When the well is shut in and gas migrates up the annulus, what is the effect on the BHP?

When a well is shut in and gas migrates up the annulus, the bottom hole pressure (BHP) is affected by the presence of gas in the annulus. As gas migrates, it can create a situation where the hydrostatic pressure from the liquid column that was counteracting the formation pressure is reduced.

Gas is significantly less dense than liquid, and as it rises, it displaces the liquid column above it. This leads to a decrease in the overall hydrostatic pressure exerted at the bottom of the well. The increased gas volume in the annulus contributes to a rise in pressure within that space. However, because the BHP is primarily a function of the hydrostatic pressure exerted by the column of liquid in the wellbore, and this liquid column is effectively being reduced in height due to gas migration, the BHP increases in response to the gas migration, even while the pressure in the annulus increases due to the gas.

In summary, when gas migrates up the annulus, it leads to an increase in the BHP due to the hydrostatic changes that occur as the liquid column is affected. This understanding of how gas behaves in annular spaces is crucial for managing well control and ensuring safety during drilling