Abstract:  With the continuous development of offshore oil fields, more and more old oil and gas fields are facing well abandonment. The well abandonment of offshore oil and gas fields may influence the marine ecological environment greatly. Besides, the technical equipment and process requirements of well abandonment are rigorous, involving huge costs, thus it has become the focus of attention. We introduce the permanent well abandonment of the production wells in Platform S in Bohai Oilfield. The platform equipment is old, all wells are with the same pressure in tubing and casing, and multi-layer casing with pressure exists. By analyzing the pressure source of the multi-layer casing, it is judged that the wellbore plugging of the old well fails after sidetracking, gas flows along the annulus of the production casing and the technology casing, and the wellhead aging causes the seal failure between the multi-layer casings. Based on the analysis, killing well program is made. In the production casing for hydraulic cutting, cement plug is used above and below the cut to plug the gas channeling, and cement seal is used fo the injection cement bridge plug to complete the effective isolation of gas channeling. Through the cutting process optimization, the overall cutting of 339.7 mm， 508 mm and 762 mm three-layer casing is completed by one operation. The successful completion is the first large-scale one in the production platform for the old wells abandonment operations in Bohai Oilfield. A set of techniques to solve the permanent well abandonment of oil and gas fields and the implementation of process management measures in complex well conditions are achieved, which can provide reference for the future well abandonment of offshore oil and gas fields.