On the whole, only one-third of the oil in an oil field can be brought to the surface. Some is forced out by gas pressure when a well is drilled, and engineers can sometimes pump water to drive out more. ----. And although plastics can be used to block stringers or even to increase the viscosity of the water, they are very expensive. It has been suggested that the problem can be solved effectively with the help of tiny bacteria that form biofilms to block fissures.