Page 81 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

Evaluation Methods and Influencing Factors CHAPTER 2 65

FIGURE 2.28
Wetting balance of gas/liquid/solid.

the liquid wettability on the surface and the weaker the gas wettability. When
the free energy of the solid surface is less than that of a certain liquid surface,
gas wets solid surface to a certain extent with respect to liquid. The lower the
surface energy of the solid is, the stronger the gas wettability.
2. Theoretical study of effect of free energy of liquid surface and solid/liquid inter-
face free energy on gas wettability
In gas-liquid-solid system, gas wettability of the solid surface results from the
interaction of three phases (see Fig. 2.28). When balance is achieved, Young’s
equation is satisfied.
σ solid-gas 5 σ solid-liquid 1 σ gas-liquid cosθ (2.31)

Transpose to:
σ solid-gas 2 σ solid-liquid 5 σ gas-liquid cosθ 5 A (2.32)

The relationship between gas-wetting parameter and surface (interface) tension
can be obtained from Eqs. (2.31) and (2.32), as shown below:

ζ 5 σ solid-gas 2 σ solid-liquid 5 A (2.33)
gas2liquid
σ gas2liquid σ gas2liquid
In the equation, σ solid-gas , σ solid-liauid , and σ gas-liauid are solid/gas interfaceten-
sion, solid/liquid interface tension, and liquid surface tension, respectively.
Surface free energy is the variable that defines the capacity property, and sur-
face tension is the variable that defines the intensive property. The solid/gas
interfacetension ðσ solid-gas Þ equals solid/gas interface free energy ðγ Þ,
solid-gas
solid/liquid interface tension ðσ solid-liauid Þ equals solid/liquid interface free
energy ðγ solid-liquid Þ, and liquid surface tension ðσ gas-liauid Þ equals free energy of
liquid surface ðγ Þ in value, as can be seen in Eq. (2.33):
gas-liquid
γ 2 γ A
ζ 52 solid2gas solid2liquid 5 (2.34)
gas-liquid γ γ
gas2liquid gas2liquid
It can been seen from Eq. (2.34):
Preferential gas-wetting: 0 , ζ # 1, γ , γ ,A , 0;
gas-liquid solid-gas gas-liquid
Intermediate gas-wetting: ζ 5 0,γ 5 γ ,A 5 0;
gas-liquid solid-gas gas-liquid