Transporting natural gas
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 transported on 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 natural gas. Wellhead natural gas may contain contaminants and hydrocarbon gas liquids (HGL) that must be removed before the natural gas can be safely delivered to the high-pressure, long-distance pipelines that transport natural gas to consumers.
A natural gas processing plant typically receives natural gas from a gathering system of pipelines from natural gas and oil wells. Natural gas processing can be complex and usually involves several processes, or stages, to remove oil, water, HGL, and other impurities such as sulfur, helium, nitrogen, hydrogen sulfide, and carbon dioxide. The composition of the wellhead natural gas determines the number of stages and the processes required to produce pipeline-quality dry natural gas. These stages and processes may be integrated into one unit or operation, be performed in a different order or at alternative locations (lease/plant), or not be required at all.
The basic stages of natural gas processing/treatment are
- Gas-oil-water separators: Pressure relief in a single-stage separator causes a natural separation of the liquids from the gases in the natural gas. In some cases, a multi-stage separation process is required 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 gas-oil-water separators. The gas flow into the separator comes directly from the wellhead. Extracted condensate is sent to storage tanks.
- Dehydration: A dehydration process removes water that may cause the formation of undesirable 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 common removal technique is to direct the natural gas 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 HGL.
- Fractionation: Fractionation separates the HGL into component liquids using the varying boiling points of the individual HGL. HGL from the processing plant may be sent to petrochemical plants, oil refineries, and other HGL consumers.
Pipelines move natural gas from production fields to markets
Natural gas transmission pipelines are wide-diameter pipelines and are often the long-distance portion of natural gas pipeline systems that connect gathering systems in producing areas, natural gas processing plants, other receipt points, and the main consumer service areas. The three types of transmission pipelines are
- 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 arrives at the communities where it will be used (usually through large pipelines), it flows into smaller diameter pipelines called mains and then into smaller service lines that go directly to homes or buildings.
Natural gas can also be stored for times of peak demand
Demand for natural gas fluctuates daily and seasonally, while production and pipeline imports are relatively constant in the short term. Storage of natural gas during periods of low demand helps to ensure that sufficient supplies of natural gas are available during periods of high demand. Natural gas is stored in large volumes in underground facilities and in smaller volumes in tanks above or below ground.
The United States uses three main types of natural gas underground storage facilities:
- Depleted natural gas or oil fields that are close to consuming areas is where most of the natural gas is stored in the United States.
- Salt caverns provide high withdrawal and injection rates relative to their working natural gas capacity. Base natural gas requirements are relatively low. Most of the salt cavern storage facilities are in salt dome formations in the Gulf Coast states. Salt caverns have also been leached from bedded salt formations in states in the Midwest, Northeast, and Southwest.
- Aquifers converted to natural gas storage reservoirs, most notably in the Midwest, where water-bearing sedimentary rock formations are overlaid with impermeable cap rock.