Page 66 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

50 Gas Wettability of Reservoir Rock Surfaces with Porous Media

wettability. However, measurement results may have errors due to hysteresis
and heterogeneity of the surface of rock samples. The Washburn method can
easily measure the contact angle of powders, but it can only qualitatively eval-
uate whether the surface of the powders is relatively wetted by gas and cannot
specifically assess the degree of gas wettability. The nature of gas is greatly dif-
ferent from that of liquid. This has significant theoretical and practical impli-
cation in terms of establishing quantitative gas wettability evaluation
methods.

2.3 ESTABLISHING GAS WETTABILITY EVALUATION
METHOD

Gas is colorless and intangible, with low viscosity and strong compressibility.
In commonly used liquid wetting evaluation methods, the contact angle
method is used to establish a visual gas wettability quantitative evaluation
method.

2.3.1 Sessile Drop Method
2.3.1.1 EVALUATION PRINCIPLE AND EVALUATION INDEX

2.3.1.1.1 Evaluation Principle
In the gas phase, if a drop of liquid falls on the surface of a solid and it does
not spread, it indicates that the solid surface is gas wettable [26,27]. The
spread of liquid drops on solid surface is the process of liquid and gas com-
peting to preferentially cover the solid surface in the micro level, and the wet-
ting process of the solid surface caused by the liquid, with respect to the gas in
the macro level. The smaller the contact angle θ liquid of liquid drops is, the
better the wetting capability of the solid surface by the liquid, with respect to
the gas, and the weaker the wetting capability of the solid surface due to gas
is, with respect to the liquid. Therefore the supplementary angle

ð180 2 θ liquid Þ of the contact angle of liquid drops on the solid surface can be
viewed as a representation of the gas wetting capability of the solid surface,
with respect to the liquid.

2.3.1.1.2 Evaluation Index
In the gas phase, the definition of wetting capability parameter ζ gas-liquid of
solid surface by gas, with respect to a certain liquid, is shown in Eq. (2.15).
The relation of wettability of the solid surface by gas with respect to a certain
liquid, and the wetting parameter ζ gas-liquid and liquid contact angle θ liquid are
displayed in Fig. 2.15.
ζ (2.15)
gas2liquid 5 cosð180 2 θ liquid Þ
Fig. 2.15 shows the value range of ζ gas2liquid is [ 2 1,1] and that gas wettability
is strengthened as liquid contact angle θ liquid and ζ increase. The visual
gas2liquid