Page 26 - Fundamentals of Enhanced Oil and Gas Recovery

14 Amirhossein Mohammadi Alamooti and Farzan Karimi Malekabadi

1.9.13 Surface Tension
Differential molecular forces make an imbalance in the boundary layer of liquid phase
and vapor phase. The force exerted on the interface of oil and gas is called surface
tension. For a pure substance the surface tension decreases by increasing temperature.
For hydrocarbon liquid and gas at equilibrium the surface tension is calculated as
follows:
" 4
#
ρ ρ

X g
σ 5 p i x i l 2y i
M l M g
i
where σ is surface tension; p is parachor, which is constant for a pure substance; x is
molar percentage of liquid components; ρ is liquid density; M l is apparent liquid
l
molecular weight; y is molar percentage of gas components; ρ is gas density; and M g
g
is apparent gas molecular weight.
When the surface tension approaches zero, gas tends to be solved in oil. In other
words, chance of miscibility increases by surface tension reduction.

1.10 RESERVOIR DRIVE MECHANISMS

A lot of forces, including capillary, viscous, and gravity forces, influence reser-
voir fluid in the porous media. Along the reservoir formation time these forces are
equalized through the reservoir. The balance of these forces determines how the fluids
flow toward the well. In other words, these forces as well as overburden pressure and
reservoir temperature are forming the reservoir energy, and whenever the energy is
not exhausted, the reservoir can produce oil naturally. The production period in
which reservoir energy is enough to produce oil naturally is called primary recovery.
During the primary period oil can be produced naturally by the primary reservoir
energy, which can be categorized in different drive mechanisms as follows: rock and
liquid expansion drive, solution gas drive, gas cap drive, water drive, gravity drainage
drive, and combination drive.

1.10.1 Rock and Liquid Expansion

This mechanism is dominant when the reservoir is undersaturated. When the pressure
decreases above the bubble point pressure, the rock and fluid expand. By decreasing
the liquid pressure formation, compaction occurs and the pore volume is willing to
be reduced. On the other hand, the liquid and individual rock grains tend to expand.
Consequently, the crude oil will come out of pores and conduct to the well.