Addition of slightly CO2-soluble, brine-soluble, surfactants to high pressure CO2 for EOR may facilitate in-situ generation of CO2-in-brine foams for mobility control. These non-ionic surfactants have been demonstrated to dissolve in CO2 to concentrations of 0.1wt% at reservoir conditions and stabilize CO2-in-brine foams in a high pressure windowed cell. One such surfactant is Huntsman SURFONIC® N, a branched nonylphenol ethoxylates with averages of 12 (N-120) or 15(N-150) ethylene oxide repeat units in the hydrophile. SURFONIC® N-120 was selected for mobility reduction studies involving flow of CO2 into brine-saturated porous media.
Transient mobility measurements were conducted using a water-wet Berea core (104mD), water-wet Bentheimer sandstone core (~1500mD), and several SACROC carbonate cores (3.6 and 8.9mD). The CO2 was injected into brine-saturated cores at superficial velocity of 10 ft/day, and surfactant was either not used (control), dissolved only in brine at 0.07wt%, dissolved only in CO2 at ~0.07wt%, or dissolved in brine and CO2 at 0.07wt%. In general, in-situ foam generation in relatively high permeability sandstone was evidenced during the first few pore volumes of CO2 injected by pressure drops that were 2-3 times greater than control tests regardless of what phase CO2 was in. Mobility reduction was more modest (20–50% increases in pressure drop) in lower permeability SACROC cores (3.6 and 8.9mD) when surfactant was dissolved in CO2. With surfactant dissolved in brine, pressure drops increased by a factor of 2–3 when CO2 was injected into an 8.9mD core.
High pressure CT imaging of in-situ foam generation was conducted by injecting high pressure CO2 into 5wt% KI-brine-saturated Berea sandstone (3-8mD). Tests with no surfactant (control), or with surfactant dissolved either brine or CO2 at ~0.07wt%. At lower superficial velocities (0.47ft/day), in-siti foam generation was obvious only when surfactant was dissolved in brine. Higher flow rates (4.7ft/day) preferential flow of CO2 through high permeability layers and viscous fingering within layers that occurred during control tests was suppressed by addition of surfactant to either CO2 or brine. The most distinct CO2 foam front occurred with surfactant dissolved in brine
