FR3065456A1 - METHOD FOR DETERMINING THE CONCENTRATION OF AT LEAST ONE CEMENT ADDITIVE PRESENT IN AQUEOUS MEDIUM AND ASSOCIATED DEVICE - Google Patents

Abstract

The invention relates to a device and a method for checking the amount of at least one cement additive which is present in an aqueous medium to which the additive has been added. The process comprises the following steps: a) adding, to the aqueous medium, a mixture comprising the additive and an iodide salt, the ratio between the amounts of additive and iodide salt present in the mixture being known; b) stabilizing the amount of iodide ions present in the aqueous medium by applying, in the aqueous medium, a stream configured so that the molar concentration of triiodide ions in the aqueous medium is greater than or equal to 10 -5 times the concentration; molar iodide ions of the aqueous medium; c) determining the amount of iodide ions present in the aqueous medium by means of an electrochemical sensor; d) calculating the amount of additive present in the aqueous medium by correlation between the amount of iodide ions determined in step c) and the ratio between the amounts of additive and iodide salt present in the mixture at step at).

Description

Holder (s): COMMISSIONER OF ATOMIC ENERGY AND ALTERNATIVE ENERGIES Public establishment.

Extension request (s)

Agent (s): BREVALEX Limited liability company.

FR 3 065 456 - A1

164 / METHOD FOR DETERMINING THE CONCENTRATION OF AT LEAST ONE CEMENT ADDITIVE PRESENT IN AN AQUEOUS MEDIUM AND ASSOCIATED DEVICE.

(© The invention relates to a device and a method for checking the quantity of at least one cement additive which is present in an aqueous medium to which the additive has been added.

The process includes the following steps:

a) adding, to the aqueous medium, a mixture comprising the additive and an iodide salt, the ratio between the amounts of additive and iodide salt present in the mixture being known;

b) stabilization of the amount of iodide ions present in the aqueous medium by application, in the aqueous medium, of a current configured so that the molar concentration of triiodide ions in the aqueous medium is greater than or equal to 10 ' ⁵ times the concentration molar in iodide ions from the aqueous medium;

c) determination of the quantity of iodide ions present in the aqueous medium by means of an electrochemical sensor;

d) calculation of the amount of additive present in the aqueous medium by correlation between the amount of iodide ions determined in step c) and the ratio between the amounts of additive and iodide salt present in the mixture in step at).

METHOD FOR DETERMINING THE CONCENTRATION OF AT LEAST ONE CEMENT ADDITIVE PRESENT IN AN AQUEOUS MEDIUM AND ASSOCIATED DEVICE

DESCRIPTION

TECHNICAL AREA

The field of the invention is that of cementing and more particularly that of controlling the concentration of one or more cement additives in an aqueous medium, for example a mud, intended to form a cement part.

In general, the present invention finds application in any field requiring the use of a cement. It is in particular very appreciable for the production of high performance cements, for example in the nuclear field, in the field of construction, in the field of cement 3D printers, etc. and in particular in HPHT (High Pressure High Temperature) and / or deep water environments, as is notably the case in the petroleum field for the production of wells.

PRIOR STATE OF THE ART

In cementing operations, the control of the properties of an aqueous medium comprising cement (this medium is generally called "mud", "grout", "suspension", "milk", "boiled" or even "slag" of cement , all these terms being grouped under the English term "slurry") is essential to obtain a cement part correctly dimensioned and having an adequate compressive strength. It is recalled that an aqueous medium comprising cement, such as a mud, a suspension, a milk, a slurry or a slag of cement, mainly comprises water, cement and additives (among which mention may be made of setting retarders, setting accelerators, dispersants, etc.). For the sake of simplification, we will use in the following the term "mud" to designate an aqueous medium comprising cement.

Accurate knowledge of the concentration of additives in the mud is of crucial importance, especially in HPHT / deep water environments, where complex temperature profiles and high background pressures significantly affect the sensitivity of the mud. It is therefore necessary to adapt the concentration of additives in the sludge according to parameters such as depth, temperature, pressure, etc.

They are technicians who directly prepare formulations adapted to needs in large tanks directly on site and add the bags of additives manually. Dosage errors can therefore occur, which can have more or less significant consequences in terms of loss of time and money. In the event of a metering error, it is indeed necessary to stop drilling and re-drill through the layer of cement which has not been correctly installed, which has the consequence of greatly slowing down operations and increase the cost of drilling.

It would be interesting to have an in situ technique for controlling the concentration of additives in a sludge, which would make it possible to check its adequacy with a specific cementation. For example, in the field of oil drilling, this would make it possible to control the formulation of additives in a mud before it is injected into the wellbore.

However, to date there are a very large number of different additives which can be used in the cementing processes, each additive having a complex chemical formulation. It is therefore impossible to envisage a method of controlling the concentration of an additive in a sludge by direct measurement without the use of complex measuring instruments which are unusable in the field, such as mass spectrometry devices. , high performance liquid chromatography (HPLC in English), inductively coupled plasma mass spectrometry (ICP-MS in English), etc.

Furthermore, there is currently no in situ method for verifying (or even quantifying) the concentration of an additive in a cement slurry which is satisfactory in terms of reliability and reproducibility of the measurement.

Current practice involves indirect thickening / rheology tests, which are time consuming and difficult to perform in remote operations.

Document [1] discloses a method for controlling the concentration of an additive in a cementing mixture in the petroleum field. The technique is based on an indirect assay method using a colored marker associated with the additive to be assayed and an analysis of the marker by spectroscopy. The downside of this method is that it cannot be used in all situations. In fact, in the case where the additive is also colored and absorbs at a wavelength close to that of the colored marker, this causes interference with the measurement of the colored marker. This is for example the case of one of the main additives to be checked, namely the retarders containing molecules of the lignosulfate family, which color the solution in dark brown, or even black, at very low concentrations. Thus, in many cases, the additives present in the mixture will make measurement by spectrophotometry impossible, because the mixture absorbs too much light. In addition, many additives containing polymers make the mixtures opaque, which blocks spectrophotometric measurement. Finally, certain additives are caused to change color depending on the pH, which itself depends on the mixture of the other additives. Consequently, the measurement obtained by this method can be erroneous in many cases and does not allow a reliable measurement to be obtained.

The authors of document [2] propose to associate a marker with an additive to be controlled and to determine the concentration of the marker, in order to then deduce therefrom the concentration of the additive. They propose according to a first embodiment to use a fluorescent marker and to use a spectroscopic method to determine its concentration. According to another embodiment, they propose to use an electrochemical marker capable of undergoing a redox reaction and of determining its concentration by measuring the electric current produced during this reaction.

The solution proposed by the authors of document [2] is promising because it is in situ, it is fast and it requires inexpensive equipment, which is transportable and miniaturizable. However, it turns out that the measurements obtained fluctuate over time and are therefore not reliable.

In view of the drawbacks of the prior art, the inventors set themselves the goal of developing a method of in situ analysis of sludge, at low cost and almost in real time, and which makes it possible to know the concentration of one or more additives in the mud analyzed and thus providing, if necessary, the possibility of modifying the composition of the mud before its use for the production of a cement part.

As we will see below, the analytical method developed by the inventors has the advantage of being able to know the concentration of one or more additives contained in an aqueous medium, this aqueous medium containing or not containing cement.

STATEMENT OF THE INVENTION

To do this, the inventors have chosen the principle of an indirect assay using a specific marker for electrochemical detection.

More particularly, the inventors have chosen to use iodide ions (Γ) as electrochemical markers, in particular sodium iodide (Nal). The choice of iodide ions, and in particular sodium iodide, is not trivial. Iodide ions are particularly suitable as electrochemical markers, because they have a high electrochemical reactivity, which makes them easily detectable. They do not modify the properties of cement, nor those of additives. They are inexpensive and readily available in large quantities. Finally, they are chemically stable over time.

Furthermore, in order to resolve the problems of reliability and reproducibility of measurements encountered in the prior art, the inventors have also chosen to stabilize the iodine in solution. In fact, iodine can be present in different species depending on the pH and / or the degree of oxidation of the aqueous medium in which it is found. In addition, the formation of certain species by electrochemistry results in slow chemical reactions which can interfere with the measurements.

For example, in the case of sodium iodide, the formation of diiode by oxidation of sodium iodide at acid pH causes a chemical reaction causing the formation of triiodide. The chemical reaction for the formation of triiodide is spontaneous and slow:

f ₂ (s) + / (aq) Z ₃ - (aq)

This chemical reaction consumes the iodine ions present on the surface of the working electrode, which disturbs the electrochemical measurements. In order to accelerate the stabilization of the chemical equilibrium and obtain a reproducible measurement of the concentration of the iodide ion (Γ), the inventors have developed an electrochemical method based on the application, in the aqueous medium, of a current configured so that the following relationship is obtained [l ₃ ]> [Γ], preferably the following relationship [I3] = [I] θ ± 25%.

Thus, the invention proposes a method for checking the quantity of at least one cement additive which is present in an aqueous medium to which the additive has been added, the method being characterized in that it comprises the following steps:

a) adding, to the aqueous medium, a mixture comprising the additive and an iodide salt, the ratio between the amounts of additive and iodide salt present in the mixture being known;

b) stabilization of the amount of iodide ions present in the aqueous medium by application, in the aqueous medium, of a current configured so that the molar concentration of triiodide ions in the aqueous medium is greater than or equal to 10 ⁵ times the molar concentration in iodide ions of the aqueous medium;

c) determination of the quantity of iodide ions present in the aqueous medium by means of an electrochemical sensor;

d) calculation of the amount of additive present in the aqueous medium by correlation between the amount of iodide ions determined in step c) and the ratio between the amounts of additive and iodide salt present in the mixture in step a). Preferably, the iodide salt is sodium iodide.

According to the invention, the iodide salt will serve as an electrochemical marker for the additive. In the presence of water, the iodide salt dissolves into an iodide ion; the amount of iodide salt introduced into the aqueous medium with the additive will therefore be the same as the amount of iodide ion present in the aqueous medium.

It should be noted that if the aqueous medium contains more than one additive marked with an iodide ion and it is desired to know the quantity of more than one of these additives, the quantity value obtained at l 'resulting from step d) will correspond to the sum of the quantities of additives marked by the electrochemical marker of the iodide ion type.

In order to know the quantity of each of the additives, the invention provides a method for verifying the quantities of at least a first and a second cement additive which are present in an aqueous medium to which the first and the second additive have been added, characterized in that it includes the following steps:

a) adding, to the aqueous medium, a first mixture comprising the first additive and a first iodide salt, the ratio between the quantities of the first additive and the first iodide salt present in the first mixture being known;

b) stabilization of the amount of iodide ions present in the aqueous medium by application, in the aqueous medium, of a current configured so that the molar concentration of triiodide ions in the aqueous medium is greater than or equal to 10 ' ⁵ times the concentration molar in iodide ions from the aqueous medium;

c) determination of the quantity of iodide ions present in the aqueous medium by means of an electrochemical sensor;

d) calculation of the quantity of the first additive present in the aqueous medium by correlation between the quantity of iodide ions determined in step c) and the ratio between the quantities of the first additive and of the first iodide salt present in the first mixture with l 'step a);

e) adding, to the aqueous medium, a second mixture comprising the second additive and a second iodide salt, the ratio between the quantities of the second additive and of the second iodide salt present in the second mixture being known;

f) stabilization of the amount of iodide ions present in the aqueous medium by application, in the aqueous medium, of a current configured so that the molar concentration of triiodide ions in the aqueous medium is greater than or equal to 10 ' ⁵ times the concentration molar in iodide ions from the aqueous medium;

g) determination of the quantity of iodide ions present in the aqueous medium by means of the electrochemical sensor;

h) calculation of the amount of the second additive present in the aqueous medium by correlation between the amount of the iodide ions determined in step g) and the ratio between the amounts of the second additive and of the second iodide salt present in the second mixture at l 'step e).

Thanks to this process, it is possible to check the quantity of each marked additive when these are added one after the other by repeating the measurement protocol after the addition of each additive. The first and second iodide salt can be chosen from sodium iodide and potassium iodide. Preferably, the first and the second iodide salt are identical. Preferably, the first and second iodide salt are sodium iodide.

During the stabilization step (s), the chemical equilibria of the different iodine species, in particular iodide ions, present in the aqueous medium are stabilized in order to prevent these iodine species from causing variations in the quantity of iodide ions during the step of determining the quantity of iodide ions present in the aqueous medium by means of the electrochemical sensor.

In the context of the present invention, the term “cement additive” means any compound (with the exception of water) capable of being added to a cement in order to modify its physicochemical properties; it may for example be a setting retarder, a setting accelerator, a dispersant, a diluent, etc.

In the context of the invention, the term "cement" includes hydraulic cements such as cements classified "CEM I", "CEM II", "CEM III", "CEM IV" and "CEM V" by the European standard. NF EN 197-1, cements activated by bases such as vitrified slag from blast furnaces and cements activated by acids such as phosphomagnesium cements.

Preferably, in the stabilization step (steps), the current is configured so that the molar concentration of triiodide ions in the aqueous medium is equal, more or less 25%, to 10 ³ times the molar concentration of iodide ions aqueous medium.

To be able to carry out measurements of a stable concentration of iodide ions, it is necessary to work close to the chemical equilibrium of the following reaction:

/ ₂ (s) + / (aq) Z ₃ - (aq)

By applying a current to the aqueous medium so that we have the relation [l ₃ ]> 10 ' ⁵ χ [Γ], preferably the relation [l ₃ ] = [Γ], to within ± 25%, we place ourselves close to this chemical equilibrium.

To obtain these relations, it is necessary to generate, in the aqueous medium, anodic current in order to oxidize enough iodide ions in diiode. This is obtained by injecting enough charge (Q) into the aqueous medium to carry out this oxidation while respecting the following Faraday equation:

where n (in moles) is the quantity of triiodide ions produced at the working electrode, Q (in Coulomb) is the total electric charge imposed on the aqueous medium, F (in Coulomb per mole) is the Faraday constant and z is the number of electrons exchanged.

For example, for an aqueous medium of 25 mL comprising a concentration of iodide ions of 5 g / L, it is possible to stabilize the amount of iodide ions present in the aqueous medium by obtaining the relationship [l ₃ ]> 10 ′ ⁵ χ [Γ] by applying a constant current of + 20 mA for 4 seconds.

According to a preferred embodiment of the invention, the stabilization (s) is (are) obtained by the application, on a first and a second electrode of the electrochemical sensor, of a plurality of electrical signals (which we will call pulses) whose polarity is reversed by alternation. In other words, stabilization is obtained by the application of at least two successive electrical signals, alternately positive and negative, or vice versa.

To obtain these electrical signals, it is possible, for example, to apply a periodic alternating current, having a triangular, sinusoidal, crenellated shape or any other shape. Preferably, the applied current is chosen alternating with a form of periodic slots.

For example, for an aqueous medium of 25 mL comprising a concentration of iodide ions of 5 g / L, it is possible to stabilize the amount of iodide ions present in the aqueous medium by obtaining the relationship [l ₃ ]> 10 ⁵ χ [ Γ] by applying an alternating current with a form of periodic slots, having 50 slots of + 20 mA and 50 slots of - 20 mA which alternate in succession for 10 seconds.

Preferably, at least one of the first and second electrodes is made of doped diamond.

Advantageously, the electrochemical sensor comprises a working electrode in doped diamond, a reference electrode and a counter electrode. Since the electrodes of an electrochemical sensor must be electrically conductive, the diamond electrode will be doped enough to make it conductive. Preferably, the doping rate will be greater than 10 ² ° cm ' ³ . An electrode which is stable over time is chosen for the counter electrode, for example a platinum or diamond electrode. For the reference electrode, an electrode is chosen which is not liable to be modified by the various additives present in the aqueous medium to be analyzed. It can for example be a platinum or diamond electrode.

Preferably, the addition of the mixture to the aqueous medium is carried out with stirring.

The aqueous medium may optionally contain cement; if it contains it, the aqueous medium may, for example, be in the form of a suspension, a mud or a cementitious milk; if it does not contain any, the cement will be introduced afterwards, after the concentration of additive (s) has been checked.

The invention also provides a device for checking the quantity of at least one cement additive which is present in an aqueous medium, a mixture comprising the additive and an iodide salt being added to the aqueous medium, the ratio between the quantities of additive and iodide salt present in the mixture being known, the device being characterized in that it comprises:

- A stabilization system configured to stabilize the quantity of iodide ions present in the aqueous medium by application, in the aqueous medium, of a current configured so that the molar concentration of triiodide ions in the aqueous medium is greater than or equal to 10 ⁵ times the molar concentration of iodide ions in the aqueous medium;

an electrochemical sensor for determining a quantity of iodide ions present in the aqueous medium;

a calculation unit for calculating the quantity of additive present in the aqueous medium by correlation between the quantity of iodide ions determined by the sensor and the ratio between the quantities of additive and iodide salt present in the mixture added to the aqueous medium .

Advantageously, the stabilization system comprises:

- a power supply configured to supply current;

at least a first and a second electrode intended to be introduced into the aqueous medium in order to apply the current thereto;

and wherein the stabilization system and the electrochemical sensor have at least two electrodes in common. Preferably, the current supplied by the power supply is in the form of a plurality of electrical pulses whose polarity is reversed by alternation.

Preferably, at least one electrode in common is made of doped diamond.

Preferably, the power supply of the stabilization system is supplied by a power supply of the electrochemical sensor and the first and the second electrode of the stabilization system are in common with the first and the second electrode of the electrochemical sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of nonlimiting example, and made with reference to the appended drawings in which:

- Figure 1 is a schematic representation of the device according to the invention according to a particular embodiment;

FIG. 2 represents the voltammograms (current as a function of the voltage) obtained with the method according to the invention for 5 different samples (a mixture of additives diluted 10 times) having an increasing Nal concentration (respectively of 0.2, 4, 6, and 8 g / L);

- Figure 3 shows the calibration points of the current peaks (A) as a function of the concentration of Nal (g / L) in a buffer solution of pH9 and in the diluted mixture studied in Figure 2;

- Figure 4 represent the voltammograms obtained with the method according to the invention for 5 other different samples (the mixture of undiluted additives) having an increasing Nal concentration (respectively of 0, 2, 4, 6, and 8 g / L);

FIG. 5 represents the calibration points of the current peaks (A) as a function of the concentration of Nal (g / L) in a buffer solution of pH 9 and in the undiluted mixture studied in FIG. 4.

DETAILED PRESENTATION OF A PARTICULAR EMBODIMENT

The objective of the invention is to present an efficient, rapid and inexpensive technical solution which makes it possible to control in the field (in situ) the concentration of at least one additive in an aqueous medium for the preparation of cement.

To do this, at least one critical additive is added to this, which one seeks to quantify an electrochemical marker (or “tag-redox” in English) whose concentration in the additive must be known upstream.

The aqueous mixture is then produced by introducing a concentration of marked additive to be determined in water. It is also possible to label several additives with a known concentration of iodide ions.

In the context of the present invention, sodium iodide (Na) is preferably used as the electrochemical marker, but it could also be, for example, potassium iodide (K1).

An iodide salt (such as sodium iodide) can be oxidized to a diiode, which can be detected by electrochemical voltammetry. To do this, electrochemical detection comprises bringing the electrodes of the electrochemical sensor into contact with the aqueous medium and measuring the difference in electrical potential between the working electrode and the reference electrode.

As is known, when a chemical species present in the aqueous medium has a redox activity, the voltammogram obtained reveals a peak whose maximum current level is given by the Randles-Sevcik equation at 25 ° C:

I _p = (2.69 X lO ^ n ^ v ^ ACD ^1/2 where _p is the intensity at the peak (A) n is the number of electrons exchanged; A is the surface of the electrode working (cm ² ); C is the concentration of the redox species (here, the electrochemical marker) (mol.cm ³ ); D is the diffusion coefficient of the redox species in the solution (here, the aqueous medium ) (cm ² / s); v is the voltage sweep speed (V / s).

From this well-known equation in the electrochemistry field, we deduce that the current peak is directly proportional to the concentration of the redox species. Thus, the electrochemical sensor can be calibrated and used in different additive formulations to quantify their concentration of sodium iodide.

However, problems of reproducibility and reliability of the measurement were revealed during the development of this technique, making it unusable under the desired experimental conditions. By way of example, the inventors have found that, by performing an electrochemical detection using sodium iodide as an electrochemical marker in a solution containing only water with a bottom salt such as potassium chloride, a variation of the intensity of the current peak greater than 10%. This phenomenon of variation of the intensity of the peak is further amplified in the presence of certain additives. However, this variation in the intensity of the peak directly causes a significant error in the concentration of the additive to be dosed.

This is due to the chemistry of iodine. Indeed, the diiode formed during the electrochemical analysis is not stable in solution and disturbs the electrochemical equilibria.

In addition, certain additives exhibit electrochemical activity at potentials close to that of the oxidation of sodium iodide to diiode. For example, in aqueous solution at a pH of 7, one can have the following electrochemical reactions:

2 / (oq) * ^ 2 (s) T 2 <? (£ ⁼ 0.5345 F)

2r _(aCi ^ I _{2 (aci)} + 2e- (E ° = 0.62 F)

3 / ”( _{a £?} ) 4 (aq) + 2e” (£ = 0.5355 F)

The impact of these additives on the result of the measurement of the iodide ion concentration is the presence of an offset on the value of the intensity of the current peak. This shift therefore artificially increases the value of the concentration of electrochemical marker and, by extension, that of the additive whose concentration is sought. This problem has been encountered with many additives having a more or less marked redox activity. The absence of correction of this offset leads to large variations in measurement errors as a function of the additives used, as well as according to their concentrations. This phenomenon therefore leads to an overvaluation of the real value of the additive to be checked.

Added to this is the aging and fouling of the working electrode. Indeed, many additives can come to agglomerate or adsorb on the surface of the working electrode. In addition, in the particular case of the detection of the electrochemical marker by voltammetry, it is necessary to carry out a potential scan. The application of certain potentials during this scanning can lead to the initiation of polymerization reactions of species present in solution by providing energy from the working electrode. Thus, it is very common to see a working electrode become clogged when it is used in complex media containing polymers. This fouling will have the impact of reducing the intensity of the peak observed by the oxidation of sodium iodide and therefore an error on the quantification of the additive to be checked.

The combination of these different problems made the results obtained by electrochemical detection very difficult to interpret and did not allow reliable and reproducible measurements to be obtained.

The inventors solved these problems by proceeding, before electrochemical detection, to stabilize the concentration of iodide ions Γ in the aqueous medium. This makes it possible to obtain a reliable and reproducible value of the concentration of sodium iodide in the presence of additives interfering with the measurement.

According to a preferred embodiment of the invention, we use, to carry out such a measurement, a device which comprises an electrochemical sensor, which is provided with:

- a diamond electrode with a doping rate> 10 ² ° cm ³ and whose surface is precisely known;

-a counter electrode stable over time, for example a platinum or diamond electrode;

a reference electrode which cannot be modified by the various additives, for example a platinum or diamond electrode.

The electrochemical sensor also comprises a means for carrying out the measurements, for example a potentiostat, which comprises a current supply.

The device according to the invention also includes a stabilization system which comprises:

- a power supply configured to supply a current, for example in the form of a plurality of electrical pulses whose polarity is reversed by alternation;

- at least a first and a second electrode;

at least two electrodes of the stabilization system and of the electrochemical sensor being in common. Preferably, current is applied to the working electrode and to the counter electrode.

Finally, the device according to the invention comprises a calculation unit for calculating the amount of additive present in the aqueous medium by correlation between the amount of iodide ions determined by the sensor and the ratio between the amounts of additive and iodide salt present in the mixture added to the aqueous medium.

As the aqueous medium to be analyzed, we use a mixture of additives, of which at least one additive is associated with an electrochemical marker of the iodide ion type at a known concentration. Preferably a stirring system is used to homogenize the mixture.

The electrodes of the sensor and those of the stabilization system are brought into contact with the mixture of additives. According to a preferred embodiment illustrated in FIG. 1, the first and the second electrode of the stabilization system are the working electrode of the sensor and the counter electrode of the sensor; the stabilization system supply is from the sensor. Thus, according to the preferred embodiment illustrated in FIG. 1, the device 1 according to the invention comprises a set of three electrodes, including a working electrode 2, a counter electrode 3 and a reference electrode 4, the set of electrodes being immersed in the aqueous medium 5 to be analyzed, which is mixed using a stirrer 7; the sensor electrode assembly is connected to the sensor potentiostat 6; the first and the second electrode of the stabilization system are respectively the working electrode 2 and the counter electrode 3 of the sensor and the supply of the potentiostat of the sensor is also the supply of the stabilization system.

Before proceeding with the electrochemical measurement of iodine in solution, we will stabilize it. For this, and according to a preferred embodiment of the invention, current pulses are applied to the working electrode, these pulses being alternately and successively of opposite polarities.

The pulse setting parameters (number of pulses, current density, duration of each pulse, etc.) depend on the concentration of iodide ion Γ present in the aqueous medium to be analyzed (therefore on the quantity of Nal introduced into the medium ). The number of pulses as well as the current density used will depend on the volume of the aqueous medium to be analyzed and on the surface of the working electrode. For example, for an aqueous medium with a volume of 20 mL, a working electrode having an area of 1 cm ² , and an amount of Nal of between 0.01 g / L and 50 g / L, preferably between 1 g / L and 10 g / L, we can use, to stabilize the solution, an alternating current configured so as to obtain between 2 and 100 pulses, alternately of opposite polarities, with a current intensity between 0.1 mA and 100 mA. For an amount of 0.1 g of sodium iodide introduced into the aqueous medium, we consider that at least one positive current pulse and one negative current pulse are required, each of a duration greater than or equal to 100 ms, with current levels of at least 1 mA for an electrode having a surface of 1 cm ² .

After stabilizing the iodine in solution, dosimetry measurements can be performed. Several techniques known to those skilled in the art can be used.

Conventionally, an electrochemical sensor must be calibrated in the aqueous media in which it is brought to work. The advantage of the method and the device according to the invention is that they make it possible to dispense with a calibration of the sensor in all possible unknown aqueous mixtures, by carrying out some measurements prior to the assay. To do this, you must know the following parameters:

- the diffusion coefficient of iodine in the aqueous medium to be analyzed;

- the diffusion coefficient of iodine in the calibration solution;

- the response of the signal in the aqueous medium to be analyzed without the presence of the electrochemical marker;

- the signal response in the calibration solution without the presence of the electrochemical marker;

- the surface of the working electrode used.

A first method for carrying out a dosage of iodide ions consists in determining the various parameters set out above. Control of these parameters makes it possible to determine an unknown concentration of an additive in an aqueous medium, that is to say in a solution in which the electrochemical sensor of the device according to the invention has not been pre-calibrated in the aqueous medium to be analyzed. Our calculation model is based on the following equations:

Ip = Ip (I) + Ip (Middle) lp (n = 0.8926 FA [/ “]

IpiMidieu) = 0.4463 FA (U) '£ („f ^{/ 2} i

where A is the surface of the working electrode (cm ² ); F is the Faraday constant (C.mol ^-1 ); v is the scanning speed (V / s); R is the ideal gas constant (JK ^-1 .mol ^-1 ); T is the temperature (K); n, is the number of electrons exchanged by the active species i; [i] is the concentration of the active species i (mol.L ^_1 ); D (r / Medium) is the diffusion coefficient of the iodide ions in the aqueous medium to be analyzed (cm ² / s); D (i / Medium) is the diffusion coefficient of the active species i in the aqueous medium to be analyzed (cm ² / s); l _p is the maximum current level (A).

A second method for carrying out a dosage of iodide ions makes it possible to dispense with the measurement of these parameters thanks to the presence of an internal standard. In this case, a first measurement can be carried out in the aqueous medium (before the addition of a mixture comprising the additive to be dosed and an iodide salt), then a second measurement is carried out after a controlled addition of the mixture comprising the additive and an iodide salt. This technique can be used for all amperometric and voltametric dosing methods. The difference in signal will allow the sensor to be calibrated before adding the marked additive.

The device according to the invention has many advantages.

The device is compact and of small dimensions; it is inexpensive and can be installed directly on the cementing units; Finally, it allows real-time control of the concentration of additives in the aqueous medium.

In addition, the device can be used regardless of the mixture of additives used. In fact, unlike conventional electrochemistry sensors, the device according to the invention does not require calibration of the sensor in each medium that one wishes to analyze.

The method and the device according to the invention make it possible to deduce a concentration of one or more additives from a known calibration. The method and the device according to the invention have been tested on mixtures containing additives used in petroleum cements. The technique has proven to be very effective. According to the results obtained, this technique is indeed more than 90% reliable and reproducible with less than 0.5% measurement errors.

We will now describe in detail the results obtained with two different aqueous media.

We used sodium iodide (Nal, purity> 99.5%) and potassium chloride (KCI, purity> 99.0%) supplied by the company Sigma-Aldrich; these compounds were used as such.

The additives used were supplied by Schlumberger and are as follows. It should be noted that the references given to the additives do not correspond to the commercial references of Schlumberger.

Reference Main effect Compound main Known as an additive interfere with measurement Al self-timer potassium pentaborate no A2 self-timer acrylamide no A3 self-timer sodium lignosulfonate Yes A4 defoamer silicate Yes AT 5 additive against gas migration latex no A6 additive reducing the cement density silica smoke no

Table 1: nomenclature and main properties of the additives used

These compounds were introduced in the proportions below and mixed to obtain a given mixture.

Reference % in weight Weight (g) Water 41.7 147.397 Al 21.7 77,559 A2 6.2 22.16 A3 1.1 4.03 AT 5 0.7 2,598 A6 23.5 83,714 A7 5.1 18,466

Table 2: mixture used to produce the aqueous medium to be analyzed

For water, we used pure deionized water with a resistivity of 18.2 MQ.cm ¹ , obtained using the Millipore DirectQ ™ 3 UV water purification device. But we could just as easily have used tap water.

We have carried out electrochemical measurements of various aqueous media by voltammetric analysis using the device as illustrated in FIG. 1. The potentiostat is an Autolab PGSTAT 302 of the brand Ecochemie; the counter electrode is a platinum mesh electrode (nominal mesh opening of 0.25 mm); the working electrode is a boron-doped diamond electrode obtained using an MPCVD reactor, the electrode having a surface of 0.82 cm ² and a doping level> 10 ²¹ at / cm ³ ; the reference electrode is a platinum wire having a diameter of 1 mm.

We performed a calibration in a mixture diluted 10 times and a calibration directly in a mixture.

Calibration in a mixture diluted 10 times:

The mixture shown in Table 2 above was diluted in pure water containing KCI to a concentration of 0.1 µM.

From this diluted mixture, we made a first sample (sample 1) and we made 4 other samples by adding respectively a quantity of Nal of 2 g / L (sample 2), 4 g / L (sample 3) , 6 g / L (sample 4) and 8 g / L (sample 5).

Each sample (except for sample 1) was first stabilized by the activation step (here, by applying a rectangular periodic alternating current comprising 50 pulses of 100 ms at ± 20 mA), then was analyzed by performing 5 linear voltammetric scans from 0 V to IV. The samples were shaken between each measurement to ensure their homogeneity.

The voltammograms obtained for each of these 5 samples are shown in Figure 2 (curves 1, 2, 3, 4, 5 respectively representing the results for samples 1, 2, 3, 4, 5) and allow the device to be calibrated of measurement. An oxidation peak is observed at around 0.8 V, which corresponds to the oxidation of the dissolved iodide.

If we analyze the current peak as a function of the Nal concentration over this range of concentrations (2, 4, 6 and 8 g / L), we obtain a linear variation.

The aqueous medium of each sample has a pH equal to 9. In order to validate this calibration, we compared these regression data with measurements carried out in a buffer solution having a pH of 9 at the same Nal concentrations. The results are shown in Figure 3.

It is noted that the points obtained for the calibration of the device according to the invention have perfect overlap with the measurement points obtained in the buffer solution, which makes it possible to conclude that the additives contained in the aqueous media have no impact on the measurement obtained according to the invention and that the measurement method according to the invention is correct.

Calibration directly in a mixture:

A direct electrochemical measurement can only be obtained if the solution is capable of conducting an electric current. This is the case of the mixture described in Table 2, since it has a conductivity at 20 ° C of 18 mS / cm ² .

We prepared a sample from the mixture described in table 2 (sample 11) and four other samples by adding to the mixture described in table 2 respectively an amount of Nal of 2 g / L (sample 12), 4 g / L (sample 13), 6 g / L (sample 14) and 8 g / L (sample 15).

As for the calibration in the mixture diluted 10 times, each sample was first of all stabilized by the activation step (here, by applying a rectangular periodic alternating current comprising 200 pulses from 100 ms to ± 20 mA, therefore 100 pulses at + 20 mA and 100 pulses at - 20 mA), then was analyzed by carrying out 5 linear voltammetric scans from 0V to IV. The samples were shaken between each measurement to ensure their homogeneity.

The results are represented in FIG. 4 (curves 11, 12, 13, 14, 15 respectively representing the results for the samples 11, 12, 13, 14, 15).

We see that there are two oxidation peaks, one around 0.7 V and the other around 0.85 V.

These two peaks could be attributed to the oxidation of iodine and further investigation is necessary to fully understand the chemistry of the reaction.

To perform a calibration from the voltammograms illustrated in Figure 4, we used the first oxidation peak since it is better defined than the second.

The pH of the samples being 9, we compared the calibration points obtained with the calibration previously obtained in a buffer solution of pH 9.

The results obtained are presented in Figure 5.

Perfect coverage is obtained with the measurement points obtained in the buffer solution and those obtained in the samples after correction, which makes it possible to conclude that the additives contained in the aqueous media analyzed have no impact on the measurement obtained according to the invention and that the measurement method according to the invention is correct.

In the context of the present invention, we are interested in the petroleum field for the production of boreholes, but high performance cements are used in many other fields, in particular that of construction. In general, the invention may be useful in all fields using cement, and in particular in the case of critical or very costly operations (large installations), where the possibility of being able to control the formulation of additives would be of great interest for guarantee the success of operations.

REFERENCES CITED [1] US 4,003,431 [2] US 2011/0127034 Al

Claims (11)

1. Method for checking the quantity of at least one cement additive which is present in an aqueous medium to which the additive has been added, characterized in that it comprises the following steps: a) adding, to the aqueous medium, a mixture comprising the additive and an iodide salt, the ratio between the amounts of additive and iodide salt present in the mixture being known; b) stabilization of the amount of iodide ions present in the aqueous medium by application, in the aqueous medium, of a current configured so that the molar concentration of triiodide ions in the aqueous medium is greater than or equal to 10 ⁵ times the molar concentration in iodide ions of the aqueous medium; c) determination of the quantity of iodide ions present in the aqueous medium by means of an electrochemical sensor; d) calculation of the amount of additive present in the aqueous medium by correlation between the amount of iodide ions determined in step c) and the ratio between the amounts of additive and iodide salt present in the mixture in step at). 2. The method of claim 1, wherein the iodide salt is sodium iodide. 3. Method for verifying the quantities of at least a first and a second cement additive which are present in an aqueous medium to which the first and the second additive have been added, characterized in that it comprises the following steps: a) adding, to the aqueous medium, a first mixture comprising the first additive and a first iodide salt, the ratio between the quantities of the first additive and the first iodide salt present in the first mixture being known; b) stabilization of the amount of iodide ions present in the aqueous medium by application, in the aqueous medium, of a current configured so that the molar concentration of triiodide ions in the aqueous medium is greater than or equal to 10 ⁵ times the molar concentration in iodide ions of the aqueous medium; c) determination of the quantity of iodide ions present in the aqueous medium by means of an electrochemical sensor; d) calculation of the quantity of the first additive present in the aqueous medium by correlation between the quantity of iodide ions determined in step c) and the ratio between the quantities of the first additive and of the first iodide salt present in the first mixture with l 'step a); e) adding, to the aqueous medium, a second mixture comprising the second additive and a second iodide salt, the ratio between the quantities of the second additive and of the second iodide salt present in the second mixture being known; f) stabilization of the amount of iodide ions present in the aqueous medium by application, in the aqueous medium, of a current configured so that the molar concentration of triiodide ions in the aqueous medium is greater than or equal to 10 ⁵ times the molar concentration in iodide ions of the aqueous medium; g) determination of the quantity of iodide ions present in the aqueous medium by means of the electrochemical sensor; h) calculation of the amount of the second additive present in the aqueous medium by correlation between the amount of the iodide ions determined in step g) and the ratio between the amounts of the second additive and of the second iodide salt present in the second mixture at l 'step e). 4. The method of claim 3, wherein the first and second iodide salt are sodium iodide. 5. Method according to any one of claims 1 to 4, in which, in the stabilization step (steps), the current is configured so that the molar concentration of triiodide ions in the aqueous medium is equal to more or less 25%, at 10 ³ times the molar concentration of iodide ions in the aqueous medium. 6. Method according to any one of claims 1 to 5, in which the stabilization is obtained by the application, on a first and a second electrode of the electrochemical sensor, of a plurality of electrical pulses whose polarity is reversed by sandwich course. 7. The method of claim 6, wherein at least one of the first and second electrodes is of doped diamond. 8. Device (1) for checking the quantity of at least one cement additive which is present in an aqueous medium (5), a mixture comprising the additive and an iodide salt being added to the aqueous medium, the ratio between the quantities of additive and iodide salt present in the mixture being known, the device being characterized in that it comprises: - A stabilization system configured to stabilize the amount of iodide ions present in the aqueous medium, by application, in the aqueous medium, of a current configured so that the molar concentration of triiodide ions in the aqueous medium is greater than or equal to 10 ⁵ times the molar concentration of iodide ions in the aqueous medium; an electrochemical sensor for determining a quantity of iodide ions present in the aqueous medium; a calculation unit for calculating the quantity of additive present in the aqueous medium by correlation between the quantity of iodide ions determined by the sensor and the ratio between the quantities of additive and iodide salt present in the mixture added to the aqueous medium . 9. Device according to claim 8, in which the stabilization system comprises: - a power supply configured to supply current; at least a first and a second electrode intended to be introduced into the aqueous medium in order to apply the current thereto; and wherein the stabilization system and the electrochemical sensor have at least two electrodes in common. 10. Device according to claim 9, in which at least one electrode in common is made of doped diamond. 11. Device according to claim 9 or claim 10, in which the power supply of the stabilization system is supplied by a power supply of the electrochemical sensor and in which the first and the second electrode of the system 10 of stabilization are in common with the first (2) and the second (3) electrode of the electrochemical sensor. S.60753 1/3