Grace Robert. Advanced Blowout and Well Control
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56
Advanced
Blowout
and
Well
Control
Step
15
Displace the kill-weight mud to the surface, keeping the drillpipe
pressure
constant.
Refemng
to
Figure
2.5,
once kill-weight mud has reached the bit
and the displacement of the
annulus
begins, conditions on the
drillpipe side of the
U-Tube
Model are constant and do not
change. Therefore, the kill-weight mud
can
be
displaced
to
the
surface by keeping the drillpipe pressure constant. Some change
in casing pressure and adjustment in the choke size
can
be
expected during this phase. If the procedure has been executed
properly, the choke
size
will
be
increased to maintain the constant
drillpipe pressure and the casing pressure
will
decline to
0
when
the kill-weight mud reaches the surface.
Step
16
With kill-weight mud to the surface, shut in the well by keeping
the casing pressure constant while slowing the pumps.
Step
17
Read and record the shut-in drillpipe pressure and the shut-in
casing pressure. Both pressures should
be
0.
Step
18
Open the well and check for flow.
Step
19
If the well is
flowing,
repeat the procedure.
Step
20
If
no flow is observed, raise the mud weight
to
include the desired
trip margin and circulate until the desired mud weight is attained
throughout the system.
The
Driller’s Method is illustrated in Example
2.3:
Classic
Pressure
Control
Procedures
51
Example
2.3
Given:
Wellbore schematic
Well
depth,
Hole size,
Drillpipe size,
8
5lS-inch surface casing
Casing internal diameter,
Fracture gmbent,
Mud weight,
Mud gradient,
=
Figure 2.6
=
10,OOOfeet
=
7 718 inches
=
4
!4
inches
=
2,OOOfeet
=
8.017inches
=
0.76psVft
=
9.6ppg
=
OSOpsi/ft
A
kick is taken
with
the drill string
on
bottom and
Shut-in drillpipe pressure,
pdp
=
200 psi
Shut-in annulus pressure,
P,
=
300
psi
Pit level increase
=
10 barrels
Normal circulation rate
=
6
bpm at
60
spm
Kill
rate
=
3
bpm at
30
spm
Circulating
pressure
at
kill
rate,
Pkv
500
psi
58
Advanced
Blowout
and
Well
Control
Pump capacity,
CP
Capacity
of
the:
Drillpipe,
cdpi
Drillpipe
casing
annulus,
Cdp
Drillpipe hole annulus,
cdpha
Note:
=
0.1
bbVstk
=
0.0142bbVft
=
0.0428bbVft
=
0.0406bbVft
For simplicity in calculation
and
illustration, no drill
collars
are
assumed. The inclusion
of
drill
collars
adds
only
an
intermediate
calculation.
Required:
Describe the
kill
procedure using the Driller’s Method.
Solution:
1.
Establish pressure versus volume
diagram
(Figure 2.4).
2.
Record
the shut-in drillpipe pressure
and
shut-in casing
pressure.
Pa
=
300 psi
3.
Establish
the pumping pressure
at
the
kill
rate
of
30
spm
using Equation 2.8:
Classic
Pressure
Control
Procedures
59
IO
BBL
lnfkrx
Depth
-
0500
fuel
Figure
2.6
-
Wellbore Schematic
P,
=
500
+
200
P,=
700
psi
4.
Bring
the
pump
to
30
spm,
maintaining
300
psi
on
the
casing
annulus.
60
Advanced Blowout and Well Control
5.
Read and record the drillpipe pressure equal
to
700
psi.
Confirm
the drillpipe pressure using Figure
2.4.
6.
Displace the annulus and all gas
which
has
entered the
wellbore, keeping the drillpipe pressure constant at
700
psi.
7.
Read
and record the drillpipe pressure equal
to
700
psi
and
casing
pressure equal
to
200
psi.
8.
Shut
in
the well, keeping the casing pressure
constant
at
200
psi.
Allow
the well to stabilize.
9.
Determine that all
gas
is
out
of
the mud.
10.
Determine the
kill
weight,
p,,
using Equation
2.9:
0.5(
10000)
+
200
”
=
0.052(10000)
PI
=
10
PPg
11.
Determine the strokes
to
the bit using Equation
2.10:
Classic Pressure
Control
Procedures
61
12.
13.
14.
15.
16.
17.
18.
19.
(0.0142)(
10000)
0.1
STB
=
Sll3
=
1,420
strokes
Raise the mud weight at the surface to 10 ppg.
Bring the pump
to
30 spm, keeping the casing pressure
constant at
200
psi.
Displace the 10-ppg mud to the bit with 1,420 strokes,
keeping the annulus pressure,
Pa,
constant at 200 psi.
The choke size must not change.
At 1,420 strokes observe and record the circulating
pressure on the drillpipe. Assume that the observed
pressure on the drillpipe is
5
13 psi.
Circulate the 10-ppg mud
to
the surfice, keeping the
drillpipe pressure constant at
5
13 psi.
Shut in and check
Pa
=
Pdp
=
0
psi. The well is dead.
Circulate and raise the mud weight
to
some acceptable
trip margin, generally between 150
to
500 psi above the
formation pressure,
pb
,
or 10.3 to 11.0 ppg.
Continue drilling
ahead.
WAIT AND WEIGHT METHOD
The alternative classical method
is
commonly
known
as
the Wait
and Weight
Method.
As the name implies, the well is shut in while the
mud density is increased
to
the kill weight
as
determined by Equation 2.9.
Therefore, the primary difference is operational in
that
the kill-weight
mud,
p,
,
is pumped while the gas
is
being displaced. The result
is
that the
well is killed in one circulation with the Wait and Weight
Method
62
whereas,
with
the Driller’s
Method,
two circulations are required.
In
the
early days of pressure control, the time required
to
increase the density of
the mud
in
the surface
system
to the kill weight was significant. During
that time, it was not uncommon for the gas
to
migrate or for the drillpipe
to become stuck. However, modem mud mixing systems have eliminated
the time factor from most operations
in
that most systems
can
raise the
density
of
the surfkce system
as
fast
as
the mud is pumped. There are
other important comparisons which
will
be
presented after the Wait and
Weight Method is presented, illustrated and discussed.
The displacement procedure for the Wait and Weight Method
is
as
follows, subsequently, each
step
will
be discussed in
detail:
Advanced
Blowout
and
Well
Control
step
1
On
each tour read and record the standpipe pressure at several
rates in
strokes
per minute (spm), including the anticipated kill
rate for each pump.
Step 2
Prior to pumping, read and record the drillpipe and casing
pressures. Determine the anticipated pump pressure at the
kill
rate using Equation 2.8:
step
3
Determine the density of the kill-weight mud, pl, using
Equation2.9 (note that no
‘‘safety
factor’’ is recommended or
included):
Classic
Pressure
Control
Procedures
63
Step
4
Determine the number
of
strokes
to
the bit by dividing the
capacity
of
the drill string in barrels
by
the capacity
of
the pump
in barrels per stroke according to Quation 2.10:
Step
5
Determine the new circulating pressure,
P,,
at the kill rate with
the kill-weight mud at the bit utilizing Quation 2.1
1:
(2.1 1)
Where:
pl
p
Pk,
Pdp
D
=
Well depth,
feet
=
Density
of
the kill-weight mud, ppg
=
Density of the original mud, ppg
=
0rigm.l
circulating pressure
at
kill rate, psi
=
Shut-in drillpipe pressure, psi
Step 6
For
a
complex drill string configuration, determine and graph the
pumping schedule for reducing the initial circulating pressure,
Pc,
determined in Step 2
to
the final circulating pressure,
P,,
determined
in Step
5.
Using Equations 2.12 and 2.13, calculate
Table
1 and create the corresponding graph.
Note:
A
“section”
of
drill string
is
the length where all the
diameters remain the same.
A new section would
start
anytime
the hole size or pipe diameter changed.
As
long
as
the
diameters
remain the same, it is one section. Therefore, each section
has
one annular capacity. The calculations begin from
the
surface.
64
Advanced
Blowout
and
Well
Control
Strokes
0
STKS
1
STKS2
sTKs3
STJ3
...
For example, if the hole size does not change
and
the string
consists
of
two
weights
of
drillpipe, heavy-weight drillpipe and
drill
collars,
four calculations would be required.
Pressure
700
8
p2
p3
PC?I
...
The table would look like the following:
(2.12a)
(2.12b)
(2.12c)
STK;S
1
(*-ph)(
STB
)
(2.13a)
Classic
Pressure
Control
Procedures
65
p3
=P,-O.O52(p,
-p)(Zdrl
+ldr2
+Idr3)+
SXS
3
(9-ph)(
STB
)
(2.13b)
(2.13~)
=
Pdp
-
0.052@,
-p)
*
(2.13d)
(1&
+
l&*
+
z,,
+.
. .
+ldJ
+
Where:
STKSl
=
Strokes to end
of
section 1
of
drill string
STKS2
=
Strokes to end
of
section 2
of
drill string
STKS3
=
Strokes to end
of
section
3
of
drill string
STB
=
Strokes to the bit
as
determined in Step
4
Where:
h
P
ldr1,2,3
cdr1.2,3
=
Capacity
of
section
of
drill string, bbVft
4,2,3
p+
Ph
cp
=
Pump capacity, bbVstroke
en
e
=
Density
of
kill-weight mud, ppg
=
Density
of
original mud, ppg
=
~ength
of
section
of
drill string, feet
=
Circulating pressure
with
kill-weight mud to
the end
of
section 1,2,3, psi
=
Shut-in drillpipe pressure, psi
=
Circulating pressure at
kill
speed determined
in Step
1,
psi
=
New circulating pressure, psi
=
Initial
displacement pressure determined
in
Step
2
using Equation
2.8,
psi