This invention relates to electrochemical sensors for determining an analyte in a fluid. There are numerous circumstances in which it is desirable to detect, measure or monitor a constituent of a fluid. Electrochemical sensors may be used for this purpose and the electrochemistry may incorporate a redox-active species whose oxidation and/or reduction is monitored as a part of the analysis.
For instance to measure pH, WO 2005/066618 disclosed an electrochemical sensor in which the electrochemical cell contains two organic compounds which are pH-sensitive redox systems and a ferrocene compound as an internal reference which is not sensitive to pH. To measure sulfide, WO2001/063094 and WO2004/011929 described an approach in which electrochemistry is coupled through a mediator compound to sulfide which is the intended analyte. This mediator compound is present in an electrochemical cell which is exposed to the sulfide. Both in the presence and absence of the sulfide analyte, an electrochemical oxidation and reduction of the mediator compound can take place when appropriate electrical potential is applied to the electrodes. However, one of the redox reactions of the mediator compound can also be brought about through a chemical reaction with the sulfide, and when this takes place there is a measurable change to the electrochemistry. Ferrocene carboxylate and sulfonate were suggested as mediator compounds in Electroanalysis Vol. 18, pages 1658-63 (2006) and in Electrochimica Acta Vol. 52, pages 499-50 (2006). A number of ferrocene sulfonates for possible use in this way have been described in Journal of Organometallic Chemistry Vol. 692, pages 5173-82 (2007). Experimental work in this area has, however, generally been confined to laboratory experiments at ambient temperature.
An issue which can arise in connection with electrochemical analytical systems is the stability of the redox active species employed, in particular stability when exposed to elevated temperatures during use. Exposure to elevated temperature may, however, be unavoidable when using an electrochemical sensor to monitor an industrial process.
One circumstance where there is exposure to temperature arises when carrying out analysis of fluids encountered downhole in a wellbore. Analysis of downhole fluids can be an important aspect of determining the quality and economic value of a hydrocarbon formation and can be applied to save costs and increase production at many stages of oil and gas exploration and production. Some chemical species dissolved in water (for example, Cl− and Na+) do not change their concentration when moved to the surface and information about their quantities may be obtained at the surface by analysis of 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+]), CO2, or H2S 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 samples taken downhole, this change may also happen due to degassing of a sample (seal failure), mineral precipitation in a sampling bottle, and (especially in case of H2S)—a chemical reaction with the sampling chamber. It should be stressed that pH, H2S and CO2 are among the most critical parameters for corrosion and scale assessment. Consequently it is of considerable importance to determine their downhole values and there have been proposals for analytical sensors to be used downhole even though this is a difficult environment for an analytical system.
Redox reactions of organic compounds solubilised in surfactant micelles have been examined, in particular for biochemical analyses carried out close to ambient temperature. One instance is Ryabov et al., J. Phys. Chem., Vol. 99, 14072 (1995) which reports voltammetry studies of ferrocene and alkyl-substituted ferrocenes in surfactants, in a biochemical context where the ferrocene redox system is coupled to glucose and glucose oxidase.