Mud acid, which is composed of HCl and HF, is commonly used to remove the formation damage in sandstone reservoirs. However, many problems are associated with HCl, especially at high temperatures.Formic-HF acids have served as an alternative of mud acid for a long period.Several factors may influence the outcome of an acidizing job in sandstone formations.In this research, effects of mineralogy, temperature, and HF concentration were studied.Various clay minerals (kaolinite, chlorite, and illite) were examined to react with formic-HF acid mixtures which contain different concentrations of HF. Coreflood experiments on sandstone cores featured by different mineralogy with dimensions of 1.5 in. × 6 in.were also conducted at a flow rate of 5 cm 3 /min.