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wells may begin to produce gas alone. Under favourable
geological conditions, these gravitational effects may result in the
appearance of a gas cap even where there was none originally and
then the reservoir operates mainly by the energy of gas released
from solution, i.e., by a solution gas drive mechanism (Fig. 2.6).

Figure 2.6 - Gas cap drive is energized by expanding gas that
fills the voids that occur after liquids are removed

The gravity drive is due to the fact that oil-bearing
formations are not horizontal but tilted. The magnitude of the static
pressure then depends upon the angle of the dip of the strata. In
low-dipping strata, the oil can also flow to the wells by gravity,
particularly in thick formations. The level of fluid in the reservoir
then drops below the capping, this being the case of movement of
fluid with a free surface. In some cases, gravity is the only type of
energy causing the oil to move to the wells. This occurs most

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