In this article, we will describe the effect of friction pressure on bottom hole pressure and equivalent circulating density while performing forward circulation.
What is forward circulation?
It is the typical circulating path which is from a mud pump into drill pipe. Mud is pumped down into drill string / BHA and come out of a bit. Then, the mud is flown up the annulus and return back to surface as you can see in the diagram below (Figure 1).
Under a static condition:
The bottom hole pressure is equal to hydrostatic pressure from the drilling fluid.
Bottom Hole Pressure (BHP) = Hydrostatic Pressure (HP)
Under a dynamic condition:
Stand pipe pressure equates to summation of pressure loss of whole system.
SPP = FrPds + FrPbha + FrPbit + FrPann
SPP = Stand Pipe Pressure
FrPds = Pressure loss in drill string
FrPbha = Pressure loss in BHA
FrPbit= Pressure loss across the bit
FrPann = Pressure loss in Annulus
The friction pressure 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 (BHP) = Hydrostatic Pressure (HP) + Pressure Loss in Annulus (FrPann)
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 = Equivalent Circulating Density, ppg
- MW = Mud Weight in the well, ppg
- FrPann = Pressure loss in Annulus, psi
- TVD = True Vertical Depth, ft
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