Page 61 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

Evaluation Methods and Influencing Factors CHAPTER 2 45

Table 2.2 Contact Angle of Water and Oil Treated With FC91 of Different Concentrations on
the Surface of Cores

Concentration (mg/L) 0 1000 2000 3000 5000 10,000
θ Water 0 65 72 80 40 40
Oil 0 64 68 76 74 52

Table 2.3 Contact Angle of Water and Oil Treated With FG40 of Different Concentrations on
the Surface of Cores

Concentration (mg/L) 0 500 1000 2000 3000 5000
θ Water 40 93 134 134 140 130
Oil 10 75 88 89 93 84

FC911 is a type of cationic fluorocarbon surfactant. From Table 2.2, it can be
seen that as the concentration of FC911increases, both contact angles of water
phase and oil phase increase at first and then decrease. When the concentra-
tion of FC911 reaches 3000 mg/L, both contact angles of water phase and oil
phase are greater than 75 degrees, and the core surface can be regarded as hav-
ing intermediate gas wettability.

FG40 is a kind of nonionic fluorocarbon surfactant. The experimental result
illustrates that water drops and oil drops present obvious spherality on the
surface of cores treated with FG40, and the core is intermediate gas wettability.
FG40 can alter the core surface to have intermediate gas wettability at very low
concentration (500 mg/L). By increasing the concentration continuously, gas
wettability is strengthened. When the concentration is 3000 mg/L, the surface
of rock samples shows strong gas wettability.
From the above conclusion, it can be seen that the contact angle method can
easily and intuitively determine whether the solid surface is gas wettable, as
well as evaluate the degree of gas wettability of the surface, based on the size
of contact angle of liquid phase. Thus it can quantitatively evaluate gas wetta-
bility. However, some problems can occur during measurement. The first is
contact angle hysteresis, which may be due to three possible reasons: surface
roughness, surface heterogeneity, and surface absorption at molecular level.
Also, the contact angle method fails to take heterogeneity of rock surface into
consideration. It is obvious that the complex composition of rocks and dif-
ferent effects of heavy surfactants in oil on wettability of sand rocks and clay
lead to uneven local wettability. Moreover, data cannot be directly obtained
to illustrate whether a permanent adhesive organic material membrane
exists on reservoir rocks that can be measured only by other wettability
measurements.