FIELD OF THE INVENTION 
     This invention relates to the utilization of tracers in connection with monitoring hydrocarbon reservoirs and/or monitoring wellbores penetrating hydrocarbon reservoirs. 
     BACKGROUND OF THE INVENTION 
     The term tracer has generally been used to denote a material which is deliberately introduced into fluid flow which is taking place. Detection of the tracer(s) downstream of the injection point(s) provides information about the reservoir or about the wellbore penetrating the reservoir. In particular, deliberate addition of tracers has been used to observe flow paths and transit times between injection wells (used for instance to inject a water flood into a reservoir) and production wells. For this application of tracers to study inter-well flow, the tracer materials have generally been dissolved in the injection water at the surface before it is pumped down the injection well. 
     Some prior documents have proposed placing tracers in a well, or adjacent to it in a perforation extending through well casing into the surrounding formation, so as to monitor flow or events within the well rather than flow between wells. U.S. Pat. No. 507,771 proposed injecting radioactive tracers into perforations and monitoring loss of tracer with a wireline tool. U.S. Pat. No. 5,892,147 and U.S. Pat. No. 6,645,769 both proposed releasing distinguishable tracers from various underground locations within a wellbore and monitoring the produced flow to detect the presence of tracer. U.S. Pat. No. 6,840,316 proposed that tracer should be released under electrical control at various points within a complex hydrocarbon well and suggests a number of possible positions for sensors (of unspecified construction) to detect the presence of tracer. 
     In these various applications a deliberately added tracer may be present at very low concentration in the produced fluid where it is detected, and a number of prior documents have been concerned with choice of tracer material and methods of detection such that the tracer is detectable at very low concentrations. Substances deliberately introduced as tracers have included radioisotopes, fluorine-containing compounds and compounds of rare earth elements. 
     Whilst there are a variety of tracers and a variety of detection methods, a number of methods for detection of tracers involve the use of laboratory instruments. For example Society of Petroleum Engineers paper SPE 124689 proposes laser spectroscopy as a method of detection. WO2007/102023 proposes the use of a tracer containing a rare metal (e.g. caesium, hafnium, silver and gold) which is then detected in a sample by means of inductively coupled plasma mass spectrometry (ICP-MS). 
     When tracers are used, especially when the tracers do not contain radioactive isotopes, it is normal that samples are taken from the produced flow and sent away to a laboratory where solvent extraction or some other preparative procedure is carried out manually to extract and/or concentrate the tracer, after which the amount of tracer is determined by an analytical method which may be a sensitive instrumental technique. In consequence there is apt to be a significant time delay between taking the sample and obtaining an analysis of tracer(s) within it. 
     SUMMARY OF THE INVENTION 
     This invention provides a method of monitoring flow within a hydrocarbon well or a hydrocarbon reservoir penetrated by a well, comprising: 
     providing one or more tracer materials at one or more subterranean locations from which tracer may enter flow produced from the well, and monitoring flow within or from the well to detect the presence of one or more tracers in the flow, 
     characterized in that the detection of tracer is carried out by an electrochemical reaction. 
     The subterranean location(s) at which tracer is provided may be within the reservoir, or sufficiently close to the reservoir that tracer may flow into the well. In some forms of this invention the location(s) at which tracer is placed may be within part(s) of the well. 
     The step of monitoring flow may entail providing electrodes connected to a source of electrical potential, bringing a sample or portion of the flow into contact with the electrodes and applying potential to the electrodes to bring about electrochemical reaction, while measuring the current flow. This may be done using one of the various forms of voltammetry in which potential applied to the electrodes is varied over a range, while measuring the current flow as potential is varied. 
     The analytical detection of tracer by means of electrochemical reaction may serve to give qualitative detection of the presence of a tracer or may serve to give a quantitative determination of the concentration of tracer in the flow. A considerable advantage of using an electrochemical reaction as the analytical method is that it can be carried out with apparatus that can be small in size, that is easy to use and transport and that does not need the support of an extensively equipped laboratory. The electrochemical reaction mixture may be exposed to ambient air. Thus, it is practical for the analytical determination of tracers to be carried out proximate to the well so that the results can be available quickly after a sample is taken. Analysis of samples could for instance be carried out in an office at the site of the well, or nearby. Conveniently this may for instance be done within 10 km of the well, possibly closer such as within 3 km. 
     It will be necessary to choose a tracer or tracers which can be detected by means of an electrochemical reaction and also arrange that the tracer(s) will be released at a sufficient concentration to be detectable in the electrochemical reaction. We have found that electrochemical redox reactions can detect some tracers at concentrations which are sufficiently low to provide a useful level of sensitivity, which is of course of particular value when it is achievable with apparatus that does not require elaborate laboratory facilities. 
     The tracer may be a redox active material, capable of undergoing a reduction or oxidation reaction within an electrochemical cell. There are a number of possibilities for redox-active tracers. 
     The tracer may be an ionic species capable of undergoing a redox reaction. One possibility is a metal ion having more than one oxidation state. For instance copper ions provided by addition of copper sulphate solution can undergo electrochemical reduction to copper metal. 
     Some inorganic anions can be used. Halides such as chloride, bromide and iodide can undergo electrochemical oxidation to chlorate, bromate or iodate respectively. Thiocyanate ions can undergo electrochemical oxidation to trithiocyanate (SCN) 3   −  while nitrate ions can undergo electrochemical reduction to nitrite. Nitrate and thiocyanate ions have the advantage that they are not normally encountered in subterranean water. 
     A number of organic molecules undergo electrochemical redox reactions and so are detectable electrochemically. Some instances are xanthine, ascorbic acid and barbituric acid. 
     In a significant development of this invention, the tracer is in the form of nanoparticles containing a metal which has more than one oxidation state. Such nanoparticles can be detected in very low concentration as will be explained further below. 
     The electrochemical reaction to determine the presence of one or more tracers in the flow may be carried out by the well established technique of cyclic voltammetry in which the potential applied to a working electrode is cycled over a sufficient range to bring about the oxidation and reduction reactions while recording the current flow as the potential is varied. The recorded current shows peaks at the potentials associated with the reduction and oxidation reactions. It is also possible that this variation in potential whilst recording current flow could be carried out over only a portion of the reduction and oxidation cycle. This would be classed as linear scan voltammetry. 
     Cyclic and linear scan voltammetry are customarily performed with a continuous variation of the applied potential over a range, keeping the rate of change sufficiently slow that the analyte is able to diffuse within the electrolyte to reach the working electrode. Further possibilities are that the applied potential is varied in steps (as in square wave voltammetry) or is varied as pulses (as in differential voltammetry for instance). A discussion of various voltammetry techniques can be found in for example Brett and Brett  Electrochemistry Principles: Methods and Applications , Oxford University Press 1993. Square wave voltammetry has been found to be effective. In this technique the potential applied to the electrodes is varied in steps superimposed on a progressive variation over a range. The resulting waveform may be such that it can be referred to as a square wave superimposed on a staircase. 
     Sensitivity of these voltammetric methods may be increased by movement of the electrolyte relative to the working electrode, so that the mass transport to the electrode is enhanced. 
     A further electrochemical technique which gives very good sensitivity to the presence of some tracer(s) is stripping voltammetry with accumulation. This technique proceeds in two stages. In the first stage the working electrode is maintained at a potential which attracts tracer to become adsorbed onto it, possibly with a redox electrochemical reaction of the tracer on the electrode. The amount of tracer which accumulates is dependent on the concentration of tracer in the solution. Then in a second stage a voltammetric scan is carried out, bringing about electrochemical reaction of the material which has been accumulated on the electrode. This voltammetric scan also strips the accumulation from the electrode. This technique can be used with metal ions (e.g. cadmium, lead, bismuth or zinc) as tracers, the metal ions being reduced during the accumulation stage and re-oxidized during the subsequent voltammetric scan. Moreover, we have found that this technique of stripping voltammetry with accumulation can be used when the tracers are nanoparticles as mentioned above. The combination of nanoparticles as tracers and stripping voltammetry with accumulation as the method of detection leads to very good sensitivity to the tracer(s). The accumulation stage will generally be longer than the subsequent detection stage. Typically it will be more than 10 times longer. 
     A further possibility for enhancing sensitivity of voltammetry is to employ an electrode which spontaneously adsorbs the tracer, possibly by means of a substance which is chemically bound to the electrode surface and has binding affinity for the tracer. Detection would again have two stages: a first stage during which the tracer would be adsorbed and so accumulate on the electrode, followed by a second stage in which a voltammetric scan is carried out to detect, and preferably quantitatively determine, the amount of tracer. 
     These methods in which tracer(s) are accumulated on the electrode before detection may also be made even more sensitive by movement of the electrolyte relative to the working electrode during the accumulation stage, so that the mass transport to the electrode is enhanced. 
     Yet another possibility is to selectively extract tracer at the surface by bringing the flow into contact with a material capable of absorbing tracer, notably an ion exchange matrix. Any tracer retained by this matrix could subsequently be extracted into a solution which was then subjected to the electrochemical reaction to detect the presence of one or more tracers. 
     Apparatus for carrying out electrochemical determinations of tracer may comprise a container for the sample under test, a plurality of electrodes to be immersed in the sample and thus form an electrochemical cell, and a potentiostat to apply potential to the electrodes and measure current flow. The potentiostat may be operated under control of a computer which also records the results obtained. 
     The electrodes may constitute a conventional three electrode arrangement with a working electrode at which the electrochemical reaction occurs, a reference electrode and a counter electrode. These could be of standard types already used in electrochemistry. 
     An alternative to the use of separate electrodes of types traditionally used in electrochemistry is to use electrodes which screen printed (or deposited in some other way) onto an insulating substrate. Such electrodes may be manufactured as disposable items for one time only use, typically having a Ag/AgCl reference electrode, a metallic counter electrode and a carbon-based working electrode all screen printed on to a single ceramic or polymeric substrate. 
     A further possibility is that the working electrode is formed from boron-doped diamond located on an (electrically insulating) area of intrinsic diamond as described in U.S. Pat. No. 7,407,566. As shown in that document, such a working electrode may be carried on an insulating support of material other than diamond which also has a counter electrode and a reference electrode deposited on it. 
     It is possible that an arrangement with more than three electrodes could be used. There could be a single reference electrode and a single counter-electrode, but a number of working electrodes with specificity to different tracers. 
     The invention may be used in connection with various tasks which can be performed by use of tracers. 
     In one form of this invention tracers are used in conventional studies of inter-well flow from one or more injection wells to one more production wells. A tracer is added to the fluid which is pumped down an injection well, and thus is provided at the subterranean location where fluid exits from the injection well into the surrounding formation. This point of exit may be in a hydrocarbon reservoir or adjacent to it so that the injected fluid can be expected to flow through the formation to a production well. Whether the fluid actually does flow to the production well and how long it takes to travel from injection well to production well are of course questions which the addition of tracer is intended to investigate. 
     In these circumstances samples may be taken from fluid produced from a production well and examined by subjecting them to the electrochemical reaction. As mentioned earlier, this examination of the samples may be carried out close to the well site rather than by shipping the samples away to a laboratory elsewhere. 
     In another form, the invention may be used to observe flow within a well. This may possibly be in the context of a simple vertical well, releasing tracer at an underground location in the well (or close to it such as in a perforation which extends into the formation) and detecting its arrival at the surface. However, using this form of the invention to observe flow within a well is of particular interest within a more complex well which has multiple entry points for fluid from the formation around the well. 
     A well with multiple entry points for hydrocarbon may be any of: 
     a well which penetrates multiple pay zones (i.e. multiple oil-bearing formations); 
     a well which extends laterally within a reservoir, so that hydrocarbon enters the well at multiple points along the lateral; 
     a well which branches below ground so as to have multiple flow paths which merge before reaching the surface. A well which branches below ground may have branches diverging at angles to the vertical or may have multiple laterals. 
     For any well architecture where hydrocarbon can enter the well at multiple points, it will be desirable to have knowledge of what is flowing into the well at the various entry points, especially if the well has been provided with valves for control over the flow from different parts of the well. 
     Tracer materials may be provided at locations distributed within such a well or at locations within the formation and close to the wellbore, such as in perforations. Whenever tracer is placed in the formation adjacent to wellbore, the location at which the tracer is placed and from which it is released into the flow is preferably not more than 1 meter upstream from the point of entry into the wellbore. 
     One possibility here is that tracer is released from supply containers, in response to commands from the surface, analogous to the proposal in U.S. Pat. No. 6,840,316. An alternative is that the tracer is encapsulated within a body of other material which is exposed to the flow and is liberated into the flow as a consequence of diffusion out of the encapsulating material and/or degradation of the body of encapsulating material, analogous to techniques for the controlled release of other oilfield chemicals from encapsulation, described for instance in U.S. Pat. No. 5,922,652, U.S. Pat. No. 4,986,354, U.S. Pat. No. 6,818,594 and U.S. Pat. No. 6,723,683. Such bodies may be formulated to release tracer when in contact with water (more accurately subterranean brine) penetrating into the well so that detection of tracer in the downstream flow provides an indication that water penetration is taking place. Bodies of material which encapsulates tracer may be secured to equipment which is put into the well at the time of well completion. Another possibility is that material encapsulating tracer is applied as a coating on such equipment, for example on the exterior of a tubular. 
     Desirably, when this invention is used for monitoring flow from a plurality of separate entry points where water penetration into a well may occur, different tracers are associated with respective different entry points. By using different tracers at different locations within the well bore, the detection of tracer can indicate the part of the well where water penetration is taking place. This is useful in the context of a complex well with control valves which can be used to regulate (for instance to shut off) flow from a part of the well penetrated by water after that part of the well subject to water penetration has been identified by means of the tracer released into the water entering the well. 
     In a yet further form, this invention is used in monitoring a hydraulic fracturing job. This well-known method of stimulating a well involves pumping thickened fluid into the well and out into the formation, creating a fracture of the formation. U.S. Pat. No. 7,032,662 proposed incorporating tracers into the fracturing fluid which is injected and subsequently testing the flow back produced from the well to determine the amount of each tracer in the flow back. That determination of tracers in the flow back would then be used to estimate the amount of injected material which has returned to the surface. In an example, this document proposed that the tracers in produced fluid are determined by GC-mass spectrometry. The present invention could be used in analogous manner to monitor a fracturing job, but using an electrochemical method in accordance with this invention to detect and quantify tracers in produced fluid. This could be done in the vicinity of the well, rather than sending samples way to a laboratory. 
     As indicated above, it is envisaged that analysis for tracer by means of an electrochemical reaction will be carried out at the surface, in the vicinity of the well. However, it is also within the scope of this invention that the electrochemical analysis for tracer could be carried out underground, using a device analogous to that shown in US 2009090176 in which electrodes are exposed to a flow of fluid to be tested for tracer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view showing two production wells surrounded by injection wells; 
         FIG. 2  diagrammatically illustrates equipment at the surface; 
         FIG. 3  shows equipment for the electrochemical determination of tracer; 
         FIG. 4  show part of a subterranean lateral, in cross-section; 
         FIG. 5  shows a well with a plurality of branches, in cross-section; 
         FIG. 6  shows a well being subjected to hydraulic fracturing; 
         FIG. 7  is a cyclic voltammogram obtained with copper sulphate in saline solution; 
         FIG. 8  is a series of square wave voltammograms obtained with barbituric acid in formation brine; 
         FIG. 9  is a plot of the peak heights in  FIG. 8  against barbituric acid concentration; 
         FIG. 10  shows voltammograms obtained by applying stripping voltammetry to cadmium sulfide nanoparticles; 
         FIG. 11  is a plot of peak height against applied potential in the deposition stage of this stripping voltammetry; 
         FIG. 12  is a plot of peak height against electrode rotation rate in the deposition stage; 
         FIG. 13  is a plot of peak height against the duration of the deposition stage; 
         FIG. 14  shows a voltammogram obtained by applying stripping voltammetry to a mixture of three kinds of sulfide nanoparticles; and 
         FIG. 15  is a plot of peak height against concentration of lead sulfide nanoparticles. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a typical arrangement where inter-well studies are required. Two production wells  10  are surrounded by six injection wells  12  used to inject water into the reservoir to drive oil towards the production wells. A question which has traditionally been addressed by use of tracers is whether the water from all of the injection wells is reaching the production wells. To investigate this, water-soluble tracers are added to the water flow into each injection well and the flow from each production well is then monitored for the presence of tracer. 
     As shown in  FIG. 2  the flow produced from each production well  10  passes through a separator  14  which separates the flow into three parts, namely gas G. liquid crude oil  0  and water W (which will be saline because it includes some brine from the formation). Samples  16  are periodically taken from the water flow W by operation of valve  18  and tested for the presence of tracer. 
     In accordance with the present invention, this testing is carried out using electrochemistry and can be done in the vicinity of the well. An example of possible apparatus is shown in  FIG. 3 . The sample  16  is placed in a beaker  20  and a set  22  of three electrodes in the form of strips deposited onto an insulating substrate, is placed dipping into the sample  16  in the beaker  20 . A potentiostat  24  is connected to the electrodes and is operated under control of computer  26  to carry out voltammetry serving to detect and quantify the tracers present. 
       FIGS. 4 and 5  illustrate a different application of tracers. The well illustrated by  FIG. 4  has a long lateral which is subdivided into sections by packers  32  around the production tube  34 . One section between two packers  32  is shown in  FIG. 4 . Within each section there is a valve arrangement which controls entry to the production tube  36 . Such a valve arrangement can be of conventional construction. As an example the arrangement shown in  FIG. 4  comprises a sliding sleeve  36  which can be moved to cover and close openings  38  for entry of fluid into the production tube  34 , in response to a command transmitted from the surface. 
     A block of material  40  is secured to the exterior of the production tube  34 . This material  40  encloses a tracer (a different tracer in each section) and is such that the tracer is not released if the material  40  is exposed to oil but is released if the material  40  comes into contact with formation water or brine. The material  40  may be water-soluble so as to release tracer as the material  40  dissolves, or maybe water permeable, allowing tracer to dissolve into water which permeates into and out of the block of material  40 . 
     Consequently, so long as oil is entering each section of the wells lateral, no tracer is released. However if water penetrates into one section, tracer will be released into the water. This tracer can be detected at the surface in the same way as described above with reference to  FIGS. 2 and 3 . 
     Detection of tracer at the surface will show that water penetration is occurring (which may of course also be apparent from an increase in the quantity of water produced) but because each section of the lateral is associated with a different tracer, identification of the tracer will also show which section of the lateral has suffered water penetration. The valve arrangement,  36 ,  38  in that section can then be shut to prevent or restrict water entry, while allowing oil production from the other sections of the lateral to continue. 
       FIG. 5  diagrammatically illustrates a complex well drilled so as to have a plurality of branches  42  which merge below ground. When the completion of the well was carried out, valves  44  were incorporated which can be operated to restrict flow from a branch if needed. Each branch  42  may be subdivided into sections by packers  32 , with valves  46  (akin to sleeve  36  in  FIG. 4 ) which can be used to shut off flow into a section of a branch. In particular, one of the valves  44 ,  46  can be operated to shut off flow if water penetration into the flow becomes significant. 
     Similarly to the arrangement in  FIG. 4 , blocks  40  of material enclosing tracers are secured to the exterior of the production tube  34 . These blocks  40  contain tracers (a different tracer in each section of each branch) which are released if the material  40  comes into contact with formation water or brine. Detection and identification of tracer at the surface will show where water penetration is occurring and the affected branch or part of a branch can be shut off by operation of the relevant valve  44  or  46 . 
       FIG. 6  illustrates the invention being used in connection with a hydraulic fracturing job. The different stages of fracturing and a range of thickeners which may be employed are well known and are not detailed here. Briefly, thickeners  50  are mixed with a supply  52  of water (as schematically indicated at  54 ) to form a fracturing fluid which is pumped into the production tube  56  of a well. Water-soluble tracer(s)  58  are also mixed into this fracturing fluid. The fracturing fluid flows out of the well as indicated at  60  and causes formation of a fracture  62 , with some fluid penetrating into the surrounding formation and depositing a filter cake  64  at the boundary of the fracture. After fracturing of the formation has taken place, fluid is allowed to flow back out of the well and is passed through an oil-water separator  14  so that there are separate flows of oil  66  and water  68 . Samples are taken from the water flow  62  and tested for the presence and amount of the tracer(s), by means of an electrochemical procedure in accordance with this invention. Detection and quantitative estimation of tracers in the samples allows the progress of flow back to be monitored and because this can be done in the vicinity of the well, the results are available with little or no delay, as flow back is progressing. 
     EXAMPLES 
     Example 1 
     One possible tracer which may be used in procedures as above is copper ions, conveniently provided as copper (II) sulfate. In order to demonstrate that this is detectable, a solution of 7 ppm copper (II) sulphate pentahydrate in a solution of 150 mM KCl in deionised water was subjected to cyclic voltammetry. A standard experimental setup was used, with a glassy carbon working electrode, a standard calomel electrode (SCE) as reference electrode and a platinum wire as counter electrode. 
       FIG. 7  shows the voltammogram obtained. It is a plot of current (in microamps) against applied potential (in volts) relative to the reference electrode. A sharp oxidative wave observed at approximately 0 volt (relative to SCE) is consistent with the oxidation of deposited Cu to Cu(I) whilst the oxidative wave at +0.15 volt is oxidation of Cu(I) to Cu(II). The scan was continued to +0.60 volt and then reversed. Two reductive waves were observed at +0.03 volt and −0.31 volt. These voltammetric signals represent the reduction of Cu(II) to Cu(I) and Cu(I) to elemental Cu respectively. 
     This demonstrates that copper ions provide a distinctive and easily identifiable voltammogram at concentrations below 10 parts copper sulphate pentahydrate per million. 
     Example 2 
     Another possible tracer is barbituric acid. A series of aliquots of this acid were added to a quantity of a formation brine (a saline solution reproducing the analysis of a North Sea formation brine). A square wave anodic voltammogram was taken after each addition had been mixed in. Voltammetry was carried out using a boron doped diamond working electrode, a standard calomel electrode (SCE) as reference and a platinum wire as counter electrode. The composition of the formation brine was: NaCl (27910 ppm), KCl (125 ppm), MgCl 2  (650 ppm), CaCl 2  (1700 ppm), SrCl 2  (250 ppm), BaCl 2  (20 ppm), and KHCO 3  (145 ppm) prepared in deionised water. The measured pH value of the formation brine was pH 7.6 (at ambient temperature). The square wave used in voltammetry had a frequency of 50 Hz; a step amplitude of 0.02 volt; and increased in potential by 0.002 volt at each step giving an overall scan rate of 0.1 volt/sec. 
       FIG. 8  shows the voltammograms obtained. In each of the voltammograms, a clearly resolvable, pronounced oxidative peak is apparent with peak potential centred at a redox potential of approximately 1 volt (relative to SCE).  FIG. 9  shows the heights of this peak in microamp (after subtracting the baseline value) plotted against barbituric acid concentration and indicates that barbituric acid is detectable at concentrations of approximately 200 ppb and above. 
     Example 3 
     Synthesis of Nanoparticles 
     Synthesis of CdS nanoparticles was performed using Schlenk techniques under nitrogen. 
     The preparation method was based on arrested precipitation of cadmium sulfide from cadmium chloride solution as disclosed by Barglik-Chory, et al Synthesis, structure and spectroscopic characterization of water-soluble CdS nanoparticles (2003)  Chemical Physics Letters,  379 (5-6), pp. 443-451 and is schematically illustrated by  FIG. 4 . The starting materials were cadmium chloride CdCl 2  and hexamethyldisilathiane (HMSDT) which has the formula (CH 3 ) 3 Si—S—Si(CH 3 ) 3 . These were used together with glutathione which served as a water-soluble capping agent so as to produce nanoparticles of cadmium sulfide with glutathione residues bound to the nanoparticles&#39; surface. Glutathione has the structure: 
     
       
                 
         
             
             
         
      
     
     To prepare the nanoparticles, 3.228 g glutathione and 0.799 g CdCl 2  were first dissolved in 176 mL deionised water and stirred for 5 mins. Subsequently, 8.5 mL tetramethylammoniumhydroxide (TMAH) and 315 mL ethanol were added and after 10 mins this precursor solution was thoroughly degassed. 0.738 mL hexamethyldisilathiane (HMSDT) was added to the degassed solution, resulting in a clear (slightly yellow) colloidal solution of glutathione-capped CdS nanoparticles. The mixture was magnetically stirred for 1 hour and the prepared particles were precipitated by adding tetrahydrofuran (THF). One day later the supernatant was decanted and the precipitate was purified by re-dispersing it as a colloidal solution in a mixture of equal parts of water and THF and then precipitating again with THF. Finally, the supernatant liquid was decanted and the precipitate was dried under vacuum (&lt;1 mbar). 
     ZnS and PbS nanoparticles were also prepared by the same procedure, using either zinc acetate or lead acetate in place of cadmium chloride. 
     Examination of these nanoparticles by scanning electron microscopy showed them to have particle diameter in a range 100-200 nanometres. 
     Example 4 
     CdS nanoparticles were dissolved at a concentration of 300 ppb in 0.1M phosphate buffer (pH7) which also contained 0.1M KCl. (Such nanoparticles can have a degree of water solubility in the presence of some anions, but here it is immaterial whether the solution of nanoparticles was a colloidal solution.) CdS nanoparticles were also dissolved at 300 ppb in formation brine of the composition given in Example 2. Each of these colloidal solutions was then subjected to stripping voltammetry using a rotatable glassy carbon working electrode (polished with 1 μm diamond paste before use) together with a standard calomel reference electrode and a platinum wire as counter electrode. The measurements were carried out using an Autolab III computer controlled potentiostat (Eco-Chemie, Netherlands). 
     To carry out the stripping voltammetry the glassy carbon (GC) electrode was held at a potential of −1.25 volt for a period of 30 seconds to electrochemically reduce and ‘deposit’ the CdS nanoparticles onto the GC electrode surface. This was the ‘deposition’ or ‘accumulation’ stage. During the deposition step the GC electrode was rotated at 1000 rpm to overcome mass-transfer limitations of the otherwise static solution, thus increasing the flow of CdS nanoparticles to the electrode surface. 
     The electrochemical deposition process, required for deposition of CdS nanoparticles prior to stripping, is believed to occur by a direct mechanism (see Merkoci et al. Nanotechnology, vol 18 (2007) article no. 035502) thus:
 
CdS+2H + +2 e   − →Cd 0 +H 2 S
 
     Following the accumulation stage, the ‘stripping/detection stage’ is invoked by scanning from −1.25 volt to +0.2 volt with a rising square wave having the same waveform as in Example 2. 
     The resulting voltammograms are shown in  FIG. 10 . Both the sample prepared in phosphate buffer (indicated  70 ) and the sample prepared in formation brine (indicated  72 ) displayed well-defined, sharp oxidative stripping peaks. The peak current maxima were at approximately −0.85 volt (relative to SCE). Notably, the peak current magnitude and the potential at which peak current is observed are approximately the same for both samples, i.e. not sensitive to the composition of the cell solution. 
     Redox signals with the same peak positions and magnitudes were reproducibly obtained over several scans and furthermore the peak position and magnitude did not change if electrodes were polished between scans. 
     In typical produced water samples there is the unavoidable presence of a low concentration of organic species. In view of this, the above voltammetry was also carried out with approximately 5% by volume of hexane, pentane or dodecane added to the formation brine to represent hydrocarbon contaminants. It was observed that the distinctive peak was still present in the voltammetric signal and that the peak position and magnitude were not affected. This indicates that these nanoparticles would be detectable in produced water samples without extensive sample preparation. 
     Example 5 
     Optimisation of Stripping Voltammetry 
     CdS nanoparticles were dissolved at a concentration of 180 ppb in formation brine of the composition given in Example 2. This solution was then subjected to stripping voltammetry generally as in the preceding example, but with a deposition/accumulation time of 60 seconds and various potentials applied to the working electrode during the accumulation stage.  FIG. 11  shows the height of the stripping current peak in microamp plotted against potential applied during accumulation. As shown in  FIG. 11 , it was observed that progressively changing the applied potential from −1 volt to −1.75 volt, which was the optimum potential, led to a considerable increase in peak height. The reduction at −2 volt was attributed to interference from electrolysis of water leading to bubble formation on the electrode. 
     The above experiment was repeated, using an applied potential of −1.75 volt in the accumulation stage, progressively increasing the rotation rate of the glassy carbon electrode.  FIG. 12  shows the height of the stripping current peak in microamp plotted against this rotation rate. As shown in  FIG. 12  it was observed that a faster rotation rate during the accumulation stage increased the stripping peak current in the detection stage. 
     The experiment was then repeated again, using an applied potential of −1.75 volt and an electrode rotation rate of 3000 rpm, with variation in the length of time given to the accumulation stage.  FIG. 13  shows the height of the stripping current peak in microamp plotted against the duration of the accumulation stage. As shown in  FIG. 13  it was observed that a ten-fold increase in the duration of the accumulation stage led to a four-fold increase in the peak current in the detection stage. Thus the best conditions tested were a potential of −1.75 volt applied during a accumulation stage lasting 6000 seconds (ten minutes) with the electrode rotated at 3000 rpm. It is possible that even greater sensitivity would be achievable with still longer times and an even faster rotation rate, but the sensitivity was very good under the conditions tested. 
     Example 6 
     Detection of Other Nanoparticles 
     An attempt was made to detect nanoparticles of cadmium, zinc and lead sulfides on a GC working electrode, but only the cadmium sulfide particles were detected. This was rectified by using bismuth chloride as a coabsorbent, as taught by Wang et al  J. Am. Chem. Soc., vol  125, pages 3214-3215 (2003) 
     Nanoparticles of CdS, ZnS and PbS were all dissolved at concentrations of 1000 ppb in formation brine of the composition given in Example 2. 500 ppb of bismuth chloride was also dissolved in this solution. Each colloidal solution was then subjected to stripping voltammetry as in the previous examples, rotating the GC electrode at 3000 rpm and applying a potential of −1.75 volt to this electrode during an accumulation stage of 120 seconds. As shown in  FIG. 14  current peaks attributed to ZnS, CdS and PbS were observed at approximately −1.02 volt, −0.72 volt and −0.52 volt (relative to SCE) respectively and a current peak attributed to bismuth (chloride) is observed at approximately −0.14 volt. It can be seen that each peak is discrete, and there is no overlapping of redox signals. Thus, each species yields a voltammetric response that is identifiable and discrete, even in the presence of the other redox active species. 
     Example 7 
     Aliquots of a stock colloidal solution of PbS nanoparticles were progressively added to formation brine (composition as in Example 2) which also contained 500 ppb bismuth chloride. The solution was subjected to stripping voltammetry using the same conditions as in the previous Example after each addition of nanoparticles. Even at the lowest concentration, which was 60 ppb, there was a clear peak current in the voltammogram showing that PbS nanoparticles are detectable at this low concentration. The stripping peak current increased after each addition of nanoparticles.  FIG. 15  is a plot of peak current (after subtracting baseline current) against PbS concentration, showing that the current is proportional to PbS concentration. The same procedure was carried out with CdS nanoparticles, showing them to be detectable at 30 ppb and also with ZnS nanoparticles showing them to be detectable at 130 ppb.
In an arrangement for monitoring of flow within a hydrocarbon well or reservoir by means of one or more tracers which are placed at subterranean locations such that they may be present in flow produced from the well, the analysis of the flow produced from the well is carried out using an electrochemical method, preferably voltammetry, to detect tracer chosen to undergo a detectable electrochemical reaction. The tracer may be provided as nanoparticles in the well fluid.