This invention concerns a drill bit for drilling a wellbore with a casing or a liner comprising

-a tubular stem made from a high strength metal or alloy;

-a top section with a cutting face comprising blades and gages, wherein said blades and gages comprise pockets and wherein cutters are mounted in said pockets.

Specifically, this drill bit is used in casing drilling applications and is meant to drill a well using a casing or liner. Once the casing or liner has been cemented, this drill bit is designed to be drilled out with an additional second drill bit. This second drill bit is of a smaller diameter, such as to be able to go through the internal diameter of the casing, and will drill out the first mentioned drill bit. It is thus important that the first drill bit comprises an easy drillable material, and preferably comprises a material that is drillable by a polycrystalline diamond compact drill bit (PDC drill bit).

This invention also concerns a method for manufacturing a drill bit for drilling a wellbore with a casing or a liner comprising the following steps:

-providing a tubular stem made from a high strength metal or alloy;

-providing a top section with a cutting face comprising blades and gages, wherein said blades and gages comprise pockets and wherein cutters are mounted in the pockets; -attaching one end of the tubular stem to one end of the top section.

With the aid of such drill bits, wellbores can be made. Such wellbores are among others used in the oil and gas industry and for geothermal applications. Such a drill bit is used for drilling a wellbore of several (thousand) metres. This means that this drill bit needs be able to perform the drilling in a good way, and further, after the drilling has taken place, it should also be possible to drill out this drill bit with a subsequent drill bit. The subsequent drill bit can be a polycrystalline diamond compact drill bit (PDC drill bit). The drill bit that is provided to be drilled out by a subsequent drill bit, comprises for example a top section made from a special type of metal or alloy that can be easily drilled out by a drill bit. The drill bit can be subject to erosion and abrasion. To protect the drillable alloy, the said metal or alloy can be coated/treated with a layer of hardfacing, such as High Velocity Oxygen Fuel, to increase its resistance thereof to abrasion and/or erosion. The special alloy is typically a nonferrous alloy and can be for example a nickel based alloy, an aluminum based alloy or a bronze based alloy. A top section that is made from said special type of alloy or metal can be easily connected to the tubular stem in a good manner, such that the drill bit does not fall apart during use. Also said special type of alloy or metal can be easily milled/machined to form the required blades and gages. To manufacture a said drill bit, a relatively large amount of said special type of metal or alloy is needed and said special type of metal or alloy is significantly more expensive than standard steel. Such a drill bit is described in US 2002189863, EP 3269919 and WO 2012088323. The proportions of said special type of metal or alloy are chosen such that the drill bit provides suitable cutting and boring of the well bore while being able to be drilled through by a subsequent drill bit.

It is therefore an object of the invention to provide a drill bit and a method for manufacturing such a drill bit, wherein the need for a said special type of metal or alloy is less, and this while the obtained drill bit still has the desired properties and can be easily made.

This object is achieved by providing a drill bit for drilling a wellbore with a casing or a liner comprising

-a tubular stem made from a high strength metal or alloy;

-a top section with a cutting face comprising blades and gages, wherein said blades and gages comprise pockets and wherein cutters are mounted in said pockets; wherein the top section comprises a head from a nonferrous alloy and a gage tubular from a medium strength metal or alloy, wherein the gage tubular comprises, according to the longitudinal direction of the gage tubular, a first end and a second end which extends opposite the first end, wherein said first end is connected to the head and said second end is connected to the tubular stem, such that the gage tubular mainly extends between the head and the tubular stem. The gage tubular preferably has a tubular shape, such that it has a longitudinal direction which extends from its first end to its second end. The top section comprises two parts, namely said head and said gage tubular. The gage tubular is made from a medium strength metal or alloy, which can be easily milled. The head is made from a nonferrous alloy such as a nickel based alloy, an aluminum based alloy or a bronze based alloy, wherein said nonferrous alloy is suitable to be drilled out by a drill bit. The nickel based alloy may for example be a nickel-copper alloy with a nickel content of between 50 and 80 percent and with a copper content of between 20 and 40 percent. Because only the head comprises said nonferrous alloy, the total amount of said nonferrous alloy is low. Said nonferrous alloy and said medium strength metal or alloy can both be easily milled, such that the blades and gages can be easily made. This drill bit can thus be made easily, is less expensive because it comprises less of said nonferrous alloy and this without adversely affecting the good qualities of the drill bit. Further, the cutting face preferably comprises nozzle bores.

The high strength metal or alloy preferably has a tensile strength higher than the tensile strength of the medium strength metal or alloy. The medium strength metal or alloy could have a tensile strength of between 40 and 70 kg/mm ² . It may be of the type ST52 which has for example a tensile strength of between 40 and 60 kg/mm ² .

The high strength metal or alloy of the tubular stem preferably has a tensile strength of at least 70 kg/mm ² , more preferably of at least 90 kg/mm ² . The tubular stem can for example be made from a high strength steel with a tensile strength of between 90 and 110 kg/mm ² .

The nonferrous alloy of the head preferably has a tensile strength of at least 40 kg/mm ² . Said tensile strength can be between 40 kg/mm ² and 90 kg/mm ² . The medium strength metal or alloy could have a tensile strength higher than the tensile strength of the nonferrous alloy, but the medium strength metal or alloy could also have the same tensile strength or a lower tensile strength than the tensile strength of the nonferrous alloy. Preferably the gage tubular mainly comprises the gages and also preferably the head mainly comprises the blades. This ensures that the part that needs to be drillable is made of the said nonferrous alloy. The said cutting face can have an inner cone, an outer shoulder and an intermediate nose between the inner cone and the outer shoulder, wherein the blades and gages extend from the inner cone to the outer shoulder.

Because said gage tubular is made from a medium strength alloy, said second end can be very easily connected to the tubular stem in a good manner. For example, one can easily provide a second end with the required thread for connecting it to the tubular stem. The tubular stem is then provided with corresponding thread. The thread can be metric thread, an ACME thread or stub ACME thread. In addition, or as sole connection, the tubular stem and the gage tubular can also be welded to each other. The connection between the tubular stem and the gage tubular can also comprise or be, a lathe connection or a lock joint.

In a very preferred embodiment the said nonferrous alloy is drillable by a PDC bit. This drill bit is preferably used in a casing drilling application and is meant to drill a well using a casing or liner. Once the casing or liner has been cemented, this drill bit can be easily drilled out by a PDC bit.

Preferably the gage tubular has a tubular shape with a certain internal diameter, wherein the corresponding outer dimensions of the head are at least the same size as said internal diameter. When a subsequent drill bit is used to drill out said drill bit of the invention, it will have an external drilling diameter which is smaller than the said internal diameter. As a result, said subsequent drill bit will only have to drill out the head. Since the head is made out of said nonferrous material, the drilling out can easily occur. Here one can use the term “drill-out diameter”. Drill-out diameter refers to the inner diameter of a casing drill bit which may be drilled through by a subsequent drill bit. Preferably the tubular stem also has a tubular shape with an internal diameter that is substantially the same as the internal diameter of the gage tubular, such that a subsequent drill bit can easily pass the tubular stem and the gage tubular before drilling out the drill bit of the invention.

Preferably the head comprises thread and the first end of the gage tubular comprises corresponding thread to connect the head and said first end by threading, and the top section comprises at least one cavity extending in both the head and gage tubular, wherein a pin extends into said cavity to further connect the head and gage tubular with each other. The connection between the head and the gage tubular is here a combination of a thread connection and at least one lock joint that uses a pin. This is a very strong connection, that will not come loose when the drill bit is used for drilling a well bore. Also this connection allows to form the blades and gages by milling, such that the cutting face can optimally perform its function. The said threads can be metric or ACME or STUB ACME threads. A pin ensures a strong connection that will not alter/come loose during milling in the manufacturing of the drill bit or during use of the drill bit. Preferably, there or several such cavities with pins. Preferably there is one cavity and pin for every pair of a blade and gage. A said cavity is preferably located at the height of a gage, this because the thickness of the top section is at this location relatively high.

Further preferably the said cavity pierces the interconnected threads. This ensures a very strong connection between the head and gage tubular.

Also further preferably, the top section comprises at least one screw which extends in both the head and the gage tubular to further connect the head and gage tubular with each other. The screw creates a very strong attachment and this by pressing both the head and gage tubular against each other. Preferably the top section comprises several screws. Preferably there is one screw for every pair of a blade and gage. The screw is preferably located at the height of a gage, this because the thickness of the top section is at this location relatively high. Further preferably, the top section comprises for every said screw at least one corresponding cavity extending in both the head and gage tubular, such that a said screw can be more easily applied.

Further preferably, according to the longitudinal direction of the drill bit, the screw extends between the threaded connection of the head and gage tubular and the top of the head, for example the said inner cone. In a specific embodiment, at the height of the screw, the head and gage tubular contact each other at an angle that lies between 30° and 60° with regard to the longitudinal direction. This angle is more preferably 45° and ensures a very strong connection between the head and gage tubular. This joint is then roughly perpendicular to the bit profile.

Said cavity and/or screw preferably extends at the height of a gage.

In a preferred embodiment, the said cutting face has an inner cone, an outer shoulder and an intermediate nose between the inner cone and the outer shoulder, wherein the blades and gages extend from the inner cone to the outer shoulder and wherein the head forms the inner cone and the head mainly forms the intermediate nose. The inner cone and the intermediate nose are the parts that need to be drillable, because these are the parts that will be drilled by a second drill bit, and this after the drill bit according to the invention has drilled the well and the casing or liner has been cemented. With this configuration it is made sure that the drill-out diameter is entirely formed from said nonferrous alloy.

Further preferably, the gage tubular mainly forms the outer shoulder and more preferably the gage tubular forms the outer shoulder. The outer shoulder does not need to be PDC drillable, because it is outside of the drilled diameter. The gage tubular can thus be made from a medium strength metal or alloy, and this without negatively influencing the required properties of the drill bit. This also means that the gage tubular forms the largest mass of the top section. Therefore there is significantly less need for the said nonferrous alloy and this in comparison with comparable drill bits according to state of art. In a specific embodiment, at the height of the first end of the gage tubular, the outer diameter of the gage tubular is larger than the corresponding outer diameter of the head. Further preferably the head comprises outer thread and the first end of the gage tubular comprises corresponding inner thread. This for connecting the head and the gage tubular by threading.

The said object of the invention is also achieved by providing a method for manufacturing a drill bit for drilling a wellbore with a casing or a liner comprising the following steps:

-providing a tubular stem made from a high strength metal or alloy;

-providing a top section with a cutting face comprising blades and gages, wherein said blades and gages comprise pockets and wherein cutters are mounted in the pockets; -attaching one end of the tubular stem to one end of the top section; wherein, to provide the said top section, a head base part from a nonferrous alloy is provided and a gage tubular base part from a medium strength metal or alloy is provided with a first end and a second end, an end of the head base part and the first end of the gage tubular base part are connected to each other and said interconnected parts are milled to form the said cutting face of the top section, such that the top section comprises a head from a nonferrous alloy that is milled from the head base part and a gage tubular from a medium strength metal or alloy that is milled from the gage tubular base part, wherein the said second end of the gage tubular base part forms the said one end of the top section that is attached to the tubular stem. Here a drill bit is formed comprising consecutively a head, a gage tubular and a tubular stem. A drill bit according to the invention as described above is thus formed. The advantages and the preferred and/or described embodiments/features therefore also apply to this method. Said head and gage tubular, together with the cutters mounted in the pockets, form the top section. The milling takes place when the head base part and the gage tubular base part are already connected to each other. This means that only the milling process determines the position of the blades and gages. Here the parts are not milled separately and this before connecting the parts, such that there is no risk that the mutual position of blades and gages is incorrect and/or that blades or gages have an incorrect shape due to incorrect connecting of the parts. The milling can be performed with the aid of a CNC milling machine. Blades, gages and the said pocket are formed by milling. Preferably also nozzle bores are formed during milling, such that said top section also comprises nozzle bores. Because only the head base part and the gage tubular are attached to each other when milling, and the tubular stem is not yet attached to the gage tubular base part, the length of the parts that go on the milling machine is minimized, such that a relatively small machine can be used for milling. Preferably the milling does not alter the connection between the head base part and the gage tubular base part, such that the connection between the head base part and the gage tubular base part is the same as the connection between the head and the gage tubular. After the milling, the cutters are mounted in the pockets, such that the top section comprises cutters.

The gage tubular base part is made from a medium strength metal or alloy, such that said gage tubular base part can be easily milled. The head base part is made from a nonferrous alloy which is suitable for being drilled by for example a PDC bit. This nonferrous alloy can be a nickel based alloy, an aluminum based alloy or a bronze based alloy, the nickel based alloy may for example be a nickel-copper alloy with a nickel content of between 50 and 80 percent and with a copper content of between 20 and 40 percent. Because only the head base part comprises said nonferrous alloy, the total amount of said nonferrous alloy is low. By providing said gage tubular base part that is made from a medium strength metal or alloy, the drill bit can still be made easily, is less expensive and this without adversely affecting the good qualities of the drill bit. The medium strength metal or alloy can be of the type ST52 which has for example a tensile strength of between 40 and 60 kg/mm ² . Preferably, to form the head base part, a piece of stock from a nonferrous alloy is provided and said piece is shaped by turning such that said end of the head base part is formed by turning and comprises thread, preferably outer thread. With the aid of thread one can easily connect the head base part with the gage tubular base part. Turning is a well-known method that can be easily performed to form thread around an axis.

Further preferably, to form the gage tubular base part, a tube from a medium strength metal or alloy is provided and said tube is shaped by turning such that said first end is formed by turning, wherein the first end comprises thread corresponding to the thread of the head base part. Preferably the first end comprises inner thread corresponding to the outer thread of the head base part. Preferably also the second end is then formed by turning and for example comprises thread. The said one end of the tubular stem then preferably comprises corresponding thread, which can also be formed by turning, such that the top section and the tubular stem are connected to each other by threading. More preferably the top section and the tubular stem are additionally connected to each other by welding and/or a lock joint.

Even more preferably the head base part and the gage tubular base part are connected to each other by threading the said corresponding threads of the head base part and the gage tubular base part. The said threads can be metric or ACME or STUB ACME threads. Here the parts are not milled separately and this before connecting the parts, such that there is no risk that the mutual position of blades and gages is incorrect and/or that blades or gages have an incorrect shape due to incorrect connecting of the parts. This enables to use a thread to connect the two parts, as a thread assembly does not easily locate precisely the assembled part angularly.

Further preferably one or more cavities are provided in the interconnected parts that extend in both the head base part and the tubular base part, wherein one or more pins are driven into said respective cavities to further connect the said parts to each other. This takes place before the milling. The milling preferably leaves said cavities and pins intact. Said cavities preferably extend at the height of the interconnected threads, such that they pierce the threads and such that they are not located at the height where the drilling out will take place. A pin ensures a strong connection between the head base part and the gage tubular base part, such that said interconnected parts can be milled without separating from each other. Preferably there or several such cavities with pins. Said pins ensure that the head base part and the gage tubular base part stay connected during milling. Preferably the position of the said one or more cavities with a pin is as such and the milling takes places as such, that there is one cavity and pin for every pair of a blade and gage. The cavity is then preferably, after the milling has taken place, located at the height of a gage.

Further, even more preferably, one or more screws are driven into both the interconnected parts to further connect said parts. This takes place before the milling. The milling preferably leaves the screws intact. A screw creates a very strong attachment and this by pressing both the head base part and gage tubular base part against each other. Preferably, several screws are driven into said interconnected parts. Preferably the position of the said one or more screws is as such and the milling takes place as such, that there is one screw for every pair of a blade and gage. Further preferably, according to the longitudinal direction of the drill bit, the screw extends between the threaded connection of the head base part and gage tubular base part, and the top of the head base part the furthest away from the end that is connected to the gage tubular base part. In a specific embodiment, at the height of the screw, the head base part and gage tubular base part contact each other at an angle that lies between 30° and 60° with regard to the longitudinal direction. This angle is more preferably 45° and ensures a very strong connection between the head base part and gage tubular base part. This joint is then roughly perpendicular to the bit profile. Before driven the one or more screws into both the interconnected parts, corresponding cavities are preferably made into both the interconnected parts. The screwing can then be performed more easily. The length and the position of the screws is preferably as such that they do not enter the part of the head that will later on be drilled out. In a very preferred embodiment the method for manufacturing a drill bit for drilling a wellbore with a casing or a liner comprising the following steps:

1) turning a piece made out of nonferrous alloy to form the head base part comprising thread;

2) turning a tube made out of a medium strength metal or alloy to form the gage tubular base part comprising a first end with thread corresponding to the thread of the head base part and a second end with preferably thread;

Said steps can be performed at the same time, or sequentially in any order.

3) after the head base part and the gage tubular base part are made, said parts are connected to each other by first threading said corresponding threads, and then adding pins and screws to further connect said parts;

4) when the head base part and the gage tubular base part are connected to each other, the milling takes place to form the cutting face with blades and gages such as to form the said top section;

5) a tube made out of a high strength metal or alloy is turned to form the tubular stem with for example an end with thread corresponding to the thread of the second end of the gage tubular base part. This tubular stem can be made at the same time as the gage tubular base part and/or the head base part or at any other time;

6) the top section and the tubular stem are connected to each other by threading the corresponding threads and/or welding;

7) Cutters are mounted in the pockets. This can be performed after the milling and before the attachment to the tubular stem. A treatment with a layer of hardfacing can be applied to the cutting face. This can be performed after the milling and before the attachment to the tubular stem.

The present invention is now explained in greater detail below with reference to the following detailed description of a preferred embodiment of a drill bit and method according to the invention. The aim of this description is solely to give illustrative examples and indicate further advantages and particularities and thus cannot be interpreted as a limitation of the field of application of the invention or of the patent rights claimed in the claims. In this detailed description, reference is made by means of reference numerals to the appended figures, wherein

-figure 1 is a section of a head base part according to the longitudinal direction; -figure 2 is a section of a gage tubular base part according to the longitudinal direction;

-figure 3 is a section of the head base part as shown in figure 1 and the gage tubular base part as shown in figure 2, wherein said parts are interconnected and wherein cavities have been made in said interconnected parts; -figure 4 is a detail of figure 3, this being the part of figure 3 indicated by a circle;

-figure 5 is a section of the interconnected parts shown in figure 3, wherein pins and screws are placed in the respective cavities;

-figure 6 is a detail of figure 5, this being the part of figure 5 indicated by a circle;

-figure 7 is a perspective view of the interconnected parts as shown in figure

5;

-figure 8 is a front view of atop section of a drill bit according to the invention; -figure 9 is a perspective view of the top section shown in figure 8; -figure 10 is a front view of a tubular stem of a drill bit according to the invention;

-figure 11 is a front view of the drill bit according to the invention;

-figure 12 is a section of the drill bit shown in figure 11, according to the longitudinal direction.

A method according to one embodiment of the invention to form a drill bit (1) according to one embodiment of the invention, is here below described and illustrated with the aid of the figures. To form the said drill bit (1), several steps are executed. First is provided a piece of stock of a nonferrous alloy, a first tube of a medium strength steel and a second tube of a high strength steel. Said piece of stock and said tubes are all subjected to turning. As a result of this turning, said piece of stock comprises outer thread (12) and forms a head base part (20) as shown in figure 1, said first tube comprises, according to its longitudinal direction (A), a first end with inner thread (13) and a second end with inner thread (14) and forms a gage tubular base part (21) as shown in figure 2 and said second tube comprises a first end with outer thread (15) and forms a tubular stem (2) as shown in figure 10.

The outer thread (12) of the head base part (20) corresponds to the inner thread (13) of the first end of the gage tubular base part (21). The next steps in the manufacturing of the drill bit (1) are the following: the head base part (20) and the gage tubular base part (21) are connected to each other by threading. Then pairs of cavities (16, 19) are made into both interconnected parts (20, 21), wherein each pair of cavities (16, 19) comprises a first cylindrically shaped cavity (16) that pierces the interconnected threads (12, 13) and a second cavity (19) that comprises two consecutively cylindrically shaped parts of a different diameter, wherein said second cavity (19) is threaded and extends between the interconnected threads (12, 13) and the top of the head base part (20). The interconnected parts (20, 21) with cavities (16, 19) are shown in figure 3, with the cavities (16, 19) shown in more detail in figure 4.

In the first cavities (16), pins (17) are driven and in the second cavities (19), screws (18) are applied. This can be seen in figure 5 and more in detail in figure 6. Figure 7 shows a perspective view of the interconnected parts (20, 21), the parts (20, 21) being connected by threading, by the pins (17) and by the screws (18). The pins (17) additionally connect the head base part (20) and the gage tubular base part (21), and they mainly ensure a good connection of the head base part (20) with regard to the gage tubular base part (21) and this especially during the milling of said interconnected parts (20, 21) (see further) and during the use of the drill bit (1) (see further). The screws (18) also additionally connect the head base part (20) and the gage tubular base part (21), and they mainly ensure a strong connection between the head base part (20) and the gage tubular base part (21), such that the connection does not come loose unwanted while machining the top section (3) or while drilling a hole or drilling out the head (10) by an additional drill bit.

The following step in the manufacturing of the drill bit (1) is the milling of said interconnected parts (20, 21). During the milling, blades (7), gages (8), nozzle bores (22) and pockets into the blades (7) and gages (8) are formed. Because of the strong connection between the said interconnected parts (20, 21), said parts (20, 21) stay connected to each other in the same position during the milling. After the milling, PCD cutters (9) are mounted in the pockets. The result is a top section (3) comprising a cutting face with an inner cone (4), an outer shoulder (5) and an intermediate nose (6) located between the inner cone (4) and the outer shoulder (5), wherein staid cutting face comprises blades (7) and gages (8) with pockets, cutters (9) mounted in the pockets and nozzle bores (22). Said top section (3) comprises a head (10), which has been shaped out of the head base part (20) and a gage tubular (11) shaped out of the gage tubular base part (21), wherein the head (10) and gage tubular (11) are connected to each other by treading of the said inner and outer thread (12, 13), the pins (17) and the screws (18). The connection between the head (10) and gage tubular (11) is thus the same as the connection between the said interconnected parts (20, 21) and this because the milling does not affect the said connections. The connection between the head (10) and gage tubular (11) is thus a very strong connection that will not come loose during the use of the drill bit (1) and in which the position of the head (10) with regard to the gage tubular (11) remains the same during use. The top section (3) is shown in the figures 8 and 9.

Then the said second end of the gage tubular base part (21) forms the end of the top section (3) and said end is threaded with the corresponding outer thread (15) of the tubular stem (2). To improve the connection between the said end and the tubular stem (2), the top section (3) and the tubular stem (2) are further welded to each other. The result is a drill bit (1) according to the invention as shown in figures 11 and 12.