Hydraulics –ECD-Bit Hydraulics

•Equivalent circulating density (ECD)

–The pressure on a formation while circulating is equal to the total annular circulating pressure losses from the point of interest to the bell nipple, plus the hydrostatic pressure of the mud.
–This force is expressed as the density of mud that would exert a hydrostatic pressure equivalent to this pressure.
Formula for ECD
ECD (lb/gal) = Pa(psi)/ 0.052 x TVD (ft)
•Excessive ECD may cause losses by exceeding fracture gradient on a well
.•It is important to optimize rheological properties to avoid excessive ECD.

Bit Hydraulics

•In addition to bit pressure loss, several other hydraulics calculations are used to optimize the drilling performance.
•These include hydraulic horsepower, impact force and jet velocity calculations.
•Hydraulic Horsepower (hhp)
–The recommended hydraulic horsepower (hhp) range for most rock bits is 2.5 to 5.0 Horsepower per Square Inch (HSI) of bit area.
–Low hydraulic horsepower at the bit can result in low penetration rates and poor bit performance.
•Hydraulic Horsepower (hhp)
–The bit hydraulic horsepower cannot exceed the total system hydraulic horsepower.
hhpb= QPBit/1,740
–Q = Flow rate (gpm)
–PBit= Bit pressure loss (psi)
•Hydraulic Horsepower (hhp)
–Hydraulic Horsepower per square inch
HSI = 1.27 x hhpb/Bit Size*2
–Bit Size = Bit diameter (in.)
–System Hydraulic HorsepowerhhpSystem= PTotalQ1,714
–PTotal= Total system pressure losses (psi)

Nozzle Velocity

–Although more than one nozzle size may be run in a bit, the nozzle velocity will be the same for all of the nozzles.
–Nozzle velocities of 250 to 450 ft/sec are recommended for most bits.
–Nozzle velocities in excess of 450 ft/sec may erode the cutting structure of the bit.
•Nozzle Velocity
Vn(ft/sec) = 417.2 x Q/(Dn1*2+ Dn2*2+ Dn3*2+ …)
–Q = Flow rate (gpm)
–Dn= Nozzle diameter (32nds in.)
–Q = Flow rate (gpm)
•Percent pressure drop at the bi
t–It is generally desired to have 50 to 65% of surface pressure used across the bit.
%PBit= PBit x 100/PTotal
•Hydraulic impact force (IF)
IF (lb) = Vn x Qr/1,930
–Vn= Nozzle velocity (ft/sec)
–Q = Flow rate (gpm)
–r = Density (lb/gal)
–Impact force per inch squaredIF (psi) = 1.27 x IF (lb)/Bit Size*2