Page 39 - Fundamentals of Enhanced Oil and Gas Recovery

An Introduction to Enhanced Oil Recovery 27

1.14.5 Areal (Sweep) Displacement Efficiency
Volumetric displacement efficiency is defined in both areal and vertical terms. These
definitions are useful and cause volumetric displacement efficiency to be expressed
based on two different independent methods. Of course, the approximation that E A is
independent of E I is not true in reality and in the reservoir.

1.14.5.1 Factors Affecting E A
Areal efficiency and displacement are controlled by four factors:
1. Injection/production well patterns
2. Reservoir permeability homogeneity
3. Mobility ratio
4. Viscosity and gravity forces
Studies that are based on physical models are analyzed using parameters 1 and 2
mentioned above. These factors include parameters that can be controlled in the labo-
ratory and have an important effect on areal displacement efficiency. Thin-shaped
models minimize the gravity segregation phenomenon and allow areal displacement
efficiency to be calculated independently from vertical efficiency. In other words, areal
efficiency is 100%. Various models of injection/production wells are used in the pro-
cess of repositioning the reservoir. Fig. 1.12 shows a number of these patterns. Spot
model 5.4 is the most widely used method in recovery and injection methods. The
rules and factors that apply to the five-spot state can also be used for other models.
This applies even if special equations are obtained for this type of pattern under its
direct influence. Permeation changes often have a special effect on areal efficiencies.
This impact may vary from one reservoir to another. Therefore developing a model
for studying and calculating this parameter is a difficult task [1].

1.14.6 Vertical Displacement Efficiency

Areal displacement efficiency is measured by calculating vertical efficiency to obtain
total efficiencies. For this purpose, simple models that are based on linear displacement
are used to show the factors affecting vertical efficiency.

1.14.6.1 Factors Affecting Vertical Displacement Efficiency
The factors influencing this displacement are:
1. Mobility ratio
2. Changes in the horizontal and vertical permeability of the layers
3. Capillary forces
4. Gravity segregation due to differences in density