Safety risks and countermeasures on the wells in underground gas storage rebuilt from depleted gas reservoir

The geological complexity results in multiple risks in the drilling engineering of underground gas storage (UGS), e.g. leakage, sticking, poor cementing quality, long period and high cost. In order to reduce these risks, a series of technical measures were taken according to the actual geological situation of the underground gas storage, such as casing program optimization, gas drilling, layered plugging under pressure, two-stage cementing, cementing slurry system improvement (double-density slurry cementing and flexible self-stress slurry system), temporary plugging and well killing technology for low-pressure carbonate reservoir, ultrafine plugging slurry system and supporting technology for old well plugging. By virtue of these measures, the accident risk of UGS well drilling and completion is reduced significantly, the cementing quality is improved and the investment cost is controlled effectively. The research results can be used as the guidance and reference for the implementation of subsequent UGS well drilling and completion engineering.