Abstract:
A method of dissociating methane hydrate deposits in-situ is provided in which a supply of oxygen, carbon dioxide, and fuel are provided, the oxygen and the carbon dioxide are mixed to form an oxidizer fluid, and the fuel is combusted downhole by reacting it with the oxidizer fluid to provide hot combustion products. The combustion products are placed in contact with a diluent fluid to produce a cooled product fluid at a temperature higher than the prevailing methane hydrate decomposition temperature. The cooled product fluid is injected into the methane hydrate deposit decomposing the hydrate and releasing natural gas.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 11/216,239; filed Aug. 30, 2005; which in turn was a continuation-in-part of U.S. patent application Ser. No. 10/891,548; filed Jul. 15, 2004, now U.S. Pat. No. 6,973,968, incorporated herein by reference, which application claimed the benefit of U.S. Provisional Application No. 60/489,876 filed Jul. 22, 2003. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a method for the production of natural gas from methane hydrate deposits. More particularly, the present invention is directed to the release of methane from methane hydrates. 
         [0004]    2. Description of the Related Art 
         [0005]    Methane hydrate deposits are abundant throughout the world and have been estimated to represent by far the greater portion of the world&#39;s fossil energy reserve. Within the United States alone, methane hydrates represent an estimated 200,000 Trillion cubic feet (Tcf) of the total 227,500 Tcf of known natural gas reserves. The methane hydrate deposits, occurring at great depths primarily in the oceans, dwarf the total known combined oil and non-hydrate gas reserves. With the United States largely dependent upon imported fuels, there is an urgent need for a method to economically produce natural gas from the abundant United States methane hydrate reserves. Unfortunately, it has not yet been demonstrated that methane can be recovered from dissociated methane hydrates economically. 
         [0006]    For in-situ dissociation, three approaches exist. One method involves heating the methane hydrate. This requires only about ten percent of the trapped gas heating value, assuming no heat losses. However, it has been found that pumping a heated fluid from the surface to the methane hydrate deposit results in such high heat loss from transporting the heated fluid downhole that essentially all of the heating value of the recovered methane is consumed to supply the needed energy expended in the recovery process. For deep deposits, all the heat is lost in transit downhole. In-situ combustion would minimize such transit heat losses but would be difficult to establish in a hydrate bed and would result in undesirably high bed temperatures. 
         [0007]    A second method for in-situ dissociation involves reducing the in-situ pressure to a value below the methane hydrate dissociation pressure. However, the dissociation energy must still be supplied to the formation. Consequently, the methane hydrate formation temperature decreases thereby requiring even lower pressures for dissociation or heating of the deposit. Accordingly, this approach typically requires mining the solid methane hydrates and pumping a slurry to the surface. Such a mining system has yet to be demonstrated to be economically feasible. 
         [0008]    Another method for in-situ dissociation involves pumping carbon dioxide downhole to displace methane from the methane hydrates by formation of carbon dioxide hydrates. However, this method has not been demonstrated as feasible as the reaction is slow at the deposit temperatures. In addition, conditions in a stable hydrate bed are appropriate for the formation of new methane hydrate from methane and water. Again, it is important in this method to raise the temperature of the deposit to minimize the reformation of methane hydrates. 
         [0009]    It has now been found that heat losses incurred in providing a heated fluid for injection downhole into a methane hydrate bed can be substantially reduced by combusting a fuel downhole within the well casing and tempering the temperature of the combustion product gases by adding fluid in sufficient quantity to produce a heated fluid of a desired temperature for injection into the hydrate bed. Further, if the fuel comprises a compound containing carbon, carbon dioxide is produced. Thus, heat released by carbon dioxide hydrate formation is available to supply a portion of the heat required for methane hydrate decomposition. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The present invention is a method and system for dissociating methane hydrate deposits by downhole production of a heated fluid. The method comprises injecting combustion products containing carbon dioxide into a methane hydrate deposit whereby at least a portion of the deposit may be heated to the temperature at which methane hydrate decomposes at the prevailing pressure. By combustion of a carbonaceous fuel downhole, the heat losses involved in piping a heated fluid downhole are avoided. Further, because the combustion products contain carbon dioxide, a hydrate forming gas, at least a portion of the heat required for methane hydrate decomposition can be supplied by the heat of formation of carbon dioxide hydrates. This works because at a given pressure, carbon dioxide hydrate formation can occur at a slightly higher temperature than that for methane hydrate dissociation. By combining downhole combustion with carbon dioxide displacement of methane, the process energy required is significantly reduced. In the method of the present invention, carbon dioxide rich combustion products provide the heat necessary to heat the deposit to a temperature above the methane hydrate decomposition temperature but below the carbon dioxide hydrate formation temperature. Thus, carbon dioxide hydrate formation is enhanced and provides its heat of formation to supply at least a portion of the decomposition heat required for methane hydrate decomposition. 
         [0011]    It also has been found that in addition to the carbon dioxide formed by fuel combustion, additional carbon dioxide can be supplied from the surface such as for sequestration as hydrate. Carbon dioxide can be part of a carbon dioxide-oxygen oxidizer stream, in admixture with the fuel gas, or as a diluent added to reduce the temperature of the combustion gases. 
         [0012]    The ability to generate heat for methane hydrate dissociation downhole resolves the problem that has limited surface heat generation for transport downhole, namely, heat losses completely or almost completely consuming all of the energy value of the methane produced. Methane dissociation from the hydrate itself according to the present invention only requires approximately 10% of the energy content of the released methane. The present invention eliminates the transport heat losses primarily because the heated fluid is produced adjacent to the methane hydrate deposit. 
         [0013]    Although it is often feasible and cost effective to use flame combustion, catalytic combustion provides a method for stabilizing combustion. Catalytic combustion offers the ability to operate without soot formation, even at stoichiometric ratios without excess oxygen. Further, the ability of catalytic combustion to operate across a wide flammability range simplifies control for a downhole hydrate heating system. 
         [0014]    In one preferred embodiment of the present invention, methane is reacted in the presence of a catalyst using a fuel rich mixture of oxygen (or air), recirculated exhaust and, optionally, added diluent (e.g. CO 2  if available or water). The catalytic reaction products are typically mixed with an oxygen-containing stream and burned. The temperature of the final combustion products is adjusted to the desired level by addition of CO 2  or water before injection into the methane hydrate bed. The injected fluids heat the deposits to the methane hydrate dissociation temperature. This dissociation temperature is a self-limiting temperature for as long as methane hydrate remains since the dissociation reaction is endothermic. The dissociation temperature of methane hydrate is below the dissociation temperature of CO 2  hydrate, yet still hot enough for a relatively high rate of CO 2  hydrate formation from CO 2  and water, a reaction not requiring the direct displacement of methane from methane hydrate by CO 2 . 
         [0015]    The method of the present invention and a device corresponding thereto offer an energy efficient means of dissociating methane hydrate. One embodiment of the present invention comprising providing the energy released by CO 2  hydrate formation offers increased energy efficiency of such a means and system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  depicts a diagrammatic representation of a methane production system according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    As depicted in  FIG. 1 , a methane production system  10  according to the present invention comprises an injection well  12  and a production well  14 . Injection well  12  and production well  14  are representative wellbores well known to one skilled in the oil and gas extraction art. Typically, the wellbores extend substantially downward from a point of access, for example, the surface  16  of a stratum  17 . The wellbores also may extend substantially downward from the surface of a body of water, sediment deposit, permafrost, or other geological formation. In  FIG. 1 , injection well  12  and a production well  14  extend downward through stratum  17 , through a hydrate formation bed  22  containing methane hydrates  24 , and downward from hydrate formation bed  22  into a gas layer  26 . As depicted in  FIG. 1 , injection well  12  is oriented substantially vertical extending downward through hydrate formation bed  22 , and transitions to a substantially horizontal orientation  38  passing beneath and proximate to hydrate formation bed  22 . Horizontal drilling of wellbores is well known to one skilled in the oil and gas extraction art. 
         [0018]    Injection well  12  houses conduit bundle  18 . Conduit bundle  18  generally comprises a plurality of separate channels for fuel  39 , oxidant  40 , and a diluent  41 , and may include additional separate channels for instrumentation or other means or reactants desired downhole. The fuel  39 , oxidant  40 , and diluent  41  pass through conduit bundle  18  into a catalytic combustor  30 . Diluent fluid provides cooling of combustor housing  32  and lowers the temperature of combustion products  34 . The cooled combustion products  34  pass downstream of the catalytic combustor  30  and enter hydrate formation bed  22  through multiple apertures  36  in the wellbore walls  37  of injection well  12  downstream of catalytic combustor  30 . As depicted in  FIG. 1 , combustion products  34  are introduced below hydrate formation bed  22 . Heated methane hydrates  24  decompose and the resultant methane gas  42  migrates into the space beneath the bed forming gas layer  26 , or into an existing gas layer  26 . The methane gas flows toward production well  14  where natural gas is withdrawn through a flow controller  44  known in the art. Where hydrate formation bed  22  is non-horizontal, as is often the case, injection well  12  and production well  14  preferably are configured such that combustion products  34  are introduced proximate to lower portion  46  of hydrate formation bed  22 . Liberated methane gas  42  flows generally upward from the lower portion  48  of gas layer  26  toward wellbottom  50  of production well  14 . 
         [0019]    In one preferred embodiment of the invention wherein methane is reacted in the presence of a catalyst using a fuel rich mixture of oxygen (or air), catalytic combustor  30  comprises a catalytic reactor disclosed in U.S. Pat. No. 6,394,791 (to Smith, et al.), incorporated herein by reference. In the third paragraph of the Summary of the Invention beginning at column 3, line 44, and in the first paragraph of the Detailed Description of the Invention beginning at column 9, line 43, the &#39;791 patent teaches a basic method and apparatus wherein a fuel-rich fuel/air mixture is contacted with a catalyst to oxidize a portion of the fuel contained therein. The catalytic reaction provides both a heat of reaction and a product stream. The catalytic reaction product stream is typically mixed with an oxygen-containing stream and burned. Further, the ability of catalytic combustion to operate across a wide flammability range simplifies control for a downhole hydrate heating system. Such characteristics are described in columns 3-4 of the &#39;791 patent. 
         [0020]    A preferred embodiment of the catalytic reactor disclosed in the &#39;791 patent, for use in a preferred embodiment of present invention, is described in the 18 th  paragraph of the Summary of the Invention beginning at column 6, line 34 of the &#39;791 patent. The method of operation disclosed does not require an ignition delay prior to complete inflammation. The combustion temperature at the stoichiometric interface between the product stream and the heated cooling fluid stream described therein is advantageously reduced. The &#39;791 patent further discloses that by transferring sufficient heat from the fuel-rich product stream to the cooling air stream before contact, the adiabatic flame temperature at the stoichiometric interface between the product stream and the cooling air stream can be reduced to a value well below the adiabatic flame temperature that would exist at the stoichiometric interface in the absence of heat transfer between the streams. The characteristics of a catalytic reactor disclosed in the &#39;791 patent would enhance a system of the present invention. The temperature of the final combustion products can be adjusted to the desired level by addition of CO 2  before injection into the methane hydrate bed. The CO 2  will make contact with the liberated water from a methane hydrate and form a CO 2  hydrate. 
         [0021]    In another embodiment of the method of the present invention, nitrogen dilution of released methane can be avoided by using an admixture of CO 2  and oxygen rather than air as an oxidant. In addition to avoiding nitrogen dilution of the released methane, this embodiment provides a method for carbon dioxide sequestration. This in turn minimizes the high energy consumption required for gas compression. In contrast, pressurizing liquids is a very low energy consumption process. Thus, in this method of the present invention, liquid carbon dioxide may be pumped to a pressure sufficient for injection into the methane hydrate deposit. Similarly, liquid oxygen may be correspondingly pressurized; however, gaseous oxygen may be preferred since only about one fifth of the amount of gas need be compressed as compared to the amount of air that would need to be compressed. The high-pressure oxygen may be mixed with the high-pressure carbon dioxide to form an oxidizer admixture. Although oxygen and carbon dioxide can be delivered downhole via separate conduits, they can be mixed before delivery downhole. Methane or other fuel is delivered downhole, preferably through a separate conduit, and then reacted with the oxidant admixture to generate a heat of combustion and a heated product stream. Advantageously, the combustion temperature is in excess of that required for injection of heated fluids into the deposit. Thus, the combustion gases are mixed with additional fluid (preferably water and/or carbon dioxide) to produce a cooled fluid for injection into contact with the hydrate deposit at a temperature above the methane hydrate decomposition temperature. 
         [0022]    Continuing with  FIG. 1 , combustion products  34  are made available proximate to hydrate formation bed  22  whereby methane hydrate  24  is dissociated both by thermal destabilization and by carbon dioxide displacement. The present invention takes advantage of the fact that at a given pressure, carbon dioxide hydrates can form at a temperature above the decomposition temperature of methane hydrates. Thus, the heat liberated by carbon dioxide hydrate formation is utilized to supply at least a portion of the heat required for the dissociation of the methane hydrate. Without the heat supplied by combustion, the temperature is too low for methane hydrate dissociation or for sufficiently rapid carbon dioxide hydrate formation by displacement of methane from methane hydrates. However, carbon dioxide may be reacted in the formation with water to form carbon dioxide hydrate and liberating the heat of reaction at a temperature above the methane hydrate stability temperature. 
         [0023]    Dissociated methane  42  is then recovered from production well  14  where natural gas is withdrawn using methods known in the art. Typically, production well  14  comprises a conduit  52  in fluid communication with the dissociated methane  42  deposited in gas layer  26 . 
         [0024]    In the present invention, any combustion method known in the art may be used. Reaction by flame combustion is suitable provided the oxygen concentration of the oxidizer gas is sufficient to support a flame. Reaction in the presence of a catalyst allows combustion regardless of oxygen concentration and is an advantageous approach. Suitable conditions for both flame and catalytic combustion are well known in the art. Catalytic combustion has been demonstrated to be especially suited to downhole combustion of heavy oil as described in U.S. Pat. No. 4,930,454 and No. 4,687,491 to Latty et al. However, the rich/lean catalytic system offers the unique advantage for methane production from hydrates in that it has been found that rich catalytic reaction proceeds at a high rate even at the lowest temperatures encountered in hydrate deposits. This allows combustion of methane, the on site fuel, without preheating of fuel or oxidizer. 
         [0025]    Catalytic combustion also offers the capability of combustion at temperature lower than typical flame temperatures thereby minimizing the possibility of over-heating the well piping and making it easier to control the temperature of the gases injected into the strata. This makes it suitable for use with either a vertical or a horizontal injection well. As is known in the art, formation fracturing may be used to improve distribution of the hot gases into the hydrate formation deposit. Use of vertical or horizontal production wells may be used as required. 
         [0026]    Inasmuch as the high-pressure natural gas produced may contain carbon dioxide, as does much gas from conventional natural gas deposits, gas processing may be required. Advantageously, produced gas containing carbon dioxide may be used as fuel. Use of high purity oxygen as the oxidizer avoids diluting the product natural gas. With high purity oxygen as the oxidizer carbon dioxide separated from the produced natural gas may be blended with the oxygen to produce the oxidizer gas. Alternately, separated carbon dioxide may be blended with the fuel. Either lowers the flame temperature on combustion. 
         [0027]    Although conventional scrubbing technology may be used to produce pipeline quality natural gas, it is advantageous to remove carbon dioxide by carbon dioxide hydrate formation. By further compressing the gas, carbon dioxide hydrates may be formed by reaction with cold seawater from the ocean depths. Methods as those disclosed in U.S. Pat. No. 5,660,603 to Elliot, et al. and U.S. Pat. No. 5,434,330 to Hnatow, et al. may be used. 
         [0028]    Although the invention has been described in considerable detail, it will be apparent that the invention is capable of numerous modifications and variations, apparent to those skilled in the art, without departing from the spirit and scope of the invention.