Composition and formation of natural gas

Composition and emissions

Natural gas is a hydrocarbon that, like oil and coal, is formed deep in the earth. It is composed of a mixture of several gaseous hydrocarbons, the main component of which is methane (70%~90%), followed by heavy hydrocarbons such as ethane, propane, butane, and CO2 (0%~8%), as well as oxygen (0%~0.2%), nitrogen (0%~5%), hydrogen sulfide (0%~5%) and traces of rare gases. The content of these components may vary greatly due to different natural gas fields. Raw, untreated natural gas is called “wet gas”, natural gas that is almost pure methane free of other hydrocarbons is called “dry gas”, and natural gas that contains a lot of hydrogen sulfide is called “sour gas”.

Natural gas is contained in underground porous rock structures, often accompanied by petroleum deposits. After natural gas is extracted, it is refined to remove impurities such as water, other gases, grit and other compounds. The separated heavy hydrocarbons, ethane, propane and butane are sold separately as Natural Gas Liquid (NGL), and other impurities are also separated, such as hydrogen sulfide, which can be purified to produce commercially valuable sulfur. Refined, pure natural gas is transported across the country through thousands of miles of U.S. pipelines. Natural gas is compressed into Compressed Natural Gas (CNG), which is neither toxic, corrosive nor carcinogenic, through these pipelines to improve delivery efficiency before being sent to where it is needed.

Combustion of natural gas emits far less CO2 and pollutants than coal and oil, making natural gas not only an environmentally friendly fuel, but also the cleanest hydrocarbon fuel currently available. High-efficiency combined-cycle power plants using natural gas emit less than half the CO2 per unit of output electricity than burning coal. Natural gas is lighter than air and is suitable as a safe fuel for a variety of residential, industrial and commercial uses. Since methane is a non-reactive hydrocarbon, its emissions are not subject to smog pollution from sunlight.

Natural gas reserves measurement: There are many ways to measure natural gas reserves, such as measuring its volume at standard temperature and pressure like ordinary gases, generally expressed in cubic feet (ft3)/cubic meters (m3). Production and distribution companies generally use thousand cubic feet (Mcf), million cubic feet (MMcf) or trillion cubic feet (Tef). Natural gas can also be measured as an energy source, generally measured and expressed in British thermal units (Btu).

1 Btu of natural gas produces the energy equivalent to heating 1 lb of water by 1℃ at standard pressure. To give the reader a relative idea, 1ft of natural gas contains about 1027Btu. For the convenience of billing, the natural gas delivered to the residence is expressed in “grams”. 1 gram card is equivalent to 100000Btu, or slightly more than 97ft3.

Formation and exploitation of natural gas

Like other fossil fuels, natural gas is derived from tiny plants and animals that lived millions of years ago. When these plants and animals die, their remnants (organic matter) sink and become covered with sediments, compressed under extreme heat and pressure below the surface, and finally transformed into gases below the crust. Porous rock structures are tightly sealed against impervious rock formations, and various geological traps are created during their formation. If the gas formed in the above process is not captured by geological traps, it will flow to the surface (and then into the atmosphere). The natural gas captured by the impermeable rocks described above is confined within the limited boundaries of the porous rock and cannot escape to the surface, gradually forming conventional natural gas deposits like sponges soaked with natural gas. Natural gas is divided into two types: associated gas (mixed with oil) and non-associated gas (the reservoir does not contain oil), among which non-associated gas is the majority.

Deep deposits usually contain mostly natural gas without oil, and in many cases only pure methane, known as thermogenic methane. This is the most accepted hypothesis of natural gas formation.

There is another way of forming methane (and natural gas)—inorganic processes. Deep below the Earth’s crust are hydrogen-rich and carbon-rich gases that may react with subsurface minerals in the absence of oxygen to produce methane (and natural gas) as they gradually rise to the surface. These reactions may also form some atmospheric constituents (including nitrogen, oxygen, carbon dioxide, argon, and water). Similar to thermogenic methane, various gases flow to the surface at extremely high pressures, potentially forming methane deposits.

Natural gas can also come from the transformation of organic matter by microorganisms. This action occurs near the surface, and the methane obtained from this pathway is called biogenic methane. Alkanogens, ie microorganisms capable of producing methane, produce methane by chemically breaking down organic matter. These microbes usually live near the surface where oxygen is deficient, and some live in most animals, including humans. Because this way of forming methane occurs near the surface, the resulting methane is often lost to the atmosphere. However, under certain circumstances, it is also possible that this methane could be captured underground and later recovered as natural gas. Landfill gas is a good example of biogenic methane. Landfill waste can be decomposed to produce considerable amounts of natural gas. Using new technology, we can already obtain these gases and add them to the natural gas supply.