Abstract

Summary The standard description of hydrofluoric acid (HF) acidizing chemistry clearly demonstrates a primary and secondary reaction of HF with aluminosilicates.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 aluminosilicates.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 HCl at temperatures above 250°F. These findings were made possible by recently applied experimental techniques including 19F nuclear magnetic resonance (NMR) spectroscopy,3 fractional pore-volume (PV) 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 HCl decomposition of the clay above 250°F. The tertiary reaction required the presence and consumption of acid to proceed. Feldspars were very stable in HCl at all temperatures, whereas every clay tested had a temperature above which it was easily decomposed by HCl. Ion-exchanging clays tended to be the least stable of the clays, whereas kaolinite was the most stable clay.

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