2.1.11 Reservoir fluids and pressure
2.1.11.1 Types of reservoir fluids
By definition, a fluid is any substance that will flow. Oil, water, and gas are all fluids; oil
and water are liquids as well as fluids; gas is a fluid but not a liquid.
2.1.11.2 Water
Oil reservoirs are composed of sediments that were deposited in the sea. Consequently,
these sedimentary beds were originally saturated with salt water. However, part of this
water was displaced by petroleum when it was formed. Salt water that remains in the
formation is called formation water.
2.1.11.3 Oil
Oil, which is lighter than water and will not mix with it, makes room for itself in the void
space of the reservoir rock by pushing the water downward. However, oil will not
displace all the original water. A film of water sticks to, or is absorbed by, the solid rock
material surrounding the pore spaces. The film of water lining the pores is called wetting
water. In other words, water is not only in the reservoir below the oil accumulation, but
also within the pores along with the oil.
2.1.11.4 Gas
Natural gas is always associated with oil produced from a reservoir. The energy supplied
by gas under pressure is probably the most valuable drive in the withdrawal of oil from
reservoirs. The industry has come a long way since the day it was general practice to
"blow" gas caps into the atmosphere, so that a well in the gas zone of a reservoir could
finally be induced to produce a little crude oil. Gas is associated with oil and water in
reservoirs in two principal ways as solution gas and as free gas in gas caps.
2.1.11.5 Fluid distribution
The oil-water contact line (the point in the reservoir where the oil and water touch) is of
prime interest to all concerned in the early development of a field because, to get
maximum production from the reservoir, the water should not be produced with the oil.
Practically all reservoirs have water in the lowest portions of the formation, and the oil
lies just above it. However, no sharp line divides the oil and water, nor is the contact line
horizontal throughout a reservoir. Actually, the oil-water contact is a zone of part water
and part oil, and this zone may be from 10 to 15 feet thick. The gas-oil contact has
somewhat the same properties. However, because oil is much heavier than gas, oil does
not tend to rise as high into the gas zone as water does into the oil zone.
2.1.11.6 Reservoir pressure
Every reservoir that contains fluids contains those fluids under pressure.
2.1.11.7 Normal pressure
Under normal conditions, the only pressure that exists in a reservoir is the pressure
caused by the water in it. Contrary to what might seem logical, all the rocks that overlie a
buried reservoir do not create pressure in the reservoir under normal circumstances.
In any case, as long as the reservoir has some ultimate outlet to the surface, the pressure
in it is caused only by the water and is considered to be normal pressure.
2.1.11.8 Abnormal pressure
Reservoirs that do not have a connection with the surface are totally surrounded by
impermeable formations. In such cases, the overlying rock formations do have a bearing
on reservoir pressure. What happens in this case is that the heavy weight of the overlying
beds presses down and squeezes the reservoir. Since the water in the reservoir cannot
escape to the surface, the reservoir pressure builds up to abnormally high amounts.
Another way in which abnormally high pressure can exist is when there is an artesian
effect (Figure 2-13). In this case, the reservoir does connect with the surface. However,
the outcrop to the surface is on the side of a hill or mountain, at an elevation much higher
than the part of the reservoir buried below the level plain. A well drilled at this point
spouts water like a fountain. The water tries to seek its own level. Such wells are called
artesian wells.
