Choke line friction pressure will increase bottom hole pressure while circulating to kill the well therefore it must be compensated in order to maintain the bottom hole pressure relatively constant. This section will describe how to compensate the choke line friction while bring pump up to speed.
Compensate The Choke Line Friction By Using Casing Pressure Gauge
At static condition – the bottom hole pressure can be described like this
Bottom Hole Pressure = Hydrostatic Pressure in Annulus + Casing Pressure
Figure 1 – Bottom hole pressure at static condition
At dynamic condition – the bottom hole pressure can be described like this
Bottom Hole Pressure = Hydrostatic Pressure in Annulus + Casing Pressure + Choke Line Friction + Annular Pressure Loss
We will use acronym like this to make it easy.
BHP = HP annulus + CP + CLF + P loss annulus
Since the pumping rate while performing well control operation is not high therefore the pressure loss in the annulus is negligible.
BHP = HP annulus + CP + CLF + P loss annulus
While circulating, CLF will increase therefore CP must be intentionally decreased at the same amount of CLF in order to keep the bottom hole pressure relatively constant.
BHP = HP annulus + ↓CP + ↑CLF
Figure 2 – Bottom Hole Pressure at Dynamic Condition
Let’s look into this example for more understanding. The well is shut in with 500 psi shut in casing pressure and 300 psi shut in drill pipe pressure. The plan is to circulate using 30 SPM.
The table below is the pre-determined CLF.
If we select 30 SPM as a kill rate, you will get 450 psi CLF therefore you need to reduce casing pressure by 450 psi to maintain the bottom hole pressure (see Figure 3). The casing pressure after the kill rate is fully established will be equal to initial shut in casing pressure minus CLF which is 50 psi.
Figure 3 – Compensate CLF by Reducing CSG pressure
Figure 4 shows simple charts representing the compensated casing pressure while bringing the pump up to kill rate at 30 SPM.
Figure 4 – Chart Showing SPM and Casing Pressure (CP)
Compensate The Choke Line Friction By Kill Line Pressure
This is another way to start circulation with constant bottom hole pressure by using kill line pressure gauge. Firstly, the circulation path is lined up to the choke line up to surface and the valves in the kill line must be opened just for reading the kill line pressure. Kill line will not be used as a circulation path. At this point, choke and kill like can read pressure separately.
Secondly, the circulation is brought up to kill rate by holding kill line pressure constant. The benefit of using the kill line pressure is that there is no friction pressure at the kill line side because no fluid is circulated through the kill line. With the maintained kill line pressure, choke pressure will decrease by the amount of CLF at a particular flow rate. The static kill line pressure will maintain the bottom constant bottom hole pressure, like casing pressure gauge on a surface BOP well control. Figure 5 demonstrates the line up and the circulation path.
Figure 5 – Compensate CLF by Using Kill Line Pressure Gauge
For some advanced subsea BOP’s, they are equipped with BOP gauges. The BOP pressure gauge can be used to maintain the bottom hole pressure constant, like the kill line pressure gauge. The process is the same as the procedure when you use kill line pressure gauge but only you monitor the BOP pressure gauge. Figure 6 shows the configuration of CLF compensation using the BOP gauge.
Figure 6 – Compensate CLF by Subsea BOP Pressure Gauge
Reference books: 
Well Control Books
