Concept of Natural Gas

Concept of Natural Gas

Gas hydrates, or clathrate hydrates, exist in a solid, ice-like form that consists of a host lattice of water molecules enclosing cavities occupied by molecules of guest gases. Common guest gases in gas hydrates include CH4, C2H6, C₃H₈, i-C₄H₁₀, CO₂, and H₂S; they also include other gases such as Ne, Ar, Kr, Xe, N₂, O₂, and hydrocarbons such as cyclopropane. Gas hydrates can be categorized into Structure I, II, and H according to the type of cavity in the host lattice. Each cubic meter of gas hydrates can hold approximately 160 m³ of natural gas at standard temperature and pressure. Gas hydrates stay stable under certain thermodynamic conditions, i.e., low temperatures and high pressures. Such a condition can be provided by geologic formations such as permafrost and suboceanic sediments. Most marine gas hydrates are formed of microbially generated gas. In general, gas hydrates can contain different guest molecules in different cages, depending on their sizes and the availability of guest molecules under given thermodynamics conditions. But methane is the prevalent gas in natural gas hydrates. Therefore, many studies under the name of gas hydrates are actually directed to methane hydrate.

Gas hydrates are important energy sources mainly due to the huge number of hydrocarbons in concentrated forms they contain. Of primary interest are the hydrates that contain combustible low molecular weight hydrocarbons such as methane, ethane, and propane. According to Makogon, there are tremendous amounts of natural gas trapped in hydrates in the permafrost and the continental shell in the ocean around the globe. Worldwide, gas hydrate was estimated to hold about 1016 kg of organic carbon in the form of methane. The surveys by the US Geological Survey (USGS) have estimated that reserves of methane in hydrate form exceed all the other fossil fuel forms of organic carbon. Therefore, naturally occurring gas hydrates on the earth, containing mostly methane, have the potential to become a major source of energy in the second half of the 21st century. Gas hydrates have aroused great interest in disciplines such as chemical engineering, chemistry, earth sciences, and environmental sciences. But in fact, gas hydrates were initially regarded as a source of problems in the energy industry because the conditions under which oil and gas are produced, transported, and processed are frequently conducive to gas hydrates formation. Recently, considerable concern over the potential threat of gas hydrates to the global environment has been raised because of the great greenhouse effect of methane. It was argued that release of the large volumes of greenhouse gas stored in hydrates into the ocean and atmosphere may have played a role in the past climate change. Besides, rapid hydrate dissociation may lead to landslides along continental margins as well as other geohazards.

In a narrow sense, natural gas is a combustible gas that is buried in deep ground and mainly composed of hydrocarbon compounds.

Natural gas is a mixture of hydrocarbon gases that occurs with petroleum deposits, principally methane together with varying quantities of ethane, propane, butane, and other gases, and is used as a fuel and in the manufacture of organic compounds. Natural gas is a gaseous mixture consisting mainly of methane trapped below ground; used extensively as a fuel.

Broadly speaking, natural gas refers to all gases naturally generated in nature. In fact, not all-natural gases from reservoirs are hydrocarbon gases. Some are exactly the nonhydrocarbon gases of high purity, such as N₂, CO₂, etc. For example, the content of H₂S in a natural gas, which is from the fourth member of Shahejie and Kongdian formations in Paleogene in Zhao Lanzhuang structure of Jizhong Depression in North China, is up to 92 %. In Sha touping gas field of Sanshui Basin in Guangdong, China, and the content of CO₂ is up to 99.75 %.

Natural gas is a naturally occurring mixture of low-molecular-weight hydrocarbons and nonhydrocarbon gases found in porous formation underground. It is nearly everywhere considered to be the gaseous phase of petroleum. However, the properties of natural gas are considerably different from liquid petroleum because there is greater distance between the molecules in natural gas than in crude oil. For example, pressure has much greater effect on the density of a natural gas than on oil’s density. The notable features of natural gases are much easily compressible and flowable.