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In the case of a gas cap drive oil recovery is somewhat
lower than with a water drive because in this case oil is displaced
by gas which is of considerably lower viscosity than oil and does
not wet the rock. In such cases, even a small decline in reservoir
pressure results in the release of gas from the oil which reduced
the phase permeability to oil. However, when the dip of the strata
is rather steep (not less than 12-15°) and other conditions are
favourable to gravitational segregation of oil and gas, the oil
recovery factor may be rather high.
Very low oil recovery factors are observed when solution
gas supplies the main driving force. In this case, a considerable
part of the energy of the expanding gas is spent on slippage to the
bottom holes without performing any useful work in displacing oil.
Besides high recovery rates, deposits with pressure drives
are usually characterised by a high production rate and a relatively
short producing life. Therefore, in the field practice, it is extremely
important already at the initial stage of production of a reservoir to
determine its natural potential and to decide on a general
development plan accordingly. It is exceedingly important to
establish the nature of the sources of reservoir energy operative in
the given reservoir, the possibility of utilising natural energy to
achieve maximum oil recovery or the necessity of supplementing
this energy artificially by injecting some kind of a driving agent
into the reservoir to secure more effective drainage.

§3 MAIN HYDRODYNAMIC CALCULATIONS FOR
DIFFERENT DRIVES

§3.1 MAIN HYDRODYNAMIC CALCULATIONS
UNDER WATER DRIVE

Zone existence of water drive is above the saturation
pressure it is characterized by constancy formation pressure, which

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