Viscosity of pure and mixed aqueous NaCl and CaCl2 solutions at 293 K to 353 K and 0.1 MPa: a simple empirical correlation parameterised with original analytical data

This paper reports more than 500 newly measured values of the dynamic viscosity of pure and mixed sodium chloride (NaCl) and calcium chloride (CaCl₂) aqueous solutions of which about half are unique. The data were acquired with a commercial rolling ball viscometer at ambient pressure, temperatures between 293 and 353 K, concentrations up to 6.0 mol/kg (NaCl) and 5.3 mol/kg (CaCl₂) as well as five different mixing ratios for the ternary system. Compared to existing values found in the literature, the present data match within mostly 5% uncertainty. Potential sources of errors are thoroughly discussed. An empirical correlation was derived from the measured data, accurately reproducing the measured data with good precision. This permits the functional dependence of dynamic viscosity on concentration and temperature of pure and mixed NaCl and CaCl₂ solutions to be easily and reliably implemented in coupled thermal–hydraulic–mechanical–chemical (THMC) numerical models aiming to predict the evolution of geothermal reservoir dynamics. Moreover, numerical sensitivity analyses were conducted exemplarily for one deep geothermal (DG) as well as one aquifer thermal energy storage (ATES) system to constrain the effect of viscosity variations and/or uncertainty on the prediction of key reservoir and operational parameters. It is demonstrated that viscosity variations systematically affect the productivity and injectivity indices (both systems) and the required pumping power (ATES), though no effect was observed for the timing of thermal breakthrough (DG) and the temperature evolution at the warm well (ATES). Not least, the stated precision of the analytical data obtained in this study proves well sufficient for the quality of numerical reservoir simulations aiming at predictions in a geothermal context.