You calculate the maximum initial shut-in casing pressure (MISICP) based on the original mud weight before you start drilling ahead. Once you drill deeper, you may increase mud weight. With new mud weight, you are not able to use the MASICP calculated by the initial weight because higher mud weight will reduce the MASCIP. The formula below demonstrates you how to adjust the MASICP with new mud weight.
Adjusted MASICP = Leak off pressure – [Shoe TVD x (MW2 – MW1)] x 0.052
Adjusted MASCIP = maximum allowable shut-in casing pressure in psi
Leak off pressure = pressure you get when you perform leak off test in psi
Shoe TVD = true vertical depth of casing shoe in ft
MW2 = current mud weight in pgg
MW1 = original mud weight in ppg
Let’s learn about it via this example.
Casing shoe depth is at 5000’MD/4500’TVD. Leak off test is performed with 9.5 ppg and the leak off pressure is 1000 psi. The current operation is drilling ahead with 12.0 ppg. Determine the adjusted MASCIP with current mud weight.
Adjusted MASICP = 1000 – [4500 x (12.0 – 9.5)] x 0.052
Adjusted MASICP = 415 psi
If you take a kick with 12.0 ppg mud in hole, you maximum allowable surface casing pressure is only 415 psi.