Patent Publication Number: US-2021180407-A1

Title: High-power capacitor for downhole electrocrushing drilling

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to downhole electrocrushing drilling and, more particularly, to high power capacitors suitable for use 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 adjacent rock to fracture. The fractured rock is carried away from the bit by drilling fluid and the bit advances downhole. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure and its features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates an elevation view of an exemplary downhole electrocrushing drilling system used in a wellbore environment; 
         FIG. 2  illustrates exemplary components of a bottom hole assembly for a downhole electrocrushing drilling system; 
         FIG. 3  illustrates a top cross-sectional view of an exemplary pulsed-power tool for a downhole electrocrushing drilling system; 
         FIG. 4  illustrates a schematic for an exemplary pulse-generating circuit for a downhole electrocrushing drilling system; 
         FIG. 5A  illustrates a side-facing view of components of an exemplary high-voltage, high-power capacitor for a downhole electrocrushing drilling system; 
         FIG. 5B  illustrates an exploded front-facing view of components of an exemplary high-voltage, high-power capacitor for a downhole electrocrushing drilling system; 
         FIG. 5C  illustrates a composite view of components of the exemplary high-voltage, high-power capacitor, shown in part in  FIGS. 5A and 5B , for a downhole electrocrushing drilling system; 
         FIG. 6  illustrates a flow chart of an example method for manufacturing a high-voltage, high-power capacitor; 
         FIG. 7  illustrates a schematic diagram of an example capacitor array for a fuse-protected capacitor in a downhole electrocrushing drilling system; 
         FIG. 8A  illustrates a cut out view of components of an example fuse for a fuse-protected capacitor in a downhole electrocrushing drilling system; 
         FIG. 8B  illustrates a cross sectional view of an example fuse for a fuse-protected capacitor in a downhole electrocrushing drilling system; 
         FIG. 8C  illustrates a cross sectional view of an example fuse, with an intermediate barrier, for a fuse-protected capacitor in a downhole electrocrushing drilling system; and 
         FIG. 9  illustrates a flow chart of exemplary method for drilling a wellbore. 
     
    
    
     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 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 capacitors that utilize a dielectric composite including nanoparticles in a polymer matrix. The dielectric composite provides a high dielectric constant and is capable of withstanding the high voltages and the harsh environment of a downhole pulsed-power system. The dielectric composite maintains a stable dielectric constant over a wide temperature range (for example, from 10 to 150 degrees Centigrade or from 10 to 200 degrees Centigrade), and physically withstands the vibration and mechanical shock resulting from the fracturing of rock during downhole electrocrushing drilling. Moreover, the capacitors may include a plurality of fuse protected branches coupled in parallel to each other, which may allow the pulse-generating circuit to continue to operate in the event that one or more branches of the capacitors fail. 
     There are numerous ways in which a dielectric composite may be implemented in a capacitor for a downhole electrocrushing pulsed-power system. Thus, embodiments of the present disclosure and its advantages are best understood by referring to  FIGS. 1 through 8C , where like numbers are used to indicate like and corresponding parts. 
       FIG. 1  is an elevation view of an exemplary electrocrushing drilling system used to form a wellbore in a subterranean formation. Although  FIG. 1  shows 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 wellbore  116  is shown as being a generally vertical wellbore, wellbore  116  may be any orientation including generally horizontal, multilateral, or directional. 
     Drilling system  100  includes drilling platform  102  that supports derrick  104  having traveling block  106  for raising and lowering drill string  108 . Drilling system  100  also includes pump  124 , which circulates electrocrushing drilling fluid  122  through a feed pipe to drill string  110 , which in turn conveys electrocrushing drilling fluid  122  downhole through interior channels of drill string  108  and through one or more orifices in electrocrushing drill bit  114 . Electrocrushing drilling fluid  122  then circulates back to the surface via annulus  126  formed between drill string  108  and the sidewalls of wellbore  116 . Fractured portions of the formation are carried to the surface by electrocrushing drilling fluid  122  to remove those fractured portions from wellbore  116 . 
     Electrocrushing drill bit  114  is attached to the distal end of drill string  108 . In some embodiments, power to electrocrushing drill bit  114  may be supplied from the surface. For example, generator  140  may generate electrical power and provide that power to power-conditioning unit  142 . Power-conditioning unit  142  may then transmit electrical energy downhole via surface cable  143  and a sub-surface cable (not expressly shown in  FIG. 1 ) contained within drill string  108  or attached to the side of drill string  108 . A pulse-generating circuit within bottom-hole assembly (BHA)  128  may receive the electrical energy from power-conditioning unit  142 , and may generate high-energy pulses to drive electrocrushing drill bit  114 . The pulse-generating circuit may include one or more capacitors and/or fuse-protected capacitors as described in further detail below with reference to  FIGS. 3-8C . 
     The pulse-generating circuit within BHA  128  may be utilized to repeatedly apply a high electric potential, for example up to or exceeding 150 kV, across the electrodes of electrocrushing drill bit  114 . Each application of electric potential may be referred to as a pulse. When the electric potential across the electrodes of electrocrushing drill bit  114  is increased enough during a pulse to generate a sufficiently high electric field, an electrical arc forms through a rock formation at the end of wellbore  116 . The arc temporarily forms an electrical coupling between the electrodes of electrocrushing drill bit  114 , allowing electric current to flow through the arc inside a portion of the rock formation at an end (such as the bottom) of wellbore  116 . 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 fluid  122 , which moves the fractured rock away from the electrodes and uphole. 
     Wellbore  116 , which penetrates various subterranean rock formations  118 , is created as electrocrushing drill bit  114  repeatedly fractures the rock formation and electrocrushing drilling fluid  122  moves the fractured rock uphole. Wellbore  116  may 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, wellbore  116  may be any hole drilled into a subterranean formation or series of subterranean formations for the purpose of geothermal power generation. 
     Although drilling system  100  is described herein as utilizing electrocrushing drill bit  114 , drilling system  100  may also utilize an electrohydraulic drill bit. An electrohydraulic drill bit may have multiple electrodes similar to electrocrushing drill bit  114 . 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 wellbore  116 . 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 wellbore  116 . When the shock wave hits and bounces off of the rock at the bottom of wellbore  116 , the rock fractures. Accordingly, drilling system  100  may utilize pulsed-power technology with an electrohydraulic drill bit to drill wellbore  116  in subterranean formation  118  in a similar manner as with electrocrushing drill bit  114 . 
       FIG. 2  illustrates exemplary components of a bottom hole assembly for downhole electrocrushing drilling system  100 . Bottom-hole assembly (BHA)  128  may include pulsed-power tool  230 . BHA  128  may also include electrocrushing drill bit  114 . For the purposes of the present disclosure, electrocrushing drill bit  114  may be referred to as being integrated within BHA  128 , or may be referred to as a separate component that is coupled to BHA  128 . 
     Pulsed-power tool  230  may be coupled to provide pulsed power to electrocrushing drill bit  114 . Pulsed-power tool  230  receives electrical energy from a power source via cable  220 . For example, pulsed-power tool  230  may receive power via cable  220  from a power source on the surface as described above with reference to  FIG. 1 , or from a power source located downhole such as a generator powered by a mud turbine. Pulsed-power tool  230  may also receive power via a combination of a power source on the surface and a power source located downhole. Pulsed-power tool  230  converts the electrical energy received from the power source into high-power electrical pulses, and may apply those high-power pulses across electrodes of electrocrushing drill bit  114 . For the purposes of the present disclosure, ground ring  250  may also be referred to generally as an electrode or more specifically as a ground electrode. In one example, pulsed-power tool  230  may apply the high-power pulses across electrode  208  and ground ring  250  of electrocrushing drill bit  114 . Pulsed-power tool  230  may also apply high-power pulses across electrode  210  and ground ring  250  in a similar manner as described herein for electrode  208  and ground ring  250 . 
     Pulsed-power tool  230  may include a pulse-generating circuit as described below with reference to  FIG. 3 . Such a pulse-generating circuit may include high-power capacitors, which are described below with reference to  FIGS. 5A-6 , and which may include fuse-protection as described below with reference to  FIGS. 7-8C . 
     Referring to  FIG. 1  and  FIG. 2 , electrocrushing drilling fluid  122  may exit drill string  108  via openings  209  surrounding each electrode  208  and each electrode  210 . The flow of electrocrushing drill fluid  122  out of openings  209  allows electrodes  208  and  210  to be insulated by the electrocrushing drilling fluid. In some embodiments, electrocrushing drill bit  114  may include a solid insulator (not expressly shown in  FIG. 1 or 2 ) surrounding electrodes  208  and  210  and one or more orifices (not expressly shown in  FIG. 1 or 2 ) on the face of electrocrushing drill bit  114  through which electrocrushing drilling fluid  122  may exit drill string  108 . 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 fluid  122  may 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 fluid  122  pressure or to direct electrocrushing drilling fluid may be included for some uses. 
     Drilling fluid  122  is typically circulated through drilling system  100  at a flow rate sufficient to remove fractured rock from the vicinity of electrocrushing drill bit  114  in sufficient quantities within a sufficient time to allow the drilling operation to proceed downhole at least at a set rate. In addition, electrocrushing drilling fluid  122  may be under sufficient pressure at a location in wellbore  116 , particularly a location near a hydrocarbon, gas, water, or other deposit, to prevent a blowout. 
     Electrodes  208  and  210  may be at least 0.4 inches apart from ground ring  250  at 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 system  100  experiences vaporization bubbles in electrocrushing drilling fluid  122  near electrocrushing drill bit  114 , the vaporization bubbles may have deleterious effects. For instance, vaporization bubbles near electrodes  208  or  210  may impede formation of the arc in the rock. Electrocrushing drilling fluids  122  may be circulated at a flow rate also sufficient to remove vaporization bubbles from the vicinity of electrocrushing drill bit  114 . 
     In addition, electrocrushing drill bit  114  may include ground ring  250 , shown in part in  FIG. 2 . Although not all electrocrushing drill bits  114  may have ground ring  250 , if it is present, it may contain passages  260  to permit the flow of electrocrushing drilling fluid  122  along with any fractured rock or bubbles away from electrodes  208  and  210  and uphole. 
       FIG. 3  illustrates a top cross-sectional view of an exemplary pulsed-power tool for a downhole electrocrushing drilling system. Pulsed-power tool  230  includes outer pipe  232 , which may form a section of an outer wall of a drill string (for example, drill string  108  illustrated in  FIG. 1 ). Pulsed-power tool  230  also includes capacitor  241  that provides a high-voltage capacitance across terminals  251  and  252 . 
     Capacitor  241  may be implemented as a capacitor in a pulse-generating circuit, such as capacitor  414  of pulse-generating circuit  400  described below with reference to  FIG. 4 . In such embodiments, terminal  251  may be coupled to electrode  208 , terminal  252  may be coupled to ground ring  250 , and capacitor  241  may help control the voltage that is applied across electrode  208  and ground ring  250  during electrocrushing drilling in a similar manner as described below for capacitor  414  of  FIG. 4 . Further, capacitor  241  may be a high-voltage capacitor that is rated for use at voltages up to, for example, 150 kV or more. The dielectric materials forming capacitors, such as capacitor  241 , are described in greater detail below with reference to  FIGS. 5A-5C . 
     Capacitor  241  is shaped to fit within the circular cross-section of pulsed-power tool  230 . Capacitor  241  is also shaped such that pulsed-power tool  230  may include fluid channels  234 . For example, as shown in  FIG. 3 , capacitor  241  may fit within capacitor housing  240 . The outer wall of capacitor housing  240  includes curved portions that align with the inner wall of pipe  232 . The outer wall of capacitor housing  240  also includes flattened portions. Accordingly, capacitor  241  may be located adjacent to at least one or more fluid channels within the circular cross-section of pipe  232  of downhole pulsed-power tool  230 . Drilling fluid  122  may pass through fluid channels  234  as drilling fluid is pumped down through a drill string as described above with reference to  FIG. 1 . 
       FIG. 4  illustrates a schematic for an exemplary pulse-generating circuit for a downhole electrocrushing drilling system. Pulse-generating circuit  400  includes power source input  401 , including input nodes  402  and  403 , and capacitor  404  coupled between input nodes  402  and  403 . Pulse-generating circuit  400  also includes switch  406 , transformer  410 , and capacitor  414 . 
     Pulse-generating circuit  400  may be implemented within pulsed-power tool  230  of  FIG. 2 . And as described above with reference to  FIG. 2 , pulsed-power tool  230  may 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 received by pulse-generating circuit  400  at power source input  401 . Switch  406  is coupled to power source input  401  and includes any suitable device to open and close the electrical path between power source input  401  and the first winding  411  of transformer  410 . For example, switch  406  may include a mechanical switch, a semiconductor switch, a magnetic switch, or any other type of switch suitable to open and close the electrical path between power source input  401  and first winding  411  of transformer  410 . Switch  406  is open between pulses and closes at the beginning of a pulse cycle. When switch  406  closes, electrical current flows through first winding  411  of transformer  410 . Second winding  412  of transformer  410  is electromagnetically coupled to first winding  411 . Accordingly, when switch  406  closes and an electrical current flows through first winding  411 , a current also flows through second winding  412 . The current through second winding  412  charges capacitor  414 , thus increasing the voltage across capacitor  414 . 
     Electrode  208  and ground ring  250  of a drill bit (for example, electrocrushing drill bit  114  illustrated in  FIGS. 1 and 2 ) may be coupled to opposing terminals of capacitor  414 . As the voltage across capacitor  414  increases, the voltage across electrode  208  and ground ring  250  also increases. Moreover, the rate at which the voltage across electrode  208  and ground ring  250  increases is a function of the capacitance value of capacitor  414 . And, as described above with reference to  FIG. 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 electrode  208  and ground ring  250 . The arc provides a temporary electrical short between electrode  208  and ground ring  250 , and thus discharges, at a high current level, the voltage built up across capacitor  414 . As described above with reference to  FIG. 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. 
     Although  FIG. 4  illustrates 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 across electrode  208  and ground ring  250 . Moreover, although  FIG. 4  illustrates capacitor  404  and capacitor  414  implemented within a particular pulse-generating circuit  400 , the capacitors 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 capacitors. As described below with reference to  FIGS. 5A-5C , the capacitors described herein may utilize dielectric materials that withstand high voltages (for example, up to 30 kV), and that withstand high temperatures (for example, up to 150 degrees Centigrade). Thus, the capacitors described herein may be suitable for use within other downhole pulsed-power applications that required a stable capacitance value at high voltages and across a large temperature range. 
     Further, although capacitor  404  and capacitor  414  are each illustrated in  FIG. 4  as a single capacitor, such capacitors may be implemented by multiple capacitors coupled in series and/or multiple capacitors coupled in parallel with each other. For example, as described below with reference to  FIG. 7 , multiple individual capacitors may be placed in parallel to form a single capacitor array with a capacitance approximately equal to the sum of the multiple individual capacitors. Further, multiple capacitors may be coupled in series to increase the total voltage rating of the capacitors. For example, five capacitors that each withstand a voltage up to 30 kV may be placed in series to provide a capacitance that withstands up to 150 kV. 
       FIG. 5A  illustrates a side-facing view of components of an exemplary high-voltage, high-power capacitor for a downhole electrocrushing drilling system. Capacitor  500  may be implemented, for example, in a pulse-generating circuit such as pulse-generating circuit  400  described above with reference to  FIG. 4 . As shown in  FIG. 5A , capacitor  500  includes a plurality of dielectric sheets  510 , a plurality of spacers  512  disposed on each side of each dielectric sheet  510 , and a plurality of electrode sheets  514 . The plurality of electrode sheets  514  is interleaved with the plurality of dielectric sheets  510 . 
     Further, conductor  502  couples every other electrode sheet  514  (for example, electrode sheets  514   a  and  514   c ) to terminal  503 , and conductor  504  couples the other electrode sheets  514  (for example, electrode sheets  514   b  and  514   d ) to terminal  505 . The interleaving of dielectric sheets  510  and electrode sheets  514  provides a summation of dielectric sheet capacitance between terminals  503  and  505  of capacitor  500  when every other electrode sheet  514  is coupled together. For example, coupling electrode sheet  514   a  and electrode sheet  514   c  together provides a summation of (i) the capacitance between electrode sheet  514   a  and electrode sheet  514   b , and (ii) the capacitance between electrode sheet  514   b  and electrode sheet  514   c . Electrode sheets  514  may also be coupled in a manner that places the capacitances provided between the respective electrode sheets in series, thus increasing the voltage capability of capacitor  500 . 
     The capacitance of capacitor  500  depends on the dielectric constant of dielectric sheets  510 . The dielectric constant of dielectric sheets  510  indicates the ability of the sheets to store electrical energy when exposed to an electric field. The dielectric constant of dielectric sheets  510  may be at least 3, at least 10, or at least 20, from 0.1 Hz to 1.0 MHz frequency, and at temperatures experienced downhole and during use of electrocrushing drill bit  114 , such as temperatures from 10 degrees Centigrade up to 200 degrees Centigrade. For example, the dielectric constant of dielectric sheets  510  may be at least 3, at least 10, or at least 20, at 0.1 kHz frequency, and at 150 degrees Centigrade. As another example, the dielectric constant of dielectric sheets  510  may be at least 3, at least 10, or at least 20, at 100 kHz frequency, and at 200 degrees Centigrade. As yet another example, the dielectric constant of dielectric sheets  510  may be at least 3, at least 10, or at least 20, at 1 MHz frequency, and at 150 degrees Centigrade. Expressed alternatively, the dielectric constant may provide capacitor  500  with a storage density of, for example, at least 0.05 Joules per cubic inch (J/in 3 ) or at least 0.5 J/in 3 . The high storage density allows capacitor  500  to provide a large capacitance in a small amount of space. Thus, capacitor  500  may be utilized to provide a large capacitance in a downhole environment where space is limited, such as in downhole pulsed-power tool  230  depicted in  FIGS. 2 and 3 . 
     The capacitance or other discharge properties of capacitor  500  may also depend upon the dielectric strength of dielectric sheets  510 . The dielectric strength indicates the electric field or voltage to which dielectric sheet  510  may be exposed before experiencing electrical breakdown. The dielectric strength of dielectric sheets  510  may be, for example, at least 300 kV/cm, at least 330 kV/cm, at least 350 kV/cm, or at least 400 kV/cm at 10 microseconds rise time. Expressed alternatively, the dielectric strength may allow capacitor  500  to resist failure at charges of, for example, at least 20 kV, at least 25 kV, or at least 30 kV. Expressed still another way, the dielectric strength may be sufficient to allow capacitor  500  to be used for at least 10 7  or at least 10 9  charge/discharge cycles in a pulse-generating circuit, similar to capacitor  414  depicted in pulse-generating circuit  400  of  FIG. 4 . 
     It is also useful for dielectric sheets  510  to be sufficiently temperature-resistant to not undergo degradation or experience other temperature-related negative effects at temperatures experienced downhole and during use of electrocrushing drill bit  114 . For instance, dielectric sheets may have a stable dielectric constant (varying less than 1%) between 10 and 150 degrees Centigrade, or between 10 and 200 degrees Centigrade, at a voltage of at least 30 kV. Accordingly, capacitor  500  may maintain a stable capacitance value at high voltage over the wide range of temperatures experienced in the downhole environment. 
     It is further useful for dielectric sheets  510  to be elastic, particularly as compared to materials that on their own possess a sufficient dielectric constant, such as the ferroelectric materials described below. For example, dielectric sheets  510  may have a Young&#39;s modulus of 0.05 GPa or less, or 0.01 GPa or less. Express another way dielectric sheets  510  may have sufficient material strength to allow capacitor  500  to withstand pressures of at least range of at least 10,000 pounds-per-square-inch (psi), at least 15,000 psi, or between 10,000 and 15,000 psi. Moreover, dielectric sheets  510  may help capacitor  500  to withstand the physical shock and vibrations that result from the repeated fracturing of subterranean rock during the drilling of a wellbore with the electrocrushing drill bit. 
     Although a single material that exhibits sufficient dielectric constant, dielectric strength, elasticity, and temperature-resistance may form dielectric sheets  510 , single materials with this appropriate combination of properties are not common. As a result, dielectric sheets may also be formed from composite materials in which one component contributes positively to one property, and another component contributes positively to another property. For instance, a simple composite material may include a matrix that provides the desired physical properties (for example, strength and elasticity) and homogeneously dispersed particles to increase the dielectric constant. This increase in dielectric constant is proportional to the stored energy of the capacitor and inversely proportional to size of the capacitor for a given energy. As an example, a capacitor produced for a given energy with a composite material with a dielectric constant of 15 may be five times smaller than a capacitor produced with the matrix material alone with a typical dielectric constant of 3. Relative to the application, a single material such as the matrix alone may not be practical due to energy requirements and size limitations. More complex composite materials may include a plurality of components to contribute different positive properties. The relative proportions or total amounts of components may be determined by the minimal and maximal proportions or total amounts that provide a dielectric constant, dielectric strength, elasticity, or temperature-resistance for the composite material as a whole. 
     In addition, the physical form of the composite material and its components may be determined by the nature of the components as well as the dielectric constant, dielectric strength, elasticity, or temperature-resistance for the composite material as a whole. Using the example above, particles embedded in a matrix may help retain the particles if they are otherwise chemically reactive, melt at downhole temperatures, or are otherwise subject to loss from the composite material. The components of the composite material may be present in other arrangements that are physically distinct on a macroscopic level, such as sheets. Using a different example, the composite material may include a blend of components such that, unlike particles in a matrix, they are not physically distinct on a macroscopic level. Such a composite material may exhibit a higher dielectric strength because of its uniformity. 
     Silicone and carbon polymers have a high elasticity and are thermally stable at temperatures experienced downhole and during use of electrocrushing drill bit  114 . These materials are also relatively cheap and safe. Thus, they may be used as a component of the composite material. Silicone and carbon polymers are also well suited to forming certain macroscopic physical structures, such as a matrix or sheets. 
     Silicone polymers used herein may have a backbone formed of repeating silicon-oxygen (S—O) monomer units. These silicon polymers may also have substituent (R) groups. In general, the silicone polymers may have the structural formula: 
     
       
         
         
             
             
         
       
     
     n may be any integer. For instance, n may be at least 10, at least 50, at least 100, at least 500, at least 1000, or at least 5000. n may be 10,000 or less, 5,000 or less, 1,000 or less, 500 or less, 100 or less, or 50 or less. n may also be between any of these endpoints. 
     One or both of R 1  and R 2  may be absent. If both present, R 1  and R 2  may be the same substituent or different substituents. R 1  or R 2  may be H, a carbon (C) containing group such as an aryl, or alkyl group, such as a single or branched polymer, a nitrogen (N)-containing group, such as an amine or imine, an oxygen (O)-containing group, such as a hydroxyl group, a halogen-containing group, or a Si-containing group, such as a siloxane or a further silicone to form a branched polymer. 
     In addition, the silicone polymers may contain the same repeating monomer unit, or they may contain at least two or a plurality of different monomer units which may repeat in sequences or randomly. 
     Carbon polymers used herein may have a backbone formed of repeating carbon monomer units with the general structural formula: 
     
       
         
         
             
             
         
       
     
     q may be any integer. For instance, q may be at least 10, at least 50, at least 100, at least 500, at least 1000, or at least 5000. q may be 10,000 or less, 5,000 or less, 1,000 or less, 500 or less, 100 or less, or 50 or less. q may also be between any of these endpoints. 
     One or both of R 3  and R 4  may be absent. If both present, R 1  and R 2  may be the same substituent or different substituents. R 3  and R 4  may be H, a C containing group such as an aryl, or alkyl group, such as a single or branched polymer, an N-containing group, such as an amine or imine, an O-containing group, such as a hydroxyl group, a halogen-containing group, or a Si-containing group, such as a silicone or siloxane. In addition, the carbon polymers may contain the same repeating monomer unit, or they may contain at least two or a plurality of different monomer units which may repeat in sequences or randomly. 
     Carbon polymers used herein may also have a backbone formed of repeating carbon-oxygen (C—O) monomer units. These silicon polymers may also have substituent (R) groups. In general, the silicone polymers may have the structural formula: 
     
       
         
         
             
             
         
       
     
     s may be any integer. For instance, s may be at least 10, at least 50, at least 100, at least 500, at least 1000, or at least 5000. s may be 10,000 or less, 5,000 or less, 1,000 or less, 500 or less, 100 or less, or 50 or less. s may also be between any of these endpoints. 
     One or both of R 5  and R 6  may be absent. If both present, R R 5  and R 6  may be the same substituent or different substituents. R 5  and R 6  may be H, a C containing group such as an aryl, or alkyl group, such as a single or branched polymer, an N-containing group, such as an amine or imine, an O-containing group, such as a hydroxyl group, a halogen-containing group, or a Si-containing group, such as a siloxane or silicone. 
     In addition, the carbon polymers may contain the same repeating monomer unit, or they may contain at least two or a plurality of different monomer units which may repeat in sequences or randomly. 
     Hybrid polymers containing a mixture of any of the silicone, carbon, or carbon-oxygen monomers may also be used. These hybrid polymers may repeat different monomer units in sequences or randomly. 
     Furthermore, the composite material may contain a blend of one or more polymers described above to form a homogenous or heterogeneous polymer matrix. 
     Silicone polymers, silicone-containing hybrid polymers, or composite materials with blends of polymers including silicone polymers or silicone containing—hybrid polymers may provide greater elasticity than pure carbon polymers. 
     Polymers as described above may form a polymer matrix simply upon setting from a liquid state, or through crosslinking. Crosslinking may occur due to chemical reaction of separate polymers with one another, such as via a condensation or addition reaction, or it may be facilitated by the use of a chemical crosslinking agent, which may chemically react with the polymers or catalyze their chemical reaction, for example by producing free radicals. Crosslinking may also be facilitated by factors that cause the polymers to chemically react with one another or with a crosslinking agent; such factors may include electromagnetic radiation, such as light, ultraviolet light, or infrared radiation. Such factors may also include heat. 
     It is further possible for a combination of monomers to both form polymers and the polymer matrix simultaneously while undergoing a polymerization reaction. 
     Other components of the composite material may be present while the polymers are forming the polymer matrix, even when they polymer matrix is formed at the same time as polymerization. This may allow the other components of the composite material to be more homogenously distributed within the composite material. 
     Substituent (R) groups for the silicone and carbon polymers may help increase the dielectric constant or dielectric strength of these components, but many silicone and carbon polymers have a dielectric constant of less than 6 and are thus not suitable for use alone in dielectric sheets  510 . Another component with a high dielectric constant may be added to increase the dielectric constant of the composite material. In addition, the dielectric strength of some silicone and carbon polymers may be too low for dielectric sheets  510 . In that case, another component may be added to increase the dialectic strength of the composite material. A single component may be added to increase both dielectric constant and dielectric strength, or a plurality of components may be used. In addition, more than one component may contribute a given property. For instance, two components may be used even when both contribute to dielectric strength. 
     Suitable components for increasing the dielectric constant of the composite material include a component with a dielectric constant of at least 30, at least 50, at least 100, at least 500, at least 1000, at least 5000, or at least 10,000, at 0.1 Hz to 1.0 MHz frequency, and at temperatures up to 150 degrees Centigrade. Such suitable components include a ferroelectric component, such as barium titanate, strontium titanate, barium neodymium titanate, barium strontium titanate, magnesium zirconate, titanium dioxide, calcium titanate, barium magnesium titanate, lead zirconium titanium, and any combinations thereof. 
     When crystalline components are used, they may have a particular crystal structure, which may affect one or more of the properties of dielectric sheets  510 . The crystal structure within the particles allows for the electronic polarization and increased energy storage capability, observed as a high dielectric constant. The crystal structure and polarization behavior of the particles may also depend on, for example, particle size and temperature. A change in crystal structure may result in a non-linear temperature coefficient of capacitance associated with a changing dielectric constant. In the case of barium titanate, larger particles (for example, greater than 1.0 micron) may experience a phase transition from tetragonal to cubic crystal structure around 125 degrees Centigrade. Approaching this transition, the dielectric constant of the crystal component can change by a factor of five to ten, which results in a change in the composite material&#39;s dielectric strength. Alternatively, barium titanate nanoparticles (for example, less than 0.5 microns) have a cubic structure throughout the temperature range experienced by the composite material during use (for example, from 10 degrees to 150 degrees Centigrade), and thus may experience no significant change in dielectric constant. 
     When the composite includes at least one component in the form of particles, it may be in the form microparticles (for example, particles with an average diameter of 1 μm to 999 μm), or nanoparticles (for example, particles with an average diameter of 1 nm to 999 nm). Nanoparticles may allow more uniform dispersal within the composite material. For instance, they allow more uniform dispersal within a polymer matrix as described above. In addition to the stable dielectric constant versus temperature described above, the nanoparticles do not represent point defects that can have a detrimental impact on the mechanical and electrical properties of the composite material. Microparticles can represent asperities in a composite material that initiate mechanical failure such as low tensile strength. Similarly, microparticles can represent voltage enhancement sites in composites which can initiate ionization and low dielectric breakdown strength. Failure initiates and propagates along the large boundary layer between the matrix and surface of the microparticles. In the case of nanoparticles, boundary layer distances are minimized and the composite material behaves closer to a single phase material. The adverse impact on mechanical and electrical properties of the matrix is minimized in the composite through the use of nanoparticles. 
     In one example, dielectric sheet  510  may be formed from a composite material including a silicone polymer matrix with between 10% and 60% by volume embedded ferroelectric nanoparticles, such as barium titanate nanoparticles or strontium titanate nanoparticles. The example ferroelectric component nanoparticles may have an average diameter of 20 nm to 150 nm, from 40 nm to 60 nm, or of 50 nm. 
     Dielectric sheets  510  may be shaped to isolate electrode sheets  514   a  and  514   c  coupled to terminal  503  from electrode sheets  514   b  and  514   d  coupled to terminal  505 . For example, as shown in both  FIG. 5A  and  FIG. 5B , the ends of dielectric sheet  510  extend past the ends of electrode sheets  514 . Accordingly, dielectric sheets  510  prevent electrical arcs from forming between electrode sheets  514   a  and  514   c  coupled to terminal  503  and electrode sheets  514   b  and  514   d  coupled to terminal  505  when a large voltage potential (for example, up to 30 kV) is applied across terminals  503  and  505  of capacitor  500 . 
     Capacitor  500  also utilizes dielectric encapsulant  522  to insulate electrode sheets  514  from each other. For example, capacitor  500  includes spacers  512  located on each of dielectric sheets  510 . Spacers  512  may include material that is placed on dielectric sheet  510  but is otherwise distinct from dielectric sheet  510 . Spacers  512  may also be an integral part of dielectric sheet  510  including, but not limited to, protrusions of a textured surface of dielectric sheet  510 . In addition, spacers  512  may include a combination of material distinct from dielectric sheet  510  and material that is an integral part of dielectric sheet  510 . Dielectric encapsulant  522  fills the space provided by spacers  512  between electrode sheets  514  and dielectric sheets  510 . Dielectric encapsulant  522  also surrounds each instance of electrode sheet  514  and dielectric sheet  510 . Dielectric encapsulant  522  may include a fluid dielectric material that surrounds the edges of each electrode sheet  514  and each dielectric sheet  510 , as well as fill the spaces between each electrode sheet  514  and dielectric sheet  510 . 
     Dielectric encapsulant  522  may include the same or similar polymer/nanoparticle composition as dielectric sheets  510 . In some embodiments, dielectric encapsulant  522  may have a dielectric constant similar to the dielectric sheets  510  but may be modified with a conductive or semi-conductive particulate filler, which may cause dielectric encapsulant  522  to have resistance that is lower than the resistance of dielectric sheets  510 . Accordingly, dielectric encapsulant  522  may reduce the electric field between two adjacent electrode sheets  514 , and thus protect capacitor  500  from failure when a large voltage potential (for example, up to 30 kV) is placed across terminals  503  and  505 . The lower resistance of the dielectric encapsulant  522  increases the charge dissipation and reduces the electrical field enhancement at the electrode ends such that dielectric encapsulant  522  may protect capacitor  500  against a voltage breakdown of dielectric sheets  510  near the edges of electrode sheets  514 . Further features of capacitor  500  that may protect capacitor  500  from failure when a large voltage potential (for example, up to 30 kV) is placed across terminals  503  and  505  are discussed below with reference to  FIG. 5B . 
       FIG. 5B  illustrates an exploded front-facing view of components of an exemplary high-power, high-voltage, capacitor for a downhole electrocrushing drilling system. The exploded view in  FIG. 5B  shows an example instance of electrode sheet  514   b . Electrode sheet  514   b  may be formed of any suitable conductive material. For example, electrode sheet  514   b  may be formed of copper, aluminum, steel, or any other suitable electrically conductive metal or metal compound. 
     As shown in the exploded front-facing view of  FIG. 5B , electrode sheet  514   b  is smaller than dielectric sheet  510 , and is located such that the edges of dielectric sheet  510  extend beyond the edges of electrode sheet  514   b . As such, dielectric sheet  510  isolates electrode sheet  514   b  on one side of dielectric sheet  510  from an opposing instance of electrode sheet  514   b  on the other side of dielectric sheet  510 . 
     Electrode sheet  514   b  illustrated in  FIG. 5B  is coupled to terminal  505  via conductor  504 . Electrode sheet  514   b  may be isolated from conductor  502  and terminal  503 . For example, electrode sheet  514   b  in  FIG. 5B  includes curve  415  to ensure a minimum distance between electrode sheet  514   b  and conductor  502 . Likewise, electrode sheets coupled to terminal  503  via conductor  502  (such as electrode sheets  514   a  and  514   c  shown in  FIG. 5A ) may include a curve to ensure a minimum distance to conductor  504 . The distance, provided by the curve, between electrode sheets  514   b  and the opposing conductors may prevent an electrical arc from forming between those electrode sheets and the opposing conductors when a large voltage potential (for example, up to 30 kV) is placed across terminals  503  and  505 , thus preventing failure of capacitor  500 . 
     Electrode sheet  514  may also include rounded edges  516 . The use of rounded edges  516  in place of, for example, ninety-degree corners, reduces the electric field enhancements that may otherwise exist at a corner of electrode sheet  514 . Rounded edges  516  may prevent punctures through dielectric sheet  510  that may result from electric field enhancements when large voltage potentials are placed across terminals  503  and  505 . 
     Although  FIG. 5B  shows rounded edges  516  having a rounded shape, electrode sheet  514  may include edges with any combination of curves and/or obtuse angles to prevent or lower potentially harmful electric field enhancements. 
       FIG. 5C  illustrates a composite view of components of the exemplary high-voltage, high-power capacitor, shown in part in  FIGS. 5A and 5B , for a downhole electrocrushing drilling system. As described above with reference to  FIG. 5A , a plurality of electrode sheets  514  (not expressly shown in the view of  FIG. 5C ) are interleaved between a plurality of dielectric sheets  510 . As also described above with reference to  FIG. 5A , every other electrode sheet may couple to terminal  503  via conductor  502 , while opposing electrode sheets  514  may couple to terminal  505  via conductor  504 . Accordingly, capacitor  500  may provide a capacitance between terminals  503  and  505 . 
     The plurality of dielectric sheets  510  and electrode sheets  514  may be placed together in a square or rectangular shape. Such a square or rectangular shape may be sized to fit within the limited space of a downhole tool. For example, capacitor  500  may be sized to fit within the dimensions of downhole pulsed-power tool  230 , similar to capacitor  241  shown in  FIG. 4 . The plurality of dielectric sheets  510  and electrode sheets  514  may also be placed together in a shape having curved or non-rectangular sides to fit within corresponding curved or non-rectangular dimensions of downhole pulsed-power tools. 
       FIG. 6  illustrates a flow chart of an example method for manufacturing a high voltage, high-power capacitor. Although method  600  describes an exemplary process for forming a high-voltage, high-power capacitor such as capacitor  500  of  FIGS. 5A-5C , method  600  may also be utilized to form other capacitors. Furthermore, method  600  may be adapted for other composite materials, such as composite materials including microparticles. 
     Method  600  starts and proceeds to step  602 , during which nanoparticles are prepared. Nanoparticles may be formed by chemical reaction of reagents, or by processing larger-sized samples of their constituent materials. For instance, nanoparticles of a ferroelectric component may be prepared by grinding larger samples of the constituent material in a nanogrinder. Nanoparticles may also be prepared by dissolving the constituent material in a solvent or allow a chemical reaction that forms the constituent material to proceed in a solvent, then precipitating the ferroelectric component as nanoparticles. Other low-temperature processes, such as non-refractory processes may also be used. 
     In step  604 , at least one polymer component is formed by polymerization of suitable monomers. Monomers may have particular substituents, or the polymer may be subjected to further chemical reactions to add, remove, or modify substituents after polymerization. 
     At step  606 , the polymer component and nanoparticle component are mixed in appropriate proportions. 
     At step  608 , the mixture of polymer component and nanoparticle component are cast into dielectric sheets and crosslinked, so that a polymer matrix with embedded nanoparticles is formed. The dielectric sheets may be cast into a mold or simply formed by pouring the mixture of polymer component and nanoparticle component onto a surface. Any chemical crosslinking agents may be added at this time, typically prior to casting. If heat or electromagnetic radiation induce crosslinking to form the polymer matrix, they may be applied to the cast mixture. The dielectric sheets may then finished, if needed. For instance, they may be cut to form dielectric sheets  510  described above with reference to  FIGS. 5A-5C . 
     Steps  604 ,  606 , and  608  may be combined for polymers that polymerize and form a polymer matrix simultaneously. 
     At step  610 , electrode sheets are formed from a conductive material. For example, electrode sheets  514  described above with reference to  FIGS. 5A-5C  may be formed by rolling a metal or metal alloy, such as copper, aluminum, or steel, to form a thin sheet or foil. If the conductive material is not sufficiently ductile to allow rolling, it may be cast as a thin sheet. 
     At step  612 , dielectric sheets are assembled with electrode sheets in a configuration suitable for capacitor  500 . For example, As described above with reference to  FIGS. 5A-5C , a plurality of dielectric sheets  510  may be interleaved with a plurality of electrode sheets  514 . Further, conductor  502  may couple every other electrode sheet  514  (for example, electrode sheets  514   a  and  514   c ) to terminal  503 , and conductor  504  may couple the other electrode sheets  514  (for example, electrode sheets  514   b  and  514   d ) to terminal  505 . 
     At step  614 , a dielectric encapsulant is prepared. For example, a fluid dielectric material, such as dielectric encapsulant  522  described above with reference to  FIG. 5A , may be prepared. In some embodiments, dielectric encapsulant  522  may include a composition equivalent to dielectric sheets  510  or a composition with tailored properties. In some embodiments, dielectric encapsulant  522  may have a dielectric constant similar to the dielectric sheets  510  but may be modified with conductive or semi-conductive particulate filler. Dielectric encapsulant  522  may have a resistance that is lower than the resistance of dielectric sheets  510 . Accordingly, dielectric encapsulant  522  may shape the electric field between opposing electrode sheets  514 , and thus protect capacitor  500  from failure when a large voltage potential is placed across terminals  503  and  505 . 
     At step  616 , an assembly of the dielectric sheets and the electrode sheets are encapsulated within the dielectric encapsulant. For example, dielectric sheets  510  and electrode sheets  514  assembled together in step  612  may be encapsulated within dielectric encapsulant  522  prepared in step  614 . In some embodiments, dielectric sheets  510  and electrode sheets  514  may be placed in a mold in a vacuum. The vacuum may pull dielectric encapsulant  522  into the space between dielectric sheets  510  and electrode sheets  514  as described above with reference to  FIG. 5A . The vacuum may also pull dielectric encapsulant  522  into the area within the mold surrounding the edges of dielectric sheets  510  and electrode sheets  514 . Crosslinking or curing may be performed after vacuum infiltration to solidify the dielectric encapsulant  522 . 
     Modifications, additions, or omissions may be made to method  600  without departing from the scope of the disclosure. For example, the order of the steps may be performed in a different manner than that described above 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. Furthermore, some steps may be omitted. For instance, nanoparticles, polymer, dielectric encapsulant, or any combination thereof may be purchased or separately formed and then otherwise used in method  600  rather than being formed during method  600  by the entity otherwise performing the other steps. 
       FIG. 7  illustrates a schematic diagram of an example capacitor array for a fuse-protected capacitor in a downhole electrocrushing drilling system. As described above with reference to  FIG. 3 , a capacitor such as capacitor  404  or capacitor  414  may be implemented by multiple capacitors coupled in parallel with each other. As shown in  FIG. 7 , a capacitor may also be formed with an array of two or more fuse-protected branches. 
     Capacitor array  702  includes branches  704   a ,  704   b ,  704   c , and  704   d . Each individual branch is coupled in parallel with the other branches. Further, each individual branch includes a fuse coupled in series with a branch capacitor. For example, branch  704   a  includes fuse  706   a  coupled in series with branch capacitor  708   a , branch  704   b  includes fuse  706   b  coupled in series with branch capacitor  708   b , branch  704   c  includes fuse  706   c  coupled in series with branch capacitor  708   c , and branch  704   d  includes fuse  706   d  coupled in series with branch capacitor  708   d . Each branch capacitor  708   a - d  may be formed in the same manner as the capacitors described above with reference to  FIGS. 4-6 . Further, although capacitor array  702  is depicted in  FIG. 7  as having four branches, capacitor array  702  may be implemented with any suitable number of branches coupled in parallel. The capacitance of each respective branch may be approximately equal to the capacitance of the branch capacitor in that respective branch. Further, the total capacitance of capacitor array  702  may be approximately equal to the sum of the capacitances of each branch coupled in parallel. 
     Fuses  706   a - d  in the respective branches  704   a - d  of capacitor array  702  protect capacitor array  702  from the failure of an individual branch capacitor. In the event that branch capacitor  708   a  fails due to an excessive voltage potential across the terminals of branch capacitor  708   a , such a failure may cause an electrical short between the terminals of branch capacitor  708   a . In such an event, the electrical short of the failed branch capacitor  708   a  will begin to discharge the charge stored on each of the other branch capacitors  708   b - d  within capacitor array  702 . Thus, a large current will temporarily flow through branch  704   a  until fuse  706   a  is blown. When fuse  706   a  blows, branch  704   a  will transition from a short circuit to an open circuit. Thus, branch  704   a  may no longer contribute to the total capacitance of capacitor array  702 . But, the short circuit of the failed capacitor  708   a  will no longer drain the charge from the other branch capacitors  708   b - d  within capacitor array  702 . As a result, the remaining branches of capacitor array  702  may continue to contribute to the capacitance of capacitor array  702 , and capacitor array  702  may continue to function as a capacitor as intended. 
     The fuse protection of each individual branch in capacitor array  702  may extend the useful life of capacitor array  702 . For example, capacitor array  702  may be implemented as a charge capacitor in a pulse-generating circuit (such as pulse-generating circuit  400  described above with reference to  FIG. 3 ) in a downhole electrocrushing drilling system. The fuse protection of individual branches of capacitor array  702  may allow capacitor array  702  to continue functioning as a charge capacitor when one or more of branch capacitors  708   a - d  fail. Accordingly, a pulse-generating circuit utilizing capacitor array  702  may continue to operate, and the downhole electrocrushing drilling system may continue to drill, despite the failure of one or more of branch capacitors  708   a - d.    
       FIG. 8A  illustrates a cut out view of components of an example fuse for a fuse-protected capacitor in a downhole electrocrushing drilling system. Different instantiations of fuse  706  may be utilized, for example, as fuses  706   a - d  described above with reference to  FIG. 7 . 
     Fuse  706  includes contact  802 , contact  804 , outer cylinder  810 , inner cylinder  820 , and filament  822 . As shown in  FIG. 8A , filament  822  is electrically coupled between contact  802  and contact  804 . Filament  822  may include an electrically conductive material such as copper wire, aluminum wire, or any other electrical conductor. During normal operation, filament  822  provides a low-resistance electrical coupling between contact  802  and  804 . As shown in  FIG. 8A , filament  822  is wrapped around inner cylinder  820 . Inner cylinder  820  may include a thermally conductive material which draws heat away from filament  822 . Inner cylinder  820  may draw sufficient heat away from filament  822  to prevent filament  822  from melting during normal operation. But, as described above with reference to  FIG. 7 , a branch capacitor in capacitor array  702  may fail and cause a short circuit condition across the branch capacitor. The short circuit condition causes an excessive current may temporarily flow through the branch capacitor and its corresponding fuse. The excessive current may cause portions of filament  822  to melt and/or vaporize, thus creating an open circuit between contacts  802  and  804 . Accordingly, fuse  706  may prevent the failed branch capacitor from short-circuiting the other branches of capacitor array  702 . 
     As described above with reference to  FIGS. 7 and 8A , fuse  706  may be utilized within capacitor array  702 , which may in turn be utilized within a pulse-generating circuit of a downhole electrocrushing drill system. In such applications, high-power electrical pulses may be applied to fuse  706 , including at times soon after fuse  706  has blown. As described below with reference to  FIG. 8B  and  FIG. 8C , fuse  706  may include elements, packed within outer cylinder  810 , that may help disperse the molten material and/or vapor that results when filament  822  melts and/or vaporizes. Such dispersion of the molten material and/or vapor prevents an electrical arc forming across the molten material and/or vapor during a subsequent high-voltage electrical pulse. 
       FIG. 8B  illustrates a cross sectional view of an example fuse for a fuse-protected capacitor in a downhole electrocrushing drilling system. As shown in  FIG. 8B , the area between inner cylinder  820  and outer cylinder  810  may be filled with beads  830  and fluid  840 . In some embodiments, the area between inner cylinder  820  and outer cylinder  810  may include air and/or a powder material in place of, or in combination with, fluid  840 . Fluid  840  may include an insulating material and thus may insulate the windings of filament  822  from each other. Beads  830  have a spherical shape. Further, beads  830  may be hollow and may be formed with a material, such as glass, that shatters when fuse  706  blows. As described directly above with reference to  FIG. 8A , when a branch capacitor of capacitor array  702  fails, the branch capacitor may generate a short circuit. As a result of a short circuit, an excessive current begins to flow through the branch capacitor and its corresponding fuse  706 . Due to the rapid rise in electrical current, portions of filament  822  melt and/or vaporize in a rapid manner. The rapid melting and/or vaporization of filament  822  causes a shock wave through fluid  840 . Further, any vaporization causes an increase in pressure within the walls of outer cylinder  810 . As a result, beads  830  may shatter. The shattering of beads  830  provides an increased volume of space within outer cylinder  810  through which the molten material and/or vapor from filament  822  may disperse. Moreover, the remaining shards of the shattered beads  830  may provide many disjointed surfaces within the walls of outer cylinder  810 . Thus, the molten material from filament  822  may disperse on the disjointed surfaces. Similarly, condensation from any vaporized portions of filament  822  may form on the disjointed surfaces. 
     The dispersion of the molten material and/or vapor on the many disjointed surfaces of the shattered beads  830  may prevent an electrical arc from forming across the molten material and/or vapor during a high-power electrical pulse that may be applied across fuse  706  after fuse  706  has blown, or to prevent a restrike during the fuse-opening pulse. Accordingly, after fuse  706  has blown, fuse  706  may maintain its operation as an open circuit despite the application of high-power electrical pulses across contacts  802  and  804 . Accordingly, capacitor array  702  may continue to function as intended within a pulse-generating circuit of a downhole electrocrushing drilling system as described above with reference to  FIGS. 7 and 8A . 
       FIG. 8C  illustrates a cross sectional view of an example fuse, with an intermediate barrier, for a fuse-protected capacitor in a downhole electrocrushing drilling system. As shown in  FIG. 8C , some embodiments of fuse  706  may include barrier  850 . Barrier  850  has a cylindrical shape and may form an intermediate barrier that separates the space between inner cylinder  820  and outer cylinder  810  into two spaces. The space between outer cylinder  810  and barrier  850  includes fluid  840  and beads  830 . The space between barrier  850  and inner cylinder  820  includes fluid  840 , but may be free of beads  830 . Thus, the insulation provided by fluid  840  to the windings of filament  822  may be uniform across the length of inner cylinder  820 . The uniform insulation may further prevent electrical arcs from forming across different windings of filament  822  when high-power electrical pulses are applied across contacts  802  and  804  of fuse  706 . 
     Similar to beads  830 , barrier  850  may be formed with a material, such as glass or a thin layer of plastic, which may shatter when fuse  706  blows. The shattering of barrier  850  and beads  830  may provide an increased volume of space within outer cylinder  810  in a similar manner as described above for beads  830  with reference to  FIG. 8B . The molten material and/or vapor from filament  822  may disperse throughout this increased area. Further, the remaining shards of the shattered barrier  850  and beads  830  may provide many disjointed surfaces within the walls of outer cylinder  810 . Molten material from filament  822  may disperse on the disjointed surfaces. Similarly, condensation from any vaporized portions of filament  822  may form on the disjointed surfaces. As described above with reference to  FIG. 8B , the dispersion of the molten material and/or vapor on the many disjointed surfaces may prevent an electrical arc from forming across the molten material and/or vapor during a high-power electrical pulse that may be applied across fuse  706  after fuse  706  has blown. 
       FIG. 9  illustrates a flow chart of exemplary method for drilling a wellbore. 
     Method  900  may begin and at step  910  a drill bit may be placed downhole in a wellbore. For example, drill bit  114  may be placed downhole in wellbore  116  as shown in  FIG. 1 . 
     At step  920 , 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 to  FIG. 4 , pulse-generating circuit  400  may be implemented within pulsed-power tool  230  of  FIG. 2 . And as described above with reference to  FIG. 2 , pulsed-power tool  230  may 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 circuit  400  within pulse-power tool  230  at power source input  401 . As further shown in  FIGS. 2 and 4 , the pulse generating circuit may be coupled to a first electrode (such as electrode  208 ) and a second electrode (such as ground ring  250 ) of drill bit  114 . 
     At step  930 , a capacitor located downhole and electrically coupled between the first electrode and the second electrode may be charged. For example, as shown in  FIG. 4 , capacitor  414  of pulse-generating circuit  400  may be coupled between a first electrode (such as electrode  208 ) and a second electrode (such as ground ring  250 ) of drill bit  114 . Moreover, when switch  406  of pulse-generating circuit  400  closes, electrical current may flow through first winding  411  of transformer  410 . Second winding  412  of transformer  410  is electromagnetically coupled to first winding  411 . Accordingly, when switch  406  closes and an electrical current flows through first winding  411 , a current also flows through second winding  412 . The current through second winding  412  may charge capacitor  414 , thus increasing the voltage across capacitor  414 . In some embodiments, capacitor  414  may be formed in a similar manner as described above for capacitor  500  with reference to  FIGS. 5A-5C . For example, the capacitor may include at least one dielectric sheet having a composite material including a polymer matrix formed from a polymer component and a nanoparticle component that increases the dielectric constant of the composite material above that of the polymer component. 
     At step  940 , an electrical arc may be formed between the first electrode and the second electrode of the drill bit. And at step  950 , the capacitor may discharge via the electrical arc. For example, as the voltage across capacitor  414  increases during step  930 , the voltage across electrode  208  and ground ring  250  also increases. As described above with reference to  FIGS. 1 and 2 , when the voltage across electrode  208  and ground ring  250  becomes sufficiently large, an arc may form through a rock formation that is in contact with electrode  208  and ground ring  250 . The arc may provide a temporary electrical short between electrode  208  and ground ring  250 , and thus may discharge, at a high current level, the voltage built up across capacitor  414 . 
     At step  960 , the rock formation at an end of the wellbore may be fractured with the electrical arc. For example, as described above with reference to  FIGS. 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 step  970 , fractured rock may be removed from the end of the wellbore. For example, as described above with reference to  FIG. 1 , electrocrushing drilling fluid  122  may move the fractured rock away from the electrodes and uphole away from the bottom of wellbore  116 . 
     Subsequently, method  900  may end. Modifications, additions, or omissions may be made to method  900  without 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 drill bit having a first electrode and a second electrode. The downhole drilling system may also have a pulse-generating circuit coupled to the first electrode and the second electrode. A capacitor within the pulse-generating circuit may include a plurality of electrode sheets and a plurality of dielectric sheets interleaved with the plurality of electrode sheets. Each of the dielectric sheets may include a composite material including a polymer matrix formed from a polymer component and a nanoparticle component that increases the dielectric constant of the composite material above that of the polymer component. 
     B. A capacitor including a first electrode sheet, a second electrode sheet, and a dielectric sheet located between the first and second electrode sheets. The dielectric sheet has a composite material including a polymer matrix formed from a polymer component and a nanoparticle component that increases the dielectric constant of the composite material above that of the polymer component. 
     C. A downhole drilling system including a drill bit having a first electrode and a second electrode. The downhole drilling system also includes a bottom-hole assembly having a pulse-generating circuit coupled to the drill bit to provide an electrical pulse to the drill bit, and a capacitor within the pulse-generating circuit. The capacitor includes a plurality of branches coupled in parallel with each other, each of the plurality of branches having a fuse and a branch capacitor coupled in series with the fuse. 
     D. A method including placing a drill bit downhole in a wellbore, providing electrical power to a pulse-generating circuit coupled to a first electrode and a second electrode of the drill bit, and charging a capacitor located downhole and electrically coupled between the first electrode and the second electrode, the capacitor having at least one dielectric sheet having a composite material including a polymer matrix formed from a polymer component and a nanoparticle component that increases the dielectric constant of the composite material above that of the polymer component. The method further includes forming an electrical arc between the first electrode and the second electrode of the drill bit, discharging the capacitor via the electrical arc, 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, B, C, and D may have one or more of the following additional elements in any combination: 
     Element 1: wherein each of the dielectric sheets has a dielectric constant of at least 3 at 0.1 Hz to 1.0 MHz frequency and 150 degrees Centigrade. Element 2: wherein the capacitor is coupled between the first electrode and the second electrode of the drill bit. Element 3: wherein the capacitor is coupled between two nodes of the pulse-generating circuit, the two nodes independent from a first node of the pulse-generating circuit connected to the first electrode and a second node of the pulse-generating circuit connected to the second electrode. Element 4: wherein the polymer matrix includes a silicone polymer having the structural formula: 
     
       
         
         
             
             
         
       
     
     wherein: n is at least 10; none, one or both of R 1  and R 2  are absent; R 1  and R 2 , if both present, are the same substituent or different substituents; and R 1  or R 2  are H, a carbon (C) containing group, a nitrogen (N)-containing group, an oxygen (O)-containing group, a halogen-containing group, or a Si-containing group. Element 5: wherein the nanoparticle component has ferroelectric nanoparticles embedded in the polymer matrix. Element 6: wherein each of the plurality of dielectric sheets has spacers protruding from a surface of the dielectric sheet. Element 7: wherein the capacitor has a dielectric encapsulant that fills space between the interleaved dielectric sheets and electrode sheets. Element 8: wherein the dielectric encapsulant has a dielectric constant less than a dielectric constant of the dielectric sheets. Element 9: wherein each of the plurality of electrode sheets has a plurality of rounded edges. Element 10: wherein the capacitor is located adjacent to at least one drilling-fluid channel within a circular cross-section of a downhole pulsed-power drilling tool. Element 11: wherein the drill bit is selected from the group consisting of an electrocrushing drill bit or an electrohydraulic drill bit. Element 12: wherein the fuse includes an outer cylinder, a filament located within the outer cylinder, an insulating fluid located within the outer cylinder, and a plurality of beads located within the outer cylinder. Element 13: wherein the plurality of beads are hollow and include glass. Element 14: wherein the plurality of beads have a spherical shape. Element 15: wherein the fuse further includes an inner cylinder located within the outer cylinder, and the filament is wrapped around the inner cylinder. Element 16: wherein the inner cylinder includes a ceramic material. Element 17: wherein the fuse further includes an intermediate barrier located between the outer cylinder and the filament. Element 18: wherein the plurality of beads are located between the intermediate barrier and the outer cylinder. Element 19: wherein the intermediate barrier includes glass. 
     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.