When the drill string is pulled out of the hole, the mud level decreases by a volume equivalent to the steel volume. If the hole does not take the calculated volume of mud, it is assumed a formation fluid has entered the wellbore. Even though gas or salt water entered the holes, the well may not flow until enough fluid has entered to reduce the hydrostatic pressure below the formation pressure.
Therefore, while pulling out the well should be filled continuously by using trip tank and differences of calculated and actual mud volume be recorded at regular interval. Similarly while running in drill string, trip tank should be used to monitor displacement volume correctly at regular intervals.
If the hole is not filled to the replace the steel volume, the fluid column in the wellbore shall go down and reduce the hydrostatic pressure. At the same time the pulling out of drill string causes a reduction in BHP due to swabbing effect. Therefore to avoid the possibility of any formation fluid entering the bore hole due to combination of above two factors the should be properly / regularly filled during tripping out.
In the field normally the practice is to fill up the hole either on a regular fill up schedule or to fill up continuously with a re-circulating trip tank. Irrespective of the practice being used an accurate method of measuring the amount of fluid actually being taken by hole should be monitored and an accurate record of actual volume v/s theoretical volume should be kept. If at any stage during pulling out it is observed that the actual filled in volume is significantly less than volume of steel that has been removed, it means that some formation fluids must have entered the well bore.
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