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production of a reservoir and also by natural conditions. A
particular mechanism of a reservoir drive may be established,
maintained, controlled and even replaced by a different
mechanism. It depends to a large extent on the rate of fluid and gas
withdrawals and on artificial measures carried out during the
exploitation of the reservoir (such as injecting a driving agent into
the reservoir). Geological conditions and the specific source of
formation energy only help to establish a particular driving
mechanism but do not only fully determine it.
Elastic drive. Condition elastic drive - excess reservoir
pressure above the saturation pressure. Bottom-hole pressure is not
less than the saturation pressure.
Oil is a single-phase state. The influx of oil to the wells is
due to the elastic properties of reservoir rocks and fluids.
In the case of an elastic-water drive, the changes observed
in the reservoir behaviour are of a different nature. A typical
feature of the elastic-water drive mechanism is a decline in
pressure during the initial period. Thereafter, if fluid withdrawals
remain constant, the rate of pressure decline tapers off. This is due
to the fact that with time, the zone of reduced pressure extends to
an ever-increasing area of the reservoir, and to maintain a given
influx of fluid by the elastic expansion of the reservoir rocks and
fluids a smaller pressure decline is required than in the initial
period. If the bottom-hole pressure is maintained constant, the flow
at the wells at first diminishes rapidly, but later on, the rate of
decrease tapers off. The elastic properties of reservoir rocks and
fluids are nicely characterised by the fact that any change in
pressure at any point in the reservoir is not transmitted through the
formation instantaneously but at a certain rate (Fig. 2.2).

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