Aluminum and Iron Precipitation During Sandstone Acidizing Using Organic-HF Acids

Abstract

Abstract Different organic-HF acid mixtures have been used to stimulate sandstone formations. Typically, they are used as alternative to regular mud acid in order to overcome its potential limitations such as rapid spending, high corrosion rate and incompatibility with sensitive clays. However, organic-HF systems are more susceptible to fluoride-based precipitations because they contain high amount of free fluoride ions. This paper focuses on identifying the type of precipitations that occur during reactions of organic-HF acids, and on determining the factors that affect these precipitations. Solutions of different organic-HF acids namely formic, acetic and citric acid, containing HF concentrations of 0.5, 1 and 1.5 wt%, were examined in this study. Aluminum chloride or iron chloride were added separately to each organic-HF acid solution to contain 1,000; 5,000 or 10,000 mg/L of aluminum or iron (III) ions, respectively. The acid mixtures were neutralized by adding sodium hydroxide. Filtered solutions were analyzed using inductively coupled plasma (ICP) to assess the ability of used acids to hold Al and Fe (III) dissolved ions, while formed solid precipitates were analyzed by X-ray diffraction (XRD). The type and amount of precipitates were found to be mainly dependent on solution pH, organic-HF type, and initial free fluoride concentration. All live organic-HF acids containing dissolved iron (III) showed no precipitation even after iron (III) level reached 10,000 mg/L. However, when solution pH value was raised, none of tested organic-HF acids were able to prevent iron fluoride precipitation. On the other hand, the main factor that controlled the aluminum-fluoride precipitation was found to be F/Al ratio. It was found that there is a critical F/Al ratio, above which the aluminum fluoride precipitation occurred.