Storage, usually in great quantities, of crude oil and natural gas after production from natural reservoirs. Large amounts of refined products are stored as well. Storage is necessary to meet seasonal and other fluctuations in demand; for efficient operation of producing equipment, pipelines, tankers, and refineries; and for emergency use.
Crude oil and refined products
Oil from producing wells is first collected in welded-steel, bolted-steel, or wooden tanks of 100 bbl (16 m3) or greater capacity. These tanks, upright cylinders with low-pitched conical roofs, provide temporary storage while the oil is awaiting shipment. Several tanks grouped together are a tank battery. Assemblages of large steel tanks, known as tank farms, are used for more permanent storage at pipeline pump stations, points where tankers load and unload, and refineries.
For offshore producing fields a number of unique storage systems have been designed. In several instances old tankers have been adapted for storage, and barges have been constructed especially for offshore storage use.
To minimize vaporization losses, lease tanks are sometimes equipped to hold several ounces pressure. At large-capacity storage sites, special tanks are generally used. Tanks with lifter or floating roofs are used to store crude oil, motor gasoline, and less volatile natural gasoline. Motor and natural gasolines are also stored in spheroid containers. Spherical containers are used for more volatile liquids, such as butane. Horizontal cylindrical containers are used for propane and butane storage. Refrigerated insulated tank systems enabling propane to be stored at a lower pressure are also in use.
Large quantities of volatile liquid-petroleum products, including propane and butane, are stored in underground caverns dissolved in salt formations and in mined caverns, gas reservoirs, and water sands. Refrigerated propane is also being stored in excavations in frozen earth and in underground concrete tanks.
Natural gas
Natural gas is stored in low-pressure surface holders, buried high-pressure pipe batteries and bottles, depleted or partially depleted oil and gas reservoirs, water sands, and several types of containers at extremely low temperature (−258°F or −161°C) after liquefaction. Low-pressure holders, which store relatively small volumes of gas, basically use either a water or a dry seal, and variations of each type exist.
In the United States gas pipeline and utility companies store large quantities of natural gas in underground reservoirs. In most cases these reservoirs are located near market areas. Underground storage permits greatly increased pipeline utilization, resulting in lower transportation costs and reduced gas cost to the consumer. Underground storage is the only economical method of storing large enough quantities of gas to meet the seasonal fluctuations in pipeline loads.
In operating storage reservoirs only a portion of the stored gas, called working gas, is normally withdrawn. The remaining gas, called cushion gas, stays in the reservoir to provide the necessary pressure to produce the storage wells at desired rates. In aquifer storages some water returns to help maintain the reservoir pressure. In aquifer storages the original hydrostatic pressure must be exceeded in order to push the water back.
Storage of liquefied natural gas throughout the world is in connection with shipment of liquefied natural gas by tanker, and is located at the loading and unloading ends of the tanker runs as well as at peak sharing facilities operated by gas pipeline and local utility companies. Storage is in insulated metal tanks, buried concrete tanks, or frozen earth excavations. In two projects using frozen earth excavations, excessive boil-off of the liquefied gas led to replacement with insulated metal tanks. See also Liquefied natural gas (LNG); Oil and gas field exploitation; Petroleum; Pipeline.
