Environmental Engineering Reference
In-Depth Information
As for the gas-liquid system, we can derive the Young-Laplace equa-
tion for the capillary pressure between CO 2(g) , and brine (w):
2
pp
−=γ
cos
,
g
w
gw
R
where we know the surface tension between the CO 2 and brine phase.
Because of capillary forces, the wetting phase will be preferentially drawn
into the small pores.
Brine-CO 2 interfacial tension
A key parameter in the expression for the capillary pressure is the brine-
CO 2 interfacial tension (
γ gw ). In many practical applications, the values of
the capillary pressure are estimated from data on other gasses (e.g., N 2 ,
CH 4 ). To see whether these estimates are valid, Nielsen et al . [9.14] car-
ried out molecular level calculations to study the effect of pressure on the
brine-CO 2 interfacial tension. The results shown in Figure 9.4.4 show
that the interfacial tension decreases strongly with pressure to a value of
γ gw
25 mN/m at conditions where CO 2 exists as a supercritical fl uid.
Other non-aqueous fl uids (e.g., N 2 , CH 4 ) have
γ gw values that are roughly
twice as large, which brings into question the practice of using data on
other non-wetting fl uids for those systems where data on the behavior of
CO 2 are not available.
The pressure dependence of
γ gw values is caused by the adsorption of
CO 2 on the water surface as described by the Gibbs adsorption equation:
d
γ
gw
g ,
=−
RT
Γ
df
ln
Γ g w is the Gibbs sur-
face excess of CO 2 relative to H 2 O, i.e., the quantity of CO 2 adsorbed on
the water surface ( Figure 9.4.4 ). This equation, which should be generally
applicable to any species in the brine-CO 2 system, shows that the addi-
tion of species that adsorb at the brine-CO 2 interface tends to lower the
interfacial tension (and, conversely, species that are excluded from the
interface tend to increase the interfacial tension). For example, it predicts
that if NaCl is excluded from the vicinity of the brine-CO 2 interface,
where f is the fugacity of CO 2 (a function of p g ) and
γ gw
should increase with brine salinity, as observed experimentally.
 
Search WWH ::




Custom Search