Fluosilicate Solubilities Affect HF Acid Compositions

Abstract

Summary The purpose of this work was to determine the solubilities of sodium and potassium fluosilicate over a broad range of HCl concentrations, ionic strengths, and temperatures. These data were required for determining optimum or safe HCl/HF treating-fluid compositions for acidizing sandstone formations containing feldspars. Solubilities of sodium and potassium fluosilicate were determined over a temperature range of 150 to 300°F. Analysis of the solubility data indicated that the solids formed were not simply Na2SiF6 and K2SiF6. Regression analyses indicated that the sodium salt could best be described as NaHSiF6 · 0.52HCl (or Na0.48H1.52SiF6· 0.52NaCl). Regression analyses indicated that the potassium salt could best be described as KHSiF6 · 0.41KCl (or K1.41H0.59SiF6 · 0.41HCl). The solubility data were then included in a HF-spending simulator to determine regions where possible precipitation of the fluosilicates could occur. Optimum HCl/HF concentrations were then determined for specific mineral and temperature conditions. Introduction The results from a comprehensive study discussed the analysis of HF returns from an acid treatment. Of particular interest was the absence of excess acid and silicon accompanied by a high abundance of both aluminum and sodium in essentially undiluted spent HF returns. This was unexpected because the primary reaction of HF with feldspar predicts that most of the fluoride is spent on silicon.