Evaluation of Chelating-Hydrofluoric Systems

Different chelating agent-HF mixture systems have been used to stimulate deep hot sandstone formations. Typically, chelating agents are used as a substitute for HCl and organic acids in HF based systems to stimulate hot and exotic oil and gas sandstone formations. Apart from reducing the risk of corrosion, secondary and tertiary reactions, the system can be deployed in formations containing high clays and carbonates as well as iron and zeolite bearing sandstone formations. However, several fields’ cases indicated precipitation of aluminum-based scale following acid treatments that include these systems. The focus of this paper is to identify the type of precipitations that occur during the reaction of chelating-HF systems and determine the factors that affect these precipitations. Solutions of different ethylenediaminetetraacetic acid EDTA containing two ammonium bifluoride ABF concentrations of 0.5 and 1.0 wt% were examined in this study. Aluminum chloride or calcium chloride was added separately to each chelating-HF solution to contain various concentrations of aluminum or calcium. The filtered liquid was analyzed using inductively coupled plasma ICP, while formed solid precipitate was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The main type of precipitate in solutions containing aluminum was identified as AlF3. The amount of precipitate was found to be dependent on solution initial aluminum and free fluoride concentrations. In other words, the main factor that controlled the aluminum-fluoride precipitation was found to be F/Al ratio and above a critical F/Al value, AlF3 precipitates.