Intermediate-Scale Experimental and Numerical Study of Multiphase CO2 Attenuation in Layered Shallow Aquifers During Leakage from Geologic Sequestration Site

Abstract:

In order to effectively predict and mitigate the potential risks from leakage of stored CO2, we must first understand the physicochemical processes that CO₂ undergoes during migration through shallow aquifers, including dissolved phase advection and dispersion as well as gas phase exsolution, multiphase flow, and dissolution. Since field sites are inherently large-scale, heterogeneous, 3-D systems, large-scale experimental data is important to validate numerical models and to make confident predictions regarding CO2 migration.

A large, highly instrumented, two-dimensional tank was built and packed with porous media to represent a portion of a layered shallow aquifer. Flow of water across the tank was established by applying a small difference in head between the two ends. A separate stream of water was then saturated with dissolved CO₂ and injected into the bottom of the tank near the upstream end. Various saturation sensors measured the spatiotemporal pattern of gas phase evolution in the tank, while an external sensor and an Ion Chromatograph were used to monitor the dissolved CO₂ concentrations at various locations in the system. The top of the tank was baffled into four sealed compartments, each of which was connected to a gas flow meter to monitor the spatiotemporal pattern of gas phase CO₂ release to the atmosphere.

Numerical simulations were also performed to better understand the fundamental physics that drove the observed CO₂ evolution processes, and to help validate a widely used code using the experimental data. The simulations were performed with the Finite Element Heat and Mass Transfer (FEHM) software that was developed at Los Alamos National Laboratory. The model domain, porous media properties, and initial conditions were set up to match those of the experiment, and the boundary conditions were adjusted to investigate the mass transfer between the dissolved and gaseous phases of CO₂ that developed within the system.

Results from both the experiments and the simulations indicate that the layer of fine material prevented any release of gas phase CO₂ to the atmosphere. In addition, exsolved gas phase CO₂ accumulated beneath the fine layer and slowly dissolved into the clean water that came from the upstream end of the tank. This led to significant attenuation of dissolved CO₂ migration throughout the system.