High Density Perforating Gun System Producing Reduced Debris
A perforating system has a perforating module comprising a unitary body of explosive. The explosive is contained within a non-explosive casing, or liner, having formed indentations and a cover thereover. The indentations, which will transform into explosively formed penetrators (EFP's) upon detonation, have a perimeter shape that allows for improved packing density, e.g., a hexagonal perimeter, which results in relatively little “dead space” wherein no perforating penetrators are generated. In operation, the module provides a relatively dense shot pattern and substantially reduced amount of post-detonation debris that could clog the perforations and/or require remedial clean-up or repeat perforation.
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This application is a continuation-in-part of and claims priority to U.S. patent application Ser. No. 11/405,148, filed on Apr. 17, 2006.
BACKGROUND OF THE DISCLOSURE1. Field of the Disclosure
The disclosure relates generally to the design of perforating tools for use in creating perforations in wellbores to improve the flow of fluids from the wellbore.
2. Description of the Related Art
Commercial development of hydrocarbon fields requires significant amounts of capital. Therefore, before field development begins, operators desire to have as much information as possible in order to evaluate the reservoir for commercial viability. Such information may be acquired at the seismic exploration phase, during well construction, prior to well completion and/or any time thereafter.
Perforation guns are used within wellbore holes to increase the permeability of the formation surrounding the wellbore. In general, perforation guns producing greater numbers of perforations are considered to be more effective than those producing fewer perforations. It is therefore often desired to maximize the number of penetrating jets within a segment of the wellbore. This may be difficult, however, because there are limitations relating to placement of the charges used for perforation. Standard shaped charges have an outer housing formed of metal or another material that encloses the high explosive charge. The shaped charge holder has openings that have typically circular perimeters. When packing the charges in an adjoining manner in the charge tube, interstitial spaces are unavoidably left between the neighboring charges as a result their shape. This packing of the charges results in significant “dead spaces,” that is, areas from which no perforating product, i.e., no jets, is/are provided, between the charges, and limits the density with which the charges can be packed.
There are a number of known styles and designs for perforation guns. There are, for example, strip guns that include a strip carrier upon which are mounted a number of capsule charges. The capsule charges are individually sealed against corrosive wellbore fluids. Also known are hollow carrier guns that have a sealed outer housing that contains unencapsulated shaped charges. In each case, the shaped charges are arranged such that they will detonate in a radially outward direction to form a specific pattern of perforations.
An alternative perforation gun design is described in U.S. Pat. No. 5,619,008 to Chawla et al. In this design, a two-layer liner serves to sheath discontinuous loadings of explosive material. The liner is configured with indentations that are each aligned with an individual loading of the explosive material. Upon detonation of the loadings of explosive material, these indentations act in the manner of a shaped charge, creating a directed jet of liner material. The indentations have a circular perimeter and are spaced apart from one another, leaving significant “dead space” between them. Following detonation and any resulting perforation, the housing that surrounds the charges is not completely destroyed and forms debris. This debris is undesirable, both because it must be removed by wireline or by other means in a secondary operation, and because it may clog the perforations that are formed by the perforation operation, thereby making the perforations less effective and sometimes necessitating repeat perforation operations. The Chawla et al. invention thus suffers from problems relating to both “dead space” and debris creation.
The present disclosure addresses the problems of the prior art.
SUMMARY OF THE DISCLOSUREThe present disclosure provides a perforating device that produces multiple perforating penetrators from a single high explosive charge. In one embodiment, the perforating module has a central rod with a surrounding cylinder of high explosive. The cylinder of high explosive is contained within a liner having formed indentations. The liner may be of any suitable material, such as a non-explosive material including, for example, an elemental metal or alloy, a composite, a ceramic, a thermoplastic or thermo set polymer, or the like. Finally, a cylindrical outer cover is disposed about the liner. In one embodiment, the indentations are linearly contiguous to one another. In another embodiment, the indentations each have a perimeter that is triangular, square, hexagonal, or octagonal and are disposed in an adjoining fashion to one another.
In operation and as a result of detonation of the explosive material, the module forms penetrators of liner material that propagate into the formation in a direction that is, in one embodiment, substantially perpendicular to the longitudinal axis of the wellbore. The module thus is capable of providing a relatively dense shot pattern with little or no “dead space” between the locations from which the penetrators are formed. This results in an effective perforation of a wellbore segment.
During the detonation, the constituent components of the module, the high explosive, the liner, and the outer cover, are largely destroyed. As a result, the amount of debris resulting from the detonation is reduced or eliminated, in contrast with the large amount of debris produced by many conventional perforation devices.
One embodiment of the disclosure includes a method for perforating a subterranean formation, comprising: lowering a plurality of perforating modules in a wellbore, wherein each of the plurality of perforating modules comprises: a central rod, wherein the central rod comprises: an exterior load bearing portion, and an interior detonation portion including a first explosive, a second explosive adapted to at least partially surround the central rod, and a liner disposed to surround the second explosive, wherein the liner is made of a non-explosive material, and wherein the liner has a plurality of concave arcuate surface indentations; and detonating the plurality of perforating modules in the wellbore.
Another embodiment of the disclosure includes a method for perforating a subterranean formation, comprising: lowering a plurality of perforating modules in a wellbore, wherein at least one of the plurality of perforating modules is separated from another of the plurality of perforating modules by a spacer, and wherein each of the plurality of perforating modules comprises: a central rod, wherein the central rod comprises: an exterior load bearing portion comprised of a ceramic, an axial passage adapted to receive hydraulic fluid disposed along the length of the exterior load bearing portion, a wire disposed within the axial passage, and an interior detonation portion including a first explosive, a second explosive adapted to at least partially surround the central rod, a liner disposed to surround the second explosive, wherein the liner is made of a non-explosive material, and wherein the liner has a plurality of concave arcuate surface indentations, wherein each of the plurality of concave arcuate surface indentation is polygonal, wherein each of the plurality of concave arcuate surface indentations is configured to face substantially perpendicular to the longitudinal axis of the wellbore, and wherein each of the plurality of concave arcuate surface is linearly contiguous with at least another of the plurality of concave arcuate surface indentations, and a cover disposed about the liner; and detonating the plurality of perforating modules in the wellbore.
In embodiments, the plurality of shallow concave surface indentations may each have a cavity. The cavity may be defined by a diameter and a depth. In one arrangement, the diameter to depth ratio is approximately not less than two to one. In another embodiment, the diameter to depth ratio is not less than six to one. In still other embodiments, the depth is no greater than a thickness of the liner. The method may also include forming a plurality of perforations in a region adjacent to the perforating modules, wherein the plurality of perforations extend substantially through a cement layer into a formation a distance from a cement face that is no greater than a diameter of the cavity. In some applications, the distance is no greater than one-half of the diameter of the cavity. Also, the method may include at least partially lining the plurality of perforations in the cement layer with a liner material.
Another embodiment of the disclosure includes a method for perforating a subterranean formation. The method may include lowering a perforating module into a wellbore having a casing incased in cement, the perforating module having at least one explosively formed penetrator forming charge and liner; positioning the plurality of perforating modules in the wellbore and adjacent to a substantially unconsolidated formation; perforating the casing and cement; and perforating the formation to a distance no greater than a diameter of the at least one explosively formed penetrator forming charge and liner, wherein the distance measured from a boundary between the cement and the formation.
For greater understanding of the disclosure, reference is made to the following detailed description of the embodiments of the present disclosure, taken in conjunction with the accompanying drawings in which reference characters designate like or similar elements throughout the several figures of the drawings.
The present disclosure relates to devices and methods for perforating wellbores. The present disclosure is susceptible to embodiments of different forms. These are shown in the drawings, and herein will be described in detail, specific embodiments of the present disclosure with the understanding that the present disclosure is to be considered an exemplification of the principles of the disclosure, and is not intended to limit the disclosure to that illustrated and described herein.
The hydrocarbon-bearing formation 16 contains two oil-bearing strata 22, 24, which are separated by a layer of water 26. A layer of water 28 also separates the lower oil stratum 24 from a stratum of gas 30. It is noted that this arrangement of strata in formation 16 is presented only by way of example and that those skilled in the art will recognize that the actual composition and configuration of formations varies.
The perforation system 10 is disposed into the wellbore 12 on a conveyance string 32. The conveyance string 32 may be of any known construction for conveying a tool into a wellbore, including a drill pipe, wireline, production tubing, coiled tubing, and the like. The perforation system 10 includes one or more perforating modules that are used to perforate portions of the surrounding formation 16. In the described embodiment, there are three perforating modules 34, 36, 38 that are secured to one another in series. There may, of course, be more or fewer than three modules, depending upon the desired length of wellbore to be perforated. Additionally, it is pointed out that there may be intermediate sections of tubing, or subs 37 (see
An exemplary individual module 40 is depicted in
The detonation layer 43 comprises, in this embodiment, a primasheet of a type known in the art for initiation of detonations. The load-bearing portion 41 of the central rod 42 may also contain an axial passage 48 along its length to contain electrical wiring (not shown) that is necessary for initiation of the detonation layer 43 which, in turn, results in detonation of the body 50 of high explosive material. The detonation layer 43 may be initiated with a control signal either manually or utilizing some preprogrammed device. For example, suitable initiating systems can include using electrical signals transmitted from the surface via wiring (not shown) in the axial passage 48 to initiate detonation by increasing hydraulic pressure in the wellbore, or by the dropping of a drop bar (not shown) into the axial passage 48, as is used conventionally with tubing conveyed perforation guns. Other initiating systems can utilize timers or well bore parameter sensitive devices (e.g., pressure, temperature, depth, etc.). Initiation systems for detonating perforating guns are known in the art and will not be discussed in further detail.
Surrounding the central rod 42 is a substantially unitary body 50 of high explosive material that explosively forms the perforating penetrators using the liner 52. Suitable high explosive materials may include, for example, conventionally-employed high explosives such as RDX, HMX and HNS. While the size of the module is not a critical aspect thereof, it may be convenient to configure the module 40 such that it is a cylinder about 12 inches in length and about 4.5 inches in diameter. However, the length and diameter may be varied according to the dimensions of the wellbore 12 or other factors. In one embodiment, a tube 51 of cardboard or a similar material is disposed between the central rod 42 and the high explosive body 50.
The liner 52 surrounds the body 50 of high explosive and is configured to form a plurality of perforating penetrators. The penetrators formed by the liner 52 may travel in a direction generally perpendicular to the longitudinal axis of the wellbore, although modifications in direction may also be achieved in other embodiments of this disclosure. In one embodiment, the liner 52 may be, in this embodiment, a cylindrical and non-explosive liner formed of a metal, such as, for example, tantalum. Alternatively, the liner 52 may be made from extruded copper, tungsten, steel, depleted uranium, aluminum, or another elemental metal or alloy. In other embodiments blends of elemental metals or alloys with materials such as lead, graphite, and zinc stearate may also be employed. In still other embodiments blends or alloys of aluminum with either titanium or hafnium may be used. Additionally, a frangible material may be used to form the liner 52 in order to further reduce the likelihood that the formed penetrator will plug the perforation created in the surrounding formation. Such may include, for example, the use of pressed, sintered metallic powders, such as those described in U.S. Pat. No. 6,012,392, which is incorporated herein by reference in its entirety, and metal/matrix composites.
The size, shape, velocity and other characteristics of the perforating penetrators formed by the liner 52 may be controlled, in part, by adjusting the surface contours and/or geometry of the liner 52. In one embodiment, a plurality of linearly contiguous indentations 54 is formed into the liner 52. As used herein, the phrase “linearly contiguous” means that the perimeters of every indentation shares at least one common side with an adjacent indentation. In some embodiments a majority of each indentation is linearly contiguous with adjacent indentations, and in other embodiments essentially all of each indentation is linearly contiguous with adjacent indentations. In one embodiment, each indentation 54 has an axis that is substantially perpendicular to the exterior surface of the liner 52, where such exterior surface is substantially parallel to the longitudinal axis of the wellbore. In other embodiments such indentation axis may be significantly greater or less than ninety degrees to the exterior surface of the liner 52 and/or to the longitudinal axis of the wellbore, in order to direct the penetrators in a specific direction, according to the purposes and goals of the perforation operation.
The shape of the cavity 60 may determine one or more characteristics of the penetrator and may dictate how the penetrator is formed.
Certain shapes (such as those for conventional shaped charges) produce a Munroe effect, whereby a small fraction (10-15%) of the liner 52 is propelled into a target to cause a narrow and deep penetration. Under the Munroe effect, the tip of the jet formed by a shaped charge liner collapse travels at tremendous velocity (8-10 km/sec). A generally conical or pyramidal cavity 60 produces a jet that forms such a penetration. For the purposes of this disclosure, a Munroe effect penetration may be defined as a penetration wherein a depth of penetration into the formation, i.e., a distance from a cement face, is generally six times or more of the diameter D of the cavity 60.
Certain other shapes produce Misznay-Schardin effect, whereby a large fraction (90-100%) of the liner 52 is propelled into a target to cause a wide and shallow perforation. Projectiles formed under the Misznay-Schardin effect are commonly called Explosively Formed Penetrators (EFPs). EFPs travel much more slowly (˜1 km/sec.) than the jet of a conventional shaped charge. A generally spherical, shallow curved hollow, a shallow pyramid indentation, or a shallow concave arcuate shaped cavity 60 forms a projectile that makes such a penetration. For the purposes of this disclosure, “shallow” means that the cavity 60 has a diameter D to depth De ratio of greater than two to one. In some embodiments, the diameter to depth ratio may be six to one or greater. The diameter of the cavity 60 may be measured across the outer perimeter 56 (i.e. diameter D), and the depth De of cavity 60 may be measured from the apex 62 and the plane of the ridges 58. In embodiments where the bottom of the cavity 60 is flattened, then the depth De may be measured between the plane of the flattened area and the plane of the ridges 58. For a non-circular shape, the diameter D may be considered the diameter of the circle that encompasses or circumscribes that shape. In other aspects, the term “shallow” may include designs wherein the depth De of the cavity 60 is approximately the same as or less than a thickness of the liner 52. For the purposes of this disclosure, a Misznay-Schardin effect penetration may be defined as a penetration wherein a depth of penetration into the formation is about one-half to one diameter of the cavity 60. In certain embodiments, the penetration may be less than one-half times the diameter of the cavity 60.
Embodiments of this disclosure contemplate the use of a variety of cavity 60 shapes as required to accomplish the desired penetration.
An alternative method for forming the high explosive body 50 is by pressing a billet to a desired length and diameter, and then machining the billet to match the hexagonal indentations 54 at the outer surface of the liner 52. A long axial hole is then drilled into the center of the billet and sized to accommodate the tube 51. As those skilled in the art are aware, a billet of high explosive is a mass of high explosive material that has been pressed or cast into cylindrical shape. Pressed billets can be machined to a desired shape, while cast billets are formed to the desired shape, such as, in this case, a cylinder with an axial passage therethrough.
Circumferentially surrounding the liner 52 is a cover 64 that protects the liner 52 and other parts of the module 40 from the harsh wellbore environment. In one embodiment, the cover 64 is a generally cylindrical construction having planar inner and outer surfaces. The cover 64 may be formed of, for example, a thermoplastic or thermoset polymer that is resistant to high wellbore temperatures. The cover 64 may be relatively thin, having a thickness of, for example, just 0.05 inch, and light in weight, such that it will not unduly interfere with the creation of the penetrators from the indentations 54 or 54′. In some embodiments, an elemental metal or alloy, composite material, thermoplastic or thermoset polymer, or glass, for example, may be used to form the cover 64. The cover 64 overlies the adjoining ridges 58 between neighboring indentations 54 or 54′ (see
Upper and lower end caps 66, 68 (see
In operation, the perforation system 10 is lowered into the wellbore 12 until the modules 34, 36, 38 of the perforation system 10 are aligned with the desired strata 22, 24, and 30, respectively, of the formation 14. The modules 34, 36, 38 of the perforation system 10 are then detonated to create penetrators that perforate the casing 18, cement 20 and formation 14. Following perforation of the formation 14, the remains of the perforation system 10 may be removed from the wellbore 12 by pulling upwardly on the conveyance string 32. It is anticipated that, in many embodiments, the perforation modules 34, 36, 38 will be substantially or totally consumed in the detonation.
During detonation of the perforation modules 34, 36, 38, directional penetrators are formed by the indentations 54, 54′. It is noted, however, that the detonation sequence of each module 34, 36, 38, begins at the top end proximate to the central rod 42 and proceeds simultaneously in axially downward and radially outward directions. Each liner indentation 54, 54′, when acted upon by the advancing detonation wave, forms a robust EFP, which is particularly well suited for making large and shallow perforation holes in sandy or soft formations. While conventional shaped charges form a relatively fast-moving, low mass jet that accomplishes the perforation, followed by a relatively slow-moving slug that thereafter carries the mass of the remaining charge liner but does not take part in the actual perforation, the EFP of the present disclosure carries essentially all of the mass of the liner 52 forming the indentation 54 or 54′. This means that the liner mass effectively forms part of the penetrator and takes an active part in the perforation, increasing the relative effectiveness thereof. In one embodiment it has been found that the perforations that result from indentations 54 or 54′ having hexagonal perimeters very closely approximate those created from indentations having circular perimeters.
Alternative to indentations having hexagonal perimeters, other perimeter shapes may be selected, most commonly polygonal shapes, such that the perimeters may be adjoined in a linearly contiguous fashion. For example, the indentations may be configured to have triangular, square, or octagonal perimeters.
It will be understood by those in the art that each perimeter shape will impart some effect on the configuration of the cavity formed by an indentation, and therefore of the penetrator that will be formed from collapse of the cavity as a result of detonation. Factors such as the fabrication method, and capabilities and limitations thereof, of the liner wherein the indentations are formed, and the material of which the liner is composed, will desirably be taken into account when selecting the perimeter shape and associated packing parameters. For example, triangular and square perimeter indentations may, because of their shape, not collapse as readily during detonation as do hexagonal perimeter indentations in a perforation module wherein all materials and detonation factors are the same. However, modification of such factors may, in some embodiments, offset such disadvantages or even turn such a tendency into an advantage.
Turning now to
Referring now to
Referring now to
Upon the firing of the perforating modules 34, 36, and 38, perforations will be formed in the casing and cement in a manner as generally shown in
In summary of the foregoing description, those skilled in the art will appreciate that the design of the perforation system 10 thus provides a number of advantages over conventional perforation systems. Included among these, first, is the fact that the linearly contiguous packing of the indentations combined with the unitary body of high explosive produces a greater number of perforating penetrators over a given axial length of a module 40 and reduced amount of “dead space,” as compared with conventional perforation systems using shaped charges and indentations that are physically separated and/or have circular perimeters. The greater number of penetrators results in a desirably greater density in the post-detonation perforation shot pattern. Second, the disclosure provides for a substantial reduction in debris formed during the perforation operation. And third, the perforation module 40 may be created or manufactured and customized relatively easily, without the need for time-consuming placement and orientation of individual shaped charges, as with conventional systems.
From the above, it should be appreciated that what has been described includes, in part, a method for perforating a subterranean formation. The method may include lowering a plurality of perforating modules in a wellbore and detonating the plurality of perforating modules in the wellbore. Each of the plurality of perforating modules may include a central rod having an exterior load bearing portion, and an interior detonation portion including a first explosive, a second explosive at least partially surrounding the central rod, and a liner surrounding the second explosive, wherein the liner has a plurality of shallow concave surface indentations. The central rod further may include an axial passage. In one arrangement, a conducting wire may be disposed within the axial passage. In another embodiment, the axial passage may receive a hydraulic fluid. In embodiments, the plurality of shallow concave surface indentations may each have a cavity. The cavity may be defined by a diameter and a depth. In one arrangement, the diameter to depth ratio is approximately not less than two to one. In another embodiment, the diameter to depth ratio is not less than six to one. In still other embodiments, the depth is no greater than a thickness of the liner.
In arrangements, the plurality of shallow concave surface indentations may each have a cavity. The method may include forming a plurality of perforations in a region adjacent to the perforating modules, wherein the plurality of perforations extend substantially through a cement layer into a formation a distance from a cement face that is no greater than a diameter of the cavity. In some applications, the distance is no greater than one-half of the diameter of the cavity. Also, the method may include at least partially lining the plurality of perforations in the cement layer with a liner material.
In embodiments, the exterior load bearing portion may comprise a frangible material. Also, each of the plurality of shallow concave surface indentations may be linearly contiguous with at least another of the plurality of shallow concave surface indentations. In some arrangements, the shallow concave surface indentations may be arcuate.
From the above, it should be appreciated that what has been described also includes, in part, a method for perforating a subterranean formation. The method may include lowering a perforating module into a wellbore having a casing incased in cement, the perforating module having at least one explosively formed penetrator forming charge and liner; positioning the plurality of perforating modules in the wellbore and adjacent to a substantially unconsolidated formation; perforating the casing and cement; and perforating the formation to a distance no greater than a diameter of the at least one explosively formed penetrator forming charge and liner, wherein the distance measured from a boundary between the cement and the formation.
The foregoing description is directed to particular embodiments of the present disclosure for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible without departing from the scope of the disclosure. Thus, it is intended that the following claims be interpreted to embrace all such modifications and changes.
Claims
1. A method for perforating a subterranean formation, comprising:
- lowering a plurality of perforating modules in a wellbore, wherein each of the plurality of perforating modules comprises: a central rod, wherein the central rod comprises: an exterior load bearing portion, and an interior detonation portion including a first explosive, a second explosive at least partially surrounding the central rod, and a liner surrounding the second explosive, wherein the liner has a plurality of shallow concave surface indentations; and
- detonating the plurality of perforating modules in the wellbore.
2. The method of claim 1, wherein the central rod further comprises:
- an axial passage.
3. The method of claim 2, wherein the central rod further comprises:
- a conducting wire disposed within the axial passage.
4. The method of claim 2, wherein the axial passage is adapted to receive a hydraulic fluid.
5. The method of claim 1, wherein the plurality of shallow concave surface indentations each have a cavity, the cavity being defined by a diameter and a depth, and wherein the diameter to depth ratio is approximately not less than two to one.
6. The method of claim 5, wherein the diameter to depth ratio is not less than six to one.
7. The method of claim 1 wherein the plurality of shallow concave surface indentations each have a cavity, the cavity being defined by a depth, wherein the depth is no greater than a thickness of the liner.
8. The method of claim 1, wherein the plurality of shallow concave surface indentations each have a cavity, and further comprising: forming a plurality of perforations in a region adjacent to the perforating modules, wherein the plurality of perforations extend substantially through a cement layer into a formation a distance from a cement face that is no greater than a diameter of the cavity.
9. The method of claim 8, wherein the distance is no greater than one-half of the diameter of the cavity.
10. The method of claim 8, further comprising at least partially lining the plurality of perforations in the cement layer with a liner material.
11. The method of claim 1, wherein the exterior load bearing portion comprises a frangible material.
12. The method of claim 1, wherein each of the plurality of shallow concave surface indentations is linearly contiguous with at least another of the plurality of shallow concave surface indentations.
13. The method of claim 1, wherein the shallow concave surface indentations are arcuate.
14. A method for perforating a subterranean formation, comprising:
- lowering a plurality of perforating modules in a wellbore, wherein at least one of the plurality of perforating modules is separated from another of the plurality of perforating modules by a spacer, and wherein each of the plurality of perforating modules comprises: a central rod, wherein the central rod comprises: an exterior load bearing portion comprised of a frangible material, an axial passage adapted to receive hydraulic fluid disposed along the length of the interior load bearing portion, a wire disposed within the axial passage, and an interior detonation portion including a first explosive, a second explosive adapted to at least partially surround the central rod, a liner disposed to surround the second explosive, wherein the liner is made of a non-explosive material, and wherein the liner has a plurality of shallow concave arcuate surface indentations, wherein each of the plurality of shallow concave arcuate surface indentation is polygonal, wherein each of the plurality of shallow concave arcuate surface indentations is configured to face substantially perpendicular to the longitudinal axis of the wellbore, and wherein each of the plurality of concave arcuate surface is linearly contiguous with at least another of the plurality of shallow concave arcuate surface indentations, and a cover disposed about the liner; and
- detonating the plurality of perforating modules in the wellbore.
15. A method for perforating a subterranean formation, comprising:
- lowering a perforating module into a wellbore having a casing incased in cement, the perforating module having at least one explosively formed penetrator forming charge and liner;
- positioning the plurality of perforating modules in the wellbore and adjacent to a substantially unconsolidated formation;
- perforating the casing and cement; and
- perforating the formation to a distance no greater than a diameter of the at least one explosively formed penetrator forming charge and liner, wherein the distance measured from a boundary between the cement and the formation.
Type: Application
Filed: Aug 14, 2009
Publication Date: Jan 7, 2010
Applicant: Owen Oil Tools LP (Houston, TX)
Inventors: Dan W. Pratt (Benbrook, TX), Manmohan Singh Chawla (University Park, MD)
Application Number: 12/541,827
International Classification: E21B 43/117 (20060101);