The present invention relates to an apparatus and method for detecting, in a fluid, a tracer that absorbs an excitation light of a first wavelength and emits an emission light of a second wavelength. In particular, but not exclusively, the present invention relates to an apparatus and method for the detecting of upconverting tracers in fluids produced from oil reservoirs . Background

Optical tracers, that is tracers that absorb excitation light of a first wavelength and emit emission light of a second wavelength, are well-known for use in various applications. A well-known example of such tracers are fluorescent tracers. Such tracers are typically detected by apparatus in which an excitation light is directed into a fluid containing the tracer and the resulting emission is measured, for example with a spectrometer. Since the light sources and

spectrometers can be bulky, and since they can also be sensitive to the sometimes challenging environmental

conditions in which tracers might be used, it is also known to use probes, typically fibre optic coupled, to deliver the excitation light to the fluid and to return the emission light to the spectrometer. A problem with such arrangements is that the quality of the light signals deteriorates as they travel along the fibre optics, particularly when high-powered light sources such as lasers are used. That becomes a

particular issue when trying to detect low levels of tracer in challenging conditions. The use of optical tracers to trace fluids from hydrocarbon reservoirs is desirable because the optical tracers are suitable for real-time, on-line detection. The provision of real-time, on-line information is desirable for allowing quick decisions to be made. The use of fluorescent tracers in such applications is hampered by the natural fluorescence of crude oil. That can mask the fluorescence of the tracer and there therefore exists a need for improved optical tracer systems, with the associated improved detection apparatus and capability, for monitoring flows from hydrocarbon reservoirs in real-time and on-line.

Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages of the prior art. In particular, preferred embodiments of the present invention seek to provide improved apparatus and methods for detecting, in a fluid, a tracer that absorbs an excitation light of a first wavelength and emits an emission light of a second wavelength.

Summary of Invention According to a first aspect of the invention, there is provided an apparatus for detecting, in a fluid, a tracer that absorbs an excitation light of a first wavelength and emits an emission light of a second wavelength, the apparatus comprising : a. a unit comprising a light source to generate the excitation light and a detector to measure the emission light; and b. a probe connected to the unit by a light guide, the probe comprising: i. a first filter to remove unwanted wavelengths from the excitation light arriving along the light guide from the light source; ii. a focusing lens to focus the excitation light through a window into the fluid and to collect the emission light originating in the fluid; iii. a second filter to remove the excitation light from the emission light before the emission light is transmitted along the light guide to the detector; and iv. a dichroic mirror to allow the excitation

light from the first filter to pass to the focusing lens and to deflect the emission light from the focusing lens to the second filter.

According to a second aspect of the invention, there is provided a method of detecting, in a fluid, a tracer that absorbs an excitation light of a first wavelength and emits an emission light of a second wavelength, the method

comprising: a. generating an excitation light with a light source; b. transmitting the excitation light along a light

guide to a probe; c. in the probe: i. filtering the excitation light to remove

unwanted wavelengths using a first filter; ii. focusing the excitation light through a

window into the fluid using a focusing lens; iii. collecting the emission light originating in the fluid using the focusing lens; iv. filtering the excitation light out of the emission light using a second filter; wherein the excitation light is passed from the first filter to the focusing lens through a

dichroic mirror that deflects the emission light to the second filter; d. transmitting the emission light along the light

guide to the detector; and e. detecting the emission light with a detector.

By providing the optics, including the first and second filters and the dichroic mirror, in the probe the problem of loss of quality during transmission along the light guide is solved. That is particularly beneficial in tracer

applications, where the detection of very low concentrations of tracer is desirable. It is particularly beneficial in challenging tracer applications, such as the detection of upconverting phosphors or low concentrations of quantum dot tracers in fluids produced from hydrocarbon reservoirs such as crude oil or produced water. For example, when using upconverting phosphors, the upconversion allows the emission light to be separated from emissions resulting from the crude oil, but the upconversion emission light may be a weak signal and difficult to detect. For example, even if a powerful, monochromatic light source such as a laser is used, effects such as auto-fluorescence in the light guide can introduce unwanted wavelengths into the excitation light which may interfere with detection of the upconversion emission light. By situating the first filter in the probe, such unwanted wavelengths are removed from the excitation light after it has been transmitted along the light guide and before it is focussed into the fluid. Similarly, signals from quantum dots or from very low concentrations of fluorescent tracers may be weak and difficult to detect. While a dichroic mirror is typically used to separate emission light from excitation light, the provision of the second filter to remove any remaining excitation light from the emission light allows the use of high powered light sources that generate so much light that, even at the separation levels of the dichroic mirror, would result in enough excitation light remaining with the emission light to interfere with the detection of the

upconversion or weak fluorescence light. By providing the filters and the dichroic mirror in the probe, the quality of the excitation light is maximised immediately before it is focused into the fluid and the quality of the emission light is maximised immediately after it is emitted from the fluid, thus improving the sensitivity of the apparatus and allowing the use of very high intensities of excitation light and the detection of very low intensities of emission light. Further advantageously, providing the optics, including the first and second filters and the dichroic mirror, in the probe, and providing the light source to generate the

excitation light and the detector to measure the emission light in a separate unit optically coupled to the optics via a light guide, allows the unit, and hence the source and the detector and associated control and processing electronics to be remotely spaced apart from the probe and the optics therein. This can be particularly useful in the sometimes challenging conditions in which an apparatus of the first aspect of the invention might be used. Examples may include high temperature, corrosive, flammable, strong

electromagnetic interference and radioactive environments, and use in inaccessible locations where size of the source and detector may inhibit use. In such challenging scenarios, the probe and associated optics may be effectively located closely adjacent to the detection site and closely adjacent to the fluid containing the tracer to be detected, avoiding the problem of loss of quality of the emission signal during transmission along the light guide, but the typically more sensitive source and detector and associated control and processing electronics may be located spaced away from the detection site and the potentially challenging environmental conditions associated with the detection site.

Preferably the light source is a laser and most preferably a laser having a power of greater than 5 mW, preferably greater than 50 mW and most preferably greater than 500 mW. While the apparatus of the invention advantageously permits the most efficient use of any given light source, the use of a high- power laser advantageously permits the apparatus to be used to detect very low levels of tracer in challenging conditions such as, for example, the detection of upconversion phosphor tracers, down-converting phosphors or low concentrations of fluorescent or luminescent tracers in crude oil.

One of the first filter or the second filter may be a long- pass filter. One of the first filter or the second filter may be a short-pass filter. In a preferred case one of the first filter or the second filter comprises a long-pass filter and the other of the first filter or the second filter comprises a short-pass filter. Alternatively, the first filter may be a narrow band pass filter. In an example with particular application to upconversion emissions the first filter comprises a long-pass filter or a narrow band pass filter and the second filter comprises a short-pass filter. However, the skilled person can readily select suitable filters, including short-pass filters, long-pass filters and band-pass filters, and select suitable cut-off points for those filters, depending on the excitation wavelength

required for a particular tracer and the emission wavelength produced by that particular tracer.

The detector is preferably a spectrometer. That is to say, the detector is adapted to resolve emission light into a plurality of wavelength bands and to measure emission light intensity in each of the said plurality of wavelength bands.

In some applications, especially for upconversion emissions, it may be preferred that the focusing lens has a focal point of not more than 500 μπι spot size in the fluid. A small spot size will result in delivering a high level of excitation light to a small region, thus maximising the intensity of emission from that region. Some emissions, such as

upconversion emissions, typically do not rise linearly with excitation power density and instead rise in a quadratic, or even cubic, way. Thus, focusing the excitation light into a small region generates greater emissions than would be generated by spreading the excitation light across a large region. However, the spot size may be altered to suit the tracer and fluid being monitored. The focusing lens may collect emission light from the same region of the fluid as the region in which the excitation light is focused. However, in some embodiments, for example if delayed fluorescence is used in a moving fluid, the focusing lens may collect the emission light from a different region of the fluid to the region in which the excitation light is focused. The emission light may be collected through the same part of the focusing lens as is used to focus the excitation light or through a different, possibly even separate, part of the focussing lens. The focussing lens may thus comprise a single lens or multiple lenses. A single lens may advantageously result in a more compact probe, while multiple lenses may advantageously permit the optimisation of different lenses for focussing the excitation light and collecting the emission light.

The window is preferably a sapphire window. Sapphire windows exhibit good transmission of the excitation and emission lights, while being chemically resistant and mechanically robust for inserting into the fluid. In a possible arrangement, the probe may comprise a housing and a probe extension projecting from the housing, with the window being provided in the probe extension, for example towards a distal end thereof. The focusing lens may

additionally be provided, or in the case of a compound lens provided at least in part, in the probe extension, for example towards a distal end thereof. The window may comprise the focusing lens or a part thereof. The remaining optics may then be provided in the housing, the housing for example containing at least the first and second filters and the dichroic mirror. In such a configuration, the probe is typically adapted such that in use the probe extension projects towards and at least the distal end thereof projects into a fluid containing a tracer to be monitored. For example, in application of the invention to inflow monitoring using tracers the probe extension projects towards and at least the distal end thereof projects into the flow.

Efficient use of the excitation and emission lights is important in permitting the advantageous detection of very low levels of tracers in challenging conditions. Minimising losses as the excitation light exits the light guide into the probe and as the emission light enters the light guide from the probe is therefore desirable. Preferably, therefore, a lens is provided in the probe to collimate the excitation light arriving from the light guide before the excitation light reaches the first filter. Preferably a further lens is provided in the probe to focus the emission light into the light guide for transmission from the probe to the detector.

The light guide transmits the excitation and the emission lights to and from the probe. The light guide transmits the excitation and the emission lights from and to the unit comprising the light source to generate the excitation light and the detector to measure the emission light. The light guide may comprise separate parts, or separate light guides, for transmitting the excitation light to the probe and for transmitting the emission light from the probe. For example, the light guide may be a coaxial light guide with one of the excitation or emission lights being transmitted in the core light guide and the other of the excitation or emission lights being transmitted in the surrounding light guide.

Alternatively, the light guide may comprise two separate light guides, which may, for example, be contained within a single tube or sheath for ease of handling. Preferably the light guide is a fibre optic light guide. Preferably the light guide is a flexible light guide. A flexible light guide may advantageously make manoeuvring and positioning of the probe more straightforward.

The apparatus and method of the invention are suitable for use in any application where it is desirable to detect low levels of tracer in challenging conditions. A particular such application is the detection of tracers in hydrocarbon fluids, such as crude oil, and fluids produced from

hydrocarbon reservoirs such as crude oil or produced water. Because such fluids may fluoresce themselves, it may be necessary or desirable to use tracers that can be

distinguished from that fluorescence.

An example of such application uses upconverting tracers. Upconverting tracers, such as upconverting phosphor tracers can be distinguished, because the upconversion emission is at a shorter wavelength than the excitation light and is

therefore separable from the fluorescence of the hydrocarbon, which occurs at a longer wavelength than the excitation light. However, upconversion emissions are weak and the use of the apparatus and method of the invention therefore has particular advantages in detecting upconverting tracers in hydrocarbon fluids such as crude oil or in detecting

upconverting tracers in fluids produced from hydrocarbon reservoirs such as crude oil or produced water. In

particular, the use of the apparatus and method of the invention may permit the delivery of a very high intensity of excitation light to a small region so as to excite as great an upconversion emission as possible and then separate that upconversion emission, which may still be at a low intensity, from the reflected excitation light to allow detection of the upconversion emission. Delayed fluorescence may be another option for distinguishing tracer fluorescence from natural crude oil fluorescence. In that case the detection may be time-gated so as to allow the natural fluorescence to

subside. However, the tracer signal may also be weak by that time so the apparatus and method of the invention may be advantageous in detecting that weak fluorescence.

Furthermore, even when detecting tracers in produced water, where natural fluorescence may not be a concern, the

desirability of using smaller quantities of tracer and making the tracer applied to a well last for a longer period of time means that it is desirable to be able to detect very low concentrations of tracers, such as fluorescent tracers. The apparatus and method of the present invention may be

advantageous for that reason also.

A particular application of the use of tracers is the tracing of fluids produced from subterranean reservoirs (for example, oil-wells) . The skilled person will be familiar with the various tracing applications in such reservoirs, including inter-well tracer tests and inflow monitoring using tracers. The tracers may be used to trace both hydrocarbon and water production from the reservoir, typically by using different tracers that follow either the water or the oil. It is desirable to use optical tracers to provide real-time

analysis if the optical tracers can be detected on-line. The apparatus and method of the invention can be used in such methods to detect tracers, such as upconverting tracers, quantum dots or photonic crystals, in the produced fluids from the reservoir. The tracers are preferably luminescent tracers. The tracers are preferably fluorescent tracers. The tracers are preferably upconverting phosphors, down- converting phosphors, quantum dots, or photonic crystals, most preferably upconverting phosphors. The tracers,

especially solid tracers such as upconverting phosphors, are preferably encapsulated to make them dispersible in either the water produced from the reservoir or the hydrocarbons produced from the reservoir. Thus, different tracer

materials, such as different upconverting phosphors, can be encapsulated in different materials, such as polymers, to create different tracers to trace either hydrocarbon or water from the reservoir. Moreover, different tracers, such as different upconverting phosphors, can be introduced into the reservoir at different points to monitor the fluids produced from those different points. The tracers are preferably released into the fluid from a polymer release system whereby the tracer is embedded in a polymer material that degrades to allow steady release of the tracer. The tracers can be monitored in real-time using the apparatus and method of the invention to provide information about the flow of the fluids in the reservoir. Thus, an aspect of the invention provides a method of monitoring a hydrocarbon reservoir, the method comprising introducing a tracer, preferably an optical tracer, more preferably a luminescent tracer, more preferably a fluorescent tracer, quantum dot, photonic crystal, down- converting phosphor or upconverting phosphor, and most preferably an upconverting phosphor, into the reservoir, producing fluids from the reservoir, detecting, preferably on-line and preferably in real-time, the tracer in the fluids using the apparatus or method of the invention, and

determining from the amount of tracer detected information about the flow of the fluids in the reservoir. The probe is preferably installed in a conduit carrying fluids from the reservoir .

It will be appreciated that features described in relation to one aspect of the invention may be equally applicable in another aspect of the invention. For example, features described in relation to the method of the invention, may be equally applicable to the apparatus of the invention, and vice versa. Some features may not be applicable to, and may be excluded from, particular aspects of the invention. Description of the Drawings

Embodiments of the present invention will now be described, by way of example, and not in any limitative sense, with reference to the accompanying drawings, of which:

Figure 1 is a view of an apparatus according to the

invention;

Figure 2 is a schematic diagram of a probe for use in the apparatus of Figure 1; and

Figure 3 is a schematic diagram of a method according to the invention for tracing fluids from a hydrocarbon reservoir. Detailed Description

In figure 1 a unit 1, which contains a light source 4 and a detector 5, is connected to but remotely spaced from a probe comprising a primary probe housing 2 and projecting stainless steel probe tube 6 by a flexible light guide 3 in the form of twin fibre optic cables. The light source 4 in this

embodiment is a 980 nm CW laser. The detector 5 in this embodiment is a thermos-electric cooled spectrometer. In use, excitation light from the light source 4 is transmitted to the probe optics in the housing 2 by a first fibre optic cable of the flexible light guide 3 and then projected through the probe tube 6 across an interface represented by the broken line into a fluid in which a tracer is being detected through a sapphire window 15 at a distal end of the projecting probe tube. Emission light received by the probe is transmitted back to the detector 5 by a second fibre optic cable of the flexible light guide 3. In figure 2, the probe optics are shown in more detail.

Within the primary probe housing 2 an optical arrangement comprises a lens 12, which collimates excitation light 11 received from a first fibre optic cable of the light guide 3 through inlet port 10. The lens 12 collimates the excitation light 11 which passes through a long-pass filter 13 with a

950 nm cut-off and through a dichroic mirror 14 with a 900 nm cut-off. A band pass filter could be substituted for the long pass filter 13. Since the excitation light 11 has a

wavelength of 980 nm from the light source 4 and the long- pass filter 13 has removed any wavelengths below 950 nm so as to remove any unwanted wavelengths that have resulted from auto-fluorescence in the light guide 3, the excitation light 11 is not deflected by the dichroic mirror 14 and passes straight through to focussing lens 16. Focussing lens 16 focuses the excitation light 11 along the projecting probe tube 6 through a sapphire window 15 at a distal end of the projecting probe tube 6 to a focal point 22 in a fluid in which a tracer is being detected. The focal point 22 in this embodiment has a spot size of about 400 μπι but may be altered to suit the tracer and fluid being monitored. The high intensity excitation light 11 causes a phosphor tracer in the fluid to emit an emission light. In the example embodiment, an upconverting phosphor tracer is used in the fluid and caused to emit an emission light having a

wavelength in the range 400 to 850 nm. The emission light 20 is emitted in all directions, and a portion is captured by the focusing lens 16 and directed to the dichroic mirror 14. Since the dichroic mirror 14 has a 900 nm cut-off, the emission light 20 is reflected by the dichroic mirror 14 onto mirror 17, which in turn reflects it through short-pass filter 18 to lens 19. Short-pass filter 18 has an 850 nm cutoff and serves to remove any reflected excitation light 11 that, due to inefficiency in the cut-off of the dichroic mirror 14, is travelling with the emission light 20. The function of the short-pass filter 18 is particularly

beneficial in this embodiment where a high-power laser is being used as the light source 4 and an upconverting phosphor is being used as the tracer. Because the intensity of the excitation light 11 is very much greater than the intensity of the emission light 20 in such circumstances, even very small inefficiencies in the cut-off at the dichroic mirror 14 can result in sufficient reflected excitation light 11 being reflected by the dichroic mirror 14 to interfere with the detection of the emission light 20. The lens 19 focuses the emission light 20 through exit port 21 into a second fibre optic cable of the light guide 3 for transmission back to the detector 5 and associated electronics in the unit 1.

The use of a light guide, such as in the example embodiment a pair of fibre optic cables, to transmit excitation light from the light source 4 in the unit 1 to the probe and in

particular to the optics in the housing 2 and to transmit emission light from the probe and in particular to the optics in the housing 2 to the detector 5 and associated electronics in the unit 1 allows the unit and the probe to be separated and spaced remotely. This may be particularly advantageous where the fluid in which the tracer to be detected is present presents a harsh environment, and for example a high

temperature environment, and the probe optics necessarily must be located at or in the vicinity of this harsh

environment to perform its function. By provision of a light guide the unit 1 containing the detector and associated electronics may then be spaced safely away from this harsh environment. Excitation light is generated in the unit 1 and passed via an optical coupling at a proximal end of the light guide 3 along the light guide 3 to the remotely spaced probe optically coupled at the distal end of the light guide 3.

Provision within the optical system of the probe of the long- pass filter 13 removes any resultant unwanted wavelengths that may have resulted from auto-fluorescence from the excitation light 11 in the light guide 3. In figure 3, a method of monitoring a hydrocarbon reservoir 110 comprises introducing a tracer 114 into the reservoir 110, producing fluid from the reservoir 110 and detecting the tracer 114 in the fluid so as to monitor the reservoir 110. The tracer 114 may for example be an upconverting phosphor encapsulated in a polymer material so as to improve its dispersibility in at least one of the reservoir fluids. The tracer 114 is introduced into the reservoir 110 in a release system 111. The release system 111 may for example be a polymer release system whereby the tracer 114 is embedded in a polymer material that degrades to allow steady release of the tracer 114. The tracer 114 is released from the release system 111 and carried by the production flow 112 of the reservoir fluids to the surface where it is detected using an apparatus and method as described above with reference to figures 1 and 2. The reservoir 110 includes a surface

facility 115 and the apparatus is installed and the method carried out at the surface facility 115. Preferably the analysis is carried out on-line in real-time.

It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only, and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims. For example, where the tracer exhibits standard fluorescence, the long-pass filter 13 may be replaced with a short-pass filter to remove unwanted long wavelengths that would interfere with detection of the fluorescence. Similarly the short-pass filter 18 may be replaced in that situation with a long-pass filter so as to allow the fluorescence to pass whilst

removing any reflected excitation light 11. The skilled person can select suitable light source wavelengths and filters, including short-pass filters, long-pass filters, band-pass filters and dichroic mirrors and select suitable cut-off points for those filters and mirrors depending on the excitation wavelength required for a particular tracer and the emission wavelength produced by that particular tracer.