U.S. patent application number 14/521662 was filed with the patent office on 2015-07-02 for sacrificial isolation ball for fracturing subsurface geologic formations.
The applicant listed for this patent is Jeffrey Stephen Epstein. Invention is credited to Jeffrey Stephen Epstein.
Application Number | 20150184486 14/521662 |
Document ID | / |
Family ID | 53481147 |
Filed Date | 2015-07-02 |
United States Patent
Application |
20150184486 |
Kind Code |
A1 |
Epstein; Jeffrey Stephen |
July 2, 2015 |
SACRIFICIAL ISOLATION BALL FOR FRACTURING SUBSURFACE GEOLOGIC
FORMATIONS
Abstract
An embodiment of a fracking ball cooperates with a ball seat to
isolate a well first portion of an earthen well drilled into the
earth's crust from a well second portion and comprises an interior
chamber to receive an explosive charge. The explosive charge may be
surrounded by a filler material that is resistant to deformation. A
pressure sensor, a circuit, and a battery are also received into
the chamber. The ball material, may comprise one of zirconium
oxide, aluminum oxide, bulk metallic glass, silicon nitride or
tungsten carbide, and the ball is resistant to deformation within
the ball seat under the application of a substantial pressure
differential across the ball and ball seat. Detonation of the
explosive charge fragments the ball to prevent the ball from
presenting an obstruction to subsequent well operations. A safety
fuse may be included to enable safe handling and transport.
Inventors: |
Epstein; Jeffrey Stephen;
(Houston, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Epstein; Jeffrey Stephen |
Houston |
TX |
US |
|
|
Family ID: |
53481147 |
Appl. No.: |
14/521662 |
Filed: |
October 23, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61898088 |
Oct 31, 2013 |
|
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|
Current U.S.
Class: |
166/193 |
Current CPC
Class: |
E21B 34/14 20130101;
E21B 43/26 20130101; E21B 33/12 20130101 |
International
Class: |
E21B 34/06 20060101
E21B034/06 |
Claims
1. A ball for use with a ball seat set in a tubular string within a
well drilled into the earth's crust to isolate a pressure within a
first portion of the well from a pressure in a second portion of
the well, the ball comprising: a spherical body of a solid material
and having an interior chamber and an exterior surface to engage
the ball seat; a battery received within the interior chamber of
the body; an explosive charge including an explosive material
received within the interior chamber of the body and coupled for
detonation by the battery; a pressure sensor received within the
interior chamber of the body in fluid communication with an
aperture extending from the exterior surface of the spherical body
to the interior chamber of the spherical body; and a circuit
received within the interior chamber and conductively coupled to
receive an electrical current from the battery, conductively
coupled to receive a signal from the pressure sensor, and
conductively coupled to generate, after a predetermined time
interval, a detonating current to detonate the explosive charge in
response to detecting a predetermined pressure sensed using the
pressure sensor; wherein detonation of the explosive charge
fragments the spherical body to limit the size of spherical body
fragments present in the well.
2. The ball of claim 1, wherein the solid material of the spherical
body is a material, that is at least in part dissolvable in one or
more fluids introduced into the well.
3. The ball of claim 1, wherein the spherical body includes a
plurality of spherical body portions that are assembled and secured
together to form the spherical body.
4. The ball of claim 3, wherein the plurality of spherical body
portions includes two hemispherical body portions.
5. The ball of claim 4, wherein the plurality of spherical body
portions are securable together using an epoxy adhesive.
6. The ball of claim 1, wherein the solid material of the spherical
body comprises at least one of: zirconium oxide, silicon nitride,
tungsten carbide, zirconia toughened alumina, bulk metallic glass
and aluminum oxide.
7. The ball of claim 1, wherein the ball further comprises; a
filler material disposed within the hollow interior of the
ball.
8. The ball of claim 7, wherein the filler material includes at
least one of sand, gel, ceramic beads and an incompressible
fluid.
9. The ball of claim 1, further comprising a fuse aperture in the
spherical body for receiving a safety fuse; wherein the safety fuse
enables the detonation of the explosive charge by current provided
from the battery.
10. A ball for landing within a ball seat set in a tubular string
to isolate a first portion of a well drilled into the earth's crust
from a second portion of the well, the ball, comprising: a
spherical body of a solid material having an exterior surface to
engage the ball seat and an interior chamber; a battery received
within the interior chamber; an explosive charge received within
the interior chamber and coupled for detonation by an electrical
current from the battery; a pressure sensor received within the
interior chamber and in fluid communication with an aperture
extending from the exterior surface to the interior chamber; and a
circuit received within the interior chamber to receive a signal
from the pressure sensor upon detection by the pressure sensor of a
predetermined pressure and to generate a detonation signal from the
battery to the explosive charge at a predetermined time interval
after the detection of the predetermined pressure by the pressure
sensor; wherein detonation of the explosive charge fragments the
ball.
11. The ball of claim 10, wherein the solid material of the
spherical body is at least in part dissolvable in one or more
fluids introduced into the well.
12. The ball of claim 10, wherein the spherical body includes a
plurality of assembled spherical body portions secured together to
form the spherical body.
13. The ball of claim 12, wherein the plurality of spherical body
portions includes two hemispherical body portions.
14. The ball of claim 13, wherein the plurality of spherical body
portions are securable together using an epoxy adhesive.
15. The ball of claim 10, wherein the solid material of the
spherical body comprises at least one of: zirconium oxide, silicon
nitride, tungsten carbide, zirconia toughened alumina, bulk
metallic glass and aluminum oxide.
16. The ball of claim 10, wherein the ball further comprises: a
filler material disposed within the hollow interior of the
ball.
17. The ball of claim 16, wherein the filler material includes at
least one of sand, gel, ceramic beads and an incompressible
fluid.
18. The ball of claim 10, further comprising a fuse aperture in the
spherical body for receiving a safety fuse; wherein the safety fuse
enables the detonation of the explosive charge by current provided
from the battery.
Description
STATEMENT OF RELATED APPLICATIONS
[0001] This application depends from and claims priority to U.S.
Provisional Application No. 61/898,088 filed on 31 Oct. 2013.
FIELD OF THE INVENTION
[0002] The present invention relates to an improved sacrificial
isolation ball for use with a ball seat to fluidically isolate a
targeted geologic zone for hydraulic fracturing operations to
enhance production of hydrocarbons from a well drilled into the
targeted geologic zone.
BACKGROUND OF THE RELATED ART
[0003] Hydraulic fracturing is the fracturing of rock by a
pressurized liquid. Some hydraulic fractures form naturally.
Induced hydraulic fracturing or hydro-fracturing, commonly known as
"fracking," is a technique in which a fluid, typically water, is
mixed with a prop-pant and chemicals to form a mixture that is
injected at high pressure into a well to create small fractures in
a hydrocarbon-bearing geologic formation along which the
hydrocarbon fluids such as gas, oil or condensate may migrate to
the well for production to the surface. Hydraulic pressure is
removed from the well, then small grains of the proppant, for
example, sand or aluminum oxide, hold the fractures open once the
formation pressure achieves an equilibrium. The technique is
commonly used in wells for shale gas, tight gas, tight oil, coal
seam gas and hard rock wells. This well stimulation technique is
generally only conducted once in the life of the well and greatly
enhances fluid removal rates and well productivity.
[0004] A hydraulic fracture is formed by pumping fracturing fluid
into the well at a rate sufficient to increase pressure downhole at
the target zone (determined by the location of the well casing
perforations) to exceed that of the fracture gradient (pressure
gradient) of the rock. The fracture gradient is defined as the
pressure increase per unit of the depth due to its density and it
is usually measured in pounds per square inch per foot or bars per
meter. The rock cracks and the fracture fluid continues further
into the rock, extending the crack still further, and so on.
Fractures are localized because pressure drop off with frictional
loss attributed to the distance from the well. Operators typically
try to maintain "fracture width," or slow its decline, following
treatment by introducing into the injected fluid a proppant--a
material such as grains of sand, ceramic beads or other
particulates that prevent the fractures from closing when the
injection is stopped and the pressure of the fluid is removed. The
propped fracture is permeable enough to allow the flow of formation
fluids to the well. Formation fluids include gas, oil, salt water
and fluids introduced to the formation during completion of the
well during fracturing.
[0005] The location of one or more fractures along the length of
the borehole is strictly controlled by various methods that create
or seal off boles in the side of the well. A well may be fracked in
stages by setting a ball seat below the geologic formation to be
fracked to isolate one or more lower geologic zones open to the
well from the anticipated pressure to be later applied to a zone
closer to the surface. A ball of a predetermined diameter is
introduced into the well at the surface and pumped downhole. When
the ball reaches the ball seat installed in the bore of a casing,
the ball seats in the ball seat to form a seal that isolates
geologic formation zones below the ball seat from the anticipated
hydraulic fracturing pressure to be exposed on a geologic formation
zone above the ball seat.
[0006] Hydraulic-fracturing equipment used in oil and natural gas
fields usually consists of a slurry blender, one or more
high-pressure, high-volume fracturing pumps (typically powerful
triplex or quintuples pumps) and a monitoring unit. Associated
equipment includes fracturing tanks, one or more units for storage
and handling of proppant, high-pressure treating iron, a chemical
additive unit (used to accurately monitor chemical addition),
low-pressure flexible hoses, and many gauges and meters for flow
rate, fluid density, and treating pressure. Chemical additives are
typically 0.5% percent of the total fluid volume. Fracturing
equipment operates over a range of pressures and injection rates,
and can reach up to 100 megapascals (15,000 psi) and 265 litres per
second (9.4 cu. ft./sec or 100 barrels per min.).
[0007] A problem that can be encountered in a fracking operation
involves the impairment to subsequent operations that can result
from the presence of the ball. After the fracking operation is
concluded, the surface pressure is restored to a pressure at which
the well will flow and produce formation fluids to the surface for
recovery. A fracking ball having a sufficiently low density can be
floated or back-flowed from the well, but a ball having a low
density may be deformed by the large pressure differential applied
across the ball and ball seat and thereby compromised during
fracturing operations. If the ball is of a material that is more
dense so that it can not be floated or back-flowed from the well to
the surface, then the ball may present an unwanted obstruction that
has to be removed from the well to prevent impairment of subsequent
well operations.
[0008] A workover operation can be implemented in which a drilling
instrument is introduced into the well to drill out and to
mechanically destroy the ball, but a workover operation requires
that a workover rig be brought to the surface location of the well
for downhole operations. The need for the rental, transportation,
rigging up and use of a rig imposes substantial delays and
substantial costs.
[0009] What is needed is a fracking ball that has a sufficient
density and resistance to deformation so that it can be used in
conjunction with a ball seat to reliably isolate geologic formation
zones below the ball seat from anticipated large fracturing
pressures applied to geologic formation zones above the ball seat
and that does not impair subsequent well operations.
BRIEF SUMMARY
[0010] One embodiment of the present invention provides a fracking
ball for sealing with a ball seat in a well. The fracking ball
contains an explosive charge for fragmenting the fracking ball
after use. The fracking ball is constructed in a manner that
provides sufficient resistance to deformation of the ball as a
large pressure differential is applied across the ball and the
engaged ball seat.
[0011] An embodiment of the present invention provides a fracking
ball that can be fragmented by detonation of an explosive charge
provided within an interior chamber of the ball to produce, upon
detonation of the explosive charge, a plurality of ball fragments
that do not interfere with subsequent well operations. In one
embodiment, the use of a ceramic spherical body provides sufficient
resistance to fracking ball deformation under large pressure
differentials across the fracking ball and ball seat applied during
fracking operations. In addition, these materials can provide for
favorable fragmentation of the ball upon detonation of the
explosive charge stored within an interior chamber of the ball to
prevent unwanted obstacles having a substantial size from
obstructing flow in the well.
[0012] In one embodiment of the ball of the present invention, a
battery, a pressure sensor and a circuit are included within the
interior chamber of the fracking ball along with the explosive
charge. The pressure sensor is disposed in fluid communication with
an exterior surface of the ball through an aperture in the ceramic
structure. The pressure sensor detects a predetermined pressure
threshold and initiates a predetermined timer delay period prior to
detonation. Upon elapse of the predetermined timer delay period, a
circuit is completed that generates an electrical current from the
battery to the explosive charge to detonate the explosive charge
and to thereby fragment the ball. In one embodiment in which the
ball is a dissolvable ball, the fragmentation of the ball
dramatically increases the aggregated surface area exposed to the
fluids in the well to provide a much more rapid rate of dissolution
as compared to a dissolvable ball that is not fragmented.
[0013] The higher fracking pressures achievable by use of
embodiments of the fracking ball of the present invention, along
with the lack of obstruction of subsequent well operations due to
fragmentation, increase the success and effectiveness of the
fracking process, lowers or eliminates workover rig rental costs,
and prevents unwanted delays in subsequent well operations after
the fracking process.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0014] FIG. 1 is a sectional view of a well drilled into the
earth's crust and illustrating a series of hydraulic fractures
disposed at a predetermined spacing to enhance production and
recovery of formation fluids from a hydraulically fractured
subsurface geologic formation.
[0015] FIG. 2 is the sectional view of the well of FIG. 1
illustrating the lack of fractures within the targeted geologic
formation prior to the creation of the hydraulic fractures and
illustrating a location of a desired placement of a ball and a ball
seat to receive the ball to thereby isolate zones deeper in the
well than the ball seat (to the right) from zones shallower in the
well than the ball seat (to the left).
[0016] FIG. 3 is a sectional elevation of an embodiment of a ball
of the present invention received in a ball seat set within the
casing of the drilled well illustrated in FIG. 2 to create an
isolating seal.
[0017] FIG. 4 is a sectional view of an embodiment of a ball of the
present invention.
[0018] FIG. 5 is a sectional view of an alternate embodiment of a
ball of the present invention.
[0019] FIG. 6 is an illustration of the fragments resulting from
the detonation of the explosive charge contained within the
interior chamber of the ball of the present invention.
[0020] FIG. 7 is an illustration of a safety feature that may be
used to enhance the safety of personnel that may handle, prepare
and deploy an embodiment of the ball of the present invention.
DETAILED DESCRIPTION
[0021] One embodiment of the present invention provides a ball
having an outer surface of sufficient smoothness to enable the ball
to seat within and to seal with a ball seat, wherein the ball has
substantial resistance to deformation by an applied pressure
differential across the seal created by the ball received within
the ball seat. The embodiment of the ball contains an explosive
device that can be detonated to destroy the ball from within and to
thereby fragment the ball into a large plurality of small
fragments. The embodiment of the ball may include a filler material
received within the hollow interior of the ball, along with the
explosive device, wherein the filler material comprises a
non-compressible fluid such as, for example, a gel, or particles or
pieces of such a small size that they can be released in the well
without concern for the particles or pieces interfering with the
function or operation of any downhole components that might be
contacted. The filler material may comprise one of sand, ceramic
beads or some other filler material that exhibits substantial
resistance to deformation and resistance to compression. The filler
material may also comprise an incompressible fluid, such as water.
It will be understood that the temperature at which the ball will
reside prior to detonation of the explosive charge should be
considered when choosing a filler material as an incompressible
fluid may result in excessive internal pressure at elevated
temperatures.
[0022] The manner in which an embodiment of the fracking ball of
the present invention is made may vary, but will generally include
the steps of providing a ceramic outer shell having a hollow
interior and, optionally, a hole through which a pressure sensor
may be inserted into the fracking ball. An embodiment of a fracking
ball of the present invention may include an explosive device and a
filler material that can be disposed within the hollow interior. In
one embodiment of the ball, a first hemispherical portion and a
second hemispherical portion are secured together to form a
spherical ball.
[0023] In one embodiment, a ceramic sphere may consists of two or
more pieces secured together to form a spherical body. In another
embodiment the ceramic sphere consists of a unitary spherical body
having a hole for insertion of a safety fuse such as, for example,
a pressure sensor, to enable the explosive charge and the
timer-controlled detonator. It will be understood that the pressure
sensor may be provided to generate a signal that enables or
initiates the circuit that ultimately delivers the detonating
current flow from the battery to the explosive charge, and that the
provision of the pressure sensor to complete and thereby enable the
fracking ball circuit would cause the pressure sensor to function
as a safety fuse without which the fracking ball would be unable to
self-destruct.
[0024] In one embodiment, the ceramic ball may comprise one of
zirconium oxide, silicon nitride, tungsten carbide, zirconia
toughened alumina, bulk metallic glass (BMG) and aluminum oxide.
The high compressive strengths of these ceramic materials enable
the fracking ball to seat in the ball seat and to cooperate with
the ball seat to isolate deeper well zones from shallower well
zones to be fracked. This requires the ball and ball seat to
withstand a very high fracking pressure on an uphole side of the
ball and a substantially lower pressure on a downhole side of the
ball. Embodiments of the ceramic fracking ball of the present
invention may be manufactured by, for example, but not by way of
limitation, isostatic pressing, hot isostatic processing (HIP),
injection molding, slip casting or gel casting techniques. In one
embodiment, a ball comprising zirconia with a very thin wall
thickness of only 0.060 inches can be gel cast and subsequently hot
isostatically pressed to increase the flexural strength of the
fracking ball so it can be seated m the ball seat to withstand very
high differential pressures while yielding less debris material
subsequent to fragmentation by the explosive charge. Less debris
material will result in a much lower probability of any debris for
fragments of a size sufficient to interfere with or obstruct
equipment to be used in fracking other, deeper or lower zones.
[0025] FIG. 1 is a sectional view of a well 20 drilled from the
surface 21 into the earth's crust 29 and illustrating a series of
proposed hydraulic fractures 26 disposed at a predetermined spacing
28 to enhance production and recovery of formation fluids from a
hydraulically fractured subsurface geologic formation 24. The
drilled well 20 may include multiple layers of surface casing as is
known in the art. The drilled well 20 may include one or more turns
or changes in direction to align the portion of the well 20 to be
perforated or otherwise to gather fluids within a known geological
structure, seam or formation 24. The fractures 26 created in the
formation 24 are generally disposed at a predetermined spacing 28
selected for optimal drainage. The targeted formation 24 may reside
between a top layer 22 and an underlying layer 23 within the
earth's crust 29. It will be understood that fluids entering the
well 20 flow according to a pressure gradient in the direction of
the arrow 27 to the surface for processing, storage or
transportation.
[0026] FIG. 2 is the sectional view of the well 20 of FIG. 1
illustrating the lack of fractures 26 (seen in FIG. 1) within the
targeted geologic formation 24 prior to the creation of the
hydraulic fractures shown in FIG. 1. FIG. 2 illustrates, using a
circle, a location of a desired placement of a ball (not shown) and
a ball seat (not shown) to receive the ball to thereby isolate a
zone 50, that is deeper in the well than the ball seat (i.e., to
the right) from a zone 51 that is shallower in the well 20 than the
ball seat (i.e. to the left). It will be understood that the ball
and ball seat are to be placed in a portion of the casing 62 that
lies within the targeted geologic formation 24 and that the
pressure at any given location within the well 20 is approximately
equal to the pressure at a wellhead 49 at the surface 21 plus the
product of the vertical elevation change 46 times the density (as
measured in units corresponding to the unit used to measure depth)
of a fluid residing in the well 20, assuming that the well 20 is
filled with the fluid.
[0027] FIG. 3 is a sectional elevation of an embodiment of a ball
10 of the present invention received in a ball seat 44 that has
been previously set within a section of a casing 62 of the drilled
well 20 (not shown in FIG. 3) illustrated in FIG. 2 to create an
isolating seal. It will be understood that a number of tools exist
for setting the ball seat 44 within the portion of the casing 62 in
which the seal is to be affected, and that those tools and the
methods of setting those tools are not within the scope of the
present invention. FIG. 3 is provided merely to illustrate the
manner in which an embodiment of a ball 10 moves through the bore
70 of the casing 62 to engage the ball seat 44 after the ball seat
44 is set in the portion of the casing 62 and after the ball 10 is
introduced into the well 20 and moved to the ball seat 44. The ball
10 and ball seat 44 together form a seal to isolate a lower portion
of the bore 71 from the upper portion of the bore 70 that is uphole
to the ball 10 and ball seat 44.
[0028] FIG. 4 is a sectional view of an embodiment of a ball 10 of
the present invention. The ball 10 of FIG. 4 comprises a hollow
interior consisting of a hollow interior 15 of a first
hemispherical portion 11 and a hollow interior 16 of a second and
matching hemispherical portion 12. The circular rim 13 of the first
hemispherical portion 11 is manufactured to correspond in shape for
mating engagement with the circular rim 14 of the second
hemispherical portion 12. Securing of the first hemispherical
portion 11 to the second hemispherical portion 12 provides a
spherical ball having an exterior surface consisting of the
exterior surface 17 of the first hemispherical portion 11 and the
exterior surface 18 of the second hemispherical portion 12.
[0029] FIG. 5 is a plan view of a hollow interior 15 of the first
hemispherical portion 11 of FIG. 4. An aperture 30 in the ceramic
hemispherical shell 11 is fluidically connected by a conduit 31 to
a pressure sensor 32. The pressure sensor 32 closes a switch upon
sensing a predetermined threshold pressure through the aperture 30
and the conduit 31.
[0030] Upon receiving the signal from the pressure sensor 32, a
timer is activated. After a predetermined amount of time from
activation, a signal is sent to a detonator to explode the
explosive charge within the fracking ball. Upon detonation of the
explosive charge 36, the outer shell of the fracking ball 10 is
fragmented.
[0031] FIG. 6 illustrates a fragmented ceramic ball 10A as it might
appear immediately after the moment of detonation of the explosive
charge 36 within a hollow interior of the fracking ball 10 to
fragment the ball 10 into numerous ball fragments 49, which are
then dispersed into well fluids moving throughout the interior bore
of the casing 62. It will be understood that such fragmentation
dramatically increases the cumulative surface area of the ball
fragments 49 exposed to the fluids in the well. This will provide a
correspondingly dramatic increase in the rate at which any
dissolvable material will degrade and dissolve in the fluids in the
well.
[0032] FIG. 7 illustrates a safety feature that may be used to
enhance the safety of personnel that may handle, prepare and deploy
an embodiment of the ball 10 of the present invention. FIG. 7
illustrates the first hemispherical portion 11 of the ball 10
having a fuse aperture 52 to receive the safety fuse (such as a
pressure sensor) 53. Upon deployment of the ball 10 from the
surface, the safety fuse 53 can be inserted into and through the
fuse aperture 52 to engage and enable a critical connection. For
example, but not by way of limitation, the safety fuse 53 may be
inserted and seated in the fuse aperture 52 to engage, within the
hollow interior 15 of the ball 10, a pair of conductive leads
bridged by the safety fuse 53 that completes an electrical circuit
that will later, after the pressure sensor 32 senses the threshold
pressure and after the delay period has run, enable the battery 40
to detonate the preliminary explosive charge 35. Alternately, the
safety fuse 53 may engage and enable the circuit 33 so that, upon
detection of the threshold pressure by the pressure sensor 32, the
circuit 33 will begin the delay period. It will be understood that
there are various ways of enabling the explosive charge using a
safety fuse 53, that multiple safety fuses 53 may be used. In one
embodiment, no safety fuse 53 is used, but this is not recommended
for obvious reasons. In the embodiment illustrated in FIG. 7, the
safety fuse 53 comprises an enlarged head 54 that limits the extent
to which the safety fuse 53 can be inserted through the fuse
aperture 52. This head 54 and the safety fuse 53 length may be
customized to precisely position the safety fuse 53 relative to the
other components 31, 32, 33, 34, 35, 36 and 40 within the fracking
ball 10.
[0033] The configuration of the well 20 and the depth at which the
ball seat 44 and the ball 10 are to be used determine the size of
the ball seat 44 and the ball 10. The range of sizes of the ball
111 may be within the range from 4.45 cm (1.75 inches) to 10 cm
(4.0 inches), or larger. The filler material, if any, may comprise
particles or beads that vary in size and material, but are
preferably in the range from 0.2 mm (0.008 inch) to 1 mm (0.04
inch) diameter. A noncompressible fluid, such as a gel, can also be
used as the filler material.
[0034] The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of
the invention. As used herein, the singular forms "a", "an" and
"the" are intended to include the plural forms as well, unless the
context clearly indicates otherwise. It will be further understood
that the terms "comprises" and/or "comprising," when used in this
specification, specify the presence of stated features, integers,
steps, operations, elements, components and/or groups, but do not
preclude the presence or addition of one or more other features,
integers, steps, operations, elements, components, and/or groups
thereof. The terms "preferably," "preferred," "prefer,"
"optionally," "may," and similar terms are used to indicate that an
item, condition or step being referred to is an optional (not
required) feature of the invention.
[0035] The corresponding structures, materials, acts, and
equivalents of all means or steps plus function elements in the
claims below are intended to include any structure, material, or
act for performing the function in combination with other claimed
elements as specifically claimed. The description of the present
invention has been presented for purposes of illustration and
description, but it is not intended to be exhaustive or limited to
the invention in the form disclosed. Many modifications and
variations will be apparent to those of ordinary skill in the art
without departing from the scope and spirit of the invention. The
embodiment was chosen and described in order to best explain the
principles of the invention and the practical application, and to
enable others of ordinary skill in the art to understand the
invention for various embodiments with various modifications as are
suited to the particular use contemplated.
* * * * *