I just got questions from my friends asking about how casing pressure and drillpipe pressure react while performing the 2 nd circulation of driller’s method.
I have this post for somebody missing about the first circulation of driller’s method – How are pressure and pit volume doing during the first circulation of the driller’s method?
During 2nd circulation of driller’s method, we need to hold casing pressure constant until kill weight mud to the bit and then we will hold drill pipe pressure constant. The reason why we do like this is that we need to maintain the constant bottom hole pressure.
Let’s take a look at details.
Hold casing pressure constant until kill weight mud to the bit
The Casing Pressure will be holding constant by adjusting choke. While circulating Kill Weight Mud (KWM) to the bit, the drill pipe pressure will decrease because of increasing in hydrostatic pressure. At the casing side, there is no change in hydrostatic pressure therefore we can maintain constant the bottom hole pressure (BHP) by holding constant casing pressure.
Hold drill pipe pressure constant
When Kill Weight Mud (KWM) reaches the bit, you need to record drill pipe pressure, called “final circulating pressure”, and hold drill pipe pressure constant until Kill Weight Mud (KWM) to the surface. You need to ensure mud weight out by measurement not by calculated strokes.
Example of pressure profile at the second circulation of driller’s method.
Reference book: 
Well Control Books
