Abstract:
System for drilling boreholes into subsurface formations. A gyrotron injects millimeter-wave radiation energy into the borehole and pressurization apparatus is provided for pressurizing the borehole whereby a thermal melt front at the end of the borehole propagates into the subsurface formations. In another aspect, a system for fracturing a subsurface formation is disclosed.

Description:
[0001]    This application claims priority to U.S. provisional application Ser. No. 61/015,394, filed Dec. 20, 2007, the contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to system and method for drilling and fracturing subsurface formations and more particularly to such a method and system using millimeter-wave radiation energy. 
         [0003]    There is a recognized need for a better technology for deep drilling into subsurface formations to access, for example, new sources of gas, oil and geothermal energy. Drilling at depths beyond 25,000 feet is increasingly difficult and costly using present rotary drilling methods. 
         [0004]    Current rotary drilling technology is a slow grinding and fluid flushing process that has been in use for over 100 years. This drilling process is further slowed by the need to frequently withdraw the drill to replace drill bits, casing/cementing, and to make diagnostic measurements of the borehole, accounting for up to 50% of the drill time. Furthermore, drilling to penetration depths beyond 25,000 feet (7,620 m) can be extremely difficult and costly because of increasing temperature, pressure and decreasing mechanical torque efficiencies with increasing depth. Advances in ground boring technology over this current state of the art are needed to make access into the earth&#39;s subsurface easier, deeper, and less costly. 
         [0005]    It is also recognized that fracturing is required in many deep underground formations to extend borehole access to deep underground energy sources, for example. It is a key element in enhanced geothermal systems to make possible the circulation of injected water into hot dry rock between injection and production wells to extract heat. Fracturing is also necessary to extract natural gas and petroleum from tight formations that are being increasingly accessed to meet growing energy demands. Currently there is a large market to stimulate natural gas and petroleum reservoirs using hydraulic fracturing. 
         [0006]    Hydraulic fracturing, known in the prior art, uses a fluid under high pressure to cause fractures to open in subsurface strata. The maximum pressure that can be obtained is limited by the mechanical pumps used to pump the fluid. Getting at increasingly deeper and tighter formations is constrained by available mechanical pumping technology. In addition, large volumes of fluid are normally required. A typical fluid is water with chemical additives optimized for fracturing. This fluid is a source of pollution that can contaminate underground drinking water sources and surface areas when it is pumped out into surface reservoirs. Thus the fluid is a significant detrimental environmental issue for many locations that can prevent exploitation of some energy formations. 
         [0007]    It is therefore an object of the present invention to provide technology for deep drilling and fracturing of subsurface formations. The approach disclosed herein can potentially increase the penetration rate for deep drilling by a factor of 10 to 100. 
       SUMMARY OF THE INVENTION 
       [0008]    In one aspect, the system for drilling boreholes into subsurface formations according to the invention includes a gyrotron for injecting millimeter-wave radiation energy into a borehole and pressurization apparatus for pressurizing the borehole such that a thermal melt front at the end of the borehole propagates into the subsurface formations. It is preferred that the millimeter-wave radiation energy is in the frequency range of 30 to 300 GHz. It is preferred that the borehole be pressurized by a combination of the pressurization apparatus and by volatilized material in the borehole. 
         [0009]    In a preferred embodiment of this aspect of the invention, a waveguide extends into the borehole. In one embodiment, the waveguide is corrugated with circumferential grooves having a spacing and depth dependent on the millimeter-wave energy wavelength. In an embodiment, there are approximately three circumferential grooves per wavelength and the grooves have a depth of approximately one-quarter wavelength. In a preferred embodiment, the waveguide is metallic. In yet another embodiment, a mirror is provided to allow substantially horizontal drilling. The mirror may be water cooled. 
         [0010]    In another aspect, the invention is a method and system for fracturing a subsurface formation. This aspect of the invention includes establishing a borehole extending to the subsurface formation and introducing a fluid into the borehole. A beam of millimeter-wave radiation energy is transmitted into the borehole to heat the fluid and to convert the fluid into a high pressure gas or super fluid that fractures the subsurface formation. In a preferred embodiment the radiation transmission is continuous to maintain a steady, high pressure. Alternatively, the radiation may be transmitted in a pulsed fashion to achieve high peak impulses to propagate fractures. In a preferred embodiment, a waveguide in the borehole is provided to transmit the millimeter-wave beam separate from delivery of the fluid. The energy beam and fluid may be combined at a location at which the pressure causes fracturing. The pulse width and repetition rate of the energy beam and the fluid flow volume are selected to optimize fracturing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0011]      FIG. 1  is a schematic illustration of an embodiment of a gyrotron ground borer system disclosed herein. 
           [0012]      FIGS. 2   a  and  2   b  are graphs of transmission against distance for millimeter-wave beam transmission at 170 and 280 GHz into boreholes. 
           [0013]      FIG. 3  is a cross-sectional view of a borehole including a metallic waveguide insertion to improve gyrotron transmission efficiency. 
           [0014]      FIG. 4  is an illustration of surface corrugation inside a metallic waveguide for low loss transmission of millimeter-wave radiation. 
           [0015]      FIG. 5  is a graph of rate of penetration against depth for conventional rotary drilling and millimeter-wave beam drilling. 
           [0016]      FIG. 6  is a cross-sectional view of an embodiment of the invention adapted for horizontal drilling. 
           [0017]      FIG. 7  is a schematic illustration of an embodiment of a high-temperature, subsurface millimeter-wave fracturing apparatus using a high power source of beam energy. 
           [0018]      FIG. 8  is a schematic illustration of an embodiment of the invention used to augment conventional hydraulic fracturing to higher pulsed pressures. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    The millimeter wave drilling system disclosed herein uses an intense beam of millimeter-wave electromagnetic energy in combination with pressure to thermally make a path through solid strata. The millimeter wave power is preferably provided by gyrotron technology. Gyrotron technology is a high power source of millimeter-wave radiation in the frequency range of approximately 30-300 GHz. These frequencies are 10 to 100 times higher than microwave frequencies. Gyrotron CW output power levels of 1 MW have been achieved at 110 and 170 GHz as part of the international fusion energy development program with electrical to millimeter-wave power conversion efficiencies of over 50% [ 2 ,  3 ]. Numbers in square brackets refer to the references appended hereto. The contents of all of these references are incorporated herein by reference. One megawatt output between 200 and 300 GHz has also been demonstrated in short pulse operation in modes that could be used for CW operation [ 4 ]. Millimeter wavelengths are ideally suited for making boreholes because they are long enough to propagate through visibly obscured paths without scattering or absorption losses, but short enough to be easily localized for spot heating. 
         [0020]    Moreover, borehole diameters in the 4 to 12 inch (10-30 cm) range are well suited to serve as waveguides to guide millimeter-wave beams over long distances. Thus the borehole acts as a guide to propagate the high power beam to greater depths as it is created. In addition, millimeter-wave diagnostics at frequencies offset from the high power beam can be superimposed on the drilling beam to monitor temperature, rate of penetration, and quality of the borehole in real time, providing information for control of the penetration process. 
         [0021]    The basic elements a of a gyrotron earth penetrator system are shown in  FIG. 1 . The gyrotron  10  is powered by a power supply  20  connected by a power cable  30 . The high power millimeter-wave beam output of the gyrotron is guided by a waveguide  40  which has a waveguide bend  80 , a window  90  to pressure seal the waveguide, and a ground inserted waveguide section  120  with opening  125  for off gas emission and pressure control. A section of the waveguide is below ground  130  to help seal the borehole. 
         [0022]    As part of the waveguide transmission line  40  there is an isolator  50  to prevent reflected power from returning to the gyrotron and an interface for diagnostic access  60 . The diagnostic access is connected to the diagnostics electronics and data acquisition  70  by low power waveguide  65 . At the window  90  there is a pressurized gas supply unit  100  connected by plumbing  110  to the window to inject a clean gas flow across the inside window surface to prevent window deposits. A second, pressurization unit  150  is connected by plumbing  140  to the waveguide opening  125  to help control the pressure in the borehole  200  and to introduce and remove borehole gases as needed. The window gas injection unit  100  is operated at a slightly higher pressure relative to the borehole pressure unit  150  to maintain a gas flow across the window surface. A branch line  145  in the borehole pressurization plumbing  140  is connected to a pressure relief valve  160  to allow exhaust of volatilized borehole material and window gas through a gas analysis monitoring unit  170  followed by a gas filter  180  and exhaust duct  185  into the atmosphere  190 . 
         [0023]    Pressure in the borehole is increased in part or in whole by the partial volatilization of the subsurface material being melted. A thermal melt front  220  at the end of the borehole  200  is propagated into the subsurface strata under the combined action of the millimeter-wave power and gas pressure leaving behind a glassy/ceramic borehole wall  210 . This wall acts as a dielectric waveguide to transmit the millimeter-wave beam to the thermal front  220 . 
         [0024]    An approximate estimate of the rate of penetration that is possible with the millimeter-wave beam drill can be made by using the known high temperature heat capacity of silicon dioxide (SiO 2 ), a major constituent of sandstone, shale, and granite. Assuming a 1 MW beam of energy completely absorbed by a 6 inch (15 cm) diameter column of SiO 2  and a temperature averaged heat capacity of 1150 Joules/kg ° C. it would take about 110 seconds to heat a 1 meter column from 20° C. (68° F.) to an average temperature 2000° C. (3630° F.), which is about 280° C. above the melting temperature of SiO 2 . Since the gyrotron beam profile is peaked on center with a profile that has a Gaussian function, the heating on the axis of the beam will be more intense than around the circumference of the bore diameter. Axial material will be volatilized creating a pressure to force the remaining melt material forward and into the sides making dense glassy walls while removing some of the material as off gas. 
         [0025]    The high pressure also facilitates the absorption efficiency of intense millimeter-wave energy by the melt front. The plasma break down threshold would be increased with pressure allowing more intense energy to be used for melting and volatilization without plasma creation that could reduce the direct energy coupling to the surface. It would also deform the melt front into a conical cavity for trapping reflections for more complete absorption. 
         [0026]    Surrounding underground strata may also be fractured due to the intense thermal stresses, which would facilitate displacement of the molten material from the borehole clearance path. The molten rock that was not removed by volatilization would be a super heated, low viscosity fluid that would be displaced into the surrounding strata rather than removed as in current drilling approaches. For this simplified example the rate of penetration (ROP) would be about 105 feet (32 m) per hour, 3-5 times faster than conventional rotary drilling at depths less than 10,000 ft (3.0 km), and 10-100 times faster at depths greater than 10,000 ft (3.0 km). There would be no need to stop for replacing drills, inserting casing, or to do borehole diagnostics since this would be accomplished in real time. The chemistry of the borehole and the location depth of valuable energy resources would be identified in real time by the off gas analysis unit. 
         [0027]    In actual practice the ROP will vary from this idealized example depending on the exact composition of the borehole material, the water content, and gyrotron beam transmission losses. The transmission losses in particular would determine how quickly the rate of penetration would fall off with distance. In waveguides with diameters larger than the wavelength, the transmission losses can be calculated for the most efficient mode of propagation (HE 11 ) by well known theory [ 5 ]. The transmission attenuation factor deceases inversely as the cube of borehole diameter (1/D 3 ) and inversely as the square of the frequency (1/f 2 ). However, there are limits to maximum borehole diameter because the power density would decrease due to the larger beam area, reducing the initial penetration rate, and the maximum frequency is limited to about 300 GHz by present megawatt gyrotron technology. Calculated transmission curves for two nonconducting glassy/ceramic wall borehole sizes and two gyrotron frequencies with an assumed wall dielectric constant of 2 are shown in  FIGS. 2   a  and  2   b . The transmission factor for 6 inch (15 cm) and 8 inch (20 cm) diameter boreholes as a function of distance into the borehole for the 170 GHz gyrotron frequency is shown in  FIG. 2   a  and for 280 GHz in  FIG. 2   b . Transmission at 170 GHz would decrease to 50% after about 1600 and 3600 ft (0.5 and 1.1 km) in 6 and 8 inch (15 and 20 cm) boreholes, respectively. At 280 GHz the 50% transmission distances would be 4300 and 9800 ft (1.3 and 3.0 km), respectively for these same boreholes. 
         [0028]    Extremely deep boreholes can be achieved by inserting into the borehole a more efficient waveguide. Metallic waveguides are more efficient than dielectric waveguides and for a given diameter larger than two wavelengths, a metallic waveguide with an internally corrugated surface has significantly better transmission efficiency than a smooth walled waveguide [ 6 ]. For 4 inch (10 cm) diameter aluminum tubing the transmission efficiency would be 90% for 25 miles (40 km) at 170 GHz. Therefore inserting into the borehole an internally corrugated aluminum waveguide would make possible extremely deep penetration that could be used beyond present limits of rotary drilling technology. 
         [0029]    The basic elements of a borehole with an inserted waveguide for millimeter-wave transmission are shown in  FIG. 3 . The borehole  200  with glassy/ceramic wall  210  and permeated glass  212  has a metallic waveguide section  230  inserted to improve the efficiency of gyrotron beam propagation. The inserted waveguide diameter is smaller than the borehole diameter to create an annular gap  214  for exhaust/extraction. The stand off distance  240  of the leading edge of metallic insert waveguide from the thermal melt front  220  of the borehole is far enough to allow the launched millimeter-wave beam divergence  232  to fill  234  the dielectric borehole  200  with the guided millimeter-wave beam. The standoff distance  240  is also far enough to keep the temperature at the metallic insert low enough for survivability. 
         [0030]    The inserted millimeter-wave waveguide also acts as a conduit for a pressurized gas flow  236  from the surface. This gas flow keeps the waveguide clean and contributes to the extraction/displacement of the rock material from the borehole. The gas flow from the surface  236  mixes  242  with the volatilized out gassing of the rock material  244  to carry the condensing rock vapor to the surface through annular space  214 . The exhaust gas flow  246  is sufficiently large to limit the size of the volatilized rock fine particulates and to carry them all the way to the surface. 
         [0031]    The approximate corrugation dimensions required on the internal metallic waveguide surface for efficient transmission at 170 GHz is shown in  FIG. 4 . There need to be about three circumferential grooves per wavelength that are about one quarter wavelength deep. The groove could be a screw thread with a v-shaped groove. At 170 GHz a thread pitch of 40 per inch (15.7 per cm) with a groove depth of 0.017 inches (0.43 mm) would work well. At 280 GHz the optimum pitch and groove depth would be 66 per inch (26 per cm) and 0.010 inch (0.25 mm), respectively. The corrugation period “w” would be 0.023 and 0.014 inches (0.58 and 0.0.36 mm) for 170 and 280 GHZ, respectively. 
         [0032]    A comparison of the rates of penetration as a function of depth that have been achieved by conventional rotary rock drilling and those estimated for millimeter-wave beam drilling by the simplified analysis presented here is shown in  FIG. 5 . Curve A is a plot of the average data for rotary drilling of gas wells in the Judge Digby field in Louisiana [ 7 ]. The ROP varies from about 25 ft/hr (7.6 m/hr) near the surface to less than 2 ft/hr (0.6 m/hr) below depths of 20,000 ft (6.1 km). An estimated ROP for a 1 MW 280 GHz millimeter-wave beam is shown by curve B for a 6 inch (5 cm) diameter borehole. The ROP varies from approximately 105 ft/hr (32 m/hr) at the surface to about 20 ft/hr (6 m/hr) at a depth of 10,000 ft (3.0 km), the fall off in ROP due to the transmission losses of the millimeter-wave beam in the glassy/ceramic walled borehole. At this depth a metallic waveguide is inserted and the rate of penetration is restored back to about 105 ft/hr (32 m/hr) at the full 1 MW power level. For the next 10,000 ft (3.0 km) depth increment to 20,000 ft (6.1 km) the rate of penetration varies in the same way as for the first 10,000 ft (3.0 km) interval after which the metallic waveguide is extended to 20,000 ft (6.1 km) and the ROP restored to again about 105 ft/hr (32 m/hr). The millimeter-wave drilling is then resumed to 30,000 ft (9.1 km) and so on. This cycle of drilling and waveguide insertion can be repeated as often as necessary to reach extreme depths not possible with rotary drilling technology. The distance interval that is covered between waveguide insertions can be varied to maximize the average penetration rate and minimize cost. 
         [0033]    The millimeter wave drilling system can be used by itself or in combination with conventional drilling. Conventional drilling can be employed where it works best. At a depth where the expense becomes prohibitory, conventional drilling could be discontinued and millimeter wave drilling could be used to extend the well depth. This approach could be carried out by placing waveguides inside the bore that was produced by conventional drilling 
         [0034]    Millimeter-wave beam drilling can also be used for horizontal drilling. When drilling vertically the beam is aimed downward by waveguide/optics at the surface and the drilling will follow this aimed direction without deviation. It is possible to change the beam direction with a special waveguide mirror system to drill horizontally or any other desired direction. The basic elements of a beam turning waveguide mirror system are shown in  FIG. 6 . The corrugated waveguide  230  inserted into the borehole has a turning mirror at its end  250  to change the direction of the millimeter-wave beam. The waveguide has a water jacket  260  with water input  270  and output  280  to direct a circulation of water  290  to the mirror to keep its temperature below damage levels as the drill direction of the borehole is changed. The angled borehole  300  will follow the direction set by the turning mirror. This system could also be used to make chambers in the borehole. In addition, it could also be used with short high peak power pulses, not long enough to cause melting, to fracture the glassy/ceramic wall of the original millimeter-wave drilled borehole to make it more permeable to the subsurface energy resources. 
         [0035]    High-temperature fracturing as disclosed herein can increase the maximum pressure and fracturing that can be achieved deep underground and could reduce the volume of fluid that is needed to cause a given amount of stimulation of an energy formation. This is accomplished by transmitting an intense millimeter-wave beam of energy into the borehole to heat a working fluid and converting it into a high pressure gas or super fluid underground at the energy formation where it is needed. Operation of this high temperature fracturing technology could be either continuous to maintain a steady high pressure as in conventional hydraulic fracturing or it could be rapidly pulsed to achieve high peak impulses that would propagate fractures in a hammer like manner. A proppant in the working fluid and high average pressure would keep the fractures open between pulses to propagate the next pressure pulse to new fracturing beyond the preceding fractures. Since the high pressure is generated by a beam of energy locally in the borehole, mechanical limits for generating high pressures are removed and pressure drops for transmitting a high pressure flow long distances are circumvented. Such an approach could make it possible to access deeper and tighter energy resources with reduced environmental impacts. 
         [0036]    One embodiment is shown in  FIG. 7 . A transmission waveguide for a millimeter-wave beam of energy  400  is inserted into the borehole  200  along with a companion conduit  403  for a fluid such as water. The output millimeter-wave beam  404  and the output fluid  405  combine below the waveguide and fluid conduit to heat the fluid to a high temperature raising the pressure. For example, water could be heated to a high temperature steam or super fluid. The super heated fluid would create stress fractures  406  in the subsurface strata that would propagate away from the borehole and accept the further flow of the high pressure gas/liquid fracturing fluid to propagate the fracturing process. 
         [0037]    A packer  407  is used to seal the borehole to the surface atmosphere to confine the high pressure thrust generated underground to propagate downward and outward into the subsurface strata. The waveguide  400  and fluid flow conduit  403  are also sealed to upward pressure flow by a window  90  in the waveguide  400  and a one way valve  408  in the fluid line  403 . The fluid is pumped by a pump  409  from an outside source of fluid  410  such as water. A gas manifold  411  in the waveguide is used to introduce a waveguide purge gas flow through a second one way valve  412  from a compressor  413  from an outside source of gas  414  such as dry air. The purge gas functions as a transparent medium for propagation of the millimeter wave energy, keeping the interior surface of the waveguide clean, and displacing some of the required fluid volume for fracturing. 
         [0038]    The high power source of energy  10  above ground such as a millimeter-wave gyrotron is connected by an above ground waveguide  40  having waveguide bends  80  as necessary to guide the energy beam to the window  90  and align it with the underground waveguide  400 . An isolator  50  is incorporated into the waveguide  40  to prevent back reflections from perturbing the of the high power source of energy  10 . 
         [0039]    The main advantages of this approach over current hydraulic fracturing methods are: 1) high pressure is generated by non-mechanical means at the borehole location where it is required, making possible higher pressures at deeper locations, and 2) the volume of required fluids is reduced because higher pressure can do more work for a given fluid volume and because some of the fluid is replaced by non-polluting gas flow. Therefore, this approach would be capable of accessing more energy formations with a smaller environmental impact. 
         [0040]    A second embodiment is shown in  FIG. 8 . In this embodiment the millimeter-wave beam is used to create pressure pulses to augment conventional hydraulic fracturing. The fluid conduit  403  extends only to just beyond the borehole sealing packer  407  to fill the entire borehole  200  below the packer  407  with the hydraulic fracturing fluid  415 . The millimeter-wave waveguide  400  is pressurized by a gas  414  through a compressor  416  to keep it clear of the hydraulic fracturing fluid  415 . At the output aperture  417  of the waveguide the millimeter-wave beam of energy  404  is absorbed by the hydraulic fracturing fluid, resulting in the generation of a high pressure pulse  418  that propagates into the substrata to promote fracturing  419 . The pulse width and repetition rate of the millimeter-wave beam can be adjusted to optimize the fracturing process. 
         [0041]    A third embodiment is the use of millimeter wave energy for fracturing rock without the use of a fluid. High pressure pulses could be employed. The same millimeter wave system that is used for the other fracturing embodiments could be utilized. The source can be a gyrotron and the transmission system can be a corrugated waveguide. The fractured rock can be removed by various means that include but are not limited to use of a fluid. 
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