Page 160 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

144 Gas Wettability of Reservoir Rock Surfaces with Porous Media

4.2.3.1.2 Specific Surface Area of Rocks
When the chemical composition of rocks is the same, the CEC enlarges with
the increasing of the specific surface area (or disperses).

4.2.3.1.3 Acidic and Alkaline Environments
When chemical composition and dispersity of rocks is the same, the CEC is
greater in alkaline environment and smaller in an acidic environment.

4.2.3.2 TEST OF CATION EXCHANGE CAPACITY
The CEC of montmorillonite with different gas-wetting alteration degrees was
characterized by the methylene blue capacity (MBC) test. The montmorillonite
treated with the gas-wetting alteration agent by distilled water was turned into
a suspension liquid with 4% mass fraction, and let the liquid to stand still for
24 hours at room temperature. It was then stirred again, and 2 mL suspension
liquid was added into a conical flask holding 10 mL distilled water, in addi-
tion to 15 mL hydrogen peroxide solution with mass fraction of 3% and
0.5 mL diluted sulphuric acid of 2.5 mol/L concentration. The solution was
boiled slowly for 10 minutes, but did not evaporate to dryness. Finally, it was
diluted to 50 mL.

Next, 0.1 mL methylene blue solution with concentration of 0.01 mol/L was
dripped into a conical flask and then shaken for 30 seconds. In a state of solid
suspension, a drop of liquid was dripped on a filter paper using a stirring rod.
When blue-green circles appeared around the dyed solid, the conical flask was
shaken for 2 minutes. Another drop was dripped on the filter paper, and
when the blue-green circle was still obvious, the titration reached the terminal
point. When the circle did not appear any longer, the process was continued
till the blue-green circles appeared on the filter paper, after shaking for 2 min-
utes. The experimental result is displayed in Fig. 4.13.
Methylene blue exchange capacity MBC of montmorillonite water solution is
calculated using the formula below:

V m
MBC 5 (4.9)
V
In the formula:
MBC—Methylene blue capacity;
V m —Consumption volume of methylene blue capacity, mL;
V—Sample volume, mL.
From Fig. 4.13, it can be seen that as the concentration of gas-wetting alter-
ation agent increases, the CEC of montmorillonite decreases rapidly, and MBC
is reduced from 2.14 in an untreated state to 0.2, and followed by a reduction
in CEC of montmorillonite after gas-wetting alteration and strengthening of
stability. Analysis showed that it could be because the gas-wetting alteration