Fact Sheet - Disposal in Salt Caverns
Introduction to Salt Caverns
Underground salt deposits are found in the continental United States and worldwide. Salt domes are large, fingerlike projections of nearly pure salt that have risen to near the surface. Bedded salt formations typically contain multiple layers of salt separated by layers of other rocks. Salt beds occur at depths of 500 to more than 6,000 feet below the surface.
Salt caverns used for oil field waste disposal are created by a process called solution mining. Well drilling equipment is used to drill a hole from the surface to the depth of the salt formation and a smaller diameter pipe called tubing is lowered through the middle of the well. This arrangement creates two pathways into and out of the well. To form a salt cavern, the well operator pumps fresh water through one of the pipes. As the fresh water comes in contact with the salt formation, the salt dissolves until the water becomes saturated with salt. Cavern space is created by the removal of the salt-laden brine.
Use of Salt Caverns for Disposal
Salt caverns have been used for several decades to store various hydrocarbon products. More recently, their use for disposal of oil field wastes has received increased attention. In the early 1990s, several Texas brine companies obtained permits to receive oil field waste, much of which was drilling waste, for disposal into caverns they had previously developed as part of their brine production operations. Through August 2002, Texas had permitted 11 caverns at 7 locations. Interest has grown for siting new commercial disposal caverns near the coast that can receive wastes from offshore operations. As of the end of 2003, only Texas has issued permits for disposal of oil field wastes in salt caverns in the United States. Louisiana adopted cavern disposal regulations in May 2003 but has not yet permitted any disposal caverns. Several disposal caverns are also operated in Canada, and, in early 2004, Mexico announced that it was developing regulations for disposal of oil-based muds and cuttings in salt caverns.
Waste Disposal Process
Wastes are brought to the cavern site in trucks and unloaded into mixing tanks where they are blended with water or brine to make a slurry. Many exploration and production (E&P) wastes are suitable for disposal in caverns, including drilling muds, drill cuttings, produced sands, tank bottoms, contaminated soil, and completion and stimulation wastes. Grinding equipment may be used to reduce particle size. The waste slurry is then pumped into the caverns. The incoming waste displaces the brine, which is brought to the surface and either sold or injected into a disposal well. Inside the cavern, the solids, oils, and other liquids separate into distinct layers: solids sink to the bottom, the oily and other hydrocarbons float to the top, and brine and other watery fluids remain in the middle.
In a 1997 study, the costs for cavern disposal of nonhazardous oil field waste, including drilling waste, ranged from $2 to $6/bbl. At that time, these costs were competitive with other oil field waste disposal companies operating in the same geographic area. Transportation of the wastes to the cavern facilities incurs an additional cost.
Caverns are appropriate for drilling wastes because they can readily accept wastes that contain excessive levels of solids. Some other forms of drilling waste disposal, like slurry injection, require much more careful control of solids size. The oil content of the injected waste is not as critical for injection, thereby reducing operational costs.
The surface footprint and chance of surface-related problems are greatly reduced from that of a land treatment or landfill operation. Wastes are placed deep underground in an impermeable and self-healing matrix of salt. No leaks or releases have been observed from the limited number of caverns used for disposal.
At the present, there are only a few permitted disposal caverns. In order to be cost-effective, cavern sites must be located relatively close to where the waste is being generated. The cost of hauling drilling waste more than 50 to 75 miles is prohibitive.
The Solution Mining Research Institute Web site provides extensive information about salt caverns: www.solutionmining.org.