BAKER HUGHES - Drilling Fluids Reference Manual

HYDRAULICS
Jet Selection
When optimizing a hydraulics program, it is sometimes desirable to maximize either the hydraulic
horsepower or the impact force within the constraints of minimum and maximum flow rates and
maximum standpipe pressure. A detailed derivation of maximizing either parameter is not given
here, however, a common assumption is that maximum hydraulic horsepower occurs when 65% of
the total available standpipe pressure is developed at the bit. Correspondingly, maximum impact
force occurs when 48% of the total available standpipe pressure is developed at the bit. It can be
easily seen that maximizing both hydraulic horsepower and impact force cannot be done
simultane ously. The calculation for maximizing either hydraulic horsepower or impact force
involves finding a total nozzle area required to develop the specified pressure loss at the bit, and
then simply selecting the required number and available nozzle sizes that give the total nozzle area.
It should be noted that the jet nozzles need not be all of the same size.
It is sometimes necessary to run a bit with large nozzles in order to circulate lost circulation
material. This frequently causes a great decrease in hydraulic horsepower or impact force.
Downhole motors, MWD and LWD equipment all contribute significantly to drill string pressure
losses. These pressure losses may be so great as to restrict the ability to optimize bit hydraulics.
However, in many cases, it is possible to maintain reasonably good hydraulics by reducing the
number of nozzles. For example, two 16 / 32 in. nozzles have approximately the same area as three
13
/ 32 in. nozzles.
To find the total jet area for maximum hydraulic horsepower or impact force,
where:
A t
Q
= ---------------------------------------------------
2.96 (---------------------------- 1238.5 )CP
1/2
ρ
A t = optimum total nozzle area, in. 2
Q = flow rate, gal/min
P = available standpipe pressure, psi
ρ = fluid density, lb m/gal
C = Constant: 0.65 for maximum HHP,
0.48 for maximum impact force,
0.59 for HHP-Impact compromise.
Once the total jet area is obtained, simple algorithms are used to back-calculate jet sizes. The
following algorithms show how jet sizes can be obtained for any number of jets.
J 1
= 1303.797 1 ⁄ 2
---------------------A t
N
1303.797
J --------------------- ⎛
2
N – 1
A ( J ⎞
= ⎜ – t 130 -------
1)
--------------⎟
⎝ 3.797⎠
2
1/2
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 9-19