Page 51 - Fundamentals of Enhanced Oil and Gas Recovery

An Introduction to Enhanced Oil Recovery 39

Mechanisms: [2]
This technique includes the following mechanisms:
1. Evaporation of the lighter components of crude oil and generating miscibility in
ample and high pressure.
2. Providing gravity and drift in a location where a huge portion of reservoir volume
is filled with gas.
3. Increasing gravity drift mechanism which happens in dipping reservoirs.

1.15.6 Required Condition for Miscible Injection
The most important condition for a successful miscible injection is either attaining
gravity equilibrium or stable displacement. These conditions are only found in a reser-
voir that has highly dipping formations with its slopes having a general high perme-
ability. Although the permeability of fractures increase the permeability of the whole
reservoir, these fractures reduce miscible drift efficiency. The main reason for the
aforementioned problem is the gradual movement of the displacing gas alongside
those fractures without having complete contact with matrix oil. Unless the layers are
very thin, the stable miscible displacement cannot be sustained in vertical layers. The
results of laboratory and field studies indicate that gravity, in this case, is dominant,
and fingering can lead to low efficiency of productivity. On the other hand, the het-
erogeneity of reservoir can intensify this issue. Other optimum conditions for miscible
injection are:
• Homogeneous formation
• High permeability of the matrix (in fractured reservoirs)
• Thick oil column
• Lack of gas cap
• Primary high pressure in reservoir (preferably more than miscible pressure)
• Appropriate properties of reservoir fluid (light oil, rich gas)

1.15.7 Immiscible Gas Injection
Immiscible gas injection is performed in two ways:

1.15.7.1 Gas Cap Injection
If a reservoir has a gas cap or when the gas cap is created by the movement of gas
molecules emitted from oil toward the upper part of reservoir, during the initial pro-
duction process gas will be injected to sustain and maintain pressure. Increasing the
pressure of gas caps will drive oil towards production wells. This process resembles the
rise of water and oil level during water injection to aquifer. Injecting gas to a gas cap
is applicable when the reservoir has high permeability, the vertical thickness of oil