6, with common control technigues as known to the skilled person.

The downhole tool 20 includes a sonic transducer system 22 which emits an ultrasonic signal 24 to provide an echo inspection of the well bore 2. It will be appreciated that inspecting the well bore 2 may include e.g. determining the contour of the well bore 2 defined by its inside wall or surface 2a, but may also include gaining data and information of properties of the casing/liner 12, e.g. the wall thickness of the circumferential steel wall 14 of the casing/liner 12, e.g. to examine corrosion of the circumferential steel wall 14.

Referring to Figs. 2a/b the downhole tool 20 according to an exemplary embodiment of the invention includes a sonic transducer system 22 which emits a directional ultrasonic signal 24 having directional components in transverse direction, i.e. in direction to the circumferential steel wall 14 of the casing/liner 12. This should not exclude that the directional ultrasonic signal 24 may also include longitudinal components, i.e. is emitted at an angle other than 900 to the longitudinal axis A and to the inside wall 12a of the circumferential steel wall 14 of the casing/liner 12. Such transmission of the ultrasonic signal 24 can e.g. be used to perform Doppler-type measurements.

The downhole tool 20 further includes a rotatable portion 26 being rotatable by an angle of equal or larger than 3600 with respect to body portion 28. Non-rotatable body portion 28 provides movement of the downhole tool 20 within the casing/liner 12. The transducer system 22 is mounted at the rotatable portion 26 in order to rotate the transducer system 22 around the longitudinal axis A of the casing/liner 12 enabling inspection of the whole circumference of the well bore 2. Alternatively, the transducer system 22 may include an array of a plurality of transducers being arranged circumferentially around the downhole tool 20 (not shown) In this example the transducer system 22 includes an ultrasonic piezoelectric transducer 30, having a sound coupling surface 32 being in contact with the fluid 18 in the casing/liner 12. The sound coupling surface 32 acts to couple the ultrasonic signal 24 into the fluid 18 enabling propagation of the ultrasonic signal from the transducer system 22 through the fluid 18 to the wall 14 of the casing/liner 12 and further.

The downhole tool 20 includes a wiper system 34 to wipe the sound coupling surface 32 of the transducer system 22 in order to keep it clean from contamination disturbing the transmission and reception of ultrasonic signals by the transducer system 22.

Referring to Fig. 3 the rotational portion 26 of the downhole tool 20 is smaller than the internal diameter of the casing/liner 12. The transducer system 22 is held at a distance d to the inside surface 12a of the circumferential steel wall 14 of the casing/liner 12 so that the emitted ultrasonic signal 24 is coupled by the sound coupling surface 32 of the respective ultrasonic piezoelectric transducer 30 directly into the fluid 18 in the casing/liner 12. The emitted ultrasonic signal 24 propagates from the sound coupling surface 32 through the fluid 18 until it reaches the inside surface 12a of the circumferential steel wall 14.

Referring to Fig. 3 and 4, most energy of the signal 24 will be reflected at the circumferential steel wall 14 of the casing/liner 12, which forms a heavy boundary. Only a small amount of energy will transmit through the circumferential steel wall 14 into the fluid 16 in the annular space 13 between the casing/liner 12 and the inside surface 2a of the well bore 2. A part of the signal energy will be reflected from the inside surface 2a of the well bore 2 and returns as an echo signal 36 back. After propagating through the fluid 16 back, again a small amount of the energy of the reflected signal 36 will pass through the circumferential steel wall 14 from outside to inside of the casing/liner 12. In other words, the loss of the ultrasonic signal when passing through the steel wall 14 is doubled because of the back propagation to the ultrasonic piezoelectric transducer 30, which is, in this example used as a transceiver. This very small echo signal 36 from the inside surface 2a of the well bore 2 is -in this example the desired signal -but will, however, be superimposed by very large reverberation 38 of the efficient reflected energy from the steel wall 14 of the casing/liner 12.

In this example, the desired echo signal 36 is then discriminated against the -in this example undesired -linear interfered signal parts 38 caused by the reverberation at the steel wall 14. For this, the distance d between the sound coupling surface 32 of the ultrasonic piezoelectric transducer 30 and the inside surface 2a of the steel wall 14 is kept constant during rotation of the rotational portion 26 and/or during longitudinal movement of the downhole tool 20 along the axis A by means of the construction of the downhole tool. E.g. the transducer system 22 is fixedly mounted excentrically at the rotational portion 26 as shown in Fig 3. By this, the propagation time of the ultrasonic signal may be used to discriminate between desired and undesired parts of the echo signal. The distance d is selected so that the expected desired echo signal 36 being reflected from the inside surface 2a of the well bore 2 arrives timely between reverberated reflections, in this example between the direct reverberated reflections 38 from the steel wall 14 and further multiple reflections, e.g. double reflections 40, from the steel wall 14. An exemplary time succession of the desired echo signal 36 and the single and double reflections 38, 40 at the steel wall 14 is schematically shown at the bottom of Fig. 3.

The selection of the proper distance d depends on the propagation speed of the ultrasonic signal within the fluids 18, 16 and the steel wall 14. In particular, the distance d is selected in dependence on the propagation speed of the sound signal, in particular in dependence on parameters including material of the fluid, pressure of the fluid, and/or temperature of the fluid.

Referring to Fig. 5 the propagation speed of a ultrasonic signal within water having a salinity of 3,5% is depicted as a function of fluid temperature for a variety of pressure values, namely 1 bar, 10 bar, 50 bar, 100 bar, 150 bar and 200 bar.

Referring to Figs. 6 and 7, in the preferred embodiment, the spectral information of the emitted signal 24 and/or of the echo signal 36, 38, 40 received by the ultrasonic piezoelectric transducer 30 is used for recognizing the origin of the received echo signal, e.g. whether being reflected from the circumferential steel wall 14 of the liner 12 or from behind the circumferential steel wall 14, e.g. from the inside surface 2a of the well bore 2. For this, the ultrasonic piezoelectric transducer 30 is frequency selective in transmission and reception. In some examples one or more freguencies are emitted and/or one or more frequencies are received to inspect the same position of the well bore Referring to Fig. 6, it is noted that for exploiting the spectral information of the emitted and/or received ultrasonic signal for the signal analysis of the echo signal parts 36, 38 it is considered that passing the circumferential steel wall 14 results in a band pass filtering of the signal, the so-called waveguide behavior.

This means, that mainly a certain spectral band will pass -here double pass -the circumferential steel wall 14. The passed frequency will be only visible in a reduced amplitude within the reflected signal, because the sum will be constant. Fig. 6 shows a frequency spectrum of a sonic pass signal through a circumferential steel wall 14 in a test configuration. The transmission band is determined at a distance of 45 mm outside the steel pipe in which the transducer 30 is located without direct contact to the circumferential steel wall 14. The transducer emits a sound signal having a flat frequency spectrum as shown by line 52. The first maximum 54 in the transmission band frequency spectrum has been determined at about 280 kHz (first harmonic) , as can be best seen in the enlarged depiction of portion E of the spectral region around the first harmonic, shown at the right in Fig. 6. Close to the circumferential steel wall 14 the corresponding maximum 56 is at about 299 kHz. The frequency spectrum also shows multiple harmonics.

In this exemplary embodiment a transmission frequency of 570 kHz is used corresponding to the second harmonic.

However, it will be appreciated that other frequencies may be used as well. In addition, the emitted ultrasonic signal can have a plurality of frequency peaks, so-called multitone bursts. This can advantageously enhance frequency discrimination.

Referring to Fig. 7 a split filter bank 60 is used for filtering of the received echo signal 36, 38, 40 and estimation of origin of the respective parts of the received echo signal 36, 38, 40. The split-spectrum method may be used, which is generally known to the person skilled in the art.

In a preferred embodiment, the discrimination methods as described in connection with Figs. 3 to S (propagation time) and Figs. 6 to 7 (spectral information) are combined.

As an example, the signal analysis may include to evaluate S the spectral behaviour of the echo signal 36, 38, 40 in a predefined time window, while the time window may depend on the distance d and/or on fluid parameters, e.g. fluid material, fluid pressure and/or fluid temperature.

If the transducer system 22 continuously rotates around the longitudinal axis A while the downhole tool 20 moves along the longitudinal axis a within the casing/liner 12 the transducer system 22 follows a helical curve and the ultrasonic signal 24 is emitted along a helical curve. By this, the echo inspection according to the invention may cover the 3-dimensional inner contour of the well bore 2 and may create a 3-dimensional image of the inner contour of the well bore 2.

Referring to Figs. 8 and 9 It is to be noted that the filtering effect of the circumferential steel wall 14 is depending on the thickness. Therefore, the invention is not only capable of inspecting the inner contour of the well bore 2, but also to inspect the casing/liner 12 itself, i.e. to inspect the circumferential steel wall 14 of the casing/liner. The echo signal also carries information on the thickness of the wall 14 of the casing/liner 12. Fig. 8 shows the signal amplitude of the received echo signal in arbitrary units (Y-axis) as a function of the propagation time of the ultrasonic signal in microseconds (X-axis) . The plots show the echo amplitude for different thicknesses of the circumferential steel wall 14, namely: Upper plot: 6 mm steel wall thickness, Mid plot: 7 mm steel wall thickness, and Bottom plot: 10 mm steel wall thickness.

The signal oscillations in the time interval 63 between about 120 and 160 microseconds are from direct reflections from the steel wall 14 of the liner 12 (echo signal part 38) and the signal oscillations in the time interval 62 between about 180 and 240 microseconds are from reflections at the inner boundary 2a of the well bore 2 (echo signal part 36) . Further signal oscillations at times at about >250 microseconds are from multiple reflections/reverberations. The signal oscillations in the time interval 62 between about 180 and 240 microseconds vary measurably with the wall thickness. Information about the wall thickness, e.g. to determine corrosion of the casing/liner 12 can be gained in response to an analysis of this signal part in the time interval 62. Generally, the wall thickness of the casing/liner 12 may be evaluated from the received echo signal. By this, corrosion, even corrosion of the outside surface 12b of the casing/liner 12 may be determined.

Fig. 10 shows an example of a software analysing tool 64.

Referring to Figs. 11 and 12 the signal analysis may include to discriminate between desired reflections and undesired reflections in response to a combination of emitted frequency and the received frequency speotrum.

Figs. 11 and 12 show an example of the spectral content of the received echo signal including a superposition of the reflections 38, 40 from the circumferential steel wall 14 and reflections 36 from the inside surface 2a of the well bore 2. The two-dimensional plot shows the intensity of the received echo signal in dependence on the freguency of the emitted signal 24 which is shown on the X-axis and the frequency of the received echo signal which is shown on the Y-axis. Fig. U shows the signal being filtered by the time window 63 in the time spectrum, i.e. filtered to emphasise the signal from the circumferential steel wall 14 and Fig. 12 shows the signal being filtered by the time window 62 in the time spectrum, i.e. filtered to emphasise the signal from the inner boundary 2a of the well bore 2 behind the circumferential steel wall 14 demonstrating the spectral difference of the echoes from the steel wall 14 or from behind the steel wall 14. The two-dimensional frequency spectrum plots as shown in Figs. 11 and 12 can in turn be used for further signal filtering to improve discrimination between the desired signal part 36 and undesired signal parts 38, 40 etc. . More generally, the discrimination between signal reflections from the steel wall 14 of the casing/liner 12 and signal reflections from the inside wall 2a of the well bore 2 may include filtering with one filter parameter or a combined (multidimensional) filtering with at least two filter parameters, wherein the filter parameters may be selected from a group of filter parameters including: I) timing of the received echo signal, e.g. by using time intervals 62, 63; ii) emit freguency of the emitted sound signal 24; iii) receive frequency of the received echo signal.

Referring to Fig. 13, a preferred embodiment of an ultrasonic piezoelectric transducer 30 is shown in more detail. It has been turned out that for the aim of this invention the use of one, two, three or more piezoelectric composite layers 70 is advantageous. The exploded schematic representation of Fig. 13 shows a preferred embodiment having three piezoelectric composite layers 70. The piezoelectric ceramic material, in this example lead zirconate titanate (PZT) , is provided in specific structures, in this example a plurality of parallel arranged cylindrical PZT-rods 72. The so structured PZT-material is embedded in a resin or polymer matrix in the form of a bulk matrix layer 74. The piezoeleotric composite layers 70 are arranged on top of each other in a sandwich-like arrangement. This configuration allows a high coupling coefficient with low overall density. This advantageously results in high pressure amplitudes combined with large bandwidth in transmit mode and vice versa in receive mode. The positive behavior may be supported by using one or more matching layers, in this example two matching layers 76, 78 to optimize the sonic transmission or coupling of the ultrasonic signal into the fluid. The front surface 78a of the front matching layer 78 forms the sound coupling surface 32 of this ultrasonic piezoelectric transducer 30. A damping layer 80 is provided on the backside of the piezoelectric composite layers 70.

Especially for lower frequency resonators the use of multiple active layers (acoustic serial, electric parallel) to reduce electric load impedance is preferred.

Such ultrasonic piezoelectric transducer 30 provides a very efficient and broadband transfer function. This facilitates discrimination between desired and undesired reflections in the echo signal 36, 38, 40, in response to the spectral behavior of the received acoustic waves of the echo signal 36, 38, 40.

Referring to Figs. 14a/b the downhole tool 20 according to a further exemplary embodiment of the invention includes two ultrasonic piezoelectric transducers 30, 30' each having a sound coupling surface 32, 32' being in contact with the fluid 18 in the casing/liner 12. The sound coupling surfaces 32, 32' each couple an ultrasonic signal 24 into the fluid 18 enabling propagation of the ultrasonic signal from the respective ultrasonic piezoelectric transducer 30, 30' through the fluid 18 to the wall 14 of the casing/liner 12 and further. The sound coupling surfaces 32, 32' are positioned radially at different positions, i.e. at different distance to the wall 14 of the casing/liner 12. Because of the different propagation times, this enables the desired signal part 36 reflected from the inner boundary 2a of the well bore 2 to be timely positioned properly, e.g. between the reverberations 38, 40 of the steel wall 14 of the liner/casing 12, at least in one of the two echo signals, even if the diameter and/or shape of the casing/liner 12 and/or of the well bore 2 vary along the longitudinal axis, thereby changing the relative timing between sigoal parts 36, 38, 40.

Referring to Fig. 15 the downhole tool 20 according to a further exemplary embodiment of the invention includes two rotatable portions 26, 26' being rotatably mounted at the body portion 28, in this example at opposite longitudinal ends of the body portion 28. Each rotatable portion 26, 26' comprises a sonic transducer system 22, 22' . The rotatable portions 26, 26' may rotate independently. According to a preferred embodiment, the two rotatable portions 26, 26' rotate in opposite rotary direotions.

Referring to Fig. 16 the method aocording to an embodiment of the present invention comprises the following steps: In a first step 82 the downhole tool 20 is initially moved to or positioned in a subsurface section of the casing/liner 12 in the well bore 2 which is to be inspected. In a second step 84 the sound signal 24 is emitted by the ultrasonic transducer 30. In a third step 86 the reflected echo signal is received, which comprises signal parts 36, 38, 40 from reflections at the wall 14 of the casing/liner, the inner boundary 2a of the well bore 2 and possibly further sound signal reflecting boundaries which may occur in the surrounding. In a fourth step 86 the received echo signal is analysed and the desired signal 36 parts are discriminated from the undesired signal parts 38, 40. In a fifth step 90 the analysis results for this inspected portion of the well bore 2 are stored. In a sixth step 92 the transducer system 22 is rotated to the next rotational position and/or the downhole tool 20 is moved longitudinally to inspect the next position again employing steps 84, 86, 88, 90. As an example there may be a combined rotation and longitudinal movement resulting in an inspection along a helical curve of movement. The 1oop of steps 84-92 may be run through many times to obtain a scanned two-or three-dimensional image of the well bore 2, which can be displayed to the user in a step 94.

It will be appreciated that the features defined herein in accordance with any aspect of the present invention or in relation to any specific embodiment of the invention may be utilized, either alone or in combination with any other feature or aspect of the invention or embodiment. In particular, the present invention is intended to cover a method for inspecting a well bore and a downhole tool configured to include any feature described herein in relation to the method and an ultrasonic piezoelectric transducer configured to include any feature described herein in relation to the method and/or the downhole tool and vice versa. It will be generally appreciated that any feature disclosed herein may be an essential feature of the invention alone, even if disclosed in combination with other features, irrespective of whether disclosed in the

description, the claims and/or the drawings.

It will be further appreciated that the above-described embodiments of the invention have been set forth solely by way of example and illustration of the principles thereof and that further modifications and alterations may be made therein without thereby departing from the scope of the invention.