Supercritical CO2 Dissolution and Mass Transfer in a Heterogeneous Pore Network under Drainage and Imbibition Conditions

Abstract:

Dissolution trapping of supercritical CO₂ (scCO₂) is usually modeled by assuming instantaneous scCO₂ dissolution and equilibrium phase partitioning. Our recent core-scale imbibition experiments show a prolonged depletion of residual scCO₂ by dissolution, implying a non-equilibrium mechanism. In our 2D sandstone-analogue micromodel experimental study, pore-scale scCO₂ dissolution was inferred from imaging (1) drainage using a pH-sensitive water dye and (2) imbibition using a scCO₂ dye. The drainage experiment was conducted by injecting scCO₂ into the dissolved-CO₂ (dsCO₂)-free water-filled pore network. The time-lapse images of non-uniform dye intensities indicating varying pH show that dsCO₂ concentration varies from zero to solubility in individual pores and pore clusters and the average concentration gradually increases with time. The different rates of dissolution in different pores/clusters can be attributed to (1) fast scCO₂ dissolution at scCO₂-water interfaces, (2) rate-limited mass transfer due to limited interface areas, and (3) a transition from rate-limited to diffusion-limited mass transfer, revealed by detailed analysis on selected pores and pore clusters. The imbibition experiments conducted by injecting deionized water at different rates show (1) water flow in channels bypassing scCO₂ at high residual saturations, (2) subsequent, slow scCO₂ depletion by dissolution and mass transfer as effluent dsCO₂ concentration varies from 0.06% to 4.44% of solubility, and (3) creation of new water flow paths by dissolution, enhancing scCO₂ depletion by coupled displacement and dissolution. Both the drainage and imbibitions experiments indicate non-equilibrium scCO₂ dissolution in the centimeter-scale pore network over 4.5 hours and up to 14 hours, respectively. The pore-scale imaging can help better understand the effects of pore-throat characteristics on scCO₂ dissolution and mass transfer during drainage and imbibition and the interplay between displacement and dissolution.