Page 62 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

46 Gas Wettability of Reservoir Rock Surfaces with Porous Media

2.2.3 Evaluating Gas Wettability by Washburn Method
2.2.3.1 EXPERIMENTAL METHOD
The Washburn method is used to measure the contact angle in quartz sand
based on the Washburn equation:
rCos θ
2
H 5 γt (2.13)
2η
The powder plunger filled in the glass tube can be regarded as a group of capillar-
ies with average radius R, and thus the Washburn equation is reversed to:
cRCosθ
2
H 5 γt (2.14)
2η
Where H is height of liquid seeping into capillary
c is irregularly oriented correction factors of capillary pore in powder
system.
γ is surface tension of liquid.
t is time taken for liquid to seep into capillary.
θ is contact angle of liquid on capillary walls.
η is viscosity of liquid.
2
From Eq. (2.14), the slope value cRγCos θ=ð2ηÞ of H -t line is obtained.
Liquids with best wettability are used normally as perfect wetting liquids, and
the contact angle on the powder is taken as zero. According to liquid viscosity
η and surface tension γ, parameter γ value can be acquired, and the relative
contact angle of every liquid phase is obtained when cR value is acquired.

2.2.3.2 EXPERIMENTAL RESULT AND DISCUSSION
If a Washburn tube with 1.0 cm inner radius and 15 cm length is adopted in
the experiment, with 0.2% NaCl solution as water phase, the surface tension is
γ water 5 70 mN/m and viscosity is η water 5 1.0 cp at 25 C room temperature;

with n-decane (n-C10) as oil phase, the surface tension is γ oil 5 23.4 mN/m
and the viscosity is η oil 5 0.92 cp [8]. Untreated quartz sand is taken as water-
wet standard, and quartz sand treated with methylsilicone oil is taken as oil-
wet standard.

First, the wettability of quartz sand with water, treated with CTAB of different
concentrations, is measured by Washburn method as displayed in Fig. 2.12.
2
2
The height square H has a linear relation with time t, and R is greater than
0.98. Linear fitting is conducted for the data as displayed in Fig. 2.12, and con-
tact angles of water treated with CTAB of various concentrations on the surface
of quartz sand are based on the Eq. (2.14). The result is displayed in
Table 2.4.

Clean quartz sand surface is strongly water-wet; it can be regarded as a stan-
dard for water-wet, and the contact angle of water on the surface is taken as