BAKER HUGHES - Drilling Fluids Reference Manual

HYDRAULICS
E t = transport efficiency, %.
Note: If the slip velocity is zero, the transport efficiency is 1, or 100%.
Cuttings Concentration
Due to the slip velocity of cuttings in the annulus, the concentration of cuttings in the annulus
depends upon the transport efficiency as well as the volumetric flow rate and the rate at which
cuttings are generated at the bit (ROP and hole size). Experience has shown that cuttings
concentration in excess of five (5) volume % can lead to a pack-off, tight hole, or stuck pipe. When
drilling in soft formations, where pipe connections in the drillstring are made as rapidly as possible,
the cuttings concentration may easily exceed 5%, if ROP is uncontrolled. The cuttings
concentration is calculated by:
where,
C a
( ROP)D 2
= ---------------------------- × 100
14.71 ( E t)Q
C = cuttings concentration, vol %
a
D = hole diameter, in.
E t = transport efficiency, %
Q = flow rate, gal/min
ROP = rate of penetration, ft/hr
The ROP is obtained by measuring the time it takes to drill a length of pipe and dividing that length
by the time. For instance, a stand length for a top-drive is 100 ft and it is drilled down in 20 min,
then the ROP is 100 ft/20 min × 60 min/hr = 300 ft/hr.
When the cuttings concentration exceeds five (5) volume %, the effect on hydrostatic pressure and
equivalent circulating density can be substantial. The change in hydrostatic pressure depends upon
the density of the cuttings as well as the concentration of the cuttings in that section of hole. The
effective fluid weight due to the cuttings concentration in that section of hole is calculated by,
where:
ρ e
=
C a
( SG c )( 8.34)
⎛--------
⎞ +
⎝100⎠
ρ⎛1
⎝
C a
– -------- ⎞
100⎠
ρ e = effective fluid weight due to cuttings concentration, lb /gal m
SG c = specific gravity of the cuttings
C a = cuttings concentration, vol %
ρ = fluid density, lb m /gal
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 9-27