What is natural gas and how is it formed?
Natural gas is a mixture of gases formed from millions of years of plant and animal decomposition deep under the earth’s surface. Natural gas contains:
- Methane (up to 97%)
- Higher carbon (C2 to C8) gases such as ethane, propane, and isobutene
- Other gases such as nitrogen, carbon dioxide, hydrogen sulfide, and helium
Natural gas can also contain contaminants like sand, water, and oil. At this initial stage, natural gas is referred to as well head quality or raw gas. Extracted from both onshore and offshore drilling activities, over 27 trillion cubic feet of natural gas is produced in the US alone. To take this raw natural resource to a finished product suitable for use in homes, utilities, refineries, or petrochemical plants requires many processing steps.
Natural gas processing
As a source of energy, natural gas has many uses including electricity generation, heating, and cooking. But before reaching its final destination in a home or city utility, natural gas can journey thousands of miles. The NG transportation highway is an intricate and extensive network of pipelines.
- First, gathering lines bring natural gas from the wells to processing plants.
- Then, transmission lines carry finished natural gas to distribution centers.
- Finally, distribution lines carry the natural gas to homes and businesses. In the US alone, this transportation highway amounts to over 2.4 million miles of pipeline. That’s enough pipelines to circle the equator 96 times!
Raw natural gas can come from gas well (natural gas only), condensate wells (natural gas with other heavier and liquid gas condensates), or crude oil well (natural gas dissolved in crude oil). Processing natural gas transforms it from raw into pipeline quality natural gas by removing the condensates, impurities, and contaminants. Processing facilities produce natural gas that meets quality standards set by pipeline transmission and distribution companies. Owners and operators of the transmission and distribution pipelines require natural gas to be within a certain calorific range (British Thermal Unit [BTU]), dry with very low water content, and free of contaminants at levels that will damage or erode the pipelines.
Contaminants in raw natural gas
Different types of contaminants need to be removed from the raw natural gas. Common contaminants include:
- Gases, helium, hydrogen sulfide, carbon dioxide, nitrogen, oxygen and water vapor
- Liquid natural gas condensate and crude oil
- Trace metals such as mercury
- Radioactive materials such as radon
Analysis of NG and NGL
Throughout NG and NGL processing, a significant amount of testing is required to meet various standards and contracts. Results from this testing determine the economic value of the field, potential end uses and customers, potential processing costs, quality control for plant operations, and finally end user costs for using the gas and liquids. The value of natural gas when purchased is determined by its energy content, which can vary significantly based upon the final composition.
Gas chromatography (GC) is typically the analytical technique used to determine the composition of the natural gas and calculate its energy content. In North America, energy content is reported as BTUs. The higher the BTU rating, the more valuable the fuel is, and the higher price it commands. An example of what this analysis looks like is shown below.
Analysis of natural gas composition is critical for producers, distributors, utilities, gas engine and appliance manufactures, and of course consumers. Multiple standard methods are used for natural gas analysis including GPA 2145, 2172, 2177, 2186, 2261, 2286, TP-17, ASTM D1945, D1946 and D3588, ISO 6974 and 6976, Din51872-5 and UOP539. Gas analyzers are the preferred tools for analysis.
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