Hydrochloric acid diffusion coefficients at acid-fracturing conditions

Abstract

The reaction of a 15% HCl solution with Indiana Limestone was investigated at 93 °C and 34.5 MPa using an annular flow reactor that was erected vertically. The laminar flow conditions were such that both free and forced convections contributed to the overall mass-transfer rate. The experimental results correlated well with theory when the forced and free convections were in the same direction (assisting flow) and when free-convection mass transfer dominated the mass-transfer process. Deviation from theory, however, occurred when free convection was in the opposite direction to forced convection (opposing flow). Correlating the experimental results with theory provided a method for the determination of the effective diffusion coefficient of HCl at the high temperature, pressure and acid concentrations encountered in acid-fracturing treatments. An HCl effective diffusion coefficient of 5.25 × 10 −5 cm 2/s was found from correlating the present experimental results with theory. Recent trend in designing acid-fracturing treatments is to use computer models to predict and optimize the etched length of the created fracture. The accuracy and reliability of such computer model predictions depend heavily on the accuracy of the values used for physical parameters such as reaction rate and diffusion coefficient of HCl. The annular flow reactor and the correlations discussed here provide a way to obtain reaction rates and diffusion coefficients at practical conditions for use in computer models and design of acid-fracturing treatments.