Formation damage is critical factor in reducing production of oil and gas. This damage effects production at any stage during life of the a well. There could be many reasons of formation damage but this work is describing formation damages in sandstone reservoir during stimulation jobs. This damage took place due to precipitation of colloidal silica, swelling clays, iron bearing cements formed in resuits of reaction with quartz, iron minekels and clay particles with acid. Reaction products enter into porous media and adversely affect well performance of reservoir. Such damages are necessarily irreversible; the substance get into porous media not necessarily come out