Page 21 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

Concept of Gas Wettability and Research Status CHAPTER 1 5

1.1.3 Preferential Wettability of Gas on Reservoir Rock
Surface
In Reservoir Physics, it is believed that liquids are preferential over gas in wet-
ting solids [4]. Research in the oil industry is based on the assumption that
liquid is a strong wetting phase in gas/liquid/rock systems, while gas is
regarded as the nonwetting phase [5]. Hence wettability of reservoir rocks is
divided into water-wet, oil-wet, and neutral-wet systems. However, a great
number of theories and experimental phenomena prove that gas wettability or
preferential wettability of solids caused by gas cannot be neglected.
When the CA of water on the surface of rock, θ water , is less than 90 degrees,
water preferentially wets rocks. Water is the wetting phase fluid, and the rock
surface is preferential water-wet, or it is called water-wet rock. The smaller
θ water is, the stronger the water-wettability of rocks. When the CA of oil on the
surface of rock, θ oil , is less than 90 degrees, oil preferentially wets rock. Oil is
the wetting phase fluid, and rock is oil-wet or called oil-wet rock. The smaller
θ oil is, the stronger the oil-wettability of rocks. Assuming the existence of this
phenomenon on the same rock surface, when θ water . 90 degrees and θ oil . 90
degrees, gas (air) can be considered to preferentially wet the rock surface,
where gas (air) is the wetting-phase fluid. The assumption has been proven by
putting this hypothesis into practice several times under certain conditions.
As early as 1976, Morrow and McCaffery [6] found that the intrinsic CA of
water was 108 degrees on a smooth, low free energy polytetrafluoroethylene
(PTFE) surface with respect to that of air. Under such a condition, where air
and water exist in this group of fluids, air preferentially wets the surface of
PTFEs.
In 1983, Penny et al. [7], used a method of “nonwetting” (i.e., increasing the
CA of water on the rock surface to 90 degrees to achieve zero-capillary pres-
sure) in fracturing transformations of gas wells to improve the relative perme-
ability. Field applications show that with the resulting enhanced fracture
length and flow conductivity, the productivity of transformed wells is
increased by 2B3 times compared to that of fractured wells transformed by
the traditional method. Penny believed that the productivity of gas well
increased due to the alteration of wettability. However, no valid experimental
data is available to prove this hypothesis.

In May 1987, Zhou Zukang et al. [8], proposed that the “wettability” of solids
for gas and that for liquid is contrary in his book Colloidal Chemistry
Foundation. He believed that if a solid is more hydrophobic then it would be
easily “wetted” by gas and would be attached to bubbles more easily. On the
contrary, the more hydrophobic the solid is, the easier it is wetted by liquid
and the more difficult it is for it to attach to bubbles. Foam flotation is the
use of gas and liquid to the solid “wetting” or “wetting capacity” difference, to
separate the ore and slag.