Page 84 - Reservoir Formation Damage

Petrophysics-Flow Functions and Parameters 67

Wettability is an important property of sedimentary formations that
affects the fluid distribution, capillary pressure, relative permeability, and
behavior of fluids in reservoirs (Dubey and Waxman, 1991). Wettability
is a measure of the preferential tendency of immiscible fluids to spread
over a solid surface (Civan and Donaldson, 1987; Grattoni et al., 1995).
Thus, the solid is called a water-wet material when water tends to spread
out to cover the solid surface, and oil-wet vice versa. Contact angle is a
good indication of the spreadability and wetting characteristics of fluids
over simple continuous surfaces. A smaller contact angle, 9 < 90°, indicates
stronger wettability, and a larger contact angle, 9 > 90°, indicates weaker
wettability. 9 ~ 90° indicates intermediate wettability and the probability
of a fluid to have exactly 9 = 90° is very small.
The wettability of porous materials may be two types: (1) uniform
or homogeneous and (2) nonuniform or heterogeneous (Cuiec, 1991;
Kovscek et al., 1992; McDougall and Sorbie, 1995). Uniformly wet porous
materials have either a completely water-wet or oil-wet pore surface
throughout the porous media. Whereas, most sedimentary formations are
nonuniform because they typically contain separate portions of water- and
oil-wet regions. Two types of wettability nonuniformity may be distinguished
in a sedimentary rock: (1) mixed-wettability and (2) fractional-wettability
(McDougall and Sorbie, 1995). Mixed-wettability describes the rocks
having only the larger pores being oil-wet and only the smaller pores
being water-wet, as indicated by McDougall and Sorbie (1995). This
mixed-wettability condition is created by oil migration preferentially into
larger pores followed by organic deposition, such as asphaltene, paraffins,
and resins, to transform the water-wet to oil-wet types (McDougall and
Sorbie, 1995). On the other hand, fractional-wettability describes the rocks
having sites of different surface characteristics due to the differences in
the type of surface mineralogy. Therefore, as depicted by McDougall and
Sorbie (1995), the water-wet and oil-wet pores may encompass over all
sizes of pores in a fractionally-wet formation.
As pointed out by Hirasaki (1991): "The wettability of a rock/brine/oil
system cannot be described by a single contact angle because it is the
multitude of contact angles at the various three-phase contact regions in
the pore spaces that determines system wettability. A complete wet-
tability description requires a morphological description of the pore space
with the contact angles as a boundary condition for the fluid distribution."
Therefore, characterization of the wettability of porous materials is a difficult
task. As stated by Robin et al. (1995): "The contact angle is a macroscopic
concept." Jerauld and Rathmell (1997) consider a formation preferentially
water-wet when the apparent (as measured) contact angle 9 < 30°, pre-
ferentially oil-wet when 9 > 150° and mixed-wet when 30° < 9 < 150°.