Page 18 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

2 Gas Wettability of Reservoir Rock Surfaces with Porous Media

δ gas-liquid

δ solid-gas θ
δ solid-liquid
FIGURE 1.1
Definition and diagram of the contact angle through liquid droplets.

“change of Gibbs free energy of a unit solid surface caused by the contact with
liquid, assuming that free energy does not change when contact with air” [1].
By comparing wettability of different liquids to certain solids, one can predict
whether a liquid in the solid pore can be displaced by another liquid. This def-
inition indicates the quantitative evaluation of wettability in an ideal system
(immiscible liquids and homogeneous solids). The actual system is rather
complicated, and another definition is normally used, which states, “relative
preferential selectivity of solid surface covered by a liquid among considered
liquids” [2]. The most common wettability is a liquid displacing air on the
solid surface from the solid surface—i.e., a liquid preferentially wetting solid.

Wettability is one of the most important features of solid surfaces, and it has
extremely extensive applications, including mineral foam floatation, oil explo-
ration, adhesion and bonding, waterproofing, and washing in industrial pro-
duction. Studies show that wettability of solid surfaces jointly depends on the
chemical constitution, micro geometry, and macro geometrical shapes [3].
Wettability is mainly affected by the chemical constitution of a solid surface.
The greater the free energy of the solid surface, the easier it is wetted by some
liquids, and vice versa. The contact angle (CA) usually works as a criterion for
wettability of a solid by a liquid. It can be explained when a line is drawn at a
tangent from the intersection of three phases—solid, liquid, gas—where the
angle between the tangent and the border of solid and liquid is the CA
(Fig. 1.1). Actually, the CA through a liquid droplet onto the surface of a solid
is the result of surface tension balance between the interfaces of solid, liquid,
and gas. The balance of liquid droplets tends to minimize the gross energy of
the system so that the liquid droplets are in a steady or metastable state on
the solid surface.

1.1.2 Wettability of Reservoir Rocks and Its Effect Factors
It is generally acknowledged that wettability of reservoir rocks falls into three
categories: water-wet, oil-wet, and intermediate-wet. The wettability of rocks
can be classified based on the size of wettability CAs or the quantity of water
or oil absorbed in reservoir rocks, while the wettability of actual reservoir
rocks is much more complicated. There are four main perspectives:

1. The reservoir rock is water-wet: Based on rock geological deposition the-
ory, it is believed that reservoir rocks are water-wet.