Doe/netl-2012/1540 Mobility And Conformance Control For Carbon Dioxide Enhanced Oil Recovery (Co2-Eor) Via Thickeners, Foams, And Gels - U.s. Department Of Energy Page 133

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11.
Effects of Process Variables on CO
Foam Mobility Reduction
2
Temperature
Because reservoir temperature is a factor that cannot be altered during a CO
flood, there are
2
relatively few core flooding studies in which temperature is varied systematically. Nonetheless it
o
is apparent that CO
foam flooding at temperatures above 80
C may require more careful design
2
than low temperature floods. There is at least one aspect of high temperature that is favorable:
decreased surfactant adsorption with increasing temperature [Ziegler and Handy, 1979].
However, the primary obstacles for the application of foams in deep, hot formations include the
decrease of surfactant solubility in brine that typically occurs with increasing temperature, the
thermal degradation of the surfactant that is enhanced with increasing temperature [Handy et al.,
1982], the slight increase in the interfacial tension between the CO
and the brine [Liu et al.,
2
o
2005], and diminished foam stability [Wang, 1984] especially at temperatures above 60
C that
must be compensated for by higher concentrations of surfactant [Liu et al., 2005].
Pressure
In general, higher pressure favors CO
foam stability. CO
becomes a more dense solvent at
2
2
higher pressure, which enhances the intermolecular associations between the CO
and the
2
hydrophobic tails of the surfactant molecules. In a micromodel study, it was found that sweep
efficiencies associated with CO
foams flowing at a pressure just below the MMP were just as
2
high as the efficiencies measured at pressures well above the MMP. Therefore, it was concluded
that the high sweep efficiencies can be accomplished using the least CO
if the foam flood is
2
conducted at ~MMP rather than much higher pressures [Chang et al., 1994].
Brine Composition
In general, for a given surfactant, increased salinity (increased TDS) may tend to destabilize
foam or, depending on the surfactant, have little effect. In general, a decrease in foam stability
may be attributable to the increased salinity breaking the foam by decreasing the electrostatic
double layer forces, or by diminishing the surfactant solubility in brine [Alkan et al., 1991].
For example, in Figure 11.1, the volume of an air-in-brine foam, in which the “base” brine (1X)
contained 10.58% NaCl and 0.76% CaCl
, tends to decrease with increasing salinity.
2
102

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