Abstract:
A method of unplugging a seat, including dissolving at least a surface of a plug seated against the seat, and unseating the plug from the seat.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application contains subject matter related to the subject matter of co-pending applications, which are assigned to the same assignee as this application, Baker Hughes Incorporated of Houston, Tex. that were all filed on Dec. 8, 2009. The below listed applications are hereby incorporated by reference in their entirety: 
     U.S. patent application Ser. No. 12/633,682, entitled NANOMATRIX POWDER METAL COMPACT; 
     U.S. patent application Ser. No. 12/633,686, entitled COATED METALLIC POWDER AND METHOD OF MAKING THE SAME; 
     U.S. patent application Ser. No. 12/633,688, entitled METHOD OF MAKING A NANOMATRIX POWDER METAL COMPACT; and 
     U.S. patent application Ser. No. 12/633,678, entitled ENGINEERED POWDER COMPACT COMPOSITE MATERIAL. 
     BACKGROUND 
     In the drilling and completion industry it is often desirable to utilize what is known to the art as tripping balls, darts, (generically plugs) for a number of different operations requiring pressure up events. As is known to one of skill in the art, tripping balls are dropped at selected times to seat in a downhole ball seat and create a seal there. The seal that is created is often intended to be temporary. After the operation for which the tripping ball was dropped is completed, the ball is removed from the wellbore by methods such as reverse circulating the ball out of the well. Doing so, however, requires that the ball dislodge from the seat. At times balls can become stuck to a seat thereby preventing it from being circulated out of the well, thereby requiring more time consuming and costly methods of removing the ball, such as, through drilling the ball out, for example. Devices and methods that allow an operator to remove a ball without resorting to such a costly process would be well received by the art. 
     BRIEF DESCRIPTION 
     Disclosed herein is a method of unplugging a seat, including dissolving at least a surface of a plug seated against the seat, and unseating the plug from the seat. 
     Also disclosed is a plug including a body having an outer surface configured to seatingly engage a seat wherein at least the outer surface of the plug is configured to dissolve upon exposure to a target environment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
         FIG. 1  depicts a cross sectional view of a plug disclosed herein within a tubular; 
         FIG. 2  depicts a cross sectional view of an alternate plug disclosed herein; 
         FIG. 3  is a photomicrograph of a powder  210  as disclosed herein that has been embedded in a potting material and sectioned; 
         FIG. 4  is a schematic illustration of an exemplary embodiment of a powder particle  12  as it would appear in an exemplary section view represented by section  4 - 4  of  FIG. 3 ; 
         FIG. 5  is a photomicrograph of an exemplary embodiment of a powder compact as disclosed herein; 
         FIG. 6  is a schematic of illustration of an exemplary embodiment of a powder compact made using a powder having single-layer powder particles as it would appear taken along section  6 - 6  in  FIG. 5 ; 
         FIG. 7  is a schematic of illustration of another exemplary embodiment of a powder compact made using a powder having multilayer powder particles as it would appear taken along section  6 - 6  in  FIG. 5 ; 
         FIG. 8  is a 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. 
     
    
    
     DETAILED DESCRIPTION 
     A detailed description of one or more embodiments of the disclosed apparatus and method are presented herein by way of exemplification and not limitation with reference to the Figures. 
     Referring to  FIG. 1 , an embodiment of a tripping ball, also described herein in a more generic term as a plug is illustrated generally at  10 . Although the plug  10  is illustrated as a ball other shapes are contemplated such as conical, elliptical, etc. The plug  10  is configured to seatingly engage with a seat  14 . The seat  14  illustrated herein includes a conical surface  18  sealingly engaged with a tubular  22 . Seating engagement of the plug  10  with the seat  14  allows the body  12  to seal to the seat  14  thereby permitting pressure to be built thereagainst. The body  12  has an outer surface  26  that is configured to dissolve upon exposure to an environment  30  that is anticipated during deployment of the plug  10 . This dissolution can include corrosion, for example, in applications wherein the outer surface  26  is part of an electrochemical cell. The dissolution of the outer surface  26  allows the body  12 , when it has become stuck, wedged or lodged to the seat  14 , to be dislodged and unsealed therefrom. This dislodging can be due, at least in part, to a decrease in frictional engagement between the plug  10  and the seat  14  as the body  12  begins to dissolve. Additionally, the dislodging is due to dimensional changes of the plug  10  as the body  12  dissolves initially from the outer surface  26 . 
     The ability to dislodge the plug  10  from the seat  14  is particularly helpful in instances where the plug  10  has become wedged into an opening  34  of the seat  14 . The severity of such wedging can be significant in cases where the body  12  has become deformed due to forces urging the plug  10  against the seat  14 . Such deformation can cause a portion  38  of the body  12  to extend into the opening  34 , thereby increasing frictional engagement between the portion  38  and a dimension  42  of the opening  34 . 
     In applications for use in the drilling and completion industries, as discussed above, wherein the plug  10  is a tripping ball the ball will be exposed to a downhole environment  30 . The downhole environment  30  may include high temperatures, high pressures, and wellbore fluids, such as, caustic chemicals, acids, bases and brine solutions, for example. By making the body  12  of a material  46  (This is not shown in any fig) that degrades in strength in the environment  30 , the body  12  can be made to effectively dissolve in response to exposure to the downhole environment  30 . The initiation of dissolution or disintegration of the body  12  can begin at the outer surface  26  as the strength of the outer surface  26  decreases first and can propagate to the balance of the body  12 . Possible choices for the material  46  include but are not limited to Magnesium, polymeric adhesives such as structural methacrylate adhesive, high strength dissolvable Material (discussed in detail later in this specification), etc. 
     The body  12  and the outer surface  26  of the plug  10  in the embodiment of  FIG. 1  are both made of the material  46 . As such, dissolution of the material  46  can leave both the body  12  and the outer surface  26  in small pieces that are not detrimental to further operation of the well, thereby negating the need to either pump the body  12  out of the tubular  22  or run a tool within the wellbore to drill or mill the body  12  into pieces small enough to remove hindrance therefrom. 
     Referring to  FIG. 2 , an alternate embodiment of a plug disclosed herein is illustrated at  110 . Unlike the plug  10  the plug  110  has a body  112  made of at least two different materials. The body  112  includes a core  116  made of a first material  117  and a shell  120  made of a second material  121 . Since, in this embodiment, an outer surface  126  (this is not shown in the figs) that actually contacts the seat  14  is only on the shell  120 , only the second material  121  needs to be dissolvable in the target environment  30 . In contrast, the first material  117  may or may not be dissolvable in the environment  30 . 
     If the first material  117  is not dissolvable it may be desirable to make a greatest dimension  124  of the core  116  less than the dimension  42  of the seat  14  to permit the core  116  to pass therethrough after dissolution of the shell  120 . In so doing the core  116  can be run, or allowed to drop down, out of a lower end of the tubular  22  instead of being pumped upward to remove it therefrom. 
     As introduced above, further materials that may be utilized with the ball as described herein are 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  FIG. 3 , a metallic powder  210  includes a plurality of metallic, coated powder particles  212 . Powder particles  212  may be formed to provide a powder  210 , 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 powder compacts  400  ( FIGS. 6 and 7 ), 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  212  of powder  210  includes a particle core  214  and a metallic coating layer  216  disposed on the particle core  214 . The particle core  214  includes a core material  218 . The core material  218  may include any suitable material for forming the particle core  214  that provides powder particle  212  that can be sintered to form a lightweight, high-strength powder compact  400  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  218  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  218  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  214  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  214  of these core materials  218  is high, even though core material  218  itself may have a low dissolution rate, including core materials  220  that may be substantially insoluble in the wellbore fluid. 
     With regard to the electrochemically active metals as core materials  218 , 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  214 , such as by improving the strength, lowering the density or altering the dissolution characteristics of the core material  218 . 
     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  214  and core material  218 , 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  214  and core material  218  have a melting temperature (T P ). As used herein, Tp includes the lowest temperature at which incipient melting or liquation or other forms of partial melting occur within core material  218 , regardless of whether core material  218  comprises a pure metal, an alloy with multiple phases having different melting temperatures or a composite of materials having different melting temperatures. 
     Particle cores  214  may have any suitable particle size or range of particle sizes or distribution of particle sizes. For example, the particle cores  214  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. 3 . In another example, particle cores  214  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. The selection of the distribution of particle core size may be used to determine, for example, the particle size and interparticle spacing  215  of the particles  212  of powder  210 . In an exemplary embodiment, the particle cores  214  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  214  may have any suitable particle shape, including any regular or irregular geometric shape, or combination thereof In an exemplary embodiment, particle cores  214  are substantially spheroidal electrochemically active metal particles. In another exemplary embodiment, particle cores  214  are substantially irregularly shaped ceramic particles. In yet another exemplary embodiment, particle cores  214  are carbon or other nanotube structures or hollow glass microspheres. 
     Each of the metallic, coated powder particles  212  of powder  210  also includes a metallic coating layer  216  that is disposed on particle core  214 . Metallic coating layer  216  includes a metallic coating material  220 . Metallic coating material  220  gives the powder particles  212  and powder  210  its metallic nature. Metallic coating layer  216  is a nanoscale coating layer. In an exemplary embodiment, metallic coating layer  216  may have a thickness of about 25 nm to about 2500 nm. The thickness of metallic coating layer  216  may vary over the surface of particle core  214 , but will preferably have a substantially uniform thickness over the surface of particle core  214 . Metallic coating layer  216  may include a single layer, as illustrated in  FIG. 4 , or a plurality of layers as a multilayer coating structure. In a single layer coating, or in each of the layers of a multilayer coating, the metallic coating layer  216  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  216 , each of the respective layers, or combinations of them, may be used to provide a predetermined property to the powder particle  212  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  214  and the coating material  220 ; the interdiffusion characteristics between the particle core  214  and metallic coating layer  216 , including any interdiffusion between the layers of a multilayer coating layer  216 ; the interdiffusion characteristics between the various layers of a multilayer coating layer  216 ; the interdiffusion characteristics between the metallic coating layer  216  of one powder particle and that of an adjacent powder particle  212 ; the bond strength of the metallurgical bond between the metallic coating layers of adjacent sintered powder particles  212 , including the outermost layers of multilayer coating layers; and the electrochemical activity of the coating layer  216 . 
     Metallic coating layer  216  and coating material  220  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  220 , regardless of whether coating 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 coating material layers having different melting temperatures. 
     Metallic coating material  220  may include any suitable metallic coating material  220  that provides a sinterable outer surface  221  that is configured to be sintered to an adjacent powder particle  212  that also has a metallic coating layer  216  and sinterable outer surface  221 . In powders  210  that also include second or additional (coated or uncoated) particles  232 , as described herein, the sinterable outer surface  221  of metallic coating layer  216  is also configured to be sintered to a sinterable outer surface  221  of second particles  232 . In an exemplary embodiment, the powder particles  212  are sinterable at a predetermined sintering temperature (T S ) that is a function of the core material  218  and coating material  220 , such that sintering of powder compact  400  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  214 /metallic coating layer  216  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  214 /metallic coating layer  216  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  400  as described herein. 
     In an exemplary embodiment, core material  218  will be selected to provide a core chemical composition and the coating material  220  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  218  will be selected to provide a core chemical composition and the coating material  220  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  220  and core material  218  may be selected to provide different dissolution rates and selectable and controllable dissolution of powder compacts  400  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  400  formed from powder  210  having chemical compositions of core material  218  and coating material  220  that make compact  400  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. 
     As illustrated in  FIGS. 3 and 5 , particle core  214  and core material  218  and metallic coating layer  216  and coating material  220  may be selected to provide powder particles  212  and a powder  210  that is configured for compaction and sintering to provide a powder compact  400  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  400  includes a substantially-continuous, cellular nanomatrix  416  of a nanomatrix material  420  having a plurality of dispersed particles  414  dispersed throughout the cellular nanomatrix  416 . The substantially-continuous cellular nanomatrix  416  and nanomatrix material  420  formed of sintered metallic coating layers  216  is formed by the compaction and sintering of the plurality of metallic coating layers  216  of the plurality of powder particles  212 . The chemical composition of nanomatrix material  420  may be different than that of coating material  220  due to diffusion effects associated with the sintering as described herein. Powder metal compact  400  also includes a plurality of dispersed particles  414  that comprise particle core material  418 . Dispersed particle cores  414  and core material  418  correspond to and are formed from the plurality of particle cores  214  and core material  218  of the plurality of powder particles  212  as the metallic coating layers  216  are sintered together to form nanomatrix  416 . The chemical composition of core material  418  may be different than that of core material  218  due to diffusion effects associated with sintering as described herein. 
     As used herein, the use of the term substantially-continuous cellular nanomatrix  416  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  420  within powder compact  400 . As used herein, “substantially-continuous” describes the extension of the nanomatrix material throughout powder compact  400  such that it extends between and envelops substantially all of the dispersed particles  414 . Substantially-continuous is used to indicate that complete continuity and regular order of the nanomatrix around each dispersed particle  414  is not required. For example, defects in the coating layer  216  over particle core  214  on some powder particles  212  may cause bridging of the particle cores  214  during sintering of the powder compact  400 , thereby causing localized discontinuities to result within the cellular nanomatrix  416 , 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  420  that encompass and also interconnect the dispersed particles  414 . 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  414 . 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  414 , generally comprises the interdiffusion and bonding of two coating layers  216  from adjacent powder particles  212  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  414  does not connote the minor constituent of powder compact  400 , 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  418  within powder compact  400 . 
     Powder compact  400  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 sintering and pressing processes used to form powder compact  400  and deform the powder particles  212 , including particle cores  214  and coating layers  216 , to provide the full density and desired macroscopic shape and size of powder compact  400  as well as its microstructure. The microstructure of powder compact  400  includes an equiaxed configuration of dispersed particles  414  that are dispersed throughout and embedded within the substantially-continuous, cellular nanomatrix  416  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  416  of sintered metallic coating layers  216  may be produced using constituents where thermodynamic phase equilibrium conditions would not produce an equiaxed structure. The equiaxed morphology of the dispersed particles  414  and cellular network  416  of particle layers results from sintering and deformation of the powder particles  212  as they are compacted and interdiffuse and deform to fill the interparticle spaces  215  ( FIG. 3 ). The sintering temperatures and pressures may be selected to ensure that the density of powder compact  400  achieves substantially full theoretical density. 
     In an exemplary embodiment as illustrated in  FIGS. 3 and 5 , dispersed particles  414  are formed from particle cores  214  dispersed in the cellular nanomatrix  416  of sintered metallic coating layers  216 , and the nanomatrix  416  includes a solid-state metallurgical bond  417  or bond layer  419 , as illustrated schematically in  FIG. 6 , extending between the dispersed particles  414  throughout the cellular nanomatrix  416  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  417  is formed in the solid state by solid-state interdiffusion between the coating layers  216  of adjacent powder particles  212  that are compressed into touching contact during the compaction and sintering processes used to form powder compact  400 , as described herein. As such, sintered coating layers  216  of cellular nanomatrix  416  include a solid-state bond layer  419  that has a thickness (t) defined by the extent of the interdiffusion of the coating materials  220  of the coating layers  216 , which will in turn be defined by the nature of the coating layers  216 , 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  400 . 
     As nanomatrix  416  is formed, including bond  417  and bond layer  419 , the chemical composition or phase distribution, or both, of metallic coating layers  216  may change. Nanomatrix  416  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  416 , regardless of whether nanomatrix material  420  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  414  and particle core materials  418  are formed in conjunction with nanomatrix  416 , diffusion of constituents of metallic coating layers  216  into the particle cores  214  is also possible, which may result in changes in the chemical composition or phase distribution, or both, of particle cores  214 . As a result, dispersed particles  414  and particle core materials  418  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  400  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  414  may comprise any of the materials described herein for particle cores  214 , even though the chemical composition of dispersed particles  414  may be different due to diffusion effects as described herein. In an exemplary embodiment, dispersed particles  414  are formed from particle cores  214  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  214 . Of these materials, those having dispersed particles  414  comprising Mg and the nanomatrix  416  formed from the metallic coating materials  216  described herein are particularly useful. Dispersed particles  414  and particle core material  418  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  214 . 
     In another exemplary embodiment, dispersed particles  414  are formed from particle cores  214  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  414  of powder compact  400  may have any suitable particle size, including the average particle sizes described herein for particle cores  214 . 
     Dispersed particles  414  may have any suitable shape depending on the shape selected for particle cores  214  and powder particles  212 , as well as the method used to sinter and compact powder  210 . In an exemplary embodiment, powder particles  212  may be spheroidal or substantially spheroidal and dispersed particles  414  may include an equiaxed particle configuration as described herein. 
     The nature of the dispersion of dispersed particles  414  may be affected by the selection of the powder  210  or powders  210  used to make particle compact  400 . In one exemplary embodiment, a powder  210  having a unimodal distribution of powder particle  212  sizes may be selected to form powder compact  2200  and will produce a substantially homogeneous unimodal dispersion of particle sizes of dispersed particles  414  within cellular nanomatrix  416 , as illustrated generally in  FIG. 5 . In another exemplary embodiment, a plurality of powders  210  having a plurality of powder particles with particle cores  214  that have the same core materials  218  and different core sizes and the same coating material  220  may be selected and uniformly mixed as described herein to provide a powder  210  having a homogenous, multimodal distribution of powder particle  212  sizes, and may be used to form powder compact  400  having a homogeneous, multimodal dispersion of particle sizes of dispersed particles  414  within cellular nanomatrix  416 . Similarly, in yet another exemplary embodiment, a plurality of powders  210  having a plurality of particle cores  214  that may have the same core materials  218  and different core sizes and the same coating material  220  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  400  having a non-homogeneous, multimodal dispersion of particle sizes of dispersed particles  414  within cellular nanomatrix  416 . 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  414  within the cellular nanomatrix  416  of powder compacts  400  made from powder  210 . 
     Nanomatrix  416  is a substantially-continuous, cellular network of metallic coating layers  216  that are sintered to one another. The thickness of nanomatrix  416  will depend on the nature of the powder  210  or powders  210  used to form powder compact  400 , as well as the incorporation of any second powder  230 , particularly the thicknesses of the coating layers associated with these particles. In an exemplary embodiment, the thickness of nanomatrix  416  is substantially uniform throughout the microstructure of powder compact  400  and comprises about two times the thickness of the coating layers  216  of powder particles  212 . In another exemplary embodiment, the cellular network  416  has a substantially uniform average thickness between dispersed particles  414  of about 50 nm to about 5000 nm. 
     Nanomatrix  416  is formed by sintering metallic coating layers  216  of adjacent particles to one another by interdiffusion and creation of bond layer  419  as described herein. Metallic coating layers  216  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  216 , or between the metallic coating layer  216  and particle core  214 , or between the metallic coating layer  216  and the metallic coating layer  216  of an adjacent powder particle, the extent of interdiffusion of metallic coating layers  216  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  416  and nanomatrix material  420  may be simply understood to be a combination of the constituents of coating layers  216  that may also include one or more constituents of dispersed particles  414 , depending on the extent of interdiffusion, if any, that occurs between the dispersed particles  414  and the nanomatrix  416 . Similarly, the chemical composition of dispersed particles  414  and particle core material  418  may be simply understood to be a combination of the constituents of particle core  214  that may also include one or more constituents of nanomatrix  416  and nanomatrix material  420 , depending on the extent of interdiffusion, if any, that occurs between the dispersed particles  414  and the nanomatrix  416 . 
     In an exemplary embodiment, the nanomatrix material  420  has a chemical composition and the particle core material  418  has a chemical composition that is different from that of nanomatrix material  420 , 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  400 , including a property change in a wellbore fluid that is in contact with the powder compact  400 , as described herein. Nanomatrix  416  may be formed from powder particles  212  having single layer and multilayer coating layers  216 . This design flexibility provides a large number of material combinations, particularly in the case of multilayer coating layers  216 , that can be utilized to tailor the cellular nanomatrix  416  and composition of nanomatrix material  420  by controlling the interaction of the coating layer constituents, both within a given layer, as well as between a coating layer  216  and the particle core  214  with which it is associated or a coating layer  216  of an adjacent powder particle  212 . Several exemplary embodiments that demonstrate this flexibility are provided below. 
     As illustrated in  FIG. 6 , in an exemplary embodiment, powder compact  400  is formed from powder particles  212  where the coating layer  216  comprises a single layer, and the resulting nanomatrix  416  between adjacent ones of the plurality of dispersed particles  414  comprises the single metallic coating layer  216  of one powder particle  212 , a bond layer  419  and the single coating layer  216  of another one of the adjacent powder particles  212 . The thickness (t) of bond layer  419  is determined by the extent of the interdiffusion between the single metallic coating layers  216 , and may encompass the entire thickness of nanomatrix  416  or only a portion thereof. In one exemplary embodiment of powder compact  400  formed using a single layer powder  210 , powder compact  400  may include dispersed particles  414  comprising Mg, Al, Zn or Mn, or a combination thereof, as described herein, and nanomatrix  416  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  420  of cellular nanomatrix  416 , including bond layer  419 , has a chemical composition and the core material  418  of dispersed particles  414  has a chemical composition that is different than the chemical composition of nanomatrix material  416 . The difference in the chemical composition of the nanomatrix material  420  and the core material  418  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  400  formed from a powder  210  having a single coating layer configuration, dispersed particles  414  include Mg, Al, Zn or Mn, or a combination thereof, and the cellular nanomatrix  416  includes Al or Ni, or a combination thereof. 
     As illustrated in  FIG. 7 , in another exemplary embodiment, powder compact  400  is formed from powder particles  212  where the coating layer  216  comprises a multilayer coating layer  216  having a plurality of coating layers, and the resulting nanomatrix  416  between adjacent ones of the plurality of dispersed particles  414  comprises the plurality of layers (t) comprising the coating layer  216  of one particle  212 , a bond layer  419 , and the plurality of layers comprising the coating layer  216  of another one of powder particles  212 . In  FIG. 7 , this is illustrated with a two-layer metallic coating layer  216 , but it will be understood that the plurality of layers of multi-layer metallic coating layer  216  may include any desired number of layers. The thickness (t) of the bond layer  419  is again determined by the extent of the interdiffusion between the plurality of layers of the respective coating layers  216 , and may encompass the entire thickness of nanomatrix  416  or only a portion thereof. In this embodiment, the plurality of layers comprising each coating layer  216  may be used to control interdiffusion and formation of bond layer  419  and thickness (t). 
     Sintered and forged powder compacts  400  that include dispersed particles  414  comprising Mg and nanomatrix  416  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  400  that have pure Mg dispersed particles  414  and various nanomatrices  416  formed from powders  210  having pure Mg particle cores  214  and various single and multilayer metallic coating layers  216  that include Al, Ni, W or Al 2 O 3 , or a combination thereof. These powders compacts  400  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. Powder compacts  400  that include dispersed particles  414  comprising Mg and nanomatrix  416  comprising various nanomatrix materials  420  described herein have demonstrated 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  400  can be further improved by optimizing powder  210 , particularly the weight percentage of the nanoscale metallic coating layers  16  that are used to form cellular nanomatrix  416 . Strength of the nanomatrix powder metal compact  400  can be further improved by optimizing powder  210 , particularly the weight percentage of the nanoscale metallic coating layers  216  that are used to form cellular nanomatrix  416 . For example, varying the weight percentage (wt. %), i.e., thickness, of an alumina coating within a cellular nanomatrix  416  formed from coated powder particles  212  that include a multilayer (Al/Al 2 O 3 /Al) metallic coating layer  216  on pure Mg particle cores  214  provides an increase of 21% as compared to that of 0 wt % alumina. 
     Powder compacts  400  comprising dispersed particles  414  that include Mg and nanomatrix  416  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  400  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  210 , including relative amounts of constituents of particle cores  214  and metallic coating layer  216 , and are also described herein as being fully-dense powder compacts. Powder compacts  400  comprising dispersed particles that include Mg and nanomatrix  416  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  400  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, powder compacts  400  comprising dispersed particles  414  that include Mg and cellular nanomatrix  416  that includes various nanomatrix materials as described herein have relatively low rates of corrosion in a 3% KCl solution at room temperature that range 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  216 . 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, powder compacts  400  comprising dispersed particles  414  that include Mg and nanomatrix  416  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. 8 , which illustrates that at a selected predetermined critical service time (CST) a changed condition may be imposed upon powder compact  400  as it is applied in a given application, such as a wellbore environment, that causes a controllable change in a property of powder compact  400  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  400  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  400  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  400  and its removal from the wellbore. In the example described above, powder compact  400  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  400  described herein and includes a cellular nanomatrix  416  of nanomatrix material  420 , a plurality of dispersed particles  414  including particle core material  418  that is dispersed within the matrix. Nanomatrix  416  is characterized by a solid-state bond layer  419 , 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  400  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. 8 . 
     Without being limited by theory, powder compacts  400  are formed from coated powder particles  212  that include a particle core  214  and associated core material  218  as well as a metallic coating layer  216  and an associated metallic coating material  220  to form a substantially-continuous, three-dimensional, cellular nanomatrix  216  that includes a nanomatrix material  420  formed by sintering and the associated diffusion bonding of the respective coating layers  216  that includes a plurality of dispersed particles  414  of the particle core materials  418 . 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  400 , 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  416 , which may be selected to provide a strengthening phase material, with dispersed particles  414 , which may be selected to provide equiaxed dispersed particles  414 , 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. A powder compact  400  made using uncoated pure Mg powder and subjected to a shear stress sufficient to induce failure demonstrated intergranular fracture. In contrast, a powder compact  400  made using powder particles  212  having pure Mg powder particle cores  214  to form dispersed particles  414  and metallic coating layers  216  that includes Al to form nanomatrix  416  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. 
     While the invention has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims. Also, in the drawings and the description, there have been disclosed exemplary embodiments of the invention and, although specific terms may have been employed, they are unless otherwise stated used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention therefore not being so limited. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.