Slow Circulation Rate is a circulation rate which will be used in well kill operation. Typically, slow circulation rate pressure (SCR) is recorded from each particular flow rate and the pressure represents pressure loss of the system while circulating. Since there are so many pressure gauges on the rig, you may get confused on the figures. In order to be at the same page for every personnel on the rig, the SCR should be recorded by the pressure gauge that we will use for killing the well.
There are a lot of reasons why we should kill the well with slow rate rather than a drilling rate as follows:
• To minimize friction pressure
• To allow time to weight up mud if you use wait and weight method
• To reduce pressure on surface equipment
• To allow degasser to separate gas from the mud
• To reduce needs for fast choke operation
• To allow personnel to think if something goes wrong
Let’s get an idea how to get pre-recorded SCR.
1. Turn pump on at slow speed as 10, 20, 30 and 40 spm
2. Record pressure without rotating or moving drillstring
3. Each pressure recorded at particular pump speed is SCR.
Please always remember that SCR that you take for each time representing pressure loss at that time. I don’t recommend you to use it to estimate the initial circulating pressure.
Let’s me explain why – There are some errors in pre-recorded SCR which can make you in a trouble.
• Pre-recorded SCR may not reflect current pressure due to drilling mud properties change.
• The depth of pre-recorded SCR is not the same as the current drilling depth.
• SCR may be changed due to unknown condition down hole as BHA mechanism, plugged jets, etc.
How can we get ICP without using pre-recorded SCR?
In order to get the initial circulating pressure, you just simply bring the pump up to speed by holding casing pressure constant until you reach kill rate. Additionally, you will be able to calculate the actual SCR by calculation.
Let’s take a look at the equation below.
ICP = SIDPP + SCR
Where; ICP is Initial Circulating Pressure.
SIDPP is Shut In Drill Pipe Pressure.
SCR is Slow Circulation Rate pressure.
Therefore, SCR = ICP – SIDPP
If you would like to check SCR, the following time is when you should check SCR.
• Check before drilling out of casing shoe
• Check after tripping back to the bottom
• Check when mud properties are changes
• Check at least two times each shift at the drilling depth at that time
Reference book: Well Control Books