A more thorough understanding of the chemistry of HF acid and its reaction products on silica and alumino-silicates is essential to the design and optimization of HF acidizing treatments. So far, results from laboratory flow tests and HF acid well returns have not been explained readily by traditional theories. To more clearly define the chemistry of HF acidizing, an indepth investigation of the reaction of HF and H2SiF6 with alumino-silicates was undertaken using 19F Nuclear Magnetic Resonance (NMR) spectroscopy.

In addition to the fluosilicic acid (H2SiF6) and A1F2+ predicted by traditional theories, 19F NMR spectroscopy shows a complex mixture of silicon and aluminum fluoride species in reacted HF acidizing solutions. During a secondary reaction of silicon fluorides with alumino-silicates, a constant F/Al ratio was maintained until the silicon fluorides had reacted completely. The distribution of the fluoride species depends on the HC1 concentration.

In this investigation, a tertiary reaction of HF acid on alumino-silicates was identified. When the silicon fluorides have reacted completely to give silica gel, the aluminum fluoride complexes continue to react on fresh alumino-silicates. The reaction causes the aluminum content to increase and the F/Al ratio and acid concentration to decrease. The final F/Al ratio is dependent upon acid strength and temperature.

Numerous HF acidizing well returns have been analyzed to verify the reactions conducted in the laboratory. The extent of the reaction of HF acid can be determined with the use of 19F NMR spectroscopy. In wells with temperatures of 150 to 200°F, the reaction of H2SiF6 is complete. Silicon content was quite low, and pH levels were 2 to 3. The F/Al ratios of the returns were 0.5 to 1.3, depending on the concentration of HC1 and HF used in the treatment. In wells less than 100°F, the secondary reaction did not go to completion. Silicon and aluminum fluoride complexes were present in the returns along with live HC1.

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