Page 47 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

Evaluation Methods and Influencing Factors CHAPTER 2 31

Wettability can range from strongly oil-wet to strongly water-wet with large
measurement range and clear boundary conditions. Sessile drop method, com-
monly used in the oil industry, adopts quartz flakes or siliceous ore particles
to simulate stone reservoirs and calcite ore particles to simulate carbonate or
reef limestone reservoirs [2 5].

2.1.1.1 TYPES OF CONTACT ANGLE
Types of contact angle include equilibrium contact angle, advancing contact
angle, and receding contact angle. In an oil-water-rock system, different con-
tact angles are used for measuring wettability by contact angle measurement
method, and it is connected with the evaluation criterion for wettability of dif-
ferent strata surfaces.
In an oil-water-rock system, when equilibrium contact angle is adopted to
evaluate wettability and if the contact angle of water on the surface of the min-
eral/rock surface is less than 90 degrees, it is water-wet. If the contact angle is
more than 90 degrees, it is oil-wet. If the contact angle is equal to 90 degrees,
it is intermediate-wet.

In an oil-water-rock system, when the advancing contact angle is adopted to
evaluate wettability and if the contact angle of water on the surface of the min-
eral/rock surface is less than 90 degrees, it is water-wet. If the contact angle is
greater than 140 degrees, it is oil-wet. If the contact angle is between 90B140
degrees, it is intermediate-wet.
In an oil-water-rock system, when the receding contact angle is adopted to
evaluate wettability and if the contact angle of water on the surface of the min-
eral/rock surface is less than 60 degrees, it is water-wet. If the contact angle is
more than 100 degrees, it is oil-wet. And if the contact angle is between
60 B100 degrees, it is intermediate-wet.

2.1.1.2 UNCERTAINTY OF INEVALUATING WETTABILITY USING CONTACT
ANGLE METHOD
The advantage of evaluating wettability through contact angle method is that
it is simple, quick, and it has a wide measurement range. It can measure
strongly water-wet to strongly oil-wet solid surfaces and has clearly defined
values and boundaries. However, the uncertainty is higher. The contact angle
is also known as equilibrium contact angle when wettability reaches equilib-
rium. The equilibrium contact angle is strictly a thermodynamic parameter,
and so far researchers have only measured its value on certain organic polymer
solid surfaces that are clean, homogeneous, and have low surface energy.
However, it is difficult to measure the equilibrium contact angle [2,6] on the
surface of normal solids such as reservoir rocks.
To begin with, the uncertainty of contact angle method is characterized by hys-
teresis. Three possible reasons for contact angle hysteresis are surface rough-
ness, surface heterogeneity, and surface penetration at a molecular level.