Newly Discovered Equilibrium Controls HF Stoichiometry
Authors: Gdanski Rick D. , Shuchart Chris E. 
Published: June 2014

Abstract
The standard description of HF acidizing chemistry suggests a stoichiometry that has been found to be deficient. This paper describes the discovery of a chemical equilibrium between silicon fluorides, aluminum fluorides, and HCl that controls the "reactivity ratios" or stoichiometry of the primary and secondary reactions of HF acid. The significance of the discovery is that it allows accurate predictions of effluent compositions in laboratory flow tests.
This discovery is also consistent with returns analyses from HF treatments recently reported in the literature. The discovery and determination of the equilibrium expression was made possible as the result of recently applied experimental techniques including 19F NMR spectroscopy and fractional porevolume flow experiments. An accurate description of the stoichiometry of the HFacidizing process is presented and verified with laboratory flow experiments and batch tests.

Introduction
The acidizing monograph indicated that the primary reaction of HF acid onalumino-silicates results in fluosilicic acid (H2SiF6) and complex aluminumfluoride species. Other works suggested the formation of free aluminum (Al3+), aluminum fluoride (AlF3), and fluoaluminic acid (H3AlF6) as reaction products. The ratio of fluoride to silicon and aluminum in the reaction products defines the reaction ratios or stoichiometry. As work continued in HF acidizing, various assumptions were made concerning the reaction products at the end of both the primary and secondary reactions. Work by Hall indicated that aluminumfluorides were still very reactive but could be used to provide a retarded HF system. Based on this information, it was concluded that H3AlF6 was not a reasonable reaction product at all.
Later, Gdanski and Peavy reported their results from analyzing HF acid returns and found a composition that (1) was inconsistent with our industry'sclassical thinking about HF and (2) could not be supported by the geochemical simulators of that time. In this particular composition, the silicon fluorides were completely reacted, as well as all of the HCl. In their place was a large amount of aluminum and a large amount of sodium. A secondary reaction was proposed, which assumed a final reaction product of AlF2+ and was written showing the reaction as occurring directly between H2SiF6 and the alumino-silicates. This proposed reaction provided reasonably good agreement for a number of the acid returns analyses performed over the following years.
Work recently published by Shuch art and Buster has revealed additional information regarding the true chemical species involved in HF acidizing chemistry. They showed that the dominant silicon fluoride species could best be described as HSiF5. Interestingly, the F/Al ratio during the primary and secondary reactions was dependent on the HCl concentration. With these developments, it became clear that the time had come to perform careful experiments to determine the properly balanced reaction of HF acid through both the primary and secondary reactions. The determination of the stoichiometry through the secondary reaction was particularly important because of recent calculations on fluosilicate precipitation.

Equipment and Method
Batch Tests. For batch tests, a specified amount of clay was added to a solution of HF or H2SiF6 in a glass container. Some tests were stirred with a magnetic stirring bar while others were left static. Tests were conducted at room temperature, 110 F, or 200 F. Samples of the suspensions were obtained at various time intervals and filtered hot. After the samples had cooled, they were analyzed for their ionic composition.
Additional batch tests were conducted as previously described, but at the various time intervals, the entire suspensions were filtered hot, and a small sample was obtained for analysis. A second portion of fresh clay was added to the solution for continued reaction. This process was continued 3 to 6times.