Unstable Density‐Driven Convection of CO₂ in Homogeneous and Heterogeneous Porous Media With Implications for Deep Saline Aquifers

Abstract

The capture of CO 2 from fossil fuel power plants and storage in deep saline aquifers has been the focus of greenhouse gas emissions mitigation. Considerable progress has been achieved in understanding CO 2 migration during storage process worldwide (Blanco et al., 2012). Large-scale CO 2 capture and storage (CCS) projects, such as the Sleipner CO 2 storage project in Norway and the Gorgon CO 2 injection project in Australia, have been constructed or are in operation (Viebahn et al., 2014). Deep saline aquifers have enormous storage potential and are economically favorable; hence, properly selecting storage sites and predicting geo-sequestration security are necessary. Stored CO 2 migrates upward and accumulates beneath a cap rock, and complex flows and reactions will occur in the porous media during this process (Fredd & Fogler, 1998; Hoefner & Fogler, 1988). Over time, a high density CO 2 solution will migrate downward, referred to as "solubility trapping" (Al-Khdheeawi et al., 2018; Boot-Handford et al., 2014). This density-driven convection can enhance CO 2 storage and reduce the risk of CO 2 leakage (Jiang et al., 2019). Fluid dissolution in porous media was rarely considered in studies conducted before the 2000s. In recent years, heterogeneous reservoirs were reported to attain higher dissolution convection mixing velocities with higher CO 2 storage capacities than homogeneous reservoirs (Hou et al., 2012; Islam & Sun, 2015; Lv et al., 2017b; Tanino & Blunt, 2012). Notably, natural formations tend to be heterogeneous or exhibit fractured zones, which can trigger viscous flow instabilities during multifluid displacement (Macminn et al., 2012). The rate of CO 2 mass transfer is closely related to the formation morphology, especially the distributions of the formation porosity and permeability (Kong & Saar, 2013). Aronne et al. (Dell'Oca et al., 2018) investigated the effect of buoyancy and permeability heterogeneity on solute dispersion and found that spatial redistribution of the velocity field can reduce the spread of high solute concentrations and event contract these areas. In some experimental works, convection velocity mapping analysis was conducted by magnetic resonance imaging (MRI) (Darbouli et al., 2016; Shattuck et al., 1995), and the onset morphology changed with increasing fluid concentration. As a nondestructive Abstract Density-driven convection mixing has been identified to enhance CO 2 trapping in deep saline aquifers storage. In this study, the dynamics of fingering instability and associated mass transfer caused by convection mixing between the movements of miscible analogue fluid pairs are investigated by magnetic resonance imaging (MRI). Two kinds of homogeneous porous media and six kinds of heterogeneous porous media with varying permeability permutations were used at ambient pressure and temperature. In homogeneous porous media, the detail analyses on the data of fingering pattern, finger number and distribution identify that the large finger sizes and high sinking velocities can be dev...

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