Abstract:
A system for recovering gas trapped within the earth includes a casing ( 24 ) sized and configured to be positioned within a borehole in the earth, the casing ( 24 ) formed of a material that is transmissive to electromagnetic energy and gas within the earth; an antenna ( 40 ) sized and configured to be positioned within the casing ( 24 ). The antenna ( 40 ) has a distal end and a proximal end and including a radiating element at the distal end of the antenna ( 40 ) which, in operation, transmits electromagnetic energy toward a desired area of the earth, and an interior channel for allowing gas to be conveyed from the distal end to the proximal end of the antenna ( 40 ).

Description:
This application claims the benefit of Provisional Application No. 60/256,367, filed Dec. 18, 2001. 

   BACKGROUND 
   The invention relates to the recovery of gas from subterranean formations in the earth. 
   Extensive and high volumes of hydrocarbon gases (e.g., methane) trapped within coal seams have been discovered in various parts of the United States. For example, large amounts of trapped methane gas have been discovered in eastern Wyoming (see, for example, “Powder River Basin Coalbed Methane Play Heats Up,” E&amp;P Perspectives, Vol. X, R57, Oct. 22, 1998 (attached herewith). Naturally occurring degradation processes, such as the biodegradation of microorganisms in the coal is believed to cause the generation of the methane gas trapped within the coal seams. 
   Methods of economic and environmentally sound gas recovery are underway. A major problem encountered is the large amount of aquifers (water) that impedes the ability to recover the gas from bore holes drilled in to the coal seam. Specifically, the in-ground water serves as a barrier to the effective removal of the gas from the bore hole. The water must be removed by a pump or redirected to allow more efficient removal of the gas. Systems of co-generation of power for pumps are being considered for the prime supply of electrical energy for the pumps. That is, the electrical power for operating gas turbines used to drive the pumps could be generated using a portion of the gas removed from the borehole. 
   SUMMARY 
   In a general aspect of the invention, a system for recovering gas trapped within the earth, the system includes a casing sized and configured to be positioned within a borehole in the earth, the casing formed of a material that is transmissive to electromagnetic energy and gas within the earth, and an antenna sized and configured to be positioned within the casing. The antenna includes a radiating element at a distal end of the antenna which, in operation, transmits electromagnetic energy toward a desired area of the earth, and an interior channel for allowing gas to be conveyed from the distal end to a proximal end of the antenna. 
   In another aspect of the invention, a method for recovering gas trapped within the earth includes the following steps. A casing is positioned within a borehole in the earth, the casing formed of a material that is transmissive to electromagnetic energy and gas within the earth. An antenna is positioned within the casing, the antenna having a distal end and a proximal end. The antenna includes a radiating element at the distal end of the antenna which, in operation, transmits electromagnetic energy toward a desired area of the earth; and an interior channel for allowing gas to be conveyed from the distal end to the proximal end of the antenna. The method further includes applying electromagnetic energy to the antenna to radiate the earth surrounding the casing; drawing gas within the earth into the interior channel of the antenna at the distal end of the antenna; and conveying the gas within the interior channel to the proximal end of the antenna. 
   Embodiments of these aspects of the invention may include one or more of the following features. 
   A product return pipe has a first end connected to the proximal end of the antenna and a removable cap attached to a second end of the product return pipe. A bellows is connected to the proximal end of the antenna. A thermocouple assembly is connected to the proximal end of the antenna. 
   The antenna is configured to operate in a frequency range between 300 KHz and 300 GHz. More particularly, the frequency range is between 1 MHz and 100 MHz (e.g., about 27 MHz). The antenna is configured to operate at a power level in a range between 3 Kwatts and 20 Kwatts (e.g., about 10 Kwatts). 
   Among other advantages, the system and method (1) reduce the negative impact of water on the in situ recovery of coal gas, such as methane from underground beds or seams of coal; and (2) provide additional or enhanced stimulation of gas production from the coal deposits. 
   The basic energy source proposed for reducing the water barrier effect and stimulating production in-situ is electromagnetics. Electromagnetic energy at frequencies as low as 60 Hz and extending into the microwave frequencies supplied by earth electrodes in the form of antennas and/or waveguides may be employed in the proposed processes. The basic idea is to introduce current into the subterranean formation to vaporize or boil the water in a specified region of the coal seam. The currents are derived from the electromagnetic field energy absorbed by the coal material and water. 
   Specific in-ground applicator structures such as rod electrodes, antennas or waveguides and transmission lines provide the induced currents in the coal seam to vaporize a given amount of water. For example, antennas in a vertical or horizontal bore hole drilled in a coal seam radiate electromagnetic energy away from the antenna into the coal creating a dry region around the bore hole/antenna structure. A pump can be used in conjunction with the antenna for water removal or the bore hole containing the antenna may be pressurized to keep the water away from the antenna/bore hole. 
   A special gas filtering system can be employed around the antenna (within or outside the bore hole) to permit gas recovery up to the antenna bore hole without water. This special filter would block liquid water and allow only gas to pass through it. 
   The dry region around the antenna borehole created by dielectric heating of the coal/water matrix is maintained by the power supplied by the antenna (e.g., 3 to 20 kilowatts on average). This dry region, maintained by either resistive (low frequency) currents or dielectric (high frequency) currents in the coal seam, allows the gas to be transferred from regions outside the casing to within the antenna case, bore hole, or adjacent recovery wells equipment with special filters and flow lines for ease of gas recovery without water. 
   The dry sheath region or zone is maintained at approximately 100° C. to ensure that there is no liquid water. 
   Thermal energy is not a requirement for the gas deposits in place. As a result of the dielectric sheath created by electromagnetic currents, the radiation fields of the antenna now extend further into the coal seam away from the antenna bore hole thereby creating an enhanced zone or region of heating and results in an enlargement of the dry zone and less impedance of gas flow to the recovery well by water. 
   Another benefit of electromagnetic heating is the enlargement of fracture zones in the coal seams by steam pressure and thermal gradients. The result is enhanced flow of methane gas to recovery wells. 
   Still another benefit of electromagnetic heating is the increased activity of microorganisms from the thermal energy deposit, especially at radio frequencies. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates the upper portion of an RF gas recovery system in accordance with the invention. 
       FIG. 2  illustrates the lower portion of the RF gas recovery system of  FIG. 1 . 
       FIG. 3  illustrates an alternative embodiment of a lower portion of the RF gas recovery system of  FIG. 1 . 
       FIG. 4  illustrates another alternative embodiment of the lower portion of the RF gas recovery system of  FIG. 1 . 
       FIG. 5  illustrates still another alternative embodiment of the lower portion of the RF gas recovery system of  FIG. 1 . 
       FIG. 6  illustrates still another alternative embodiment of the lower portion of the RF gas recovery system of  FIG. 1 . 
       FIG. 7  illustrates still another alternative embodiment of the lower portion of the RF gas recovery system of  FIG. 1 . 
   

   Like reference symbols in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
   Referring to  FIGS. 1 and 2 , the upper portion of an RF gas recovery system  10  is shown for radiating electromagnetic energy into a coal seam deposited with the ground  12  and extracting gas released by the heating generated by the electromagnetic energy. In particular, gas recovery system  10  includes an outer casing  14  disposed within a borehole  16  drilled deep within the ground. The outer casing  14  houses a coaxial RF applicator  18  that includes a coaxial transmission line  20  extending from the upper end of the antenna at the surface of the earth to a distal end of the antenna. The coaxial transmission line  20  includes a center conductor  22  positioned coaxially within an outer conductor  24 . In this embodiment, center conductor  22  and outer conductor  24  have diameters of about 1 inch and 2.9 inches, respectively, and have lengths greater than 30 feet. In general, the length of the RF applicator  18  and the outer casing  14  can be between 8 and 200 feet. Insulative spacers (e.g., Teflon)  26  are spaced along the length of the center conducter  22  to maintain its coaxial position relative to the outer conductor  24 . Furthermore, due to the relative long length of RF applicator  18 , support collars  27  are spaced periodically along the length of outer conductor  24 . The upper end of the coaxial transmission line  20  is connected to an RF generator (not shown) via an RF coax line  30 . The upper ends of center conductor  22  and outer conductor  24  of coaxial transmission line  20  include expansion joints in the form of bellows  31  and  32 , respectively. 
   As shown in  FIG. 2 , in this embodiment, the distal end of the RF applicator includes a dipole antenna  40  extending between 5–6 feet from the end of coaxial transmission line  20 . Dipole antenna  40  has a diameter larger than coaxial transmission line  20 . A collar  41  is attached at the transition between dipole antenna  40  and coaxial transmission line  20  to provide mechanical support and to ensure a gas-tight seal between outer conductor  24  of transmission line  20  and outer conductor  43  of the dipole antenna. Dipole antenna  40  includes a tapered section  45  which serves as an impedance transformer between the coaxial transmission line and antenna. 
   In operation, dipole antenna  40  receives RF energy from the RF generator via coaxial transmission line  20  and radiates the coal seam deposit in the surrounding earth. As will be described in greater detail below, the radiated RF energy heats the coal and, in particular, vaporizes or boils the water in a specified region of the coal seam. By removing the water from the coal seam, methane and other gases trapped within the coal seam are released and more easily removed. 
   Center conductor  22  of transmission line  20  is dual-purposed. The center conductor not only serves as a part of the structure for heating the water in the coal seam, it also provides an inner passage  42  for conveying the gas to the surface of the earth for processing. The gas enters inner passage  42  through intake  48 . To remove the gas, a product return pipe  44  having a removable plug  46  extends from the end of center conductor  22  at bellows  32 . 
   RF gas recovery system  10  also includes a thermocouple assembly  50  having a thermocouple coil  52  connected to bellows  32 . Thermocouple coils serve as a filter to “choke” or prevent the flow of low frequency currents to flow. Outer casing  14  also includes input pipes  56  through which nitrogen gas is introduced within the casing. The nitrogen gas is much less flammable than oxygen and, therefore, provides a much safer environment for introducing high current levels from RF applicator  18 . 
   The operation of this particular embodiment will now be described. In general, RF applicator  18  is placed within borehole  16  at a depth in a range between eight and 200 feet (e.g., 100 feet) at a location approximately central to a coalbed. RF energy at a power between 3 and 20 KW (here, 10 KW), at a frequency of 27.12 megahertz (MHz) is provided to dipole antenna  40  from the RF generator. When the temperature at the applicator well  20  reaches about 100 degrees C., the radiation power can be cycled down to a lower power level sufficient for maintaining the temperature until the temperature of the borehole  16  cools to a predetermined threshold (e.g., 90 degrees C.) and then the power is cyled back to 10 KW. The cycling of radiation power may be referred to generally as modulating the power, or modulating the radation energy. Such modulation may also include cessation of the process. 
   It is also appreciated that the applicator well target temperatures implemented in the process may be slected to accommodate the temperature tolerance of the components of RF oil recovery system  10  (e.g., a 150 degree C. tolerance of the coaxial transmission line  20 ). It is also appreciated that the frequency of the radiated energy from the RF generator can be selected according to FCC regulations, and according to principles well known in the art, including the dielectric heating characteristics of particular media. The energy may include radio frequency energy and microwave energy. In this context, radio frequency energy has a frequency in the range between 300 kilohertz (KHz) and 300 MHz, and microwave energy has a frequency in a range between 300 MHz and 300 GHz. 
   The RF energy is transmitted from the RF generator to dipole antenna  40  via coaxial transmission line  20 . Dipole antenna  40  induces currents within the coal seam causing resistive and/or dielectric heating of the surrounding region of the coal seam. The heating vaporizes or boils the water in the coal seam creating a dry region. The dry region within the coal seam is maintained by resistive hearing (low frequency) currents or dielectric (high frequency) currents and allows the trapped methane gas to be released. The released methane gas flows within outer casing  14  of oil recovery system  10  and to inner passage  42  of center conductor  22  via intake  48  where the methane gas is conveyed to the surface of the earth for processing. In particular applications, a gas filtering system can be positioned around RF applicator  14  (within or outside the bore hole) to permit gas recovery through inner passage  42  without water. The gas filtering system blocks liquid water and allows only the gas to pass through it. 
   Other embodiments are within the scope of the claims. For example, although RF applicator  14  includes dipole antenna  40 , other antenna configurations are equally applicable for use with the RF applicator. For example, referring to  FIG. 3 , RF applicator  14  can include an antenna  70  which is in the form of an extension of coaxial transmission line  20 . 
   The applicators described in conjunction with  FIGS. 2 and 3  are designed to provide a predetermined impedance characteristic, for example, to provide a high level of coupling into the coal seam. However, in other embodiments, changing the impedance characteristics of the RF applicator may be desirable. For example, dielectric characteristic of the subterranean formation may differ or change as the water is converted to steam. In such embodiments, the applicator may include a tuning mechanism. 
   Referring to  FIG. 4 , for example, a shorting link antenna  80  is connected to the distal end of coaxial transmission line  20 . In essence, shorting link antenna  80  is a dipole antenna having a looped end  82  and shorting link  84  positioned across the end. An insulated push rod  86  is connected to shorting link  84  such that, in operation, it can be used to move the shorting link and adjust the electrical length of the antenna. A remotely controlled, non-conducting hydraulic actuator  90  is provided to move push rod  86 . In the embodiment shown, a center conductor transition  92  is provided between coaxial transmission line  20  and a center conductor  94  of antenna  80 . It is important to note that because antenna  80  has a looped end, center conductor  94  has a section offset from the axis of coaxial transmission line  20 . 
   In addition, collinear array antennas, such as those described in U.S. Pat. Nos. 4,583,589, 5,065,819, and 6,097,985, all of which are incorporated herein by reference, are also well-suited for use in RF applicator  14 . In addition, the “RF choke” structures described in these references may be desirable for use to prevent the flow of certain frequencies. 
   The applicators described above in conjunction with  FIGS. 2–4  are often referred to as electric antennas. Such antennas are well suited for applications requiring a strong near electric field. In other applications, magnetically coupled antennas may be more suitable. Because the amplitude of the near field is relatively less than that of an electrically coupled antenna, the risk of electric arcing is reduced, thereby increasing safety. 
   For example, referring to  FIGS. 5 and 6 , in still other embodiments, helical antennas  100  and  110  include multi-turn links surrounded by an other helix. Specifically,  FIGS. 5 and 6  show a twenty-turn link  102  and three-turn link  112 , respectively. Multi-turn links are multi-turn loops surrounded by an outer helix  104  which, in turn, surrounds outer conductor  43  and is floating (i.e., has no ground plane). Outer helix  104  is excited in the To mode by the multi-turn links. Excitation in this manner is similar to exciting a rectangular waveguide in the TE 10  mode with an electric monopole positioned along the centerline of a broad wall of the waveguide. Further details of antennas having this combination of elements can be found in U.S. Pat. No. 6,097,985. 
   Referring to  FIG. 7 , a helical antenna  130 , similar to that of the helical antenna  100  (shown in  FIG. 5 ) includes a floating outer helix  132 , which unlike outer helix  104  of antenna  100  is positioned concentrically within outer conductor  43 . 
   Whether electrically coupled or magnetically coupled antennas, the applicators are designed to maximize the impedance match between the applicator and surrounding media. 
   Still other embodiments are within the scope of the claims.