Slip Element and Assembly for Oilfield Tubular Plug

Abstract

A slip assembly for anchoring a sealing device, such as a bridge plug, frac plug, or packer, in an oilfield tubular. The slip assembly includes a polymeric slip cone and slip stop disposed along a mandrel. Plural wedge shaped slip elements, which each have plural teeth for selectively engaging the interior circumference of the tubular, are pressure molded of powered iron that is sintered. The plural slip elements are spaced apart from each other such that the sum length of the spaces between them along the interior circumference of the tubular accounts for greater than thirty percent of the circumference. The teeth of the plural slip elements are driven against the interior circumference of the tubular when the slip cone and the slip stop are driven toward one another along the mandrel by a setting tool.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to plug tools for petroleum well tubulars. More particularly, the present invention relates to pressure sealing bridge plugs, ‘frac’ plugs and packers that employ all polymeric components, except for the use of sintered metal slips that are spaced apart to facilitate setting and removal by drilling after deployment.

Description of the Related Art

Oil and gas well construction begins with a wellbore drilled into the ground to a predetermined depth. The wellbore is lined with a steel well casing, which is commonly cemented in place within the wellbore. Before the well is placed into production, the casing is perforated at one or more depths to enable well fluids to flow from the formation into the well casing. Various tools may be run down the casing to develop the well and to commence production of hydrocarbon minerals, and to maintain the well over the years. Depending on the petroleum fluid bearing formation into which the well is drilled, various sequences of tools may be used. For example, in the case of well that has well fluids dispersed into a porous formation, hydraulic fracturing may be employed to facilitate the migration of well fluids into the well casing.

A common requirement in well development and servicing is the need to seal the casing, or other petroleum tubular, against pressure to control movement of fluids between two sides of a particular location in the tubular. For example, in the case of hydraulic fracturing operations, it is necessary to plug the casing below the perforations into which fracturing fluid is pumped in order to assert the requisite pressure needed to fracture the formation. This type of tool is referred to as a “frac” plug. More generically, the term “bridge plug” may be used. Another pertinent term of art is the “packer”, which is a specialized plug used in an annulus between tubulars. Generally, these may simply be referred to as “plugs”. Hydraulic fracturing is just one example, and those skilled in the art are familiar with numerous applications for the use of pressure sealing plugs in oil and gas well operations.

A plug is typically lowered into the tubular using a line, such as a wire line, and is then set into place using a setting tool. There are various setting tools available, but a common mode of operation is a setting tool that engages the plug, and then applies downward force from above the plug while pulling upwardly on a central mandrel of the plug. This action generates high compressive forces within the plug, and this force is used to compress a sealing member of the plug against the interior of the tubular. Such plugs generally incorporate plural slips that bite into the interior circumference of the tubular as the plug is set, and serve to hold the plug in position and also to hold compressive forces on the sealing element to perfect the seal while the pressure operations are undertaken.

Once the pressure operation is completed, the plug generally must be removed so that subsequent operations can be undertaken. A common method to accomplish this is to drill the plug out of the tubular using a well drilling bit such as the common tricone drill bit. The drill bit is lowered to the plug location and then run to grind the plug into small pieces, which may either be pumped to the surface using a liquid, or allowed to fall to the bottom of the wellbore. This brings into issue the choice of plug materials, as well as the physical configuration of the plug. There is also a significant cost factor because plugs are widely employed and are generally single use tools. Simple metal plugs are low cost, but take longer to drill out and are harder on the drill bits. Drill bits are not indestructible and gradually wear, so it is preferable to manufacture plugs out of materials that can be easily drilled, that can be drilled quickly, and that fragment into relatively small pieces. Polymeric materials have been employed, but present challenges with respect to strength and cost factors. Particularly with respect to the requirement that the slips bite into the steel tubular material sufficiently to hold the plug and resist high differential pressures.

Thus it can be appreciated that there is a need in the art for a petroleum tubular pressure plug that enables the pressure sealing requirements, and is readily removable by drilling.

SUMMARY OF THE INVENTION

The need in the art is addressed by the apparatus of the present invention. The present disclosure teaches a slip assembly for anchoring a sealing device that is assembled about a mandrel and set in place in a tubular with a setting tool, where the tubular has an interior circumference. The slip assembly includes a slip cone fabricated from a polymeric material this is disposed along the mandrel, and a slip stop fabricated from a polymeric material also disposed along the mandrel. The assembly also includes plural wedge shaped slip elements that each have plural teeth disposed on an arcuate surface for selectively engaging the interior circumference of the tubular, which are pressure molded of powered iron that is sintered. The plural slip elements are disposed between the slip cone and the slip stop, and are spaced apart from each other. The teeth of the plural slip elements are driven against the interior circumference of the tubular when the slip cone and the slip stop are driven toward one another along the mandrel by the setting tool. And wherein, the sum length of the spaces between the plural slip elements along the interior circumference of the tubular accounts for greater than thirty percent of the interior circumference length of the tubular.

In a specific embodiment of the foregoing apparatus, the powered iron further contains carbon, and the arcuate surface of the plural slip elements is surface hardened. In a refinement to this embodiment, the plural slip elements are molded from powdered iron comprising carbon in the range of 0.6 to 0.9 percent by weight, and also comprising powered copper in the range from 1.5 to 3.9 percent by weight, and, the pressure molded plural slip elements are heat sintered to a temperature above the melting point of copper. In another refinement, the arcuate surface of the plural slip elements are surface hardened by oxy-gas flame and rapidly cooled to yield a case surface with a Rockwell C-scale hardness value that is greater than fifty-five.

In a specific embodiment of the foregoing apparatus, the sum length of the spaces between the plural slip elements along the interior circumference of the tubular accounts for greater than fifty percent of the interior circumference length of the tubular.

In a specific embodiment, the foregoing apparatus further includes plural pins disposed between the plural slip elements and the slip cone to rigidly fix the plural slip elements to the slip cone, and the plural pins have a shear strength selected to shear under force of the setting tool. In another specific embodiment, the foregoing apparatus further includes plural pins disposed between the plural slip elements and the slip stop to rigidly fix the plural slip elements to the slip stop, and the plural pins have a shear strength selected to shear under force of the setting tool.

In a specific embodiment, the foregoing apparatus further includes plural pins disposed between the plural slip elements and the slip stop and the slip cone to rigidly fix the plural slip elements to the slip stop and the slip cone, and the plural pins have a shear strength selected to shear under force of the setting tool. In a refinement to this embodiment, the plural pins are fabricated from fiber reinforced polymeric material.

In a specific embodiment of the foregoing apparatus, the slip cone and slip stop polymeric materials are fiber reinforced, and the mandrel is fabricated from a fiber reinforced polymeric material.

The present disclosure teaches a slip assembly for anchoring a sealing device that is assembled about a fiber reinforced polymeric mandrel and set in place in a tubular with a setting tool, the tubular having an interior circumference. The slip assembly includes a slip cone fabricated from a fiber reinforced polymeric material, and a slip stop fabricated from a fiber reinforced polymeric material, both disposed along the mandrel. The assembly also includes plural slip elements, each configured in a wedged shape, that each have plural teeth disposed on an arcuate surface thereof for selectively engaging the interior circumference of the tubular. The plural slip elements are pressure molded from powdered iron including carbon in the range of 0.6 to 0.9 percent by weight, and also including powered copper in the range from 1.5 to 3.9 percent by weight. The pressure molded plural slip elements are heat sintered to a temperature above the melting point of copper, and the arcuate surface of the plural slip elements are surface hardened by oxy-gas flame and rapidly cooled to yield a case hardened surface having a Rockwell C-scale hardness value greater than fifty-five. The assembly further includes plural pins, fabricated from a fiber reinforce polymeric material, disposed between the plural slip elements and the slip stop and the slip cone to rigidly fix the plural slip elements to the slip stop and the slip cone. The plural pins have a shear strength selected to shear under force of the setting tool. The plural slip elements are spaced apart from each other such that the sum length of the spaces between the plural slip elements along the interior circumference of the tubular accounts for greater than thirty percent of the interior circumference length of the tubular. In operation, the teeth of the plural slip elements are driven against the interior circumference of the tubular when the slip cone and the slip stop are driven toward one another along the mandrel by the setting tool.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view drawing of a bridge plug according to an illustrative embodiment of the present invention.

FIG. 2 is a partial exploded view of a bridge plug according to an illustrative 15 of the present invention.

FIGS. 3A, 3B, 3C, and 3D are a front view, side view, top view and bottom view drawing, respectively, of a bridge plug slip according to an illustrative embodiment of the present invention.

FIGS. 4A and 4B are a top view and section view drawing, respectively, of a bridge plug slip cone according to an illustrative embodiment of the present invention.

FIGS. 5A and 5B are a top view and section view drawing, respectively, of a bridge plug slip stop according to an illustrative embodiment of the present invention.

FIG. 6 is a side section view drawing of a slip stop assembly in a well casing according to an illustrative embodiment of the present invention.

FIG. 7 is a top section view drawing of a slip stop assembly in a well casing according to an illustrative embodiment of the present invention.

FIG. 8 is a side section view drawing of a deployed slip stop assembly in a well casing according to an illustrative embodiment of the present invention.

FIG. 9 is a top section view drawing of a slip stop assembly in a well casing according to an illustrative embodiment of the present invention.

FIG. 10 is a top section view drawing of a deployed slip stop assembly in a well casing according to an illustrative embodiment of the present invention.

FIG. 11 is a top section view drawing of a deployed slip stop assembly in a well casing according to an illustrative embodiment of the present invention.

FIG. 12 is a top section view drawing of a deployed slip stop assembly in a well casing according to an illustrative embodiment of the present invention.

DESCRIPTION OF THE INVENTION

Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention.

While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope hereof and additional fields in which the present invention would be of significant utility.

In considering the detailed embodiments of the present invention, it will be observed that the present invention resides primarily in combinations of steps to accomplish various methods or components to form various apparatus and systems. Accordingly, the apparatus and system components and method steps have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the disclosures contained herein.

In this disclosure, relational terms such as first and second, top and bottom, upper and lower, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.

The present disclosure teaches an improved slip and slip assembly suitable for use in sealing plugs utilizes in petroleum well drilling, development, and production operations. Such plugs include, but are not limited to, bridge plugs, ‘frac’ plugs, and packers. A common mode of operation is to run the plug down hole, such as into a steel well casing, with a setting tool attached. When a predetermined depth is reached, the plug is set by the setting tool to provide a pressure seal at the predetermined depth. After the pressure operation, such as pumping high-pressure hydraulic fracture fluid into a geologic formation, is completed, the plug needs to be removed to clear the way for subsequent down-hole operations. Most plugs are removed from the well casing, (generically a “tubular”) by drilling.

Well drilling is accomplished with a drill bit. While well drill bits are manufactured from durable components that are capable of cutting rather hard materials, the drill bits do wear out. Drill bits are relatively expensive. There is also a time factor involved with respect to how long it takes to drill out a plug. As would be expected, operators prefer a plug that is readily drilled out with minimal wear and tear on the drill bit, and in minimal operating time. It is further noteworthy that since the plug is a one-time use, disposable item, low cost is a paramount requirement in the plug design. The materials that are used to build the plug directly affect all of these foregoing factors. Iron and steel are strong and inexpensive, but are difficult to drill out. Composite materials range higher in cost, and present strength limitations in the plug design.

A sealing plug comprises several components to achieve its intended function. An elastomeric sealing member is provided, which is compressed against the interior circumference of the tubular to achieve the pressure sealing function. In lay terms, the sealing member is a ‘rubber’ donut disposed on a mandrel. The sealing member is held along the mandrel between a pair of sealing shoes that are shaped to engage and compress the sealing member in an efficient manner. The sealing member is squeezed between the sealing shoes. In addition to the sealing function of the plug, the plug also requires a holding function that fixedly locates the plug during the pressure operation. The forces against the holding function are high, and can reach tens of thousands of pounds. The holding function is achieved using a pair of slip assemblies, one above and one below the sealing member. As the sealing member is compressed, the slips are also driven against the interior circumference of the tubular, so as to ‘bite’ into the steel and hold the plug in place, even after the setting tool is removed.

The slip assemblies in a plug engage the interior circumference of the tubular is a ratchet-like fashion, meaning that they are directional. The upper slip bites into the tubular at an angle that resists upward movement, and the lower slip bites into the tubular at an angle that resists downward movement. This arrangement facilitates the functions of holding the sealing member in compression, thereby sealably engaging the tubular. The slips are driven into the tubular using an inclined plane arrangement. The slips are generally wedged shape and engage a slip cone, and together, these form the inclined plane. A slip stop is located against each slip assembly opposite of the sealing member, and serve as the anchor points from the setting operation. The bottom slip stop is fixed to the mandrel, and the upper slip stop is moved toward the lower slip stop by the setting tool. Essentially, the setting tool pulls upwardly in the mandrel while pushing downwardly on the upper slip stop, thereby squeezing the plug assembly in compression.

In an illustrative embodiment of this disclosure, the various components disposed along the mandrel, as well as the mandrel itself, are fabricated from polymeric materials, some or which are fiber reinforced. This includes the mandrel, seal shoes, slip cones, slip stops, spacers and certain other components. For example, epoxy or phenolic resins can be used. And, there is a range of other organic plastics that are suitable. Reinforcing fibers may be glass, carbon, aramid, or other suitable non-metallic fibers. All of these materials are relatively easy to drill out after the plug is deployed. However, in the case of the slips, the use of purely polymeric material is challenging because the hardness and compressive strength is generally insufficient to accommodate the compressive forces at the time the plug is set and biting forces into the steel tubulars. There have been a number of prior art attempts to incorporate hardened teeth into polymeric slips, and these are disclosed together with the filing of this disclosure. While a hybrid approach may function adequately, they drive up cost and make such plugs less competitive in the marketplace.

The present disclosure presents new slip designs, as well as slip assembly designs, that effectively trade between strength, cost, and drillability. A slip assembly is generally the combination of plural slips, a slip cone, and a slip stop, which together enable the requisite setting, holding, and drilling aspects of pressure plug utilization. Generally, a single plug as two slip assemblies disposed along its mandrel, although other numbers of slip assemblies can certainly be employed. Powdered metal slip components are employed which present a suitably hard biting surface to engage the interior of the tubular, but which are more easily drilled out due in part to the porous nature of sintered metals. Surface hardening of the biting surface may be employed to ensure the slips can adequately grip the interior of the tubular. Sintered iron slips are particularly cost effective.

In addition to the use of sintered metal slips, the present disclosure also teaches that the spacing between slips may be configured to facilitate drilling after the plug has been deployed. In the prior art, plural slips are assembled together in the fashion of slices of pie, one squarely against the next. If such slips are engaged by a drill bit, such as the familiar tricone drill bit, the slips tend to support one another, forcing the drill to grind through them all, taking time and wearing the drill bit. It should be noted that generally, an operator does not distinguish between bits of debris from a drilled out plug that are pumped upwardly in the drilling liquid or that falls downwardly to the bottom of the well casing. It is more important that the plug is removed quickly and efficiently so that subsequent well operations maybe undertaken. For this to occur, the drill bit needs to effectively grind the plug into suitable small pieces that either get pumped up, or fall down. By providing an adequate spacing between the slips, the drill bit can shift and rotate the slips to more quickly facilitate the crumbling of the plug assembly into debris of suitable size.

Reference is directed to FIG. 1, which is an exploded view drawing of a bridge plug 2 according to an illustrative embodiment of the present invention. Unless otherwise stated, all of the components in FIG. 1 are fabricated from polymeric materials, which may be fiber reinforced. As mentioned hereinbefore, these include epoxy resins, phenolic resins, and other suitable organic plastics. Reinforcing fibers may be glass, carbon, aramid, or other suitable non-metallic fibers. A cylindrical mandrel 4 forms the central core of the plug 2, and includes plural apertures for engaging assembly pins (not shown), which fix the various elements together. All of the other elements (6, 8, 10, 14, 16, 18, 20, 22, and 26) comprise an aperture sized to slide onto the mandrel 4. These elements generally slide together in a surface to surface manner. At the bottom of the mandrel 4 is a lower slip assembly 23, which includes a “mule shoe” bottom slip stop 26, which serves at the end of the plug assembly 2, and which is first inserted into the tubular as the plug 2 is lowered down hole. The lower slip assembly 23 further includes plural slips 24, which are inserted between the bottom slip stop 26 and the bottom slip cone 22 that is also a part of the lower slip assembly 23. This arrangement will be more fully discussed hereinafter.

The next group of components slid onto the mandrel 4 is the seal assembly 17, which includes the sealing member 18 and a pair of seal shoes 16, 20. The seal member 18 is a synthetic rubber that is suitable for sealably engaging the interior circumference of a steel tubular. The pair of seal shoes 16, 20 are conical members that advantageously engage the seal member to affect the compression thereof. Above the top seal shoe 16 is the upper slip assembly 11, which comprises the upper slip cone, plural slips 12, and an upper slip stop 10. Additional stops 6, 8 are employed in this embodiment as spacers to facilitate common parts usage between different size plugs. A single upper slip stop could also have been employed.

Reference is directed to FIG. 2, which is a partial exploded view of a bridge plug according to an illustrative embodiment of the present invention. This view provides further details of the lower slip assembly 24 from FIG. 1. In FIG. 2, the seal assembly 17 is also presented. Note the configuration of the lower slip cone 22 and its relationship with the plural slips 24, which comprises individual slip elements 28. When the slip assembly 23 is assembled together, the slips 28 are supported between the lower slip stop 26 and the lower slip cone 22. As the plug is set, the lower slip stop 26 is driven toward the lower slip cone 22, and this causes the outward radial movement of the slips 28 into the tubular (not shown). It is necessary to retain the slips 28 in place at the time of manufacture, and through deployment into a down hole tubular. In prior designs, one or more grooves are cut into the arcuate exterior face of each slip, which engaged one or more piano-wires wrapped about the circumference of the plug to hold the slips in place. In the present illustrative embodiment, the slips 28 are provided with certain holes into which location pins are inserted, and this will be more fully described hereinafter. In addition, polyolefin bands may be placed over slips on the assembled plug, which are then heat-shrunken, to present a safe and smoothed exterior surface of the plug assembly.

Reference is directed to FIGS. 3A, 3B, 3C, and 3D, which are a front view, side view, top view, and bottom view drawing, respectively, of a bridge plug slip 28 according to an illustrative embodiment of the present invention. The configuration of the slip 28 is generally wedge-shaped, with an arcuate exterior face 38, which corresponds to the interior circumference of the size tubular that the plug is designed to fit. The exterior face 38 also comprises plural teeth 30, which are angled in a sawtooth fashion to engaged the interior surface of a tubular (not shown) in a directional manner, similar to the function of a ratchet. The interior surface 40 of the slip 28 is also formed as an arcuate surface, which is configured to engage the conical shape of the slip cone (not shown). A pin hole 34 is formed through the slip 28, which serves as a location for insertion of a pin (not shown) between the slip 28 and the slip cone (not shown). In addition, a pair of holes 36 are formed into the top of the slip 28 for receiving a pair of pins (not shown) to engage the slip stop (not shown). The arrangement of holes 34, 36 and pins (not shown) enable the attachment of the slips 28 to the slip assembly. A groove 32 is formed vertically along the interior arcuate surface 40 of the slip. The groove 32 engages a ridge on the slip cone (not shown), and serves to both locate the slips 28 with respect to the slip cone (not shown) and also to guide the movement of the slips 28 during the plug setting operation. This arrangement will be more fully described hereinafter.

The material of construction and finishing techniques of the slips in FIG. 3A-D are significant features of the illustrative embodiment. The slip 28 body needs adequate strength to endure the forces applied during the setting operation and the pressure holding operation, yet it is desirable that the slip 28 be readily drillable. In addition, the teeth 30 must have sufficient hardness to bite into the steel tubular. These goals are advantageously achieved through use of sintered metal powder as the material of construction of the slips 28. Sintered metal is comprised of powered metal that is fused into a solid shape through a sintering operation, which yields an object with porous consistency. The porosity results in a material that is more readily drillable as compared to a solid objet made from the same material. There is also a significant cost savings in that the complex shape of the slip 28 is formed through molding operations as opposed to machining operations. Although, some minor finishing operations may still be required with the sintered part. In the illustrative embodiment, the slips 28 are pressure molded to produce a ‘green’ casting, which is then heat sintered to permanently fuse the powdered metal. In addition, a case hardening technique may be applied to the exterior surface 38, particularly including the plural teeth 30, to ensure that the hardness is sufficient to bite into the steel tubular surface.

In an illustrative embodiment, the metal powder used to mold the slips 28 is primarily iron power, but with the addition of copper and carbon. The copper serves to enable the heat sintering operation at temperatures just over the melting point of copper. The carbon improves hardness of the sintered part, and particularly enables a heat-treating operation to case harden the exterior face 38 and teeth 30 of the slips 28. The slips 28 should have a surface hardness of at least fifty-five on the Rockwell-C hardness scale, with the range of RHC 55 to 60 being a suitable engineering specification. Oxy-gas flame hardening with a rapid quench is a suitable approach to surface hardening, however, other case hardening techniques may be applied as well.

In an illustrative embodiment, the specification for the sintered metal is selected from the Metal Powder Industries Federation Standards, particularly MPIF Standard 35 for PM Structural Parts (ISBN No. 978-0-9853397-1-5 (2012)). The Material Designation is FC-0208-65HT, which is a powdered metal comprised of 1.5 to 3.9% copper, 0.6 to 0.9% carbon, with the balance being powdered iron. The parts are pressure molded and heat sintered, per the MPIF guidelines. Note that the minimum carbon content specification of 0.6% places the material into the alloy range for high carbon steel, which is particularly suitable for the case hardening operation applied to the teeth 30 of the slips 28. With adequate hardening techniques, the case may be converted to martensite, a hardened steel, yet the interior of the slips remains a porous sintered material that is more readily drillable, as discussed hereinbefore. Oxy-gas flame or induction hardening can be employed.

Reference is directed to FIGS. 4A and 4B, which are a top view and section view drawing, respectively, of a bridge plug slip cone 14, 22 according to an illustrative embodiment of the present invention. The slip cone 14, 22 illustrated in this drawing figure is suitable to serve as both the upper 14 and lower 22 slip cone, although, the two slip cones may differ to accommodate differences between the upper and lower slip assembly positions. The slip cone 14, 22 body 40 has a aperture 44 formed there through to accommodate the support mandrel (not shown). There is an upper conical portion 42, which serves as an inclined plane to drive the slips (not shown) into the tubular (not shown) when the plug is set. Plural ridges 46 are formed along the conical portion 42, which engage the slips (not shown) and serves to located and guide the slips during the setting operation. The number of ridges 46 corresponds to the number slips in the slip assembly, which in this embodiment is eight, although different numbers of slips may certainly be employed. Plural holes 48 are formed into the conical portion 42 and serve to engage locator pins (not show) between slip cones 14, 22 and the slips (not shown). In this embodiment, the holes 48 are aligned with the ridge 46, however, this correspondence is not required. The aforementioned locator pins (not shown) could be positioned elsewhere on the slip cones 14, 22.

The choice of materials used for the slip cones 14, 22 in FIG. 4A-B is a design choice. As was discussed hereinbefore, polymeric materials, which may be fiber reinforced are suitable choices. As mentioned hereinbefore, these include epoxy resins, phenolic resins, and other suitable organic plastics. Reinforcing fibers may be glass, carbon, aramid, or other suitable non-metallic fibers. In an illustrative embodiment, the slip cones 14, 22 are fabricated from e-glass reinforced phenolic resin that is cavity molded. The -glass fibers range in length from 0.5 to 10 inches. Carbon fiber roving is another option. Glass spheres may be added to the phenolic resin. E-glass is alumino-borosilicate glass with less than 1% alkali oxides, and is a common glass used in fiber reinforced polymers. However, other fiber reinforced plastic (FRP), such as epoxy, vinyl ester, polyester thermosetting plastic, or phenol formaldehyde may be used. Hereinafter, the material used for the fiber reinforced organic plastics will be referred to as “FRP”, which means any suitable fiber reinforced plastic, or suitable substitute, as described herein.

Reference is directed to FIGS. 5A and 5B, which are a top view and section view drawing, respectively, of a bridge plug slip stop 50 according to an illustrative embodiment of the present invention. In practical implementations of the slip assembly according to the illustrative embodiments, the slip stops may take many forms to accommodate other elements and functions of the plug. For example, the bottom slip stop 26 in FIGS. 1 and 2 serves as mule shoe that guides the assembly down hole. The embodiment of FIGS. 5A-B is an exemplar of a generic slips stop 50. The slip stop 50 comprises an FRP body 52 that presents a donut configuration with a center aperture 54 to accommodated the plug mandrel (not shown). The slip stop includes plural holes 56 formed therein to accept locator pins (not shown), which connect the slip stop 50 to the plural slips (not shown). Further explanation of the slip stop arrangements will be more fully developed hereinafter.

Reference is directed to FIG. 6, which is a side section view drawing of a slip assembly in a well casing 60 according to an illustrative embodiment of the present invention. The slip assembly comprises the slip stop 50, the slip cone 14, and plural slips 28. The slip assembly is located on a plug mandrel 4, which is drawn in phantom to aid in clarity. FIG. 6 illustrates the slip assembly as it is provided from the manufacturer and before it is set into the tubular 60. In this drawing, only two opposing slips 28 are presented to aid in clarity of the drawing. Note that the slip cone 14 has plural ridges 46 along its conical portion. Also note that the grooves 32 on the interior of each slip 28 engages the ridge 46, which thereby locates and guides the movement of the slips 28 along the slip cone 14 when the assembly is set (not shown in this figure). The slips 28 are fixed to the slip cone 14 using one pin 64 per slip 28. The pins 64 are fabricated from FRP and are fixed with 5000 psi epoxy into the holes 48 in the slip cone and the holes 34 in the slips 28. This arrangement fixedly attaches the slips 28 to the slip cone 14. In one embodiment, the pins 64 are one quarter inch in diameter.

The slips 28 in FIG. 6 are fixed to the slip stop 50 in a similar manner as they are attached to the slip cone 14. The slips 28 are fixed to the slip stop 50 using two pins 62 per slip 28. The pins 62 are fabricated from FRP and are fixed with 5000 psi epoxy into the holes 56 in the slip stop 50 and the holes 36 in the slips 28. This arrangement fixedly attaches the slips 28 to the slip stop 50. In one embodiment, the pins 62 are one-eighth inch in diameter. All of the pins 62, 64 are selected to have a shear strength that readily fails when the slip assembly is set into the tubular 60. Compressive forces from the setting tool (not shown) drives the slip stop 50 and the slip cone 14 toward one another. This action causes the slips 28 to ride up the conical portion of the slip cone 14 along the ridge 46 and groove 32, which action induces the shear forces on the pins 62, 64.

Reference is directed to FIG. 7, which is a top section view drawing of a slip assembly in a well casing 60 according to an illustrative embodiment of the present invention. FIG. 7 corresponds with FIG. 6 in that FIG. 7 illustrates two of the slips 28 in solid line, and the other six slips 29 are illustrated in phantom. The slip cone 14 is presented, including its central aperture 44 for engaging the mandrel (not shown). The plural ridges 46 on the slip cone 14 are visible, and well as the plural pin holes 48 along the conical portion 42 of the slip cone 14. With respect to the two slips 28 that are presented in solid line, the plural pin holes 56 can be seen as well as the end of the interior groove 32 on the slips 28. Also note the separation between the arcuate exterior surface 38 of the slips 28, and the interior circumference of the tubular 60.

Reference is directed to FIG. 8, which is a side section view drawing of a deployed slip assembly in a well casing 60 according to an illustrative embodiment of the present invention. FIG. 8 corresponds with FIG. 6, where FIG. 8 shows the slip assembly after it has been set into the tubular 60. Note the force arrows 65, which are presented to show the direction the setting tool (not shown) forces act upon the slip cone 14 and slip stop 50. This action drives the two together, which causes the slips 28 to ride the slip cone 14 outwardly and engage the interior circumference of the tubular 60. When the setting operation occurs, the slip cone pins 64 shear, as do the slip stop pins 62. This action separates the components of the slip assembly, which is beneficial for the drilling operation. The slips 28 bite into the tubular 60 is a ratcheting fashion, such that the opposing slip assemblies (only one shown in FIG. 8), bindingly retain pressure on the seal assembly (not shown). Thusly, the slips 28 remain engaged with the tubular 60, and the seal assembly (not shown) remains compressibly sealed against the tubular 60 as well. This condition is maintained until the plug is drilled out of the tubular, or is otherwise removed. Note that a suitable setting tool for a 5.5 inch well casing would drive the forces 65 at 3500 psi. A four-inch stroke length setting tool would be sufficient for the illustrative embodiment plug assembly. The typical setting distance for a two-slip assembly plug ranges from 1.5 to 3.0 inches. The plug of the illustrative embodiment has been tested to exceed 25,000 psi differential pressure.

Reference is directed to FIG. 9 and FIG. 10, which are a top section view drawings of a slip stop assembly in a well casing 60 according to an illustrative embodiment of the present invention. FIG. 9 shows the slips 28 prior to being set in the tubular 60, and FIG. 10 shows the slips 28 after being set into the tubular 60. The slip cone 14 and its conical portion 42 and central aperture 44 are also presented in these drawings. Note that the setting operation increases the spacing between adjacent slips 28.

Reference is directed to FIG. 11, which is a top section view drawing of a deployed slip stop assembly in a well casing 60 according to an illustrative embodiment of the present invention. The illustrative embodiment slip assembly comprises eight slips 28. Note that the slips are not tightly spaces, as for example slices of pie, but rather are sized to provide spacing there between. This is a beneficial feature of the illustrative embodiment. Note in FIG. 11, that two of the slips have been rotated 68, 70. The clearance provided between adjacent slips enables such rotation. And, such rotation allows a drill bit to more easily and freely grind the plug assembly into small enough debris to either be pump out or to fall to the bottom of the tubular. It has been determined that a spacing which provides greater than 29% spacing along the interior circumference of the tubular 60 is sufficient to enable free rotation of the slips 28 during the drilling operation. A suitable design criteria is to size the slips and spaces such that the sum length of the spaces between the plural slip elements 28 along the interior circumference of the tubular 60 accounts for thirty percent, and up to fifty percent, of the interior circumference length of the tubular 60. With this spacing arrangement, the drill bit will be considerably more efficient at drilling out a set plug assembly.

Reference is directed to FIG. 12, which is a top section view drawing of a deployed slip assembly in a well casing 70 according to an illustrative embodiment of the present invention. This embodiment illustrates an example where ten slips 76 are employed. The drawing illustrates three of the slips 78 as having been rotated by the drilling operation. The drawing also illustrates a slip cone 72 with a conical portion 72, and a central aperture 74. The number of slips, width, and spacing is a design choice, and can range from three to many.

Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications and embodiments within the scope thereof.

It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention.

Claims

1. A slip assembly for anchoring a sealing device that is assembled about a mandrel and set in place in a tubular with a setting tool, the tubular having an interior circumference, the slip assembly comprising:

a slip cone fabricated from a polymeric material and disposed along said mandrel;

a slip stop fabricated from a polymeric material and disposed along said mandrel;

plural slip elements, each configured in a wedged shape, and having plural teeth disposed upon an arcuate surface thereof for selectively engaging the interior circumference of the tubular, and wherein said plural slip elements are pressure molded of powered iron that is sintered, and wherein

said plural slip elements are disposed between said slip cone and said slip stop, and spaced apart from each other, and wherein

said teeth of said plural slip elements are driven against the interior circumference of the tubular when said slip cone and said slip stop are driven toward one another along said mandrel by the setting tool, and wherein

the sum length of the spaces between said plural slip elements along the interior circumference of the tubular accounts for greater than thirty percent of the interior circumference length of the tubular.

2. The slip assembly of claim 1, and wherein:

said powered iron further contains carbon, and wherein

said arcuate surface of said plural slip elements is surface hardened.

3. The slip assembly of claim 2, and wherein:

said plural slip elements are molded from powdered iron comprising carbon in the range of 0.6 to 0.9 percent by weight, and also comprising powered copper in the range from 1.5 to 3.9 percent by weight, and wherein

said pressure molded plural slip elements are heat sintered to a temperature above the melting point of copper.

4. The slip assembly of claim 2, and wherein:

said arcuate surface of said plural slip elements are surface hardened by oxy-gas flame and rapidly cooled to yield a case surface having a Rockwell C-scale hardness value greater than fifty-five.

5. The slip assembly of claim 1, and wherein:

said sum length of the spaces between said plural slip elements along the interior circumference of the tubular accounts for greater than fifty percent of the interior circumference length of the tubular.

6. The slip assembly of claim 1, further comprising:

plural pins disposed between said plural slip elements and said slip cone to rigidly fix said plural slip elements to said slip cone, and wherein

said plural pins have a shear strength selected to shear under force of the setting tool.

7. The slip assembly of claim 1, further comprising:

plural pins disposed between said plural slip elements and said slip stop to rigidly fix said plural slip elements to said slip stop, and wherein

said plural pins have a shear strength selected to shear under force of the setting tool.

8. The slip assembly of claim 1, further comprising:

plural pins disposed between said plural slip elements and said slip stop and said slip cone to rigidly fix said plural slip elements to said slip stop and said slip cone, and wherein

said plural pins have a shear strength selected to shear under force of the setting tool.

9. The slip assembly of claim 8, and wherein:

said plural pins are fabricated from fiber reinforced polymeric material.

10. The slip assembly of claim 1, and wherein:

said slip cone and slip stop polymeric materials are fiber reinforced, and wherein

the mandrel is fabricated from a fiber reinforced polymeric material.

11. A slip assembly for anchoring a sealing device that is assembled about a fiber reinforced polymeric mandrel and set in place in a tubular with a setting tool, the tubular having an interior circumference, the slip assembly comprising:

a slip cone fabricated from a fiber reinforced polymeric material and disposed along the mandrel;

a slip stop fabricated from a fiber reinforced polymeric material and disposed along the mandrel;

plural slip elements, each configured in a wedged shape, and having plural teeth disposed upon an arcuate surface thereof for selectively engaging the interior circumference of the tubular, and wherein said plural slip elements are pressure molded from powdered iron comprising carbon in the range of 0.6 to 0.9 percent by weight, and also comprising powered copper in the range from 1.5 to 3.9 percent by weight, and wherein

said pressure molded plural slip elements are heat sintered to a temperature above the melting point of copper, and wherein

said arcuate surface of said plural slip elements are surface hardened by oxy-gas flame and rapidly cooled to yield a case surface having a Rockwell C-scale hardness value greater than fifty-five;

plural pins, fabricated from a fiber reinforce polymeric material, disposed between said plural slip elements and said slip stop and said slip cone to rigidly fix said plural slip elements between said slip stop and said slip cone, and wherein

said plural pins have a shear strength selected to shear under force of the setting tool, and wherein

said plural slip elements are spaced apart from each other such that the sum length of the spaces between said plural slip elements along the interior circumference of the tubular accounts for greater than thirty percent of the interior circumference length of the tubular, and wherein

said teeth of said plural slip elements are driven against the interior circumference of the tubular when said slip cone and said slip stop are driven toward one another along said mandrel by the setting tool.