Abstract
This study simulated the natural convection of dissolved carbon dioxide (CO_2) in a small-scale heterogeneous saline formation using the state module ECO2N equation in the TOUGHREACT model. A one-way downscaling approach that involves using a series of sub-models in simulation procedures was proposed to efficiently simulate problems with high-scale discrepancies. This study evaluated the effects of different degrees of small-scale permeability variations on the vertical migration of dissolved CO_2. The sequential Gaussian simulation model was used to generate unconditional random permeability fields for different natural logarithm of permeability (lnk) variations (i.e., lnk variances and correlations in x and z directions). The results showed an identical transition zone of dissolved CO_2 near the top boundary, where a constant CO_2 gas saturation was specified. The local permeability variations can trigger fingerings and enhance the vertical convection of the dissolved CO_2. The number of fingerings depends on the variations in permeability near the front interface of the dissolved CO_2 (i.e., the bottom edge of the transition zone for the dissolved CO_2). However, the fingering patterns and developments are constrained by the permeability variations along the fingering paths. At the same mean lnk permeability the convection fluxes increase with an increase in lnk variances. However, an increase in lateral correlations (i.e., increase in the correlation lengths in the x direction) can slightly reduce the convection fluxes at the same lnk variance. The highly variable flux rates of the dissolved CO_2 occur early and the variations in the flux rate decrease with time.
Highlights
Carbon dioxide (CO2) geological sequestration in deep saline formations is a feasible approach for mitigating CO2 emissions
This study focused on exploring the small-scale physical phenomena involved in dissolved CO2 transport such as fingering, channeling and lateral mixing for anisotropic and heterogeneous saline formations
When the statistical structure of the random field (RF) was applied to Cases 8 and 9 (Table 2), the fingering development was significant in Case 8 compared with the fingerings in Cases 2 and 9
Summary
Carbon dioxide (CO2) geological sequestration in deep saline formations is a feasible approach for mitigating CO2 emissions. Hassanzadeh et al (2007) performed a linear stability analysis with scaling behavior to examine the onset of natural convection in a homogeneous conceptual frame. Pau et al (2010) developed a numerical model for analyzing the instability of density-driven convection during CO2 sequestration in homogeneous media. Natural geological formations typically involve different degrees of heterogeneity in material permeability, and such variations can considerably influence density-driven natural convection (Royer and Flores 1994; Flett et al 2007; Lindeberg and Wessel-Berg 2011). Ranganathan et al (2012) conducted numerical simulations to assess natural CO2 convective processes in heterogeneous media. Different simulation conditions such as domain sizes, boundary conditions and random permeability field distributions were applied in their study. This downscaling approximation is expected to physically connect small-scale CO2 natural convection simulations to the large-scale model
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