In CO2 floods the large viscosity contrast between the reservoir and injected fluids induces an unfavorable mobility ratio that results in early breakthrough and consequently decreases oil sweep efficiency. There is evidence that the presence of an aqueous surfactant solution will produce a foam with CO2. This not only controls the gas mobility but may also selectively reduce the gas mobility through regions of differing permeabilities. This could be a very favorable phenomenon in EOR processes if it occurs in the reservoir. The purpose of this study is to evaluate foam application from these perspectives.

This study concerns a series of steady-state CO2-foam flow experiments in Berea sandstone cores at typical reservoir conditions. The effects on foam mobility of flow rate, foam quality, surfactant type, surfactant concentration, and rock permeability were investigated. Four commercial surfactants (Chevron Chaser™ CD 1040, CD 1045, CD 1050 and Shell Enordet 2001) at 500 ppm and 2500 ppm concentrations (in 2 wt% brine) were used. In each run the contribution of each parameter on foam mobility was evaluated independently.

Our results have shown that the mobility of 80% quality foam increased with increasing total flow rate for all four surfactants. As evidenced by increased pressure drop, foam was generated in all runs, including those in which the total Darcy flow velocity was as low as 0.04 fit/d. Selective mobility reduction, in which the measured mobility was reduced further in high permeability cores, was observed for some but not all surfactants at certain flow rates. Some sequences of experiments were performed in which the flow rate of one phase was held constant, while that of the second was increased from one run to the next. Between experiments, in each sequence, it was observed that pressure drop depended little on gas velocity. Our results also showed that foam mobility increased with increasing liquid velocity in all the runs. We also found that the foam mobility increased slightly as foam quality increased up to about 85 % and then rose quite rapidly as foam became dryer, up to about 98 % CO2. Foam hysteresis was also investigated in 36 experimental runs and was found to be absent in almost all cases within a reasonable experimental error.

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