Page 186 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

170 Gas Wettability of Reservoir Rock Surfaces with Porous Media

It can be seen from Fig. 5.5,
2σ water-gas cosθ
2ðσ solid-gas 2 σ solid-water Þ 2σ water-gas cosθ gas
P c 5 P 2 2 P 1 5 5 5
r r r
(5.2)

In formula (5.2), P c is capillary force, and P 1 and P 2 are pressures in the front
and back of gas/water interface of gas-displacing front, respectively. σ solid-gas is
the tension of solid/gas interface, and σ solid-water is the tension of solid/water
interface. σ water-gas is the surface tension of water, and r is the radius of
capillaries. θ gas is the advancing angle of gas-displacing-water front, and θ is
the supplementary angle of θ gas .
Under nongas-wetting conditions, θ gas Að90 ; 180 Š,cosθ gas # 0, the direction

of capillary force P c , which is opposite to the direction of gas-displacing-
water, is the flow resistance of gas. Within this range, as gas wettability increases
or θ gas decreases, the direction of P c does not change. When the value of
P c decreases, the fluidity of water is enhanced and the flow resistance of
gas reduces.
Under neutral gas wettability conditions, θ gas 5 90 , capillary force P c is zero.

Under preferential gas wettability conditions, θ gas A½0 ; 90 Þ, cosθ gas $ 0, the
direction of capillary force P c , which is the same as the direction of gas-
displacing-water, is the force of gas flow. Within this range, as gas wettability
increases or θ gas decreases, the direction of P c does not change. When the
value of P c increases, the fluidity of water is enhanced and flow resistance of
gas is reduced.
The change in gas wettability changes the percolation environment of the gas
around the bottom of gas wells. As gas wettability increases, water fluidity
increases, and permeation resistance of gas is reduced. This is beneficial for
removing water-blocking effect, and recovering or improving the capacity of
condensed gas reservoir.

5.1.2.2 EXPERIMENT METHOD
The apparatus and materials used for gas-displacing-water and water-
displacing-gas experiments conducted with straight capillaries of different gas
wettability are the same. The gas wettability degree of the inner wall of the
capillary treated with gas-wetting alteration agent solutions (0.04%, 2%, and
8%) of different concentrations is measured with the bubble capture method.
When the capillary micro experiment is processed, air is taken as the displa-
cing phase and distilled water as the displaced phase. The experimental
procedure of gas-displacing-water is the same as that of water-displacing-gas
in capillaries at room temperature, and the displacement speed of gas is
0.3 mL/h.