Page 138 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

122 Gas Wettability of Reservoir Rock Surfaces with Porous Media

4.1 GAS ADSORPTION CAPABILITY OF ROCKS
4.1.1 Basic Theory of Gas Adsorption of Rocks
4.1.1.1 ADSORPTION PHENOMENA AND CLASSIFICATION
When gas molecules collide with a rock surface, some molecules remain on
the surface for a period of time due to the effect of residual force field, causing
the density of gas molecules on the rock surface to increase and density of gas
phase to decrease. This phenomenon is called gas adsorption on the rock sur-
face. Normally, the solid substance is called adsorbent and the adsorbed gas is
called adsorbate.
Adsorption is the result of interaction between solid surface molecules or
atoms and gas molecules. Adsorption can be classified into two types: physical
adsorption and chemical adsorption, depending on the forces of attraction
between the adsorbate and adsorbent. The former is due to van der Waals
forces of attraction, while the latter is due to a chemical bond. This results in
different properties of the two kinds of adsorption (Table 4.1).

4.1.1.2 ADSORPTION CAPACITY AND ITS MEASUREMENT
Adsorption capacity q is normally expressed in terms of volume V (nor-
mally converted to the volume under standard condition) of the gas
adsorbed by adsorbent of unit mass, or amount of substance n, as
follows:
V 0 n
q 5 或 q 5 (4.1)
m m

The two most commonly used methods are introduced briefly: gravimetric
method and continuous-flow chromatography.

Table 4.1 Physical Adsorption and Chemical Adsorption
Property Physical Adsorption Chemical Adsorption
Adsorption Vanderwaal Van der Waal Chemical bond
force
Selectivity No Yes
Adsorption heat Approximate heat of Approximate heat of
liquefaction (0 20 kJ/mol) reaction (80 400 kJ/mol)
Adsorption rate Quick, easy to balance, Slow, difficult to balance,
doesn’t require activation and requires activation
energy energy
Adsorption layer Monomolecular layer or Monomolecular layer
polymolecular layer
Reversibility Reversible Irreversible