CARBON SEQUESTRATION IN BASALTIC ROCKS: EXPERIMENTAL RESULTS AND GEOCHEMICAL MODELING

Basaltic rocks have potential as reservoirs for carbon dioxide (CO₂) sequestration because of their high capacity for mineral trapping of CO₂ in the form of divalent metal carbonate minerals such as calcite, magnesite, siderite, and dolomite. We carried out a suite of geochemical models and experiments, reacting basalt with CO₂-charged fluids over a range of conditions from 50°C to 150°C and from 100 bars to 300 bars. Experiments were conducted in flexible gold-cells with a serial online fluid sampling capability that allows for monitoring reaction progress. Experiments were also carried out in fixed-volume titanium reaction cells to investigate batch reactions. We conclude that basalt has a high reactivity to supercritical CO₂ and carbonic acid. Moreover, batch experiments reacting olivine basalt with supercritical CO₂ at 100°C, 300 bars P_CO2 for two months produced almost complete alteration of the rock with 90% of the divalent cations (primarily Mg, Fe, and Ca) having been incorporated into carbonates.

In experiments with basalt and CO₂-charged fluids that were under-saturated with respect to CO₂, initial reaction caused a rapid drop in fluid pH and an increase in dissolved cations including Ca, Mg, Fe, and Mn then a decline toward steady-state values. An asymptotic CO₂ decrease to steady-state concentrations was found to follow a repeatable pattern after re-injection of CO₂ into the experiment. These results showed that the basalt continued to have a capacity to react with CO₂.

Predictions based on reaction path simulations suggested that calcite would initially precipitate then dissolve following the sequential addition of P_CO2 and would be later replaced by dolomite and siderite at final equilibrium. In contrast to model prediction, SEM analyses of the solid reaction products found only Fe- and Mg-bearing phases. These results are consistent with important kinetic barriers to forming dolomite. Continued experiments are underway to optimize conditions for basalt reaction with CO₂.