MXPA06007846A - Electro-chemical sensor - Google Patents

Abstract

An electro-chemical sensor is described having two molecular redox systems sensitive to the same species and having an detector to detect relative shifts in the voltammograms of the two redox systems.

Description

ELECTROCHEMICAL SENSOR The invention relates to a chemical sensing tool for use at the bottom of the well and methods for analyzing fluids produced from underground formations. More specifically, it refers to an electrochemical sensor for downhole pH and ion content analysis of effluents produced from underground formation.

BACKGROUND OF THE INVENTION Analyzing the representative samples of downhole fluids is an important aspect in determining the quality and economic value of a hydrocarbon formation. The operations to date obtain a flux analysis from the bottom of the well, usually through wire recording using a training tester such as the MDT ™ tool from Schlumberger Oilfield Services. However, more recently, it is suggested to analyze fluids from the bottom of the well, either through sensors permanently or almost permanently installed in a sounding or through a sensor mounted on the sounding column. The last method, if implemented successfully, has the advantage of obtaining data while drilling, while the previous installation could be part of a control system for drilling and hydrocarbon production thereof. To obtain an estimate of the downhole fluid composition, the MDT tools use an optical probe to estimate the amount of hydrocarbons in the samples collected from the formation. Other sensors use resistivity measurements to discern various components of the formations fluids. Particularly, the knowledge of the water chemistry of the well bottom formation (produced) is necessary to save costs and increase production in all stages of gas and oil production and exploration. The knowledge particularly of water chemistry is important for a number of key processes of hydrocarbon production, including: Prediction and valuation of corrosion and mineral scale; Strategy for oil / water separation and water reinjection; - Understanding of container compartment / flow units; Characterization of water saturation; Derivation of the water cut Rw; and Evaluation at the bottom of the H2S division, water or oil (if used for H2S measurement). Some chemical species dissolved in water (such as, for example, Cl "and Na +) do not change their concentration when they are removed for the surface either as a part of a flow through a well, or as a sample taken from the bottom of the well .

Consequently, the information about their quantities can be obtained from samples from the bottom of the well and in some cases the surface samples of a flow. However, the state of the chemical species, such as H + (pH = -log [H + concentration], CO2, or H2S) can change significantly as they travel to the surface.The change occurs mainly due to a difference in temperature and pressure between the bottom of the well and the environment of the surface.In case of sampling, this change can also happen due to degassing of a sample (seal failure), precipitation of mineral in a sample bottle, and (especially in case of H2S) - a chemical reaction with the sampling chamber It must be understood that pH, H2S, or CO2 are among the most critical parameters for scale and corrosion assessment, consequently, it is of considerable importance to have their bottomhole values precisely The concentration of protons or their logarithm pH can be considered as the most critical parameter in water chemistry, it determines the speed of many important chemical reactions as well as the solubility of chemical compounds in water, and (by extension) in hydrocarbon. Therefore, there is and will continue to be a demand for downhole chemical measurements. However, no downhole chemical measurement currently performed on a gas and oil producing well has been reported so far, through many different methods and tools that have been proposed in the relevant literature. The general bottomhole measurement tools for applications in the oil field are known as such. Examples of such tools are found in U.S. Patent Nos. 6,023, 340; 5.51 7.024; and 5,351, 532 or in International Patent Application WO 99/00575. An example of a probe for potentiometric measurements of groundwater containers is also published as: Solodov, I. N. , Velichkin, V. I. , Zotov, A.V. et al., "Distribution and Geochemistry of Contaminated Subsurface Waters in Fissured Volcanogenic Bed Rocks of the Karachai Lake Area, Chelyabinsk, Southern Urals" in: Lawrence Berkeley Laboratory Report 36780 / UC-603 (1 994b), RAC-6, Ca, USES. The known state of the art in the field of high temperature potentiometric measurements and tooling is described, for example, in published UK patent application GB-2362469 A. A number of chemical analysis tools are known from chemical laboratory practice. Such known analysis tools include, for example, the various types of chromatography, spectral and electrochemical analysis. In particular, the potentiometric method has been widely used for water composition measurements (pH, Eh, H2S, CO2, Na \ CI, etc.) both in the laboratory and in the groundwater quality control field. U.S. Patent No. 5,223, 1 1 7 discloses a two-term thermostatic microsensor having an internal reference using molecular self-assembly to form a system in which the reference electrode and the indicator electrode are both on the electrode The reference molecule is described as a redox system that is insensitive to pH, while the indicator molecule is formed by a redox system based on hydroquinone having a potential that changes with pH. The reference and indicator molecule are prepared by self-assembly in gold microelectrodes (Au) In the known microsensor, a pH reading is derived from peak readings of the voltagrams. However, they are often not suitable for application in drilling with demands for solidity, stability and low maintenance and energy consumption rarely satisfying. It is therefore an object of the present invention to provide apparatuses and methods for performing electrochemical measurements in hydrocarbon wells during drilling and production. More specifically, it is an object of the present invention to provide robust sensors for molecularly selective electrochemical measurements, in particular pH measurements.

BRIEF DESCRIPTION OF THE INVENTION The invention achieves its objects by providing an electrochemical sensor having a measuring electrode with at least two receptors sensitive to the same species. In a preferred variant of the invention, the sensors are a redox system, based, for example, on anthraquinone chemistry. The substrate on which the redox system is mounted is preferably based on carbon in one of its elementary forms such as graphite, carbon powder, diamond. In a variant of the invention, the substrate can be derived nanotubes, including multi-walled nano-tubes. An electrochemical technique using a method or sensor according to the present invention can be applied, for example, as part of a production registration tool or an open hole forming testing tool (such as the Modular Dynamic Tester, MDT ™). In the latter case, the technique can provide a real-time water sample validation at the bottom of the well or downhole pH measurement which, in turn, can be used to predict mineral scale and corrosion assessment. These and other features of the invention, preferred embodiments and variants thereof, possible applications and advantages will be appreciated and understood by those skilled in the art from the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS 1 shows a schematic diagram of the main elements of a known voltammetric sensor; 2 shows a schematic diagram of the main elements of a known electrochemical microsensor and its operation; 3 shows a schematic diagram of a well-bottom well probe using potentiometric sensors; 4A illustrates the surface structure of a measuring electrode according to an example of the invention; 4B illustrates the surface structure of a measuring electrode with internal reference electrode according to another example of the invention; 4C illustrates the redox reaction of a measuring electrode according to another example of the invention using multi-walled carbon nanotube. 4D illustrates the redox reaction of a measuring electrode with internal reference electrode according to another example of the invention using multi-walled carbon nanotube; 4E illustrates a diagram of the geometrical surface of the electrode of 4B; 5 is a perspective view, partially cut away, of a sensor according to a sample of the present invention in a tool for the bottom of the well; 6 shows recorded voltammograms of an electrochemical microsensor according to the present invention at three different pH values; 7A illustrates the maximum potential change for atranquinone, diphenyl-p-phenylenediamine and a combination of the two redox systems; 7B-C are diagrams of maximum potential against pH for the redox systems of . 4C and 4D, respectively, over the range of pH 1.0 to pH 12.0 to 293K at various conditions; 8 illustrates an example of a sensor according to the invention as part of a cable formation test apparatus in a sounding; 9 shows a probe and the lower part of a sounding column including the lower hole assembly, with a sensor according to the invention; and 1 0 shows a sensor located downstream of a venturi-type flow meter, according to the invention.

DETAILED DESCRIPTION OF THE INVENTION The theory of voltammetry and its application to measurements of surface water at ambient temperatures both develop well. The method is based on the measurement of the electromotive force (e.m.f.) or potential E in a potentiometric cell that includes measuring and reference electrodes (medium cells). 1 shows the general components of a known voltammetric cell 1 0. A measuring electrode 1 1 is inserted into a 1 3 solution. This electrode consists of an internal medium (for example, Ag wire covered by an AgCl salt) in a solution of a fixed pH (for example, 0.1 M HCl in some pH electrodes), and a selective membrane of 1 1 1 ion (as selective membrane of glass H + in glass electrode pH). The reference electrode 1 2 also contains an internal average element (typically the same AgClAg) inserted in a concentrated KCI solution (for example, 3M) / gel saturated with Ag +, which diffuses (or flows) through the reference junction (liquid) 1 21. The selective electrode of ion 1 1 measures the potential that originates due to the difference in activity or concentration of a corresponding ion (H + in case of pH) in the internal solution and in the measured solution. This potential is measured against the reference potential at the reference electrode 12, which is set due to a constant composition of a reference solution / gel. The electrodes can be separated (separate middle cells), or combined into one ("combination electrode"). The e. mf measure is a total function of the temperature and activity of an ion / 'th, to which the measuring electrode is selective: [1] E = E ° + (k * T) * log (a,), where E is the measured electromotive force (em. f.) of the cell (all potentials are in V), a, corresponds to the activity of the ion / 'th and is proportional to its concentration. E ° is the standard potential (at temperature T) corresponding to the value E in a solution with the activity of the ion / th equal to one. The term in parentheses is the so-called Nernstian slope in a diagram of E as a function of log (ai). This slope (or constant "k") together with the constant (E °) of the cell (electrode) is determined experimentally through a calibration procedure using standard solutions with known activities of the ion / 'th. For electrodes without damaging of good quality this slope must be very close to the theoretical, equal to (R * T / F * z), where F is the Faraday constant (23061 cal / mol), R is the gas constant ( 1.9872 cal / mol K), zi is the charge of the ion / 'th. The Nernst equation [1] can be rewritten for the pH sensors, that is, log a (H +) as [2] E0.5 = K- (2,303 RTm / nF) pH where E0 5 is the average wave potential of the Redox system included, K is an arbitrary constant, R is the ideal gas constant, m is the number of protons and n is the number of electrons transferred in the redox reaction. The microsensor of U.S. Pat. UU do not. 5,223,117 is illustrated in FIG. 2. FIG. 2A shows a schematic electrochemical sensor with an opposite electrode 21 and a relatively much smaller Au substrate (by a factor of 1000) 22 carrying two molecular species M and R. The R species forms an insertion reference electrode, and the species M is an indicator electrode with specific receptors or sensitivity for a third species L. The schematic linear sweep voltammogram in the upper half of FIG. 2C shows the difference in the current peaks for oxidation in the normal state. When the third species L joins M (FIG.2B), this difference increases as illustrated by the change of peaks in the lower half of FIG. 2C, thus providing a measurement for the concentration of L in the solution surrounding the sensor. In the context of the present invention, it is important to note that R is specifically selected for being insensitive to species L, eg, pH. In FIG. 3, elements of a well-known well analysis tool 30 as used by Solodov et al. (See background) are illustrated schematically. The body of the tool 30 is connected to the surface through the cable 31 that transmits energy and signals. A computer console 32 controls the tool, monitors its activity and records the measurements. The tool 30 includes a sensor head with a number of selective electrochemical probes 33 each sensitive to a different molecular species. Also housed in the body of the tool are additional drive parts 34 operating the head, a test system 35 and transceivers 36 for converting the measurements into a data stream and for communicating such data stream to the surface. The electrodes are located at the bottom of the probe and include those for pH, Eh (or ORP), Ca2 + (pCa), Na + (pNa), S2"(pS), N H4 + (pNH4), and reference electrode (RE) The partial pressure of H2S can be calculated from pH and pS readings. detail aspects and elements of the present invention The present invention introduces a new molecular system in which the redox characteristics of two molecules are combined, thus leading to a considerably higher accuracy and, in turn, ability to deploy at the bottom of the Well In a preferred embodiment for a pH sensitive sensor an anthraquinone is homogeneously derived on carbon particles (AQC) The AQC system is derived using 2 g of carbon powder (1.5 μm in average diameter) mixed with a 10 cm3 solution containing 5 mM of Fast Red AL Salt (anthraquinone-1-diazonium chloride) to which 50 mM is added of hypophosphoric acid (50%). The reaction is allowed to remain with occasional stirring at 5 ° C for 30 minutes, after which it is filtered by suction of water. The excess acid is removed by rinsing with distilled water and with the powder rinsing finely with acetonitre to remove any unreacted diazonium salt in the mixture. It is then air dried by placing it inside a ventilation hood for a period of 12 hours and finally it is stored in an airtight container. In a similar manner, phenanthrenequinone (PAQ) It is prepared as a second molecular species to undergo a redox reaction. Alternatively, N, N'-diphenyl-p-phenylenediamine (DPPD) nailed to carbon particles undergoes a redox process as jo: The binding of DPPD to carbon is achieved by mixing 4 g of carbon powder with 25 μL of solution of 0.1 M HCl + 0.1 M KCI and 20 mM DPPD in acetone. The reaction mixture is stirred continuously for 2 hours in a beaker and then filtered after which it is rinsed with distilled water to remove excess acid and chloride. It is then air dried by placing it inside a ventilation hood for 12 hours and finally stored in an airtight container. In a static environment where the surface of the sensor is not exposed to a flow, it is possible to immobilize DPPD crystals insoluble in water directly on the surface of the electrode. However in the sounding environment it is preferred to bind the sensitive molecules through a chemical bond to such a surface. The derived carbon powders are immobilized in an abrasive manner on a basic flat pyrolytic graphite (BPPG) electrode before the volumetric characterization following a procedure described by Scholz, F. and Meyer, B., "Voltammetry of Solid Microparticles Immobilized on Electrode Surfaces in Electroanalytical Chemistry "ed. A.J. Bard, and I. Rubenstein, Marcel Dekker, New York, 1998, 20, 1. initially the electrode is polished with glass polishing paper (H00 / 240) and then with silicon carbide paper (P1000C) for uniformity. The carbons are first mixed and then immobilized on BPPG by gently rubbing the electrode surface on a fine quality filter paper containing the functionalized carbon particles. The resulting modified electrode surface is illustrated schematically by FIG. 4A showing an electrode 41 with DPPD and AQC attached. It is further advantageous to add an internal pH reference including a redox pair independent of pH to increase the stability of any voltammetric reading, thus surrounding uncertainties caused by embedding of the external reference electrode. In the configuration, the sensor includes two reference electrodes. A stable reference molecule is, for example, K5MO (CN) or polyvinylferrocene (PVF) both have a stable redox potential (K5MO (CN) 8 at approximately 521 mV) which is sufficiently separated from the expected change of redox signals from the two indicator species over the pH range of interest. As shown in Table 1, both the reduction and oxidation potentials of K5MO (CN) 8 are almost constant throughout the entire pH range of interest. TABLE 1 Reference species based on Mo can be retained in the solid substrate through ionic interactions with co-existing cationic polymer, such as poly (vinylpyridine), which is embedded in the solid phase. Other independent pH species, such as ferrocyanide, are less suitable since the redox peaks are obscured by the redox measuring system signals. In FIG. 4B the electrode 42 carries bound molecules AQC and PAQ together with PVF as an internal reference molecule. The most common forms of conductive carbon used in electrode fabrication are vitreous carbon, carbon fibers, natural gas carbon black, various forms of graphite, carbon paste and carbon epoxy. An additional form of carbon, which has seen a great expansion in its use in the field of electrochemistry since its discovery in 1991, is the carbon nanotube (CNT). The structure of CNTs approximates rolled sheets of graphite and can be formed as either multi-walled or single-walled tubes. Single wall carbon nanotubes (SWCNTs) are a unique, hollow graphite tube. The multi-walled carbon nanotubes (MWCNTs) on the other hand consist of several concentric tubes fitted one inside the other. Prior activation methods for attaching an active redox species to carbon or graphite surfaces can be extended through chemical reduction of aryldiazonium salts with hypophosphoric acid, to include covalent derivative of MWCNTs by anthraquinone-1-diazonium chloride and 4-nitrobenzenediazonium tetrafluoroborate . This results in the synthesis of 1-antatraquinonyl-MWCNTs (AQ-MWCNTs) and 4-nitrophenyl-MWCNTs (NB-MWCNTs) as shown in FIGS. 4C and 4D, respectively. The respective substrates 46 and 47 are multi-walled carbon nanotubes. The derivative MWCNT preparation process includes the following steps: first 50 mg of MWCNTs are stirred in 10 cm3 of a 5 μM solution of either Fast Red AL (anthraquinone-1-diazonium chloride) or Fast Red GG (4 -nitrobenzenediazonium tetrafluoroborate), to which 50 cm3 of hydrophosphoric acid (H3PO2, 50% w / w in water) is added. Then the solution is allowed to remain at 5 ° C for 30 minutes with gentle agitation. After which, the solution is filtered by suction of water to remove any unreacted species from the MWCNT surface. The additional rinse with deionized water is carried out to remove any excess acid and finally with acetonitrile to remove any unreacted diazonium salt from the mixture. Derived MWCNTs are then air dried by placing them inside a ventilation hood for a period of 12 hours after which they are stored in an airtight container before being used. The untreated multi-walled nanotubes can be purchased from commercial vendors, for example from Nano-Lab I nc of Brighton, MA, USA in 95% purity with a diameter of 30 + / 15- and a length of 5-20 μm. The reduction of diazonium salts using hypophosphoric acid as demonstrated is a versatile technique for the voluminous graphite powder derivative and MWCNTs. This has the advantage over previous methods including the direct electrochemical reduction of arildiazonium salts on the electrode surface, or our chemically activated method allows the possibility of economically mass production of chemically derived nanotubes for a variety of applications. In addition, the derivative of MWCNTs proposes the possibility of sensor miniaturization down the nano scale. In FIG. 4E a possible geometric configuration or diagram is shown for the surface of the sensor 40 which is exposed to the sounding fluid. The surface includes an operating electrode 43 as described in FIGS. 4A or 4B, together with the (external) reference electrode 44 and an opposing electrode 45. A scheme of a microsensor 50 incorporating a modified surface prepared according to the procedure described above is shown in FIG. 5. The body 51 of the sensor is fixed in the final section of an opening 52. The body carries the surface of the electrode 51 1 and contacts 512 which provide connection points to the voltage supply and measurement through a small channel 521 in the bottom of the opening 52. A sealing ring 513 protects the contact and electronic points of the sounding fluid passing under operating conditions through the sample channel 53. It is an advantage of the new sensor to include two measuring or measuring electrodes or molecules measuring two emf or potentials with reference to the same reference electrode and being sensitive to the same species or molecule in the environment. As a result the sensitivity towards a change in the concentration of the species increases. Using the previous example of AQC and DPPA and the pH (or concentration H +, the Nernst equation applicable to the new sensor is the sum of the equations describing the individual measurement electrodes.) Thus, the combination of the medium wave potential E0.5 (AQC) for anthraquinone [3] Eo s (AQC) = K (AQC) - (2,303 RTm / nF) pH with the mean wave potential E0 5 (DPP) for N, N'-diphenyl-p-phenylenediamine [4] ] E0.5 (DPPD) = K (DPPD) - (2,303 RTm / nF) Ph produces the average wave potential E0.5 (S) for the combined system: Eo.s (S) = Eo.s (AQC) + E0.5 (DPPD) = [5] (K (AQC) + K (DPPD)) - 2 * (2,303 RTm / nF) pH = K (S) -2 * (2,303 RTm / nF) pH Where K ( S) is the sum of the two constants K (AQC) and K (DPPD) Since the change of the potential with a change in pH depends on the second term, the (theoretical) sensitivity of the sensor has doubled. additional (third) redox system sensitive to the same species would in principle increase the additional sensitivity. ecta changes in the peak location of the voltammogram, however, more efforts are anticipated to be required to resolve coating peaks in such a three-molecule system. FIG. 6 shows results in the range of pH solutions (pH 4.6, 0.1 M acetic acid + 0.1 M sodium acetate regulator, pH 6.8, 0.025 M disodium hydrogen phosphate + 0.025 M potassium dihydrogen phosphate buffer, pH 9.2 , 0.05M sodium tetraborate regulator). The figure presents the corresponding square wave voltammograms when the initial potential was sufficiently negative to have both DPPD and AQ in their reduced forms.

FIG. 7A represents the relationship between the redox potential and pH for both DPPD (-) and AQ (*). The diagram reveals a linear response of pH 4 to 9 with a corresponding unit gradient of 59 mv / pH (at 25 ° C) that is consistent with an electron n, m proton transfer where n and m are probably equal to two. By combining the two individual curves in a manner as described in equation [5], a new function (-) is derived with a higher sensitivity for the species to be detected. For the two activated MWCNT species described above, the peak potential using cyclic voltammetry (CV) is found to be pH dependent. This voltammetric behavior is consistent with previous studies of carbon powder covalently modified with 1-antathquinonyl groups and can be attributed to the reduction / oxidation of two protons, two electrons from the 1-antatraquinonyl portion to the corresponding hydroquinone species. When NB-MWCTNs are studied a complicated voltammetric pattern can be observed. When first exploring a reductive way a large electrochemically irreversible peak is observed (marked as system I), the exact peak potential on which the studied pH depends. When the scanning direction reverses and sweeps in an oxidative manner a new peak at more positive potentials than the irreversible peak is observed, such a repeat cycle is found to have an electrochemically irreversible manner as the corresponding reduction wave is observed. This system is marked as I I system. Again the exact peak potential of the I I system is found to vary with the pH studied. This behavior is consistent with the reduction mechanism of the nitro portion in aqueous media as exemplified by nitrobenzene in FIG. 4D. It is noted that all subsequent characterization procedures for NB-MWCTNs are carried out in system I I, which corresponds to the arylnitroso / arylhydroxylamine reversible pair, after several initial scans are performed to form this redox pair. When investigating the pH effect of AQ-MWCNTs and NB-MWCNTs over the range pH 1.0 to pH 12.0 using CV and square wave voltammetry (SWV) at room temperature as well as the behavior of AQ-MWCNTs at elevated temperatures up to 70 ° C. SWV is used because it gives us a sharp, sharp peak in a single sweep. As the concomitant proton loss / gain occurs in the oxidation / reduction of AQ-MWCNTs or NB-MWCNTs (see FIGS.4C and 4D, respectively) the peak potential depends on the local proton concentration, ie pH, as described by the Nernst equation [6]: nF where m and n, the number of protons and electrons transferred respectively, are both probably equal to two in the case of AQ-MWCNTs and the arylnitroso / arylhydroxylamide pair in the case of NB- MWCNTs. The formulation [6] of the Nernst equation is equivalent to that of equations [1] and [2]. At room temperature the potential peaks for both AQ-MWCNTs and NB-MWCNTs are found to change to more negative potentials with increasing pH as predicted. A corresponding diagram of peak potential versus pH is found to be linear over the full pH range studied in all cases (see FIGS 7B and 7C, respectively) and a comparison of the gradient of the peak vs. peak potential diagrams. pH is found to be close to the ideal value of unit 58.1 mV / pH with the exception of the irreversible peak (system I) for NB-MWCNTs that is found to change by only unit 37.6 mV / pH. The response of AQ-MWCNTs to pH at elevated temperatures up to 70 ° C is studied using SWV. Note that the pH of the solutions used can vary with temperature, and for this purpose three IUPAC regulators with a known pH at each temperature studied are used. These are regulators of pH 4.6, pH 6.8 and pH 9.2. The Nernst equation predicts that the peak potential should change to more negative values as the temperature increases due to the temperature dependence of the formal potential (E0P¡CO). FIG. 7D therefore reveals that as the temperature increases the peak potential is changed to more negative values. However, in contrast to the behavior of carbon powder covalently derived from the anthraquinonyl (Aqcarbon) moiety, where the peak currents increase steadily with increasing temperature after an initial increase in peak current up to ca 40 ° C, the currents peak for AQ-MWCNTs gradually decreases with increasing temperature. This behavior has also previously been observed for MWCNT agglomerates at elevated temperatures. The temperature invariance of derived MWCNTs is not fully understood but has a potential advantage for pH sensors that are required for use in high temperature environments. In FIG. 7E illustrates the effect of variable pH at room temperature for molecular anthraquinone in the solution phase against the AQ-MWCNTs immobilized on a bppg electrode. Solutions of 1 mM anthraquinone are prepared at each pH and are studied using cyclic voltammetry on a simple bppg electrode. The variation of peak potential with pH for both cases over the range of pH 1.0 to 14.0 are studied with additional experiments carried out at pH 1 0.5, pH 13.0 and pH 14.0. The plot of peak potential against pH for both 1 mM anthraquinone in solution and for immobilized AQ-MWCNTs reveals that, in the case of AQ-MWCNTs, a linear response is observed over the entire pH range studied. However for the anthraquinone in the solution phase, the diagram is not more linear above ca. PH 10.5 (FIG 7E). This can be attributed to the pKa for the removal of the first proton, pKa ^ of the reduced form of anthraquinone (see FIG 4C) in solution being ca. pKa-? = 1 0. The pKa for the removal of the second proton is ca pKa2 = 12. At pHs higher than pH 1 0 the reduced form of anthraquinone can be deprotonated causing a change in the peak potential variation with pH. No such linearity deviation is observed for AQ-MWCNTs. From this it can be concluded that the deviation over the surface of MWCNTs can change the pKa of the anthraquinonyl moiety. This clearly demonstrates that the deviation in MWCNTs is proved advantageous for the analytical detection of pH as the pH window to be used is broadened favorably for AQ-MWCNTs derived in comparison to free anthraquinone in solution. The analysis of the peak potential as a function of pH at each temperature shows good agreement between the theoretically predicted and experimental values showing that the mechanism can easily be used as a simple, economical pH probe, operating over a wide range of temperatures. The new probe can be placed inside several tools and installations as described in the following examples. In FIGs. 8-1 1 The sensor is shown in several possible applications at the bottom of the well. In FIG. 8, a forming test apparatus 810 held on a cable 812 is shown within a sounding 814. The apparatus 81 0 is a well-known modular dynamic tester (MDT, Schlumberger Marking) as described in US Pat. UU No. co-owner 3,859,851 for Urbanosky, U.S. Pat. UU No. 3,780,575 for Urbanosky and Patent No. 4,994,671 for Safinya et al., With this known tester being modified by introduction of an electrochemical analyzer sensor 816 as described in detail above (FIG 8). The modular dynamics tester comprises body 820 of about 30 m in length and containing a main flow line bus or conduit 822. Analysis tool 816 communicates with flow line 822 through opening 817. In addition to the new sensor system 816, the test apparatus comprises an optical fluid analyzer 830 within the lower part of the flow line 822. The flow through the flow line 822 is driven by means of a pump 832 located toward the upper end of flow line 822. Hydraulic arms 834 and armrests 835 join external to body 820 and carry a sample probe tip 836 to sample fluid. The base of the probe tip 836 is isolated from the sounding 814 by an O-ring 840, or other sealing devices, eg, packers. Before the completion of a well, the modular dynamics tester is lowered into the well in the cable 812. After reaching a target depth, i.e. layer 842 of the formation to be sampled, the hydraulic arms 834 extend to engage the sample probe tip 836 with the formation. The O-ring 840 on the base of the sample probe 836 forms a seal between the sounding side 844 and the formation 842 in which the probe 836 is inserted and prevents the sample probe 136 from acquiring fluid directly from the inner hole 814. Once the sample probe 836 is inserted into the array 842, an electrical signal is passed down the cable 812 from the surface to start the pump 832 and the sensor systems 816 and 830 to begin sampling a fluid sample from the formation 842. Electrochemical detector 816 is adapted to measure the pH and ion content of the formation effluent. A bottle (not shown) inside the MDT tool can be initially filled with a calibration solution to ensure on-site (downhole) calibration of sensors. The MDT module can also contain a tank with a larger volume of calibration solution and / or cleaning solution that can be pumped periodically through the sensor volume for cleaning and re-calibration purposes. Electrochemical probes in a downhole tool type MDT can be used for absolute measurements of downhole parameters that differ significantly from those measured in surface samples (such as pH, Eh, dissolved H2S, CO2). This correction of surface values are important for the validation of the water chemistry model. An additional possible application of the new sensor and separation system is in the measurement field while drilling (MWD). The principle of MWD measurements is known and described in a vast amount of literature, including for example U.S. Patent No. 5,445,228, entitled "Metadata and apparatus for formation sampling during the drilling of a hydricarbon web". In FIG. 9, a sounding 91 1 and the lower part of a drill string 912 including the bottom-hole-assembly (BHA) 910 are shown. BHA has perforator bit 913 at its apex. It also includes drill collars that are used to assemble equipment additional such as a telemetry aid 914 and a sensor aid 915. The telemetry aid provides a telemetry link to the surface, for example, through mud-pulse telemetry. The sensor auxiliary includes the new electrochemical analysis unit 91 6 as described above. The analysis unit 916 collects drilling fluid through a small hole 917 protected from debris and other particles by a metal mesh. During the drilling operation the sounding fluid enters the gap 917 and is subsequently analyzed using sensor unit 91 6. The results are transmitted from the data acquisition unit to the telemetry unit 914, converted into telemetry signals and transmits to the surface. A third application is illustrated in FIG. 1 0. It shows a Venturi 101 0 flow meter, well known in the industry and described for example in U.S. Patent No. 5,736,650. Mounted on the production pipe or housing 1 012, the flow meter is installed at a location within well 101 1 with a wired connection 1013 to the surface following known procedures as described for example in U.S. Patent No. 5,829,520 . The flow meter consists essentially of a constriction or conduit 1 014 and two pressure branches 101 8, 1 01 9 conventionally located at the inlet and the position of maximum constriction, respectively. Usually the Venturi flow meter is combined with a 1015 hydrometer located also upstream or downstream. The new electrochemical analysis unit 1 01 6 is preferably located downstream of the Venturi to take advantage of the mixing effect that the Venturi has on the flow. A hole 1 017 protected by a metal mesh provides an entrance to the unit. During production the drilling fluid enters the hole 1017 and is subsequently analyzed using sensor unit 1016. The results are transmitted from the data acquisition unit to the surface via wires 101 3. Various embodiments and applications of the invention have been described. The descriptions are intended to be illustrative of the present invention. It will be apparent to those skilled in the art that modifications can be made to the invention as described without departing from the scope of the claims set forth below.

Claims (13)

1. REIVI NDICATIONS 1. An electrochemical sensor comprising at least two redox systems sensitive to the same species.
2. 2. The sensor of claim 1, characterized in that the species are protons.
3. The sensor of claim 1, characterized in that the at least two redox systems have a peak or maximum redox reaction at different voltages.
4. The sensor of claim 1, characterized in that the at least two redox systems are mounted on the same conductive substrates.
5. 5. The sensor of claim 4, characterized in that the at least two redox systems are mounted on a carbon based substrate.
6. The sensor of claim 5, characterized in that the at least two redox systems are mounted on a carbon powder substrate.
7. The sensor of claim 5, characterized in that the at least two redox systems are mounted on a diamond-based substrate.
8. The sensor of claim 7, characterized in that the at least two redox systems are mounted on a multi-walled nanotube-based substrate.
9. The sensor of claim 1, comprising a detector adapted to measure the redox potential of said at least two redox systems in the presence of the species and to convert measurements into a signal indicative of the concentration of said species.
10. 10. An electrochemical sensor for determining the concentration of a cular species in a fluid comprising a first redox system sensitive to said species and a second redox system sensitive to said species; voltage supply and electric current detector for voltamogram measurements; and an analyzer to detect relative changes in said voltamogram measurements. eleven .
11. A downhole tool for measuring characteristic parameters of sounding effluents comprising an electrochemical sensor according to claim 1.
12. 12. A bottomhole formation sampling tool for measuring characteristic parameters of sounding effluents comprising an electrochemical sensor according to claim 1.
13. 13. A downhole tool for measuring characteristic parameters of sounding effluents comprising an electrochemical sensor according to claim 1 mounted on a permanently installed part of the sounding.