Method for real-time monitoring of the pH of a fluid, using an optical measuring system and a colored pH indicator

Abstract

The present invention relates to a method and system for determining the pH of a fluid flowing in a measurement zone, using a colored pH indicator, and to at least one optical measurement system for measuring absorbance as a function of wavelength. Abstract figure: 1A

Description

Title of the invention: Method for real-time monitoring of the pH of a fluid, using an optical measuring system and a colored pH indicator. Technical field

[0001] The present invention relates to the field of monitoring the evolution over time of the pH of a fluid circulating in a medium, by means of an absorbance measurement as a function of wavelength and a colored pH indicator. In particular, the present invention finds a specific application for monitoring the evolution over time of the pH of a fluid in cases where the fluid's pH is variable over time, potentially over a wide pH range, and / or under high / high pressure conditions.

[0002] pH is a measure of the acidity or basicity of a solution. It is determined by the concentration of H+ ions (protons) in the solution. Acids release protons into the water, thus lowering the pH, while bases accept protons, raising the pH. A pH less than 7 indicates an acidic solution, a pH of 7 is neutral, and a pH greater than 7 is basic.

[0003] pH is often a key parameter in the study of physicochemical reactions in solution. For understanding these reactions, for environmental monitoring, or for tracking industrial processes, measuring this parameter can be essential.

[0004] In the field of geosciences, fluid / rock interactions are central to many natural processes. Understanding and modeling them generally involves a laboratory investigation phase, using coreflow experiments (rock samples—also called cores—swept by a fluid). This is often a critical step for establishing and validating the models that describe or predict these interactions. pH is generally one of the major parameters that govern these phenomena, and its measurement is essential. Several tools exist today for this measurement; however, most of these tools operate on samples and are not feasible in real time, which risks affecting the measurement.Indeed, the pH of a solution is highly dependent on the kinetics of the ongoing reaction and on certain parameters such as temperature and pressure, which can change between the time of sampling and the time of measurement. A typical example concerns the application of geological CO2 storage. When studying the interaction of a rock with acidified water, it is important to measure the pH at the core outlet without varying the pressure and temperature; however, this is difficult if the measurement involves sampling with delayed analysis.

[0005] In addition to the representativeness of laboratory measurements, online and real-time pH measurement is essential for process monitoring in several geoscience applications: oil and gas, CO2 sequestration, underground H2 storage, and geothermal energy. In these cases, underground pH measurements are crucial for monitoring geochemical conditions and the structural integrity of borehole systems. Real-time pH measurements under these conditions are essential. They allow, for example, the detection and prediction of corrosion deterioration of borehole components, which can compromise the safety and continuous operation of borehole systems. Previous technique

[0006] The two classic methods for measuring pH are those which use pH meters or colored indicators.

[0007] A pH meter is an electronic device equipped with a glass electrode. It measures the potential difference between a reference electrode and an electrode sensitive to H+ ions. This method is suitable for precise measurements in laboratories or industrial settings. It can be used online but has the disadvantage of not being suitable for high-pressure environments, even when using specific electrodes reinforced to better withstand the pressure. A second drawback of this method is that the required volumes are relatively large (a minimum of several ml), which is limiting for certain processes.

[0008] The second family of methods used in the laboratory to measure pH are those which use colored indicators.

[0009] An acid-base indicator is a species whose acidic (IndH) and basic (Ind) forms have different colors in solution. This property results in a variation of the observed color of a solution depending on its pH. When the indicator is added to a solution, it participates in a proton transfer equilibrium:

[0010] IndH + H2O = Ind- + H3O +

[0011]

[0012] „ _ LM pH = pKA + ^Si

[0013] The IndH and Ind forms exhibit different colors depending on the pH, which means that their absorption spectra are different. This color difference arises from a difference in electron delocalization in the two forms.

[0014] Several techniques allow the use of colored indicators to measure pH:

[0015] 1. Direct observation in solution: the indicator is added to the solution Then the resulting color is compared to a standard scale. This method, which is not very effective, is not widely used. used in the laboratory has the disadvantage of being approximate and limited by subjectivity and slight color variations in some cases.

[0016] 2. pH paper: In this case, a paper impregnated with indicators changes from The color is determined by the pH and compared to a scale. This solution, often used in laboratories, also lacks precision, making it difficult to distinguish between similar values. Furthermore, it is unsuitable for extreme pH levels.

[0017] 3. Acid-base titration: An indicator is used to identify the The equivalence point reveals the pH or concentration of the substances involved. The drawback of this technique is that it requires a precise protocol, time, and does not directly provide the pH of the initial solution.

[0018] 4. Spectrophotometry: This is a quantitative measurement of The absorbance of the color to precisely determine the pH. This method is more precise than previous ones, allows the detection of color changes not detectable to the naked eye or even with pH paper, but has the disadvantage of 1) requiring specialized equipment and 2) impractical, based on existing and current protocols, for rapid or field analyses.

[0019] Each method meets specific needs, but the choice depends on the required level of precision and the constraints on the measurement (isolated sample, process monitoring, online, real-time, delayed measurement, number of measurements to be performed, required volumes, etc.). For one-off measurements outside of a process, several available techniques can be applied. However, when the need is to monitor a process online, in real time, and under imposed and sometimes very restrictive conditions (HPHT, small volume, high measurement frequency, etc.), the choice becomes very limited.

[0020] New methods (optical, electrochemical, etc.) are increasingly used and meet certain needs. In particular, the use of spectrophotometry with colored indicators has been well developed. For example, US patent 7,402,424 B2 proposes a procedure that, based on this method, allows pH measurement under relatively high pressure and temperature conditions, which is well suited to monitoring reactions in reactors or closed chambers. However, this technique has the disadvantage of not being usable online, given the large volume required and the protocol used.

[0021] pH measurement can also be implemented using potentiometric probes. These probes are generally usable within specific pressure ranges.

[0022] Patent application EP4058798 discloses a method for determining the acidity of an acidic aqueous solution, employing a pH-sensitive dye and determining a UV-visible absorbance spectrum of the dye present in the This solution requires preparation of the aqueous solution, with several mixing steps, which does not allow for online and real-time use.

[0023] The recent development of the use of miniaturized systems which can be instrumented and connected by optical fibers to conventional instruments which until now could only be used on a one-off basis and on large sample volumes makes the spectrophotometry technique particularly interesting for online process monitoring.

[0024] The present invention makes it possible to overcome these drawbacks.

[0025] The objective of this invention is to enable online monitoring of the pH of effluents (aqueous solutions) at the outlet of the core, under the temperature and pressure conditions of the experiment.

[0026] The advantage of the methodology presented here is that it allows for the online measurement of pH simultaneously: Online, In real time, Under high pressure and high temperature (HPHT) conditions, involving small volumes, over a wide pH range, and easy to implement. Summary of the invention

[0027] The invention relates to a method for determining the pH of a fluid flowing in a measurement zone, using a colored pH indicator, and at least one optical measurement system for measuring absorbance as a function of wavelength. The method comprises at least the following steps:

[0028] A) A first correspondence law is determined relating a ratio between an absorbance and a concentration of said pH color indicator in a mixture comprising said fluid and said color indicator as a function of the pH of said mixture in the following manner:

[0029] a. by means of at least said optical measuring system, an absorbance is measured as a function of wavelength for a first plurality of mixtures of said fluid and the colored pH indicator, said mixtures having a different pH value from one mixture to another, and a first plurality of absorption spectra are determined corresponding to the spectral response of said colored pH indicator for each of said mixtures of said first plurality of mixtures;

[0030] b. 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 of said wavelength, and first absorbance values ​​are determined at said points of intersection between said curve and each of said absorption spectra of said first plurality of absorption spectra;

[0031] c. said first correspondence law is constructed by means of a regression method, preferably a linear regression, of said first determined absorbance values, of a concentration of said coloured indicator of predetermined pH for each of said mixtures of said first plurality of mixtures, and of said pH corresponding to each of said absorption spectra of said first plurality of absorption spectra;

[0032] B) for at least one time step of a plurality of time steps sampling a period of circulation of said fluid in said measuring zone, a pH of a mixture comprising said fluid and said pH color indicator is determined in the following manner, said mixture circulating in said measuring zone at said time step, said pH color indicator having a predetermined concentration for said time step:

[0033] (i) by means at least of said optical measuring system, measurement is taken in said measurement zone an absorbance as a function of the wavelength of said mixture comprising said fluid and said pH color indicator in said predetermined concentration for said time step, and at least one absorption spectrum corresponding to the spectral response of said pH color indicator in said mixture for said time step is determined;

[0034] (ii) an absorbance value is determined at the intersection between said curve and said absorption spectrum determined for said time step;

[0035] (iii) from said predetermined concentration of said colored pH indicator in said mixture for said time step, of said absorbance value determined for said time step, and of said first correspondence law, a pH of said mixture circulating in said measurement zone (MZ) is determined at said time step.

[0036] According to one embodiment, when said pH color indicator is of the type having an isobestic point, said concentration of said pH color indicator is determined in one of said mixtures of said first plurality of mixtures and / or in said mixture circulating in said measuring zone at said at least one time step in the following manner:

[0037] I) A second correspondence law is determined relating an absorbance at an isobestic point relative to said pH color indicator as a function of a concentration of said pH color indicator in a mixture comprising said fluid and said pH color indicator in the following manner:

[0038] • 1.1) using at least said optical measuring system, a Absorbance as a function of wavelength for a second plurality of mixtures said fluid and said pH color indicator, said mixtures having a different concentration value in said pH color indicator from one mixture to another, and a second plurality of absorption spectra is obtained, each corresponding to one of said concentration values ​​in said pH color indicator;

[0039] • 1.2) second absorbance values ​​are determined at an isobestic point relating to said colored pH indicator for each of said absorption spectra of said second plurality of absorption spectra;

[0040] • 1.3) said second correspondence law is constructed by means of a regression linear, of said second absorbance values ​​determined for said second plurality of absorption spectra, and of said concentration values ​​of said pH color indicator of said mixtures of said second plurality of mixtures,

[0041] II) a concentration of said colored pH indicator is determined in said mixture of said first plurality of mixtures and / or in said mixture circulating in said measuring zone at said time step in the following manner:

[0042] • II. 1) from said absorption spectrum determined for said mixture of said first plurality of mixtures and / or said absorption spectrum determined for said time step, an absorbance value is determined at said isobestic point relative to said colored pH indicator;

[0043] • II.2) from said absorbance value determined at said isobestic point and From the said second law of correspondence, the said concentration of the said colored pH indicator is determined in the said mixture of the said first plurality of mixtures and / or in the said mixture circulating in the said measurement zone at the said time step.

[0044] According to one implementation, said optical measurement system includes at least one light source for emitting radiation in at least the absorption wavelength range of said color indicator, and a spectrometer for measuring the light intensity of said radiation transmitted through said measurement area at least in said predefined wavelength range.

[0045] According to one aspect, said optical measurement system further comprises at least one measurement cell connected to said light source and said spectrometer, and in which said fluid is located.

[0046] According to one embodiment, said measurement zone is disposed downstream of a porous medium, such as a sample of a rock from an underground formation, in which said fluid circulates.

[0047] According to one embodiment, step B) is repeated for a plurality of time steps of said succession of time steps of a circulation of said fluid in said measurement zone.

[0048] According to one implementation, said process is implemented further by means of a fluid circulation system to circulate said fluid at least in said porous medium and said measurement zone, said fluid circulation system comprising a first and a second pump, said first and second pumps preferably being capable of delivering a flow rate with high precision, a sample holder cell in which said porous medium is disposed, and preferably a thermostatically controlled bath or oven to control the temperature of said fluid in said measurement zone.

[0049] Advantageously, the pH color indicator is selected from the following list: cresol red, malachite green, thymol blue, methyl yellow, bromothymol blue, bromophenol blue, bromocresol green, methyl red, bromocresol purple, thymol blue, phenolphthalein, indigo carmine, universal buffer mixtures

[0050] Furthermore, the invention relates to a system for determining the pH of a fluid circulating in a measurement zone, said system comprising a light source, a spectrometer, and means for processing and analyzing at least some of the measurements made by said spectrometer, said system being suitable for implementing the method according to one of the preceding characteristics.

[0051] Other features and advantages of the process according to the invention will become apparent from the following description of non-limiting examples of embodiments, with reference to the figures attached and described below. List of figures

[0052] [Fig.1A]

[0053] Fig. 1A represents an embodiment of the optical measurement system according to the invention.

[0054] [Fig.1B]

[0055] Fig. 1B represents another embodiment of the optical measurement system.

[0056] [Fig.2]

[0057] Fig. 2 illustrates an embodiment of an optical measurement system and means of circulation, according to an implementation of the invention.

[0058] [Fig.3]

[0059] Figure 3 shows the absorption spectra determined for a plurality of mixtures of the fluid of interest and the colored pH indicator, according to one embodiment of the invention.

[0060] [Fig.4]

[0061] Figure 4 presents an example of the first correspondence law, according to an implementation of the invention.

[0062] [Fig.5]

[0063] Figure 5 presents, for an example, the measurement of pH by the method according to the invention, and according to a prior art.

[0064] [Fig.6]

[0065] Figure 6 shows the absorption spectra determined for a plurality of mixtures of the fluid of interest and the colored pH indicator, according to one embodiment of the invention.

[0066] [Fig.7]

[0067] Figure 7 illustrates an example of a second correspondence law according to an implementation of the invention. Description of the implementation methods

[0068] The invention relates to a method for measuring a time evolution of the pH of a fluid circulating in a measurement zone, by means of at least an optical measurement system for measuring absorbance as a function of wavelength and a colored pH indicator.

[0069] According to a principal embodiment of the invention that can be applied in the field of geosciences, the measurement zone can be located downstream of a sample of a porous medium through which the fluid of interest is circulated. In particular, the porous medium sample can be a rock sample from an underground formation, for example, obtained by coring; this is then referred to as a "coreflood" test. The method according to the invention can then be used to monitor the evolution of the pH of the fluid of interest at the outlet of the porous medium sample, in order to understand fluid / rock interactions, or to enable the detection and prediction of risks (deterioration by corrosion, for example) to the components present in the fluid circuit.According to one implementation, the method can be used to monitor the evolution over time of a chemical reaction, by tracking the evolution of the pH in the fluid following the addition of a reagent upstream of the measurement zone.

[0070] Alternatively, the invention can be implemented during experiments characterizing the loss of injectivity of geothermal fluids due to bacterial activity. Measuring the pH at the outlet of the medium allows for a better understanding of the mechanisms of injectivity loss and facilitates the search for remediation solutions.

[0071] Furthermore, the present invention can be implemented in the context of CO2 storage. For this application, core flood experiments can be carried out in the laboratory to identify the conditions suitable for the geological trapping of CO2. However, the acidification of the water by CO2 during these experiments can cause the dissolution of part of the rock. Thus, combining the pH data measured at the core outlet with the pressure data measured along the medium makes it possible to decouple the trapping phenomena from those of dissolution (for example).

[0072] Advantageously, the invention can also be useful for monitoring groundwater. For this application, monitoring carried out in real time, or in the laboratory on a real sample under hydrothermal conditions representative of those on site, makes it possible to detect contamination by industrial, agricultural (pesticides, fertilizers), or acidic pollutants. This monitoring (and therefore pH measurement) is crucial for taking immediate measures to protect drinking water resources.

[0073] Furthermore, the present invention can be implemented for water treatment. For this application, measuring pH during laboratory experiments on membrane water filtration allows for a better understanding of filter clogging phenomena and, in particular, how to remedy them (for example, by co-injecting an acid).

[0074] The invention can be implemented for any analogous application.

[0075] By "pH color indicator" (or acid-base color indicator), we mean a compound whose color depends on the pH of the solution in which it is found.

[0076] According to a preferred embodiment of the invention, the pH indicator can be Bromotymol Blue, denoted BBT. Indeed, such an indicator has the advantage of being discriminating over a pH range between 4 and 8, which is a relatively wide range, corresponding to the pH of fluids predominantly used in industry, particularly in the field of the main variant of the invention. Furthermore, BBT exhibits an isobestic point, which can be used in the first preferred embodiment of the invention described below. An isobestic point in spectroscopy is a specific wavelength at which the absorbance of two or more chemical species is identical, regardless of their concentration. This phenomenon is often observed in chemical reactions where intermediate species and final products absorb light similarly at this wavelength.This allows us to monitor the progress of a reaction without being affected by variations in the concentration of the different species present.

[0077] Alternatively, the pH indicator may be chosen from the following list (particularly depending on the expected acidity): cresol red, malachite green, thymol blue, methyl yellow, bromophenol blue, bromocresol green, methyl red, bromocresol purple, thymol blue, phenolphthalein, indigo carmine, mixtures of universal buffers, or any similar indicator. Preferably, the pH indicator may be one of the indicators listed above and having an isobestic point. By way of example, indicators having an isobestic point may be chosen from: - Bromothymol blue - This indicator has an isobestic point at approximately 602 nm, which is useful for tracking pH changes in a solution without being affected by the concentration of species. - Phenolphthalein - It has an isobestic point at 550 nm during the transition between its colorless and pink forms, which is convenient for acid-base titrations. - Bromophenol blue - this indicator has an isobestic point at 590 nm, used to observe pH transitions in biological and chemical applications.

[0078] These indicators are commonly used in laboratories to monitor chemical reactions and pH changes, thanks to their isobestic property which allows for a more stable and reliable measurement.

[0079] In the following description, the use of BBT is illustrated by way of example, the other coloured indicators being selectable.

[0080] According to the invention, the optical measurement system comprises: - a light source for emitting radiation in at least the absorption wavelength range of the pH color indicator, such as, for example, in the visible range for BBT. Such a range is sufficient for implementing the method according to the invention. According to one embodiment of the invention, a light source can be used to emit radiation in at least the absorption wavelength range of the pH color indicator and the fluid of interest, so as to be able to measure a complete absorption spectrum of the fluid of interest, or alternatively, a light source can be used to emit radiation in the ultraviolet and visible range (UV-VIS), so as to be able to use the light source interchangeably for any type of pH color indicator. - a spectrometer to measure light intensity as a function of wavelength in at least the wavelength range of the light source.

[0081] Fig. 1A schematically presents an embodiment of the optical measurement system SMO according to the invention, comprising a light source SL for emitting radiation RE, in a measurement 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 measurement zone ZM along an optical path of length d.

[0082] Figure 1B schematically presents another embodiment of the SMO optical measurement system according to the invention, incorporating the same elements as those of Figure 1A (the common elements will not be described again), and further schematically showing an optional ST temperature sensor for measuring the temperature of the fluid FL in the measurement zone ZM. Such a temperature sensor can be used to monitor temperature conditions of the implementation of the process according to the invention. Indeed, it is preferable that the temperature remain constant during the implementation of the process according to the invention and that it be substantially equal to that of the fluid during calibration.

[0083] According to one example of an implementation of the invention, the light source can be a halogen-deuterium source, such as the AvaLight-DH-S-BAL model from Avantes (Netherlands) and / or the spectrometer can be a high-sensitivity fiber spectrometer, such as the AvaSpec-HS2048XL-EVO model from Avantes (Netherlands), or any similar light source.

[0084] Advantageously, the optical measurement system may include a measuring cell connected upstream to the light source and downstream (the upstream and downstream directions being considered with respect to the radiation emitted by the light source) to the spectrometer, and in which is located the fluid whose absorbance is to be measured as a function of wavelength. According to one embodiment, the measuring cell may have two inlets and two outlets, to allow connection with the light source and the spectrometer, but also with a fluid circulation system described below.

[0085] Advantageously, the optical measurement system may include means for at least processing and analyzing measurements performed by the spectrometer, in order to determine at least an absorption spectrum from the measured light intensity. According to one embodiment, the means for processing and analyzing the light intensity measured by the spectrometer may include a computer on which Avasoft software from Avantes (Netherlands) is installed to determine absorption spectra from the measured light intensity. Advantageously, the optical measurement system may include 400 µm core optical fibers for connecting the light source to the measurement cell, and the measurement cell to the spectrometer.Advantageously, the optical measurement system may further include means for transmitting (e.g., via electrical wire, optical fiber, or a wireless communication system) the measurements taken by the spectrometer to the means for processing and analyzing the light intensity.

[0086] According to one embodiment of the invention, the method may include a preliminary step of calibrating the optical measuring system. Such a calibration step may include measuring the light intensity as a function of the wavelength transmitted through a reference fluid. In one embodiment, purified water may be used as the reference fluid. In one embodiment of this invention, the calibration step may include the following steps:

[0087]

[0088]

[0089]

[0090]

[0091]

[0092]

[0093]

[0094] - the emission by the light source of radiation through the reference fluid within a measurement zone; - the detection by the spectrometer of the radiation which has passed through the reference fluid in the measurement area and the generation of a light intensity as a function of the wavelength of the radiation which has passed through the reference fluid. From this calibration, the absorbance A of a fluid can be determined using a formula of the type: A (À) = -ln(^) (2) ' where X is the wavelength, Is(À ) is the light intensity as a function of the wavelength of the radiation transmitted through the fluid considered, and / q(2) is the light intensity as a function of the wavelength of the radiation transmitted through the reference fluid. According to one embodiment of the invention, the process can further be implemented using a fluid circulation system comprising: - a first pump, intended for circulating the fluid of interest in the measurement area: the first pump is preferably a pump capable of delivering a flow rate with high precision (such as a high-performance liquid chromatography pump, known as HPLC pumps), preferably capable of delivering a flow rate in the range of 0-2 ml / min; - a second pump, preferably identical to the first pump (but operating at lower flow rates), intended for circulating the colored pH indicator in the measurement area: - a sample holder cell, in which, for example, a core sample from a porous medium can be placed, and, optionally, - a pressure regulator to impose line pressure throughout the fluid circuit; and / or - a thermostatically controlled bath or oven, for controlling (maintaining) the temperature of the fluid in the measurement area. Advantageously, the fluid circulation system in the porous medium may also include: - a flow meter to monitor the pump's flow rate; and / or - a differential pressure sensor to measure the pressure drop along the porous medium; and / or - means of connecting and controlling the fluid, such as valves and 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 flow, and / or pressure and / or temperature measurements. Advantageously, the means for processing and analyzing flow, and / or pressure and / or temperature measurements may be the same as the means for processing and analyzing measurements made using any embodiment of the optical system. - A differential pressure sensor at the terminals of the measuring cell to ensure the absence of particles that may accumulate in the cell and alter the UV signal.

[0095] In a preferred embodiment 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 oven to control the temperature of the fluid in the measurement zone. Using a thermostatically controlled oven or bath in which the measuring cell and the rock are stored ensures that the fluid in the measuring cell has the same temperature as the fluid circulating in the rock. Alternatives can be implemented, such as controlling the temperature of the injected fluid and performing 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.

[0096] Figure 2 shows an embodiment of an optical measurement system and circulation means suitable for implementing the process according to its main variant, comprising a pump PI for circulating, via lines C (represented by solid arrows), the fluid of interest through a rock sample (not shown) placed in a sample holder PE, and then injecting a pH indicator into the fluid of interest, in the line C between the sample holder PE and a measurement cell CE, via a pump P2. The measurement cell CE is further 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 measurement cell CE, and the bypass BP are immersed in a thermostatically controlled bath BT, allowing control of the fluid temperature.Such a CE measuring cell allows for optical and temperature measurement within a measurement zone through which a fluid flows.

[0097] The method according to the invention comprises at least the following steps.

[0098] 1) Determination of a correspondence law relating a ratio between an absorbance and a concentration of colored indicator in a fluid-colored indicator mixture as a function of the pH of the mixture

[0099] 1.1) Measurement of a plurality of absorption spectra

[0100] 1.2) Definition of a curve intersecting the absorption spectra

[0101] 1.3) Construction of the correspondence law by regression

[0102] 2) Determination of the pH of the mixture

[0103] 2.1) Measurement of an absorption spectrum of a colored fluid-indicator mixture

[0104] 2.2) Determination of an absorbance value at the intersection with the curve

[0105] 2.3) Determination of the pH of the mixture Step 1) can be carried out only once, before the implementation of step 2).

[0106] The implementation of the above steps assumes that the concentration of pH indicator in the fluid-indicator mixtures formed during step 1) is predetermined. In a first preferred embodiment of the invention, which will be described later, the concentration of pH indicator in the mixtures formed in step 1) can be determined beforehand.

[0107] It should also be noted that it is preferable that the temperature conditions be substantially constant (by way of non-limiting example, variations of 1°C maximum) when implementing steps 1), 2) of the process according to the invention.

[0108] The steps of the process according to the invention are described below.

[0109] 1) Determination of a correspondence law relating a ratio between a absorbance and the concentration of a colored indicator in a fluid-colored indicator mixture as a function of the pH of the mixture

[0110] In this step, the aim is to determine a correspondence law relating the ratio between absorbance and the concentration of a colored indicator in a fluid-colored indicator mixture, as a function of the pH of the mixture. This correspondence law will subsequently be called the "first correspondence law" to better distinguish it from the second correspondence law, which will be described later.

[0111] 1.1) Measurement of a plurality of absorption spectra

[0112] According to the invention, during this substep, using at least the optical measuring system, at least one absorbance is measured as a function of wavelength for a first plurality of mixtures of the fluid of interest and the pH indicator, the mixtures having a different pH value from one mixture to another, and a first plurality of absorption spectra (i.e., a curve representing the evolution of absorbance as a function of wavelength) corresponding to the spectral response of the pH indicator for each of said mixtures in the first plurality of mixtures is determined. In other words, at the end of this step, an absorption spectrum representative of the indicator's signature is obtained. colored pH in each of the mixtures having different pH values. Or, put another way, we thus obtain the variations in absorbance as a function of wavelength due to the presence of the colored pH indicator in each of the mixtures considered. For example, [Fig. 6], which will be described in more detail below, presents a series of SI, SN absorption spectra representative of the spectral response of the colored pH indicator, in this case BBT, for a first plurality of mixtures as defined above. We can observe that the signature of BBT is characterized by: • A first PKI peak around 440 nm, associated with the acidic form and whose amplitude depends on both the pH and the concentration of BBT; • A second PK2 peak around 600 nm associated with the basic form and whose amplitude also depends on both the pH and the concentration of BBT; • An isobestic point IB which appears at 501 nm. The absorbance at this point is independent of pH but varies with the concentration of BBT.

[0113] Advantageously, the invention can be implemented with a low dilution ratio of the fluid of interest to the colored indicator. For example, this low dilution ratio can result in a quantity of fluid of interest at least twenty times greater than the quantity of colored indicator. This low dilution ratio minimizes the dilution of the fluid of interest during the co-injection of the colored indicator and thus reduces the pH of the mixture of fluid of interest plus colored indicator to that of the fluid of interest. This assumption has been verified for ratios of the quantity of fluid of interest to the quantity of colored indicator greater than 20.

[0114] For the implementation of this substep, the optical measurement system and fluid circulation means described in [Fig. 2] can be used, for example. Thus, the pH indicator can be injected, via pump P2, into a pipe C through which the fluid of interest whose pH is to be measured flows (here, at the outlet of a porous medium). According to one embodiment of the invention, the injection of the pH indicator can be carried out in the form of a pulse (injection for a very short duration, for example, not limited to the order of 1 minute) or in the form of a step function (starting an injection at a given time step and then maintaining the injection rate over time). Furthermore, the fluid circulation means of [Fig. 2] have the advantage of allowing a constant fluid temperature to be maintained by means of a thermostatically controlled bath.

[0115] According to an embodiment in which the absorption spectrum of the fluid of interest and the absorption spectrum of the pH color indicator (in the mixtures of the first plurality of mixtures or in the mixture at the time step considered in step 2.1 described below) interfere with each other (partially or totally), the absorption spectrum of the fluid of interest alone can be measured beforehand, by means of at least of the optical measurement system, and we can subtract the absorption spectrum of the fluid of interest alone from the absorption spectrum of the mixture considered, in order to obtain the absorption spectrum representative of the spectral response of the colored pH indicator in the mixture considered.

[0116] According to an embodiment of the invention in which the colored pH indicator is injected into the fluid of interest in the form of a pulse, the absorption spectrum of the fluid of interest alone can be measured between two pulses.

[0117] It is quite clear that if the absorption spectrum of the fluid of interest and the absorption spectrum of the colored pH indicator do not interfere with each other, there is no need to subtract the absorption spectrum of the fluid of interest alone from the absorption spectrum of the mixture considered, since the presence of the fluid of interest has no impact on the absorption spectrum of the colored pH indicator.

[0118] Advantageously, the number of fluid mixtures having distinct pH values ​​is at least 5, preferably at least 10, most preferably at least 15.

[0119] It is quite clear that the pH range covered by the mixtures of the first plurality of mixtures can be advantageously chosen according to the pH values ​​expected for the fluid of interest mentioned above. For example, in the case of the main embodiment of the invention, the fluid injected and / or present in the porous medium has a pH typically between 4 and 8.

[0120] According to one embodiment of the invention, the pH measurement of each of the mixtures of the first plurality of mixtures can be carried out by means of a potentiometric probe, or by means of any similar measuring means.

[0121] 1.2) Definition of a curve intersecting the absorption spectra

[0122] According to the invention, during this substep, a curve is defined intersecting at a single point each of the absorption spectra of the first plurality of absorption spectra determined as described above, and such that this curve is a bijective function of absorbance and wavelength (in other words, this curve is such that to any value of absorbance there is only one corresponding value of wavelength and vice versa).

[0123] In other words, an absorbance curve is defined as a function of the intersecting wavelength: - the set of absorption spectra: this allows us to cover the entire pH range of the mixtures generated in sub-step 1.1); - at a single point: this helps to ensure the uniqueness of the pH values ​​determined in step 2) described below. - and such that to an absorbance value there corresponds only one wavelength value and vice versa: this also contributes to the uniqueness of the pH values ​​determined in step 2) described below.

[0124] Figure 3 shows the absorbance A as a function of wavelength X in nm for SI to SN absorption spectra (only the SI and SN spectra are annotated for clarity of the figure) determined for the first plurality of mixtures of the fluid of interest and the colored pH indicator (in this case, BBT), the mixtures having a different pH value from one mixture to another, as well as an example of a curve, in the form of a straight line DI, meeting the above criteria applied to the SI to SN absorption spectra.It can be observed that the line DI thus defined intersects at a single point all the absorption spectra SI to SN, and in portions of these spectra where a single absorbance value is associated with a single wavelength value, and vice versa. In this figure, the SI absorption spectrum corresponds to the lowest pH and the SN spectrum corresponds to the highest pH.

[0125] According to an embodiment in which said pH color indicator is BBT, a curve according to the invention can be defined such that it intersects the absorption spectra of the first plurality of absorption spectra in a wavelength range between 580 nm and 640 nm. Indeed, as shown in [Fig. 3] already described, in this wavelength range, the absorption spectra of the first plurality of absorption spectra associated with BBT exhibit the peak with the largest maximum and are highly dissociated from one another, thus ensuring the uniqueness conditions.

[0126] Preferably, the defined curve can be a straight line. This implementation is advantageous because it is then very quick to determine the intersection of the line with each of the absorption spectra, notably faster than with a curve represented by a more complex function, such as a polynomial function of degree 2 or higher. This allows for a real-time determination of the pH evolution of a fluid, as described below. However, it is clear that any other curve meeting the above criteria can be defined.

[0127] It should be noted that, as described above, prior art methods conventionally collect absorbance values ​​along a vertical line passing through the absorbance maxima of absorption spectra. As can be seen in [Fig. 3], this implementation does not allow coverage of such a wide pH range as that according to the invention, since there is no vertical line passing through all the maxima of the SI to SN absorption spectra determined for the entire pH range of interest.

[0128] 1.3) Construction of the correspondence law by regression

[0129] During this substep, according to the invention, the first correspondence law is constructed by means of a regression method applied to a ratio between the absorbance values ​​determined at the points of intersection of the absorption spectra of the first plurality of absorption spectra with the curve defined as described above and the concentrations of predetermined pH colored indicator for each of the mixtures of the first plurality of mixtures, as well as at the pH corresponding to each of the absorption spectra of the first plurality of absorption spectra.

[0130] We thus obtain a first law of correspondence relating a ratio between an absorbance and a concentration of coloured indicator in a fluid-coloured indicator mixture as a function of the pH of the mixture.

[0131] This substep assumes that the pH indicator concentration values ​​are predetermined for each of the mixtures in the first plurality of mixtures. As will be described below, the pH indicator concentration values ​​for each of the mixtures in the first plurality of mixtures can be determined by means of the first preferred embodiment of the invention, comprising determining a second correspondence law relating an absorbance at an isobestic point relative to the pH indicator to the pH indicator concentration in a fluid-pH indicator mixture. This first preferred embodiment of the invention is described later.

[0132] Figure 4 shows an example of a first PLC correspondence law, determined by an exponential regression method from A / C0 ratio values ​​between absorbance and concentration, as a function of the pH (pHm) of a plurality of mixtures, represented by the filled disks. The curve determined for this example has the equation: A / C0 = 0.0284 pHm6 3.

[0133] The first step according to the invention can be performed only once beforehand. Once the first correspondence law has been determined, steps 2) and 3) of the method according to the invention, described below, can be repeated for a plurality of time steps sampling a period during which the fluid of interest flows in the measurement zone. Steps 2) and 3) are described below for one time step. 2) Determination of the pH of the mixture

[0134] During this step, for a time step of a plurality of time steps sampling a period of circulation of said fluid in said measurement zone (also called residence time in the cell), a pH is determined of a mixture comprising the fluid of interest and the pH indicator, the mixture circulating in the measurement zone at the time step considered, said mixture resulting from an injection upstream of the measurement zone of the pH indicator into the fluid, the indicator colored pH indicator having a predetermined concentration for said time step. In other words, at any instant, the pH of the mixture (fluid of interest and colored indicator) flowing in the measurement zone is determined, based on a predetermined concentration of colored indicator.

[0135] 2.1) Measurement of an absorption spectrum of a colored fluid-indicator mixture

[0136] During this substep, using at least said optical measuring system, an absorbance is measured in said measuring zone as a function of the wavelength of the mixture comprising the fluid and the colored pH indicator at the predetermined concentration for said time step (at the desired time), and at least one absorption spectrum representative of the presence of said colored pH indicator in said mixture is determined for said time step.

[0137] 2.2) Determination of an absorbance value at the intersection with the curve

[0138] During this substep, an absorbance value is determined at the intersection between the curve determined in substep 1.2) and the absorption spectrum determined for said time step in substep 2.1).

[0139] This amounts to superimposing the curve determined in step 1.2) onto the absorption spectrum determined in substep 2.1), and recording the absorbance at the point of intersection between the curve and the spectrum. In other words, the point of intersection between the curve and the absorption spectrum is determined. This substep can be performed numerically, using the processing and analysis means of the system according to the invention. 2.3) Determination of the pH of the mixture

[0140] From the predetermined concentration of colored pH indicator in the mixture for the time step considered, the absorbance value determined in substep 2.2) for the time step considered, and the first correspondence law, the pH of the mixture circulating in the measurement zone at the time step considered is determined.

[0141] The first correspondence law is exploited here as a kind of abacus: it is sufficient to have an absorbance value at the intersection with the curve defined in sub-step 1.2) to find, by correspondence, the pH of the mixture.

[0142] This substep assumes that the pH indicator concentration value is predetermined for the fluid-indicator mixture flowing through the measurement zone at the considered time step. This can be achieved by means of the first preferred embodiment of the invention, comprising determining a second correspondence law relating an absorbance at an isobestic point relative to the pH indicator to the pH indicator concentration in the fluid-indicator mixture. This first preferred embodiment of the invention is described later.

[0143] This substep can be carried out digitally, using the processing and analysis means of the system according to the invention. Preferred implementation

[0144] According to a first preferred embodiment of the invention, when the colored pH indicator is of the type having an isobestic point, the concentration of colored pH indicator in one of the mixtures of the first plurality of mixtures and / or in the mixture circulating in the measuring zone at the time step considered can be determined in the following manner:

[0145] I) A second correspondence law is determined, the second correspondence law relating an absorbance at an isobestic point relative to the colored pH indicator as a function of a concentration of colored pH indicator in a mixture comprising the fluid and the colored pH indicator, in the following manner:

[0146] 1.1) using at least the optical measuring system, an absorbance is measured depending on the wavelength for a second plurality of mixtures of the fluid and the colored pH indicator, the mixtures having a different colored pH indicator concentration value from one mixture to another, and a second plurality of absorption spectra are determined, each corresponding to one of the colored pH indicator concentration values;

[0147] 1.2) absorbance values ​​are determined (called second values absorbance) at an isobestic point relative to the colored pH indicator for each of the absorption spectra of the second plurality of absorption spectra;

[0148] 1.3) the second correspondence law is constructed by means of a regression linear, of the second absorbance values ​​determined for the second plurality of absorption spectra, and of the pH color indicator concentration values ​​of the mixtures of the second plurality of mixtures;

[0149] II) a concentration of colored pH indicator is determined in one of the mixtures of the first plurality of mixtures and / or in the mixture circulating in the measurement zone at the time step considered in the following manner:

[0150] II. 1) from the absorption spectrum determined for the mixture of the first plurality of mixtures considered and / or said absorption spectrum determined for the time step considered, an absorbance value is determined at the isobestic point relative to the colored pH indicator;

[0151] II.2) from the absorbance value determined at the isobestic point and the second law of correspondence, we determine the concentration of colored pH indicator in the mixture of the first plurality of mixtures and / or in the mixture circulating in the measurement zone at the time step considered.

[0152] Determining the second correspondence law of this first preferred implementation can be carried out as a preliminary step to the process according to the invention, at least before step 1) described above when the aim is to determine the concentrations of coloured indicator of the first plurality of mixtures, and at least before step 2) described above when the aim is to determine the concentrations of coloured pH indicator in the mixture at the time step considered.

[0153] This implementation of the invention makes it possible to improve the accuracy of pH determination.

[0154] In this first preferred embodiment, the fact that the absorbance at the isobestic point of an absorption curve of a mixture comprising a fluid and a colored pH indicator is a function of the concentration of the colored pH indicator only is exploited.

[0155] This is illustrated in [Fig. 6], which shows the absorbance A as a function of wavelength X in nm for SI to SN absorption spectra, already shown in [Fig. 3], determined for mixtures of different pH but having the same BBT concentration. Regardless of the pH of the fluid of interest, the absorbance at the two peaks PKI, PK2 characteristic of BBT depends on the pH of the mixture and the BBT concentration for all wavelengths, except at the isobestic point IB where the absorbance depends only on the color indicator concentration.

[0156] Thus, at the end of substep 1.1), an absorption spectrum (i.e., a curve representing the evolution of absorbance as a function of wavelength) is obtained for each mixture in the second plurality of mixtures with distinct concentrations. The concentration of the colored indicator in each of the mixtures in the second plurality of mixtures can be determined by means of a potentiometric probe measurement.

[0157] Furthermore, for the implementation of substep 1.1), the temperature of a mixture is very preferably substantially the same from one mixture to another. For this purpose, the fluid circulation means described in [Fig. 2] can be used, which allow the fluid temperature to be controlled by means of a thermostatically controlled bath. Advantageously, the number of fluid mixtures having distinct colored indicator concentration values ​​is at least 5, and preferably 10, very preferably 15.

[0158] According to an embodiment of the invention in which the pH color indicator is BBT, during the performance of substep 1.2) and / or substep II.1) of the first preferred embodiment of the invention, the absorbance measurement can be performed at a wavelength of 501 nm. This corresponds in fact to the wavelength of the isobestic point of BBT.

[0159] According to an embodiment of step 1.3 of this first preferred implementation of the invention, using the absorbances determined at the isobestic points for the second plurality of samples with color indicator concentration distinct and using linear regression, we can determine the slope a and the y-intercept b of a line such that:

[0160] = aCQ + b

[0161] where A^ is the absorbance determined at the isobestic point, and CQ is the pH indicator concentration. This yields a second correspondence law relating the absorbance at the isobestic point to the indicator concentration in a mixture. Figure 7 shows such a second correspondence law (DLC) relating the absorbance determined at the isobestic point Aiso to the pH indicator concentration C0.

[0162] According to an embodiment in which the pH indicator is BBT, the second correspondence law relating the absorbance determined at the isobestic point A,^ to the concentration Co of the indicator can be written according to a formula of the type:

[0163] Aiw = 3194 Co +0.0046

[0164] This first preferred implementation of the invention can be carried out digitally, using the processing and analysis means of the system according to the invention.

[0165] The invention further relates to a system for determining the pH of a fluid (FL) flowing through a measurement zone (ZM), the system comprising a light source, a spectrometer, and means for processing and analyzing at least some of the measurements taken by the spectrometer, the system being intended for implementing the method as described above. The means for processing and analyzing the measurements implement steps 1 to 2, and optionally, the first preferred implementation. Such a system can be used online, in real time, under high pressure and high temperature conditions, involving small volumes, over a wide pH range, and is easy to implement. The system can conform to Figures 1 and 2, as described above. Examples

[0166] The characteristics and advantages of the method according to the invention will become clearer upon reading the application example below.

[0167] For example, the determination of pH by the method according to the invention is compared to the pH determined by a prior art method (potentiometric measurement). To implement the invention, BBT is injected into a plurality of fluids of interest having different pH values, and steps 1 and 2 of the method according to the invention are carried out.

[0168] Figure 5 illustrates, for this comparative example, the pH of the mixture obtained by the process according to the invention, denoted pHm, as a function of the pH measured by a prior art method, denoted pHf. The squares represent the measurements, followed by a regression The linear correlation coefficient (COR) allows for the determination of the correlation coefficient between the two measurement methods. In the illustrated example, the slope of the correlation is very close to 1 (0.985). Thus, the invention enables a precise determination of the pH of the fluid of interest.

Claims

1. Demands A method for determining the pH of a fluid (FL) flowing in a measurement zone (ZM), using a colored pH indicator, and at least one optical measurement system (SMO, SL, SP) for measuring absorbance (A) as a function of wavelength (L), characterized in that said method comprises at least the following steps: A. A first correspondence law is determined relating a ratio (A / CO) between an absorbance (A) and a concentration (CO) of said colored pH indicator in a mixture comprising said fluid and said colored indicator as a function of the pH of said mixture in the following manner: a. by means of at least said optical measurement system (SMO, SL, SP), an absorbance (A) is measured as a function of wavelength (L) for a first plurality of mixtures of said fluid and of the colored pH indicator, said mixtures having a different pH value from one mixture to another, and a first plurality of absorption spectra (SI, SN) corresponding to the spectral response of said colored pH indicator is determined for each of said mixtures of said first plurality of mixtures; b. a curve (DI) is defined intersecting each of said absorption spectra (SI, SN) 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 said wavelength (L), and first absorbance values ​​are determined at said points of intersection between said curve (DI) and each of said absorption spectra (SI, SN) of said first plurality of absorption spectra; c. said first correspondence law is constructed by means of a regression method, preferably a linear regression, of said first determined absorbance values, of a concentration (CO) of said color indicator of predetermined pH for each of said mixtures of said first plurality of mixtures, and of said pH corresponding to each of said absorption spectra (SI, SN) of said first plurality of absorption spectra;

2. B. For at least one time step of a plurality of time steps sampling a period of circulation of said fluid in said measurement zone, a pH of a mixture comprising said fluid and said pH color indicator is determined in the following manner, said mixture circulating in said measurement zone (MZ) at said time step, said pH color indicator having a predetermined concentration (CO') for said time step: a. by means of at least said optical measuring system (SMO, SL, SP), an absorbance (A') is measured in said measuring zone (ZM) as a function of the wavelength (L) of said mixture comprising said fluid and said pH colour indicator in said predetermined concentration (CO) for said time step, and at least one absorption spectrum (S') corresponding to the spectral response of said pH colour indicator in said mixture for said time step is determined; b. an absorbance value is determined at the intersection between said curve (DI) and said absorption spectrum (S') determined for said time step; c. from said predetermined concentration (CO') of said colored pH indicator in said mixture for said time step, of said absorbance value determined for said time step, and of said first correspondence law, a pH of said mixture circulating in said measurement zone (ZM) is determined at said time step. A method according to any one of the preceding claims, wherein, when said pH color indicator is of the type having an isobestic point, said concentration (CO, CO') of said pH color indicator is determined in one of said mixtures of said first plurality of mixtures and / or in said mixture circulating in said measuring zone (ZM) at said at least one time step in the following manner: I) A second correspondence law is determined relating an absorbance (A2) at an isobestic point relative to said pH color indicator as a function of a concentration (CO, CO') of said pH color indicator in a mixture comprising said fluid and said pH color indicator in the following manner: • 1.1) by means of at least said optical measuring system, absorbance is measured as a function of wavelength for a second plurality of mixtures of said fluid and said pH color indicator, said mixtures having a different concentration value in said pH color indicator from one mixture to another, and a second plurality of absorption spectra is obtained, each corresponding to one of said concentration values ​​in said pH color indicator; • 1.2) Second absorbance values ​​(A2) are determined at an isobestic point (IB) relative to said colored pH indicator for each of said absorption spectra of said second plurality of absorption spectra; • 1.3) we construct the said second law of correspondence by means of linear regression, of said second absorbance values ​​(A2) determined for said second plurality of absorption spectra, and of said concentration values ​​of said colored pH indicator of said mixtures of said second plurality of mixtures, II) a concentration (CO, CO') of said colored pH indicator is determined in said mixture of said first plurality of mixtures and / or in said mixture circulating in said measurement zone (MZ) at said time step in the following manner: • II. 1) from said absorption spectrum (SI, SN) determined for said mixture of said first plurality of mixtures and / or from said absorption spectrum (S') determined for said time step, an absorbance value (A2) is determined at said isobestic point relative to said colored pH indicator; • II.2) from said absorbance value (A2) determined audit isobestic point and of the said second law of correspondence, the said concentration (CO, CO') in the said colored pH indicator is determined in the said mixture of the said first plurality of mixtures and / or in the said mixture circulating in the said measurement zone (ZM) at the said time step.

3. A method according to any one of the preceding claims, wherein said optical measurement system (SMO, SL, SP) comprises at least one light source (SL) for emitting radiation in at least the absorption wavelength range of said color indicator, and a spectrometer (SP) for measuring a light intensity of said radiation transmitted through said measurement 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, IEC, CE2, CE3) connected to said light source (SL) and said spectrometer (SP), and in which said fluid is located.

5. A method according to any one of the preceding claims, wherein said measuring zone (ZM) is disposed downstream of a porous medium, such as a rock sample from an underground formation, in which said fluid (FL) flows.

6. A method for determining a time evolution of the pH of a fluid (FL) circulating in a measurement zone (ZM), wherein step B) is repeated for a plurality of time steps of said succession of time steps of a circulation of said fluid in said measurement zone.

7. A method according to claim 6, wherein said method is further implemented by means of a fluid circulation system (PI, P2, BP, V, PE, BT) for circulating said fluid at least in said porous medium and said measurement zone, said fluid circulation system (PI, P2, BP, V, PE) comprising a first and a second pump (PI, P2), said first and second pumps (PI, P2) preferably being capable of delivering a flow rate with high precision, a sample holder cell (PE) in which said porous medium is disposed, and preferably a thermostatically controlled bath (BT) or an oven for controlling the temperature of said fluid in said measurement zone.

8. A method according to any one of the preceding claims, wherein the pH color indicator is selected from the following list: cresol red, malachite green, thymol blue, methyl yellow, bromotymol blue, bromophenol blue, bromocresol green, methyl red, bromocresol purple, thymol blue, phenolphthalein, indigo carmine, universal buffer mixtures

9. System for determining the pH of a fluid (FL) flowing in a measurement zone (ZM), said system comprising a light source (SL), a spectrometer (SP), and means for processing and analyzing at least some of the measurements made by said spectrometer (SP), said system being suitable for carrying out the method according to one of the preceding claims.

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