Strategic Petroleum Reserve, United States
█ WILLIAM J. ENGLE
The Strategic Petroleum Reserve (SPR), located in the United States and operated by the Department of Energy (DOE), is the largest emergency supply system of oil in the world. To enhance national security, in a Presidential Order signed November 13, 2001, President George W. Bush ordered the U.S. Department of Energy (DOE) to fill the Strategic Petroleum Reserve. The oil will come from royalty-in-kind transfers between the Department of the Interior and the DOE.
The SPR is designed to act as a "first line of defense" against a reduction of oil supplies to the United States. The president can release the stored oil as necessary—including a release to stabilize prices. In addition to the SPR, there are also a Naval Petroleum Reserve and special emergency reserves of home heating oil maintained in storages tanks (some rented from commercial sources in other areas of the country).
The Strategic Petroleum Reserve (SPR) contains a mixture of oil from domestic and foreign sources.
The SPR is presently made up of four underground storage facilities located in salt domes along the coastal regions of Louisiana and Texas and has a total storage capacity of 700 million barrels of oil. These sites were chosen from among the more than 400 potential areas along the Gulf Coast of the southern United States after careful review of their relative geologic characteristics.
A salt dome is a body of rock salt surrounded by layers of sedimentary rock. Geologic characteristics considered in selecting storage sites include: 1) area geologic activity, 2) structural size, 3) existence of a trapping mechanism, 4) salt geometry, 5) salt composition, and 6) surface conditions.
Geologic activity in the area of potential storage sites must be well understood. The coastal plains along the Gulf Coast tend to be in a perpetual state of either subsidence or uplift and the rate of such relative change must be measurable and predictable.
Structural size is a significant factor. Oil is stored in cylindrically shaped caverns constructed within the salt body that are typically 200 feet in diameter and approximately 2,000 feet in height or larger. A storage dome may consist of from one to more than twenty caverns in a three-dimensional pattern. Salt domes along the Gulf Coast typically range between being one half to five miles in diameter and may be over 20,000 feet in vertical height.
Fluid naturally flows through permeable strata just as water passes through a sponge. Oil will seek the highest possible level due to its relatively low specific gravity and would float to the surface if not otherwise trapped. A salt dome must be overlain by a trapping mechanism in order to be an environmentally safe and an economically secure storage site, Cap rock is a stratum of rock lacking permeability that can act as a trapping mechanism. However, not all salt domes are overlain by cap rock.
Salt domes are usually formed as the lighter salt rises through sedimentary strata above in a plastic state from a deeper source while forming irregular shaped and sometimes freestanding columns. The three dimensional geometry of the salt diapir must be profiled in order to facilitate the design of the storage cavern pattern.
Ideally the salt dome is composed of homogenous halite free of shale or other sedimentary rock. The presence of irregularities in composition may effect cavern construction and containment integrity.
Surface conditions play a role in site selection and project design, construction and ease of operation. Typically such sites are located in marsh areas or beneath standing water. Proximity to existing infrastructure supporting oil import, delivery and water handling is a major cost and operational consideration.
Though geologically complex, salt domes have proven to be a reliably safe and economically competitive means of storing oil for future use and play a key role in national energy management and supply.
█ FURTHER READING:
U.S. Department of Energy. "Fossil.Energy.gov Petroleum Reserves." < http://www.fe.doe.gov/program_reserves.html ">> (March 2, 2003).