Abstract:
A method and system of producing hydrocarbons from a hydrocarbon containing subterranean hydrate reservoir is disclosed. Waste heat is captured and transferred to a hydrocarbon bearing hydrate formation to dissociate hydrates into natural gas and water. The waste heat can be heat generated from surface facilities such as a Gas To Liquids (GTL) plant, a Liquefied Natural Gas (LNG) plant, an electric or power generation plant, and an onshore or offshore facility producing other conventional or unconventional hydrocarbons from a subterranean reservoir. Alternatively, the waste heat can be obtained from subterranean reservoirs such as hydrocarbon containing producing wells and geothermal wells producing heated water.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the production of hydrocarbons from subterranean hydrocarbon containing hydrate reservoirs. 
       BACKGROUND OF THE INVENTION 
       [0002]    Natural gas hydrates (NGH or clathrate hydrates of natural gases) form when water and certain gas molecules are brought together under suitable conditions of relatively high pressure and low temperature. Under these conditions, the ‘host’ water molecules will form a cage or lattice structure capturing a “guest” gas molecule inside. Large quantities of gas are closely packed together by this mechanism. For example, a cubic meter of methane hydrate contains 0.8 cubic meters of water and typically 164 but up to 172 cubic meters of methane gas. While the most common naturally occurring clathrate on earth is methane hydrate, other gases also form hydrates including hydrocarbon gases such as ethane and propane as well as non-hydrocarbon gases such as CO 2  and H 2 S. 
         [0003]    NGH occur naturally and are widely found in sediments associated with deep permafrost in Arctic and alpine environments and in continental margins at water depths generally greater than 500 meters (1600 feet) at mid to low latitudes and greater than 150-200 meters (500-650 feet) at high latitudes. The thickness of the hydrate stability zone varies with temperature, pressure, composition of the hydrate-forming gas, underlying geologic conditions, water depth, and other factors. 
         [0004]    Worldwide estimates of the natural gas potential of methane hydrates approach 700,000 trillion cubic feet—a staggeringly large figure compared to the 5,500 trillion cubic feet that make up the world&#39;s currently proven gas reserves. Most of the methane hydrate research to date has focused on basic research as well as detection and characterization of hydrate deposits. Extraction methods that are commercially viable and environmentally acceptable are still at an early stage of development. Developing a safe and cost effective method of producing methane hydrate remains a significant technical and economic challenge for the development of hydrate deposits. 
         [0005]    Hydrate dissociation is a strongly endothermic process (i.e., in order to take place, the hydrate must draw in heat from the surrounding environment). The amount of heat available from surrounding geologic strata is often limited and the rate of heat flow is often slow. Initial thinking for hydrate production had been to provide dedicated external heat sources (for example steam boilers) to inject heat (for example hot water or steam) into the hydrate reservoir in order to provide sources of heat to support the endothermic dissociation process and provide consequently higher production rates of hydrocarbons. This is commonly called thermal stimulation. Economic analyses at the time were based on the cost of generating the steam or hot water in dedicated machinery, and showed this technique to be uneconomic. 
         [0006]    Research since that time has turned to production of hydrate reservoirs using depressurization (with endothermic heat provided by the earth itself). This gives understandably lower production rates than those obtainable by direct heating (thermal stimulation) because heat inflow is subject to the aforementioned geologic limits. 
       SUMMARY OF THE INVENTION 
       [0007]    A method of producing hydrocarbons from a hydrocarbon bearing subterranean hydrate reservoir is disclosed. Waste heat from a physically proximate but otherwise unrelated facility or equipment is captured and transferred to a fluid to create a heated fluid. The heated fluid is transferred to a hydrocarbon bearing hydrate formation and heat is transferred to hydrates contained in the hydrate reservoir causing the hydrates to dissociate into natural gas and water. The dissociated natural gas and water is transported to a production facility where the natural gas is processed. Examples of the heated fluid that may be injected in the hydrate formation, includes by way of example, and not limitation: hot fresh or saline water, steam, hot hydrocarbon gases or liquids, CO 2  or nitrogen. 
         [0008]    In one embodiment, the waste heat is heat generated at a heat generating facility on land or on or near the surface of a body of water. By way of example, and not limitation, such heat generating surface facilities can include a Gas To Liquids (GTL) plant, a Liquefied Natural Gas (LNG) plant, an electric or power generation plant, and an onshore or offshore facility producing other conventional or unconventional hydrocarbons from a subterranean reservoir. The phrase “unconventional hydrocarbons” refers to, for example, gas shale, tight gas, coal bed methane, oil shale and oil sand. 
         [0009]    In another alternative, the waste heat is generated from subterranean formations. For example, the waste heat can be produced by subterranean geothermal wells. Another alternative is to use waste heat which is heat captured from produced fluids from a hydrocarbon producing reservoir. Ideally, the waste heat from the produced fluids is transferred to fluids which are injected into the hydrate reservoir to induce hydrate dissociation. Produced fluids may also be directly injected. 
         [0010]    Also disclosed is a system for producing natural gas from a hydrocarbon containing subterranean hydrate reservoir. The system includes a surface facility creating waste heat, a fluid which is heated by the waste heat to produce a heated fluid, a hydrocarbon containing hydrate reservoir, a first conduit carrying the heated fluid to the hydrocarbon containing hydrate reservoir causing hydrates to dissociate into natural gas and water, a second conduit for carrying the dissociated natural gas and water from the hydrocarbon containing hydrate reservoir to a production facility. 
         [0011]    Non-limiting examples of such surface facilities include a Gas To Liquids (GTL) plant, a Liquefied Natural Gas (LNG) plant, an electric generation plant and an onshore or offshore facility producing conventional or unconventional hydrocarbons from a subterranean reservoir. 
         [0012]    Alternatively, a system is described which includes a subterranean source of waste heat rather than the surface facility. As one example, the subterranean source of waste heat may be heated rocks of a geothermal source. As another example, the subterranean source of waste heat may be hydrocarbon producing reservoir and the fluid which is heated is water which receives heat from produced fluids of the hydrocarbon producing reservoir. Heat from the produced fluid may be transferred, such as by using a heat exchanger, to the fluid that is to be reinjected to cause dissociation of hydrates. Produced fluids may also be directly injected. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    These and other objects, features and advantages of the present invention will become better understood with regard to the following description, appended claims and accompanying drawing where: 
           [0014]      FIG. 1  is a schematic drawing of a first embodiment wherein waste heat from a surface facility is transported to and used to enhance hydrocarbon recovery from a hydrate reservoir; and 
           [0015]      FIG. 2  is a schematic drawing of a second embodiment in which heat from a geothermal well and from a subterranean formation is used to enhance hydrocarbon recovery from a hydrate reservoir. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    Available waste heat can be advantageously used as a source for providing thermal stimulation. This waste heat could come, for example, via heat exchangers from a production facility&#39;s power generation system, compressors, or from conventional or unconventional oil and gas production. Unconventional oil and gas production refers to, for example, gas shale, tight gas, coal bed methane, oil shale and oil sands. Larger sources of waste heat could, for example, be provided by co-located power plants or chemical plants such as Gas To Liquids (GTL) or Liquefied Natural Gas (LNG) plants. Co-location with a GTL plant may be particularly beneficial because the GTL process is highly exothermic and GTL plants require large supplies of methane both for fuel an as a raw material. The GTL plant generates large amounts of waste heat, which would be directed into the hydrate reservoir, which would generate large amounts of methane gas at high rates to feed the GTL plant. Additional synergistic benefits would arise from GTL co-location if the hydrate reservoir is geographically remote from potential consumers. GTL products (synthetic liquid fuels) are much more easily transported from remote locations than natural gas. The same principles would apply to an LNG plant. 
         [0017]    Further, the availability of a ‘heat sink’ in the hydrate reservoir might mean that GTL plant construction and operation costs could be reduced, due to possible elimination or at least reduction in size of the traditional plant, i.e., cooling system (cooling towers and/or fin fan heat exchangers). 
         [0018]    Also, the large quantities of fresh water liberated during hydrate dissociation and produced to the surface during hydrate production would be available to be routed to the GTL or LNG plant and used as a fluid for heat exchange. 
         [0019]      FIG. 1  shows a first embodiment of a system  20  for producing hydrocarbons from a hydrate formation  22 . An overlying formation  28  is disposed above the hydrate formation  22 , acting as a top seal and also providing limited geologic heat to support the endothermic dissociation of hydrate formation  22 . Located below hydrate formation  22  is a supporting formation  30  that provides the majority of geologic heat to support the endothermic dissociation of hydrate formation  22 . An hour glass shaped dissociation zone  26  is formed between producer well  36  and hydrate formation  22 . Producer well  36  can be either vertical (as shown) or any other orientation. Producer well  36  provides a lower pressure region surrounding producer well  36  which allows hydrates in hydrate formation  22  to dissociate and flow into the wellbore. To enhance the dissociation of hydrates in hydrate formation  22 , a number of injector wells  34  provide a heated fluid, such as heated water or steam to hydrate formation  22 . Ideally, the heat will pass into hydrate formation  22  and cause natural gas and water to be formed from the dissociation of natural gas hydrate formation  22 . Injector wells  34  can terminate in the hydrate reservoir or continue to a deeper geobody with suitable characteristics to contain the spent heated fluid. 
         [0020]    Fluids are communicated from dissociated zone  26  by way of producer well  36 . The produced fluids are ideally separated by a separator  40  and passed to hydrocarbon and water deliver lines  42  and  44  for further treatment, storage, transport, or use as a heat exchange fluid in plants  50 ,  60 ,  70  or  80 . 
         [0021]    The heated fluid ideally may come from sources for which heat otherwise may be problematic to dispose. As a first example, utilizing waste heat from a Gas To Liquids (GTL) plant  50 , via heat exchangers and piping (not shown), may provide thermal stimulation to hydrate formation  22 . A second exemplary embodiment may use waste heat from a Liquefied Natural Gas (LNG) plant  60 , via heat exchangers and piping (not shown), to provide thermal stimulation to hydrate formation  22 . A third exemplary embodiment may utilize waste heat from an electric generation plant  70  (for example a gas-fired steam turbine plant or gas turbine cogeneration plant, via heat exchangers and piping (not shown), to provide thermal stimulation to hydrate formation  22 . A fourth exemplary embodiment employs heat delivered via heated water from oil and/or gas production facilities or structures  80  (for example from power generation systems, compressors or produced gas and oil coolers), via heat exchangers and piping (not shown), to provide thermal stimulation of hydrate formation  22 . 
         [0022]    Injection of heated fluid into hydrate formation  22  could be achieved for example by use of pumps  40  or by gravity flow. 
         [0023]      FIG. 2  illustrates other alternatives for providing heat to a hydrocarbon containing hydrate reservoir  122 . Hydrate reservoir  122  bounds a generally hour glass shaped dissociated zone  126  through which a producer well  136  passes. Dissociated zone  126  contains hydrocarbons and water and other constituents that are released from hydrate reservoir  122 . Producer well  136  delivers water and hydrocarbons to a separator  140 . Water and hydrocarbons can be separated and delivered through hydrocarbon and water conduits  142  and  144 . 
         [0024]    A geothermal source  150  of water located above or below the hydrate reservoir  122  may be tapped to obtain a heated source of water. The heated water can then be returned to the surface where it pumped down injector well  134  into hydrate formation  122 . The heat from the water assists in the dissociation of hydrocarbons and water from hydrate formation  122  and release into dissociated zone  126 . Alternatively, the heated water could be delivered directly to well  134  by an auxiliary conduit  136  without the need to first return the water to the surface. Or else, heat from the heated water or steam may be passed to or exchanged with the fluid to be injected. 
         [0025]    As another alternative source of heat available to provide heat to hydrate reservoir  122 , producing hydrocarbon reservoir  260  delivers hot produced fluids to a producing well  262 . The produced fluids, i.e., hydrocarbons such as oil and natural gas, along with produced water, are passed up producing well  262  to a platform containing a separator  280 . Again, the produced fluids may be separated into hydrocarbons and water and exported by way of hydrocarbon conduit  282  and  284 . A heat exchanger  280  may be created around the tubing in producer well  262 . Water produced at the surface, or else otherwise available such as sea water, may be passed down to the lower portion of heat exchanger  280  and the water allowed to pass upwardly along hydrate reservoir  122 . The heat from the produced fluids in producer well  262  will transfer from the heat exchanger into the passing water and then into hydrate formation  122 . Again, the introduced “waste heat” from reservoir  260  is used to enhance production from hydrate formation  122  by inducing dissociation of the hydrates. 
         [0026]    While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to alteration and that certain other details described herein can vary considerably without departing from the basic principles of the invention.