AlCl 3 has been used as a retarding agent for regular mud acid. It has been studied in the lab and tested in the field. However, the mechanism of AlCl 3 retardation has never been determined and the reactions of fines control acid (15 wt% HCl, 1.5 wt% HF, and 5 wt% AlCl 3 ·6H 2 O) with clay minerals and sandstones at different conditions have never been fully examined. To enhance the acid performance and to minimize formation damage, a systematic investigation to the interactions between the fines control acid and clay minerals in sandstone reservoirs has been provided in this study. Also, for the first time, 19 F Nuclear Magnetic Resonance (NMR) spectroscopy was used to determine the reaction of fines control acid with clay minerals.Solubility tests were performed to evaluate the retardation of the fines control acid when reacted with kaolinite, bentonite and illite. Inductively Coupled Plasma (ICP) and 19 F NMR were utilized to analyze the concentrations and ratios of key cations and components in the supernatant while the Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) techniques were used to identify the reaction products and explore the possibility of the presence of any precipitations. Coreflood tests on sandstone cores featured with different mineralogy were also conducted at different conditions. The core effluent samples were analyzed by ICP to determine the concentrations of Si, Al, Ca, Fe, and Mg.This study showed AlCl 3 can retard the reaction of HF with kaolinite, bentonite or illite at 75°F in fines control acid. Even with 5 wt% AlCl 3 ·6H 2 O added into the acid system, no AlF 3 precipitate was observed in any of the solubility tests. 19 F NMR results showed several aluminum fluorides species that were not identified before. H 2 SiF 6 and HSiF 5 were also recognized in the spent fines control acid. Based on these new results, new mechanisms were developed to better understand the retarding effect of this acid system, and how this acid reacts with different clays.