Page 31 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

Concept of Gas Wettability and Research Status CHAPTER 1 15

efficient cleanup additives; developed a new type of hydrochloric acid acidiz-
ing system with heat resistance and salt tolerance mixed in proportion by fluo-
rocarbon, organosilicon, hydrogenated surfactants, and polymeric alcohol; and
evaluated the cleanup performance. The experimental results show that the
hydrochloric acid cleanup addictive system can greatly reduce surface tension
and interfacial tension, with the relative wettability measured using the
Washburn dynamic method. The mass concentration of the composite cleanup
system is 100 mg/L, the surface tension and interfacial tension is 22.23 and
1.2 mN/m, respectively, and the CA is 78 degrees. It is close to intermediate-
wet. The cleanup efficiency is up to 97.2%.

In the same year, Su Huan et al. [31], analyzed changes in reservoir wettability
and its effects on oil recovery using indoor tests. The natural core wettability
measurement and oil displacement test conducted with oil-displacement
agents indicates that the oil displacement agent improves the oil-displacement
efficiency by around 10%. It cannot convert the wettability of rocks into
water-wet cores but can make them into intermediate-wet cores. Analysis of
capillary force shows that when water displacement is conducted till only
residual oil remains, the capillary force contributes to resistance by blocking
oil flow from tiny pores. Calculations of capillary force reveal that when the
wettability angle is 90 degrees, the capillary force blocking oil flow is the least,
and it is easy for oil to peel off from the rock surface. At the moment, the rock
exhibits intermediate-wet characteristics from the wettability perspective. That
is to say, the oil-displacement agent changes the wettability of rocks into
intermediate-wet that can ultimately improve recovery. Moreover, the oil-
displacement agent does not only change the wettability of reservoir rocks, but
also effectively reduces the injection gradient pressure, increases the injection
rate making for pressure reducing and injection increasing, and effectively
improves recovery.
In 2012, Fu Meilong et al. [32], discussed the factors affecting change in wetta-
bility and analyzed the mechanism of enhancing oil recovery by altering wetta-
bility on the basis of extensive investigations. It is believed that the internal
factors for wettability changes are rock mineral compositions, crude oil compo-
nents, strata water compositions, and pH value, and the external factors are
temperature and aging time. The core flow test shows that the oil recovery of
intermediate-wet core is the highest in water displacement, followed by a strong
water-wet core, and then strong oil-wet core, which is the lowest. The change in
wettability from oil-wet to water-wet makes the curve of water-oil permeability
move to the right, thereby decreasing the saturation of residual oil by 25% and
increasing the range of oil-water co-permeability. Finally, the recovery percent
of reservoir is increased. Surfactants are added to change the wettability of reser-
voir rocks, which is a more feasible option in improving oil recovery.
In recent years, there have been several studies on “neutral-wet” or “intermedi-
ate-wet” surfaces, increasing recovery and improving the effects of water plug-
ging. “Gas wettability” was studied by Chinese researchers in 2006.