(1) During the drilling and drilling operations, a self-sealing device must be installed at the wellhead to prevent objects from falling into the well. (2) During drilling and well cleaning operations, install a filter screen at the outlet of the water supply pipeline and return water pipeline on the pump truck to ensure that impurities do not enter the well. (3) During drilling, cementing, and well completion operations, ensure that the wellbore is thoroughly cleaned, and prevent any objects from falling into the well during wellhead construction. (4) Before performing drilling and cleaning operations, thoroughly assess the debris or substances present in the wellbore and select appropriate tools for the task.
Well control:
Well D12-P3 completed fracturing on 26 July. From 27 July to 8 August, fluid was discharged, with a total discharge volume of 1,752 m³ and a return rate of 39.6%. During this period, gas was observed at the outlet, with a combustible gas concentration of 28–38%. The gas was ignitable but not continuous. From 9th to 13th August, the casing was reinserted, and the sand-flooding completion string was lowered to 3,342.39 metres before encountering resistance. After sand-flooding to 3,348 metres, a large amount of nitrogen gas was discharged from the outlet. Sand-flooding was stopped, and multiple attempts to depressurise the well failed.
Cause Analysis (1) Sand discharge during blowout may lead to the formation of a sand bridge in the horizontal section, resulting in incomplete well control. After the sand bridge is breached, gas in the wellbore may surge upward, causing fluid return at the wellhead; (2) After well control, the wellbore maintains a certain pressure. Following pressure diffusion or relief after well closure, if the observation period is too short, misjudgment may occur, leading to the mistaken identification of fluid return at the wellhead as overflow, resulting in repeated well control operations. Preventive Measures and Recommendations (1) After fracturing, adjust the blowout procedures based on actual site conditions to ensure rapid fluid removal while controlling sand production, preventing sand accumulation in the wellbore, and reducing sand blockages in the tubing; (2) Select appropriate well control fluid for well control operations. After pressure diffusion or pressure relief, accurately assess the wellhead conditions. (3) If nitrogen gas or liquid is observed at the outlet after sand bridge dislodgement, cease sand removal and well control operations, connect the process flow to continue fluid discharge, and allow nitrogen gas from the fracturing fluid to exit the wellbore. (4) If wellhead pressure is exceptionally high and well control is difficult, it is recommended to use pressure-bearing equipment for tubing replacement or continuous tubing equipment for circulating well control.
