Grace Robert. Advanced Blowout and Well Control
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86
problem of keeping the hole full of fluid
has
been emphasized for many
years. Pressure control problems and blowouts associated with trips
continue to
be
a
major occurrence. A lack of training and understanding
contributes
to
these circumstances. Classical pressure control procedures
apply
to
drilling operations, not
to
tripping operations.
All
of the
modeling and technology used in pressure control
was
developed based on
a
drilling model
as
opposed
to
a
tripping model. Therefore, the
technology that applies to pressure control problems which occur during
drilling operations does not apply to pressure control problems which
occur during tripping operations. As
a
result, when pressure control
problems occur while tripping, drilling procedures are
often
applied,
confusion reigns, and disaster results.
Advanced
Blowout
and Well
Control
TRIP SHEETS
AND
FILLING
PROCEDURES
Prior
to
a trip, it is assumed that the well is under control and that
a
trip
can
be made safely
if
the full hydrostatic is maintained. Pressure
control problems which occur during
a
trip are generally the result of
swabbing or
a
simple failure
to
keep the hole
full.
In
either case,
recognition and prevention of the problem is much easier than the cure.
Accurate "trip sheets" must be kept whenever productive horizons have
been penetrated or on the last trip before entering the transition zone or
pay interval. The trip
sheet
is simply
a
record
of
the actual amount
of
mud used to keep the hole full while the drill
string
is being pulled
compared to the theoretical quantity required
to
replace the pipe
that
is
being removed. Properly monitoring the tripping operation and utilizing
the trip sheet will forewarn the crew of potential well control problems.
In
order to
fill
and monitor the hole properly, the drillpipe must be
slugged
dry
with a barite pill. Difficulty in keeping the pipe
dry
is not an
acceptable excuse for fdure
to
fill
the hole properly. If the first pill
fails
to
dry
the drillpipe, pump a second pill heavier
than
the first. If the pipe
is
dry
for some time and then pulls wet, pump another pill.
A
common
question is
"HOW
frequently should the hole be
The
basic factors
determining frequency are regulations, critical nature of the well, and the
wellbore geometry.
Often,
special field rules or regulatory commissions will specify
the method to be used to maintain the hydrostatic in particular fields or
areas. Certainly, these rules are
to
be
observed. It is acceptable
to
Pressure
Control
Procedures
while
Tripping
87
deviate from the established procedure with appropriate cause
or
when the
procedure to
be
used
is widely accepted
as
being more definitive
than
that
established by regulation. Certainly, the condition of some wells
is
more
critical
than
that
of others. The critical nature may be related
to
location,
depth, pressure, hydrocarbon composition
or toxic nature of the formation
fluids, to name a few.
To
illustrate the significance of the wellbore geometry, consider
Example 3.1
:
Example
3.
I
Given:
Well depth,
D
=
10,OOOfeet
Mud density, P
=
15PPl3
Mud gradient,
p,,,
=
0.78psdft
Length
of
stand,
Lsld
=
93
feet
Displacement
of
4
%-inch drillpipe,
Dsp4112
=
0.525
bbVstd
2
7/8-inch drillpipe,
Dsp2
718
=
0.337 bbVstd
Capacity
of
12
%-inch hole less pipe displacement,
C,
=
0.14012 bbVft
4
%-inch hole less pipe displacement,
C2
=
0.0
1829 bbVft
Stands
of
pipe to
be
pulled
=
lostands
Wellbore configuration
1
:
4
%-inch drillpipe in 12 %-inch hole
Wellbore configuration 2:
2 7/8-inch drillpipe in
4
%-inch hole
88
Advanced
Blowout
and
Well
Control
Required:
Compare
the
loss in hydrostatic resulting from pulling
10
stands
without filling from wellbore configuration
1
and
wellbore
configuration
2.
Solution:
Determine the displacement
for
10
stands
for wellbore
configuration
1.
Displacement
=
(DSP,)(number
of
stands)
(3.1)
Where:
DSP,
=
Displacement
of
the drill string, bbls/std
Displacement
=
(0.525)(
10)
Displacement
=
5.25
bbls
Determine hydrostatic
loss
for
wellbore configuration
1.
pm(
Displacement)
Loss
=
(3
4
c,
Where:
Pm
=
Mud
gradient, psilft
Cl
=
Hole capacity less pipe displacement, bbVft
(0.78)(5.25)
0.1
40
12
Loss
=
Loss
=
29
psi
Determine the displacement
for
10
stands
for wellbore
configuration
2.
Pressure
Control
Procedures while
Tripping
89
Displacement
=
(DSP,)(number
of stands)
Displacement
=
(0.3
3
7)
(
1
0)
Displacement
=
3.37
bbls
Determine hydrostatic loss for wellbore configuration 2.
p,(
Displacement)
Loss
=
c2
(0.78X3.37)
0.01829
Loss
=
Loss
=
144
psi
The loss
in
hydrostatic for the first wellbore configuration
is
obviously insigmficant for most drilling operations. The loss of almost
150
psi in hydrostatic for the second case is much more significant. Often
the trip margin, which
is
the difference between the mud hydrostatic and
the formation pore pressure,
is
no more
than
150 psi. Therefore, all
things being
equal,
filling after 10
stands
would be acceptable in the first
instance while continuous filling would
be
in order in the second case.
PERIODIC
FILLING
PROCEDURE
Periodic filling, which is filling the hole after pulling
a
specified
number of
stands,
is the
minimum
requirement and is usually
accomplished utilizing a pump stroke counter according
to
a schedule.
The periodic filling procedure is
as
follows:
90
Periodic Filling Procedure:
Advanced
Blowout
and
Well
Control
Step
1
Determine the pump capacity,
cp,
bbls/stk.
Step
2
Determine the drill string displacement,
DSP,
,
bbls/std, for each
section of drill string.
Step
3
Determine the number of
stands
of each section of drill string
to
be pulled prior
to
filling the hole.
Step
4
Determine the theoretical number of pump strokes required
to
fill
the hole after pulling the number
of
stands determined in Step
3.
Step
5
Prior to reaching the critical interval, begin maintaining a record
of
the number
of
pump strokes required to fill the hole after
pulling the number of stands determined in Step
3.
Maintain the
data
in tabular form, comparing the number of
stands pulled
with
the
actual
strokes required
to
fill the hole, the
number
of
strokes theoretically required and the number of
strokes required on the previous trip.
This
data,
in tabular
form,
is the trip
sheet.
The trip sheet should be posted and maintained
on the rig floor.
Step
6
Mix and pump a barite slug in order to pull the drill string dry.
Wait for the hydrostatic inside the drill string to equalize.
Step
7
Pull the specified number
of
stands.
Pressure
Control
Procedures While
Tripping
91
step 8
Zero the stroke counter.
Start
the pump. Observe the return of mud
to the surface and record
in
the appropriate column the actual number
of
strokes required to bring mud
to
the surface.
step
9
Compare the
actual
number
of
strokes required to bring mud to
the
surface
to
the number
of
strokes required on the previous trip
and
the number
of
strokes theoretically determined.
The trip sheet generated by the periodic filling procedure is
illustrated in Example
3.2:
Example
3.2
Given:
Pump capacity,
cp
=
0.1 bbVstk
Drillpipe displacement,
DSP,
=
0.525
bbVstd
Drillpipe
=
4
%-inch 16.60
#le
Drillpipe pulled between filling
=
10
stands
Actual strokes required
as
illustrated in Table 3.1
Strokes required on previous trip
as
illustrated in Table
3.1
Required:
Illustrate the proper trip sheet for a periodic filling procedure.
Solution:
Strokes per
10
stands
DSP,
(number
of
stands)
Stroks
=
CP
(3.3)
Where:
DSP,
=
Displacement
of
the drill string, bbl/std
92
Advanced
Blowout
and
Well
Control
Cumulative
Stands
Pulled
10
20
30
c~
=
Pump capacity, bbVstk
Actual Strokes Theoretical Strokes
Required Strokes Required on
Previous Trip
55 52.5 56
58
52.5 57
56 52.5 56
(0.525)(10)
0.1
strokes
=
Strokes
=
52.5
per
10
stands
I
I I
I
....
....
....
....
The periodic filling procedure represents the minimum acceptable
filling procedure. The “flo-sho,” or drilling fluid return indicator, should
not be used
to
indicate when circulation is established.
It is preferable
to
zero the stroke counter,
start
the pump
and
observe the
flow
line returns.
As
a matter of practice, the displacement should be determined for each
different section of the drill string. The number
of
stands
of
each section
to be pulled between fillings may vary. For example, the hole should be
filled after each stand of drill collars since
the
drill collar displacement is
usually approximately five times drillpipe displacement.
CONTINUOUS FILLING PROCEDURE
In
critical well situations, continuous filling
is
recommended using
a
trip
tank.
A trip
tank
is
a
small-volume
tank
(usually less
than
60
barrels) which permits the discerning of fractions
of
a
barrel. The better
arrangement is with the trip
tank
in
full
view
of
the driller or floor crew
Pressure
Control
Procedures
while Tripping
93
and rigged with a
small
centrifbgal pump for filling the
tank
and
continuously circulating the mud inside the
tank
through the bell nipple or
drillpipe annulus and back into the trip
tank.
When
that
mechanical
arrangement is
used,
the hydrostatic will never drop.
The procedure would be
that
the hole would be filled
continuously. After each
10
(or some specified number
of)
stands, the
driller would observe, record
and
compare the volume pumped from the
trip tank into the hole with
the
theoretical volume required. The procedure
for continuously filling the hole using the trip
tank
would be
as
follows:
Step
1
Determine the drill string displacement,
DSP,,
bblsktd, for
each
section of drill string.
Step
2
Determine the number
of
stands of each section
of
drill
string to
be pulled prior to checking the trip
tank.
Step 3
Determine the theoretical number
of
barrels required
to
replace
the drill string pulled from the hole. Fill the hole after pulling the
number of stands determined in Step
2.
Step
4
Prior to reaching the critical interval, begin maintaining
a
record
of
the number
of
barrels required
to
maintain
the hydrostatic
during the pulling of the number
of
stands
determined in Step
2.
Maintain the
data
in tabular form, comparing the number of
stands pulled
with
the cumulative volume required to maintain the
hydrostatic, the volume required
as
theoretically determined and
the volume required
on
the previous
trip.
This
data, in tabular
form, is the trip sheet and it should be posted and maintained on
the rig
floor.
94
Advanced
Blowout
and
Well
Control
Step
5
Mix and pump
a
barite slug in order
to
pull the drill string dry.
Wait for the hydrostatic inside the drill string
to
equalize.
Step
6
Fill the trip
tank
and isolate the hole from the mud pits.
step
7
Start
the centrifugal pump and observe the return of mud
to
the
trip
tank.
Step
8
With the centrifugal pump circulating the hole, pull the number
of
stands
specified in Step
2.
step
9
After pulling the number of stands specified in Step 2, observe
and record the number of barrels of mud transferred from the trip
tank
to
the hole.
step
10
Compare the number
of
barrels of mud transferred from the trip
tank
to
the hole during
this
trip with the same volume transferred
during the previous trip and the volume theoretically determined.
The trip
sheet
generated by the continuous filling procedure is
illustrated in Example
3.3:
Example
3.3
Given:
Drill pipe
=
4
%-inch
16.60
#/e
Drillpipe displacement,
DSP,
=
0.525bbVstd
Stands
pulled between observations
=
10 stands
Trip
tank
capacity is
60
barrels in %-barrel increments
Pressure Control Procedures While
Tripping
95
Actual volume required
as
illustrated in Table
3.2
Volume required on previous trip
as
illustrated
in
Table
3.2
Required:
Illustrate the proper trip sheet for
a
confinuous filling procedure.
Solution:
Determine volume per
10
stands.
Displacement
=
(DSP,Xnumber
of
stands)
Displacement
=
(0.525)(
10)
Displacement
=
5.25
barrels per
10
stands
The proper trip
sheet
for
periodic filling
is
illustrated
as
Table
3.2:
Table
3.2
Trip Sheet
Generally, the
actual
volume
of
mud required
to
keep the hole full
exceeds the theoretical calculations. The excess can
be
as
much
as
50%.
On rare occasions, however, the actual volume requirements to
keep
the
hole
fidl
are consistently less than those theoretically
determined.
Therefore, it is vitally important that the trip sheets
from
previous trips be