Effect of Formation Heterogeneity on CO₂ Dissolution in Subsurface Porous Media

Dissolution trapping is one of the most dominant mechanisms for the secure storage of CO2 injected in porous subsurface formations saturated with brine. This trapping mechanism is enhanced by convective mixing, which occurs due to the gravitational instability between the different fluid layers in the aquifer. The reservoir permeability also plays a crucial role in the dissolution rate and overall fluid flow dynamics during the density-driven convection in porous media. This study investigates the role of complex heterogeneity, i.e., irregular permeability distribution in CO2 dissolution, using a novel experimental approach to create medium permeability heterogeneity in Hele-Shaw cells. Complex subsurface transport phenomena such as a preferential dissolution path, CO2 sweep efficiency, changes in finger morphology, and CO2 concentration distribution are visualized by creating heterogeneous media. Experimental results showed that reservoir permeability heterogeneity causes significant channeling effects and poor sweep efficiency. A scaling relationship between average finger growth rate (Gr) and permeability (k) was obtained as Gr [m s(-1)] = 266.8k [m(2)] + 1.20 x 10(-6). Furthermore, the mass of CO2 dissolved is calculated using the spectrophotometric method to characterize the convective instability. The convective flux was analyzed by comparing the experimental dissolution flux with the theoretical diffusion flux, calculating a maximum Sherwood number of 6.8. The study's findings improve the current understanding of the CO2 convection morphology in heterogeneous media, allowing better assessment of long-term CO2 storage.