KINETICS OF TERTIARY REACTIONS OF HYDROFLUORIC-ACID ON ALUMINOSILICATES

The standard description of hydrofluoric acid (HF) acidizing chemistry clearly demonstrates a primary and secondary reaction of HF with alum inosilicates.(1) Field experience has taught our industry that possibl e precipitation during the secondary reaction can adversely affect tre atment success.(2) This statement has been particularly true in format ions with high K-feldspar content or formations having temperatures ab ove 300 degrees F. Recent work has also reported the existence of a th ird, or tertiary, reaction of HF with aluminosilicates.(3) This paper reports how the rate law and kinetics for this tertiary reaction are d etermined on kaolinite and feldspar over a broad temperature range. Th is document also discusses the discovery of how most clays were therma lly unstable to HCl at temperatures above 250 degrees F. These finding s were made possible by recently applied experimental techniques inclu ding F-19 nuclear magnetic resonance (NMR) spectroscopy,(3) fractional pore-volume (PV) flow experiments,(4) and an accurate knowledge of th e HF stoichiometry.(5) The tertiary reaction of aluminum fluorides, Al Fx (where x is the average F/Al ratio), with clay was slow below 200 d egrees F and was dominated by HCl decomposition of the clay above 250 degrees F. The tertiary reaction required the presence and consumption of acid to proceed. Feldspars were very stable in HCl at all temperat ures, whereas every clay tested had a temperature above which it was e asily decomposed by HCl. Ion-exchanging clays tended to be the least s table of the clays, whereas kaolinite was the most stable clay.