Abstract:
An underground reservoir is provided comprising an injection well and a production well. The production well has a horizontal section oriented generally perpendicularly to a generally linear and laterally extending, upright combustion front propagated from the injection well.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority benefit under 35 U.S.C. Section 119(e) to U.S. Provisional Patent Ser. No. 61/245,321 filed on Sep. 24, 2009 the entire disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     Embodiments of the invention relate to a method for recovering hydrocarbons with in situ combustion. 
     BACKGROUND OF THE INVENTION 
     In situ combustion (ISC) processes are applied for the purpose of recovering oil from light oil, medium oil, heavy oil and bitumen reservoirs. In the process, oil is heated and displaced to an open production well for recovery. Historically, in situ combustion involves providing spaced apart vertical injection and production wells within a reservoir. Typically, an injection well will be located within a pattern of surrounding production wells. An oxidant, such as air, oxygen enriched air or oxygen, is injected through an injection well into a hydrocarbon formation, allowing combustion of a portion of the hydrocarbons in the formation in place, i.e., in-situ. The heat of combustion and the hot combustion products warm the portion of reservoir adjacent the combustion front and drive (displace) hydrocarbons toward offset production wells. 
     One difficulty associated with applying in situ combustion as a stand alone recovery method in heavy oil and bitumen reservoirs is the lack of mobility of the oil. For example, in situ combustion involves the injection of an oxidant into a formation. The oil in place serves as a fuel for the combustion front once ignition has occurred. As with any burning process, heat, oxygen, and fuel must be readily available to sustain combustion. In heavy oil and bitumen reservoirs this process is interrupted by the fact that the oil in the reservoir is not mobile. Therefore, combustion gas products (CO, CO 2 , H 2 S, etc.) and mobilized oil can become trapped in the reservoir which leads to the suffocation of the combustion front. Therefore, a need exists for a method of initiating enhanced communication between the injection and production wells along with a method for extracting both oil and gas from the reservoir for in situ combustion processes. 
     SUMMARY OF THE INVENTION 
     In one embodiment, a method of conducting in situ combustion in an underground reservoir, includes: forming at least one injection well disposed in the underground reservoir, wherein the injection well includes a vertically deviated well, a first horizontal injector well and a second horizontal injector well, wherein the first and second horizontal injector wells can vary from 30° to 120° from the vertically deviated well, wherein the injection well including the first and second horizontal injector wells are at least 5 meters above a hydrocarbon producing zone, wherein the distal ends of the first and second horizontal injector wells include a toe portion, wherein the opposite ends of the first and second horizontal injector wells include a heel portion, wherein the heel portions connect the first and second horizontal portions to the vertically deviated well; forming a first production well having a first substantially horizontal producer portion and a first substantially vertical producer portion disposed in the underground reservoir, wherein the distal end of the horizontal producer portion includes a toe portion, wherein the opposite end of the horizontal portion includes a heel portion, wherein the heel portion connects the first horizontal producer portion to the first vertical portion of the first production well; forming a second production well having a second substantially horizontal producer portion and a second substantially vertical producer portion disposed in the underground reservoir, wherein the distal end of the horizontal producer portion includes a toe portion, wherein the opposite end of the horizontal portion includes a heel portion, wherein the heel portion connects the second horizontal producer portion to the second vertical portion of the second production well, wherein the second production well is located lower in the reservoir than the first production well; injecting an oxidant into the injection well to establish a combustion front of ignited hydrocarbons to propagate a combustion front through the reservoir; recovering hydrocarbons from the reservoir via the second production well due to gravity drainage; and recovering combustion gas from the reservoir via the first production well. 
     In another embodiment, a method of conducting in situ combustion in an underground reservoir, includes: forming at least one injection well disposed in the underground reservoir, wherein the injection well includes a vertically deviated well, a first horizontal injector well and a second horizontal injector well; forming a first production well having a first substantially horizontal producer portion and a first substantially vertical producer portion disposed in the underground reservoir; forming a second production well having a second substantially horizontal producer portion and a second substantially vertical producer portion disposed in the underground reservoir; injecting an oxidant into the injection well to establish a combustion front of ignited hydrocarbons which propagate a combustion front through the reservoir; and recovering hydrocarbons through the production well. 
     In another embodiment, a method of conducting in situ combustion in an underground reservoir, includes: forming at least one injection well disposed in the underground reservoir, wherein the injection well includes a vertically deviated well, a first horizontal injector well and a second horizontal injector well; forming a first production well having a first substantially horizontal producer portion and a first substantially vertical producer portion disposed in the underground reservoir; forming a second production well having a second substantially horizontal producer portion and a second substantially vertical producer portion disposed in the underground reservoir; heating the reservoir surrounding the injection well, wherein the heating occurs without igniting oil in the reservoir and with operations conducted through the injection well; initiating in situ combustion after heating the reservoir, within the initiating includes injecting an oxidant into the injection well to establish a combustion front of ignited hydrocarbons which propagate a combustion front through the reservoir; and recovering hydrocarbons through the production well. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with further advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic section of an injection well and a series of production wells according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to embodiments of the present invention, one or more examples of which are illustrated in the accompanying drawing. Each example is provided by way of explanation of the invention, not as a limitation of the invention. It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations that come within the scope of the appended claims and their equivalents. 
     Referring to  FIG. 1 , an underground reservoir  108  contains an injection well  106  and a series of production wells  100 ,  102 ,  104  disposed therein. The “x-axis” is parallel to the earth surface  109 . The “y-axis” is orthogonal to the x-axis and vertical to the earth surface  109 . The “z-axis” is orthogonal to both the x-axis and the y-axis. 
     The injection well  106  is a single well with a vertically deviated well from the surface, i.e., along the y-axis, with multiple wells at angles varying from 30° to 120° from the vertically drilled well into the reservoir along the x-axis and/or the y-axis and/or the z-axis. The configuration of the injection well is similar to a fishbone configuration. Depicted in  FIG. 1 , the injection well defines a first horizontal injector well  124  and a second horizontal injector well  126 . The first and second horizontal injector wells  124  and  126 , respectively, may progress through the reservoir at angles which differ from the original angle facilitating the best placement of the well within the reservoir. In an embodiment, the injection well may contain multiple horizontal injector wells. Furthermore, the horizontal injector portions  124  and  126  increase potential area for communication between the injection well  106  and the production wells relative to only utilizing vertical injection wells where lateral area for establishing communication is limited. The injection well  106  along with the first horizontal injector well  124  and second horizontal injector well  126  are at least 5 meters above the bottom pay zone. 
     The reservoir  108  contains at least two production wells each having a vertical producer portion and a substantially horizontal producer portion completed via horizontal drilling techniques known in the art. The horizontal producer portions of the production wells can be placed at the base of the reservoir pay zone, where at least one or more of the horizontal producer portions are arranged parallel or perpendicular to one or more of the horizontal producer portions situated vertically beneath the other wells. In an embodiment, as depicted in  FIG. 1 , the reservoir contains two horizontal producer wells  103  and  105  situated along the z-axis above a single perpendicular horizontal producer well  101  situated along the x-axis. 
     The production wells  100 ,  102 ,  104  have the general shape of a foot, and are defined by a “toe” portion  110 ,  114 ,  118  and a “heel” portion  112 ,  116 ,  120 . The toe portion is located at the distal end of the horizontal producer portion, while the heel portion is located at the intersection of the horizontal producer portion and vertical producer portion. The production wells contain slots at various desired locations along the horizontal producer portion to facilitate production of fluids from the reservoir. The slots are narrowly cut either axially or transversely in the wall of the horizontal producer portion. The slots are made sufficiently narrow to exclude particles greater than a selected size, while allowing flow into or out of the wellbore. The number of slotted wall sections, the size of the slots, and the location of the slots are solely dependent on operational requirements and desires. 
     In situ combustion cannot be applied directly to an immobile reservoir without prior stimulation due to inadequate initial communication between the injection well and the production well. The cold heavy oil and/or bitumen in the formation cause this lack of communication resulting in an inability to produce combustion gas products or mobile oil from the reservoir. The inability to vacate the products from the reservoir ultimately results in the suffocation of the combustion front and termination of the process. Cyclic steam stimulation (CSS), also known as the huff-and-puff method, is typically applied to heavy-oil reservoirs to boost recovery and can ultimately initiate the required communication between the injection and production wells. During the primary production phase, the cyclic steam stimulation method assists natural reservoir energy by melting the oil so it will more easily move through the formation. 
     Preheating the formation  108  around the fishbone injection well configuration  106  with steam, for example, may facilitate in establishing initial communication between the fishbone injection well configuration  106  and the production wells  100 ,  102 ,  104 . In an embodiment of the huff-and-puff method, a predetermined amount of steam is injected into the fishbone injection well configuration, which has been drilled or converted for injection purposes. In another embodiment, a predetermined amount of steam is injected into the fishbone injection well configuration and one or more of the injection wells. In another embodiment, a predetermined amount of steam is injected into one or more of the injection wells. Once the pay zone between the wells has been heated (&gt;90° F.), the well is then shut in to allow the steam to heat or “soak” the producing formation around the well. After a sufficient time has elapsed to allow adequate heating, the injection well is back in production until the heat is dissipated with the production fluids. The huff phase (steam injection), the soak phase, and the puff phase (production phase) are repeated as necessary to heat the formation around the fishbone injection well configuration and to establish fluid communication between the injection well and the production wells for in situ combustion. 
     Once communication is established, the in situ combustion process may begin. In operation, the in situ combustion process begins with the injection of an oxidant  122  through the injection well  106  to initiate combustion. Air is usually used; however it may be substituted directly with oxygen or with recycled gases enriched with oxygen. Water may also be injected continuously or as slugs along with an oxidant to improve the combustion process. Continuous gas injection and cold water circulation in the injection well can be used to minimize combustion damage to the well. 
     The major driver for recovery of oil through the combustion process will be gravity drainage. For example, as the combustion front propagates from the injection well at the top of the formation, oil and gas drain to the base of the reservoir. Specifically, combustion is initiated and maintained by the injection of an oxygen containing gas at the top of the reservoir into the injection well  106 , with mobilized oil draining to lower horizontal producer wells, i.e.,  101 ,  103 ,  105 . 
     The preferred embodiment of the present invention has been disclosed and illustrated. However, the invention is intended to be as broad as defined in the claims below. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described in the present invention. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims below and the description, abstract and drawings not to be used to limit the scope of the invention.