Patent Grant
7353879
The present application is related to U.S. patent application Ser. No. 10/803,668, now U.S. Pat. No. 7,093,664 issued on Aug. 22, 2006, and entitled “One-Time Use Composite Tool Formed of Fibers and a Biodegradable Resins”, which is owned by the assignee hereof, and is hereby incorporated by reference herein.
Not applicable.
Not applicable.
The present invention relates to biodegradable downhole tools and methods of removing such tools from wellbores. More particularly, the present invention relates to downhole tools or components thereof comprising an effective amount of biodegradable material such that the tool or the component desirably decomposes when exposed to a wellbore environment, and methods and systems for decomposing such downhole tools in situ.
A wide variety of downhole tools may be used within a wellbore in connection with producing hydrocarbons or reworking a well that extends into a hydrocarbon formation. Downhole tools such as frac plugs, bridge plugs, and packers, for example, may be used to seal a component against casing along the wellbore wall or to isolate one pressure zone of the formation from another. Such downhole tools are well known in the art.
After the production or reworking operation is complete, these downhole tools must be removed from the wellbore. Tool removal has conventionally been accomplished by complex retrieval operations, or by milling or drilling the tool out of the wellbore mechanically. Thus, downhole tools are either retrievable or disposable. Disposable downhole tools have traditionally been formed of drillable metal materials such as cast iron, brass and aluminum. To reduce the milling or drilling time, the next generation of downhole tools comprises composites and other non-metallic materials, such as engineering grade plastics. Nevertheless, milling and drilling continues to be a time consuming and expensive operation. Therefore, a need exists for disposable downhole tools that are removable without being milled or drilled out of the wellbore, and for methods of removing disposable downhole tools without tripping a significant quantity of equipment into the wellbore. Further, a need exists for disposable downhole tools that are removable from the wellbore by environmentally conscious methods and systems.
The present invention relates to a disposable downhole tool or a component thereof comprising an effective amount of biodegradable material such that the tool or the component desirably decomposes when exposed to a wellbore environment. In an embodiment, the biodegradable material comprises a degradable polymer. The biodegradable material may further comprise a hydrated organic or inorganic solid compound. The biodegradable material may also be selected to achieve a desired decomposition rate when the tool is exposed to the wellbore environment. In an embodiment, the tool or component is self-degradable. In an embodiment, the disposable downhole tool further comprises an enclosure for storing a chemical solution that catalyzes decomposition of the tool or the component. The tool may also comprise an activation mechanism for releasing the chemical solution from the enclosure. In various embodiments, the disposable downhole tool comprises a frac plug, a bridge plug, a packer, or another type of wellbore zonal isolation device.
In another aspect, the present invention relates to a method for performing a downhole operation wherein a disposable downhole tool is installed within a wellbore comprising desirably decomposing the tool or a component thereof in situ via exposure to the wellbore environment. In an embodiment, the tool or a component thereof is fabricated from an effective amount of biodegradable material such that the tool or the component desirably decomposes when exposed to the wellbore environment. The method may further comprise selecting the biodegradable material to achieve a desired decomposition rate of the tool or the component. In various embodiments, the method further comprises exposing the tool or the component to an aqueous fluid before the tool is installed in the wellbore or while the tool is installed within the wellbore. In an embodiment, at least a portion of the aqueous fluid is released from a hydrated compound within the tool when the compound is exposed to the wellbore environment. The method may further comprise catalyzing decomposition of the tool or the component by applying a chemical solution onto the tool, either before, during, or after the downhole operation. In various embodiments, the chemical solution is applied to the tool by dispensing the chemical solution into the wellbore; by lowering a frangible object containing the chemical solution into the wellbore and breaking the frangible object; by extending a conduit into the wellbore and flowing the chemical solution through the conduit onto the tool; or by moving a dart within the wellbore and engaging the dart with the tool to release the chemical solution.
In yet another aspect, the present invention relates to a system for applying a chemical solution to a disposable downhole tool or a component thereof that desirably decomposes when exposed to a wellbore environment; wherein the chemical solution catalyzes decomposition of the tool or the component. The chemical may be a caustic fluid, an acidic fluid, an enzymatic fluid, an oxidizer fluid, a metal salt catalyst solution or a combination thereof. In an embodiment, the system further comprises an enclosure for containing the chemical solution. The system may also include an activation mechanism for releasing the chemical solution from the enclosure. In various embodiments, the activation mechanism may be mechanically operated, hydraulically operated, electrically operated, timer-controlled, or operated via a communication means. In various embodiments, the enclosure is disposed on the tool, lowered to the tool on a slick line, or dropped into the wellbore to engage the tool. In an embodiment, the system further comprises a conduit extending into the wellbore to apply the chemical solution onto the tool.
In still another aspect, the present invention relates to a method for desirably decomposing a disposable downhole tool or a component thereof installed within a wellbore comprising releasing water from a compound within the tool upon exposure to heat in the wellbore environment, and at least partially decomposing the tool or the component by hydrolysis.
While the exemplary operating environment of
Structurally, the biodegradable downhole tool 100 may take a variety of different forms. In an embodiment, the tool 100 comprises a plug that is used in a well stimulation/fracturing operation, commonly known as a “frac plug.”
One or more components of the frac plug 200, or portions thereof, are formed from biodegradable materials. More specifically, the frac plug 200 or a component thereof comprises an effective amount of biodegradable material such that the plug 200 or the component desirably decomposes when exposed to a wellbore environment, as further described below. In particular, the biodegradable material will decompose in the presence of an aqueous fluid in a wellbore environment. A fluid is considered to be “aqueous” herein if the fluid comprises water alone or if the fluid contains water. The biodegradable components of the frac plug 200 may be formed of any material that is suitable for service in a downhole environment and that provides adequate strength to enable proper operation of the plug 200. The particular material matrix used to form the biodegradable components of the frac plug 200 may be selected for operation in a particular pressure and temperature range, or to control the decomposition rate of the plug 200 or a component thereof. Thus, a biodegradable frac plug 200 may operate as a 30-minute plug, a three-hour plug, or a three-day plug, for example, or any other timeframe desired by the operator.
Nonlimiting examples of biodegradable materials that may form various components of the frac plug 200, or another biodegradable downhole tool 100, include but are not limited to degradable polymers. A polymer is considered to be “degradable” herein if the degradation is due to, inter alia, chemical and/or radical process such as hydrolysis, oxidation, or UV radiation. The degradability of a polymer depends at least in part on its backbone structure. For instance, the presence of hydrolyzable and/or oxidizable linkages in the backbone often yields a material that will degrade as described herein. The rates at which such polymers degrade are dependent on the type of repetitive unit, composition, sequence, length, molecular geometry, molecular weight, morphology (e.g., crystallinity, size of spherulites, and orientation), hydrophilicity, hydrophobicity, surface area, and additives. Also, the environment to which the polymer is subjected may affect how it degrades, e.g., temperature, presence of moisture, oxygen, microorganisms, enzymes, pH, and the like.
Suitable examples of degradable polymers that may form various components of the disposable downhole tools 100 include but are not limited to those described in the publication of Advances in Polymer Science, Vol. 157 entitled “Degradable Aliphatic Polyesters” edited by A. C. Albertsson. Specific examples include homopolymers, random, block, graft, and star- and hyper-branched aliphatic polyesters. Polycondensation reactions, ring-opening polymerizations, free radical polymerizations, anionic polymerizations, carbocationic polymerizations, coordinative ring-opening polymerization, and any other suitable process may prepare such suitable polymers. Specific examples of suitable polymers include polysaccharides such as dextran or cellulose; chitin; chitosans; proteins; aliphatic polyesters; poly(lactides); poly(glycolides); poly(ε-caprolactones); poly(hydroxybutyrates); poly(anhydrides); aliphatic polycarbonates; poly(orthoesters); poly(amino acids); poly(ethylene oxides); and polyphosphazenes. Of these suitable polymers, aliphatic polyesters and polyanhydrides are preferred.
Aliphatic polyesters degrade chemically, inter alia, by hydrolytic cleavage. Hydrolysis can be catalyzed by either acids or bases. Generally, during the hydrolysis, carboxylic end groups are formed during chain scission, and this may enhance the rate of further hydrolysis. This mechanism is known in the art as “autocatalysis,” and is thought to make polyester matrices more bulk eroding.
Suitable aliphatic polyesters have the general formula of repeating units shown below:
where n is an integer between 75 and 10,000 and R is selected from the group consisting of hydrogen, alkyl, aryl, alkylaryl, acetyl, heteroatoms, and mixtures thereof. Of the suitable aliphatic polyesters, poly(lactide) is preferred. Poly(lactide) is synthesized either from lactic acid by a condensation reaction or more commonly by ring-opening polymerization of cyclic lactide monomer. Since both lactic acid and lactide can achieve the same repeating unit, the general term poly(lactic acid) as used herein refers to Formula I without any limitation as to how the polymer was made such as from lactides, lactic acid, or oligomers, and without reference to the degree of polymerization or level of plasticization.
The lactide monomer exists generally in three different forms: two stereoisomers L- and D-lactide and racemic D,L-lactide (meso-lactide). The oligomers of lactic acid, and oligomers of lactide are defined by the formula:
where m is an integer: 2≦m≦75. Preferably m is an integer: 2≦m≦10. These limits correspond to number average molecular weights below about 5,400 and below about 720, respectively. The chirality of the lactide units provides a means to adjust, inter alia, degradation rates, as well as physical and mechanical properties. Poly(L-lactide), for instance, is a semicrystalline polymer with a relatively slow hydrolysis rate. This could be desirable in downhole operations where a slower degradation of the degradable material is desired. Poly(D,L-lactide) may be a more amorphous polymer with a resultant faster hydrolysis rate. This may be suitable for other downhole operations where a more rapid degradation may be appropriate. The stereoisomers of lactic acid may be used individually or combined in accordance with the present invention. Additionally, they may be copolymerized with, for example, glycolide or other monomers like ε-caprolactone, 1,5-dioxepan-2-one, trimethylene carbonate, or other suitable monomers to obtain polymers with different properties or degradation times. Additionally, the lactic acid stereoisomers can be modified by blending, copolymerizing or otherwise mixing high and low molecular weight polylactides; or by blending, copolymerizing or otherwise mixing a polylactide with another polyester or polyesters.
Plasticizers may also be present in the polymeric degradable materials comprising the disposable downhole tools 100. Suitable plasticizers include but are not limited to derivatives of oligomeric lactic acid, selected from the group defined by the formula:
where R is a hydrogen, alkyl, aryl, alkylaryl, acetyl, heteroatom, or a mixture thereof and R is saturated, where R′ is a hydrogen, alkyl, aryl, alkylaryl, acetyl, heteroatom, or a mixture thereof and R′ is saturated, where R and R′ cannot both be hydrogen, where q is an integer: 2≦q≦75; and mixtures thereof. Preferably q is an integer: 2≦q≦10. As used herein the term “derivatives of oligomeric lactic acid” includes derivatives of oligomeric lactide.
The plasticizers may be present in any amount that provides the desired characteristics. For example, the various types of plasticizers discussed herein provide for (a) more effective compatibilization of the melt blend components; (b) improved processing characteristics during the blending and processing steps; and (c) control and regulate the sensitivity and degradation of the polymer by moisture. For pliability, plasticizer is present in higher amounts while other characteristics are enhanced by lower amounts. The compositions allow many of the desirable characteristics of pure nondegradable polymers. In addition, the presence of plasticizer facilitates melt processing, and enhances the degradation rate of the compositions in contact with the wellbore environment. The intimately plasticized composition should be processed into a final product in a manner adapted to retain the plasticizer as an intimate dispersion in the polymer for certain properties. These can include: (1) quenching the composition at a rate adapted to retain the plasticizer as an intimate dispersion; (2) melt processing and quenching the composition at a rate adapted to retain the plasticizer as an intimate dispersion; and (3) processing the composition into a final product in a manner adapted to maintain the plasticizer as an intimate dispersion. In certain preferred embodiments, the plasticizers are at least intimately dispersed within the aliphatic polyester.
A preferred aliphatic polyester is poly(lactic acid). D-lactide is a dilactone, or cyclic dimer, of D-lactic acid. Similarly, L-lactide is a cyclic dimer of L-lactic acid. Meso D,L-lactide is a cyclic dimer of D-, and L-lactic acid. Racemic D,L-lactide comprises a 50/50 mixture of D-, and L-lactide. When used alone herein, the term “D,L-lactide” is intended to include meso D,L-lactide or racemic D,L-lactide. Poly(lactic acid) may be prepared from one or more of the above. The chirality of the lactide units provides a means to adjust degradation rates as well as physical and mechanical properties. Poly(L-lactide), for instance, is a semicrystalline polymer with a relatively slow hydrolysis rate. Poly(D,L-lactide) is an amorphous polymer with a faster hydrolysis rate. The stereoisomers of lactic acid may be used individually combined or copolymerized in accordance with the present invention.
The aliphatic polyesters may be prepared by substantially any of the conventionally known manufacturing methods such as those described in U.S. Pat. Nos. 6,323,307; 5,216,050; 4,387,769; 3,912,692; and 2,703,316, which are hereby incorporated herein by reference in their entirety.
Poly(anhydrides) are another type of particularly suitable degradable polymer useful in the disposable downhole tools 100. Poly(anhydride) hydrolysis proceeds, inter alia, via free carboxylic acid chain-ends to yield carboxylic acids as final degradation products. The erosion time can be varied over a broad range of changes in the polymer backbone. Examples of suitable poly(anhydrides) include poly(adipic anhydride), poly(suberic anhydride), poly(sebacic anhydride), and poly(dodecanedioic anhydride). Other suitable examples include but are not limited to poly(maleic anhydride) and poly(benzoic anhydride).
The physical properties of degradable polymers depend on several factors such as the composition of the repeat units, flexibility of the chain, presence of polar groups, molecular mass, degree of branching, crystallinity, orientation, etc. For example, short chain branches reduce the degree of crystallinity of polymers while long chain branches lower the melt viscosity and impart, inter alia, elongational viscosity with tension-stiffening behavior. The properties of the material utilized can be further tailored by blending, and copolymerizing it with another polymer, or by a change in the macromolecular architecture (e.g., hyper-branched polymers, star-shaped, or dendrimers, etc.). The properties of any such suitable degradable polymers (e.g., hydrophobicity, hydrophilicity, rate of degradation, etc.) can be tailored by introducing select functional groups along the polymer chains. For example, poly(phenyllactide) will degrade at about ⅕th of the rate of racemic poly(lactide) at a pH of 7.4 at 55° C. One of ordinary skill in the art with the benefit of this disclosure will be able to determine the appropriate functional groups to introduce to the polymer chains to achieve the desired physical properties of the degradable polymers.
In various embodiments, the frac plug 200 or a component thereof is self-degradable. Namely, the frac plug 200, or portions thereof, are formed from biodegradable materials comprising a mixture of a degradable polymer, such as the aliphatic polyesters or poly(anhydrides) previously described, and a hydrated organic or inorganic solid compound. The degradable polymer will at least partially degrade in the releasable water provided by the hydrated organic or inorganic compound, which dehydrates over time when heated due to exposure to the wellbore environment.
Examples of the hydrated organic or inorganic solid compounds that can be utilized in the self-degradable frac plug 200 or self-degradable component thereof include, but are not limited to, hydrates of organic acids or their salts such as sodium acetate trihydrate, L-tartaric acid disodium salt dihydrate, sodium citrate dihydrate, hydrates of inorganic acids or their salts such as sodium tetraborate decahydrate, sodium hydrogen phosphate heptahydrate, sodium phosphate dodecahydrate, amylose, starch-based hydrophilic polymers, and cellulose-based hydrophilic polymers. Of these, sodium acetate trihydrate is preferred.
In operation, the frac plug 200 of
The frac plug 200 is then lowered by the cable 118 to the desired depth within the wellbore 120, and the packer element assembly 230 is set against the casing 125 in a conventional manner, thereby isolating zone A as depicted in
After the frac plug 200 is set into position as shown in
If additional well stimulation/fracturing operations will be performed, such as recovering hydrocarbons from zone C, additional frac plugs 200 may be installed within the wellbore 120 to isolate each zone of the formation F. Each frac plug 200 allows fluid to flow upwardly therethrough from the lowermost zone A to the uppermost zone C of the formation F, but pressurized fluid cannot flow downwardly through the frac plug 200.
After the fluid recovery operations are complete, the frac plug 200 must be removed from the wellbore 120. In this context, as stated above, at least some components of the frac plug 200, or portions thereof, are formed from biodegradable materials. More specifically, the frac plug 200 or a component thereof comprises an effective amount of biodegradable material such that the plug 200 or the component desirably decomposes when exposed to a wellbore environment. In particular, these biodegradable materials will decompose in the presence of an aqueous fluid in a wellbore environment. A fluid is considered to be “aqueous” herein if the fluid comprises water alone or if the fluid contains water. Aqueous fluids may be present naturally in the wellbore 120, or may be introduced to the wellbore 120 before, during, or after downhole operations. Alternatively, the frac plug 200 may be exposed to an aqueous fluid prior to being installed within the wellbore 120. Further, for those embodiments of the frac plug 200 or a component thereof that are self-degradable, an aqueous fluid is released by the hydrated organic or inorganic solid compound as it dehydrates over time when heated in the wellbore environment. Thus, the self-degradable frac plug 200 or component thereof is suitable for use in a non-aqueous wellbore environment.
Accordingly, in an embodiment, the frac plug 200 is designed to decompose over time while operating in a wellbore environment, thereby eliminating the need to mill or drill the frac plug 200 out of the wellbore 120. Thus, by exposing the biodegradable frac plug 200 to wellbore temperatures and an aqueous fluid, at least some of its components will decompose, causing the frac plug 200 to lose structural and/or functional integrity and release from the casing 125. The remaining components of the plug 200 will simply fall to the bottom of the wellbore 120. In various alternate embodiments, degrading one or more components of a downhole tool 100 performs an actuation function, opens a passage, releases a retained member, or otherwise changes the operating mode of the downhole tool 100.
In choosing the appropriate biodegradable materials for the frac plug 200 or a component thereof, one should consider the degradation products that will result. These degradation products should not adversely affect other operations or components. The choice of biodegradable materials also can depend, at least in part, on the conditions of the well, e.g., wellbore temperature. While no upper temperature limit is known to exist, lactides have been found to be suitable for lower temperature wells, including those within the range of 60° F. to 150° F., and polylactides have been found to be suitable for wellbore temperatures above this range. Also, poly(lactic acid) may be suitable for higher temperature wells in the range of from about 350° F. to 500° F. Some stereoisomers of poly(lactide) or mixtures of such stereoisomers may be suitable for even higher temperature applications. In certain embodiments, the subterranean formation F has a temperature above about 180° F., and self-degradable frac plugs 200 are most suitable for use where the formation F has a temperature in excess of about 200° F. to facilitate release of the water in the hydrated organic or inorganic compound.
As stated above, the biodegradable material forming components of the frac plug 200 may be selected to control the decomposition rate of the plug 200 or a component thereof. However, in some cases, it may be desirable to catalyze decomposition of the frac plug 200 or the component by applying a chemical solution to the plug 200. The chemical solution comprises a caustic fluid, an acidic fluid, an enzymatic fluid, an oxidizer fluid, a metal salt catalyst solution or a combination thereof, and may be applied before or after the frac plug 200 is installed within the wellbore 120. Further, the chemical solution may be applied before, during, or after the fluid recovery operations. For those embodiments where the chemical solution is applied before or during the fluid recovery operations, the biodegradable material, the chemical solution, or both may be selected to ensure that the frac plug 200 or a component thereof decomposes over time while remaining intact during its intended service.
The chemical solution may be applied by means internal to or external to the frac plug 200. In an embodiment, an optional enclosure 275 is provided on the frac plug 200 for storing the chemical solution 290 as depicted in
As depicted in
As depicted in
Referring now to
Removing a biodegradable downhole tool 100, such as the frac plug 200 described above, from the wellbore 120 is more cost effective and less time consuming than removing conventional downhole tools, which requires making one or more trips into the wellbore 120 with a mill or drill to gradually grind or cut the tool away. Further, biodegradable downhole tools 100 are removable, in most cases, by simply exposing the tools 100 to a naturally occurring downhole environment over time. The foregoing descriptions of specific embodiments of the biodegradable tool 100, and the systems and methods for removing the biodegradable tool 100 from the wellbore 120 have been presented for purposes of illustration and description and are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously many other modifications and variations are possible. In particular, the type of biodegradable downhole tool 100, or the particular components that make up the downhole tool 100 could be varied. For example, instead of a frac plug 200, the biodegradable downhole tool 100 could comprise a bridge plug, which is designed to seal the wellbore 120 and isolate the zones above and below the bridge plug, allowing no fluid communication in either direction. Alternatively, the biodegradable downhole tool 100 could comprise a packer that includes a shiftable valve such that the packer may perform like a bridge plug to isolate two formation zones, or the shiftable valve may be opened to enable fluid communication therethrough.
While various embodiments of the invention have been shown and described herein, modifications may be made by one skilled in the art without departing from the spirit and the teachings of the invention. The embodiments described here are exemplary only, and are not intended to be limiting. Many variations, combinations, and modifications of the invention disclosed herein are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited by the description set out above, but is defined by the claims which follow, that scope including all equivalents of the subject matter of the claims.
| Number | Name | Date | Kind |
|---|---|---|---|
| 2238671 | Woodhouse | Apr 1941 | A |
| 2703316 | Schneider | Mar 1955 | A |
| 3173484 | Huitt et al. | Mar 1965 | A |
| 3195635 | Fast | Jul 1965 | A |
| 3302719 | Fischer | Feb 1967 | A |
| 3364995 | Atkins et al. | Jan 1968 | A |
| 3366178 | Malone et al. | Jan 1968 | A |
| 3455390 | Gallus | Jul 1969 | A |
| 3784585 | Schmitt et al. | Jan 1974 | A |
| 3828854 | Templeton et al. | Aug 1974 | A |
| 3868998 | Lybarger et al. | Mar 1975 | A |
| 3912692 | Casey et al. | Oct 1975 | A |
| 3960736 | Free et al. | Jun 1976 | A |
| 3968840 | Tate | Jul 1976 | A |
| 3998744 | Arnold et al. | Dec 1976 | A |
| 4068718 | Cooke, Jr. et al. | Jan 1978 | A |
| 4169798 | DeMartino | Oct 1979 | A |
| 4187909 | Erbstoesser | Feb 1980 | A |
| 4387769 | Erbstoesser et al. | Jun 1983 | A |
| 4417989 | Hunter | Nov 1983 | A |
| 4470915 | Conway | Sep 1984 | A |
| 4526695 | Erbstoesser et al. | Jul 1985 | A |
| 4715967 | Bellis et al. | Dec 1987 | A |
| 4716964 | Erbstoesser et al. | Jan 1988 | A |
| 4743257 | Tormala et al. | May 1988 | A |
| 4809783 | Hollenbeck et al. | Mar 1989 | A |
| 4843118 | Lai et al. | Jun 1989 | A |
| 4848467 | Cantu et al. | Jul 1989 | A |
| 4957165 | Cantu et al. | Sep 1990 | A |
| 4961466 | Himes et al. | Oct 1990 | A |
| 4986353 | Clark et al. | Jan 1991 | A |
| 4986354 | Cantu et al. | Jan 1991 | A |
| 4986355 | Casad et al. | Jan 1991 | A |
| 5082056 | Tackett, Jr. | Jan 1992 | A |
| 5131472 | Dees et al. | Jul 1992 | A |
| 5216050 | Sinclair | Jun 1993 | A |
| 5224540 | Streich et al. | Jul 1993 | A |
| 5271468 | Streich et al. | Dec 1993 | A |
| 5294469 | Suzuki et al. | Mar 1994 | A |
| 5390737 | Jacobi et al. | Feb 1995 | A |
| 5439055 | Card et al. | Aug 1995 | A |
| 5439059 | Harris et al. | Aug 1995 | A |
| 5460226 | Lawson et al. | Oct 1995 | A |
| 5479986 | Gano et al. | Jan 1996 | A |
| 5540279 | Branch et al. | Jul 1996 | A |
| 5591700 | Harris et al. | Jan 1997 | A |
| 5607017 | Owens et al. | Mar 1997 | A |
| 5607905 | Dobson, Jr. et al. | Mar 1997 | A |
| 5685372 | Gano | Nov 1997 | A |
| 5689085 | Turner | Nov 1997 | A |
| 5698322 | Tsai et al. | Dec 1997 | A |
| 5701959 | Hushbeck et al. | Dec 1997 | A |
| 5765641 | Shy et al. | Jun 1998 | A |
| 5839515 | Yuan et al. | Nov 1998 | A |
| 5849401 | El-Afandi et al. | Dec 1998 | A |
| 5984007 | Yuan et al. | Nov 1999 | A |
| 5990051 | Ischy et al. | Nov 1999 | A |
| 6102117 | Swor et al. | Aug 2000 | A |
| 6131661 | Conner et al. | Oct 2000 | A |
| 6135987 | Tsai et al. | Oct 2000 | A |
| 6143698 | Murphey et al. | Nov 2000 | A |
| 6161622 | Robb et al. | Dec 2000 | A |
| 6162766 | Muir et al. | Dec 2000 | A |
| 6189615 | Sydansk | Feb 2001 | B1 |
| 6209646 | Reddy et al. | Apr 2001 | B1 |
| 6218343 | Burts, Jr. | Apr 2001 | B1 |
| 6220349 | Vargus et al. | Apr 2001 | B1 |
| 6242390 | Mitchell et al. | Jun 2001 | B1 |
| 6318460 | Swor et al. | Nov 2001 | B1 |
| 6323307 | Bigg et al. | Nov 2001 | B1 |
| 6328105 | Betzold | Dec 2001 | B1 |
| 6378606 | Swor et al. | Apr 2002 | B1 |
| 6387986 | Moradi-Araghi et al. | May 2002 | B1 |
| 6394185 | Constien | May 2002 | B1 |
| 6422314 | Todd et al. | Jul 2002 | B1 |
| 6444316 | Reddy et al. | Sep 2002 | B1 |
| 6481497 | Swor et al. | Nov 2002 | B2 |
| 6494263 | Todd | Dec 2002 | B2 |
| 6527051 | Reddy et al. | Mar 2003 | B1 |
| 6554071 | Reddy et al. | Apr 2003 | B1 |
| 6599863 | Palmer et al. | Jul 2003 | B1 |
| 6655459 | Mackay | Dec 2003 | B2 |
| 6666275 | Neal et al. | Dec 2003 | B2 |
| 6667279 | Hessert et al. | Dec 2003 | B1 |
| 6669771 | Tokiwa et al. | Dec 2003 | B2 |
| 6681856 | Chatterji et al. | Jan 2004 | B1 |
| 6710019 | Sawdon et al. | Mar 2004 | B1 |
| 6761218 | Nguyen et al. | Jul 2004 | B2 |
| 6837309 | Boney et al. | Jan 2005 | B2 |
| 7036587 | Munoz et al. | May 2006 | B2 |
| 7080688 | Todd et al. | Jul 2006 | B2 |
| 7178596 | Blauch et al. | Feb 2007 | B2 |
| 20010016562 | Muir et al. | Aug 2001 | A1 |
| 20020036088 | Todd | Mar 2002 | A1 |
| 20020125012 | Dawson et al. | Sep 2002 | A1 |
| 20030060374 | Cooke, Jr. | Mar 2003 | A1 |
| 20030114314 | Ballard et al. | Jun 2003 | A1 |
| 20030130133 | Vallmer | Jul 2003 | A1 |
| 20030168214 | Sollesnes | Sep 2003 | A1 |
| 20030213601 | Schwendemann et al. | Nov 2003 | A1 |
| 20030234103 | Lee et al. | Dec 2003 | A1 |
| 20040014607 | Sinclair et al. | Jan 2004 | A1 |
| 20040040706 | Hossaini et al. | Mar 2004 | A1 |
| 20040152601 | Still et al. | Aug 2004 | A1 |
| 20040231845 | Cooke, Jr. | Nov 2004 | A1 |
| 20050006095 | Justus et al. | Jan 2005 | A1 |
| 20050056425 | Grigsby et al. | Mar 2005 | A1 |
| 20050126785 | Todd | Jun 2005 | A1 |
| 20050205265 | Todd et al. | Sep 2005 | A1 |
| 20060105917 | Munoz, Jr. | May 2006 | A1 |
| 20060283597 | Schriener et al. | Dec 2006 | A1 |
| Number | Date | Country |
|---|---|---|
| 0681087 | May 1995 | EP |
| WO 0057022 | Sep 2000 | WO |
| WO 0102698 | Jan 2001 | WO |
| WO 2004007905 | Jan 2004 | WO |
| WO 2004037946 | May 2004 | WO |
| WO 2004038176 | May 2004 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20050205266 A1 | Sep 2005 | US |
