Page 164 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

148 Gas Wettability of Reservoir Rock Surfaces with Porous Media

From Fig. 4.15, it can be seen that fluorine carbon chains in gas-wetting alter-
ation agent wrap around and coat the surface of the montmorillonite grains,
presenting a spherical structure. Along with the result of scanning electron
microscope experiment, when gas-wetting alteration agent is adsorbed on the
montmorillonite surface, the fluoroalkyl side chains are arranged on the mont-
morillonite surface in an orderly manner. While avoiding contact with any
outside medium, the wettability of the treated montmorillonite changes from
preferential gas-wetting to hydrophobic-oleophobic preferential gas-wetting,
and the stability of the montmorillonite is maintained.

4.3 CONDUCTIVITY OF ROCKS
Recovery can be increased substantially if gas-wetting alteration of rocks is
realized. However, the effect of change of wettability on the conductivity of
rocks cannot be ignored. It affects the position and distribution of fluids in
pores, changes the electrical property of rocks, and finally impacts the accuracy
of electrical logging used to identify oil-water layers [18,19].
Some foreign researchers studied the relationship between rock wettability and
electrical resistance. Keller measured the electrical resistance of oil-wet and
water-wet cores using the air-dry method. The electrical resistance of cores
gradually increased as water saturation decreased. However, reducing water
saturation by the air-dry method leads to a change in salt concentration and
generates a huge error. Goddard measured the electrical resistance of rocks in
sandstone samples, taking mercury as the nonwetting fluid. As mercury satura-
tion decreases, electrical resistance increases, and is almost infinite [20] when
mercury saturation is constrained. Sweeney and Jennings measured the effect
of wettability on the electrical resistance of carbonate cores. It was found that
the change in electrical resistance of oil-wetting rocks exhibited two different
characteristics. In some rocks, even though saturation of salt solution was very
high, electrical resistance was still quite high; for other kinds of cores, electrical
resistance gradually increased when water saturation decreased. When water
saturation decreased to about 35%, electrical resistance rose rapidly [21].
Mungan and Moore studied the effect of wettability on electrical resistance
with synthesized polytetrafluoroethylene cores, and got results similar to the
results of Sweeney and Jennings. When salt saturation is reduced to a certain
specific value, part of the salt solution in cores becomes discontinuous, and
electrical resistance rises faster [22].
This section conducts laboratory experiments to study conductivity change,
compares the change of the electrical resistance of water-wetting and gas-
wetting cores under different gas saturations, and studies the relationship and
effects of electrical resistance and gas wettability of rocks, based on gas-wetting
alteration of water-wetting artificial sandstone cores using developed gas-
wetting alteration agents. Table 4.16 shows the parameters of artificial sand-
stone cores that were used for the experiment. At the same time, the cores
underwent pretreatment, including salt-leaching, oil-washing and weighing.