Overview of Salt Caverns

Salt caverns have been used for several decades to store various hydrocarbon products. In the past few years, nine facilities in the United States have been permitted to dispose nonhazardous oil field wastes in salt caverns. Two of these are also permitted to dispose of naturally occurring radioactive materials (NORM) contaminated oil field wastes. Several other disposal caverns have been permitted in Canada and Europe. Argonne National Laboratory has completed studies analyzing the technical feasibility, legality, economics, and health risks associated with disposing both nonhazardous oil-field wastes (NOW) and NORM contaminated oil-field wastes in salt caverns. Complete documentation of these studies can be found in:

Salt domes and bedded salt occur in many areas within the United States. If the salt is sufficiently thick, salt caverns can be formed through solution mining. These caverns are either created incidentally as a result of salt recovery or intentionally to create an underground chamber. This chamber has many potential uses (e.g., for storing hydrocarbon products and for waste disposal).

Salt caverns are subject to the requirements of the Safe Drinking Water Act's Underground Injection Control (UIC) program. Many states have their own UIC injection regulations. All injection wells are assigned to one of five classes; salt caverns for disposing of oil field wastes and wells for disposing of produced water are Class II wells. In 1988, the U.S. Environmental Protection Agency (EPA) published a list of those oil-field wastes that were exempt from regulation as hazardous wastes under Subtitle C of the Resource Conservation and Recovery Act (RCRA). Efforts are currently underway to obtain clarification from the EPA that all exempted oil field wastes can be injected into Class II wells, including salt caverns.

The types of oil field wastes that are planned for disposal in salt caverns are those that are most troublesome to dispose of through regular Class II injection wells because they contain excessive levels of solids. The solids-containing oil field wastes most likely to be disposed of in salt caverns include used drilling fluids, drill cuttings, completion and stimulation wastes, produced sand, tank bottoms, scales, and soil contaminated by crude oil or produced water. The difference between nonhazardous oil field wastes (NOW) and naturally occurring radioactive material (NORM) is the presence of radionuclides above a state-specified action level.

The presence of these radionuclides does not change the waste's exempt status under RCRA as long as the waste itself, exclusive of the radiological components, is an exempt waste. Therefore, most oil field NORM waste can be classified as a nonhazardous waste.

Currently, there are no federal regulations that specifically address handling and disposing of NORM wastes. In the absence of federal regulations, individual states have taken the responsibility for developing their own regulatory programs for NORM.

The location and design of waste disposal caverns plays an important role in ensuring long-term waste isolation from the surface or groundwater resources. Hundreds of caverns have been used for safely storing hydrocarbons. Several factors should be considered in selecting sites for disposal of oil-field wastes in caverns including: distance to populated areas; proximity to other industrial facilities; current and future use of adjacent properties; handling of brine and other displaced fluid; proximity to environmentally sensitive wetlands, waters, and fresh water aquifers; proximity to the salt boundary; and proximity to other existing and abandoned surface activities such as neighboring caverns for brine or hydrocarbon storage.

The first step in salt cavern disposal is waste emplacement. During emplacement, oil field waste is injected as a slurry of waste and a carrier fluid (brine or fresh water). As the waste is injected, the cavern acts as an oil/water/solids separator. The heavier solids fall to the bottom of the cavern and form a pile. Any free oils or hydrocarbons float to the top of the cavern. Clean brine displaced by the slurry is removed from the cavern and either sold as a product or disposed in an injection well. When the cavern is filled, the operator removes a hydrocarbon pad from the top of the cavern and plugs the cavern.

During the filling process, pressure in the cavern is monitored, and monitoring is continued after cavern closure. During the post-closure period, the pressure in the cavern will increase because of geothermal heating and salt creep. Several release scenarios were identified by which contaminants could potentially leave the salt cavern and impact the environment: failure of the cavern seal, release through cracks or leaky interbeds, and partial collapse of the roof.

Risk assessment calculations were then performed for the scenarios identified to evaluate human health risks (cancer and non-cancer risks) associated with exposure to the discharged wastes.

Based on the assumptions that were developed for the Argonne studies, even if all caverns fail, the human health risks associated with NOW wastes are very low (excess cancer risks of 10-7 to 10-16) and hazard indices (referring to non-cancer health effects) of 10-3 to 10-8. These cancer risks are well below the EPA acceptable target range (10-4 to 10-6) and the non-cancer hazard indices are much less than 1.0. For NORM wastes, the chemical risks would be the same as those for NOW material, and the excess cancer risk from the radiological components of NORM would range from 10-13 to 10-22. Because these risks are extremely small, the potential for human health risks associated with using salt caverns for disposal of either NOW or NORM is, therefore, low.

In addition to regulatory issues and potential human health effects, economic issues were addressed by the Argonne studies. During 1997, when data were collected on waste disposal costs, disposal caverns for NOW wastes were cost-competitive in the Texas waste disposal market. However, disposal costs may rise in the near future as the Railroad Commission of Texas adopts regulations governing cavern disposal. It is unclear how the increased costs will affect the competitiveness of disposal caverns in the future because costs at competing waste disposal facilities are unlikely to remain constant.
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