An isolation device and methods of removing the isolation device are provided. The isolation device includes at least a first material that is capable of dissolving via galvanic corrosion when an electrically conductive path exists between the first material and a different metal or metal alloy in the presence of an electrolyte. According to an embodiment, the isolation device is used in an oil or gas well operation. Several factors can be adjusted to control the rate of dissolution of the first material in a desired amount of time.

US 2014/0027128 A1 discloses a downhole flow inhibition tool including at least a first component and a mating component at least a portion of one of the first component and the mating component is dissolvable in a target environment to reduce flow inhibition upon dissolution of the at least a portion.

US 2013/0300066 A1 discloses a seal including a metal composite that has a cellular nanomatrix that includes a metallic nanomatrix material, a metal matrix disposed in the cellular nanomatrix, and a disintegration agent; an inner sealing surface; and an outer sealing surface disposed radially from the inner sealing surface. The seal can be prepared by combining a metal matrix powder, a disintegration agent, and metal nanomatrix material to form a composition; compacting the composition to form a compacted composition; sintering the compacted composition; and pressing the sintered composition to form the seal.

US 2010/0294510 A1 discloses a dissolvable downhole tool. The tool includes, a dissolvable body constructed of at least two materials and at least one of the at least two materials is a reactive material, and a first material of the at least two materials being configured to substantially dissolve the dissolvable body and a second material configured to control reaction timing of the first material.

US 2013/327540 A1 discloses a downhole assembly with controlled degradation including a body having a cavity therein and is formed from a first material having a first electrode potential. An insert is disposed in the cavity, the insert electrically coupled to the body and formed from a second material having a second electrode potential, with the first electrode potential being more negative than the second electrode potential.

US 2013/0327450 A1 discloses a wellbore isolation device comprising: at least a first material, wherein the first material: is a metal or a metal alloy; and is capable of at least partially dissolving when an electrically conductive path exists between the first material and a second material and at least a portion of the first and second materials are in contact with an electrolyte, wherein the second material: is a metal or metal alloy; and has a greater anodic index than the first material. A method of removing the wellbore isolation device comprises: contacting or allowing the wellbore isolation device to come in contact with an electrolyte; and allowing at least a portion of the first material to dissolve.

The features and advantages of certain embodiments will be more readily appreciated when considered in conjunction with the accompanying figures. The figures are not to be construed as limiting any of the preferred embodiments.

• Fig. 1 depicts a well system containing more than one isolation device.
• Fig. 2 depicts an isolation device according to an example, not part of the scope of protection.
• Fig. 3 depicts an isolation device according to an embodiment of the invention.

As used herein, the words "comprise," "have," "include," and all grammatical variations thereof are each intended to have an open, non-limiting meaning that does not exclude additional elements or steps.

It should be understood that, as used herein, "first," "second," "third," etc., are arbitrarily assigned and are merely intended to differentiate between two or more materials, isolation devices, wellbore intervals, etc., as the case may be, and does not indicate any particular orientation or sequence. Furthermore, it is to be understood that the mere use of the term "first" does not require that there be any "second," and the mere use of the term "second" does not require that there be any "third," etc.

As used herein, a "fluid" is a substance having a continuous phase that tends to flow and to conform to the outline of its container when the substance is tested at a temperature of 71 °F (22 °C) and a pressure of one atmosphere "atm" (0.1 megapascals "MPa"). A fluid can be a liquid or gas.

Oil and gas hydrocarbons are naturally occurring in some subterranean formations. In the oil and gas industry, a subterranean formation containing oil or gas is referred to as a reservoir. A reservoir may be located under land or off shore. Reservoirs are typically located in the range of a few hundred feet (shallow reservoirs) to a few tens of thousands of feet (ultra-deep reservoirs). In order to produce oil or gas, a wellbore is drilled into a reservoir or adjacent to a reservoir. The oil, gas, or water produced from a reservoir is called a reservoir fluid.

A well can include, without limitation, an oil, gas, or water production well, or an injection well. As used herein, a "well" includes at least one wellbore. A wellbore can include vertical, inclined, and horizontal portions, and it can be straight, curved, or branched. As used herein, the term "wellbore" includes any cased, and any uncased, open-hole portion of the wellbore. A near-wellbore region is the subterranean material and rock of the subterranean formation surrounding the wellbore. As used herein, a "well" also includes the near-wellbore region. The near-wellbore region is generally considered to be the region within approximately 100 feet radially of the wellbore. As used herein, "into a well" means and includes into any portion of the well, including into the wellbore or into the near-wellbore region via the wellbore.

A portion of a wellbore may be an open hole or cased hole. In an open-hole wellbore portion, a tubing string may be placed into the wellbore. The tubing string allows fluids to be introduced into or flowed from a remote portion of the wellbore. In a cased-hole wellbore portion, a casing is placed into the wellbore that can also contain a tubing string. A wellbore can contain an annulus. Examples of an annulus include, but are not limited to: the space between the wellbore and the outside of a tubing string in an open-hole wellbore; the space between the wellbore and the outside of a casing in a cased-hole wellbore; and the space between the inside of a casing and the outside of a tubing string in a cased-hole wellbore.

It is not uncommon for a wellbore to extend several hundreds of feet or several thousands of feet into a subterranean formation. 100 feet is equivalent to 30.48 meters. The subterranean formation can have different zones. A zone is an interval of rock differentiated from surrounding rocks on the basis of its fossil content or other features, such as faults or fractures. For example, one zone can have a higher permeability compared to another zone. It is often desirable to treat one or more locations within multiples zones of a formation. One or more zones of the formation can be isolated within the wellbore via the use of an isolation device to create multiple wellbore intervals. At least one wellbore interval corresponds to a formation zone. The isolation device can be used for zonal isolation and functions to block fluid flow within a tubular, such as a tubing string, or within an annulus. The blockage of fluid flow prevents the fluid from flowing across the isolation device in any direction and isolates the zone of interest. In this manner, treatment techniques can be performed within the zone of interest.

Common isolation devices include, but are not limited to, a ball and a seat, a bridge plug, a packer, a plug, and wiper plug. It is to be understood that reference to a "ball" is not meant to limit the geometric shape of the ball to spherical, but rather is meant to include any device that is capable of engaging with a seat. A "ball" can be spherical in shape, but can also be a dart, a bar, or any other shape. Zonal isolation can be accomplished via a ball and seat by dropping or flowing the ball from the wellhead onto the seat that is located within the wellbore. The ball engages with the seat, and the seal created by this engagement prevents fluid communication into other wellbore intervals downstream of the ball and seat. As used herein, the relative term "downstream" means at a location further away from a wellhead. In order to treat more than one zone using a ball and seat, the wellbore can contain more than one ball seat. For example, a seat can be located within each wellbore interval. Generally, the inner diameter (I.D.) of the ball seats is different for each zone. For example, the I.D. of the ball seats sequentially decreases at each zone, moving from the wellhead to the bottom of the well. In this manner, a smaller ball is first dropped into a first wellbore interval that is the farthest downstream; the corresponding zone is treated; a slightly larger ball is then dropped into another wellbore interval that is located upstream of the first wellbore interval; that corresponding zone is then treated; and the process continues in this fashion - moving upstream along the wellbore - until all the desired zones have been treated. As used herein, the relative term "upstream" means at a location closer to the wellhead.

A bridge plug is composed primarily of slips, a plug mandrel, and a rubber sealing element. A bridge plug can be introduced into a wellbore and the sealing element can be caused to block fluid flow into downstream intervals. A packer generally consists of a sealing device, a holding or setting device, and an inside passage for fluids. A packer can be used to block fluid flow through the annulus located between the outside of a tubular and the wall of the wellbore or inside of a casing.

Isolation devices can be classified as permanent or retrievable. While permanent isolation devices are generally designed to remain in the wellbore after use, retrievable devices are capable of being removed after use. It is often desirable to use a retrievable isolation device in order to restore fluid communication between one or more wellbore intervals. Traditionally, isolation devices are retrieved by inserting a retrieval tool into the wellbore, wherein the retrieval tool engages with the isolation device, attaches to the isolation device, and the isolation device is then removed from the wellbore. Another way to remove an isolation device from the wellbore is to mill at least a portion of the device or the entire device. Yet, another way to remove an isolation device is to contact the device with a solvent, such as an acid, thus dissolving all or a portion of the device.

However, some of the disadvantages to using traditional methods to remove a retrievable isolation device include: it can be difficult and time consuming to use a retrieval tool; milling can be time consuming and costly; and premature dissolution of the isolation device can occur. For example, premature dissolution can occur if acidic fluids are used in the well prior to the time at which it is desired to dissolve the isolation device.

A novel method of removing an isolation device includes using galvanic corrosion to dissolve at least a portion of the isolation device. The rate of corrosion can be adjusted by selecting the materials used, the electrolyte used, the concentration of free ions available in the electrolyte, and the distance between the two materials of the galvanic system.

Galvanic corrosion occurs when two different metals or metal alloys are in electrical connectivity with each other and both are in contact with an electrolyte. As used herein, the phrase "electrical connectivity" means that the two different metals or metal alloys are either touching or in close enough proximity to each other such that when the two different metals are in contact with an electrolyte, the electrolyte becomes electrically conductive and ion migration occurs between one of the metals and the other metal, and is not meant to require an actual physical connection between the two different metals, for example, via a metal wire. It is to be understood that as used herein, the term "metal" is meant to include pure metals and also metal alloys without the need to continually specify that the metal can also be a metal alloy. Moreover, the use of the phrase "metal or metal alloy" in one sentence or paragraph does not mean that the mere use of the word "metal" in another sentence or paragraph is meant to exclude a metal alloy. As used herein, the term "metal alloy" means a mixture of two or more elements, wherein at least one of the elements is a metal. The other element(s) can be a non-metal or a different metal. An example of a metal and non-metal alloy is steel, comprising the metal element iron and the non-metal element carbon. An example of a metal and metal alloy is bronze, comprising the metallic elements copper and tin.

The metal that is less noble, compared to the other metal, will dissolve in the electrolyte. The less noble metal is often referred to as the anode, and the more noble metal is often referred to as the cathode. Galvanic corrosion is an electrochemical process whereby free ions in the electrolyte make the electrolyte electrically conductive, thereby providing a means for ion migration from the anode to the cathode - resulting in deposition formed on the cathode. Metals can be arranged in a galvanic series. The galvanic series lists metals in order of the most noble to the least noble. An anodic index lists the electrochemical voltage (V) that develops between a metal and a standard reference electrode (gold (Au)) in a given electrolyte. The actual electrolyte used can affect where a particular metal or metal alloy appears on the galvanic series and can also affect the electrochemical voltage. For example, the dissolved oxygen content in the electrolyte can dictate where the metal or metal alloy appears on the galvanic series and the metal's electrochemical voltage. The anodic index of gold is -0 V; while the anodic index of beryllium is -1.85 V. A metal that has an anodic index greater than another metal is more noble than the other metal and will function as the cathode. Conversely, the metal that has an anodic index less than another metal is less noble and functions as the anode. In order to determine the relative voltage between two different metals, the anodic index of the lesser noble metal is subtracted from the other metal's anodic index, resulting in a positive value.

There are several factors that can affect the rate of galvanic corrosion. One of the factors is the distance separating the metals on the galvanic series chart or the difference between the anodic indices of the metals. For example, beryllium is one of the last metals listed at the least noble end of the galvanic series and platinum is one of the first metals listed at the most noble end of the series. By contrast, tin is listed directly above lead on the galvanic series. Using the anodic index of metals, the difference between the anodic index of gold and beryllium is 1.85 V; whereas, the difference between tin and lead is 0.05 V. This means that galvanic corrosion will occur at a much faster rate for magnesium or beryllium and gold compared to lead and tin.

The following is a partial galvanic series chart using a deoxygenated sodium chloride water solution as the electrolyte. The metals are listed in descending order from the most noble (cathodic) to the least noble (anodic). The following list is not exhaustive, and one of ordinary skill in the art is able to find where a specific metal or metal alloy is listed on a galvanic series in a given electrolyte.

• PLATINUM
• GOLD
• ZIRCONIUM
• GRAPHITE
• SILVER
• CHROME IRON
• SILVER SOLDER
• COPPER - NICKEL ALLOY 80-20
• COPPER - NICKEL ALLOY 90-10
• MANGANESE BRONZE (CA 675), TIN BRONZE (CA903, 905)
• COPPER (CA102)
• BRASSES
• NICKEL (ACTIVE)
• TIN
• LEAD
• ALUMINUM BRONZE
• STAINLESS STEEL
• CHROME IRON
• MILD STEEL (1018), WROUGHT IRON
• ALUMINUM 2117, 2017, 2024
• CADMIUM
• ALUMINUM 5052, 3004, 3003, 1100, 6053
• ZINC
• MAGNESIUM
• BERYLLIUM

The following is a partial anodic index listing the voltage of a listed metal against a standard reference electrode (gold) using a deoxygenated sodium chloride water solution as the electrolyte. The metals are listed in descending order from the greatest voltage (most cathodic) to the least voltage (most anodic). The following list is not exhaustive, and one of ordinary skill in the art is able to find the anodic index of a specific metal or metal alloy in a given electrolyte. Anodic index Metal Index (V) Gold, solid and plated, Gold-platinum alloy -0.00 Rhodium plated on silver-plated copper -0.05 Silver, solid or plated; monel metal. High nickel-copper alloys -0.15 Nickel, solid or plated, titanium an s alloys, Monel -0.30 Copper, solid or plated; low brasses or bronzes; silver solder; German silvery high copper-nickel alloys; nickel-chromium alloys -0.35 Brass and bronzes -0.40 High brasses and bronzes -0.45 18% chromium type corrosion-resistant steels -0.50 Chromium plated; tin plated; 12% chromium type corrosion-resistant steels -0.60 Tin-plate; tin-lead solder -0.65 Lead, solid or plated; high lead alloys -0.70 2000 series wrought aluminum -0.75 Iron, wrought, gray or malleable, plain carbon and low alloy steels -0.85 Aluminum, wrought alloys other than 2000 series aluminum, cast alloys of the silicon type -0.90 Aluminum, cast alloys other than silicon type, cadmium, plated and chromate -0.95 Hot-dip-zinc plate; galvanized steel -1.20 Zinc, wrought; zinc-base die-casting alloys; zinc plated -1.25 Magnesium & magnesium-base alloys, cast or wrought -1.75 Beryllium -1.85

Another factor that can affect the rate of galvanic corrosion is the temperature and concentration of the electrolyte. The higher the temperature and concentration of the electrolyte, the faster the rate of corrosion. Yet another factor that can affect the rate of galvanic corrosion is the total amount of surface area of the least noble (anodic metal). The greater the surface area of the anode that can come in contact with the electrolyte, the faster the rate of corrosion. The cross-sectional size of the anodic metal pieces can be decreased in order to increase the total amount of surface area per total volume of the material. The anodic metal or metal alloy can also be a matrix in which pieces of cathode material is embedded in the anode matrix. Yet another factor that can affect the rate of galvanic corrosion is the ambient pressure. Depending on the electrolyte chemistry and the two metals, the corrosion rate can be slower at higher pressures than at lower pressures if gaseous components are generated. Yet another factor that can affect the rate of galvanic corrosion is the physical distance between the two different metal and/or metal alloys of the galvanic system.

A method of removing a wellbore isolation device according to the present invention is defined by claim 1. Dependent claims relate to preferred embodiments.

Any discussion of the embodiments regarding the isolation device or any component related to the isolation device (e.g., the electrolyte) is intended to apply to all of the method embodiments.

Turning to the Figures, Fig. 1 depicts a well system 10. The well system 10 can include at least one wellbore 11. The wellbore 11 can penetrate a subterranean formation 20. The subterranean formation 20 can be a portion of a reservoir or adjacent to a reservoir. The wellbore 11 can include a casing 12. The wellbore 11 can include only a generally vertical wellbore section or can include only a generally horizontal wellbore section. A tubing string 15 can be installed in the wellbore 11. The well system 10 can comprise at least a first wellbore interval 13 and a second wellbore interval 14. The well system 10 can also include more than two wellbore intervals, for example, the well system 10 can further include a third wellbore interval, a fourth wellbore interval, and so on. At least one wellbore interval can correspond to a zone of the subterranean formation 20. The well system 10 can further include one or more packers 18. The packers 18 can be used in addition to the isolation device to create the wellbore interval and isolate each zone of the subterranean formation 20. The isolation device can be the packers 18. The packers 18 can be used to prevent fluid flow between one or more wellbore intervals (e.g., between the first wellbore interval 13 and the second wellbore interval 14) via an annulus 19. The tubing string 15 can also include one or more ports 17. One or more ports 17 can be located in each wellbore interval. Moreover, not every wellbore interval needs to include one or more ports 17. For example, the first wellbore interval 13 can include one or more ports 17, while the second wellbore interval 14 does not contain a port. In this manner, fluid flow into the annulus 19 for a particular wellbore interval can be selected based on the specific oil or gas operation.

It should be noted that the well system 10 is illustrated in the drawings and is described herein as merely one example of a wide variety of well systems in which the principles of this disclosure can be utilized. It should be clearly understood that the principles of this disclosure are not limited to any of the details of the well system 10, or components thereof, depicted in the drawings or described herein. Furthermore, the well system 10 can include other components not depicted in the drawing. For example, the well system 10 can further include a well screen. By way of another example, cement may be used instead of packers 18 to aid the isolation device in providing zonal isolation. Cement may also be used in addition to packers 18.

According to an embodiment, the isolation device is capable of restricting or preventing fluid flow between a first wellbore interval 13 and a second wellbore interval 14. The first wellbore interval 13 can be located upstream or downstream of the second wellbore interval 14. In this manner, depending on the oil or gas operation, fluid is restricted or prevented from flowing downstream or upstream into the second wellbore interval 14. Examples of isolation devices capable of restricting or preventing fluid flow between zones include, but are not limited to, a ball and seat, a plug, a bridge plug, a wiper plug, a packer, and a plug in a base pipe. A detailed discussion of using a plug in a base pipe can be found in US patent 7,699,101. The portion of the isolation device that includes at least the first material and the second material can be the mandrel of a packer or plug, a spacer ring, a slip, a wedge, a retainer ring, an extrusion limiter or backup shoe, a mule shoe, a ball, a flapper, a ball seat, a sleeve, or any other downhole tool or component of a downhole tool used for zonal isolation.

As depicted in the drawings, the isolation device can be a ball 30 (e.g., a first ball 31 or a second ball 32) and a seat 40 (e.g., a first seat 41 or a second seat 42). The ball 30 can engage the seat 40. The seat 40 can be located on the inside of a tubing string 15. The inner diameter (I.D.) of the first seat 41 can be less than the I.D. of the second seat 42. In this manner, a first ball 31 can be dropped or flowed into wellbore. The first ball 31 can have a smaller outer diameter (O.D.) than the second ball 32. The first ball 31 can engage the first seat 41. Fluid can now be temporarily restricted or prevented from flowing into any wellbore intervals located downstream of the first wellbore interval 13. In the event it is desirable to temporarily restrict or prevent fluid flow into any wellbore intervals located downstream of the second wellbore interval 14, then the second ball 32 can be dropped or flowed into the wellbore and will be prevented from falling past the second seat 42 because the second ball 32 has a larger O.D. than the I.D. of the second seat 42. The second ball 32 can engage the second seat 42. The ball (whether it be a first ball 31 or a second ball 32) can engage a sliding sleeve 16 during placement. This engagement with the sliding sleeve 16 can cause the sliding sleeve to move; thus, opening a port 17 located adjacent to the seat. The port 17 can also be opened via a variety of other mechanisms instead of a ball. The use of other mechanisms may be advantageous when the isolation device is not a ball. After placement of the isolation device, fluid can be flowed from, or into, the subterranean formation 20 via one or more opened ports 17 located within a particular wellbore interval. As such, a fluid can be produced from the subterranean formation 20 or injected into the formation.

Referring to Figs. 2 - 3, the isolation device comprises at least a first material 51, wherein the first material partially or wholly dissolves when an electrically conductive path exists between the first material 51 and a second material 52. The first material 51 and the second material 52 are metals or metal alloys. The metal or metal alloy can be selected from the group consisting of, lithium, sodium, potassium, rubidium, cesium, beryllium, calcium, strontium, barium, radium, aluminum, gallium, indium, tin, thallium, lead, bismuth, scandium, titanium, vanadium, chromium, manganese, thorium, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, praseodymium, silver, cadmium, lanthanum, hafnium, tantalum, tungsten, terbium, rhenium, osmium, iridium, platinum, gold, neodymium, gadolinium, erbium, oxides of any of the foregoing, graphite, carbon, silicon, boron nitride, and any combinations thereof. Preferably, the metal or metal alloy is selected from the group consisting of magnesium, aluminum, zinc, beryllium, tin, iron, nickel, copper, oxides of any of the foregoing, and combinations thereof. According to an embodiment, the metal is neither radioactive, nor unstable.

According to an embodiment, the first material 51 and the second material 52 are different metals or metal alloys. By way of example, the first material 51 can be magnesium and the second material 52 can be iron. Furthermore, the first material 51 can be a metal and the second material 52 can be a metal alloy. The first material 51 and the second material 52 can be a metal and the first and second material can be a metal alloy. The first material and the second material form a galvanic couple and wherein the first material is the anode and the second material is the cathode of the couple. Stated another way, the second material 52 is more noble than the first material 51. In this manner, the first material 51 (acting as the anode) partially or wholly dissolves when in electrical connectivity with the second material 52 and when the first and second materials are in contact with the electrolyte.

The methods include allowing at least a portion of the first material or at least some of the pieces of the first material to dissolve. The step of allowing can be performed after the step of contacting or allowing the first material to come in contact with the electrolyte. At least a portion of the first material 51 can dissolve in a desired amount of time. The desired amount of time can be predetermined, based in part, on the specific oil or gas well operation to be performed. The desired amount of time can be in the range from about 1 hour to about 2 months, preferably about 5 to about 10 days. There are several factors that can affect the rate of dissolution of the first material 51. According to an embodiment, the first material 51 and the second material 52 are selected such that the at least a portion of the first material 51 dissolves in the desired amount of time. By way of example, the greater the difference between the second material's anodic index and the first material's anodic index, the faster the rate of dissolution. By contrast, the less the difference between the second material's anodic index and the first material's anodic index, the slower the rate of dissolution. By way of yet another example, the farther apart the first material and the second material are from each other in a galvanic series, the faster the rate of dissolution; and the closer together the first and second material are to each other in the galvanic series, the slower the rate of dissolution. By evaluating the difference in the anodic index of the first and second materials, or by evaluating the order in a galvanic series, one of ordinary skill in the art will be able to determine the rate of dissolution of the first material in a given electrolyte.

Another factor that can affect the rate of dissolution of the first material 51 is the proximity of the first material 51 to the second material 52. A more detailed discussion regarding different embodiments of the proximity of the first and second materials is presented below. Generally, the closer the first material 51 is physically to the second material 52, the faster the rate of dissolution of the first material 51. By contrast, generally, the farther apart the first and second materials are from one another, the slower the rate of dissolution. It should be noted that the distance between the first material 51 and the second material 52 should not be so great that an electrically conductive path ceases to exist between the first and second materials. According to an embodiment, any distance between the first and second materials 51/52 is selected such that the at least a portion of the first material 51 dissolves in the desired amount of time.

Another factor that can affect the rate of dissolution of the first material 51 is the concentration of the electrolyte and the temperature of the electrolyte. A more detailed discussion of the electrolyte is presented below. Generally, the higher the concentration of the electrolyte, the faster the rate of dissolution of the first material 51, and the lower the concentration of the electrolyte, the slower the rate of dissolution. Moreover, the higher the temperature of the electrolyte, the faster the rate of dissolution of the first material 51, and the lower the temperature of the electrolyte, the slower the rate of dissolution. One of ordinary skill in the art can select: the exact metals and/or metal alloys, the proximity of the first and second materials, and the concentration of the electrolyte based on an anticipated temperature in order for the at least a portion of the first material 51 to dissolve in the desired amount of time.

Fig. 2 depicts the isolation device 30 according to an example, not part of the scope of protection. According this example, the first material 51 forms a matrix of the portion of the wellbore device that contains the first material 51 and the second material 52. It is to be understood that the entire isolation device, for example, when the isolation device is a ball or ball seat, can be made of at least the first material and second material. Moreover, only one or more portions of the isolation device can be made from at least the first and second materials. As can be seen in Fig. 2, the second material 52 can be in the form of pieces, wherein the pieces of the second material are embedded within the matrix of the first material 51. The exact number or concentration of the pieces of the second material 52 can be selected and adjusted to control the dissolution rate of the first material 51 such that at least the portion of the first material 51 dissolves in the desired amount of time. For example, the higher the concentration of pieces of second material 52 that are embedded within the matrix of the first material 51, generally the faster the rate of dissolution. Moreover, the pieces of the second material 52 can be uniformly distributed throughout the matrix of the first material 51. This example can be useful when a constant rate of dissolution of the first material is desired. The pieces of the second material can also be non-uniformly distributed throughout the matrix of the first material such that different concentrations of the second material are located within different areas of the matrix. By way of example, a higher concentration of the pieces of the second material can be distributed closer to the outside of the matrix for allowing an initially faster rate of dissolution; whereas a lower concentration of the pieces can be distributed in the middle and inside of the matrix for allowing a slower rate of dissolution. By contrast, a higher concentration of the pieces of the second material can be distributed in the middle and/ or inside of the matrix for allowing a faster rate of dissolution at the end of dissolution; whereas a lower concentration of the pieces can be distributed closer to the outside of the matrix for allowing an initially slower rate of dissolution. Of course the concentration of pieces of the second material can be distributed in a variety of ways to allow for differing rates of dissolution of the first material matrix.

According to the invention, a third material is included in the portion of the isolation device (not shown in Fig. 2). The third material is a bonding agent for bonding the pieces of the second material into the matrix of the first material 51. This embodiment can be useful during the manufacturing process to provide a suitable bond between the matrix of the first material 51 and pieces of the second material 52. Preferred manufacturing processes can include casting, forging, hot- and/or cold-working, metal injection molding, but would exclude powder compaction and sintering. According to the invention, the portion of the isolation device is made via casting. Preferably, the portion of the isolation device is also modified with a heat treatment. In one embodiment, the heat treatment involves precipitation heat treatment where the alloy is heated to allow the precipitation of the constituent ingredients that are held in a solid solution. The precipitation heat treatment temperature can be in the range from 300 °F to 500 °F (149 °C to 260 °C) for 1 to 16 hours. For example, a forged metal alloy can be heated for 24 hours at 350 °F (177 °C). In another example, cast parts are heated for 1 to 2 hours at 400 °F to 500 °F (204 °C to 260 °C), followed by slow cooling. The precipitation heat treatment could follow a solution heat treatment. A solution heat treatment involves heating the metal alloy to a temperature at which certain ingredients of the alloy go into solution, and then quenching so as to hold these ingredients in solution during cooling. The solution heat treatment temperature can be in the range from 650 °F to 1050 °F (343 °C to 566 °C) for 10 to 24 hours.

Examples of materials suitable for use as a bonding third material include, but are not limited to, copper, platinum, gold, silver, nickel, iron, chromium, molybdenum, tungsten, stainless steel, zirconium, titanium, indium, and oxides of any of the foregoing. Preferably, the third material includes a metal and/or a non-metal that is different from the metals making up the first and second materials 51/52. In one example, the first material is aluminum, the second material is iron, and the third material is iron oxide. In another example, the first material is magnesium, the second material is carbon, and the third material is iron oxide. It may be desirable to use the oxide of the metal to create a better bond between the first and second materials 51/52. The third material can be coated onto the pieces of the second material 52. A layer of the third material can be located between the surfaces of the pieces of the second material and the matrix of the first material with the surfaces of pieces of the second material being physically separated from the matrix of the first material via the layer of third material. The coating of third material can form a metal or metal oxide interface with the surface of each of the pieces of the second material 52 with the matrix of the first material 51. Accordingly, after manufacture, there will be a layer of the third material 53 located between the surfaces of the pieces of the second material 52 and the matrix of the first material 51. The thickness of the layer of the third material can be selected to provide the desired bond strength between the pieces of the second material 52 and matrix of the first material 51. For example, if the layer is too thin, then there may be an insufficient amount of third material to create a good bond, and if the layer is too thick, then the layer may become mechanically weak and mechanical failure can occur at the interface between the third material 53 and the first or second materials or failure could also occur within the layer of third material. Preferably, the thickness of the layer of third material is in the range of about 10 nanometers to about 100 nanometers. In another embodiment, the thickness of the third material is less than 10 nanometers. In another embodiment, the thickness of the third material is 100 nanometers to 5,000 nanometers.

Fig. 3 depicts the isolation device according to the invention. As depicted in Fig. 3, the isolation device can comprise pieces of the first material 51, pieces of the second material 52, and the third material 53. Although this embodiment depicted in Fig. 3 illustrates the isolation device as a ball, it is to be understood that this embodiment and discussion thereof is equally applicable to an isolation device that is a bridge plug, packer, etc. In order for galvanic corrosion to occur (and hence dissolution of at least a portion of the first material 51), both, the first and second materials 51/52 need to be capable of being contacted by the electrolyte. Preferably, at least a portion of one or more pieces of the first material 51 and the second material 52 form the outside of the isolation device, such as a ball 30. In this manner, at least a portion of the first and second materials 51/52 are capable of being contacted with the electrolyte.

According to the invention, the third material 53 physically separates at least a portion of a surface of one or more pieces of the first material 51 from at least a portion of a surface of one or more pieces of the second material 52. This is useful when it is desired to use the distance between the first and second materials 51/52 as a way to control the rate of dissolution of the first material 51. The third material 53 may also limit the ionic conductivity or the electrical conductivity between the first and second materials 51/52. According to an embodiment, the third material 53 is in the form of pieces. The third material can be selected from the group consisting of metals, non-metals, sand, plastics, ceramics, and polymers. Preferably, the third material includes a metal and/or a non-metal that is different from the metals making up the first and second materials 51/52. The pieces of the third material 53 can be located between one or more of the pieces of the first and second materials 51/52. The size and shape of the pieces of the third material 53 can be selected to provide a desired distance of the physical separation of the first and second materials 51/52. By way of example, the thicker the cross-sectional size of the piece of third material 53, the greater the reduction of the ionic and/or electrical conductivity between the pieces of the first material 51 and the pieces of the second material 52. Conversely, the smaller the thickness of the third material, the smaller the reduction of the ionic and/or electrical conductivity between the pieces of the first and second materials 51/52. The pieces of the third material 53 can also separate two or more pieces of the first material 51 and/or two or more pieces of the second material 52. The size of the pieces of the third material 53 can be the same or different. The pieces of third material having different thicknesses can be distributed throughout the portion of the isolation device in a variety of ways to provide different rates of dissolution. For example, larger-sized pieces can be located towards the outside of the portion of the isolation device; whereas smaller-sized pieces can be located towards the middle and/or inside. This embodiment could provide an initially slower rate of dissolution due to the initially greater distance between the first and second materials 51/52 and a faster rate of dissolution later due to a decreased distance between the first and second materials 51/52. Of course, the distribution of different sized pieces of the third material 53 can vary and be selected to provide the desired rates of dissolution of at least some of the pieces of the first material 51.

The concentration and distribution patterns of pieces of the third material 53 can also be selected to provide the desired rate of dissolution of at least some of the pieces of the first material 51 such that at least some of the pieces of the first material dissolve in the desired amount of time. For example, generally, the higher the concentration of the third material, the slower the rate of dissolution, and the lower the concentration of the third material, the faster the rate of dissolution. Moreover, the pieces of the third material 53 can be uniformly distributed throughout the portion of the isolation device containing the first, second, and third materials. This embodiment (assuming a relatively uniform size of the pieces of third material) can be used to provide a relatively constant rate of dissolution of the pieces of the first material 51. The pieces of the third material 53 can also be non-uniformly distributed throughout the portion of the isolation device. By way of example, a higher concentration of the pieces of the third material can be distributed closer to the outside of the portion of the isolation device for allowing an initially slower rate of dissolution; whereas a lower concentration of the pieces can be distributed in the middle and inside for allowing a faster rate of dissolution. By contrast, a higher concentration of the pieces of the third material can be distributed in the middle and/ or inside of the matrix for allowing a slower rate of dissolution at the end of dissolution; whereas a lower concentration of the pieces can be distributed closer to the outside for allowing an initially faster rate of dissolution.

The pieces of the first material 51 and the pieces of the second material 52 can be bonded together via a third material as described above with reference to Fig. 2. In this manner, the pieces of first material and pieces of the second material can be bonded together to form the portion of the isolation device. The device of Fig. 3 can also be manufactured and optionally subjected to the heat treatments described above.

The size, shape and placement of the pieces of the first and second materials 51/52 can also be adjusted to control the rate of dissolution of the first material 51. By way of example, generally the smaller the cross-sectional area of each piece, the faster the rate of dissolution. The smaller cross-sectional area increases the ratio of the surface area to total volume of the material, thus allowing more of the material to come in contact with the electrolyte. The cross-sectional area of each piece of the first material 51 can be the same or different, the cross-sectional area of each piece of the second material 52 can be the same or different, and the cross-sectional area of the pieces of the first material 51 and the pieces of the second material 52 can be the same or different. Additionally, the cross-sectional area of the pieces forming the outer portion of the isolation device and the pieces forming the inner portion of the isolation device can be the same or different. By way of example, if it is desired for the outer portion of the isolation device to proceed at a faster rate of galvanic corrosion compared to the inner portion of the device, then the cross-sectional area of the individual pieces comprising the outer portion can be smaller compared to the cross-sectional area of the pieces comprising the inner portion. The shape of the pieces of the first and second materials 51/52 can also be adjusted to allow for a greater or smaller cross-sectional area.

According to an embodiment, at least the first material 51 and second material 52 are capable of withstanding a specific pressure differential for a desired amount of time. As used herein, the term "withstanding" means that the substance does not crack, break, or collapse. The pressure differential can be the downhole pressure of the subterranean formation 20 across the device. As used herein, the term "downhole" means the location of the wellbore where the portion of the isolation device is located. Formation pressures can range from about 1,000 to about 30,000 pounds force per square inch (psi) (about 6.9 to about 206.8 megapascals "MPa"). The pressure differential can also be created during oil or gas operations. For example, a fluid, when introduced into the wellbore 11 upstream or downstream of the substance, can create a higher pressure above or below, respectively, of the isolation device. Pressure differentials can range from 100 to over 10,000 psi (about 0.7 to over 68.9 MPa). According to another embodiment, the isolation device is capable of withstanding the specific pressure differential for the desired amount of time. The desired amount of time can be at least 30 minutes. The desired amount of time can also be in the range of about 30 minutes to 14 days, preferably 30 minutes to 2 days, more preferably 4 hours to 24 hours.

As discussed above, the rate of dissolution of the first material 51 can be controlled using a variety of factors. According to an embodiment, at least the first material 51 includes one or more tracers (not shown). The tracer(s) can be, without limitation, radioactive, chemical, electronic, or acoustic. As depicted in Fig. 3, each piece of the first material 51 can include a tracer. A tracer can be useful in determining real-time information on the rate of dissolution of the first material 51. For example, a first material 51 containing a tracer, upon dissolution can be flowed through the wellbore 11 and towards the wellhead or into the subterranean formation 20. By being able to monitor the presence of the tracer, workers at the surface can make on-the-fly decisions that can affect the rate of dissolution of the remaining first material 51.

Such decisions might include to increase or decrease the concentration of the electrolyte. As used herein, an electrolyte is any substance containing free ions (i.e., a positive- or negative-electrically charged atom or group of atoms) that make the substance electrically conductive. The electrolyte can be selected from the group consisting of, solutions of an acid, a base, a salt, and combinations thereof. A salt can be dissolved in water, for example, to create a salt solution. Common free ions in an electrolyte include sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺), chloride (Cl^-), hydrogen phosphate (HPO₄^2-), and hydrogen carbonate (HCO₃^-). The concentration (i.e., the total number of free ions available in the electrolyte) of the electrolyte can be adjusted to control the rate of dissolution of the first material 51. According to an embodiment, the concentration of the electrolyte is selected such that the at least a portion of the first material 51 dissolves in the desired amount of time. If more than one electrolyte is used, then the concentration of the electrolytes is selected such that the first material 51 dissolves in a desired amount of time. The concentration can be determined based on at least the specific metals or metal alloys selected for the first and second materials 51/52 and the bottomhole temperature of the well. Moreover, because the free ions in the electrolyte enable the electrochemical reaction to occur between the first and second materials 51/52 by donating its free ions, the number of free ions will decrease as the reaction occurs. At some point, the electrolyte may be depleted of free ions if there is any remaining first and second materials 51/52 that have not reacted. If this occurs, the galvanic corrosion that causes the first material 51 to dissolve will stop. In this example, it may be necessary to cause or allow the first and second materials to come in contact with a second, third, or fourth, and so on, electrolyte(s).

It may be desirable to delay contact of the first and second materials 51/52 with the electrolyte. The isolation device can further include a coating 60 on the outside of the device. The coating can be a compound, such as a wax, thermoplastic, sugar, salt, or a conducting polymer and can include chromates, phosphates, and polyanilines. The coating can be selected such that the coating dissolves in wellbore fluids, melts at a certain temperatures, or cracks and falls away. Upon dissolution or melting, at least the first material 51 of the isolation device is available to come in contact with the electrolyte. The coating 60 can also be porous to allow the electrolyte to come in contact with some of the surface of the first and second materials 51/52.

It may also be desirable to selectively dissolve certain portions of the first material 51 at different times or at different rates. By way of example, it may be desirable to dissolve the top portion of the isolation device first and then dissolve the bottom portion at a later time. This can be accomplished, for example, by introducing a first electrolyte into the wellbore to come in contact with the first and second materials 51/52. There are many operations, such as stimulation operations involving fracturing or acidizing techniques, or tertiary recovery operations involving injection techniques, in which this may be desirable. After the desired operation has been performed, the bottom of the isolation device can be contacted by produced formation fluids. The formation fluids can contain a sufficient concentration of free ions to allow the dissolution of the remaining first material 51.

The methods include the step of contacting or allowing the wellbore isolation device to come in contact with the electrolyte. The step of contacting can include introducing the electrolyte into the wellbore 11. The step of allowing can include allowing the isolation device to come in contact with a fluid, such as a reservoir fluid. The methods can include contacting or allowing the device to come in contact with two or more electrolytes. If more than one electrolyte is used, the free ions in each electrolyte can be the same or different. A first electrolyte can be, for example, a stronger electrolyte compared to a second electrolyte. Furthermore, the concentration of each electrolyte can be the same or different. It is to be understood that when discussing the concentration of an electrolyte, it is meant to be a concentration prior to contact with either the first and second materials 51/52, as the concentration will decrease during the galvanic corrosion reaction. Tracers can be used to help determine the necessary concentration of the electrolyte to help control the rate and finality of dissolution of the first material 51. For example, if it is desired that the first material 51 dissolves to a point to enable the isolation device to be flowed from the wellbore 11 within 5 days and information from a tracer indicates that the rate of dissolution is too slow, then a more concentrated electrolyte can be introduced into the wellbore or allowed to contact the first and second materials 51/52. By contrast, if the rate of dissolution is occurring too quickly, then the first electrolyte can be flushed from the wellbore and a less concentrated electrolyte can then be introduced into the wellbore.

The methods can further include the step of placing the isolation device in a portion of the wellbore 11, wherein the step of placing is performed prior to the step of contacting or allowing the isolation device to come in contact with the electrolyte. More than one isolation device can also be placed in multiple portions of the wellbore. The methods can further include the step of removing all or a portion of the dissolved first material 51 and/or all or a portion of the second material 52 or the substance 60, wherein the step of removing is performed after the step of allowing the at least a portion of the first material to dissolve. The step of removing can include flowing the dissolved first material 51 and/or the second material 52 or substance 60 from the wellbore 11. According to an embodiment, a sufficient amount of the first material 51 dissolves such that the isolation device is capable of being flowed from the wellbore 11. According to this embodiment, the isolation device should be capable of being flowed from the wellbore via dissolution of the first material 51, without the use of a milling apparatus, retrieval apparatus, or other such apparatus commonly used to remove isolation devices. According to an embodiment, after dissolution of the first material 51, the second material 52 or the substance 60 has a cross-sectional area less than 0.05 square inches, preferably less than 0.01 square inches.

Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below.