Page 75 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

Evaluation Methods and Influencing Factors CHAPTER 2 59

2.4 FACTORS INFLUENCING GAS WETTABILITY
Establishment of gas wettability evaluation methods provides methods for
researching gas wettability. In the previous section, the glass slides were treated
with different concentrations of cationoid fluorocarbon polymerZonyl8740
solution to obtain different gas wettability levels (preferential gas-wetting,
intermediate-wetting, and nongas-wetting) in the gas-water system, and the
degree of gas wettability gradually enhances as the Zonyl8740 solution con-
centration increases. In gas-oil system (neutral kerosene or n-hexadecane),
though it shows nongas-wettability characteristics, the gas wettability on the
surface of the glass slide indicates a similar pattern to the gas-water system.
Thus for the glass slide with gas wettability in different degrees, the surface
(interface) energy affecting gas wettability is studied with established gas wet-
tability evaluation methods and corresponding mathematical models.
Furthermore, the effects of roughness of the solid surface on gas wettability are
researched deeply.

2.4.1 Process and Essence of Gas Wettability
Wettability refers to the procedure of one fluid replacing another immiscible
fluid on a solid surface. Thus wetting effect must involve three phases, of
which two are fluid phases. Wetting phenomenon is the macro behavior of
micro characteristics, including solid surface structure and nature, fluid surface
(interface) property, and interaction of the molecule of solid/fluid phase [32].
Among the three phases involved in wettability, due to the different nature of
two phases, the interface can be divided into five types: gas/liquid, gas/solid,
liquid/liquid, solid/liquid, and solid/solid interface. Those interfaces that par-
ticipate in a gas phase are called the surface, namely, liquid phase surface and
solid phase surface.
In a gas-liquid-solid system, the wetting process can be divided into three
cases: adhesional wetting, soaking, and spreading, which have an effect on dif-
ferent actual situations. The essence and conditions for spontaneous progress
of the three wetting procedures are discussed separately below.

1. Adhesional wetting refers to the process of liquid drops changed from
noncontact to contact with solids in a gas-liquid-solid system and the pro-
cess of liquid/gas interface and solid/gas interface altered into solid/liquid
interface (see Fig. 2.24). For instance, whether raindrops stick to clothes in
daily life and dispersed liquid can return to the surface of rocks.
The condition of spontaneous progress can be obtained through thermody-
namic analysis of adhesional wetting. Suppose the contact area is unit value,
the reduction value of free energy W a in this system is:
W a 5 γ 1 γ 2 γ (2.16)
SG LG SL
In the equation, γ —is gas/solid interface free energy.
SG