In the process of recovering oil and gas, water is also withdrawn from underground formations. The American Petroleum Institute estimates that in stripper oil well operations approximately nine barrels (1 bbl = 42 gallons) of water are recovered for each barrel of oil. This water-oil mixture is usually separated by floatation or gravity separation in tank batteries, heat separation, skimming pits, or some combination of these methods. The remaining water, called produced water or oilfield brine, can be either deep-well injected or discharged to surface waters, as permitted.