Note: Descriptions are shown in the official language in which they were submitted.
W091/11587 PCT/US~1/00~2~
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METHOD AND APPARATUS FOR SEALING PIPE PERFORATIONS
Backqround of the Invention
5This invention relates to ball sealers for
plugging perforations in a pipe and more particularly to
ball sealers which will selectively bridge across
per~orations that are receiving a disproportionately large
amount of .well treatment fluid being injected into a
wellbore.
During the drilling and in the operation of oil,
water, or gas wells, it is often necessary to treat the
borehole or earth formations penetrated by the borehole
with a variety of treatment processes including fracturing,
acidizing, or the like where fluids and materials are
pumped into the wellhore from the surface and thence
through casing perforation openings downhole into earth
formations.
. In such treating operations, it often occurs that
a disproportionately large amount of the treating fluid or
pumpable material passes through one or more of the several
perforations in the casing.
The flow of a disproportionately large amount of
treatin~ material through one or a few perforations in the
casing may be attributable to the higher permeability of
the formation adjacent to those perforations. If the
treating fluid may be easily pumped through one or a few
perforations, it is often impossible to pump enough fluid
into the well to build up sufficient hydrostatic pressure
in the wellbare to force fluid or treating material through
the perforations communicating with less pPrmeable
formations or generally impermeable sections af the earth
~ormations.
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One solution to the above-recited problem
involves temporarily plugging at least some of the
perforations communicating with the permeable sections of
earth formations durin~ the inject:ion of treatment
materials so that the hydrostatic pressure in the wellbore
is permitted to develop to the extent that treatment fluids
and materials are forced into the less permeable sections
of the earth formation through other perforations which
remain open. Ball sealers have been developed in the
industry for accomplishing this selective pluq~ing process
to solve this fluid loss problem.
These ball sealing elements are usually made of
rubber or of a hard-core material surrounded by a resilient
outer covering. The balls are inserted into the well as
fluid is-pumped through the per~orations. The balls ar2
carried along by the flowing stream of fluid and seat
against the casing perforations through which the
preponderance of fluid passes, i.e., those perforations
communicating with permeable sections of earth formation.
Once seated against a perforation, the ball sealer element
plugs the per~oration and is held in place by the pressure
against it of the fluid in the casing to thereby prevent
passage of the fluid in the casing through the plugged
perforations. Such ball sealers are shown in U.S. Pa ent
No. 2,754,910, issued July 17, 1956t to Derrick; U.S.
Patent No. 3,011,548, issued December 5, 1961, to Holt;
U.S. Patent No. 2,933,136, issued April 19, 1960, to Ayers
et al., and U.S. Patent No. 4,702j316, issued October 27,
1987, to Chung et al. Patent No. 4,702,316 shows a ball
sealer composed of a polymer compound covered with an
elastomer.
One disadvantage to the ball sealers in the
patents listed above is that the pIugging ball or element
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becomes lodged in the perforation so that when hydrostatic
pressure in the wellbore is reduced, the ball sealer
remains positioned in the perforation. Thus, the formation
adjacent such permanently sealed perforations is no longer
in communication with the wellbore, which would not only
prevent txeating materials from reaching those portions of
the 'ormation, but would also result in a decrease in
production from that portion of the well served by those
perforations.
Another problem encountered with ball sealers is
that perforations are not always round, and a spherical
ball may not be effective to bridge across the psrforation
opening which may have been formed as an irregular opening
or later becomes split or cracked as a result of stress and
chemical action in the wellbore. In any e~ent, ball
sealers of a conventional, spherically fixed configuration
do not effectively seal such irregular openings. U.S.
Patent No. 3,376,934 issued April 9, 1968, to Bertram
proposes a solution to such a problem by providing a
partially spheroidal body and a flexible skirt of fluid
impervious material attached to and extending outwardly
about the body to overspread the wall surface adjacent the
perforation. This apparatus also is subject to becoming
deformed to the extent that it may become permanently
lodged in the perforation to thereby permanently close off
such opening. One solution to this permanent sealing
problem is suggested in U.S. Patent No. 4,716,964, issued
January 5, l988, to Ertsloesser et al., wherain the ball
sealer is made of degradable material. This system
requires that the chemical environment of the wellbore be
maintained compatible with the materials of which the balls
are made. Degradation of the material is also dependent on
the wellbore fluid chemistry.
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It is, therefore, an object of the present
invention to provide a sealer device which can be pumped
under fluid pr~ssure into plug~ing contact with wellbore
perforations that are re~eiving a disproportionat~ flow of
fluids and which sealer devices will then release
them~elves from such plugging contact upon decrease in
fluid pressure in the wellbore.
summary of The Invention
With this and other obj ects in view, the present
invention relates to a ball sealer for sealing off
per~orations in a wellbore wherein the sealers are
comprised of an impermeable outer deformable shell defining
a central core portion, which core portion is filled with
nondeformable particulate matter that is sized to ~low i~to
the shape assumed by the deformable outer shell. While the
particulate matter is small enough to flow with and thereby
accommodate a change in the shape of the outer shell, it is
large enough so that as it consolidates under the force of
fluid flow pressure pushing the sealer against a
per~oration, it will cause the impermeable outer shell to
bridge over the perforation opening whe~ the fluid flowing
into the casing perforation forces the ball sealer against
and somewhat into the opening. Since the particulate
matter is nondeformable, it will not hold or store
compressed energy when pushed into the opening. Therefore,
when ~luid pressure is reduced and the force against the
sealer is thereby reduced, the particulate matter will
become unconsolidated to relax the bridge and permit the
entrapped energy in the deformed outer shell to expel the
ball from the perforation Qpening.
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Brief Descripkion Of The Drawin~s
Figure 1 is a diagrammatic sectional view of a
perforated oil well with ball sealers being pumped into the
well;
Figure 2 is a partially cut away view of a ball
sealer;
Figure 3 shows a cross-sectional view o~ a ball
sealer engaging a perforation in a well casing; and
Figure 4 shows a perspGctive view of a prior art
ball sealer positioned in an irregular perforation in a
casing wall.
Detailed Description Of The Preferred Embodiments
Referring first to Figure 1 of the drawings, a
casing 12 is run to the bottom of the well and cemented as
at 14 around the outside at least to a distance above the
producing formations 16, as shown. The casin~ 12 and the
cement 1~ are then perforated by any one of various means
to provid a fluid communication channel between the
producing formations and the interior of the casing. If
the well does not come into pr~duction, it is th~n a common
practice to treat the well by some process which will open
up the producing formation to allow a ready passage of
formation fluids into the wellhore. Such xemedial
treatment operations may also be employed in an older
pxoducing well when the production therefrom has diminished
to an uneconomical level. In any event, such treatment
processes typically include acidizing, hydraulic
fracturing, or the like which involve pumping a treating
material down the casing and into the producing formakion
through the perforations 18 which extend through the casing
and into the earth formations. Exceedingly high pressures
are sometime~; used in such treatment operations with
pressures of lO,000 psi not being unusual. It is well
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WO91/11587 ) ~ ~, 3 ~!; PCT/US91/0022~
recognized that under these conditions, treating materials
will preferentially flow through certain of the perforation
more readily than through others. It is apparent then that
only that part of the formation which is receiving this
preferential flow is being s~1bjected to the intended
treatment. It, therefore, becomes desirable to selectively
close off those perforations through which the highly
disproportionate share of materials are flowing so that the
treatment materials will be forced to act on the formation
adjacent to the other perforations.
In order to accomplish this, balls ~2 are
introduced into the treating materials which are being
pumped into the casing 12. The wellbore shown in Figure l
utilizes a tubing string 2~ which is suspended in the
wellbore from the sur~ace and having an open lower end
thereof terminating near the producing formations lS. A
packer 26 is provided about the outside of the tubing 24
and is arranged to seal the annular space between the
tubing 2~ and the casing 12, above the perforations 18 in
the casing. The treating material is pumped down and out
the end of the tubing 24 and through the perforations 18 in
the casing and cement into the adjacent formation. The
balls 22 are introduced through a lubricator 28 at the
surface and are moved down the tubing 24 with the treating
materials which are entering thD tubing through the
pipe 32. The balls are forced seleotively to engage the
perforations such as at 34 through which the major portion
of treating materials are flowing, leaving open those
perforations through which the treating materials are not
being injectecl. These balls seal off the perforations just
so Iong as the pressure within the tubing and casing is
greater than the pressure in the formation. When the
pressure is reduced at the surface, the ball sealers will
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be released ~rom engagement with the perforations.
Thereafter, flow will be established through all of the
perforations.
During the treating process, the plugs are
carried by the fluid skream to the particular perforation
through which the treating ~aterial is entering the
formation and the sealing action can be determined r2adily
by the increase in pressure at the well head. ~he plugs
can be admitted or introduced a~s desired and move readily
with the material traveling at a rate such that it can be
easily determined when they will arrive at the sealing
position and the plugs can be admitted one or two or as
many at a time as needed according to the pressure rise and
fall within the casing. During the pumping of treating
material, the pressure will constantly rise until such time
as the material is injected into the formation. At that
time, the pressure will drop, indicating that the formation
has broken down, and at this time, plugs will be introduced
into the fluid stream to plug the perforations opposite the
existing permeability. When this occurs, the pressure will
again rise, indicating that the pressure is being exerted
against another part of th~ formation where little or no
permeability exists. When this part of the formation
breaks, the pressure may again drop, at which time more
plugs may be admitted to plug those perforations through
which fluid is now moving This procedure can be followed
until as many formation breaks are obtained as desired, or
until all of the perforations are plugged. This allows
control of the fluid entry into tha formation of a treating
material by the sealing off of perforations adjacent to the
more permeable part of the producing formation.
one of the problems encountered when using such
ball sealers in a treating operation is that the
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perforations are not always of a uniform circular
configuration. Therefore, a spherical ball, typically
ha~ing a hard solid nylon core covered with a deformable
material suoh as rubber or the like, may be forced against
the perforation to oover the greatest portion of the
perforation which it can cover, ~ut because the shape is
irregular, elongated, cracked, etc., openings will extend
beyond the uni~orm circular face of the ball. This
prevents a complete seal of the perforation and may prevent
the pressure buildup which is necessary to treat the
formation. Thus, it is desirable to have a sealer which
will be e~fective to cover substantially the entire open
area constituting the perforation. Such a prior art sealer
is shown in Figure 4 wherein a typical ball sealer lO is
shown projecting into an irregular perforation ll in a
casing ~2. It is readily seen that a substantial amount of
fluid flow leakage might be possible around a sealing
configuration as that shown in Figure 4, such as through
the space 13 formed between the ball lO and irregular
opening ll.
Another problem which exists in prior art ball
sealers is that when the ball sealer is made of a so~ter
yet solid rubber material, the ball will tend to enter the
perforation and become lodged therein to permanently seal
off the perforation. This is because the softer solid ball
will deform as it enters the perforation under the
- differPntial pressure forces occurring in the treating
operation and will be squeezed under compression into the
perforation to create a seal. When the pressure is
relieved, the compressional energy will remain trapped in
the ball, holding the ball within and against the internal
wall surface of the perforation opening, and will not
permit release of the ball.
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Referring now to Figure 2 of the drawings, the
ball sealer in ac~ordance with the present invention is
shown having a generally spherical oonfiguration in its
natural state under ambient conditions. An outer shell 42
is constructed of a durable yet: flexible and impermeable
material such as rubber to form a deformable bladder around
a core portion 4~ which is filled with particulate
m~tter 45, as shown in Figure 2. This particulate matter
46 may be comprised of beads of material such as nylon or
other substantially nondeformable material. A graded
material works well in that the individual particles tend
to move readily relative to one another as not to assume a
fixed relationship. Spherical beads would provide the
ultimate mobility to the particulate core material with the
size of the particles or beads being determinative of the
degree of mobility. Basically, the smaller the bead, the
more fluid like the core will be. On the other hand, very
fine core particles will tend not to ~orm a ~ridge across
the opening o~ the perforation but rather will tend to flow
through the opening. Therefore, a compromise between the
desired functional qualities of ~luidity and ability to
bridge will determine the size of core particle. The span
of the perforation opening will provide the primary
parameter in determining such particle size. A rule of
thumb which is used when designing treatment processes, for
example, a gravel pack, is to size the particulate matter
to be greater than one-sixth the diameter of the
- perforation to be closed by the bridging effect of gravel.
In a fracturing process, the particles are sized
to be less than one-sixth the size of the perforation to
ensure that the particles will flow throu~h- the
perforation. Standard new perforations are nominally about
10 mm in diameter. When corrosion and wear are taken into
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account, 12 mm would be a good estimate for the size of old
perforations. In the present ball sealer application where
the particulate matter is confined within the shell
enclosure, the particulate material will tend to
consolidate into a bridge more easily than in 1005e
condition and thus could be somewhat smaller in size than
the rule of thumb, one-sixth perforation diameter used for
gravel packs or the like. A si~e range of 1.5 to 3 mm or
6 to 12 mesh would be an appropriate size for the
particulate matter ~6 tFigure 2) within the shell 42. The
outside diameter o~ the shell would be sized to be
approximately 22 mm or more when the per~orations are about
12 mm.
In the preferred embodiment, the core of the ball
sealers further comprises a temporary binder material such
as a wax or similar material to ~ind the beads or particles
together while the cover is formed about the core. The
temporary binder material preferably has a melting
temperature lower than the operating températures downholP
but high enough to form a workable solid at about room
temperature. Thereafter, the temporary binder materiai
forms a liquid in the interstices of the beads or particles
within the cover. The melted binder may ~orm a lubricant
causing the beads to easily slide relative to one another.
Moreover, the liquid binder would be more capable of
resisting the downhole compressive forces than air or other
gaseous media and would therefore prevent the cover from
deforming into the interstices of the beads.
The weight of the ball sealers, or more
particularly, the spPcific gravity of the ball sealers is
an important design criterion since the ball sealers are
intended to flow with the well treatment fluid. If the
ball sealers were too heavy or too light, they would be
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less inclined to flow with the fluid and plug the
perforations. Therefore, the ball sealers should have
approximately the same density as the well treatment ~luid
so as to be relative neutrally buoyant therein (i.e. the
ball sealers should not necessarily float to the top or
sink to the bottom). However, under some circumstances it
may be preferable to provide the ball sealers with a small
positive buoyancy (float relative to the fluid) and in
other situations to provide ball sealers with a negative
buoyancy ~sink in the ~luid).
The particles and the temporary binder which form
the core are particularly selected so as to form ball
sealers having a predetermined specific gravity. It is
conventional in the art to provide sealers having a variety
of specific gravities generally in the range of l.0 to l.3
to accommodate the variety of well treatment fluids that
may be used. Based on such figures, the ball sealers of
the preferred embodiment having a diam~ter of approximately
7/8 inch would weigh generally between 0.2 and 0.26 ounces.
Turning now to an operation utilizing the ball
sealers of the present in~ention, if it was determined that
certain formations were taking treating materials in
disproportion to the total flow volume, the sealers would
be introduced into the flow stream through the lubri-
cator 2~ at the surface. These ball sealers 28 would then
move by pumpin~ through the tubing 2~ to the borehole area
below the packer ~6. The balls 22 tend to move with the
flow stream to the perforations taking the highest flow and
thereby become forced against such perforation opening.
Be~ng larger in size than the perforation, the balls will
seat against the opening, and under the in~luence of the
differential pressure between the inside of the casing and -
the outer or formation side of the casing, acting against
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the impermeabls outer shell 42, the sealers will try to
flow into the perforation. Due to the manner in which this
sealer is constructed, the sealer will partially flow into
the perforation 18 as shown in Figure 3.
The particles ~6 making up the core o~ the ball
sealer 22 will migrate or flow with the changing shape of
the outer rubber casing 42 as it spreads over and into the
perforation under the influence of hydraulic forces actiny
on the ball 22. Although in a relaxed or ambient state the
shell 42 assumes a round shape, the thickness and nature of
material making up the shell 2 is such that the shape of
the sealer may readily change under the applied ~orces of
the hydraulic system in which it is operating.
With the thief zones plugged, the other
perforations will then receive entry of the treating
fluids, and upon completion of the treating process, the
hydraulic pressure on the system will be reduced. The
sealer shell 42 will then no longer be pressed against and
into the per~oration so that the shell will be free to
assume its static configuration of alball. The beads not
being compressed into the perforation opening will be free
to flow with and follow the shape of the shell 42. This
process of returning to its static state will permit the
sealer to fall away from the casing wall and eventually may
even fall or sink to the bottom of the hole as at 36 in
Figure 1.
While particular embodiments o~ the present
invention have been shown and described, it is apparent
that changes and modifications may be made without
departing from this invention in its broader aspects, and
therefore, the aim in the appended claims is to cover all
such changes and modifications as fall within the true
j spirit and scope of the invention.
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