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Nanotechnology in hydrogen sulfide detection

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Hydrogen sulfide (H2S) leaks can cause problems that affect both workers and equipment in the drilling industry. The explosive gas naturally occurs in oil and natural gas deposits. The lesser risk from H2S, corrosion of metal, paint, and epoxy, can be prevented with the use of special coating. The greater risk, the risk to the health of industry workers, can be prevented with detection equipment. More recently, nanotechnology has been tested to detect H2S in the air.

Presence in reservoirs

The presence of H2S in reservoir fluids is a major problem for the petroleum industry and is associated with reservoir souring, iron sulfide deposition, poor sweep efficiency, and increased corrosion. Its occurrence may cause the early abandonment of many oil and gas reservoirs by increased costs, reduced revenue, and environmental concerns. In many cases, reservoirs that initially did not contain sulfide have become sour as a result of operations. This progressive increase in sulfide levels is most notable in reservoirs that were flooded with seawater. (50980-PA)

Health effects of hydrogen sulfide

From http://www.safetydirectory.com/hazardous_substances/hydrogen_sulfide/fact_sheet.htm: H2S poses multiple health risks to people working around it, ranging from watery eyes to nausea or migraines, coma, and even death. The gas is classed as a chemical asphyxiant, in the same category as carbon monoxide and cyanide gases. It inhibits cellular respiration and uptake of oxygen, causing biochemical suffocation. Common exposure symptoms include:

Working with hydrogen sulfide

Most countries have legal limits in force that govern the maximum allowable levels of exposure to hydrogen sulfide in the working environment. A typical allowed exposure limit in multiple countries is 10 ppm. While the distinctive odor of H2S is easily detected, it causes olfactory fatigue, therefore one cannot rely on the nose as a warning device. The only way to accurately determine H2S exposure levels is to measure the amount in the air. With a vapor density of 1.19, H2S is about 20 percent heavier than air, so this invisible gas will collect in depressions in the ground and in confined spaces.

Traditional detection methods

Paper strips infused with lead acetate have been commonly used to measure air samples for H2S levels. This method has been improved upon by soaking the paper in mercuric chloride or silver nitrate. Mercuric chloride paper strips are sensitive and reliable for measurement of hydrogen sulfide in air with a sensitivity of 0.7 µg/L. Strips impregnated with silver nitrate are suitable for determining H2S concentrations in the range of 0.001–50 ppm. Potentiometric titration with a sulfide ion-selective electrode as an indicator has been used to measure hydrogen sulfide in the air at ppb levels. This method has been shown to have very good accuracy and precision. Passive card monitoring has been used to detect hydrogen sulfide in workplace environments. Badges worn in a worker’s breathing zone that change color based on exposure to toxic gases also detect hydrogen sulfide. The sensitivity for the hydrogen sulfide badges is 10 ppm/10 minutes with a color range of white to yellow, which indicates H2S presence. Other colorimetric methods for monitoring hydrogen sulfide include hand-held colorimetric tubes. Air is drawn through the tube and the presence of hydrogen sulfide reacts with a chemical reagent in the glass tube and causes a color change.

Electrochemical sensors

Electrochemical and metal oxide sensors are the most commonly used sensors for hydrogen sulfide. They consist of a diffusion barrier that is porous to gas but not to liquid, an acid electrolyte reservoir (usually sulfuric or phosphoric acid), a sensing electrode, a counter electrode, and (in three-electrode designs), a third reference electrode (Figure 1). Gas diffusing into the sensor reacts at the surface of the sensing electrode, which catalyzes a specific reaction. Depending on the sensor and the gas being measured, the gas is either oxidized or reduced at the surface of the sensing electrode. This reaction causes the potential of the sensing electrode to rise or fall with respect to the counter electrode. The current generated is proportional to the amount of reactant gas present. This two-electrode detection principle presupposes that the potential of the counter electrode remains constant. In reality, the surface reactions at each electrode cause them to polarize, and significantly limit the concentrations of reactant gas they can measure. In three electrode designs, it is the difference between the sensing and reference electrode that is actually measured. In the three-electrode models, the reference electrode is shielded from any reaction, thus maintaining a constant potential and providing a true point of comparison. With this arrangement, the change in potential of the sensing electrode is solely due to the concentration of the reactant gas.

Metal oxide semiconductor sensors

With metal oxide semiconductor (MOS) sensors , changing the temperature of the sensing element can alter its sensitivity. MOS sensors are designed to respond to the widest possible range of toxicity, which can be helpful in situations where unknown toxic gases may be present and a simple go/no-go determination of the presence of toxic contaminants is sufficient. Sensitivity of the sensing element to a particular gas is mathematically predictable, so a commonly used strategy is to pre-program the instrument with a number of theoretical specific response curves.

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