How does the casing shoe pressure respond when the well is shut in and gas moves up the annulus?

When the well is shut in and gas starts to migrate up the annulus, the casing shoe pressure is affected primarily by the expansion of the gas. As gas rises through the annulus, it expands due to the decrease in pressure as it moves upwards. This expansion results in an increase in volume, which causes a corresponding increase in pressure at the casing shoe.

It is important to note that the pressure at the casing shoe reflects the pressure exerted by the fluids (including gas) in the annulus. Therefore, as more gas accumulates and exerts pressure upward, the casing shoe pressure will rise. This principle is grounded in fundamental fluid dynamics and well control concepts, where the pressure in a confined space increases with the introduction of gas.

In scenarios where gas migrates into the annulus upon a shut-in, the pressure response at the casing shoe is thus directly linked to this phenomena of gas expansion and accumulation, leading to an observable increase in casing shoe pressure.