Kent Merrill, General Motors Inc.
Commonly known as sour gas, hydrogen sulfide (H2S) is a highly toxic chemical agent found in petroleum drilling, production and refining operations. As secondary and tertiary recovery efforts are undertaken on older reservoirs, H2S is being found where previously it was not recorded. The gas is present in many crudes and is produced in common refinery processes such as cracking and desulfurization. Hydrogen sulfide is formed primarily by decomposition of organic matter containing sulfur. The gas occurs naturally in many geologic formations throughout the United States and is both toxic and corrosive. This colorless gas has the characteristic odor of rotten eggs and can be readily detected by the human nose. One can usually smell concentrations of less than 10 parts per million (ppm), with concentrations of 700 to 1000 ppm being fatal, even if exposure is brief. It is important to realize that continuous exposure to low concentrations of H2S (approximately 50 ppm) deaden the olfactory nerves, causing the sense of smell to become an ineffective detection tool. A variety of both federal and state regulations apply to the petroleum industry in areas known or suspected to contain sour gas. Federal regulations basically are designed to protect the employee and are handled through such agencies as the Occupational Safety and Health Administration (OSHA), the National Institute of Occupational Safety and Health (NIOSH), and the U.S. Geological Survey (USGS). Most state regulations are designed to protect the general public and require certain precautions be followed to minimize the chance of accidental public exposure to H2S. Probably the best known state regulation is Texas Railroad Commission Rule 36, whose degree of operator compliance depends on the radius of exposure based on the calculated concentration of H2S and the rate at which it is expected to flow from a well. Due to the occurrence and toxicity of hydrogen sulfide and the array of regulations concerning sour gas areas, a method of monitoring H2S becomes essential. Factors to consider in selecting a monitoring system include reliability and accuracy, ease of operation and maintenance, response time and expected lifespan of the system components. Another important concern of any H2S detector is proper placement of the sensor units, with special care taken to ensure all installation and location guidelines are clearly understood.