Page 36 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

20 Gas Wettability of Reservoir Rock Surfaces with Porous Media

reservoirs. The summary of this study is based on superhydrophobic materials
and methodology.

Currently, the wettability of solid surface is usually characterized by the CA
of liquid on the solid surface. The definition of CA is the tangent made by a
line intersecting the point of solid/liquid/gas three-phase to gas-liquid inter-
face, and the inclined angle between the tangent and solid-liquid interfacial
lines thus constitutes the CA. When the CA of water or oil droplets fall on
solid surface, the angle is almost 0 degree. These types of solid surfaces are
called super hydrophilic or super oleophilic surfaces, respectively. When the
CA of water or oil droplets on solid surface is larger than 150 degrees, then
these types of solid surfaces are called superhydrophobic or superoleophobic
surfaces, respectively. When the CAs of water and oil droplets with the sur-
face are all larger than 150 degrees, then it is called a super-amphiphobic
surface.
It has generally been acknowledged that the larger the CA is, the higher the
surface hydrophobicity is. Although CA is the most common standard used to
measure hydrophobicity of solid surface, the dynamic process is also consid-
ered to completely estimate the hydrophobic effect. Normally, it is measured
by a roll angle, α. The roll angle is defined as the critical angle when a certain
mass of water droplets begin to roll on an inclined surface. The value of roll
angle α is always used to characterize the lag in CAs when they contact solid
surfaces. The smaller the α value is, the stronger the ability of droplets to roll-
off the solid surface, the smaller the lag phase of CA on the solid surface, and
vice versa. Super-hydrophobic surfaces in their true sense possess larger static
CAs and smaller roll angles.

Studies indicate that the wettability of a solid surface is determined jointly by
the chemical composition, micro geometry, and macro geometry of the sur-
face. Furthermore, external fields, such as light, electricity, heat, and certain
chemical solutions, also affect the wettability of solid surfaces. However, micro
structure and free surface energy are two major factors affecting the wettability
of solid surfaces. The greater the free energy of a solid surface, the easier it is
for some liquids to spread on it. On the contrary, the lesser the free energy of
a solid surface, the harder it is for fluids to wet it. Thus, chemical methods are
usually used to modify this free energy of surfaces while producing a super-
hydrophobic surface. Chemical application is a relatively universal way to
modify solid surfaces with materials possessing low surface energy. However,
simply modifying the surface with a chemical to change surface tension will
make the water droplets hit the solid surface at a 120 degrees CA. Therefore,
introducing surface roughness during the making of a super-hydrophobic sur-
face is required. As early as the 1970s, it was known that micron-sized rough
structures on the surface of a lotus leaf are the key to hydrophobicity and self-
cleansing. Jiang Lei’s research group believes that nano-structures also exist in
the micro-structural mastoids on the lotus leaf surface. This type of hierarchi-
cal structure with a combination of a micro-structure and nano-structure