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

In modeling of geological carbon storage, dissolution of supercritical CO2 (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 degrees 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 CO2 (dsCO(2)) in water effluent varies from 0.38% to 2.72% of CO2 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. (C) 2016 Elsevier Ltd. All rights reserved.