Canadian Drilling School

What is H2S?

Hydrogen sulfide is the chemical compound with the formula H2S. It is a colorless gas with the characteristic foul odor of rotten eggs; it is heavier than air, very poisonous, corrosive, flammable and explosive.


Common Sources of H2S

Hydrogen sulfide is a naturally-occurring chemical compound that is usually formed as anaerobic bacteria decompose organic material in the absence of oxygen. It is as a result of this anaerobic digestion that the H2S in sewer and swamp gasses are formed, giving these places their characteristic sulfurous odours.

The decomposition of certain organic materials such as lignosulfonates in drilling fluids by these sulfate-reducing bacteria have been reported to be one of the reasons why some sweet wells later turn sour after drilling activities have been completed.

H2S is produced in the human body, not only through bacterial activity within the alimentary canal, but also in extremely small quantities by some cells to perform a number of neurotransmission roles. It may be that H2S even plays some protective role preventing cardiovascular disease.

H2S is present in some farming, pulp and paper, coal mining, tanning, and smelting activities, which may be responsible for up to 10% of total emissions worldwide.

H2S is present in the volcanic gases emanating from some hot springs, volcanoes, and subsea vents. However, by far the most significant source of H2S in Western Canada is encountered in the oil and gas industry. The source of all this noxious material is certain geological formations beneath our Western Sedimentary Basin.

H2S Properties

Because we humans naturally produce H2S in our body, we have developed enzymes to enable us oxidize it into harmless sulfates. This means our bodies are capable of tolerating the presence of minuscule concentrations of H2S almost indefinitely.

Nevertheless, we face a problem when we encounter concentrations of H2S high enough to completely overwhelms our body’s ability to safely oxidize this compound, leaving it free to attack our central nervous system. The IDLH of H2S is 100 ppm, while the short-term exposure limit is 15 ppm.

For this reason, H2S is a controlled substance that is extremely poisonous to humans in everything but trace concentrations and has been rated as being comparable in toxicity to hydrogen cyanide, which was one of the chemical agents used by the Germans during WW 1 and WW 11.

This toxic nature of H2S is the main reason why it is mandatory for every oilfield worker in Western Canada attend a full day of instruction and practicum in H2S every three years.

The H2S UN number is 1053.

Since our industry has developed a number of ways to safely work with H2S, it is imperative that every oil and gas worker be given consistent training in each of these methods. Therefore, the only universally recognized certification program in Western Canada is Enform's H2S Alive course.

This is the reason on-line H2S courses cannot meet the adequate training criteria of the H2S committee and are insufficient for most oil and gas jobs.

H2S is colourless and has a refractive index very close to air. This means we cannot use our eyes to detect it.

H2S is a heavier than air gas which has a boiling point of -60.4 Deg C and a vapor density of 1.1363 relative to air. This means it is typically encountered as a gas that usually falls to low-lying, poorly ventilated areas. Nevertheless, when mixed with lighter-than-air gasses, or if its temperature is higher than the ambient temperature, it can rise in air.

H2S is highly explosive. The lowest concentration of a H2S / air mixture that is capable of producing a flash fire when exposed to an ignition source is 4% (LEL). Concentrations of less than 4% are too lean to explode.

Although the UEL of H2S is 46%, a release of H2S in concentrations too rich to burn will undoubtedly become explosive as the concentration of the cloud diminishes. This means concentrations greater than 46% are still considered explosive

The auto ignition temperature is 232 Deg C, which is well below the 700 Deg C of a lit cigarette.

H2S burns in air with a blue flame, producing SO2 and water. Sulfur dioxide is also a very poisonous gas with a IDLH set at 100ppm and a short term exposure limit of 5ppm.

H2S is explosive in the presence of strong nitric acid.

H2S is soluble in water and many other liquids. When dissolved in water, H2S will form hydrosulfuric acid and is one reason why someone exposed to H2S will suffer eye irritation. Dissolved sulfides can exist as H2S, HS-, or S-- ions, depending upon the pH of the solution.

Contaminated standing water will initially be clear, but will sometimes become cloudy as elemental sulfur precipitates out.

H2S combines with the soft tissue in the throat and lungs to form sodium sulfide, a caustic material that can irritate the affected organs.

H2S is corrosive to metals, forming metal sulfides. Since these insoluble metal sulfides are characteristically dark brown or black, it is not uncommon for a sludge which is rich in metallic ions to take on this dark colour when contaminated with H2S. Blackened tubulars can be a warning sign of the presence of H2S.

H2S smells in very low concentrations like rotten eggs; however, the nature of the odour will vary as the H2S concentration increases until the olfactory receptors in our nose become paralyzed at approximately 100ppm. Further, we must remember our ability to smell even at very low concentrations will soon be lost because of olfactory fatigue.

This means we cannot use our nose as a H2S detector and victims are often unaware of its presence before being knocked down.

H2S Detection

Detector tubes containing lead acetate, which discolour to a dark gray as the lead acetate is converted to lead sulfide in the presence of H2S, are used to give continual monitoring. Electronic monitors, using a Wheatstone bridge, can give continuous detection.

pH - pS ion monitoring of drilling fluids by mudloggers and mud engineers can provide an advance warning of the presence of H2S in drilling fluids

H2S Alive Course

The H2S Alive course is designed to bring lessons learned from past mistakes and teach workers how to protect themselves and their co-workers in the workplace.

The course identifies five methods of ensuring a safe jobsite and teaches us how to identify likely danger zones.

The practicum includes inspecting and using gas detection equipment and breathing apparatus according to the manufacturer's instructions.