Fluctuations in the borehole pressure has been generally recognized as one of the major problems in aerated mud drilling due to its detrimental impact on borehole stability and formation damage. This paper focuses on analyses of bottomhole pressure fluctuations due to circulation break and backpressure variation. The pressure fluctuations due to circulation break is characterized by the gradient loss defined as ECD less EMW. The pressure fluctuation due to backpressure change is evaluated using the pressure instability factor defined as the ratio of change in the bottomhole pressure to the change in the surface choke pressure.

Sensitivity analyses with an analytical model indicate that the gradient loss decreases with depth, increases with both liquid pumping rate and gas injection rate, and decreases with backpressure. The analyses also show that the pressure instability factor increases with depth, decreases with both liquid pumping rate and gas injection rate, and increases with backpressure. At a given depth and backpressure, it is the combination of liquid and gas flow rate, rather than the injection GLR, that determines the pressure instability factor. Following a change in the backpressure, the transition time for borehole pressure to stabilize at a new level depends on several factors including well depth, well geometry, and liquid pumping rate.

This paper presents an easy-to-use method for predicting pressure fluctuations and identifies key factors affecting the pressure stability in aerated mud drilling. The results can be applied to designing of hydraulics for Underbalanced drilling (UBD) and managed-pressure drilling (MPD) wells.

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