Page 142 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

126 Gas Wettability of Reservoir Rock Surfaces with Porous Media

If two kinds of gas molecules, A and B, are adsorbed on the rock surface, or
adsorbed gas molecule A reacts with the rock surface and the generated prod-
uct B is also adsorbed, the adsorption rate of A is:

r a 5 k a p A ð1 2 θ A 2 θ B Þ (4.3)
In formula p A —partial pressure of A;

θ A —coverage of A on rock surface;
θ a —coverage of A on rock surface;
θ B —coverage of B on rock surface;
θ B —coverage of B on rock surface;
k a —coefficient of A’s adsorption rate;
k a —coefficient of A’s adsorption rate.

The desorption rate of A is r d 5 k d θ A , if balanced, then r a 5 r d
The desorption rate of A is r d 5 k d θ A , if balanced, then r a 5 r d
θ A
5 ap A (4.4)
1 2 θ A 2 θ B
In the formula, a 5 . Similarly, when B is balanced, the following relation-
k a
k d
ship is established:
θ B 0
5 a p B (4.5)
1 2 θ A 2 θ B
In formula p —partial pressure of B
B
Combining the above formulae (4.4) and (4.5), the following are obtained:
ap A
θ A 5 (4.6)
0
1 1 ap A 1 a p B
0
a p A
θ B 5 (4.7)
0
1 1 ap A 1 a p B
It can be seen from formula (4.6) and formula (4.7) that when p increases,
B
θ A decreases. That is, the existence of gas B can block the adsorption of gas A.
Similarly, the adsorption of gas A is also blocked by gas B. Formula (4.6) and
formula (4.7) can be easily applied to various gas adsorption scenarios. For
the B component gas with partial pressureP B , the Langmuir isotherm can be
normally recorded as:
a B p B
θ B 5 P (4.8)
1 1 a B p
B B
Alhough Langmuir isotherm describes the relationship between θ and p, it also
represents the relationship between V and p, because θ 5 V (V is volume of
V m
adsorbed gas).