Page 144 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

128 Gas Wettability of Reservoir Rock Surfaces with Porous Media

surface tension lower than the surface energy of the contacted solid, it spreads
on the solid surface, resulting in wetting. From the physical chemistry of sur-
faces theory, it can be seen that solids with higher surface energy are easily wet
by liquids. Therefore, it can be deduced that the relationship of wettability of
solid surface and surface free energy is:
In gas-water-solid system, the lower the surface free energy of the solid, the
poorer water wettability.
In gas-oil-solid system, the lower the surface free energy of the solid, the
poorer oil wettability.
In gas-liquid-solid system, the lower the surface free energy of the solid,
the poorer gas wettability.

4.1.2.2 GAS ADSORPTION EXPERIMENT
4.1.2.2.1 Gas-wetting Alteration of Substrate
Shafrin et al. proposed that wettability of a solid surface is determined by the
nature and stacking of surface atoms or exposed atom groups, and have noth-
ing to do with the nature and arrangement of interior atoms and molecules
[2]. Thus, a solid surface can be modified by materials with low surface energy
and altered to gas-wetting.
Artificial core slices were placed in a flask filled with a gas-wetting alteration
agent in an aqueos solution of different concentrations at room temperature.
The solution was left to stand still after being vibrated strongly at room tem-
perature. It was then subjected to strong vibration once every hour. The liquid
sample was taken out in intervals to measure the solution concentration with
a spectrophotometer nutil solution concentration no longer changes, and this
was regarded as the adsorption equilibrium.
When the artificial core slices were adsorbed in equilibrium, they were washed
with distilled water for three times, and then dried in an oven for 24 hours at

a constant temperature of 100 C. A JC2000D3 contact angle measuring instru-
ment was used to measure the contact angle of oil and water phases on the
surface, respectively. n-hexadecane and distilled water, were used as test solu-
tions for oil phase and water phase respectively. The droplet volume was 5 μL.
Three different points were selected for each sample measurement, and the
average value was taken.
Based on the measured contact angle, the surface energy of the core slices
before and after undergoing adsorption of the gas-wetting alteration agent was
calculated using Owens two-liquid method, and the surface tension, disper-
sion force, and polarity force of distilled water were 72.8, 21.8 and 51.0 mJ/
2
2
m , respectively, and that of n-hexadecane were 27.6, 27.6 and 0 mJ/m ,
respectively. The result is displayed in Table 4.2.
It can be seen from Table 4.2 that when gas wetting alteration agent was
adsorbed, the wettability of the artificial core surface showed an obvious