Page 89 - Fundamentals of Enhanced Oil and Gas Recovery

77
Enhanced Oil Recovery Using CO 2

have an associated gas cap or have low bubble point pressure, this mechanism has high
effectiveness. Indeed, the swelling phenomenon is important for two reasons. Firstly,
the residual oil saturation decreases as it is inversely proportional to swelling factor. The
residual oil saturation is an important point in relative permeability curves and deter-
mines ultimate recovery. Secondly, as the crude oil swells, it drives trapped oil droplets
out of pores, leading to a drainage process. It also increases oil saturation, which actually
increases oil relative permeability. All of these result in increased oil recovery. In com-
parison to all common nonhydrocarbon gases used for immiscible gas injection, CO 2
promotes oil relative permeability to higher degrees [14].
Most common correlations used for the swelling factor of CO 2 -saturated oil mix-
tures are as follows:

3.2.2.2.1 Welker [43]
In their model, the swelling factor is a linear function of CO 2 solubility. Although
their correlation is simple, it is mainly applicable to crude oils with API gravities from
20 to 40 API. The formula was also developed for dead oils at temperature equal to

80 F. Eq. (3.12) represents the Welker [43] correlation for the swelling factor as given
as follows:
3 3
1:96525 3 Solðm =m Þ
SF 5 1:0 1 (3.12)
1000
Welker [43] examined their correlation over 13 crude oils and reported an average
deviation of 0.01. Chung et al. [40] have also reported a good fit of their data on the
Welker [43] correlation (Fig. 3.8).

1.5
Bartlett oil
1.4 Merey oil
Trix-Liz oil
Swelling factor 1.3
MC kittrick oil

1.2

3
3
1.96525 × (Sol(m /m ))
1.1 SF = 1.0 +
1000
Ref. [43]
1.0
0 200 400 600 800 1000
Solubility (scf/bbl)
Figure 3.8 Correlation for swelling factor [43].