Correct initial circulating pressure is very important for well control operation because the correct ICP figure will tell you about the balance point while circulating the influx out using Driller’s method. Many people get confused when they try to get the right ICP because they tend to forget about a safety factor that is added while bring the pump to speed. Hence, the ICP with unintentionally added safety margin may fracture formations downhole because people will tend to add more safety factor while circulating. It means that excessive safety factor added into the system will make the well control operation worse.
The example below will demonstrate you how to establish correct ICP
Well shut in after the influx is detected.
Stabilized shut in casing pressure = 600 psi
The float is bumped and the SIDPP is 450 psi
Figure 1 – Pressure at Shut-In Condition
The pump is brought up to speed at 30 SPM for well kill operation by holding casing pressure constant
Casing pressure = 700 psi
Drill pipe pressure = 1,300 psi
Figure 2 – Pressure after Pump is brought to Speed.
Is 1,300 psi the correct Initial Circulating Pressure (ICP)?
The answer is NO.
If you don’t watch the pressure carefully, you will maintain the 1,300 psi as circulating pressure and you may be add safety factor over this figure. You can see that you may end up having extra safety factor which may cause formation fracture.
Look at casing pressure in Figure 2. It shows 700 psi which is 100 psi over the stabilized casing pressure (600 psi) when the well is shut in. The 100 psi represents an overbalanced therefore the circulating pressure at 1,300 psi must be subtracted with the overbalance in order to get the correct ICP
Initial Circulating Pressure (ICP) = Drill Pipe Pressure – Overbalance
Initial Circulating Pressure (ICP) = 1,300 – 100 = 1,200 psi
Figure 3 – Correct ICP
Always Know Overbalance
Reference books: Well Control Books