Pore-scale supercritical CO2 dissolution and mass transfer under imbibition conditions

Abstract In modeling of geological carbon storage, dissolution of supercritical CO 2 (scCO 2 ) is often assumed to be instantaneous with equilibrium phase partitioning. In contrast, recent core-scale imbibition experiments have shown a prolonged depletion of residual scCO 2 by dissolution, implying a non-equilibrium mechanism. In this study, eight pore-scale scCO 2 dissolution experiments in a 2D heterogeneous, sandstone-analog micromodel were conducted at supercritical conditions (9 MPa and 40 °C). The micromodel was first saturated with deionized (DI) water and drained by injecting scCO 2 to establish a stable scCO 2 saturation. DI water was then injected at constant flow rates after scCO 2 drainage was completed. High resolution time-lapse images of scCO 2 and water distributions were obtained during imbibition and dissolution, aided by a scCO 2 -soluble fluorescent dye introduced with scCO 2 during drainage. These images were used to estimate scCO 2 saturations and scCO 2 depletion 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 scCO 2 by dissolution. The average mass fraction of dissolved CO 2 (dsCO 2 ) in water effluent varies from 0.38% to 2.72% of CO 2 solubility, indicating non-equilibrium scCO 2 dissolution in the millimeter-scale pore network. In general, the transient depletion rate decreases as trapped, discontinuous scCO 2 bubbles and clusters within water-flow paths dissolve, then remains low with dissolution of large bypassed scCO 2 clusters at their interfaces with longitudinal water flow, and finally increases with coupled transverse water flow and enhanced dissolution of large scCO 2 clusters. The three stages of scCO 2 depletion, common to experiments with time-independent water-flow paths, are revealed by zoom-in image analysis of individual scCO 2 bubbles and clusters. The measured relative permeability of water, affected by scCO 2 dissolution 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 scCO 2 dissolution becomes less important when water flow is relatively low and the time scale for dissolution is large, and more pronounced when heterogeneity is strong.