Page 23 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

Concept of Gas Wettability and Research Status CHAPTER 1 7

results indicate that wettability has significant effects on critical condensate sat-
uration and relative permeability. Under certain saturation conditions, relative
permeability increases dramatically when the CA increases to 85 degrees
(intermediate gas-wetting) from 0 degree (strong liquid-wetting). This means
that changing the wettability of reservoirs near well walls plays an important
role in improving the productivity of condensate gas reservoir. Hence it is
believed that changing the wettability of a porous medium into preferential
gas wetting from preferential liquid wetting will significantly improve the pro-
ductivity of gas wells.
The group subsequently changed the wettability of porous media reservoirs
from “preferential liquid wetting” to “preferential gas wetting” through fluoro-
carbon polymers FC754 and FC722 in the laboratory. The degree of gas wetta-
bility was evaluated by capillary rise tests, spontaneous imbibition
experiments, and core flow experiments. The results of capillary rise tests indi-
cate that the wettability of a gas/water/rock system can be changed from strong
water-wet to neutral gas-wetting, and after treatment with FC722, it can be
changed to preferential wetting. Spontaneous imbibition experiments indicate
that greater the gas recovery rate of core through self-absorption-oil of gas-wet
alternation compared to gas recovery rate through self-absorption-water is real-
ized, and self-absorption-water quantity and self-absorption-oil quantity
would be zero for the core, when it is treated by FC722. The threshold capil-
lary pressure in the core flow experiment increases with the increase in the
treating agent concentration of fluorocarbon polymer, and this further con-
firms that gas wettability of treated core is also enhanced [11].
Tang et al. [12], studied the effects of gas wettability alternation on the relative
permeability of a gas/liquid system in 2000. They processed Berea sandstone
and chalk core with fluorocarbon polymers FC722 and FC759 and conducted
flow tests using single and parallel cores. Results show that when the core is
processed with hydrophobic and oleophobic fluorocarbon polymer, it changes
from preferential liquid-wet to preferential gas-wet surface. The fluidity and
relative permeability of the liquid phase increases, and the fluidity of gas
phase increases. While the relative permeability may increase or decrease at
certain saturation levels of wettability, the saturation of residual fluid
decreases. However, temperature resistance of the treatment agent of two kinds

of gas-wet alternated states is not high—it is merely 90 C.
Li Kewen et al. [13], put forward a quantitative evaluation method for gas wet-
tability of a “gas/liquid/solid” system in 2003. When capillary pressure and
relative permeability is known at a certain wettability saturation, a quantitative
assessment can be done using the evaluation method for gas wettability. The
value of wettability index of a “gas/liquid/rock” system is greater than that of
an “oil/water/rock” system. However, this method has not yet been applied
for complicated processes.

Seethepalli et al. [14], conducted wettability, interfacial tension, and adsorp-
tion experiments using carbonate rocks for weak bases, surfactants, and an