One commonly used technique for extracting every last drop of oil from an underground reservoir is called "water flood injection". This diagram illustrates the idea:

Concerns have been expressed about the effect of sequestering water in these flood-injected reservoirs both in terms of the possible effects on the water cycle and ultimately whether such water can be recovered as drinking water or water that can again flow in our rivers without contaminating them. Currently, it appears that most of the injected water is pumped into large dikes for semi-permanent storage.

The petrochemical industry uses between 2 and 4 barrels of water for every barrel of oil produced.