New Fluosilicic Acid System Removes Deep Clay Damage

Abstract

Summary 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 with field tests, that show that fluosilicic acid can be injected, by itself, into a sandstone reservoir without causing any damage. In fact, 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, which were injecting 11 and 15 m3/d, respectively, are sustaining injection rates at or above the desired quota of 30 m3/d 5 months after the treatments. The treatments were monitored with a real-time monitoring program, which showed that the skin factor has dropped from about 30/40 to zero. Besides having a lower cost than conventional hydrofluoric acid (HF) mixtures, the main advantage of the new acid system is that it reacts mostly with clays and feldspars, but it almost does not react with quartz. Thus, it can dissolve damage without weakening the rock structure, which makes it suitable for deep damage removal. The low cost of H2SiF6, which is a byproduct of sodium fluoride manufacturing, also makes it a viable option for routine acidizing operations that normally would require the use of an HF acid system.