The standard description of HF acidizing chemistry clearly demonstrates a primary and secondary reaction of HF with alumino-silicates.1  Field experience has taught our industry that possible precipitation during the secondary reaction can adversely affect treatment success.2  This statement has been particularly true in formations with high K-feldspar content or formations having temperatures above 300°F. Recent work has also reported the existence of a third, or tertiary reaction of HF with alumino-silicates.3 

This paper reports how the rate law and kinetics for this tertiary reaction are determined on kaolinite and feldspar over a broad temperature range. This document also discusses the discovery of how most clays were thermally unstable to HC1 at temperatures above 250°F. These findings were made possible by recently applied experimental techniques including l9FNMR Spectroscopy,3  fractional pore volume flow experiments,4  and an accurate knowledge of the HF stoichiometry.5 

The tertiary reaction of aluminum fluorides, AlFx (where x is the average F/Al ratio), with clay was slow below 200°F and was dominated by HC1 decomposition of the clay above 250°F. The tertiary reaction required the presence and consumption of acid to proceed. Feldspars were very stable in HC1 at all temperatures while every clay tested had a temperature above which it was easily decomposed by HC1. Ion-exchanging clays tended to be the least stable of the clays, while kaolinite was the most stable clay.

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