Experimental study on effects of geologic heterogeneity in enhancing dissolution trapping of supercritical CO₂

Abstract

Dissolution trapping is one of the primary mechanisms that enhance the storage security of supercritical carbon dioxide (scCO 2 ) in saline geologic formations. When scCO 2 dissolves in formation brine produces an aqueous solution that is denser than formation brine, which leads to convective mixing driven by gravitational instabilities. Convective mixing can enhance the dissolution of CO 2 and thus it can contribute to stable trapping of dissolved CO 2 . However, in the presence of geologic heterogeneities, diffusive mixing may also contribute to dissolution trapping. The effects of heterogeneity on mixing and its contribution to stable trapping are not well understood. The goal of this experimental study is to investigate the effects of geologic heterogeneity on mixing and stable trapping of dissolved CO 2 . Homogeneous and heterogeneous media experiments were conducted in a two-dimensional test tank with various packing configurations using surrogates for scCO 2 (water) and brine (propylene glycol) under ambient pressure and temperature conditions. The results show that the density-driven flow in heterogeneous formations may not always cause significant convective mixing especially in layered systems containing low-permeability zones. In homogeneous formations, density-driven fingering enhances both storage in the deeper parts of the formation and contact between the host rock and dissolved CO 2 for the potential mineralization. On the other hand, for layered systems, dissolved CO 2 becomes immobilized in low-permeability zones with lowdiffusion rates, which reduces the risk of leakage through any fault or fracture. Both cases contribute to the permanence of the dissolved plume in the formation.

Critical Dynamics of Gravito-Convective Mixing in Geological Carbon Sequestration

“…(7)). Fast mixing and spreading towards higher depths improve the long-term storage of dissolved CO 2 throughout the aquifer6, because CO 2 is likely to remain in aquifer due to residual and mineral trapping even if the pressure were to drop due to a failure of the cap rock.…”

“…Geological sequestration of carbon dioxide (CO 2 ) has been proposed as a technology to reduce the amount of CO 2 emitted into the atmosphere123456. The main processes controlling the trapping of CO 2 during geological sequestration are storage of supercritical CO 2 in a gas cap, CO 2 dissolution in brine, known as solubility trapping78, residual trapping due to hysteresis910, and mineral trapping by CO 2 precipitation as secondary carbonates11.…”

“…These studies concluded that permeability heterogeneity could have a significant impact on the onset, rate, and characteristic behavior of density-driven flow of CO 2 . On the other hand, theoretical26, numerical27, and experimental6 results suggest that density-driven flow may not always lead to significant convective mixing especially in layered systems containing low-permeability zones; CO 2 may actually become immobilized in these zones. In order to quantify the effects of vertical heterogeneity, Green and Ennis-King28 developed a simple model consisting of a random and uncorrelated distribution of horizontal, impermeable barriers with a given overall volume fraction (0.04% and 0.15%).…”

Pore-scale supercritical CO2 dissolution and mass transfer under drainage conditions

“…Local scCO 2 dissolution may also affect enhanced dissolution by convection. So far, all studies of convection-enhanced dissolution have assumed the presence of a boundary layer of dsCO 2 at solubility at the top of study domain in numerical simulations (e.g., Ennis-King and Paterson, 2005;Riaz et al, 2006;Pau et al, 2010) or have introduced gaseous CO 2 or an analogue fluid into an aperture in 2D Hele-Shaw cells or packed sands or glass beads in 1D columns or 2D sand tanks (Kneafsey and Pruess, 2010;Agartan et al, 2015). The non-equilibrium dissolution may result in dsCO 2 concentration in the boundary layer that slowly increases with time before reaching solubility, leading to the non-instantaneous dissolution at certain spatial scales.…”

“…The non-equilibrium dissolution may result in dsCO 2 concentration in the boundary layer that slowly increases with time before reaching solubility, leading to the non-instantaneous dissolution at certain spatial scales. This slow process will affect the development of a stable, diffusion-dominant boundary layer of dsCO 2 -containing brine, the onset and initialization of unstable fingers of dsCO 2 originating from the boundary layer, and the evolution, merging, and decay of these fingers in both homogeneous and heterogeneous porous media (Ennis-King and Paterson, 2003;Kneafsey and Pruess, 2010;Neufeld et al, 2010;Backhaus et al, 2011;Aggelopoulos and Tsakiroglou, 2012;MacMinn et al, 2012;Agartan et al, 2015).…”