Recent models for sandstone acidizing have shown that the secondary reaction of fluosilicic acid (H2SiF6) with clays plays a significant role in the removal of formation damage. The conventional wisdom is that possible precipitation during the secondary reactions can adversely affect treatment success. However, this paper presents laboratory experiments with Berea cores and two Brazilian sandstone cores, as well as field tests, which show that fluosilicic acid can be injected, by itself, into a sandstone reservoir without causing any damage at all. Quite the contrary, mixtures of fluosilicic acid with a proper amount of hydrochloric acid (HCl) or an organic acid such as acetic acid (HAc) have been able to improve the performance of two Brazilian water injection wells by removing clay damage. These two wells, that were injecting 11 and 15 m3/d, are sustaining injection rates at or above the desired quota of 30 m3/d, five months after the treatments. The treatments were monitored with a real-time monitoring program, that showed that the skin factor has dropped from about 30/40 to zero. Besides having a lower cost than conventional HF mixtures, the main advantage of the new acid system is that it reacts mostly with clays and feldspars, but almost does not react with quartz. This way, it can dissolve damage without weakening the rock structure, what makes it suitable for deep damage removal. The low cost of H2SiF6, which is a byproduct of sodium fluoride manufacturing, makes it also a viable option for routine acidizing operations, that would normally require the use of an HF acid system.

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