Natural gas basics

Like oil, natural gas is also a mixture of hydrocarbons, containing many different compounds. The largest component of natural gas is methane, a compound with one carbon atom and four hydrogen atoms (CH4). In addition to methane, natural gas in the reservoir also contains smaller amounts of heavier hydrocarbons called natural gas liquids (NGLs), such as ethane (C2H6), propane (C3H8) and butane (C4H10), and non-hydrocarbon gases, such as carbon dioxide and water vapour.

The formation of natural gas is, essentially, the same as the formation of oil, and takes a very long time – in the order of millions of years. In some places, natural gas is found in large cracks and pore spaces (reservoirs) between layers of overlying rock. Such gas is often called conventional natural gas and can be produced using traditional drilling and compression techniques. In other places, natural gas occurs in the tiny pore spaces within some formations of shale, sandstone and other types of sedimentary rock. This natural gas is called shale gas or tight gas and can be produced only by using novel techniques, such as fracturing. A type of natural gas that is found in coal deposits is called coalbed methane. Shale gas, tight gas and coalbed methane are collectively called unconventional natural gas, as they need some unconventional techniques to produce. Natural gas that occurs with deposits of crude oil in the pore spaces is called associated natural gas. Figure 6.1 gives a schematic view of various types of natural gas formation.

Natural gas is usually measured on either a volumetric basis or an energy content basis. The volumetric measures are given in cubic feet (CF or ft3) or cubic metres (CM), or the multiples of cubic feet or cubic metres, such as thousands of cubic feet (MCF), millions of cubic feet (MMCF) and billions of cubic feet (BCF) or thousands of cubic metres (MCM), millions of cubic metres (MMCM) and billions of cubic metres (BCM), and so on, usually measured at a “standard” temperature of 60.0°F (15.6°C) and pressure of 14.73 psi (one atmosphere).