Assessment of CO sub(2)-soluble non-ionic surfactants for mobility reduction using mobility measurements and CT imaging

The addition of CO sub(2)-soluble, brine-soluble surfactants to the high pressure CO sub(2) can facilitate the in-situ generation of CO sub(2)-in-brine foams for conformance and/or mobility control. These non-ionic surfactants dissolve in CO sub(2) to concentrations of roughly 0.02-0.10 wt% at typical CO sub(2) enhanced oil recovery (EOR) conditions and, upon mixing with brine in a closed, agitated, windowed vessel, stabilize CO sub(2)-in-brine foams. Branched nonylphenol ethoxylates containing an average of 12 (Huntsman SURFONIC super( registered ) N-120) or 15 (Huntsman SURFONIC super( registered ) N-150) ethylene oxide (EO) repeat units, and a branched tridecyl alcohol ethoxylate with 9 EO repeat units (Huntsman SURFONIC super( registered ) 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 sub(2)-soluble, in accordance with the Bancroft rule for generating CO sub(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 sub(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 sub(2), only in brine, or in both brine and CO sub(2). The surfactant concentration is ~0.07 wt% in the CO sub(2) (the maximum concentration capable of dissolving in CO sub(2)) or in the brine. The transient differential pressure drop during the injection of three pore volumes of CO sub(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 sub(2), 79-300% when the surfactant is dissolved in the brine, and 220-330% if surfactant is present in both the brine and CO sub(2). These results indicate that the greatest mobility reduction is achieved with the surfactant in both brine and CO sub(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 sub(2). CT scanning of in-situ foam generation is conducted by injecting high pressure CO sub(2) into a 5 wt% KI brine-saturated water-wet Berea sandstone (4-8 mD). Tests are performed with no sur