[001] This invention relates to a corrodible downhole article comprising an aluminium alloy, a method for making such an article 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] One way of producing 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. Such fracking balls may be made from aluminium, magnesium, polymers or composites. The seats are usually made of steel.

[006] An essential characteristic of the material from which the fracking ball (ie a corrodible downhole article) is formed is that it dissolves or corrodes under the conditions in the well or borehole. Such corrodible articles need to corrode at a rate which allows them to remain useable for the time period during which they are required to perform their function, but that allows them to corrode or dissolve afterwards.

[007] As fracking ball technology has developed, valves have been designed which have a smaller overlap (ie contact point) between the steel seat and the dissolvable fracking ball. An example of such a valve is shown in Figure 1. [008] This reduction in the surface area of the contact between the seat and the fracking ball results in an increase in the pressure being applied at this contact point. Thus, there is a need for fracking balls which are able to withstand these increased pressures, for example by development of materials (eg alloys) having improved strength whilst still maintaining the desired corrosion properties (eg high corrosion rate, uniformity of corrosion). It is also desirable if the alloys are processable by extrusion. Such alloys are also sought for use in other downhole applications where corrodible articles are required.

[009] US 2009/0226340 A1 relates to products used in oilfield exploration, production and testing which are made from degradable aluminium alloys. The alloys are formed by adding one or more elements to an aluminium or aluminium alloy melt. Ga, In and Zn are described as possible additives, but the amounts of these elements are not mentioned.

[0010] US 2010/0209288 A1 describes aged-hardenable and degradable aluminium alloys, and their use in wellbore environments. Aluminium alloys including 0.5-8.0 wt% Ga, 0.5-8.0 wt% Mg, less than about 2.5 wt% In and less than about 4.5 wt% Zn are mentioned.

[0011] WO 2014/113058 A2 relates to a degradable ball sealer for use in hydraulic fracturing. The degradable ball sealer includes an aluminium alloy containing gallium, carbon particles and salt particles.

[0012] Statement of invention

[0013] This invention relates to a corrodible downhole article comprising an aluminium alloy, wherein the aluminium alloy comprises (a) 3-15 wt% Mg, (b) 0.01-5 wt% In, (c) 0-0.25 wt% Ga, and (d) at least 60 wt% Al.

[0014] 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. Thus, in the context of the inventive alloy, any elements mentioned are in their metallic form (and not, for example, present as a salt). [0015] In particular, the aluminium alloy may comprise Mg in an amount of 3-13 wt%, more particularly 4-12 wt%, even more particularly 5-11 wt%.

[0016] More particularly, the aluminium alloy may comprise In in an amount of 0.05-

5 wt%, even more particularly 0.1-4 wt%.

[0017] In particular, the aluminium alloy may comprise In in an amount of 0.1-1.5 wt%, more particularly 0.2-1.3 wt%, even more particularly 0.3-1.2 wt%. In some embodiments, the alloy may comprise these amounts of In when it comprises Sn in an amount of 0-0.1 wt%, more particularly 0-0.05 wt%, even more particularly when it is substantially free of Sn.

[0018] In particular, the aluminium alloy may comprise In in an amount of 0.5-4.5 wt%, more particularly 0.75-4.0 wt%. In some embodiments, the alloy may comprise these amounts of In when it comprises Sn in an amount of 0.5-4.5 wt%, more particularly 0.75-4.0 wt%.

[0019] More particularly, the aluminium alloy may comprise one or more compounds which are capable of forming an intermetallic phase. In particular, the one or more compounds may be selected from Ni, Fe, W, Zr and Au. More particularly, the one or more compounds may be Ni and/or Fe.

[0020] More particularly, the aluminium alloy may comprise Fe in an amount of 0-2.5 wt%, even more particularly 0.1-2.0 wt%, more particularly 0.5-1.8 wt%, even more particularly 0.5-1.50 wt%.

[0021] In particular, the aluminium alloy may comprise Ni in an amount of 0-10 wt%, more particularly 0.1 -7.5 wt%, even more particularly 0.5-6 wt%, more particularly 1-

6 wt%.

[0022] More particularly, the aluminium alloy may comprise Zn in an amount of 0-15 wt%, even more particularly 0.3-14 wt%, more particularly 1-13 wt%, even more particularly 3-10 wt%. [0023] For example, the aluminium alloy may comprise (a) 5-11 wt% Mg, (b) 0.3-1.2 wt% In, (c) 0-0.25 wt% Ga, (d) 0-1.8 wt% Fe, (e) 0-6 wt% Ni, and (f) 1-13 wt% Zn. In a further embodiment, the aluminium alloy may comprise (a) 5-11 wt% Mg, (b) 0.3- 1.2 wt% In, (c) 0-0.25 wt% Ga, (d) 0-1.5 wt% Fe, (e) 0-0.5 wt% Ni, and (f) 0.3-10 wt% Zn.

[0024] In particular, the aluminium alloy may comprise Ga in an amount of 0-0.1 wt%, more particularly 0-0.05 wt%. In some embodiments, the aluminium alloy may be substantially free of Ga.

[0025] More particularly, the aluminium alloy may comprise Mn in an amount of 0- 0.1 wt% more particularly 0-0.05 wt%. In some embodiments, the aluminium alloy may be substantially free of Mn.

[0026] In particular, the aluminium alloy may comprise Si in an amount of 0-0.2 wt%, more particularly 0-0.1 wt%, even more particularly 0-0.05 wt%. In some embodiments, the aluminium alloy may be substantially free of Si.

[0027] More particularly, the aluminium alloy may comprise Bi in an amount of 0-0.2 wt%, even more particularly 0-0.1 wt%, more particularly 0-0.05 wt%. In some embodiments, the aluminium alloy may be substantially free of Bi.

[0028] In particular, the aluminium alloy may comprise Cu in an amount of 0-0.2 wt%, more particularly 0-0.1 wt%, even more particularly 0-0.05 wt%. In some embodiments, the aluminium alloy may be substantially free of Cu.

[0029] More particularly, the aluminium alloy may comprise Ca in an amount of 0-0.2 wt%, even more particularly 0-0.1 wt%, more particularly 0-0.05 wt%. In some embodiments, the aluminium alloy may be substantially free of Ca.

[0030] In particular, the aluminium alloy may comprise carbon in an amount of 0-1 wt%, more particularly 0-0.5 wt%, even more particularly 0-0.1 wt%. In some embodiments, the aluminium alloy may be substantially free of carbon. [0031] In particular, in some embodiments the aluminium alloy may comprise an element that is known to act as a corrosion rate modifier, e.g. a rare earth element other than Y, such as Ce. In the context of the invention, the rare earth elements are defined as the fifteen lanthanides, plus Y. The aluminium alloy may comprise the corrosion rate modifier, in an amount of 0-1 wt%, more particularly 0-0.5 wt%, even more particularly 0-0.1 wt%. In some embodiments, the aluminium alloy may be substantially free of the corrosion modifier.

[0032] More particularly, the aluminium alloy may comprise Ti in an amount of 0-0.5 wt%, even more particularly 00.5-0.2 wt%. In some embodiments, the aluminium alloy may be substantially free of Ti.

[0033] In particular, the content of Al in the aluminium alloy may be at least 65wt%, more particularly at least 70wt%. In some embodiments, the remainder of the alloy may be aluminium and incidental impurities.

[0034] In particular, the aluminium alloy may have a corrosion rate of at least 300 mg/cm ² /day, more particularly at least 500 mg/cm ² /day, in some embodiments at least 1000 mg/cm ² /day, in 3 % KCI at 93 °C (200 F). More particularly, the corrosion rate, in 3 % KCI at 93 °C may be less than 15,000 mg/cm ² /day.

[0035] In particular, the aluminium alloy may be heat treatable and/or extrudable. More particularly, the aluminium alloy may be heat treated and/or extruded.

[0036] In particular, the corrodible downhole article may be a downhole tool or a wellbore isolation device. More particularly, the wellbore isolation device may be a fracking ball, plug/plug component, packer or other tool assembly, even more particularly a fracking ball. In particular, the fracking ball may be substantially spherical in shape.

[0037] This invention also relates to a method of making a corrodible downhole article comprising an aluminium alloy, the method comprising the steps of:

(a) melting aluminium, Mg, In and optionally Ga, to form a molten aluminium alloy comprising 3-15 wt% Mg, 0.01-5 wt% In, 0-0.25 wt% Ga, and at least 60 wt% Al, (b) mixing the resulting molten aluminium alloy,

(c) casting the aluminium alloy or producing an aluminium alloy powder, and

(d) forming the aluminium alloy into a corrodible downhole article.

[0038] In particular, the method may be for producing an aluminium 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 melting step (a). More particularly, the melting step may be carried out at a temperature of 660°C (ie the melting point of pure aluminium) or more, even more particularly less than 2470°C (the boiling point of pure aluminium). In particular, the temperature range during melting and/or forming of a molten aluminium alloy may be 600°C to 850°C, more particularly 700°C to 800°C, even more particularly about 750°C.

[0039] More particularly, in step (a) the resulting alloy may be fully molten. In particular, prior to melting in step (a) the alloy components may be present in elemental form or as one or more alloys.

[0040] In particular, in step (c) the casting may comprise pouring the molten aluminium 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. More particularly, in step (c) the producing an aluminium alloy powder may be by casting and then grinding, or by atomisation.

[0041] More particularly, step (d) may comprise one or more of: compacting, additive manufacturing, extruding, forging, rolling, and machining. In particular, compacting may comprise forming a Metal Matrix Composite (MMC).

[0042] In particular, after step (c) and either before or after step (d) the method may comprise the step of heat treating the alloy. The heat treatment may be by any technique known in the art in relation to aluminium alloys.

[0043] In addition, this invention relates to a method of hydraulic fracturing comprising the use of a corrodible downhole article 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 may be a fracking ball, plug, packer or tool assembly, even more particularly a fracking ball. In particular, the fracking ball may be substantially spherical in shape. In some embodiments, the corrodible downhole article, more particularly the fracking ball, may consist essentially of the aluminium alloy described above.

[0044] This invention will be further described by reference to the following Figures which are not intended to limit the scope of the invention claimed, in which:

Figure 1 shows an example of the typical geometry of a fracking ball on a seat,

Figure 2 shows a graph of corrosion rate as a function of In content for three alloy compositions, and

Figure 3 shows a graph of the force withstood in load with ball on seat testing as a function of In content for two alloy compositions.

[0045] Examples

[0046] Alloy preparation

[0047] Aluminium alloy compositions were prepared by combining the components in the amounts listed in Table 1 below (the balance being aluminium and incidental impurities) and then melting them. These components were then melted by heating at a temperature in the range 600 °C-900 °C (dependent upon the alloy components). Each melt was then cast into a billet.

[0048] Corrosion testing

[0049] In order to simulate the corrosion performance in a well, the material was corrosion tested by measuring weight loss in an aqueous solution of 3 wt% potassium chloride at a constant temperature of 93 °C (200 F). These results are shown in Table 1 below. The results demonstrate that the alloys of the invention achieve the desired corrosion rates.

[0050] In addition, three further alloy compositions were prepared as follows:

(i) 1 wt% Fe, 5 wt% Ni, 5 wt% Zn, 10 wt% Mg, X wt% In, remainder Al,

(ii) 1 wt% Fe, 5 wt% Ni, 10 wt% Zn, 10 wt% Mg, X wt% In, remainder Al, and

(iii) 1 wt% Fe, 3 wt% Ni, 6 wt% Zn, 5 wt% Mg, X wt% In, remainder Al.

[0051] Various alloys were produced where the amount of In (ie the X value) was varied from 0-1.2 wt%. These alloys were then subjected to corrosion testing. The results of this testing are shown in Figure 2, which demonstrates the effect of In addition on corrosion behaviour.

[0052] “Ball on seat” testing

[0053] 23.5 mm diameter balls were manufactured by machining alloy billets. The ball on seat test is shown in Figure 1 which utilises a steel seat for the ball test. The seat angle was 30° and the overlap between the aluminium alloy ball and the steel ball seat is approximately 1.5 %, where % overlap = (1 -(Diameter _seat /Diameter _ba n)) x 100). Each ball was then forced through the steel seat using a Zwick universal testing machine, utilising uniaxially applied compressive load, which gives a maximum force in kN. Where a particular alloy was tested, these results are shown in Table 1 below. These results demonstrate that the alloys of the invention achieve the required force values.

[0054] In addition, two further alloy compositions were prepared as follows:

(i) 1 wt% Fe, 4 wt% Ni, 8 wt% Zn, 10 wt% Mg, X wt% In, remainder Al, and

(ii) 1 wt% Fe, 3 wt% Ni, 6 wt% Zn, 5 wt% Mg, X wt% In, remainder Al.

[0055] Various alloys were produced where the amount of In (ie the X value) was varied from 0-1.1 wt%. These alloys were then subjected to ball on seat testing. The results of this testing are shown in Figure 3, which demonstrates that force can be maintained within the desired range at varying amounts of In.

Table 1

Table 1 ctd