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Abstract: Basaltic rocks are potential repositories for sequestering carbon dioxide (CO2) because of their capacity for trapping CO2 in carbonate minerals. We carried out a series of thermodynamic equilibrium models and high pressure experiments, reacting basalt with CO2-charged fluids over a range of conditions from 50 to 200°C at 300bar. Results indicate basalt has a high reactivity to CO2 acidified brine. Carbon dioxide is taken up from solution at all temperatures from 50 to 200°C, 300bar, but the maximum extent and rate of reaction occurs at 100°C, 300bar. Reaction path simulations utilizing the geochemical modeling program CHILLER predicted an equilibrium carbonate alteration assemblage of calcite, magnesite, and siderite, but the only secondary carbonate identified in the experiments was a ferroan magnesite. The amount of uptake at 100°C, 300bar ranged from 8% by weight for a typical tholeite to 26% for a picrite. The actual amount of CO2 uptake and extent of rock alteration coincides directly with the magnesium content of the rock suggesting that overall reaction extent is controlled by bulk basalt Mg content. In terms of sequestering CO2, an average basaltic MgO content of 8% is equivalent to 2.6×108 metrictonCO2/km3 basalt. [Copyright &y& Elsevier]

Injection of carbon dioxide into subsurface geologic reservoirs during geologic carbon sequestration (GCS) introduces an oxidizing supercritical CO 2 phase into a subsurface geologic environment that is typically reducing. The resulting redox disequilibrium provides the chemical potential for the reduction of CO 2 to lower free energy organic species. However, redox reactions involving carbon typically require the presence of a catalyst. Iron oxide minerals, including magnetite, are known to catalyze oxidation and reduction reactions of C-bearing species. If the redox conditions in the reservoir are modified by redox transformations involving CO 2 , such changes could also affect mineral stability, leading to dissolution and precipitation reactions and alteration of the long-term fate of CO 2 in GCS reservoirs. We present experimental evidence that reservoirs with reducing redox conditions are favorable environments for the relatively rapid abiotic reduction of CO 2 to organic molecules. In these experiments, an aqueous suspension of magnetite nanoparticles was reacted with supercritical CO 2 under pressure and temperature conditions relevant to GCS in sedimentary reservoirs (95–210 °C and ∼100 bars of CO 2 ). Hydrogen production was observed in several experiments, likely caused by Fe(II) oxidation either at the surface of magnetite or in the aqueous phase. Heating of the Fe(II)-rich system resulted in elevated P H 2 and conditions favorable for the reduction of CO 2 to acetic acid. Implications of these results for the long-term fate of CO 2 in field-scale systems were explored using reaction path modeling of CO 2 injection into reservoirs containing Fe(II)-bearing primary silicate minerals, with kinetic parameters for CO 2 reduction obtained experimentally. The results of these calculations suggest that the reaction of CO 2 with reservoir constituents will occur in two primary stages (1) equilibration of CO 2 with organic acids resulting in mineral–fluid disequilibrium, and (2) gradual dissolution of primary minerals promoting significant CO 2 reduction through the release of Fe(II). The reduction of CO 2 is identified as a new trapping mechanism that could significantly enhance the long-term stability of GCS reservoirs. Identification of reservoir characteristics that promote CO 2 redox transformations could be used as an additional factor in screening geologic reservoirs for GCS. [ABSTRACT FROM AUTHOR]

In order to evaluate the chemistry and kinetics of mineral carbonation reactions under conditions relevant to subsurface injection and storage of CO2, olivine alteration was studied at 60°C and 100bar CO2 pressure, including olivine dissolution and the formation of carbonate minerals. Batch experiments were performed with olivine (Fo92), water, CO2, and NaCl inside gold cells contained within rocking autoclaves. Two reproducible experiments yielded an initial (1h) dissolution rate of 9.50±0.10×10−11 and a long-term (10–70days) rate of 1.69±0.23×10−12 molcm−2 s−1. The long-term rate is consistent with previously published rate laws at 4.5