The overall recovery factor (efficiency) RF of any secondary or tertiary oil recovery method is the product of a combination of three individual efficiency factors as given by the following generalized expression:

In terms of cumulative oil production, Equation 14-5 can be written as:

where RF = overall recovery factor

N _s = initial oil in place at the start of the flood, STB

N _P = cumulative oil produced, STB

E _D = displacement efficiency

E _A = areal sweep efficiency

E _V = vertical sweep efficiency

The displacement efficiency E _D is the fraction of movable oil that has been displaced from the swept zone at any given time or pore volume injected. Because an immiscible gas injection or waterflood will always leave behind some residual oil, E _D will always be less than 1.0.

The areal sweep efficiency E _A is the fractional area of the pattern that is swept by the displacing fluid. The major factors determining areal sweep are:

• Fluid mobilities

• Pattern type

• Areal heterogeneity

• Total volume of fluid injected

The vertical sweep efficiency E _V is the fraction of the vertical section of the pay zone that is contacted by injected fluids. The vertical sweep efficiency is primarily a function of:

• Vertical heterogeneity

• Degree of gravity segregation

• Fluid mobilities

• Total volume injection

Note that the product of EA EV is called the volumetric sweep efficiency and represents the overall fraction of...