THE KINETICS OF FLUID-ROCK INTERACTION IN CO2 STORAGE SITES: CONSTRAINTS FROM MEASUREMENTS ON NATURAL CO2 CHARGED WATERS

Reactions between CO₂-charged brines and reservoir minerals might either enhance the long-term storage of CO₂ in geological reservoirs or facilitate leakage by corroding caprocks and fault seals. Modelling the progress of such reactions is frustrated by uncertainties in the absolute mineral surface reaction rates and the significance of other rate limiting steps in natural systems. Here we use the chemical evolution of groundwater from the Jurassic Navajo sandstone, part of a leaking natural accumulation of CO₂ at Green River, Utah, in the Colorado Plateau, to place constraints on the rates and potential controlling mechanisms of the mineral-fluid reactions, under elevated CO₂ pressures, in a natural system.

The progress of individual reactions, inferred from changes in groundwater chemistry is modelled using mass balance techniques. Mineral modes, in conjunction with published surface area measurements and flow rates estimated from hydraulic head measurements, are then used to quantify the kinetics of mineral dissolution.

Maximum estimated dissolution rates for plagioclase and K-feldspar are 2x10^-14 and 4x10^-16 mol·m^-2·s^-1, respectively. Fluid ion-activity products are close to equilibrium (e.g. ΔG_r for plagioclase between 2 and 10 kJ/mol) and lie in the region in which mineral surface reaction rates show a strong dependence on ΔG_r. Local variation in ΔG_r is attributed to the injection and disassociation of CO₂ which initially depresses mineral saturation in the fluid, promoting feldspar dissolution. With progressive flow through the aquifer feldspar hydrolysis reactions consumed H⁺ and liberate solutes to solution which increase mineral­­ saturation in the fluid and rates slow as a consequence.

The measured plagioclase dissolution rates at low ΔG_r of 2x10^-14 mol·m^-2·s^-1 would be compatible with far-from-equilibrium rates of ~1x10^-13 mol·m^-2·s^-1 as observed in some experimental studies. This suggests that the discrepancy between field and laboratory reactions rates may in part be explained by the differences in the thermodynamic state of natural and experimental fluids, with field-scale reactions occurring close to equilibrium whereas most laboratory experiments are run far-from-equilibrium.