Transporting natural gas involves several steps
Transporting natural gas from the wellhead to consumers requires many infrastructure assets and processing steps, and it includes several physical transfers of custody.
Natural gas delivery infrastructure can be grouped into three categories:
Processing natural gas for transportation by pipeline
Natural gas fed into the mainline natural gas transportation system in the United States must meet specific quality measures so that the pipeline network (or grid) can provide uniform quality gas. Natural gas produced at the wellhead may contain contaminants and natural gas liquids, which must be removed before the natural gas can be safely delivered to the high-pressure, long-distance pipelines that transport natural gas to consumers.
Processing the wellhead natural gas into pipeline-quality dry natural gas can be complex and usually involves several processes to remove oil, water, natural gas liquids, and other impurities such as sulfur, helium, nitrogen, hydrogen sulfide, and carbon dioxide.
A natural gas processing plant typically receives natural gas from a gathering system of pipelines from natural gas wells and sends out processed gas to one or more major pipeline networks. Liquids removed at the processing plant may be sent to petrochemical plants, refineries, and other gas liquids consumers.
The number of stages and the type of techniques used to create pipeline-quality natural gas depends on the composition of the natural gas produced at the well. In some cases, several of the stages shown below may be integrated into one unit or operation, performed in a different order or at alternative locations (lease/plant), or not required at all. There are several stages of natural gas processing/treatment:
- Gas-oil-water separators: Pressure relief will cause a natural separation of gases from oil in a single-stage separator. In some cases, a multi-stage separation process is needed to separate the different fluid streams.
- Condensate separator: Condensates are most often removed from the natural gas stream at the wellhead with separators much like the gas-oil-water separator described above. The gas flow into the separator comes directly from the wellhead. Extracted condensate is sent to storage tanks.
- Dehydration: A dehydration process is needed to eliminate water that may cause the formation of hydrates and water condensation in pipelines.
- Contaminant removal: Nonhydrocarbon gases such as hydrogen sulfide, carbon dioxide, water vapor, helium, nitrogen, and oxygen must also be removed from the natural gas stream. The most commonly used technique is to first direct the flow though a vessel containing an amine solution. Amines absorb hydrogen sulfide and carbon dioxide from natural gas and can be recycled and regenerated for repeated use.
- Nitrogen extraction: Once the hydrogen sulfide and carbon dioxide are reduced to acceptable levels, the natural gas stream is routed to a Nitrogen Rejection Unit (NRU), where it is further dehydrated using molecular sieve beds.
- Methane separation: The process of demethanizing the natural gas stream can occur as a separate operation in a natural gas processing plant or as part of the NRU operation. Cryogenic processing and absorption methods are some of the ways used to separate methane from natural gas liquids (NGL).
- Fractionation: Fractionation is used to separate NGL into component liquids using the varying boiling points of the individual hydrocarbons in the NGL stream.
Pipelines move natural gas from production fields to markets
A natural gas transmission line is a wide-diameter and often, long-distance portion of a natural gas pipeline system, located between the gathering system (production area), the natural gas processing plant, and the other receipt points and the principal consumer service area(s). There are three types of transmission pipelines:
- Interstate natural gas pipelines operate and transport natural gas across state borders.
- Intrastate natural gas pipelines operate and transport natural gas within a state border.
- Hinshaw natural gas pipelines receive natural gas from interstate pipelines and deliver it to consumers for consumption within a state border.
When natural gas gets to the communities where it will be used (usually through large pipelines), it flows into smaller pipelines called mains. Small lines, called services, connect to the mains and go directly to homes or buildings where the natural gas will be used.
Natural gas can also be stored for times of peak demand
Underground natural gas storage provides pipelines, local distribution companies, producers, and pipeline shippers with an inventory management tool, seasonal supply backup, and access to natural gas needed to avoid imbalances between receipts and deliveries on a pipeline network.
There are three main types of natural gas underground storage facilities used in the United States today:
Depleted natural gas or oil fields
Most of the existing natural gas storage in the United States is in depleted natural gas or oil fields that are close to consumption centers.
Salt caverns provide high withdrawal and injection rates relative to their working gas capacity. Base gas requirements are relatively low. Most salt cavern storage facilities have been developed in salt dome formations located in the Gulf Coast states. Salt caverns have also been leached from bedded salt formations in states in the Midwest, Northeast, and Southwest.
In some areas, most notably in the Midwest, natural aquifers have been converted to natural gas storage reservoirs. An aquifer is suitable for natural gas storage if the water bearing sedimentary rock formation is overlaid with an impermeable cap rock.