The so-called “Hang's Mudding-off” technique is critical to keeping the dynamic pressure balance and guaranteeing the sufficient safe operation time in wellbore. However, for lack of dynamic mathematical model analysis methods for reflecting the changes of annulus liquid level in the borehole during the “Hang's Mudding-off” operation, the actual operation is basically conducted blindly without reasonable engineering basis. According to the actual conditions, a mathematical model for the safe time of the “Hang's Mudding-off” was, for the first time, built up by using the dynamic borehole leakage analysis method. Then, the integral results of leak-off rates were calculated. Finally, the calculation results were verified by field cases. It is shown that the calculation results are highly accordant with the actual data, indicating the reliability of the mathematical model. The safe operation time can be increased by increasing mud amount or reducing mud density appropriately. With this model, the safe time of “Hang's Mudding-off” operation can be calculated accurately. This research result is of great significance to avoiding well control risk of absorption wells, optimizing the “Hang's Mudding-off” technique and reducing project cost.
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