Page 16 - Fundamentals of Enhanced Oil and Gas Recovery

4 Amirhossein Mohammadi Alamooti and Farzan Karimi Malekabadi

1.5 PERMEABILITY

The rock’s potential to conduct single phase fluid is called permeability, mea-
sured in Darcy. Ranges of permeability vary from 0.1 in tight limestone to more than
1000 md in loose sands. Higher permeability values allow fluids to flow fast through
porous media. The term is independent of fluid type. This concept was founded by
Henry Darcy. The following equation is recognized as Darcy’s equation, showing the
momentum equation in porous media:
kAΔP
q 5
μΔx

where q is volumetric rate, k is permeability, A is surface area, ΔP is differential
pressure along the media, μ is fluid viscosity, and Δx is the length of media. In SI
2
unit, unit of permeability is m equaled to 1.013E 1 12 Darcy. The mentioned
equation is applicable under linear, laminar, steady state condition, and exclusively
for incompressible fluid and homogenous media. For multiphase flow the relative
permeability function is applicable. Also for turbulent flow, a parameter is consid-
ered for adjusting Darcy equation. This parameter should be considered in gas wells
with high velocity.

1.6 WETTABILITY

When two immiscible fluids are in contact with solid surface, a contact angle is
created between them, showing the tendency of fluids to spread on solid surface. The
tendency of fluids to adhere to the solid surface is called wettability, and the fluid tending
to have maximum contact surface with solid is called wetting fluid. Wettability is one of
the main forces in reservoirs determining the fluid distribution in porous media. The
contact angle between two immiscible liquidsisanindex for the degree of wettability.
A zero contact angle shows the completely wetting phase, and a degree of 180 illustrates
the completely nonwetting phase. As the wetting fluids tend to spread on solid surface,
the small pores in porous media are occupied by wetting fluids, and the large pore throats
are filled by nonwetting fluids. Therefore in water-wet porous media, water adheres to
small pores and oil flows in open channels. This distribution occurs because of attractive
forces between wetting phase and solid surface and repulsive forces between nonwetting
phase and solid surface.
The forces at the boundary of oil water can be drawn as oil solid interfacial
energy, water solid interfacial energy, and oil water interfacial tension, which are
shown in Fig. 1.1.