New fluosilicic acid system removes deep clay damage

Recent models for sandstone acidizing have shown that the secondary reactio n of fluosilicic acid (H2SiF6) with clays plays a significant role in the r emoval of formation damage. The conventional wisdom is that possible precip itation during the secondary reactions can adversely affect treatment succe ss. However, this paper presents laboratory experiments with Berea cores an d two Brazilian sandstone cores, as well as with field tests, that show tha t fluosilicic acid can be injected, by itself, into a sandstone reservoir w ithout causing any damage. In fact, mixtures of fluosilicic acid, with a pr oper amount of hydrochloric acid (HCl) or an organic acid such as acetic ac id (HAc), have been able to improve the performance of two Brazilian water- injection wells by removing clay damage. These two wells, which were inject ing 11 and 15 m(3)/d, respectively, are sustaining injection rates at or ab ove the desired quota of 30 m(3)/d 5 months after the treatments. The treat ments 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 low er cost than conventional hydrofluoric acid (HF) mixtures, the main advanta ge 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 wi thout weakening the rock structure, which makes it suitable for deep damage removal. The low cost of H2SiF6 which is a byproduct of sodium fluoride ma nufacturing, also makes it a viable option for routine acidizing operations that normally would require the use of an HF acid system.