The beginning of the 20th century in the United States was marked by what historians call the Texas oil boom. Its other name was the Gusher Age. The gushers were oil wells where the oil and gas were under so much pressure; they sprang out when the well got drilled. Today, the gusher age is a thing of the past. We know how to handle gas and liquids under massive pressure. Also, oil producers have developed ways to extract as much oil and gas from a reservoir as possible even after the initial pressure subsides.
The three stages of oil recovery
The hydrocarbon resources associated with gushers of the Texas oil boom would now be called primary oil recovery. The gushing itself would now be called a blowout and drillers nowadays have blowout preventers to stop the oil from springing up in a fountain. The initial production of oil from a well that relies on the internal pressure of the reservoir to push the hydrocarbons to wellhead is called primary recovery.
This primary recovery can yield about a quarter to 35% of a reservoir’s reserves on average. After that, internal pressure declines and the well needs additional stimulation so the oil can rise to the surface. This extra stimulation comes in the form of water or gas. Waterflooding, as the name suggests, involves sending substantial amounts of water down the well to push the lighter oil to the surface. Gas injection does the same: the gas expands in the well and displaces the oil, which gets forced to flow up.
These are both production approaches that work well at conventional wells in formations with excellent permeability characteristics, meaning it is easy to extract the oil from the rock. They also work well with lighter (less viscous) crude oil grades. But not all oil-bearing geology is the same and not all oil is the same. For heavier oil and oil trapped in less permeable rock, drillers have developed other, more complex, methods of extraction.
All the methods used to boost oil production after the primary phase are collectively known as enhanced oil recovery (EOR).