Page 52 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

36 Gas Wettability of Reservoir Rock Surfaces with Porous Media

curves coincide (or almost coincide), the measured rock sample is oil-wet.
And if the oil-phase curves don’t coincide, the measured rock sample is water-
wet. The third is the relative permeability curve loop identification method.
The form of the relative permeability curve has a lot to do with microscopic
distribution of fluid. The wetting hysteresis formed by the change in fluid satu-
ration order affects the microscopic distribution of fluid, due to how the rela-
tive permeability curve tested by displacement method significantly differs
from that of imbibitions method. If the oil-phase loops are separated and
water-phase loops coincide, the measured rock sample is water-wet. However,
if the oil-phase loops coincide and water-phase loops both are separated, the
measured rock sample is oil-wet.
Relative permeability is the basis for numerous qualitative wettability measuring
methods, but it can be only applied to distinguish between strongly water-wet
and strongly oil-wet cores. It is difficult to measure subtle changes in wettability
through this method. This method is recommended for special measurement in
case wettability is not present to compensate for any data deficiencies.

2.1.5 Microscopic Examination
Microscopic examination is sometimes used to visualize the flow dynamics in
an ideal porous medium at the pore level during the water displacement pro-
cess. For strongly water-wet systems, water sticks to the sand surface, residual
oil stays on the water film, and smaller spherical oil drops are located at the
center of pores. If the system is intermediate-wet, oil and water are both in
contact with the rock surface and can be located in small pores. If the system
is oil-wet, the distribution feature of oil is the same as that of water in the
water-wet system [15,16].

2.1.6 Flotation
The flotation method can be used to rapidly evaluate wettability, but it only
works for strong wetting systems. Recommended by API, the flotation method
is used to determine the influence of surfactants on wettability. Oil, water, and
sand are poured into a glass bottle that is shaken, and the sand is then
observed. If the system is strongly water-wet, the sand sinks to the bottom,
and the sand in the oil forms small tighter clusters surrounded by a water
film. If the system is oil-wet, part of the sand gets suspended on the oil-water
interface, oil-wet sands congeal in water, and small oil droplets stick to the
sand’s surface [17,18].
Oil flotation is first used, and then it is replaced by gas flotation used for min-
eral screening, as displayed in Fig. 2.4. The effect of gravity is neglected for
simplicity. If particles are moved into water dh, the solid-liquid interface
enlarges dA, and the solid-gas interface reduces dA. At the same time, gas-
0
liquid interface changes dA 5 cosθUdA. S represents solid phase, L refers to liq-
uid phase, and A is area.