Page 50 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

34 Gas Wettability of Reservoir Rock Surfaces with Porous Media

reservoir rocks. However, it is not sensitive when approaching intermediate-
wet. When contact angle is between 60B120 degrees, not all fluids will auto-
matically displace the other fluids, resulting in errors and interferences.
Furthermore, when the contact angle exceeds the limiting contact angle which
doesn’t generate spontaneous imbibitions depends on the initial saturation of
cores which directly affects wettability measurement of cores. McCaffery and
Morrow [9,10] measured spontaneous imbibitions of pure fluid in artificial
Teflon cores with known contact angle. It was found that octyl ether (θ 5 49
degrees) is not imbibed into dry cores but can be imbibed into cores saturated
with 30% original octyl ether. There is a larger error in measured result. If the
measured core contains soctyl ether, it is water-wet when measured with octyl
ether. If the measured core is originally dry, it is intermediate-wet when mea-
sured with octyl ether.
The Amott method is not sensitive to an intermediate-wet condition, which
significantly affects gas wettability evaluation. Therefore it is not suitable for
gas wettability evaluation.

2.1.3 USBM Method
The USBM method was proposed in 1969 by Donaldson et al. [11,12] to mea-
sure average wettability of cores. The underlying principle of this method is
that one fluid displaces another, and the effort required by a wetting-phase
fluid to displace a nonwetting phase fluid is less than that required for reverse
displacement. Furthermore, it has been proved that the effort required is in
direct proportion to the corresponding area under the capillary pressure curve
[13]. The wettability of pore medium is expressed with the obtained imbibi-
tions and displacement capillary pressure curve and the logarithm W 5 lg
(A 1 /A 2 ) of the ratio of the area under the curve, namely wettability. In the
equation, A1 and A2 are the areas that fall under oil-displacing and saline
water-displacing curves, respectively. When W is more than zero, the core is
water-wet. When W is less than zero, the core is oil-wet. Wettability close to
zero indicates that the core is intermediate-wet. The greater the absolute value
of W is, the greater the wettability bias [1].
When evaluating wettability, if the influence of pore geometric shapes is simi-
lar, experimental results are reproducibile, as USBM method draws support
from capillary pressure curve to determine wettability and the shape of capil-
lary pressure curve depends on the distribution of wettability of porous
medium and pore size.
The USBM method is more sensitive to intermediate-wet samples compared to
the Amott method. It can make use of reservoir fluid and is less time-
consuming. However, it can only determine the average wettability of porous
medium samples and cannot tell whether the sample is partially wet or mixed
wet. However, the Amott method is more sensitive to the case.
Experimental procedure for USBM method is as follows [11].