Page 72 - Principles of Applied Reservoir Simulation 2E

Part I: Reservoir Engineering Primer 57

S or residual oil saturation
Displacement efficiency can approach 100% if residual oil saturation can be
driven to zero. One of the goals of enhanced oil recovery processes such as
micellar-polyrner flooding or miscible flooding is to reduce residual oil satura-
tion and increase displacement efficiency.
The definition of displacement efficiency can be modified to include the
effects of swelling. Swelling is represented by using surface volume rather than
reservoir volume in the definition of displacement efficiency. The volume
conversion is achieved by dividing reservoir volume by formation volume factor.
For example, the displacement efficiency of a waterflood is
V S . V S S. S
p oi p or 01 or
17
E _ oi __ oa ._ oi oa <7 2)
° ~ ~ '

where

B oi oil FVF at the beginning of waterflood
B oa oil FVF at the waterflood pressure
Notice that oil formation volume factor is a maximum at the bubble point
pressure of the oil. If the waterflood is conducted at or just above bubble point
pressure, the value of B^ will be maximized and the residual oil term will be
minimized. The resulting displacement efficiency for a waterflood is then
maximized.
Displacement efficiency is a measure of how effectively mobile hydrocar-
bons can be recovered. Although the above definitions of displacement
efficiency have been given for oil, similar definitions can be provided for gas.
In addition to displacement efficiency, volumetric factors are needed to
determine overall recovery efficiency. Areal and vertical sweep efficiencies are
defined by
„ swept area
E =
A - , - (7.3)
total area
and