Assessment of CO2-soluble non-ionic surfactants for mobility reduction using mobility measurements and CT imaging

Abstract The addition of CO 2 -soluble, brine-soluble surfactants to the high pressure CO 2 can facilitate the in-situ generation of CO 2 -in-brine foams for conformance and/or mobility control. These non-ionic surfactants dissolve in CO 2 to concentrations of roughly 0.02–0.10 wt% at typical CO 2 enhanced oil recovery (EOR) conditions and, upon mixing with brine in a closed, agitated, windowed vessel, stabilize CO 2 -in-brine foams. Branched nonylphenol ethoxylates containing an average of 12 (Huntsman SURFONIC ® N-120) or 15 (Huntsman SURFONIC ® N-150) ethylene oxide (EO) repeat units, and a branched tridecyl alcohol ethoxylate with 9 EO repeat units (Huntsman SURFONIC ® TDA-9) are selected for the mobility and computed tomography (CT) studies detailed in this paper. These foam-stabilizing surfactants are much more brine-soluble than CO 2 -soluble, in accordance with the Bancroft rule for generating CO 2 -in-brine foam. Transient mobility measurements are conducted using several mixed wettability SACROC carbonate cores of low permeability (13–16 mD), and a high permeability water-wet Bentheimer sandstone core (1550 mD). The CO 2 is injected into a brine-saturated core at a constant rate, yielding superficial velocities of 60.96 cm/day or 304.8 cm/day. Surfactant was either not used, dissolved only in CO 2 , only in brine, or in both brine and CO 2 . The surfactant concentration is ~0.07 wt% in the CO 2 (the maximum concentration capable of dissolving in CO 2 ) or in the brine. The transient differential pressure drop during the injection of three pore volumes of CO 2 into the core indicate that the average total pressure drop across the core during the experiment increases by an average of 25–120% when the surfactant is dissolved in the CO 2 , 79–300% when the surfactant is dissolved in the brine, and 220–330% if surfactant is present in both the brine and CO 2 . These results indicate that the greatest mobility reduction is achieved with the surfactant in both brine and CO 2 , and the foams that are generated with surfactant dissolved in the brine alone tend to provide greater mobility reduction than when the surfactant is dissolved only in CO 2 . CT scanning of in-situ foam generation is conducted by injecting high pressure CO 2 into a 5 wt% KI brine-saturated water-wet Berea sandstone (4–8 mD). Tests are performed with no surfactant, surfactant dissolved in brine at 0.03 wt%, in CO 2 at 0.07 wt%, or in both brine and CO 2 . CT images indicate that in the absence of surfactant, sweep efficiency is very low primarily because CO 2 tends to flow through high permeability bedding planes. The use of CO 2 -soluble surfactants to form CO 2 -in-brine foam within a sandstone core is verified via CT imaging. At low and high superficial velocity values of 14.33–143.3 cm/day, in-situ foam generation and propagation, as indicated by piston-like flow of the CO 2 through the core, is most evident when surfactant was dissolved in the brine. While there is some evidence of foam formation when Huntsman SURFONIC ® N-120 or Huntsman SURFONIC ® N-150 is present in the CO 2 , very distinct foam formation and propagation occurs when Huntsman SURFONIC ® TDA-9 is dissolved in CO 2 .