Abstract: Nanpu buried-hill hydrocarbon reservoirs are buried deeply. This results in that wells drilled to develop these reservoirs have complicated borehole structure, longer drilling time and extremely high cost. To further enhance economic performances of oil exploration and development, tests for window side-tracking completion in existing ?177.8 mm casing were conducted. Well NP280C is the first well with window side-tracking in ?177.8 mm in the Nanpu Oilfield. Significant window side-tracking depth and smaller sizes in borehole of the second completion brought multiple challenges to window side-tracking operations. By selecting optimal window side-tracking processes for casing in deep formation, slim hole expansion techniques and completion with expandable tailpipe hanger+direct connection with casing, a series of drilling and completion techniques have been developed to guarantee successful implementation of casing window side-tracking in Well NP280C. These techniques may provide necessary technical support for re-vitalization of existing wells for deep buried-hill reservoirs in Nanpu Oilfield and may also provide technical references for side-tracking operation in existing wells in similar reservoirs.