Page 49 - Fundamentals of Enhanced Oil and Gas Recovery

An Introduction to Enhanced Oil Recovery 37

H
FG = Δρ gH
if FG< <FC gravity drainage will happened
H
FG<FC and oil pro. Stared
FG = Δ ρ gH
Gas
Water
FG: Gravity force Gas
FC: Capillary force Water

Figure 1.19 Gravity drainage mechanisms.

1.15.4.3 Gas Cap Expansion Mechanism
A gas cap might be formed under primary reservoir conditions, while a secondary cap
can be shaped by free dissolved gas from oil due to reduced pressure resulting from oil
recovery. In this case, it is necessary that the pressure of the reservoir be reduced to a
lower pressure than the bubble point pressure. Reservoirs designed to produce gas cap
by expansion mechanism usually have a lower pressure drop compared to reservoirs
that are under the influence of a dissolved gas energy mechanism.

1.15.4.4 Water Flooding Mechanism
In reservoirs with aquifer, the energy required to drive the oil is either produced by
the expansion of a large amount of water that is condensed in the adjacent (lower)
aquifer pores, or it is provided by the hydrostatic pressure of an aquifer extending up
to the surface. Oil production by aquifer energy is usually good for low-viscosity oils,
provided that it is produced properly from the reservoir, but this method is not effi-
cient for reservoirs with high viscosity oil.

1.15.4.5 Rock and Fluid Density Mechanism
The production of fluid from a reservoir increases the pressure difference between the
upper layers of the reservoir and the porosity pressure and, as a result, reduces the vol-
ume of reservoir porosity and leads to increased oil production. If the compressibility
coefficient of the structure is precisely determined, the reduction of porosity volume
can be calculated. The production of oil by this mechanism would be high, provided
that the density of the formation is high. Most of the reservoirs with high compress-
ibility coefficients are shallow reservoirs that are not too dense.