GB2402738A - Scale detection - Google Patents

Abstract

A method of detecting the formation of scale within, for example, a well bore, comprises deploying an optical fibre into the well bore, launching light into the fibre and detecting light emerging from the fibre. Any scale forming on the fibre will exert stresses on the fibre and possibly distort the fibre. The stresses and distortion alter the optical characteristics of the fibre, so that changes appear in the emerging light, such as increased attenuation or changes in the wavelength of Brillouin scattered light. The emerging light is therefore detected and monitored for any such changes, as these provide an early warning indication of the presence of scale within the well bore. This can be enhanced by providing the fibre with a coating that will cause scale to be preferentially deposited on the fibre. The coating may comprise one or more materials likely to precipitate as scale within a given well bore, such as barium sulphate.

Description

TITLE OF THE INVENTION

SCALE DETECTION

BACKGROUND OF THE INVENTION

The present invention relates to a method of and apparatus for detecting the formation of scale, particularly but not exclusively within the well bore of an oil well.

Scale presents a significant problem in the production of oil from underground hydrocarbon reservoirs. It is a crystalline deposit which can form on virtually any part of a well bore, including the casing, tubing and perforations. Scale can arise from the presence of water in and around a well bore. The water often carries large quantities of ions, the type of which depends on the origin of the water and the mineral phases with which it has come into contact. Thus near-surface water tends to carry different ions from deep subsurface water. If two differing waters come together, the ions can form atoms and then seed crystals, which grow into scale deposits. The temperature and pressure variations within the well environment can also affect scale formation.

Mineral solubility in water typically depends on temperature and pressure, so if water is near-saturated with a dissolved mineral and then undergoes an appropriate change in environmental conditions, the mineral can precipitate and be deposited as scale.

Common scales include sulphates and carbonates such as calcium carbonate, calcium sulphate and barium sulphate.

Once scale deposition begins, it typically progresses very rapidly. In extreme cases the well bore can become completely blocked by scale in a matter of hours. Oil production then ceases until it is possible to clear the scale and unblock the well bore.

This is very expensive, both in teens of the cost of removing the scale, and the loss of production time.

Consequently, much time and effort has been devoted to developing and improving techniques for scale removal, and also ways of inhibiting the deposition and redeposition of scale. Removal methods include the use of drilling and impact tools to break up the scale, chemical cleaners that attack and dissolve the scale, and jet blaster tools that spray fluids, sometimes containing abrasive particles, against the scale at high pressure. The prevention of scale is commonly achieved by dilution methods, in which a supply of fresh water is continuously delivered to parts of the well bore, and by injecting chemical inhibitors that block the growth of scale by preventing the ions from bonding.

However, while scale removal can reopen a well bore and allow production to recommence, it is an expensive, complex process, which may result in damage to the lo tubing and other components of the well. Scale prevention can also be costly, and treatment with chemical inhibitors has a limited lifetime.

Therefore, it would be highly beneficial to be able to detect the start of scale formation, so that removal can be undertaken quickly, before the well is entirely put out of production, and also before the deposit becomes so extensive that it is very difficult to remove. A potential technique which suggests itself is to detect the relevant ions as soon as they become present. However, this is a chemically complex problem, because the ions can be difficult to detect. For example, identification of barium is complicated by its similarity to sodium, which typically does not contribute to scaling.

There are, however, several scale detection methods which have been put into practse. The analysis of downhole samples can reveal the presence of scale. Gamma ray log measurements can identify barium sulphate scale, because naturally radioactive radium Ra226 precipitates with it. The sudden onset of water production from a well can suggest that scale formation is likely to begin or has begun, particularly if accompanied by a drop in oil production. Chemical analysis of the water can be performed to look for scaling ions. However, all of these methods take time to perform, and are non-continuous. Preferably, scale detection should be rapid and capable of indicating the presence of scale as soon as possible, so that removal and prevention steps can be undertaken before the problem increases.

To address these issues, a scale management system that allows permanent monitoring has been proposed. A downhole electrochemical sensor is used to obtain measurements of pH and chloride ion concentration, which are combined with measurements of temperature, pressure and flow to detect build-up of carbonate, and also to control delivery of chemical cleaners and inhibitors. The system requires many parameters to be monitored, and is limited to the detection of carbonate scales.

A similarly complex approach is that of chemical modelling that can predict the nature and extent of scaling. Data relating to elementalconcentration analysis, temperature, pressure and gas-phase composition are required inputs to modelling lo programs, together with information pertaining to reservoir performance and expected water breakthrough. Predictions far into the future can be made, but, particularly for new reservoirs, highly accurate chemical composition data is required, which is seldom available, and must be collected.

A review of these scale detection techniques, together with a discussion of is scale formation and removal, can be found in Crabtree et al [1].

Therefore, there is a need for a simple and reliable method of detecting scale, that is preferably inexpensive, and gives an early warning of both when and where scale begins to deposit. This would be of great benefit to oil and gas producers, as it would enhance scale management and reduce costs and lost revenue.

5U)11IAIIY UF I HE 1N LN I ION Accordingly, a first aspect of the present invention is directed to a method of detecting scale, comprising: deploying an optical fibre into a region of interest; launching light into the optical fibre; detecting light subsequently returning from the optical fibre; and monitoring the detected light to identify any change arising from one or more changes in optical characteristics of the optical fibre caused by scale formation on the optical fibre.

If scale is deposited on the outer surface of an optical fibre, it produces stresses lo in the fibre, and can also distort it. These stresses and distortions change various optical characteristics of the fibre, thus altering the way in which the fibre carries light. This shows up in light transmitted by the fibre, and can be identified because light resuming from the fibre will differ in some respect from light returning before the scale was deposited. This arrangement therefore provides a very simple and straightforward technique of directly detecting scale in well bores. There is no requirement for complex modelling, or the measurement of many parameters. Results obtained are indications of actual scale formation, rather than mere predictions.

Further, the method utilises a technology already proven to be effective in environments such as oil well bores, namely optical fibre. An efficient early warning detection system is provided, because scale forming on the fibre will produce a detectable effect in the early stages of deposition, so that appropriate steps for removal or inhibition can be taken before the scale has had an opportunity to proliferate extensively.

The method may further comprise using time measurements of the returning light to calculate distance along the optical fibre at which any change occurs, the distance indicating a location of scale formation. This is readily achieved by measuring the time period between launch of the light and the detection of the returning light, and using the speed of light within the fibre to determine the distance.

Thus, it is possible to rapidly determine precisely when and where scale is forming.

In a preferred embodiment, the emerging light is detected at an end of the optical fibre into which the light is launched. Detection can therefore be performed using one end of the fibre only. This is clearly useful, because one end of a fibre deployed, for example, down a well bore, is located near the bottom of the bore, and is s inaccessible. It is possible to arrange a fibre which extends down into the well bore and back up again, thus giving access to both ends of the fibre, but this needs twice the length of fibre and more complex deployment mechanisms, so the single ended arrangement is preferred.

According to some embodiments, the emerging light arises from back o scattering within the optical fibre. For example, the identifying any change may comprise identifying a shift in wavelength and/or a change in amplitude of one or more components of the emerging light arising from Brillouin scattering within the optical fibre.

Alternatively, the identifying any change may comprise identifying attenuation of the emerging light arising from one or more stresses in the optical fibre. In this context, the optical fibre may have a filament wrapped around it to enhance stress formation within the optical fibre. The optical fibre may be operable as an optical time domain reflectometer. This is an established technique for measuring losses in optical fibres that can be readily adapted for the monitoring of the present invention.

to In an alternative embodiment, the optical fibre comprises one or more fibre Bragg gratings, and the emerging light arises from reflection from the one or more Bragg gratings. In this case, the identifying any change may comprise identifying a shift in wavelength of light reflected from the one or more Bragg gratings. This approach requires structuring of the fibre to provide the gratings, but offers the Is advantage that a much larger amount of light can be returned back up the fibre compared to that from the various backscattering mechanisms of other embodiments.

Thus an increased amount of emerging light is available for detection and monitoring, givmg an improved signal-to-noise ratio and hence enhanced accuracy. Also, less sensitive detectors can be used to detect the emerging light.

Advantageously, all or part of the optical fibre is covered with an outer layer before deploying the optical fibre, the outer layer operable to make scale Connation preferential to the optical fibre compared to surrounding parts of the region of interest.

This improves the early warning aspect of the invention, because if scale forming conditions develop within the region of interest, scale will form on the fibre sooner than on the surrounding components, so the onset of scaling will be detectable before any damage is done to the components.

The outer layer may comprise material likely to be deposited as scale in the region of interest. The outer layer may comprise barium sulphide or another material lo that promotes scale deposition. Scale develops rapidly on existing crystals of scale material, which act as a seed to further crystal growth. Therefore, if it is known which minerals are liable to contribute to scaling in a given region of interest, the optical fibre can be coated with the relevant material to increase sensitivity to specific ions as they become present. Such information is often available, for example, from knowledge of the local geology in the case of an oil well bore.

The optical fibre may be deployed by pumping the fibre into a tube situated in the region of interest. Fibre deployment by pumping the fibre into a tube using a hydraulic system is a convenient and established way of positioning a fibre downhole.

In the present case, this advantage can be particularly exploited by using a tube that is fluid permeable, so that the fibre can be exposed to any water carrying scale-forming components. For example, the tube may comprise a plurality of perforations closed with a degradable substance arranged to degrade and hence open the perforations after deployment of the optical fibre. This allows the hydraulic pumping to operate efficiently when required, followed by good exposure of the fibre.

Advantageously, the optical fibre may be deployed into a well bore. Scale formation can cause major problems in oil wells, so a simple, rapid and efficient scale detection method is of great benefit. The present invention is particularly suited to the down hole environment because of its use of optical fibres, which are well-proven for sensing applications in the oil industry.

However, the invention is by no means limited to well bores, and can be usefully implemented in many scale-prone situations. For example, the optical fibre may be deployed in a pipeline.

A second aspect of the present invention is directed to apparatus for detecting scale, comprising: an optical fibre for deploying into a region of interest; an optical source operable to generate light and launch the light into the optical fibre; a photodetector operable to detect light subsequently emerging from the optical fibre; and a processor operable to monitor the detected light to identify any change arising from one or more changes in optical characteristics of the optical fibre caused by scale lo formation on the optical fibre.

The processor may be further operable to make time measurements of the emerging light and use the time measurements to calculate distance along the optical fibre at which any change occurs, the distance indicating a location of scale formation.

The photodetector may be arranged to detect emerging light at an end of the optical fibre into which the light from the optical source is launched.

The photodetector may be operable to detect emerging light arising from back scattering within the optical fibre. Then, the identifying any change may comprise identifying a shift in wavelength and/or a change in amplitude of one or more components of emerging light arising from Brillouin scattering within the optical fibre, or may alternatively comprise identifying attenuation of the emerging light arising from one or more stresses in the optical fibre. In the latter case, the apparatus may be configured to be operable as an optical time domain reflectometer. Further, the apparatus may further comprise a filament wrapped around the optical fibre to enhance stress formation within the optical fibre.

Alternatively, the optical fibre may comprise one or more fibre Bragg gratings, and the photodetector may be operable to detect emerging light arising from back reflection from the one or more Bragg gratings. The identifying any change may then comprise identifying a shift in wavelength of light reflected from the one or more Bragg gratings.

Advantageously, all or part of the optical fibre is covered with an outer layer, the outer layer acting to make scale formation preferential to the optical fibre compared to surrounding parts of the region of interest. The outer layer may comprise material likely to be deposited as scale in the region of interest, such as barium s sulphide or another material that promotes scale deposition.

The apparatus may further comprise a tube for situating in the region of interest into which the optical fibre may be pumped for deployment. The tube may be fluid permeable. For example, the tube may comprise a plurality of perforations closed with a degradable substance arranged to degrade and hence open the perforations after lo deployment of the optical fibre.

According to various embodiments, the region of interest may be a well bore, or a pipeline.

A third aspect of the present invention is directed to an optical fibre for deployment into a region of interest, and having an outer layer that, in use, acts to is make scale formation preferential to the optical fibre compared to surrounding parts of the region of interest. The outer layer may comprise material likely to be deposited as scale, such as barium sulphide or another material that promotes scale deposition.

Another aspect of the present invention is directed to a method for deploying a sensing optical fibre into a region of interest, comprising providing a deployment tube that passes at least partially into the region of interest; deploying the sensing optical fibre within the deployment tube; enabling fluid communication through the deployment tube between the region of interest and the sensing optical fibre; and sensing a parameter with the sensing optical fibre. The deployment tube may be fluidpermeable, and it may include plurality of perforations closed with a degradable substance arranged to degrade and hence open the perforations after deployment of the optical fibre. The degradation may be caused by heat, time, or another fluid.

Alternatively, the sensing optical fibre may protrude from the deployment tube into the region of interest. In any of these embodiments, the sensing optical fibre may be -9 - pumped into the deployment tube, and the region of interest may be a well bore or a pipeline. -l o-

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings in which: Figure I shows a vertical cross-sectional schematic representation of a well-bore, with an optical fibre deployed within the well bore; Figure 2 shows the well bore of Figure 1, with a deposit of scale; Figure 3 shows a cross-section through an optical fibre with a deposit of scale on 0 its surface; Figure 4 shows a schematic representation of apparatus suitable for implementing methods according to embodiments of the present invention; Figure 5 shows an example spectrum of backscattered light emerging from an optical fibre, including changes to components arising from Brillouin scattering which i 5 are monitored according to a first embodiment of the invention; Figure 6 shows a cross-section through an optical fibre with microbends produced by deposits of scale on its surface; Figure 7 shows a schematic representation of a ray propagating in the core of an optical fibre with a microbend; Figure 8a shows an example output from an optical time domain reflectometer, together with an output obtained according to a second embodiment of the present invention from a fibre containing two microbends caused by scale on the fibre surface; Figure 8b shows an external view of an optical fibre adapted for enhancement of microbending according to an embodiment of the present invention; Figure 9 shows a schematic representation of a fibre Bragg grating as utilised in a third embodiment of the present invention; Figure 10 shows a typical spectrum of the reflection/transmission of a fibre Bragg grating; Figure lla shows a cross-sectional view of an optical fibre having an outer coating according to a first example of a third embodiment of the present invention; Figure I lb shows a cross- sectional view of an optical libre having an outer coating according to a second example of a third embodiment of the present invention; and Figure 12 shows a vertical cross-sectional schematic representation of a well bore, with an optical fibre deployed within the well bore in accordance with an embodiment of the present invention; Figure 13 shows a cross- sectional schematic representation of a portion of a well lo bore tubing and an optical fibre deployed in accordance with an embodiment of the present invention; Figure 14 shows a cross-sectional schematic representation of a portion of a well bore tubing and an optical fibre deployed in accordance with a further embodiment of the present invention; and Figure 15 shows a cross-sectional schematic representation of a portion of the tubing ofFigures 13 and 14.

DE AILED DESCRIPTION

Figure l shows a simplified schematic vertical cross-sectional view of an oil well, illustrating some basic features. The oil well, or well bore 10 comprises an outer casing 12 sunk into the ground 14, and penetrating through a hydrocarbon reservoir 16, the contents of which are to be extracted using the well 10. Disposed within the casing 12 is a tubing 18, which is used to carry oil upwards from the reservoir 16 to the surface. The tubing 18 may comprise production tubing or coiled tubing, among others. The tubing 18 is open at its lower end (or has ports proximate the lower end), lo and may be held in place within the casing 12 by a packer 22. A well head 20 is located at the top end of the tubing 18, and contains equipment used to extract the oil, such as pumping equipment, valves and the like (not shown). Oil flows from the reservoir 16 into the casing 12 and then up the tubing 18 by way of perforations 24 extending through side walls of the casing 12 and into the reservoir 16.

Also shown in Figure 1 is an optical fibre installation. An optical fibre 26, which may be arranged within a protective fibre deployment tube, runs down the length of the inner surface of the tubing 18, and is attached thereto by a number of attachments 28. A control unit 30 connected to the upper end of the optical fibre 26 is provided outside the well bore 10. The control unit 30 typically contains equipment such as a laser or similar optical source to generate light which propagates along the fibre 26, and a detector to detect light emerging from the fibre 26.

Optical fibres arranged in this manner are used in well bores as sensors and detectors. Light is launched into the fibre and subsequently detected when emerging from within the fibre. The amount of light detected, and its wavelength, phase, etc. can be affected by physical conditions within the well bore, which gives a way of measuring these conditions. For example, temperature affects the quantity of light which is back- scattered up the fibre by Raman scattering, so monitoring of the Stokes and anti-Stokes components in detected back-scattered light gives a measurement of temperature within the well bore. Alternatively, optical fibres can be used to convey light to and from separate optical sensors situated within the well bore.

Optical fibres have been well-proven for use in oil wells. They are robust, able to withstand the environmental rigours to which they are exposed when deployed at depth underground, and also safe for use in a combustible environment.

Figure 2 shows the well bore 10 of Figure 1, at some later point in time when scale has begun to form. In this case, scale 32 is shown lining the inner surface of tubing 18, and in some places has engulfed the fibre 26. The tubing is not yet completely blocked, but even a thin layer of scale reduces the volume of the tubing significantly, so that the rate at which oil can be brought to the surface is greatly reduced.

Scale, for purposes of this application, means the build-up of any solid particles, including wax, hydrates, asphaltines, and any other solid formation known to block pipes.

Although not shown in Figure 2, scale can similarly be deposited over virtually any of the other components of the well bore, leading to decreased or interrupted production by reducing volumes, clogging valves and the like. Also, scale can form in the rock formation containing the oil reservoir, which decreases the permeability of the formation and inhibits fluid flow.

The present invention uses optical fibre, with all its known advantages for down-hole use, to detect the formation of scale. A deposit of scale on the surface of a fibre will alter the optical characteristics of the fibre; in other words, the quantity and/or quality of light transmitted by the fibre is changed by the scale. A system arranged to monitor this light and look for such changes will therefore be able to detect the formation of scale very rapidly.

Figure 3 shows a longitudinal cross-sectional view through a simple optical fibre 40. The fibre 40 comprises a waveguiding core 42 of a material such as silica having a refractive index no The core is surrounded by a cladding 44 with a refractive index n2, where none. The refractive index difference between the core 42 and the claddmg 44 allows light to be guided along the fibre in the known manner. The whole fibre 40 is encased in a protective outer jacket 46, which reduces damage to the fibre 40. Also, a small deposit of scale 48 has formed on the fibre. The deposition of a layer of a given material over a substrate of a different material generally results in the creation of stresses at the interface between the two. The stresses can cause deformation of the substrate. In the present case, the scale forms a layer over a fibre substrate, and produces stresses in the fibre. These stresses, and the possible deformation of the fibre, can change various optical characteristics of the fibre and therefore alter the way in which the fibre transmits, scatters and reflects light. Various o embodiments of the invention relate to the detection of these changes as an identification of scale formation.

APPARATUS

Apparatus suitable for Implementing methods according to various embodiments of the present invention will now be described in general terms.

Figure 4 shows a schematic depiction of the apparatus. A length of optical fibre 50 is suitable for deployment within a well bore. An optical source 52, for example a laser, light emitting diode or a rare earth doped fibre source, is operable to generate light. The light may be continuous wave or pulsed, depending on the application. In Figure 4, a pulse 54 is shown as an example; the pulses may vary depending on the type of system and the special resolution required between measurement points. Any optical source capable of generating pulses of an appropriate duration and wavelength can be used.

The light is coupled in a front end of the optical fibre 50 and propagates along the fibre. It will undergo various interactions as it propagates, either with the fibre material and/or with features of the fibre. These mteractons typically result in some of the light being returned and hence redirected back along the fibre 50, counter to the original propagation direction. Therefore, a fibre coupler 56, such as a wavelength division multiplexer or a fused fibre coupler, is provided at the front end of the fibre 50. The back-propagating light is diverted by the coupler 56 to an output fibre which is optically coupled to a photodetecting device 58 so the light emerging from the fibre is detected by the photodetecting device 58. The photodetecting device 58 detects the emerging back-propagating light, and generates an electrical signal representing the detected light. The signal is fed to a processor 60 where it can be analysed and stored.

Optionally, the signal can also be displayed on a display device 62, for viewing by an operator.

The analysis comprises looking for any changes over time in the emerging light that have been produced by scale forming on the optical fibre. This can be 0 performed in any convenient manner capable of identifying such a change, using various processing, calculating or analysing approaches. For example, a comparison of the signal derived from light emerging at a particular time with that from an earlier time will reveal any differences that have developed in the interim. Therefore, when the fibre is first deployed, a measurement of the emerging light can be made and stored in the processor as a reference measurement representing the performance of a fibre with no scale. Repeated further measurements can then be made and each compared with the reference measurement. Alternatively, repeated measurements may be made, and each measurement compared with the previous measurement. The comparison can be performed by simple calculation, such as subtracting one measurement from another, or by more complex analytical methods.

A further technique for identifying changes in the emerging light is to establish some steady state operating condition of the scale-less fibre, such as an amount of light at a particular wavelength, and monitor the signal to identify when the condition moves from the steady state value, or, more usefully, moves outside a defined acceptable range around the steady state value. Defining a range provides some tolerance to temporary perturbations to the system caused by factors other than scale, and which are therefore not of interest.

The analysis can further include a determination of the location along thefibre of any factor, i.e. a scale deposit in this case, which causes a change in the detected signal. In the case of pulsed hght, a pulse propagates along the fibre, and various back- directed components are generated along the length of the fibre. The time taken for each component to then arrive at the detector depends on the position d along the fibre at which it was generated, because the light has had to travel forwards to the position s d, and then back the same distance to the detector. The speed of light in the fibre is know to be cln, where c is the speed of light in a vacuum and n is the refractive index of the fibre. Therefore, if the time between launching the pulse into the fibre and detecting the returning component is measured, the position d of whatever gave rise to the component can be calculated from d = ctl2n. In this way, the position of a scale lo formation along the fibre can be derived.

In the alternative of using continuous wave light, it can be possible to use wavelength information to identify the location of scale formation.

Therefore, the present invention provides a simple yet powerful method of rapidly detecting both the formation of scale within the well bore, and its location.

FIRST EMBODIMENT

The first embodiment relies on the optical phenomenon known as Brillouin scattering. A pulse of light propagating along the fibre will continuously interact with the fibre material, and part of the light will be back-scattered back along the fibre, eventually emerging at the detector.

Figure 5 shows a typical spectrum of the emerging light, as a plot of wavelength against intensity I. The dotted line represents the spectrum of the original transmitted pulse. The solid line represents the detected back-scattered light.

Note that the spectra have been normalised; in reality the intensity of the back- 2s scattered light is very low compared to that of the original pulse.

The back-scattered spectrum has a number of features. A central peak 70 having the same wavelength as the original pulse arises from Rayleigh scattering.

Brillouin scattering, which is scattering of the light from acoustic phonons (sound waves) produces a small side peak 72 on each side of the central peak 70, at _ 1 -17 wavelengths XB} and XB2. These peaks have an Intensity IB. Finally, Raman scattering, resulting from interaction of the light with vibrational modes of the molecules making up the fibre (scattering from optical phonons), gives two smaller side peaks 74 shifted significantly from the central peak 70.

If the optical fibre is subjected to stress, both the wavelength and intensity of the Brillouin scattering peaks will shift. This is indicated by the arrows on Figure 5.

Therefore, monitoring of the Brillouin components of the emerging and detected back scattered light with a suitable photodetector will reveal the presence of stress if the wavelength and/or the intensity changes. In a scale-prone environment, such a change 0 is extremely likely to be caused by scale forming on the fibre. Thus, this arrangement can be used to detect scale. If suitable time measurements of the pulses are made, as described above, the location of the scale can also be determined. The nature of the Brillouin scattering, in that it arises naturally from all points of the fibre, allows this embodiment of the fibre scale detector to be operated as a distributed system, in which scale can be detected at any point along the fibre length.

Temperature changes will also cause shifts in the Brillouin components.

However, the simultaneous measurement of frequency and amplitude shift enables the discrimination of the temperature from stress effects.

SECOND EMBODIMENT

A second embodiment of the invention is based on looking for changes in fibre perfonnance brought about by distortion of the fibre when scale is deposited on the fibre surface.

Figure 6 shows a cross-sectional view of an optical fibre 80 comprising a core 82 and a cladding 84, and having small deposits of scale crystals 86 on its surface.

Adherence of the scale to the fibre, and the uneven nature of the deposits, has stressed and distorted the fibre, forming so-called microbends 87, 89. It is well known that stresses caused by microbends and other distortions can cause significant optical loss in a fibre. Therefore, detection of such loss by monitoring light emerging from the pore gives a way of detecting scale.

Figure 7 is a schematic view of the core 90 of part of an optical fibre. The optical fibre has been subjected to a microbend through an angle a. Light propagates along the core 90; this is represented by the ray 92 in Figure 7. The ray 92 propagates by undergoing repeated total internal reflections at the boundary between the core 90 and the surrounding cladding. In this case, the ray 92 is incident on the boundary at an internal angle (3. After the microbend, however, the internal angle is reduced to O' = - a. If (3' is less than the critical angle for total internal reflection at the lo interface, the ray will be refracted out of the fibre and lost. Also, the change in angle causes energy to be coupled into a different propagation mode, which may be a lossy mode so that energy is lost. These losses can be significant.

Losses from distortions such as microbends, and hence the microbends themselves, and the scale causing the microbends, can be detected using the technique of optical time domain reflectometry (OTDR). This depends on detecting light back propagating along the fibre, in the way described with respect to Figure 4.

A pulse of hght with energy E(0) is launched into the front end of the fibre. As the pulse propagates, it suffers attenuation due to absorption, so that after propagating a distance d, the pulse has a reduced energy E(d) = E(0)exp(-ad), where a is the absorption coefficient of the fibre.

At each point along the fibre, a certain portion of the pulse energy SR is lost by Rayleigh scattering. Of this portion, a fraction f will be scattered such that it can propagate back down the fibre. The amount of backscattered energy at point d can therefore be written as SRfE(d). The backscattered light is then itself subject to attenuation as it propagates back to the front end of the fibre. Therefore, the light emerging from the fibre that is due to backscatter from point d is Eb(d) = SRfE(cl)exp(-ad). If the amount of emerging light is measured, and time measurements are used to derive the distance d from d = ct/2n, a plot such as that shown in Figure 8a can be obtained.

Figure 8a is a plot of the loss of the fibre in decibels with distance d along the fibre. The solid line shows a typical trace such as might be obtained from an optical fibre newly deployed within a well bore, with no scale. The loss increases with increasing fibre length. A spike caused by Fresncl reflection marks the far end of the s fibre. The local gradient of the line is related to the value of a at that point. In a down- hole situation, losses in the fibre may vary over time with changes in temperature and pressure. However, these changes are typically nonlocalised, so that the curve will generally be fairly smooth.

In contrast, microbends produced by scale formation will give rise to highly 0 localised increased loss from the fibre. This situation is represented by the dotted line in Figure 8a. Localised loss is seen at points do and d2, thus indicating scale formation at these positions.

Continuous OTDR sensing therefore provides a rapid way of identifying scale formation, because it is merely necessary to process the detected light to provide a plot of the type shown in Figure 8a, and monitor for any unexpected local loss in transmission. Moreover, the measurement is distributed because Rayleigh scattering occurs at all points along the fibre, so scale can be detected and pinpointed to anywhere along the fibre.

In order to enhance micro-bending effects, a filament can be wrapped around the optical fibre. The filament may comprise, for example, a second optical fibre, a strand of some suitable material, or other thread- like item. The wrapped geometry promotes the bending and the attenuation. Figure 8b shows an external view of a length of optical fibre 94 having a filament 96 twisted around it in this way.

HIIID Knin UDI! Idol The first and second embodiments described above can utilise standard optical fibre, because the physical phenomena relied upon (Brillouin and Rayleigh scattering) are based on the interaction of light with the normal fibre material. A third embodiment, however, takes advantage of an optical device known as a fbre Bragg grating.

Figure 9 shows a schematic illustration of a section of optical here 100 comprising a Bragg grating 102. The grating is situated in the core 104 of the fibre 100, and comprises a series of alternating portions of high and low refractive index, represented by the shaded and unshaded parts in Figure 9. The width and spacing of the portions defines the grating period A. The grating is typically created, or written, by fabricating a fibre with a core made of photosensitive material, and then exposing the core to a pattern of ultraviolet light corresponding to the desired pattern of the lo grating. The parts exposed to the light have their refractive index increased.

A Bragg grating operates by reflecting light at a particular wavelength. The wavelength is defined by the grating period A, and the level of reflectivity by the strength of the grating, which is the difference in refractive index between the high and low index portions.

Figure 10 shows a graph of the transmissitivity T/reflectivity R of a typical fibre Bragg grating as a function of wavelength it. The dotted line represents reflectivity, and indicates that, at the central wavelength of the grating, about 60% of incident light is reflected from the grating.

If the fibre containing the grating is distorted, for example by being stressed by scale deposition, the grating period will be altered. This causes the reflectivity peak of the grating to shift. Therefore, to operate a fibre with a grating as a scale sensor, it is necessary to deploy the fibre into a well bore, launch into it light that will be reflected by the grating in its unstressed condition, and monitor the amount of back-reflected light at this wavelength. Continuous wave light is generally used with Bragg gratings, in contrast to the pulsed light used in the previous embodiments. Any change in the amount of light indicates a shift in the Bragg grating wavelength caused by a change in grating period. This may have been caused by scale depositing on the fibre surface in the vicinity of the grating. However, temperature changes can also cause alterations in the performance of the grating (partly because refractive index is temperature dependent), so it is preferable to be able to dstinguish between the two effects. This could be achieved, for example, by separately monitoring temperature within the well bore using a known method, and comparing temperature measurements with measurements from the scale detector.

The fibre Bragg grating is a discrete feature of the fibre, so it provides a localised single point scale detector. This is in contrast with the distributed detection of the first and second embodiments. However, a multi-point detector can be provided by writing a series of gratings along the length of the fibre. In some cases, this may provide sufficient resolution. If each grating has the same peak reflectivity 0 wavelength, time measurements will be needed to distinguish between the components of the reflected light originating from the various gratings. An alternative arrangement is to write a series of gratings having different peak wavelengths. Light having a broad bandwidth covering all the wavelengths is then launched into the fibre, and the wavelength of each reflected and detected component will then reveal the positional information used to locate scale formations.

FOURTH EMBODIMENT

An aim of the present invention is to provide early warning of scale formation, by detecting the onset of deposition as quickly as possible. For detection to occur at all, scale must form on the fibre itself. Therefore, a way of facilitating early detection is to arrange for scale to form preferentially on the fibre compared to surrounding components of the well bore, such as the casing and the tubing. According to a fourth embodiment of the present invention, this is achieved by providing the fibre with an outer layer that will encourage the deposition of scale. Such a layer can be used in conjunction with any of the other embodiments.

The outer layer may comprise a coating of material likely to be deposited as scale in the particular well bore being monitored. Scale formation increases with increasing quantity of scale, so the coating acts as a seed for scale formation, and encourages further scale to form on the fibre if the correct ions are present. If, therefore, it is probable that barium sulphide is going to be a major constituent of future scale in a well, a coating of barium sulphide may usefully be applied to the fibre to enhance early detection. Probable scale materials for a particular well can be determined from a geological survey of the area.

s An alternative to a coating of seeding material is an outer layer that gives a rough surface to the fibre. Scale formation can be initiated by surface defects, by a process known as heterogeneous nucleation. Thus, the chance of scale forming on the fibre is increased. The surface roughness can be provided in a number of ways, such as a hairy polypropylene braid jacket, or an outer layer moulded with corrugations, 0 indentations and/or protrusions, or abrading the surface of an outer protective jacket of the fibre.

Figure lla shows a cross-sectional view of an optical fibre having a coating of seeding material. Figure 1 l(b) shows a cross-sectional view of an optical fibre having a rough coating 1 12.

FURTHER EMBODIMENTS

As described above, the first, second and third embodiments have all relied on the detection of back-propagating light. In other words, the fibre scale detector can be regarded as a single ended fibre sensor, because access is only required to one end of the fibre. However, in certain circumstances, it may be preferable to operate a double ended sensor, whereby light is launched into one end of the fibre, and transmitted hght emitted from the far end of the fibre is monitored. In the case of Bragg gratings, transmitted components can be detected instead of reflected components, if the gratings are configured for an intermediate level of reflection so that sufficient light is Is transmitted for accurate monitoring. Alternatively, the fibre may be arranged to give access to both ends for some other purpose, but be operated as a scale detector on the basis of single ended sensing. Access to both ends of the sensor can be arranged by deploying the fibre in a suitable U-shaped deployment tube which provides a path deep downside the well bore and back to the surface again.

FIBRE DEPLOYMENT

As will be clear from the foregoing description, it is necessary for scale to form on the fibre itself to be detected and located. Therefore, the fibre is deployed in such a way that it is exposed to the relevant fluids.

Figures l and 2 show an optical fibre deployed within the tubing of a well bore. In this arrangement, the fibre will detect scale forming within the tubing.

However, scale can fomm virtually anywhere within a well bore, so one or more fibres may usefully be positioned elsewhere.

0 Figure 12 (which uses the same numbering as Figures 1 and 2 for ease of understanding) shows a schematic representation of a well bore 10 in which a fibre 26 is positioned down the outer surface of a tubing 18, within the annulus between the tubing 18 and the outer casing 12 of the well bore 10. This embodiment is particularly useful when oil and gas production is via the annulus.

A fibre may instead be located against the inner surface of the casing 12, or against the outer surface of the casing 12. This latter arrangement may detect scale within the surrounding geological formation and hence potentially provides a particularly early warning system that identifies the onset of scaling before scale penetrates into the well bore structure.

In any position, a fibre can be deployed within the well bore in an exposed state. This gives direct contact with water over the length of the fibre. However, the exposed fibre is at risk of damage, and generally needs to be pemmanently positioned within the well bore, for example by fastening it in some way to the tubing or casing as the well is constructed.

As an altemative, the fibre can be deployed into a well bore inside a deployment tube. This protects the fibre, and may be a non-permanent arrangement that allows the fibre to be removed and/or replaced. A known technique for deploying fibres in this way is to use a hydraulic arrangement to pump the fibre through the deployment tube, which is attached in position inside the well bore [2]. However, the deployment tube is closed to allow the hydraulic system to operate efficiently.

Therefore, once deployed inside the tube, the here is protected from any water that may enter the well bore and cause scale.

To address this issue while still taking advantage of the benefits offered by pumping optical fibres into place, it is proposed that alternative configurations of deployment tube be used with the present invention. For example, the tube may comprise one or more portions that are fluid-permeable, or which otherwise expose a fibre within the tube to fluid flowing past the tube.

Figure 13 shows a first example embodiment of such a configuration. A lo portion of a tubing 120 of a well bore is shown in cross-section, with a deployment tubing 122 arranged down the outer surface of the tubing 120 in a U-shape. This shape allows a fibre 124 to be pumped down into the well bore in a loop, so that both ends of the fibre 124 are at the surface. The fibre is to be used to detect scale within the tubing 120, so the deployment tube 122 passes through the wall of the tubing 120, runs for some distance inside the tubing 122, and then passes back out through the tubing wall.

Thus a portion of the fibre 124 is carried inside the tubing 120. Part of the deployment tube 126 within the tubing 120 is water permeable, in that it has a plurality of perforations 128. When the deployment tube 126 is first installed, the perforations 128 are blocked with a degradable substance, such as wax, that will deteriorate with time, heat, exposure to fluid flow, or some other external factor. This allows the fibre 124 to be hydraulically pumped through the deployment tube 122 when the perforations 128 are closed, and then be exposed to the fluid flowing in the tubing 120 as the perforations 128 open, so that scale can be detected. In one embodiment, the deterioration of the material blocking the perforations 128 is triggered by a specific fluid pumped into the tube. In an alternative embodiment, the perforations 128 are not blocked, but are small enough to offer enough resistance to enable the fiber to be deployed therethrough via pumping. In another alternative embodiment, at least the section of the deployment tubing 122 passing within the tubing 120 is permeable. In this case, such part of the tubing 122 can be made permeable to a particular fluid of interest.

Figure 14 shows an alternative embodiment. Again, a portion of tubing 120 is shown. In this case, a deployment tube 130 is positioned against the outer surface of the tubing 120. The deployment tube 130 then passes through the wall of the tubing 120, and terminates inside the tubing 120. A fibre 132 is delivered into the deployment tube 130 such that an end 134 of the fibre 132 protrudes out of the end of the deployment tube 130 in the path of production flow within the tubing 120. The fibre end 134 is hence exposed to water and other fluids, and can detect scale. The fibre lo 132 should be loose in the direction of flow. For instance, in Figure 14, it is assumed that flow is downward and that therefore the fibre 132 "hangs" downward inside the tubing 120. If flow is in the opposite or upward direction, then the deployment tube would be inserted into the tubing 120 so as to enable the fibre 132 to be suspended in the upward direction within the flow (the deployment tube 130 would have a u shape).

At the point or points where the deployment tube passes through the wall of the tubing 120, it is preferable that some sealing be provided, to prevent the passage of fluid between the tubing and the annulus. Figure IS shows, in cross-section, an example of how this can be achieved. A hole 140 is formed through the wall of the tubing 142, with a recess forming a larger bore at one end of the hole. A tubular sealing fitting 144 is disposed within the recess. The deployment tube 146 is passed through the fitting 144 and the hole l 40. The fitting 144 which is made of rubber or plastic so that it grips tightly against the sides of the hole l 40 and the deployment tube 146 to provide a fluid-tight seal. Metal sealing may be preferable in some situations.

The embodiments of Figures 13 and 14 can both be adapted for scale detection within the annulus, by arranging the deployment tube wholly outside the tubing.

Alternatively, the deployment tube may be positioned wholly within the tubing, with its upper end within the tubing, and one or more portions passing through the tubing wall to carry the fibre into the annulus. Perforations can be provided at any point along any configuration of deployment tube, both insde and outside the tubing.

Alternatively, the deployment tube can be located to pass through the casing instead of the tubing, or possibly through both, to give scale detection at a plurality of positions.

The embodiments of Figures 13 and 14 may also be used when the fibre is adapted to sense other parameters, such as chemical sensing, that require it to be in contact with the relevant fluids.

FURTHER EMBODIMENTS

As described above, the first, second and third embodiments have all relied on lo the detection of emerging back-propagating light at the same end of the fibre into which the light is launched. In other words, the fibre scale detector can be regarded as a single ended fibre sensor, because access is only required to one end of the fibre.

However, in certain circumstances, it may be preferable to operate a double ended sensor, whereby light is launched mto and detected at both ends of the fibre. This gives two sets of measurements that can be compared or averaged for enhanced accuracy. Alternatively, the fibre may be arranged to give access to both ends for some other purpose, but be operated as a scale detector on the basis of single ended sensing. Access to both ends of the sensor can be arranged by deploying the fibre in a suitable U-shaped which provides a path deep down inside the well bore and back to the surface again.

Also, the invention is not limited to the detection of scale within well bores. It is equally applicable to many other situations in which scale can develop and in which detection of that scale is desired. To achieve this, it is merely necessary to deploy the scale-sensing optical fibre into a region of interest in which there is a need or desire to monitor scale. For example, the invention may advantageously be implemented to monitor scale formation in pipelines. These include subsea pipelines such as are used to transport extracted oil and gas from an offshore well to the mainland or other offshore installation, or to supply oil and gas to consumers, and also pipelines used to transport fluids in various industrial processes, some of which use materials that are prone to producing scale. In the case of pipeline monitoring, the fbre could be pumped directly into the pipeline, instead of through a deployment tube.

REFERENCES

[1] Mike Crabtree, David Eslinger, Phil Fletcher, Matt Miller, Ashley Johnson and George King, "Fighting scale - removal and prevention", Schlumberger

Oilfield Review, pages 30-45, Autumn 1999

[2] US RE37,283 E

Claims (47)

1. A method of detecting scale, comprising: deploying an optical fibre into a region of interest; launching light into the optical fibre; detecting light subsequently emerging from the optical fibre; and monitoring the detected light to identify any change arising from one or more changes in optical characteristics of the optical fibre caused by scale formation on the optical fibre.

2. A method according to claim 1, and further comprising using time measurements of the emerging light to calculate distance along the optical fibre at which any change occurs, the distance indicating a location of scale formation.

3. A method according to claim 1 or claim 2, in which the emerging light is detected at an end of the optical fibre into which the light is launched.

4. A method according to claim 3, in which the emerging light arises from back- scattering within the optical fibre.

5. A method according to claim 4, in which the identifying any change comprises identifying a shift in wavelength and/or a change in amplitude of one or more components of the emerging light arising from Brillouin scattering within the optical fibre.

6. A method according to claim 4, in which the identifying any change comprises identifying attenuation of the emerging light arising from one or more stresses in the optical fibre.

7. A method according to claim 6, in which the optical fibre has a filament wrapped around it to enhance stress formation within the optical fibre.

8. A method according to claim 6 or claim 7, in which the optical fibre is operable as an optical time domain reflectometer.

9. A method according to claim 3, in which the optical fibre comprises one or more fibre Bragg gratings, and the emerging light arises from reflection from the one or more Bragg gratings.

10. A method according to claim 9, in which the identifying any change comprises identifying a shift in wavelength of light reflected from the one or more Bragg gratings.

11. A method according to any preceding claim, in which all or part of the optical fibre is covered with an outer layer before deploying the optical fibre, the outer layer operable to make scale formation preferential to the optical fibre compared to surrounding parts of the region of interest.

12. A method according to claim 11, in which the outer layer comprises material likely to be deposited as scale in the region of interest.

13. A method according to claim 12, in which the outer layer comprises barium sulphide.

14. A method according to claim l to 12, in which the optical fibre is deployed by pumping the fibre into a tube situated in the region of interest.

15. A method according to claim 14, in which the tube is fluid-permeable.

16. A method according to claim 15, in which the tube comprises a plurality of perforations closed with a degradable substance arranged to degrade and hence open the perforations after deployment of the optical fibre.

17. A method according to any one of claims I to 16, in which the optical fibre is deployed into a well bore.

18. A method according to any one of claims 1 to 16, in which the optical fibre is 0 deployed into a pipeline.

19. Apparatus for detecting scale, comprising: an optical fibre for deploying into a region of interest; an optical source operable to generate light and launch the light into the optical fibre; a photodetector operable to detect light subsequently emerging from the optical fibre; and a processor operable to monitor the detected light to identify any change arising from one or more changes in optical characteristics of the optical fibre caused by scale formation on the optical fibre.

20. Apparatus according to claim 19, in which the processor is further operable to make time measurements of the emerging light and use the time measurements to calculate distance along the optical fibre at which any change occurs, the distance indicating a location of scale formation.

21. Apparatus according to claim 19 or claim 20, in which the photodetector is arranged to detect emerging light at an end of the optical fibre into which the light from the optical source is launched.

22. Apparatus according to claim 21, in which the photodetector is operable to detect emerging light arising from back-scattering within the optical fibre.

s

23. Apparatus according to claim 22, in which the identifying any change comprises identifying a shift in wavelength and/or a change in amplitude of one or more components of emerging light arising from Brillouin scattering within the optical fibre.

lo

24. Apparatus according to claim 21, in which the identifying any change comprises identifying attenuation of the emerging light arising from one or more stresses in the optical fibre.

25. Apparatus according to claim 24, and further comprising a filament wrapped around the optical fibre to enhance stress formation within the optical fibre.

26. Apparatus according to claim 24 or claim 25, and configured to be operable as an optical time domain reflectometer.

27. Apparatus according to claim 21, in which the optical fibre comprises one or more fibre Bragg gratings, and the photodetector is operable to detect emerging light arising from reflection from the one or more Bragg gratings.

28. Apparatus according to claim 27, in which the identifying any change comprises identifying a shift in wavelength of light reflected from the one or more Bragg gratings.

29. Apparatus according to any one of claims 19 to 28, in which all or part of the optical fibre is covered with an outer layer, the outer layer acting to make scale forrnation preferential to the optical fibre compared to surrounding parts of the region of interest.

30. Apparatus according to claim 29, in which the outer layer comprises material s likely to be deposited as scale in the region of interest.

31. Apparatus according to claim 29 or claim 30, in which the outer layer comprises barium sulphide.

lo

32. Apparatus according to any one of claims 19 to 31, and further comprising a tube for situating in the region of interest into which the optical fibre may be pumped for deployment.

33. Apparatus according to claim 32, in which the tube is fluid-permeable.

34. Apparatus according to claim 33, In which the tube comprises a plurality of perforations closed with a degradable substance arranged to degrade and hence open the perforation after deployment of the optical fibre.

35. Apparatus according to any one of claims l9 to 34, in which the region of interest is a well bore.

36. Apparatus according any one of claims 19 to 34, in which the region of interest is a pipeline.

37. An optical fibre for deployment into a region of interest, and having an outer layer that, in use, acts to make scale formation preferential to the optical fibre compared to surrounding parts of the region of interest.

38. An optical fibre according to claim 37, in which the outer layer comprises material likely to be deposited as scale.

39. An optical fibre according to claim 38, in which the outer layer comprises s barium sulphide.

40. A method for deploying a sensing optical fibre into a region of interest, comprising: providing a deployment tube that passes at least partially into the region of interest; lo deploying the sensing optical fibre within the deployment tube; enabling fluid communication through the deployment tube between the region of interest and the sensing optical fibre; and sensing a parameter with the sensing optical fibre.

is

41. A method according to claim 40, in which the deployment tube is fluid- permeable.

42. A method according to claim 41, in which the deployment tube comprises a plurality of perforations closed with a degradable substance arranged to degrade and hence open the perforations after deployment of the optical fibre.

43. A method according to claim 42, in which the degradation is caused by one of heat, time or a fluid.

44. A method according to claim 40, in which the sensing optical fibre protrudes from the deployment tube into the region of interest.

45. A method according to any one of claims 40 to 44, in which the sensing optical fibre is pumped into the deployment tube.

46. A method according to any one of claims 40 to 45, in which the region of interest is a well bore.

s

47. A method according to any one of claims 40 to 45, in which the region of interest is a pipeline.