Patent Description:
<CIT> discloses methods and apparatuses for directly charging capacitors in a down-hole pulsed power system used for electrocrushing drilling. An above ground power supply is directly connected to the capacitors. The power supply can be a switching power supply, a DC supply, or an AC supply. Capacitor voltage is monitored and controlled. The system reduces noise caused by coupling control signal cables and the power cable, and does not have the ground swing control problems of other charging schemes. The power may alternatively be provided by microwave transmission.

<CIT> discloses a downhole device for rotary drilling, including a power generator installed at the end of a series of rods; a pulse generator which is mechanically and electrically connected to said electricity generator; an electric drilling tool; and an electrical sliding switch system. The device has use in the field of drilling by electrical discharge.

<CIT> discloses a rock formation drill bit assembly with electrodes. The assembly includes a drill bit including a hollow portion that extends along a longitudinal axis of the drill bit. The hollow portion extends from a first end to a second end opposing the first end. Cutters as positioned on the first end. The cutters are configured to cut the rock formation resulting in a rock core protruding from the rock formation into the hollow portion. The rock core includes a circumferential surface. Multiple electrodes are positioned within an inner circumferential surface of the hollow portion. The multiple electrodes are configured to apply electrical discharge across multiple locations on the circumferential surface of the rock core. The electrical discharge causes the rock core to fracture.

<CIT> discloses a system and method using plasma channel drilling by which material is removed from a body of material, e.g. to create a bore hole. High voltage, high energy, rapid rise time electrical pulses are delivered many times per second to an electrode assembly in contact with the material body to generate therein elongate plasma channels which expand rapidly following electrical breakdown of the material causing the material to fracture and fragment.

Some embodiments disclosed herein are directed to a system for drilling a borehole. In an embodiment, the system includes a tubular string, and a drill bit coupled to the tubular string. In addition, the system includes a plasma inducing apparatus coupled to the drill bit, and a power conversion assembly coupled to the tubular string. The plasma inducing apparatus is configured to generate plasma from electric current generated within the power conversion assembly.

In other embodiments the system includes a tubular string, and a bottom hole assembly coupled to the tubular string. The bottom hole assembly includes a downhole motor, a power conversion assembly configured to generate electric current from operation of the downhole motor, a drill bit, and an electrode assembly coupled to a downhole end of the drill bit. The electrode assembly is configured to generate plasma when energized with electric current from the power conversion assembly.

Other embodiments disclosed herein are directed to a method of drilling a borehole. In an embodiment, the method includes: (a) rotating a drill bit about a central axis; (b) engaging the drill bit with a subterranean formation during (a); (c) generating electric current downhole; (d) generating plasma from a plasma inducing apparatus coupled to the drill bit during (b) using the electric current generated in (c); (e) weakening the subterranean formation with the plasma during (d); and (f) extending the borehole within a subterranean formation as a result of (a)-(e). Embodiments described herein comprise a combination of features and characteristics intended to address various shortcomings associated with certain prior devices, systems, and methods. The foregoing has outlined rather broadly the features and technical characteristics of the disclosed embodiments in order that the detailed description that follows may be better understood. The various characteristics and features described above, as well as others, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. It should be appreciated that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes as the disclosed embodiments.

For a detailed description of various exemplary embodiments, reference will now be made to the accompanying drawings in which:.

The following discussion is directed to various exemplary embodiments. However, one skilled in the art will understand that the examples disclosed herein have broad application, and that the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment.

Certain terms are used throughout the following description and claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness.

In the following discussion and in the claims, the terms "including" and "comprising" are used in an open-ended fashion, and thus should be interpreted to mean "including, but not limited to. " Also, the term "couple" or "couples" is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices, components, and connections. In addition, as used herein, the terms "axial" and "axially" generally mean along or parallel to a central axis (e.g., central axis of a body or a port), while the terms "radial" and "radially" generally mean perpendicular to the central axis. For instance, an axial distance refers to a distance measured along or parallel to the central axis, and a radial distance means a distance measured perpendicular to the central axis. Any reference to up or down in the description and the claims will be made for purposes of clarity, with "up", "upper", "upwardly" "upstream", "uphole" meaning toward the surface of the borehole and with "down", "lower", "downwardly" "downstream" or "downhole" meaning toward the terminal end of the borehole, regardless of the borehole orientation. As used herein, the terms "approximately," "about," "substantially," and the like mean within <NUM>% (i.e., plus or minus <NUM>%) of the recited value. Thus, for example, a recited angle of "about <NUM> degrees" refers to an angle ranging from <NUM> degrees to <NUM> degrees. As used herein, the term "elongate" when used to refer to a body, means that the longitudinal or axial length of the body is longer than its lateral or radial width.

As previously described, the cost of drilling or forming a subterranean borehole may be directly related to the ROP of the drill bit forming the borehole. Thus, it is generally desirable to increase the ROP of a borehole drilling operation so as to reduce the costs associated therewith. A given drill bit may have a higher ROP for formations that are weaker or that present less resistance to shearing, puncturing, etc. as a result of engagement of the drill bit. Thus, it may be desirable to weaken the subterranean formation prior to or simultaneously with engaging the formation with the drill bit so as to increase the ROP during a drilling operation. Accordingly, examples disclosed herein include drill bits and associated drilling systems or assemblies that include electrode assemblies that are configured to weaken a subterranean formation that is to be engaged by the drill bit and thereby increase the ROP during a drilling operation.

In the specific embodiments disclosed herein, drill bits are described for drilling or forming a borehole in a subterranean formation for accessing hydrocarbons (e.g., oil, gas, condensate, etc.). However, it should be appreciated that the drill bits and associated systems described herein may be employed within any system for forming a subterranean borehole, regardless of the purpose of such a borehole formation. For instance, in some embodiments, the disclosed drill bits (and/or the associated drilling systems) may be utilized to form a subterranean borehole for accessing other resources (e.g., such as ground water), or to form a pathway through a subterranean formation for conduits, cables, fluids, and/or other mechanisms or substances. Further, in some embodiments, embodiments of the disclosed drill bits and/or drilling systems may be utilized to form bores or holes in other mediums (that is, other than a subterranean formation). For instance, in some embodiment, embodiments of the disclosed drill bits may be utilized to drill holes in teeth (e.g., such as for dental applications), walls, structures, etc. Thus, any specific reference to the forming of boreholes for accessing subterranean hydrocarbon resources is merely meant to provide one example implantation of the disclosed embodiments, and should not be interpreted as limiting all potential uses thereof.

Referring now to <FIG>, a schematic view of an embodiment of a system <NUM> for drilling a borehole <NUM> in a subterranean formation <NUM> is shown. In general, system <NUM> includes surface equipment <NUM>, a tubular drill string <NUM>, and a bottom-hole assembly (BHA) <NUM>.

In this example, drill string <NUM> includes a plurality of elongate pipe joints connected together end-to-end. In some embodiments, the elongate pipe joints may be threadably coupled to one another; however, any suitable coupling mechanism or method may be used to join the elongate pipe joints in various embodiments. The drill string <NUM> may be supported by and extended from the surface equipment <NUM> into borehole <NUM>. During operations, drill string <NUM> may both support the BHA <NUM> within borehole <NUM> and provide a flow path for fluids, such as, for instance, drilling mud, into the borehole <NUM> during drilling operations. In some embodiments, drill string <NUM> may comprise any other suitable tether (e.g., such as wireline, slickline, e-line, coiled tubing, etc.) for supporting BHA <NUM> within borehole <NUM> that may or may not also comprise or define a fluid flow path therethrough.

The BHA <NUM> is coupled to a distal or downhole end of the drill string <NUM> within borehole <NUM>. In this embodiment, BHA <NUM> includes a central or longitudinal axis <NUM>, a downhole motor <NUM>, a power conversion assembly <NUM>, and a drill bit <NUM>. Generally speaking, the power conversion assembly <NUM> is axially positioned between the downhole motor <NUM> and drill bit <NUM>.

During drilling operations, drill bit <NUM> is rotated with weight-on-bit (WOB) applied to drill the borehole <NUM> through the earthen formation <NUM>. In this embodiment, drill bit <NUM> is rotated by the downhole motor <NUM>. In other embodiments, surface equipment <NUM> may include additional components for rotating tubular string <NUM> and drill bit <NUM> (e.g., such as a rotary table, top drill, power swivel, etc.). In still other embodiments, the drill bit <NUM> may be rotated by a combination of the downhole motor <NUM> and additional, surface-mounted components (e.g., such as those noted above).

Referring still to <FIG>, while drilling borehole <NUM>, a suitable drilling fluid is pumped under pressure from the surface <NUM> through the drill string <NUM>. The drilling fluid flows down drill string <NUM>, through the BHA <NUM>, and is ultimately discharged at the bottom of borehole <NUM> through nozzles (not shown) in face of drill bit <NUM> (described in more detail below). Thereafter, the drilling fluid circulates uphole to the surface <NUM> through an annular space or annulus <NUM> radially positioned between tubular string <NUM> and the sidewall of borehole <NUM>.

Further, during these operations and as will be described in more detail below, power conversion assembly <NUM> generates electric current, which is utilized to selectively generate plasma at one or more electrode assemblies <NUM> disposed on the face of drill bit <NUM>. The plasma creates cracks and fractures within the formation <NUM> proximal drill bit <NUM> so as weaken the formation <NUM>, thereby offering the potential to increase the ROP of the drilling operation. Additional details of these operations as well as embodiments of the BHA <NUM> are discussed in more detail below.

Referring now to <FIG>, in some embodiments downhole motor <NUM> may comprise progressive cavity or positive displacement motor that is driven via the flow of pressurized drilling fluid therethrough. In particular, the downhole motor <NUM> includes a rotor <NUM> rotatably disposed within a stator <NUM>. The rotor <NUM> includes a shaft formed with one or more helical vanes or lobes extending along its length. In addition, the stator <NUM> is formed of an elastomer liner bonded to the inner wall of the stator housing that defines helical lobes complementary to that of the lobe or lobes of the rotor <NUM>. During operations, pressurized drilling fluid is flowed between the rotor <NUM> and stator <NUM>, thereby driving rotor <NUM> to rotate within the stator <NUM> in an eccentric manner. More particularly, the rotor <NUM> generally orbits about the central longitudinal axis of the stator <NUM>, which is coaxially aligned with central axis <NUM>, while simultaneously rotating about a central axis (not shown) of the rotor <NUM>.

A driveshaft assembly <NUM> is coupled between a downhole end of rotor <NUM> and the drill bit <NUM>. Drive shaft assembly <NUM> includes one or more shafts, joints (e.g., universal joints), connectors (not shown), or combinations thereof that transfer torque from the rotor <NUM> to drill bit <NUM>. Thus, driveshaft assembly <NUM> converts the precessional or orbital motion of the rotor <NUM> into rotation of drill bit <NUM> about central axis <NUM>. In addition, while not specifically shown, it should be appreciated that driveshaft assembly <NUM> may also include one or more bearing assemblies for reducing friction and generally supporting the rotational motion of driveshaft assembly <NUM> and drill bit <NUM> during drilling operations.

It should be appreciated that the design of downhole motor <NUM> may be varied in other embodiments. For instance, in some embodiments downhole motor <NUM> may be configured to rotate rotor <NUM> concentrically about axis <NUM> (e.g., rather than precessionally or eccentrically as previously described above). Accordingly, the design of driveshaft assembly <NUM> may also be varied so as to correspond with the design and arrangement of downhole motor <NUM> during drilling operations.

Referring still to <FIG>, as previously described above, power conversion assembly <NUM> is axially disposed between downhole motor <NUM> and drill bit <NUM> within BHA <NUM>. The components of power generation assembly <NUM> may be generally disposed circumferentially about driveshaft assembly <NUM>. In addition, while not specifically shown, a fluid flow path may be defined through driveshaft assembly <NUM> and/or between driveshaft assembly <NUM> and the power conversion assembly <NUM> to communicate drilling fluid flowing through the downhole motor <NUM> to the drill bit <NUM>, where is it then emitted from one or more nozzles (not shown) in the drill bit <NUM>.

Generally speaking, power conversion assembly <NUM> generates electric current from the rotation of rotor <NUM> within downhole motor <NUM>, and then supplies that electric current to the drill bit <NUM> so as to selectively generate plasma (or "plasmatic discharges") from the electrode assemblies <NUM> during drilling operations. In addition, as will be described in more detail below, power conversion assembly <NUM> may also multiply or increase a voltage of the generated electric current, so as to achieve a desired power discharge via the electrode assemblies <NUM>. In this embodiment, power conversion assembly <NUM> includes an alternator <NUM>, a power storage assembly <NUM>, an inverter <NUM>, a transformer <NUM>, a voltage multiplier and rectifier <NUM>, and a power distribution assembly <NUM>.

Alternator <NUM> generates a flow of electric current utilizing the rotational motion of the rotor <NUM> and/or driveshaft assembly <NUM> during drilling operations. In particular, in some embodiments, alternator <NUM> includes a rotor <NUM> that is rotatably coupled to driveshaft assembly <NUM> so that as driveshaft assembly <NUM> is rotated about central axis <NUM>, rotor <NUM> is also rotated about the central axis <NUM>. Alternator <NUM> also includes one or more coils <NUM> wound circumferentially about the rotor <NUM>. During drilling operations, as the driveshaft assembly <NUM> rotates about the central axis <NUM> (e.g., via the orbiting motion of rotor <NUM> within downhole motor <NUM> as previously describe above), the rotor <NUM> rotates within the coils <NUM>, which thereby generate a magnetic field that in turn induces an electric current flow within the coils <NUM>.

Power storage assembly <NUM> is disposed downhole of alternator <NUM> and stores electric power generated by alternator <NUM>. In particular, power storage assembly <NUM> includes a plurality power storage devices <NUM> (e.g., batteries, capacitors, etc.), electrically coupled to one another and to the coils <NUM> within alternator <NUM>. In this embodiment, the power storage devices <NUM> are batteries (e.g., <NUM> Volt batteries, <NUM> Volt batteries, etc.). Thus, power storage devices <NUM> may also be referred to herein as "batteries <NUM>. " The batteries <NUM> may be coupled to one another in series (e.g., such that a positive terminal of each battery <NUM> is electrically coupled to a negative terminal of another of the batteries <NUM>), or in parallel (e.g., such that all of the positive terminals of batteries <NUM> are coupled to one another and all of the negative terminals of batteries <NUM> are coupled to one another). The choice between series connection or parallel connection between the batteries <NUM> may be driven by a desired output voltage from the power storage assembly <NUM> to the other components within power conversion assembly <NUM>, the power storage capacity of the batteries <NUM>, etc..

In this embodiment, the batteries <NUM> within power storage assembly <NUM> are elongate cylindrical bodies that are parallel to and radially offset from central axis <NUM>. More specifically, the batteries <NUM> are uniformly circumferentially spaced about central axis <NUM> and driveshaft assembly <NUM>. However, it should be appreciated that batteries <NUM> may have alternative shapes or forms, and/or the batteries <NUM> may have alternative arrangements or orientations within the power conversion assembly <NUM> in other embodiments.

Referring still to <FIG>, inverter <NUM> is positioned downhole of and electrically coupled to the power storage assembly <NUM>. Thus, during drilling operations, electric current flows from batteries126 of power storage assembly <NUM> to inverter <NUM>. The electric current produced from batteries <NUM> may be direct current (DC). Generally speaking, during operations, inverter <NUM> converts the DC current provided from batteries <NUM> to alternating current (AC). In general, inverter <NUM> may comprise any suitable circuit(s) and/or other mechanisms for affecting the conversion of DC current to AC current.

Transformer <NUM> is positioned downhole of inverter <NUM> and increases the voltage of the AC current emitted from inverter <NUM> to a higher, desired voltage. In some embodiments, the transformer <NUM> may receive an input current (e.g., from inverter <NUM>) having a voltage of about <NUM> to <NUM> V (AC) and may produce an output current having a voltage of about 1kV (AC) to about <NUM> kV (AC). In some specific embodiments, the transformer <NUM> may receive an input current having a voltage of about <NUM> V (AC) and produce an output current having a voltage of about <NUM> kV (AC), or may receive an input current having a voltage of about <NUM> V (AC) and produce an output current having a voltage of about <NUM> kV (AC). While not specifically shown, it should be appreciated that transformer <NUM> may, in some embodiments, comprise one or more coils or windings that create a varying magnetic field when energized with an electric current (e.g., such as an electric current supplied from inverter <NUM>), so as to induce an output electric current (e.g., an output AC electric current) at a different (e.g., in this case higher) voltage than the input electric current.

Voltage multiplier and rectifier <NUM> is disposed downhole of and electrically coupled to transformer <NUM>. Thus, during drilling operations, the AC electric current output from transformer <NUM> is supplied to voltage multiple and rectifier <NUM>. In some embodiments, the voltage multiplier and rectifier <NUM> may comprise a Cockcroft-Walton generator, and thus, may be generally referred to herein as a "generator <NUM>. " During drilling operations, generator <NUM> generates a high voltage DC current based on the AC current received from transformer <NUM>. In addition to effectively converting the AC electric current from transformer <NUM> into DC current, the DC current output from generator <NUM> also has a higher voltage than the input AC current supplied from transformer <NUM>. In some embodiments, the DC current output from generator <NUM> has a voltage potential of approximately <NUM> kV or greater (e.g., approximately 50kV). In addition, in some embodiments, the DC current output from generator <NUM> has a current of approximately <NUM> mA (however, currents above and below <NUM> mA are also contemplated herein).

The relatively high output DC electric current from the generator <NUM> is then supplied to the power distributor <NUM>. Power distributor <NUM> may comprise one or more circuits, controllers, and/or other devices that selectively emit the output electric current from generator <NUM> to the electrode assemblies <NUM> coupled to drill bit <NUM>. In particular, in some embodiments, power distributor <NUM> provides electric current to the electrode assemblies <NUM> in a desired sequential order or pattern. In some embodiments, the sequence or sequential order for providing electric current to the various electrode assemblies <NUM> is tailored and configured to weaken a portion or surface of the formation <NUM> prior to (or simultaneous with) engaging that surface or portion of the formation <NUM> with the drill bit <NUM>. In some embodiments, the speed in which the energization sequence for the electrode assemblies <NUM> is carried out may be dictated or based on a rotational speed of the drill bit <NUM> (e.g., about axis <NUM>) during drilling operations.

In at least some embodiments, power distributor <NUM> rapidly transfers or applies a relatively high voltage electric current to the electrode assemblies <NUM>. For instance, in some embodiments, the power distributor <NUM> transfers or applies about <NUM> volts per nanosecond (V/ns) or greater to the electrode assemblies <NUM> during drilling operations. In some embodiments, the power distributor <NUM> transfers or applies greater than or equal to about <NUM> V/ns to the electrode assemblies <NUM> during drilling operations. Without being limited to this or any other theory, a relatively rapid transfer of higher voltage electric current to the electrode assemblies <NUM> may allow for relatively low energy, high voltage pulses to be generated within the liquids filling the borehole <NUM>, regardless of the conductivity of the liquids.

Referring now to <FIG> and <FIG>, in some embodiments, power distributor <NUM> includes a plurality of electrical contacts 138a, 138b that are coupled to the electrode assemblies <NUM> within drill bit <NUM>. In particular, in the embodiment shown in <FIG>, power distributor <NUM> includes a first electrical contact 138a coupled to a first electrode assembly 160a disposed within drill bit <NUM>, and a second electrical contact 138b coupled to a second electrode assembly 160b within drill bit <NUM>. The electrical contacts 138a, 138b are coupled to the electrode assemblies 160a, 160b via a pair of communication paths <NUM>, which may comprise any suitable mechanism or device configured to conduct electrical current therethrough (e.g., such as a wire, cable, conductive trace, etc.). The electrical contacts 138a, 138b are circumferentially arranged or spaced about central axis <NUM>. In some embodiments, the contacts 138a, 138b are uniformly-circumferentially spaced about axis <NUM>. Thus, in the embodiment shown in <FIG>, the two electrical contacts 138a, 138b are circumferentially spaced about <NUM>° from one another about axis <NUM> (i.e., electrical contacts 138a, 138b radially oppose one another across central axis <NUM>). However, as will be described in more detail below, the arrangement, number, and spacing of the electrode assemblies <NUM> on drill bit <NUM> may be varied in different embodiments.

Referring still to <FIG> and <FIG>, power distributor <NUM> also includes a conductive tip <NUM>. The power distributor <NUM> is coupled to driveshaft assembly <NUM> and/or drill bit <NUM> so that the rotation of driveshaft assembly <NUM> and drill bit <NUM> about axis <NUM> also drives a relative rotation between the tip <NUM> and the electrical contacts 138a, 138b. In particular, in some embodiments, the electrical contacts 138a, 138b may rotate about central axis <NUM> along with drill bit <NUM> and driveshaft assembly <NUM>, relative to the conductive tip <NUM>. The conductive tip <NUM> may be spaced (e.g., in an axial direction with respect to central axis <NUM>) from the electrical contacts 138a, 138b, and may be energized with electric current from the generator <NUM>. Thus, during rotation of the drill bit <NUM> and the relative rotation of the electrical contacts 138a, 138b, the tip <NUM> is progressively brought into close proximity to each of the contacts 138a, 138b. When tip <NUM> is sufficiently close the contacts 138a, 138b, electric current "jumps" from the tip <NUM> to the corresponding electrical contact 138a, 138b via an arc <NUM> (e.g., such as shown between the tip <NUM> and electrical contact 138a in <FIG>). Thereafter, the electric current flows from the electrical contact to the corresponding electrode assemblies 160a, 160b in drill bit <NUM> via the conductive paths <NUM>. In some embodiments, the tip <NUM> may physically engage with contacts 138a, 138b so as to conduct electrical current therebetween during drilling operations.

Generally speaking, each electrode assembly 160a, 160b includes a pair of outwardly extending electrodes <NUM> spaced apart from one another. When electric current is conducted to the electrode assemblies 160a, 160b via conductive paths <NUM> (e.g., such as when electric current is conducted from the tip <NUM> to the corresponding electrical contacts 138a, 138b as described above), the electric current may be conducted into at least one of the electrodes <NUM> whereby it may again "jump" to the other electrode <NUM> via an arc <NUM>. Arc <NUM> may be referred to herein as a plasmatic discharge or plasma that generates increased temperatures and pressures. Thus, the electrode assemblies 160a, 160b (as well as electrode assemblies <NUM> discussed more broadly herein and shown in <FIG>, <FIG>, and <FIG>) may be referred to herein as "plasma inducing" devices or apparatuses that generate plasma (e.g., arc <NUM>). During drilling operations, the electrodes <NUM> may be disposed relatively close to a surface of the formation <NUM> within borehole <NUM>, such as, for instance within <NUM> or less, or within <NUM> or less. Large gradients accompanying the formation of plasma <NUM> may also induce shock waves <NUM> and cavitation within the fluid disposed in the borehole <NUM> (e.g., drilling fluid, water, etc.). The induced shockwaves <NUM> impact formation <NUM> and thereby form fractures <NUM> (e.g., cracks, micro-cracks, etc.). In some embodiments, the shockwaves <NUM> may apply elevated pressures to the formation <NUM> that are greater than or equal to <NUM> GPa. As a result, the formation <NUM> is generally weakened so that drill bit <NUM> may more easily shear, puncture, etc. the formation <NUM> and therefore extend borehole <NUM> during drilling operations.

According to the claimed invention, the average electrical power for generating plasma <NUM> between the select pairs of electrodes <NUM> in electrode assemblies 160a, 160b is less than 20kW, or may be less than 5kW (e.g., such as from about <NUM> W to about <NUM> kW). Also, the plasma <NUM> may be generated rapidly between the electrodes <NUM>, with instantaneous (or near instantaneous) power release of about <NUM> MW or greater, and may have an energy release of about <NUM> Joules (J) to about <NUM> kJ.

In addition, the electrical pulse or current conducted to the electrode assemblies 160a, 160b via conductive paths <NUM> may be either monopoloar or bipolar. In some embodiments, the electrical or current conducted to the electrode assemblies 160a, 160b is monopolar and of the electrode <NUM> of each electrode assembly 160a, 160b may receive electric current having a voltage of about <NUM> kV to about <NUM> kV. In some embodiments, one of the electrodes <NUM> of each electrode assembly 160a, 160b may be coupled to a ground potential. In some embodiments, the electrical current conducted to electrode assemblies 160a, 160b may be bipolar, and one electrode <NUM> within each electrode assembly 160a, 160b may receive a positively biased electric current, while the other electrode <NUM> of each electrode assembly 160a, 160b may receive a negatively biased electric current, wherein the positive and negative biases are made with reference to a ground potential.

In some embodiments, the duration of the plasmatic discharges (e.g., arcs <NUM>) may occur relatively quickly between electrodes <NUM>. For instance, in some embodiments, the duration of the plasmatic discharges between electrodes <NUM> may be <NUM> nanoseconds (ns) or less, or from about <NUM> ns to about <NUM> microsecond (µs). Additionally, in some embodiments, the plasmatic discharges between electrodes <NUM> may occur at frequencies of about <NUM> to about <NUM>.

In general, drill bit <NUM> may be any suitable type or design of drill bit for forming borehole <NUM> in subterranean formation <NUM>. For instance, drill bit <NUM> may be a fixed cutter drill bit (e.g., which is sometimes referred to as a "drag bit") that shears portions of the formation <NUM> to extend borehole <NUM>. In some embodiments, drill bit <NUM> may be a rolling cone drill bit <NUM> that punctures and crushes the formation <NUM> to extend borehole <NUM>. In still other embodiments, drill bit <NUM> may be another form of drill bit (e.g., including hybrid designs incorporating elements of a fixed cutter and rolling cone drill bit). In the following discussion, a drill bit that may be used as drill bit <NUM> within BHA <NUM> according to some embodiments is described in more detail; however, as noted above, it should be appreciated that the drill bit <NUM> may comprise a number of different designs that may differ from those specifically discussed below.

Referring now to <FIG>, a drill bit <NUM> that may be used as drill bit <NUM> within BHA <NUM> according to some embodiments is shown. In this embodiment, drill bit <NUM> includes a so-called fixed cutter drill bit that is configured to shear off portions of a subterranean formation (e.g., formation <NUM>) to extend a borehole (e.g., borehole <NUM>) therein.

Generally speaking, drill bit <NUM> has a central or longitudinal axis <NUM>, a first or uphole end 250a, and a second or downhole end 250b. Central axis <NUM> of bit <NUM> is coaxially aligned with central axis <NUM> of BHA <NUM> when bit <NUM> is coupled within BHA <NUM> as drill bit <NUM> (see e.g., <FIG> and <FIG>). Drill bit <NUM> is configured to rotate about axis <NUM> in a cutting direction represented by arrow <NUM>. In addition, bit <NUM> includes a bit body <NUM> extending axially from downhole end 250b, a threaded connection or pin <NUM> extending axially from uphole end 250a, and a shank <NUM> extending axially between pin <NUM> and body <NUM>. Pin <NUM> couples bit <NUM> to BHA <NUM> (see e.g., <FIG>). Bit body <NUM>, shank <NUM>, and pin <NUM> are coaxially aligned with axis <NUM>, and thus, each has a central axis coincident with axis <NUM>.

The portion of bit body <NUM> that faces the formation at downhole end 250b includes a bit face <NUM> provided with a cutting structure <NUM>. Cutting structure <NUM> includes a plurality of blades <NUM>, <NUM>, <NUM>, which extend from bit face <NUM>. In this embodiment, the plurality of blades <NUM>, <NUM>, <NUM> are uniformly circumferentially-spaced on bit face <NUM> about bit axis <NUM>.

In this embodiment, blades <NUM>, <NUM>, <NUM> are integrally formed as part of, and extend from, bit body <NUM> and bit face <NUM>. In particular, blades <NUM>, <NUM>, <NUM> extend generally radially along bit face <NUM> and then axially along a portion of the periphery of bit <NUM>. Blades <NUM>, <NUM>, <NUM> are separated by drilling fluid flow courses or junk slots <NUM>. Each blade <NUM>, <NUM>, <NUM> has a leading edge or side 291a, 292a, 293a, respectively, and a trailing edge or side 291b, 292b, 293b, respectively, relative to the direction of rotation <NUM> of bit <NUM>.

Referring still to <FIG>, each blade <NUM>, <NUM>, <NUM> includes a cutter-supporting surface <NUM> for mounting a plurality of cutter elements <NUM>. In particular, cutter elements <NUM> are arranged adjacent one another in a radially extending row along the leading edge 291a, 292a, 293a of each blade <NUM>, <NUM>, <NUM>. In this embodiment, each cutter element <NUM> is a generally cylindrical member that includes a relatively hard material for engaging with and shearing portions of a subterranean formation (e.g., formation <NUM>) during operations. In some embodiments, the cutter elements <NUM> may comprise polycrystalline diamond.

Bit body <NUM> further includes gage pads <NUM> of substantially equal axial length measured generally parallel to bit axis <NUM>. Gage pads <NUM> are circumferentially-spaced about the radially outer surface of bit body <NUM>. Specifically, one gage pad <NUM> intersects and extends from each blade <NUM>, <NUM>, <NUM>. In this embodiment, gage pads <NUM> are integrally formed as part of the bit body <NUM>. In general, gage pads <NUM> can help maintain the size of the borehole by a rubbing action when cutter elements <NUM> wear slightly under gage. Gage pads <NUM> also help stabilize bit <NUM> against vibration.

Referring specifically now to <FIG>, a cross-section of drill bit <NUM> is shown that shows a profile of with a first blade <NUM>; however, it should be appreciated that each of the blades <NUM>, <NUM>, <NUM> is generally configured the same, such that the portions and components of the profile of blade <NUM> are also present along the blades <NUM>, <NUM>. In this embodiment, the profile of blades <NUM>, <NUM>, <NUM> (as shown by the representation of the profile of blade <NUM> in <FIG>) may generally be divided into three regions conventionally labeled cone region 299a, shoulder region 299b, and gage region 299c. Cone region 299a includes the radially innermost region of bit body <NUM>, and extends from bit axis <NUM> to shoulder region 299b. In this embodiment, cone region 299a is generally concave. Adjacent cone region 299a is the generally convex shoulder region 299b. The transition between cone region 299a and shoulder region 299b, typically referred to as the nose 299d. Moving radially outward, adjacent shoulder region 299b is the gage region 299c which extends substantially parallel to bit axis <NUM> at the outer radial periphery of composite blade profile <NUM>. Gage pads <NUM> define the gage region 299c and an outer radius of bit body <NUM>. Cutter elements <NUM> are provided in cone region 299a, shoulder region 299b, and gage region 299c.

As is also best shown in <FIG>, bit <NUM> includes an internal plenum <NUM> extending axially from uphole end 250a through pin <NUM> and shank <NUM> into bit body <NUM>. Plenum <NUM> permits drilling fluid to flow from the tubular string <NUM> (see e.g., <FIG> and <FIG>) into bit <NUM>. Flow passages <NUM> extend from plenum <NUM> to downhole end 250b. As best shown in <FIG> and <FIG>, nozzles <NUM> are seated in the lower end of each flow passage <NUM>. The nozzles <NUM> and corresponding flow passages <NUM> distribute drilling fluid around cutting structure <NUM> to flush away formation cuttings and to remove heat from cutting structure <NUM>, and more particularly cutter elements <NUM>, during drilling.

Referring again to <FIG>, the plurality of electrode assemblies <NUM> are disposed about the cutting structure <NUM>. As best shown in <FIG> and <FIG>, in this embodiment the electrode assemblies <NUM> are disposed within the cone region 299a and the shoulder region 299b. In this embodiment, no electrode assemblies <NUM> are included within the gage region 299c; however, it should be appreciated that in other embodiments, one or more of the electrode assemblies <NUM> may be included within the gage region 299c. Also, in some embodiments (e.g., such as the embodiment of <FIG>), the electrode assemblies <NUM> may be recessed within the cutting structure <NUM> so as to protect electrodes <NUM> from impacting the formation (e.g., formation <NUM>) or other components or features during a drilling operation.

In addition, as is best shown in <FIG>, in this embodiment the electrode assemblies <NUM> are disposed at different radial positions relative to central axes <NUM>, <NUM> such that each electrode assembly <NUM> traces or sweeps through a different orbit <NUM> about axis <NUM>, <NUM> as drill bit <NUM> rotates about axes <NUM>, <NUM> in the cutting direction <NUM>. In particular, each orbit <NUM> is radially spaced from the other orbits <NUM>, so that each electrode assembly <NUM> interacts with a different portion of the formation <NUM> (see e.g., <FIG>) during drilling operations. In this embodiment, there are total of four electrode assemblies <NUM> so that during operations, the electrode assemblies trace four different orbits <NUM> that are radially spaced moving radially outward form the central axes <NUM>, <NUM>. In some embodiments, the electrode assemblies <NUM> are arranged so that the orbits <NUM> are generally uniformly radially spaced; however, in other embodiments, one or more of the orbits <NUM> traced by the electrode assemblies <NUM> may not be evenly radially spaced from one another.

Referring now to <FIG> and <FIG>, in some embodiments the conductive paths <NUM> electrically coupling electrode assemblies <NUM> to power distribution assembly <NUM> are routed (e.g., at least partially) through the plenum <NUM> of drill bit <NUM>. In addition, while not specifically shown, it should be appreciated that conductive paths <NUM> may also be routed through additional bores or tunnels extending from plenum to electrode assemblies <NUM>. In some embodiments, conductive paths <NUM> may extend through one or more of the flow passages <NUM> in addition to the plenum <NUM>. In still other embodiments, conductive paths <NUM> may extend through tunnels or pathways within drill bit <NUM> that do not extend through and/or intersect with the plenum <NUM> or flow passages <NUM>.

Referring now to <FIG>, an embodiment of a method <NUM> for drilling a borehole (e.g., such as borehole <NUM> in <FIG>) is shown. In describing the features of method <NUM>, reference will be made to the features of system <NUM> shown in <FIG>; however, it should be appreciated that method <NUM> may be performed with other system and assemblies that may be different from those described above for system <NUM>. Thus, reference to system <NUM> and its components and features (e.g., BHA <NUM>, drill bit <NUM>, drill bit <NUM>, etc.) is merely meant to describe particular embodiments of method <NUM> and should not be interpreted as limiting all potential embodiments of method <NUM>.

Initially, method <NUM> begins by rotating a drill bit about a central axis at block <NUM>. For instance, a drill bit (e.g., drill bit <NUM>, <NUM>) may be rotated about a central axis of the drill bit and/or of a bottom hole assembly (e.g., BHA <NUM>, central axis <NUM>). Next, method <NUM> includes engaging the drill bit with a subterranean formation during the rotating at block <NUM>. In some embodiments, the engaging at block <NUM> may comprise shearing the formation with a cutting structure of the drill bit (e.g., cutting structure <NUM> of drill bit <NUM>), and/or puncturing the formation with the drill bit (e.g., such as for a rolling cone drill bit).

Next, method <NUM> includes generating plasma with a plasma inducing apparatus coupled to the drill bit during the engaging at block <NUM>. For instance, in some embodiments, the plasma inducing apparatus may comprise an electrode assembly (e.g., electrode assembly <NUM>) coupled to the drill bit, and generating plasma at block <NUM> may comprise flowing electric current to the electrode assembly. In some embodiments, the plasma inducing apparatus (e.g., electrodes <NUM>) may be coupled to a downhole end (e.g., downhole end 250b and cutting structure <NUM> of drill bit <NUM>) of the drill bit.

Method <NUM> next includes weakening the subterranean formation with the plasma during the generating at block <NUM>. For instance, in some embodiments weakening the subterranean formation may comprise forming cracks (e.g., cracks <NUM> in <FIG>) in the subterranean formation) as a result of the plasma generated at <NUM>. Method <NUM> also includes extending a borehole within the subterranean formation at block <NUM>. In some embodiments, extending the borehole at <NUM> may directly result from the rotating, engaging, generating, and weakening of blocks <NUM>, <NUM>, <NUM>, <NUM>, previously described.

The embodiments disclosed herein have included drill bits and associated drilling systems or assemblies (e.g., system <NUM>, BHA <NUM>, drill bit <NUM>) including electrode assemblies (e.g., electrodes <NUM> within electrode assemblies <NUM>) configured to weaken a subterranean formation that is to be engaged by the drill bit and thereby increase the ROP during a drilling operation. Thus, through use of the embodiments disclosed herein, the time required to drill a borehole may be reduced, so that the costs associated with such a drilling operation may also be reduced.

While the embodiments described herein have included electrode assemblies (e.g., electrode assemblies <NUM>) coupled to a downhole end of a drill bit (e.g., drill bit <NUM>, <NUM>, etc.), it should be appreciated that other embodiments may position electrode assemblies in different locations within system <NUM> either in lieu of or in addition to the electrode assemblies coupled to the bit as described above. For instance, in some embodiments, system <NUM> may include a reamer cutter disposed along or uphole of BHA <NUM> that includes one or more electrode assemblies that may be configured substantially the same as the electrode assemblies <NUM> described above.

Claim 1:
A system (<NUM>) for drilling a borehole (<NUM>) in a subterranean formation (<NUM>), the system comprising:
a tubular string (<NUM>);
a drill bit (<NUM>, <NUM>) coupled to the tubular string (<NUM>) and comprising a bit body (<NUM>) having a bit face (<NUM>) positioned along the bit body (<NUM>);
and a plasma inducing apparatus coupled to the drill bit (<NUM>, <NUM>) and comprising an electrode assembly (<NUM>) coupled to a downhole end of the drill bit (<NUM>, <NUM>), wherein the electrode assembly (<NUM>) comprises one or more pairs of electrodes (<NUM>) positioned along the bit face (<NUM>) of the drill bit (<NUM>, <NUM>) and configured to emit an electrical discharge between the one or more pairs of electrodes (<NUM>) to generate plasma for weakening the subterranean formation (<NUM>), the electrical discharge having an average electrical power that is less than <NUM> kilowatts (kW); and
a power conversion assembly (<NUM>) coupled to the tubular string (<NUM>), wherein the plasma inducing apparatus is configured to generate plasma from electric current generated within the power conversion assembly (<NUM>);
wherein the drill bit (<NUM>, <NUM>) comprises a cutting structure (<NUM>) positioned along the bit face (<NUM>) and comprising a plurality of cutter elements (<NUM>) configured to cut into the subterranean formation (<NUM>) as the subterranean formation (<NUM>) is weakened by the electrical discharge emitted between the one or more pairs of electrodes (<NUM>).