Grace Robert. Advanced Blowout and Well Control
Подождите немного. Документ загружается.
66
Advanced Blowout
and
Well
Control
For
a
drill string composed
of
only
one weight
of
drillpipe and one
string
of
heavy-weight drillpipe
or
drill collars, the pumping
schedule
can
be determined using Equation 2.14:
STKS
-
25(STB)
25psi
pC-Pm
--
(2.14)
Step
7
Raise the density
of
the mud in the suction pit to the
kill
weight
determined in Step
3.
Step 8
Bring the pump
to
a
kill
speed,
keeping the casing pressure
constant
at
the shut-in casing pressure.
This
step
should require
less than five minutes.
step
9
Once the pump is
at
a satisfactory kill
speed,
read and record the
drillpipe pressure. Adjust the pumping schedule accordingly.
Veri@ the drillpipe pressure using the diagram established in
Step 1. Displace the kill-weight mud
to
the bit pursuant
to
the
pumping schedule established in Step
6
as
revised in
this
step.
step
10
Displace the kill-weight mud
to
the surface, keeping the drillpipe
pressure constant.
Step 11
Shut
in
the well, keeping the casing pressure constant and observe
that
the drillpipe pressure and the casing pressure are
0
and the
well
is
dead.
Classic
Pressure
Control
Procedures
67
step 12
If the surface pressures are not
0
and the well is not dead,
continue
to
circulate,
keeping
the drillpipe pressure constant.
Step
13
Once the well is dead,
raise
the mud weight in the suction pit to
provide the desired trip margin.
Step
14
Drill ahead.
Discussion of each step in detail follows:
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.
This
is
the same discussion
as
presented after Step
1
of the
Driller’s
Method.
Experience has
shown
that
one of the most
difficult aspects of any
kill
procedure is bringing the pump to
speed without permitting
an
additional influx or fracturing the
casing shoe.
This
problem is compounded by attempts
to
achieve
a
precise kill rate. There is nothing magic about the kill
rate
used
to circulate out
a
kick.
In
the early
days
of pressure control,
surface
facilities were inadequate to bring
an
influx
to
the surface
at
a
high pump speed. Therefore, one-half normal speed became
the arbitrary rate of choice for circulating the influx to the
surface. However, if only one rate such
as
the one-half speed is
acceptable, problems
can
arise when the pump speed is slightly
less or slightly more
than
the precise one-half speed. The reason
for the potential problem
is
that the circulating pressure at
rates
other
than
the kill rate is
unknown.
Refer to fhrther discussion
after
Step
4.
The best procedure is to record and graph several flow rates and
corresponding pump pressures
as
illustrated
in
Figure
2.4.
It is
assumed
in
Examples
2.3
and
2.4
that
the kill speed used is
30
68
Advanced
Blowout
and Well
Contml
strokes per minute. However, the
actual
pump speed used need
not be exactly
30
strokes per minute. The drillpipe pressure
corresponding
to
the actual pump
speed
being used could be
verified readily using Figure 2.4.
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:
This
is the same discussion
as
presented after Step 2 of the
Driller’s
Method.
It
is
not uncommon for the surface pressures
to
fluctuate slightly due
to
temperature, migration or gauge
problems. Therefore, for future reference it
is
important
to
record
the surhx pressures immediately prior
to
commencing pumping
operations.
When in doubt, shut in the well! It
seems
that
the prevailing
impulse is
to
continue circulating regardless of the consequences.
If the condition
of
the well
has
deteriorated since it was shut in, it
deteriorated during the pumping phase.
When
in doubt, shut in
the well, read the surface pressures, compare
with
the original
pressures and evaluate the situation prior
to
firther operations. If
something is
wrong
with the displacement procedure being used,
the situation is less likely
to
deteriorate while shut in and more
likely
to
continue
to
deteriorate if pumping is continued. The well
was
under control when initially shut in.
Step
3
Determine the density
of
the kill-weight mud,
p,,
using
Equation2.9 (note that no “safety factor” is recommended or
included):
Classic
Presmre
Control
Procedures
69
pnrD
+
‘dp
”
=
0.0520
Safety factors are discussed in Chapter
4.
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
Equation 2.10:
Sections
of
different weights
of
drillpipe, drill collars, or heavy-
weight drillpipe may
be
added
or
deleted from Equation 2.10
simply by adding to or subtracting from the numerator
of
Equation 2.10 the product
of
the capacity and the length
of
the
section.
Step
5
Determine the new circulating pressure,
Pcn,
at the kill rate with
the kill-weight mud utilizing Equation 2.1
1
:
The new circulating pressure with the kill-weight mud will
be
slightly greater
than
the recorded circulating pressure at the kill
speed
since
the frictional pressure losses are a function
of
the
density
of the mud.
In
Equation 2.11 the frictional pressure loss
is
considered
a
direct function of the density.
In
reality, the
frictional pressure
loss
is
a
function
of
the density
to
the
0.8
power. However, the difference
is
insignificant.
70
Advanced
Blowout
and
Well
Control
Step
6
For
a
complex drill string configuration, determine and graph the
pumping schedule for reducing the
initial
circulating pressure,
P,,
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.
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.
Determining
this
pump schedule is
a
most critical phase. Use
of
these equations is illustrated in Examples
2.3
and
2.4.
Basically,
the circulating drillpipe pressure is reduced systematically
to
offset the increase in hydrostatic introduced by the kill-weight
mud and ultimately to keep the bottomhole pressure constant.
The systematic reduction in drillpipe pressure must be attained by
reducing
the
casing pressure by the scheduled amount and waiting
four
to
five seconds for the pressure transient
to
reach the
drillpipe pressure gauge. Efforts
to
control the drillpipe pressure
directly by manipulating the choke are usually unsuccessful due
to the time
lag.
The key
to
success is
to
observe several gauges at
the same time. The sequence is usually to observe the choke
position, the casing pressure and drillpipe pressure. Then
concentrate on the choke position indicator while slightly opening
the choke. Next, check the choke pressure gauge for the reduction
in
choke pressure. Continue
that
sequence
until
the designated
amount of pressure
has
been bled from the annulus pressure
gauge. Finally, wait
10
seconds and read the result on the
drillpipe pressure gauge. Repeat the process until the drillpipe
pressure
has
been adjusted appropriately.
Classic
Pressure
Control
Procedures
71
Step
7
Raise the density
of
the mud in the suction pit to the kill weight
determined in
Step
3.
Step
8
Bring the pump
to
a
kill
speed,
keeping the casing pressure
constant at the shut-in casing pressure. This step should require
less
than
five minutes.
Step
9
Once the pump is at a satisfistow kill speed, read and record the
drillpipe pressure. Adjust the pumping schedule accordingly.
VerrfL the drillpipe pressure using the diagram established in
Step
1.
Displace the kill-weight mud to the bit pursuant to the
pumping schedule established in
Step
6
as
revised in this
step.
As
discussed in the Driller’s
Method,
the
actual
kill speed used
is
not critical. Once the
actual
kill
speed
is established at
a
constant
casing pressure
equal
to the shut-in casing pressure, the drillpipe
pressure read is correct. The pumping schedule must
be
adjusted
to
reflect
a
pump
speed
different from the pump speed used
to
construct the table
and
graph. The adjustment
of
the
table
is
accomplished by reducing arithmetically the initial drillpipe
pressure by the shut-in drillpipe pressure and re-making the
appropriate calculations. The graph is more easily adjusted. The
circulating drillpipe pressure marks the beginning point. Using
that point, a line
is
drawn
which is parallel
to
the line drawn in
Step
6.
The new line becomes the correct pumping schedule. The
graph
of
pump pressure versus volume constructed in
Step
1
is
used to
confirm
the calculations.
If
doubt arises during the pumping procedure, the well should be
shut in by keeping the casing pressure constant while slowing the
pump. The shut-in drillpipe pressure, shut-in casing pressure,
and volume pumped should
be
used
to
evaluate the situation. The
pumping procedure
can
be continued by bringing the pump
to
speed
keeping the casing pressure constant, reading the drillpipe
pressure, plotting the point on the pumping schedule graph, and
72
Advanced Blowout
and
Well
Control
establishmg a new line parallel to the original.
These points are
clarified in Examples 2.3 and 2.4.
Keeping the casing pressure constant
in
order to establish the
pump
speed
and
correct
circulating drillpipe pressure
is
an
acceptable procedure provided that the time period
is
short and
the dux is not near the surface.
The time period should never
be more
than
five minutes.
If
the influx is near the surface, the
casing pressures will
be
changing very rapidly.
In
that case, the
time period should be one
to
two
minutes.
Step
10
Displace the kill-weight mud to the
surface,
keeping the drillpipe
pressure
constant.
Step
11
Shut in the well, keeping the casing pressure constant and observe
that
the drillpipe pressure and the casing pressure are
0
and the
well
is
dead.
Step 12
If
the surface pressures are not
0
and the well is not dead,
continue to circulate, keeping the drillpipe pressure constant.
Step
13
Once the well is dead, raise the mud weight in the suction pit
to
provide the desired trip
margin.
Step
14
Drill
ahead.
The Wait and Weight Method is illustrated in Example 2.4:
Exarnr.de
2.4
Given:
Wellbore schematic
=
Figure2.6
Classic
Pressure
Control
Procedures
73
Well depth,
D
=
10,OOOfeet
Hole size,
Dh
=
7
718
inches
Drillpipe size,
Dp
=
4
!4
inches
8
518-inch
surf'ace casing
=
2,OOOfeet
Casing internal diameter,
0,.
=
8
.O
17
inches
Fracture gradient,
Fg
=
0.76psiIft
Mud weight,
P
=
9.6PPl.3
Mud gradient,
Pm
=
OSOpsiIft
A
kick is taken
with
the drill string on bottom
and
Shut-in drillpipe pressure,
pdp
=
200
psi
Shut-in annulus pressure,
Pa
=
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,
Pk,v
=
500
psi
Pump capacity,
cp
=
0.1
bbVstk
74
Advanced
BIavout
and
Well
Control
Capacity
of
the:
Drillpipe,
cdp.
=
0.0142 bbVft
Drillpipe casing annulus,
CW
=
0.0428bbVft
Drillpipe hole annUluS,
CdplM
=
0.0406bbVft
Note:
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
Wait
and Weight
Method.
Solution:
1. Establish pressure versus volume diagram using
Figure 2.4.
2.
Record the shut-in drillpipe pressure and shut-in casing
pressure:
Pdp
=
200
psi
Pa
=3OOpsi
3.
Establish the pumping pressure at the kill rate
of
30
spm
using Equation 2.8:
P,
=
500
+
200
P,=
700
psi
Classic
Pressure
Control
Procedures
15
4.
Determine
the
kill
weight,
pl
,
using Equation
2.9:
PIP+
pdp
”
=
0.052
D
0.5(
10000)
+
200
”
=
0.052(
10000)
h
=
10
PPS
5.
Determine the strokes to the bit using Equation 2.10:
(0.0 142)(10000)
0.1
sm
=
STB
=
1,420
strokes
6.
Determine the new circulating pressure,
P,,
at the
kill
rate
with
the kill-weight mud utilizing Equation 2.1
1:
P,
=
Pdp
-
O.O52(p,
-p)D
+
-
Pk,
(9
P,
=
200- 0.052(10-9.6)(10000)+