Understand shoe pressure – Top of Gas Kick Below the Shoe

I would like to address about the shoe pressure concept while while circulating gas influx out of wellbore. This is the fist scenario – Top of the gas kick below the casing shoe

The hydrostatic pressure above the casing shoe remains constantly because the fluid column is the same. If we don’t let casing pressure increase, the over all hydrostatic pressure in the well will decrease because gas expansion when it is being circulated. In order to maintain the bottom hole pressure constant, casing pressure will increase to balance the loss of hydrostatic pressure in the wellbore due to gas expansion.

You can express in term of an equation below;

Bottom Hole Pressure = Casing Pressure + Hydrostatic Pressure in the annulus

Let’s take a look at casing shoe and we can write the relationship as follows;

Pressure at shoe = Hydrostatic Pressure above shoe + Casing Pressure

If the gas influx is still below the casing shoe, the hydrostatic pressure above the shoe remains constant and the casing pressure will increase due to gas expansion during circulation; therefore, pressure at shoe will increase.  The shoe pressure will reach the highest pressure when the gas influx reaches the casing shoe because this is the point which gas expansion is at the maximum value while still has whole hydrostatic column above shoe.

Conclusion: Shoe pressure will increase until the top of the bubble reaches the casing shoe.

Ps – assumptions : water base mud is used and kick is gas.

Reference book: Well Control Books