The type and extent of natural gas processing depend on the original gas composition and the specifications of the consumer. Most produced gas must be treated before the consumer can use it, and there were nearly 2,000 gas processing plants operating around the world as of mid-2006. These plants range from relatively simple plants, where oil, impurities (if present), and water are removed from the produced gas, to complex plants. In the latter, various hydrocarbon compounds are separated from the gas stream and large quantities of gas, liquids, and water are handled, as per figure below.
Gas processing is necessary for the following reasons:
- Sales gas specifications. Customers demand that gas delivered to them meets certain compositional and pressure specifications.
- Pipeline transport. Pipelines, especially those that aggregate gas from multiple gas fields, often specify the composition of the feed gas to maintain pipeline flow and reduce corrosion. Produced field gas may be processed to remove solids (such as sand), water, CO2, and H2S. CO2 and H2S are highly corrosive compounds, and they may have to be removed prior to transportation or further processing, or both.
- Liquids recovery. NGLs—ethane, propane, butane, and condensates—that can be recovered as liquids on the surface are often removed from the natural gas stream and sold separately. Petrochemical and other consumers often buy NGLs directly from the gas producer, which may transport NGLs to them by ship or dedicated pipelines.
- LNG (and GTL) feedstock specifications. LNG plants have rigid and tight specifications for their feed gas. Any impurities in the gas, especially water, CO2, and heavier hydrocarbons, will seriously impact their LNG production. Thus, gas producers selling to LNG plants will be required to pretreat all feed gas to the LNG plants.
Water, which is often produced with natural gas, must be removed to prevent corrosion and icing.