18 SPRING 2012 SAUDI ARAMCO JOURNAL OF TECHNOLOGY ABSTRACT acid is the main reactant with formation rocks, HCl acid is inevitably added into the mixture to reduce HF consumption as well as to maintain an acidic environment and subsequently prevent precipitations of HF reaction byproducts. Despite the reasonable success of mud acid, critical drawbacks associated with using the mud acid have limited its use. One of the most potential limitations of mud acid is rapid spending, especially at elevated temperatures, which results in subsequent precipi- tations of reaction products following secondary and tertiary reactions, and limits acid penetration in the formation as well 6, 9 . A combination of these precipitations, matrix uncon- solidation, the presence of HCl acid sensitive clays and high corrosion rates has resulted in the variable success rate of mud acid stimulation treatments or, even worse, in further formation damage 9-11 . Several attempts have been made to tackle the potential problems likely to be encountered during strong acid treatments of producing and injection wells. These include adjustment of HCl/HF acid ratios and use of retarded mud acids 5, 9, 12, 13 ; however, these methods remain prone to the same mud acid problems at elevated temperature values. Several research studies have revealed that retarded mud acids provided only marginal reduction in the mud acid reaction rate with aluminosilicates, and their reactions created new precipitation types. For example, fluoboric based retarded mud acid will form a potassium based precipitate, KBF 4 , when it reacts with either illite or K-feldspars. The aluminum chloride (AlCl 3 ) HF retarded system is very sus- ceptible to aluminum fluoride (AlF 3 ) precipitation 13 . Another approach to improve the effectiveness of sandstone acidizing involved the use of organic-HF acids 14-19 . Organic acids, with and without HF, have been widely used over the years to treat sandstone and carbonate formations, respectively. They were preferred over many other acid systems due to their retarded nature and low corrosion rates, especially in high temperature formations 18 . In addition, organic acids are often used as an additive to control iron and aluminum precipitation by buffering or chelating action, or both. The use of organic-HF acids has become imperative in formations con- taining HCl acid sensitive clays, such as illite 14 . The two main organic-HF acids in use are based on acetic and formic acids. Typically, the acetic and formic acids are used at concentrations less than 13 wt% and 9 wt%, respectively. This is because these acids are expensive and some of their reaction products Different organic-hydrofluoric (HF) acid mixtures have been used to stimulate sandstone formations. Typically, they are used as an alternative to overcome potential limitations associated with regular mud acid, such as rapid spending, high corrosion rate and incompatibility of hydrochloric (HCl) acid with sensi- tive clays. Consequently, organic-HF acid systems are more susceptible to fluoride-based precipitations because they contain a high amount of free fluoride ions. This article focuses on identifying the types of precipitations that occur during reactions of organic-HF acids and 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 (AlCl 3 ) or iron chloride (FeCl 3 ) was added separately to each organic-HF acid solution so they contained 1,000, 5,000 and 10,000 mg/L, respectively, of either aluminum or iron (Fe) (III) ions. The acid mixtures then were neutralized by adding sodium hydroxide. Filtered solutions were analyzed using inductively coupled plasma (ICP) to assess the ability of used acids to hold aluminum and Fe (III), while formed solid precipitates were analyzed by X-ray diffraction (XRD). The type and amount of precipitates were found to be mainly dependent on pH solution, organic-HF acid types and initial free fluoride concentrations. All live organic-HF acids containing dissolved Fe (III) showed no precipitation even after the Fe (III) level reached 10,000 mg/L; however, when the pH solution value was raised, none of the tested organic-HF acids was able to prevent iron fluoride (FeF 3 ) precipitation. On the other hand, the main factor that controlled the aluminum fluoride (AlF 3 ) precipitation was found to be the flouride/aluminum (F/Al) ratio. It was found that there is a critical F/Al ratio above which AlF 3 precipitation occurred. This article also discusses the effect of other factors, such as temperature, on both AlF 3 and Fe (III) fluoride precipitations. INTRODUCTION Mud acids containing different hydrochloric/hydrofluoric (HCl/HF) acid ratios have been extensively used to dissolve primarily clays and feldspars, and to a less extent silica, and therefore increase well productivity or injectivity 1-8 . While HF Aluminum and Iron Precipitation during Sandstone Acidizing Using Organic-HF Acids Authors: Bader G. Al-Harbi, Dr. Mohammed N. Al-Dahlan, Dr. Mohammed H. Al-Khaldi and Dr. Mahmoud Doklah