Lecture 2 con't part (4)

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.