Chemical Study of Organic-HF Blends Leads to Improved Fluids

Abstract

Abstract Organic-HF blends have successfully stimulated sandstone formations with corrosion concerns, HCl-sensitive mineralogy, and/or crude oil incompatibilities, three conditions in which the use of typical HCl-based fluids can result in severe damage. Despite the success of such blends, very little work has been published on the reactivity of organic-HF fluids with aluminosilicates. Recent work on the chemistry of HF acidizing focused on HCl-HF systems. This paper discusses the chemistry of organic-HF systems as determined by laboratory reactivity tests, 19F nuclear magnetic resonance (NMR) studies, and organic-HF flowback analyses. Laboratory reactivity testing revealed severe precipitation problems associated with the currently used acetic-HF and formic-HF systems. Flowback analyses after organic-HF field treatments fully supported the laboratory results. A 19F NMR spectroscopic study of the fluoride distribution throughout the primary, secondary, and tertiary reactions helped explain the precipitation and when it might occur. From this work, new organic-HF systems were developed that prevent precipitation while maintaining all the advantages associated with the acetic-HF and formic-HF fluids. The retarded nature of these systems is discussed in this paper. Introduction Common problems associated with HCl-based fluids include high reactivity, high corrosivity, sludging tendencies when fluids contact crude oils, and the HCl sensitivity of clay, minerals and zeolites. These problems are aggravated at higher temperatures. Organic acids, such as acetic acid, formic acid, and combinations have overcome many of these problems. The recent realization that organic acids act like fresh water has led to the addition of 5% NH4Cl to organic acids used in formations containing clays and zeolites that are capable of ion exchange and/or susceptible to swelling or fines migration. Acetic-HF and formic-HF fluids were developed because of the problems associated with HCl-based HF fluids. However, very few studies have investigated the chemistry of these systems. Recently, we discovered that HCl-HF fluids obey a chemical equilibrium during the primary and secondary reactions. As a result, the fluoride distribution, and in particular the F/Al ratio, depends upon the acid (H+) concentration. For example, a solution containing 13.5% HCl-1.5% HF reacted with alumino-silicates will have a F/Al ratio of approximately 1.3 during the primary and secondary reactions. However, at the low acid (H+) concentrations of organic acids (pH 1 through 4), the F/Al ratio of organic-HF fluids during these reactions would be 2.5 to 3.0. The dominant aluminum fluoride species in spent organic-HF solutions would be AlF2+ and AlF3. The acid dependence on the F/Si ratio is only slight. In HCl-based fluids, the F/Si ratio is considered to be constant at 5.0; the dominant species is HSiF5. In organic-HF systems, the F/Si ratio is about 5.5, and the dominant species are H2SiF6 and HSiF5. Based on the fluoride distribution, the primary reaction (Eq. 1) and secondary reaction (Eq. 2) for organic-HF fluids can be written in the general terms below. (1) (2) As reported in a previous paper, flowback analyses from acetic-HF and formic-HF field treatments showed that aluminum fluoride precipitation had occurred. Laboratory spending tests and flow tests showed that AlF3 precipitated from the acetic-HF and formic-HF fluids quickly upon spending because of the high F/Al ratio and the poor solubility of AlF3.P. 675^