WO2015107332A1

WO2015107332A1 - Determining sensitivity profiles for das sensors

Info

Publication number: WO2015107332A1
Application number: PCT/GB2015/050055
Authority: WO
Inventors: David Hill; Kevin Boone
Original assignee: Optasense Holdings Limited
Priority date: 2014-01-14
Filing date: 2015-01-13
Publication date: 2015-07-23
Also published as: GB201400537D0; GB2522061A

Abstract

This application relates to fibre optic distributed acoustic sensing (DAS) applied using a sensing optical fibre (104) that is at least partly embedded in a cement structure (203) and addresses a problem of different acoustic sensitivities at different parts of the fibre (203). A method of producing an acoustic sensitivity profile is described which involves monitoring (501) temperature along the length of the optical fibre during curing of the surrounding cement structure. A temperature profile(301, 302) can then be generated (502) for the optical fibre based on said monitored temperature and used to generate (506) the acoustic sensitivity profile. The temperature profile indicates how well thermally coupled the fibre is to the surrounding cement, which also indicates how well acoustically coupled the fibre will be when the cement is cured. The sensitivity may be based on the rate of temperature change (303, 304) and/or magnitude of temperature change (T1, T2) during curing. The sensitivity profile can then be used to calibrate of adjust (604) measurement signals obtained using the optical fibre in a DAS sensor. The temperature during curing may be monitored using fibre optic distributed sensing such as DTS or DAS techniques.

Description

DETERMINING SENSITIVITY PROFILES FOR DAS SENSORS

This invention relates to methods and apparatus for determining and/or using sensitivity profile for a DAS fibre optic sensor, in particular for a DAS fibre optic sensor embedded within a cement structure and especially for a downwell DAS sensor.

Distributed acoustic sensing (DAS) is a known technique in which an optical fibre is interrogated using optical radiation and radiation which is backscattered from within the fibre is detected and analysed to determine information about any disturbances acting on the fibre, such as resulting from incident acoustic stimuli. By analysing the received backscattered radiation in separate time bins, based on the time after launch of the interrogating radiation, the fibre can be divided into a plurality of discrete longitudinal sensing portions where the disturbances acting on each sensing portion can be separately identified. DAS therefore can effectively divide a single optic fibre into a plurality of discrete acoustic sensing portions. DAS is typically based on Rayleigh scattering from intrinsic scattering sites inherent in the optic fibre and thus does not rely on deliberately introduced reflection sites such as formed by fibre Bragg gratings or the like. The sensing function is thus distributed through the full length of the fibre and the size and location of the individual sensing portions can be varied by varying the properties of the interrogating radiation, which is typically one or more distinct pulses, and/or the time bins used in processing the detected backscatter.

DAS has been used in a number of applications such as perimeter security and monitoring of linear assets such as pipelines. DAS has also been applied to provide downhole monitoring of wellbores, such as production wells in the oil and gas industry and/or injections wells, e.g. for carbon dioxide sequestration.

DAS provides a number of advantages for downwell use. A single interrogator unit can be provided at the proximal end of the optical fibre, i.e. at the well head, to interrogate the optical fibre. This avoids the need to supply power to downwell sensors. The interrogation is optical and thus safe for downwell use. If an optical fibre is provided along the whole length of the well then the whole well can be monitored effectively simultaneously. Optical fibre is also relatively inexpensive and thus can be deployed permanently in a well setting. In typical well formation a well bore is drilled and then an outer casing is installed down the wellbore, with sections of casing being welded together as they are inserted. Once the outer casing is in place cement is provided between the outer casing and the edge of the well bore, for at least a significant part of the well. The cement holds the casing in place and also seals the wellbore outside of the casing thus preventing any flow path along the well path other than through the casing.

Where an optical fibre for DAS is to be permanently installed in a well it may typically be attached to the outside of the outer casing during well formation. Thus the fibre, in a suitable protective cover, is attached to the sections of casing at the well head as they are forced into the well bore. This means that the fibre typically has good coupling to the outer casing and the presence of the fibre does not interfere with any subsequent stages of well completion. This also means that the optical fibre will be cemented in place during the well completion.

DAS sensors deployed in such a downwell setting may be used for a variety of different monitoring applications, in particular for monitoring operation of the completed well, e.g. to determine flow conditions and/or identify any anomalies in use. It has also been proposed that such a downwell DAS sensor could also be used in conducting seismic surveys, in particular for various forms of vertical seismic profiling (VSP). VSP is a known technique for geophysical surveying for instance for surveying of an oil or gas reservoir. Typically at least one seismic source is used to generate a seismic stimulus that travels through the region of interest to an array of sensors deployed in a well bore. In conventional seismic surveying an array of geophones (or hydrophones if the well is fluid filled) is inserted into the well to be used as the sensor array. Recently however it has been surprising found that a DAS sensor can provide sufficiently good results to be used in VSP or other seismic surveys. DAS has the advantage that sensing can be provided over the whole depth of the well at the same time, whereas a typical geophone array may be able to cover only part of the well depth and thus multiple tool repositions are required to cover the whole well. Also the use of geophone arrays requires the sensor tool to be lowered within the well outer casing in order to conduct the survey, requiring a well intervention. The use of DAS on an optical fibre which is permanently installed in the well does not require any well intervention. Using a DAS sensor with an optical fibre installed downwell as described above for seismic surveying thus offers several advantages compared to the conventional approach. However it has been found that such a DAS sensor can suffer from variations in the sensitivity of the various sensing portions to the seismic signals. It appears that with a fibre installed downwell some sensing portions of the fibre may be more or less sensitive to the incident seismic waves than other sensing portions. This effect can reduce the effective signal to noise ratio of the DAS sensor for seismic surveying. This problem can be at least partly addressed by increasing the number of seismic shots for any given survey, i.e. the number of times that a given stimulus is applied and the results detected, and applying seismic stacking techniques to the data.

DAS has also been proposed for other monitoring purposes, e.g. structural monitoring or the like, where the sensing fibre is at least partially embedded within or surrounded by a cement structure and similar sensitivity issues may be encountered in such applications.

Embodiments of the present invention relate to methods and apparatus for DAS sensing which at least mitigate the above mentioned problem, in particular to methods and apparatus that allow for a sensitivity/calibration profile to be obtained for a DAS sensor which uses a sensing optical fibre which is at least partly embedded within a cement structure.

Thus according to the present invention there is provided a method of producing an acoustic sensitivity profile for a distributed acoustic sensor having a first optical fibre at least partly embedded within a cement structure, the method comprising:

monitoring temperature along the length of the first optical fibre during curing of said cement structure;

generating a temperature profile for the length of said first optical fibre based on said monitored temperature; and

generating the acoustic sensitivity profile based on said temperature profile.

The sensitivity of a fibre embedded within a cement structure to acoustic signals travelling through the cement structure depends, at least partly, on the acoustic coupling of the optical fibre to the structure. Thus an optical fibre cable which is in intimate contact with a surrounding cement structure which fully fills the desired area to the correct density may be more sensitive than one where there are voids or other defects in the cement around the fibre and thus the acoustic coupling of the optical fibre to the structure. Embodiments of the present invention are based on the realisation that characteristics of the cement structure surrounding the optical fibre will not only effect the acoustic coupling in the cured structure but may also effect the thermal coupling during the curing process, i.e. the setting or solidification process of the cement in the cement structure. It will be appreciated by one skilled in the art that curing of cement will typically generate temperature changes in the structure during the curing process. Typical cements exhibit exothermic curing processes and thus heat will be generated during curing or setting. The extent to which this generated heat is conducted to the optical fibre will depend on the characteristics of the cement surrounding the fibre. A continuous volume of cement having the correct or intended density will generate a certain quantity of heat during curing/setting. If the optical fibre (or at least the protective jacket or cable outer within which the optical fibre is disposed) is in intimate contact with the surrounding cement along its length this generated heat will conduct relatively easily to the optical fibre, i.e. the heat generated will couple well to the fibre. Were a similar volume of cement to contain significant voids or a less dense packing of cement the amount of heat generated, and its rate of conduction to the optical fibre may be lower, especially if the optical fibre is in less intimate contact with the cement. Embodiments of the present invention thus use thermal coupling of the cement to the fibre as an indicator for acoustic coupling in the cured structure.

Monitoring temperature along the length of the first optical fibre may comprise monitoring for any temperature changes along the length of the fibre, e.g. for any temperature changes at each of a plurality of points along the length of the optical fibre and/or for discrete portions of the optical fibre. The absolute temperature along the length of the fibre, especially for a fibre that is several kilometres in length may be different at different points due to the local ambient conditions, especially in a well setting where the depth may change along the length of the fibre. Thus the starting temperature for the curing process may vary along the length of the fibre. It is temperature changes which are monitored. In some embodiments temperature changes may be monitored by monitoring an indication of absolute local temperature and determining any changes. However as will be explained in more detail later other may monitoring technique may only be sensitive to relative temperature changes and thus can monitor temperature changes from an unknown starting temperature. Such techniques can be used in embodiments of the invention. The temperature profile produced may indicate the rate of temperature change during curing. The rate of temperature change can indicate how well thermally coupled the optical fibre is to the cement structure. The acoustic sensitivity profile may therefore be based on the rate of temperature change. The temperature profile may indicate the rate of temperature increase during curing. As mentioned above typical cement curing processes are exothermic and thus the rate of the temperature increase during curing may be of interest. In some embodiments the temperature profile may further indicate the rate of a subsequent temperature decrease. In some embodiments just the rate of a temperature decrease could be monitored.

Generating the acoustic sensitivity profile may comprise determining the rate of temperature change during curing for a given portion of fibre and deriving a sensitivity value based on said rate of temperature change wherein a greater rate of temperature change results in a value indicating a greater sensitivity.

Additionally or alternatively the temperature profile may indicate the magnitude of a temperature change during curing. If the optical fibre has good thermal coupling to the cement then more of the heat generated may be conducted to the optical fibre and thus the overall magnitude of the temperature change may be greater. Also if a given volume of cement has significant voids there may be an overall lower density in that volume and thus less heat may be generated during curing leading to a lower overall volume change. The acoustic sensitivity profile may therefore be based on the magnitude of a temperature change during curing. The temperature profile may indicate the magnitude of a temperature increase during curing.

Generating the acoustic sensitivity profile may therefore comprise determining the magnitude of temperature change during curing for a given portion of fibre and deriving a sensitivity value based on said magnitude of temperature change wherein a greater magnitude of temperature change results in a value indicating a greater sensitivity.

It will of course be appreciated that the acoustic sensitivity profile may be based on one or more of the rate of temperature increase, magnitude of temperature increase, rate of temperature decrease, magnitude of temperature decrease, overall or average rate of temperature change, overall or average magnitude of temperature excursion or processed variants thereof. Generating the acoustic sensitivity profile may comprise comparing the temperature profiles for a plurality of discrete longitudinal portions of said first optical fibre to determine a relative sensitivity value for each of said discrete portions. Thus the various parameters of the temperature profile discussed above may be compared for different portions, i.e. at different positions along the length, of the first optical fibre in order to determine a relative sensitivity. The discrete longitudinal portions may correspond to discrete sensing portions of the first optical fibre when used as a distributed acoustic sensor. The temperature profile may also indicate the duration of any temperature variation due to the curing process. Generating the acoustic sensitivity profile may also take this duration into account, i.e. the acoustic sensitivity profile may be generated as a function of the overall duration and/or adjusted to take account of any variations in overall duration. It is possible that different parts of the cement structure may cure at different rates due to different local ambient conditions, this may affect the rate of heat generation and thus the rate of temperature change and/or maximum temperature excursion experienced by the optical fibre at that position.

In some embodiments the temperature along the length of the first optical fibre may be monitored by discrete temperature sensors which are placed at suitable intervals along the length of the first optical fibre. In some embodiments such temperature sensors may be required for other purposes in any case. However where such additional temperature sensors are not required the use of additional sensors increases the cost and increased complexity in terms of powering the sensors and data transfer.

In some embodiments therefore fibre optic sensing techniques are used to monitor the temperature. The fibre optic sensing may be performed using the first optical fibre itself as a sensing fibre. In some embodiments however the first optical fibre may be one of a plurality of optical fibres which are embedded into the cement structure, e.g. as a bundle of optical fibres in a cable structure. Any of the optical fibres in such a bundle could be used for monitoring temperature, although there may be some advantages in using the first optical fibre, i.e. the optical fibre that is intended for later use as a DAS sensing fibre. In some embodiments where there are a plurality of optical fibres temperature monitoring may be applied to more than one optical fibre. Thus in some embodiments monitoring temperature along the length of the first optical fibre comprises: repeatedly interrogating a DTS optical fibre to provide a distributed temperature sensor; wherein said DTS optical fibre is one of said first optical fibre or a second optical fibre deployed substantially along the path of the first optical fibre within the cement structure.

Distributed temperature sensing (DTS) is a well known fibre optic distributed sensing technique as would be well understood by one skilled in the art. An optical fibre is repeatedly interrogated with suitable optical radiation and the backscattered radiation is detected and analysed. Typically the radiation which is subject to Raman or Brillouin scattering is detected and analysed to determine the temperature of the fibre. DTS can provide an indication of the absolute temperature along the length of the optical fibre. Thus in some embodiments monitoring temperature along the length of the first optical fibre comprises monitoring the absolute temperature. The monitored temperature can then be used to derive the temperature profile.

In some embodiments therefore DTS may be employed on the first optical fibre during the curing process to derive a sensitivity profile than can be later used when the first optical fibre is used for DAS.

Additionally or alternatively to using DTS, in some embodiments monitoring

temperature along the length of the first optical fibre comprises: repeatedly

interrogating a sensing optical fibre with one or more pulses of coherent radiation; detecting radiation which is Rayleigh backscattered from within the optical fibre; and analysing said backscattered radiation to determine any changes in the temperature of said sensing optical fibre; wherein the sensing optical fibre is one of said first optical fibre or a second optical fibre deployed substantially along the path of the first optical fibre within the cement structure. In these embodiments the techniques of distributed acoustic sensing (DAS) are used to monitor for any temperature changes. As will be understood by one skilled in the art DAS techniques typically detect radiation which is Rayleigh backscattered from coherent interrogating radiation. Optical time domain reflectometry (OTDR) techniques are used to divide the detected backscatter in various time bins that correspond to different longitudinal sensing portions of the fibre. For broadband radiation propagating in an optical fibre the overall intensity of Rayleigh backscatter gradual drops with increasing distance into the optical fibre. With coherent interrogating radiation however the backscatter is subject to a random intensity modulation due to interference effects from the intrinsic scattering sites in the optical fibre, e.g. inhomogenities and the like. This interference modulation is random but, in the absence of any environmental disturbances acting on the optical fibre, the backscatter from any given sensing portion will be the same in response to repeated interrogations (assuming the optical properties of the interrogating radiation does not change). If however a sensing portion of the optical fibre is subject to a disturbance, such as a strain due to an incident acoustic wave, the distribution of the scattering sites may vary and thus the backscatter may vary. Analysing the detected backscatter from repeated interrogations thus allows the detection of any dynamic changes acting on the fibre. Typically these techniques are used to detect any vibrations or like, e.g. due to incident acoustic stimuli. It has been appreciated however that temperature changes will also produce a detectable change in backscatter properties. Typically any such temperature changes were regarded as noise and removed from the acoustic measurement signal. However it has been appreciated that using the techniques of DAS it is possible to detect temperature changes. This technique is not suitable for determining absolute temperature and thus can only indicate that a temperature change is occurring.

However as mentioned above this is suitable for embodiments of the present invention.

Rayleigh based temperature sensing, which may be referred to a distributed

temperature gradient sensing (DTGS) has the advantage of being able to resolve temperature changes with a high temperature resolution, better than DTS, and provides a fast indication of any temperature changes. DTS requires averaging over many interrogations, whereas DTGS can indicate temperature changes at the repeat rate of the interrogations.

In some embodiments the temperature monitoring may comprise applying both DTS and DTGS, possibly on different optical fibres, one of which may be the first optical fibre. In other embodiments DTGS and DTS may be conducted on the same optical fibre, for instance by using a series of interrogation for DTS interspersed with a series of interrogations suitable for DTGS, possible using wavelength divisional multiplexing techniques or by interrogating the optical fibre with suitable interrogating radiation such that the detected backscatter radiation can be analysed for both DTS and DTGS. In some embodiments the first optical fibre is disposed in a wellbore and the cement structure comprises cement between a well casing and the sides of the wellbore.

The acoustic sensitivity profile obtained may then be later used when using the first optical fibre for DAS. Thus aspects of the invention also provide a method of distributed acoustic sensing using a first optical fibre that is at least partly embedded within a cement structure, the method comprising:

repeatedly interrogating the first optical fibre to provide a plurality of discrete longitudinal sensing portions, each sensing portion providing a measurement signal indicative of acoustic disturbances acting on said sensing portion; and

modulating the measurements signals from said sensing portions based on an acoustic sensitivity profile;

said acoustic sensitivity profile having been generated according to any of the methods discussed above.

Thus in another aspect of the invention there is a provided a method of distributed acoustic sensing using a first optical fibre that is at least partly embedded within a cement structure, the method comprising:

wherein said acoustic sensitivity profile is based on a temperature profile indicating temperature changes that occurred along the length of the first optical fibre during curing of said cement structure.

The acoustic sensitivity profile may be obtained using any of the variants of the method discussed above.

In some embodiments the first optical fibre may be disposed in a wellbore and the cement structure may comprise cement between a well casing and the sides of the wellbore. The method may therefore comprise detecting seismic waves originating from outside of the wellbore. It will be appreciated that the acoustic sensitivity profile is an indication of how good the acoustic coupling between the first optical fibre and the cement structure is. Thus the sensitivity profile can be applied to calibrate or modulate detected acoustic waves that reach the first optical fibre through the cement structure, i.e. acoustic waves that originate from outside the well bore. It will be appreciated that in use the DAS sensor using the first optical fibre may be used to monitor various well processes that generate acoustic sounds from within the well. For such signals the acoustic sensitivity profile may not be appropriate. However there are various applications where DAS is used to detect signals that originate from outside the well and thus travel to the optical fibre via the cement structure. The acoustic sensitivity profile can be used in these situations to adjust the detected signals from different parts of the first optical fibre to take account of any difference in acoustic sensitivity.

This is particularly the case in performing a seismic survey. Thus the method may further comprise stimulating the earth surrounding the wellbore as part of a seismic survey. The seismic survey may, for example, comprise a vertical seismic profile survey although the same technique may be applied to a variety of seismic surveys or other seismic monitoring, e.g. fracture monitoring or microseismics. In another aspect therefore there is provided a method of performing a seismic survey of an area of interest around a wellbore comprising stimulating the area of interest with one or more seismic source and using the method of distributed acoustic sensing described above, using an acoustic sensitivity profile, to detect seismic waves reaching the wellbore.

In general, aspects of the invention relate to a method of producing an acoustic sensitivity profile for a distributed acoustic sensor having a first optical fibre at least partly embedded within a cement structure, the method comprising:

taking a temperature profile for said first optical fibre generated by monitoring any temperature changes along the length of said first optical fibre during curing of said cement structure; and

generating the acoustic sensitivity profile based on said temperature profile.

Aspects of the invention also relate to apparatus. Thus in a further aspect there is provide an apparatus for generating an acoustic sensitivity profile for a distributed acoustic sensor having a first optical fibre at least partly embedded within a cement structure, the apparatus comprising:

an optical source for repeatedly interrogating the first optical fibre with one or more pulses of coherent radiation;

a detector for detecting radiation which is backscattered from within the optical fibre;

a data analyser for analysing said backscattered radiation to determine at least one of the temperature of said sensing optical fibre and any changes in temperature of said sensing optical fibre; and

a processor configured to generate a temperature profile for the length of said first optical fibre based on said detected temperature and/or temperature change and generate the acoustic sensitivity profile based on said temperature profile.

The apparatus according to this aspect offers all of the advantages discussed above and the processor may be configured to implement any of the method variants discussed above.

In general aspects of the invention relate to a method of determining acoustic coupling of a first optical fibre at least partly embedded within a cement structure to said cement structure comprising monitoring the thermal coupling of the first optical fibre to the cement during curing of the cement surrounding the first optical fibre and deriving said acoustic coupling from said thermal coupling.

Aspects of the invention can also be seen as the use of a temperature profile obtained for a first optical fibre which is at least partly embedded within a cement structure, said temperature profile obtained during curing of said cement structure, as an acoustic sensitivity profile for acoustic measurements obtained using said first optical fibre as a sensing fibre for distributed acoustic sensing for acoustic signals transmitted to the first optical fibre via the cement structure. The invention will now be described by way of example only, with reference to the accompanying drawings, of which:

Figure 1 illustrates the general components of a DAS sensor;

Figure 2a and 2b illustrate the principles of a DAS sensor on a downwell fibre used for seismic surveying;

Figures 3a and 3b illustrate the principles of different acoustic coupling between the fibre and cement;

Figure 4 illustrates the principles of different thermal profiles;

Figure 5 shows a flowchart illustrating the method of determining an acoustic sensitivity profile according to an embodiment of the invention; and

Figure 6 shows a flowchart illustrating the method of using an acoustic sensitivity profile according to an embodiment of the invention.

Figure 1 shows a schematic of a distributed fibre optic sensing arrangement. A length of sensing fibre 104 is removably connected at one end to an interrogator 106. The output from interrogator 106 is passed to a signal processor 108, which may be co- located with the interrogator or may be remote therefrom, and optionally a user interface/graphical display 110, which in practice may be realised by an appropriately specified PC. The user interface may be co-located with the signal processor or may be remote therefrom.

The sensing fibre 104 can be many kilometres in length and can be at least as long as the depth of a wellbore which may be at least 1.5km long. The sensing fibre may be a standard, unmodified single mode optic fibre such as is routinely used in

telecommunications applications without the need for deliberately introduced reflection sites such a fibre Bragg grating or the like. The ability to use an unmodified length of standard optical fibre to provide sensing means that low cost readily available fibre may be used. However in some embodiments the fibre may comprise a fibre which has been fabricated to be especially sensitive to incident vibrations. In use the fibre 104 is deployed in area of interest to be monitored, for instance deployed to lie along the length of a wellbore, such as in a production or injection well as will be described.

In operation the interrogator 106 repeatedly launches interrogating electromagnetic radiation, which may for example comprise a series of optical pulses having a selected frequency pattern, into the sensing fibre. The optical pulses may have a frequency pattern as described in GB patent publication GB2,442,745 the contents of which are hereby incorporated by reference thereto. Note that as used herein the term "optical" is not restricted to the visible spectrum and optical radiation includes infrared radiation and ultraviolet radiation. As described in GB2,442,745 the phenomenon of Rayleigh backscattering results in some fraction of the light input into the fibre being reflected back to the interrogator, where it is detected to provide an output signal which is representative of acoustic disturbances in the vicinity of the fibre. The interrogator therefore conveniently comprises at least one laser 1 12 and at least one optical modulator 1 14 for producing a plurality of optical pulses separated by a known optical frequency difference. The interrogator also comprises at least one photodetector 1 16 arranged to detect radiation which is Rayleigh backscattered from the intrinsic scattering sites within the fibre 104. The signal from the photodetector is processed by signal processor 108. As will be understood by one skilled in the art optical time domain reflectometry (OTDR) techniques are used to divide the detected backscatter in various time bins that correspond to different longitudinal sensing portions of the fibre. As the interrogating radiation is coherent the backscatter is subject to a random intensity modulation due to interference effects from the intrinsic scattering sites in the optical fibre, e.g.

inhomogenities and the like. This interference modulation is random but, in the absence of any environmental disturbances acting on the optical fibre, the backscatter from any given sensing portion will be the same in response to repeated interrogations (assuming the optical properties of the interrogating radiation does not change). If however a sensing portion of the optical fibre is subject to a disturbance, such as a strain due to an incident acoustic wave, the distribution of the scattering sites may vary and thus the backscatter may vary. Analysing the detected backscatter from repeated interrogations thus allows the detection of any dynamic disturbances acting on the fibre. Any changes in the intensity of backscatter may be detected and used as an indication of a disturbance on the fibre. In some embodiments however, especially where each interrogation involves multiple pulses at different frequencies, the backscatter may be analysed to determine any changes in phase of an interference signal.

Thus the signal processor may demodulates the returned signal based on the frequency difference between the optical pulses, for example as described in

GB2,442,745. The signal processor may also apply a phase unwrap algorithm as described in GB2,442,745. The phase of the backscattered light from various sections of the optical fibre can therefore be monitored. Any changes in the effective path length from a given section of fibre, such as would be due to incident pressure waves causing strain on the fibre, can therefore be detected. The distributed acoustic sensor is thus able to detect any disturbance, e.g. vibration, caused by an incident pressure waves such as an acoustic stimulus. As used in this specification the term acoustic shall be used generally to refer to any type of pressure wave or stimulus and, for the avoidance of doubt, shall include seismic waves.

The form of the optical input and the method of detection allow a single continuous fibre to be spatially resolved into discrete longitudinal sensing portions. That is, the acoustic signal sensed at one sensing portion can be provided substantially independently of the sensed signal at an adjacent portion. Such a sensor may be seen as a fully distributed or intrinsic sensor, as it uses the intrinsic scattering processes inherent in an optical fibre and thus distributes the sensing function throughout the whole of the optical fibre. The spatial resolution of the sensing portions of optical fibre may, for example, be approximately 10m, which for a continuous length of fibre deployed down the entire length of a 4km production well say provides 400

independent acoustic channels or so deployed along the entire length of the well which can provide effectively simultaneous monitoring of the entire length of the wellbore.

As the sensing optical fibre is relatively inexpensive the sensing fibre may be deployed in a wellbore location in a permanent fashion as the costs of leaving the fibre in situ are not significant. Fibre optic cable is relatively robust and once secured in place can survive for many years in the downwell environment. The fibre is therefore

conveniently deployed in a manner which does not interfere with the normal operation of the well. Typically a suitable fibre may be installed during the stage of well construction, such as shown in Figures 2a and 2b. Typically producing or injection wells are formed by drilling a bore hole 201 and then forcing sections of metallic casing 202 down the bore hole. The various sections of the casing are joined together as they are inserted to provide a continuous outer casing. After the production casing has been inserted to the depth required the void between the borehole and the casing is backfilled with cement 203, at least to a certain depth, to prevent any flow other than through the well itself. As shown in Figures 2a and 2b the optical fibre to be used as the sensing fibre 104 may be clamped to the exterior of the outer casing 202 as it is being inserted into the borehole. In this way the fibre 104 may be deployed in a linear path along the entire length of the wellbore and subsequently cemented in place for at least part of the wellbore. It has been found that an optical fibre which is constrained, for instance in this instance by passing through the cement back fill, exhibits a different acoustic response to certain events to a fibre which is unconstrained. An optical fibre which is constrained may give a better response than one which is unconstrained and thus it may be beneficial to ensure that the fibre in constrained by the cement. It will therefore be seen that in the completed well the sensing optical fibre 104 is at least partly embedded with the cement structure that extends between the casing 202 and sides of the wellbore 201.

The fibre protrudes from the well head and is connected to interrogator 106, which may operate as described above.

Interrogator 106 may be permanently connected to the fibre 104 to provide continual acoustic/seismic monitoring and may monitor a range of well operations. In some embodiments however the interrogator is removably connected to the fibre 104 when needed to perform a geophysical survey but then can be disconnected and removed when the survey is complete. The fibre 104 though remains in situ and thus is ready for any subsequent survey. The fibre is relatively cheap and thus the cost of a permanently installed fibre is not great. Having a permanently installed fibre in place does however remove the need for any sensor deployment costs in subsequent surveys and removes the need for any well intervention. This also ensures that in any subsequent survey the sensing is located in exactly the same place as for the previous survey. This readily allows for the acquisition and analysis of seismic data at different times to provide a time varying seismic analysis. To perform geophysical monitoring, one or more seismic sources 204, for example Vibroseis™ trucks are located with a desired offset from the wellbore and used to excite the ground at the surface as illustrated in Figure 2a. There may be several seismic sources exciting the ground at the same time, at the same or different locations although only one source is shown in Figure 2a for clarity. Depending on the type of geophysical survey the seismic source 204 may apply a stimulus with a time varying frequency pattern and when analysing the data from the DAS sensor a frequency correlation may be applied to isolate the seismic signals of interest from background noise etc. Multiple different arrangements of seismic source may be used. For example for performing a zero-offset vertical seismic profile (ZO-VSP) the seismic source may be located generally above the wellbore, but outside of the wellbore. In a Walk-away vertical seismic profile (WA-VSP) the seismic source may be progressively moved further away from the well bore. The seismic source may also be used to induce tube waves in the well casing. The different types of survey can be used to monitor different aspects of the well, for example in a carbon dioxide sequestration well a ZO-VSP may be used to monitor C0₂ containment, a WA-VSP may be used to track the C0₂ injection plume and tube wave monitoring may be used to monitor casing integrity. The stimulus applied by the seismic source 204 may be very energetic and thus the signals incident on the potions of fibre at the top of the well will also be energetic.

However the signals at deeper sections of the fibre may be significantly attenuated and may be relatively faint. Thus the DAS sensor ideally has a large dynamic range. To help cope with a wide dynamic range the sampling speed of the photodetector 1 16 and initial signal processing is at a high rate so as to reduce the amount of variation between any two samples. The can aid in subsequent reconstruction of the form of the incident seismic signal. However once the general form of the signal is known a high data rate may not be required and thus the signal processor 108 may decimate the processed data to reduce further processing and storage requirements. As the skilled person will appreciate the sampling speed of the backscatter signal should in general be high enough to provide the desired spatial resolution. For instance if the spatial sensing portions are 10m in length then the time between successive samples should be such that if a first sample corresponds to backscatter from a first section of fibre then the second sample should correspond to backscatter from a second section of fibre no more than 10m away from the first section of fibre. Thus the time between samples should be no more than the time taken for light to move 20m in the fibre (i.e. the time for the interrogating radiation to move 10m further into the fibre and the backscattered light to travel the additional 10m back toward the front of the fibre). Taking the speed of light as 3 x 10⁸ms^"1 and the refractive index of the fibre as 1.5 this requires a sample rate of about 10MHz. In most DAS systems, to avoid the cost associated with high speed components the sample rate would be set around this minimum required rate - especially as the minimum spatial resolution is set by the form of the interrogating radiation and thus a higher sample rate would usually not lead to a better spatial resolution. In an embodiment of the DAS sensor used in the present invention however the sample rate may be at least eight times greater than the minimum sample rate required given the size of the sensing portions. For instance the sample rate may be of the order of 80-100MHz. Each sample may therefore be processed to determine an indication of the acoustic signal, before at least some samples are combined to form a composite sample for the sensing portion. By oversampling in this way and processing the samples before combination then any very intense signals can be identified.

As will be appreciated in a DAS sensor which relies on interference a large strain may cause a variation in path length that leads to a phase change of greater than 2ττ. If this was to occur between samples it would be difficult to correctly determine the incident signal. This embodiment of the present invention avoids such a problem and hence improves the dynamic range of the DAS sensor.

The signals from a given shot, i.e. given form of seismic stimulus, can be detected from each of the longitudinal sensing portions of the optical fibre (assuming the signals have not been completely attenuated). Thus it is possible to receive a signal from each sensing portion of fibre along the entire depth of the well. The result will be a series of signals indicating the seismic signals detected over time in each longitudinal section of the fibre. The sensing fibre thus effectively acts as a series of point seismometers but one which can cover the entire length of the wellbore at the same time, unlike a conventional geophone array. Further as the optical fibre can be installed so as to not interfere with normal well operation no well intervention is required. An additional advantage to leaving the fibre in situ is the ability to perform time-lapse geophysical surveys. The optical fibre will be located in the same place each time that a survey is performed and, as the position of the acoustic channels along the fibre are determined by the interrogator, the acoustic channels may have exactly the same position from survey to survey. Thus, the results of two surveys which are conducted using the same fibre but conducted at different times can be directly compared to determine any changes occurring over time. The ability to directly correlate the results of surveys conducted at different times is an advantage of using DAS sensors with permanently deployed fibres.

Embodiments of the present invention therefore provide the ability to monitor the response from the entire length of the wellbore, or at least as much of the wellbore as is of interest, in response to a single shot of the seismic source. In practice, however, the seismic stimulus may be applied a plurality of times and the acoustic response from the wellbore monitored in the response to each shot, i.e. instance of the stimulus. With a DAS sensor, as no tool redeployment is necessary, the repeat shots may be acquired relatively quickly. The data from each shot can then be processed using seismic stacking techniques to improve the signal to noise ratio. Thus, data from a plurality of shots using DAS can be acquired in a fraction of the time that would be needed to acquire the same number of shots from each of the different well depth positions required with conventional geophones. It will be appreciated however that the conditions of the well and the surrounding area will evolve over time, which means that the conditions of shot may change over time. This can impact the accuracy of the resulting seismic stacking procedure.

It will be appreciated that in such a seismic survey the signals of interest originate from outside of the wellbore and travel through the surrounding earth formation to the optical fibre via the cement 203 between the wellbore and the casing. It has been found that the sensitivity of the optical fibre can vary along its length and it is believed that this variation in sensitivity may be, at least partly, due to differences in the acoustic coupling between the optical fibre 104 and the surrounding cement. These variations in sensitivity may arise due to variations in the quality of the cement in the local area of the optical fibre which effect how well the optical fibre is mechanically coupled to the cement.

Figures 3a and 3b illustrate the principles of variations in mechanical and acoustic coupling of an optical fibre which is, at least partly, embedded within a cement structure. At this point it should be noted that the terms cement structure refers to any structure where a cement binder is used to set and form at least part of a resulting structure. It will be appreciated that various different types of cement exist and ay be used in different applications. It will also be appreciated that some cement structures may comprise one or more particulate materials disposed within a cement matrix, e.g. concrete has particulate material embedded within cement. For the avoidance of doubt the term cement structure as used herein include structures formed out of concrete or aggregates including cement.

Figure 3a illustrates a section of the optical fibre 104 and the surrounding cement 203. Figure 3a shows an example where the cement 203 completely surrounds the optical fibre 203 and the fibre is in intimate contact with the surrounding cement along the length of this section. The cement may be relatively uniform and may have the intended density. In the context of the well setting this is the intended arrangement of the cement - which will therefore provide a good seal for outside of the well. In this example the fibre 104 is in good mechanical contact with the cement 203 and any pressure waves travelling through the cement will couple well to the optical fibre.

By contrast figure 3b illustrates an example where the cement 203 comprises a number of voids 301 , 302 and does not completely fill the volume around the optical fibre.

Such voids may be caused, for example by gas bubbles or other contaminants trapped within the cement and/or uneven distribution of the cement in the volume and/or problems in the mixing of the cement. One skilled in the art of oil wells will appreciate that the quality of the cement may vary along the length of the well. The presence of the voids 301 , 302 may interfere with the transmission of pressure waves through the cement structure and thus interfere with the acoustic coupling of the optical fibre to the cement. In this example the optical fibre is illustrated as passing through at least one void 302. In this illustrated section of optical fibre the mechanical coupling of the optical fibre to the cement 203 is thus worse than the example illustrated in figure 3a. The acoustic sensitivity of a sensing portion in the example of figure 3b may therefore be lower than in the example of figure 3a. It will of course be appreciated that an optical fibre 104 will normally be contained in one or more jacket/protective layers in a fibre optic cable structure, possible with a plurality of optical fibres in the same cable structure. Thus the optical fibre may not itself be directly coupled to cement. It is therefore the coupling between the cable outer layer and the surrounding cement which is important. However as the structure of the cable is fixed the coupling between the cable outer layer and the cement is what effects the coupling between the cement and the optical fibre. Thus references herein to the coupling between the optical fibre and the cement or contact between the optical fibre and the cement shall be taken as meaning references to the coupling/contact of the cable structure containing the optical fibre. This variation in sensitivity can impact on the signal to noise ratio of DAS sensor using the optical fibre 104. This can particularly be the case for seismic surveys such as VSP. To a certain extent this problem can be addressed by increasing the number of seismic shots and performing seismic stacking techniques. However, the time and cost of the survey will be related to the number of shots required and thus there is a general imperative to use as few shots as possible. There will be time and personnel costs in performing additional shots which ideally may be minimised. In addition in some instances well processes may be shut down during the survey to avoid extraneous noise and it may be desired to keep the duration of the shut-down short. In addition, as mentioned above, the environmental conditions will in any case change over time and this the longer the survey takes, because of the number of shots required, the more errors that may be introduced from environmental changes.

Embodiments of the present invention therefore provide methods and apparatus for determining an acoustic sensitivity profile for the optical fibre, i.e. an indication of how the acoustic coupling of the optical fibre to the surrounding cement structure may vary along the length of first optical fibre. The acoustic sensitivity may be used to adjust or calibrate the measurement signals from the various sensing portions of optical fibre when used to detect acoustic/seismic disturbances that originate from outside the well. For seismic surveying this can reduce the number of shots required to achieve a desired Signal to Noise ration (SNR) with the attendant advantages.

It has been appreciated that optical fibre 104 will clearly be located in situ as the cement 203 is introduced into the well. The optical fibre will thus be present in situ as the cement cures, i.e. sets or hardens. The curing process will involve a temperature change within the cement structure - typical cements exhibit an exothermic curing process and thus heat is generate as the cement cures. Embodiments of the present invention monitor the temperature at various points or sections along the length of the optical fibre during the curing process. By monitoring how the temperature of the optical fibre changes during the curing process a thermal profile indicative of the thermal coupling of the optical fibre to the cement can be determined. It has been appreciated by the present inventor(s) that the thermal coupling is good proxy for the acoustic coupling and thus the thermal profile can be used to determine an acoustic sensitivity profile.

Referring back to figure 3 consider the temperature evolution of the example illustrated in figure 3a as the cement cures. The whole volume is filled with cement which is intimately coupled to the optical fibre. Thus a certain amount of heat will be generated during the curing process and at least some heat will be conducted to the optical fibre leading to a certain temperature rise. In the situation illustrated in figure 3b there are voids in the cement as it cures. Thus there is less cement in the same volume than in the example of figure 3a and thus the overall amount of heat generated will be expected to be lower. Also there are some areas where the conduction path to the optical fibre is interrupted, e.g. by an air bubble. Air is poor conductor and thus the rate of heat transfer to the optical fibre may be lower. This means that the rate of temperature increase and/or the magnitude temperature excursion may be greater for the example of figure 3a than the example of figure 3b. Embodiments of the present invention therefore monitor for temperature changes at the optical fibre during the curing process and use the temperature profile to generate an acoustic sensitivity profile. Figure 4 illustrates the principles of determining a thermal profile and generating a sensitivity profile. It will be appreciated that figure 4 illustrates an idealised example for the purpose of explanation only.

Figure 4 illustrates a plot of temperature change, ΔΤ, against time during the curing process for an optical fibre embedded within a cement structure. Figure 4 illustrates a first thermal profile illustrated by solid line 301 and a second thermal profile illustrated by dashed line 302. It can be seen that both thermal profiles indicate an increase in temperature over the period to a maximum value that then subsequently reduces. It can be seen that the thermal profile indicated by solid line 301 reaches a greater magnitude of temperature increase than the profile indicated by dashed line 302, i.e. the temperature excursion T1 is greater than the temperature excursion T2. The rate of increase of profile 301 is also greater than that for profile 302, as illustrated by extrapolated average rates of increase 303 and 304 respectively. The temperature profile 301 may therefore represent the temperature profile for a portion of optical fibre according to the example shown in figure 3a whereas the temperature profile 302 may represent the temperature profile for a portion of optical fibre according to the example shown in figure 3b.

In some embodiments therefore the temperature profile may be used to generate an acoustic sensitivity profile according to a method as generally illustrated by the flow chart of figure 5.

In a first step, 501 , the temperature along the length of the optical fibre may be monitored.

In some embodiments the temperature along the length of the first optical fibre may be monitored by discrete temperature sensors which are placed at suitable intervals along the length of the first optical fibre. Depending on the application in which the optical fibre is to be used for DAS there may be a requirement for discrete temperature sensor for other purposes in any case. However where such additional temperature sensors are not required in general the use of additional sensors would increase the cost and complexity of the overall sensor arrangement in terms of powering the sensors and reading data from the sensor. The provision of such discrete temperature sensors could even reduce the overall acoustic coupling.

In some embodiments therefore fibre optic sensing techniques are used to monitor the temperature. The fibre optic sensing may be performed using the optical fibre 104 itself as a sensing fibre. In some embodiments however the optical fibre 104 may be one of a plurality of optical fibres which are embedded into the cement structure, e.g. as a bundle of optical fibres in a cable structure. Any of the optical fibres in such a bundle could be used for monitoring temperature, although there may be some advantages in using the optical fibre 104 that is intended for later use as a DAS sensing fibre. In some embodiments where there are a plurality of optical fibres temperature monitoring may be applied to more than one optical fibre.

The temperature along the length of the optical fibre could be monitored using distributed temperature sensing (DTS). DTS is a well known fibre optic distributed sensing technique as would be well understood by one skilled in the art. An optical fibre is repeatedly interrogated with suitable optical radiation and the backscattered radiation is detected and analysed. Typically the radiation which is subject to Raman or Brillouin scattering is detected and analysed to determine the temperature of the fibre. DTS can provide an indication of the absolute temperature along the length of the optical fibre. Thus monitoring the temperature may comprise attaching a suitable DTS interrogator unit to the optical fibre 104, or another optical fibre that runs along the same path, and applying DTS during the cement curing.

Additionally or alternatively to using DTS, in some embodiments the techniques of distributed acoustic sensing (DAS) are used to monitor for any temperature changes. As described above DAS techniques typically detect radiation which is Rayleigh backscattered from coherent interrogating radiation. Typically these techniques are used to detect any vibrations or like, e.g. due to incident acoustic stimuli. It has been appreciated however that temperature changes will also produce a detectable change in backscatter properties in the Rayleigh backscatter and thus detection of coherent Rayleigh backscatter can be used to detect dynamic temperature changes. In conventional DAS the effects due to any such temperature changes were regarded as noise and removed from the acoustic measurement signal. However it has been appreciated that using the techniques of DAS it is possible to detect temperature changes. This technique is not suitable for determining absolute temperature and thus can only indicate that a temperature change is occurring. However as mentioned above this is suitable for embodiments of the present invention.

Rayleigh based temperature sensing, which may be referred to a distributed temperature gradient sensing (DTGS) has the advantage of being able to resolve temperature changes with a high temperature resolution, better than DTS, and provides a fast indication of any temperature changes. DTS requires averaging over many interrogations, whereas DTGS can indicate temperature changes at the repeat rate of the interrogations.

In some embodiments the temperature monitoring may comprise applying both DTS and DTGS, possibly on different optical fibres. In other embodiments DTGS and DTS may be conducted on the same optical fibre, for instance by using a series of interrogation for DTS interspersed with a series of interrogations suitable for DTGS, possible using wavelength divisional multiplexing techniques or by interrogating the optical fibre with suitable interrogating radiation such that the detected backscatter radiation can be analysed for both DTS and DTGS. However the monitoring is performed at step 502 a thermal profile may be determined for each of a plurality of sensing portions of the optical fibre, e.g. a profile along the lines illustrated in figure 4. Conveniently each of the sensing portions may correspond to the sensing portions likely to be used later for DAS and an advantage of using DGTS for temperature monitoring on the fibre 104 is that the fibre may be interrogated with the same interrogating radiation and using the same time analysis bins as will be later used when using the fibre for DAS.

The temperature profile may then be used to determine at least one of the rate of temperature increase during curing, step 503 or the magnitude of the temperature excursion, step 504. The rate and/or extent of subsequent temperature decrease could also be determined.

In some instances the overall duration of curing may be determined at step 505. The cement may cure at different rate depending on local conditions which may impact on the local rate of heat generation. The thermal profiles for each sensing portion may therefore be normalised according to the local curing conditions.

Using at least some of these determined parameters an acoustic sensitivity value may be determined for each sensing portion. The acoustic sensitivity value may be a value related to the relative sensitivity. Thus a first thermal profile for a first sensing portion that shows a greater rate of temperature increase and/or greater temperature excursion than a second thermal profile for a second sensing portion may lead to a sensitivity value for the first sensing portion than indicates greater acoustic sensitivity than for the second sensing portion. Depending on the particular application the sensitivity value could be greater for the more sensitive sensing portions or the sensitivity value could be lower for the more sensitive portions indicating that a lower calibration factor is required. It will of course be appreciated that the starting temperature may be different for different sensing portions, but by looking at temperature changes and adjusting for different curing conditions it will be possible to derive sensitivity profiles for different sensing portions. In some embodiments the sensitivity value may principally be a relative value comparing the sensitivity of differing portions of the optical fibre.

However in some instances the temperature profile could be compared to some reference or expected temperature profile(s), possibly adjusted for the particular combination of cement material, local environment and fibre optic cable structure, to generate a measure of absolute acoustic sensitivity.

The sensitivity profile can then be used to calibrate or adjust the measurement signals from the sensing portions of the optical fibre when used as part of a DAS sensor for detecting acoustic/seismic signals that reach the optical fibre via the cement structure.

Figure 6 shows a flow chart that illustrates this general method. At step 601 the optical fibre is used with a DAS interrogator unit as part of a DAS sensor. In embodiments where the DAS sensor is used for seismic surveying the method may comprise controlling at least one seismic source to stimulate the area of interest (step 602). It will be appreciated however that the method can be used for detecting any signals that originate from outside the cement structure. At step 603 a measurement signal is derived for each of the plurality of sensing portions of the optical fibre, the

measurement signal being indicative of the detected acoustic/seismic signals. At step 604 the acoustic sensitivity profile may be used to adjust or calibrate these

measurement signals. In effect the sensitivity profile may be used as a calibration value which is used to adjust the magnitude of any detected signals. For instance, where the sensitivity value is lower for the more acoustically sensitive portions the sensitivity value may be used as a multiplier for the amplitude of the signals, although more sophisticated processing may of course be used. In essence the measurement signals may be weighted in some way in accordance with the estimated sensitivity of the relevant sensing portion. For seismic surveying there may be a number of seismic shots and the method may include seismic stacking of the data. However because the sensitivity variations have been reduced fewer seismic shots are required to achieve a desired SNR.

In general therefore embodiments of invention involve a method of determining acoustic coupling of an optical fibre to a cement structure by monitoring the thermal coupling of the optical fibre to the cement during curing of the cement surrounding the first optical fibre and deriving said acoustic coupling from said thermal coupling.

Embodiments also involve use of a temperature profile obtained for an optical fibre which is at least partly embedded within a cement structure, the temperature profile being obtained during curing of the cement structure, as an acoustic sensitivity profile for acoustic measurements obtained using the optical fibre as a sensing fibre for distributed acoustic sensing for acoustic signals transmitted to the first optical fibre via the cement structure.

The discussion above has focussed on the use of the optical fibre in a downwell setting however the same principle would apply to an optical fibre embedded in any cement structure for DAS where the fibre is provided in situ during curing of the cement structure.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim, "a" or "an" does not exclude a plurality, and a single feature or other unit may fulfil the functions of several units recited in the claims. Any reference numerals or labels in the claims shall not be construed so as to limit their scope

Claims

1. A method of producing an acoustic sensitivity profile for a distributed acoustic sensor having a first optical fibre at least partly embedded within a cement structure, the method comprising:

generating the acoustic sensitivity profile based on said temperature profile.

2. A method as claimed in claim 1 wherein monitoring temperature along the length of the first optical fibre comprises monitoring for any temperature changes.

3. A method as claimed in any preceding claim wherein the temperature profile indicates the rate of temperature change during curing.

4. A method as claimed in claim 3 wherein the temperature profile indicates the rate of temperature increase during curing.

5. A method as claimed in claim 4 wherein the temperature profile further indicates the rate of a subsequent temperature decrease.

6. A method as claimed in any of claims 3 to 5 wherein the acoustic sensitivity

profile is based on the rate of temperature change.

7. A method as claimed in claim 6 wherein generating the acoustic sensitivity profile comprises determining the rate of temperature change during curing for a given portion of fibre and deriving a sensitivity value based on said rate of temperature change wherein a greater rate of temperature change results in a value indicating a greater sensitivity.

8. A method as claimed in any preceding claim wherein the temperature profile indicates the magnitude of a temperature change during curing.

9. A method as claimed in claim 8 wherein the temperature profile indicates the magnitude of a temperature increase during curing.

A method as claimed in claim 8 or claim 9 wherein the acoustic sensitivity profil is based on the magnitude of a temperature change during curing.

A method as claimed in claim 10 wherein generating the acoustic sensitivity profile comprises determining the magnitude of temperature change during curing for a given portion of fibre and deriving a sensitivity value based on said magnitude of temperature change wherein a greater magnitude of temperature change results in a value indicating a greater sensitivity.

A method as claimed in any preceding claim wherein generating the acoustic sensitivity profile comprises comparing the temperature profiles for a plurality of discrete longitudinal portions of said first optical fibre to determine a relative sensitivity value for each of said discrete portions.

A method as claimed in claim 12 where said discrete longitudinal portions correspond to discrete sensing portions of the first optical fibre when used as a distributed acoustic sensor.

14. A method as claimed in any preceding claim wherein the temperature profile indicates the duration of any temperature variation due to the curing process.

15. A method as claimed in claim 14 wherein the acoustic sensitivity profile is

generated to take any variations in duration along the length of the first optical fibre into account.

16. A method as claimed in any preceding claim wherein monitoring temperature along the length of the first optical fibre comprises:

repeatedly interrogating a sensing optical fibre with one or more pulses of

coherent radiation;

detecting radiation which is Rayleigh backscattered from within the optical fibre; and

analysing said backscattered radiation to determine any changes in the

temperature of said sensing optical fibre; wherein said sensing optical fibre is one of said first optical fibre or a second optical fibre deployed substantially along the path of the first optical fibre within the cement structure.

A method as claimed in any of claims 1 to 15 wherein monitoring temperatu along the length of the first optical fibre comprises monitoring the absolute temperature.

A method as claimed in any preceding claim wherein monitoring temperature along the length of the first optical fibre comprises:

repeatedly interrogating a DTS optical fibre to provide a distributed temperature sensor;

wherein said DTS optical fibre is one of said first optical fibre or a second optical fibre deployed substantially along the path of the first optical fibre within the cement structure.

A method as claimed in any preceding claim wherein said first optical fibre is disposed in a wellbore and the cement structure comprises cement between a well casing and the sides of the wellbore.

A method of distributed acoustic sensing using a first optical fibre that is at least partly embedded within a cement structure, the method comprising:

repeatedly interrogating the first optical fibre to provide a plurality of discrete

longitudinal sensing portions, each sensing portion providing a measurement signal indicative of acoustic disturbances acting on said sensing portion; and

said acoustic sensitivity profile having been generated according to the method of any of claims 1 to 19.

21. A method of distributed acoustic sensing using a first optical fibre that is at least partly embedded within a cement structure, the method comprising:

wherein said acoustic sensitivity profile is based on a temperature profile

indicating temperature changes that occurred along the length of the first optical fibre during curing of said cement structure.

22. A method as claimed in claim 20 or claim 21 wherein said first optical fibre is disposed in a wellbore and the cement structure comprises cement between a well casing and the sides of the wellbore, the method comprises detecting seismic waves originating from outside of the wellbore.

23. A method as claimed in claim 22 comprising stimulating the earth surrounding the wellbore as part of a seismic survey.

24. A method as claimed in claim 23 wherein said seismic survey comprises a

vertical seismic profile survey.

25. A method of performing a seismic survey of an area of interest around a wellbore comprising stimulating the area of interest with one of more seismic source and using the method of distributed acoustic sensing as claimed in claim 20 to detect seismic waves reaching the wellbore.

26. A method of producing an acoustic sensitivity profile for a distributed acoustic sensor having a first optical fibre at least partly embedded within a cement structure, the method comprising:

generating the acoustic sensitivity profile based on said temperature profile.

27. An apparatus for generating an acoustic sensitivity profile for a distributed

acoustic sensor having a first optical fibre at least partly embedded within a cement structure, the apparatus comprising: an optical source for repeatedly interrogating the first optical fibre with one or more pulses of coherent radiation;

A method of determining acoustic coupling of a first optical fibre at least partly embedded within a cement structure to said cement structure comprising monitoring the thermal coupling of the first optical fibre to the cement during curing of the cement surrounding the first optical fibre and deriving said acoustic coupling from said thermal coupling.

The use of a temperature profile obtained for a first optical fibre which is at least partly embedded within a cement structure, said temperature profile obtained during curing of said cement structure, as an acoustic sensitivity profile for acoustic measurements obtained using said first optical fibre as a sensing fibre for distributed acoustic sensing for acoustic signals transmitted to the first optical fibre via the cement structure.