Squeezing Oil out of Rocks
Imagine trying to force oil through a rock. Can't be done, you say?
Actually, it can.
In fact, oil droplets can squeeze through the tiny pores of underground rock on their own, pushed by the tremendous pressures that exist deep beneath the surface. How does this happen?
Imagine a balloon, blown up to its fullest. The air in the balloon is under pressure. It wants to get out. Stick a pin in the balloon and the air escapes with a bang!
Oil in a reservoir acts something like the air in a balloon. The pressure comes from millions of tons of rock lying on the oil and from the earth's natural heat that builds up in an oil reservoir and expands any gases that may be in the rock. The result is that when an oil well strikes an underground oil reservoir, the natural pressure is released - like the air escaping from a balloon. The pressure forces the oil through the rock and up the well to the surface.
If there are fractures in the reservoir -- fractures are tiny cracks in the rock -- the oil squeezes into them. If the fractures run in the right direction toward the oil well, they can act as tiny underground "pipelines" through which oil flows to a well.
Oil producers need to know a lot about an oil reservoir before they start drilling a lot of expensive wells. They need to know about the size and number of pores in a reservoir rock. They need to know how fast oil droplets will move through these pores. They need to know where the natural fractures are in a reservoir so that they know where to drill their wells.
Today, scientists have invented many new ways to learn about the characteristics of an oil reservoir.
They have developed ways to send sound waves through reservoir rock. Sound waves travel at different speeds through different types of rocks. By listening to soundwaves using devices called "geophones," scientists can measure the speed at which the sound moves through the rock and determine where there might be rocks with oil in them.
Scientists also measure how electric current moves through rock. Rocks with a lot of water in the tiny pores will conduct electricity better than rocks with oil in the pores. Sending electric current through the rock can often reveal oil-bearing rocks.
Finally, oil companies will look at the rocks themselves. An exploratory well will be drilled, rock samples, called "cores," will be brought to the surface. Scientists will look at the core samples under a microscope. Often they can see tiny oil droplets trapped inside the rock.
When companies are convinced that they have found the right kind of underground rock formation that is likely to contain oil, they begin drilling production wells. When the wells first hit the reservoir, some of the oil begins coming to the surface immediately.
Many years ago, when oil field equipment wasn't very good, it was sometimes difficult to prevent the oil from spurting hundreds of feet out the ground. This was called a "gusher." Today, however, oil companies install special equipment on their wells called "blowout preventors," that prevent "gushers", like putting a cork in a bottle.
When a new oil field first begins producing oil, Nature does most of the work. The natural pressures in the reservoir force the oil through the rock pores, into fractures, and up production wells. This natural flow of oil is called "primary production." It can go on for days or years. But after a while, an oil reservoir begins to lose pressure, like the air leaving a balloon. The natural oil flow begins dropped off, and oil companies use pumps (like the drawing at the very top of the page) to bring the oil to the surface.
In some fields, natural gas is produced along with the oil. In some cases, oil companies separate the gas from the oil and inject it back into the reservoir. Like putting air back into a balloon, injecting natural gas into the underground reservoir keeps enough pressure in the reservoir to keep oil flowing.
Eventually, however, the pressure drops to a point where the oil flow, even with pumps and gas injection, drops off to a trickle. Yet, there is actually a lot of oil left in the reservoir. How much? In many reservoirs, as many as 3 barrels can be left in the ground for every 1 barrel that is produced. In other words, if oil production stopped after "primary production," almost 3/4ths of the oil would be left behind!
That's why oil producers often turn to "secondary recovery" processes to squeeze some of this remaing oil out of the ground.
What are "secondary recovery" processes?