A NUMERICAL SIMULATION STUDY OF SUBSURFACE ARSENOPYRITE DISSOLUTION AND ARSENIC MIGRATION UNDER GEOLOGIC CARBON STORAGE CONDITIONS

Geologic carbon storage (GCS) is the process to store large amount of carbon dioxide (CO₂) captured from stationary CO₂ emission sources in deep subsurface for permanent storage. GCS is widely recognized as a promising strategy to reduce the emissions of greenhouse gas (GHG) to the atmosphere. However, the potential for mobilization of arsenic (As) from their parent minerals in deep subsurface because of induced dissolution due to CO₂ injection, remains a concern. In this study, A TOUGHREACT model was developed to investigate the potential of arsenopyrite (FeAsS) dissolution in a hypothetical deep saline aquifer rich in arsenopyrite in the presence of Fe(III)-bearing minerals under geologic carbon storage conditions. An average reservoir temperature of 50°C, an average reservoir pressure of 18.7 MPa and a CO₂ injection rate of 0.1 MMT/year were used in this study. Numerical simulation results show that pH decreased as a result of CO₂ dissolution after injection of CO₂, which led to release of Fe³⁺from Fe(III)-bearing minerals. The oxidative Fe³⁺ caused dissolution of arsenopyrite and release of As(III) to brine in the saline aquifer. The model also simulated the rate of As(III) migration from the saline aquifer to a shallow aquifer above the saline aquifer. The As(III) migrated toward the shallow aquifer through a permeable borehole 23 m away from the CO₂ injector. Model simulations show that the As(III) contamination front migrated to 182 m above the saline aquifer at t = 133 days through the borehole, which corresponds to an average migration rate of 1.37 m/day. Model simulations also show that the rate of As(III) contamination front migration was not significantly affected by borehole permeability change. Those results suggest that a shallow aquifer, 1810 m above the shallow aquifer as designed in model simulations, has the potential to be impacted by As (III) contamination if an As-rich layer is present at or close to the CO₂ injection interval, and oxidative species like Fe (III) and oxygen are present. This study investigates a worst-case scenario (from the perspectives of both site configuration and abundance of arsenopyrite) and the results should not be interpreted as evidences making subsurface CO₂ storage projects unfeasible.