Evaluation of the dissolution kinetics of NaCl particles in aqueous drilling fluids viscosified with bentonite

Abstract The salt dissolution in aqueous drilling fluids is one of the main problems founded in well drilling of salt layers. The dissolution may cause the well enlargement, the salt layer outflow towards the drilling column or a drill immobilization resulted by the accumulation of saline gravels in the well bottom. In this study, the dissolution of sodium chloride (NaCl) particles and its influence on the physicochemical properties of fluids viscosified with bentonite were evaluated. In the rheological study of suspensions, the effect of time in the apparent viscosity was evaluated by the analysis of different bentonite and salt concentrations. The dissolved salt in the dispersion provided an apparent viscosity reduction and, regardless of the added salt mass, the shear stress values obtained were lower than those values in the pure bentonite dispersions. The dissolution kinetic was studied in a stirred tank, where experimental profiles of salt concentration in the liquid phase were obtained. The experimental data were used to evaluate the mass transfer coefficient in different conditions and to validate the mathematical model. The rate of salt particles dissolution was affected by both the fluid's bentonite and initial salt concentration. The mathematical model was used to simulate the process by the application of the mass conservation equations applied on both solid and fluid phase. The global mass transfer coefficients were estimated, and its values were in the range of 0.250 × 10 −4 to 0.792 × 10 −4  m/s. The experimental results were compared with those provided by the model with an error less than 10%.