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The term “oil pools” refers to deposits of petroleum as though there were underground lakes of oil. A more accurate term, however, is one that is often used in the petroleum industry: oil sands.
The deposits are, in other words, more often like piles of sand or porous rock that have been saturated with oil. Oil does not really flow rapidly through sand or rock, of course, and so it must be pushed or driven. Natural flow is when the reservoir delivers fluid to the well bore. Sufficient pressure energy is needed to lift the fluid to the surface.
There are three kinds of natural drives, as the forces that push the oil are called. Each drive involves the gas and water that are almost always found with oil.
First is the dissolved-gas drive. Gas is dissolved in the oil. As it expands, it exerts the pressure which pushes the oil through rock or sand. Recovery is slow when this type of drive is encountered.
Second is the gas-cap drive. Gas has not only dissolved in the oil: a large amount of it has formed above the oil. As the gas expands, it pushes the oil through the rock or sand at a more rapid rate than when only dissolved gas is present.
Third is the water drive, in which there is a large amount of water below the oil. Pressure forces the water upward into the oil-bearing rock or sand and moves the oil ahead of it.
If none of these drives is present, the oil must be pumped to the surface. This is more expensive than when natural drives are present, since power for the pumping must be supplied.
Artificial lift – is when insufficient pressure energy exists. In this case the well may require assistance by the application of artificial lift. This provides all or portion of the vertical lift pressure losses.
Gravity drive is less common, although it’s an ideal recovery mechanism. The hydrostatic pressure on the oil column and pressure of the gas cap provide the drive to a producing well system. For this drive to be effective it requires maximum structural dip, low oil viscosity, good vertical and horizontal permeability.
Compaction drive. The oil in the reservoir (pore space) is compressed by the weight of overlying sediments and pressure of the fluid is withdrawn from the reservoir, then the pressure depletion can be compensated by the overlying sediments compacting lower sediments. This impact creates a reduction on porosity and thus a potential compression effect.
Combination drive. The production of fluids in the majority of reservoirs is not controlled by one but often by several drive mechanisms in combination.
In the early days of the oil industry, new wells often came in as gushers. In these wells great underground pressure forced the oil upward without any control, and it was necessary to wait until it dropped enough for the oil to flow at a normal rate before any of it could be recovered. Of course, a great deal of oil wasted when a well gushed in this way.
Today, with the great increase in the price of oil – and therefore the value – great care is taken to prevent gushers. They are indeed quite rare, thanks to the use of modern technology. The petroleum industry today is very much concerned about acquiring the greatest possible amount of oil from each deposit without unnecessary waste.
Fig.6. Drive Mechanisms:
1. water drive 2. dissolved-gas drive 3. gas-cap drive
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