Grace Robert. Advanced Blowout and Well Control
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96
kept for future reference. Whatever the
fill
pattern, it must be recorded
faithfully for future comparison.
Advanced Blowout and Well
Control
TRIPPING INTO THE
HOLE
Not always, but in specific instances, it is prudent
to
monitor
displacement while tripping in the hole
to
insure
that
fluid displacement is
not excessive. The
best
means of measuring the displacement going in the
hole is
to
displace directly into the isolated trip
tank.
A
trip sheet exactly
like Table
3.2
would
be
maintained.
All
too
often crews are relaxed and
not
as
diligent
as
necessary on the trip in the hole.
As
a result, industry
history
has
recorded several instances where excessive displacement went
unnoticed and severe pressure control problems resulted.
Calculations and experience prove
that
swabbing can occur while
tripping out or
in.
Swab pressures should
be
calculated,
as
additional trip
time can be more costly and hazardous
than
insignificant swab pressures.
Swab pressures
are
real and should
be
considered. If a well is swabbed in
on a trip
in the hole, the influx will
most
probably be inside the drillpipe
rather than the annulus because the frictional pressure is greater inside the
Further, the potential for problems does not disappear once the bit
is on bottom. The pit level should
be
monitored carefully during the first
circulation after reaching bottom. The evolution of the trip gas from the
mud
as
it
is
circulated to the
surface
may reduce the
total
hydrostatic
sufficiently
to
permit
a
kick.
Special attention is due when using inverted oilemulsion systems.
Historically, influxes into oil muds are difficult to detect. Because gas is
infinitely soluble in oil, significant quantities of gas may pass undetected
by the
usual
means until the pressure is reduced
to
the bubble point for
that particular hydrocarbon mixture. At that time, the
gas
can flash out
of solution,
unload
the annulus and result
in
a
kick.
drill string.
Pressure
Control
Pmedures
While Tripping
97
SHUT-IN
PROCEDURE
WELL
KICKS
WHILE TRIPPING
the correct procedure is
as
follows:
When
a
trip
sheet
is
maintained
and the well fails to
fill
properly,
step
1
The hole
is
observed not
to
be
filling properly. Discontinue the
trip and check for flow.
Step
2
If the well
is
observed
to
be
flowing, space out
as
may be
necessary.
step
3
Stab a full opening valve in the drillpipe and shut in the drillpipe.
step
4
Open the choke line, close the blowout preventers and close the
choke, pressure permitting.
Step
5
Observe and record the shut-in drillpipe pressure, the shut-in
annulus pressure and the volume of formation fluid that has
invaded the wellbore.
step
6
Repeat Step
5
at
15-minUte
intervals.
Step
7
Prepare to strip or snub back to bottom.
The following is
a
discussion
of
each step:
98
Advanced
Blowout
and
Well
Control
Step
1
The hole is observed not to be filling properly. Discontinue the
trip and check for flow.
If the hole is not filling properly, it should be checked for flow.
The
observation period is
a
hnction of experience in the
area,
the
productivity of the productive formation, the depth
of
the well and
the mud
type.
Under most conditions,
15
minutes is sufficient.
In
a
deep well below
15,000
feet or if oil-based mud is being
used,
the observation period should be extended to
a
minimum
of
30
minutes.
If the well is not observed
to
be
flowing, the trip can be continued
with the greatest caution. If a periodic filling procedure is being
used, the hole should be filled after each stand and checked for
flow until the operation
returns
to
normal. If, after pulling
another designated number of stands the well continues to fill
improperly, the trip should be discontinued. If the well
is
not
flowing when the trip is discontinued, the bit may be returned
cautiously
to
bottom.
In
the event
that
the bit is returned to
bottom, the displacement of each stand should be monitored
closely and the well should be checked for flow after
each
stand.
If at any time the well is observed
to
be flowing, the trip should be
discontinued.
It
is well
known
that
in
many areas of the world
trips are made with the well flowing; however, these should be
considered isolated instances and special cases.
step
2
If the well is observed to be flowing, space out as may be
necessary.
The
drill
string should
be
spaced out to insure that there is not
a
tool joint
in
the rams. If
that
is not
a
consideration, a tool joint is
normally spotted at the connection position.
step
3
Stab
a
full
opening
valve
in
the drillpipe and shut in the drillpipe.
Pressure
Contrvl
Pmcedums
mile
Tripping
99
The drillpipe should
be
shut in first. It is well
known
that
the ball
valves normally used to shut
in
the
drillpipe are difficult
to
close
under
flow
or
pressure.
Step
4
Open
the
choke line, close the blowout preventers and close the
choke, pressure permitting.
This
step represents
a
soft
shut-in. For
a
discussion
of
the
soft
shut-in
as
opposed
to
the
hard
shut-in, refer
to
the shut-in
procedure for the Driller’s
Method
in Chapter
2.
Observe and record the shut-in drillpipe pressure, the shut-in
annulus pressure and the volume
of
formation fluid that
has
invaded the wellbore.
Step
6
Repeat
Step
5
at 15-minute intervals.
If
the well
has
been swabbed
in,
the bubble should
be
below the
bit,
as
illustrated
in
Figure
3.1.
In
that
event,
the
shut-in drillpipe
pressure
will
be
equal
to
the shut-in casing pressure.
The
pressures and influx must
be
monitored
at
15-minute intervals
in
order
to
insure that the well is effectively shut in, to establish the
true reservoir pressure and to monitor bubble rise. Bubble
rise
is
discussed
in
chapter
4.
Step
7
Prepare
to
strip or snub back to
bottom.
Stripping is
a
simple
operation.
However,
stripping
dues
require
a
means of bleeding and accurately measuring small volumes
of
mud
as
the drill string is
stripped
in the hole.
A
trip
tank
is adequate
for
measuring the mud volumes bled.
In
the alternative,
a
service company
pump
truck
can
be
rigged up
to
the annulus and the
dispb&
can
be
measured
into
its
displacement
tanks.
If
the
gain
is
large and the
100
pressures are high, stripping through the rig equipment may not be
desirable.
This
point
is
more thoroughly discussed in Snubbing
Operations in Chapter
6.
Advanced Blowout
and
Well Control
Figure
3.
I
-
Influx
Swabbed
In
Once shut in, the well is under
control. Properly done, the surface pressure
on
the well should
be
less
than
500
psi, which
is almost insignificant. With the well shut
in,
there
is
adequate time to consider alternatives
without danger of disaster. The well that
has
been swabbed in is
a
well that was under
control when the bit
started
off
bottom.
Properly done, it should be a simple procedure
to return the well
to
fill control.
Once the well is shut in and under
control, the options are
as
follows:
STRIPPING IN THE HOLE
Stripping in is not complicated if a
few
simple rules and concepts are observed.
The procedure is
as
follows:
step
1
Install a back-pressure valve on top of
the
safety
valve in the drill string.
Open the
safety
valve.
Step
2
Determine the displacement in barrels
per stand of the drill string
to
be
stripped into the hole. Consider
that
the inside
of
the drill string will
be
void.
Pressure Contml Procedures While
Tripping
101
Step
3
Determine the anticipated increase in surface pressure when the
bit enters the influx according to Equation
3.4:
Where:
DSP,
=
Displacement
of
drill string, bbl/std
ChU
Pm
=
Mud gradient, psi/ft
Pf
=
Influx gradient, psi/fi
=
Capacity of drill string annulus, bbVf€
Where:
%
r,
=
Bottomhole pressure, psi
Tb
=
Bottomhole temperature, "Rankine
'b
=
Compressibility factor
=
Specific gravity of gas
Step
4
Determine the anticipated top
of
the influx, TOI, pursuant
to
Equation
3.6:
Influx
Volume
TOI
=
D-
'h
(3.6)
Where:
D
=
Well depth,
feet
ch
=
Hole capacity,
bbVft
102
Advanced
Blowout
and
Well
Control
step
5
Prepare
to
lubricate the drill string with water
as
it passes through
the surface equipment.
Step
6
Lower one stand into the hole.
step
7
At the same time, bleed and precisely measure the displacement
determined in Step
2.
Step
8
Shut in the well.
step
9
Read and record in tabular form the shut-in casing pressure.
Compare the shut-in casing pressures before and after the stand
was lowered into the hole.
Note: The shut-in casing pressure should remain constant until
the bit reaches the influx.
step
10
Repeat Step
9
until the bit reaches the top
of
the influx
as
determined in Step
4.
Step
11
When the bit reaches the top
of
the influx
as
determined in Step
4,
the shut-in surface pressure will increase even after the proper
volume of mud is released.
Read
and record the
new
shut-in casing pressure and compare
with the original shut-in casing pressure and the anticipated
increase in shut-in casing pressure
as
determined in
Step
2.
Pressure
Control
Procedures
While
Tripping
103
The new shut-in casing pressure should not
be
greater than the
original shut-in casing pressure plus the anticipated increase.
Step
12
Repeat Step 11 until the bit is on bottom.
Step 13
Once the bit is on
bottom,
circulate out the influx using the
Driller’s Method
as
outlined in Chapter 2.
Step
14
If necessary, circulate and raise the mud weight and trip out
of
the
hole.
Each step will be discussed
as
appropriate:
step 1
Install a back-pressure valve on top of the safety valve in the drill
string. Open the safety valve.
Step
2
Determine the displacement in barrels per stand of the drill string
to be stripped into the hole. Consider
that
the inside of the drill
string will
be
void.
Step
3
Determine the anticipated increase in surface pressure when
the
bit enters the inf‘lux according
to
Equation
3.4:
When the bit enters the influx, the influx will become longer
because
it
will then occupy the annular
area
between the drill
string and the hole
as
opposed to
the
open hole. Therefore, the
volume of mud which
is
bled from the well will be replaced by the
104
Advanced
Blowout
and
Well
Control
increased length of the influx. Provided that the volume of mud
bled from the well is exactly
as
determined, the result
is
that the
surface pressure will increase automatically by the difference
between the hydrostatic of the mud and the hydrostatic
of
the
influx, and the bottomhole pressure
will
remain constant. No
additional influx will
be
permitted.
Step
4
Determine the anticipated top of the influx, TOI, pursuant to
Equation
3.6:
Influx
Volume
TOI
=
D-
Ch
It
is
necessary to anticipate the depth at which the bit will enter
the influx. As discussed, the annular pressure will suddenly
increase when the bit enters the influx.
step
5
Prepare to lubricate the drill string with water
as
it passes
through
the surface equipment.
Lubricating the string
as
it
is
lowered into the hole will reduce the
weight required to cause the drill string
to
move.
In
addition? the
lubricant
will
reduce the wear
on
the equipment used for
stripping.
Step
6
Lower one stand
into
the hole.
It
is
important that the drill
string
be
stripped into the hole,
stand
by stand.
Step
7
At the same time, bleed
and
precisely measure the displacement
determined in Step
2.
p*essu*e
Control
h&m
mile
Tripping
105
It
is vital that the volume of mud removed is exactly replaced
by
the drill string
that
is stripped into the well.
step
8
Shut in the well.
Step
9
Read
and record
in
tabular form the shut-in casing pressure.
Compare the shut-in casing pressures More and after the
stand
was
lowered into the hole.
Note:
The shut-in casing pressure should remain
constant
until
the bit reaches the influx.
It
is
important
to
monitor
the
surf$ce
pressure after
each
stand.
Prior
to
the bit entering the influx,
the
suri'ace
pressure should
remain constant. However, if
the
influx
is
gas
and begins
to
migrate
to the surface, the surface pressure will slowly begin to
increase.
The
rate
of
increase
in
surfhce pressure indicates
whether the increase is caused by influx migration or penetration
of the influx. If the increase is due to influx penetration, the
pressure
will
increase
rapidly during
the
stripping
of
one stand. If
the increase is due to influx migration, the increase is almost
imperceptible for
a
single stand. Bubble migration and the
procedure for stripping in
the
hole
with
intlux
migratim
is
discussed in Chapter
4.
step
10
Repeat Step
9
until the bit reaches
the
top
of
the influx
as
detennined in Step
4.
step
11
When
the
bit
reaches
the
top
of
the
influx
as
determined
in Step
4,
the shut-in
surface
pressure
will increase
even
after the proper
volume of mud
is
released.