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Open Access Publications from the University of California

Pore-scale supercritical CO2dissolution and mass transfer under imbibition conditions

  • Author(s): Chang, C
  • Zhou, Q
  • Kneafsey, TJ
  • Oostrom, M
  • Wietsma, TW
  • Yu, Q
  • et al.

© 2016 Elsevier Ltd. In modeling of geological carbon storage, dissolution of supercritical CO2(scCO2) is often assumed to be instantaneous with equilibrium phase partitioning. In contrast, recent core-scale imbibition experiments have shown a prolonged depletion of residual scCO2by dissolution, implying a non-equilibrium mechanism. In this study, eight pore-scale scCO2dissolution experiments in a 2D heterogeneous, sandstone-analog micromodel were conducted at supercritical conditions (9MPa and 40°C). The micromodel was first saturated with deionized (DI) water and drained by injecting scCO2to establish a stable scCO2saturation. DI water was then injected at constant flow rates after scCO2drainage was completed. High resolution time-lapse images of scCO2and water distributions were obtained during imbibition and dissolution, aided by a scCO2-soluble fluorescent dye introduced with scCO2during drainage. These images were used to estimate scCO2saturations and scCO2depletion rates. Experimental results show that (1) a time-independent, varying number of water-flow channels are created during imbibition and later dominant dissolution by the random nature of water flow at the micromodel inlet, and (2) a time-dependent number of water-flow channels are created by coupled imbibition and dissolution following completion of dominant imbibition. The number of water-flow paths, constant or transient in nature, greatly affects the overall depletion rate of scCO2by dissolution. The average mass fraction of dissolved CO2(dsCO2) in water effluent varies from 0.38% to 2.72% of CO2solubility, indicating non-equilibrium scCO2dissolution in the millimeter-scale pore network. In general, the transient depletion rate decreases as trapped, discontinuous scCO2bubbles and clusters within water-flow paths dissolve, then remains low with dissolution of large bypassed scCO2clusters at their interfaces with longitudinal water flow, and finally increases with coupled transverse water flow and enhanced dissolution of large scCO2clusters. The three stages of scCO2depletion, common to experiments with time-independent water-flow paths, are revealed by zoom-in image analysis of individual scCO2bubbles and clusters. The measured relative permeability of water, affected by scCO2dissolution and bi-modal permeability, shows a non-monotonic dependence on saturation. The results for experiments with different injection rates imply that the non-equilibrium nature of scCO2dissolution becomes less important when water flow is relatively low and the time scale for dissolution is large, and more pronounced when heterogeneity is strong.

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