Accumulation of Oil and Gas Reservoir Fluids
In all types of traps, oil and gas seldom fill up all the rocks in the reservoir. Most of the time, salt water occupies the remaining space. Much of the salt water lies underneath the oil and gas, but salt water usually coats the grains of the rock where the oil and gas occur as a well. This salt water is the ancient seawater that existed when the formation was originally created. When hydrocarbons move into a reservoir, they have to move or displace the salt water that is already in the rock pores, but they cannot displace all of the water. In fact, pore spaces can be anywhere from 10 to 50 percent salt water even in the midst of oil and gas.
Oil, gas and water in a reservoir generally do not occur in a haphazard manner. Instead, gas is in the highest part, oil is below the gas, and salt water is below the oil. The arrangement of gas, oil and water happens because the three fluids have different a mass (they do not weigh the same).
After a well is drilled and completed (and if necessary, stimulated); attention turns to producing oil and gas from the reservoir. Some reservoirs contain mostly gas and others mostly oil. Let's consider those that contain mainly oil first.
In order for oil to flow to the surface, there must be a drive force associated with it in the reservoir. Fortunately, at the time oil was forming and accumulating in the reservoirs, pressure energy in the gas and salt water associated with the oil was also being stored. This gas or water pressure (or both) is what drive oil through and from the pores of the reservoir, into a well, and to the surface.
Let's visualize a porous and permeable rock layer buried deeply beneath the surface, overlain by an impermeable layer, and covering a very large area. In the highest part of the porous layer lies an oil deposit that is relatively small in size compared to the rest of the layer. Occupying the remainder of the pores of the layer, and lying in contact with the oil, resides a vast amount of salt water. This huge quantity of salt water occurs under pressure and provides a source of energy for driving oil to the surface.
In most cases, oil in a reservoir has gas dissolved in it. This gas comes out of the oil when pressure drops as the oil is withdrawn through wells drilled into the reservoir. As the gas escapes from the oil, it expands. The expanding gas drives oil through the reservoir toward the wells and assists in lifting it to the surface. Reservoirs in which oil is lifted by dissolved gas escaping and expanding from the oil are termed dissolved-gas drive reservoirs. Unfortunately, dissolved-gas drive is not very effective in terms of the amount of oil it allows to be recovered. Typically what happens is that oil flows to the surface only at the beginning. Rather quickly, reservoir pressure drops to a value where it can push oil only to the well and not the surface. At this time, some type of pump or other artificial method must be applied to recover more oil. However, a point will eventually be reached where no more oil can be recovered, even with artificial lift. In fact, statistics show that only 5 to 30 percent of the oil originally in place in a dissolved-gas drive reservoir can be recovered. Any portions of the remaining 70 to 95 percent can be produced only by additional recovery techniques.
Gas - Cap Drive
Some oil reservoirs have so much gas that all of it cannot be dissolved in the oil. This extra gas forms a layer, or cap, over the oil. In gas - cap reservoirs, two gas drivers are actually at work. One is the gas dissolved in the oil; the other is the gas in the cap. As oil is produced through wells drilled into the reservoir, the dissolved gas comes out of the oil, expands, and helps drive the oil to the surface. In addition, the gas cap expands to push oil to the surface. Gas - cap drive is a more effective drive than dissolved - gas alone. A reservoir with a gas - cap may yield from 20 to 50 percent of the oil originally in it.
A combination drive, a drive in which both free gas in a cap and water are available in the reservoir, is the most common type of drive. In combination drives, the gas and water expand and displace oil to the surface. In addition to the free gas in the cap, gas dissolved in the oil, which escapes and expands when wells are produced, is a factor in combination drives. In reservoirs that contain hydrocarbons primarily in the form of free gas, the drive force can be thought of as a combination drive. Even though the hydrocarbons in the reservoir are mostly gas, frequently hydrocarbon liquids (often called gas liquids) and water are associated with them. While wells drilled into such reservoirs are termed gas wells, the expanding gas and water will also drive any liquid hydrocarbons as well. Of course in reservoirs where no massive amounts of water are associated with the gas and liquid hydrocarbons, then a gas - cap drive would be in force.