BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
V
w
=
⎡ C
Ph
⎢
⎣W
a
m
C
dp
+
W − W
m
0.052
d
⎤
⎥
⎦
⎡ 0.0538 0.0142
250
⎢
+
⎣ 10 .6 10 .6 − 8.34
=
0.052
⎤
⎥
⎦
= 54 .6
⎡ ⎛ V ⎞⎤
⎡ ⎛ 54 ⎞⎤
P
max
= 0.052⎢
m d
⎢ ⎜ ⎟ =
0.0142
⎥
⎢
⎜ ⎟
⎣ ⎝ C
dp ⎠⎥⎦
⎣ ⎝ ⎠⎦
( ) ⎜
w
W −W
⎟⎥
= 0.052 ( 10.6 − 8.34) 446. 9
V
a
⎛ C
a
= P
⎜
h
⎝ 0.052×
W
m
⎞ ⎛ 0.0538 ⎞
⎟ = 250⎜
⎟ = 244.0
⎠ ⎝ 0.052×
10.6 ⎠
where,
V w = bbl of light fluid to be pumped into drillpipe
P h = desired pressure reduction at 8000 ft
C a = annular capacity between drillpipe and casing in bbl/ ft
W m = fluid weight (lb m /gal)
C dp = Capacity of drillpipe in bbl/ft
P max = pressure required to displace required volume of light fluid
V a = amount of light fluid which will flow back to obtain equilibrium
W d = weight of the displacement fluid.
A reduction of 250 psi in bottom hole pressure would result in an equivalent drilling fluid density
of 10.0 lb m /gal at total depth.
Caution should be exercised with this method, since a drop in fluid level could reduce hydrostatic
pressure to a value less than formation pressure allowing a kick. In the above example, fluid level
is lowered 454 feet which would result in an equivalent drilling fluid density of 9.0 lb m /gal at the
casing shoe.
If the well flows, it will be necessary to circulate out through the choke utilizing a balanced
bottomhole pressure technique. Caution should be exercised if reverse circulation is attempted
since all circulating pressure losses below the casing shoe (annular friction loss from casing to bit
and friction losses through nozzles, collars, drillstring, etc.) would be imposed at the casing shoe
and could induce lost returns.
The primary advantages of this technique are that drilling fluid properties do not have to be
altered and hydrostatic pressure can be reduced slowly, minimizing the possibility of hole
sloughing or kicks. Also, with this method the displacing fluid does not normally come into
contact with the wellbore.
If remedial procedures do not free stuck pipe, it may be necessary to wash over the fish. It should
be noted that wash pipe would have greater tendency to stick than the drillstring, therefore,
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-41