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Nanotechnology in hydrogen sulfide detection
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.
Health effects of hydrogen sulfide
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:
|0.00011-0.00033||Typical background concentrations|
|0.01-1.5||Odor threshold (when rotten egg smell is first noticeable to some). Odor becomes more offensive at 3-5 ppm. Above 30 ppm, odor described as sweet or sickeningly sweet.|
|2-5||Prolonged exposure may cause nausea, tearing of the eyes, headaches or loss of sleep. Airway problems (bronchial constriction) in some asthma patients.|
|20||Possible fatigue, loss of appetite, headache, irritability, poor memory, dizziness.|
|50-100||Slight conjunctivitis ("gas eye") and respiratory tract irritation after 1 hour. May cause digestive upset and loss of appetite.|
|100||Coughing, eye irritation, loss of smell after 2-15 minutes (olfactory fatigue). Altered breathing, drowsiness after 15-30 minutes. Throat irritation after 1 hour. Gradual increase in severity of symptoms over several hours. Death may occur after 48 hours.|
|100-150||Loss of smell (olfactory fatigue or paralysis).|
|200-300||Marked conjunctivitis and respiratory tract irritation after 1 hour. Pulmonary edema may occur from prolonged exposure.|
|500-700||Staggering, collapse in 5 minutes. Serious damage to the eyes in 30 minutes. Death after 30-60 minutes.|
|700-1000||Rapid unconsciousness, "knockdown" or immediate collapse within 1 to 2 breaths, breathing stops, death within minutes.|
|1000-2000||Nearly instant death|
Source: United States Department of Labor
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 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. 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.
Optical Sensing Technology
Optical H2S detectors use an operating principle similar to optical hydrocarbon detectors, which are a field-proven solution used in industrial applications. In optical H2S sensors, the signal is absorbed by the H2S gas as it passes through the detector’s optical path. The sensor will record the signal reduction and a micro-processor will calculate a corresponding gas value.
Detection methods using nanotechnology
Nanotechnology, while still in its infancy in the oil and gas industry, can provide more accurate H2S readings while using less energy than traditional methods.
Nanomaterials, particularly metal oxide, have been inventively researched for sensing applications because of their large surface-area-to-volume ratios, which enhance the effect surface reactions and adsorption have on their electrical properties. Additionally, nanomaterials have the ability to be heated to high temperatures using minimal power. While research has shown that tungsten oxide nanomaterials perform well as sensors for hydrogen sulfide, the sensors must be heated to obtain sufficient sensitivity and response times. As shown in Piantanida, M.,Veneziani, M., Fantoni, R.F., et al, microheaters using heat pulses (versus continuous heating) heated to 300°C can draw a response in seconds with a response magnitude of over 500 times that at room temperature, all with minimal power consumption.
Advances in nano-enhanced material construction have effectively dealt with the challenges of traditional MOS sensors. The appearance and operating principle of NE-MOS sensors are identical to that of traditional MOS sensors, but NE-MOS sensors benefit from a mechanically conformed array of sensing components known as “nanotubes” applied to the resistive film perfectly aligned, symmetric, and extremely concentrated manner during manufacturing. Traditional materials are produced using a process that leaves gaps and creates irregularities, but nano-enhanced materials allow for increased overall sensing capability, faster response, and much higher stability.
Because of their advanced design, NE-MOS sensors can, in some applications, respond faster than both electrochemical and traditional MOS sensors. NE-MOS response to T50 (the time it takes for the detector to reach 50 % of full-scale concentrations) can be as quick as 10 seconds. It responds to T90 in approximately 15 seconds. The latest design has also been able to include electronic temperature and humidity compensation, enabling NE-MOS detectors to function efficiently in even the most extreme climate conditions.
Scientists at Rice University developed nanoscale detectors to analyze hydrogen sulfide levels in crude oil, to detect and avoid sour crude—crude with an H2S concentration of 1 percent or more—which is toxic to workers and corrosive to pipelines and transportation vessels. The nanoreporters, based on probes originally developed by University of Oregon chemists to test H2S levels in biological and water samples, use thermally stable, soluble, highly mobile carbon black with fluorescent properties that change in the presence of H2S. The particles can then be analyzed using a spectrometer to determine the contamination level. Adding polyvinyl alcohol (PVA) stabilized the nanoreporters in temperatures as high as 100°C. Because the detection method is sensitive, large amounts of nanaoreporters are not needed to be pumped downhole, which makes detection simpler for workers.
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Noteworthy papers in OnePetro
Awad, M., and Macwan, N. 2012. H2S Risk Management. Abu Dhabi International Petroleum Conference and Exhibition, Abu Dhabi, 11-14 November. SPE-162613-MS. http://dx.doi.org/10.2118/162613-MS.
Battle, J. A., and Russell, J. 1977. Plastic Coatings Can Be Used Successfully In Hydrogen Sulfide Environments. Presented at the SPE Symposium on Sour Gas and Crude, Tyler, Texas, USA.14-15 November, . SPE-6661-MS. http://dx.doi.org/10.2118/6661-MS.
Bouhroum, A., Marx, C., and Schade, W. 1990. Effect of H2S on Coatings. Midland, Texas, USA. Presented at the Permian Basin Oil and Gas Recovery Conference, 8-9 March. SPE-20110-MS. http://dx.doi.org/10.2118/20110-MS.
Carter, D. R., and Adams, N. J. 1979. Hydrogen Sulfide In The Drilling Industry. SPE Deep Drilling and Production Symposium, Amarillo, Texas, USA, 1-3 April. SPE-7854-MS. http://dx.doi.org/10.2118/7854-MS.
Elshahawi, H., and Hashem, M. N. 2005. Accurate Measurement of the Hydrogen Sulfide Content in Formation Fluid Samples-Case Studies. SPE Annual Technical Conference and Exhibition, Dallas, Texas, 9-12 October. SPE-94707-MS. http://dx.doi.org/10.2118/94707-MS.
Hill, D. G., and DeMott, D. N.1977. Effect Of Hydrogen Sulfide On The Inhibition Of Oil Field Tubing In Hydrochloric Acid. Presented at the SPE Symposium on Sour Gas and Crude, Tyler, Texas, USA, 14-15 November. SPE-6660-MS. http://dx.doi.org/10.2118/6660-MS.
Hirezi, G. J., Al-Khelaiwi, F. T., and Al-Khamis, M. N. 2012. H2S Early Notification System for Production Pipelines: A Pilot Test. Abu Dhabi International Petroleum Conference and Exhibition, Abu Dhabi, 11-14 November. SPE-161062-MS. http://dx.doi.org/10.2118/161062-MS.
Mickelson, W., Sussman, A., Zhou, Q., et al. 2013. An Innovative Wireless H2S Sensor Based On Nanotechnology To Improve Safety In Oil & Gas Facilities. SPE Offshore Europe Oil and Gas Conference and Exhibition, Aberdeen, UK, 3-6 September. SPE-166544-MS. http://dx.doi.org/10.2118/166544-MS.
Naranjo, E., and Kornbech, M. 2008. Hydrogen Sulfide Detection in Offshore Platforms. SPE Middle East Heath, Safety, Security, and Environment Conference and Exhibition, Doha, Qatar, 20-22 October. doi:10.2118/120932-MS.
Roth, D., White, B., Benavides, et al, R. 2001, January 1. Automated Chemical Control of H2S Content of Natural Gas. SPE Production and Operations Symposium, Oklahoma City, 24-27 March. SPE-67247-MS. http://dx.doi.org/10.2118/67247-MS.
Samuels, A. 1974. H2S Need Not Be Deadly, Dangerous, Destructive. Presented at the SPE Symposium on Sour Gas and Crude, Tyler, Texas, USA, 11-12 November. SPE-5202-MS. http://dx.doi.org/10.2118/5202-MS.
Zea, L., Cooper, D., and Kumar, R. 2011. Hydrogen Sulfide Absorption Phenomena in Brine/Oil Mixtures. SPE Journal 16 (4): 931 – 939. http://dx.doi.org/10.2118/145401-PA.
AAPG Wiki. 2014. Wellsite safety. http://wiki.aapg.org/Wellsite_safety#Hydrogen_sulfide.
IADC. 2014. Hydrogen Sulfide. IADC Drilling Lexicon. (8 July 2014 revision), http://www.iadclexicon.org/hydrogen-sulfide/.
Lee, J., Mubeen, S., Hangarter, C.M. 2011. Selective and Rapid Room Temperature Detection of H2S Using Gold Nanoparticle Chain Arrays. Electroanalysis 23 (11): 2623 – 2628. http://dx.doi.org/10.1002/elan.201100295.
Montoya, L.A. , Pearce, T.F., Hansen, R.J., et al. 2013. Development of Selective Colorimetric Probes for Hydrogen Sulfide Based on Nucleophilic Aromatic Substitution. J. Org. Chem. 78 (13): 6550–6557. http://dx.doi.org/10.1021/jo4008095.
Wikipedia. 2014. Hydrogen sulfide (23 September 2014 revision), http://en.wikipedia.org/wiki/Hydrogen_sulfide.
Adebayo, D., Okafor, A. 2012. Hydrogen sulfide : sources, detection, and health hazards. New York : Nova Science Publishers.
American National Standards Institute. 1990. Installation, operation, and maintenance of hydrogen sulfide detection instruments. Research Triangle Park, NC : Instrument Society of America.
Carpenter, M.A., Mathur, S., Kolmakov, A. 2013. Metal Oxide Nanomaterials for Chemical Sensors. New York: Springer.
National Research Council. Committee on Medical and Biologic Effects of Environmental Pollutants. Subcommittee on Hydrogen Sulfide. 1979. Hydrogen sulfide. Baltimore : University Park Press.