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FIELD OF THE INVENTION 
       [0001]    This invention relates to an modified process for hydrocarbon recovery from an underground reservoir by in situ combustion and employing a horizontal production well. 
       BACKGROUND OF THE INVENTION 
       [0002]    Commonly assigned U.S. Pat. No. 5,626,191 issued May 6, 1997 (hereinafter the &#39;191 patent) discloses an in situ combustion processes for producing hydrocarbon from an underground hydrocarbon reservoir utilizing (i) at least one injection well placed relatively high in an oil reservoir for injecting an oxidizing gas into the hydrocarbon formation, and (ii) a production well for producing liquefied or gasified hydrocarbon from the hydrocarbon reservoir. The production well has a vertical section which is in communication with a horizontal leg extending substantially perpendicularly outwardly from the vertical section and having a “toe” portion and a “heel” portion. The horizontal leg is completed relatively low in the reservoir, and at a “heel” portion thereof is in communication with the vertical section. Air, or other oxidizing gas, such as oxygen-enriched air, is injected through the injection well into the hydrocarbon reservoir, typically via perforations in the upper part of a vertical injection well, located in the vicinity of the “toe” of the horizontal leg of the production well. The horizontal leg of the production well is oriented generally perpendicularly to a generally quasi-vertical combustion front of combusting hydrocarbon which is produced upon ignition of a portion of the hydrocarbon in the reservoir proximate the injection well. Such combustion front is supplied with oxidizing gas via the injection well. The “toe” of the horizontal leg portion is positioned in the path of the advancing combustion front. The resulting combustion front propagates from the “toe” of the horizontal leg along the horizontal leg in the direction of and towards the “heel” portion. During this process heated hydrocarbon in the reservoir in advance of the moving combustion front becomes liquefied or gasified and flows into the horizontal leg, and from such leg thereafter removed to the surface via the vertical section of the production well. This process of U.S. Pat. No. 5,626,191 is called “THAI™”, an acronym for “toe-to-heel air injection”, and a registered trademark of Archon Technologies Ltd., a subsidiary of Petrobank Energy and Resources Ltd, Calgary, Alberta, Canada. 
         [0003]    U.S. Pat. No. 6,412,557, also commonly assigned, discloses a similar but modified process having the added step of placing a hydrocarbon upgrading catalyst along, within, or around the horizontal leg to substantially decrease the viscosity of the hydrocarbon and upgrade the quality of the hydrocarbon and increasing the flow of hydrocarbon from the reservoir into the horizontal leg of the production well for subsequent removal to surface. Such modified process is known in the industry by the trademark CAPRI™, likewise a registered trademark of Archon Technologies Ltd. 
         [0004]    WO2005121504 (PCT/CA2005/000833) published Dec. 12, 2005, also commonly assigned, teaches a similar process to that of THAI™, further comprising the additional step of providing injection tubing inside the production well within the vertical section and substantially along the length of the horizontal leg to a position proximate the “toe” thereof, for the purpose of injecting a non-oxidizing medium comprising steam, water, or a non-oxidizing gas via said tubing to the “toe” region of said horizontal leg. The injection of such non-oxidizing medium into the “toe” region of the horizontal leg has the effect of displacing any oxidizing gas in such area and thus preventing combustion of upgraded hydrocarbon which has flowed into the horizontal leg, and further increases the ambient pressure in the horizontal leg so as to prevent or reduce further inflow of oxidizing gas from the injection well which is injecting oxidizing gas into the hydrocarbon reservoir. 
         [0005]    Disadvantageously, in each of the above prior art methods for recovering liquefied and/or gasified hydrocarbons from a hydrocarbon formation oxidizing gas is needed to be injected proximate the toe of the horizontal leg, and remote from the vertical section of the production well. Such site of injection of oxidizing gas is remote from the vertical section of the production well, the surface of the production well being the location where oxidizing gas is typically generated. The injection and vertical section of the production wells can be separated by one (1) kilometer or more. Thus such prior art methods thus typically require transport of the oxidizing gas to the site of the injection well via piping from the production well, or alternatively require installation of equipment at the injection well site to permit generation of oxidizing gases for subsequent injection. Such requires clear access, via clearcutting, and/or increased space at the injection well site to accommodate additional oxidizing gas delivery and/or generation and compression facilities, thereby increasing the environmental “footprint” and impact of drilling operations on the environment, and also typically results in increased cost. 
         [0006]    Thus a need thus exists for a modified process of THAI™ and CAPRI™ wherein such drawbacks are eliminated. 
       SUMMARY OF THE INVENTION 
       [0007]    The method of the present invention is to a modified in situ hyrdrocarbon recovery process that instead of injecting oxidizing gas near the “toe” portion of the horizontal leg injects oxidizing gas in or near the producing vertical section of the producing well (ie at the “heel” portion). The modified process obviates the need for a separate drilling/production pad for oxidizing gas injection, thereby reducing cost and decreasing detrimental environmental impact of in situ recovery methods. 
         [0008]    Advantageously, the process of the present invention in a particular third embodiment described below further eliminates the need for a separate oxidizing gas injection well, in that in such refinement the vertical section of the production well also serves as the injection well, thereby reducing well drilling costs and reducing the capital costs. 
         [0009]    Specifically, rather than being a “toe-to heel” process, the process of the present invention is a “heel-to-toe” process. The oxidizing gas injection point is modified to be at the “heel” as opposed to the “toe” so that the combustion front moves in the opposite direction from that of the THAI™ process, namely from the direction of the “heel” of the horizontal well towards the “toe”. 
         [0010]    In the present invention three regions of the reservoir are developed relative to the position of the combustion zone. Near the “heel” and after the passage of the combustion front away from the “heel” lies the burned oil-depleted zone which results after injection of the oxidizing gas and after the combustion front has advanced for a period outwardly and away from the injection well and the “heel” portion of the horizontal leg. Such burned zone is filled substantially with oxidizing gas. Next lies the coke zone, which is essentially the area within the reservoir which the oxidizing gas has been able to then penetrate in the reservoir, and is essentially the area at which the combustion front exists (the combustion which occurs being that of the remaining coke which is the hydrocarbon then remaining after the lighter hydrocarbons within such reservoir and ahead of such combustion front have been liquefied or gasified and have flowed into the horizontal leg and thereafter removed to surface. Lastly, towards the “toe” of the horizontal well lies the region of the reservoir containing hydrocarbons which the combustion front is advancing toward. 
         [0011]    At higher oxidant injection rates, reservoir pressure increases and oxidizing gas in the burned zone, containing residual oxygen, can be forced into the horizontal leg of the production well. This is prevented in the process of the present invention by injecting, either for a limited time, or continuously, a medium such as a non-oxidizing gas such as carbon dioxide, and/or steam or water, to increase the pressure within the horizontal leg of the production well. 
         [0012]    Accordingly, in one broad aspect of the process of the present invention, to realize the advantage of being able to inject the oxidizing gas proximate or in the vertical section of the production well, a modified process for recovering liquefied or gasified hydrocarbon from an underground hydrocarbon reservoir is disclosed, comprising the steps of: 
         [0013]    (a) providing at least one production well having a substantially horizontal leg positioned relatively low in said reservoir, said horizontal leg having at one end thereof a heel portion and at an opposite end thereof a toe portion, said horizontal leg adapted to permit inflow of hydrocarbon into an interior of said horizontal leg, said production well having a substantially vertical section connected to said horizontal leg proximate said heel portion thereof; 
         [0014]    (b) providing production tubing inside said production well extending within said vertical section and within at least a portion of said horizontal leg to collect said hydrocarbon which flows into said horizontal leg; 
         [0015]    (c) injecting periodically or continuously a medium into the horizontal leg proximate the heel portion thereof, wherein said medium is selected from the group of mediums comprising alone or in combination, a non-oxidizing gas such as carbon dioxide, steam, or water; 
         [0016]    (d) supplying an oxidizing gas to said underground reservoir, at least initially, at a location of or proximate said vertical section of said production well; 
         [0017]    (e) igniting hydrocarbon within said hydrocarbon reservoir proximate said injection well, so as to cause combustion of a portion of said hydrocarbon in said hydrocarbon reservoir proximate said vertical section and thereby create a combustion front which advances outwardly and away from said injection well in at least a direction along said horizontal leg and towards said toe portion thereof; 
         [0018]    (f) causing heated hydrocarbon from said reservoir to flow from upper regions thereof and collect in said horizontal leg; and 
         [0019]    (g) removing from the production well, via said production tubing, said hydrocarbon which has flowed into said horizontal leg. 
         [0020]    Regarding step (g) above, the removal of the hydrocarbon from the production well via the production tubing is typically without pumping, but may require pumping in order to be removed from the horizontal leg if sufficient quantities of inert gases such as gasified hydrocarbon, carbon dioxide or nitrogen do not flow into the horizontal leg and thus the production tubing under significant ambient pressure of the hydrocarbon formation, as may occur during a start-up period. The normal mechanism of producing oil by reducing the mixed-fluid density with gases is called ‘gas lift’. 
         [0021]    In a first refinement/embodiment of the above process of the present invention, the injection of the oxidizing gas proximate the vertical section of the production well is accomplished via the drilling of a separate injection well proximate the vertical section of the production well so as to permit the oxidizing gas to be injected into the formation via such injection well proximate the production well. In this manner, and advantageously, the same drilling pad can then be used for drilling both the production well and the injection well, thus saving on expense and cost of well drilling. 
         [0022]    Additionally, and advantageously, because the injection well is situated proximate the production well which typically has power generation equipment used for production, oxidizing gas can usually and more easily be obtained and immediately injected into the injection well, which would not otherwise be capable of being done were the injection well positioned remote from the vertical section of the production well as in the prior art. 
         [0023]    In a second embodiment of the invention, the injection well is a side entry well within the vertical section of the production well, thus again allowing the injection well to be situated proximate the injection well so as to achieve the above benefits, as well as the additional benefit in that the upper portion of the vertical section of the production well can be used when drilling the side entry well, thus further reducing drilling costs. 
         [0024]    Specifically, in such second preferred embodiment, the present invention comprises a process for recovering liquefied or gasified hydrocarbon from an underground hydrocarbon formation comprising the steps of: 
         [0025]    (a) providing at least one production well having a substantially horizontal leg positioned relatively low in said formation, said horizontal leg having at one end thereof a heel portion and at an opposite end thereof a toe portion situated in the formation slightly lower in elevation than said heel portion, said horizontal leg adapted to permit inflow of liquefied hydrocarbon into an interior of said horizontal leg, said production well having a substantially vertical section connected to said horizontal leg proximate said heel portion thereof; 
         [0026]    (b) providing production tubing inside said production well extending downwardly within said vertical section and along said horizontal leg to said toe portion, to collect said hydrocarbon which flows into said horizontal leg; 
         [0027]    (c) providing injection tubing in said production well, said injection tubing extending downwardly in said vertical section to said heel portion; 
         [0028]    (d) injecting a medium into said production well via said injection tubing wherein said medium is selected from the group of mediums comprising alone or in combination, a non-oxidizing gas, steam, water, or carbon dioxide; 
         [0029]    (e) providing an injection well as a side track re-entry from said vertical section of said production well, which injection well extends into the hydrocarbon formation; 
         [0030]    (f) supplying an oxidizing gas to a portion of said hydrocarbon formation via said injection well; 
         [0031]    (g) igniting said hydrocarbon in said hydrocarbon formation proximate said vertical section so as to cause combustion of a portion of said hydrocarbon in said hydrocarbon formation and thereby create a combustion front which advances outwardly and away from said vertical section in at least a direction along said horizontal leg and towards said toe portion thereof; and 
         [0032]    (h) removing from the production well, via said production tubing, hydrocarbon which has flowed into said horizontal leg. 
         [0033]    In a third preferred embodiment the present invention comprises a method of producing hydrocarbon from a hydrocarbon reservoir whereby the necessity of an injection well for injecting the oxidizing gas is completely eliminated, thus reducing the cost of implementing the in situ process of the present invention. 
         [0034]    Specifically, in such third and preferred embodiment of the present invention, the vertical section of the production well is perforated to permit an oxidizing gas (which is provided to such vertical section) to escape into the hydrocarbon formation proximate the vertical section. In such manner, the need to drill a separate injection well is eliminated. 
         [0035]    Again, as part of the method of the present invention, a medium in the form of a non-oxidizing gas such as carbon dioxide, steam or water is injected either continuously or intermittently into the production well via injection tubing, which extends to the heel portion of the production well. A series of “packers” located in the production well may be provided to isolate the oxidizing gas supplied to the vertical section of the production well from the heel portion of the horizontal leg of the production well to which the non-oxidizing medium is supplied. 
         [0036]    Thus in such third preferred embodiment, the method of the present invention comprises a process for recovering liquefied or gasified hydrocarbon from an underground hydrocarbon reservoir, comprising the steps of: 
         [0037]    (a) providing at least one production well having a substantially horizontal leg positioned relatively low in said reservoir, said horizontal leg having at one end thereof a heel portion and at an opposite end thereof a toe portion, said horizontal leg adapted to permit inflow of liquefied hydrocarbon into an interior of said horizontal leg, said production well having a substantially vertical section connected to said horizontal leg proximate said heel portion thereof; 
         [0038]    (b) providing production tubing in said production well, extending from a surface of said production well to at least said heel portion of said production well to collect said hydrocarbon which flows into said horizontal leg; 
         [0039]    (c) providing injection tubing in said production well, said injection tubing extending downwardly in said vertical section to a position extending into at least said heel portion of said horizontal leg; 
         [0040]    (d) injecting a medium into the production well, wherein said medium is selected from the group of mediums comprising alone or in combination, a non-oxidizing gas such as carbon dioxide, steam, or water; 
         [0041]    (e) providing perforations in said vertical section of said production well at a position above said heel portion; 
         [0042]    (f) supplying an oxidizing gas to said vertical section and thus to a portion of said hydrocarbon reservoir via said perforations in said vertical section; 
         [0043]    (g) igniting said hydrocarbon in said hydrocarbon reservoir proximate said vertical section so as to cause combustion of a portion of said hydrocarbon in said hydrocarbon reservoir and thereby create a combustion front which advances outwardly and away from said vertical section in at least a direction along said horizontal leg and towards said toe portion thereof; and 
         [0044]    (h) causing heated hydrocarbon from said reservoir to flow from upper regions thereof and collect in said horizontal leg; and 
         [0045]    (i) removing from the production well, via said production tubing, said hydrocarbon which has flowed into said horizontal leg. 
         [0046]    Advantageously, the third embodiment of the present invention also eliminates the need as in the prior art to “close off” (using a cement plug or the like) the horizontal leg of each production well when a series of production wells are situated end to end and when the vertical section of a first production well is subsequently converted to an injection well (see. US &#39;191, col 6, lines 47-col 7, line 9 and  FIGS. 14D-F  thereof). The in situ method of the present invention, in particular the third embodiment, is a method of further reducing the cost of in situ recovery by reducing the number of steps, including not only eliminating the need to drill injection wells but also eliminating the necessity of “closing off” other wells as is necessary in the in situ methods of the prior art, as exemplified in US &#39;191 above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0047]    In the accompanying drawings, which illustrate a number of exemplary embodiments of the invention: 
           [0048]      FIG. 1A  is a perspective schematic view of a prior-art in situ recovery arrangement in a hydrocarbon reservoir, showing air injection wells situated at the toe of each of corresponding horizontal legs of associated production wells; 
           [0049]      FIG. 1B  is a cross section through one injection well and associated production well shown in  FIG. 1A ; 
           [0050]      FIG. 2A  is a schematic cross-section (not to scale) through one injection well and associated production well of a first embodiment of the present invention, using the method of the present invention of causing a combustion front to propagate in the direction of the “toe” of the horizontal leg of the production well, at a point in time close to the time of ignition of the hydrocarbon and the initial propagation of the combustion front; 
           [0051]      FIG. 2B  is a similar cross-section to that of  FIG. 2A , likewise not to scale, at a subsequent point in time when the combustion front has propagated for a time and moved closer to the “toe” portion of the horizontal leg of the production well; 
           [0052]      FIG. 2C  is a similar cross-section to that of  FIG. 2B , likewise not to scale, at a still further point in time when the combustion front has further propagated and moved even closer to the “toe” portion of the horizontal leg of the production well; 
           [0053]      FIG. 3  is a schematic partial cross-section through a hydrocarbon reservoir containing a hydrocarbon-containing formation, which shows a second embodiment of the method of the present invention, namely a production well and associated side entry injection well (not to scale) and further depicting the method of the present invention of causing a combustion front to propagate in the direction of the “toe” of the horizontal leg of the production well, at a point in time close to the time of ignition of the hydrocarbon and the initial propagation of the combustion front; 
           [0054]      FIG. 4  is a schematic partial cross-section through a hydrocarbon reservoir containing a hydrocarbon-containing formation, which shows a third preferred embodiment of the present invention, namely a cross-section through a production well (not to scale) employing the method of the present invention of causing a combustion front to propagate in the direction of the “toe” of the horizontal leg of the production well, at a point in time close to the ignition of the hydrocarbon and the initial propagation of the combustion front; and 
           [0055]      FIG. 5  is a perspective schematic view of an in situ recovery method of  FIG. 4 , showing the third and preferred embodiment of the method of the present invention for recovering hydrocarbons from a hydrocarbon reservoir 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0056]      FIG. 1A  shows a schematic, semi-transparent view of an arrangement of wells utilized in the prior art for in situ recovery of hydrocarbon from a subsurface hydrocarbon reservoir or formation  10 . 
         [0057]    Specifically,  FIG. 1A  schematically depicts the prior art method of in situ recovery of hydrocarbon disclosed in U.S. Pat. No. 5,626,191, comprising locating a series of production wells  12 , each comprising a substantially vertical section  16  and a substantially horizontal leg  16 , having a “toe” portion  18  and a “heel” portion  20 . The horizontal leg  16  of production well  12  is located at a lower region of hydrocarbon formation  10 , and is substantially porous to allow ingress of fluids. A series of injection wells  22  are provided, situated at a region proximate the “toe” and extending downwardly into the formation  10 , with perforations in the upper reaches of the oil-bearing reservoir. 
         [0058]      FIG. 1B  shows a schematic cross-section through an injection well  22  and associated production well  12  of  FIG. 1A . 
         [0059]    In the prior art in situ recovery process depicted in  FIGS. 1A  &amp; B, an oxidizing gas  24 , such as air (which contains oxygen), oxygen, or oxygen-enhanced air, is injected into the formation  10  via each of injection wells  22 , so as to permit a portion of the hydrocarbon in formation  10  to be combusted. Specifically, a portion of the hydrocarbon in hydrocarbon formation  10  in the region of the injection well  22  when supplied with the oxidizing gas  26  is caused to be ignited and caused to combust, thereby forming and creating within formation  10  a substantially vertical and laterally-extending combustion front  26 . Such combustion front  26 , by way of heat conduction and creation of heated combusted gases within formation  10 , heats hydrocarbons in the formation  10  directly ahead and in advance of combustion front  26 , causing the more volatile hydrocarbon compounds in formation  10  to gasify and further cause upgrading of a portion of the hydrocarbon solids or bitumens in the formation simultaneously increasing their viscosity so as to create mobile liquefied hydrocarbons  30 . The remaining heavier hydrocarbons, particularly coke, remain, which provide fuel for the advancing combustion front  26  and sustain the advance of the combustion front  26  and the in situ combustion and hydrocarbon upgrading process. Then mobile liquefied hydrocarbons  30  and gasified components (some of which may subsequently condense as liquids  30 ), then flow downwardly by action of gravity through the formation and are collected in a lowermost region of the formation  10  by flowing into horizontally-extending horizontal leg  16  of the production well  12 . Horizontal leg  16  of production well  12  generally has, at least for a limited time, a gas pressure therein less than that of the formation  10  (due to removal of collected liquid hydrocarbons  30  as well as gaseous hydrocarbons therefrom), Such reduced gaseous pressure in horizontal leg  16  as opposed to within formation  10  in advance of combustion front  26  assists in liquid and gaseous hydrocarbon inflow from hydrocarbon formation  10  into the horizontal leg  16 . At other times, due to injection of medium  52  via injection tubing  50  (discussed below) into horizontal leg  16 , horizontal leg  16  may at times may have a gaseous pressure close to, or even in excess of the gas pressure within formation  10 . 
         [0060]    Importantly, in the prior art method of in situ recovery as shown in  FIGS. 1A  &amp; B and described above, injection wells  22  are situated proximate the “toe” of the horizontal leg  16 , and oxidizing gas injected into the formation at these locations via the injection wells  22 . The combustion front  26  which receives oxidizing gas  24  is thus caused to progress outwardly from the injection well  22 , and perpendicular to and along the horizontal wells  16  in a direction from the “toe” portion to the “heel” portion. 
         [0061]    Disadvantageously with this prior art method, not only need a drilling pad  32  be created for the production well  12 , but an additional and separate drilling pad need be created for the injection well  22 , and such separate injection well  22  need be drilled into such formation. In addition, oxygen creation and injection equipment (not shown) must be hauled to and installed at the surface of such injection well  22 , as such injection well is remote from the surface of production well  12 . Both of such requirements add significantly to the cost of carrying out the prior art methods of in situ recovery of hydrocarbons. 
         [0062]      FIGS. 2A-2C  herein show a modified (first) in situ recovery process, which is expressly adapted to eliminate at least one of the above expenses in the prior art methods of in situ hydrocarbon recovery, namely the expense of creating a separate drilling pad for the injection well  22 . 
         [0063]    Specifically, as seen in  FIGS. 2A-2C , a single drilling pad  32  is created by way of clearing of trees and other obstacles, and a single drill platform erected thereon. A production well  12  is drilled using conventional drilling techniques, comprising a vertical section  14 , and a further horizontal leg  22  in communication with vertical section  14 . The horizontal leg  16  has a “toe” portion  18  and a “heel” portion  20  where it meets vertical section  14 . The production well  12  is completed by the usual process of casing well  12 , and further by the insertion within such production well  12  of production tubing  40 , which extends downwardly in vertical section  14  to such heel portion  20  and preferably along the horizontal leg  16 , preferably to toe portion  18  thereof, such production tubing  40  having an open end  42  within said horizontal leg  16 . Production tubing  40  is typically coiled tubing as is conventionally used in drilling operations. 
         [0064]    Additional injection tubing  50 , likewise typically coiled tubing as is conventionally used in drilling operations, is further provided for injection of a medium  52  into production well  12 , such medium  52  comprising a non-oxidizing gas, preferably carbon dioxide due to its diluent effect on hydrocarbons, or alternatively or in combination steam or water or other non-combustible flowable medium. As seen from  FIGS. 2A-2C , injection tubing  50  extends into the “heel” portion  20  of horizontal leg  16 . At least one isolation packer  54  is provided to allow medium  52  to be injected, if desired, in a pressurized state from time to time or continuously injected, so as to pressurize from time to time or continuously if desired, horizontal leg  16  to assist in forcing liquefied hydrocarbon  30  into production tubing  40  and inhibiting entry of oxidizing gas into the horizontal leg  16 . 
         [0065]    Using the single drilling pad  32 , a further injection well  22  is drilled, extending into at least the upper region of the hydrocarbon formation  10 . Injection well  22  typically has perforations  75  in a lower end thereof to permit infusion and injection of an oxidizing gas  24  such as air or oxygen into the hydrocarbon-containing region of hydrocarbon formation  10 . 
         [0066]    The method of the present invention, in the first embodiment shown in  FIGS. 2A-2C , thereafter operates as follows: 
         [0067]    Oxidizing gas  24  is injected into formation  10  via injection well  22 . Advantageously, equipment (not shown) used to create oxidizing gas  24  and inject such oxidizing gas  24  need not be located remote from the production well  12 , but instead can, by virtue of the method of the present invention, be located proximate to production well  12 , and in particular if desired may be located on drilling pad  32  or closely proximate thereto, thereby eliminating the need for clearing and creating a separate drilling pad at a remote site such as would occur if the injection well  22  were located towards the “toe” of horizontal well  16 . Also, operation and maintenance of the oxidizing gas supply equipment can conveniently be conducted at the oil-treating site located near well  12 . Hydrocarbons proximate the injection well  22  are ignited, and due to the supply of oxidizing gas  24 , a combustion front  26  is created, which in the method shown in  FIGS. 2A-2C , advances as a substantially vertical laterally extending front (see also  FIG. 5  herein) from the “heel”  20  of horizontal leg  16  towards the “toe”  18 . Viscous and high viscosity hydrocarbons, including bitumen, in the hydrocarbon formation  10  in advance of the advancing combustion front  26 , due to heat which is generated, are caused to upgrade and become liquid, and in the process become less viscous. Some hydrocarbons in the formation  10  in advance of the front  26  will gasify. Liquified hydrocarbons  30  and gasified hydrocarbons, now being mobile, flow downwardly and into horizontal leg  16  which is made porous (ie has apertures  60  in an upper portion thereof) to permit infusion of such hydrocarbons  30  and thus collection of such hydrocarbons  30 . 
         [0068]    Such process continues as combustion front  26  progresses and thus “sweeps” from the “heel” portion  20  to the “toe”  18  of horizontal leg  16 . 
         [0069]    Notably, prior to generation of combustion front  26 , hydrocarbon formation  10  is preferably initially preheated by injection of a heated non-oxidizing medium  52  such as steam, which is injected into the horizontal leg  16  of production well  12  via injection tubing  40 , and removed via production tubing  50  or alternatively via annulus  80  in vertical section  16  if isolation packers  54  are not present. Pre-injection of a heated medium has the benefit of heating the production well  12  and its production components thereby increasing the flowability of liquefied hydrocarbons  30  which flow into horizontal leg  16  of production well  12 . This procedure is useful in bitumen reservoirs because cold oil that may enter the horizontal leg  16  will be very viscous and will flow poorly, possible plugging the horizontal leg  16 . For formations  10  with mobile oil, extensive pre-ignition steaming is not required for the purpose of heating the oil so that it will flow, however, it can be useful to reduce oil saturations near the oxidizing gas injection well  22  and to raise the hydrocarbon temperature to achieve ignition thereof. Other ignition methods may be employed such as the injection of easily ignitable fuels such as linseed oil, or by injection of hot combustion gas. For bitumen reservoirs, steam is also injected via injector well  22  and may also be injected into the reservoir  10  in the region between the injector well  22  and the toe  18  of the horizontal well  16  to warm the oil and increase its mobility prior to initiating injection of oxidizing gas  24  into the reservoir formation  10 . 
         [0070]    After initiation of combustion and combustion front  26 , a non-oxidizing medium  52  in the form of steam, a non-oxidizing gas such as carbon dioxide, or water, is injected, either continuously or sporadically via injection tubing  50  into horizontal leg  16 , which due to isolation packers  54 , can be pressurized. The purpose of such non-oxidizing medium  52  is for a number of reasons. Firstly, increased pressure within horizontal leg  16  reduces or prevents oxidizing gas  24  infusing into horizontal leg  16  from formation  10  which could otherwise detrimentally, in combination with liquefied and gaseous hydrocarbons therein, form an explosive mixture with potentially explosive consequences, or alternatively react with oxygen directly so as to form coke which could otherwise seal the horizontal leg  16  of production well  12 . The consequence of having hydrocarbon (oil) and oxygen together in a wellbore is combustion and potentially an explosion with the attainment of high temperatures, perhaps in excess of 1000° C. This can cause irreparable damage to the wellbore, including the failure of the sand retention screens (not shown). The presence of oxygen and wellbore temperatures over 425° C. must be avoided for safe and continuous oil production operations. Secondly, injection of medium  52  can serve to pressurize horizontal leg  16  and assist in driving liquefied and gaseous hydrocarbons  30  collected in horizontal leg  16  into the open end  42  of production tubing  40 , thereby assisting in drawdown of such liquids  30  and producing such hydrocarbons  30  from producing well  12 . Thirdly, medium  52  when injected via injection tubing  50  can be heated. Advantageously, means for heating such medium  52  are, in this method, conveniently capable of being located at the surface of production well  12  and on or near drilling pad  32 . Lastly, where the injected medium  52  is carbon dioxide, injection thereof into horizontal well  16  serves as not only a convenient carbon “sink” to allow disposal of such greenhouse gas, but further due to the diluent properties on carbon dioxide on liquid hydrocarbons  30 , reduces the viscosity thereof and thus aids in the drawdown of collected liquid hydrocarbons  30  via production tubing  40 . 
         [0071]    As seen from  FIGS. 2A-2C , during the advance of combustion front  26 , coke is deposited in the reservoir  10  and serves as fuel for the in situ combustion process. Hot combustion gases  70  advance into formation  10  heating the hydrocarbon therein and any connate water that is present. A portion of these hydrocarbons liquefies and such liquefied hydrocarbons  30  flow, along with combustion gases, into the horizontal leg  16  through the perforations  60 , as shown in  FIGS. 2A-2C . The liquefied hydrocarbons  30  flow along and to the “toe”  18  of horizontal leg  16  and enter the open end  42  of production tubing  40  therein, and flow back and then upward to the surface. The process is stable and continuous, with the combustion front  26  continuously advancing towards the “toe”  18  of the horizontal leg  16 . 
         [0072]    The oxidizing gas  24 , typically air, oxygen or oxygen-enriched air, is injected into the upper part of the reservoir  10 . Coke that was previously laid down consumes the oxygen so that only oxygen-free gases contact the oil ahead of the coke zone at the combustion front  26 . Combustion gas temperatures of typically 600° C. and as high as 1000° C. are achieved from the high-temperature oxidation of the coke fuel. In the mobile oil zone  80  in advance of the combustion front  26 , these hot gases  70  and steam heat the oil to over 400° C., partially cracking the oil, vaporizing some components and greatly reducing the oil viscosity. The heaviest components of the oil, such as asphaltenes, remain on the rock and will constitute the coke fuel later when the combustion front  26  arrives at that location. In the mobile oil zone  80 , gases and oil drain downward into the horizontal leg  16 , drawn by gravity and at times by the low-pressure sink of the horizontal leg  16  when unpressurized. The coke zone at the combustion front  26  and the mobile oil zone  80  move laterally from the direction from the heel  20  towards the toe  18  of the horizontal well  16 . The burned zone section  100  behind the combustion front is depleted of liquids (oil and water) and is filled with oxidizing gas  24 . The section of the horizontal well  16  opposite this burned zone  100  is in jeopardy of receiving oxygen or oxidizing gas  24  which will combust the oil present inside horizontal well  16  creating extremely high wellbore temperatures that would damage the steel casing and especially the sand screens that are used to permit the entry of fluids  30  but exclude sand. If the sand screens fail, unconsolidated reservoir sand will enter the horizontal wellbore  16  and necessitate shutting for cleaning-out and remediation with cement plugs. This operation is very difficult and dangerous since the horizontal wellbore  16  can contain explosive levels of oil and oxygen. 
         [0073]    The method of the present invention contemplates a number of ways to prevent influx of oxidizing gas  24  from the formation  10  into the horizontal leg  16 . A first method is to reduce the injection rate of the oxidizing gas  24  in order to reduce the reservoir pressure in formation  10 . A second method is to reduce the liquefied hydrocarbon  30  drawdown rate via the production tubing  40  (ie reduce the production rate via production tubing  40 ) to thereby increase wellbore pressure in horizontal leg  16 . Both of these methods result in the reduction of hydrocarbon production rates, which is economically detrimental. An alternative and preferred method is that as described previously herein, namely the injection of non-oxidizing medium  52  into horizontal leg  16  via injection tubing  50 , which is believed to have little effect on gravity draining of hydrocarbon liquids into horizontal well  16 . In any event, such injection of medium  52  may be done periodically and only for a time sufficient to reduce concentrations of oxygen within horizontal leg  16  to less-than-explosive concentrations. In a typical operation, a thermocouple string can be placed along the horizontal section, or within, and the occurance of elevated temperatures will signal the intrusion of oxidizing gas so that water of steam may be added via tubing  52  to reduce well-bore temperatures, dilute the oxygen present and increase wellbore pressure to inhibit further oxidizing gas entry. 
         [0074]      FIG. 3  schematically illustrates a further more preferable embodiment of the method of the present invention, having similar components to those identified in  FIGS. 2A-2C , and having similar methodology. Again, an oxidizing gas in injected into formation  10  via injection well  22 , and a combustion front  26  created which “sweeps” from heel  20  to toe  18  of horizontal leg  16 , causing liquefied hydrocarbons  30  as well as gasified hydrocarbons to flow into horizontal leg  16  and be delivered to surface via production tubing  40 . 
         [0075]    Notably, however, the important and sole distinction in the method of in situ recovery shown in  FIG. 3  over the method previously discussed and as shown in  FIGS. 2A-2C  is that injection well  22  in the method depicted in  FIG. 3  is formed as a side entry well from within vertical section  16  of production well  12 . 
         [0076]    Advantageously, using the method depicted in  FIG. 3 , injection well  22  is less expensive to drill as an upper portion of such injection well has already been drilled as it is common with vertical section  16  of production well  12 . 
         [0077]    Accordingly, not only are cost savings realized in locating the injection well  22  at the location of and in close proximity to the production well  12  and its associated equipment and no separate drill pad  32  needed to be created, but in addition, well drilling costs are reduced when drilling injection well  22 . 
         [0078]      FIG. 4  depicts a third and most preferred embodiment of the method of the present invention for carrying out in situ recovery of hydrocarbon. Such method, like the first embodiment of the method of the present invention depicted in  FIGS. 2A-2C , and like the second embodiment of the invention depicted in  FIG. 3 , includes as an integral component of the method the creation of a combustion front  26  which “sweeps” from “heel”  20  to “toe”  18  of horizontal leg  16 , thereby causing liquid hydrocarbons  30  to be collected in horizontal leg  16 , and thereafter drawndown by production tubing  40  and produced to surface. 
         [0079]    Importantly, however, in this third embodiment of the method of the present invention shown in  FIG. 4 , there is no step of drilling an injection well  22 . Instead, perforations  110  are made in the vertical section  16  of production well  12 , and an oxidizing gas  24  injected into such vertical section  16  and thus into formation  10 . Oxidizing gas  24  is prevented from injection into horizontal leg  16  by the presence of isolation packers  54  which effectively separate produced liquefied hydrocarbons in horizontal leg  16  from oxidizing gas  24  such as oxygen, thereby preventing formation of explosive mixtures. Injection tubing  50  still serves, like in earlier embodiments, to permit sporadic or continuous injection of non-oxidizing gas  52  into horizontal leg  16  to prevent oxidizing gas  24  within the burned zone  80  of the formation from permeating into horizontal leg  16 . 
         [0080]    Advantageously, using the method depicted in  FIG. 4 , the cost of drilling an injection well  22  is completely eliminated. Accordingly, with the method depicted in  FIG. 4 , not only are cost savings realized and environmental impact reduced in being able to have oxidizing injection apparatus at the production well and only on a single drill pad  32  at the production well which is otherwise the case in prior art methods which require creation of a separate drill pad and additional clearing for oxidizing gas creation and injection equipment (not shown), but in addition substantial cost savings are achieved by elimination the necessity to drill any injection well. 
         [0081]      FIG. 5  depicts how the method of  FIG. 4  (ie the third embodiment of the method of the present invention) may be deployed with a series of production wells  12  in a hydrocarbon formation  10 , using a combustion front  26  which advances from “heel”  20  to “toe”  18 . 
         [0082]    Although the disclosure describes and illustrates preferred embodiments of the method of the present invention, it is understood that the invention is not limited to these particular embodiments. Many variations and modifications will now occur to those skilled in the art. For a full definition of the invention, reference is to be made to the appended claims.

Summary:
A modified method of in situ recovery of hydrocarbon from an underground hydrocarbon-containing formation. An “L” shaped production well, having a vertical upper section, and a lower horizontally-extending leg which is positioned low in the hydrocarbon formation, is provided. The horizontal leg connects to the vertical section of the production well at a heel portion and has a toe portion at an opposite end thereof. An oxidizing gas is injected into the formation proximate the vertical section of the production well. A vertical combustion front is created which is caused to sweep outwardly therefrom and laterally within the formation above the horizontal leg, from the heel to the toe of the horizontal leg, causing hydrocarbons in the formation above the horizontal leg to be upgraded and liquify, and thereafter to drain downwardly into the horizontal leg which is permeable, where such liquified hydrocarbons are then delivered to surface via production tubing. A non-oxidizing gas is injected into the heel portion of the horizontal leg via injection tubing contained within the vertical section of the production well. Benefits of the modified method of in situ recovery include decreased costs and lessened environmental impact.