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In primary natural resource recovery, the initial approach to produce oil is generally via natural reservoir pressure or simple mechanical pumps used to raise oil to the surface. Most oil wells today have to be pumped. Primary oil recovery can only produce a small fraction of the oil in a reservoir. The need for secondary oil recovery methods arise from this fact. Waterflooding can be both effective and economical.
Secondary recovery of remaining oil from a proven reservoir can be obtained by injecting water ('waterflooding)', to maintain reservoir pressure and push oil out of the rock. Prolonged oil production can be achieved effectively once the primary production has tapered off. This is called secondary recovery.
 A well planned oil field can go a long way toward the reliability and overall cost effectiveness of a waterflooding project. One method uses centrally located wells that, once primary and secondary production declines, will be used as injection wells.
These injection wells use water to force the remaining oil reserves toward the extremities of the oil field. Such techniques can raise production on these outer wells to near initial production numbers as well as allow for the recovery of up to 80% of the remaining reserves.
Oil reservoirs suitable for secondary recovery projects have been produced for several years. It takes time to inject sufficient water to fill enough of the void spaces to begin to move the oil.
Another method involves increasing the viscosity of the water, allowing for drillers to gain 'control' over where the water flows. This allows for the introduction of water into areas of the reservoir in which it naturally wouldn't flow. By forcing water into the crevices around an oil reserve, oil can be 'moved' toward the production zone. 
To locate underground structures that may contain oil, geologists survey the Earth with devices such as gravity meters or magnetometers. With seismographs similar to those used to measure earthquakes, they explore what they cannot see by sending sound waves underground. (How long they take to return helps determine the depth and characteristics of the rock layers through which they travel.) Using computer models, geologists create three-dimensional pictures of sedimentary basins. With CAT scans and magnetic resonance imaging (MRI) – the same technology doctors use to peer inside the human body – they visualize the flow of oil through the rocks.
Oil and natural gas are often discovered in dome-shaped structures called "anticlines." But oil does not reside in underground lakes as is commonly supposed. Instead, it is trapped in rocks with holes like sponges. Natural gas is dissolved in the oil or separates and rests atop the oil as a separate layer.
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