The following are the major non-hydrocarbon components of natural gas and crude oil production.
(1) Sulphur Compounds.
Pure Sulphur and Sulphur compounds are generally present in reservoirs and are contaminants. The main compounds are Hydrogen Sulphide (H2S), Carbonyl Sulphide (C0S), and Mercaptan (RSH), but the most unwanted is definitely Hydrogen Sulphide. For your information, with mercaptan, the “R” represents an alkyl group CH3.
Hydrogen Sulphide is a highly toxic and corrosive gas having an extremely odorous smell in small concentration but please note that, never trust your sense of smell when dealing with Hydrogen Sulphide, therefore, remember to wear your own personal Hydrogen Sulphide detector when working in Hydrogen Sulphide area.
Therefore, it must be removed from petroleum products after downstream processing of the crude oil and from natural gas.
In general, contract requirements for the purchase of LNG normally state that the final product should contain less than 0.003% to 0.005% Hydrogen Sulphide.
Water (H2O) is one of the universal contaminate in a reservoir. Water exists in a well stream in both liquid and vapour states. Free water is removed from oil and gas streams by means of a production separator. One of the biggest problems caused by water entrainment in natural gas is the formation of hydrates.
A hydrate is a crystalline solid structure formed when the gas is cooled after the reservoir or in pipelines. Hydrates can be removed by inhibitors that lower the freezing point of water and dissolve the ice structure or by absorption techniques, which removes the water from the gas.
(3) Carbon Dioxide.
Carbon dioxide (CO2) can exist in large concentrations in a well stream. It is very corrosive in the presence of water and it lowers the heating value of natural gas because of its low boiling point (-1100F at 14.7 psia) and therefore must be removed. Carbon Dioxide is removed along with Hydrogen Sulphide in many of the desulphurization processes with no double processing is required.
Nitrogen (N2) appears naturally in natural gas and comprises some 80% of the air we breathe. It is a colorless, odorless gas and to all intents, an inert gas that does not react with hydrocarbons present in crude oil or natural gas. It simply occupies volume or space. Nitrogen lowers the heat value of gas.
Helium (He) is commercially produced during the liquefaction of natural gas. Some natural gas streams contain up to 8% helium by volume. When natural gas is cooled to a liquid phase, helium remains in the vapor phase due to its low boiling point (-268.5°C) which makes helium when extracted an excellent refrigerant. Extracted Helium is a valuable by-product of natural gas, but left in the gas, it lowers the heat value.
Oxygen (O2) is not a natural contaminant but appears due to leakage of air into low pressure production systems. Oxygen can be very corrosive in the presence of water vapor and when mixed with hydrocarbon, can be explosive. In is necessary to support life and as a result, when equipment is shutdown for entry, the atmosphere must be tested for an adequate Oxygen content, but, the Oxygen must be removed before re-commissioning.
Other miscellaneous compounds found include metallic compounds such as sodium, calcium and magnesium that are cations in the brine that accompanies the well fluids. A second group of metals include Vanadium, Nickel, Cobalt and Iron.
Crude oil also contains solid particles such as asphaltenes and resins that are suspended in the crude. These products contribute to emulsion stabilization in field processing if they settle out at the interfaces but they may also cause foaming. Wax deposits can also be present due to the lowering of temperatures until the wax comes out of solution or as a result of flashing of the crude oil in flow equipment.
Naturally occurring radioactive materials (NORM) are also present in reservoir rock structures and are associated with well fluid production. Radioactive scale can contaminate downhole tubing and surface production equipment. This problem needs to be identified and tested for prior to equipment entry.
Arsenic and Mercury are two elements occurring in nature that cause problems in the natural gas industry. These materials are both toxic and may also cause corrosion and catalyst poisoning.
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