Mixtures of HCl-HF, known as mud acid, have been extensively used in sandstone matrix stimulation. However, mud acid treatments were applied with poor success rate. This is mainly due to complex acid/rock interactions and other factors, such as the presence of HCl-sensitive clays and poorly consolidated formations. In addition, precipitation of reaction products of sandstone minerals dissolution in HF, especially at elevated temperatures, is another common factor limiting the mud acid effectiveness. These limitations associated with the use of typical HCl-HF mixtures warrant the use of organic-HF acids. These mixtures are used as an alternative to the HCl-HF blends to stimulate formations with HCl-sensitive clays and corrosion concerns. Various organic-HF blends have been extensively used; however, the studies of their interactions with sandstone rocks have been relatively limited. This work, for the first time, provides a systematic investigation of the chemical interactions of various sandstone minerals with three different organic-HF acids over a wide range of parameters.

Organic-HF systems based on acetic, formic, and citric acids were studied. Using static solubility tests coupled with chemical analysis of spent acid solutions, the dissolution of different sandstone minerals in each system was evaluated as a function of organic acid/HF weight ratio, reaction time, and temperature values of 50 and 75°C. The Scanning Electron Microscopy (SEM) and the X-ray fluorescence (XRF) techniques were used to explore the possibility of the presence of any precipitated reaction products.

Results based on this study showed that all organic-HF acids tested have a retarded nature with sand and clay minerals when compared to regular HCl-HF mixtures. In general, the dissolution of different sand and clay minerals in each organic-HF system is dependent on the type, ratio of the organic acid/HF used, and the temperature value. The reaction rate of different organic-HF acids with sand and clay minerals increased when the temperature value was increased from 50 to 75°C.

Findings based on this study addresses various chemical interactions and precipitation mechanisms involved with the reaction of organic-HF acids with sand and clay minerals. This work highlights the effects of the initial acid concentration, temperature, and soaking time on the performance of organic-HF acids. In addition, it provides new insights into the field application, and potential limitations of organic-HF acids.

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