Patent Publication Number: US-8118095-B2

Title: In situ combustion processes and configurations using injection and production wells

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     None 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     None 
     FIELD OF THE INVENTION 
     Embodiments of the invention relate to methods and systems for oil recovery with in situ combustion. 
     BACKGROUND OF THE INVENTION 
     In situ combustion offers one approach for recovering oil from reservoirs in certain geologic formations. With in situ combustion, an oxidant injected through an injection well into the reservoir reacts with some of the oil to propagate a combustion front through the reservoir. This process heats the oil ahead of the combustion front. Further, the injection gas and combustion gasses drive the oil that is heated toward an adjacent production well. 
     Success of the in situ combustion in a heavy oil or bitumen environment depends on stability of the combustion front and ability to ensure that oxidation occurring is an exothermic reaction. Amount of beneficial thermal cracking of the oil to make the oil lighter tends to increase with higher temperatures from the oxidation. Further, oxidation of the oil by an endothermic reaction can create hydrogen bonding and result in undesired increases in viscosity of the oil. 
     Various factors attributed to failure of the in situ combustion include loss of ignition, lack of control, and inadequate reservoir characterization. For maximum recovery of the oil, the combustion front must be able to stay ignited in order to sweep across the entire reservoir. Due to issues such as formation heterogeneity influencing the combustion front, prior approaches often result in instability of the combustion front, premature extinguishing of the combustion front, or inability to achieve or maintain desired temperatures. 
     Therefore, a need exists for improved methods and systems for oil recovery with in situ combustion. 
     SUMMARY OF THE INVENTION 
     In one embodiment, a method of conducting in situ combustion includes forming an injection well that extends in length deviated from vertical in at least a first direction and at two locations having a vertical offset from each other. The method further includes forming a plurality of production wells that each extend in length deviated from vertical with orientation misaligned relative to the first direction and at least one of the production wells deviated from vertical in a second direction. Injecting oxidant into the injection well to propagate combustion enables recovering hydrocarbons through the production wells. 
     According to one embodiment, a method of conducting in situ combustion includes forming an injection well that extends in length deviated from vertical and forming a production well that extends in length deviated from vertical toward the injection well. Heating a reservoir surrounding the injection well along a section of the injection well where vertically deviated occurs without igniting oil in the reservoir and with operations conducted through the injection well. Further, the method includes initiating the in situ combustion after heating the reservoir and recovering hydrocarbons through the production well. The initiating includes injecting oxidant into the injection well and may be achieved spontaneously or by using an ignition device. 
     For one embodiment, a method of conducting in situ combustion includes injecting oxidant into an injection well to propagate combustion and recovering hydrocarbons through a plurality of production wells. The production wells define heels at where the production wells turns toward horizontal and toes at where the production wells terminates distal to the heels. The injecting oxidant occurs along longitudinal sections of the injection well that are closer to the toes of the production wells than the heels of the production wells, are spaced from one another closer to surface than the toes of the production wells, and come closest to the production wells intermediately along the longitudinal sections. 
    
    
     
       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. 
         FIG. 1  is a three dimensional schematic of injection and production wells in a formation, according to one embodiment of the invention. 
         FIG. 2  is a schematic top view of the injection and production wells shown in  FIG. 1 , according to one embodiment of the invention. 
         FIG. 3  is a three dimensional schematic of a multilateral injection well and dual production wells in a formation, according to one embodiment of the invention. 
         FIG. 4  is a schematic sectional side view of the injection and production wells shown in  FIG. 3 , according to one embodiment of the invention. 
         FIG. 5  is a three dimensional schematic of heated horizontal injection and production wells in a formation, according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention relate to in situ combustion. Configurations of injection and production wells facilitate the in situ combustion. The wells define vertically deviated lengths that have different orientations from one another. Further, heating processes such as resistive heating and cyclic steam stimulation may take place in one or both of the injection and production wells to precondition a reservoir prior to the in situ combustion. 
       FIGS. 1 and 2  illustrate an injection well  100  and a production well  102  disposed in a formation  104 . Vertical from a surface  105  of earth is represented in a “y” direction with “x” and “z” directions being orthogonal to each other and the y-direction. For some embodiments, the injection well  100  includes a horizontal injector portion  106  that may extend lengthwise in the z-direction. Further, the production well  102  may include a horizontal producer portion  108  that may extend lengthwise in the x-direction. 
     Direction of deviation from vertical for the horizontal injector portion  106  relative to direction of deviation from vertical for the horizontal producer portion  108  defines an angle θ. While the angle θ is shown to be about 90°, the angle may be between 20° and 160°, such as between 80° and 100°. For example, the horizontal producer portion  108  may extend in the x-direction while the horizontal injector portion  106  may extend in orientation midway between the x-direction and the z-direction creating the angle θ of 45°. 
     Further, angle of deviation from the y-direction for the horizontal injector portion  106  and/or the horizontal producer portion  108  may be between 20° and 160°, between 80° and 100°, or about 90°. The angle of deviation from the y-direction defines slant toward horizontal corresponding to 90°. In comparison to exemplary less horizontally oriented slanting shown in  FIGS. 3 and 4 , both the horizontal injector portion  106  and the horizontal producer portion  108  deviate from the y-direction by about 90°. 
     The production well  102  defines a heel  110  at where the production well  102  turns toward horizontal and a toe  112  at where the horizontal producer portion  108  terminates distal to the heel  110 . In some embodiments, the horizontal injector portion  106  is closer to the toe  112  of the production well  102  than the heel  110  of the production well  102 . In operation, oxidant  114  injected into the formation  104  along the horizontal injector portion  106  propagates a combustion front  116  from the toe  112  of the production well  102  to the heel  110  of the production well  102 . Examples of the oxidant  106  include oxygen or oxygen-containing gas mixtures. Injection of the oxidant occurs at multiple spaced locations or continuous along the horizontal injector portion  106 . 
     For some embodiments, the horizontal injector portion  106  is closer to the surface  105  than the toe  112  of the production well  102 . The toe  112  of the production well  102  may terminate prior to reaching beneath the horizontal injector portion  106  or may extend beneath the horizontal injector portion  106  such that the horizontal injector portion  106  and the horizontal producer portion  108  cross one another, spaced one on top of another. As the combustion front  116  progresses through the formation  104 , combustion gasses (e.g., CO 2  and CO) and hydrocarbons  118  warmed by the in situ combustion drain downward by gravity into the horizontal producer portion  108  and are recovered via the production well  102 . 
     In some embodiments, the injection well  100  comes closest to the production well  102  intermediately along the horizontal injector portion  106  and may come within 5 to 30 meters of the production well  102 . Fluid communication exists between the horizontal injector portion  106  and the toe  112  of the production well  102  upon initiating the in situ combustion. Spacing between the horizontal injector portion  106  and the toe  112  of the production well  102  enables this communication that is necessary for the in situ combustion to progress through the formation  104 . Further, the horizontal injector portion  106  increases potential area for the communication relative to utilizing only vertical injection wells where lateral area for establishing communication is limited. 
     Location of entry for the hydrocarbons  118  into the horizontal producer portion  108  changes along the horizontal producer portion  108  as the combustion front  116  moves through the formation  104 . After the combustion front  116  passes over part of the horizontal producer portion  108 , oil no longer flows into the part of the horizontal producer portion  108  that is disposed behind the combustion front and in clean sands devoid of oil. Inflow of the hydrocarbons  118  ahead of the combustion front  116  toward the heel  110  of the production well  102  is limited to a region of mobile oil caused by the in situ combustion. 
     Pressure from the injection and the combustion gasses act to drive the mobile oil down toward the horizontal producer portion  108 . Existence of differential pressures from the injection and the combustion gasses relative to inside the production well  102  augments gravity drainage into the production well  102 . The horizontal injector portion  106  and the horizontal producer portion  108  orientation relative to one another ensures that the combustion front  116  remains stable and allows draining of the hydrocarbons  118  into the production well  102  without significant bypassing of the mobile oil below the production well  102 . 
     With the horizontal injector portion  106 , injection is not limited to any finite reservoir thickness in the formation  104  since areal coverage can extend laterally. Lateral extent of the areal coverage creates the pressure gradient discussed herein across the combustion front  116  without loss of the gradient along the z-direction of the combustion front  116 . Quantity of the oxidant  114  able to be injected into the formation  104  corresponds to available outlets into the formation that due to the horizontal injector portion  106  are also not limited by any finite reservoir thickness. The horizontal injector portion  106  thereby permits sufficient rate of oxidant injection into the formation  104  to result in high temperature oxidation or exothermic reactions during the in situ combustion. Given that increase in oxidant supply tends to raise temperatures for the in situ combustion, the rate of oxidant injection possible through the horizontal injector portion  106  thus also enables thermally upgrading the mobile oil while in the formation  104  to lighter oil. 
     Further, the areal coverage provided by the horizontal injector portion  106  ensures sweep efficiency for the combustion front  116  across the formation  104 . Heterogeneities in the formation  104  such as an impermeable body  120  can result in gas channeling or otherwise influence transmission of the oxidant  114  through the formation  104 . Any composition of relatively lower porosity within the formation  104  may provide the impermeable body  120 . The horizontal injector portion  106  provides the oxidant  114  on multiple sides of the impermeable body  120  that could otherwise inhibit the oxidant reaching the combustion front  116  beyond one of the sides of the impermeable body  120 . In this manner, the horizontal injector portion  106  mitigates change to the combustion front  116  due to the impermeable body  120 . 
       FIGS. 3 and 4  show a multilateral injection well  300  and first and second production wells  301 ,  302  in a formation  304 . Configurations illustrated for the wells  300 ,  301 ,  302  exemplify suitable variations of foregoing described aspects. Selection of appropriate variations depends on reservoir particulars, such as size and shape, within the formation  304 . The injection well  300  defines a first lateral wellbore  306  and a second lateral wellbore  307 . The first and second production wells  301 ,  302  have respective first and second horizontal portions  308 ,  309  deviated about 90° from vertical. Drilling techniques employed to create any of the wells  300 ,  301 ,  302  can create fish-bone patterns, multilaterals, slant wells, or horizontal wells deviated about 90° from vertical. 
     The first and second production wells  301 ,  302  both recover hydrocarbons during the in situ combustion generated by oxidant injection through the injection well  300 . Some embodiments include additional production wells and/or injection wells. Regardless of a production well to injection well ratio, at least one production and injection well pair defines a configuration as set forth herein. 
     Referring to  FIG. 4 , the deviation from vertical (the y-direction) for the first and second lateral wellbores  306 ,  307  is less than 90°. The lateral wellbores  306 ,  307  thus slant downward while extending lengthwise in the z-direction. The first lateral wellbore  306  permits injecting into the formation  304  above the second lateral wellbore  307 . Relative to using the second lateral wellbore  307  alone, the first lateral wellbore  306  increases areal coverage in the y-direction in addition to the z-direction and also increases surface area available for injection. 
     Further, the first and second horizontal portions  308 ,  309  extend lengthwise in an offset direction from the x-direction. With reference to the angle θ shown in  FIG. 2 , misalignment between the offset direction, in which the production wells  301 ,  302  extend in length deviated from vertical, and the z-direction, in which the injection well  300  extends lengthwise deviated from vertical, defines an angle of less than 90°. 
       FIG. 5  shows a heated horizontal injection well  500  and a heated horizontal production well  502  in a formation  504 . Only one of the injection well  500  or the production well  502  may be heated for some embodiments. Further, the heated horizontal injection well  500  and/or the heated horizontal production well  502  provide exemplary heating of the formation  504  prior to conducting the in situ combustion as may occur with any embodiments described herein. 
     Start-up represents a potential problem for the in situ combustion since inefficient ignition processes due to lack of adequate initial communication between the injection well  500  and the production well  502  can promote endothermic reactions instead of the exothermic reactions. When cold, bitumen in the formation  504  tends to block the communication between the injection well  500  and the production well  502 . Heating the formation  504  around a vertically deviated section  506  of the injection well  500  and/or a vertically deviated section  508  of the production well  502  reduces viscosity of the bitumen and makes the bitumen mobile. 
     This reduction in viscosity results in decrease of initial oil saturation around the injection well  500 . In addition, the reduction in viscosity allows for the combustion gasses and the mobile oil to be produced through the production well  502 . Heating the deviated sections  506 ,  508  of the wells  500 ,  502  enables heating of a lateral portion of the formation  504 . Ability to heat the lateral potion of the formation increases heating efficiency and increases areal extent of the bitumen capable of being heated to establish communication as desired. Since the communication depends on proximity of the injection well  500  to the production well  502 , the heating further permits greater separation of the injection well  500  from the production well  502 . 
     In some embodiments, a conductive element  550  conveys current (i) to resistive heating elements  551  disposed along the vertically deviated section  506  of the injection well  500 . The heating elements  551  heat the formation  504  by thermal conduction. Heating of the formation with the resistive heating elements  551  may take place over an extended period of time, such as at least 100 days or at least 300 days. 
     Cyclic steam stimulation provides another option for heating the reservoir  504  surrounding the vertically deviated section  506  of the injection well  500 . While both the steam stimulation and the heating with the elements  551  are depicted, one or both such techniques may be utilized prior to the in situ combustion. For the steam stimulation, a steam generator  552  converts a water input  554  into steam. An injector output  556  from the steam generator  552  directs the steam through the injection well  500  into the formation  504 , where the steam is held in place to allow for heat of the steam to transfer into the cold bitumen. Once this initial heat transfer takes place, additional steam is injected into the injection well  500 . This process of injecting steam is repeated as necessary to heat the formation around the vertically deviated section  506  of the injection well  500 . 
     Similar to the injection well  500 , heating of the vertically deviated section  508  of the production well  502  may utilize resistive based elements  560  and/or the cyclic steam stimulation. The resistive based elements  560  may be disposed only proximate a toe  512  of the production well  502  where possible to heat the bitumen between the injection well  500  and the production well  502 . A producer output  558  of the steam generator  552  may repeatedly introduce steam pulses into the production well  502  for preheating the formation  504  prior to performing the in situ combustion. 
     For some embodiments, the in situ combustion described herein may take place after processes for steam assisted gravity drainage (SAGD). For example, injecting steam into the injection well  100  shown in  FIG. 1  may heat and drive oil into the production well  102  where the oil is recovered. Once recovery of the oil using this steam injection diminishes beyond economical returns, the in situ combustion commences as a follow-up recovery operation. 
     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 herein. 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 are not to be used to limit the scope of the invention.