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

Abstract

In modeling of geological carbon storage, dissolution of supercritical CO 2 (scCO) is often assumed to be instantaneous with equilibrium phase partitioning. In contrast, recent corescale 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-analogue 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 timelapse 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 3 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.

Characterizing the Dissolution Rate of CO2-Brine in Porous Media under Gaseous and Supercritical Conditions

“…With similar core characteristics, the injection flow rate was much lower than the velocities in the dissolution experiment in the literature [9]. Therefore, the dissolution mechanism dominated during the brine imbibition and CO 2 gradually dissolved in the pore space as the brine injection continued.…”

“…However, characteristics of flow development indicated that brine tended to avoid the gaseous CO 2 clusters, implying lower dissolution rates and less probability of dissolution events. In the previous study, imbibition of unsaturated brine was more focused and had a lower sweep efficiency [9]. Taking the efficiency into account, the comparison of our results indicates that the supercritical CO 2 demonstrates its superiority to improve the trapping efficiency of CO 2 sequestration during the imbibition.…”

“…When the brine filled the upstream pores, some of the downstream bubbles happened to snap-off, and the detached part then rapidly dissolved (shown in Figure 2 R6 and R7). The flow development and the diffusion and the coupled water flow and dissolution at the pore scale enhanced the dissolution rate at the pore-network scale, shortening the total depletion time of CO 2 [9,27]. After the major CO 2 bulk dissolved, the remaining CO 2 bubbles were trapped in the unconnected pores.…”

“…In previous studies, the dissolution process was widely investigated between the non-wetting phase (air/non-aqueous phase liquid) and the nearby wetting phase (water/brine) in porous media. For the observation focused on a single pore or a cluster and connected pore throats, the dissolution is dominated by molecular diffusion and mass transfer at the CO 2 -water interfacial area [9]. Besides, in a 2-D model, the observed convection and dissolution rates are mainly an effect of dispersion due to the transport velocity of the solvent phase.…”

Intermediate‐Scale Experimental Study to Improve Fundamental Understanding of Attenuation Capacity for Leaking CO2 in Heterogeneous Shallow Aquifers

“…More recently, these findings have been applied to the field of GCS. For instance, Chang et al (2016) showed that the depletion of supercritical CO 2 occurred more slowly when the CO 2 saturation was higher, but that the dependence of the mass transfer rate on saturation was not necessarily monotonic. Likewise, Jiang et al (2017) showed that the rate of CO 2 gas dissolution into brine was slow when gas saturation was high, then the rate increased as gas saturation decreased, and then decreased again as the last of the gas disappeared.…”