Method for monitoring the concentration of a chemical compound in a fluid over time, using an optical measuring system and a temperature sensor

Abstract

The invention relates to a method for determining a change over time in the concentration of a chemical compound in a fluid, using an optical measuring system and a temperature sensor. For each predefined temperature of a plurality of predefined temperatures, an intermediate model (MTc1, MTc5) of the change in absorbance as a function of concentration is constructed by means of linear regression and from measurements of absorption spectra carried out for a plurality of samples of the fluid at distinct concentrations of the chemical compound. From the leading coefficients and the ordinates at the origin of each intermediate model (MTc1, MTc5), a model of the change in absorbance as a function of concentration and temperature is determined by linear regression.Then, a time evolution of a concentration of the chemical compound is determined using the model thus determined and from a measurement of an absorption spectrum and a temperature. Figure 4b to be published.

Description

Title of the invention: Method for monitoring over time the concentration of a chemical compound in a fluid, using an optical measuring system and a temperature sensor Technical field

[0001] The present invention relates to the field of monitoring the evolution over time of a concentration of a chemical compound in a fluid circulating in a medium, by means of an absorbance measurement as a function of the wavelength and a measurement of the temperature of the fluid. In particular, the present invention finds a particular application for monitoring the evolution over time of the concentration of a chemical compound in a fluid in the case where the temperature of the fluid is variable over time.

[0002] For example, in the field of geosciences (geothermal energy, CO2 storage, enhanced oil recovery, etc.), laboratory measurements are often carried out to measure a chemical compound present in a fluid circulating in a rock sample, in order to identify any interactions / exchanges that have taken place between the fluid and the porous medium. Such studies aim to study and understand the physical and geochemical phenomena involved in the rock.

[0003] Until recently, the dosages (of salts, surfactants, minerals, oils, polymers, etc.) were carried out by collecting the effluents in tubes (using fraction collectors) which were then analyzed manually and in a time-delayed manner. This method has several disadvantages: - Time-consuming analysis (manual measurement); - Risk of aging / alteration of solutions before dosing; - Averaged measurements on large sample volumes often required by the measurement technique; - Delayed measurement result, not allowing real-time adaptation of the measurement experience if necessary; - Low measurement frequency.

[0004] Monitoring the concentration of a chemical compound is increasingly carried out using spectrometers equipped with cells suitable for online measurement (i.e. continuous, or even in real time) and which can cover different types of analysis (dosage, monitoring of reaction kinetics, turbidity, etc.) of solutions of interest for geoscience themes (for example, interaction of salts, minerals, surfactants, and / or polymers with rock).

[0005] In particular, ultraviolet-visible spectroscopy (often called spectroscopy UV-VIS) is one of the most widely used methods for characterizing fluids and measuring the chemical species present. UV-Vis spectrometry is a spectroscopy technique using photons with wavelengths in the ultraviolet (200 nm - 400 nm) and visible (400 nm - 800 nm) ranges. When exposed to radiation in this wavelength range, absorbing molecules undergo an electronic transition. Measuring the absorbance (or optical density) due to this transition for each wavelength provides the UV-Vis spectrum of the solution, which is defined as the variation in absorbance as a function of different wavelengths.

[0006] The analysis of this absorption spectrum makes it possible to access qualitative information, by detecting the presence of certain substances but above all quantitative information by determining the concentration of the absorbing species. Prior art

[0007] The following document will be cited during the description:

[0008] Malik, M., Chan, KH, & Azimi, G. (2021). Quantification of nickel, cobalt, and manganese concentration using ultraviolet-visible spectroscopy. RSC Advances, 11 (45), 28014-28028.

[0009] Among the quantitative analysis methods, we know methods based on the Beer-Lambert law. The Beer-Lambert law is an empirical relationship which establishes that, at a given wavelength X, the absorbance Aa of a solution is proportional to the concentration c of the absorbing species, as well as to the length of the path or optical path d (distance over which the light crosses the sample). More precisely, for a clear solution containing a single absorbing species this law is written:

[0010] ^-c(l)

[0011] where is the extinction coefficient for the wavelength considered. This coefficient does not depend on the concentration of the solution or the thickness crossed by the light; on the other hand, it depends on the nature of the solution (absorbent species and solvent), and on the temperature.

[0012] Under ambient conditions (temperature around 20°C), the effect of temperature is often neglected. But generally, at a given temperature T, the spectrum of a solution containing a single species that absorbs in the wavelength range considered has a maximum absorbance Amax (T) at a maximum wavelength Xmax (T). The pair (Xmax; Amax) characterizes the absorbing chemical species at temperature T. Classically, for this type of single-component solutions (i.e. comprising only a single chemical compound that responds in the UV-Vis), to determine the concentration of a given compound, the wavelength chosen is the wavelength Xmax of the peak Amax of the absorption spectrum (see for example example the document Malik et al., 2021). This choice certainly makes it possible to minimize the uncertainty on the absorbance, but due to the logarithmic law linking absorbance to light intensity, stray lights and other diffusion and fluorescence phenomena, it considerably limits the dynamics of the measurement (measurable concentration range).

[0013] Furthermore, a saturation phenomenon is observed beyond a critical concentration whose value depends on the compound to be analyzed. More precisely, beyond a critical concentration, the absorbance no longer evolves in a linear manner with respect to the concentration and tends towards the same maximum value regardless of the concentration in place, making it impossible to differentiate the concentrations of solutions whose concentrations exceed this critical value. In other words, beyond a critical concentration, the absorbance saturates and becomes independent of the concentration.

[0014] Thus, the use of this analysis method based on the Beer-Lambert law is classically restricted to solutions of low concentrations, which is very limiting. For high concentrations, a dilution can be carried out beforehand, but this step, although it remains possible for one-off measurements, considerably complicates the measurement procedure when it involves an online assay where thousands of measurements must be carried out on often unknown concentrations. Added to this is the fact that dilution is an additional source of error in the measurement.

[0015] Thus, this analysis method based on the Beer-Lambert law is limited to very dilute single-component solutions, when used online, for two reasons: - reduced measurement dynamics (concentration range) due to the logarithmic law. - need to automate data processing for online analysis. Indeed, hundreds or even thousands of spectra often need to be analyzed.

[0016] Patent application FR 22 / 01215 (filing number) is known in particular, which relates to a method for determining a change over time in a concentration of a chemical compound in a fluid, by means of an optical measuring system. More precisely, in this method, a model of the change in absorbance as a function of concentration is constructed in the following manner: a plurality of absorption spectra of the chemical compound are measured, each corresponding to a concentration of the chemical compound, a curve is defined intersecting each of the absorption spectra at a single point and such that the curve is a bijective function of the absorbance and the wavelength, the model of the change in absorbance as a function of concentration is constructed from the absorbance values at intersection points, and the concentration relative to each absorption spectrum. Then, a change in time of a concentration of the chemical compound is determined using the model thus determined. However, this method does not allow monitoring over time of the concentration of the fluid when the temperature of the fluid is variable over time. Indeed, for this method, the model is constructed at a given temperature, and can only be applied in the case where the temperature of the fluid remains constant over time.

[0017] The present invention makes it possible to overcome these drawbacks. In particular, the present invention relates to a method for monitoring over time the evolution of a concentration of a chemical compound in a fluid, by means of an optical measurement system and a temperature sensor. More specifically, the present invention relates to a method for analyzing absorption spectra that is valid for a wide range of concentrations and / or a wide range of temperatures of the fluid, while allowing real-time application. In particular, the present invention does not require the use of dilution, but nevertheless allows a significant broadening of the measurement range, without compromising the accuracy of the measurement. In addition, the present invention makes it possible to monitor the concentration of a chemical compound in a fluid over time, even in the event of variations in the temperature of the fluid over time. Summary of the invention

[0018] The present invention relates to a method for determining a change over time in a concentration of a chemical compound in a fluid circulating in a measurement zone, by means of at least one optical measurement system for measuring an absorbance as a function of a wavelength of said fluid and a temperature sensor for measuring a temperature of said fluid, said chemical compound of said fluid being the only chemical compound of said fluid or the only chemical compound of said fluid whose concentration varies or the only chemical compound of said fluid having a non-zero absorbance in a range of wavelengths absorbed by said chemical compound, said method comprising at least the following steps:

[0019] A) for each predefined temperature of a plurality of predefined temperatures, an intermediate model of the evolution of said absorbance as a function of said concentration for said predefined temperature is constructed in the following manner:

[0020] i) by means of at least said optical measuring system, an absorbance is measured as a function of the wavelength for a plurality of samples of said fluid at said predefined temperature and having distinct concentrations of said chemical compound, and a first plurality of absorption spectra relating to said chemical compound are obtained, each corresponding to one of said concentrations of said chemical compound;

[0021] ii) a curve is defined intersecting each of said absorption spectra of said first plurality of absorption spectra at a single point of intersection and such that said curve is a bijective function of said absorbance and said wavelength;

[0022] iii) constructing said intermediate model of the evolution of said absorbance as a function of said concentration for said predefined temperature by means of a linear regression applied to first absorbance values at said points of intersection between said curve and each of said absorption spectra of said first plurality of absorption spectra, and at said concentration corresponding to each of said absorption spectra of said first plurality of absorption spectra;

[0023] B) from direction coefficients and origin ordinates determined for each of said intermediate models of the evolution of said absorbance as a function of said concentration constructed for each of said predefined temperatures, a first function representative of a variation of said direction coefficient as a function of said temperature and a second function representative of a variation of said origin ordinate as a function of said temperature are determined by linear regression, and said model of the evolution of said absorbance as a function of said concentration and said temperature is constructed according to a formula of the type: A = Ma ( T ) Jn ( C ) + MHT)

[0024] where T is said temperature of said fluid and C is said concentration of said fluid;

[0025] C) determining an evolution over time of a concentration of said chemical compound in said fluid circulating in said measurement zone in the following manner:

[0026] a) by means of at least said optical measuring system and said temperature sensor, an absorbance as a function of the wavelength and a temperature are measured in said measuring zone respectively for a succession of time steps and a second plurality of absorption spectra relating to said chemical compound are obtained, each corresponding to a time step as well as a temperature measured for each time step; and

[0027] b) for each of said absorption spectra of said second plurality of absorption spectra, a second absorbance value is determined at the intersection between said curve and said absorption spectrum, and, by means of said model of the evolution of said absorbance as a function of said concentration and said measured temperature, and from said second absorbance value and said measured temperature, said concentration of said chemical compound is deduced for said time step.

[0028] According to one implementation of the invention, said curve may be a straight line.

[0029] According to one implementation of the invention, said optical measuring system can include at least one light source to emit radiation into at least one at least in a predetermined wavelength range, and a spectrometer for measuring a light intensity of said radiation transmitted through said measuring area at least in said predefined wavelength range.

[0030] According to one implementation of the invention, said optical measuring system may further comprise at least one measuring cell connected to said light source and to said spectrometer, in which said fluid may be located.

[0031] According to an implementation of the invention, when said chemical compound is the only chemical compound of said fluid of which said concentration varies and in the absence of a step of calibrating said optical measurement system by means of a reference fluid corresponding to said fluid to the exclusion of said chemical compound, a pre-processing step can be applied to the absorbance measurements as a function of the wavelength to determine an absorption spectrum of said chemical compound, comprising at least one subtraction of said absorption spectrum of said additional chemical compound previously recorded.

[0032] According to one implementation of the invention, said measuring zone can be arranged downstream of a porous medium, such as a sample of a rock from an underground formation, in which said fluid circulates.

[0033] According to one implementation of the invention, said chemical compound can be chosen from the following list: a surfactant, a salt, a hydrocarbon compound, a polymer.

[0034] According to an implementation of the invention, said method can be implemented by means of a fluid circulation system for circulating said fluid at least in said porous medium and said measurement zone, said fluid circulation system comprising a pump, preferably a pump capable of delivering a flow rate with high precision, a sample cell in which said porous medium is arranged, and preferably a thermostatically controlled bath or an oven for controlling the temperature of said fluid in said measurement zone.

[0035] The invention further relates to a system for determining a change over time in a concentration of a chemical compound in a fluid, said system comprising a light source, a spectrometer, and means for processing and analyzing measurements made by said spectrometer, said system being capable of implementing the method as described above.

[0036] The invention further relates to a computer program product downloadable from a communications network and / or recorded on a computer-readable medium and / or executable by a processor, comprising program code instructions for implementing steps ii, and / or iii), and / or B), and / or b), of the method as described above, when said program is executed on a computer.

[0037] Other characteristics and advantages of the method and system according to the invention, ap will appear on reading the following description of non-limiting examples of embodiments, with reference to the figures appended and described below. List of figures [Fig 1]

[0038] [Fig.l] schematically presents an embodiment of the optical measuring system and a temperature sensor capable of implementing the method according to the invention. [Fig 2a]

[0039] [Fig.2a] schematically presents an embodiment of an optical measuring system, a temperature sensor and fluid circulation means suitable for implementing the method according to its main variant. [Fig 2b]

[0040] [Fig.2b] schematically presents another embodiment of an optical measuring system, a temperature sensor and fluid circulation means suitable for implementing the method according to its main variant. [Fig 2c]

[0041] [Fig.2c] shows an embodiment of a measuring cell that can be used for the embodiments of [Fig.2a] or [Fig.2b]. [Fig 3]

[0042] [Fig. 3] illustrates, for a first example of application, a plurality of absorption spectra measured at a predefined temperature during step 1.1) of the method according to the invention described below, as well as the curve resulting from step 1.2) of the method according to the invention described below. [Fig 4a]

[0043] [Fig.4a] illustrates, for the application example of [Fig.3], an intermediate model of the evolution of the absorbance as a function of the concentration constructed during the application of step 1.3) of the method according to the invention described below at the predefined temperature of [Fig.3]. [Fig 4b]

[0044] [Fig.4b] illustrates, for the application example of [Fig.3], five intermediate models of the evolution of absorbance as a function of concentration constructed by repeating steps 1.1) to 1.3) of the method according to the invention described below, applied to five predefined temperatures. [Fig 4c]

[0045] [Fig.4c] illustrates, for the application example of [Fig.3], the evolution of the direction coefficients and the ordinates at the origin as a function of the temperature determined from the five intermediate models of [Fig.4b]. [Fig 5]

[0046] [Fig.5] illustrates, for the application example of [Fig.3], a plurality of absorption spectra measured during step 3.1) of the method according to the invention described below, as well as the curve resulting from step 1.2) of the method according to the invention. [Fig 6]

[0047] [Fig.6] illustrates, for the application example of [Fig.3], a curve representing the evolution over time of the concentration of a chemical compound, resulting from step 3.2) of the method according to the invention described below. [Fig 7]

[0048] [Fig.7] illustrates a plurality of absorption spectra measured during step 1.1) of the method according to the invention described below, applied to a second application example. [Fig 8]

[0049] [Fig.8] illustrates, for the application example of [Fig.7], a curve representing the evolution over time of the concentration of a chemical compound, resulting from step 3.2) of the method according to the invention described below. Description of the embodiments

[0050] The invention relates to a method for measuring a change over time in a concentration of a chemical compound in a fluid circulating in a measurement zone, by means of at least one optical measuring system for measuring an absorbance as a function of the wavelength and a temperature sensor for measuring a temperature of the fluid.

[0051] According to a main variant of the invention which can find its application in the field of geosciences, the measurement zone can be located downstream of a sample of a porous medium in which the fluid comprising the chemical compound of interest is circulated. In particular, the sample of porous medium can be a sample of a rock from an underground formation, for example taken by coring. The method according to the invention can then have the objective of monitoring the evolution of the concentration of the chemical compound of interest at the outlet of the sample of porous medium into which a solution comprising the chemical compound is injected at a known concentration, in order to understand the fluid / rock interactions.According to one implementation, the method can be used to follow the evolution over time of a chemical reaction, by following the evolution of the concentration of a chemical compound of a solution following the addition of a reagent upstream of the measurement zone. A particular application can be the determination of the pH of a solution which does not respond in the UV, by using a specific reagent responding in the UV.

[0052] Alternatively, the measurement zone may also be a portion of the ambient air of a confined or semi-confined space so as to monitor the evolution of a pollutant concentration. It is clear that these confined or semi-confined spaces must be temperature-controlled.

[0053] According to the invention, the chemical compound whose concentration evolution over time in the fluid of interest is sought to be monitored is either: - the unique chemical compound of the fluid of interest: in other words, the fluid considered only comprises a single chemical compound (we then speak of a single-component fluid); or - the only chemical compound of the fluid of interest having a non-(substantially) zero absorbance in the range of wavelengths absorbed by the chemical compound considered: in other words, the fluid considered may include several chemical compounds (we then speak of a multi-component fluid), but such that the absorption spectrum of the chemical compound of interest is dissociated from the absorption spectra of the other chemical compounds (we will subsequently speak of additional chemical compounds). By "substantially" we mean "at least to the measurement error". By "range of wavelengths absorbed by the chemical compound" we mean the range of wavelengths for which the absorbance of the chemical compound is non-zero.In other words, for this alternative, the absorption spectra of the additional chemical compounds exhibit (substantially) zero absorbance values for the wavelength values for which the absorption spectrum of the chemical compound of interest exhibits non-(substantially) zero absorbance values; or . - the single chemical compound of the fluid of interest whose concentration varies: in other words, here again, the fluid considered may include several chemical compounds, and the absorption spectra of these additional chemical compounds may overlap at least in part with the absorption spectrum of the chemical compound of interest, but their concentration is invariant over time.

[0054] In the first two alternatives described above (single-component fluid and multi-component fluid with dissociated absorption spectra), no pre-processing of the measured absorption spectrum is necessary since the absorption spectrum of the chemical compound of interest is directly accessible.

[0055] In the third alternative (multi-component fluid with absorption spectra that can overlap, and such that the concentration of the additional compounds is invariant over time), a pre-treatment step can be applied to the measured absorption spectrum, as will be described below, so as to dissociate the absorption spectrum of the chemical compound of interest from the measured absorption spectrum. Indeed, the absorption spectrum measured in the case of a multi-component fluid corresponds to the sum of the absorption spectra of all the chemical compounds in the fluid, according to the law of additivity of absorbances.

[0056] According to an implementation of the main variant of the invention, the chemical compound of interest can be chosen from the following list: a surfactant, a salt, a hydrocarbon compound, a polymer. These are in fact chemical compounds of interest in geosciences, in particular in geothermal energy, for the storage of CO2, or for enhanced oil recovery. These chemical compounds are often part of fluids injected into an underground formation, whether to store them (CO2, natural gas, etc.), or as a sweeping fluid to recover an energy resource (hydrocarbons, heat, etc.) present in the porous underground formation. Knowledge of the evolution over time of the concentration of these chemical compounds makes it possible to identify any interactions / exchanges that have taken place between the injected fluid and the porous medium.

[0057] According to one implementation of the invention, the optical measuring system may comprise: - a light source for emitting radiation at least in a wavelength range of interest for the chemical compound of interest. By wavelength range of interest is meant a wavelength range in which the characteristic absorption spectrum of the chemical compound of interest has non-zero absorbance values. According to an implementation of the main variant of the invention according to which the chemical compound of interest is a surfactant, a salt, a hydrocarbon compound, or a polymer, the wavelength range of the light source may be the ultraviolet wavelength range (denoted UV), and - a spectrometer for measuring light intensity as a function of wavelength in at least the wavelength range of interest for the chemical compound of interest.

[0058] [Fig.l] schematically shows an embodiment of the optical measuring system SMO according to the invention, comprising a light source SL for emitting radiation RE, in a measuring zone ZM comprising a fluid FL, and a spectrometer SP for measuring the absorbance as a function of the wavelength of the radiation RT having passed through the fluid FL in the measuring zone ZM along an optical path of length d. This figure also schematically shows a temperature sensor ST for measuring the temperature of the fluid FL in the measuring zone ZM.

[0059] According to an exemplary implementation of the invention, the light source may be a halogen-deuterium source, such as the AvaLight-DH-S-BAL model from Avantes (Netherlands) and / or the spectrometer may be a high-sensitivity fiber spectrometer, such as the AvaSpec-HS2048XL-EVO model from Avantes (Netherlands), or any similar light source.

[0060] Advantageously, the optical measuring system may comprise a measuring cell connected upstream to the light source and downstream (the upstream and downstream directions being considered relative to the radiation emitted by the light source) to the spectrometer, and in which is located the fluid for which an absorbance is to be measured as a function of the wavelength. According to one implementation, the measuring cell may comprise two inlets and two outlets, to allow the connection with the light source and the spectrometer, but also with a fluid circulation system described below.

[0061] Advantageously, the optical measurement system may comprise means for processing and analyzing the light intensity measured by the spectrometer, in order to determine an absorption spectrum from the measured light intensity. According to an exemplary implementation, the means for processing and analyzing the light intensity measured by the spectrometer may comprise a computer on which the Avasoft software from the company Avantes (Netherlands) is installed to determine the absorption spectra from the measured light intensity. Advantageously, the optical measurement system may comprise 400 pm core optical fibers to connect the light source to the measuring cell, and the measuring cell to the spectrometer.Advantageously, the optical measuring system may further comprise means for the transmission (for example by electrical wire, by optical fiber or by a wireless communication system) of the measurements carried out by the spectrometer to the means for the processing and analysis of the light intensity.

[0062] According to one implementation of the invention, the method may comprise a step, possibly prior, of calibrating the optical measurement system. Such a calibration step may comprise a measurement of a light intensity as a function of the wavelength transmitted through a reference fluid. According to an implementation according to which the fluid of interest is a single-component liquid, purified water may be used as the reference fluid. According to the alternatives according to which the fluid of interest comprises, in addition to the chemical compound of interest, additional chemical compounds as defined above, the reference fluid may contain only the additional chemical elements, present according to their respective concentrations in the fluid of interest. According to one embodiment of this implementation of the invention, the calibration step may comprise the following steps:

[0063] - the emission by the light source of radiation through the fluid of reference within a measurement area;

[0064]

[0065]

[0066]

[0067]

[0068]

[0069]

[0070]

[0071] - the detection by the spectrometer of the radiation having passed through the reference fluid in the measurement zone and the generation of a light intensity as a function of the wavelength of the radiation having passed through the reference fluid. From this calibration, the absorbance A of a fluid can be determined according to a formula of the type: -ln(— where X is the wavelength, 7S(À) is the light intensity as a function of the wavelength of the radiation transmitted through the fluid considered, and I0(À ) is the light intensity as a function of the wavelength of the radiation transmitted through the reference fluid. It is clear that for the application of step 1) described below, the preliminary step of calibrating the optical measuring system is carried out for the plurality of predefined temperatures of step 1). According to at least one implementation of the main variant of the invention, the method can be further implemented by means of a fluid circulation system comprising: - a pump, preferably a pump capable of delivering a flow rate with high precision (such as for example a pump for high-performance liquid chromatography, known as HPLC pumps), preferably capable of delivering a flow rate in the range 0-2 ml / min; - a sample cell, in which the porous medium is placed, and, optionally, - a pressure regulator allowing the line pressure to be imposed throughout the fluid circuit; and / or - a thermostatically controlled bath or oven, for controlling the temperature of the fluid in the measurement zone. Advantageously, the fluid circulation system in the porous medium may further comprise: - a flow meter to control the pump flow; and / or - a differential pressure sensor for measuring the pressure drop along the porous medium; and / or - means of connection and control of the fluid, such as valves, pipes; and / or - an additional temperature sensor (in addition to the temperature sensor according to the invention); and / or - a bypass, for example in the form of a pipe, allowing the porous medium to be bypassed; and / or - means for processing and analyzing the flow rate, and / or pressure and / or temperature measurements. Advantageously, the means for processing and analyzing the flow rate and / or pressure and / or temperature measurements may be the same as the means for processing and analyzing the light intensity measurements made by means of any embodiment of the optical system.

[0072] In a preferred implementation of the method according to the invention, the measuring cell of the optical measuring system, the sample holder, and optionally the bypass of the fluid circulation system, can be immersed in a thermostatically controlled bath or an oven making it possible to control the temperature of the fluid in the measuring zone. The use of an oven or a thermostatically controlled bath in which the measuring cell and the rock are stored makes it possible to ensure that the fluid in the measuring cell has the same temperature as that circulating in the rock. Alternatives can be implemented, such as for example controlling the temperature of the injected fluid and carrying out the injection into the measuring cell at a high flow rate to ensure that the temperature does not change between the reservoir in which the fluid is stored and its arrival in the measuring cell.

[0073] [Fig.2a] shows an embodiment of an optical measuring system and circulation means suitable for implementing the method according to its main variant, comprising a pump P for circulating, via pipes C represented by arrows, the fluid of interest in a rock sample (not shown) placed in a sample holder PE, then in a measuring cell CE, which is itself connected to a light source SL and a spectrometer SP via optical fibers F, and to a temperature probe ST. In addition, in this design, the sample holder PE, the measuring cell CE and the bypass BP are immersed in a thermostatically controlled bath BT allowing the temperature of the fluid to be controlled. Such a measuring cell CE allows optical measurement and temperature measurement in a measurement zone in which a fluid circulates.

[0074] [Fig.2b] shows another embodiment of an optical measuring system and circulation means suitable for implementing the method according to its main variant, distinguished from the embodiment of [Fig.2a] in that it comprises two measuring cells CEI, CE2: one measuring cell CEI arranged upstream of the rock sample (not shown) arranged in the sample holder PE and another measuring cell CE2 arranged downstream of the rock sample. The measuring cell CEI arranged upstream of the sample has the sole purpose of verifying the stability of the solution injected into the rock sample, while the measuring cell CE2 arranged at the outlet of the rock sample has the purpose of dosing the effluents leaving the medium. The measuring cells CEI, CE2 are each connected to temperature probes temperature STI, ST2, as well as to the light source SL and to the spectrometer SP by optical fibers F. The fluid circulation system comprises a pump P upstream of the upstream measuring cell CEI, pipes C to connect the pump P to the upstream measuring cell CEI, the upstream measuring cell CEI to the sample placed in the sample holder PE, and the sample to the downstream measuring cell CE2. A pressure regulator RP for imposing the line pressure throughout the fluid circuit is placed at the outlet of the assembly. The fluid circulation system further comprises a bypass BP to bypass the rock sample, controlled by a valve V. The bypass BP is particularly useful for carrying out the application of step 1.1) described below for each of the two measuring cells CEI, CE2, by means of a single injection of a fluid sample.Furthermore, in this design, the PE sample holder, the CEI, CE2 measuring cells and the LP bypass are immersed in a BT thermostatic bath allowing the fluid temperature to be controlled. This embodiment is particularly suitable for implementing the method according to the invention in the case of HPHT (High Pressure High Temperature) conditions, as will be demonstrated in the second application example below. This can be useful in many geoscience applications (geothermal energy, CO2 storage, hydrogen storage, etc.) because the temperature of the fluid circulating in a formation as well as the pressure to which it is subjected increase with depth and can reach very high values.In order to study fluid / rock interactions at the laboratory scale and correctly describe the chemical reactions involved, it is important to place oneself in temperature and pressure conditions representative of the application concerned.

[0075] [Fig.2c] shows an embodiment of a measuring cell CE3, for example which can be used for the embodiments of [Fig.2a] or [Fig.2b], comprising a channel CL of length d connected to the pipes C in which the fluid FL circulates, to a spectrometer SP and to a light source via optical fibers F, as well as to a temperature probe ST whose section constituting the end is flush with the fluid circulating in this channel. This type of measuring cell CE3 makes it possible to carry out a measurement of the absorbance as a function of the wavelength relative to the fluid FL circulating in the channel CL as well as a measurement of the temperature T of this same fluid. According to the embodiment illustrated, the channel CL is substantially perpendicular to the pipes C and to the temperature probe ST.In a non-limiting manner, in this figure, the temperature of the circulating fluid is controlled using a thermostatic bath BT in which the measuring cell CE3 is immersed. Advantageously, the length d of the channel can be adjusted, depending on the range of concentrations to be measured. In fact, this length constitutes the length of the optical path on which the absorbance depends (see equation (1) below): the longer the path . The longer the optical path, the more we can access low concentrations.

[0076] The method according to the invention comprises at least the following steps described below.

[0077] 1) Construction of a plurality of intermediate models of the evolution of absorbance as a function of concentration for predefined temperatures.

[0078] This involves constructing, for each predefined temperature of a plurality of predefined temperatures, an intermediate model of the evolution of the absorbance as a function of the concentration for the predefined temperature. In other words, this involves constructing a plurality of intermediate models of the evolution of the absorbance as a function of the concentration, each corresponding to a predefined temperature of a plurality of predefined temperatures. In other words, this involves calibrating models of the evolution of the absorbance as a function of the concentration at different fixed temperatures.

[0079] According to an implementation of the invention, the number of predefined temperatures of the plurality of predefined temperatures is at least 3, preferably 5, very preferably 10. It is clear that the range of predefined temperatures can be chosen according to the minimum and maximum temperatures expected during the implementation of step 3) of the method according to the invention. For example, if the method according to the invention is implemented within the framework of an experiment for reinjection of geothermal fluid into the Dogger (geothermal aquifer exploited in the Paris region), a range of predefined temperatures will be taken with temperatures between 57 and 85°C (values associated with those measured on site). Advantageously, for a given range of predefined temperatures, it is possible, for example, to use a step of 5°C (in particular for the aforementioned range, which is relatively narrow), or a step of 10°C between each predefined temperature.The person skilled in the art knows how to choose an ad hoc step depending on the predefined temperature range.

[0080] Steps 1.1) to 1.3) described below are implemented for each predefined temperature of the plurality of predefined temperatures. In other words, steps 1.1) to 1.3) are repeated for each predefined temperature of the plurality of predefined temperatures. Hereinafter, the current temperature will be used for an iteration of steps 1.1) to 1.3) at a given predefined temperature.

[0081] 1.1 Acquisition of a plurality of absorption spectra for a plurality of concentrations of the chemical compound of interest and for a predefined temperature

[0082] During this step, by means of at least the optical measuring system, an absorbance is measured as a function of the wavelength for a plurality of samples of the fluid, the samples having distinct concentrations of the chemical compound of interest and being at the predefined temperature, and a first plurality of absorption spectra each corresponding to one of the concentrations of the chemical compound of interest. In other words, this step aims to determine an absorption spectrum (i.e. a curve representing the evolution of the absorbance as a function of the wavelength) for each fluid sample having a concentration of the chemical compound of interest distinct from another sample of the plurality of samples. It is clear that the concentration of chemical compound of each sample is known. The temperature is identical from one sample to another, and equal to the predefined temperature. To implement this step, it is possible, for example, to use the fluid circulation means described in figures 2a or 2b, which make it possible to control the temperature of the fluid by means of a thermostatically controlled bath.Advantageously, the number of fluid samples having distinct concentrations of the chemical compound is at least 5, and preferably 10, very preferably 15. It is clear that the range of concentrations covered by the samples can be chosen according to the concentrations expected during the implementation of step 3). For example, in the case of the main variant of the invention, it is conventional to want to follow the evolution over time of the concentration at the outlet of a porous medium of a solution of known concentration of a chemical compound, injected at the inlet of the porous medium.Thus, the range of concentrations covered by the fluid samples can advantageously vary between a zero concentration and the concentration of the solution which will be at the inlet of the porous medium, preferably between a zero concentration and a maximum value higher (for example 50%) than the concentration of the solution at the inlet of the porous medium, to be able to measure concentrations at the outlet of the porous medium exceeding the inlet concentration due to a phenomenon of temporary retention (by adsorption or other) in the porous medium.

[0083] Depending on the alternatives in which the fluid considered is a single-component fluid or a multi-component fluid with dissociated spectra as described above, the absorbance curves as a function of the measured wavelength correspond directly to the absorption spectra of the chemical compound of interest.

[0084] According to an implementation according to which the fluid considered comprises one or more additional chemical compounds compared to the chemical compound of interest, the absorption spectra of which overlap at least in part with the absorption spectrum of the chemical compound of interest and the concentration of which is known and invariant over time, it is possible to carry out a preprocessing of the measurement of the absorbance as a function of the wavelength in order to obtain the absorption spectrum of the chemical compound of interest. According to an embodiment of this implementation of the invention, it is possible to carry out a preprocessing of the measurement of the absorbance as a function of the wavelength in the following manner: for each additional chemical compound, the spectrum is measured absorption of a sample of a solution comprising this additional chemical compound according to its known concentration in the fluid of interest, and the absorption spectrum of this additional chemical compound is subtracted from the absorption spectrum measured for the fluid of interest. Note that, when a preliminary calibration step of the optical measurement system has been carried out using a reference fluid formed from the additional chemical compounds according to their respective concentrations as described above (i.e. for a reference fluid corresponding to the fluid of interest, excluding the chemical compound of interest), it is not necessary to carry out a pre-processing of the measurement of the absorbance as a function of the wavelength, since the measured light intensity is in fact corrected by the light intensity of the reference fluid, according to the formula of equation (2).

[0085] According to an implementation of the main variant of the invention, step 1.1) can be implemented by means of an optical measurement system and a fluid circulation system as described in [Fig.2b], the fluid circulation system being configured such that the valve V makes it possible to bypass the porous medium via the bypass BP. In this way, it is possible to measure, with a single fluid circulation system, an absorbance as a function of the wavelength for each fluid sample and at each measuring cell CEI, CE2, in order to construct an intermediate model of the evolution of the absorbance as a function of the concentration and at a fixed temperature for each measuring cell CEI, CE2 as described below. This model is valid at the current predefined temperature.

[0086] [Fig. 3] illustrates a plurality of absorption spectra resulting from the application of step 1.1) to an example application which will be described below. In particular, this figure presents six curves (only curves S1 and S6 are annotated for reasons of clarity) representing the evolution of the absorbance A as a function of the wavelength L in nm, each curve resulting from a measurement carried out for a fluid having a concentration of a given single chemical compound. In addition, all the spectra were obtained on samples having the same predefined temperature (in this case Tel = 50°C for this figure). In this figure, the absorption spectrum S1 corresponds to the lowest concentration and the spectrum S6 corresponds to the highest concentration.

[0087] 1.2) Definition of an absorbance curve as a function of wavelength for a predefined temperature

[0088] During this step, a curve is defined intersecting at a single point each of the absorption spectra of the plurality of absorption spectra measured in step 1.1) and such that this curve is a bijective function of the absorbance and the wavelength (in other words, this curve is such that to any absorbance value there corresponds only a single wavelength value and vice versa).

[0089] In other words, we define a curve of absorbance as a function of the intersecting wavelength: - the entire absorption spectra: this allows the entire predefined concentration range to be covered; - at a single point: this helps to contribute to the uniqueness of the concentration values determined during step 3) described below. - and such that an absorbance value corresponds to only one wavelength value and vice versa: this also contributes to the uniqueness of the concentration values determined during step 3) described below.

[0090] Very preferably, the defined curve may be a straight line. This mode of implementation is advantageous, because it is then very quick to determine the intersection of the straight line with each of the absorption spectra, in particular faster than with a curve represented by a more complex function, such as for example a polynomial function of degree at least 2. This makes it possible to contribute to a real-time determination of the evolution of the concentration of a chemical compound in a medium during step 3) described below. But it is clear that any other curve meeting the above criteria can be defined.

[0091] The [Fig.3] described above shows an example of a curve, in the form of a straight line DI, meeting the above criteria applied to the absorption spectra SI, S6 measured in step 1.1). In particular, it can be observed that the straight line DI thus defined intersects at a single point all of the absorption spectra SI, S6, and in portions of these spectra where a single absorbance value is associated with a single wavelength value, and vice versa.

[0092] For the purposes of step 1.3) described below, it is advantageous to collect the absorbance values at the intersection between the curve thus defined in step 1.2) and each of the absorption spectra of the plurality of absorption spectra determined at the end of step 1.1).

[0093] 1.3) Construction of an intermediate model of the evolution of absorbance in function of concentration for a predefined temperature

[0094] During this step, an intermediate model of the evolution of the absorbance as a function of the concentration is constructed for a predefined temperature from the absorbance values at the points of intersection between the curve defined in step 1.2) and the absorption spectra determined in step 1.1), and from the concentration corresponding to each of these absorption spectra.

[0095] Indeed, each absorption spectrum determined in step 1.1) corresponds to a known concentration of the chemical compound of interest. For an absorption spectrum, we can then associate the absorbance value at the point of intersection between this spectrum and the curve from step 1.2), the concentration associated with this spectrum. We thus obtain for each absorption spectrum a pair formed by an absorbance and a concentration.

[0096] According to the invention, for a predefined temperature, an intermediate model of the evolution of the absorbance as a function of the concentration is constructed by searching by linear regression for a function making it possible to best approximate the absorbance values at the points of intersection between the curve defined in step 1.2) and the absorption spectra determined in step 1.1), as a function of their associated concentration values. The intermediate model of the evolution of the absorbance as a function of the concentration for a predefined temperature is thus a straight line.

[0097] [Fig.4a] presents an example of MTcl model of the evolution of absorbance A as a function of concentration C in g / 1 (on logarithmic scale), determined by linear regression from the pairs (represented by points in this figure) formed by an absorbance value and a concentration value and obtained at the predefined temperature (denoted Tel hereafter) of the fluid.

[0098] At the end of this step applied to a given predefined temperature, we in fact obtain, by linear regression, a direction coefficient and an ordinate at the origin of the intermediate model of the evolution of said absorbance as a function of the concentration constructed for the current predefined temperature.

[0099] At the end of the repetition of steps 1.1) to 1.3) for each predefined temperature of the plurality of predefined temperatures, a direction coefficient and an ordinate at the origin are obtained for each of the intermediate models of the evolution of said absorbance as a function of the concentration constructed for the plurality of predefined temperatures.

[0100] [Fig.4b] presents an example of five intermediate models MTcl, MTc5 (only the models MTcl and MTc5 are annotated for reasons of clarity of the figure) of evolution of the absorbance A as a function of the concentration C (in logarithmic scale) relative to five predefined temperature values (denoted Tel to Tc5 thereafter), constructed by regression from the pairs (represented by points in this figure) formed by an absorbance value A and a concentration value C.

[0101] 2) Construction of a model of the evolution of absorbance as a function of the concentration

[0102] During this step, it is a question of constructing a model of the evolution of the absorbance as a function of the concentration, from the leading coefficients and the ordinates at the origin determined for each of the intermediate models of the evolution of the absorbance as a function of the concentration constructed for each of the predefined temperatures of step 1).

[0103] According to the invention, a first function Ma(T) is determined by linear regression. representative of a variation of the direction coefficient as a function of temperature and a second function Mb(T) representative of a variation of the ordinate at the origin as a function of temperature, from the direction coefficients and the ordinates at the origin determined for each of the intermediate models of the evolution of the absorbance as a function of the concentration constructed for each of the predefined temperatures, that is to say from the direction coefficients and the ordinates at the origin determined for each of the predefined temperatures.

[0104] According to the invention, the model of the evolution of the absorbance is then constructed as a function of the concentration and the temperature according to a formula of the type:

[0105] A = Ma(r).ln(C)+M^T)(3)

[0106] where T is the temperature of the fluid and C is the concentration of the fluid. In other words, a model of the evolution of the absorbance as a function of the concentration is determined, the coefficients of which vary as a function of the temperature of the fluid. From such a formula, whatever the temperature of the fluid, the concentration of the fluid can be determined from an absorbance measurement.

[0107] [Fig.4c] illustrates the evolution of the direction coefficient MaTc and the ordinate at the origin MbTc as a function of the predefined temperature Te of the intermediate models of [Fig.4b] determined for the predefined temperatures Tel to Tc5.

[0108] 3) Determination of the evolution over time of the concentration of the compound chemical

[0109] The model of the evolution of the absorbance as a function of the concentration and the temperature having been determined at the end of the previous step, this model can be used to determine an evolution over time of the concentration of the chemical compound of interest of the fluid circulating in a measurement zone at a given temperature, which can be variable or constant.

[0110] 3.1) Acquisition of absorption spectra for a succession of time steps

[0111] At least by means of the optical measuring system and the temperature sensor as described above, the temperature of the fluid is measured in the measuring zone (in which the fluid itself comprising the chemical compound of interest circulates), as well as an absorbance as a function of the wavelength for a succession of time steps, and a second plurality of absorption spectra relating to said chemical compound are obtained, each corresponding to a time step and to a temperature.

[0112] In other words, during this step, an absorbance is measured over time as a function of the wavelength in the measurement zone comprising the fluid containing the chemical compound of interest, as well as the temperature of this fluid in this same measurement zone.

[0113] According to an implementation of the main variant of the invention, a measurement of the absorbance can be carried out as a function of the wavelength and the temperature in a measurement zone downstream of the porous medium sample every ten seconds, preferably every second. Such time steps allow continuous (or online, or even real-time) monitoring of the evolution of the concentration of a chemical compound in a fluid circulating in a porous medium.

[0114] According to an implementation according to which the fluid present in the medium comprises only one chemical compound or an additional chemical compound whose absorption spectrum is dissociated from the absorption spectrum of the chemical compound of interest, the measurement of the absorbance as a function of the wavelength leads directly to the absorption spectrum of the chemical compound of interest.

[0115] According to an implementation according to which the fluid present in the medium comprises one or more additional chemical compounds compared to the chemical compound of interest, the absorption spectra of which are at least partly superimposed on the absorption spectrum of the chemical compound of interest and the concentration of which is known and invariant over time, it is possible to carry out a pre-processing of the measurement of the absorbance as a function of the wavelength as described above in order to obtain the absorption spectrum of the chemical compound of interest.

[0116] [Fig.5] illustrates a plurality of SN absorption spectra measured during step 3.1) for a measured fluid temperature varying by step (Tm = 17, 26, 40, 50, 60 and 70 °C) and for a succession of time steps. This figure further shows the superposition of the DI line determined during step 1.2) described above and which is used in step 3.2) described below.

[0117] Thus, whatever the temperature, at the end of this step, a plurality of absorption spectra relating to the chemical compound of interest are obtained, each absorption spectrum being associated with a temperature and corresponding to a time step of the succession of time steps.

[0118] 3.2) Determination of the concentration of the chemical compound for each step of time

[0119] During this step, for each of the absorption spectra each corresponding to a time step, an absorbance value is determined at the intersection between the curve defined in step 1.2) and the absorption spectrum considered. Then, using the model of the evolution of the absorbance as a function of the concentration and the temperature, from this absorbance value and the temperature value measured at the same time step, the concentration of the chemical compound of interest for the time step considered is deduced.

[0120] In other words, during this step, we look for the intersection between the curve defined in step 1.2) and each absorption spectrum measured in step 3.1), and from the absorbance value at this intersection and the measurement of the temperature of the fluid, we deduce the concentration of the chemical compound for this time step using the model determined in step 2), in particular equation (3) described above.

[0121] This step can be illustrated using [Fig.5], which shows the superposition on all the absorption spectra measured in step 3.1) of the DI line determined during step 1.2). For each spectrum measured at a given time step, the absorbance value is determined at the intersection of the DI line and the spectrum considered, and using the model according to equation (3) described above, which uses the functions Ma(T) and Mb(T) determined at the end of step 2), and knowing the temperature of the fluid, the concentration value corresponding to this absorbance value is deduced. A concentration value is then obtained for the time step considered.

[0122] By repeating this operation for a succession of time steps, we obtain a curve as illustrated in [Fig.6], which presents the evolution over time t (in minutes) of the concentration C in g / 1 in the chemical compound of interest, each time step being characterized by the measured fluid temperature Tm in °C.

[0123] It is clear that steps 3.1) and 3.2) can be implemented at the end of each time step, for a determination of the concentration of the chemical compound of interest in real time.

[0124] At least some of the steps of the method according to the invention, in particular steps 1.2) and / or 1.3) and / or 2) and / or 3.2), can be implemented by means of equipment (for example a computer workstation, i.e. a computer) comprising data processing means (a processor) and data storage means (a memory, in particular a hard disk), as well as an input and output interface for entering data and returning results.

[0125] Furthermore, the invention relates to a computer program product downloadable from a communication network and / or recorded on a computer-readable medium and / or executable by a processor, comprising program code instructions for implementing at least steps 1.2) and / or 1.3) and / or 2) and / or 3.2) of the method as described above, when said program is executed on a computer.

[0126] The invention further relates to a system for determining a change over time in a concentration of a chemical compound in a fluid, the system comprising a light source, a spectrometer, a temperature sensor, as well as means for processing and analyzing measurements made by the spectrometer, the system being intended for implementing the method as described above. Examples

[0127] The characteristics and advantages of the method according to the invention will appear more clearly on reading the application examples below. First example

[0128] This first application example falls within the field of geosciences, and aims more particularly to follow the evolution over time of a tracer at the outlet of a porous medium. In order to verify the feasibility of the dosage method under high temperature conditions explained above, a passive tracer (no chemical interaction with the medium) was chosen. It is a pure potassium iodide (Kl) solution dissolved in purified water. The porous medium considered is a carbonate-type rock. The rock sample has a diameter of 1 cm, a length of 2.4 cm, and a permeability of 280 mD. During this dosage experiment at the outlet of the rock, a solution of known concentration c0 (c0 = 5 g / L) in Kl is injected at a constant flow rate of 0.1 ml / min into a rock initially saturated with this same solution.

[0129] The experimental setup for this implementation of the method according to the invention is that presented in [Fig.2b]. This experimental setup comprises an optical measurement system, formed by a light source SL, for this example emitting in the UV (potassium iodide absorbing in this wavelength range) and a spectrometer SP to measure a light intensity, for this example in the UV. The measuring cell CEI placed upstream of the sample makes it possible to check the stability of the solution injected into the sample, while the measuring cell CE2 placed at the outlet of the sample is intended to measure the effluents at the outlet of the medium. The measuring cells CEI, CE2 are each connected to the light source SL and to the spectrometer SP by optical fibers F with a core of 400 pm.The fluid circulation system further includes a bypass BP to bypass the rock sample, controlled by a valve V, to allow the implementation of step 1.1) described above for each measuring cell CEI, CE2 at the same time. During this experiment, the temperature of the thermostatic bath BT was first set at 17°C and then varied in stages along the injection until reaching 70°C. The line pressure was kept constant at 3 bar.

[0130] The implementation of the method according to the invention for this application example, by means of the experimental setup described above, is as follows:

[0131] a) Construction of a model of the evolution of absorbance as a function of concentration and temperature: for a predefined temperature TCi of the thermostatic bath, six solutions comprising potassium iodide Kl at known concentrations and covering the concentration range [0; 8c0] are injected one by one into the measuring cells CEI, CE2, without passing through the porous medium placed in the sample holder PE, by means of the bypass BP. In this case, the concentration is the same in the two measuring cells CEI, CE2. For each of the solutions and on each measuring cell CEI, CE2, the transmitted intensity I is measured. The absorbance associated with the concentration in place is then deduced from I and Io which is the intensity previously measured on a reference fluid, here purified water (MQ water). For example, for this purpose, the porous medium and the measuring cells CEI, CE2 can be saturated with the reference fluid, which is here the MQ water solution. The intensity Io associated with this solution can then be measured in the two measuring cells CEI, CE2.

[0132] [Fig.3], already described above, shows the six absorption spectra SI, S6 measured for each concentration, as well as the DI line defined when applying step 1.2) described above, intersecting the absorption spectra SI, S6 at a single point and in such a way that a single absorbance value corresponds to a single wavelength value and vice versa.

[0133] [Fig.4a] presents the intermediate model MTcl of the evolution of absorbance for a first predefined temperature Tcl=50°C as a function of the concentration C (in logarithmic scale) determined by regression from the absorbance values at the intersection of the straight line DI with the absorption spectra SI, S6, as well as the concentrations associated with each spectrum SI, S6.

[0134] Steps 1.1) to 1.3) of the method according to the invention are repeated for four other predefined temperatures Tc2, Tc3, Tc4, and Tc5, always considering the same six potassium iodide solutions of known concentrations. [Fig.4b] presents the intermediate models of the evolution of the absorbance A as a function of the concentration C (in logarithmic scale) associated with each of the temperatures Tel, Tc2, Tc3, Tc4, and Tc5. As for the intermediate model MTcl, each intermediate model MTci associated with the predefined temperature Tci (i=2, 3, 4, 5) was determined by linear regression from the absorbance values at the intersection of the straight line DI with the absorption spectra SI, S6, obtained at the temperature Tci as well as the concentrations associated with each spectrum SI, S6.

[0135] [Fig.4c] shows the evolution of the coefficients MaTc (slope) and MbTc (ordinates at the origin) of the five intermediate models MTci (i=l, 2, 3, 4, 5) of [Fig.4b] as a function of the predefined temperature Te. We then determine by regression two functions Ma(T) and Mb(T) respectively from the different values of the coefficients MaTc and the intermediate models MTci (i=l, 2, 3, 4, 5) determined for their respective predefined temperature Tci (i=l, 2, 3, 4, 5).

[0136] The final model of the evolution of absorbance as a function of concentration and temperature is then obtained according to the formula of equation (3) described above, by means of the two functions Ma(T) and Mb(T) thus determined.

[0137] b) Monitoring the evolution of the tracer concentration at the outlet of the rock: Using the fluid circulation system of the experimental setup, a Kl solution of known concentration c0 = 5 g / 1 is continuously injected at a constant flow rate (0.1 cc / min for this example) into the rock. The absorbance is then continuously determined in thetwo measuring cells CEI, CE2, the measurement in CEI being used only for the validation of the concentration at the sample inlet. [Fig.5] already described above shows the SN absorption spectra measured at the level of the measuring cell CE2 at the outlet of the porous medium, as well as the straight line DI already presented in [Fig.3]. It shows an evolution of the spectra over time due to the evolution of the temperature. [Fig.6] illustrates the evolution over time t of the concentration C determined at the outlet of the sample, at the end of the application of step 3.2 of the method according to the invention. It can be observed that the fluid leaving the rock sample and passing through the downstream measuring cell CE2 has a constant concentration of Kl (in the wavelength range of interest) and as expected equal to c0 which is the concentration of Kl of the injected fluid.Note that in this application example, we use the measurements made at the upstream CEI measuring cell to verify that the fluid passing through the upstream CEI cell has a constant Kl concentration (equal to c0). Second example

[0138] This second application example also falls within the field of geosciences, and aims more particularly to demonstrate the validity of the process under high pressure conditions.

[0139] For this example, the porous medium, the experimental setup, the fluid studied and the injection flow rate are identical to the previous example. The temperature of the bath (and therefore of the fluid) was maintained at 50°C. Only the line pressure (i.e. the pressure of the fluid circulating in the sample) was modified during the injection.

[0140] [Fig.7] shows the SN absorption spectra measured at the level of the CE2 measuring cell at the outlet of the porous medium, as well as the DI line already shown in [Fig.3]. This figure shows that whatever the line pressure, all the spectra measured during the injection are superimposed.

[0141] [Fig.8] illustrates the evolution over time t (in minutes) of the concentration C determined at the outlet of the sample, following the application of step 3.2 of the method according to the invention. This figure also shows the evolution of the fluid temperature Tf and the fluid pressure Pf as a function of time t. It can be observed that the fluid leaving the rock sample and passing through the downstream measuring cell CE2 has a constant concentration of Kl (in the wavelength range of interest), and therefore not dependent on the line pressure.

[0142] Thus, the method according to the invention allows, thanks to a simple and rapid method, real-time monitoring of the evolution of the concentration of a chemical compound in a fluid, and this in a wide range of concentrations and for a fluid temperature which can vary over time. Indeed, the method according to the invention allows measurement of the concentration of a chemical compound: - Even when the chemical compound is present in high concentration, particularly in a concentration beyond which the Beer-Lambert law can no longer be applied. - Whatever the fluid temperature in the range, provided it is known. - And this, what's more, regardless of the line pressure, as demonstrated in the second application example above.

[0143] Furthermore, the method according to the invention makes it possible, once the model of the evolution of the absorbance as a function of the concentration and the temperature has been constructed, to determine the concentration of the chemical compound of interest after each acquisition of an absorption spectrum, and thus to follow online the evolution of the concentration of the chemical compound of interest.

Claims

Claims

1. Method for determining a change over time in a concentration (C) of a chemical compound in a fluid (FL) circulating in a measurement zone (ZM), by means of at least one optical measurement system (SMO, SL, SP) for measuring an absorbance (A) as a function of a wavelength (L) of said fluid (FL) and a temperature sensor (ST) for measuring a temperature of said fluid (FL), said chemical compound of said fluid (FL) being the only chemical compound of said fluid (FL) or the only chemical compound of said fluid (FL) in which said concentration (C) varies or the only chemical compound of said fluid (FL) having a non-zero absorbance (A) in a range of wavelengths absorbed by said chemical compound, characterized in that: A) for each predefined temperature of a plurality of predefined temperatures, an intermediate model (MTcl, MTc5) of the evolution of said absorbance (A) as a function of said concentration (C) is constructed for said predefined temperature in the following manner: i) by means of at least said optical measuring system (SMO, SL, SP), an absorbance (A) is measured as a function of the wavelength (L) for a plurality of samples of said fluid (FL) at said predefined temperature and having distinct concentrations of said chemical compound, and a first plurality of absorption spectra (SI, S6) relating to said chemical compound is obtained, each corresponding to one of said concentrations of said chemical compound; ii) a curve (DI) is defined intersecting each of said absorption spectra (SI, S6) of said first plurality of absorption spectra at a single point of intersection and such that said curve (DI) is a bijective function of said absorbance (A) and of said wavelength (L); iii) constructing said intermediate model (MTcl, MTc5) of the evolution of said absorbance (A) as a function of said concentration (C) for said predefined temperature by means of a linear regression applied to first absorbance values at said points of intersection between said curve (DI) and each of said absorption spectra (SI, S6) of said first plurality of absorption spectra, and at said concentration (C) corresponding to each of said absorption spectra (SI, S6) of said first plurality of

2.

3. absorption spectra; B) from direction coefficients (MaTc) and origin ordinates (MbTc) determined for each of said intermediate models (MTcl, MTc5) of the evolution of said absorbance (A) as a function of said concentration (C) constructed for each of said predefined temperatures, a first function Ma(T) representative of a variation of said direction coefficient (MaTC) as a function of said temperature (T) and a second function Mb(T) representative of a variation of said origin ordinate (MbTC) as a function of said temperature (T) are determined by linear regression, and said model (M) of the evolution of said absorbance (A) as a function of said concentration (C) and said temperature (T) is constructed according to a formula of the type: A = Ma (T ) Jn ( C ) + Mb(T) where T is said temperature of said fluid (FL) and C is said concentration of said fluid (FL); C) determining an evolution over time of a concentration (C) of said chemical compound of said fluid (FL) circulating in said measurement zone (ZM) in the following manner: a) by means of at least said optical measuring system (SMO, SL, SP) and said temperature sensor (ST), an absorbance (A) as a function of the wavelength (L) and a temperature (TM) are measured in said measuring zone (ZM) respectively for a succession of time steps and a second plurality of absorption spectra (SN) relating to said chemical compound are obtained, each corresponding to a time step, as well as a measured temperature (TM) for each time step; and b) for each of said absorption spectra (SN) of said second plurality of absorption spectra (SN), a second absorbance value is determined at the intersection between said curve (DI) and said absorption spectrum (SN), and, by means of said model of the evolution of said absorbance (A) as a function of said concentration (C) and said measured temperature (T), and from said second absorbance value and said measured temperature (TM), said concentration (C) of said chemical compound is deduced for said time step. Method according to one of the preceding claims, in which said curve (DI) is a straight line. Method according to one of the preceding claims, wherein said optical measuring system (SMO, SL, SP) comprises at least one light source (SL) for emitting radiation in at least one predetermined wavelength range, and a spectrometer (SP) for measuring a light intensity of said radiation transmitted through said measuring zone (ZM) at least in said predefined wavelength range.

4. Method according to claim 3, wherein said optical measuring system (SMO, SL, SP) further comprises at least one measuring cell (CE, CEI, CE2, CE3) connected to said light source (SL) and to said spectrometer (SP), and in which said fluid is located.

5. Method according to one of the preceding claims, in which, when said chemical compound is the only chemical compound of said fluid (FL) of which said concentration (C) varies and in the absence of a step of calibrating said optical measuring system (SMO, SL, SP) by means of a reference fluid corresponding to said fluid to the exclusion of said chemical compound, a pre-processing step is applied to the absorbance measurements (A) as a function of the wavelength (L) to determine an absorption spectrum of said chemical compound, comprising at least one subtraction of said absorption spectrum of said additional chemical compound previously recorded.

6. Method according to one of the preceding claims, in which said measuring zone (ZM) is arranged downstream of a porous medium, such as a sample of a rock from an underground formation, in which said fluid (FL) circulates.

7. A method according to claim 6, wherein said chemical compound is selected from the following list: a surfactant, a salt, a hydrocarbon compound, a polymer.

8. Method according to one of claims 6 to 7, wherein said method is implemented by means of a fluid circulation system (P, BP, V, PE, BT) for circulating said fluid at least in said porous medium and said measuring zone, said fluid circulation system (P, BP, V, PE) comprising a pump (P), preferably a pump (P) capable of delivering a flow rate with high precision, a sample cell (PE) in which said porous medium is arranged, and preferably a thermostatically controlled bath (BT) or an oven for controlling the temperature of said fluid in said measuring zone.

9. System for determining an evolution over time of a concentration of a chemical compound in a fluid, said system comprising a light source (SL), a spectrometer (SP), and means for processing and analyzing measurements carried out by said spectrometer (SP), said method being suitable for implementing the method according to one of the preceding claims.

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