Abstract: The casing milling of Well SU7 in Suqiao Gas Storage was exposed to a series of risks due to its deep and slim hole.For example,returning of iron filings was difficult,drill pipe sticking tended to happen easily and the fishing head was lost.To solve these problems,milling parameters and drilling fluid property parameters were determined according to the milling speed control equation and the iron fillings suspension force calculation equation.In addition,the driving mode based on hydraulic swivel was adopted,and the bottom hole assembly was optimized.Thus,a casing milling technology suitable for deep and slim holes was developed.This technology was applied in Well SU7,no downhole failure occurred during 50.0 m continuous casing milling,and the casing milling efficiency more than doubled.The results demonstrated that this casing milling technology is suitable for deep and slim holes and can meet the milling requirements of plugged casing sections in abandoned wells in Suqiao Gas Storage,improve milling efficiency and avoid downhole failure.This techniqueis applicable to the casing milling of oil and gas wells having similar conditions.