Page 43 - Fundamentals of Enhanced Oil and Gas Recovery

An Introduction to Enhanced Oil Recovery 31

category of the practical procedure applied to recover oil from the reservoir. In
general, oils with light or medium combinations or oils in which medium combi-
nations have a greater portion than the other combinations are more suitable for
miscible thrust operation.
4. Temperature and pressure properties of the reservoir
A miscible thrust operation can be carried out successfully only if the pressure
of the reservoir is adequately controlled, because pressure is a crucial element for
the miscibility condition to remain stable between oil and the solvent and between
the solvent and the injected gas.
5. Saturated fluids in the reservoir rock
The saturated fluids within the reservoir affect the miscibility thrust in certain
ways. Yet these fluids must be identified before launching the miscibility thrust
project. In most cases, these fluids are produced within the reservoir before the
project initiates, and therefore their essence can easily be discerned and their satu-
ration can be obtained. In most reservoirs capillarity force makes the residual oil
remain trapped in narrow and tight spaces.
6. Dispersion
Dispersion coefficient affects the miscible displacement. The fingering
phenomenon occurs when miscible thrust does not stimulate mobility. Whether
longitudinal or latitudinal, dispersion coefficient affects the fingering phenomenon.
Latitudinal dispersion has more severe effects on the fingers formed since it affects
a vaster area and starts to move from the fingers, while longitudinal dispersion
does not affect them so severely, as it covers a limited area.

1.15 MOBILITY RATIO CONTROL
Mobility ratio control is defined as any procedure that aims to reduce the
mobility of the displacing fluid or the injector fluid within a reservoir. Mobility can
improve the volumetric displacement in a process. It is usually analyzed in terms of
mobility ratio. When mobility declines, volumetric displacement efficiency
increases.
Some of mobility control techniques involve addition of some chemicals to
the injector fluid. These chemicals increase the apparent viscosity in the injector
fluid or reduce the efficient permeability of the injector fluid. The chemicals
employed for this purpose include polymers when the injector fluid is water and
foams when the injector fluid is gas. In some cases the mobility is controlled
through WAG [1].