Chilingarian G.V. et al. Surface Operations in Petroleum Production, II
Подождите немного. Документ загружается.
205
small the liner perforations can be made, because they have to be cut into the pipe.
Usually, the smallest size of the liner slots ranges from 0.12 to 0.015 in.
Wire-wrapped screens cost approximately three times as much as the slotted
liners, but they do offer some advantages over liners. These screens are typically
made of stainless steel and are corrosion and erosion resistant. Depending upon the
pipe size, a wire-wrapped screen can have up to fifteen times more area open to flow
than a slotted liner. Screens can be manufactured with much smaller slot widths
than liners (as small as 0.002 in.) and, in some instances, can control sand
production without a gravel pack behind them
if
the formation is able to form a
bridge. This sand control technique is used in recompletions when it is difficult or
impossible to gravel pack.
METHODS
OF
GRAVEL-PACKING
Openhole
gravel
pucks
In an openhole gravel pack, casing is set above the productive interval. The hole
is
then underreamed across the productive zone and a liner or screen is set in this
interval. Gravel is then placed between the open hole and the liner or screen (see
Fig. 6-11). If properly designed, this completion method provides a high degree of
sand control and maximum productivity because restrictive perforations are not
used. Proper placement of gravel is of utmost importance.
The underreaming involves the use of a hole opener or underreamer. This device
has cutter arms that can be hydraulically extended from the body of the tool once
Fig.
6-11.
Schematic diagram
of
a gravel pack in underreamed open hole.
206
the tool is below the casing shoe. The wellbore diameter can be increased up to
approximately two-fold by underreaming. By increasing the hole diameter, a thicker
gravel pack is obtained which will result in better sand control and greater
productivity. It is recommended that the gravel pack thickness in openhole gravel-
packed completions be at least 2 in. (Borden et al., 1982). The average thickness
used is
3
in. and the liner should be positioned by centering devices installed
through its full length to make sure that it is concentrically located in the hole
(Wright and Solum, 1967).
Cased-hole gravel packing
According to Saucier (1974), it is extremely important that the perforation
tunnels be filled with gravel and not formation sand. Perforations act to restrict the
flow
of
fluids into the wellbore. Any additional restrictions that are incurred by
having perforation tunnels filled with formation sand can cause large pressure drops
through the perforations with a resulting decrease in productivity. To insure that all
perforations and perforation tunnels are open, it is advisable to wash the perfora-
tions with a circulation washer tool prior to gravel packing (see Figs. 6-12.A,B).
Cased-hole gravel packs can be placed in either a “single-stage’’ or a
“
two-stage’’
operation (Suman, 1975). Either method is aimed at a tight pack in the liner-casing
annulus and in the perforation tunnels. In all cased-hole gravel packs, it is
recommended that the gravel annulus be
3/4
in. thick (Borden et al., 1982).
“Single stage” method
In “single-stage’’ cased-hole gravel packing, gravel is pumped into the liner-cas-
ing annulus either through a crossover tool or down the casing-workstring annulus.
Periodically, the flow of returns should be restricted, thereby forcing flow out of the
perforations. Inasmuch as flow out of the perforations is required, it is necessary
that the carrier fluid be able to leak-off into the formation. This can cause
formation damage. If liner vibration is used during the gravel packing, leak-off into
the formation can be eliminated or minimized (Solum, 1984).
“Two
-stage
’’
method
“TWO-stage’’ gravel packing involves packing the perforation tunnels and the
liner-casing annulus in separate operations. The perforation tunnels can either be
packed using a squeeze packer or by using an open-ended tubing squeeze method.
An open-ended tubing squeeze involves running open-ended tubing into the well-
bore opposite the perforations. Gravel is then pumped down the tubing and
pressure is held against the tubing-casing annulus. Reciprocation of the tubing
string may aid in forcing gravel into the perforations (Borden et al., 1982). Vibration
can also be used (Solum, 1984).
After the perforations have been packed, the liner or screen is run into the
wellbore. The annulus can then be gravel packed by either reverse circulating gravel
slurry down the annulus with returns coming up the tubing, or the gravel slurry can
be pumped down the tubing and through a crossover tool with returns coming up
the annulus. The most frequently used method is
“
single-stage”.
201
Fig.
6-12.
A.
Circulation washer tool. (Courtesy
of
Solum
Oil Tool Corporation.)
208
FIRST
PASS
Downward jetting action
picks up sand and re-
turns
it
through annulus.
Cleans up inside casing.
SECOND PASS
One
full
turn to left at
tool closes downward jet
and washing takes place
through side jets, as
shown.
WITHDRAWAL
When washing is com-
plete, one full turn to
right at tool reopens
downward jet. Tool is
then pulled dry.
Fig.
6-12.
B.
Operation
of
circulation washer. (Courtesy
of
Solum
Oil
Tool
Corporation.)
209
DEVIATED WELLS
Gravel packing highly-deviated wells requires additional design considerations to
increase the chances of obtaining a successful completion (Maly et al.,
1974;
Gruesbeck et al.,
1979;
Shryock,
1980;
and Elson et al.,
1984).
According to Maly et
al.
(1974),
packing efficiency decreases drastically in wells that are deviated beyond
60'
(see Fig.
6-13).
Laboratory tests indicate that the problem in deviated well is
due to the formation of gravel dunes as
it
is being packed. As the dune forms, the
carrier fluid flows preferentially into the tailpipe-liner annulus and deposit ad-
ditional gravel on the dune. As this process continues, a sand-off will eventually
occur (Elson et al.,
1984).
In an effort to eliminate the gravel dunes, Solum
(1984)
used liner vibration which prevented their formation and in all tests there were no
sand-offs or bridging.
Several investigators have concluded that restricting the flow of fluids into the
tailpipe-liner annulus will improve gravel placement in highly-deviated wells (Maly
et al.,
1974;
Gruesbeck et al.,
1979;
Shryock,
1980;
and Elson et al.,
1984).
Maly et
al.
(1974)
suggested that this flow could be restricted by either placing a tight
bladder seal inside the screen or by placing deformable baffles along the tailpipe. In
long liners, this proved impractical due to the high packing pressures. Shryock
(1980)
recommended restricting flow into the tailpipe-liner annulus by having a
tailpipe OD/liner
ID
ratio of
0.8
or greater.
GRAVEL-PACKING TOOLS AND EQUIPMENT
Blender
The blender is used to combine gravel and carrier fluid in a specified ratio and
pump the slurry downhole. The gravel is fed into the blender tank by a screw or
\CONVENTIONAL
\\TOOLS
--
’0
A
30
60
90
120
HOLE ANGLE, DEGREES
FROM
VERTICAL
Fig.
6-13.
Effect
of
hole angle on gravel packing efficiency with
and
without unipack for a one-pass pack
in a l/lO-scale laboratory model. Conditions: equivalent flow rate-130-150 gal/min; fluid-tap
water; gravel concentration-2-4 lb/gal. (After Maly et
al.,
1974, p. 22; courtesy
of
the Society
of
Petroleum Engineers.)
210
FLUlDlGRAVEL
ClSE CONTROL
VELlFLUlD MIX
CENTRIFUGAL
VEL PASSES
CENTRIFUGAL
PUMP AND
VALVES
FROM
MUD
TANKS
TRIPLEX PUMP
Fig. 6-14. Conventional gravel blending unit. Gravel breakup occurs when fluid and gravel are mixed in
the tank and pass through centrifugal pump and triplex pump valves before injection into the well.
(Courtesy
of
Solum Oil Tool Corporation.)
bucket conveyor. The carrier fluid is also pumped at a controlled rate into the
blender tank (see Fig.
6-14).
This unit mixes the fluid and gravel in the tank
by
use
of
a centrifugal
pump.
The gravel/fluid ratios are controlled by the conveyor rate
of
Fig. 6-15. Cylinder-type gravel-packing method. There is
no
control
of
(1)
fluid/gravel ratio and
(2)
fluid
velocity. (Courtesy of Solum Oil
Tool
Corporation.)
211
gravel input; however, gravel crushing occurs as the slurry passes through the
centrifugal and triplex pumps.
Gravel pots
Gravel pots can also be used to mix carrier fluid and gravel and inject the slurry
into the wellbore. This system utilizes two pressure vessels that can be filled with
gravel from a hopper through a sealable top port. Once the gravel is placed in the
vessel, the top port is sealed and fluid pressure is applied to the vessel. Gravel is
then gravity drained into the fluid stream by opening a valve at the bottom
of
the
vessel as the pressure is maintained in the vessel.
As
one pot is draining gravel, the
second one is filled with gravel.
As
the slurry does not pass through any pumps,
gravel crushing is minimized. It should be noted, however, that the system has the
disadvantage of not being able to control gravel/fluid ratios (see Fig. 6-15).
Positive-injection gravel blending unit
Positive-injection gravel blender was developed by Solum Oil Tool Corporation
in order to have accurate gravel/fluid ratio control and not crush the gravel. This is
accomplished by adding the gravel at a controlled rate on the downstream side
of
the centrifugal and triplex pumps (see Fig. 6-16). The unit is reported to induce no
gravel crushing, whereas conventional blenders crush an average of 17.5%
of
the
gravel (Solum, 1983; see Fig. 6-17).
Surface equipment
The normal gravel-injection equipment flow pattern at the wellsite is shown in
Fig. 6-18. One important function of the surface equipment is to filter all the fluid
just before it is mixed with the gravel. The filtering system should be located
between the fluid storage tank and the blending unit; this insures filtration of solids
down to
2
pm in size. Often, the storage tanks are contaminated, thereby con-
taminating the pre-filtered fluid.
The instrumentation and schematic of the fluid-gravel flow pattern with after-
the-pump injection is shown in Fig.
6-19.
The basic gauges to control the surface
operation of the gravel are (1) meter (bbl/min),
(2)
ratiometer (reading in lb/gal),
and
(3)
a discharge pressure gauge (reading in psi). These instruments, receiving
their data from the various components, as shown in Fig. 6-19, enable the blending
operator to have complete control of the gravel-packing operation.
Crossover tool
A
crossover tool is a device that
is
attached between the bottom of the working
string (drillpipe
or
tubing) and the top of the liner or screen during the gravel-pack-
ing operation. The device allows for the slurry to be pumped down the working
21
2
GRAVELIFLUID
RATIOMETER
ID
FROM
MUD TANKS TO SHAKER SCREEN ,NJECTION
CHAMBER
Fig. 6-16. Schematic diagram
of
a positive-injection gravel blending unit. Accurate ratiometer controls
fluid/gravel ratio. Gravel is not broken by passing through mixing tank, centrifugal pump, and triplex
pump valves. (Courtesy
of
Solum Oil Tool Corporation.)
Fig.
6-17.
Photograph
of
positive-injection gravel-blending unit. (Courtesy
of
Solum Oil Tool Corpora-
tion.)
213
INJECT10
N
CHAMBER
-WELLHEAD
Fig.
6-18.
Wellsite fluid and gravel
flow
pattern. Gravel
is
injected after the pump. (Courtesy of Solum
Oil Tool Corporation.)
string and then out the crossover ports into the openhole-liner or casing-liner
annulus. Fluid returns then pass through the slots in the liner back into the
wellbore. The fluid returns travel up the washpipe and then back through the
FLUID TANK
8
B
L
I
M
I
N
METER
GRAVEL INJECTION
MOTOR OR RAM
DISCHARGE
PRESSURE
GAUGE
(PSI1
GRAVELlFLUlD
DISCHARGE
TO
WELL
Fig.
6-19.
Basic instrumentation schematic for fluid and gravel control by blender operator. Gravel/fluid
ratios can be controlled very accurately. (Courtesy of
Solum
Oil Tool Corporation.)
214
Fig.
6-20.
Crossover
tool.
(Courtesy
of
Solurn Oil
Tool
Corporation.)
crossover, which directs the returns into the casing-working string annulus (see Fig.
Shryock
(1979)
has identified the crossover port as an area where gravel crushing
occurs. The gravel shattering was determined to be due to high velocity impinge-
ment against the casing as the slurry exits the crossover ports. If the crossover ports
are located opposite open hole, then erosion and mixing of formation material and
gravel is likely to occur. By increasing the size or number of crossover ports, the
stream velocity of the slurry as it exits the ports is reduced. This reduction in
velocity minimizes gravel shattering and wellbore erosion.
The writers have also noticed stress cracking from gravel impingement on the
steel surface at the base of the crossover port. According to Solum (1983), a
minimum
of
8% gravel crushing occurs as gravel strikes this steel surface at normal
pumping rates. Solum Oil Tool Corporation has developed a crossover tool that has
a 3-in. resilient rubber cushion at the base of the crossover. It is reported that this
tool reduces gravel breakup to less than 1%.
6-20).