Feldspar dissolution kinetics and Gibbs free energy dependence in a CO 2-enriched groundwater system, Green River, Utah

Abstract

Reactions between CO 2-charged brines and reservoir minerals might either enhance the long-term storage of CO 2 in geological reservoirs or facilitate leakage by corroding cap rocks 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 CO 2 at Green River, Utah, in the Colorado Plateau, USA, to place constraints on the rates and potential controlling mechanisms of the mineral–fluid reactions, under elevated CO 2 pressures, in a natural system. The progress of individual reactions, inferred from changes in groundwater chemistry is modelled using mass balance techniques. The mineral reactions are close to stoichiometric with plagioclase and K-feldspar dissolution largely balanced by precipitation of clay minerals and carbonate. 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 feldspar dissolution. Maximum estimated dissolution rates for plagioclase and K-feldspar are 2 × 10 − 14 and 4 × 10 − 16 mol m − 2 s − 1 , respectively. Fluid ion-activity products are close to equilibrium (e.g. Δ G 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 Δ G r. Local variation in Δ G r is attributed to the injection and disassociation of CO 2 which initially depresses silicate mineral saturation in the fluid, promoting feldspar dissolution. With progressive flow through the aquifer feldspar hydrolysis reactions consume 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 Δ G r of 2 × 10 − 14 mol m − 2 s − 1 would be compatible with far-from-equilibrium rates of ~ 1 × 10 − 13 mol m − 2 s − 1 as observed in some experimental studies. This suggests that the discrepancy between field and laboratory reaction 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.