Drilling personnel need to fully understand about the lag time. Lag time is time delay from pressure adjustment made on a choke valve or a choke HCR to show up on the drill pipe pressure gauge.
Does it sound confuse? Let’s me explain more about it. When you adjust a choke position, you will not be able to see changes on the drill pipe gauge right away because drilling fluid is compressible so you need to be patient and wait a little bit until you see the changes on the drill pipe gauge.
How can we know how many seconds for the lag time?
We can roughly estimate the lag time about 2 seconds per 1,000 MD. This number is just a rule of thumb. If you want the actual lag time, you can determine it by performing a choke drill. You really need to know about the lag time otherwise you can get confused a lot when you attempt to adjust drill pipe pressure.
Reference book: Well Control Books