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1. WO2018130816 - CORRODIBLE DOWNHOLE ARTICLE

Note: Text based on automatic Optical Character Recognition processes. Please use the PDF version for legal matters

[ EN ]

CORRODIBLE DOWNHOLE ARTICLE

[001] This invention relates to a magnesium alloy suitable for use as a corrodible downhole article, a method for making such an alloy, an article comprising the alloy and the use of the article.

[002] Background

[003] The oil and gas industries utilise a technology known as hydraulic fracturing or "fracking". This normally involves the pressurisation with water of a system of boreholes in oil and/or gas bearing rocks in order to fracture the rocks to release the oil and/or gas.

[004] In order to achieve this pressurisation, valves may be used to block off or isolate different sections of a borehole system. These valves are referred to as downhole valves, the word downhole being used in the context of the invention to refer to an article that is used in a well or borehole.

[005] Downhole plugs are one type of valve. A conventional plug consists of a number of segments that are forced apart by a conical part. The cone forces the segments out until they engage with the pipe bore. The plug is then sealed by a small ball. Another way of forming such valves involves the use of spheres (commonly known as fracking balls) of multiple diameters that engage on pre-positioned seats in the pipe lining. Downhole plugs and fracking balls may be made from aluminium, magnesium, polymers or composites.

[006] A problem with both types of valve relates to the ductility of the material used to make them. Corrodible magnesium alloys such as those used to make downhole valves have limited ductility due to their hexagonal crystal structure. These alloys can exhibit significant crystallographic texture (ie crystals aligned in a particular direction) when used in their wrought form, such as when they are

extruded. This can further limit ductility, especially in the transverse direction. These factors mean that the ductility of dissolvable magnesium alloys is lower than is desirable.

[007] The applicant's earlier patent application, GB2529062A, relates to a magnesium alloy suitable for use as a corrodible downhole article. This document discloses an alloy comprising 3.7-4.3wt% Y, 0.2-1.0wt% Zr, 2.0-2.5wt% Nd and 0.3-1.0wt% rare earths having a maximum elongation (ie ductility) of 21%, a corrosion rate of around 1100mg/cm2/day in 3% KC1 at 93°C (200F) and a 0.2% proof stress of around 200MPa. The range of uses of these magnesium alloys can be limited by their ductility.

[008] CN 106086559 describes magnesium alloys comprising Gd and/or Y as well as Ni. However, the atomic percentage amounts of Y and/or Gd in these alloys correspond to weight percentages which are greater than 2wt% Y and/or greater than 7wt% Gd. CN 104152775 relates to a magnesium alloy comprising 86.7wt% Mg, 2.2wt% Ni, 5.8wt% Gd and 5.3% Nd.

[009] A material which provides the desired corrosion characteristics, but with improved ductility, has been sought.

[0010] Statement of invention

[0011] This invention relates to a magnesium alloy suitable for use as a corrodible downhole article, wherein the alloy comprises:

(a) 2-7wt% Gd,

fb) 0-2wt% Y,

(c) 0-5.0wt% Nd, and

(d) at least 80wt% Mg,

and has an elongation as measured by ASTM B557M-10 of at least 22%.

[0012] In relation to this invention, the term "alloy" is used to mean a composition made by mixing and fusing two or more metallic elements by melting them together, mixing and re-solidifying them.

[0013] The term "rare earth metals" is used in relation to the invention to refer to the fifteen lanthanide elements, as well as Sc and Y.

[0014] Plugs and fracking balls made from the magnesium alloys of the invention can find a broader range of uses.

[0015] In particular, the alloy may have an elongation as measured by ASTM B557M-10 of at least 23%, more particularly at least 24%, even more particularly at least 25%.

[0016] In particular, the magnesium alloy may comprise rare earth metals other than Gd in a total amount of less than 5wt%, more particularly in a total amount of less than 3wt%, even more particularly in a total amount of less than lwt%. In some embodiments, the magnesium alloy may comprise rare earth metals other than Gd in a total amount of less than 0.5wt%, more particularly less than 0.1wt%. In particular embodiments, the magnesium alloy may be substantially free of rare earth metals other than Gd. More particularly, the rare earth metals other than Gd may comprise Y and/or Nd, even more particularly they may be Y and/or Nd.

[0017] More particularly, the magnesium alloy may comprise Gd in an amount of 3-6wt%, even more particularly in an amount of 4.0-6.0wt%. In some embodiments, the magnesium alloy may comprise Gd in an amount of 4.5-5.5wt%, more particularly 4.6-4.9wt%.

[0018] More particularly, the magnesium alloy may comprise Zr in an amount of up to 1.0wt%. In some embodiments, the magnesium alloy may comprise Zr in an amount of 0.01-0.5wt%, more particularly in an amount of 0.02-0.2wt%, even more particularly in an amount of 0.05-0.10wt%. In some embodiments, the magnesium alloy may be substantially free of Zr.

[0019] In particular, the magnesium alloy may comprise one or more elements which promote corrosion. More particularly, the one or more elements may be one or more transition metals. In particular, the magnesium alloy may comprise one or more of Ni, Co, Ir, Au, Pd, Fe or Cu. These elements are known in the art to promote the corrosion of magnesium alloys. The magnesium alloy may comprise 0-2wt% in total of one or more of Ni, Co, Ir, Au, Pd, Fe or Cu, more particularly 0.1-2wt%, even more particularly 0.2-1.0wt%. In some embodiments, the magnesium alloy may comprise 0.4-0.8 wt% in total of one or more of Ni, Co, Ir, Au, Pd, Fe or Cu, more particularly 0.5-0.7wt%.

[0020] In particular, the magnesium alloy may comprise 0-2wt% Ni, more particularly 0.1-2wt%, even more particularly 0.2-1.0wt%. In some embodiments, the magnesium alloy may comprise Ni in an amount of 0.4-0.8 wt%, more particularly 0.5-0.7wt%.

[0021] More particularly, the magnesium alloy may comprise Y in an amount of less than lwt%, even more particularly less than 0.5wt%, more particularly less than 0.1wt%. In some embodiments, the magnesium alloy may be substantially free of Y.

[0022] In particular, the magnesium alloy may comprise Nd in an amount of less than 2wt%. More particularly, the magnesium alloy may comprise Nd in an amount of less than lwt%, even more particularly less than 0.5wt%, more particularly less than 0.1wt%. In some embodiments, the magnesium alloy may be substantially free of Nd.

[0023] More particularly, the magnesium alloy may comprise Al in an amount of less than lwt%, even more particularly less than 0.5wt%, more particularly less than 0.1wt%. In some embodiments, the magnesium alloy may be substantially free of Al.

[0024] In particular, the magnesium alloy may comprise Ce (for example, in the form of mischmetal) in an amount of less than lwt%, even more particularly less than 0.5wt%, more particularly less than 0.1wt%. In some embodiments, the magnesium alloy may be substantially free of Ce.

[0025] More particularly, the remainder of the alloy may be magnesium and incidental impurities. In particular, the content of Mg in the magnesium alloy may be at least 85wt%, more particularly at least 0wt%, even more particularly at least 92wt%.

[0026] A particularly preferred composition of the first embodiment is a magnesium alloy comprising rare earth metals other than Gd in atotal amount of less than 2wt%, Gd in an amount of 4.0-6.0wt%, Zr in an amount of 0.02-0.2wt%, Ni in an amount of 0.1-0.8wt% and Mg in an amount of at least 90wt%.

[0027] In particular, the magnesium alloy may have a corrosion rate of at least 50mg/cm2/day, more particularly at least 75mg/cm2/day, even more particularly at least 100mg/cm2/day, in 3% KCl at 38°C (100F). In particular, the magnesium alloy may have a corrosion rate of at least 50mg/cm2/day, more particularly at least 250mg/cm2/day, even more particularly at least 500mg/cm2/day, in 15% KCl at 93°C (200F). More particularly, the corrosion rate, in 3% KCl at 38°C or in 15% KCl at 93°C (200F), may be less than 15,000mg/cm2/day.

[0028] In particular, the magnesium alloy may have a 0.2% proof stress of at least 75MPa, more particularly at least lOOMPa, even more particularly at least 125MPa, when tested using standard tensile test method ASTM B557-10. More particularly, the 0.2% proof stress may be less than 700MPa. The 0.2% proof stress of a material is the stress at which material strain changes from elastic deformation to plastic deformation, causing the material to deform permanently by 0.2% strain.

[0029] In addition, this invention relates to a wrought magnesium alloy having the composition described above.

[0030] This invention also relates to a corrodible downhole article, such as a downhole tool, comprising the magnesium alloy described above. In some embodiments, the corrodible downhole article is a fracking ball, plug, packer or tool assembly. In particular, the fracking ball may be substantially spherical in shape. In some embodiments, the fracking ball consists essentially of the magnesium alloy described above.

[0031] This invention also relates to a method for producing a magnesium alloy suitable for use as a corrodible downhole article comprising the steps of:

(a) heating Mg, Gd, and optionally one or more of Y and Nd, to form a molten magnesium alloy comprising 2-7wt% Gd, 0-2wt% Y, 0- 5.0wt% Nd, and at least 80wt% Mg,

(b) mixing the resulting molten magnesium alloy, and

(c) casting the magnesium alloy.

[0032] In particular, the method may be for producing a magnesium alloy as defined above. Any other required components in the resulting alloy (for example, those listed in the preceding paragraphs describing the alloy) can be added in heating step (a). More particularly, the heating step may be carried out at a temperature of 650°C (ie the melting point of pure magnesium) or more, even more particularly less than 1090°C (the boiling point of pure magnesium). In particular, the temperature range may be 650°C to 850°C, more particularly 700°C to 800°C, even more particularly about 750°C. More particularly, in step (b) the resulting alloy may be fully molten.

[0033] The casting step normally involves pouring the molten magnesium alloy into a mould, and then allowing it to cool and solidify. The mould may be a die mould, a permanent mould, a sand mould, an investment mould, a direct chill casting (DC) mould, or other mould.

[0034] After step (c), the method may comprise one or more of the following additional steps: (d) extruding, (e) forging, (f) rolling, (g) machining.

[0035] The composition of the magnesium alloy can be tailored to achieve a desired corrosion rate falling in a particular range. The desired corrosion rate in 15% KCl at 93°C can be in any of the following particular ranges: 50-100mg/cm2/day; 100-250mg/cm2/day; 250-500mg/cm2/day; 500-1000mg/cm2/day; 1000-3000mg/cm2/day; 3000-4000 mg/cm2/day; 4000-5000mg/cm2/day; 5000-10,000mg/cm2/day; 10,000-15,000 mg/cm2/day.

[0036] The method of the invention may also comprise tailoring compositions of the magnesium alloys such that the cast magnesium alloys achieve desired corrosion rates in 15% KCl at 93°C falling in at least two of the following ranges: 50 to 100mg/cm2/day; 100-250mg/cm2/day; 250-500mg/cm2/day; 500-1000mg/cm2/day; 1000-3000mg/cm2/day; 3000-4000 mg/cm2/day; 4000-5000mg/cm2/day; 5000-10,000mg/cm2/day; and 10,000-15,000 mg/cm2/day.

[0037] This invention also relates to a magnesium alloy suitable for use as a corrodible downhole article which is obtainable by the method described above.

[0038] In addition, this invention relates to a magnesium alloy as described above for use as a corrodible downhole article.

[0039] This invention also relates to a method of hydraulic fracturing comprising the use of a corrodible downhole article comprising the magnesium alloy as described above, or a downhole tool as described above. In particular, the method may comprise forming an at least partial seal in a borehole with the corrodible downhole article. The method may then comprise removing the at least partial seal by permitting the corrodible downhole article to corrode. This corrosion can occur at a desired rate with certain alloy compositions of the disclosure as discussed above. More particularly, the corrodible downhole article my be a fracking ball, plug, packer or tool assembly. In particular, the fracking ball may be substantially spherical in shape. In some embodiments, the fracking ball may consist essentially of the magnesium alloy described above.

[0040] This invention will be further described by reference to the following Figure which is not intended to limit the scope of the invention claimed, in which:

Figure 1 shows a graph of ductility against Gd content in wt%.

[0041] Examples

[0042] Magnesium alloy compositions were prepared by combining the components in the amounts listed in Table 1 below. These compositions were then melted by heating at 750°C. The melt was then cast into a billet and extruded to a rod.


20 - 0.6 4.8 - 0.02 146 228 26.8

21 - 0.6 5.4 - 0.01 152 236 23.0

22+ - 0.6 6.0 - 0.02 147 232 20.2

23+ - 0.6 7 - 0.02 152 239 18.8

24 - 0.6 8 - 0.02 158 241 12.8

RE includes all Rare Earth elements, including yttrium, but excluding gadolinium

Comparative examples

Table 1

[0043] This data clearly shows that the examples of the invention surprisingly show a significantly improved elongation/ductility. This is confirmed by viewing this data in the form of the graph of Figure 1.