WO2021176192A1

WO2021176192A1 - Luminescent carbon-based nanoparticles and methods of monitoring hydrocarbon reservoirs

Info

Publication number: WO2021176192A1
Application number: PCT/GB2021/050001
Authority: WO
Inventors: Thomas William BURNS; Felicity Jane ROBERTS; Victoria Louise WHITTLE
Original assignee: Johnson Matthey Public Limited Company
Priority date: 2020-03-02
Filing date: 2021-01-04
Publication date: 2021-09-10
Also published as: GB202002958D0; GB2592742A; GB202100036D0

Abstract

A luminescent carbon-based nanoparticle tracer material for monitoring a hydrocarbon reservoir, the luminescent carbon-based nanoparticle tracer material comprising a plurality of luminescent carbon- based nanoparticles, each carbon-based nanoparticle comprising: a carbon-based core having a diameter in a range 1 to 100 nm; and a surface functionalised for dispersion of the luminescent carbon-based nanoparticles within an aqueous hydrocarbon reservoir fluid, wherein the luminescent carbon-based nanoparticles are fabricated to emit a luminescence signal at an emission wavelength which remains within 85 to 115% of its initial intensity at said emission wavelength when dispersed within an aqueous fluid for a time period between 20 and 70 days at a temperature in a range 60°C to 90°C.

Description

LUMINESCENT CARBON-BASED NANOPARTICLES AND METHODS OF

MONITORING HYDROCARBON RESERVOIRS

Field

The present specification relates to luminescent carbon-based nanoparticles and their usage as tracers which are suitable for monitoring hydrocarbon reservoirs, methods of fabricating such luminescent carbon-based nanoparticle tracers, and to methods of monitoring hydrocarbon reservoirs using such luminescent carbon-based nanoparticle tracers. It is envisaged that the luminescent carbon-based nanoparticle tracers may also be suitable for other applications, particularly, but not exclusively, those where the carbon-based nanoparticle tracers are exposed to elevated temperatures for extended time periods.

Background

The use of tracers to monitor aspects of the performance of hydrocarbon wells is an established technique. The tracers may be water tracers, in that they are predominantly soluble or dispersible in water, oil tracers, in that they are soluble or dispersible in the hydrocarbons, or partitioning tracers, in that they are soluble or dispersible between both the water and hydrocarbon phases. Some tracing methods will employ more than one type of tracer and use the difference in behaviour to deduce properties of the hydrocarbon formation. For example, partitioning and water tracers may be injected into a production well along with injected water and then monitored as they are subsequently produced from the well. The time difference between the production of the water tracers, which are produced with the returning injected water, and the partitioning tracers, whose production is delayed by their interaction with the hydrocarbons in the formation, can be used to deduce parameters relating to the local remaining hydrocarbon content of the formation. Alternatively, applications may use only water tracers. For example, water tracers may be introduced in an injection well and their presence monitored at adjacent production wells in order to obtain information about the flux of water from the injection well to the production well. In addition to injection techniques, it is also known to introduce tracers into a well by including them in articles placed into the well. By detecting the rate of tracer production over time, information can be deduced about the performance of the hydrocarbon well.

Tracers should be detectable in small to very small quantities, for example at levels below 100 parts per billion (ppb), preferably at levels of 50 ppb or lower, more preferably at levels of 10 ppb or lower, and most preferably in the parts per trillion (ppt) range (that is, at levels less than 1 ppb). The levels are determined on a mass/mass basis. The tracers should also be environmentally acceptable with low toxicity for insertion into the ground and usage, for example, in reservoir applications, but they must also be species that are not naturally present in the ground in such quantities as to contaminate the results of a tracer study.

Typical detection methods include gas chromatography - mass spectrometry (GC-MS), gas chromatography - mass spectrometry - mass spectrometry (GC-MS-MS), liquid chromatography - mass spectrometry (LC-MS), liquid chromatography - mass spectrometry - mass spectrometry (LC- MS-MS) and high-performance liquid chromatography (HPLC), which can typically detect very low concentrations of the tracers in the produced fluids. It is desirable that tracers should be detectable in low quantities and also that they can be reliably distinguished from other tracers and species which are naturally present in reservoir fluids.

Tracers may comprise or include a luminophore, that is a material that can emit energy upon excitation with energy of certain wavelengths. Known luminophores include fluorophores (i.e., materials that exhibit fluorescence) and phosphors (i.e., materials that exhibit phosphorescence). The presence of such a luminescent tracer may be determined by optical spectroscopy. Detection via luminescence (e.g. emission spectroscopy) can be advantageous over tracers which require detection via chromatography and mass spectrometry in that the analysis can take less time, have a lower cost, and can easily be performed on location in real time. Furthermore, the analysis techniques required can be made simpler and do not require a highly trained person. Other benefits are less consumables, greater portability, and gases, such as methane and helium, are not required. That is, the use of luminescent tracers provides an opportunity to perform the tracer analysis quickly in the field (i.e., on-site analysis) without requiring samples to be transported to a laboratory for more complex chromatography and mass spectrometry analysis.

While luminescent tracers have some advantageous features as outlined above, current luminescent tracers have some drawbacks for certain reservoir monitoring applications. Known fluorophores may have undesirable properties relating to their performance as tracers or to their environmental impact. For example, known fluorophores based on metals are frequently toxic or environmentally damaging and as a result tend to require an inert coating such as a silica coating. Furthermore, known fluorophores based on fluorescent dyes (molecular organic species) tend to be unstable in hydrocarbon well conditions.

Luminescent nanoparticles (e.g. fluorescent nanoparticles) have attracted recent attention for reservoir monitoring applications. Examples include semiconductor quantum dots, metallic quantum dots, carbon nanoparticles, and upconverting nanoparticles. Nanoparticles are advantageous over larger microparticles in that they remain dispersed within reservoir fluids for longer time periods without settling out. Furthermore, nanoparticles can be provided at higher concentrations in terms of number of particles per unit volume of fluid, leading to better detection limits. Further still, nanoparticles can have improved chemical stability in hydrocarbon well conditions compared to luminescent organic molecules and can be selected to have low toxicity. For example, the use of fluorescent carbon nanoparticles as tracers for hydrocarbon reservoir monitoring is discussed in US9891170.

Carbon dots are small carbon-based nanoparticles (for example less than 100 nm in size, and generally less than 10 nm in size) which have been found to exhibit useful luminescent properties, and in particular fluorescent properties, together with low toxicity and high chemical stability that make them potentially attractive for reservoir tracer applications. Carbon dots are also known in the literature as carbon quantum dots, C-dots, carbon nanoparticles, amorphous carbon dots, graphitic carbon dots, graphene quantum dots or graphene dots. Novel carbon quantum dot (CQD) based fluorescent tracers have been proposed for production and well monitoring. They may be structured or have surface modifications to exhibit high dispersibility in water. Their use as aqueous phase tracers has been discussed for example in US9891170.

The present invention seeks to provide improved luminescent carbon-based nanoparticle tracers for use in hydrocarbon well monitoring and in particular for use in monitoring produced water from hydrocarbon wells. It is envisaged that the luminescent carbon-based nanoparticle tracers may also be suitable for other applications, particularly, but not exclusively, those where the carbon-based nanoparticle tracers are exposed to elevated temperatures for extended time periods. Summary of Invention

In order to be useful in hydrocarbon reservoir monitoring applications, a tracer should be thermally stable in that it should be stable at the temperatures typically encountered in hydrocarbon wells, which may be, for example, 60 to 90°C.

While carbon nanoparticle tracers can be more chemically stable than fluorescent dyes in hydrocarbon well conditions, the present inventors have found that the luminescence signal from certain types of luminescent carbon nanoparticles tends to degrade (e.g. reduce in intensity) when exposed to elevated temperatures in their aqueous phases for extended time periods. As such, the usefulness of such luminescent nanoparticles in hydrocarbon reservoir monitoring applications is limited in this regard when extended time periods are required.

In addition, the present inventors have also found that the luminescence signal from certain types of luminescent carbon nanoparticles tends to remain high but has a high degree of instability when exposed to elevated temperatures for extended time periods. For example, known methods of synthesising luminescent carbon nanoparticles have been found to yield luminescent carbon nanoparticles which exhibit an increase in luminescence intensity over an initial time period when exposed to elevated temperatures and this can also be followed by a subsequent decrease in luminescence intensity. While the luminescence signal remains sufficiently high over a certain time period to be detectable and used qualitatively, extracting quantitative or even semi-quantitative information from such an unstable signal can be difficult or impossible. As such, the usefulness of such luminescent nanoparticles in hydrocarbon reservoir monitoring applications is limited in this regard.

Further still, the present inventors have also found that the luminescence signal from certain types of luminescent carbon nanoparticles tends to shift when dispersed in an aqueous phase and exposed to elevated temperatures for extended time periods. This could be a shift in the frequency of the emission maximum (e.g. blue shifting) and/or a change in the emission band shape. This may also be accompanied by a change in intensity of the signal. Again, this instability in luminescence can make it difficult or impossible to extract quantitative or even semi-quantitative information and thus reduces the usefulness of such carbon nanoparticles in hydrocarbon reservoir monitoring applications.

In relation to the above, it should be noted that hydrocarbon reservoir tracers can be either qualitative, quantitative, or semi-quantitative. Usually, it is desirable to have quantitative or semi- quantitative tracing as this provides the most information about the hydrocarbon reservoir. In particular:

• Qualitative tracers provide an indication of whether fluid is coming from an area. o This can be achieved if the tracer survives the downhole environment, i.e. if some of the tracer's response does not fully degrade in the downhole environment - some degradation is tolerable for qualitative analysis.

• Quantitative tracers provide an indication of whether fluid is coming from an area, and also provide a means to quantify the flow from that area. o This can only be achieved if the tracer's response is uniform throughout its lifetime within the well - significant degradation or non-uniformity in the tracer's response is not tolerable. • Semi-quantitative tracers provide an indication of whether fluid is coming from an area, and also provide a means to proportionally quantify the flow from that area, with respect to other areas. o This can only be achieved if the tracer's response is uniform throughout its lifetime within the well.

The present inventors have found that commercially available carbon-based nanoparticles are not suitable for use in hydrocarbon reservoir monitoring applications, at least when quantitative or semi- quantitative information is required, due to fluorescence signal instability over extended time periods at elevated temperatures in an application environment and/or in a test environment simulating a hydrocarbon reservoir environment. Furthermore, the present inventors have found that carbon- based nanoparticles which they have synthesised themselves using prior art methods of synthesis are not suitable for the same reasons of signal instability over extended time periods at elevated temperatures.

As such, the present inventors have investigated new ways of manufacturing carbon-based nanoparticles in order to increase the thermal stability of the fluorescence signal from the carbon- based nanoparticles at elevated temperature for extended time periods within a hydrocarbon reservoir environment. Several different fabrication routes have been developed. The present specification thus provides a range of luminescent carbon nanoparticle tracers which exhibit a stable luminescence signal over extended time periods at elevated temperatures and are suitable for hydrocarbon reservoir tracing applications, preferably quantitative or semi-quantitative hydrocarbon reservoir monitoring applications. It is envisaged that the carbon-based nanoparticle tracers may also be suitable for other applications, particularly, but not exclusively, those where the luminescent carbon-based nanoparticle tracers are dispersed in an aqueous phase and exposed to elevated temperatures for extended time periods.

In particular, the present specification provides a luminescent carbon-based nanoparticle tracer material for monitoring a hydrocarbon reservoir, the luminescent carbon-based nanoparticle tracer material comprising a plurality of luminescent carbon-based nanoparticles, each carbon-based nanoparticle comprising: a carbon-based core having a diameter in a range 1 to 100 nm; and a surface functionalised for dispersion of the luminescent carbon-based nanoparticles within an aqueous hydrocarbon reservoir fluid, wherein the luminescent carbon-based nanoparticles are fabricated to emit a luminescence signal at an emission wavelength which remains within 85 to 115% of its initial intensity at said emission wavelength when dispersed within an aqueous fluid for a time period between 20 and 70 days at a temperature in a range 60°C to 90°C.

In relation to the above, it should be noted that unlike organic molecular dyes, it is not possible to precisely define luminescent carbon-based nanoparticles in terms of a specific molecular formula. Furthermore, the luminescent properties of the carbon-based nanoparticles can result from characteristics of the bulk core material, the surface functionalisation, or a combination of the two. Further still, while the carbon-based nanoparticle material contains carbon, optionally in graphitic form, it may also comprise one or more further atomic or molecular species and/or the carbon may be present, at least in part, in molecular form depending on the specific synthesis conditions used to fabricate the carbon-based nanoparticle material. In addition, the present specification describes a number of different ways in which luminescent carbon-based nanoparticles can be fabricated to emit a stable luminescence signal (e.g. a stable photoluminescence or a stable fluorescence signal) for extended time periods at elevated temperatures. As such, in the present case the luminescent carbon- based nanoparticle tracer materials can most clearly be defined in terms of their emission characteristics which are readily testable.

The luminescent carbon-based nanoparticle materials as described herein have been specifically engineered to improve the performance of such materials in reservoir tracing applications. In particular, the emission properties of the carbon-based nanoparticle materials have been modified to improve thermal stability over extended time periods at elevated temperatures. In this regard, it should be noted that luminescent carbon-based nanoparticles have been used in a wide range of applications to date, particularly in biological applications. However, there is no requirement for the carbon-based nanoparticle materials used in those applications to have a stable luminescence in the type of conditions experienced within a hydrocarbon reservoir. In fact, the present inventors have found that prior art luminescent carbon-based nanoparticle materials exhibit a luminescence which decays significantly and/or is unstable in terms of intensity and/or wavelength when subjected to elevated temperatures over extended time periods in deionised water or produced reservoir fluids. As such, a significant amount of work has been performed to engineer a new range of luminescent carbon-based nanoparticle materials with emission properties tailored for long timescale monitoring of hydrocarbon reservoirs.

The present specification also provides a method of monitoring a hydrocarbon reservoir using the luminescent carbon-based nanoparticle tracer material as described herein, the method comprising: introducing the luminescent carbon-based nanoparticle tracer material into a hydrocarbon well, pipeline or formation; producing an aqueous fluid from the hydrocarbon well, pipeline or formation; and analysing the aqueous fluid to detect a luminescence signal from the luminescent carbon- based nanoparticle tracer material.

Due to the stability of the luminescence signal of the carbon-based nanoparticle tracer material, the analysis can be quantitative or semi-quantitative rather than merely qualitative. That is, the luminescence signal can be used not only to determine the presence of flow from a specific region of the hydrocarbon reservoir but also to determine the quantity of flow, or at least to proportionally quantify the flow, from that region.

The present specification also provides a method of fabricating the luminescent carbon-based nanoparticle tracer material as described herein, the method comprising: synthesising the luminescent carbon-based nanoparticle tracer material using a hydrothermal or solvothermal process at a temperature above of 100°C and a pressure above 1 atmosphere; and applying a post synthesis thermal treatment to the luminescent carbon-based nanoparticle tracer material to increase the stability of the luminescence signal from the luminescent carbon-based nanoparticle tracer material.

Hydrothermal or solvothermal processes for synthesising luminescent carbon-based nanoparticle material are known in the art using a variety of reactants, solvents, and specific synthesis conditions. Other fabrication routes are also known in the art such as acid oxidation in which a carbon material, such as carbon black, soot, or activated charcoal, is heated in concentrated nitric acid or in a concentrated nitric acid/sulfuric acid solution. However, the present inventors have found that such methods result in carbon-based nanoparticle tracer materials which have an unstable luminescence signal when exposed to elevated temperatures for extended time periods. For carbon-based nanoparticle materials fabricated via a hydrothermal/solvothermal route, the present inventors have found that applying a post synthesis thermal treatment to the carbon-based nanoparticle material, such as heating in aqueous liquid at a temperature between 50 and 100°C for at least 3 days, increases the stability of the luminescence signal. The result is that the treated carbon-based nanoparticle tracer material has a luminescence signal intensity immediately after the treatment which subsequently remains within 85 to 115% of this luminescence signal intensity when dispersed within an aqueous fluid in use for a time period between 20 and 70 days at a temperature in a range 60°C to 90°C. As previously indicated, this is highly advantageous as it is possible to use the material for quantitative, or at least semi-quantitative, monitoring of fluid flow from different regions of a hydrocarbon reservoir and over extended time periods. This is difficult or impossible to do with prior art fluorescent carbon-based nanoparticle materials which exhibit an unstable fluorescence signal over extended time periods in a hydrocarbon reservoir environment. It is also difficult or impossible to do with organic fluorescent dyes which also exhibit an unstable fluorescence signal over extended time periods in a hydrocarbon reservoir environment. Extended monitoring of hydrocarbon reservoirs is possible using other techniques such as gas chromatography - mass spectrometry. Flowever, such analysis techniques are time consuming and expensive and involve taking a sample to a laboratory for testing. Other issues with chromatography-mass spectrometry systems are that they are, bulky, heavy, lack portability, require gas, such as methane and helium, and require stable conditions which may require heating or cooling. They also require trained personnel to operate and troubleshooting and resolving issues can be time-consuming. In addition, water tracers require solid phase extraction followed by further functionalisation to be detected. A large number of consumables are required including chemicals, solvents, syringes, and a separating column. In contrast, a luminescent tracer may be determined by optical spectroscopy which allows fast, simple, on site monitoring in real time by a person who has minimal training. Previously, such techniques using fluorescent carbon-based nanoparticle materials could only give qualitative information, generally over short-term testing. The present invention allows such techniques to give semi-quantitative and/or fully quantitative information using fluorescent carbon-based nanoparticle materials.

Brief Description of the Drawings

For a better understanding of the present invention and to show how the same may be carried into effect, certain embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 shows a plot illustrating the stability of carbon nanoparticle emission for carbon nanoparticle material fabricated (sample 1) via an acid oxidation method and then dispersed in deionised water and exposed to temperatures of 60°C and 90°C for a period of 84 days (all data is normalised to day 0) - the emission is unstable and accordingly this material is not suitable for quantitative or semi- quantitative analysis of hydrocarbon reservoir fluids over extended time periods;

Figure 2 shows a plot illustrating the stability of carbon nanoparticle emission for two carbon nanoparticle materials (samples 2 and 3) fabricated via a solvothermal method and then dispersed in deionised water and either kept at room temperature or exposed to a temperature of 90°C fora period of 79 days (all data is normalised to day 0) - the luminescence emission is unstable, particularly for the heated samples, and accordingly these materials are not suitable for quantitative or semi-quantitative analysis of hydrocarbon reservoir fluids over extended time periods; Figure 3 shows a plot illustrating the stability of carbon nanoparticle emission for sample 2 material fabricated via a solvothermal method and then dispersed in produced water from a hydrocarbon reservoir and either kept at room temperature or exposed to temperatures of 60°C and 90°C for a period of 61 days (note that the sample at 90°C was unfortunately compromised after 29 days although the instability in emission is already demonstrated by that time) - as in deionised water, the emission is also unstable in produced water, particularly for the heated samples, and accordingly this material is not suitable for quantitative or semi-quantitative analysis of hydrocarbon reservoir fluids over extended time periods;

Figure 4 shows a plot illustrating the stability of carbon nanoparticle emission for sample 3 material fabricated via a solvothermal method and then dispersed in produced water from a hydrocarbon reservoir and either kept at room temperature or exposed to temperatures of 60°C and 90°C for a period of at least 61 days - as in deionised water, the emission is also unstable in produced water, particularly for the heated samples, and accordingly this material is not suitable for quantitative or semi-quantitative analysis of hydrocarbon reservoir fluids over extended time periods;

Figure 5 shows a plot illustrating the stability of carbon nanoparticle emission for samples of carbon nanoparticle materials synthesised via a solvothermal method according to samples 2 and 3 but then subsequently subjected to a post synthesis thermal treatment by dispersing the carbon nanoparticle materials in deionised water and heated to 90°C for 7 days prior to testing by heating samples dispersed in deionised water at 90°C for a period of 72 days - emission remained stable throughout the analysis period indicating that this material is suitable for quantitative or semi-quantitative analysis of hydrocarbon reservoir fluids over extended time periods;

Figure 6 shows a plot illustrating the stability of carbon nanoparticle emission for samples of carbon nanoparticle materials synthesised via a solvothermal method according to samples 2 and 3 but then subsequently subjected to a post synthesis thermal treatment by dispersing in produced water and heated to 90°C for 5 days prior to testing by heating samples dispersed in produced water at 90°C for a period of 24 days - emission remained stable throughout the analysis period indicating that this material is suitable for quantitative or semi-quantitative analysis of hydrocarbon reservoir fluids over extended time periods;

Figure 7 shows a plot illustrating the stability of carbon nanoparticle emission for sample 3 synthesised and treated as per Figure 6 but subjected to testing by heating a sample at 90°C in produced water for an extended period of 56 days - emission remained stable throughout the analysis period indicating that this material is suitable for quantitative or semi-quantitative analysis of hydrocarbon reservoir fluids over extended time periods;

Figure 8 shows a plot illustrating the stability of carbon nanoparticle emission for a carbon nanoparticle material (sample 4) fabricated via a different solvothermal method than sample 2 or sample 3 and then dispersed in produced water and either kept at room temperature or exposed to a temperatures of 60°C or 90°C for a period of 42 days (all data is normalised to day 0) - the luminescence is unstable, particularly for the heated samples, and accordingly this material is not suitable for quantitative or semi-quantitative analysis of hydrocarbon reservoir fluids over extended time periods;

Figure 9 shows a plot illustrating the stability of carbon nanoparticle emission for sample 4 as per Figure 8 but where the material is subsequently subjected to a post synthesis thermal treatment by dispersing in produced water and heating to 90°C for 7 days prior to subjecting the sample to testing by heating a sample at 90°C for a period of 35 days in produced water - emission remained stable throughout the analysis period indicating that this material is suitable for quantitative or semi- quantitative analysis of hydrocarbon reservoir fluids over extended time periods;

Figure 10 shows a plot illustrating the stability of carbon nanoparticles for sample 5 fabricated via a solvothermal method; and

Figure 11 shows a plot illustrating the stability of carbon nanoparticles for sample 6 fabricated via a solvothermal method.

These figures represent a sub-set of the samples and data produced by the present inventors which is illustrative of the present invention.

Detailed Description

As described in the summary section, the present specification provides a luminescent carbon nanoparticle tracer material for monitoring a hydrocarbon reservoir, the luminescent carbon nanoparticle tracer material comprising a plurality of luminescent carbon nanoparticles, each carbon nanoparticle comprising: a carbon core having a diameter in a range 1 to 100 nm; and a surface functionalised for dispersion of the luminescent carbon nanoparticles within an aqueous hydrocarbon reservoir fluid, wherein the luminescent carbon nanoparticles are fabricated to emit a luminescence signal at an emission wavelength which remains within 85 to 115% (advantageously within 90 to 110%) of its initial intensity at said emission wavelength when dispersed within an aqueous fluid for a time period between 20 and 70 days (advantageously at least 30, 40, 50 or 60 days) at a temperature in a range 60°C to 90°C (advantageously at least 70, 80 or 90°C).

The luminescent carbon nanoparticle materials as described herein have been specifically engineered to improve the performance of such materials in reservoir tracing applications. In particular, the nanoparticle materials have been modified to improve thermal stability of the emission signal over extended time periods at elevated temperatures.

Further still, advantageously the luminescent nanoparticle tracer materials as described herein generate a luminescence signal with a well-defined narrow peak having a full-width-half-maximum (FHWM) of less than 100 nm, more preferably less than 50 nm, 30 nm, or 20 nm. Such a well-defined emission peak enables improved detection and quantification in produced fluids from hydrocarbon reservoirs and also allows the usage of multiple tracers in a single well.

For certain applications, advantageously the luminescent carbon nanoparticle tracer materials emit a luminescence signal with an emission maximum greater than 500 nm, more preferably greater than 550 nm. In this regard, one problem is that fluid from hydrocarbon reservoirs generally contains organic species which are naturally fluorescent at wavelengths below 500 nm. Certain luminescent carbon nanoparticles also emit at wavelengths below 500 nm. Accordingly, the signal from the luminescent nanoparticle tracers can be difficult to detect reliably due to interference from the fluorescent organic species occurring naturally within the reservoir fluid. For example, the carbon- based nanoparticles described in US9891170 exhibit a peak fluorescence intensity occurring at an emission wavelength of 440 to 475 nm and this may limit the effective use of such carbon nanoparticles without significant fluid preparation as is described in US9891170. Furthermore, the naturally occurring fluorescent organic species are not limited to the oil phase with some of the organic species exhibiting appreciable water solubility and thus may also be present in produced water. As a consequence, even complete separation of oil and water phases in the produced fluid will not prevent this effect. As such, preferably the luminescence signal of the carbon nanoparticle material has a maximum intensity at a wavelength greater than 500 nm, more preferably greater than 550 nm, and thus is less prone to interference from fluorescent organic species occurring naturally within the reservoir fluid. The emission band will be over a range of wavelengths and ideally the bulk of the band is outside the background fluorescence.

The surface of the luminescent carbon nanoparticles can be functionalised for dispersion/solubility in water, hydrocarbon, or both water and hydrocarbon. However, it is preferred that the core surface is functionalised for dispersion of the luminescent nanoparticles in water to be used as a water tracer. The examples of fluorescent carbon nanoparticles described herein inherently have such a functionalisation as a result of their fabrication method.

The present specification also provides a method of monitoring a hydrocarbon reservoir using the luminescent carbon nanoparticle tracer materials as described herein. The method comprises: introducing the luminescent carbon nanoparticle tracer material into a hydrocarbon well, pipeline or formation; producing a fluid (e.g. water) from the hydrocarbon well, pipeline or formation; and analysing the fluid to detect a luminescence signal from the luminescent carbon nanoparticle tracer material.

As described previously, due to the stability of the luminescence signal of the carbon nanoparticle tracer material, the analysis can be quantitative or semi-quantitative rather than merely qualitative. That is, the luminescence signal can be used not only to determine the presence of flow from a specific region of the hydrocarbon reservoir but also to determine the quantity of flow, or at least to proportionally quantify the flow, from that region.

The luminescent carbon nanoparticle tracer material can be introduced into the hydrocarbon well, pipeline or formation in a number of different ways including: injecting an aqueous solution of the luminescent carbon nanoparticle tracer material into the hydrocarbon well, pipeline or formation; adsorbing the luminescent carbon nanoparticle tracer material onto a proppant which is introduced into the hydrocarbon well, pipeline or formation; and/or forming a solid matrix in which the luminescent carbon nanoparticle tracer material is dispersed and introducing the solid matrix into the hydrocarbon well, pipeline or formation.

The luminescent carbon nanoparticle tracer materials can be fabricated using a method comprising: synthesising the luminescent carbon nanoparticle tracer material using a hydrothermal or solvothermal process at a temperature above of 100°C and a pressure above 1 atmosphere; and applying a post synthesis thermal treatment to the luminescent carbon nanoparticle tracer material to increase the stability of the luminescence signal from the luminescent carbon nanoparticle tracer material.

As previously described, the present inventors have found that applying a post synthesis thermal treatment to the carbon nanoparticle material increases the stability of the luminescence signal. The result is that the treated carbon nanoparticle tracer material has a luminescence signal intensity immediately after the treatment which subsequently remains within 85 to 115% of this luminescence signal intensity when dispersed within an aqueous fluid in use for a time period between 20 and 70 days at a temperature between 60°C and 90°C. As previously indicated, this is highly advantageous as it is possible to use the material for quantitative, or at least semi-quantitative, monitoring of fluid flow from different regions of a hydrocarbon reservoir and over extended time periods.

The post-synthesis thermal treatment may comprise dispersing the luminescent carbon nanoparticle tracer material in an aqueous liquid and heating the aqueous liquid at a temperature between 50 and 100°C for at least 3, 4, 5, 6, or 7 days (optionally no more than 14 days). The treatment can be in deionised water or another aqueous liquid such as produced water from a reservoir. Unlike the hydrothermal / solvothermal synthesis conditions, the post-synthesis thermal treatment can be performed at atmospheric pressure and temperatures below the boiling point of the aqueous fluid.

The hydrothermal / solvothermal synthesis (pre-treatment) can be performed using a range of different reactants and reaction conditions. Several methods are known in the art and the present inventors have also developed several variants of known methods. For example, the hydrothermal / solvothermal synthesis of the luminescent carbon nanoparticle material can use, as reactants, a molecular carbon source, such as an organic acid, citric acid, amino acid, or a peptide-bond containing compound (e.g. a dipeptide or a tripeptide). These may be disposed in a solvent such as in a formamide-based solvent (e.g. formamide, N,N-dimethylformamide, N-methylformamide). The reactants may further include one or more of: an amide, such as a urea or thiourea; a mild acidic component, such as phosphoric acid; and a fluorescent dye or surface functionalising molecule for the carbon nanoparticles (preferably red emitting, preferably water soluble) such as a fluorescent compound with a xanthene core (e.g. sulforhodamine B). Furthermore, the hydrothermal / solvothermal synthesis can be performed at a temperature in a range 150 to 250°C, e.g. in a sealed vessel, for a time period of between 15 minutes and 1 hour. The pressure for the hydrothermal / solvothermal synthesis can be at least 5 bar, at least 10 bar, and optionally no greater than 100 bar or no greater than 50 bar. Certain examples utilize a pressure in a range 10 to 20 bar, optionally 12 to 17 bar.

Alternatively, the hydrothermal / solvothermal synthesis of the luminescent carbon nanoparticle material can be considered to be based on the use of a molecular carbon source, which can include a combination of one or more of an organic acid, an amino acid, a peptide-bond containing compound (e.g. dipeptide or tripeptide), a fluorescent dye (e.g. a compound with a xanthene core), a urea or a thiourea analogue.

Specific solvothermal synthesis methods which have been used by the present inventors include, as a non-exhaustive list, the following:

(1) A 15% (w/v) citric acid solution in N-methylformamide was heated in a sealed reactor vessel in a microwave at 200°C for 30 minutes.

(2) A 15% (w/v) citric acid and urea solution in N-methylformamide was heated in a sealed reactor vessel in a microwave at 200°C for 30 minutes.

(3) A 15% (w/v) citric acid and thiourea solution in N-methylformamide was heated in a sealed reactor vessel in a microwave at 200°C for 30 minutes.

(4) To 15% (w/v) citric acid and urea solution in N-methylformamide, 0.1M phosphoric acid was added to make a solution of approximately pH 4. The solution was heated in a sealed reactor vessel in a microwave at 200°C for 30 minutes. (5) To 15% (w/v) citric acid and thiourea solution in N-methylformamide, 0.1M phosphoric acid was added to make a solution of approximately pH 4. The solution was heated in a sealed reactor vessel in a microwave at 200°C for 30 minutes.

(6) To 15% (w/v) citric acid and 15% (w/v) urea solution in N-methylformamide, phosphoric acid was added to make a 3% (w/v) solution. The solution was heated in a sealed reactor vessel in a microwave at 200°C for 30 minutes.

(7) To 15% (w/v) citric acid and 15% (w/v) thiourea solution in N-methylformamide, phosphoric acid was added to make a 3% (w/v) solution. The solution was heated in a sealed reactor vessel in a microwave at 200°C for 30 minutes.

(8) Citric acid (4.7 g), phosphoric acid (1.27 g), urea (2.26 g) and sulforhodamine B (1.4 g; sample 3-0.14 g) were dissolved in formamide (20 mL). The mixture was then placed into a microwaveable reactor which was then sealed. The reactor was heated in a microwave at 180°C for 30 minutes.

(9) Citric acid (4.7 g), phosphoric acid (1.27 g), urea (2.26 g) and sulforhodamine B (0.14 g) were dissolved in formamide (20 mL). The mixture was then placed into a microwaveable reactor which was then sealed. The reactor was heated in a microwave at 180°C for 30 minutes.

(10)A glutathione in formamide solution (14% (w/v)) was prepared. 20 mL of this was added to a microwaveable reactor which was then sealed. The reactor was heated in a microwave at 180°C for 30 minutes.

The aforementioned solvothermal synthesis techniques 1 to 7 have also been repeated using formamide in place of N-methylformamide. In the aforementioned solvothermal synthesis techniques, the product can be subsequently processed as follows.

After cooling to room temperature, the reaction mixture is transferred to a conical flask. 60 mL of acetone is added, and the mixture cooled using an ice bath. After 1 hour the mixture is centrifuged at 10000 RCF for 20 minutes. The supernatant is discarded. The pellet is dispersed in acetone (30 mL) and centrifuged at 10000 RCF for 20 minutes. These steps are repeated once more. The pellet is then dispersed in deionised water (100 mL). This is then vacuum filtered using a 1 kD membrane filter.

Such synthesis routes have been found to produce carbon nanoparticles which can exhibit enhanced luminescence intensity over extended time periods at elevated temperatures. Furthermore, these synthesis routes have been found to produce carbon nanoparticles which are red-emitting (i.e. emit a luminescence signal at wavelengths, or at least with a peak wavelength, greater than 500 nm or greater than 550 nm) in addition to improving the emission intensity of the luminescence signal over extended time periods at elevated temperatures. As such, these examples show that various fabrication chemistries can produce carbon nanoparticles with sufficient emission intensity and the correct wavelength. However, stability of the emission signal is still problematic using such synthesis routes. Hence, the further development of a post-synthesis treatment step to improve the stability of the emission signal from such materials over extended time periods at elevated temperatures.

In addition to the above, carbon nanoparticles can be functionalised to alter their thermal stability and/or chemical stability and water dispersibility. Examples include amidation which is achieved by initially forming doped carbon nanoparticles. Dopants can include, for example, N, P, and S. The carbon nanoparticles are then purified and dispersed in deionised water. Depending upon the type of functionalisation molecule for the amidation of the carbon nanoparticles, an appropriate pH is chosen. Heating may also be applied. Examples of suitable functionalisation molecules for the amidation include taurine and b-alanine. Functionalisation was achieved under basic conditions and heating was shown to accelerate or enhance the formation. There is a large range of carbon nanoparticles which may be suitable for functionalisation. There is also a large range of other functionalisation materials which may be used for the amidation. It has been found that functionalisation of the carbon nanoparticles to achieve amidation of the nanoparticles can improve emission intensity at elevated temperatures and extended time periods. Functionalisation can also improve chemical stability and water dispersibility.

In contrast to previously discussed hydrothermal / solvothermal synthesis techniques, acid oxidation has also been investigated as a way of producing luminescent carbon nanoparticles with enhanced emission intensity over extended time periods at elevated temperatures. This involves heating the carbon in either a concentrated nitric acid or a concentrated nitric/sulfuric acid mixture. Acid oxidation can either be used as a synthesis method to make fluorescent carbon nanoparticles from nonfluorescent carbon material, or as a post-synthesis treatment applied to fluorescent carbon nanoparticles (synthesised by another method) to improve their emission characteristics.

Acid oxidation involves heating the carbon in either aqueous nitric acid (30-70 wt% solution), or in a concentrated nitric acid/sulfuric acid solution (made by mixing 70% nitric acid and 97% sulfuric acid in different ratios). The nonfluorescent carbon materials are stirred in the acid at temperatures between 80-120°C for 6-24 hours to produce fluorescent carbon nanoparticles, while the fluorescent carbon nanoparticles are stirred in acid at room temperature - 100 °C for up to several days to improve emission characteristics.

The acid mixture is thought to have a number of different effects on the carbon. Firstly, it functionalises the carbon surface with carboxylic acid, epoxy and hydroxyl groups, making the nonfluorescent carbon water soluble. Other surface groups are oxidised. Secondly, it can also break down the carbon structure, leading to smaller carbon particles, which may have a higher quantum yield. The attack of the acid on the carbon structure is also thought to create surface defects, making previously nonfluorescent carbon fluorescent. Nitric acid is also thought to incorporate nitrogen into the carbon structure, modifying the particle to give a red shift in emission. While not being bound by theory, it is possible that the loss of fluorescence in other types of carbon nanoparticles is due to reactions and changes of the carbon nanoparticle surface on heating, including oxidation.

A number of more detailed examples are set out below to illustrate different aspects of the present invention. It should be noted that in the following examples the emission intensity/stability is measured at a given wavelength and is excited by the same wavelength each time a measurement is taken. As such, emission stability is a function of intensity and emission wavelength. For example, at least in theory an emission peak could shift in position, but keep the same intensity, which would make it less useful in tracing application than examples of the present invention. It should also be noted that examples of the present invention have been developed which achieve the ability to scale the fabrication of luminescent carbon nanoparticle material for industrial synthesis. This contrasts with prior art methods which are only suitable for synthesising small quantities (< 1 gram) of carbon nanoparticle material.

Examples

Acid Oxidation Fabrication Method (Sample 1)

Synthesis

60 mg Enasco Imerys powder (carbon black) was added to a solution of 8 mL sulphuric acid (97%) and 8 mL nitric acid (70%). The mixture was sonicated for 1.5 hours. The reaction flask was fitted with a reflux condenser and heated at 100°C for 24 hours. After cooling to room temperature, the reaction solution was added to a 250 mL conical flask. This was cooled using an ice bath. 30 mL deionised water was added dropwise. An aqueous solution sodium carbonate (1 g / mL) was added dropwise, until a pH of 7 was obtained. The neutralised mixture was then centrifuged at 8000 RCF for 20 minutes. The product was the supernatant solution.

Analysis

The carbon nanoparticle product material was dispersed in deionised water, exposed to temperatures of 60°C and 90°C for a period of 84 days, and the emission from the carbon nanoparticles was periodically measured during this time period. It should be noted that whilst the samples were exposed to stability testing at elevated temperatures, the temperature at which the analysis was undertaken was ambient. Figure 1 shows a plot illustrating the emission intensity with all data normalised to the emission intensity at day 0. As can be seen, emission intensity initially increases over the first 30 to 40 days before decreasing back to an intensity similar to the initial emission intensity by the end of the 84-day analysis period.

Results indicate that the emission intensity remains sufficiently high over an extended time period to reliably detect the presence of the luminescent carbon nanoparticles and be used as a qualitative tracer. However, due to the instability of the emission at elevated temperatures for extended time periods, it is difficult or impossible to use this carbon nanoparticle material as a quantitative or semi- quantitative tracer in hydrocarbon reservoir monitoring applications, at least without significant data correction and/or uncertainties in the results and conclusions.

Solvothermal Fabrication Method (Samples 2 and 3)

Synthesis

Citric acid (4.7 g), phosphoric acid (1.27 g), urea (2.26 g) and a xanthene containing compound e.g., sulforhodamine B (Sample 2-1.4 g; Sample 3-0.14 g) were dissolved in formamide (20 mL). The mixture was then placed into a microwaveable reactor which was then sealed. The reactor was heated in a microwave at 180°C for 30 minutes. After being cooled to room temperature the reaction mixture was transferred to a conical flask. 60 mL of acetone was added, and the mixture cooled using an ice bath. After 1 hour the mixture was centrifuged at 10000 RCF for 20 minutes.

The supernatant was discarded. The pellet was dispersed in acetone (30 mL) and centrifuged at 10000 RCF for 20 minutes. These steps were repeated once more. The pellet was then dispersed in deionised water (100 mL). This was then vacuum filtered using a 1 kD membrane filter.

Analysis

The carbon nanoparticle product material was dispersed in deionised water, exposed to temperatures of room temperature, 60°C, and 90°C for a period of 79 days, and the emission from the carbon nanoparticles was periodically measured during this time period. Figure 2 shows a plot illustrating the emission intensity with all data normalized to the emission intensity at day 0. As can be seen, emission intensity initially increases rapidly over the first 5 to 10 days in the heated samples, before stabilising for the remainder of the analysis period. In contrast, the emission from the samples kept at room temperature slowly drifts upwards in intensity over the analysis period.

Results indicate that the emission intensity remains sufficiently high over an extended time period to reliably detect the presence of the luminescent carbon nanoparticles and be used as a qualitative tracer. However, due to the instability of the emission intensity at elevated temperatures for extended time periods, it is difficult to use this carbon nanoparticle material as a quantitative or semi- quantitative tracer in hydrocarbon reservoir monitoring applications, particularly over the first 10 days of use, at least without significant data correction and/or uncertainties in the results and conclusions.

While the results in Figure 2 are for the carbon nanoparticle dispersed in deionised water, Figure 3 shows a plot illustrating the stability of carbon nanoparticle emission for Sample 2 when dispersed in produced water from a hydrocarbon reservoir. Samples were subjected to room temperature, 60°C, or 90°C conditions for a period of 61 days in the produced water. Results are similar to those in deionised water. That is, emission intensity initially increases over the first few days in the heated samples, before stabilising for the remainder of the analysis period. In contrast, the emission from the sample kept at room temperature initially decreased slightly before slowly drifting upwards in intensity and then stabilising over the remainder of the analysis period. It should be noted that the sample at 90°C was unfortunately compromised after 29 days, although the trend in emission characteristics is already demonstrated by that time.

Again, these results indicate that the emission intensity remains sufficiently high over an extended time period in produced water to reliably detect the presence of the luminescent carbon nanoparticles and be used as a qualitative tracer in hydrocarbon reservoir applications. However, due to the instability of the emission intensity at elevated temperatures for extended time periods, it is difficult to use this carbon nanoparticle material as a quantitative or semi-quantitative tracer in hydrocarbon reservoir monitoring applications, particularly over the first few days of use, at least without significant data correction and/or uncertainties in the results and conclusions.

Figure 4 shows a plot illustrating the stability of carbon nanoparticle emission for Sample 3 when dispersed in produced water from a hydrocarbon reservoir. Emission intensity of the heated samples initially increases, then decreases, prior to stabilising after about 20 days. Emission intensity for the unheated samples follows a similar trend.

Again, these results indicate that the emission intensity remains sufficiently high over an extended time period in produced water to reliably detect the presence of the luminescent carbon nanoparticles and be used as a qualitative tracer in hydrocarbon reservoir applications. However, due to the instability of the emission intensity at elevated temperatures for extended time periods, it is difficult to use this carbon nanoparticle material as a quantitative or semi-quantitative tracer in hydrocarbon reservoir monitoring applications, particularly over the first 20 days of use, at least without significant data correction and/or uncertainties in the results and conclusions.

Solvothermal Fabrication Method (Samples 2 and 3) with Post-Synthesis Thermal Treatment in Deionised Water

Fabrication

Luminescent carbon nanoparticle material was fabricated as previously described for Samples 2 and 3. The luminescent carbon nanoparticle material was than subjected to a post-synthesis thermal treatment by dispersing the samples in deionised water and heating to 90°C for 7 days.

Analysis

The thermally treated samples of carbon nanoparticle material dispersed in deionised water were exposed to a temperature of 90°C for a period of 72 days and the emission from the carbon nanoparticles was periodically measured during this time period. Figure 5 shows a plot illustrating the emission intensity with all data normalised to the emission intensity at day 0 of the analysis period. As can be seen, emission intensity remained within a few percent of the emission intensity at day 0 of the analysis period indicating a high degree of emission stability. Accordingly, this material is suitably adapted to enable quantitative or semi-quantitative analysis of hydrocarbon reservoir fluids over extended time periods with a higher degree of accuracy and without requiring complicated data processing to account for variations in emission response.

Solvothermal Fabrication Method (Samples 2 and 3) with Post-Synthesis Thermal Treatment in Produced Water from a Hydrocarbon Reservoir

Fabrication

Luminescent carbon nanoparticle material was fabricated as previously described for Samples 2 and 3. The luminescent carbon nanoparticle material was than subjected to a post-synthesis thermal treatment by dispersing the samples in produced water from a hydrocarbon reservoir and heating to 90°C for 5 days.

Analysis

The thermally treated samples of carbon nanoparticle material dispersed in produced water were exposed to a temperature of 90°C for a period of 24 days and the emission from the carbon nanoparticles was periodically measured during this time period. Figure 6 shows a plot illustrating the emission intensity with all data normalised to the emission intensity at day 0 of the analysis period. As can be seen, emission intensity remained within a few percent of the emission intensity at day 0 of the analysis period indicating a high degree of emission stability. Accordingly, this material is suitably adapted to enable quantitative or semi-quantitative analysis of hydrocarbon reservoir fluids over extended time periods with a higher degree of accuracy and without requiring complicated data processing to account for variations in emission response.

Figure 7 shows a plot illustrating the stability of carbon nanoparticle emission for Sample 3 synthesised and treated as per Figure 6 but subjected to 90°C temperature conditions in produced water for an extended period of 56 days. The results confirm that emission remains stable for this longer time period indicating that this material is suitable for quantitative or semi-quantitative analysis of hydrocarbon reservoir fluids over this extended time period.

Solvothermal Fabrication Method (Sample 4¾

Synthesis

A glutathione in formamide solution (14% (w/v)) was prepared. 20 mL of this was added to a microwaveable reactor which was then sealed. The reactor was heated in a microwave at 180°C for 30 minutes. After being cooled to room temperature the reaction mixture was transferred to a conical flask. 60 mL of acetone was added, and the mixture cooled using an ice bath. After 1 hour the mixture was centrifuged at 10000 RCF for 20 minutes.

Analysis

The carbon nanoparticle product material was dispersed in produced water, exposed to temperatures of room temperature, 60°C, and 90°C for a period of 42 days, and the emission from the carbon nanoparticles was periodically measured during this time period. Figure 8 shows a plot illustrating the emission intensity with all data normalised to the emission intensity at day 0. As can be seen, emission intensity initially increases rapidly over the first few days in the heated samples, before stabilising and drifting slowly downwards for the remainder of the analysis period. In contrast, the emission from the sample kept at room temperature increases over the first 7 days, then drops over the subsequent period, before slowly drifting upwards in intensity over the remainder of the analysis period.

Results indicate that the emission intensity remains sufficiently high over an extended time period to reliably detect the presence of the luminescent carbon nanoparticles and be used as a qualitative tracer. However, due to the instability of the emission intensity, particularly at elevated temperatures, it is difficult to use this carbon nanoparticle material as a quantitative or semi-quantitative tracer in hydrocarbon reservoir monitoring applications at least without significant data correction and/or uncertainties in the results and conclusions.

Solvothermal Fabrication Method (Sample 4) with Post-Synthesis Thermal Treatment in Produced Water from a Hydrocarbon Reservoir

Fabrication

Luminescent carbon nanoparticle material was fabricated as previously described for Sample 4. The luminescent carbon nanoparticle material was than subjected to a post-synthesis thermal treatment by dispersing the samples in produced water from a hydrocarbon reservoir and heating to 90°C for 7 days.

Analysis

The thermally treated sample of carbon nanoparticle material dispersed in produced water was exposed to a temperature of 90°C for a period of 35 days and the emission from the carbon nanoparticles was periodically measured during this time period. Figure 9 shows a plot illustrating the emission intensity with all data normalised to the emission intensity at day 0 of the analysis period. As can be seen, although there is a slight drift downwards in emission intensity, the emission intensity remained within a few percent of the emission intensity at day 0 of the analysis period indicating a high degree of emission stability. Accordingly, this material is suitably adapted to enable quantitative or semi-quantitative analysis of hydrocarbon reservoir fluids over extended time periods with a higher degree of accuracy and without requiring complicated data processing to account for variations in emission response.

Solvothermal Fabrication Method (Samples 5 and 6)

Synthesis

Citric acid (4.7 g), urea (2.3 g), formamide (Sample 5 = 0 mL, Sample 6 = 0.97 mL) and a xanthene containing compound e.g., sulforhodamine B (0.2 g) were dissolved in ethanol (20 mL). The mixture was then placed into a microwaveable reactor which was then sealed. The reactor was heated in a microwave at 180°C for 30 minutes. After being cooled to room temperature the reaction mixture was transferred to a round-bottomed flask and evaporated to dryness.

The solid was dispersed in deionised water (50 mL). After 10 minutes, this was then vacuum filtered using a 1 kD membrane filter.

Analysis

The carbon nanoparticle product materials were separately dispersed into deionised water, produced water and brine and exposed to temperatures of room temperature and 90°C for a period of 92 days, and the emission from the carbon nanoparticles was periodically measured during this time period. Figure 10 shows a plot illustrating the emission intensity with all data normalized to the emission intensity at day 0, for Sample 5. As can be seen, emission intensity is unstable up to day 12, with some samples showing both an increase and decrease in intensity during that period. From day 12 to day 30 there is a steady increase in intensity with intensity stabilising after that point to within ±5 % after that point, up until day 92.

Figure 11 shows a plot illustrating the emission intensity with all data normalized to the emission intensity at day 0, for Sample 6. As can be seen, emission intensity is unstable up to day 12, with samples showing both an increase and decrease in intensity during that period. From day 12 to day 47 there is a steady increase in intensity with intensity stabilising after that point to within ±5 % after that point, up until day 92.

Summary

Luminescent carbon nanoparticle tracers as described herein have been engineered for reservoir tracer applications in which the tracers experience elevated temperatures for days, weeks, months, or years before being analysed. A tracer is injected into a well, or placed in/on proppant beads, or placed into an article which allows controlled release. The tracer may spend days, weeks, or months moving through the reservoir underground before it is analysed. Underground, the tracer will experience a range of conditions, including elevated temperature. The luminescent carbon nanoparticle tracers as described herein are quantified by measuring the luminescence/fluorescence intensity, therefore this must remain constant, even when exposed to high temperatures for long periods of time. Many types of luminescent carbon-based nanoparticles have been synthesised from different starting materials by several different methods. Flowever, most are very unstable, with the fluorescence intensity falling to <60% of its initial value after 2 weeks at 90°C. In contrast, the luminescent carbon-based nanoparticles as described herein do not show a catastrophic drop in fluorescence intensity and in some instances show an increase in fluorescence intensity, at least over a certain period of use. Further improvements have been made by providing methods of achieving very stable emission frequency/intensity over extended time periods and elevated temperatures which neither decreases nor increases significantly. Such nanoparticles are suitable for reservoir tracing and can be useful for quantitative or semi-quantitative analysis of produced fluids. As such, these nanoparticle tracers can provide a new class of tracer, with the amount of tracer in produced water quantified by fluorescence intensity. The tracers can be used to enable on-site tracer analysis, which can also be developed further to allow on-line analysis. It is also envisaged that the luminescent carbon nanoparticles may be useful for other applications where thermal stability over extended time periods is required or advantageous.

One final issue which the present inventors have noted is that use of certain prior art luminescent compounds, e.g. certain rhodamine compounds, such as rhodamine WT, on their own as reservoir tracers, i.e. not incorporated into a nanoparticle, can be adsorbed onto the rock surrounding the wellbore and are not thermally stable within the wellbore for prolonged periods. Consequently, excessive amounts of these rhodamine compounds are required for tracing, which can cause issues of discolouration of produced fluids and some other environmental issues.

By binding a fluorescent compound to a nanoparticle carrier, potential problems of adsorption and instability may be avoided, enabling a larger range of luminescent compounds to be utilized. Following on from this, the present inventors also consider that modifying certain luminescent compounds to increase their solubility, e.g. by functionalizing with one or more solubilizing groups, can also reduce or eliminate the adsorption and stability issues. In this regard, the present inventors consider that using sulforhodamine B, rather than a less soluble rhodamine compound, will increase the tracer's stability, increase water solubility, reduce adsorption and eliminate the need to use excessive amounts fortracing. As such, while the focus of the appended claims is on carbon nanoparticle tracers, it is envisaged that sulforhodamine B may be used on its own as a tracer without requiring it to be combined with a nanoparticulate material. Other rhodamine derivatives which include one or more solubilizing substituents, e.g. one or more sulfonic acid groups, may also be useful in this respect.

While this invention has been particularly shown and described with reference to certain examples, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

Claims

1. A luminescent carbon-based nanoparticle tracer material for monitoring a hydrocarbon reservoir, the luminescent carbon-based nanoparticle tracer material comprising a plurality of luminescent carbon-based nanoparticles, each carbon-based nanoparticle comprising: a carbon-based core having a diameter in a range 1 to 100 nm; and a surface functionalised for dispersion of the luminescent carbon-based nanoparticles within an aqueous hydrocarbon reservoir fluid, wherein the luminescent carbon-based nanoparticles are fabricated to emit a luminescence signal at an emission wavelength which remains within 85 to 115% of its initial intensity at said emission wavelength when dispersed within an aqueous fluid for a time period between 20 and 70 days at a temperature in a range 60°C to 90°C.

2. A luminescent carbon-based nanoparticle tracer material according to claim 1, wherein the luminescence signal remains within 90 to 110% of its initial intensity.

3. A luminescent carbon-based nanoparticle tracer material according to any preceding claim, wherein the time period is at least 30, 40, 50 or 60 days.

4. A luminescent carbon-based nanoparticle tracer material according to any preceding claim, wherein the temperature is at least 70, 80 or 90°C.

5. A luminescent carbon-based nanoparticle tracer material according to any preceding claim, wherein the luminescent nanoparticles are fabricated to emit the luminescence signal with an emission maximum greater than 500 nm.

6. A luminescent carbon-based nanoparticle tracer material according to any preceding claim, wherein the luminescence signal has a full-width-half-maximum of less than 100 nm.

7. Use of the luminescent carbon-based nanoparticle tracer material according to any preceding claim for monitoring a hydrocarbon reservoir.

8. A method of monitoring a hydrocarbon reservoir using the luminescent carbon-based nanoparticle tracer material according to any one of claims 1 to 6, the method comprising: introducing the luminescent carbon-based nanoparticle tracer material into a hydrocarbon well, pipeline or formation; producing an aqueous fluid from the hydrocarbon well, pipeline or formation; and analysing the aqueous fluid to detect a luminescence signal from the luminescent carbon- based nanoparticle tracer material.

9. A method according to claim 8, wherein the luminescent carbon-based nanoparticle tracer material is introduced into the hydrocarbon well, pipeline or formation by one or more of: injecting an aqueous suspension of the luminescent carbon-based nanoparticle tracer material into the hydrocarbon well, pipeline or formation; adsorbing the luminescent carbon-based nanoparticle tracer material onto or into a proppant which is introduced into the hydrocarbon well, pipeline or formation; and forming a solid matrix in which the luminescent carbon-based nanoparticle tracer material is dispersed and introducing the solid matrix into the hydrocarbon well, pipeline or formation.

10. A method of fabricating the luminescent carbon-based nanoparticle tracer material according to any one of claims 1 to 6, the method comprising: synthesising the luminescent carbon-based nanoparticle tracer material using a hydrothermal or solvothermal process at a temperature above of 100°C and a pressure above 1 atmosphere; and applying a post-synthesis thermal treatment to the luminescent carbon-based nanoparticle tracer material to increase the stability of the luminescence signal from the luminescent carbon-based nanoparticle tracer material.

11. A method according to claim 10, wherein the post-synthesis thermal treatment comprises dispersing the luminescent carbon- based nanoparticle tracer material in an aqueous liquid and heating the aqueous liquid at a temperature between 50 and 100°C for at least 3 days.

12. A method according to claim 10 or 11, wherein the post synthesis thermal treatment is at atmospheric pressure.

13. A method according to any one of claims 10 to 12, wherein the hydrothermal or solvothermal synthesis of the luminescent carbon-based nanoparticle tracer material uses a molecular carbon source in a formamide-based solvent.

14. A method according to any one of claims 10 to 13, wherein the hydrothermal or solvothermal synthesis of the luminescent carbon-based nanoparticle tracer material uses one or more of: an organic acid; citric acid; an amino acid; a peptide- bond containing compound; a fluorescent dye; a urea; or a thiourea.

15. A method according to any one of claims 10 to 14, wherein the hydrothermal or solvothermal synthesis is performed at a temperature in a range 150 to 250°C for a time period of between 15 minutes and 1 hour.