The major constituents of natural gas are paraffinic hydrocarbons, principally Methane with smaller quantities of Ethane, Propane and Butane; Nitrogen, Carbon Dioxide and hydrogen sulfide may also be present. Natural gas is found in many parts of the world in sedimentary rock basins, where it was probably formed from the decay of large accumulations of organic matter that were covered by sediment and subjected to intense heat and pressure over long time periods. Gas fields occur where the gas has migrated and collected in porous strata beneath an anticline formed by an impervious layer of rock. The gas is found at pressures of 10 to 200 bar, and both oil and water may be present in the strata.
Natural gas is extracted from the reservoir through multiple wells, the rate of extraction depending on the rate of flow through the rocks, and other factors. Gas withdrawn from most geological formations is saturated with water vapor at the prevailing temperatures and pressures and steps must be taken to avoid the formation, downstream of the well head, of solid hydrates (by the reaction of methane and water), which, at the elevated pressures, can occur at temperatures well above the freezing point of water. An inhibitor such as Methanol is injected into the gas at the well head to avoid hydrate formation. The subsequent treatment depends on the type of gas, but in most cases involves the use of primary separators to remove liquids and any particulate matter. There is also a need to avoid the formation, in the transmission pipes, of liquid hydrocarbons by retrograde condensation, as the pressure falls. The well head pressure is therefore reduced to the working level, liquids are removed and the gas is finally dried to adjust the water vapor dew point by direct contact with glycol.
Wet gases that contain substantial amounts of liquid hydrocarbons are treated to reduce the propane and higher hydrocarbon content for economic reasons and to facilitate transmission. This can be achieved by cooling the gas to –35°C and scrubbing it with a refrigerated wash oil. The wash oil is then regenerated by steam stripping to recover the liquid hydrocarbon 'condensates.'
Sour gases are treated to reduce the carbon dioxide and hydrogen sulfide contents to the required levels and to meet statutory limits. A number of processes using chemical absorbents such as solutions of ethanolamines, or potassium carbonate are used; the gas is contacted with the absorbent in a packed tower to remove the unwanted compounds, and the absorbent is regenerated by heating.
Natural gas is transmitted through an extensive network of high pressure pipelines., of up to 1.0 m, or more diameter, that operate at 70 bar pressures. Recompression stations at intervals of 60 tm may be required on long distance pipelines.
Natural gas is also transported in large quantities in the liquid form at –160°C by ship, e.g., from Alaska, etc. to Japan. The natural gas is purified using solid absorbents to remove the last traces of carbon dioxide, water vapor and sulfur compounds prior to cooling and Liquefaction in a cascade refrigeration cycle. The liquid, LNG, is stored either above ground, or in ground insulated tanks at the terminals, a variety of equipment being used for revaporization of the liquid prior to use.
Barenblatt, G. I., Entov V. M. and Ryzhik, (1990) Theory of Fluid Flows Through Natural Beds. Flower Academic Publishers.
Bereez, E. and Balla-Achs, M. (1983) Studies in Inorganic Chemistry No. 4. Gas Hydrates. Elsevier.
Selby, R. C. (1985) Elements of Petroleum Geology. Freeman and Co. The Gas Engineers Handbook, (1966). The Industrial Press.