SWITCHES FOR DOWNHOLE ELECTROCRUSHING DRILLING

A downhole drilling system is disclosed. The downhole drilling system may include a bottom-hole assembly having a pulse-generating circuit and a switching circuit within the pulse-generating circuit, the switching circuit comprising a solid-state switch. The downhole drilling system may also include a drill bit having a first electrode and a second electrode electrically coupled to the pulse-generating circuit to receive a pulse from the pulse-generating circuit.

TECHNICAL FIELD

The present disclosure relates generally to downhole electrocrushing drilling and, more particularly, to switches utilized in downhole electrocrushing drilling.

BACKGROUND

Electrocrushing drilling uses pulsed power technology to drill a borehole in a rock formation. Pulsed power technology repeatedly applies a high electric potential across the electrodes of an electrocrushing drill bit, which ultimately causes the surrounding rock to fracture. The fractured rock is carried away from the bit by drilling fluid and the bit advances downhole.

DETAILED DESCRIPTION

Electrocrushing drilling may be used to form wellbores in subterranean rock formations for recovering hydrocarbons, such as oil and gas, from these formations. Electrocrushing drilling uses pulsed-power technology to repeatedly fracture the rock formation by repeatedly delivering high-energy electrical pulses to the rock formation. In some applications, certain components of a pulsed-power system may be located downhole. For example, a pulse-generating circuit may be located in a bottom-hole assembly (BHA) near the electrocrushing drill bit. The pulse-generating circuit may include one or more switches. For example, the pulse-generating circuit may include one or more solid-state switches. As another example, the pulse-generating circuit may include one or more magnetic switches. Such switches may be capable of withstanding the high voltages and the high currents utilized in the pulsed-power system. Moreover, such switches may be capable of withstanding harsh environment of a downhole pulsed-power system. The switches may operate over a wide temperature range (for example, from 10 to 150 degrees C. or from 10 to 200 degrees C.), and may physically withstand the vibration and mechanical shock resulting from the fracturing of rock during downhole electrocrushing drilling.

There are numerous ways in which solid-state switches and magnetic switches may be implemented in a downhole electrocrushing pulsed-power system. Thus, embodiments of the present disclosure and its advantages are best understood by referring toFIGS. 1 through 8, where like numbers are used to indicate like and corresponding parts.

FIG. 1is an elevation view of an exemplary electrocrushing drilling system used to form a wellbore in a subterranean formation. AlthoughFIG. 1shows land-based equipment, downhole tools incorporating teachings of the present disclosure may be satisfactorily used with equipment located on offshore platforms, drill ships, semi-submersibles, and drilling barges (not expressly shown). Additionally, while wellbore116is shown as being a generally vertical wellbore, wellbore116may be any orientation including generally horizontal, multilateral, or directional.

Drilling system100includes drilling platform102that supports derrick104having traveling block106for raising and lowering drill string108. Drilling system100also includes pump124, which circulates electrocrushing drilling fluid122through a feed pipe to drill string110, which in turn conveys electrocrushing drilling fluid122downhole through interior channels of drill string108and through one or more orifices in electrocrushing drill bit114. Electrocrushing drilling fluid122then circulates back to the surface via annulus126formed between drill string108and the sidewalls of wellbore116. Fractured portions of the formation are carried to the surface by electrocrushing drilling fluid122to remove those fractured portions from wellbore116.

Electrocrushing drill bit114is attached to the distal end of drill string108. In some embodiments, power to electrocrushing drill bit114may be supplied from the surface. For example, generator140may generate electrical power and provide that power to power-conditioning unit142. Power-conditioning unit142may then transmit electrical energy downhole via surface cable143and a sub-surface cable (not expressly shown inFIG. 1) contained within drill string108or attached to the side of drill string108. A pulse-generating circuit within bottom-hole assembly (BHA)128may receive the electrical energy from power-conditioning unit142, and may generate high-energy pulses to drive electrocrushing drill bit114.

The pulse-generating circuit within BHA128may be utilized to repeatedly apply a high electric potential, for example up to or exceeding 150 kV, across the electrodes of electrocrushing drill bit114. Each application of electric potential may be referred to as a pulse. When the electric potential across the electrodes of electrocrushing drill bit114is increased enough during a pulse to generate a sufficiently high electric field, an electrical arc forms through a rock formation at the bottom of wellbore116. The arc temporarily forms an electrical coupling between the electrodes of electrocrushing drill bit114, allowing electric current to flow through the arc inside a portion of the rock formation at the bottom of wellbore116. This electric current flows until the energy in a given pulse is dissipated. The arc greatly increases the temperature and pressure of the portion of the rock formation through which the arc flows and the surrounding formation and materials. The temperature and pressure is sufficiently high to break the rock into small pieces. The vaporization process creates a high-pressure gas which expands and, in turn, fractures the surrounding rock. This fractured rock is removed, typically by electrocrushing drilling fluid122, which moves the fractured rock away from the electrodes and uphole.

As electrocrushing drill bit114repeatedly fractures the rock formation and electrocrushing drilling fluid122moves the fractured rock uphole, wellbore116, which penetrates various subterranean rock formations118, is created. Wellbore116may be any hole drilled into a subterranean formation or series of subterranean formations for the purpose of exploration or extraction of natural resources such as, for example, hydrocarbons, or for the purpose of injection of fluids such as, for example, water, wastewater, brine, or water mixed with other fluids. Additionally, wellbore116may be any hole drilled into a subterranean formation or series of subterranean formations for the purpose of geothermal power generation.

Although drilling system100is described herein as utilizing electrocrushing drill bit114, drilling system100may also utilize an electrohydraulic drill bit. An electrohydraulic drill bit may have multiple electrodes similar to electrocrushing drill bit114. But, rather than generating an arc within the rock, an electrohydraulic drill bit applies a large electrical potential across two electrodes to form an arc across the drilling fluid proximate the bottom of wellbore116. The high temperature of the arc vaporizes the portion of the fluid immediately surrounding the arc, which in turn generates a high-energy shock wave in the remaining fluid. The electrodes of electrohydraulic drill bit may be oriented such that the shock wave generated by the arc is transmitted toward the bottom of wellbore116. When the shock wave hits and bounces off of the rock at the bottom of wellbore116, the rock fractures. Accordingly, drilling system100may utilize pulsed-power technology with an electrohydraulic drill bit to drill wellbore116in subterranean formation118in a similar manner as with electrocrushing drill bit114.

FIG. 2illustrates exemplary components of the bottom hole assembly for downhole electrocrushing drilling system100. Bottom-hole assembly (BHA)128may include pulsed-power tool230. BHA128may also include electrocrushing drill bit114. For the purposes of the present disclosure, electrocrushing drill bit114may be referred to as being integrated within BHA128, or may be referred to as a separate component that is coupled to BHA128.

Pulsed-power tool230may be coupled to provide pulsed power to electrocrushing drill bit114. Pulsed-power tool230receives electrical energy from a power source via cable220. For example, pulsed-power tool230may receive power via cable220from a power source on the surface as described above with reference toFIG. 1, or from a power source located downhole such as a generator powered by a mud turbine. Pulsed-power tool230may also receive power via a combination of a power source on the surface and a power source located downhole. Pulsed-power tool230converts the electrical energy received from the power source into high-power electrical pulses, and may apply those high-power pulses across electrode208and ground ring250of electrocrushing drill bit114. Pulsed-power tool230may also apply high-power pulses across electrode210and ground ring250in a similar manner as described herein for electrode208and ground ring250. Pulsed-power tool230may include a pulse-generating circuit as described below with reference toFIG. 3.

Referring toFIG. 1andFIG. 2, electrocrushing drilling fluid122may exit drill string108via openings209surrounding each electrode208and each electrode210. The flow of electrocrushing drill fluid122out of openings209allows electrodes208and210to be insulated by the electrocrushing drilling fluid. In some embodiments, electrocrushing drill bit114may include a solid insulator (not expressly shown inFIG. 1 or 2) surrounding electrodes208and210and one or more orifices (not expressly shown inFIG. 1 or 2) on the face of electrocrushing drill bit114through which electrocrushing drilling fluid122may exit drill string108. Such orifices may be simple holes, or they may be nozzles or other shaped features. Because fines are not typically generated during electrocrushing drilling, as opposed to mechanical drilling, electrocrushing drilling fluid122may not need to exit the drill bit at as high a pressure as the drilling fluid in mechanical drilling. As a result, nozzles and other features used to increase drilling fluid pressure may not be needed. However, nozzles or other features to increase electrocrushing drilling fluid122pressure or to direct electrocrushing drilling fluid may be included for some uses.

Drilling fluid122is typically circulated through drilling system100at a flow rate sufficient to remove fractured rock from the vicinity of electrocrushing drill bit114in sufficient quantities within a sufficient time to allow the drilling operation to proceed downhole at least at a set rate. In addition, electrocrushing drilling fluid122may be under sufficient pressure at a location in wellbore116, particularly a location near a hydrocarbon, gas, water, or other deposit, to prevent a blowout.

Electrodes208and210may be at least 0.4 inches apart from ground ring250at their closest spacing, at least 1 inch apart at their closest spacing, at least 1.5 inches apart at their closest spacing, or at least 2 inches apart at their closest spacing. If drilling system100experiences vaporization bubbles in electrocrushing drilling fluid122near electrocrushing drill bit114, the vaporization bubbles may have deleterious effects. For instance, vaporization bubbles near electrodes208or210may impede formation of the arc in the rock. Electrocrushing drilling fluids122may be circulated at a flow rate also sufficient to remove vaporization bubbles from the vicinity of electrocrushing drill bit114.

In addition, electrocrushing drill bit114may include ground ring250, shown in part inFIG. 2. Although not all electrocrushing drill bits114may have ground ring250, if it is present, it may contain passages260to permit the flow of electrocrushing drilling fluid122along with any fractured rock or bubbles away from electrodes208and210and uphole.

FIG. 3illustrates a schematic for an exemplary pulse-generating circuit for a downhole electrocrushing drilling system. Pulse-generating circuit300may include power source input301, including input terminals302and303, and capacitor304coupled between input terminals302and303. Pulse-generating circuit300may also include switching circuit306, transformer310, and capacitor314.

As described above with reference toFIG. 2, power source input301may receive electrical energy from a power source located on the surface or located downhole. Pulse-generating circuit300may convert the received energy into high-power electrical pulses that are applied across electrodes208or electrodes210and ground ring250of electrocrushing drill bit114. As described above with reference toFIG. 1andFIG. 2, the high-power electrical pulses at the electrodes are utilized to drill wellbore116in subterranean formation118.

Switching circuit306may include any suitable device to open and close the electrical path between power source input301and the first winding311of transformer310. For example, switching circuit306may include a mechanical switch, a solid-state switch, a magnetic switch, a gas switch, or any other type of switch suitable to open and close the electrical path between power source input301and first winding311of transformer310. Switching circuit306may be open between pulses. When switching circuit306is closed, electrical current flows through first winding311of transformer310. Second winding312of transformer310may be electromagnetically coupled to first winding311. Accordingly, transformer310generates a current through second winding312when switching circuit306is closed and current flows through first winding311. In some embodiments, one or both of first winding311and second winding312may include multiple magnetically coupled windings that are coupled in series or in parallel. For example, second winding312may include multiple individual windings that are coupled in series to increase the voltage across second winding312. As another example, second winding312may include multiple individual windings that are coupled in parallel to increase the current provided by second winding312for a given current through first winding311. Similarly, transformer310may include multiple isolated transformers with their respective outputs coupled in series to produce a higher voltage output, or with their outputs coupled in parallel to produce a higher current output.

The current through second winding312charges capacitor314, thus increasing the voltage across capacitor314. Electrode208and ground ring250may be coupled to opposing terminals of capacitor314. Accordingly, as the voltage across capacitor314increases, the voltage across electrode208and ground ring250increases. And, as described above with reference toFIG. 1, when the voltage across the electrodes of an electrocrushing drill bit becomes sufficiently large, an arc forms through a rock formation that is in contact with electrode208and ground ring. The arc provides a temporary electrical short between electrode208and ground ring250, and thus discharges, at a high current level, the voltage built up across capacitor314. As described above with reference toFIG. 1, the arc greatly increases the temperature of the portion of the rock formation through which the arc flows and the surrounding formation and materials. The temperature is sufficiently high to vaporize any water or other fluids that might be touching or near the arc and may also vaporize part of the rock itself. The vaporization process creates a high-pressure gas which expands and, in turn, fractures the surrounding rock.

AlthoughFIG. 3illustrates a schematic for a particular pulse-generating circuit topology, electrocrushing drilling systems and pulsed-power tools may utilize any suitable pulse-generating circuit topology to generate and apply high-voltage pulses to across electrode208and ground ring250. Such pulse-generating circuit topologies may utilize one or more switching circuits such as switching circuit306. Moreover, althoughFIG. 3illustrates switching circuit306implemented within a particular pulse-generating circuit300, the switches described herein may be utilized within any other type of pulse-generating circuit, within any other pulsed-power tool, or within any other suitable application implementing high-voltage switches.

FIG. 4illustrates a schematic for an exemplary switching circuit for a downhole electrocrushing drilling system. Switching circuit401may be implemented with one or more solid state switches. For example, switching circuit401may be implemented with solid-state switch410and solid-state switch415. As illustrated inFIG. 4, solid-state switches410and415may be controlled by a control signal at terminal407. When activated, solid-state switches410and415pass an electrical current between terminals402and404.

As shown inFIG. 4, switching circuit401may be implemented with solid-state switches410and415coupled in series with each other between terminals402and404. Switching circuit401may also be implemented with any suitable number of solid-state switches coupled in series and/or in parallel between terminals402and404. For example, switching circuit401may include one, two, four, ten, or more solid-state switches coupled in series between terminals402and404. Moreover, one, two, four, ten, or more additional solid-state switches may be coupled in parallel with each respective solid-state switch that is coupled in series between terminals402and404.

Switching circuit401may be configured to handle high voltages and high currents present in a pulsed-power system for downhole electrocrushing drilling. For example, switching circuit401may be configured to operate with up to 40 kV or more across terminals402and404. Further, switching circuit401may be configured to pass up to 10 kA or more when activated. The voltage rating of switching circuit401may be based on the number of solid-state devices coupled in series between terminals402and404. For example, as shown inFIG. 4, solid-state switches410and415may be coupled in series with each other between terminals402and404. Accordingly, each of solid-state switch410and solid-state switch415may have a voltage rating of up to 20 kV or more to provide switching circuit401with a total voltage rating of up to 40 kV or more. The current rating of switching circuit401may be based on the number of solid-state devices coupled in parallel along the path between terminals402and404. Thus, each of solid-state switches410and415shown inFIG. 4may have a current rating of 10 kA to provide switching circuit401with a current rating of 10 kA. In other implementations of switching circuit401, one or more solid-state switches with current ratings of less than 10 kA may be placed in parallel to achieve a total current rating of 10 kA or more.

Switching circuit401may also include grading resistors. For example, switching circuit401may include resistor420and resistor425. Resistor420may be coupled in parallel with solid-state switch410between terminals402and403. Similarly, resistor425may be coupled in parallel to solid-state switch415between terminals403and404. Resistors420and425grade the voltage across terminals402and404such that the voltage across terminals402and404of switching circuit401is evenly divided across solid-state switch410and solid-state switch415. Switching circuit401may also include capacitor430coupled in parallel with solid-state switch410, and capacitor435coupled in parallel with solid-state switch415. Accordingly, capacitor430dampens any transient voltage spikes across solid-state switch410that occurs during operation of switching circuit401. Likewise, capacitor435dampens any transient voltage spikes across solid-state switch415that occurs during operation of switching circuit401. Such devices that dampen transient voltages may also be referred to as a protection circuits or as snubber circuits.

Solid-state switches410and415, and any other solid-state switches utilized in switching circuit401, may be implemented with any suitable type of solid-state switch. For example, the solid-state switches410and415implemented in switching circuit401may be silicon-carbide or gallium-arsenide switches. Such solid-state switches are capable of withstanding the high voltages and the high currents utilized in the pulsed-power system. Moreover, such solid-state switches are capable of withstanding harsh environment of a downhole pulsed-power system. The solid-state switches may operate over a wide temperature range (for example, from 10 to 150 degrees C. or from 10 to 200 degrees C.), and may physically withstand the vibration and mechanical shock resulting from the fracturing of rock during downhole electrocrushing drilling. Solid-state switches410and415may also be silicon switches, which may operate of a temperate range of 10 to 125 degrees C. and may physically withstand the vibration and mechanical shock resulting from the fracturing of rock during downhole electrocrushing drilling.

FIG. 5illustrates a side expanded view of certain components of an exemplary switching circuit for a downhole electrocrushing drilling system. As described above with reference toFIG. 4, switching circuit401may include solid-state switch410coupled in series with solid-state switch415. As shown inFIG. 5, solid-state switch410may be implemented in a disc shape with contact411located on a first side of the disc and contact412located on an opposing side of the disc. Similarly, solid-state switch415may be implemented in a disc shape with contact416located on a first side of the disc and contact417located on an opposing side of the disc. Contact411of solid-state switch410electrically couples to terminal402of switching circuit401, and contact417of solid-state switch415electrically couples to terminal404of switching circuit401. Further, solid-state switch410and solid-state switch415may be mechanically clamped together such that contact412of solid-state switch410electrically couples directly to contact416of solid state switch415. Accordingly, any parasitic resistance due to the coupling between solid-state switch410and solid-state switch415is minimized.

FIG. 6illustrates a top cross-sectional view of an exemplary pulsed-power tool for a downhole electrocrushing drilling system. Pulsed-power tool230includes outer pipe232that forms a section of an outer wall of a drill string (for example, drill string108illustrated inFIG. 1). As shown in the top cross-sectional view ofFIG. 6, solid-state switch410of switching circuit401is sized and shaped to fit within pulsed-power tool230, which as described above with reference toFIG. 2, may form part of BHA128. Although not expressly shown in the top cross-sectional view ofFIG. 6, other components of switching circuit401(for example, other solid-state switches, grading resistors, capacitors) may also be shaped to fit within pulsed-power tool230. For example, components of switching circuit401may fit within inner channel236of pulsed-power tool230.

The downhole electrocrushing drilling system in which pulsed-power tool230is incorporated may be configured to drill, for example, eight-and-a-half inch wellbores. The outer diameter of pulsed-power tool230may have a smaller outer diameter than the wellbore. As an example, for an eight-and-a-half inch wellbore, pulsed-power tool230may have a seven-and-a-half inch outer diameter. Further, pulsed-power tool230includes one or more fluid channels234within the circular cross-section of outer pipe232, through which drilling fluid122passes as the fluid is pumped down through a drill string (for example, drill string108) as described above with reference toFIG. 1. Accordingly, to fit within inner channel236of pulsed-power tool230, some embodiments of solid-state switch410may have a diameter of approximately five to six inches. In some embodiments, the components of switching circuit401such as solid-state switch410may have a smaller or larger size depending on the diameter of the wellbore, the corresponding outer diameter of pulsed-power tool230, and the size of inner channel236.

FIG. 7illustrates a schematic for an exemplary switching circuit for a downhole electrocrushing drilling system. Switching circuit700includes magnetic switch701coupled between terminals710and720. Magnetic switch701includes primary coil715, secondary coil735, and core716.

Primary coil715and core716operates as a magnetic switch by alternating between providing a small inductance value and a large inductance value depending on whether core716is saturated or not saturated. The inductance of magnetic switch701is represented by the following equation:

where μ0equals the permeability of free space (i.e., 8.85*10−12farads/meter), μ equals relative permeability, n equals the number of turns of primary coil715per meter, L equals the length of primary coil715in meters, and A equals the cross section area of the primary coil715in square meters. Core716includes a magnetic material that has a high relative permeability (for example, from two-thousand gausses up to ten-thousand gausses or more) when core716is not saturated, and a low relative permeability (for example, approximately one gauss) when core716is saturated. For example, core716may include a cobalt-iron alloy such as supermendur, which may include approximately forty-eight percent cobalt, approximately forty-eight percent iron, and approximately two percent vanadium by weight. The supermendur material maintains its high relative permeability across a wide range of temperatures (for example, from 10 to 150 degrees C. or from 10 to 200 degrees C.), and thus withstands the high temperatures of a downhole environment. As other examples, core716may include a ferrite material or Metglas, which includes a thin amorphous metal alloy ribbon which may be magnetized and demagnetized.

In operation, a switching cycle of magnetic switch701begins with core716in a non-saturated state. In the non-saturated state, magnetic switch701has a large inductance (for example, 50 to 400 mH). A voltage ramp is then be applied to terminal710. The current in the magnetic switch rises according to the following equation:

where dI/dt equals the rise in current over time, V is the voltage applied to magnetic switch701, and L is the inductance of magnetic switch701. As shown by Equation 2, the large inductance of magnetic switch701will cause the current through magnetic switch701to rise slowly over time. After a period of time, the voltage-time product (for example, the voltage across magnetic switch701multiplied by the time of the voltage ramp) increases to a value at which the magnetic material of core716saturates. When the magnetic material of core716saturates, the relatively permeability of core716decreases down to, for example, approximately one gauss. Thus, according to Equation 1 above, the inductance of magnetic switch701also decreases. For example, magnetic switch701may have an inductance that drops to approximately 5 to 50 uH when core716saturates. In accordance with Equation 2, the current through magnetic switch701begins to rise more quickly when the inductance of magnetic switch701decreases. Accordingly, when core716saturates, magnetic switch701operates as a closed switch, and the electrical energy at terminal710is rapidly transferred to terminal720.

As shown inFIG. 7, magnetic switch701includes secondary coil735in addition to primary coil715. Secondary coil735is coupled to reset-pulse generator730, which is configured to provide a reset signal to secondary coil735. For example, reset-pulse generator730may provide a pulsed reset waveform. Reset-pulse generator730may also be referred to more generally as a reset generator and may provide either a pulsed reset waveform or a constant current for a period of time through secondary coil735, either of which may cause core716to come out of saturation. When core716returns to a non-saturated state, the inductance of magnetic switch701returns to a high value, and thus operate as an open switch. AlthoughFIG. 7illustrates reset-pulse generator730coupled to secondary coil735to provide a reset pulse that pulls core716out of saturation, a reset pulse may be applied to magnetic switch701in any suitable manner. For example, a reset pulse may also be applied directly to primary coil715to pull core716out of saturation.

In some embodiments of a downhole electrocrushing drilling system, each of the switching circuits utilized in a pulse-generating circuit, such as pulse-generating circuit300illustrated inFIG. 3, may include magnetic switches such as magnetic switch701illustrated inFIG. 7. In such embodiments, the pulse-generating circuit may be free of solid-state switches. The magnetic switches described herein may withstand the harsh environment of the downhole drilling system. Thus, the use of magnetic switches may further improve the mean time to failure (MTTF) of pulse-generating circuits, and the time and costs of repairs may be reduced.

FIG. 8illustrates a top cross-sectional view of an exemplary pulsed-power tool for a downhole electrocrushing drilling system. Switching circuit700may serve, for example, as a switching circuit in a pulse-generating circuit similar to switching circuit306in pulse-generating circuit300depicted inFIG. 3. Switching circuit700may be shaped and sized to fit within the circular cross-section of pulsed-power tool230, which as described above with reference toFIG. 2, may form part of BHA128. For example, switching circuit700may be shaped and sized to fit within inner channel236. Moreover, switching circuit700may be enclosed within encapsulant810. Encapsulant810includes a thermally conductive material. For example, encapsulant810may include APTEK 2100-A/B, which is a two component, unfilled, electrically insulating urethane system for the potting and encapsulation of electronic components, and may have a thermal conductivity of 0.17 W/mK. Encapsulant810adjoins an outer wall of one or more fluid channels234. As described above with reference toFIG. 1, drilling fluid122passes through fluid channels234as drilling fluid is pumped down through a drill string. Encapsulant810transfers heat generated by switching circuit700to the drilling fluid that passes through fluid channels234. Thus, encapsulant810prevents switching circuit700from overheating to a temperature that degrades the relative permeability of core716(shown inFIG. 7) within switching circuit700when core716is in a non-saturated state.

FIG. 9illustrates a flow chart of exemplary method for drilling a wellbore.

Method900may begin and at step910a drill bit may be placed downhole in a wellbore. For example, drill bit114may be placed downhole in wellbore116as shown inFIG. 1.

At step920, electrical power may be provided to a pulse-generating circuit coupled to a first electrode and a second electrode of the drill bit. For example, as described above with reference toFIG. 3, pulse-generating circuit300may be implemented within pulsed-power tool230ofFIG. 2. And as described above with reference toFIG. 2, pulsed-power tool230may receive power from a power source on the surface, from a power source located downhole, or from a combination of a power source on the surface and a power source located downhole. The power may be provided to pulse-generating circuit400within pulse-power tool230at power source input301. As further shown inFIGS. 2 and 3, the pulse generating circuit may be coupled to a first electrode (such as electrode208) and a second electrode (such as ground ring250) of drill bit114.

At step930, a switch located downhole within the pulse-generating circuit may close to charge a capacitor that is electrically coupled between the first electrode and the second electrode. For example, switching circuit306may close to generate an electrical pulse and may be open between pulses. Switching circuit306may include a solid-state switch (such as solid-state switches410and415ofFIG. 4) or a magnetic switch (such as magnetic switch701ofFIG. 7). As described above with reference toFIG. 3, switching circuit306may switch to close the electrical path between power source310and the first winding311of transformer310. When switching circuit306is closed, electrical current flows through first winding311of transformer310. Second winding312of transformer310may be electromagnetically coupled to first winding311. Accordingly, transformer310generates a current through second winding312when switching circuit306is closed and current flows through first winding311. The current through second winding312charges capacitor314, thus increasing the voltage across capacitor314. Capacitor314of pulse-generating circuit300may be coupled between a first electrode (such as electrode208) and a second electrode (such as ground ring250) of drill bit114. Accordingly, as the voltage across capacitor314increases, the voltage across electrode208and ground ring250increases.

At step940, an electrical arc may be formed between the first electrode and the second electrode of the drill bit. And at step950, the capacitor may discharge via the electrical arc. For example, as the voltage across capacitor314increases during step930, the voltage across electrode208and ground ring250also increases. As described above with reference toFIGS. 1 and 2, when the voltage across electrode208and ground ring250becomes sufficiently large, an arc may form through a rock formation that is in contact with electrode208and ground ring250. The arc may provide a temporary electrical short between electrode208and ground ring250, and thus may discharge, at a high current level, the voltage built up across capacitor314.

At step960, the rock formation at an end of the wellbore may be fractured with the electrical arc. For example, as described above with reference toFIGS. 1 and 2, the arc greatly increases the temperature of the portion of the rock formation through which the arc flows as well as the surrounding formation and materials. The temperature is sufficiently high to vaporize any water or other fluids that may be touching or near the arc and may also vaporize part of the rock itself. The vaporization process creates a high-pressure gas which expands and, in turn, fractures the surrounding rock.

At step970, fractured rock may be removed from the end of the wellbore. For example, as described above with reference toFIG. 1, electrocrushing drilling fluid122may move the fractured rock away from the electrodes and uphole away from the bottom of wellbore116.

Subsequently, method900may end. Modifications, additions, or omissions may be made to method900without departing from the scope of the disclosure. For example, the order of the steps may be performed in a different manner than that described and some steps may be performed at the same time. Additionally, each individual step may include additional steps without departing from the scope of the present disclosure.

Embodiments herein may include:A. A downhole drilling system including a bottom-hole assembly having a pulse-generating circuit and a switching circuit within the pulse-generating circuit. The switching circuit includes a solid-state switch. The downhole drilling system also includes a drill bit having a first electrode and a second electrode electrically coupled to the pulse-generating circuit to receive a pulse from the pulse-generating circuit.B. A downhole drilling system including a bottom-hole assembly having a pulse-generating circuit and a switching circuit within the pulse-generating circuit. The switching circuit includes a magnetic switch. The downhole drilling system also includes a drill bit having a first electrode and a second electrode electrically coupled to the pulse-generating circuit to receive a pulse from the pulse-generating circuit.C. A method, including placing a drill bit downhole in a wellbore and providing electrical power to a pulse-generating circuit coupled to a first electrode and a second electrode of the drill bit. The method also includes closing a switch located downhole within the pulse-generating circuit to charge a capacitor that is electrically coupled between the first electrode and the second electrode, forming an electrical arc between the first electrode and the second electrode of the drill bit, and discharging the capacitor via the electrical arc. Further, the method includes fracturing a rock formation at an end of the wellbore with the electrical arc and removing fractured rock from the end of the wellbore.

Each of embodiments A and B may have one or more of the following additional elements in any combination:

Element 1: wherein the solid-state switch is a silicon-carbide switch. Element 2: wherein the solid-state switch is one of a gallium-arsenide switch and a silicon switch. Element 3: wherein the solid-state switch is located within a circular cross-section of the bottom-hole assembly. Element 4: wherein the switching circuit includes a plurality of solid-state switches coupled together in parallel. Element 5: wherein the switching circuit includes a plurality of solid-state switches coupled together in series. Element 6: wherein the switching circuit further includes an additional solid-state switch coupled in parallel with each respective solid-state switch of the plurality of solid-state switches coupled together in series. Element 7: wherein the downhole drilling system further includes a plurality of grading resistors, each of the plurality of grading resistors coupled in parallel to a corresponding solid-state switch of the plurality of solid-state switches. Element 8: wherein the downhole drilling system further includes a plurality of capacitors, each of the plurality of capacitors coupled in parallel to a corresponding solid-state switch of the plurality of solid-state switches. Element 9: wherein the drill bit is one of an electrocrushing drill bit and an electrohydraulic drill bit. Element 10: wherein the magnetic switch includes a primary coil and a supermendur core. Element 11: wherein the magnetic switch includes a primary coil and a Metglas core. Element 12: wherein the pulse-generating circuit includes a plurality of switching circuits, each of the plurality of switching circuits including a magnetic switch. Element 13: wherein the downhole drilling system further includes a reset generator coupled to the magnetic switch. Element 14: wherein the magnetic switch further includes a secondary coil coupled to receive a constant current from the reset generator to transition the core from a saturated state to a non-saturated state. Element 15: wherein the magnetic switch further includes a secondary coil coupled to receive a reset pulse from the reset generator to transition the core from a saturated state to a non-saturated state. Element 16: wherein the magnetic switch is located within a circular cross-section of the bottom-hole assembly. Element 17: wherein the downhole drilling system further includes a thermally conductive encapsulant surrounding the magnetic switch. Element 18: wherein the thermally conductive encapsulant adjoins the outer wall of a drilling fluid channel within the circular cross-section of the bottom-hole assembly. Element 19: wherein the drill bit is integrated within the bottom-hole assembly. Element 20: wherein a reset pulse is applied to a secondary coil of the magnetic switch to transition the core from a saturated state to a non-saturated state. Element 21: wherein a constant current is applied to a secondary coil of the magnetic switch to transition the core from a saturated state to a non-saturated state.

Although the present disclosure has been described with several embodiments, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompasses such various changes and modifications as falling within the scope of the appended claims.