Page 23 - Petroleum and Gas Field Processing

Figure 5 An example of a sandstone lense-type reservoir.

cause for this abrupt change in formation porosity. An example
of this type of reservoirs is shown in Figure 5.
6. Combination Reservoirs: In this case, combinations of folding,
faulting, abrupt changes in porosity, or other conditions that
create the trap, from this common type of reservoir.

1.3.2 Reservoir Drive Mechanisms
At the time oil was forming and accumulating in the reservoir, the pressure
energy of the associated gas and water was also stored. When a well is drilled
through the reservoir and the pressure in the well is made to be lower than the
pressure in the oil formation, it is that energy of the gas, or the water, or both
that would displace the oil from the formation into the well and lift it up to
the surface. Therefore, another way of classifying petroleum reservoirs,
which is of interest to reservoir and production engineers, is to characterize
the reservoir according to the production (drive) mechanism responsible for
displacing the oil from the formation into the wellbore and up to the surface.
There are three main drive mechanisms:
I. Solution-Gas-Drive Reservoirs: Depending on the reservoir
pressure and temperature, the oil in the reservoir would have
varying amounts of gas dissolved within the oil (solution gas).
Solution gas would evolve out of the oil only if the pressure is
lowered below a certain value, known as the bubble point
pressure, which is a property of the oil. When a well is drilled

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