Abstract: In the absence of vacant slots, selecting old wells with depleted productivity for the borehole disposal, and then relying on existing jacket facilities to sidetrack directional wells or extended reach wells, which can economically and effectively develop small structures around existing offshore oil and gas fields. Considering that the drag torque during the sidetracking process is the key factor restricting the extension limit of the sidetracking hole,by designing the most optimized well trajectory, selecting the best sidetracking plan, and maximizing the disposal of the old borehole to obtain the best sidetracking position, it is the key to reduce the drag torque and rig load during sidetracking drilling and completion. It is also the key to ensuring the safe and efficient completion of sidetracking of deep wells or extended reach wells. In the East China Sea area, by carrying out technical research related to the disposal of old boreholes before sidetracking, the sidetracking plan is optimized, the casing cutting position and the casing salvage construction technology during the disposal of the old well are optimized, and the fishing tools are optimized and processed. Using two wells with exhausted production capacity for disposal and reuse of old boreholes, two ultra-deep extended reach wells are drilled successfully. Through the disposal and reuse of old boreholes, we have accumulated rich operating experiences for the salvage and recovery of long-distance and large-size casing operations, a complete set of new technologies for the disposal of old boreholes casings in the East China Sea has been formed, and valuable technical experiences have been accumulated for the future use of sidetracking adjustment wells in old well slots.