Kill mud weight

Kill mud is the mud required to kill the well in case the Kick has taken place. To calculate the kill mud weight required, first we need to calculate formation pressure due to which kick has taken place.

kill mud weight

At equilibrium, bottom hole pressure becomes equal to formation pressure and there is no further inflow of formation fluid into the borehole. Also surface shut-in pressures become stable at this point.

Formation pressure=BHP = Hydrostatic pressure on drill string side
+ Shut in drill pipe pressure
= 0.052 * MW * TVD + SIDPP

While killing the well, formation pressure should be balanced by the hydrostatic pressure due to kill mud

Hydrostatic pressure due to kill mud = Formation pressue
0.052*TVD*Kill mud weight = Formation pressure= 0.052 * MW * TVD + SIDPP
Kill mud weight= MW + (SIDPP + 200) / (0.052*TVD)

Here we have added 200 psi of overbalance for safety margin which may differ based on the situation. After calculating the kill mud weight, well kill method which suits the well condition is chosen.

Example:- While Drilling 8 ½’’ hole with 11.2 ppg mud at 9860 ft a kick has taken place which was observed as gain in pit volume. To confirm, flow check was performed. During flow check well was found to be flowing and was immediately shut. Following are the shut in pressure readings:-
SIDPP= 400 psi
SICP=500 psi
calculate formation pressure and kill mud weight using overbalance of 100 psi.

shut in pressure
Sol. Formation pressure = 0.052 * MW * TVD + SIDPP
= 0.52*11.2*9860 + 400
= 6142psi

Kill mud weight= (6142+100)/ (0.052*9860)
= 12.17ppg = 12.2 ppg (rounded-off)

Here we have used drill string side pressures (SIDPP and mud hyrdostatic pressure on drill string side) to calculate formation pressure and kill mud weight, which is generally the case. But in some special cases, SIDPP can’t be seen at the surface or SIDPP is zero, such problems will be dealt in upcoming articles.

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