Using a Well-Controlled Heterogeneous Permeability Field to Study Its Role on Miscible Density-Driven Convection in Porous Media and the Implications to Geological Carbon Sequestration

In this work, we report on a new experimental method to construct well-controlled heterogeneous permeability fields to study miscible density-driven convection in quasi-2D porous media, which has implications for geological carbon sequestration (GCS) in saline aqui-fers. Dissolution trapping is a critical CO₂ trapping mechanism in GCS, which can be accelerated by miscible density-driven convection. Previous studies on the role of permeability heterogeneity on miscible density-driven convection relied heavily on numerical simulation due to the challenges encountered by conventional experimental methods in the construction of a well-controlled heterogeneous permeability field. To solve this challenge, we developed a 3D-printing-assisted method to construct well-controlled heterogeneous permeability fields. In particular, the borders between different permeability regions in a sand box were extracted with digital image processing and then 3D printed. Silica sands having desired permeability values were placed into the corresponding space in the 3D-printed frame to construct the heterogeneous permeability field in the sand box. Unlike conventional experimental methods that rely on stacking cubic sediment blocks, the 3D-printing-assisted method can print the boundaries between different permeability regions with arbitrary lengths and directions, thereby enabling the construction of smooth boundaries between permeability regions, eliminating preferential flow paths in the orthotropic directions, and ensuring the isotropic properties of the constructed permeability field. We further studied the impact of permeability heterogeneity, characterized by the permeability’s coefficient of variation (COV) and spatial correlation length, on miscible density-driven convection, providing the first laboratory evidence for the role of different combinations of permeability’s COV and spatial correlation length in regulating convective dissolution by the fingering and channeling mechanisms. A high-permeability COV combined with a large spatial correlation length of permeability increases the length scale of the branching at the penetration front and the uncertainty on the dissolution mass flux across the top boundary. More CO₂ can be stored in a moderately heterogeneous saline aquifer by convective dissolution than in a homogeneous saline aquifer during the same period of time after the onset of convective dissolution, which is caused by the higher average mass flux in the heterogeneous aquifer. However, the permeability heterogeneity does not further enhance CO₂ storage when the saline aquifer is highly heterogeneous. With this work, we also extend the empirical correlation between the Sherwood number and Rayleigh number from homogeneous permeability fields to heterogeneous permeability fields.