You may now already that the equivalent circulating density is a combination between current mud weight and frictional pressure. However, this post I would like to share more detail in regard to the bottom hole pressure when the well is in static and dynamic condition when the forward circulation is performed.
For forward circulation, we circulated from mud pump into drill pipe and the mud is returned from the annulus as you can see in the diagram below.
Under a static condition: The bottom hole pressure is equal to hydrostatic pressure from the drilling fluid.
BHP = Hydrostatic Pressure
Under a dynamic condition: Stand pipe pressure equates to summation of pressure loss of whole system.
Stand pipe pressure = Pressure loss in drill string + Pressure loss in BHA + Pressure loss across the bit + pressure loss in the annulus
The pressure loss acts opposite way while fluid is being moved; therefore, if you look at the annulus side, you will be able to determine the bottom hole pressure at the dynamic condition as per the equation below.
Bottom hole pressure = Hydrostatic Pressure + pressure loss in the annulus
Under the dynamic condition, the only effect on the bottom hole pressure is the pressure loss in the annulus. This is the reason why the ECD while forward circulation is expressed like this;
ECD = Current Mud Weight + [(pressure loss in the annulus) ÷ (0.052 x TVD of the well)]