V.W.L. Skates, R.E.G. van Hal
Keywords: H2S, nanoparticles, chemical sensor, harsh conditions
Summary:Hydrogen sulfide is found naturally in surface water and well water but also in many oil and gas fields. We present a novel method to measure hydrogen sulfide using a colorimetric method. A metal ion-based reagent reacts with the sulfide to form metal sulfide nanoparticles, causing the solution to change color. Many metals are suitable candidates at room temperature. Bismuth was chosen because the optical absorption is independent of particle size, high color intensity, stability of the sulfide after reaction (especially at elevated temperatures) and single ionic state. The effective mass approximation theory is used to confirm that the optical absorption spectrum is independent of small variations in particle size. The solution is stabilized with poly(acrylic acid) polymer both to prevent precipitation and to control particle size. A linear relation between optical absorbance and hydrogen sulfide concentration was observed, and it is proven that the reagent is reproducible and independent of the reaction environment (mixing method, mixing rate, etc.). The new reagent is cost effective and consists of benign chemicals. It has no known cross-sensitivities and is therefore suitable for environmental applications. Furthermore, the reagent can withstand the harsh temperature and pressure requirements of the oil and gas industry. Water based samples can be mixed directly with the reagent, reacting instantaneously. Gas samples can be mixed with reagent in a closed bottle by shaking or spraying the reagent several times through the sample. A more elegant method is the use of a small-scale device to create a slug flow regime. In this flow regime, two immiscible fluids flow together as a series of slugs of one phase separated by the other. In this case, reagent forms the continuous, wall-wetting phase, enabling inline optical measurement, and sulfide containing sample forms the dispersed phase. Reagent and sample move in alternating regular subvolumes, creating a continuously refreshed area for mass transfer via convection within slugs and diffusion between slugs. Sulfide is extracted from the sample slugs and mixed into the reagent slugs in this manner. This method is advantageous because it enables measurement with both gas and oil samples. Experimental results demonstrate that gas measurements can be done at elevated temperature and pressure. For measurement with oil, the channel used must be made from a material ensuring that the channel remains hydrophilic and only reagent-wet. Size and geometry of the channel also affect the range of oils with different fluid properties accepted into the correct flow regime with reagent. However, experiments with oil samples have shown that no color transfer from sulfide-free oil to reagent is expected and that the use of slug flow is an effective method in exchanging hydrogen sulfide between sample and reagent.