Abstract
HCl is generated in subsurface brines exposed to high CO2 pressures via reactions such as CO2 + NaCl + H2O ↔ NaHCO3 + HCl. The extent of partitioning of HCl between a concentrated chloride brine and supercritical CO2 (scCO2) has not been measured before and it cannot be estimated with the accuracy sufficient for predictive modeling of subsurface processes. The partitioning of HCl between 4.92 mol/kg aqueous solutions of NaCl and scCO2 was measured at 100 and 150 °C and at pressures between 9 and 16.2 MPa. At P = 15 MPa, the concentrations of HCl in the scCO2 phase were 7 × 10–4 and 4 × 10–3 mol/L at 100 and 150 °C, an increase by more than 5 and 3 orders of magnitude, respectively, relative to the concentrations in water vapor in the absence of CO2. These levels of HCl may accelerate reactions with reservoir rocks, natural seals (shale caprocks) and downhole materials, potentially enhancing or impairing operation and long-term performance of subsurface technologies such as geologic carbon sequestration/storage, enhanced geothermal systems using compressed CO2, and scCO2 extraction of hydrocarbons, and other resources in the presence of chloride brines.