Economists sometimes describe a fuel in terms of its energy return on energy invested (EROEI), a measure of how much energy must be used up to acquire, process, and deliver the fuel in a useful form. OPEC oil, for example, is typically estimated to have an EROEI of 12 to 18, which means that 12 to 18 barrels of oil are produced at the wellhead for every barrel of oil consumed during their production. In this calculation, tar sands look awful: they have an EROEI of 4 to 7. (Steaming out the bitumen also requires a lot of water. Environmentalists ask, with some justification, where it all is going to come from.)