Activity: Gas Migration











Tutorial prepared by Eltun Talibov, Drilling Engineer at State Oil Company of AZERBAIJAN REPUBLIC (SOCAR). 

For any questions:   LinkedIn contact: Eltun Talibov ,  Email: Eltun-talibov98@mail.ru


Which Pressure Should Be Kept Constant During Gas Migration in a Shut-In Well and WHY?

 

Let’s first visualize the scene:

Once the well is shut in on a kick, i.e. the BOP closed, and allowed to stabilize, and if drill string is in the hole, two pressures can be observed:

·         Shut in drill pipe pressure – SIDPP

·         Shut in casing pressure – SICP




All of a sudden, the same amount of increase on both gauges is observed. This indicates that this is a gas kick migrating up and increasing pressure in the wellbore. And you aren’t done with calculations or weighing up the kill mud yet or due to some problems, kill operation can’t be commenced. Here are the questions:

What is the safest course of action? How can Bottom Hole Pressure be kept constant in this case?

 The answer is:

Keep SIDPP constant. The reason why SIDPP must be kept constant is that SIDPP shows how much underbalanced the well is, i.e. SIDPP is the difference between the formation pressure (FP) and the hydrostatic pressure of the mud in the drill pipe. Although SICP indicates the amount of underbalance too, we must remember that the annulus is not a full column of clean mud, but also contains the influx (here a gas kick). So, if keeping SICP constant is attempted, gas expansion in the annulus may cause hydrostatic pressure to drop and create a chance for a second influx.

Note: You have to be pro-active here:

 While opening choke little, wait for the delay on the drill pipe, don’t open until you see SIDPP start dropping and then close (the well may go underbalance).




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