Dynamic MAASP and Kill Speed: Understanding Pressure Losses During Well Control
When calculating the Maximum Allowable Annular Surface Pressure (MAASP), we typically do so under static conditions. However, once the pumps are started during a well kill operation, we immediately face annular pressure losses. These losses, which occur as fluids move through the wellbore, add to the pressure and increase the risk of fracturing or breaking down exposed formations. To minimize this risk, it’s important to manage these dynamic pressures by controlling the kill rate (pump rate), which helps to reduce the additional pressure applied during the kill process.
The Role of Dynamic Pressure Losses
In deep, slim-hole wells, annular pressure losses (APL) can become significant, even at slow kill rates. If not properly managed, this pressure could cause the shoe (the bottom part of the casing) to break down. If you know the annular pressure loss at the shoe, you can adjust the casing pressure accordingly to offset this loss. However, this action reduces the MAASP by the same amount, leading to what is known as the Dynamic MAASP.
Calculating the dynamic MAASP can be complex, especially when determining annular pressure loss accurately. As a result, dynamic MAASP is not commonly used in surface stack operations where APL is harder to measure.
Subsea Wells and Choke Line Friction
Subsea wells are a different story. In these wells, there can be significant dynamic pressure losses through the choke line. Much like annular pressure loss, these losses contribute to the overall pressure in the wellbore. The good news is that choke line friction is easy to measure, which makes it possible to account for it when adjusting casing pressure.
To avoid exceeding MAASP, it’s common practice to reduce the casing pressure by the amount of choke line friction as the pumps are brought online during a kill operation. This adjustment effectively reduces the dynamic MAASP by the same amount as the choke line friction.
Calculating Dynamic MAASP
The equation for calculating the Dynamic MAASP is simple:
Dynamic MAASP (psi) = Initial MAASP (psi) – Choke Line Friction (psi)
Example Calculation:
Let’s look at an example of a subsea well.
- Initial MAASP = 1638 psi
- Choke Line Friction at 40 SPM = 160 psi
To calculate the Dynamic MAASP when bringing the pumps to 40 strokes per minute (SPM) at the start of the kill:
Dynamic MAASP = Initial MAASP (1638 psi) – Choke Line Friction (160 psi)
Dynamic MAASP = 1638 psi – 160 psi = 1478 psi
So, after adjusting for choke line friction, the new dynamic MAASP pressure would be 1478 psi.
Conclusion
Managing dynamic pressures during a well kill operation is critical to preventing damage to the wellbore and formations. By understanding how to adjust the MAASP for annular pressure losses and choke line friction, operators can maintain control of the well while minimizing the risks associated with pressure increases.
For more information on dynamic MAASP, you may refer to my Well Control Manual at: https://learn-well-control.com/product/well-control-manual-by-edwin-ritchie/
