Page 19 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

Concept of Gas Wettability and Research Status CHAPTER 1 3

(A) (B)

Oil-wet surface Water-wet surface Water-wet surface Oil-wet surface

Oil Water-wet surface Water

FIGURE 1.2
Different types of wettability in reservoir rocks. (A) Porphyritic wettability; (B) Mixed wettability.

2. The reservoir rock is oil-wet: Based on the formation of oil and gas
reservoirs, adsorption of surface active substances in crude oil on the
surface of rock makes oil and gas reservoirs oil-wet.
3. Reservoir rocks exhibit porphyritic wettability: On the surface of the
same rock samples, different types of wettability coexist due to different
material constituents, and the mineral composition on the water-wet
and oil-wet surfaces is randomly distributed. As far as individual pores,
some part of their surface is strongly water-wet. The rest of it may be
strongly oil-wet. Furthermore, the oil-wet surface is not necessarily con-
tinuous (Fig. 1.2A).
4. The reservoir rock has mixed wettability: Wettability is different in pores
of different sizes. Small pores keep water-wet without oil in the pores.
Due to the sand surface of big pores being exposed to crude oil for such
a long time that it is oil-wet, oil can flow continuously in the channel
formed by big pores (Fig. 1.2B).
According to statistics from actual data, about 27% of reservoir rocks are
water-wet, 28% are oil-wet, and the rest are intermediate-wet. Hence the wetta-
bility of actual reservoir rocks is uncertain and needs further exploration.
The principle factors affecting wettability of reservoir rocks are relatively com-
plicated and mainly include the following:
1. The effect of mineral constituents of rocks on wettability.
Clay minerals in rock composition, especially montmorillonite and
argillaceous cement, enhance water-wet properties of rocks. The wetta-
bility of various mineral constituents is different.
2. The effect of reservoir fluid properties on wettability.
On the same solid surface, the CA differs for two different fluid combi-
nations. On the surface of different minerals, the CAs of the same fluid
are also different. For example, water is preferentially wettable against air
on the water-wet rocks’ surface in a water/air system, while air preferen-
tially wets rocks’ surface when the other two fluids, mercury and air, are
on the surface of rocks with similar properties.
3. The effect of surfactants or surface active materials on wettability.