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frequently in closed-type deposits after prolonged exploitation
when the energy of gas drive has been depleted.
Combination drive. Depletion and water drives can be
characterized as pure drive mechanisms; however, another drive is
one that can best be described as a combination drive. One such
drive has a gas cap above the oil and water below it. Both the gas
cap and the water drive the oil into and up the wellbore to the
surface. Another type of combination drive has gas dissolved in
the oil with water below it. Both the water and the gas coming out
of solution drive the oil to the surface.

Oil recovery under different drives.
The oil recovery factor of a reservoir, defined as the
ratio between recovered oil and the initial reserve of oil.
According to the experimental and statistical field data,
the recovery factor may have the following values depending on
the expulsive forces operative in the reservoir:
elastic-water drive ….. 0.4-0.7
solution gas drive …... 0.05-0.3
water drive …………. 0.5-0.8
gas cap drive ……….. 0.1-0.4
gravity drive ……….. < 0.1
The highest recovery factor is assured by the water drive
since in this case the oil is displaced by water, the viscosity of
which under reservoir conditions may be greater than that of the
oil. In any case, the viscosity of water is many times greater than
that of gas. It is known that the greater the viscosity of the
displacing agent in relation to that of the oil, the greater is oil
recovery. When water drive reservoirs are produced for a long
time (until the water breaks through into the wells), there is a
single-phase movement of the oil in the oil-bearing part of the
reservoir as it is displaced by the water acting, as it were, like a
piston.

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