The standard description of HF acidizing chemistry clearly demonstrates a primary and secondary reaction of HF with alumino-silicates.1  Field experience has taught us that possible precipitation during the secondary reaction can adversely affect treatment success,2  especially in formations with high K-feld-spar content or 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 the determination of the rate law for the secondary reaction and the kinetics for that reaction on kaolinite and feldspar over a broad temperature range. In addition, the kinetics of fluosilicate precipitation during the secondary reaction were studied and found to be fast but not instantaneous. These findings were made possible by recently applied experimental techniques including 19F NMR spectroscopy, 3 fractional pore volume flow experiments,4  and an accurate knowledge of the HF stoichiometry.5  Laboratory findings have been verified based on detailed analysis of flowback samples from field treatments.6 

The reaction rate for the secondary reaction is much slower on feldspars than on clays. At temperatures below 125°F, the secondary reaction rate is very slow, which makes spent HF susceptible to fluosilicate precipitation if the HF is mixed with brines as it often is for disposal or injection well treatments. The difference in the kinetics between clays and feldspar allows one to design treatments at lower temperatures based on just the clay fraction. At temperatures above 150°F, the average mineralogy, clay mineralogy, and feldspar mineralogy should each be evaluated separately for potential fluosilicate precipitation. The fluid of choice is the one that passes all three conditions.

The rate law indicates that the speed of the reaction is independent of HCl concentration and would proceed to completion even in the absence of HCl. Chemically, one can accomplish this effect by using water as a proton source and by adjusting the stoichiometry based on a previously reported equilibrium.5  The kinetics for fluosilicate precipitation were fast but not instantaneous. At very high temperatures, the secondary reaction is fast enough that fluosilicate precipitations can largely be avoided. Unfortunately, the silica gel precipitation then occurs so close to the wellbore that damage to permeability will likely occur. The equations recently developed for the HF acidizing process have been incorporated into a fully kinetic geochemical radial flow simulator. Calculations performed with this simulator have confirmed that a good knowledge of the formation mineralogy and acidizing kinetics is necessary for proper optimization of HF acidizing treatments.

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