Pressure Losses in the Circulating System
In order to optimise the hydraulics of any system it is therefore essential that the pressure losses in that system are understood and can be quantified.
Since the returning mud at the flowline is at atmospheric pressure, the discharge pressure delivered by the pump has been totally dissipated throughout the system. The pressure drops may be denoted by:
(i) Psc — the pressure loss in the surface connections (e. g. standpipe, kelly
hose). This is generally small in comparison to other components (<100
psi).
(ii) Pd — the pressure loss in the drillstring (i. e. inside the drillpipe and drill
collars).
(iii) Pb — the pressure loss through the bit nozzles. This is where most of the
pressure drop should occur for efficient drilling
(iv) Pa — the pressure drop in the annulus.
The total pressure drop (Pt) can be written:
P = P + P + P + P or
t sc d b a
P = P + P
tbs
where Ps = pressure loss in the system (Ps = Psc + Pd + Pa). The system pressure loss (parasitic loss) must be controlled so that most of the total pressure delivered by the pump is used across the bit. All of these losses can be quantified using Sections 4 and 5 of this set of notes.
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Generally, laminar flow occurs in the annulus, while turbulent flow occurs in the drill string. Turbulent flow is generally avoided in the annulus since it may cause washouts in the formation by erosion.