Processing wellhead natural gas
After natural gas is collected at the well site (wellhead natural gas), it must be processed before it can travel in pipelines to customers or to storage.
Natural gas delivery infrastructure has three categories:
- Processing
- Transportation
- Storage
Wellhead natural gas is transported from wells through gathering pipelines to natural gas processing plants where contaminants and hydrocarbon gas liquids (HGLs) are removed. The resulting dry natural gas can be safely transported in high-pressure, long-distance pipelines (the grid) to consumers.
Natural gas processing can be complex and usually involves several steps to remove oil; water; HGLs; and other impurities such as sulfur, helium, nitrogen, hydrogen sulfide, and carbon dioxide. The impurities in wellhead natural gas determine the number of stages and the processes required to produce pipeline-quality dry natural gas. These steps and processes may all happen at the same time, may be performed in a different order or at different locations (such as a lease separator, field facilities, or natural gas processing plants), or not be required at all.
Basic stages of natural gas processing and treatment
- Gas-oil-water separators: Pressure relief in a single-stage separator causes the liquids and gases to naturally separate 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 with separators at the wellhead, much like gas-oil-water separators. The natural gas flows into the separator directly from the wellhead. The extracted condensate is sent to storage tanks.
- Dehydration: This process removes water that may condense in pipelines and cause undesirable hydrates to form.
- 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 through 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: This process can be a separate operation in a natural gas processing plant or part of the NRU operation. Cryogenic processing and absorption methods are two of the ways to separate methane from HGLs.
- Fractionation: HGLs are separated into component liquids using the varying boiling points of the individual HGL. HGLs from the processing plant may be sent to petrochemical plants, oil refineries, or other HGL consumers.
Transporting natural gas from production fields to markets
Natural gas transmission pipelines are wide-diameter pipes and are often the long-distance portion of natural gas pipeline systems that connect gathering systems in producing areas, natural gas processing plants, and other receipt points to the main consumer service areas.
Three types of transmission pipelines
- Interstate natural gas pipelines operate and transport natural gas across state lines.
- Intrastate natural gas pipelines operate and transport natural gas within a state.
- Hinshaw natural gas pipelines receive natural gas from interstate pipelines and deliver it to consumers within a state.
When natural gas arrives 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 into homes or buildings.
Storing natural gas for times of peak demand
Demand for natural gas fluctuates daily and seasonally, but production and pipeline imports are relatively constant in the short term. Natural gas storage during low demand helps to ensure that enough natural gas is available during high demand. Large volumes of natural gas are stored in underground facilities and smaller volumes are stored in tanks above or below ground.
The United States uses three main types of underground natural gas storage facilities
- Depleted natural gas or oil fields—Most natural gas storage is in depleted natural gas or oil fields that are close to areas where the natural gas will be used.
- Salt caverns—Most of the salt cavern storage facilities are in salt dome formations in the states bordering the Gulf of America. Salt caverns have also been leached from bedded salt formations in states in the Midwest, Northeast, and Southwest.
- Aquifers—Water aquifers that have water-bearing sedimentary rock formations overlaid with impermeable cap rock are used as natural gas storage reservoirs, especially in the Midwest.
