well productivity
Spectacular Wells
Result From Alternate
Path Technology
Improvements in alternate path [shunt] technology make long-interval and multiple-zone completions easy and lead to higher production rates and much longer well life without screen failures.
series of spectacular wells recently completed
using alternate path (shunt) apparatus and procedures indicates that the technology has come of
age and provides major advantages in many situations. The days of production limited by sand control measures appear to be over. Recent improvements in alternate
path technology have eliminated most of the problems causing reduced productivity. Long-interval and multiple-zone
completions are now easy. In addition to much higher production rates, much longer well life without screen failures
or plugging is the rule. The proof lies in a series of spectacular wells including completions with gravel packs, fracpacks, and horizontal wellbores.
A
Beginnings In 1991
Alternate path (shunt) technology has been available to the
industry in some form since 1991. Initially it was used only
for gravel packing; recent advances have expanded applicability from specialty gravel packing to a general, all-purpose
technology with huge benefits. Its flexibility permits use in
extremely long intervals and highly variable formations
allowing completions previously considered difficult or
impossible, such as gravel packs and frac-packs on multiple
intervals without staging and horizontal well frac-packs. In
addition, screen failures from sand production seem to have
been eliminated, well productivity has generally been about
double that obtained with other procedures, and reduced
production rates over time associated with screen plugging
are no longer observed. The earliest horizontal well gravel
packed with this technology has reached the 5-million-bbl
mark, with minimal decline in productivity index. Technical
aspects, recent developments, current and projected applications are discussed below.
Technical Aspects
Alternate path technology provides slurry bypasses through
the premature annulus bridges which frequently form during gravel packing or frac packing and halt the packing
process. The bypasses are provided by rectangular tubes or
shunts usually attached to the screen, extending from
above the top of the screen down to the rathole. Exit ports
are placed at more or less regular intervals along the tubes.
When a bridge builds in the annulus, the tubes become the
path of least resistance and the slurry bypasses the bridge,
exiting from the shunt wherever a void exists in the annulus. The pack builds in the annulus when the carrier fluid
flows through the screen or into the formation. As the gravel pack builds, it grows back into the exit ports, which are
designed so that the presence of packing sand in a port
diverts trailing slurry on downstream. The shunts therefore
act as a series of small crossovers, used in turn, with each
responsible for packing only a few feet of interval. Fig. 1
illustrates the concept.
Ideal packing fluids are gels with good sand suspension
capabilities. The high viscosity of gels allows substantial
fracturing and helps provide flow resistance in the exit ports
to prevent bridge formation within the shunts.
The ability to deliver nearly the entire pump pressure for
squeezing, fracture extension, and pack dehydration
through exit ports at each 2–3 ft segment of the completion
interval eliminates the classical gel packing problem of loose
packs. The resulting packs are complete, much tighter than
normally found (even with water packing), and extend into
every fracture and perforation which can take fluid.
Low viscosity fluids can be used with shunts, but the
improvement in the packs then extends only over a few feet
of interval, and most of the potential benefits are lost.
by Lloyd Jones, Consultant
WELL PRODUCTIVITY
Fig. 1. This alternate path sequence shows bridge building in the annulus (left), slurry bypass through exit ports (center), and finished
pack (right) which diverts slurry on downstream.
Recent Developments
In 1996, Schlumberger obtained a license from Mobil,
with the intention of further developing the technology.
Since that time, major advances have adapted the technology for applications in frac-packing and horizontal wells.
Improvements include:
■ Tungsten-carbide-lined exit ports to handle the large
quantities of sand placed in horizontal wells or when
frac-packing.
■ Seamless shunt tubes which can handle 5,000 psi differential pressure or more, as compared to about 2,000 psi
with welded tubes.
■ Larger shunts for fracturing which deliver three times
the previous slurry volume.
■ Shrouds to protect and centralize the shunt screens in
horizontal wells.
In a parallel effort, opposing cup packers have been
adapted to provide subsequent water shutoff capability in
long or stratified pay intervals.
A wellbore simulation system ranging up to 2,000-ft
long was developed to test and demonstrate the viability of
the new apparatus. Studies and early case histories have
been reported in several technical papers.2,4 Results have
been used to advance packing procedures until it is now
feasible to frac-pack through shunts at rates up to 25
bbl/min and gravel pack horizontal wells 4,000 ft or longer.
Applications
Gravel Packing. Alternate path technology was originally
developed for gravel packing difficult wells. Mobil, the initial developer, and several other operators have used the
shunts for about 250 completions since 1990. Mobil has
reported an average sand placement increase of 160% and
an approximate doubling of productivity index. Mobil also
reported using the technique for gravel packing horizontal
wells.1
Statoil and Conoco have used the apparatus to complete very long intervals in the North Sea. This eliminated
staging, with huge savings in rig time and well production
up to a month earlier than obtained previously. The companies used the technology in the Heidrun field to develop
some of the most productive wells ever seen,6 with production rates ranging from 30,000 to 43,000 b/d. A major
advantage was obtained by drilling through the pay at a
high angle to expose long completion intervals. The operators took great care in assuring open perforations and controlling fluid losses. Reportedly, clear evidence of early
bridging was observed in the recorded pumping pressures,
followed by long periods where packing took place entirely
through the shunts.
Most operators have observed that the deterioration in
well productivity, previously common in gravel packed
wells, has been nearly eliminated with shunts. This probably is the result of complete, or nearly complete, packs
which halt the movement of formation sand into perforation tunnels and also protect the screens from plugging
and erosion associated with production of fines.
Figs. 2 and 3 show gravel pack logs taken immediately
after gravel packs on two similar zones in the Gulf of
Mexico. Fig. 2 is a log from a well without shunts and Fig.
3 is a log from a shunt-packed well. Both gravel packs
were placed with gels at pump rates of about 4 bbl/min
and about 5 lb/gal of 40/60 sand. These results are typical
of wells gel packed with and without shunts. As might be
expected, superior well performance was observed from
the shunt-packed well.
Frac-Packing. Shunt technology is almost ideally suited
for frac-packing. When a shunt system is placed in the
squeeze position, the slurry dehydrates as with any fracturing operation until the pack in the fracture builds back
into the annulus at some high leak-off location. Then,
instead of a sand out, the slurry bypasses the bridge
through the shunts and finishes packing the downstream
interval. At best, the fracture grows until the entire interval
is fractured and packed. At worst, the slurry dehydrates
through the screen, and the pack is completed with no
more fracture growth. In either case, all of the annulus is
gravel packed; all of the fracture is packed; and any perfo-
WELL PRODUCTIVITY
GR
Fig. 2. Gravel pack log is typical of a conventional gel-packed well.
rations or locations taking fluid to the formation are also
packed.
It is believed that fracturing creates a “halo” around the
casing and that almost all of the perforations communicate with that halo. In most frac-packing operations, an
upstream bridge probably forms before the lower portion
of the halo and associated perforations are packed. With
shunts, they will be packed. Since much longer intervals
(including multiple pay zones) can be successfully fracpacked with shunts, the end result is higher rate wells.
Early results with the upgraded apparatus have been
spectacular. One major operator did a comparative study
of frac-pack results with and without shunts.3 The operator completed a number of wells with two producing
zones using:
■ Staged frac-packs without shunts followed by commingling.
■ Staged frac-packs using shunts only in the top interval
followed by commingling.
■ Frac-packs over the entire interval using shunts.
As seen in Fig. 4, the wells completed with shunts had
productivity indexes about double those without. Fig. 5
shows the pumping record from one of the wells. When
the pump pressure began to spike, indicating slurry delivery through the shunts, the operator reduced the rate to
drop the treating pressure to a comfortable level. A new
GR
Fig. 3. Log covers a zone similar to Fig. 3 that was gel-packed
with shunts.
fracture apparently developed at about that time, and slurry pumping continued until a final sandoff occurred.
About 40% of the total proppant placement was through
the shunts.
In a U.S. Gulf Coast application2, a 220-ft interval was
frac-packed at 18 bbl/min using three large shunts. Fig. 6
shows pressure and temperatures recorded during the
pumping operation. The substantial rise in surface pressure relative to bottomhole pressure (BHP) indicates
shunt activation. The crossover of pressures taken at the
bottom and top of the interval indicates opening of a new
fracture face at the bottom of the interval and slurry delivery to that location through the shunts. The temperature
records at the top and bottom of the interval coincide with
the pressure observations. This well has produced about
70 MMscf/d, tubing limited, for a year. Formation drawdown is around 200 psi. Similar wells completed without
shunts have produced about 35 MMscf/d. The presumption is that the fracture extension and large increase in the
number of perforations packed is a major factor in the
high productivity.
Another set of several dozen completions fits somewhere between regular gravel packing and frac packing.
David Bryant5 of Mobil has been a pioneer in developing
apparatus and applications for shunt technology. He soon
WELL PRODUCTIVITY
Conventional
Conv + Shunts
Shunts
15
10
5
0
1
2
3
4
5
6
7
8
9
10
Well
Fig. 4. Wells completed with shunts have productivity indices
about double those without.
discovered that excellent gravel packs could be obtained
with gels in the squeeze position and adopted that as his
normal procedure. Subsequently, he has developed a
database of dozens of completions done in that manner at
standard gravel packing pump rates. At various technical
gatherings he indicated that skin effects from these wells
overlay those from standard frac packs. Although this is
like comparing potato soup to mashed potatoes, the end
result is not unreasonable. Bryant obtains narrow and
short fractures extending along much or most of his intervals, and packs all of the perforations taking fluid. With a
regular frac pack where shunts are not used, the fracture
is much wider and longer, but upstream bridging near the
end of the packing process probably prevents packing of
the halo and many perforations, with the result that well
productivity is often comparable for the two procedures.
ALLFRAC JOB
10,000
8,000
Final Wellbore
Screenout
First Wellbore Screenout
Shunt Tubes Start Working
40
30
6,000
4,000
20
Rate Reduced to
13 bpm
2,000
0
22:00
50
22:05
Rate
Water Shut-Off. Substantial water shut-off using shunts is
accomplished by running shunts through packer bodies to
lower intervals.4,7 Until now, all jobs done with this technique have used opposing cup packers, which have shunts
mounted through them and are assembled into the string
wherever desired. All packing below the first packer is
accomplished entirely through the shunts, making it necessary to use tungsten carbide inserts immediately below
the packers. To restrict flow of water from lower intervals,
a plug is placed in the production string adjacent to the
packer when water production begins. It is required that
the shunt or shunts through the packers be packed tightly
at the end of the gravel pack job. This normally requires
re-pressuring the pack several times until no more slurry
can be forced into the well. This, in turn, requires use of a
carrier fluid which will suspend the sand while re-stressing of the pack is taking place.
The system works well so long as the above guidelines
are followed. Although small amounts of water can still be
produced through the pack in the shunt tubes, their small
cross-sectional area reduces flow to a trickle. Up to three
packers have been used in this manner, commingling four
zones over a 600-ft interval.
The ability to gravel pack through packers provides several options for exotic well completions. Some of these
will be discussed later.
Horizontal Wells. Much effort has been put into development of a viable process for gravel packing horizontal
wellbores when leak-off is present.1 Originally, it was
thought that the shunts would make such horizontal well
gravel packing an easy task. After much research, this has
proven true. However, variables that are insignificant over
limited pay intervals come into play here, and it has been
found that certain requirements must be met to have a
successful job.
It has long been known that inertial and boundary layer
effects cause much of the sand to by pass small shunt ports
and concentrate downstream. In vertical wells and short
high angle or horizontal wells this phenomenon is not significant. However, in a long horizontal wellbore, the sand
concentration can become so high that the shunt can plug.
There is also a tendency for certain ports to pack substantial lengths in horizontal wellbores, with the result
that these ports can be enlarged to a point where the shunt
sands off through the enlarged opening.
Both of these problems were overcome using large (1⁄4in.) elongated ports lined with tungsten carbide. Then it
was discovered that only minimal sand settling can be tolerated during a horizontal job. This requires use of gels
with good suspension qualities, such as viscoelastic surfactants, XC gels, or cross-linked polymers.
To satisfactorily test horizontal packing processes, it was
necessary to develop an exceedingly long wellbore simulator. The one devised uses slotted plastic pipe (120 ten-thousandths slots/foot) to simulate a permeable formation, and
one shunt placed inside to test the distance which a single
shunt can pack with a designated set of slurry characteristics and pumping parameters. A 1,000-ft simulator has
been packed many times with 40/60, 20/40, and 16/30 sand
using a single shunt. Smaller sand sizes or use of lower
fluid viscosities allow longer packs. A 2,000-ft length was
recently packed using a new manifold connector design
which permits delivery of slurry through tubes which have
no leak off to short tube segments which pack the wellbore.
This design seems to relax the fluid requirements somewhat. In either circumstance, it is now feasible to design
horizontal gravel packs 4,000 ft or more in length.
For field use in open holes, the wire-wrapped shunt
screens are enclosed in a highly perforated shroud which
provides both protection and screen centralization. Gravel
packing takes place through the holes in the shroud. The
Pressure (psi)
NPI (PI/KH)*1000
20
10
22:10
22:15
0
22:20
Time (hh:mm:ss)
Surface Pressure (psi)
BHP
Rate (bpm)
Fig. 5. Record of a shunt-packed well shows initiation of flow
through the shunts, rate reduction and final screenout.
WELL PRODUCTIVITY
3,500
Upper BHP
10,000
Final screen-out
3,000
9,000
8,000
Pressure (psi)
Surface pressure
Lower BHP
5,000
Time (minutes)
Lower BHT
30
220
25
200
20
180
15
160
Injection rate
10
5
140
120
Upper BHT
0
0
25
50
75
100
Time (minutes)
125
150
Temperature, °F
Injection Rate (bpm)
5
2,500
7,000
6,000
100
175
Fig. 6. Gulf Coast well record indicates shunt activation and
opening of a new fracture face.
capability to use shunts with regular wire wrapped screens
in these wells reduces or eliminates the plugging problems
which have plagued operators using stand-alone screens
or earlier methods for gravel packing horizontal wells.
The shunts also allow simultaneous wellbore stimulation while packing. Frac-packing of long horizontal wells
in a single step is entirely feasible and allows consideration of shorter horizontal segments. Extra planning is
required for availability of materials, etc., since sand
placement volumes can be large. The first well completed
using this approach is an unqualified and spectacular success. The ability to accomplish this kind of completion
without staging turns a difficult, expensive, and potentially
dangerous 2-week ordeal into a 1-day job.
Acidizing to remove filter cake can either be done after
screen placement but before gravel packing by using an
acid base gel carrier fluid, or after the job as is now customary.
A number of horizontal wells have been gravel packed
at this time using shunts. The longer jobs, from 500 to
1,100 ft, have been in Equatorial Guinea by Mobil.1
Shunts with regular wire- wrapped screens and shrouds
were used because re-entry into the wells is not feasible
and, from prior experience with shunt packing, Mobil
believed that screen plugging and failures would not
occur. This has proven to be the case. All seven of their
wells have been highly successful deepwater subsea completions, helping Mobil and its partners reach high production levels in record time. Although one well has
watered out, the rest have produced at rates up to 15,000
b/d through 4-in. tubing with productivity indexes as high
as 100. The first of these has produced over 5 MMbbl in 19
months. Fig. 7 shows a typical pumping data record from
one of the horizontal completions. Packing proceeded normally at first. Then a bridge formed, packing through the
shunt began, and the pump rate was reduced to maintain
a reasonable pressure. Packing continued until more than
100% of the sand required to fill the calculated annular
2,000
1,500
1,000
6
Initial screen-out
down shunts
flow diverted
Slurry at
open hole
4
3
2
Slurry at
cross over
1
500
Surface pressure
Total flowrate
0
15:25 15:35 15:45 15:55
Injection rate (bpm)
Pressure (psi)
11,000
0
-1
16:05 16:15 16:25 16:35 16:45
Time (hh:mm)
Fig. 7. Pumping data from gravel packing horizontal completions
shows the pattern of bridge formation, shunt packing and
screen out.
space had been placed. About 40% of the total sand was
placed through the shunts in this example.
The formation sands in these wells are of variable quality, with occasional shale breaks. Completion failures in
this type of environment have been common with prior
packing procedures. Sand placement for these seven wells
ranged from 108% to 140% of calculated annular space.
Much of the sand was placed through the shunts, as indicated by the pumping records. Recent improvements in
procedure have eliminated the need for acidizing following sand placement. There is no reason to believe that any
of the wells will fail or experience plugging prior to production of all the associated reserves, or that successes
using this technology will be unique to Equatorial Guinea.
Projected Applications
Based on the early string of successes in horizontal wells, it
appears feasible to drill horizontal segments from existing
wellbores with coiled tubing, and then gravel pack or frac
pack them with shunt screens, also placed with coiled tubing. This potential application should allow using current
wellbores to harvest modest reserves left behind at the normal end of reservoirs’ lives. The coiled tubing unit for this
application has been successfully tested and a shrouded
shunt screen has been designed to fit 4-3⁄4-in. holes.
Another set of options can be considered with the isolation packer system. Since it is easy to pack or frac-pack
separate intervals through separate shunt tubes, various
new completion scenarios can be considered. These
include using different sized tubes for treating and packing different intervals, variable rate pumping schemes to
initiate different fractures, or sleeves, plugs, and valves to
produce from different segments of horizontal wells.
Elimination of staging in placing of multiple fractures
in horizontal wells in hard rock applications also appears
feasible. This could be accomplished with packers, as suggested above, or less expensively by altering the pumping
procedures. In this application, continuous gravel packs are
not required, so the string can be altered to reduce costs.
WELL PRODUCTIVITY
Conclusions
Until now, the petroleum industry has had to live with a
Catch 22 whenever gravel packing has been required. The
choice was to use gel carrier fluids, which can pack more
perforations and develop substantial fractures, but often
leave poor packs in the annulus; or to use water, which
can provide excellent annulus packs in more or less vertical holes, but limits fracturing to a few feet of vertical
interval and has limited capacity to pack perforations. In
horizontal holes, gel packing has previously led to premature bridging and incomplete packs, while water packing
requires almost complete sealing off of the wellbore while
packing, and later removal of the seal, if possible, so that
production can take place.
With alternate path packing, the best of both worlds is
obtained, providing annulus packing better than with
water, while reaping the benefits of superior perforations
and fracture packing with viscous gels. In addition, the
use of visco-elastic surfactant carrier fluids, which break
with exposure to hydrocarbons,8 eliminates residual damage inherent to polymeric fluids.
In horizontal holes, completely sealing off the wellbore
to assure circulation is no longer required since leak-off
control is provided at the shunt ports, and almost unlimited lengths can be packed with proper designs.
Stimulation can be accomplished before, after, or simultaneously with packing.
The ability to gravel- and frac-pack multiple intervals
and almost unlimited lengths promises to make extremely
high rate wells the rule rather than the exception. By
drilling through producing formations at an angle to expose
more wellbore, and then packing or frac-packing with
shunts, wells with negative skin effects and much higher
production rates should be common in the future. ●
REFERENCES
1. Jones, L.G., Tibbles, R.J., Myers, L.G., Bryant, D.W., Hardin, J., and Hurst, G.: “Gravel
Packing Horizontal Wellbores with Leak-Off Using Shunts,” SPE 38640 presented at the SPE
Annual Conference, San Antonio, Texas (Oct. 5-8, 1997).
2. Jones, L.G., Tibbles, R.J., Myers, L.G., Crowder, Scott, and Kaberlein, Michael J.: “Fracturing
and Gravel Packing with Alternate Paths,” Presented at SPE 68th Annual Western Regional
Meeting, Bakersfield, Calif. (May 1-15, 1998).
3. Shepard, Don, and Toffanin, Ezio: “Frac-Packing Using Conventional and Alternative Path
Technology,” SPE 39478 presented at the Formation Damage Control Symposium,
Lafayette, La. (Feb. 18-19, 1997).
4. Hurst, Gary D., Waters, Frank W., and Baker, Chris G.: “Shunt Tube Gravel Packing—A Cost
Effective Solution for Gravel Packing and Commingling Longer Producing Intervals in
Trinidad,” Presented at 12th Biennial Business and Technology Conference and Exhibition,
Port of Spain, Trinidad (Feb. 10-13, 1998).
5. Bryant, D.W., and Jones, L.G.: “Completion and Production Results from Alternate Path Gravel
Packed Wells,” SPE 27359, SPE Formation Damage Symposium, Lafayette, La. (1994).
6. Landrum,W.R., Burton, R.C., MacKinlay, W.M., Erlandsen, A., and Vigen, A.: “Results From
the Heidrun Field Cased-Hole Gravel Packs,” JPT (September 1996), pp 848-852.
7. Bryant, D.W.: “Tool for Blocking Axial Flow in Gravel-Packed Well Annulus,” U.S. Patent No.
5,588,487 (Dec. 31, 1996).
8. Samuel, Mathew, et al: “Polymer-Free Fluid for Hydraulic Fracturing,” SPE 38622 presented
at the SPE Annual Meeting, San Antonio, Texas (October 1997).
ABOUT THE AUTHOR
■ Lloyd Jones is the inventor of the ALLPAC and
ALLFRAC systems. He holds 43 U.S. patents, mostly in
the area of well completions. Jones worked 33 years for
Mobil and now operates from Dallas as a consultant.
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