Choke Line Friction pressure (CLF) has directly affect on the bottom hole pressure while performing well kill operation in a deepwater operation. You may be able to circulate the kick out of the well without breaking the shoe down with the current mud weight however; the well can be fractured when the kill weight mud reaches the surface due to excessive CLF.
How Kill Weight Mud and CLF Will Break the Shoe
When the kill mud is circulated from the bit to surface as per the second circulation of driller’s method, drill pipe pressure is held constant and the choke is gradually opened. Once the choke is in the fully open position, the back pressure due to choking back the well is gone but the CLF is still there. With the heavier weight, the CLF with KWM will be more than the CLF with the original mud. If the hydrostatic pressure is more than the fracture pressure, the formation will be broken down. Let’s take a look at the following calculation to get clearer picture of this topic.
Example: The well information is listed below;
- Water depth = 5,000 ft
- Shoe depth = 15,000 ft TVD
- Hole depth = 25,000 ft TVD
- Kill mud weight = 12.5 ppg
- Choke line friction pressure @ 25 spm = 500 psi with 12.5 ppg
- Shoe fracture pressure = 13.0 ppg
- Neglect annular pressure loss in the well due to low flow rate
Casing pressure is 0 psi due to fully opened choke, the shoe pressure be determined by the following calculation process.
Figure 1 – Shoe Pressure with Original Mud Weight
BHP @ shoe = MW + ((Casing Pressure + Choke Line Friction) ÷ 0.052 ÷ Shoe TVD)
BHP @ shoe = 12.5 + ((0+500) ÷ 0.052 ÷ 15,000) = 13.2 ppg (round up figure)
Figure 2 – Shoe is fractured due to Choke Line Friction
Without any surface casing pressure (Figure 2), the bottom hole pressure (13.2 ppg) is still exceed the shoe fracture gradient (13.0 ppg). Therefore, the formation will be broken. As you can see from the example, the excessive CLF while kill weight mud is coming out to surface can fracture the formation.
How To Prevent the Effect of Choke Line Friction Pressure
There are few ways to prevent this issue as listed below;
- Use the large choke diameter as much as possible – This should be done at the rig selection stage. It will cost additional cost if you want the existing choke line to be upgraded.
- Reduce kill rate – Reduce flow rate will cut down choke line friction due to square relationship. However, the circulating time will increase.
- Circulate both choke line and kill line at the same time – The flow will distribute to both choke and kill line; therefore, this option will still maintain the same flow rate but the frictional pressure is reduced by approximately 75% due to square relationship. There are few disadvantages too. The first one is that without BOP sensor, you need to back off the friction pressure manually and you need to know the frictional pressure by circulating through choke and kill line together. You also have the potential to loss of redundancy if both lines are plugged off.
Reference books: Well Control Books