Deflating the Balloon
A familiar oil-well image is an uncontrolled “gusher” spewing oil high into the sky to rain down on the joyous drillers. Oil gushes because it is under pressure in the ground.
Oil gusher from an old Ohio Oil Co. derrick, about 1910.
In a reservoir, oil and water fill the open pores between the grains of the reservoir rock, much as a drink fills the spaces between ice cubes in a glass. The weight of all the rock on top of a reservoir applies pressure on the reservoir rock, as well as on the oil, water, and gas within it. When a well penetrates a reservoir, the pressure in the reservoir drives the oil and gas toward the lower pressure of the open well. As the oil or gas is withdrawn, reservoir pressure falls as the volume of oil between the grains of the reservoir rock is reduced. Somewhat like a balloon whose neck is opened, the reservoir gradually “deflates.” In some very large oil fields, this deflation has actually caused the land surface to subside. In some areas of Long Beach, California, for example, subsidence due to oil withdrawal exceeds 10 feet.
As the pressure in the reservoir falls, the flow of oil into the well slows to a trickle – and, eventually, to nothing. Oil produced under this natural driving pressure is the “primary recovery” or “primary production” of a field.
Squeezing Out More Oil – Fracturing Rocks and Sweeping Up with Waterfloods
Two new technologies developed in the 1950s let producers force more oil out of newly discovered and existing fields. In the first process, called hydraulic fracturing, powerful pumps at the surface inject a fluid, commonly with the consistency of a milkshake, into the oil-producing reservoir rocks. The pressure exerted by the fluid, is great enough to fracture the rocks around the well, and sand grains injected with the fluid keep the cracks propped open once the pumping stops. The newly opened fractures make the reservoir rocks more porous, and oil can flow more easily into the well.
Water pumped into adjacent wells forces oil to the oil well.
In the second technique, called waterflooding, water is injected into the reservoir rocks to maintain reservoir pressure as the oil is withdrawn, and to sweep the oil out of the reservoir rocks and toward the well. In the most commonly used technique, the five-spot pattern, water is pumped into the reservoir rocks at four wells arrayed around a central producing well in an arrangement like the five dots on the face of a domino or die.
Using techniques to maintain reservoir pressure and drive the oil out of the reservoir rocks is called “secondary” recovery or production. With hydraulic fracturing and waterflooding, Illinois’ total oil production rose to about 82.3 million barrels in 1956, a peak from which it has been declining almost continuously ever since.
Cross Section view of In-Ground Pipe
In this cross section view of a pipe in the earth, the arrows show the oil-water mixture as it moves through perforations in the rock layers and into the well bore. Finally, it is pumped to the surface through a production pipe located inside the well casing pipe.
Getting the Oil Out
Deep in the well is a pump that is connected to the surface by long steel rods. When the pump jack at the surface is rocking up and down, it opens and closes valves in the submerged pump. Each stroke brings a cup or two of fluid up to the surface. This fluid, generally a mixture of oil and water, must be separated by an oil-water separator. The separator takes advantage of the fact that oil floats on water. Once separated, the oil is stored in large tanks before it is transferred by pipeline or truck to the refinery and the water is safely pumped back into the ground.