Abstract:
An electro-chemical sensor is described having one or more redox species sensitive to a certain analyte and one or more redox species that are insensitive to that analyte, for the purpose of making measurements in a downhole environment in aquifers or oilfield reservoirs.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of, and claims the benefits of priority from application Ser. No. 10/585,263, entitled “ELECTRO-CHEMICAL SENSOR,” filed in the United States of America. Application Ser. No. 10/585,263 is a national phase application (371) of Application Number PCT/GB2004/005397, entitled “ELECTRO-CHEMICAL SENSOR,” filed under the PCT on Dec. 22, 2004 and claims priority to GB0400325.7, entitled “ELECTRO-CHEMICAL SENSOR,” filed in the United Kingdom on Jan. 8, 2004. Accordingly, this application also claims the benefits of priority from Application Number GB0400325.7. 
     All of which are commonly assigned to assignee of the present invention and hereby incorporated by reference in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     Embodiments of the present invention relate to a chemical sensor for sensing properties of a fluids. More specifically, but not by way of limitation, certain embodiments of the present invention may provide an electro-chemical sensor for pH and ion content analysis of fluids. Merely by way of example, some aspects of the present invention may be used in a downhole environment, such as aquifer and oilfield reservoir and may be used to analyse substances produced from and/or found in an aquifer or an oilfield reservoir. 
     Analyzing samples representative of downhole fluids is an important aspect of determining the quality and economic value of a hydrocarbon formation. Similarly, analyzing properties of liquids associated with an aquifer may be important in aquifer analysis in the hydrocarbon, water production industries and/or resource management. 
     In the hydrocarbon industry, analysis operations may obtain an analysis of downhole fluids usually through wireline logging using a formation tester such as the MDT™ tool of Schlumberger Oilfield Services. However, more recently, it was suggested to analyze downhole fluids either through sensors permanently or quasi-permanently installed in a wellbore or through sensors mounted on the drillstring. The latter method, if successfully implemented, has the advantage of obtaining data while drilling, whereas the former installation could be part of a control system for wellbores and hydrocarbon production therefrom. 
     To obtain an estimate of the composition of downhole fluids, the MDT tools may 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, knowledge of downhole formation (produced) water chemistry is needed to save costs and increase production at all stages of oil and gas exploration and production. Knowledge of particularly the water chemistry is important for a number of key processes of the hydrocarbon production, including:
         Prediction and assessment of mineral scale and corrosion;   Strategy for oil/water separation and water re-injection;   Understanding of reservoir compartmentalization/flow units;   Characterization of water break-through;   Derivation of the water cut R w ; and   Evaluation of downhole the H 2 S partition the oil and or water (if used for H 2 S measurements).       

     Some chemical species dissolved in water (like, for example, Cl −  and Na + ) do not change their concentration when removed to the surface either as a part of a flow through a well, or as a sample taken downhole. Consequently information about their quantities may be obtained from downhole samples and in some cases surface samples of a flow. However, the state of chemical species, such as H +  (pH=−log[concentration of H + ]), CO 2 , or H 2 S may change significantly while tripping to the surface. The change occurs mainly due to a difference in temperature and pressure between downhole and surface environment. In case of sampling, this change may also happen due to degassing of a sample (seal failure), mineral precipitation in a sampling bottle, and (especially in case of H 2 S)—a chemical reaction with the sampling chamber. It should be stressed that pH, H 2 S, or CO 2  are among the most critical parameters for corrosion and scale assessment. Consequently it is of considerable importance to know their downhole values precisely. 
     The concentration of protons or its logarithm pH can be regarded as the most critical parameter in water chemistry. It determines the rate of many important chemical reactions as well as the solubility of chemical compounds in water, and (by extension) in hydrocarbon. 
     Hence, there is and will continue to be a demand for downhole chemical measurements. However, no downhole chemical measurements actually performed in an oil and gas producing well have been reported so far, though many different methods and tools have been proposed in the relevant literature. 
     General downhole measurement tools for oilfield applications are known as such. Examples of such tools are found in the U.S. Pat. Nos. 6,023,340; 5,517,024; and 5,351,532 or in the International Patent Application WO 99/00575. An example of a probe for potentiometric measurements of ground water reservoirs is further 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 Lake Karachai Area, Chelyabinsk, Southern Urals” in: Lawrence Berkeley Laboratory Report 36780/UC-603(1994b), RAC-6, Ca, USA. 
     The known state of the art in the field of high temperature potentiometric measurements and tool is described for example in the 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, electro-chemical and spectral analysis. Particularly, the potentiometric method has been widely used for the measurements of water composition (pH, Eh, H 2 S, CO 2 , Na 30  , Cl −  etc.) both in the laboratory and in the field of ground water quality control. U.S. Pat. No. 5,223,117 discloses a two-terminal voltammetric microsensor having an internal reference using molecular self-assembling to form a system in which the reference electrode and the indicator electrode are both on the sensor electrode. The reference molecule is described as a redox system that is pH-insensitive, while the indicator molecule is formed by a hydro-quinone based redox system having a potential that shifts with the pH. Both, reference molecule and indicator molecule layers are prepared by self-assembly on gold (Au) microelectrodes. In the known microsensor, a pH reading is derived from peak readings of the voltammograms. 
     The laboratory systems, however, are often not suitable for wellbore application with demands for ruggedness, stability and low maintenance and energy consumption being rarely met. 
     It is therefore an object of the present invention to provide apparatus and methods to perform electro-chemical measurements in hydrocarbon wells during drilling and production. More specifically, it is an object of the present invention to provide robust sensors for ion selective electro-chemical measurements, in particular pH measurements. 
     SUMMARY OF THE INVENTION 
     Some embodiments of the present invention may provide an electro-chemical sensor comprising one or more redox species that are sensitive to an analyte and one or more redox centers that are insensitive to the analyte. In certain aspects, the sensor is configured for use in a downhole environment, including but not limited to, hydrocarbon reservoirs and/or aquifers. 
     In one embodiment of the present invention, the sensor may comprise a redox system, based for example on anthraquinone redox chemistry. 
     The substrate onto which the redox system is mounted may be based on carbon in one of its elementary forms such as graphite, carbon powder, diamond. In a variant of the invention, the substrate may be derivatised nanotubes, including multi-walled nanotubes. 
     An electro-chemical technique using a method or sensor in accordance with the present invention may be applied for example as part of a production logging tool, an open hole formation tester tool (such as the Modular Dynamic Tester, MDT™), an aquifer analyzing tool and/or the like. In certain aspects, the technique according to certain embodiments of the present invention may provide a downhole real-time water sample validation or downhole pH measurement which may be used for predicting mineral scale, corrosion assessment and/or the like. 
     These and other features of the invention, embodiments and variants thereof, possible applications and advantages may become appreciated and understood by those skilled in the art from the following detailed description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a schematic diagram of the main elements of a known voltametric sensor; 
         FIG. 2  A-C shows a schematic diagram of the main elements of a known electro-chemical microsensor and its operation; 
         FIG. 3  shows a schematic diagram of a known downhole probe using potentiometric sensors; 
         FIG. 4A  illustrates the surface structure of a measuring electrode in accordance with an example of the invention; 
         FIG. 4B  illustrates the surface structure of a measuring electrode with internal reference electrode in accordance with another example of the invention; 
         FIG. 4C  illustrates the redox reaction of a measuring electrode in accordance with another example of the invention using multi-walled carbon nanotubes; 
         FIG. 4D  illustrates the redox reaction of a measuring electrode with internal reference electrode in accordance with another example of the invention using multi-walled carbon nanotube; 
         FIG. 4E  illustrates the geometrical surface layout of the electrode of  FIG. 4B ; 
         FIG. 5  is a perspective view, partially cut-away, of a sensor in accordance with an example of the present invention in a downhole tool; 
         FIG. 6  shows voltammograms recorded from an electro-chemical microsensor in accordance with the present invention at three different pH values; 
         FIG. 7A  illustrates the shift of the peak potential for anthraquinone, diphenyl-p-phenylenediamine and a combination of the two redox systems; 
         FIGS. 7B-C  are plots of peak potential against pH for the redox systems of  FIGS. 4C and 4D , respectively, over the pH range pH 1.0 to pH 12.0 at 293 K at various conditions; 
         FIG. 7D  illustrates change in peak potential with temperature. 
         FIG. 7E  illustrates the effect of varying pH at room temperature for molecular anthraquinone in the solution phase versus the AQ-MWCNTs immobilized onto a bppg electrode. 
         FIG. 8  illustrates an example of a sensor in accordance with the invention as part of a wireline formation testing apparatus in a wellbore; 
         FIG. 9  shows a wellbore and the lower part of a drill string including the bottom-hole-assembly, with a sensor in accordance with the invention; and 
         FIG. 10  shows a sensor located downstream of a venturi-type flowmeter, in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The theory of voltammetry and its application to surface water measurements at ambient temperatures are both well developed. The method is based on the measurement of the electromotive force (e.m.f.) or potential E in a potentiometric cell which includes measuring and reference electrodes (half-cells). 
       FIG. 1  shows the general components of a known voltammetric cell  10 . A measuring electrode  11  is inserted into a solution  13 . This electrode consists of an internal half element (for example, Ag wire covered by an AgCl salt) in a solution of a fixed pH (for example, 0.1M HCl in some pH electrodes), and an ion-selective membrane  111  (like glass H +  selective membrane in pH glass electrode). The reference electrode  12  also contains an internal half-element (typically the same AgCl; Ag) inserted in a concentrated KCl (for example 3M) solution/gel saturated with Ag + , which diffuses (or flows) through the reference (liquid) junction  121 . 
     The ion-selective electrode  11  measures the potential that arises because of 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 on the reference electrode  12 , which is fixed because of a constant composition of a reference solution/gel. The electrodes may be separated (separate half cells), or combined into one (“combination electrode”). 
     The measured e.m.f. is an overall function of the temperature and the activity of an ith ion, to which the measuring electrode is selective:
 
 E=E   0 +( k*T )*log( a   i ),  [1]
 
where E is the measured electromotive force (e.m.f.) of the cell (all potentials are in V), a i  corresponds to the activity of the ith ion and is proportional to its concentration. E 0  is the standard potential (at temperature T) corresponding to the E value in a solution with the activity of ith ion equal to one. The term in parenthesis is the so-called Nernstian slope in a plot of E as a function of log(a i ). This slope (or the constant “k”) together with the cell (electrode) constant (E 0 ) is experimentally determined via a calibration procedure using standard solutions with known activities of ith ion. For good quality undamaged electrodes this slope should be very close to the theoretical one, equal to (R*T/F*z), where F is the Faraday constant (96485 kJ/mole), R is the gas constant (8.313 j/mole K), z i  is the charge of ith ion.
 
     The Nernst equation [1] can be rewritten for pH sensors, i.e. log a(H + ) as
 
 E   0.5   =K −(2.303 RTm/nF )pH  [2]
 
where E 0.5  is the half-wave potential of the redox system involved, 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. No. 5,223,117 is illustrated in  FIG. 2 .  FIG. 2A . shows a schematic electro-chemical sensor with a counter electrode  21  and a relatively much smaller (by a factor of 1000) Au substrate  22  that carries two molecular species M and R. The R species forms an inert reference electrode, and 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 the oxidization in the normal state. When the third species L binds to M ( FIG. 2B ), this difference increases as illustrated by the shift of peaks in the lower half of  FIG. 2C , thus providing a measure for the concentration of L in the solution surrounding the sensor. In the context of the present invention, it is important to note that the R is specifically selected to be insensitive to the species L, e.g. pH. 
     In  FIG. 3 , there are schematically illustrated elements of a known downhole analyzing tool  30  as used by Solodov et al (see background). The body of the tool  30  is connected to the surface via a cable  31  that transmits power and signals. A computer console  32  controls the tool, monitors its activity and records measurements. The tool  30  includes a sensor head with at number of selective electro-chemical probes  33  each sensitive to a different molecular species. Also housed in the body of the tool are further actuation parts  34  that operate the head, a test system  35  and transceivers  36  to convert measurements into a data stream and to communicate such data stream to the surface. The electrodes are located at the bottom part of the probe and include those for pH, Eh (or ORP), Ca 2+  (pCa), Na +  (pNa), S 2−  (pS), NH 4   +  (pNH 4 ), and reference electrode (RE). H 2 S partial pressure may be calculated from pH and pS readings. 
     In the following aspects and elements of the present invention are described in detail. 
     The present invention introduces a new molecular system in which the redox features of two molecules are combined, thus leading to a considerably higher accuracy and, in turn, downhole deployability. 
     In a preferred embodiment for a pH sensitive sensor an anthraquinone is homogenously derivatised onto carbon particles (AQC) 
     
       
                 
         
             
             
         
      
     
     The AQC system is derived using 2 g of carbon powder (1.5 μm in mean diameter) mixed with a 10 cm 3  solution containing 5 mM Fast Red AL Salt (Anthraquinone-1-diazonium chloride) to which 50 mM hypophosphorous acid (50%) is added. The reaction is allowed to stand with occasional stirring at 5° C. for 30 minutes, after which it is filtered by water suction. Excess acid is removed by washing with distilled water and with the powder being finally washed with acetonitrile to remove any unreacted diazonium salt in the mixture. It is then air dried by placing inside a fume hood for a period of 12 hours and finally stored in an airtight container. 
     In a similar manner, phenanthrenequinone (PAQ) 
     
       
                 
         
             
             
         
      
     
     Is prepared as a second molecular species to undergo a redox reaction 
     Alternatively, N,N′-diphenyl-p-phenylenediamine (DPPD) spiked onto carbon particles undergoes a redox process as shown below: 
     
       
                 
         
             
             
         
      
     
     The bonding of DPPD onto carbon is achieved by mixing 4 g of carbon powder with 25 mL of 0.1M HCl+0.1M KCl and 20 mM DPPD solution in acetone. The reaction mixture is stirred continuously for 2 hours in a beaker and then filtered after which it was washed with distilled water to remove excess acid and chloride. It is then air dried by placing inside a fume hood for 12 hours and finally stored in an airtight container. 
     In a static environment where the sensor surface is not exposed to a flow, it is possible to immobilize water insoluble DPPD crystals directly onto the electrode surface. However in the wellbore environment it is preferred to link the sensitive molecules via a chemical bond to such surface. 
     The derivatised carbon powders are immobilized onto a basal plane pyrolytic graphite (BPPG) electrode prior to voltammetric characterization following a procedure described by Scholz, F. and Meyer, B., “Voltammetry of Solid Microparticles Immobilised 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 smoothness. The derivatised carbons are first mixed and then immobilized onto the BPPG by gently rubbing the electrode surface on a fine qualitative filter paper containing the functionalized carbon particles. 
     The resulting modified electrode surface is schematically illustrated by  FIG. 4A  showing an electrode  41  with bonded DPPD and AQC. 
     It is further advantageous to add an internal pH reference involving a pH independent redox couple to increase the stability of any voltammetric reading, hence circumventing uncertainties caused by drift of the external reference electrode. In the configuration, the sensor includes two reference electrodes. 
     A suitable reference molecule is, for example, K 5 Mo(CN) 8  or various ferrocene containing molecules which both have a stable redox potential (K 5 Mo(CN) 8  at around 521 mV) that is sufficiently separated from expected shifting of redox signals of the two indicator species over the pH range of interest. As shown in Table 1 that both the oxidation and reduction potentials of K 5 Mo(CN) 8  are fairly constant across the entire pH range of interest. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 pH 
                 AQ OX   
                 AQ RED   
                 DPPD OX   
                 DPPD RED   
                 Mo— OX   
                 Mo— RED   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 4.6 
                 −0.440 
                 −0.448 
                 0.202 
                 0.224 
                 0.524 
                 0.524 
               
               
                 6.8 
                 −0.576 
                 −0.580 
                 0.094 
                 0.082 
                 0.528 
                 0.522 
               
               
                 9.2 
                 −0.710 
                 −0.674 
                 −0.204 
                 −0.372 
                 0.512 
                 0.508 
               
               
                   
               
             
          
         
       
     
     The Mo-based reference species can be retained in the solid substrate via ionic interactions with co-existing cationic polymer, such as poly (vinyl pyridine), that was spiked into the solid phase. Other pH independent species, such as ferrocyanide are less suitable as the redox peaks are obscured by the signals of the measuring redox system. 
     In  FIG. 4B  the electrode  42  carries bonded molecules AQC and PAQ together with PVF as an internal reference molecule. 
     The most common forms of conducting carbon used in electrode manufacture are glassy carbon, carbon fibres, carbon black, various forms of graphite, carbon paste and carbon epoxy. One further form of carbon, which has seen a large expansion in its use in the field of electrochemistry since its discovery in 1991 is the carbon nanotube (CNT). The structure of CNTs approximates to rolled-up sheets of graphite and can be formed as either single or multi-walled tubes. Single-walled carbon nanotubes (SWCNTs) constitute a single, hollow graphite tube. Multi-walled carbon nanotubes (MWCNTs) on the other hand consist of several concentric tubes fitted one inside the other. 
     The above activation methods for binding a redox active species to graphite or carbon surfaces can be extended via the chemical reduction of aryldiazonium salts with hypophosphorous acid, to include the covalent derivatization of MWCNTs by anthraquinone-1-diazonium chloride and 4-nitrobenzenediazonium tetrafluoroborate. This results in the synthesis of 1-anthraquinonyl-MWCNTs (AQ-MWCNTs) and 4-nitrophenyl-MWCNTs (NP-MWCNTs) as shown in  FIGS. 4C and 4D , respectively. The respective substrates  46  and  47  are multi-walled carbon nanotubes. 
     The preparation process of the derivatised MWCNT involves the following steps: first 50 mg of MWCNTs are stirred into 10 cm 3  of a 5 mM solution of either Fast Red AL (anthraquinone-1-diazonium chloride) or Fast Red GG (4-nitrobenzenediazonium tetrafluoroborate), to which 50 cm 3  of hypophosphorous acid (H 3 PO 2 , 50% w/w in water) is added. Next the solution is allowed to stand at 5° C. for 30 minutes with gentle stirring. After which, the solution is filtered by water suction in order to remove any unreacted species from the MWCNT surface. Further washing with deionized water is carried out to remove any excess acid and finally with acetonitrile to remove any unreacted diazonium salt from the mixture. The derivatised MWCNTs are then air-dried by placing them inside a fume hood for a period of 12 hours after which they are stored in an airtight container prior to use. Untreated multi-walled nanotubes can be purchased from commercial vendors, for example from Nano-Lab Inc of Brighton, Mass., USA in 95% purity with a diameter of 30+/−15 nm and a length of 5-20 μm. 
     The reduction of diazonium salts using hypophosphorous acid as demonstrated is a versatile technique for the derivatization of bulk graphite powder and MWCNTs. This has the advantage over previous methods involving the direct electro-chemical reduction of aryldiazonium salts onto the electrode surface, as the chemically activated method allows the possibility for inexpensive mass production of chemically derivatised nanotubes for a variety of applications. Furthermore the derivatization of MWCNTs proffers the possibility of sensor miniaturization down to the nano-scale. 
     Another way of immobilizing the redox active compounds onto the working electrode terminal would be by packing a mixture of the compounds and carbon powder effectively into a recessed working electrode cavity without a binding substance. The carbon powder could be mixed with the pH-sensitive and reference chemicals and ground finely with a mortar and pestle. Then the empty recess might be filled with the powder mix which would be mechanically compacted. The resulting void in the working electrode recess would then be refilled and compacted again. This would be repeated several times until the recess is full. The material would be pressed such that the particles are packed into a dense matrix. In order to achieve stability against liquid or gas flow across the surface, the carbon press electrode might be protected by a porous frit, a fiber mesh, a non-conducting wire mesh or a porous polymer film. 
     Although packing of the redox active compounds into a single electrode area (as discussed above) provides a means of forming the sensor it can be envisaged that immobilization of two or more species into various distinct electrodes may provide improved signals and more facile manufacturing. This can be especially thought of when the compounds are chemically attached to the electrode surface via a covalent linkage. In this case a single monolayer of compounds will be formed on the surface. It can be envisaged that if the pH sensitive and pH insensitive compounds are bulky or undergo differing immobilization rates then formation of the monolayer will favor one or other of the compounds such that the signal is dominated by a single compound and hence the sensor is inoperable. In these cases immobilization of each compound onto separate electrodes would overcome the problem, as the immobilization procedure for each would not be under competitive control. It can therefore be proposed that a sensor in which two or more working electrodes, with different electroactive species immobilized on each surface, is utilized and cross connected such that only a single voltammetric sweep is required. 
     In  FIG. 4E  there is shown a possible geometric configuration or layout for the sensor surface  40  which is exposed to the wellbore fluid. The surface includes a working electrode  43  as described in  FIG. 4A  or  4 B, together with the (external) reference electrode  44  and a counter electrode  45 . 
     A schematic of a microsensor  50  incorporating a modified surface prepared in accordance with the procedure described above is shown in  FIG. 5 . The body  51  of the sensor is fixed into the end section of an opening  52 . The body carries the electrode surface  511  and contacts  512  that provide connection points to voltage supply and measurement through a small channel  521  at the bottom of the opening  52 . A sealing ring  513  protects the contact points and electronics from the wellbore fluid that passes under operation conditions through the sample channel  53 . 
     It is an advantage of the new sensor to include two measuring or indicator electrodes or molecules measuring two e.m.f 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 shift in the concentration of the species increases. Using the above example of AQC and DPPD and the pH (or H +  concentration, the Nernst equation applicable to the new sensor is the sum of the equations describing the individual measuring electrodes. Thus, combining the half wave potential E 0.5  (AQC) for anthraquinone
 
 E   0.5 (AQC)= K (AQC)−(2.303 RTm/nF )pH  [3]
 
with the half wave potential E 0.5 (DPPD) for N,N′-diphenyl-p-phenylenediamine
 
 E   0.5 (DPPD)= K (DPPD)−(2.303 RTm/nF )pH  [4]
 
yields the half wave potential E 0.5 (S) for the combined system:
 
 E   0.5 ( S )= E   0.5 (AQC)+ E   0.5 (DPPD)=( K (AQC)+ K (DPPD))−2*(2.303 RTm/nF )pH= K ( S )−2*(2.303 RTm/nF )pH  [5]
 
     Where K(S) is the sum of the two constants K(AQC) and K(DPPD). As the shift of the potential with a change in pH depends on the second term, the (theoretical) sensitivity of the sensor has doubled. 
     The use of a further (third) redox system sensitive to the same species would in principle increase the sensitivity further. As the method detects shifts in the peak location of the voltammogram, however, more efforts are anticipated to be required to resolve overlapping peaks in such a three-molecule system. 
       FIG. 6  shows results in a range of pH solutions (pH 4.6, 0.1M acetic acid+0.1M sodium acetate buffer; pH 6.8, 0.025M disodium hydrogen phosphate+0.025M potassium dihydrogen phosphate buffer; pH 9.2, 0.05M disodium tetraborate buffer). The figure presents the corresponding square wave voltammograms when the starting potential was sufficiently negative to have both DPPD and AQ in their reduced forms. 
       FIG. 7A  depicts the relationship between the redox potential and pH for both the DPPD (▪) and AQ (♦). The plot reveals a linear response from pH 4 to 9 with a corresponding gradient of ca 59 mV/pH unit (at 25° C.) which is consistent with an n electron, m proton transfer where n and m are likely to be equal to two. By combining the two individual curves in a manner as described in equation [5], a new function (▴) is derived with a superior 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-dependant. This voltammetric behavior is consistent with previous studies of carbon powder covalently modified with 1-anthraquinonyl groups and can be attributed to the two-electron, two-proton reduction/oxidation of the 1-anthraquinonyl moiety to the corresponding hydroquinone species. 
     When NB-MWCNTs is studied a more complicated voltammetric pattern can be observed. Upon first scanning in a reductive fashion a large, electro-chemically irreversible peak is observed (labeled as system I), the exact peak potential of which depends on the pH studied. When the scan direction is reversed and swept in an oxidative fashion a new peak at more positive potentials than the irreversible peak is observed, which upon repeat cycling was found to behave in an electro-chemically reversible fashion as the corresponding reduction wave was observed. This system is labeled as system II. Again the exact peak potential of system II is found to vary with the pH studied. This behavior is consistent with the reduction mechanism, of the nitro moiety in aqueous media as exemplified by nitrobenzene in  FIG. 4D . It is worth noting that all subsequent characterization procedures for NB-MWCNTs are carried out on system II, which corresponds to the reversible arylnitroso/arylhydroxylamine couple, after several initial scans are performed to form this redox couple. 
     When investigating the effect of pH 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 was used because it provides us with a sharp, well-defined peak in a single sweep. As concomitant proton loss/gain occurs on oxidation/reduction of AQ-MWCNTs or NB-MWCNTs (see  FIGS. 4C  and  4 D respectively) the peak potential depends on the local proton concentration, i.e. pH, as described by the Nernst equation [6]: 
                     E   peak     =       E   formal   0     -         2.3   ⁢           ⁢   RTm     nF     ⁢   pH               [   6   ]               
where m and n, the number of protons and electrons transferred respectively, are both likely to be equal to two in the case of AQ-MWCNTs and the arylnitroso/arylhydroxylamine couple in the case of NB-MWCNTs. The formulation [6] of the Nernst equation is equivalent to those of equations [1] and [2].
 
     At room temperature the peak potentials for both AQ-MWCNTs and NB-MWCNTs are found to shift to more negative potentials with increasing pH as predicted. A corresponding plot of peak potential against pH was found to be linear over the entire pH range studied in all cases (see  FIGS. 7B and 7C , respectively) and a comparison of the gradient of the plots of peak potential vs. pH are found to be close to the ideal value of 58.1 mV/pH unit with the exception of the irreversible peak (system I) for NB-MWCNTs which was found to shift by only 37.6 mV/pH unit. 
     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 may vary with temperature, and so to this end three IUPAC buffers with a known pH at each temperature studied were employed. These are the pH 4.6, pH 6.8 and pH 9.2 buffers. The Nernst equation predicts that the peak potential should shift to more negative values as the temperature is increased due to the temperature dependence of the formal potential (E peak   0 ).  FIG. 7D  does indeed reveal that as the temperature is increased the peak potential is shifted to more negative values. However, in contrast to the behavior of carbon powder covalently derivatised with the anthraquinonyl moiety (AQcarbon) where the peak currents increase steadily with increasing temperature after an initial increase in peak current up to ca 40° C., the peak currents for AQ-MWCNTs gradually decreases with increasing temperature. This behavior has also been previously observed for MWCNT agglomerates at elevated temperatures. The temperature invariance of derivatised MWCNTs is not fully understood but has a potential advantage for pH sensors which are required for use in elevated temperature environments. 
     In  FIG. 7E  there is illustrated the effect of varying pH at room temperature for molecular anthraquinone in the solution phase versus the AQ-MWCNTs immobilized onto a bppg electrode. 1 mM anthraquinone solutions are prepared at each pH and studied using cyclic voltammetry at a bare bppg electrode. The variation of peak potential with pH for both cases over the pH range 1.0 to 14.0 are studied with additional experiments carried out at pH 10.5, pH 13.0 and pH 14.0. The plot of peak potential versus pH for both 1 mM anthraquinone in solution and for the 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 plot is no longer linear above ca. pH 10.5 ( FIG. 7E ). This can be attributed to the pKa for the removal of the first proton, pKa 1 , of the reduced form of anthraquinone (see  FIG. 4C ) in solution being ca. pKa 1 =10. The pKa for the removal of the second proton is ca pKa 2 =12. At higher pHs than pH 10 the reduced form of anthraquinone may be deprotonated causing a change in the variation of peak potential with pH. No such deviation from linearity is observed for the AQ-MWCNTs. From this it can be concluded that derivatization onto the surface of the MWCNTs may change the pK a  of the anthraquinonyl moiety. This clearly demonstrates that derivatization onto MWCNTs proves advantageous to the analytical sensing of pH as the pH window for use is favorably widened for derivatised AQ-MWCNTs compared to free anthraquinone in solution. 
     Analysis of the peak potential as a function of pH at each temperature shows good agreement between the experimental and theoretically predicted values thereby showing the mechanism can be readily used as a simple, inexpensive pH probe, which works over a wide range of temperatures. 
     The novel probe may be placed inside various wellbore tools and installations as described in the following examples. 
     In  FIGS. 8-11  the sensor is shown in various possible downhole applications. 
     In  FIG. 8 , there is shown a formation testing apparatus  810  held on a wireline  812  within a wellbore  814 . The apparatus  810  is a well-known modular dynamic tester (MDT, Mark of Schlumberger) as described in the co-owned U.S. Pat. No. 3,859,851 to Urbanosky U.S. Pat. No. 3,780,575 to Urbanosky and U.S.Pat. No. 4,994,671 to Safinya et al., with this known tester being modified by introduction of an electro-chemical analyzing sensor  816  as described in detail above ( FIG. 8 ). The modular dynamics tester comprises body  820  approximately  30 m long and containing a main flowline bus or a flowline  822 . The sensor  816  communicates with the flowline  822  via opening  817 . In addition to the sensor  816 , the testing apparatus comprises an optical fluid analyser  830  within the lower part of the flowline  822 . The flow through the flowline  822  is driven by means of a pump  832  located towards the upper end of the flowline  822 . Hydraulic arms  834  and counterarms  835  are attached external to the body  820  and carry a sample probe tip  836  for sampling fluid. The base of the probing tip  836  is isolated from the wellbore  814  by an o-ring  840 , or other sealing devices, e.g. packers. 
     Before completion of a well, the modular dynamics tester is lowered into the well on the wireline  812 . After reaching a target depth, i.e., the layer  842  of the formation which is to be sampled, the hydraulic arms  834  are extended to engage the sample probe tip  836  with the formation. The o-ring  840  at the base of the sample probe  836  forms a seal between the side of the wellbore  844  and the formation  842  into which the probe  836  is inserted and prevents the sample probe  136  from acquiring fluid directly from the borehole  814 . 
     Once the sample probe  836  is inserted into the formation  842 , an electrical signal is passed down the wireline  812  from the surface so as to start the pump  832  and the sensor systems  816  and  830  to begin sampling of a sample of fluid from the formation  842 . The electro-chemical detector  816  is adapted to measure the pH and ion-content of the formation effluent. 
     A bottle (not shown) within the MDT tool may be filled initially with a calibration solution to ensure in-situ (downhole) calibration of sensors. The MDT module may also contain a tank with a greater volume of calibration solution and/or of cleaning solution which may periodically be pumped through the sensor volume for cleaning and re-calibration purposes. 
     Electro-chemical probes in an MDT-type downhole tool may be used for the absolute measurements of downhole parameters which significantly differ from those measured in samples on the surface (such as pH, Eh, dissolved H 2 S, CO 2 ). This correction of surface values are important for water chemistry model validation. 
     A further possible application of the novel sensor and separation system is in the field of measurement-while-drilling (MWD). The principle of MWD measurements is known and disclosed in a vast amount of literature, including for example U.S. Pat. No. 5,445,228, entitled “Method and apparatus for formation sampling during the drilling of a hydrocarbon well”. 
     In  FIG. 9 , there is shown a wellbore  911  and the lower part of a drill string  912  including the bottom-hole-assembly (BHA)  910 . The BHA carries at its apex the drill bit  913 . It includes further drill collars that are used to mount additional equipment such as a telemetry sub  914  and a sensor sub  915 . The telemetry sub provides a telemetry link to the surface, for example via mud-pulse telemetry. The sensor sub includes the novel electro-chemical analyzing unit  916  as described above. The analyzing unit  916  collects fluids from the wellbore via a small recess  917  protected from debris and other particles by a metal mesh. 
     During drilling operation wellbore fluid enters the recess  917  and is subsequently analyzed using sensor unit  916 . The results are transmitted from the data acquisition unit to the telemetry unit  914 , converted into telemetry signals and transmitted to the surface. 
     A third application is illustrated in  FIG. 10 . It shows a Venturi-type flowmeter  1010 , as well known in the industry and described for example in the U.S. Pat. No. 5,736,650. Mounted on production tubing or casing  1012 , the flowmeter is installed at a location within the well  1011  with a wired connection  1013  to the surface following known procedures as disclosed for example in the U.S. Pat. No. 5,829,520. 
     The flowmeter consists essentially of a constriction or throat  1014  and two pressure taps  1018 ,  1019  located conventionally at the entrance and the position of maximum constriction, respectively. Usually the Venturi flowmeter is combined with a densitometer  1015  located further up- or downstream. 
     The novel electro-chemical analyzing unit  1016  is preferably located downstream from the Venturi to take advantage of the mixing effect the Venturi has on the flow. A recess  1017  protected by a metal mesh provides an inlet to the unit. 
     During production wellbore fluid enters the recess  1017  and is subsequently analyzed using sensor unit  1016 . The results are transmitted from the data acquisition unit to the surface via wires  1013 . 
     A further possible application for an embodiment of the present invention is in production logging. In order to determine the producing zones of a well, the well is traversed using a logging tool. For vertical and near vertical wells, the tool is allowed to move under gravity, controlled by a cable from the wellhead. For highly deviated wells, the tool is pushed/pulled using either coiled tubing from the surface, or a tractor powered via a cable from the surface. 
     A typical tool string comprises sensors for taking a series of measurements aimed at determining the flow distribution in the well, in terms of phase fractions and position. Measurements include spinners to determine a local velocity (distribution), and fluid fraction measurement probes—for example electrical or optical probes. These measurements are often used in combination in order to maximize the information gained from each pass of the well. 
     In certain aspects, a pH sensor according to an embodiment of the present invention, may be mounted onto this tool, and used to measure pH along a well. The pH of the aqueous phase may be determined by its composition, temperature and pressure and may reveal information on the influx of fluids into the well as well as the movement of fluids within the reservoir. As well as revealing information on fluid influxes and flows within the reservoir, the pH measurement may also be used to assess those parts of the production system that are being exposed to high concentrations of acid gases (for which the associated aqueous phase will have a low pH—typically less than about a pH of 4), and are thus prone to corrosion. In certain aspects, this information may be used to determine the strategy for minimizing and/or mitigating corrosion, e.g. through the selective placement of corrosion inhibitors. U.S. Pat. No. 6,451,603 to G. M. Oddie describes how sensors might be incorporated within the blades of the spinners within a production logging tool and is hereby incorporated by reference in its entirety for all purposes. In certain aspects, a sensor in accordance with an embodiment of the present invention, may be incorporated within the blades of the spinners of a production and may provide for increasing mass transfer to the surface of the sensor. 
     In another aspect, a sensor in accordance with an embodiment of the present invention, may be used in the monitoring of fluids pumped into a well for the purposes of fracturing, matrix treatments such as acidizing, or treatments for wellbore consolidation. pH is an important parameter that controls the property of some of these fluids, and monitoring its value may provide a means of assuring the quality of the treatment, particularly where fluids may be blended in surface modules, prior to being pumped downhole. 
     In addition to using surface monitoring for pumping fluids into a well, pH might be monitored, in accordance with an embodiment of the present invention, on the returns, when a well is brought back on production following the pumping of a treatment fluid. With the contrast in pH between a treatment fluid and the reservoir fluids, the efficacy of the treatment, and the placement of the treatment fluids, may be assessed. 
     In yet further aspects, a pH sensor, in accordance with an embodiment of the present invention, may be mounted on surface pumping units or blenders, or form part of a separate monitoring module, placed in-line and/or the like. In still further aspects, a sensor in accordance with an embodiment of the present invention may be deployed downhole on a coiled tubing unit, where the coiled tubing may be used to convey fluids downhole, and where the pH sensor may be located at the coiled tubing head, or as part of a measurement sub conveyed by the coiled tubing unit, and provide information on the state of the fluids downhole. 
     Another application of an embodiment of the present invention may be in the monitoring of underground bodies of water for the purposes of resource management. From monitoring wells drilled into the aquifers, one or more sensors, in accordance with an embodiment of the present invention, may be deployed on a cable from the surface—either for short duration (as part of a logging operation) or longer term (as part of a monitoring application). In certain aspects of the present invention, a pH sensor, in accordance with an embodiment of the present invention, may be used in the monitoring of aquifers, where long term unattended monitoring of pH is required, e.g. in the monitoring of shallow groundwater on top of CO2 storage, where the pH in the shallow groundwater may indicate whether CO2, injected into a deeper aquifer for the purposes of CO2 sequestration, is escaping to the surface. The pH sensor may be interfaced with a data-logger and the measurements from the sensor stored for later retrieval, may be transmitted to surface for direct analysis and/or the like. 
     In addition, in certain aspects, the deployment of the pH sensor within producing wells on a cable may provide information on produced water quality. In further aspects, the pH sensor may be deployed in injection wells, e.g. when water is injected into an aquifer for later retrieval, where pH may be used to monitor the quality of the water being injected or retrieved. 
     There may be a significant heterogeneity in the composition, and hence the pH, of waters produced from a reservoir: reflecting both the vertical and horizontal variations that exist in rock and fluid composition. These variations may arise from natural processes during the formation of a basin or may come about through the injection of fluids to improve oil recovery, e.g., CO2, surface waters or treatment fluids. Monitoring pH using an embodiment of the present invention beyond the wellhead in surface or subsea pipelines may provide information on the nature of the flow within a reservoir —providing information on events such as water breakthrough or the like—and/or may give warning when corrosion may become an issue because of abnormally low pH that may be due to unreacted acid treatments returning to surface or because of the natural production of the acid gases H2S or CO2. A pH sensor in accordance with an embodiment of the present invention may be deployed beyond the wellhead, permanently or temporarily, within pipelines, or located at the manifolds where pipeline flows are brought together or divided. 
     In wells where reservoir pressures are insufficient, electrosubmersible pumps (“ESPs”) can be deployed within the well to increase production. These pumps are deployed from surface with a power cable and fluid injection lines. In certain aspects of the present invention, a pH sensor in accordance with an embodiment of the present invention may be deployed permanently on the ESP and may provide pH information that may be used to interpret fluid composition. In such aspects, the sensor may provide warning of potential materials failure from acid corrosion or the like. In addition to this application, alternative means of deployment of a sensor in accordance with an embodiment of the present invention may be within a permanent monitoring system, may be a part of a completion of a well and/or the sensors may be deployed through a casing of the wellbore to monitor the fluids outside of the casing, e.g. in assessing zonal isolation or the like. 
     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 may be made to the invention as described without departing from the scope of the claims set out below.