Abstract
Storage of anthropogenic CO2 in geological formations relies on a caprock as the primary seal preventing buoyant super-critical CO2 escaping. Although natural CO2 reservoirs demonstrate that CO2 may be stored safely for millions of years, uncertainty remains in predicting how caprocks will react with CO2-bearing brines. This uncertainty poses a significant challenge to the risk assessment of geological carbon storage. Here we describe mineral reaction fronts in a CO2 reservoir-caprock system exposed to CO2 over a timescale comparable with that needed for geological carbon storage. The propagation of the reaction front is retarded by redox-sensitive mineral dissolution reactions and carbonate precipitation, which reduces its penetration into the caprock to ∼7 cm in ∼105 years. This distance is an order-of-magnitude smaller than previous predictions. The results attest to the significance of transport-limited reactions to the long-term integrity of sealing behaviour in caprocks exposed to CO2.
Highlights
Storage of anthropogenic CO2 in geological formations relies on a caprock as the primary seal preventing buoyant super-critical CO2 escaping
Fundamental uncertainties in the prediction of coupled reactive transport in low permeability clay-rich caprocks include a poor understanding of the relationship between reaction-induced changes in porosity and the pore-network structure, and the consequences for the rates of solute transport[18], the uncertain mineral surface areas and kinetics of the mineral reactions in natural settings[19], the rates of which may be limited by the intrinsic surface reaction rates of the minerals or by rates of solute transport[20], and the reaction pathways in natural settings, including the role of acid redox reactions as a CO2 sink[21], which will retard the rates of diffusive CO2 transport
U-Th geochronology of carbonate veining in associated faults attests to CO2 out-gassing regionally for ca. 400 ka and locally for at least 114 ka, providing an independent constraint of the duration over which the Carmel Formation caprock has been exposed to CO2-charged brines[25]
Summary
Storage of anthropogenic CO2 in geological formations relies on a caprock as the primary seal preventing buoyant super-critical CO2 escaping. Fundamental uncertainties in the prediction of coupled reactive transport in low permeability clay-rich caprocks include a poor understanding of the relationship between reaction-induced changes in porosity and the pore-network structure, and the consequences for the rates of solute transport[18], the uncertain mineral surface areas and kinetics of the mineral reactions in natural settings[19], the rates of which may be limited by the intrinsic surface reaction rates of the minerals or by rates of solute transport[20], and the reaction pathways in natural settings, including the role of acid redox reactions as a CO2 sink[21], which will retard the rates of diffusive CO2 transport We address this fundamental gap in our knowledge of the long-term impacts of CO2-charged fluids on caprock integrity by examining caprock recovered from a natural CO2 reservoir. The results attest to the significance of transport-limited reactions to the long-term integrity of sealing behaviour in caprocks exposed to CO2
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