This invention relates generally to downhole tools for use in wellbores and methods of drilling such apparatus out of wellbores, and more specifically, to such tools having drillable components made at least partially of composite or non-metallic materials, such as engineering grade plastics, composites, and resins.  This invention relates particularly to improvements in preventing undesired extrusion of packer seal elements between segmented non-metallic packer element shoes, alternatively referred to as back-up shoes, back-up rings, retaining shoes, packer shoes, or retaining rings, used to provide support to expandable packer elements used in drillable, essentially nonmetallic packer and bridge plug type tools.  This invention is especially suitable for use with such segmented non-metallic packer element retaining shoes used in extreme temperature and differential pressure environments which tend to make expandable packer element seals more prone to extrusion, related damage, and possibly failure. 
In the drilling or reworking of oil wells, a great variety of downhole tools are used.  For example, but not by way of limitation, it is often desirable to seal tubing or other pipe in the casing of the well, such as when it is desired to pump cement or other slurry down the tubing and force the cement or slurry around the annulus of the tubing or out into a formation.  It then becomes necessary to seal the tubing with respect to the well casing and to prevent the fluid pressure of the slurry from lifting the tubing out of the well or for otherwise isolating specific zones in a well.  Downhole tools referred to as packers and bridge plugs are designed for these general purposes and are well known in the art of producing oil and gas. 
When it is desired to remove many of these downhole tools from a wellbore, it is frequently simpler and less expensive to mill or drill them out rather than to implement a complex retrieving operation.  In milling, a milling cutter is used to grind the packer or plug, for example, or at least the outer components thereof, out of the wellbore.  Milling is a relatively slow process, but milling with conventional tubular strings can be used to remove packers or bridge plugs having relative hard components such as erosion-resistant hard steel.  One such packer is disclosed in U.S. Pat.  No.  4,151,875 to Sullaway, assigned to the assignee of the present invention and sold under the trademark EZ Disposal(copyright) packer. 
In drilling, a drill bit is used to cut and grind up the components of the downhole tool to remove it from the wellbore.  This is a much faster operation than milling, but requires the tool to be made out of materials which can be accommodated by the drill bit. 
Typically, soft and medium hardness cast iron are used on the pressure bearing components, along with some brass and aluminum items.  Packers of this type include the Halliburton EZ Drill(copyright) and EZ Drill(copyright) SV squeeze packers. 
The EZ Drill(copyright) SV squeeze packer, for example, includes a lock ring housing, upper slip wedge, lower slip wedge, and lower slip support made of soft cast iron.  These components are mounted on a mandrel made of medium hardness cast iron.  The EZ Drill(copyright) bridge plug is also similar, except that it does not provide for fluid flow therethrough. 
All of the above-mentioned packers are disclosed in Halliburton Servicesxe2x80x94Sales and Service Catalog No.  43, pages 2561-2562, and the bridge plug is disclosed in the same catalog on pages 2556-2557. 
The EZ Drill(copyright) packer and bridge plug and the EZ Drill(copyright) SV packer are designed for fast removal from the wellbore by either rotary or cable tool drilling methods.  Many of the components in these drillable packing devices are locked together to prevent their spinning while being drilled, and the harder slips are grooved so that they will be broken up in small pieces.  Typically, standard xe2x80x9ctri-conexe2x80x9d rotary drill bits are used which are rotated at speeds of about 75 to about 120 rpm.  A load of about 5,000 to about 7,000 pounds of weight is applied to the bit for initial drilling and increased as necessary to drill out the remainder of the packer or bridge plug, depending upon its size.  Drill collars may be used as required for weight and bit stabilization. 
Such drillable devices have worked well and provide improved operating performance at relatively high temperatures and pressures.  The packers and bridge plugs mentioned above are designed to withstand pressures of about 10,000 psi (700 kg/cm2) and temperatures of about 425xc2x0 F. (220xc2x0 C.) after being set in the wellbore.  Such pressures and temperatures require using the cast iron components previously discussed. 
However, drilling out cast iron components requires certain techniques.  Ideally, the operator employs variations in rotary speed and bit weight to help break up the metal parts and re-establish bit penetration should bit penetration cease while drilling.  A phenomenon known as xe2x80x9cbit trackingxe2x80x9d can occur, wherein the drill bit stays on one path and no longer cuts into the downhole tool.  When this happens, it is necessary to pick up the bit above the drilling surface and rapidly recontact the bit with the packer or bridge plug and apply weight while continuing rotation.  This aids in breaking up the established bit pattern and helps to re-establish bit penetration.  If this procedure is used, there are rarely problems.  However, operators may not apply these techniques or even recognize when bit tracking has occurred.  The result is that drilling times are greatly increased because the bit merely wears against the surface of the downhole tool rather than cutting into it to break it up. 
In order to overcome the above long-standing problems, the assignee of the present invention introduced to the industry a line of drillable packers and bridge plugs currently marketed by the assignee under the trademark FAS DRILL(copyright).  The FAS DRILL(copyright) line of tools has a majority of the components made of non-metallic engineering grade plastics to greatly improve the drillability of such downhole tools.  The FAS DRILL(copyright) line of tools has been very successful and a number of U.S. patents have been issued to the assignee of the present invention, including U.S. Pat.  No.  5,271,468 to Streich et al.,  U.S. Pat.  No.  5,224,540 to Streich et al.,  and U.S. Pat.  No.  5,390,737 to Jacobi et al, all of which are incorporated herein by reference. 
Notwithstanding the success of the FAS DRILL(copyright) line of drillable downhole packers and bridge plugs, the assignee of the present invention discovered that certain metallic components still used within the FAS DRILL(copyright) line of packers and bridge plugs at the time of issuance of the above patents were preventing even quicker drill-out times under certain conditions or when using certain equipment.  Exemplary situations include milling with conventional jointed tubulars and in conditions in which normal bit weight or bit speed could not be obtained.  Other exemplary situations include drilling or milling with non-conventional drilling techniques such as milling or drilling with relatively flexible coiled tubing. 
When milling or drilling with coiled tubing, which does not provide a significant amount of weight on the tool being used, even components made of relatively soft steel, or other metals considered to be low strength, create problems and increase the amount of time required to mill out or drill out a downhole tool, including such tools as the assignee""s FAS DRILL(copyright) line of drillable non-metallic downhole tools. 
Furthermore, packer shoes and optional back-up rings made of a metallic material are employed not so much as a first choice but due to the metallic shoes and back-up rings being able to withstand the temperatures and pressures typically encountered by a downhole tool deployed in a borehole. 
To address the preceding shortcomings, the assignee hereof filed a U.S. patent application on May 5, 1995, Ser.  No.  08/442,448, which issued on May 30, 1996, as U.S. Pat.  No.  5,540,279 (the ""279 patent), describing and claiming an improved downhole tool apparatus preferably utilizing essentially all non-metallic materials such as engineering grade plastics, resins, or composites.  The ""279 patent describes a wellbore packing-type apparatus making use of essentially only non-metallic components in the downhole tool apparatus for increasing the efficiency of alternative drilling and milling techniques in addition to conventional drilling and milling techniques and further provides a segmented non-metallic back-up ring in lieu of a conventional metallic packer shoe having a metallic supporting ring.  The tool discussed in the ""279 patent preferably employs the general geometric configuration of previously known drillable non-metallic packers and bridge plugs such as those disclosed in the aforementioned U.S. Pat.  Nos.  5,271,468, 5,224,540, and 5,390,737, while replacing essentially all of the few remaining metal components of the tools disclosed in the aforementioned patents with non-metallic materials which can still withstand the pressures and temperatures found in many wellbore applications.  In the ""279 patent, the apparatus also includes specific design changes to accommodate the advantages of using essentially only plastic and composite materials and to allow for the reduced strengths thereof compared to metal components.  Additionally, the ""279 embodiment comprises a center mandrel and slip means disposed on the mandrel for grippingly engaging the wellbore when in a set position, a packing means disposed on the mandrel for sealingly engaging the wellbore when in a set position, the slip means comprising a slip wedge positioned around the center mandrel, a plurality of slip segments disposed in an initial position around the mandrel and adjacent to the slip wedge, and retaining means for holding the slip segments in an initial position.  The slip segments expand radially outwardly upon being set so as to grippingly engage the wellbore.  Hardened inserts can be molded, or otherwise installed into the slips, and can be made of, by way of example, a ceramic material. 
In the preferred embodiment of the ""279 patent, the slip means includes a slip wedge installed on the mandrel and the slip segments, whether retained by a retaining band or whether retained by an integral ring portion, have co-acting planar, or flat portions, which provided a superior sliding bearing surface especially when the slip means are made of a non-metallic material such as engineering-grade plastics, resins, phenolics, or composites. 
Furthermore, in the ""279 patent, prior art packer element shoes and back-up rings, such as those referred to as elements 37, 38, 44, and 45 in the U.S. Pat.  No.  5,271,468 patent, were replaced by a non-metallic packer shoe having a multitude of co-acting non-metallic segments and at least one retaining band, and preferably two non-metallic bands, for holding the shoe segments in place after initial assembly and during the running of the tool into the wellbore and prior to the setting of the associated packer element within the wellbore. 
Notwithstanding the success of the invention described in the ""279 patent, in that tools made in accordance thereto are able to withstand the stresses induced by relatively high differential pressures and high temperatures found within wellbore environments, the assignee of the present invention discovered that when using packer-type tools in high temperature environments, such as temperatures, for example, exceeding 250xc2x0 F., there was a possibility for the non-metallic segmented packer element back-up shoes, also referred to as back-up rings, to allow the packer element to extrude through gaps that are designed to form between the back-up ring segments upon the segments being forced radially outward toward the wellbore surface when the packer element was activated.  Upon certain conditions, the larger O.D. packer elements, and smaller O.D. packer elements upon being subjected to elevated pressures and temperatures, were subject to being extruded through these gaps thereby possibly damaging the packer element and jeopardizing the integrity of the seal between the wellbore and the packer elements. 
To address the issue of unwanted extrusion, the assignee of the present invention filed a patent application on Mar. 29, 1996, which issued as U.S. Pat.  No.  5,701,959 (the ""959 patent) on Dec. 30, 1997, which is incorporated herein by reference.  The ""959 invention, like the ""279 invention, includes a non-metallic shoe having a multitude of co-acting non-metallic segments and at least one retaining band, and preferably two retaining bands for holding the shoe segments in place after initial assembly and during the running of the tool into the wellbore and prior to the sealing of the associated packer element within the wellbore.  The invention described in the ""959 patent provides a disk to act as a gap-spanning, structural member.  The shoe segments described in the ""959 patent include disk pockets on an inner surface thereof.  Each disk pocket is centered over the gap that it is to bridge, so that a pocket for a single disk comprises two half pockets located on adjacent shoe segments.  The disk in the ""959 patent was designed to span the gap between adjacent segments that increases in size when the packer element is set in the wellbore. 
Although the inventions described in the ""959 and ""279 patents work well for their intended purpose, there is a further need for an easily drillable downhole packer-type tool apparatus preferably being made at least partly, if not essentially entirely, of nonmetallic, such as, but not limited to, composite components, and which include expandable packer elements to be partially retained by non-metallic segmented packer element shoes, or retaining rings that prohibit, or at least significantly reduce, unwanted extrusion of packer elements between gaps of such segmented shoes or segmented rings.  While the invention described in the ""279 patent works well in many cases, there is still a need for a retaining shoe that will prohibit, or at least limit, unwanted extrusion of the packer element in high pressure, high temperature wells of up to 350xc2x0 F. and 10,000 psi. 
The present invention provides a downhole packer apparatus for preventing the extrusion of a packer element assembly installed about a packer mandrel.  The packer mandrel has a longitudinal central axis and a slip means disposed on the packer mandrel for grippingly engaging a wellbore, and preferably a casing in the wellbore, when the packer apparatus is moved from an unset to a set position.  A packer element assembly is disposed about the packer mandrel and includes at least one packer element to be axially retained about the packer mandrel.  The invention also includes at least one packer element assembly retaining shoe disposed about the packer mandrel for axially retaining the packer element assembly and for preventing extrusion of the packer element assembly when the packer apparatus is set into position.  The retaining shoe includes an inner shoe and an outer shoe.  The inner shoe is comprised of a plurality of inner shoe segments.  Adjacent ones of the inner shoe segments have gaps therebetween which may be zero when initially installed but which will expand from the initial installed position, wherein the gaps may be zero or slightly greater than zero, to a greater width when the packer apparatus is set into position, thus moving the inner shoe to an expanded position.  The inner shoe may comprise a generally cylindrical body portion which may engage the packer mandrel when the packer apparatus is in its unset position, and a fin sloping radially outwardly from the body portion.  Each inner shoe segment thus comprises a body portion having a fin portion sloping radially outwardly therefrom. 
The outer shoe of the retaining shoe is comprised of a plurality of outer shoe segments.  Adjacent ones of the outer shoe segments will spread apart so that the width of a gap therebetween will expand as the retaining shoe moves from its initial position, wherein the outer shoe segments and the wellbore define a space therebetween, to an expanded position, wherein the retaining shoe engages the wellbore.  The expanded position of the retaining shoe corresponds to the set position of the packer apparatus in the wellbore.  In the expanded position of the retaining shoe, the retaining shoe engages the wellbore and prevents, or at least limits, extrusion of the packer element assembly.  Wellbore is understood to mean either a wellbore in an openhole completion or a casing disposed in a wellbore in a cased completion, unless the context indicates otherwise. 
Additional objects and advantages of the invention will become apparent as the following detailed description of the preferred embodiment is read in conjunction with the drawings which illustrate the preferred embodiment of the present invention.