I still have the simple but interesting question about hydrostatic pressure which you can apply this calculation into drilling/cementing operation. The question I got about how much pressure we will see at cement head in case of float shoe fail.
Given situation as shown in figure above. This situation is in vertical well.
1) Inside 16” shoe, from 0’ to 1975’ = 9.3 ppg mud
From 1975’ to 2000’ = 16.0 ppg mud
2) Outside 16” shoe, From 0’ to 1500’ = 11.6 ppg cement (lead)
From 1500’ to 2000’ = 16.0 ppg cement (tail)
Given the conditions above and assuming the cement is still liquid, how much pressure will we see at cement head in case of float shoe fail?
** Note : you need to understand how to calculate hydrostatic pressure in order to fully understand this question **
Using U-tube concept: Bottom hole pressure both sides are the same.
Let’s work out at annulus side which is heavier due to cement in the annulus
Pressure at bottom hole in annulus = hydrostatic pressure of lead cement + hydrostatic pressure of tail cement
Pressure at bottom hole in annulus = 0.052×11.6×1500 + 0.052x16x(2000-1500) =1320.8 psi
Since, hydrostatic pressure in the annulus is more than hydrostatic pressure in 16″ casing; therefore, there will be pressure in the cement head in order to balance u-tube.
We can simply write equation as follows;
Bottom hole pressure = hydrostatic pressure inside 16” casing + surface pressure at cement head
1320.8 = 0.052x16x(2000-1975) + 0.052×9.3×1975 + surface pressure at cement head
Surface pressure at cement head = 1321 – 976 = 345 psi