BAKER HUGHES - Drilling Fluids Reference Manual

HYDRAULICS
value, such gains must not be made at the expense of some other detrimental effect. For example,
maximizing penetration rate may not necessarily result in the lowest cost per foot of drilled hole.
Other factors such as reduced bit life or pack-off of drilled cuttings in the annulus may increase
cost. As with all hydraulic parameters, bit hydraulics should be used in an overall hydraulic
evaluation of the drilling operation. Optimizing bit hydraulics tends to be more critical with water
based drilling fluids than with invert emulsion drilling fluids.
Nozzle Velocity
The velocity of the drilling fluid is determined by the circulation rate and the total area of the
nozzles in the bit. Even if the jet nozzles are of different sizes, the velocity is the same through
each. In other words, the velocity is independent of the individual diameters of the nozzles due to
the mechanics of fluid flow through an orifice.
First calculate the total nozzle area.
2 2
2
( J 1 ) + (J 2 ) + ... + ( J n )
A t
= ------------- ---------------------------------------------
1303.8
where:
A t = total nozzle area, in 2 .
J n = nozzle size, 32nds of an inch (i.e., 11, 12, etc.)
Then nozzle velocity:
Q
Vn
=
3.117A
t
where:
V n = nozzle velocity, ft/sec
Q = flow rate, gal/min
A t = total nozzle area, in. 2
Pressure-Drop Across Bit
When the drilling fluid passes through the jet nozzles, a pressure-drop due to friction losses occurs.
This pressure-drop has a number of important applications, such as optimizing hydraulic
horsepower or impact force of the fluid on the formation and improving chip removal and bit
cleaning. The bit nozzles are designed to maximize the portion of pressure loss in the circulating
system that may be used to perform work on the formation. Analysis of pressure-drop through an
orifice such as a jet nozzle can be complicated; however, nozzle coefficients have been empirically
determined that account for turbulence and velocity through the orifice. The total pressure-drop
across the bit is derived from an energy balance called Bernoulli's Equation and includes a nozzle
coefficient of 0.95. It is calculated by,
P n
=
---------------------------------------------------------------------
156( ρ )Q 2
J 1 ) 2 2
[(
+ ( J 2 ) + ... + ( J n ) 2
] 2
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 9-17