Abstract:
A system for producing gas from a gas hydrate formation includes a first electrode and a second electrode. The first electrode is disposed in proximity of a first region of the formation, and the second electrode is disposed within a second region of the formation. The second electrode is separated from the first electrode by an electro-conductive path through the formation. An extraction well extends within the formation and intersects the electro-conductive path. The well comprises one or more perforations in fluid communication with the formation. A voltage source is connected to the electrodes and operates to produce a voltage difference across the electrodes. A method for extracting gases from a gas hydrate formation includes the step of establishing a voltage difference across two or more electrodes in a hydrate formation to thermally react with the hydrate formation and release gas from the formation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This is a continuation-in part of U.S. patent application Ser. No. 10/279,431, filed Oct. 24, 2002, which claims the benefit of U.S. Provisional Application No. 60/335,701, filed Oct. 26, 2001, the entire disclosures of which are incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to the production of natural gas, and more particularly to a method and system for producing natural gas from gas reserves with the aid of electric current.  
       BACKGROUND  
       [0003]     The U.S. Department of Energy estimates that the ocean floor and arctic permafrost regions contain several trillion cubic feet of methane gas (also referred to as natural gas) in the form of methane hydrates. Methane hydrates are clathrate compounds which are inclusion complexes formed at high pressures and low temperatures, existing as solid crystalline structures. In these structures, methane gas molecules are surrounded or included by a cage of water molecules. Methane hydrates are typically found on the ocean floor in sediments which are stable at depths of approximately 300 meters.  
         [0004]     There is increasing interest in the development of methods to extract methane gas from formations containing methane hydrates. The production of methane gas is viewed as one means for lessening global dependency on oil and other fuels containing large amounts of carbon. Efforts to increase methane gas production are also motivated by an expanding natural gas infrastructure and growing interest in natural gas from public utility companies. At least one extraction technique, solvent injection, has been proposed and tested to extract methane gas from methane hydrates. Although solvent injection has shown promise, the technique is difficult to apply uniformly through a formation, and may not be suitable for deep formations. As a result, currently proposed techniques for extracting methane gas from methane hydrate formations leave much to be desired.  
       SUMMARY OF THE INVENTION  
       [0005]     In a first aspect of the invention, a system for extracting gases from a gas hydrate formation includes a first electrode and a second electrode. The first electrode is disposed in proximity to a first region of the formation, and the second electrode is disposed within a second region of the formation. The second electrode is separated from the first electrode by an electro-conductive path through the formation. An extraction well extends within the formation in proximity to the electro-conductive path. The well comprises one or more perforations in fluid communication with the formation. A voltage source is connected to the first and second electrodes and operates to produce a voltage difference across the first and second electrodes.  
         [0006]     In one embodiment of the invention, a system includes a first electrode in proximity to a first region of a formation containing methane hydrates on the ocean floor. A second electrode is disposed within a second region of the formation. The second electrode is separated from the first electrode by an electro-conductive path through the methane hydrate formation. An extraction well extends within the formation in proximity to the electro-conductive path. The well comprises one or more perforations in fluid communication with the formation. A voltage source is connected to the first and second electrodes and operates to produce a voltage difference across the first and second electrodes. Upon operation of the voltage source, resistance in the formation causes the voltage difference between the electrodes to generate heat energy which is sufficient to thermally react with the methane hydrates thereby releasing methane gas from the formation. The methane gas is formed at elevated pressure, which drives the gas into the extraction well. The methane gas may be recovered and stored on a barge or other ocean vessel. Once on the barge, the gas may be used to fuel an electric generator. Alternatively, the methane gas may be conveyed by undersea piping to a facility on land e.g. for distribution.  
         [0007]     In a second aspect of the invention, a method for extracting gas from a formation containing gas hydrates includes the step of placing two or more electrodes in proximity to the formation and drilling an extraction well into the formation. The extraction well has one or more perforations to connect the interior of the well with the formation. A source of voltage is connected to the electrodes, and a voltage difference is established across the electrodes to produce an electrical current through the formation. The current through the formation is adjusted to thermally react with the gas hydrates in the formation and release gases from the gas hydrates. Gases released from the gas hydrates are drawn into the extraction well. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0008]     The foregoing summary as well as the following description will be better understood when read in conjunction with the figures in which:  
         [0009]      FIG. 1  is a schematic of a system for producing gas from a gas hydrate formation in accordance with the present invention.  
         [0010]      FIG. 2  is a schematic of a system for producing gas from a gas hydrate formation in accordance with the present invention, where the system is employed on an ocean vessel to extract gas from a gas hydrate formation on the ocean floor.  
         [0011]      FIG. 3  is a schematic of an alternate system for producing gas from a gas hydrate formation in accordance with the present invention, where the system is employed on an ocean vessel to extract gas from a gas hydrate formation on the ocean floor. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]     Referring to the drawing figures in general, and to  FIG. 1  specifically, a system  10  for producing gas from a formation containing gas hydrates is shown in schematic form in accordance with the present invention. The system  10  is installed in the vicinity of a gas hydrate formation  8 . Two or more electrodes, such as a first electrode  20  and a second electrode  30 , are placed in or around the gas hydrate formation  8  and connected with a voltage source  12 . Electric current is applied between the electrodes  20 ,  30  and across the gas hydrate formation  8  to produce an electric field  40  across the hydrate formation. The electric field  40  is applied to the formation to release gas from the gas hydrates. The release of gas from the gas hydrates is primarily carried out through resistive heating. The electric field  40  gradually produces heat in the formation  8  based on electrical resistivity of the sediments and materials in the formation  8 . As heat is generated, the temperature around the gas hydrates increases until the hydrates are destabilized, releasing the gas from the hydrate molecules. A gas extraction means  50  is placed within the hydrate formation  8  to capture and convey the released gas to a gas collection system  60 .  
         [0013]     The system  10  may be used in a variety of applications to produce gas from gas hydrate deposits. For purposes of this description, the system  10  will be shown and described in the context of methane gas production, with the understanding that the invention can be applied to a variety of different gas hydrate formations containing varying amounts of methane and other gases. The present invention is operable in different formations of varying compositions, and may be used for releasing and collecting gases other than methane gas. In addition, while this description refers to methane gas, it is understood that the gas released from a formation will likely contain a mixture of methane gas and other gases.  
         [0014]     The present invention can be practiced using a multiplicity of electrodes placed in vertical, horizontal or angular orientations and configurations. The arrangement of components in a given installation will vary depending on the location and local geology of the hydrate formation. As stated earlier, methane hydrate formations have been studied in arctic permafrost regions as well as in sediment layers on or beneath the ocean floor. Hydrate formations may exist as large relatively flat homogeneous formations, or may be interrupted by outcrops of non-hydrate material. Therefore, the electrodes may be positioned in a number of arrangements in or around the formation.  
         [0015]     Referring now to  FIG. 2 , a system  110  in accordance with one embodiment of the present invention is operable to produce methane gas from a methane hydrate formation  108  along the sea floor. The system  110  includes a high-voltage electric power source  112  supported above the formation. The components of system  110  may be located on land or supported on a ship, rig, barge, vessel, or other means in proximity to the formation. In  FIG. 2 , the system is shown on a barge  115 . By means of an insulated cable  122 , the relatively positive terminal, or anode, of the power source  112  is connected to a first electrode  120 . Depending on the geology of the sea floor, and the proximity of the methane hydrate formation to the sea floor surface, the first electrode  120  may be suspended above the sea floor, rest on the sea floor or be installed beneath the sea floor through a fissure, crevice or bore hole that penetrates beneath the sea floor in proximity to the hydrate formation. For purposes of  FIG. 2 , it will be assumed that a significant volume of stabilized methane hydrate is exposed on the sea floor in a substantially flat layer, allowing the first electrode  120  to rest on the sea floor.  
         [0016]     A gas collection well  150  is drilled into the formation  108  to recover methane gas released from the formation during operation of the system  110 . The collection well  150  includes a perforated metallic liner  151  which extends down into the formation  108 . The perforated liner  151  has one or more perforations that connect the interior of the collection well  150  in fluid communication with the interior of the formation  108 . Since the hydrate formation  108  is exposed on the sea floor, the liner  151  extends from the top of the well  150  into the formation. In hydrate formations that are buried under a layer of overburden material, the well  150  may include a solid casing that extends through the overburden. The specific construction of the well is not germane to the invention, and will largely depend on the geologic conditions around the hydrate formation. Preferably, the collection well  150  is completed in accordance with conventional undersea drilling practices.  
         [0017]     The relatively negative terminal on the power source  112 , or cathode, is connected to a second electrode  130  placed within the methane hydrate formation  108 . The second electrode  130  may have several forms and be positioned in the formation in several ways. For example, the second electrode could be lowered through large cracks or fissures in the formation. In the preferred embodiment, the second electrode  130  is associated with the gas collection well  150 . The second electrode  130  may be a separate component installed inside the collection well  150  or in the proximity of the collection well. Alternatively, the second electrode  130  may be part of the collection well  150  itself. In the embodiment shown in  FIG. 2 , the perforated metallic liner  151  serves as the second electrode  130 . An insulated cable  132  connects the liner  151  with the relatively negative terminal on the power source  112 . The top portion of the well  150  forms an electro-conductive path between the insulated cable  132  and the second electrode  130 . In this arrangement, an electric field  140  is generated through the formation  108  when a voltage drop is created across the electrodes  120 ,  130 . The gas collection well  150  may be installed to depths of 500 meters or greater to reach the hydrate formation.  
         [0018]     Thus far, the first electrode  120  above the formation has been shown connected to the relatively positive terminal, or anode, of the power source  112 , and the second electrode  130  within the formation has been shown connected to the relatively negative terminal, or cathode, of the power source. There is nothing that precludes the first electrode  120  from being connected to the cathode of the power source  112 , and nothing to preclude the second electrode  130  from being connected to the anode of the power source, however. Therefore, the electrode in the formation may be connected with either terminal of the voltage source  112 .  
         [0019]     The electrical resistance of the sediment in the formation is sufficiently low to allow the passage of current through the formation between the first and second electrodes  120 ,  130 . Although the resistivity of the formation  108  is substantially higher than that of the seawater above the electrodes, the current passes directly through the formation because this path is much shorter than any path through the overlying seawater to “ground.” In the preferred embodiment, the second electrode  130  is connected with an insulating break  153  that substantially prevents short circuiting of current up through the well casing.  
         [0020]     To create the electric field  140  and commence resistive heating in the formation, a voltage drop is produced across the electrodes  120 ,  130 . The voltage may be a straight DC voltage or a DC-biased signal with a ripple component produced under modulated AC power. Alternatively, the periodic voltage may be established using pulsed DC power. The voltage may be produced using any technology known in the electrical art. For example, voltage from an AC power supply may be converted to DC using a diode rectifier. The ripple component may be produced using an RC circuit.  
         [0021]     The choice of AC power or DC power depends on many variables, and each option has advantages. One advantage of AC is that AC systems have less potential for corrosion on the electrode than DC. The use of AC also has limitations, including a limited effectiveness at deeper depths. Losses in steel well casings dissipate energy. This dissipation increases with depth, and will typically limit the use of AC to depths of approximately 5,000 feet below the top of the well. Use of AC can be applied at greater depths, but resistive heating may be very limited. Therefore, for well casings and liners extending greater than 5,000 feet, straight DC power is preferable. AC power is desirable in shallower well installations, where losses are less of a factor.  
         [0022]     Where DC power is used to induce destabilization of methane hydrates, the process of producing and recovering methane gas may be enhanced through electro-osmosis and ion migration. In addition, electrochemical reactions such as the production of oxygen and hydrogen may assist in the production of methane. Electrochemical reactions can also create methanol and ethane through oxidation and reduction. The electric potential required for carrying out thermal destabilization of methane hydrates will vary depending on pressure and temperature conditions at the formation, and the size of the desired electric field.  
         [0023]     Referring now to  FIG. 3 , a system  210  in accordance with the present invention includes a high-voltage electric power source  212  located on a barge  215 , and a first electrode  220  incorporated into the structure of the barge. The first electrode  220  is connected to a relatively positive terminal, or anode, of the power source  212 . A gas collection well  250  is drilled into a methane hydrate formation  208 , similar to the embodiment described above. The collection well  250  includes a perforated metallic liner  251  which extends down into the formation  208  and serves as a second electrode  230 . An insulated cable  232  connects the liner  251  with the relatively negative terminal on the power source  212 .  
         [0024]     Based on the foregoing, persons skilled in the art will understand the advantages of system  210  over prior methods for producing gas from gas hydrates. The first electrode  220  is integrally connected with the barge  215 , while the second electrode  230  is a stationary electrode. The position of the first electrode can be adjusted by navigating the barge in different positions relative to the second electrode  230 . By moving the first electrode, the position and intensity of the electric field can be modified. The ability to move electrodes maximizes the range of application of the electric field. Theoretically, the position of the field can be adjusted through an angle of up to 360 degrees around a single stationary electrode. The same benefits may be achieved on land by mounting electrodes on vehicles. For example, it is anticipated that the present invention may be applied in arctic permafrost regions, with electrodes mounted on heavy track machines or all-terrain vehicles. The ability to reposition the electric field greatly reduces the number of bore holes and electrodes that must be installed, since an electric field can be applied over a relatively large area by maneuvering a small number of electrodes around the formation.  
         [0025]     Gas may be captured or collected using a variety of piping arrangements in accordance with the present invention. In  FIG. 2 , the well  150  is connected to a riser pipe or conduit  152  which connects to a storage tank  160  on the barge  115 . In this arrangement, gas can be collected on the barge and transported to shore. The conduit  152  may require special reinforcements or materials suitable for withstanding pressures and currents associated with deep sea installations. These structural reinforcements and materials are generally known and therefore will not be described in detail herein. In addition to storing the gas on the barge  115 , the gas may be used to fuel an electric generator  170  installed on the barge. In this type of system, gas may be piped from the extraction well into a storage tank on the barge, and subsequently fed to a boiler to generate steam. Electricity generated on the barge may then be exported to the mainland by undersea cables. The gas may also be piped from the extraction well directly to land. In  FIG. 3 , the well  250  is connected to undersea piping  252  which transports the gas to a bulk storage plant, power generator, or other facility located on land.  
         [0026]     The terms and expressions which have been employed herein are used as terms of description and not of limitation. There is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof. It is recognized, therefore, that various modifications are possible within the scope and spirit of the invention. Accordingly, the invention incorporates variations that fall within the scope of the following claims.