Ball sealers and method of preparation

A ball sealer consists of an inner spherical rigid core of a desired density and an outer resilient or compliant continuous layer of uniform thickness which is comprised of a pair of hemispheric caps positioned about and secured to the core and to each other. The ball sealer is formed by molding a first cap in the form of a hemisphere having a central cavity adapted to receive one-half of an inner core. The cap is placed in a cavity in the bottom plate of a mold and the outer surface of an inner core or the inside of the cavity of the cap is coated with an adhesive that will bind the cap to the core. The core is then placed within the cavity of the cap. A cavity plate having a hemispheric cavity is then positioned on top of the bottom plate of the mold with exposed upper half of the inner core received within the cavity of the cavity plate. The second cap is then molded in situ about the inner core and bonded to the first cap and the inner core to form a continuous outer layer of resilient and compliant material encapsulating the inner core.

The present invention relates to ball sealers. More particularly, it 
relates to ball sealers which can be used as diverting agents in the 
treatment of wells having a perforated casing and to a method of preparing 
such ball sealers. 
BACKGROUND OF THE INVENTION 
Ball sealers are small spheres having a resilient or compliant outer layer 
which are used in the oil and gas industry in connection with efforts to 
increase the production rate of wells through acid treatment or hydraulic 
fracturing. 
Generally when ball sealers are used, an initial charge of treating fluid 
is first injected into the well. Then a number of ball sealers, which is 
less than the calculated total number of perforations in the casing of the 
well, is placed in the well. Next, additional treating fluid is injected 
under pressure and the ball sealers are carried by the flow of the fluid 
to those perforations in the well casing which are in the area of least 
resistance to flow. The ball sealers seat upon those perforations and 
divert the flow of the treating fluid to the remaining open perforations. 
The ball sealers are retained seated upon the perforations by the pressure 
differential of the treating fluid across the perforations. when the 
injection of treating fluid under pressure stops, the pressure 
differential across the perforations drops and the ball sealers become 
unseated. 
The use of the ball sealers which divert the treating fluid from the area 
of least resistance to flow makes it possible for the treating fluid to 
reach areas of higher resistance which are normally untreated. The usual 
result is an increased production rate of hydrocarbons from the well. 
In the Erbstoesser U.S. Pat. No. 4,102,401, a ball sealer is disclosed 
which has a syntactic foam inner core and an outer resilient layer of 
rubber or a similar elastomeric material. The Erbstoesser ball sealers are 
prepared by compressing an uncured rubber cover about a spherical inner 
core of syntactic foam in an arbor press and curing the rubber. Ball 
sealers prepared in this manner do not always have an outer layer of 
uniform and predictable thickness which is considered to be essential to 
insure good performance. 
The ball sealer of the Erbstoesser patent has a density which is less than 
that of the treating fluid with which it is used. However, in the past, 
ball sealers have also been used which have a density which is higher than 
that of the treating fluids. The use of the low density sealers appears to 
be the more promising. 
In order to be effective, ball sealers of either high or low density must 
have outer coatings or coverings which are sufficiently resilient or 
compliant to seat upon and to seal the jet formed perforations in the well 
casing. They must also have rigid inner cores which resist extrusion into 
or through the perforation. Otherwise, the ball sealers could penetrate 
into the pay zone and permanently damage the flow characteristics of the 
well. In addition, the ball sealers must be chemically inert in the 
environment in which they are used and resist the stresses caused by the 
hydrostatic pressure of the fluid in the well bore and the pumping 
pressures. 
SUMMARY OF THE INVENTION 
It is the primary object of the present invention to disclose a novel ball 
sealer having a spherical inner core and an outer resilient or compliant 
layer of uniform thickness. 
It is a further object of the invention to disclose a method of preparing 
such ball sealers. 
The ball sealer of the present invention consists of an inner spherical 
rigid core of the desired density and an outer resilient or compliant 
continuous layer of uniform thickness which is comprised of a pair of 
hemispheric caps positioned about and secured to the core. 
In the preferred method of the present invention, a first cap is molded in 
the form of a hemisphere having a central cavity adapted to receive 
one-half of an inner core. The cap is placed in a cavity in the bottom 
plate of a mold and the outer surface of an inner core and/or the inside 
of the cavity of the cap is coated with an adhesive that will bind the cap 
to the core. The core is then placed within the cavity of the cap. A plate 
having a hemispheric cavity is then positioned on top of the bottom plate 
of the mold with exposed upper half of the inner core received within the 
cavity of the plate. The second cap is then molded in situ about the inner 
core and bonded to the first cap and inner core to form a continuous outer 
layer of resilient and compliant material encapsulating the inner core. 
The resulting outer layer is of uniform thickness with no gaps or voids.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As seen in FIGS. 1 and 2 of the drawings, the ball sealer 10 has a 
spherical inner core 11 surrounded by a layer 12 of a uniform thickness of 
resilient or compliant elastomeric material. The preferred inner core is a 
syntactic form of the type described in U.S. Pat. No. 4,102,401. However, 
polymethylpentene, polyurethane foam and other plastics can be used if 
they have desired strength, density and other properties. The preferred 
elastomeric material of the outer layer is either a nitrile rubber having 
a durometer of about 40 to about 95 on a Shore A scale or a polyurethane 
having a similar durometer. Other elastomers, including polychloroprene, 
polyacrylates, fluoro elastomers and epichlorohydrin rubber can also be 
employed. 
Referring specifically to FIG. 2, it can be seen that the outer layer 12 is 
comprised of two caps, 13 and 14 having a central cavity, 13a and 14a, 
respectively. As seen in FIG. 2, the walls 13b and 14b of the caps are of 
equal and uniform thickness throughout. 
Still referring to FIG. 2, it can be seen that there is a first bond 15 
between the caps 13, 14 and the inner core 11 and a second bond 16 joining 
the edges or lips 13c and 14c of the caps to each other. The first bond 15 
is preferably formed by an adhesive which will form a strong bond between 
the material of the outer surface of the inner core 11 and the material of 
the inner surfaces of the cavities 13a, 14a of the caps 13, 14. The 
adhesive employed for this purpose when the core is of a syntactic form 
and the caps are of nitrile rubber is a general purpose elastomer bonding 
agent which has the ability to bond vulcanized elastomers. A suitable 
adhesive is Chemlok 234B available from the Hughson Chemical Division, 
Lord Corporation, Erie, Pa. Other adhesives which have the desired 
properties may also be used. 
The second bond 16 which is between the edges or lips 13c and 14c of the 
caps may be of the same material as the caps or may be a conventional 
adhesive. When the caps 13 and 14 are of nitrile rubber the bond may also 
be of nitrile rubber and it can be formed by treating the lip 13c with 
diluted uncured nitrile rubber before forming the cap 14 in situ. When the 
cap 13 is of polyurethane the bond 16 is preferably formed by use of an 
adhesive that will bond castable polyurethane to other materials and 
polyurethanes. A commercially available product which is suitable for this 
purpose is a mixture of Thixon 403 and Thixon 404 which is available from 
the Dayton Coatings and Chemical Division, Whittaker Corporation, West 
Alexandria, Ohio. Other types of adhesives can also be employed which will 
form a strong, continuous bond between the lips of vulcanized lower cap 
and the material of the upper cap which is formed in situ. 
In some instances it may be possible by choice of the core material and the 
cap material to eliminate the use of the adhesive to form the bond 15 
between the inner core 11 and the cap 14 or the bond 16 between the lips 
13c and 14c. However, in most instances the bonds 15 and 16 formed by the 
molding of the cap 14 in situ will not be strong enough and an adhesive 
will be required. 
The preferred method of preparing the ball sealers will now be described in 
connection with FIGS. 3 and 4. 
In FIG. 3 can be seen a single cavity of a multiple cavity transfer mold 17 
of the type which can be used to mold the caps 13. Mold 17 has a bottom 
plate 18, a transfer pot and cavity plate 19, having a transfer sprue 20 
and a plunger plate 21. To mold the caps 13, the bottom plate 18 and the 
transfer pot and cavity plate 19 are assembled as shown in FIG. 3 and the 
thus formed mold cavity charged by introducing uncured elastomer formula 
through the sprue 20. The mold is then closed by putting the plunger plate 
21 in place. It is then pressurized and heated until the elastomer is 
cured. The exact conditions used to cure the elastomer will, of course, 
depend upon the specific formula being employed. When the elastomer is an 
uncured nitrile rubber including sulfur as a curing agent, the pressure 
preferably will be about 2500 psi and the temperature about 300.degree. F. 
for about 20 minutes. If castable polyurethane is the elastomer, the resin 
is preferably heated to about 190.degree. and the hardening agent is 
heated to about 230.degree. F. and the two are mixed before introduction 
into mold. The mold temperature is maintained at about 212.degree. F. to 
effect the cure. 
In FIG. 4 is shown a single cavity of a multiple cavity mold 22 which can 
be used to mold the cap 14 in situ. The mold 22 has a bottom plate 23 
having a cap-receiving cavity 24. A cap 13 and an inner core 11 are 
positioned in the cavity 24 with the core 11 in the cavity 13a of the cap 
13. Positioned upon the bottom plate 23 is a transfer pot and cavity plate 
25. Plate 25 is aligned with the bottom plate 23 so that the upper half of 
the core 11 is centrally positioned in a cap-forming cavity 26 of the 
plate 25. Communicating with the cavity 26 is a transfer spure 27. When 
the plates 23 and 25 are assembled as seen in FIG. 4, with the cap 13 in 
the cap-receiving cavity 24 and the inner core 11 properly positioned in 
cavities 13a and 26, the mold is charged by introducing the uncured 
elastomer through the sprue 27. The mold is then closed by lowering a 
plunger plate 28 into position and the elastomer is cured, usually with 
heat and pressure, to form the cap 14. The bond 15 between the cap 13 and 
the inner core 11, if not previously formed, and the bond 16 between the 
lips on edges 13c and 14c are also formed while the ball sealer components 
are in the mold 22. When the curing process is complete, the mold 22 is 
opened and the ball sealer 11 removed and trimmed, if necessary. 
EXAMPLE 1 
A. Molding of Caps 
A 49 cavity mold of the type shown in FIG. 3 is charged with seven ounces 
of the following uncured nitrile rubber formula which upon curing has a 
durometer of about 50: 
______________________________________ 
Formula 50NO3 
(Molded Dimensions, Inc., Port Washington WI) 
Ingredient Parts 
______________________________________ 
Nitrile Resin 100.00 
Diphenyl-p-phenylene Diamine 
3.00 
Zinc Oxide 5.00 
Stearic Acid .50 
Furnace Coal Black 65.00 
Dioctyl Phthalate 10.00 
Sulfur 1.50 
Flow Agent 1.50 
Benzothiazyl Disulfide 
1.50 
Tetraethyl Thiuram Disulfide 
.20 
Total 188.20 
______________________________________ 
The mold was closed, pressurized at 2,500 pounds per square inch and cured 
at 300.degree. F. for twenty minutes. The caps were removed from the mold 
and tumbled lightly to remove flash. 
B. Priming the Cores and Caps 
Inner cores of syntactic foam were first heated at 350.degree. F. for 
forty-eight hours, then vapor degreased for three minutes with inhibited 
trichloroethylene. The caps, which had been prepared as described in 
paragraph A, were washed in methylethylketone, the lips lightly buffed and 
then washed again with methylethylketone. The inner cores were primed 
using a general purpose elastomer bonding agent (Chemlok 234B diluted in 
xylene) and the lips of the caps only were primed using as the primer the 
uncured nitrile rubber formula diluted in toluene. 
C. Preparation of the Ball Sealers 
The caps, prepared and primed as described in paragraph B, were placed in 
the cap-receiving cavities of the bottom plate of a mold of the type shown 
in FIG. 4, making sure that the caps were all level to the top of the 
plate surface. The inner cores, which had beenn primed as described in B, 
were placed into the caps and the bottom plate of the mold covered with 
the transfer pot and cavity plate. The mold cavities were then filled 
through the transfer sprue with additional uncured nitrile rubber. The 
plunger plate was positioned in place and the mold closed and pressurized 
at 2500 pounds per square inch at 300.degree. F. for twenty minutes. The 
mold was then opened and the ball sealers removed, hand trimmed, quality 
checked and found to be free of cracks and voids. The outer layer was 
found to be of uniform thickness and securely bound to the inner core. 
EXAMPLE 2 
The process of Example 1 was repeated using the following uncured nitrile 
rubber formula which upon curing has a durometer of 70, in place of the 
nitrile rubber formula of Example 1: 
______________________________________ 
Formula 70NO25 
(Molded Dimensions, Inc., Port Washington WI) 
Ingredient Parts 
______________________________________ 
Nitrile Resin 100.00 
Diphenyl-p-phenylene Diamine 
3.00 
Zinc Oxide 5.00 
Stearic Acid .50 
Furnace Coal Black 50.00 
Dioctyl Phthalate 22.00 
Sulfur 1.50 
Flow Agent 1.50 
Benzothiazyl Disulfide 
1.50 
Tetraethyl Thiuram Disulfide 
.20 
Total 185.20 
______________________________________ 
The ball sealers obtained were found to be free of cracks and voids and to 
have a layer of uniform thickness of nitrile rubber securely bound to the 
inner core. 
EXAMPLE 3 
A. Molding of Caps 
A 49 cavity mold of the type shown in FIG. 3 was treated with a mold 
release agent and charged with a mixture containing 100 parts of 
polyurethane resin (Adiprene L-83 available from du Pont Chemical, 
Wilmington, Del.), 10.3 parts of hardening agent (4,4' 
methylene-bis-(2-chloroaniline)), and 3 parts of a blue pigment. The 
mixture was heated to 190.degree. F. prior to introduction into the mold, 
the mold was closed and the mixture cured at 212.degree. F. The caps were 
then removed, post cured and washed with methylethylketone. 
B. Priming of the Caps and Cores 
The lips of the caps were buffed, washed again with methylethylketone and 
coated with two coats of a two component adhesive system suitable for 
bonding castable polyurethane compounds (Thixon 403 and Thixon 404). The 
caps were then permitted to dry. The inner cores of syntactic foam were 
heated at 350.degree. F. for forty-eight hours and vapor degreased with 
inhibited trichloroethylene. They were primed with a 50% solution of the 
previously described adhesive in toluene. 
C. Preparation of the Ball Sealers 
The caps were put into the cap-receiving cavities of the bottom plate of a 
mold of the type shown in FIG. 4 with the caps level to the mold. Inner 
cores were placed in each of the caps and the caps and inner cores were 
covered with the transfer pot and cavity plate. The mold cavities were 
then filled via the transfer sprue with the same polyurethane formula used 
to prepare the caps. The plunger plate was placed in position and the 
temperature of the mold raised to 212.degree. F. for twenty minutes. 
Before unloading the mold, the finished parts were rotated in the cavities 
to remove flash and the balls were preheated to 220.degree. F. to 
240.degree. F. Upon cooling the ball sealers were quality checked, found 
to be free of cracks and voids and to have a uniform thickness of 
polyurethane securely bound to the inner core. 
While for purposes of illustration a specific embodiment of the ball sealer 
and a method of its preparation have been described, it will be 
appreciated by those skilled in the art that a number of modifications and 
changes may be made without departing from the spirit of the scope of the 
present invention. For example, in place of the transfer molding process 
described, other molding processes such as injection molding can be used. 
In addition, other elastomers than those mentioned, including 
thermoplastic elastomers, and other adhesives may be employed if desired. 
Therefore, it is to be understood that the scope of the invention is to be 
limited only by the claims which follow.