There are several terms/acronyms about maximum surface pressure in well control such as MASP, MISICP and MAASP. These terms sometimes confuses a lot of people hence this article will explain each term and demonstrate how to use it.
Leak Off Test (LOT)
The first factor you need to understand is Leak of test pressure (LOT). LOT is the surface pressure that breaks down formation at a casing shoe for each section of the well.
Leak off test pressure formula is listed below;
Leak off test pressure, psi = Surface pressure to break formation, psi + Hydrostatic pressure, psi
Typically, leak off test pressure is describe in equivalent mud density term therefore the formulas will be like this
Leak off test pressure, ppg = (Surface pressure to break formation, psi ÷ 0.052 ÷ shoe TVD, ft) + Mud weight, ppg
Maximum Allowable Surface Pressure (MASP)
Maximum Allowable Surface Pressure (MASP) is based on surface equipment rating and most of the time, the MASP is determined by a percentage of the casing burst pressure. Generally, 80% is used for derating from the original casing burst pressure however it can be less than 80% if the well is very old and the casing is in very bad shape.
MASP, psi = percentage of casing burst x casing burst pressure, psi
Maximum Initial Shut-In Casing Pressure (MISICP)
Maximum Initial Shut-In Casing Pressure (MISICP) is the maximum casing pressure before fracturing the casing shoe when the well is shut due to well control. MISICP formula is listed below;
MISICP, psi = (Leak Off Test pressure, ppg – current mud weight, ppg) x 0.052 x Casing shoe TVD, ft
Maximum Allowable Annular Surface Pressure (MAASP)
Maximum Allowable Annular Surface Pressure (MAASP) is the maximum annular pressure which will cause formation break down. MAASP can be in a static condition and a dynamic condition (circulating).
At the static condition, MAASP will be same as MISICP and the equation is listed below;
MAASP, psi = (Leak Off Test pressure, ppg – current mud weight, ppg) x 0.052 x Casing shoe TVD, ft
At the dynamic condition, due to friction pressure in the annulus while circulating, it is very difficult to calculate an accurate MAASP therefore it is not recommended to determine the dynamic MAASP while circulating the kick out of the well. Furthermore, you should NOT use MASSP at the static condition while circulating. For example, you determine the static MASSP of 1000 psi and while circulating, casing pressure can go more than 1000 psi. If you try to lower the casing pressure down by misleading the interpretation of this value, the additional kick will go into the well and finally it will make the well control situation even worse.
Example: 9-5/8” casing was set at 8,500MD/8,000’TVD.
9-5/8” casing : L-40, 43.5 lb/ft, burst pressure = 6,330 psi, collapse pressure =3,810 psi
Leak off test at 9-5/8” casing shoe = 15.0 ppg equivalent mud weight
Current hole depth is 12,000’MD/10,000’TVD and current mud weight is 10.0 ppg
20% de-rate burst pressure
Figure 1 – Well Schematic
Determine: MASP, MASSP, MISICP with current mud weight. What will happen if the current mud weight is 12.0 ppg?
Maximum Allowable Surface Pressure (MASP) = 0.8 x 6330 psi = 5064 psi
Maximum Initial Shut-In Casing Pressure (MISICP) = (15 – 10) x 0.052 x 8,000 = 2,080 psi
Maximum Allowable Annular Surface Pressure (MAASP) at the static condition is equal to MISICP.
Maximum Allowable Annular Surface Pressure (MAASP) = (15 – 10) x 0.052 x 8,000 = 2,080 psi
At dynamic condition, you need to determine the frictional pressure to get an accurate dynamic MAASP.
For this case, if the well is shut in due to well control, the weakest point is at the shoe because it will be fractured before the surface equipment fails.
If the mud weight increases to 12.0 ppg, MISICP and static MAASP will reduce.
MISICP = static MAASP = (15 – 12) x 0.052 x 8,000 = 1,248 psi.
- MAASP in a static condition is the same as MISICP.
- MASP depends on how the surface equipment looks like. It may be derated due to corrosion, age, etc and it can be the weakest point of the well.
- The higher the mud weight is, the lower MAASP and MISICP are.
Reference books: Well Control Books