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

Abstract

The addition of CO2-soluble, brine-soluble surfactants to the high pressure CO2 can facilitate the in-situ generation of CO2-in-brine foams for conformance and/or mobility control. These non-ionic surfactants dissolve in CO2 to concentrations of roughly 0.02–0.10 wt% at typical CO2 enhanced oil recovery (EOR) conditions and, upon mixing with brine in a closed, agitated, windowed vessel, stabilize CO2-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 CO2-soluble, in accordance with the Bancroft rule for generating CO2-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 CO2 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 CO2, only in brine, or in both brine and CO2. The surfactant concentration is ~0.07 wt% in the CO2 (the maximum concentration capable of dissolving in CO2) or in the brine. The transient differential pressure drop during the injection of three pore volumes of CO2 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 CO2, 79–300% when the surfactant is dissolved in the brine, and 220–330% if surfactant is present in both the brine and CO2. These results indicate that the greatest mobility reduction is achieved with the surfactant in both brine and CO2, 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 CO2. CT scanning of in-situ foam generation is conducted by injecting high pressure CO2 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 CO2 at 0.07 wt%, or in both brine and CO2. CT images indicate that in the absence of surfactant, sweep efficiency is very low primarily because CO2 tends to flow through high permeability bedding planes. The use of CO2-soluble surfactants to form CO2-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 CO2 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 CO2, very distinct foam formation and propagation occurs when Huntsman SURFONIC® TDA-9 is dissolved in CO2.