March
2001 Vol. 222 No. 3
Feature Article
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WELL CONTROL / INTERVENTION
Well control during well intervention Part 2
Practical application of through-tubing well control operations
Concentric-tube well servicing
systems do not require the use of balance-weight fluids to maintain
pressure and flow control; however, when conditions dictate that well
pressure be at balance at the surface, well control practices must be
employed which focus on fluid hydrostatics
Alex
Sas-Jaworsky II, Sas Industries, Inc., Houston
oncentric-tube
well servicing technology is employed in numerous applications where
pressure and fluid containment must be controlled at the surface. By
design, the primary mechanism for maintaining pressure and fluid
containment in these types of concentric-tube services is the
purpose-built well control stack working in combination with fluid
flow control devices in the surface treatment piping. As such,
concentric-tube well servicing systems have a unique advantage in that
they are not bound by use of balance-weight fluids to maintain
pressure and flow control, as employed in conventional jointed-pipe
rig systems.
However, when
service conditions or job objectives dictate that well pressure be at
balance at the surface, well control practices that focus on fluid
hydrostatics must be employed. Based on the processes used in
conventional jointed-pipe rig systems, coiled tubing (CT) and
hydraulic workover (HWO) well control practices performed concentric
to existing well tubulars are more complex, and require greater
attention in design and implementation.
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Fig.
5. In general, the shut-in wellbore will reach equilibrium
with the fluids stratifying into columns as a function of
fluid density. This condition is in contrast to conventional
well control operations in which rheology of the kill fluid is
similar to rheology of fluid circulated in the wellbore, with
the difference in density attributed to volume of influx
fluid. |
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For typical
well intervention operations conducted in completed wellbores, the
capability to conduct a fluids-pumping, well control program is
contingent upon the types of fluids and equipment available onsite. If
the initial purpose of the well intervention service is to implement
operations in an underbalanced condition, the pumping equipment and
fluids available onsite may be inadequate to conduct an effective
fluids-pumping kill program, and well control must rely exclusively on
the functions of the well control stack. However, for intervention
services prepared to conduct a pumped-fluid, well control program,
numerous options become available and enhance the safe implementation
of this type of operation.
Well
Intervention / Well Control Practices
When preparing
for implementing well control programs within stabilized shut-in
wells, an estimate of the resident well fluid volume fractions can
assist in preparing the fluid circulation / displacement program. In
general, the shut-in wellbore will reach equilibrium with the produced
fluids stratifying into columns as a function of fluid density, Fig.
5. This condition is in contrast to conventional well control
operations where the rheology of the kill fluid is similar to the
rheology of the fluid circulated in the wellbore, with the difference
in apparent density attributed to the volume of influx fluid.
Typically, the return of pressure balance in this condition is
achieved by increasing the density of the circulated fluid to the
required kill weight and displacing the lighter density volume of
influx fluids.
However,
because of the variable composition of the resident wellbore fluids,
the pumping program for well-intervention well control generally will
involve two separate fluid circulation processes. The first
circulation is typically conducted using an available fluid of a given
density prior to pumping the dedicated kill-weight fluid.
This "pre-kill"
circulation program serves as the means to displace the resident
wellbore fluids with a fluid of uniform density and may be either the
pumped fluid prescribed for the well intervention service or a
specially formulated fluid. The pre-kill fluid displacement process
provides valuable information for the design and possible modification
of the kill-weight fluids pumping program.
After the
pre-kill circulation program is completed, the pumping program should
be interrupted to allow for static pressure readings to be taken in
the wellbore annulus and within the workstring. Note that with
pressure check valves installed in the BHA, assistance from the pumps
will be required (at the lowest pump rate possible) to open the check
valves and allow for pressure communication to the workstring. These
static pressure readings are then compared to the predicted pressures
at completion of the pre-kill circulation program. Variances observed
in this comparison are used to determine the corrective action
required prior to commencing the final kill-weight fluid circulation
program.
The most common
fluid circulation-type well control programs used in the oil and gas
industry are described as the "Drillers" method and
the "Wait and Weight" method. In essence, these well control
practices must be conducted in a manner that maintains a constant
bottomhole pressure (CBP) during the fluid circulation program.
The CBP
practice is intended to provide the desired balance of formation
pressure through a combination of hydrostatic pressure created within
the wellbore for given fluid columns and the pressure held at surface
through a prescribed choke pressure schedule. Using CBP well control
practices, the formation pressure is held at, or slightly above,
balance pressure during the circulation program, restricting any
additional influx of reservoir fluids into the wellbore. The fluid
circulation practices employed using the Drillers method and the
Wait and Weight method are described in greater detail below.
Drillers
Method
The Drillers
method is a fluid circulation practice typically used to displace
formation fluid influx from the wellbore annuli. To conduct the
Drillers method properly, the workstring BHA nozzle must be
positioned at the designated kill depth. In cases where the circulated
fluid density is lower than the required kill-weight density, this
pumping program will not result in hydrostatic pressure balance of the
well. However, when performed properly, this practice ensures that the
fluid in the wellbore is of uniform density upon conclusion. After
this circulation stage is completed, the surface pressure is combined
with the hydrostatic pressure exerted by the pumped-fluid column,
providing a check to confirm the required kill-weight fluid density.
The pumping
technique employed by the Drillers method is summarized as
described below. Once the BHA nozzle is positioned at the designated
kill depth, the stabilized shut-in workstring pressure (SITP) and
shut-in annulus pressure (SIAP) are recorded. While maintaining the
choke pressure at SIAP, the pump is brought up to the desired pump
rate. Once the pump rate is established, circulating pressure (CP)
observed at the fluid pump is recorded and serves as the control
pressure for this circulation procedure. The circulating pressure is
held constant throughout this fluid pumping stage, with adjustments
performed at the surface choke. This operation is continued through
displacement of at least 1-1/2 annuli volumes or until no formation
fluid influx is observed in the surface returns, whichever is greater.
Wait and
Weight Method
The Wait and
Weight method is a fluid circulation practice that is intended to
achieve hydrostatic pressure balance with the formation pressure
through the circulation of one complete wellbore fluid volume. In well
intervention applications, the Wait and Weight practice is typically
performed as the second circulation of the Drillers method,
where the kill-weight density fluid is pumped to displace the lighter
density fluid previously circulated in the wellbore.
As with the
Drillers method, the Wait and Weight method is conducted with
the BHA nozzle positioned at the designated kill depth. Upon
conclusion of this circulation stage, the observed static surface
pressure in both the workstring and wellbore annulus should be zero.
The pumping
technique employed by the Wait and Weight method is summarized below.
The BHA nozzle is positioned at the designated kill depth within the
wellbore, with stabilized shut-in workstring pressure (SITP) and
shut-in annulus pressure (SIAP) recorded. Based on observed SITP, the
required kill-weight density fluid is confirmed. The volume of
kill-weight density fluid needed to complete the kill operation is
prepared and the final stage of the pumping program is initiated.
While
maintaining the choke pressure at the SIAP observed prior to
initiating this step, the pump is brought up to the desired kill rate.
Note that pump rate is held constant throughout this pumping
technique. With the pump rate established, the pre-kill circulating
pressure (PKCP), representing the pressure losses for the system when
circulating the lighter-density fluid pumped during the previous
circulation step must be recorded.
For CT
operations, the pumping program must also accommodate a period of
pre-kill pump time to allow the kill-weight density fluid to travel
through the length of tubing remaining on the service reel to the
tubing guide arch. Once the kill-weight density fluid reaches the
tubing guide arch (CT) or the circulating swivel (HWO), the pumping
pressure is recorded as the initial circulating pressure (ICP). At
this point, the kill-weight density fluid begins to displace the
vertical workstring segment deployed within the wellbore.
As the
kill-weight density fluid moves down the workstring segment to the BHA
nozzle, the observed pump pressures should decrease with respect to
the ICP due to the difference in fluid densities. Once the kill-weight
density fluid reaches the BHA nozzle, the frictional pressure losses
occurring within the workstring are stabilized and recorded as the
final circulating pressure (FCP). At this point, the FCP serves as the
controlled minimum pumping pressure for the remainder of the
circulation procedure.
As the pumping
program continues, the kill-weight density fluid is circulated into
the wellbore annuli, causing the hydrostatic pressure exerted on the
formation to increase. The increase in hydrostatic pressure is
compensated by decreasing the surface choke pressure in adjustments
corresponding to the kill-weight fluid displacement schedule. This
operation is continued for at least one complete annuli volume, or
until no contaminated fluid is observed in the surface returns,
whichever is greater.
The kill-weight
fluid pumping program must also take into account the elevation of the
tubing guide arch (Fig. 6) or circulating swivel (Fig. 7) with respect
to the reference point selected in the wellbore, as well as the
difference in elevation with respect to the returns line. The
difference in elevation must be considered when determining the
effectiveness of the hydrostatic pressure balance in the wellbore when
pumping operations are halted.
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Fig.
6. The kill weight fluid pumping program must take into
account the tubing guide archs elevation with respect to
the reference point selected in the wellbore, as well as
difference in elevation with respect to returns line. The
difference in elevation must be considered when determining
effectiveness of hydrostatic pressure balance in the wellbore
when pumping operations are halted. |
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Fig.
7. As with the coiled tubing example, kill weight fluid
pumping program must also take into account elevation of the
circulating swivel with respect to the reference point
selected in the wellbore. |
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Onsite
Preparation For Well Control Operations
When
implementing well intervention operations, the following general
guidelines are offered to assist in preparation of the well control
program.
- Select a
reference point within the surface tree to correlate back to KB
measurements.
- Establish
the top-of-fluid column height at the flow cross or flow tee in the
riser assembly where fluid returns are expected to be taken.
- Obtain
measured height of all well control stack components as taken from
the selected reference point. Estimate height of the top of the
tubing guide arch (CT) or circulating swivel (HWO).
- Correct
completion KB measurements to selected reference point in the
surface tree.
- Confirm TVD
measurement from the designated top-of-fluid column height in the
riser to the top of the exposed formation interval. This height is
used to calculate average kill-weight fluid density.
- Correct KB
tubular lengths in the wellbore and calculate fluid volumes needed
to fill the workstring and wellbore annuli with the BHA nozzle at
kill depth. In addition, calculate fluid volume displaced by the
workstring deployed in the wellbore.
- Correct
wellbore tubular and formation interval KB measurements to TVD
measurements for calculations of height of fluid fill-up to obtain
hydrostatic column pressure.
- Select a
reference point within the surface tree or riser to correlate CT or
HWO "zero" point to wellbore KB depths.
Coming
Installments
Part 3
Well control pumping options to minimize induced formation damage.
Part 4
Proper selection of coiled tubing surface and downhole well control
equipment.
The author
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Alexander
Sas-Jaworsky II founder and principal engineer of SAS
Industries, Inc., formed in 1995 and specializing in mechanical
testing, training and service consultation for all aspects of
applied coiled tubing technology. He began his career with Conoco
after receiving a BS in petroleum engineering at the University of
Southwestern Louisiana in 1982, and he worked for various coiled
tubing companies before and while attending college. He worked in
several Conoco divisions as a production engineer before
transferring to the Conoco Houston Production Technology group in
December 1990 as worldwide concentric workover consultant for
coiled tubing and snubbing. He is a registered professional
engineer in Louisiana and Texas, SPE member, and serves on API
committee 3/subcommittee16 as chairman of the well intervention
well control task group. |
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Part
1 |
Part
3 |
Part
4 |
Part
5 |
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