Page 48 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

32 Gas Wettability of Reservoir Rock Surfaces with Porous Media

Morrow [7] believes that the roughness and geometrical shape of pores affect
the oil/water/solid contact line and change apparent contact angle. Generally,
surface roughness decreases the apparent contact angle of water-wet rocks,
while increasing that of oil-wet rocks.
Next, the contact angle method fails to account for heterogeneity of rock and
mineral surfaces and is measured considering the surface of a single mineral
crystal. However, local uneven wetting is caused due to complex composition
of rocks and different effects of heavy surfactants in oil on the wettability of
sandstone and clay.
Finally, it is not possible to directly obtain data on the presence of permanent
adhesive organic material membrane on reservoir rocks, which can only be
measured through other wettability measurement methods.
Due to surface roughness, heterogeneity, complex geometric form, and surface
penetration at molecular level, the contact angle method cannot be used for
assessing wettability of porous reservoir rocks, etc.

2.1.2 Amott Method
The Amott method got its name from the proposer’s family name. The essen-
tial fundamental of measurement is that the wetting-phase fluid accesses the
core by spontaneous imbibition to displace nonwetting-phase fluid and com-
bines imbibitions and forced displacement to measure the average wettability
[8] of the core. Therefore the Amott method provides quantitative evaluation
[9] of mean wettability of reservoir rocks by various fluids.

The experimental procedure for Amott method is as follows:
1. The core is saturated with saline water.
2. Oil displaces saline water in the core to produce bound water.
3. The core is then put in the water absorption device for water absorption
and oil extraction for more than 20 hours.
4. When water absorption and oil extraction are completed, the core is put
in a centrifuge for water displacing oil experiment for 1 hour. The cen-
trifugal force should be 1800 times of gravity.
5. After water displaces the oil completely, the core is put in oil absorption
device for oil absorption and water extraction for over 20 hours.
6. After oil absorption and water extraction have been completed, the core
is put in a centrifuge for oil displacing water experiment.
The above experimental procedure is illustrated in Fig. 2.1. The capillary pres-
sure of ordinate can be worked out using data including rotational speed of
centrifuge, rotation radius, and density difference of oil and water.
Amott indices for water and oil can be worked out using relevant data
obtained from Fig. 2.1 using the following equations:
I AðwÞ 5 ΔS wð3Þ =½ΔS wð3Þ 1 ΔS wð4Þ Š (2.1)