Publications

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Sandia National Laboratories has developed technology enabling novel downhole electrochemical assessment in extreme downhole environments. High-temperature high-pressure (HTHP) electrodes selectively sensitive to hydrogen (H+), chloride (Cl-), iodide (I-) and overall ionic strength (Reference Electrode+-) have been demonstrated in representative geothermal environments (225°C and 103 bar in surrogate geothermal brine). This 2-year program is a collaboration effort between Sandia and Thermochem, Inc. with the goal of taking the prototype sensors and developing them into a commercial product that is operable up to 300°C and 345 bar. The Sandia-developed prototype HTHP chemical sensor package creates a capability that has never been possible to date. This technology is desired by the geothermal industry to fill a gap in available downhole real-time measurements. Only limited sensors are available that operate at the extreme temperatures and pressures found in geothermal wells. For the purpose of this paper, high temperature is defined as temperatures exceeding 200°C and high pressure is defined as pressures exceeding 35 bar. Chemical sensors exceeding these parameters and sized appropriately for downhole applications do not exist. The current Thermochem two-phase downhole sampling tool (rated to 350 °C) will be re-configured to accept the sensors. A downhole tool with an integrated pH real-time sensor capable of operation at 300°C and 345 bar does not exist and as such, the developed technology will provide the geothermal industry with data that would otherwise not be possible such as vertical in-situ pH-profiling of geothermal wells. The pH measurement was chosen as the first chemical sensor focus since it is one of the fundamental measurements required to understand downhole chemistry, scaling and corrosion processes.

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The electroreduction of Er3+ in propylene carbonate, N,N-dimethylformamide, or a variety of quaternary ammonium ionic liquids (ILs) was investigated using [Er(OTf)3] and [Er(NTf2)3]. Systematic variation of the ILs' cation and anion, Er3+ salt, and electrode material revealed a disparity in electrochemical interactions not previously seen. For most ILs at a platinum electrode, cyclic voltammetry exhibits irreversible interactions between Er3+ salts and the electrode at potentials significantly less than the theoretical reduction potential for Er3+. Throughout all solvent-salt systems tested, a deposit could be formed on the electrode, though obtaining a high purity, crystalline Er0 deposit is challenging due to the extreme reactivity of the deposit and resulting chemical interactions, often resulting in the formation of a complex, amorphous solid-electrolyte interface that slowed deposition rates. Comparison of platinum, gold, nickel, and glassy carbon (GC) working electrodes revealed oxidation processes unique to the platinum surface. While no appreciable reduction current was observed on GC at the potentials investigated, deposits were seen on platinum, gold, and nickel electrodes.

Understanding the connectivity of fracture networks in a reservoir and obtaining an accurate chemical characterization of the geothermal fluid are vital for the successful operation of a geothermal power plant. Tracer experiments can be used to elucidate fracture connectivity and in most cases are conducted by injecting the tracer at the injection well, manually collecting liquid samples at the wellhead of the production well, and sending the samples off for laboratory analysis. This method does not identify which specific fractures are the ones producing the tracer; it is only a depth-averaged value over the entire wellbore. Sandia is developing a high-temperature wireline tool capable of measuring ionic tracer concentrations and pH downhole using electrochemical sensors. The goal of this effort is to collect real-time pH and ionic tracer concentration data at temperatures up to 225 °C and pressures up to 3000 psi.

Nitrogen and phosphorus atoms are constituents of some of the most toxic chemical vapors. Nitrogen-phosphorus gas chromatograph detectors (NPDs) rely on selective ionization of such compounds using ionization temperatures typically greater than 600°C. NPDs have previously been reported to be 7 × 104× and 105× more sensitive for nitrogen and phosphorus, respectively, than for carbon. Presented here is an investigation of the structure-based selectivity of a microfabricated nitrogen-phosphorus detector (μNPD). The μNPD presented here is smaller than a dime and can be placed in a system that is 1/100th the size of a commercial NPD. Comparison of responses of such devices to homologous anilines (p-methoxyaniline, p-fluoroaniline, and aniline) revealed that detection selectivity, determined by the ratio of μNPD to nonselective flame ionization detector (FID) peak areas, is correlated with acid disassociation pKa values for the respective analine. Selectivity was determined to be greatest for p-methoxyaniline, followed by p-fluoroaniline, with aniline having the smallest response. The limit of detection for a nitrogen containing chemical, p-methoxyaniline, using the μNPD was determined to be 0.29 ng compared to 59 ng for a carbon chemical containing no nitrogen or phosphorus, 1,3,5-trimethybenzene. The μNPD presented here has increased detection for nitrogen and phosphorus compared to the FID and with a slight increase in detection of carbon compounds compared to commercial NPD's sensitivity to nitrogen and carbon.

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