PATENT DOCUMENT

Abstract:
The method of the present invention deploys controlled electrolytic material, CEM, into the existing perforations to seal them and then using a bottom hole assembly, BHA, that isolates a portion of the wellbore to deliver a material that removes the CEM at a predetermined rate so that the BHA can be used to refracture the recently opened perforation. Additional new perforations can be made and fractured during the process.

Full Description:
FIELD OF THE INVENTION 
     The field of this invention is refracturing and more particularly in wells originally fractured using bridge plugs and perforating guns sequentially deployed in a direction toward the surface where subsequent conditions after production dictate additional fracturing to be appropriate. 
     BACKGROUND OF THE INVENTION 
     In one known fracturing technique a perforating gun is run in on wireline above a composite bridge plug, typically in a long horizontal run in a cased wellbore. The plug is set and released from the bottom hole assembly and the perforating gun is moved far enough away from the plug to avoid damaging the plug before the gun is fired. The wireline is then retrieved and the well is stimulated. This process is repeated in a direction toward the surface with additional plugs and perforation services until the interval is finished having the perforations in place and stimulation treatments performed. Some time after stimulation, the composite bridge plugs are milled or drilled out and the production completion is installed and production starts. 
     At some point the production rate drops off or undesirable sand or water or other materials are produced and the decision is made that additional stimulation treatments are needed. The problem is that the entire payzone now has multiple sets of perforations, so isolation of one or more perforated sections without a significant decrease in flow area is difficult. 
     In the past ball sealers have been used that are pumped into a wellbore to isolate perforations that are producing undesirable materials. These barriers were meant to stay in position while production continued through other perforations. Ball sealers are described in U.S. Pat. Nos. 5,253,709; 5,309,995; 4,505,334 and 4,881,599. Also related to such sealing techniques are U.S. Pat. Nos. 7,380,600; 6,380,138; 5,990,051; 4,716,964; 7,775,278; 7,565,929; and 4,428,424 (plugging with cement that necessitates a refracture). US Publication 2010/0186297 uses fluids triggered to plug perforations with a magnetic field. 
     Coiled tubing run bottom hole assemblies that can isolate a portion of the wellbore for fluid delivery downhole are described in US Publication 20100126725. 
     More recently, controlled electrolytic materials have been described in US Publication 2011/0136707 and related applications filed the same day. The related applications are incorporated by reference herein as though fully set forth. The listed published application specification and drawings are literally included in this specification to provide an understanding of the materials considered to be encompassed by the term “controlled electrolytic materials.” 
     Lightweight, high-strength metallic materials are disclosed that may be used in a wide variety of applications and application environments, including use in various wellbore environments to make various selectably and controllably disposable or degradable lightweight, high-strength downhole tools or other downhole components, as well as many other applications for use in both durable and disposable or degradable articles. These lightweight, high-strength and selectably and controllably degradable materials include fully-dense, sintered powder compacts formed from coated powder materials that include various lightweight particle cores and core materials having various single layer and multilayer nanoscale coatings. These powder compacts are made from coated metallic powders that include various electrochemically-active (e.g., having relatively higher standard oxidation potentials) lightweight, high-strength particle cores and core materials, such as electrochemically active metals, that are dispersed within a cellular nanomatrix formed from the various nanoscale metallic coating layers of metallic coating materials, and are particularly useful in wellbore applications. These powder compacts provide a unique and advantageous combination of mechanical strength properties, such as compression and shear strength, low density and selectable and controllable corrosion properties, particularly rapid and controlled dissolution in various wellbore fluids. For example, the particle core and coating layers of these powders may be selected to provide sintered powder compacts suitable for use as high strength engineered materials having a compressive strength and shear strength comparable to various other engineered materials, including carbon, stainless and alloy steels, but which also have a low density comparable to various polymers, elastomers, low-density porous ceramics and composite materials. As yet another example, these powders and powder compact materials may be configured to provide a selectable and controllable degradation or disposal in response to a change in an environmental condition, such as a transition from a very low dissolution rate to a very rapid dissolution rate in response to a change in a property or condition of a wellbore proximate an article formed from the compact, including a property change in a wellbore fluid that is in contact with the powder compact. The selectable and controllable degradation or disposal characteristics described also allow the dimensional stability and strength of articles, such as wellbore tools or other components, made from these materials to be maintained until they are no longer needed, at which time a predetermined environmental condition, such as a wellbore condition, including wellbore fluid temperature, pressure or pH value, may be changed to promote their removal by rapid dissolution. These coated powder materials and powder compacts and engineered materials formed from them, as well as methods of making them, are described further below. 
     Referring to  FIGS. 1-5 , a metallic powder  10  includes a plurality of metallic, coated powder particles  12 . Powder particles  12  may be formed to provide a powder  10 , including free-flowing powder, that may be poured or otherwise disposed in all manner of forms or molds (not shown) having all manner of shapes and sizes and that may be used to fashion precursor powder compacts  100  ( FIG. 16 ) and powder compacts  200  ( FIGS. 10-15 ), as described herein, that may be used as, or for use in manufacturing, various articles of manufacture, including various wellbore tools and components. 
     Each of the metallic, coated powder particles  12  of powder  10  includes a particle core  14  and a metallic coating layer  16  disposed on the particle core  14 . The particle core  14  includes a core material  18 . The core material  18  may include any suitable material for forming the particle core  14  that provides powder particle  12  that can be sintered to form a lightweight, high-strength powder compact  200  having selectable and controllable dissolution characteristics. Suitable core materials include electrochemically active metals having a standard oxidation potential greater than or equal to that of Zn, including as Mg, Al, Mn or Zn or a combination thereof. These electrochemically active metals are very reactive with a number of common wellbore fluids, including any number of ionic fluids or highly polar fluids, such as those that contain various chlorides. Examples include fluids comprising potassium chloride (KCl), hydrochloric acid (HCl), calcium chloride (CaCl 2 ), calcium bromide (CaBr 2 ) or zinc bromide (ZnBr 2 ). Core material  18  may also include other metals that are less electrochemically active than Zn or non-metallic materials, or a combination thereof. Suitable non-metallic materials include ceramics, composites, glasses or carbon, or a combination thereof. Core material  18  may be selected to provide a high dissolution rate in a predetermined wellbore fluid, but may also be selected to provide a relatively low dissolution rate, including zero dissolution, where dissolution of the nanomatrix material causes the particle core  14  to be rapidly undermined and liberated from the particle compact at the interface with the wellbore fluid, such that the effective rate of dissolution of particle compacts made using particle cores  14  of these core materials  18  is high, even though core material  18  itself may have a low dissolution rate, including core materials  20  that may be substantially insoluble in the wellbore fluid. 
     With regard to the electrochemically active metals as core materials  18 , including Mg, Al, Mn or Zn, these metals may be used as pure metals or in any combination with one another, including various alloy combinations of these materials, including binary, tertiary, or quaternary alloys of these materials. These combinations may also include composites of these materials. Further, in addition to combinations with one another, the Mg, Al, Mn or Zn core materials  18  may also include other constituents, including various alloying additions, to alter one or more properties of the particle cores  14 , such as by improving the strength, lowering the density or altering the dissolution characteristics of the core material  18 . 
     Among the electrochemically active metals, Mg, either as a pure metal or an alloy or a composite material, is particularly useful, because of its low density and ability to form high-strength alloys, as well as its high degree of electrochemical activity, since it has a standard oxidation potential higher than Al, Mn or Zn. Mg alloys include all alloys that have Mg as an alloy constituent. Mg alloys that combine other electrochemically active metals, as described herein, as alloy constituents are particularly useful, including binary Mg—Zn, Mg—Al and Mg—Mn alloys, as well as tertiary Mg—Zn—Y and Mg—Al—X alloys, where X includes Zn, Mn, Si, Ca or Y, or a combination thereof. These Mg—Al—X alloys may include, by weight, up to about 85% Mg, up to about 15% Al and up to about 5% X. Particle core  14  and core material  18 , and particularly electrochemically active metals including Mg, Al, Mn or Zn, or combinations thereof, may also include a rare earth element or combination of rare earth elements. As used herein, rare earth elements include Sc, Y, La, Ce, Pr, Nd or Er, or a combination of rare earth elements. Where present, a rare earth element or combinations of rare earth elements may be present, by weight, in an amount of about 5% or less. 
     Particle core  14  and core material  18  have a melting temperature (T P ). As used herein, T P  includes the lowest temperature at which incipient melting or liquation or other forms of partial melting occur within core material  18 , regardless of whether core material  18  comprises a pure metal, an alloy with multiple phases having different melting temperatures or a composite of materials having different melting temperatures. 
     Particle cores  14  may have any suitable particle size or range of particle sizes or distribution of particle sizes. For example, the particle cores  14  may be selected to provide an average particle size that is represented by a normal or Gaussian type unimodal distribution around an average or mean, as illustrated generally in  FIG. 1 . In another example, particle cores  14  may be selected or mixed to provide a multimodal distribution of particle sizes, including a plurality of average particle core sizes, such as, for example, a homogeneous bimodal distribution of average particle sizes, as illustrated generally and schematically in  FIG. 6 . The selection of the distribution of particle core size may be used to determine, for example, the particle size and interparticle spacing  15  of the particles  12  of powder  10 . In an exemplary embodiment, the particle cores  14  may have a unimodal distribution and an average particle diameter of about 5 μm to about 300 μm, more particularly about 80 μm to about 120 μm, and even more particularly about 100 μm. 
     Particle cores  14  may have any suitable particle shape, including any regular or irregular geometric shape, or combination thereof. In an exemplary embodiment, particle cores  14  are substantially spheroidal electrochemically active metal particles. In another exemplary embodiment, particle cores  14  are substantially irregularly shaped ceramic particles. In yet another exemplary embodiment, particle cores  14  are carbon or other nanotube structures or hollow glass microspheres. 
     Each of the metallic, coated powder particles  12  of powder  10  also includes a metallic coating layer  16  that is disposed on particle core  14 . Metallic coating layer  16  includes a metallic coating material  20 . Metallic coating material  20  gives the powder particles  12  and powder  10  its metallic nature. Metallic coating layer  16  is a nanoscale coating layer. In an exemplary embodiment, metallic coating layer  16  may have a thickness of about 25 nm to about 2500 nm. The thickness of metallic coating layer  16  may vary over the surface of particle core  14 , but will preferably have a substantially uniform thickness over the surface of particle core  14 . Metallic coating layer  16  may include a single layer, as illustrated in  FIG. 2 , or a plurality of layers as a multilayer coating structure, as illustrated in  FIGS. 3-5  for up to four layers. In a single layer coating, or in each of the layers of a multilayer coating, the metallic coating layer  16  may include a single constituent chemical element or compound, or may include a plurality of chemical elements or compounds. Where a layer includes a plurality of chemical constituents or compounds, they may have all manner of homogeneous or heterogeneous distributions, including a homogeneous or heterogeneous distribution of metallurgical phases. This may include a graded distribution where the relative amounts of the chemical constituents or compounds vary according to respective constituent profiles across the thickness of the layer. In both single layer and multilayer coatings  16 , each of the respective layers, or combinations of them, may be used to provide a predetermined property to the powder particle  12  or a sintered powder compact formed therefrom. For example, the predetermined property may include the bond strength of the metallurgical bond between the particle core  14  and the coating material  20 ; the interdiffusion characteristics between the particle core  14  and metallic coating layer  16 , including any interdiffusion between the layers of a multilayer coating layer  16 ; the interdiffusion characteristics between the various layers of a multilayer coating layer  16 ; the interdiffusion characteristics between the metallic coating layer  16  of one powder particle and that of an adjacent powder particle  12 ; the bond strength of the metallurgical bond between the metallic coating layers of adjacent sintered powder particles  12 , including the outermost layers of multilayer coating layers; and the electrochemical activity of the coating layer  16 . 
     Metallic coating layer  16  and coating material  20  have a melting temperature (T C ). As used herein, T C  includes the lowest temperature at which incipient melting or liquation or other forms of partial melting occur within coating material  20 , regardless of whether coating material  20  comprises a pure metal, an alloy with multiple phases each having different melting temperatures or a composite, including a composite comprising a plurality of coating material layers having different melting temperatures. 
     Metallic coating material  20  may include any suitable metallic coating material  20  that provides a sinterable outer surface  21  that is configured to be sintered to an adjacent powder particle  12  that also has a metallic coating layer  16  and sinterable outer surface  21 . In powders  10  that also include second or additional (coated or uncoated) particles  32 , as described herein, the sinterable outer surface  21  of metallic coating layer  16  is also configured to be sintered to a sinterable outer surface  21  of second particles  32 . In an exemplary embodiment, the powder particles  12  are sinterable at a predetermined sintering temperature (T S ) that is a function of the core material  18  and coating material  20 , such that sintering of powder compact  200  is accomplished entirely in the solid state and where T S  is less than T P  and T C . Sintering in the solid state limits particle core  14 /metallic coating layer  16  interactions to solid state diffusion processes and metallurgical transport phenomena and limits growth of and provides control over the resultant interface between them. In contrast, for example, the introduction of liquid phase sintering would provide for rapid interdiffusion of the particle core  14 /metallic coating layer  16  materials and make it difficult to limit the growth of and provide control over the resultant interface between them, and thus interfere with the formation of the desirable microstructure of particle compact  200  as described herein. 
     In an exemplary embodiment, core material  18  will be selected to provide a core chemical composition and the coating material  20  will be selected to provide a coating chemical composition and these chemical compositions will also be selected to differ from one another. In another exemplary embodiment, the core material  18  will be selected to provide a core chemical composition and the coating material  20  will be selected to provide a coating chemical composition and these chemical compositions will also be selected to differ from one another at their interface. Differences in the chemical compositions of coating material  20  and core material  18  may be selected to provide different dissolution rates and selectable and controllable dissolution of powder compacts  200  that incorporate them making them selectably and controllably dissolvable. This includes dissolution rates that differ in response to a changed condition in the wellbore, including an indirect or direct change in a wellbore fluid. In an exemplary embodiment, a powder compact  200  formed from powder  10  having chemical compositions of core material  18  and coating material  20  that make compact  200  is selectably dissolvable in a wellbore fluid in response to a changed wellbore condition that includes a change in temperature, change in pressure, change in flow rate, change in pH or change in chemical composition of the wellbore fluid, or a combination thereof. The selectable dissolution response to the changed condition may result from actual chemical reactions or processes that promote different rates of dissolution, but also encompass changes in the dissolution response that are associated with physical reactions or processes, such as changes in wellbore fluid pressure or flow rate. 
     In an exemplary embodiment of a powder  10 , particle core  14  includes Mg, Al, Mn or Zn, or a combination thereof, as core material  18 , and more particularly may include pure Mg and Mg alloys, and metallic coating layer  16  includes Al, Zn, Mn, Mg, Mo, W, Cu, Fe, Si, Ca, Co, Ta, Re, or Ni, or an oxide, nitride or a carbide thereof, or a combination of any of the aforementioned materials as coating material  20 . 
     In another exemplary embodiment of powder  10 , particle core  14  includes Mg, Al, Mn or Zn, or a combination thereof, as core material  18 , and more particularly may include pure Mg and Mg alloys, and metallic coating layer  16  includes a single layer of Al or Ni, or a combination thereof, as coating material  20 , as illustrated in  FIG. 2 . Where metallic coating layer  16  includes a combination of two or more constituents, such as Al and Ni, the combination may include various graded or co-deposited structures of these materials where the amount of each constituent, and hence the composition of the layer, varies across the thickness of the layer, as also illustrated in  FIG. 2 . 
     In yet another exemplary embodiment, particle core  14  includes Mg, Al, Mn or Zn, or a combination thereof, as core material  18 , and more particularly may include pure Mg and Mg alloys, and coating layer  16  includes two layers as core material  20 , as illustrated in  FIG. 3 . The first layer  22  is disposed on the surface of particle core  14  and includes Al or Ni, or a combination thereof, as described herein. The second layer  24  is disposed on the surface of the first layer and includes Al, Zn, Mg, Mo, W, Cu, Fe, Si, Ca, Co, Ta, Re or Ni or a combination thereof, and the first layer has a chemical composition that is different than the chemical composition of the second layer. In general, first layer  22  will be selected to provide a strong metallurgical bond to particle core  14  and to limit interdiffusion between the particle core  14  and coating layer  16 , particularly first layer  22 . Second layer  24  may be selected to increase the strength of the metallic coating layer  16 , or to provide a strong metallurgical bond and promote sintering with the second layer  24  of adjacent powder particles  12 , or both. In an exemplary embodiment, the respective layers of metallic coating layer  16  may be selected to promote the selective and controllable dissolution of the coating layer  16  in response to a change in a property of the wellbore, including the wellbore fluid, as described herein. However, this is only exemplary and it will be appreciated that other selection criteria for the various layers may also be employed. For example, any of the respective layers may be selected to promote the selective and controllable dissolution of the coating layer  16  in response to a change in a property of the wellbore, including the wellbore fluid, as described herein. Exemplary embodiments of a two-layer metallic coating layers  16  for use on particles cores  14  comprising Mg include first/second layer combinations comprising Al/Ni and Al/W. 
     In still another embodiment, particle core  14  includes Mg, Al, Mn or Zn, or a combination thereof, as core material  18 , and more particularly may include pure Mg and Mg alloys, and coating layer  16  includes three layers, as illustrated in  FIG. 4 . The first layer  22  is disposed on particle core  14  and may include Al or Ni, or a combination thereof. The second layer  24  is disposed on first layer  22  and may include Al, Zn, Mg, Mo, W, Cu, Fe, Si, Ca, Co, Ta, Re or Ni, or an oxide, nitride or a carbide thereof, or a combination of any of the aforementioned second layer materials. The third layer  26  is disposed on the second layer  24  and may include Al, Mn, Fe, Co, Ni or a combination thereof. In a three-layer configuration, the composition of adjacent layers is different, such that the first layer has a chemical composition that is different than the second layer, and the second layer has a chemical composition that is different than the third layer. In an exemplary embodiment, first layer  22  may be selected to provide a strong metallurgical bond to particle core  14  and to limit interdiffusion between the particle core  14  and coating layer  16 , particularly first layer  22 . Second layer  24  may be selected to increase the strength of the metallic coating layer  16 , or to limit interdiffusion between particle core  14  or first layer  22  and outer or third layer  26 , or to promote adhesion and a strong metallurgical bond between third layer  26  and first layer  22 , or any combination of them. Third layer  26  may be selected to provide a strong metallurgical bond and promote sintering with the third layer  26  of adjacent powder particles  12 . However, this is only exemplary and it will be appreciated that other selection criteria for the various layers may also be employed. For example, any of the respective layers may be selected to promote the selective and controllable dissolution of the coating layer  16  in response to a change in a property of the wellbore, including the wellbore fluid, as described herein. An exemplary embodiment of a three-layer coating layer for use on particles cores comprising Mg include first/second/third layer combinations comprising Al/Al 2 O 3 /Al. 
     In still another embodiment, particle core  14  includes Mg, Al, Mn or Zn, or a combination thereof, as core material  18 , and more particularly may include pure Mg and Mg alloys, and coating layer  16  includes four layers, as illustrated in  FIG. 5 . In the four layer configuration, the first layer  22  may include Al or Ni, or a combination thereof, as described herein. The second layer  24  may include Al, Zn, Mg, Mo, W, Cu, Fe, Si, Ca, Co, Ta, Re or Ni or an oxide, nitride, carbide thereof, or a combination of the aforementioned second layer materials. The third layer  26  may also include Al, Zn, Mg, Mo, W, Cu, Fe, Si, Ca, Co, Ta, Re or Ni, or an oxide, nitride or carbide thereof, or a combination of any of the aforementioned third layer materials. The fourth layer  28  may include Al, Mn, Fe, Co, Ni or a combination thereof. In the four layer configuration, the chemical composition of adjacent layers is different, such that the chemical composition of first layer  22  is different than the chemical composition of second layer  24 , the chemical composition is of second layer  24  different than the chemical composition of third layer  26 , and the chemical composition of third layer  26  is different than the chemical composition of fourth layer  28 . In an exemplary embodiment, the selection of the various layers will be similar to that described for the three-layer configuration above with regard to the inner (first) and outer (fourth) layers, with the second and third layers available for providing enhanced interlayer adhesion, strength of the overall metallic coating layer  16 , limited interlayer diffusion or selectable and controllable dissolution, or a combination thereof. However, this is only exemplary and it will be appreciated that other selection criteria for the various layers may also be employed. For example, any of the respective layers may be selected to promote the selective and controllable dissolution of the coating layer  16  in response to a change in a property of the wellbore, including the wellbore fluid, as described herein. 
     The thickness of the various layers in multi-layer configurations may be apportioned between the various layers in any manner so long as the sum of the layer thicknesses provide a nanoscale coating layer  16 , including layer thicknesses as described herein. In one embodiment, the first layer  22  and outer layer ( 24 ,  26 , or  28  depending on the number of layers) may be thicker than other layers, where present, due to the desire to provide sufficient material to promote the desired bonding of first layer  22  with the particle core  14 , or the bonding of the outer layers of adjacent powder particles  12 , during sintering of powder compact  200 . 
     Powder  10  may also include an additional or second powder  30  interspersed in the plurality of powder particles  12 , as illustrated in  FIG. 7 . In an exemplary embodiment, the second powder  30  includes a plurality of second powder particles  32 . These second powder particles  32  may be selected to change a physical, chemical, mechanical or other property of a powder particle compact  200  formed from powder  10  and second powder  30 , or a combination of such properties. In an exemplary embodiment, the property change may include an increase in the compressive strength of powder compact  200  formed from powder  10  and second powder  30 . In another exemplary embodiment, the second powder  30  may be selected to promote the selective and controllable dissolution of in particle compact  200  formed from powder  10  and second powder  30  in response to a change in a property of the wellbore, including the wellbore fluid, as described herein. Second powder particles  32  may be uncoated or coated with a metallic coating layer  36 . When coated, including single layer or multilayer coatings, the coating layer  36  of second powder particles  32  may comprise the same coating material  40  as coating material  20  of powder particles  12 , or the coating material  40  may be different. The second powder particles  32  (uncoated) or particle cores  34  may include any suitable material to provide the desired benefit, including many metals. In an exemplary embodiment, when coated powder particles  12  comprising Mg, Al, Mn or Zn, or a combination thereof are employed, suitable second powder particles  32  may include Ni, W, Cu, Co or Fe, or a combination thereof. Since second powder particles  32  will also be configured for solid state sintering to powder particles  12  at the predetermined sintering temperature (T S ), particle cores  34  will have a melting temperature T AP  and any coating layers  36  will have a second melting temperature T AC , where T S  is less than T AP  and T AC . It will also be appreciated that second powder  30  is not limited to one additional powder particle  32  type (i.e., a second powder particle), but may include a plurality of additional powder particles  32  (i.e., second, third, fourth, etc. types of additional powder particles  32 ) in any number. 
     Referring to  FIG. 8 , an exemplary embodiment of a method  300  of making a metallic powder  10  is disclosed. Method  300  includes forming  310  a plurality of particle cores  14  as described herein. Method  300  also includes depositing  320  a metallic coating layer  16  on each of the plurality of particle cores  14 . Depositing  320  is the process by which coating layer  16  is disposed on particle core  14  as described herein. 
     Forming  310  of particle cores  14  may be performed by any suitable method for forming a plurality of particle cores  14  of the desired core material  18 , which essentially comprise methods of forming a powder of core material  18 . Suitable powder forming methods include mechanical methods; including machining, milling, impacting and other mechanical methods for forming the metal powder; chemical methods, including chemical decomposition, precipitation from a liquid or gas, solid-solid reactive synthesis and other chemical powder forming methods; atomization methods, including gas atomization, liquid and water atomization, centrifugal atomization, plasma atomization and other atomization methods for forming a powder; and various evaporation and condensation methods. In an exemplary embodiment, particle cores  14  comprising Mg may be fabricated using an atomization method, such as vacuum spray forming or inert gas spray forming. 
     Depositing  320  of metallic coating layers  16  on the plurality of particle cores  14  may be performed using any suitable deposition method, including various thin film deposition methods, such as, for example, chemical vapor deposition and physical vapor deposition methods. In an exemplary embodiment, depositing  320  of metallic coating layers  16  are performed using fluidized bed chemical vapor deposition (FBCVD). Depositing  320  of the metallic coating layers  16  by FBCVD includes flowing a reactive fluid as a coating medium that includes the desired metallic coating material  20  through a bed of particle cores  14  fluidized in a reactor vessel under suitable conditions, including temperature, pressure and flow rate conditions and the like, sufficient to induce a chemical reaction of the coating medium to produce the desired metallic coating material  20  and induce its deposition upon the surface of particle cores  14  to form coated powder particles  12 . The reactive fluid selected will depend upon the metallic coating material  20  desired, and will typically comprise an organometallic compound that includes the metallic material to be deposited, such as nickel tetracarbonyl (Ni(CO) 4 ), tungsten hexafluoride (WF 6 ), and triethyl aluminum (C 6 H 15 Al), that is transported in a carrier fluid, such as helium or argon gas. The reactive fluid, including carrier fluid, causes at least a portion of the plurality of particle cores  14  to be suspended in the fluid, thereby enabling the entire surface of the suspended particle cores  14  to be exposed to the reactive fluid, including, for example, a desired organometallic constituent, and enabling deposition of metallic coating material  20  and coating layer  16  over the entire surfaces of particle cores  14  such that they each become enclosed forming coated particles  12  having metallic coating layers  16 , as described herein. As also described herein, each metallic coating layer  16  may include a plurality of coating layers. Coating material  20  may be deposited in multiple layers to form a multilayer metallic coating layer  16  by repeating the step of depositing  320  described above and changing  330  the reactive fluid to provide the desired metallic coating material  20  for each subsequent layer, where each subsequent layer is deposited on the outer surface of particle cores  14  that already include any previously deposited coating layer or layers that make up metallic coating layer  16 . The metallic coating materials  20  of the respective layers (e.g.,  22 ,  24 ,  26 ,  28 , etc.) may be different from one another, and the differences may be provided by utilization of different reactive media that are configured to produce the desired metallic coating layers  16  on the particle cores  14  in the fluidize bed reactor. 
     As illustrated in  FIGS. 1 and 9 , particle core  14  and core material  18  and metallic coating layer  16  and coating material  20  may be selected to provide powder particles  12  and a powder  10  that is configured for compaction and sintering to provide a powder compact  200  that is lightweight (i.e., having a relatively low density), high-strength and is selectably and controllably removable from a wellbore in response to a change in a wellbore property, including being selectably and controllably dissolvable in an appropriate wellbore fluid, including various wellbore fluids as disclosed herein. Powder compact  200  includes a substantially-continuous, cellular nanomatrix  216  of a nanomatrix material  220  having a plurality of dispersed particles  214  dispersed throughout the cellular nanomatrix  216 . The substantially-continuous cellular nanomatrix  216  and nanomatrix material  220  formed of sintered metallic coating layers  16  is formed by the compaction and sintering of the plurality of metallic coating layers  16  of the plurality of powder particles  12 . The chemical composition of nanomatrix material  220  may be different than that of coating material  20  due to diffusion effects associated with the sintering as described herein. Powder metal compact  200  also includes a plurality of dispersed particles  214  that comprise particle core material  218 . Dispersed particle cores  214  and core material  218  correspond to and are formed from the plurality of particle cores  14  and core material  18  of the plurality of powder particles  12  as the metallic coating layers  16  are sintered together to form nanomatrix  216 . The chemical composition of core material  218  may be different than that of core material  18  due to diffusion effects associated with sintering as described herein. 
     As used herein, the use of the term substantially-continuous cellular nanomatrix  216  does not connote the major constituent of the powder compact, but rather refers to the minority constituent or constituents, whether by weight or by volume. This is distinguished from most matrix composite materials where the matrix comprises the majority constituent by weight or volume. The use of the term substantially-continuous, cellular nanomatrix is intended to describe the extensive, regular, continuous and interconnected nature of the distribution of nanomatrix material  220  within powder compact  200 . As used herein, “substantially-continuous” describes the extension of the nanomatrix material throughout powder compact  200  such that it extends between and envelopes substantially all of the dispersed particles  214 . Substantially-continuous is used to indicate that complete continuity and regular order of the nanomatrix around each dispersed particle  214  is not required. For example, defects in the coating layer  16  over particle core  14  on some powder particles  12  may cause bridging of the particle cores  14  during sintering of the powder compact  200 , thereby causing localized discontinuities to result within the cellular nanomatrix  216 , even though in the other portions of the powder compact the nanomatrix is substantially continuous and exhibits the structure described herein. As used herein, “cellular” is used to indicate that the nanomatrix defines a network of generally repeating, interconnected, compartments or cells of nanomatrix material  220  that encompass and also interconnect the dispersed particles  214 . As used herein, “nanomatrix” is used to describe the size or scale of the matrix, particularly the thickness of the matrix between adjacent dispersed particles  214 . The metallic coating layers that are sintered together to form the nanomatrix are themselves nanoscale thickness coating layers. Since the nanomatrix at most locations, other than the intersection of more than two dispersed particles  214 , generally comprises the interdiffusion and bonding of two coating layers  16  from adjacent powder particles  12  having nanoscale thicknesses, the matrix formed also has a nanoscale thickness (e.g., approximately two times the coating layer thickness as described herein) and is thus described as a nanomatrix. Further, the use of the term dispersed particles  214  does not connote the minor constituent of powder compact  200 , but rather refers to the majority constituent or constituents, whether by weight or by volume. The use of the term dispersed particle is intended to convey the discontinuous and discrete distribution of particle core material  218  within powder compact  200 . 
     Powder compact  200  may have any desired shape or size, including that of a cylindrical billet or bar that may be machined or otherwise used to form useful articles of manufacture, including various wellbore tools and components. The pressing used to form precursor powder compact  100  and sintering and pressing processes used to form powder compact  200  and deform the powder particles  12 , including particle cores  14  and coating layers  16 , to provide the full density and desired macroscopic shape and size of powder compact  200  as well as its microstructure. The microstructure of powder compact  200  includes an equiaxed configuration of dispersed particles  214  that are dispersed throughout and embedded within the substantially-continuous, cellular nanomatrix  216  of sintered coating layers. This microstructure is somewhat analogous to an equiaxed grain microstructure with a continuous grain boundary phase, except that it does not require the use of alloy constituents having thermodynamic phase equilibria properties that are capable of producing such a structure. Rather, this equiaxed dispersed particle structure and cellular nanomatrix  216  of sintered metallic coating layers  16  may be produced using constituents where thermodynamic phase equilibrium conditions would not produce an equiaxed structure. The equiaxed morphology of the dispersed particles  214  and cellular network  216  of particle layers results from sintering and deformation of the powder particles  12  as they are compacted and interdiffuse and deform to fill the interparticle spaces  15  ( FIG. 1 ). The sintering temperatures and pressures may be selected to ensure that the density of powder compact  200  achieves substantially full theoretical density. 
     In an exemplary embodiment as illustrated in  FIGS. 1 and 9 , dispersed particles  214  are formed from particle cores  14  dispersed in the cellular nanomatrix  216  of sintered metallic coating layers  16 , and the nanomatrix  216  includes a solid-state metallurgical bond  217  or bond layer  219 , as illustrated schematically in  FIG. 10 , extending between the dispersed particles  214  throughout the cellular nanomatrix  216  that is formed at a sintering temperature (T S ), where T S  is less than T C  and T P . As indicated, solid-state metallurgical bond  217  is formed in the solid state by solid-state interdiffusion between the coating layers  16  of adjacent powder particles  12  that are compressed into touching contact during the compaction and sintering processes used to form powder compact  200 , as described herein. As such, sintered coating layers  16  of cellular nanomatrix  216  include a solid-state bond layer  219  that has a thickness (t) defined by the extent of the interdiffusion of the coating materials  20  of the coating layers  16 , which will in turn be defined by the nature of the coating layers  16 , including whether they are single or multilayer coating layers, whether they have been selected to promote or limit such interdiffusion, and other factors, as described herein, as well as the sintering and compaction conditions, including the sintering time, temperature and pressure used to form powder compact  200 . 
     As nanomatrix  216  is formed, including bond  217  and bond layer  219 , the chemical composition or phase distribution, or both, of metallic coating layers  16  may change. Nanomatrix  216  also has a melting temperature (T M ). As used herein, T M  includes the lowest temperature at which incipient melting or liquation or other forms of partial melting will occur within nanomatrix  216 , regardless of whether nanomatrix material  220  comprises a pure metal, an alloy with multiple phases each having different melting temperatures or a composite, including a composite comprising a plurality of layers of various coating materials having different melting temperatures, or a combination thereof, or otherwise. As dispersed particles  214  and particle core materials  218  are formed in conjunction with nanomatrix  216 , diffusion of constituents of metallic coating layers  16  into the particle cores  14  is also possible, which may result in changes in the chemical composition or phase distribution, or both, of particle cores  14 . As a result, dispersed particles  214  and particle core materials  218  may have a melting temperature (T DP ) that is different than T P  As used herein, T DP  includes the lowest temperature at which incipient melting or liquation or other forms of partial melting will occur within dispersed particles  214 , regardless of whether particle core material  218  comprise a pure metal, an alloy with multiple phases each having different melting temperatures or a composite, or otherwise. Powder compact  200  is formed at a sintering temperature (T S ), where T S  is less than T C , T P , T M  and T DP . Dispersed particles  214  may comprise any of the materials described herein for particle cores  14 , even though the chemical composition of dispersed particles  214  may be different due to diffusion effects as described herein. In an exemplary embodiment, dispersed particles  214  are formed from particle cores  14  comprising materials having a standard oxidation potential greater than or equal to Zn, including Mg, Al, Zn or Mn, or a combination thereof, may include various binary, tertiary and quaternary alloys or other combinations of these constituents as disclosed herein in conjunction with particle cores  14 . Of these materials, those having dispersed particles  214  comprising Mg and the nanomatrix  216  formed from the metallic coating materials  16  described herein are particularly useful. Dispersed particles  214  and particle core material  218  of Mg, Al, Zn or Mn, or a combination thereof, may also include a rare earth element, or a combination of rare earth elements as disclosed herein in conjunction with particle cores  14 . 
     In another exemplary embodiment, dispersed particles  214  are formed from particle cores  14  comprising metals that are less electrochemically active than Zn or non-metallic materials. Suitable non-metallic materials include ceramics, glasses (e.g., hollow glass microspheres) or carbon, or a combination thereof, as described herein. 
     Dispersed particles  214  of powder compact  200  may have any suitable particle size, including the average particle sizes described herein for particle cores  14 . 
     Dispersed particles  214  may have any suitable shape depending on the shape selected for particle cores  14  and powder particles  12 , as well as the method used to sinter and compact powder  10 . In an exemplary embodiment, powder particles  12  may be spheroidal or substantially spheroidal and dispersed particles  214  may include an equiaxed particle configuration as described herein. 
     The nature of the dispersion of dispersed particles  214  may be affected by the selection of the powder  10  or powders  10  used to make particle compact  200 . In one exemplary embodiment, a powder  10  having a unimodal distribution of powder particle  12  sizes may be selected to form powder compact  200  and will produce a substantially homogeneous unimodal dispersion of particle sizes of dispersed particles  214  within cellular nanomatrix  216 , as illustrated generally in  FIG. 9 . In another exemplary embodiment, a plurality of powders  10  having a plurality of powder particles with particle cores  14  that have the same core materials  18  and different core sizes and the same coating material  20  may be selected and uniformly mixed as described herein to provide a powder  10  having a homogenous, multimodal distribution of powder particle  12  sizes, and may be used to form powder compact  200  having a homogeneous, multimodal dispersion of particle sizes of dispersed particles  214  within cellular nanomatrix  216 , as illustrated schematically in  FIGS. 6 and 11 . Similarly, in yet another exemplary embodiment, a plurality of powders  10  having a plurality of particle cores  14  that may have the same core materials  18  and different core sizes and the same coating material  20  may be selected and distributed in a non-uniform manner to provide a non-homogenous, multimodal distribution of powder particle sizes, and may be used to form powder compact  200  having a non-homogeneous, multimodal dispersion of particle sizes of dispersed particles  214  within cellular nanomatrix  216 , as illustrated schematically in  FIG. 12 . The selection of the distribution of particle core size may be used to determine, for example, the particle size and interparticle spacing of the dispersed particles  214  within the cellular nanomatrix  216  of powder compacts  200  made from powder  10 . 
     As illustrated generally in  FIGS. 7 and 13 , powder metal compact  200  may also be formed using coated metallic powder  10  and an additional or second powder  30 , as described herein. The use of an additional powder  30  provides a powder compact  200  that also includes a plurality of dispersed second particles  234 , as described herein, that are dispersed within the nanomatrix  216  and are also dispersed with respect to the dispersed particles  214 . Dispersed second particles  234  may be formed from coated or uncoated second powder particles  32 , as described herein. In an exemplary embodiment, coated second powder particles  32  may be coated with a coating layer  36  that is the same as coating layer  16  of powder particles  12 , such that coating layers  36  also contribute to the nanomatrix  216 . In another exemplary embodiment, the second powder particles  232  may be uncoated such that dispersed second particles  234  are embedded within nanomatrix  216 . As disclosed herein, powder  10  and additional powder  30  may be mixed to form a homogeneous dispersion of dispersed particles  214  and dispersed second particles  234 , as illustrated in  FIG. 13 , or to form a non-homogeneous dispersion of these particles, as illustrated in  FIG. 14 . The dispersed second particles  234  may be formed from any suitable additional powder  30  that is different from powder  10 , either due to a compositional difference in the particle core  34 , or coating layer  36 , or both of them, and may include any of the materials disclosed herein for use as second powder  30  that are different from the powder  10  that is selected to form powder compact  200 . In an exemplary embodiment, dispersed second particles  234  may include Fe, Ni, Co or Cu, or oxides, nitrides or carbides thereof, or a combination of any of the aforementioned materials. 
     Nanomatrix  216  is a substantially-continuous, cellular network of metallic coating layers  16  that are sintered to one another. The thickness of nanomatrix  216  will depend on the nature of the powder  10  or powders  10  used to form powder compact  200 , as well as the incorporation of any second powder  30 , particularly the thicknesses of the coating layers associated with these particles. In an exemplary embodiment, the thickness of nanomatrix  216  is substantially uniform throughout the microstructure of powder compact  200  and comprises about two times the thickness of the coating layers  16  of powder particles  12 . In another exemplary embodiment, the cellular network  216  has a substantially uniform average thickness between dispersed particles  214  of about 50 nm to about 5000 nm. 
     Nanomatrix  216  is formed by sintering metallic coating layers  16  of adjacent particles to one another by interdiffusion and creation of bond layer  219  as described herein. Metallic coating layers  16  may be single layer or multilayer structures, and they may be selected to promote or inhibit diffusion, or both, within the layer or between the layers of metallic coating layer  16 , or between the metallic coating layer  16  and particle core  14 , or between the metallic coating layer  16  and the metallic coating layer  16  of an adjacent powder particle, the extent of interdiffusion of metallic coating layers  16  during sintering may be limited or extensive depending on the coating thicknesses, coating material or materials selected, the sintering conditions and other factors. Given the potential complexity of the interdiffusion and interaction of the constituents, description of the resulting chemical composition of nanomatrix  216  and nanomatrix material  220  may be simply understood to be a combination of the constituents of coating layers  16  that may also include one or more constituents of dispersed particles  214 , depending on the extent of interdiffusion, if any, that occurs between the dispersed particles  214  and the nanomatrix  216 . Similarly, the chemical composition of dispersed particles  214  and particle core material  218  may be simply understood to be a combination of the constituents of particle core  14  that may also include one or more constituents of nanomatrix  216  and nanomatrix material  220 , depending on the extent of interdiffusion, if any, that occurs between the dispersed particles  214  and the nanomatrix  216 . 
     In an exemplary embodiment, the nanomatrix material  220  has a chemical composition and the particle core material  218  has a chemical composition that is different from that of nanomatrix material  220 , and the differences in the chemical compositions may be configured to provide a selectable and controllable dissolution rate, including a selectable transition from a very low dissolution rate to a very rapid dissolution rate, in response to a controlled change in a property or condition of the wellbore proximate the compact  200 , including a property change in a wellbore fluid that is in contact with the powder compact  200 , as described herein. Nanomatrix  216  may be formed from powder particles  12  having single layer and multilayer coating layers  16 . This design flexibility provides a large number of material combinations, particularly in the case of multilayer coating layers  16 , that can be utilized to tailor the cellular nanomatrix  216  and composition of nanomatrix material  220  by controlling the interaction of the coating layer constituents, both within a given layer, as well as between a coating layer  16  and the particle core  14  with which it is associated or a coating layer  16  of an adjacent powder particle  12 . Several exemplary embodiments that demonstrate this flexibility are provided below. 
     As illustrated in  FIG. 10 , in an exemplary embodiment, powder compact  200  is formed from powder particles  12  where the coating layer  16  comprises a single layer, and the resulting nanomatrix  216  between adjacent ones of the plurality of dispersed particles  214  comprises the single metallic coating layer  16  of one powder particle  12 , a bond layer  219  and the single coating layer  16  of another one of the adjacent powder particles  12 . The thickness (t) of bond layer  219  is determined by the extent of the interdiffusion between the single metallic coating layers  16 , and may encompass the entire thickness of nanomatrix  216  or only a portion thereof. In one exemplary embodiment of powder compact  200  formed using a single layer powder  10 , powder compact  200  may include dispersed particles  214  comprising Mg, Al, Zn or Mn, or a combination thereof, as described herein, and nanomatrix  216  may include Al, Zn, Mn, Mg, Mo, W, Cu, Fe, Si, Ca, Co, Ta, Re or Ni, or an oxide, carbide or nitride thereof, or a combination of any of the aforementioned materials, including combinations where the nanomatrix material  220  of cellular nanomatrix  216 , including bond layer  219 , has a chemical composition and the core material  218  of dispersed particles  214  has a chemical composition that is different than the chemical composition of nanomatrix material  216 . The difference in the chemical composition of the nanomatrix material  220  and the core material  218  may be used to provide selectable and controllable dissolution in response to a change in a property of a wellbore, including a wellbore fluid, as described herein. In a further exemplary embodiment of a powder compact  200  formed from a powder  10  having a single coating layer configuration, dispersed particles  214  include Mg, Al, Zn or Mn, or a combination thereof, and the cellular nanomatrix  216  includes Al or Ni, or a combination thereof. 
     As illustrated in  FIG. 15 , in another exemplary embodiment, powder compact  200  is formed from powder particles  12  where the coating layer  16  comprises a multilayer coating layer  16  having a plurality of coating layers, and the resulting nanomatrix  216  between adjacent ones of the plurality of dispersed particles  214  comprises the plurality of layers (t) comprising the coating layer  16  of one particle  12 , a bond layer  219 , and the plurality of layers comprising the coating layer  16  of another one of powder particles  12 . In  FIG. 15 , this is illustrated with a two-layer metallic coating layer  16 , but it will be understood that the plurality of layers of multi-layer metallic coating layer  16  may include any desired number of layers. The thickness (t) of the bond layer  219  is again determined by the extent of the interdiffusion between the plurality of layers of the respective coating layers  16 , and may encompass the entire thickness of nanomatrix  216  or only a portion thereof. In this embodiment, the plurality of layers comprising each coating layer  16  may be used to control interdiffusion and formation of bond layer  219  and thickness (t). 
     In one exemplary embodiment of a powder compact  200  made using powder particles  12  with multilayer coating layers  16 , the compact includes dispersed particles  214  comprising Mg, Al, Zn or Mn, or a combination thereof, as described herein, and nanomatrix  216  comprises a cellular network of sintered two-layer coating layers  16 , as shown in  FIG. 3 , comprising first layers  22  that are disposed on the dispersed particles  214  and a second layers  24  that are disposed on the first layers  22 . First layers  22  include Al or Ni, or a combination thereof, and second layers  24  include Al, Zn, Mn, Mg, Mo, W, Cu, Fe, Si, Ca, Co, Ta, Re or Ni, or a combination thereof. In these configurations, materials of dispersed particles  214  and multilayer coating layer  16  used to form nanomatrix  216  are selected so that the chemical compositions of adjacent materials are different (e.g. dispersed particle/first layer and first layer/second layer). 
     In another exemplary embodiment of a powder compact  200  made using powder particles  12  with multilayer coating layers  16 , the compact includes dispersed particles  214  comprising Mg, Al, Zn or Mn, or a combination thereof, as described herein, and nanomatrix  216  comprises a cellular network of sintered three-layer metallic coating layers  16 , as shown in  FIG. 4 , comprising first layers  22  that are disposed on the dispersed particles  214 , second layers  24  that are disposed on the first layers  22  and third layers  26  that are disposed on the second layers  24 . First layers  22  include Al or Ni, or a combination thereof; second layers  24  include Al, Zn, Mn, Mg, Mo, W, Cu, Fe, Si, Ca, Co, Ta, Re or Ni, or an oxide, nitride or carbide thereof, or a combination of any of the aforementioned second layer materials; and the third layers include Al, Zn, Mn, Mg, Mo, W, Cu, Fe, Si, Ca, Co, Ta, Re or Ni, or a combination thereof. The selection of materials is analogous to the selection considerations described herein for powder compact  200  made using two-layer coating layer powders, but must also be extended to include the material used for the third coating layer. 
     In yet another exemplary embodiment of a powder compact  200  made using powder particles  12  with multilayer coating layers  16 , the compact includes dispersed particles  214  comprising Mg, Al, Zn or Mn, or a combination thereof, as described herein, and nanomatrix  216  comprise a cellular network of sintered four-layer coating layers  16  comprising first layers  22  that are disposed on the dispersed particles  214 ; second layers  24  that are disposed on the first layers  22 ; third layers  26  that are disposed on the second layers  24  and fourth layers  28  that are disposed on the third layers  26 . First layers  22  include Al or Ni, or a combination thereof; second layers  24  include Al, Zn, Mn, Mg, Mo, W, Cu, Fe, Si, Ca, Co, Ta, Re or Ni, or an oxide, nitride or carbide thereof, or a combination of any of the aforementioned second layer materials; third layers include Al, Zn, Mn, Mg, Mo, W, Cu, Fe, Si, Ca, Co, Ta, Re or Ni, or an oxide, nitride or carbide thereof, or a combination of any of the aforementioned third layer materials; and fourth layers include Al, Mn, Fe, Co or Ni, or a combination thereof. The selection of materials is analogous to the selection considerations described herein for powder compacts  200  made using two-layer coating layer powders, but must also be extended to include the material used for the third and fourth coating layers. 
     In another exemplary embodiment of a powder compact  200 , dispersed particles  214  comprise a metal having a standard oxidation potential less than Zn or a non-metallic material, or a combination thereof, as described herein, and nanomatrix  216  comprises a cellular network of sintered metallic coating layers  16 . Suitable non-metallic materials include various ceramics, glasses or forms of carbon, or a combination thereof. Further, in powder compacts  200  that include dispersed particles  214  comprising these metals or non-metallic materials, nanomatrix  216  may include Al, Zn, Mn, Mg, Mo, W, Cu, Fe, Si, Ca, Co, Ta, Re or Ni, or an oxide, carbide or nitride thereof, or a combination of any of the aforementioned materials as nanomatrix material  220 . 
     Referring to  FIG. 16 , sintered powder compact  200  may comprise a sintered precursor powder compact  100  that includes a plurality of deformed, mechanically bonded powder particles as described herein. Precursor powder compact  100  may be formed by compaction of powder  10  to the point that powder particles  12  are pressed into one another, thereby deforming them and forming interparticle mechanical or other bonds  110  associated with this deformation sufficient to cause the deformed powder particles  12  to adhere to one another and form a green-state powder compact having a green density that is less than the theoretical density of a fully-dense compact of powder  10 , due in part to interparticle spaces  15 . Compaction may be performed, for example, by is statically pressing powder  10  at room temperature to provide the deformation and interparticle bonding of powder particles  12  necessary to form precursor powder compact  100 . 
     Sintered and forged powder compacts  200  that include dispersed particles  214  comprising Mg and nanomatrix  216  comprising various nanomatrix materials as described herein have demonstrated an excellent combination of mechanical strength and low density that exemplify the lightweight, high-strength materials disclosed herein. Examples of powder compacts  200  that have pure Mg dispersed particles  214  and various nanomatrices  216  formed from powders  10  having pure Mg particle cores  14  and various single and multilayer metallic coating layers  16  that include Al, Ni, W or Al 2 O 3 , or a combination thereof, and that have been made using the method  400  disclosed herein, are listed in a table as  FIG. 18 . These powders compacts  200  have been subjected to various mechanical and other testing, including density testing, and their dissolution and mechanical property degradation behavior has also been characterized as disclosed herein. The results indicate that these materials may be configured to provide a wide range of selectable and controllable corrosion or dissolution behavior from very low corrosion rates to extremely high corrosion rates, particularly corrosion rates that are both lower and higher than those of powder compacts that do not incorporate the cellular nanomatrix, such as a compact formed from pure Mg powder through the same compaction and sintering processes in comparison to those that include pure Mg dispersed particles in the various cellular nanomatrices described herein. These powder compacts  200  may also be configured to provide substantially enhanced properties as compared to powder compacts formed from pure Mg particles that do not include the nanoscale coatings described herein. For example, referring to  FIGS. 18 and 19 , powder compacts  200  that include dispersed particles  214  comprising Mg and nanomatrix  216  comprising various nanomatrix materials  220  described herein have demonstrated a room temperature compressive strengths of at least about 37 ksi, and have further demonstrated room temperature compressive strengths in excess of about 50 ksi, both dry and immersed in a solution of 3% KCl at 200° F. In contrast, powder compacts formed from pure Mg powders have a compressive strength of about 20 ksi or less. Strength of the nanomatrix powder metal compact  200  can be further improved by optimizing powder  10 , particularly the weight percentage of the nanoscale metallic coating layers  16  that are used to form cellular nanomatrix  216 . For example,  FIG. 25  shows the effect of varying the weight percentage (wt. %), i.e., thickness, of an alumina coating on the room temperature compressive strength of a powder compact  200  of a cellular nanomatrix  216  formed from coated powder particles  12  that include a multilayer (Al/Al 2 O 3 /Al) metallic coating layer  16  on pure Mg particle cores  14 . In this example, optimal strength is achieved at 4 wt % of alumina, which represents an increase of 21% as compared to that of 0 wt % alumina. 
     Powder compacts  200  comprising dispersed particles  214  that include Mg and nanomatrix  216  that includes various nanomatrix materials as described herein have also demonstrated a room temperature sheer strength of at least about 20 ksi. This is in contrast with powder compacts formed from pure Mg powders which have room temperature sheer strengths of about 8 ksi. 
     Powder compacts  200  of the types disclosed herein are able to achieve an actual density that is substantially equal to the predetermined theoretical density of a compact material based on the composition of powder  10 , including relative amounts of constituents of particle cores  14  and metallic coating layer  16 , and are also described herein as being fully-dense powder compacts. Powder compacts  200  comprising dispersed particles that include Mg and nanomatrix  216  that includes various nanomatrix materials as described herein have demonstrated actual densities of about 1.738 g/cm 3  to about 2.50 g/cm 3 , which are substantially equal to the predetermined theoretical densities, differing by at most 4% from the predetermined theoretical densities. 
     Powder compacts  200  as disclosed herein may be configured to be selectively and controllably dissolvable in a wellbore fluid in response to a changed condition in a wellbore. Examples of the changed condition that may be exploited to provide selectable and controllable dissolvability include a change in temperature, change in pressure, change in flow rate, change in pH or change in chemical composition of the wellbore fluid, or a combination thereof. An example of a changed condition comprising a change in temperature includes a change in well bore fluid temperature. For example, referring to  FIGS. 18 and 20 , powder compacts  200  comprising dispersed particles  214  that include Mg and cellular nanomatrix  216  that includes various nanomatrix materials as described herein have relatively low rates of corrosion in a 3% KCl solution at room temperature that ranges from about 0 to about 11 mg/cm 2 /hr as compared to relatively high rates of corrosion at 200° F. that range from about 1 to about 246 mg/cm 2 /hr depending on different nanoscale coating layers  16 . An example of a changed condition comprising a change in chemical composition includes a change in a chloride ion concentration or pH value, or both, of the wellbore fluid. For example, referring to  FIGS. 18 and 21 , powder compacts  200  comprising dispersed particles  214  that include Mg and nanomatrix  216  that includes various nanoscale coatings described herein demonstrate corrosion rates in 15% HCl that range from about 4750 mg/cm 2 /hr to about 7432 mg/cm 2 /hr. Thus, selectable and controllable dissolvability in response to a changed condition in the wellbore, namely the change in the wellbore fluid chemical composition from KCl to HCl, may be used to achieve a characteristic response as illustrated graphically in  FIG. 22 , which illustrates that at a selected predetermined critical service time (CST) a changed condition may be imposed upon powder compact  200  as it is applied in a given application, such as a wellbore environment, that causes a controllable change in a property of powder compact  200  in response to a changed condition in the environment in which it is applied. For example, at a predetermined CST changing a wellbore fluid that is in contact with powder contact  200  from a first fluid (e.g. KCl) that provides a first corrosion rate and an associated weight loss or strength as a function of time to a second wellbore fluid (e.g., HCl) that provides a second corrosion rate and associated weight loss and strength as a function of time, wherein the corrosion rate associated with the first fluid is much less than the corrosion rate associated with the second fluid. This characteristic response to a change in wellbore fluid conditions may be used, for example, to associate the critical service time with a dimension loss limit or a minimum strength needed for a particular application, such that when a wellbore tool or component formed from powder compact  200  as disclosed herein is no longer needed in service in the wellbore (e.g., the CST) the condition in the wellbore (e.g., the chloride ion concentration of the wellbore fluid) may be changed to cause the rapid dissolution of powder compact  200  and its removal from the wellbore. In the example described above, powder compact  200  is selectably dissolvable at a rate that ranges from about 0 to about 7000 mg/cm 2 /hr. This range of response provides, for example the ability to remove a 3 inch diameter ball formed from this material from a wellbore by altering the wellbore fluid in less than one hour. The selectable and controllable dissolvability behavior described above, coupled with the excellent strength and low density properties described herein, define a new engineered dispersed particle-nanomatrix material that is configured for contact with a fluid and configured to provide a selectable and controllable transition from one of a first strength condition to a second strength condition that is lower than a functional strength threshold, or a first weight loss amount to a second weight loss amount that is greater than a weight loss limit, as a function of time in contact with the fluid. The dispersed particle-nanomatrix composite is characteristic of the powder compacts  200  described herein and includes a cellular nanomatrix  216  of nanomatrix material  220 , a plurality of dispersed particles  214  including particle core material  218  that is dispersed within the matrix. Nanomatrix  216  is characterized by a solid-state bond layer  219  which extends throughout the nanomatrix. The time in contact with the fluid described above may include the CST as described above. The CST may include a predetermined time that is desired or required to dissolve a predetermined portion of the powder compact  200  that is in contact with the fluid. The CST may also include a time corresponding to a change in the property of the engineered material or the fluid, or a combination thereof. In the case of a change of property of the engineered material, the change may include a change of a temperature of the engineered material. In the case where there is a change in the property of the fluid, the change may include the change in a fluid temperature, pressure, flow rate, chemical composition or pH or a combination thereof. Both the engineered material and the change in the property of the engineered material or the fluid, or a combination thereof, may be tailored to provide the desired CST response characteristic, including the rate of change of the particular property (e.g., weight loss, loss of strength) both prior to the CST (e.g., Stage  1 ) and after the CST (e.g., Stage  2 ), as illustrated in  FIG. 22 . 
     Referring to  FIG. 17 , a method  400  of making a powder compact  200  is described. Method  400  includes forming  410  a coated metallic powder  10  comprising powder particles  12  having particle cores  14  with nanoscale metallic coating layers  16  disposed thereon, wherein the metallic coating layers  16  have a chemical composition and the particle cores  14  have a chemical composition that is different than the chemical composition of the metallic coating material  16 . Method  400  also includes forming  420  a powder compact by applying a predetermined temperature and a predetermined pressure to the coated powder particles sufficient to sinter them by solid-phase sintering of the coated layers of the plurality of the coated particle powders  12  to form a substantially-continuous, cellular nanomatrix  216  of a nanomatrix material  220  and a plurality of dispersed particles  214  dispersed within nanomatrix  216  as described herein. 
     Forming  410  of coated metallic powder  10  comprising powder particles  12  having particle cores  14  with nanoscale metallic coating layers  16  disposed thereon may be performed by any suitable method. In an exemplary embodiment, forming  410  includes applying the metallic coating layers  16 , as described herein, to the particle cores  14 , as described herein, using fluidized bed chemical vapor deposition (FBCVD) as described herein. Applying the metallic coating layers  16  may include applying single-layer metallic coating layers  16  or multilayer metallic coating layers  16  as described herein. Applying the metallic coating layers  16  may also include controlling the thickness of the individual layers as they are being applied, as well as controlling the overall thickness of metallic coating layers  16 . Particle cores  14  may be formed as described herein. 
     Forming  420  of the powder compact  200  may include any suitable method of forming a fully-dense compact of powder  10 . In an exemplary embodiment, forming  420  includes dynamic forging of a green-density precursor powder compact  100  to apply a predetermined temperature and a predetermined pressure sufficient to sinter and deform the powder particles and form a fully-dense nanomatrix  216  and dispersed particles  214  as described herein. Dynamic forging as used herein means dynamic application of a load at temperature and for a time sufficient to promote sintering of the metallic coating layers  16  of adjacent powder particles  12 , and may preferably include application of a dynamic forging load at a predetermined loading rate for a time and at a temperature sufficient to form a sintered and fully-dense powder compact  200 . In an exemplary embodiment, dynamic forging included: 1) heating a precursor or green-state powder compact  100  to a predetermined solid phase sintering temperature, such as, for example, a temperature sufficient to promote interdiffusion between metallic coating layers  16  of adjacent powder particles  12 ; 2) holding the precursor powder compact  100  at the sintering temperature for a predetermined hold time, such as, for example, a time sufficient to ensure substantial uniformity of the sintering temperature throughout the precursor compact  100 ; 3) forging the precursor powder compact  100  to full density, such as, for example, by applying a predetermined forging pressure according to a predetermined pressure schedule or ramp rate sufficient to rapidly achieve full density while holding the compact at the predetermined sintering temperature; and 4) cooling the compact to room temperature. The predetermined pressure and predetermined temperature applied during forming  420  will include a sintering temperature, T S , and forging pressure, P F , as described herein that will ensure solid-state sintering and deformation of the powder particles  12  to form fully-dense powder compact  200 , including solid-state bond  217  and bond layer  219 . The steps of heating to and holding the precursor powder compact  100  at the predetermined sintering temperature for the predetermined time may include any suitable combination of temperature and time, and will depend, for example, on the powder  10  selected, including the materials used for particle core  14  and metallic coating layer  16 , the size of the precursor powder compact  100 , the heating method used and other factors that influence the time needed to achieve the desired temperature and temperature uniformity within precursor powder compact  100 . In the step of forging, the predetermined pressure may include any suitable pressure and pressure application schedule or pressure ramp rate sufficient to achieve a fully-dense powder compact  200 , and will depend, for example, on the material properties of the powder particles  12  selected, including temperature dependent stress/strain characteristics (e.g., stress/strain rate characteristics), interdiffusion and metallurgical thermodynamic and phase equilibria characteristics, dislocation dynamics and other material properties. For example, the maximum forging pressure of dynamic forging and the forging schedule (i.e., the pressure ramp rates that correspond to strain rates employed) may be used to tailor the mechanical strength and toughness of the powder compact. The maximum forging pressure and forging ramp rate (i.e., strain rate) is the pressure just below the compact cracking pressure, i.e., where dynamic recovery processes are unable to relieve strain energy in the compact microstructure without the formation of a crack in the compact. For example, for applications that require a powder compact that has relatively higher strength and lower toughness, relatively higher forging pressures and ramp rates may be used. If relatively higher toughness of the powder compact is needed, relatively lower forging pressures and ramp rates may be used. 
     For certain exemplary embodiments of powders  10  described herein and precursor compacts  100  of a size sufficient to form many wellbore tools and components, predetermined hold times of about 1 to about 5 hours may be used. The predetermined sintering temperature, T S , will preferably be selected as described herein to avoid melting of either particle cores  14  or metallic coating layers  16  as they are transformed during method  400  to provide dispersed particles  214  and nanomatrix  216 . For these embodiments, dynamic forging may include application of a forging pressure, such as by dynamic pressing to a maximum of about 80 ksi at pressure ramp rate of about 0.5 to about 2 ksi/second. 
     In an exemplary embodiment where particle cores  14  included Mg and metallic coating layer  16  included various single and multilayer coating layers as described herein, such as various single and multilayer coatings comprising Al, the dynamic forging was performed by sintering at a temperature, T S , of about 450° C. to about 470° C. for up to about 1 hour without the application of a forging pressure, followed by dynamic forging by application of isostatic pressures at ramp rates between about 0.5 to about 2 ksi/second to a maximum pressure, P S , of about 30 ksi to about 60 ksi, which resulted in forging cycles of 15 seconds to about 120 seconds. The short duration of the forging cycle is a significant advantage as it limits interdiffusion, including interdiffusion within a given metallic coating layer  16 , interdiffusion between adjacent metallic coating layers  16  and interdiffusion between metallic coating layers  16  and particle cores  14 , to that needed to form metallurgical bond  217  and bond layer  219 , while also maintaining the desirable equiaxed dispersed particle  214  shape with the integrity of cellular nanomatrix  216  strengthening phase. The duration of the dynamic forging cycle is much shorter than the forming cycles and sintering times required for conventional powder compact forming processes, such as hot isostatic pressing (HIP), pressure assisted sintering or diffusion sintering. 
     Method  400  may also optionally include forming  430  a precursor powder compact by compacting the plurality of coated powder particles  12  sufficiently to deform the particles and form interparticle bonds to one another and form the precursor powder compact  100  prior to forming  420  the powder compact. Compacting may include pressing, such as isostatic pressing, of the plurality of powder particles  12  at room temperature to form precursor powder compact  100 . Compacting  430  may be performed at room temperature. In an exemplary embodiment, powder  10  may include particle cores  14  comprising Mg and forming  430  the precursor powder compact may be performed at room temperature at an isostatic pressure of about 10 ksi to about 60 ksi. 
     Method  400  may optionally also include intermixing  440  a second powder  30  into powder  10  as described herein prior to the forming  420  the powder compact, or forming  430  the precursor powder compact. 
     Without being limited by theory, powder compacts  200  are formed from coated powder particles  12  that include a particle core  14  and associated core material  18  as well as a metallic coating layer  16  and an associated metallic coating material  20  to form a substantially-continuous, three-dimensional, cellular nanomatrix  216  that includes a nanomatrix material  220  formed by sintering and the associated diffusion bonding of the respective coating layers  16  that includes a plurality of dispersed particles  214  of the particle core materials  218 . This unique structure may include metastable combinations of materials that would be very difficult or impossible to form by solidification from a melt having the same relative amounts of the constituent materials. The coating layers and associated coating materials may be selected to provide selectable and controllable dissolution in a predetermined fluid environment, such as a wellbore environment, where the predetermined fluid may be a commonly used wellbore fluid that is either injected into the wellbore or extracted from the wellbore. As will be further understood from the description herein, controlled dissolution of the nanomatrix exposes the dispersed particles of the core materials. The particle core materials may also be selected to also provide selectable and controllable dissolution in the wellbore fluid. Alternately, they may also be selected to provide a particular mechanical property, such as compressive strength or sheer strength, to the powder compact  200 , without necessarily providing selectable and controlled dissolution of the core materials themselves, since selectable and controlled dissolution of the nanomatrix material surrounding these particles will necessarily release them so that they are carried away by the wellbore fluid. The microstructural morphology of the substantially-continuous, cellular nanomatrix  216 , which may be selected to provide a strengthening phase material, with dispersed particles  214 , which may be selected to provide equiaxed dispersed particles  214 , provides these powder compacts with enhanced mechanical properties, including compressive strength and sheer strength, since the resulting morphology of the nanomatrix/dispersed particles can be manipulated to provide strengthening through the processes that are akin to traditional strengthening mechanisms, such as grain size reduction, solution hardening through the use of impurity atoms, precipitation or age hardening and strength/work hardening mechanisms. The nanomatrix/dispersed particle structure tends to limit dislocation movement by virtue of the numerous particle nanomatrix interfaces, as well as interfaces between discrete layers within the nanomatrix material as described herein. This is exemplified in the fracture behavior of these materials, as illustrated in  FIGS. 23 and 24 . In  FIG. 23 , a powder compact  200  made using uncoated pure Mg powder and subjected to a shear stress sufficient to induce failure demonstrated intergranular fracture. In contrast, in  FIG. 24 , a powder compact  200  made using powder particles  12  having pure Mg powder particle cores  14  to form dispersed particles  214  and metallic coating layers  16  that includes Al to form nanomatrix  216  and subjected to a shear stress sufficient to induce failure demonstrated transgranular fracture and a substantially higher fracture stress as described herein. Because these materials have high-strength characteristics, the core material and coating material may be selected to utilize low density materials or other low density materials, such as low-density metals, ceramics, glasses or carbon, that otherwise would not provide the necessary strength characteristics for use in the desired applications, including wellbore tools and components. 
     Somewhat related to such materials are US Publication 2010/0041155; 8,021,721 and 8,006757. 
     The present invention relates in part to a method of using the controlled electrolytic materials (CEM) as described above to seal existing perforations to facilitate refracturing of a zone that had been previously fractured by using a bottom hole assembly (BHA) that isolates a now plugged perforation that was plugged with CEM to remove it in a predetermined time interval so that the same perforation can be refractured. Additionally new perforations can also be made such as with a jet tool before the CEM covered perforations are removed by fluid delivered to the BHA. Depending on the placement of the BHA with respect to the perforation to be refractured the refracturing can occur in the annulus surrounding the BHA or through the coiled or other tubing supporting the BHA. This versatility allows the refracturing to occur in either bottom up or top down directions or, stated differently for horizontal boreholes, toward the surface or away from the surface. These and other aspects of the present invention will be more apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be found in the appended claims. 
     SUMMARY OF THE INVENTION 
     The method of the present invention deploys CEM into the existing perforations to seal them and then using a BHA that isolates a portion of the wellbore to deliver a material that removes the CEM at a predetermined rate so that the BHA can be used to refracture the recently opened perforation. Additional new perforations can be made and fractured during the process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is prior art a photomicrograph of a powder  10  as disclosed herein that has been embedded in an epoxy specimen mounting material and sectioned; 
         FIG. 2  is a prior art schematic illustration of an exemplary embodiment of a powder particle  12  as it would appear in an exemplary section view represented by section  2 - 2  of  FIG. 1 ; 
         FIG. 3  is a prior art schematic illustration of a second exemplary embodiment of a powder particle  12  as it would appear in a second exemplary section view represented by section  2 - 2  of  FIG. 1 ; 
         FIG. 4  is a prior art schematic illustration of a third exemplary embodiment of a powder particle  12  as it would appear in a third exemplary section view represented by section  2 - 2  of  FIG. 1 ; 
         FIG. 5  is a prior art schematic illustration of a fourth exemplary embodiment of a powder particle  12  as it would appear in a fourth exemplary section view represented by section  2 - 2  of  FIG. 1 ; 
         FIG. 6  is a prior art schematic illustration of a second exemplary embodiment of a powder as disclosed herein having a multi-modal distribution of particle sizes; 
         FIG. 7  is a prior art schematic illustration of a third exemplary embodiment of a powder as disclosed herein having a multi-modal distribution of particle sizes; 
         FIG. 8  is a prior art flow chart of an exemplary embodiment of a method of making a powder as disclosed herein; 
         FIG. 9  is a prior art photomicrograph of an exemplary embodiment of a powder compact as disclosed herein; 
         FIG. 10  is a prior art schematic of illustration of an exemplary embodiment of the powder compact of  FIG. 9  made using a powder having single-layer coated powder particles as it would appear taken along section  10 - 10 ; 
         FIG. 11  is a prior art schematic illustration of an exemplary embodiment of a powder compact as disclosed herein having a homogenous multi-modal distribution of particle sizes; 
         FIG. 12  is a prior art schematic illustration of an exemplary embodiment of a powder compact as disclosed herein having a non-homogeneous, multi-modal distribution of particle sizes; 
         FIG. 13  is a prior art schematic illustration of an exemplary embodiment of a powder compact as disclosed herein formed from a first powder and a second powder and having a homogenous multi-modal distribution of particle sizes; 
         FIG. 14  is a prior art schematic illustration of an exemplary embodiment of a powder compact as disclosed herein formed from a first powder and a second powder and having a non-homogeneous multi-modal distribution of particle sizes. 
         FIG. 15  is a prior art schematic of illustration of another exemplary embodiment of the powder compact of  FIG. 9  made using a powder having multilayer coated powder particles as it would appear taken along section  10 - 10 ; 
         FIG. 16  is a prior art schematic cross-sectional illustration of an exemplary embodiment of a precursor powder compact; 
         FIG. 17  is a prior art flow chart of an exemplary embodiment of a method of making a powder compact as disclosed herein; 
         FIG. 18  is a prior art table that describes the particle core and metallic coating layer configurations for powder particles and powders used to make exemplary embodiments of powder compacts for testing as disclosed herein; 
         FIG. 19  a prior art plot of the compressive strength of the powder compacts of  FIG. 18  both dry and in an aqueous solution comprising 3% KCl; 
         FIG. 20  is a prior art plot of the rate of corrosion (ROC) of the powder compacts of  FIG. 18  in an aqueous solution comprising 3% KCl at 200° F. and room temperature; 
         FIG. 21  is a prior art plot of the ROC of the powder compacts of  FIG. 18  in 15% HCl; 
         FIG. 22  is a prior art schematic illustration of a change in a property of a powder compact as disclosed herein as a function of time and a change in condition of the powder compact environment; 
         FIG. 23  is a prior art electron photomicrograph of a fracture surface of a powder compact formed from a pure Mg powder; 
         FIG. 24  is a prior art electron photomicrograph of a fracture surface of an exemplary embodiment of a powder metal compact as described herein; 
         FIG. 25  is a prior art plot of compressive strength of a powder compact as a function the amount of a constituent (Al 2 O 3 ) of the cellular nanomatrix; 
         FIG. 26  shows a plug and perforate well that is in need of refracturing; 
         FIG. 27  is the view of  FIG. 26  with the CEM balls in position to block the existing perforations; 
         FIG. 28  is the view of  FIG. 27  showing the BHA that is used to selectively open blocked perforations with a corrosive material so that refracturing can take place. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 26  shows a horizontal zone  500  that has been perforated before leaving perforations  501 - 507  near a heel  508 . A production string  510  extends from a wellhead  512  at the surface  514  which can be subsea. The well in  FIG. 26  is in need of re-stimulation. In order to accomplish this, the existing perforations  501 - 507  need to first be sealed and then selectively opened so that refracturing can take place through them or alternatively new perforations can be created in the zone  500 . 
       FIG. 27  shows the controlled electrolytic materials (CEM) as described above preferably in the form of spheres  516  being pumped to the perforations  501 - 507  until such time as those perforations are plugged up as detected by a surface pressure buildup at the surface  514 . 
     After that is accomplished a surface acid tank  517  in conjunction with pump  518  is used to deliver the corrosive material through a coiled tubing unit  520  that features a gooseneck  522  through a lubricator  524 . The coiled tubing  525  supports a bottom hole assembly  526  that has one of several configurations. As shown in  FIG. 28  in one embodiment there is a resettable packer  528  with either a lower end outlet  530  or a side outlet through a circulation sub above the packer  528  as shown schematically by arrow  530 . The CEM is delivered to all perforations  501 - 507  with the packer  528  unset. 
     If using the bottom outlet  530  the packer is set above a target perforation that happens to be plugged with the CEM spheres  516  and the corrosive material from tank  517  is delivered to the zone such as  507  which is the lowermost zone. To do this the packer  528  is set between zones  506  and  507  and the corrosive material opens the perforation  507  in a predetermined time whereupon the frac fluid can be pumped through the coiled tubing  525  to the exit  530  to now refracture the perforations  507  through the coiled tubing  525 . 
     In the event the circulating sub  530  is used then the packer is initially located below perforations  507  and acid from tank  517  in a measured amount is spotted at perforations  507  but is stopped short of perforations  506  due to precise measuring of the amount of acid needed to cover the perforations  507 . After waiting the predetermined time for the CEM balls to be removed, the frac fluid is delivered through the annulus  532  while the coiled tubing  525  is closed off at the surface  514  such as by operating valves on the coiled tubing unit  520 . The packer  528  is released and relocated to just below perforations  506  and the process is repeated for a bottom up order for the refracturing. 
     Alternatively, a spaced pair of packers  528  can be used with a circulation sub in between them. When doing this the amount of acid from the tank  517  does not need to be as accurately measured because the possibility of reaching the next adjacent perforation with the acid is eliminated with the pair of packers  528  rather than leaving the other perforations open to acid flow when using a single packer and trying to spot the acid adjacent a single target formation. With the spaced packer the refracturing can occur in any order. 
     Those skilled in the art will appreciate that using CEM allows occluding the existing perforations in a plug and perforate well so that the perforations can be sequentially opened in a known amount of time with a corrosive material spotted adjacent the isolated perforation or the perforation adjacent the packer. Refracturing follows after a known amount of time has passed with acid exposure to the CEM to sufficiently open the perforation for refracturing. In the preferred way the single resettable packer is used in conjunction with the circulation sub so that the packer is set below a perforation of interest and acid is delivered through the string in a predetermined amount so that the acidic material just reaches the perforation in interest. After a predetermined amount of time the fracturing takes places through the surrounding annulus with the coiled tubing closed off so as to reduce friction losses and the potential for sand buildup in the wellbore such as when using the alternate configuration of refracturing through the coiled tubing itself. The use of the CEM material allows precise control of the amount of time it will take to sufficiently undermine the CEM plug at the perforation in question so that the refracturing can proceed. Some parts of the CEM plug can be pushed into the perforation with the refracturing without adversely affecting the access for the refracturing. 
     The above description is illustrative of the preferred embodiment and many modifications may be made by those skilled in the art without departing from the invention whose scope is to be determined from the literal and equivalent scope of the claims below: