(92f) Effect of Tail Structure of the Ability of CO2-Soluble Surfactants to Stabilize CO2-in-Brine Emulsions

In the tertiary method of oil extraction, the very low viscosity of liquid CO2 causes viscous fingering, which dramatically reduces the sweep efficiency of CO2 floods. In this study we used CO2-soluble surfactants that are capable of stabilizing CO2-in-brine emulsions. These emulsions or foams will form as the CO2+surfactant solution mixes with the brine already in the reservoir, thereby eliminating the need for alternating injections of neat CO2 and brine-based surfactant solution. The foams enhance the apparent viscosity of the injected CO2, and thereby greatly reduce the mobility of CO2.

We successfully made white, opaque, CO2-in-brine emulsions of foams under typical reservoir MMP and temperature, using several commercially available, inexpensive, non-ionic surfactants. In each case, we only used the amount of surfactant capable of dissolving in the CO2 at typical MMP conditions, and prepared the emulsions with synthetic or reservoir brine (rather than water). Visual observations of foam formation and collapse were made using a high pressure, high temperature, windowed, agitated, variable volume view cell that permitted direct observation of the entire contents of the sample volume.

The surfactants fell into three categories; water-soluble branched alkylphenol ethoxylates, linear alkylphenol ethoxylates, and branched alkyl ethoxylates. At 25 oC, the solubility experiments of these surfactants in liquid CO2 at ~1300 psia (~9 MPa) is 0.02 ? 0.10 wt%. For foam stability investigation, equal volumes of liquid CO2 and brine (5wt% NaCl) were mixed vigorously with these surfactants 40 minutes at 1300psi and 25 oC using a slotted impeller spinning at 2500rpm. A white cloudy foam (cell sizes of 100 microns or less) filled the entire sample volume. As the emulsions collapsed, they yielded a clear aqueous zone below the emulsion as the brine drains from the continuous films of surfactant-stabilized brine, while a clear CO2 zone formed above the emulsion as the droplets of CO2 coalesced.

Among these three major types of surfactants that were evaluated, the alkylphenol ethoxylates with composed of branched alkyl tails with numerous isomeric forms were the best foaming agents. After five hours, no clear zone of CO2 emerged above these emulsions, and 5-30 vol% of the brine phase remained within the continuous films of the emulsion with the remainder of the brine in an excess brine phase below the emulsion.