Disposing of oil field waste remains a crucial topic both from the standpoint of industry costs and environmental concerns. To educate the public on a secure method for dealing with this waste stream, the US Department of Energy's National Petroleum Technology Office (NPTO) has produced a brochure on salt caverns as disposal sites.
Several sites are in operation. In the mid-1990s, the Texas Railroad Commission (TRC) issued permits for six such disposal sites for nonhazardous oil field wastes (NOW). Of these, four are being used-three in West Texas bedded salts and one in an East Texas salt dome. NOW streams include drilling mud, drill cuttings, produced sand, tank bottoms, contaminated soil, completion and stimulation wastes, etc. US oil and gas exploration and production operations annually generate more than 360 million bbl of drilling wastes, 20 billion bbl of produced water, and 12 million bbl of associated wastes, according to a 1985 API survey.
TRC also permitted two disposal sites for naturally occurring radioactive material (NORM) in 1999, both in West Texas bedded salts.
Saskatchewan, Alberta, Germany, and the Netherlands are other places in which NOW salt caverns have been permitted or are under consideration, says DOE.
Salt caverns abound
In the US, solution mining has created over 1,000 salt caverns since the 1940s, primarily for storing hydrocarbons such as propane, butane, ethane, ethylene, fuel oil, gasoline, natural gas, and crude oil. The largest US storage operation is the Strategic Petroleum Reserve, which holds 560 million bbl of crude in 62 caverns at four sites in Louisiana and Texas.
Solution mining involves drilling a hole into the salt, setting casing above the salt, and then dissolving the salt with fresh water until a big enough cavern forms. As waste displaces the brine in the cavern, the brine can be sold for use in drilling fluids or for extracting salt or other chemicals from it.
The Underground Injection Control program, under the Safe Drinking Water Act, regulates wells into which wastes and other fluids are injected. These are Class II injection wells, and many states have assumed the authority to administer them. Texas, New Mexico, and Louisiana are developing regulations to deal with NOW injection in salt caverns.
DOE cites its conventionally mined cavern in a bedded salt formation in New Mexico for housing nuclear wastes as a prime example of its confidence that salt caverns make low-risk storage sites.
DOE says the $1.95-6/bbl NOW disposal cost range for caverns compares well with other disposal methods: $5.50-16/bbl for land spreading, $2.25-3.25/bbl for landfill or pit disposal, $2.50-2.75/bbl for evaporation, and $8.50-11/bbl for treatment and injection. NORM waste disposal companies charge more than $100/bbl.
Although some leaks are possible, especially if pressure increases once the cavern is full and sealed, calculated health risks are below the US Environmental Protection Agency's acceptable ranges.
To learn more, there is NPTO's website: www.npto.doe.gov/saltcaverns.