Permeability heterogeneity effects on density-driven CO2 natural convection and carbon sequestration efficiency

Underground storage of CO2 in saline aquifers has emerged as a promising strategy for mitigating global warming. The dissolution of CO2 in saline aquifers induces the descent of high-density CO2-saturated brine to the reservoir bottom, driven by gravity-induced natural convection. The manifestation of this convection is linked to the heterogeneity of the reservoir. We thus analyzed the impact of heterogeneous permeability distribution, which represent various geological structures, on the CO2 plume morphology and CO2 storage efficiency. In a homogeneous high-permeability core, high convection rate resulting in wider CO2 fingers. For heterogeneous cores, the convective mixing rate depends on average permeability, while the morphology of fingers is determined by local permeability. Abrupt permeability changes (from low permeability to high permeability) led to CO2 finger channeling, resulting in reduced sweep efficiency. In heterogeneous layered core, the permeability of the upper layer dominated convection, while vertically decreasing permeability (reversed-rhythm) was more conducive to the dissolution capture of CO2. However, increasing the number of rhythm layer, the larger permeability distribution range and the number of vertical bedding layers enhanced the convective instability, which was unconducive to efficient CO2 sequestration. Besides, a power relationship between the Rayleigh number and the Sherwood number was identified. The power-law exponent b = 1.07 was higher than the value (b≈0.984) obtained by two-dimensional plate experiments. In conclusion, reversed-rhythm reservoirs with high permeability exhibit a preference for CO2 dissolution trapping. Our study provides detailed insights into the processes of CO2 dissolution for heterogenous reservoir. The results should further facilitate the extensive implementation of Carbon Capture and Storage projects.