WO2020123918A1

WO2020123918A1 - Alloy plugs for abandoned wells

Info

Publication number: WO2020123918A1
Application number: PCT/US2019/066192
Authority: WO
Inventors: Meng QU; Yucun Lou; Jahir Pabon; Sepand Ossia; Agathe Robisson; Qin Yu; Xianjun Pei
Original assignee: Schlumberger Technology Corporation; Schlumberger Canada Limited; Services Petroliers Schlumberger; Schlumberger Technology B.V.
Priority date: 2018-12-13
Filing date: 2019-12-13
Publication date: 2020-06-18

Abstract

A method is provided for plugging a wellbore having a casing and cement surrounding the casing and traversing a formation, which involves configuring and using at least one tool located in the wellbore to deliver composite material to a target area in the wellbore, wherein the composite material includes metal alloy and an exothermal reactant. The at least one tool is further configured and used to apply heat or spark to the composite material in the target area to ignite the exothermal reactant of the composite material and melt the metal alloy of the composite material. The melted metal alloy of the composite material is permitted to solidify to form a plug at the target area in the wellbore.

Description

ALLOY PLUGS FOR ABANDONED WELLS

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application claims the benefit of U.S. Provisional Application No. 62/779,246, filed on December 13, 2018; the contents of which are incorporated herein, in its entirety, by reference.

FIELD

[0002] The subject disclosure relates to wellbore plugs for abandoned hydrocarbon wells.

BACKGROUND

[0003] Wells for the production of hydrocarbons such as oil are created by using a drill bit supported by a drill rig to drill a borehole into an earth formation. After the borehole is drilled, sections of steel pipe, also referred to as casings, having diameters slightly smaller than the diameter of the borehole are placed in the borehole. The casings are fixed in the borehole using cement which is pumped into an annulus between the casing and the formation. The cement not only provides structural integrity to the casings, but isolates zones in the earth formation from one another. After drilling and casing, the well is“completed” by making perforations in the casing through which the hydrocarbons can pass from the surrounding formation into production tubing. Various techniques may then be used to produce the hydrocarbons from the formation.

[0004] Over the course of time, when the production of a hydrocarbon well declines to the extent that it no longer profitably produces hydrocarbons, it is common to abandon the well. In abandoning the well, production tubing is removed, and a determination is made regarding the condition of the cement in the annulus. If the cement is not deemed to be in excellent condition, it is common practice to remove the casing and the annulus cement and to fill or plug the remaining borehole with cement in order to prevent interzonal and surface communication, and contamination, as environmental factors are important, particularly in offshore settings. This process is commonly referred to as“plug and abandonment”. The cost of removing the casing and the annulus cement can be significant, e.g., millions of U.S. dollars, particularly in offshore wellbores. One reason for the significant cost is that removal of the casing and annulus cement is notoriously complicated and requires very heavy and expensive rig equipment for pulling the casing out of the wellbore.

[0005] The most common material used for plug and abandonment is Portland cement, which is placed in the well as a slurry that hardens in due time. A cement plug consists of a volume of cement that fills a certain length of casing or open hole to prevent vertical migration of fluids. Cement satisfies the essential criteria of an adequate plug; it is durable, has low permeability, and is inexpensive. Furthermore, it is easy to pump in place, has a reasonable setting time and is capable of tight bonding to the formation and well casing surface. It also has a sufficient mechanical strength under compression, although its tensile characteristics are its major weakness.

SUMMARY

[0006] This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

[0007] According to one aspect, a method is provided for plugging a wellbore having a casing and cement surrounding the casing and traversing a formation, which involves configuring and using at least one tool located in the wellbore to deliver composite material to a target area in the wellbore, wherein the composite material includes metal alloy and an exothermal reactant. The at least one tool is further configured and used to apply heat or spark to the composite material in the target area to ignite the exothermal reactant of the composite material and melt the metal alloy of the composite material. The melted metal alloy of the composite material is permitted to solidify to form a plug at the target area in the wellbore.

[0008] In embodiments, the metal alloy of the composite material includes bismuth or antimony.

[0009] In embodiments, the composite material includes at least one trace metal element. For example, the at least one trace metal element can be selected from the group consisting of silver, zinc, copper, tin, and combinations thereof. [0010] In embodiments, the exothermal reactant of the composite material can include a metal power component and an oxidizer component. For example, the metal powder component can be selected from the group consisting of aluminum, boron, magnesium, zinc, silicon, titanium, and combinations thereof. The oxidizer component can include at least one metal oxide selected from the group consisting of iron oxide (FeiCb, FeO), copper oxide (CuO, CU2O), bismuth oxide (B12O3), silicon oxide (S1O2), cobalt oxide (CoO), titanium oxide (T1O2), chromium oxide ((¾0₃), and combinations thereof.

[0011] In embodiments, the exothermal reactant of the composite material can include thermite or thermate in particulate form.

[0012] In embodiments, the composite material can have a core-shell structure having a core surrounded by a shell, wherein the core is a metal alloy particle and the shell includes particles of an exothermal reactant. In embodiments, the metal alloy particle of the core can include bismuth or antimony, and the exothermal reactant of the shell can include thermite or thermate.

[0013] In embodiments, a ratio of shell thickness to core size can be selected or controlled for plugging the wellbore.

[0014] In embodiments, a ratio of shell thickness to core size can be selectively varied and controlled over a lengthwise dimension of the target area to provide a desired heating or melting or cooling or solidification profile over the target area.

[0015] In embodiments, the exothermal reactant of the shell can be selectively varied and controlled over a lengthwise dimension of the target area to provide a desired heating or melting or cooling or solidification profile over the target area.

[0016] In embodiments, the composite material comprises metal alloy particles mixed with particles of an exothermal reactant. For example, the particles of an exothermal reactant can include a metal power component and an oxidizer component.

[0017] In embodiments, a weight ratio or volume ratio of the exothermal reactant particles to the metal alloy particles of the composite material can be selectively controlled over the target area to provide a heating or melting or cooling or solidification profile over the target area. [0018] In embodiments, the exothermal reactant of the exothermal reactant particles can be selectively varied and controlled over a lengthwise dimension of the target area to provide a desired heating or melting or cooling or solidification profile over the target area.

[0019] In embodiments, the composite material comprises a housing that defines an interior cavity, wherein the housing is formed by the metal alloy, and the interior cavity is loaded with exothermal reactant in particulate form.

[0020] In embodiments, the at least one tool can be configured to deploy at least one media layer disposed below the composite material, wherein the at least one media layer includes granular material which provide space for melted alloy to flow into. For example, the granular material of the at least one media layer can be selected from the group consisting of sand, proppant, cement or alloy particles and combinations thereof.

[0021] In embodiments, the composite material can further include a surface modification agent. For example, the surface modification agent can be selected from the group consisting of phenolic resin, high temperature epoxy resin, borox, borate and combinations thereof.

[0022] In embodiments, the method can further include configuring and using a tool located in the wellbore to prepare a wellbore-formation wall interface at the target area of the wellbore, and injecting a fluid that contacts the wellbore - formation wall interface prior to delivering the composite material to the target area for plug formation, wherein the fluid includes a surface modification agent.

[0023] In embodiments, the at least one tool includes a v-shaped catcher that is operably disposed adjacent the bottom end of the target area.

[0024] In embodiments, the method can further include configuring and using a tool located in the wellbore to prepare a wellbore-formation wall interface at the target area of the wellbore, wherein the wellbore-formation wall interface is configured such that the plug formed from solidification of the melted alloy has a truncated conical cross-sectional profile or piece-wise conical cross-sectional profile. [0025] In embodiments, at least one property of the composite material can be determined according to downhole conditions at the target area.

[0026] Systems and downhole tools for plugging a wellbore are also described and/or claimed.

[0027] Additional aspects, embodiments, objects and advantages of the disclosed methods and systems and downhole tools may be understood with reference to the following detailed description taken in conjunction with the provided drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The subject disclosure is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of the subject disclosure, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein:

[0029] Figure l is a schematic of a system for plugging an offshore wellbore;

[0030] Figure 2 is a schematic diagram of a plug tool of the system of Figure 1, which is located in the wellbore and adapted for plugging the wellbore;

[0031] Figure 3 is a flow chart of a workflow for plugging a wellbore, for example, using the tool assembly of Figure 2;

[0032] Figure 4 is a schematic diagram of a composite material for forming a wellbore plug, which include particles have a core-shell structure with an alloy core and a shell of smaller-size exothermal reactant particles (which can be in non-limiting examples, thermite or thermate particles);

[0033] Figures 5 A to 5C are schematic diagrams of composite materials for forming a wellbore plug, each including a mixture of expandable alloy particles and exothermal reactant particles (which can be in non-limiting examples, thermite or thermate particles); [0034] Figure 6 is a schematic diagram of a model of a core-shell structure of a composite material for forming a wellbore plug;

[0035] Figures 7 A to 7D are diagrams that show the temperature evolution of a composite material based on the core-shell structure of Figure 6 (where a BiSn core is coated with A1+TΪ02 as the thermite shell) during melting and solidification to form a metal alloy plug;

[0036] Figure 8 shows three plots of the yield strength of a plug formed from a composite material of thermite and BiSn (no sand), the yield strength of a plug formed from a composite material of thermite and BiSn and sand (7wt% SiCh or sand), and the yield strength of a plug formed from a composite material of thermite and BiSn and sand (fully mixed), respectively;

[0037] Figure 9 depicts plots that show the change to yield strength of BiSn when aged at 110°C for two weeks;

[0038] Figures 10A and 10B are pictures that show examples that use high temperature phenolic resin and high temperature epoxy resin to improve wetting and bonding between a composite alloy (which can be used for a wellbore plug) and rock;

[0039] Figure 11 is a schematic diagram of another plug tool of the system of Figure 1, which is located in the wellbore and adapted for plugging the wellbore;

[0040] Figure 12 includes two panels (one atop the other) that describe laboratoary experiments that mimic downhole plug formation. The top panel of Figure 12 shows a media layer spaced below the target area where the plug is to be formed. The bottom panel of Figure 12 shows a target area where the plug is to be formed without any media layer below the target area;

[0041] Figures 13 A and 13B are plots that result from the experiments depicted in Figure 12, which compare alloy melting with and without a media layer, where the media layer is a sand layer as an example;

[0042] Figure 14A is a schematic diagram of a plug tool that is deployed in a wellbore and has a flat catcher; [0043] Figure 14B is a schematic diagram of another plug tool that is deployed in a wellbore and has a V-shaped catcher;

[0044] Figure 15 is a plot of the overall melted alloy fraction (liquidous phase of alloy fraction) as a function of time during the heating by thermite reaction and subsequent cooling process for the flat catcher design of Figure 14A and the V-shaped catcher design of Figure 14B;

[0045] Figures 16 and 17 are schematic diagrams where an alloy is used as a housing with thermite material filled and packed inside the housing; the thermite can be ignited, and the heat of thermite reaction can directly reach the alloy housing to melt into liquid to form the wellbore plug;

[0046] Figure 18 is a schematic diagram that depicts an alloy wellbore plug with a truncated conical geometry to provide interlocking with its surroundings (casing and/or cement and/or formation) in the downhole environment; and

[0047] Figures 19 and 20 are schematic diagrams that depicts alloy wellbore plugs with piecewise truncated conical geometry to provide interlocking with the plugs surroundings (casing and/or cement and/or formation) in the downhole environment.

DETAILED DESCRIPTION

[0048] The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the subject disclosure only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the subject disclosure. In this regard, no attempt is made to show structural details in more detail than is necessary for the fundamental understanding of the subject disclosure, the description taken with the drawings making apparent to those skilled in the art how the several forms of the subject disclosure may be embodied in practice.

Furthermore, like reference numbers and designations in the various drawings indicate like elements.

[0049] Cement-based materials are commonly used in conventional plug and abandonment (P&A) processes to form a permanent sealing plug. However, cementitious materials may shrink during their reaction with water (i.e. setting). Thus, the formed cement solid might not form a tight seal between the plug and the formation, especially in tight formations. The cement-based plug is also relatively brittle which makes it easy to crack under pressure over a long period of time. Presence of other contaminants such as oil and mud can also affect the seal between cement and the well. Finally, cement plugs may react with downhole chemicals such as CO2 and the long-term integrity of the plug may be compromised over time.

[0050] Bismuth alloy plugs were developed which aim to resolve the above issues, taking advantage of the unique property of bismuth which expands when it changes phase from liquid into solid. In most cases, the alloy particles delivered into the well are melted downhole and re solidified into a solid plug. As the alloy expands upon solidification, it can provide a good seal between any surface including both metal casing and formation rock. These alloys are relatively ductile with reasonably high strength when compared to cement. The heat source for melting the alloy can be either electric heating or chemical heating such as thermite, thermate or any other exothermal reactant materials. However, in these methods, the melting of the alloy is typically achieved by local heating tools either in the center, or around the alloy particle/pellet reservoir. That is, the melting of the alloy particles can be highly inhomogeneous with large temperature gradient within the melt pool. It is also possible that some of the particles far away from the heating tool may not be completely melted. The uneven melting may induce defects such as voids or not fully melted regions into the plug. A large temperature gradient during melting may also induce large amount of residual stress. Lastly, uneven melting can cause different microstructure and grain formation in an alloy, which may lead to different local mechanical properties. These heterogeneity properties may affect the long-term sealing capability of the plug.

[0051] In embodiments, the present disclosure is directed to methods, apparatus and systems that employ a composite material for forming a plug in a wellbore. The composite material can employ a chemical -based heating reactant component in particulate form that coats alloy particles. Alternatively or additionally, the composite material can employ a chemical-based heating reactant component in particulate form that is distributed in alloy particles as part of a composite matrix. Alternatively, the composite material can be a housing formed from an alloy with an interior cavity that holds a chemical -based heating reactant component. The chemical- based heating reactant component can be thermite, thermate or other exothermal reactants that when ignited by heat provides sufficient heat to melt the alloy of the composite material. In embodiments, the composite material can provide for more uniform melting of the alloy to form a solid plug with more homogeneous mechanical properties. Furthermore, one or more trace metal elements can possibly be added to the composite material to improve the mechanical properties of the plug formed from the composite material.

[0052] Turning now to Figure 1, a system 100 for plugging an offshore wellbore 102 is shown. The wellbore 102 traverses a formation 104 having a surface at seabed 106. A ship 108 is shown floating above the wellbore 102, and a cable or coil 110 (e.g., a wireline, slickline or coiled tube) is shown extending from the ship down into the wellbore 102. Mounted on the cable or coil 110 is a tool string that includes a laser tool 112 and a plug tool 114. The laser tool 112 is operable to remove the casing and cement and possibly part of the formation over a portion of the wellbore that will be plugged and form the desired wellbore - formation wall interface for the plug. The plug tool 114 is operable to form the plug at the wellbore - formation wall interface prepared by the laser tool 112. The laser tool 112 is located above the plug tool 114 so that the laser tool 112 can form the desired wellbore - formation wall interface, and then the tool string may be pulled upward to locate the plug tool adjacent the wellbore - formation wall interface. Alternatively, the laser tool 112 may be run separately from the plug tool 114 so that the laser tool 112 is first deployed from the ship to prepare the wellbore - formation wall interface. When the preparation is completed, the laser tool 112 is withdrawn, and the plug tool 114 is deployed. In other embodiments, milling, water jet cutting and other cutting modalities can be used to form the wellbore-formation wall interface of the plug.

[0053] Figure 2 is a schematic diagram of a plug tool 114 located in the wellbore 102 of Figure 1 and adapted for plugging the wellbore 102. The tool 114 can include a packer 201, which can be deployed such that it extends around a portion of the tool near the top of the tool and engaging the casing in the wellbore 102. Also shown is a fluid path including an inlet 202a located above the packer 201, a pump 202b, and a fluid outlet 202c located below the packer 201. The tool also includes a storage chamber 203 which stores the composite material that is used to form the plug. The storage chamber 203 can be adapted to release the composite material (denoted by arrow 204) into the target area of the wellbore 102 occupied by a one or more heating electrodes 205 located at the bottom of the tool. A catcher 206 is disposed at or near the bottom of the tool. In embodiments, the catcher 206 can be deploy such that it extends around a bottom portion of the heating electrode(s) 205 and engages the casing in the wellbore 102. In this manner, the catcher 206 can be configured to hold the composite material released from the storage chamber 203 in the target area of the wellbore 102 occupied by the one or more heating electrodes 205 thereabove. The tool 1 14 may also include a controller 207 (labeled “Control”) operably coupled to both the heating electrode(s) 205 and the pump 202b. The controller 207 can be adapted to supply electrical current to the heating electrode(s) 205 to heat the composite material in the target area and ignite the exothermal reactant of the composite material to provide sufficient heat to melt the alloy of the composite material. The controller 207 can also be configured to operate the pump 202b to supply fluid to apply confinement pressure to the composite material as the plug is formed if desired.

[0054] A method that employs the system of Figures 1 and 2 to plug a wellbore is shown in Figure 3. Note that other wellbore plugging tools and systems can also be used as part of the methods and systems of the present disclosure.

[0055] At 301, a portion (or layer(s)) of the formation that will interface to the plug (when formed in situ) is identified. Such formation portion (or layer(s)) may be identified by review of logs of the well and/or the formation previously generated in order to explore, drill the well and/or otherwise exploit the formation.

[0056] At 303, a measured depth MDp in the wellbore that corresponds to the formation portion (or layer(s)) identified in 301 is determined. The measured depth MDp may be determined by review of logs of the well and/or the formation previously generated in order to explore, drill the well and/or otherwise exploit the formation. The measured depth MDp can be represented by a value or range of values of measured depth corresponding to a length in the wellbore. Note that measured depth differs from true vertical depth in the wellbore in all but vertical wells.

[0057] At 305, one or properties of the composite material that is suited to form the plug that seals to the formation at the measured depth MDp of the wellbore can be determined. For example, the properties of the composite material can specify the type of alloy, the amount of the alloy, the amount of the exothermal reactant, the amount of one or more trace metal additives included in the composite material, and possibly other properties. Composite material that matches such composite material properties can be selected or otherwise obtained and loaded into the storage chamber 203 of the plug tool. One or more properties of the desired wellbore - formation wall interface for the plug at the measured depth MDp can also be determined at 305. For example, the properties of the wellbore - formation wall interface can specify the geometry (including the desired profile or shape and extent) of the interface. The properties of the composite material and wellbore - formation wall interface can be based on downhole conditions at the measured depth MDp, such as pH of wellbore fluid, temperature, pressure, maximum allowed temperature (which is based on the boiling point of water at the pressure of the measured depth MDp and intended to avoid vaporization induced fracture of the formation portion/layer(s) at the measured depth MDp), or possibly other downhole conditions of the formation or wellbore for the measured depth MDp. Such downhole conditions can be determined using a variety of sensing modalities, including wellsite analysis, analysis at a remote laboratory, or possibly downhole fluid analysis module that is part of the tool. The downhole conditions can also possibly be determined from historical production log data.

[0058] At 307, the laser tool (or other cutting tool) is deployed at the measured depth MDp and operated to remove the casing and cement and possibly part of the formation over a portion of the wellbore at the measured depth MDp that will be plugged, and thus form the desired wellbore - formation wall interface for the plug at the measured depth MDp.

[0059] At 309, the plug tool is deployed at or near the measured depth MDp in the wellbore.

[0060] At 311, the plug tool is operated to release the composite material from the storage chamber 203 into the target area of the wellbore at the measured depth MDp. This target area is located at the wellbore - formation wall interface at the measured depth MDp as prepared by the laser tool (or other cutting tool) and is occupied by the heating electrode(s) 205 located at the bottom of the tool above the catcher 206.

[0061] At 313, the plug tool (e.g., controller 207) is operated such that the heating electrode(s) 205 heats the composite material in the target area of the wellbore at the measured depth MDp and ignites the exothermal reactant of the composite material to provide sufficient heat to melt the alloy of the composite material.

[0062] At 315, the melted alloy is allowed to cool and solidify and form a plug in-situ that contacts and seals to the wellbore - formation wall interface at the measured depth MDp. The plug tool (e.g., controller 207 and pump 202b) can also be operated to pump fluid to apply confinement pressure to the composite material as the plug is formed if desired.

[0063] In embodiments, the alloy can expand as it solidifies and the heater electrode(s) 205 may be“frozen in” by the alloy. Thus, in one embodiment, the heater electrodes(s) 205 can be mounted on a detachable mount that may be left behind. Alternatively, the heater electrode(s) 205 may be configured with a tension joint that may be broken off. Replacement electrodes may be provided on the tool for later use in another borehole. In other embodiments, the controller 207 can apply current to the heater electrode(s) 205 to permit the tool with its electrodes to be pulled out.

[0064] In embodiments, the composite material that is used to form the plug can be a hybrid composite of alloy particles and thermite, which can be configured to achieve more uniform melting of the alloy particles. Furthermore, the hybrid composite of alloy particles and thermite can be configured to form a solid plug with homogeneous properties.

Thermite-alloy hybrid material

[0065] In embodiments, the composite material that is used to form the plug can include particles having a core-shell structure as illustrated in Figure 4. The core of each particle is an alloy powder or alloy particles. The core is surrounded or coated by a shell of fine thermite powder or thermite particles. The shell can include a binding agent that binds the thermite of the shell to the alloy core. When temperature reaches a critical value (which is higher than room temperature), the thermite of the shell can ignite, which generates a large amount of heat to melt the alloy core. This approach does not require an additional melting tool, can better control the melting process by controlling the thickness of the particle shells, and can avoid the potential damage of formation due to thermal expansion. [0066] Note that the type of alloy for the core, other properties of the alloy core (such as characteristic dimension(s) of the alloy cores), and properties of the thermite shell (such as thickness of the thermite shell) can be selected based upon downhole conditions at the desired measured depth (target area) where the plug will be formed. The alloy of the core can be any alloy(s) which expands during solidification, such as bismuth and its alloys, antimony and its alloys. Different trace elements, such as silver, zinc, copper, tin, can be added into the alloy(s) of the core to achieve desired thermal and mechanical properties. The thermite of the shell can be made with metal powder fuel and metal oxides as an oxidizer component. The metal powder fuel can be made of aluminum, boron, magnesium, zinc, silicon, and titanium. The oxidizer component can be made from a large variety of metal oxides, such as iron oxide (Fe203, FeO), copper oxide (CuO, Cu20), bismuth oxide (Bi203), silicon oxide (Si02), cobalt oxide (CoO), titanium oxide (Ti02), and chromium oxide (Cr203). In one embodiment, the oxidizer component is made from one or more metal oxides which will not form a metal that is easily corroded in the downhole environment at the desired measured depth (target area). Examples of such metal oxides include bismuth oxide, copper oxide, silicon oxide, titanium oxide and chromium oxide. Thermate particles made with metal fuel (typically metal salt), metal oxide and sulfur can also be used as the coating material. In embodiments, the binding agent for the shell can be either organic or aqueous liquid based. For example, suitable organic binding agents can include, but are not limited to, phenolic polymer, polyamide, furan polymer, epoxy, silicate, and wax. An aqueous-based binding agent can be selected such that it will fully degrade when heated. Examples of suitable aqueous-based binding agents can include, but are not limited to, polyvinyl alcohol, polysaccharide, guar gum, starch, mud, latex binder such as styrene-butadiene latex, and acrylic latex. The thermite and binding agent mixture of the shell can be applied onto alloy particles using methods such as spray painting and other coating methods. Alternatively, thermite particles can also be coated directly on melted alloy droplets which solidify together with the alloy into solids.

[0067] In some embodiments, the metal and oxidizer component of the thermite can be bound together before being coated on the metal alloy, to ensure uniform mixing between the metal fuel and oxidizer in the thermite reaction. The alloy particle size and alloy to thermite ratio can be selected or tuned to achieve optimized particle packing density, thermite reaction and alloy melting. [0068] In embodiments, the oxidizer component of the thermite can include one or more metals that will be compatible and will not easily corrode under the downhole environment at the desired measured depth (target area). In embodiments, the thermite compositions can be configured to generate little or no gas during the thermite reaction, and have an exothermal reaction temperature comparable or lower than the boiling point of the metal alloy to minimize gas generated during the thermite reaction. For example, the boiling point of Bi58Sn42 alloy is 1630°C, and the boiling point of pure bismuth is 1560°C. Relatively low reaction temperature thermite compositions can include, but are not limited to A1 + TiCh (1479°C), A1 + S1O2

(1616°C), B+CnCb (644°C), Ti+CnCb (1541°C), where the metal + metal oxide of the listed thermite compositions include reaction temperatures in parentheses.

[0069] A non-limiting example of the composite material used for plug formation includes eutectic bismuth tin (Bi58Sn42) alloy particles for the core and aluminum-titanium oxide thermite particles for the shell. In this example, aluminum-titanium oxide thermite particles can be coated onto the BiSn alloy particles. The aluminum-titanium oxide thermite particles can be smaller in size than the BiSn alloy particles as shown in Figure 4. When ignited, the aluminum-titanium oxide thermite particles undergo a reduction oxidation (Redox) reaction 4A1+3TΪ0₂ Ά 2Ah0₃+3Ti, which releases heat at or near 1500°C. The near 1500°C reaction provides heat to melt the BiSn alloy particles, which subsequently cool and expand and solidify to form a solid plug. The formed plug includes both titanium and alumina that result from the thermite reaction. The titanium has high strength with good corrosion resistance while the alumina of the plug may provide extra reinforcement.

[0070] In another embodiment shown in Figures 5A to 5C, the composite material used for plug formation can include alloy particles and thermite particles that have similar size and density. In this embodiment, the thermite particles and alloy particles can be mixed directly (instead of coating thermite particles on alloy). Furthermore, the metal fuel and oxidizer component of the thermite particles can be bound together using a binder to ensure continuous thermite bum. The melting and solidification rate of the alloy plug can be selected or tuned as desired by selecting or tuning the weight ratio (or volume ratio) of the thermite particles to the alloy particles. [0071] In embodiments, the configuration of the tool can be adapted such that the weight ratio (or volume ratio) of the thermite particles to the alloy particles of the composite material that is delivered to the target area and forms the plug achieves a desired heating, melting and/or cooling (or solidification) profile. For example, it is normally harder for the alloy delivered to the top and bottom ends of the target area to fully melt and form a dense plug. Thus, the weight ratio (or volume ratio) of thermite particles to alloy particles can be relatively higher for the composite material that is delivered to the top and bottom ends of the target area as compared to a relatively lower weight ratio (or volume ratio) of thermite particles to alloy particles for the composite material that is delivered to the middle section of the target area. In other embodiments, the desired heating and melting profile can be achieved by controlled distribution of thermite particles of varying exothermal energy over the target area. For example, thermite with a relatively higher exothermal energy can be part of the composite material that is delivered to the top and bottom ends of the target area, while thermite with a relatively lower exothermal energy can be part of the composite material that is delivered to the middle section of the target area. In another example, slower solidification in the middle of the target area can be desired as compared to the top and bottom ends of the target area. This can allow the ends of the target area to solidify first to confine the middle section for maximum expansion during the solidification of the middle section. This can be achieved by controlling the distribution of the weight ratio (or volume ratio) of the thermite particles to the alloy particles of the composite material that is delivered to the target area over the length of the target area (and the resulting plug), an example of which is shown schematically in Figure 5C.

[0072] With respect to the core-shell structure illustrated in Figure 4, the thickness of thermite coating can be determined through modeling and simulations based upon specified downhole conditions. Figure 6 is a schematic of a model of the core-shell structure. Figures 7A to 7D are diagrams that show the temperature evolution of a composite material based on the core-shell structure of Figure 6 (where a BiSn core is coated with A1+TΪ02 as the thermite shell) during the melting and solidification to form a metal alloy plug that seals to a rock formation. The thickness ratio of the coating relative to the size (radius or diameter) of the metal alloy core can be controlled and selected for different application temperatures and rock formations. In this example, the core is taken to be BiSn alloy particle and the shell is A1+TΪ02 thermite. The ignition point of the thermite is around 300°C, and the energy density is 5.48MJ/L. When the thermal diffusivity of rock diffusivity is 9.6x10 ⁷m²/s, the minimum thickness/radius ratio is calculated as 0.35 in order to allow the core-shell structures of the composite material to maintain a self-sustaining reaction. The average temperature of the procedure is around 1060°C. The thickness ratio of the coating relative to the size (radius or diameter) of the metal alloy core can also be controlled and selectively varied over the lengthwise dimension of the target area to provide a desired heating, melting and/or cooling (or solidification) profile. Additionally or alternatively, the exothermal reactant of the shell of the particles (e.g., the exothermal energy of the reactant of the shell) can also be controlled and selectively varied over the lengthwise dimension of the target area to provide a desired heating, melting and/or cooling (or solidification) profile.

[0073] The model also has the capability to study the effect of different thermite types. If the metal and metal oxide of the thermite is changed to B+(¾q₃, this decreases the ignition temperature to around 200°C, and the energy density to 3.49MJ/L. The thickness/radius ratio can be reduced to 0.27 in order to allow the core-shell structures of the composite material to maintain a self-sustaining reaction. The average temperature during the reaction is around 600°C.

[0074] Additionally, the composite material can include highly thermally conductive particles, the size of no more than 1/3 of the average size of the alloy particles, or elongated shape, which can be added to increase the thermal conductivity of the mixture. These highly thermally conductive particles will not change the packing ratio of the alloy particles, but they will increase the thermal conductivity of the interstitial space between particles.

Addition of Agents

[0075] One or more additional agents or additives can be added to the thermite-alloy composite material to modify the mechanical, thermal and corrosion-resistant properties of the plug formed from the composite material. For example, oxide particles can also be added into the thermite- alloy composite material for extra reinforcement. Figure 8 shows three plots of yield strength of a plug formed from a composite material of thermite and BiSn (no sand), yield strength of a plug formed from a composite material of thermite, BiSn and sand (7wt% S1O2 or sand) without proper mixing, and yield strength of a plug formed from a composite material of thermite, BiSn and sand fully mixed with sand particles more evenly distributed in the alloy, respectively. The plot of Figure 8 shows that by adding sand to the composite material, the yield strength of the resulting plug formed by the composite material is about 7-13% higher than that for the composite material of thermite and BiSn (no sand). In this manner, the strength of the composite material can be selected or tuned by optimizing the alloy-sand ratio and the properties of the filler (e.g. sand or proppant) including the size distribution, surface properties and the mechanical properties of the fillers. In addition, the thermal expansion coefficient can also be modified to reduce the thermal mismatch between a plug and the formation, reduce creep and improve the corrosion-resistant by adding these particles. Inorganic elongated particles (fibers) may also strengthen the plug and stabilize its behavior at high temperature (decrease creep), based on the principal of composites.

[0076] Moreover, one or more trace elements can possibly be added to the composite material used to form the plug in order to suppress structural breakdown of the plug over time as the plug ages in the downhole environment. It is known that during aging, the microstructure of many alloys changes over time. Using eutectic bismuth-tin alloy (Bi58Sn42) as an example, the two- phase BiSn’s micro structure coarsens overtime at elevated temperatures. The coarsening of the microstructure can directly affect the mechanical properties of the alloy. Figure 9 depicts plots that show the change to the yield strength of BiSn when aged at 110°C for two weeks. BiSn also has lower mechanical strength and creeps significantly at high temperatures, which limit its application temperature range. The aging effect on alloys can be minimized by adding different trace elements to minimize grain growth over time. The mechanical strength and creep resistance can be improved by adding trace elements that can minimize grain size and prevent creep through grain boundaries. Examples of the trace elements include copper (less than 5 wt%), antimony (less than 10%, preferably 2-6%), silver (less than 5 wt%, preferably 0.25 - 1 wt%), zinc (preferably less than 1 wt%) and iron (less than 2.5wt%). For example, adding 1% copper decrease the coarsening speed of phases in BiSn thus minimizing the property change over time in an aging (i.e., high temperature) downhole environment.

[0077] The plug can seal better by improving adhesion between the alloy of the plug and the rock formation. In embodiments, one or more surface modification agents can be added to the composite material that is used to form the plug. The addition of such surface modification agents can improve the adhesion between the formed plug and the rock formation. For example, the surface modification agents can include, but are not limited to, phenolic resin, high temperature epoxy resin, borox, and borate. Figures 10A and 10B shows examples that use high temperature phenolic resin and high temperature epoxy resin to improve wetting and bonding between a composite alloy (which can be used for a wellbore plug) and rock. Additionally or alternatively, the surface modification agent(s) may be pre-injected in fluid that contacts the wellbore - formation wall interface prior to delivering the composite material to the target area for plug formation.

Heater design for efficient melting and heat transfer

[0078] In embodiments, the plug tool can be adapted to employ one or more layers of media spaced below the target area where the plug is to be formed. For example, one or more layers of media 211 can be supported by the catcher 206 of the plug tool 114 as shown in Figure 1 1. The media layer(s) 211 are configured such that the composite material is delivered into the target area over and on top of the media layer(s) 211. The media layer(s) include granular material such as sand, proppant, cement or alloy particles, which can provide space for melted alloy to flow into and provide for efficient alloy melting and heat transfer. The top panel of Figure 12 shows a media layer spaced below the target area where the plug is to be formed. When the alloy near the heater electrode (center of the target area space) melts, it can flow downward into the media layer and provide space for un-melted solid particles to move closer to the heater. The bottom panel of Figure 12 shows a target area where the plug is to be formed without any media layer below the target area. In this case, when the alloy near the heater electrode (center of the target area space) melts, there is little space for the melted alloy to move and provide space for the un-melted particles to move closer to the heater. Thus, the alloy melting will rely on horizontal heat transfer. When the latent heat of the alloy is large, much higher energy is required to fully melt the alloys. The plots of Figures 13 A and 13B compare alloy melting with and without a media layer, where the media layer is a sand layer as an example. It shows that with sand as a media layer it only takes 10 minutes to fully melt the alloy at 250°C. Without the sand layer, the alloy on the bottom of the target area does not fully melt after being heated at 300°C for 2 hours.

[0079] The thickness of the media layer can also be selected or tuned to control the expansion extent of expandable alloys. For example, where formation rocks have relatively low strength, a thicker media layer can help reduce the radial expansion of the alloy during solidification and help reduce the tensile forces applied to the formation rock in order to avoid rock fracture. In applications where increased or maximum expansion is needed, and the formation has higher strength, a thinner media layer can be used such that most of the alloy expansion goes in the radial direction.

[0080] At the bottom of the tool, the heat released from the thermite reaction can transfer radially outward as well as to the region below the bottom of the tool. At the bottom part of the tool, the heat transferred to the surrounding alloy will be relatively lower compared to the alloy surrounding the middle region of the heater electrode where the heat is mainly transferred in the radial direction. With a flat catcher depicted in Figure 14 A, the alloy situated at or near the contact point of the catcher and the wellbore-formation wall interface will be difficult to melt due to lower heat and fast heat loss. Figure 14B depicts a“V” shaped catcher design where the wall of the catcher extends upward an inclination angle Q relative to the central axis of the heater electrode and the tool. This“V” shaped catcher design better matches the thermal temperature profile between the heater and alloy during the heating process, as compared to the flat bottom catcher design of Figure 14A. Note that less volume of alloy needs to be melted with this“V” shape catcher design. Figure 15 depicts the overall melted alloy fraction (liquidous phase of alloy fraction) as a function of time during the heating by thermite reaction and the subsequent cooling process for the flat catcher design of Figure 14A and the V-shaped catcher design of Figure 14B. Higher peak values of melted alloy fraction with the“V”-shaped catcher is achieved due to more effective heat transfer from the thermite reaction to the alloy. The angle Q and the location of the V-shaped catcher can be selected with the aid of computational fluid dynamics (CFD) simulations to meet different melting requirements.

Other Composite Structures for forming alloy plug

[0081] In other embodiments, the alloy of the plug can be made as a housing with a cavity that holds thermite material as shown in Figures 16 and 17. The thermite material can be ignited, for example by a heater or electrical spark, to initiate a thermite reaction that heats and melts the alloy of the housing. The heat from the thermite reaction can be efficiently transferred to the expandable alloy for melting. After melting, the reactants of the thermite and the alloy can mix together, cool and solidify to form the plug. The alloy can be expandable such that it expands as it cools and solidifies to form the plug. The thermite mixture can be casted into the plug as shown in Figure 17, or it can be solid thermite blocks with expandable alloys casted around it as a whole plug tool as shown in Figure 16. The thermite can be ignited by a heater or an electrical spark melting through the thin alloy layer as shown in Figure 17, or by directly igniting the thermite block in the casted alloy, as shown in Figure 16.

Designing interlocking geometry for metal-plug

[0082] A plug used in well abandonment functions to prevent the downhole fluid leaking to the ground. In the long-term, the adhesion or contact force between plug and formation rock may be decreased due to creep, aging and corrosion. In embodiments, different plug shapes and profiles that can be self-locked and prevent leaking are described. Figure 18 depicts a plug with a truncated conical cross-sectional profile which can be used for deep wells where the fluid pressure is high. If the fluid pressure increases on the bottom, the plug will be pushed upwards and the contact pressure between the plug and formation will increase. This is expected to help in preventing bottom fluid from leaking through the plug-wellbore interface. If there are stringent constraints on how large the plug diameter can become, then a plug with a piece-wise conical cross-sectional profile as illustrated in Figures 19 and 20 can be used. In embodiments, the geometries of the plugs depicted in Figures 18 to 20 can be configured by controlled operation of the laser tool, or other cutting tools that form the wellbore-formation wall interface. In these cases, the profile of the wellbore-formation wall interface can provide the desired cross-sectional profile of the plug as depicted in Figures 18 to 20.

Thermal analysis and thermo-mechanical modeling package for plug design optimization

[0083] For the different plug material formulations, heater designs and plug geometries described herein, the melting, heat transfer and solidification of the alloy varies. This difference will directly affect the interaction and sealing between the plug and formation rock or casing. Various melting behavior and plug geometry will also generate different residual stresses between the formed plug and formation rock (or casing). In embodiments, a lumped-parameter thermal network and thermo-mechanical model can be used to model and simulate the melting and solidification time and temperature profile of the plug, and to determine the residual stress generated. The modeling and simulation can be used for pre-job planning and plug design optimization. [0084] The subject disclosure describes composite material systems, methods, systems and workflows which improve the melting of the alloy of a wellbore plug by implementing chemical- based heating elements uniformly into alloy particles to provide a more homogeneous melting and solidification during plug formation. Furthermore, composite material systems, methods, systems and workflows are described to improve the mechanical properties alloy-based wellbore plugs by (1) forming hybrid alloys; (2) selecting or tuning the aging effect on the alloy by adding trace elements; (3) adding surface modification agents to improve the wetting between alloys and formation rock or metal casing; (4) improving heater design for efficient melting and solidification; and (5) designing a shape or profile of a metal plug that can interlock with its surroundings (casing and/or cement and/or formation rock) to prevent leaking.

[0085] Some of the modeling and simulation methods and processes described above can be performed by a processor. The term“processor” should not be construed to limit the

embodiments disclosed herein to any particular device type or system. The processor may include a computer system. The computer system may also include a computer processor (e.g., a microprocessor, microcontroller, digital signal processor, or general purpose computer) for executing any of the methods and processes described above.

[0086] The computer system may further include a memory such as a semiconductor memory device (e g., a RAM, ROM, PROM, EEPROM, or Flash-Programmable RAM), a magnetic memory device (e.g., a diskette or fixed disk), an optical memory device (e.g., a CD- ROM), a PC card (e.g., PCMCIA card), or other memory device.

[0087] Some of the methods and processes described above can be implemented as computer program logic for use with the computer processor. The computer program logic may be embodied in various forms, including a source code form or a computer executable form. Source code may include a series of computer program instructions in a variety of programming languages (e.g., an object code, an assembly language, or a high-level language such as C, C++, or JAVA). Such computer instructions can be stored in a non-transitory computer readable medium (e.g., memory) and executed by the computer processor. The computer instructions may be distributed in any form as a removable storage medium with accompanying printed or electronic documentation (e.g., shrink wrapped software), preloaded with a computer system (e.g., on system ROM or fixed disk), or distributed from a server or electronic bulletin board over a communication system (e.g., the Internet or World Wide Web).

[0088] Alternatively or additionally, the processor may include discrete electronic components coupled to a printed circuit board, integrated circuitry (e.g., Application Specific Integrated Circuits (ASIC)), and/or programmable logic devices (e.g., a Field Programmable Gate Arrays (FPGA)). Any of the methods and processes described above can be implemented using such logic devices.

[0089] Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from this invention. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. § 112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words‘means for’ together with an associated function.

Claims

WHAT IS CLAIMED IS:

1. A method for plugging a wellbore having a casing and cement surrounding the casing and traversing a formation, comprising:

using at least one tool located in the wellbore to deliver composite material to a target area in the wellbore, wherein the composite material includes metal alloy and an exothermal reactant;

using the at least one tool to apply heat or spark to the composite material in the target area to ignite the exothermal reactant of the composite material and melt the metal alloy of the composite material; and

permitting the melted metal alloy of the composite material to solidify to form a plug at the target area in the wellbore.

2. A method according to claim 1, wherein:

the metal alloy of the composite material comprises bismuth or antimony.

3. A method according to claim 1, wherein:

the composite material includes at least one trace metal element.

4. A method according to claim 3, wherein:

the at least one trace metal element is selected from the group consisting of silver, zinc, copper, tin, and combinations thereof.

5. A method according to claim 1, wherein:

the exothermal reactant of the composite material comprises a metal power component and an oxidizer component.

6. A method according to claim 5, wherein:

the metal powder component is selected from the group consisting of aluminum, boron, magnesium, zinc, silicon, titanium, and combinations thereof.

7. A method according to claim 5, wherein:

the oxidizer component includes at least one metal oxide selected from the group consisting of iron oxide (FeiCb, FeO), copper oxide (CuO, CU2O), bismuth oxide (B12O3), silicon oxide (S1O2), cobalt oxide (CoO), titanium oxide (T1O2), chromium oxide ((¾0₃), and combinations thereof.

8. A method according to claim 1, wherein:

the exothermal reactant of the composite material comprises thermite or thermate in particulate form.

9. A method according to claim 1, wherein:

the composite material comprises a core-shell structure having a core surrounded by a shell, wherein the core is a metal alloy particle and the shell includes particles of an exothermal reactant.

10. A method according to claim 9, wherein:

the metal alloy particle of the core comprises bismuth or antimony.

11. A method according to claim 9, wherein:

the exothermal reactant of the shell comprises thermite or thermate.

12. A method according to claim 9, further comprising:

a ratio of shell thickness to core size is selected or controlled for plugging the wellbore.

13. A method according to claim 9, wherein:

a ratio of shell thickness to core size is selectively varied and controlled over a lengthwise dimension of the target area to provide a desired heating or melting or cooling or solidification profile over the target area.

14. A method according to claim 9, wherein:

the exothermal reactant of the shell is selectively varied and controlled over a lengthwise dimension of the target area to provide a desired heating or melting or cooling or solidification profile over the target area.

15. A method according to claim 1, wherein:

the composite material comprises metal alloy particles mixed with particles of an exothermal reactant.

16. A method according to claim 15, wherein:

the particles of an exothermal reactant comprises a metal power component and an oxidizer component.

17. A method according to claim 15, wherein:

a weight ratio or volume ratio of the exothermal reactant particles to the metal alloy particles of the composite material is selectively controlled over the target area to provide a heating or melting or cooling or solidification profile over the target area.

18. A method according to claim 15, wherein:

the exothermal reactant of the exothermal reactant particles is selectively varied and controlled over a lengthwise dimension of the target area to provide a desired heating or melting or cooling or solidification profile over the target area.

19. A method according to claim 1, wherein:

the composite material comprises a housing that defines an interior cavity, wherein the housing is formed by the metal alloy, and the interior cavity is loaded with exothermal reactant in particulate form.

20. A method according to claim 1, wherein: the tool is configured to deploy at least one media layer disposed below the composite material, wherein the at least one media layer includes granular material which provide space for melted alloy to flow into.

21. A method according to claim 20, wherein:

the granular material of the at least one media layer is selected from the group consisting of sand, proppant, cement or alloy particles and combinations thereof.

22. A method according to claim 1, wherein:

the composite material further includes a surface modification agent.

22. A method according to claim 22, wherein:

the surface modification agent is selected from group consisting of phenolic resin, high temperature epoxy resin, borox, borate and combinations thereof.

23. A method according to claim 1, further comprising:

using a tool located in the wellbore to prepare a wellbore-formation wall interface at the target area of the wellbore; and

injecting a fluid that contacts the wellbore - formation wall interface prior to delivering the composite material to the target area for plug formation, wherein the fluid includes a surface modification agent.

24. A method according to claim 1, wherein:

the tool includes a v-shaped catcher that is operably disposed adjacent the bottom end of the target area.

25. A method according to claim 1, wherein:

using a tool located in the wellbore to prepare a wellbore-formation wall interface at the target area of the wellbore, wherein the wellbore-formation wall interface is configured such that the plug formed from solidification of the melted alloy has a truncated conical cross-sectional profile or piece-wise conical cross-sectional profile.

26. A method according to claim 1, wherein:

at least one property of the composite material is determined according to downhole conditions at the target area.

27. A system for plugging a wellbore having a casing and cement surrounding the casing and traversing a formation, comprising:

at least one tool that is deployed in the wellbore and configured to deliver composite material to a target area in the wellbore, wherein the composite material includes metal alloy and an exothermal reactant, wherein the at least one tool is further configured to apply heat to the composite material in the target area to ignite the exothermal reactant of the composite material and melt the metal alloy of the composite material, and to permit the melted metal alloy of the composite material to solidify to form a plug at the target area in the wellbore.