Back to Basics: The Importance of the Carbonate Buffer in Multi-Phase (CO2-H2O) Water-Rock Systems - Critical Considerations for Predictions, Experiments and Field Applications

In CO₂-bearing water-rock systems, incomplete characterization and definition of groundwater can lead to erroneous predictions of rock-water interactions. In particular, speciation, pH and mineral saturation can be very sensitive to natural bicarbonate buffer in groundwater. A 2% error in identifying initial bicarbonate content of groundwater, prior to adding anthropogenic CO₂, could produce as much as a 1 pH unit error when calculating in-situ pH. Such an error greatly affects calculation of saturation indices of carbonates, for example, predicting mineral dissolution when in fact carbonates are precipitating. In closed system laboratory experiments, we recreate the geochemical conditions of a multi-phase fluid-rock basin at steady state, wherein brine, rock, and immiscible supercritical CO₂ coexist. We define key parameters that need to be measured in the fluid to accurately model these systems and demonstrate how to use these data with off-the-shelf geochemical codes to accurately predict experimental results. One set of experiments evaluates the sensitivity of dawsonite saturation at 75^oC and 200 bar in 1 molal NaCl brine + albite + supercritical CO₂. Initial bicarbonate in this simple system is constrained by equilibrium with atmospheric CO₂ and final bicarbonate (1.325 molal) is measured by coulometric titration. Dawsonite is undersaturated in both experiment and calculation, and dawsonite solubility is not sensitive to initial bicarbonate. In contrast, using previously published experimental data (5.5 molal NaCl brine, arkosic sandstone, shale, and supercritical CO₂ at 200^oC and 200 bars, Kaszuba et al., 2005), we demonstrate that calculated siderite and magnesite saturation and pH are sensitive to initial bicarbonate. Siderite and magnesite are undersaturated when using near-zero initial bicarbonate and oversaturated when using 20 mmolal initial bicarbonate in calculations. Our findings are critical for predictions, field monitoring and measurement, and verification of enhanced oil recovery, in-situ oil shale production, and geologic sequestration of CO₂.