Investigation on arsenopyrite dissolution and As (III) migration under geologic carbon storage conditions: A numerical simulation approach

Geologic carbon 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 heavy metals, including arsenic (As), from their parent minerals in the subsurface because of induced dissolution due to carbon dioxide injection, remains a concern. In this study, A TOUGHREACT model was developed to investigate the potential of arsenopyrite dissolution in a deep arsenopyrite-rich formation in the presence of Fe(III)-bearing minerals under geologic carbon storage conditions (a CO₂ injection rate of 0.1 MMT/year, an average reservoir temperature of 50°C and an average reservoir pressure of 18.7 MPa). The model shows that after injection of CO₂, pH decreased as a result of CO₂ dissolution, which led to the release of Fe³⁺from Fe(III)-bearing minerals. The oxidative Fe³⁺ released from the dissolution of Fe(III) bearing minerals caused dissolution of arsenopyrite and release of As(III). Therefore, dissolution of arsenopyrite is possible if Fe(III)-bearing minerals are present at or close to the arsenopyrite-rich formation. The model also simulated the rate of As(III) migration from the arsenopyrite-rich formation to a shallow aquifer above the formation. 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 As-rich layer at t = 133 days through the borehole given a CO₂ injection rate of 0.1 MMT/yr (3.17 kg/s), 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 observations suggest that an aquifer, 1810 m above the As-rich layer as designed in model simulations, has the potential to be impacted by As contamination if an As-rich layer is present at or close to the CO₂ injection interval. It is important to note that this study investigates a worst-case scenario (from the perspectives of both site selection and abundance of arsenopyrite) and the results should not be interpreted as evidences making subsurface CO₂ storage projects unfeasible.