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Effective Circulating Density
The density of the drilling fluid doesn’t remain constant through its cycle. For example, the
weight of suspended cuttings in the annulus normally increases the effective density of the mud
and therefore the hydrostatic pressure imposed at the bottom of the hole.
An important factor in consideration of the true bottom hole pressure is the effective backpressure
imposed on the bottom due to annular pressure losses. When circulating through an open flow
line, the measured mud pressure at the surface (casing pressure) will be zero. Since a certain
amount of pump pressure was required to circulate the drilling mud, those pressure losses must be
accounted for.
Frictional effects in the annulus present a restriction to fluid flow, and a certain amount of pump
pressure required to overcome this restriction. This restriction acts in the same way as a closed-in
choke applying a back pressure to bottom of the hole during circulation is termed the Bottom
Hole Circulating Pressure (BHCP), and its equivalent mud density is termed as the Effective
Circulating Density (ECD).
The extent of the flow restriction and pressure losses is dependent upon the total depth, annular
dimension, fluid viscosity, and flow regime (laminar or turbulent). By using the conventional
Bingham model for drilling fluids, the pressure losses can be approximated using:
Pla = L x Yp + PV x L x V
A x ( I.D. - O.D. ) B x ( I.D - O.D. )
Where:
Pla = annular pressure loss psi
L = measured length of section ft
YP = yield point lb/100 ft
I.D – O.D. = hole (or casing I.D minus pipe (or collar) O.D inch
PV = plastic viscosity cps
V = annular velocity ft/min
A = 225 for drillpipe, 200 for annulus
B =90000 for drillpipe, 60000 for annulus
This equation provides pressure losses in a pipe or annulus of containing fluid moving in laminar
flow, and tends to give slightly inflated values.
Annular Velocity (ft/min) = 24.51 x gallon per minute
( I.D - O.D. )
When using tapered string or in partially cased holes, the total pressure loss will be the sum of the
pressure losses calculated for individual annular segments
ECD = W + Pla
0.0519 x D
Where:
ECD = effective circulating density lb.gal
Pla = total annular pressure loss psi
W = mud density lb/gal
D = vertical depth of the well ft
BHCP = Pla + ( W x D x 0.0519 )
= ECD x D x 0.0519
Where:
BHCP = bottom hole circulating pressure psi
So that the BHCP (this means ECD value) should be less than the fracturing pressure
(known from a leak of test) to prevent formation break down.
Notice that in circulating pressure losses the actual measured length of the flow path is used. The
sum of these will be the total measured depth of the well. When converting this pressure loss to
an equivalent mud density the vertical depth must be used since a hydrostatic column of fluid is
being considered.
Using Power Law Model annular pressure losses (usually used in down hole calculation) can be
defined as:
Pla = L
300 (I.D - O.D)
Where:
Pla = annular pressure loss psi
L = measured length of section ft
= shear stress lb/ ft
I.D – O.D. = hole (or casing I.D minus pipe (or collar) O.D inch
Current well parameters:
M.WT = 9.6 ppg = lb/gal
PV = 29
YP = 21
Flow rate = 2831 l/min
= 2831/60
= 47.1833 l/sec
= 47.1833/0.06308 gal/min
= 748 gal/min
Calculate annular velocity & pressure loss for each drill string section
Apply the result of the sum of pressure loss at
ECD = W + Pla
0.0519 x D
5” DP Section in 13 3/8” csg:
Av = 24.51 x 748 = 18333.48
(12.347) - (5) 127.44841
= 143.85021 ft/min
Pla = 1026 x 3.281 x 21 + 29 x 1026 x 3.281 x 143.85021
200 x (12.347 - 5 ) 60000 x (12.347– 5 )
= 48.109722 + 4.3360146
Pla4 = 52.445736 psi
ECD = W + Pla1+Pla2+Pla3+Pla4
0.0519 x D
ECD = 9.6 + Pla1+Pla2+Pla3+Pla4
0.0519 x 3274 x 3.281
ECD = 9.6 + 172.47528
0.0519 x 3274 x 3.281
ECD = 9.9093 lb/gal
The density of the drilling fluid doesn’t remain constant through its cycle. For example, the
weight of suspended cuttings in the annulus normally increases the effective density of the mud
and therefore the hydrostatic pressure imposed at the bottom of the hole.
An important factor in consideration of the true bottom hole pressure is the effective backpressure
imposed on the bottom due to annular pressure losses. When circulating through an open flow
line, the measured mud pressure at the surface (casing pressure) will be zero. Since a certain
amount of pump pressure was required to circulate the drilling mud, those pressure losses must be
accounted for.
Frictional effects in the annulus present a restriction to fluid flow, and a certain amount of pump
pressure required to overcome this restriction. This restriction acts in the same way as a closed-in
choke applying a back pressure to bottom of the hole during circulation is termed the Bottom
Hole Circulating Pressure (BHCP), and its equivalent mud density is termed as the Effective
Circulating Density (ECD).
The extent of the flow restriction and pressure losses is dependent upon the total depth, annular
dimension, fluid viscosity, and flow regime (laminar or turbulent). By using the conventional
Bingham model for drilling fluids, the pressure losses can be approximated using:
Pla = L x Yp + PV x L x V
A x ( I.D. - O.D. ) B x ( I.D - O.D. )
Where:
Pla = annular pressure loss psi
L = measured length of section ft
YP = yield point lb/100 ft
I.D – O.D. = hole (or casing I.D minus pipe (or collar) O.D inch
PV = plastic viscosity cps
V = annular velocity ft/min
A = 225 for drillpipe, 200 for annulus
B =90000 for drillpipe, 60000 for annulus
This equation provides pressure losses in a pipe or annulus of containing fluid moving in laminar
flow, and tends to give slightly inflated values.
Annular Velocity (ft/min) = 24.51 x gallon per minute
( I.D - O.D. )
When using tapered string or in partially cased holes, the total pressure loss will be the sum of the
pressure losses calculated for individual annular segments
ECD = W + Pla
0.0519 x D
Where:
ECD = effective circulating density lb.gal
Pla = total annular pressure loss psi
W = mud density lb/gal
D = vertical depth of the well ft
BHCP = Pla + ( W x D x 0.0519 )
= ECD x D x 0.0519
Where:
BHCP = bottom hole circulating pressure psi
So that the BHCP (this means ECD value) should be less than the fracturing pressure
(known from a leak of test) to prevent formation break down.
Notice that in circulating pressure losses the actual measured length of the flow path is used. The
sum of these will be the total measured depth of the well. When converting this pressure loss to
an equivalent mud density the vertical depth must be used since a hydrostatic column of fluid is
being considered.
Using Power Law Model annular pressure losses (usually used in down hole calculation) can be
defined as:
Pla = L
300 (I.D - O.D)
Where:
Pla = annular pressure loss psi
L = measured length of section ft
= shear stress lb/ ft
I.D – O.D. = hole (or casing I.D minus pipe (or collar) O.D inch
Current well parameters:
M.WT = 9.6 ppg = lb/gal
PV = 29
YP = 21
Flow rate = 2831 l/min
= 2831/60
= 47.1833 l/sec
= 47.1833/0.06308 gal/min
= 748 gal/min
Calculate annular velocity & pressure loss for each drill string section
Apply the result of the sum of pressure loss at
ECD = W + Pla
0.0519 x D
5” DP Section in 13 3/8” csg:
Av = 24.51 x 748 = 18333.48
(12.347) - (5) 127.44841
= 143.85021 ft/min
Pla = 1026 x 3.281 x 21 + 29 x 1026 x 3.281 x 143.85021
200 x (12.347 - 5 ) 60000 x (12.347– 5 )
= 48.109722 + 4.3360146
Pla4 = 52.445736 psi
ECD = W + Pla1+Pla2+Pla3+Pla4
0.0519 x D
ECD = 9.6 + Pla1+Pla2+Pla3+Pla4
0.0519 x 3274 x 3.281
ECD = 9.6 + 172.47528
0.0519 x 3274 x 3.281
ECD = 9.9093 lb/gal