Page 36 - Fundamentals of Enhanced Oil and Gas Recovery

24 Amirhossein Mohammadi Alamooti and Farzan Karimi Malekabadi

accounts for this contact area. The sweep or volumetric displacement is also a macro-
scopic displacement, which includes the portion of the volume of the pore space that
is occupied by the injected fluid or is affected by it. Total displacement efficiency, E c ,
in a displacement process is defined by multiplying the microscopic displacement, E D ,
in macroscopic displacement, E v .
In general, the level of reservoir fluid contact during the displacement process is
controlled by four factors:
1. Characteristics of the injected fluid
2. Properties of displaced fluid
3. Petrophysical properties of reservoir rock
4. Injection and production wells placement

1.14.2 Macroscopic Displacement Mechanism
Macroscopic displacement refers to a displacement where an injector fluid can replace
the fluid inside the pores of the reservoir rock and cause the fluid to move from the
reservoir and thus be positioned around the rock.
The macroscopic efficiency is one of the solutions used to determine displacement
efficiency when flooding takes place in a specific volume of reservoir. The process of
oil displacement almost always changes with time; hence the macroscopic displace-
ment efficiency will also vary accordingly. There are a number of factors affecting this
efficiency, some of which are:
• Mobility of the displacing fluid in comparison with that of the displaced fluid
• Homogeneity or heterogeneity of the reservoir
• Arrangement of injection and production wells
• Reservoir rock matrix
1. Mobility of the displacing fluid in comparison with that of the displaced fluid
The mobility ratio of the displacing and displaced fluids is a relative phenome-
non that occurs in the porous medium. If the displacing fluid moves more than
the displaced fluid, the latter will move forward, and thus viscosity fingering will
occur and areal sweep efficiency will be highly dependent on the mobility of the
two fluids.
2. Homogeneity or heterogeneity of the reservoir
If heterogeneity in the hydrocarbon layer of the reservoir is caused by factors
such as porosity, permeability, and cementitious properties of the reservoir, it will
prevent fluid movement across the reservoir from being homogeneous, causing a
significant effect on macroscopic displacement efficiency. In parentheses, homoge-
neous limestones and sand formations generally have wide fluctuations in terms of
porosity and permeability. Also many formations have small and large fractures that
result in heterogeneity in the reservoir. When a fracture occurs in the reservoir,