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CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/817,428, filed Aug. 12, 2013, which is a national stage entry of PCT application US2011/048325, filed Aug. 18, 2011, which claims priority of U.S. Provisional Patent Application Ser. No. 61/374,778, filed Aug. 18, 2010, all of which is incorporated herein by reference in its entirety and for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to methods and systems for production of hydrocarbons from various subsurface formations. 
         [0003]    Steam-assisted gravity drainage (SAGD) is used to recover hydrocarbons from a subsurface formation from fields where the hydrocarbons from a subsurface formation is extremely dense or has high viscosity. In this regard, steam from a horizontal wellbore is used to decrease the viscosity and to cause the hydrocarbons from a subsurface formation to drain into a second horizontal wellbore. 
       SUMMARY OF THE INVENTION 
       [0004]    Various embodiments of the present invention provide for improved delivery of thermal energy, or heat, to increase the efficiency of recovery of hydrocarbons from a subsurface formation using horizontal wellbores. 
         [0005]    In one aspect, the invention relates to a method comprising heating a heat transfer fluid; circulating the heat transfer fluid into a vertical bore to a heat exchanger; advancing feedwater into the vertical bore to the heat exchanger, wherein the heat exchanger is configured to transfer heat from the heat transfer fluid to the feedwater to generate steam; transmitting the steam from the heat exchanger into a horizontal wellbore to cause heating of a subterranean region; and returning the heat transfer fluid from the heat exchanger to the surface. 
         [0006]    In another aspect, the invention relates to a system comprising a vertical bore; a heat exchanger positioned at a down-hole position of the vertical bore; a horizontal wellbore leading from the down-hole position of the vertical bore; a heat transfer fluid loop system for circulating heated heat transfer fluid into a vertical bore to the heat exchanger; a feedwater feed system to provide feedwater into the vertical bore to the heat exchanger, wherein the heat exchanger is configured to transfer heat from the heated heat transfer fluid to the feedwater to generate steam; wherein the steam is transmitted from the heat exchanger into the horizontal wellbore to cause heating of a subterranean region; and wherein the heat transfer fluid loop system is configured to return the heat transfer fluid from the heat exchanger to the surface. 
         [0007]    In another aspect, the invention relates to a method comprising heating a heat transfer fluid; circulating the heat transfer fluid into a subterranean horizontal wellbore; advancing feedwater into the subterranean horizontal wellbore, wherein heat transfer from the heated heat transfer fluid to the feedwater generates steam for causing heating of a subterranean region; and returning the heat transfer fluid from the horizontal wellbore to the surface, wherein the horizontal wellbore is divided into a plurality of steam chambers, at least one of the steam chambers having a heat exchanger to facilitate transfer of heat from the heat transfer fluid to the feedwater. 
         [0008]    In another aspect, the invention relates to a system comprising a subterranean horizontal wellbore; a heat transfer fluid loop system for circulating heated heat transfer fluid into the horizontal wellbore; a feedwater feed system to provide feedwater into the horizontal wellbore, wherein heat transfer from the heated heat transfer fluid to the feedwater generates steam for causing heating of a subterranean region; and wherein the heat transfer fluid loop system is configured to return the heat transfer fluid from the horizontal wellbore to the surface, and wherein the horizontal wellbore is divided into a plurality of steam chambers, at least one of the steam chambers having a heat exchanger to facilitate transfer of heat from the heat transfer fluid to the feedwater. 
         [0009]    In another aspect, the invention relates to a method comprising heating a heat transfer fluid; circulating the heat transfer fluid into a subterranean horizontal wellbore; causing transfer of heat from the heat transfer fluid to a subterranean region; returning the heat transfer fluid from the horizontal wellbore to the surface, wherein the horizontal wellbore includes one or more heat exchangers to facilitate transfer of heat directly from the heat transfer fluid to the subterranean region. 
         [0010]    In another aspect, the invention relates to a system comprising a subterranean horizontal wellbore; a heat transfer fluid loop system for circulating heated heat transfer fluid into the horizontal wellbore, wherein heat is transferred directly from the heated heat transfer fluid to a subterranean region; and wherein the heat transfer fluid loop system is configured to return the heat transfer fluid from the horizontal wellbore to the surface, and wherein the horizontal wellbore includes one or more heat exchangers to facilitate transfer of heat directly from the heat transfer fluid to the subterranean region. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a cross-sectional view of a horizontal wellbore arrangement in accordance with an embodiment of the present invention; 
           [0012]      FIG. 2  is a cross-sectional view of a horizontal wellbore arrangement in accordance with another embodiment of the present invention; 
           [0013]      FIG. 3  is a schematic illustration of a down-hole heat exchanger; 
           [0014]      FIG. 4  is a schematic illustration of another embodiment of a down-hole heat exchanger; 
           [0015]      FIG. 5  is a cross-sectional view of a horizontal wellbore arrangement in accordance with another embodiment; 
           [0016]      FIG. 6  is a cross-sectional view of a horizontal wellbore arrangement in accordance with another embodiment; 
           [0017]      FIG. 7  is a cross-sectional view of a horizontal wellbore arrangement in accordance with another embodiment; and 
           [0018]      FIG. 8  is a cross-sectional view of a horizontal wellbore arrangement in accordance with another embodiment. 
       
    
    
       [0019]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and may herein he described in detail. The drawings may not be to scale. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined b the appended claims. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Concerns over depletion of available hydrocarbon resources and concerns over declining overall quality of produced hydrocarbons have led to development of processes for more efficient recovery, processing and/or use of available hydrocarbon resources. In situ processes may be used to remove hydrocarbon materials from subterranean formations. Chemical and/or physical properties of hydrocarbon material in a subterranean formation may need to be changed to allow hydrocarbon material to be more easily removed from the subterranean formation. The chemical and physical changes may include in situ reactions that produce removable fluids, composition changes, solubility changes, density changes, phase changes, and/or viscosity changes of the hydrocarbon material in the formation. A heat transfer fluid may be, but is not limited to, a gas, a liquid, an emulsion, a slurry, and/or a stream of solid particles that has flow characteristics similar to liquid flow. 
         [0021]    In some embodiments, an expandable tubular may be used in a wellbore. Expandable tubulars are described in, for example, U.S. Pat. No. 5,366,012 to Lohbeck and U.S. Pat. No. 6,354,373 to Vercaemer et al., each of which is incorporated by reference as if fully set forth herein. 
         [0022]    Heaters may be placed in wellbores to heat a formation during an in situ process. Examples of in situ processes utilizing downhole heaters are illustrated in U.S. Pat. No. 2,634,961 to Ljungstrom; U.S. Pat. No. 2,732,195 to Ljungstrom; U.S. Pat. No. 2,780,450 to Ljungstrom; U.S. Pat. No. 2,789,805 to Ljungstrom; U.S. Pat. No. 2,923,535 to Ljungstrom; and U.S. Pat. No. 4,886,118 to Van Meurs et al.; each of which is incorporated by reference as if fully set forth herein. 
         [0023]    Heat may be applied to the oil shale formation to pyrolyze kerogen in the oil shale formation. The heat may also fracture the formation to increase permeability of the formation. The increased permeability may allow formation fluid to travel to a production well where the fluid is removed from the oil shale formation. 
         [0024]    A heat source may be used to heat a subterranean formation. Heaters may be used to heat the subterranean formation by radiation and/or conduction. 
         [0025]    The heating element generates conductive and/or radiant energy that heats the casing. A granular solid fill material may be placed between the casing and the formation. The casing may conductively heat the fill material, which in turn conductively heats the formation. 
         [0026]    In typical SAGD hydrocarbons from a subsurface formation recovery, the steam is generated on the surface and transmitted to the horizontal wellbore. The great distance traveled by the steam can result in degradation of the steam through heat loss. Thus, the steam that is delivered to the hydrocarbons from a subsurface formation field, for example, may not be a high-quality steam, resulting in reduced hydrocarbons from a subsurface formation recovery. 
         [0027]    Embodiments of the present invention are directed to various methods and systems for recovering resources using horizontal wellbore in geological strata from a vertical position. The geological structures intended to be penetrated in this fashion may be coal seams, in situ gasification or methane drainage, or in hydrocarbons from a subsurface formation bearing strata for increasing the flow rate from a pre-existing wellbore. Other possible uses for the disclosed embodiments may be for use in the leaching of uranium ore from underground formation or for introducing horizontal channels for feedwater and steam injections, for example. Those skilled in the art will understand that the various embodiments disclosed herein may have other uses which are contemplated within the scope of the present invention. 
         [0028]    Referring first to  FIG. 1 , a cross-sectional view of a horizontal wellbore arrangement  100  in accordance with an embodiment of the present invention is illustrated. In accordance with the arrangement  100  of  FIG. 1 , heat loss is reduced through the use of a down-hole heat exchange system  110 . Certain embodiments of a down-hole heat exchanger  110  are described in greater detail below with reference to  FIGS. 3 and 4 . Of course, those skilled in the art will comprehend that embodiments of the present invention are not limited to use a particular heat exchanger and various other heat exchangers are contemplated within the scope of the present invention. 
         [0029]    In accordance with the embodiment illustrated in  FIG. 1 , the down-hole heat exchange system  110  is positioned within a first wellbore  130 . In various embodiments, the depth of the heat exchanger may be varied according to various factors, such as cost and environmental conditions. For example, in various embodiments, the depth of the first horizontal wellbore  130  may be between several hundred feet and several thousand feet. 
         [0030]    In the embodiment of  FIG. 1 , the first wellbore  130  includes concentric strings formed to allow various fluids to flow therethrough. Feed feedwater is injected into the first wellbore  130  through a string  120 . The down-hole heat exchange system  110  is configured to flash the hot feedwater into steam, and the steam is directed into the hydrocarbons from a subsurface formation through, for example, perforations in the wellbore  130 . The perforations  180  are schematically illustrated in  FIG. 1  at the entrance to the horizontal portion of the first wellbore  130 . Steam is directed into the horizontal portion of the first wellbore  130 , and into the geologic strata around the horizontal portion of the first wellbore  130 . 
         [0031]    The steam adds thermal energy to the hydrocarbons from a subsurface formation and serves to reduce the viscosity of the hydrocarbons from a subsurface formation deposit, causing the hydrocarbons from a subsurface formation to flow downward due to gravity. The downward flowing hydrocarbons from a subsurface formation are captured in a second wellbore, which is a production wellbore  140 . The hydrocarbons from a subsurface formation captured in the production wellbore  140  are transported to one or more tanks  199  on the surface, for example, through a production line  190 . 
         [0032]    In the embodiment of  FIG. 1 , as wellbore as in the various other embodiments described herein, the horizontal wellbores and the various strings or pipes may be formed of coiled tubing. Coiled tubing is well known to those skilled in the art and refers generally to metal piping that is spooled on a large reel. Coiled tubing may have a diameter of between about one inch and about 3.25 inches. Of course, those skilled in the art will understand that the various embodiments are not limited to coiled tubing, nor to any particular dimensions of tubing. 
         [0033]    Referring again to  FIG. 1 , a heated heat transfer fluid is delivered through a heat transfer fluid inlet string  112 . In the illustrated embodiment, the heat transfer fluid inlet string  112  is the center-most string in the concentric configuration. The heated heat transfer fluid is provided from the surface to a position within the wellbore. The heated heat transfer fluid is pumped through the heat transfer fluid inlet string  112  at a very high flow rate to minimize loss of heat to the feedwater. In one embodiment, the heat transfer fluid inlet string  112  is a tube having a diameter of approximately 0.75 inches or more. In other embodiments, the heat transfer fluid inlet string  112  may be sized according to factors such as pump capability, distance between surface and the horizontal portion of the wellbore, and the type of heat transfer fluid, for example. 
         [0034]    Additionally, hot feedwater is injected into a separate string  120  of the concentric configuration. The feedwater may be injected at a superheated temperature to maximize the thermal energy delivered to the hydrocarbons from a subsurface formation. In the illustrated embodiment, the hot feedwater string  120  is the outermost string in the concentric configuration. 
         [0035]    At a certain depth of the wellbore, the heated heat transfer fluid in the heat transfer fluid inlet string  112  flashes the hot feedwater into high-quality steam which is directed into the first wellbore  130  ( FIG. 1 ) through a wellbore  126  and perforations  180 . A purging valve  124  may allow low-quality steam and scale to be directed into a sump. 
         [0036]    After transfer of heat from the heat transfer fluid to the feedwater, the cooled transfer fluid is returned to the surface through a cold heat transfer fluid outlet string  114 . A layer of insulation  128  may be provided between the heat transfer fluid inlet string  112  and the cold heat transfer fluid outlet string  114 . In the concentric tubing configuration, the cold heat transfer fluid outlet string  114 . In one embodiment, the concentric tubing configuration has an outer diameter of between 2.5 and 3 inches, and in a particular embodiment has an outer diameter of 2.875 inches, but can be larger depending on each concentric tubing configuration. 
         [0037]    In certain embodiments, the heat transfer fluid may be circulated through a closed-loop system. In this regard, a heater may be configured to heat a heat transfer fluid to a high temperature. The heater may be positioned on the surface and is configured to operate on any of a variety of energy sources. For example, in one embodiment, the heater  111  operates using combustion of a fuel that may include natural gas, propane or methanol. The heater  111  can also operate on electricity. 
         [0038]    The heat transfer fluid is heated by the heater to a very high temperature. In this regard, the heat transfer fluid should have a very high boiling point. In one embodiment, the heat transfer fluid is molten salt with a boiling temperature of approximately 1150° F. Thus, the heater heats the heat transfer fluid to a temperature as high as 1150° F. In other embodiments, the heat transfer fluid is heated to a temperature of 900° F. or another temperature. Preferably, the heat transfer fluid is heated to a temperature that is greater than 700° F. 
         [0039]    A heat transfer fluid pump is preferably positioned on the cold side of the heater. The pump may be sized according to the particular needs of the system as implemented. Additionally, a reserve storage flask containing additional heat transfer fluid is included in the closed loop to ensure sufficient heat transfer fluid in the system. 
         [0040]    The concentricity of the various strings in the first wellbore  130  is illustrated in the cross-sectional view illustrated in  FIG. 1  and taken along I-I. In the illustrated embodiment, the hot heat transfer fluid is carried downward through an innermost string  112 , and the cooled transfer fluid is returned upward through the second innermost string  114 . A layer of insulation is provided between the two innermost strings to prevent heat transfer from the heated heat transfer fluid to the cooled transfer fluid being returned. Feed feedwater is carried downward through the outermost string  120 . In this regard, the feed feedwater may absorb some residual heat from the cooled transfer fluid being returned. 
         [0041]    Referring now to  FIG. 2 , a cross-sectional view of a horizontal wellbore arrangement  100   a  in accordance with another embodiment of the present invention is illustrated. The embodiment illustrated in  FIG. 2  is similar to that illustrated in  FIG. 1 , but with a single wellbore bore. In this regard, a single vertical wellbore bore splits into two horizontal wellbores  130 ,  140 . In this regard, the concentricity of the strings includes the production line  190 , as illustrated in  FIG. 2  and taken along II-II. In the illustrated embodiment, the hot heat transfer fluid is carried downward through an innermost string  112 , and the cooled transfer fluid is returned upward through the second innermost string  114 . A layer of insulation is provided between the two innermost strings to prevent heat transfer from the heated heat transfer fluid to the cooled transfer fluid being returned. Feed feedwater is carried downward through the third innermost string  120 . Finally, the outmost string  190 , which may only be partially concentric, is used to carry the produced resource to the surface. 
         [0042]    Referring now to  FIG. 3 , a schematic illustration of a down-hole heat exchanger is illustrated. At the down-hole heat exchanger  110  shown in  FIG. 3 , inlet tubing  112  connects to a heat exchanger tubing  302  within a steam chamber portion  126  of the downhole heat exchanger  110 . The heat transfer fluid from the inlet tubing  112  passes through heat exchanger tubing. Heat from heat exchanger tubing  302  vaporizes the feed feedwater in string  120  within steam chamber portion  126 . Vapor enters the steam chamber portion  126  so that the steam is evenly distributed and maintained at high quality or even superheated by heat from the downward-extending heat exchanger tubing  302 . After passing through downhole heat exchanger  110  and the heat exchanger tubing  302 , return heat transfer fluid ascends in the an outlet tubing  114 . 
         [0043]    A packer assembly  303  with a feed valve  304  controls the rate of feedwater into downhole heat exchanger  110 . In one embodiment, the feed valve  304  responds to the pressure differences between the feed feedwater at the base of the feed feedwater string  120  and the vapor pressure within the steam chamber portion  126  so that vapor quality is maintained at a high value. 
         [0044]    In one embodiment, scale buildup on heat exchanger tubing  302  is reduced because of the narrow diameter of this tubing which causes the scale to periodically slough off. This sloughed-off scale may then build up at the base of heat exchanger  110 . A purging valve  124  may be periodically opened to drain this accumulated scale into a sump of the wellbore. 
         [0045]    Referring now to  FIG. 4 , a schematic illustration of another embodiment of a down-hole heat exchanger is illustrated. The down-hole heat exchanger  210  of  FIG. 4  is similar to the down-hole heat exchanger  110  of  FIG. 3 . In the embodiment of  FIG. 4 , a line  223  containing hot heat transfer fluid may extend below the heat-exchange point. In this regard, heat transfer from the heat transfer fluid to the hot feedwater or steam may be provided deeper into the vertical bore of the wellbore. 
         [0046]    Referring now to  FIG. 5 , a cross-sectional view of a horizontal wellbore arrangement  400  in accordance with another embodiment of the present invention is illustrated. 
         [0047]    In the embodiment of  FIG. 5 , a first wellbore  430  includes concentric strings formed to allow various fluids to flow therethrough. A heat transfer fluid is pumped into the first wellbore  430  through a closed loop system  410 . Hot heat transfer fluid is pumped into the first wellbore  430  through a hot heat transfer fluid line  412 , and cooled transfer fluid is returned through a return line  414 . In order to minimize heat loss from the hot heat transfer fluid, insulation  428  may be provided between the hot heat transfer fluid line  412  and the return line  414 . A boiler  411  heats the heat transfer fluid for pumping into the wellbore. The closed loop system  410  may include other components, such as a pump and a reservoir of heat transfer fluid. The heat transfer fluid circulates substantially through the entire length of the horizontal first wellbore  430 . 
         [0048]    Hot feedwater is pumped into the first wellbore  430  through a line  420 . In the horizontal portion, the hot feedwater line  420  is positioned above the heat transfer fluid lines  412 ,  414 . Heat transfer from the heat transfer fluid lines  412 ,  414  to the hot feedwater line  420  and flashed on the heat exchanger produces steam which is injected into the hydrocarbons from a subsurface formation deposit. Additionally, heat from the heat transfer fluid lines  412 ,  414  may be directly transferred to the hydrocarbon formation surrounding the first wellbore  430 . 
         [0049]    As noted above, the steam adds thermal energy to the hydrocarbons from a subsurface formation and serves to reduce the viscosity of the hydrocarbons from a subsurface formation, causing the hydrocarbons from a subsurface formation to flow downward due to gravity. The downward flowing hydrocarbons from a subsurface formation are captured in a second wellbore, which is a production wellbore  440 . The hydrocarbons from a subsurface formation captured in the production wellbore  440  are transported to one or more tanks  499  on the surface, for example, through a production line  490 . 
         [0050]    The heated heat transfer fluid is pumped through the heat transfer fluid inlet string  412  at a very high flow rate to minimize loss of heat to the sea feedwater. In one embodiment, the heat transfer fluid inlet string  412  is a tube having a diameter of approximately 0.75 inches or more. In other embodiments, the heat transfer fluid inlet string  412  may be sized according to factors such as pump capability, distance between surface and the horizontal portion of the pump, and the type of heat transfer fluid, for example. 
         [0051]    After transfer of heat from the heat transfer fluid to the feedwater, the cooled transfer fluid is returned to the surface through a cold heat transfer fluid outlet string  414 . A layer of insulation  428  may be provided between the heat transfer fluid inlet string  412  and the cold heat transfer fluid outlet string  414 . In the concentric configuration, the cold heat transfer fluid outlet string  414  is an annulus. In one embodiment, the annulus has an outer diameter of between 2.5 and 3 inches, and in a particular embodiment has an outer diameter of 2.875 inches. 
         [0052]    The heat transfer fluid is heated by the heater to a very high temperature. In this regard, the heat transfer fluid should have a very high boiling point. In one embodiment, the heat transfer fluid is molten salt with a boiling temperature of approximately 1150° F. Thus, the heater heats the heat transfer fluid to a temperature as high as 1150° F. In other embodiments, the heat transfer fluid is heated to a temperature of 900° F. or another temperature. Preferably, the heat transfer fluid is heated to a temperature that is greater than 700° F. The heat transfer fluid deemed appropriate by those skilled in the art that may be injected into the wellbore such as diesel oil, gas oil, molten sodium, and synthetic heat transfer fluids, e.g., THERMINOL 59 heat transfer fluid which is commercially available from Solutia, Inc., MARLOTHERM heat transfer fluid which is commercially available from Condea Vista Co., and SYLTHERM and DOWTHERM heat transfer fluids which are commercially available from The Dow Chemical Company. 
         [0053]    A heat transfer fluid pump is preferably positioned on the cold side of the heater  411 . The pump may be sized according to the particular needs of the system as implemented. Additionally, a reserve storage flask containing additional heat transfer fluid is included in the closed loop to ensure sufficient heat transfer fluid in the system. 
         [0054]    Various embodiments of the concentricity of the various strings in the first wellbore  430  are illustrated in the cross-sectional view illustrated in  FIG. 5  and taken along V-V. In the illustrated embodiments, the hot heat transfer fluid is carried downward through an innermost string  412 , and the cooled transfer fluid string  414  may be the second innermost ring, followed by the feedwater string  420 . In another illustrated embodiment, the cooled transfer fluid string  414  and the feedwater string  420  may be switched. A layer of insulation is provided between the two innermost strings to prevent heat transfer from the heated heat transfer fluid. 
         [0055]    In the embodiment illustrated in  FIG. 5 , the horizontal portion of the first wellbore  430  is divided into a plurality of steam chambers  450 . The steam chambers are separated by packers  452  which contain a valve to facilitate equalization of steam pressure in each steam chamber  450 . Further, each chamber  450  may include a heat exchanger  454  to facilitate transfer of heat between the heat transfer fluid in the inlet string  412  and the feed feedwater. The separation of the horizontal portion into a plurality of chambers  450 , combined with the heat exchangers  454 , improves the distribution and quality of steam in the horizontal portion, thereby increasing the production of hydrocarbons from a subsurface formation, for example. The heat exchangers may include heat exchanger tubing similar to the tubing  302  described above with reference to  FIG. 3 . 
         [0056]    Referring now to  FIG. 6 , a cross-sectional view of a horizontal wellbore arrangement  400   a  in accordance with another embodiment of the present invention is illustrated. The embodiment illustrated in  FIG. 6  is similar to that illustrated in  FIG. 5 , but with a single wellbore bore. In this regard, a single vertical wellbore bore splits into two horizontal wellbores  430 ,  440 . In this regard, the concentricity of the strings includes the production line  490 , as illustrated in  FIG. 6  and taken along VI-VI. In the illustrated embodiment, the hot heat transfer fluid is carried downward through an innermost string  112 , and the cooled transfer fluid and the feed feedwater are transported in the second and third strings. A layer of insulation is provided between the two innermost strings to prevent heat transfer from the heated heat transfer fluid. Finally, the outmost string  490 , which may only be partially concentric, is used to carry the produced resource to the surface. 
         [0057]    Referring now to  FIG. 7 , a cross-sectional view of a horizontal wellbore arrangement in accordance with another embodiment is illustrated. The horizontal wellbore arrangement  500  includes a first wellbore  530  for providing thermal energy to the hydrocarbons from a subsurface formation and a production wellbore  540  for delivering recovered hydrocarbons from a subsurface formation to the surface. In the embodiment of  FIG. 7 , the heat transfer fluid is pumped into the first wellbore  530  through a closed loop system  510 . Hot heat transfer fluid is pumped into the first wellbore  530  through a hot heat transfer fluid line  512 , and cooled transfer fluid is returned through a return line  514 . In order to minimize heat loss from the hot heat transfer fluid, insulation  528  may be provided between the hot heat transfer fluid line  512  and the return line  514 . A boiler  511  heats the heat transfer fluid for pumping into the wellbore. The closed loop system  510  may include other components, such as a pump and a reservoir of heat transfer fluid. The heat transfer fluid circulates substantially through the entire length of the horizontal first wellbore  530 . 
         [0058]    In the embodiment of  FIG. 7 , there is no need for hot feedwater to be injected into the wellbore. Instead, thermal energy by conductive and/or ambient heat is directly transferred from the heat transfer fluid lines  512 ,  514  to the hydrocarbons from a subsurface formation surrounding the first wellbore  530 . In this regard, the hydrocarbons from a subsurface formation captured by the production wellbore  540  have a significantly higher hydrocarbon-to-feedwater ratio. The horizontal wellbore includes heat exchangers  550  to facilitate the direct transfer of conductive and/or ambient heat from the heat transfer fluid to the hydrocarbons from a subsurface formation deposit. 
         [0059]    The concentricity of the various strings in the first wellbore  530  is illustrated in the cross-sectional view illustrated in  FIG. 7  and taken along VII-VII. In the illustrated embodiment, the hot heat transfer fluid is carried downward through an inner string  512 , and the cooled transfer fluid is returned upward through the outer string  514 . A layer of insulation is provided between the two strings to prevent heat transfer from the heated heat transfer fluid to the cooled transfer fluid being returned. 
         [0060]    Referring now to  FIG. 8 , a cross-sectional view of a horizontal wellbore arrangement  500   a  in accordance with another embodiment of the present invention is illustrated. The embodiment illustrated in  FIG. 8  is similar to that illustrated in  FIG. 7 , but with a single wellbore bore. In this regard, a single vertical wellbore bore splits into two horizontal wellbores  530 ,  540 . In this regard, the concentricity of the strings includes the production line  590 , as illustrated in  FIG. 8  and taken along VIII-VIII. In the illustrated embodiment, the hot heat transfer fluid is carried downward through an inner string  512 , and the cooled transfer fluid is transported in an outer string. A layer of insulation is provided between the two strings to prevent heat transfer from the heated heat transfer fluid. Finally, the outermost string  590 , which may only be partially concentric, is used to carry the produced resource to the surface. 
         [0061]    Thus, embodiments described herein generally relate to systems, methods, and heaters for treating a subsurface formation. Embodiments described herein also generally relate to heaters that have novel components therein. Such heaters can be obtained by using the systems and methods described herein. 
         [0062]    In certain embodiments, the invention provides one or more systems, methods, and/or heaters. In some embodiments, the systems, methods, and/or heaters are used for treating a subsurface formation. 
         [0063]    In some embodiments, an in situ heat treatment system for producing hydrocarbons from a subsurface formation includes a plurality of wellbores in the formation; piping positioned in at least two of the wellbores; a fluid circulation system coupled to the piping; and a heat supply configured to heat a heat transfer fluid continually circulated through the piping to heat the temperature of the formation to temperatures that allow for hydrocarbon production from the formation. 
         [0064]    In some embodiments, a method of heating a subsurface formation includes heating a heat transfer fluid using heat exchange with a heat supply; continually circulating the heat transfer fluid through piping in the formation to heat a portion of the formation to allow hydrocarbons to be produced from the formation; and producing hydrocarbons from the formation. 
         [0065]    In some embodiments, a method of heating a subsurface formation includes passing a heat transfer fluid from a surface boiler to a heat exchanger; heating the heat transfer fluid to a first temperature; flowing the heat transfer fluid through a heater section to a sump, wherein heat transfers from the heater section to a treatment area in the formation; gas lifting the heat transfer fluid to the surface from the sump; and returning at least a portion of the heat transfer fluid to the vessel. 
         [0066]    In further embodiments, features from specific embodiments may be combined with features from other embodiments. For example, features from one embodiment may be combined with features from any of the other embodiments. 
         [0067]    In further embodiments, treating a subsurface formation is performed using any of the methods, systems, or heaters described herein. 
         [0068]    In further embodiments, additional features may be added to the specific embodiments described herein. 
         [0069]    The foregoing description of embodiments has been presented for purposes of illustration and description. The foregoing description is not intended to be exhaustive or to limit embodiments of the present invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of various embodiments. The embodiments discussed herein were chosen and described in order to explain the principles and the nature of various embodiments and its practical application to enable one skilled in the art to utilize the present invention in various embodiments and with various modifications as are suited to the particular use contemplated. The features of the embodiments described herein may be combined in all possible combinations of methods, apparatus, modules, systems, and computer program products.

Summary:
Systems and methods for delivery of thermal energy to horizontal wellbores are disclosed. In one embodiment, a method comprises heating a heat transfer fluid; circulating the heat transfer fluid into a vertical bore to a heat exchanger; advancing feedwater into the vertical bore to the heat exchanger, wherein the heat exchanger is configured to transfer heat from the heat transfer fluid to the feedwater to generate steam; transmitting the steam from the heat exchanger into a horizontal wellbore to cause heating of a subterranean region; and returning the heat transfer fluid from the heat exchanger to the surface. The method may further comprise collecting liquefied formation in a second horizontal wellbore; and transmitting the liquefied formation to the surface through a production line.