Strategic Petroleum Reserve (Geologic considerations) (World of Earth Science)
The Strategic Petroleum Reserve (SPR) operated by the United States Department of Energy is the largest emergency supply system of its kind in the world. The SPR presently consists 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 in SPR storage and location. Oil is stored in cylindrically shaped caverns constructed within the salt body that are typically 200 ft (61 m) in diameter and approximately 2,000 ft (610 m) 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 0.5 to 5 miles in diameter and may be over 20,000 ft (6,096 m) 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.
See also Petroleum detection; Petroleum, economic uses of; Petroleum extraction; Petroleum, history of exploration