Page 41 - Fundamentals of Enhanced Oil and Gas Recovery

An Introduction to Enhanced Oil Recovery 29

(A) (B)
Displaced
Displacing phase
phase

Displaced Displacing
phase phase

Figure 1.13 (A) Gravity segregation in the case of lighter injection fluid, (B) gravity segregation in
the case of heavier injection fluid.

Craig et al. explain factors affecting gravity segregation as follows:
1. Gravity segregation intensifies with increasing vertical and horizontal permeability;
2. Gravity segregation intensifies with increasing density difference of the displacing
fluid;
3. Gravity segregation intensifies with increasing mobility ratio;
4. Gravity segregation intensifies with decreasing flow rate due to fingering
phenomenon;
5. Gravity segregation decreases with increasing viscosity ratio.

1.14.6.3 Gravity Segregation in Dipping Reservoirs
The difference in density between the displacing and displaced fluids has a significant
effect on the displacement process in dipping reservoirs. When the reservoir is dipped,
gravity force can improve displacement. For example, in the case of the oil being
driven by a fluid that has a lower density, gravity force will lead to displacement
stability. Therefore if the speed of displacement is sufficiently low, gravity prevents fin-
gering from occurring between solvent/oil. Similarly, in the down-dip injection of
water, gravity stabilizes the movement of the front and prevents fingering.

1.14.6.4 Effect of Vertical Heterogeneity and Mobility Ratio on Vertical
Displacement Efficiency
Vertical permeability changes usually occur in reservoirs. Fig. 1.14 shows permeability
and thickness variations in the vertical direction. In this figure, the reservoir is divided
into several layers with different characteristics. Geological models have ideal condi-
tions because they do not have different characteristics in the direction of vertical
layers. In other words, real-life reservoirs are layered as depicted in the figure.
Permeation changes in the vertical direction or, in other words, the layered nature of
reservoirs, lead to a decrease in efficiency at the time of fingering [1].