Abstract:
A method of dissolving a tool includes, exposing an outer surface of the tool to an environment reactive with the tool, reacting the tool with the environment, applying stress to the tool, concentrating stress on the tool at stress risers in the outer surface, and initiating fracturing the tool at the stress risers.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    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. and are all being filed on Dec. 8, 2009. The below listed applications are hereby incorporated by reference in their entirety:
   U.S. Patent Application Attorney Docket No. MTL4-49581-US (BAO0372US), entitled NANOMATRIX POWDER METAL COMPACT;   U.S. Patent Application Attorney Docket No. OMS4-50039-US (BAO0386US), entitled COATED METALLIC POWDER AND METHOD OF MAKING THE SAME;   U.S. Patent Application Attorney Docket No. MTL4-50132-US (BAO0389US), entitled METHOD OF MAKING A NANOMATRIX POWDER METAL COMPACT;   U.S. Patent Application Attorney Docket No. MTL4-50132-US (BAO0390US) entitled ENGINEERED POWDER COMPACT COMPOSITE MATERIAL;   U.S. Patent Application Attorney Docket No. BSC4-49779-US (BAO0370US) entitled TELESCOPIC UNIT WITH DISSOLVABLE BARRIER;   U.S. Patent Application Attorney Docket No. WBI4-49156-US (BAO0374US) entitled MULTI-COMPONENT DISAPPEARING TRIPPING BALL AND METHOD FOR MAKING THE SAME; and   U.S. Patent Application Attorney Docket No. WBI4-49118-US (BAO0373US) entitled DISSOLVING TOOL AND METHOD.   
 
     
    
     BACKGROUND 
       [0009]    In the subterranean drilling and completion industry there are times when a downhole tool located within a wellbore becomes an unwanted obstruction. Accordingly, downhole tools have been developed that can be deformed, by operator action, for example, such that the tool&#39;s presence becomes less burdensome. Although such tools work as intended, their presence, even in a deformed state can still be undesirable. Devices and methods to further remove the burden created by the presence of unnecessary downhole tools are therefore desirable in the art. 
       BRIEF DESCRIPTION 
       [0010]    Disclosed herein is a method of dissolving a tool. The method includes, exposing an outer surface of the tool to an environment reactive with the tool, reacting the tool with the environment, applying stress to the tool, concentrating stress on the tool at stress risers in the outer surface, and initiating fracturing the tool at the stress risers. 
         [0011]    Further disclosed herein is a dissolvable tool. The tool includes, a body having at least one stress riser configured to concentrate stress thereat to accelerate structural degradation of the body through chemical reaction under applied stress within a reactive environment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
           [0013]      FIG. 1  depicts a quarter cross sectional view of a dissolvable tool disclosed herein; 
           [0014]      FIG. 2  depicts a partial sectioned view of an alternate embodiment of a dissolvable tool disclosed herein; 
           [0015]      FIG. 3  depicts a partial sectioned view of an alternate embodiment of a dissolvable tool disclosed herein; 
           [0016]      FIG. 4  depicts a quarter cross sectional view of an alternate embodiment of a dissolvable tool disclosed herein; 
           [0017]      FIG. 5  is a photomicrograph of a powder as disclosed herein that has been embedded in a potting material and sectioned; 
           [0018]      FIG. 6  is a schematic illustration of an exemplary embodiment of a powder particle as it would appear in an exemplary section view represented by section  6 - 6  of  FIG. 5 ; 
           [0019]      FIG. 7  is a photomicrograph of an exemplary embodiment of a powder compact as disclosed herein; 
           [0020]      FIG. 8  is a schematic illustration of an exemplary embodiment of the powder compact of  FIG. 7  made using a powder having single-layer powder particles as it would appear taken along section  8 - 8 ; 
           [0021]      FIG. 9  is a schematic of illustration of another exemplary embodiment of the powder compact of  FIG. 7  made using a powder having multilayer powder particles as it would appear taken along section  8 - 8 ; and 
           [0022]      FIG. 10  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 
       [0023]    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. 
         [0024]    Referring to  FIG. 1 , a quarter cross sectional view of an embodiment of a dissolvable tool disclosed herein is illustrated generally at  10 . The tool  10 , includes a body  14  illustrated in this embodiment as a ball, however, alternate embodiments are contemplated such as, an ellipsoid, a cylinder or a polyhedron, for example. The body  14  has a surface  18  that has a plurality of stress risers  22 . The stress risers  22  illustrated herein are indentations, however, alternate embodiments may employ stress risers  22  with other configurations, such as, cracks or foreign bodies, for example. Additionally, alternate embodiments may employ any number of stress risers  22  including embodiments with just a single stress riser  22 . The stress risers  22  are configured to concentrate stress at the specific locations of the body  14  where the stress risers  22  are located. This concentrated stress initiates micro-cracks that once nucleated propagate through the body  14  leading to fracture of the body  14 . The stress risers  22  can, therefore, control strength of the body and define values of mechanical stress that will result in failure. Additionally, exposure of the body  14  to environments that are reactive with the material of the body  14  accelerates reaction of the body  14 , such as chemical reactions, for example, at the locations of the stress risers  22 . This accelerated reaction will weaken the body  14  further at the stress riser  22  locations facilitating fracture and dissolution of the tool  10 . 
         [0025]    In an application, such as in the downhole hydrocarbon recovery industry, for example, the tool  10  may be a tripping ball. The ball  10  can be dropped or pumped within a wellbore (not shown), where it seals with a seat allowing pressure to be applied thereagainst to actuate a mechanism, such as a fracturing valve, for example, to open ports in the wellbore to facilitate treatments, like fracturing or acid treating, of a formation. In this application the downhole environment may include high temperatures, high pressures, and caustic chemicals such as acids, bases and brine solutions, for example. By making the body  14  of a material, such as, a lightweight, high-strength metallic material usable in both durable and disposable or degradable articles as disclosed in greater detail starting in paragraph [0031] below, the body  14  can be made to decrease in strength from exposure to the downhole environment. The initiation of dissolution or disintegration of the body  14  in the environment will decrease the strength of the body  14  and will allow the body  14  to fracture under stress, such as mechanical stress, for example. Examples of mechanical stress include stress from hydrostatic pressure and from a pressure differential applied across the body  14  as it is seated against a seat. The fracturing can break the body  14  into many small pieces that are not detrimental to further operation of the well, thereby negating the need to either pump the body  14  out of the wellbore or run a tool within the wellbore to drill or mill the ball into pieces small enough to remove hindrance therefrom. 
         [0026]    The stress risers  22  of  FIG. 1  are indentations that have a plurality of flat surfaces  26 , with three surfaces  26  being shown, that extend from the surface  18  to a vertex  30 . The vertex  30 , being defined as a sharp intersection of the three surfaces  26 , concentrates stress thereat. An additional stress concentration also occurs along lines  34  defined by the intersections of any two of the surfaces  26 . Although the stress risers  22  shown here are indentations defined by flat surfaces  26 , alternate embodiments may employ other stress risers  22  as will be described below. 
         [0027]    Referring to  FIG. 2 , a partial cross sectional view of an alternate embodiment of a dissolvable tool disclosed herein is illustrated generally at  110 . The tool  110  has a body  114  that includes a plurality of stress risers  122  defined by cracks that extend radially inwardly from a surface  118  of the body  114 . 
         [0028]    Referring to  FIG. 3 , a partial cross sectional view of an alternate embodiment of a dissolvable tool disclosed herein is illustrated generally at  210 . The tool  20  has a body  214  that includes a plurality of stress risers  222  defined by foreign bodies  224  embedded therein. The foreign bodies  224  extend radially inwardly from a surface  218  of the body  214 . The foreign bodies  224  can be any material other than the material from which the body  214  is made, however, making the foreign bodies  224  from a material more reactive with the anticipated environment may be desirable to accelerate the weakening of the body  214  further. 
         [0029]    Referring to  FIG. 4 , a quarter cross sectional view of an alternate embodiment of a dissolvable tool disclosed herein is illustrated generally at  310 . The tool  310  has a body  314  made of a shell  316  defining a surface  318 . The shell  316  has a plurality of stress risers  322  that are shown in this embodiment as conical indentations that extend radially inwardly from the surface  318  to a vertex  330 . The vertex  330  is located within the shell  316  and does not extend radially inwardly of an inner surface  334  of the shell  316 . The body  314  may be hollow, may be filled with a fluid  338 , may have a core  342  made of a fluidized material, such as a powder, that may provide some support to the shell  316  while easily dissolving within the environment once the shell  316  is fractured, or may have a solid core  346  made of a softer material than the shell  316 . 
         [0030]    The shell  316  of the tool  310  primarily determines the strength thereof. As such, once micro-cracks form in the shell  316  the compressive load bearing capability is significantly reduced leading to rupture shortly thereafter. Consequently, the stress risers  322  can accurately control timing of strength degradation of the tool  310  once the tool  310  is exposed to a reactive environment. 
         [0031]    Materials for the body  14 ,  114 ,  214 ,  314 , may include, 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. 
         [0032]    Referring to  FIG. 5 , a metallic powder  410  includes a plurality of metallic, coated powder particles  412 . Powder particles  412  may be formed to provide a powder  410 , 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  600  ( FIGS. 8 and 9 ), as described herein, that may be used as, or for use in manufacturing, various articles of manufacture, including various wellbore tools and components. 
         [0033]    Each of the metallic, coated powder particles  412  of powder  410  includes a particle core  414  and a metallic coating layer  416  disposed on the particle core  414 . The particle core  414  includes a core material  418 . The core material  418  may include any suitable material for forming the particle core  414  that provides powder particle  412  that can be sintered to form a lightweight, high-strength powder compact  600  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  418  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  418  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  414  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  414  of these core materials  418  is high, even though core material  418  itself may have a low dissolution rate, including core materials  420  that may be substantially insoluble in the wellbore fluid. 
         [0034]    With regard to the electrochemically active metals as core materials  418 , 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  418  may also include other constituents, including various alloying additions, to alter one or more properties of the particle cores  414 , such as by improving the strength, lowering the density or altering the dissolution characteristics of the core material  418 . 
         [0035]    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  414  and core material  418 , 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. 
         [0036]    Particle core  414  and core material  418  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  418 , regardless of whether core material  418  comprises a pure metal, an alloy with multiple phases having different melting temperatures or a composite of materials having different melting temperatures. 
         [0037]    Particle cores  414  may have any suitable particle size or range of particle sizes or distribution of particle sizes. For example, the particle cores  414  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. 5 . In another example, particle cores  414  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  415  of the particles  412  of powder  410 . In an exemplary embodiment, the particle cores  414  may have a unimodal distribution and an average particle diameter of about 5 μm to about 300 μm, more particularly about 80 nm to about 120 μm, and even more particularly about 100 μm. 
         [0038]    Particle cores  414  may have any suitable particle shape, including any regular or irregular geometric shape, or combination thereof. In an exemplary embodiment, particle cores  414  are substantially spheroidal electrochemically active metal particles. In another exemplary embodiment, particle cores  414  are substantially irregularly shaped ceramic particles. In yet another exemplary embodiment, particle cores  414  are carbon or other nanotube structures or hollow glass microspheres. 
         [0039]    Each of the metallic, coated powder particles  412  of powder  410  also includes a metallic coating layer  416  that is disposed on particle core  414 . Metallic coating layer  416  includes a metallic coating material  420 . Metallic coating material  420  gives the powder particles  412  and powder  410  its metallic nature. Metallic coating layer  16  is a nanoscale coating layer. In an exemplary embodiment, metallic coating layer  416  may have a thickness of about 25 nm to about 2500 nm. The thickness of metallic coating layer  416  may vary over the surface of particle core  414 , but will preferably have a substantially uniform thickness over the surface of particle core  414 . Metallic coating layer  416  may include a single layer, as illustrated in  FIG. 6 , 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  416  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  416 , each of the respective layers, or combinations of them, may be used to provide a predetermined property to the powder particle  412  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  414  and the coating material  420 ; the interdiffusion characteristics between the particle core  414  and metallic coating layer  416 , including any interdiffusion between the layers of a multilayer coating layer  416 ; the interdiffusion characteristics between the various layers of a multilayer coating layer  416 ; the interdiffusion characteristics between the metallic coating layer  416  of one powder particle and that of an adjacent powder particle  412 ; the bond strength of the metallurgical bond between the metallic coating layers of adjacent sintered powder particles  412 , including the outermost layers of multilayer coating layers; and the electrochemical activity of the coating layer  416 . 
         [0040]    Metallic coating layer  416  and coating material  420  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  420 , regardless of whether coating 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 coating material layers having different melting temperatures. 
         [0041]    Metallic coating material  420  may include any suitable metallic coating material  20  that provides a sinterable outer surface  421  that is configured to be sintered to an adjacent powder particle  412  that also has a metallic coating layer  416  and sinterable outer surface  421 . In powders  410  that also include second or additional (coated or uncoated) particles  432 , as described herein, the sinterable outer surface  421  of metallic coating layer  416  is also configured to be sintered to a sinterable outer surface  421  of second particles  432 . In an exemplary embodiment, the powder particles  412  are sinterable at a predetermined sintering temperature (T S ) that is a function of the core material  418  and coating material  420 , such that sintering of powder compact  600  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  414 /metallic coating layer  416  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  414 /metallic coating layer  416  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  600  as described herein. 
         [0042]    In an exemplary embodiment, core material  418  will be selected to provide a core chemical composition and the coating material  420  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  418  will be selected to provide a core chemical composition and the coating material  420  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  420  and core material  418  may be selected to provide different dissolution rates and selectable and controllable dissolution of powder compacts  600  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  600  formed from powder  410  having chemical compositions of core material  418  and coating material  420  that make compact  600  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. 
         [0043]    As illustrated in  FIGS. 5 and 7 , particle core  414  and core material  418  and metallic coating layer  416  and coating material  420  may be selected to provide powder particles  412  and a powder  410  that is configured for compaction and sintering to provide a powder compact  600  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  600  includes a substantially-continuous, cellular nanomatrix  616  of a nanomatrix material  620  having a plurality of dispersed particles  614  dispersed throughout the cellular nanomatrix  616 . The substantially-continuous cellular nanomatrix  616  and nanomatrix material  620  formed of sintered metallic coating layers  416  is formed by the compaction and sintering of the plurality of metallic coating layers  416  of the plurality of powder particles  412 . The chemical composition of nanomatrix material  620  may be different than that of coating material  420  due to diffusion effects associated with the sintering as described herein. Powder metal compact  600  also includes a plurality of dispersed particles  614  that comprise particle core material  618 . Dispersed particle cores  614  and core material  618  correspond to and are formed from the plurality of particle cores  414  and core material  418  of the plurality of powder particles  412  as the metallic coating layers  416  are sintered together to form nanomatrix  616 . The chemical composition of core material  618  may be different than that of core material  418  due to diffusion effects associated with sintering as described herein. 
         [0044]    As used herein, the use of the term substantially-continuous cellular nanomatrix  616  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  620  within powder compact  600 . As used herein, “substantially-continuous” describes the extension of the nanomatrix material throughout powder compact  600  such that it extends between and envelopes substantially all of the dispersed particles  614 . Substantially-continuous is used to indicate that complete continuity and regular order of the nanomatrix around each dispersed particle  614  is not required. For example, defects in the coating layer  416  over particle core  414  on some powder particles  412  may cause bridging of the particle cores  414  during sintering of the powder compact  600 , thereby causing localized discontinuities to result within the cellular nanomatrix  616 , 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  620  that encompass and also interconnect the dispersed particles  614 . 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  614 . 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  614 , generally comprises the interdiffusion and bonding of two coating layers  416  from adjacent powder particles  412  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  614  does not connote the minor constituent of powder compact  600 , 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  618  within powder compact  600 . 
         [0045]    Powder compact  600  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  600  and deform the powder particles  412 , including particle cores  414  and coating layers  416 , to provide the full density and desired macroscopic shape and size of powder compact  600  as well as its microstructure. The microstructure of powder compact  600  includes an equiaxed configuration of dispersed particles  614  that are dispersed throughout and embedded within the substantially-continuous, cellular nanomatrix  616  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  616  of sintered metallic coating layers  416  may be produced using constituents where thermodynamic phase equilibrium conditions would not produce an equiaxed structure. The equiaxed morphology of the dispersed particles  614  and cellular network  616  of particle layers results from sintering and deformation of the powder particles  412  as they are compacted and interdiffuse and deform to fill the interparticle spaces  415  ( FIG. 5 ). The sintering temperatures and pressures may be selected to ensure that the density of powder compact  600  achieves substantially full theoretical density. 
         [0046]    In an exemplary embodiment as illustrated in  FIGS. 5 and 7 , dispersed particles  614  are formed from particle cores  414  dispersed in the cellular nanomatrix  616  of sintered metallic coating layers  416 , and the nanomatrix  616  includes a solid-state metallurgical bond  617  or bond layer  619 , as illustrated schematically in  FIG. 8 , extending between the dispersed particles  614  throughout the cellular nanomatrix  616  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  617  is formed in the solid state by solid-state interdiffusion between the coating layers  416  of adjacent powder particles  412  that are compressed into touching contact during the compaction and sintering processes used to form powder compact  600 , as described herein. As such, sintered coating layers  416  of cellular nanomatrix  616  include a solid-state bond layer  619  that has a thickness (t) defined by the extent of the interdiffusion of the coating materials  420  of the coating layers  416 , which will in turn be defined by the nature of the coating layers  416 , 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  600 . 
         [0047]    As nanomatrix  616  is formed, including bond  617  and bond layer  619 , the chemical composition or phase distribution, or both, of metallic coating layers  416  may change. Nanomatrix  616  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  616 , regardless of whether nanomatrix material  620  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  614  and particle core materials  618  are formed in conjunction with nanomatrix  616 , diffusion of constituents of metallic coating layers  416  into the particle cores  414  is also possible, which may result in changes in the chemical composition or phase distribution, or both, of particle cores  414 . As a result, dispersed particles  614  and particle core materials  618  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  614 , regardless of whether particle core material  618  comprise a pure metal, an alloy with multiple phases each having different melting temperatures or a composite, or otherwise. Powder compact  600  is formed at a sintering temperature (T S ), where T S  is less than T C , T P , T M  and T DP . 
         [0048]    Dispersed particles  614  may comprise any of the materials described herein for particle cores  414 , even though the chemical composition of dispersed particles  614  may be different due to diffusion effects as described herein. In an exemplary embodiment, dispersed particles  614  are formed from particle cores  414  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  414 . Of these materials, those having dispersed particles  614  comprising Mg and the nanomatrix  616  formed from the metallic coating materials  416  described herein are particularly useful. Dispersed particles  614  and particle core material  618  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  414 . 
         [0049]    In another exemplary embodiment, dispersed particles  614  are formed from particle cores  414  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. 
         [0050]    Dispersed particles  614  of powder compact  600  may have any suitable particle size, including the average particle sizes described herein for particle cores  414 . 
         [0051]    Dispersed particles  614  may have any suitable shape depending on the shape selected for particle cores  414  and powder particles  412 , as well as the method used to sinter and compact powder  410 . In an exemplary embodiment, powder particles  412  may be spheroidal or substantially spheroidal and dispersed particles  614  may include an equiaxed particle configuration as described herein. 
         [0052]    The nature of the dispersion of dispersed particles  614  may be affected by the selection of the powder  410  or powders  410  used to make particle compact  600 . In one exemplary embodiment, a powder  410  having a unimodal distribution of powder particle  412  sizes may be selected to form powder compact  600  and will produce a substantially homogeneous unimodal dispersion of particle sizes of dispersed particles  614  within cellular nanomatrix  616 , as illustrated generally in  FIG. 7 . In another exemplary embodiment, a plurality of powders  410  having a plurality of powder particles with particle cores  414  that have the same core materials  418  and different core sizes and the same coating material  420  may be selected and uniformly mixed as described herein to provide a powder  410  having a homogenous, multimodal distribution of powder particle  412  sizes, and may be used to form powder compact  600  having a homogeneous, multimodal dispersion of particle sizes of dispersed particles  614  within cellular nanomatrix  616 . Similarly, in yet another exemplary embodiment, a plurality of powders  410  having a plurality of particle cores  414  that may have the same core materials  418  and different core sizes and the same coating material  420  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  600  having a non-homogeneous, multimodal dispersion of particle sizes of dispersed particles  614  within cellular nanomatrix  616 . 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  614  within the cellular nanomatrix  616  of powder compacts  600  made from powder  410 . 
         [0053]    Nanomatrix  616  is a substantially-continuous, cellular network of metallic coating layers  416  that are sintered to one another. The thickness of nanomatrix  616  will depend on the nature of the powder  410  or powders  410  used to form powder compact  600 , as well as the incorporation of any second powder  430 , particularly the thicknesses of the coating layers associated with these particles. In an exemplary embodiment, the thickness of nanomatrix  616  is substantially uniform throughout the microstructure of powder compact  600  and comprises about two times the thickness of the coating layers  416  of powder particles  412 . In another exemplary embodiment, the cellular network  616  has a substantially uniform average thickness between dispersed particles  614  of about 50 nm to about 5000 nm. 
         [0054]    Nanomatrix  616  is formed by sintering metallic coating layers  416  of adjacent particles to one another by interdiffusion and creation of bond layer  619  as described herein. Metallic coating layers  416  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  416 , or between the metallic coating layer  416  and particle core  414 , or between the metallic coating layer  416  and the metallic coating layer  416  of an adjacent powder particle, the extent of interdiffusion of metallic coating layers  416  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  616  and nanomatrix material  620  may be simply understood to be a combination of the constituents of coating layers  416  that may also include one or more constituents of dispersed particles  614 , depending on the extent of interdiffusion, if any, that occurs between the dispersed particles  614  and the nanomatrix  616 . Similarly, the chemical composition of dispersed particles  614  and particle core material  618  may be simply understood to be a combination of the constituents of particle core  414  that may also include one or more constituents of nanomatrix  616  and nanomatrix material  620 , depending on the extent of interdiffusion, if any, that occurs between the dispersed particles  614  and the nanomatrix  616 . 
         [0055]    In an exemplary embodiment, the nanomatrix material  620  has a chemical composition and the particle core material  618  has a chemical composition that is different from that of nanomatrix material  620 , 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  600 , including a property change in a wellbore fluid that is in contact with the powder compact  600 , as described herein. Nanomatrix  616  may be formed from powder particles  412  having single layer and multilayer coating layers  416 . This design flexibility provides a large number of material combinations, particularly in the case of multilayer coating layers  416 , that can be utilized to tailor the cellular nanomatrix  616  and composition of nanomatrix material  620  by controlling the interaction of the coating layer constituents, both within a given layer, as well as between a coating layer  416  and the particle core  414  with which it is associated or a coating layer  416  of an adjacent powder particle  412 . Several exemplary embodiments that demonstrate this flexibility are provided below. 
         [0056]    As illustrated in  FIG. 8 , in an exemplary embodiment, powder compact  600  is formed from powder particles  412  where the coating layer  416  comprises a single layer, and the resulting nanomatrix  616  between adjacent ones of the plurality of dispersed particles  614  comprises the single metallic coating layer  416  of one powder particle  412 , a bond layer  619  and the single coating layer  416  of another one of the adjacent powder particles  412 . The thickness (t) of bond layer  619  is determined by the extent of the interdiffusion between the single metallic coating layers  416 , and may encompass the entire thickness of nanomatrix  616  or only a portion thereof. In one exemplary embodiment of powder compact  600  formed using a single layer powder  410 , powder compact  600  may include dispersed particles  614  comprising Mg, Al, Zn or Mn, or a combination thereof, as described herein, and nanomatrix  616  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  620  of cellular nanomatrix  616 , including bond layer  619 , has a chemical composition and the core material  618  of dispersed particles  614  has a chemical composition that is different than the chemical composition of nanomatrix material  616 . The difference in the chemical composition of the nanomatrix material  620  and the core material  618  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  600  formed from a powder  410  having a single coating layer configuration, dispersed particles  614  include Mg, Al, Zn or Mn, or a combination thereof, and the cellular nanomatrix  616  includes Al or Ni, or a combination thereof. 
         [0057]    As illustrated in  FIG. 9 , in another exemplary embodiment, powder compact  600  is formed from powder particles  412  where the coating layer  416  comprises a multilayer coating layer  416  having a plurality of coating layers, and the resulting nanomatrix  616  between adjacent ones of the plurality of dispersed particles  614  comprises the plurality of layers (t) comprising the coating layer  416  of one particle  412 , a bond layer  619 , and the plurality of layers comprising the coating layer  416  of another one of powder particles  412 . In  FIG. 9 , this is illustrated with a two-layer metallic coating layer  416 , but it will be understood that the plurality of layers of multi-layer metallic coating layer  416  may include any desired number of layers. The thickness (t) of the bond layer  619  is again determined by the extent of the interdiffusion between the plurality of layers of the respective coating layers  416 , and may encompass the entire thickness of nanomatrix  616  or only a portion thereof. In this embodiment, the plurality of layers comprising each coating layer  416  may be used to control interdiffusion and formation of bond layer  619  and thickness (t). 
         [0058]    Sintered and forged powder compacts  600  that include dispersed particles  614  comprising Mg and nanomatrix  616  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  600  that have pure Mg dispersed particles  614  and various nanomatrices  616  formed from powders  410  having pure Mg particle cores  414  and various single and multilayer metallic coating layers  416  that include Al, Ni, W or Al 2 O 3 , or a combination thereof. These powders compacts  600  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  600  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  600  that include dispersed particles  614  comprising Mg and nanomatrix  616  comprising various nanomatrix materials  620  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  600  can be further improved by optimizing powder  410 , particularly the weight percentage of the nanoscale metallic coating layers  416  that are used to form cellular nanomatrix  616 . Strength of the nanomatrix powder metal compact  600  can be further improved by optimizing powder  410 , particularly the weight percentage of the nanoscale metallic coating layers  416  that are used to form cellular nanomatrix  616 . For example, varying the weight percentage (wt. %), i.e., thickness, of an alumina coating within a cellular nanomatrix  616  formed from coated powder particles  412  that include a multilayer (Al/Al 2 O 3 /Al) metallic coating layer  416  on pure Mg particle cores  414  provides an increase of 21% as compared to that of 0 wt % alumina. 
         [0059]    Powder compacts  600  comprising dispersed particles  614  that include Mg and nanomatrix  616  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. 
         [0060]    Powder compacts  600  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  410 , including relative amounts of constituents of particle cores  414  and metallic coating layer  416 , and are also described herein as being fully-dense powder compacts. Powder compacts  600  comprising dispersed particles that include Mg and nanomatrix  616  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. 
         [0061]    Powder compacts  600  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  600  comprising dispersed particles  614  that include Mg and cellular nanomatrix  616  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  416 . 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  600  comprising dispersed particles  614  that include Mg and nanomatrix  616  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. 10 , which illustrates that at a selected predetermined critical service time (CST) a changed condition may be imposed upon powder compact  600  as it is applied in a given application, such as a wellbore environment, that causes a controllable change in a property of powder compact  600  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  600  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  600  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  600  and its removal from the wellbore. In the example described above, powder compact  600  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  600  described herein and includes a cellular nanomatrix  616  of nanomatrix material  620 , a plurality of dispersed particles  614  including particle core material  618  that is dispersed within the matrix. Nanomatrix  616  is characterized by a solid-state bond layer  619  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  600  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. 10 . 
         [0062]    Without being limited by theory, powder compacts  600  are formed from coated powder particles  412  that include a particle core  414  and associated core material  418  as well as a metallic coating layer  416  and an associated metallic coating material  420  to form a substantially-continuous, three-dimensional, cellular nanomatrix  616  that includes a nanomatrix material  620  formed by sintering and the associated diffusion bonding of the respective coating layers  416  that includes a plurality of dispersed particles  614  of the particle core materials  618 . 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  600 , 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  616 , which may be selected to provide a strengthening phase material, with dispersed particles  614 , which may be selected to provide equiaxed dispersed particles  614 , 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  600  made using uncoated pure Mg powder and subjected to a shear stress sufficient to induce failure demonstrated intergranular fracture. In contrast, a powder compact  600  made using powder particles  412  having pure Mg powder particle cores  414  to form dispersed particles  614  and metallic coating layers  416  that includes Al to form nanomatrix  616  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. 
         [0063]    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.