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PRIORITY CLAIM 
       [0001]    The present application claims priority under §119 to U.S. Provisional Application No. 61/708,235 filed Oct. 1, 2012. The entire disclosure of the foregoing application is hereby incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to a method for enhancing the production of oil from subterranean oil reservoirs. In particular, the present invention provides an improved system that uses an electric current passing through the oil reserves to enhance oil recovery. 
       BACKGROUND OF THE INVENTION 
       [0003]    Recovery of heavy oil poses many challenges related to lack of oil mobility both in the formation and during artificial lift. Steam assisted gravity drainage is commonly used to improve or enable heavy oil production. 
         [0004]    Methods for inducing electrical current into oil-bearing formations to improve recovery of oil reserves are described in U.S. Pat. Nos. 3,782,465, 4,495,990, 6,877,556, and 7,325,604, issued Feb. 5, 2008, the entire contents of which are incorporated herein by reference. 
       SUMMARY OF THE INVENTION 
       [0005]    In light of the foregoing, the present invention addresses provides a method and apparatus for overcoming shortcomings of the prior art. To direct current flow through the oil bearing formation it is desirable to limit the extent of at least one electrode to the oil bearing formation. In accordance with one embodiment of this invention, this is accomplished by placing electrical isolating barriers projecting into the oil bearing formation from the casing at the top and the bottom of oil bearing formation. If the production casing does not extend past the bottom boundary of the oil bearing formation then only one isolating barrier is needed on top of the formation. 
         [0006]    According to one aspect, the present invention provides a method for recovering oil from an oil-bearing subterranean formation employing a hollow well casing having an inflow section providing fluid flow from the oil-bearing formation into the casing. The method includes the step of providing an isolation opening extending around the circumference of the casing to provide a separation of the inflow section from an adjacent section of the casing. A portion of the oil-bearing formation disposed on the outside of the isolation opening is removed to form a void in the oil-bearing formation adjacent the isolation opening. A material having less electrical conductivity than the casing is inserted into the void and the isolation opening to form an insulative barrier. 
         [0007]    According to another aspect, the present invention also provides a recovery apparatus for extracting oil using electrical enhanced oil recovery from an oil-bearing formation comprising a wellhead having a hollow casing with multiple sections extending through at least one non-oil-bearing formation into the oil bearing formation. The casing has an inflow section with at least one inflow opening in registry with the oil-bearing formation for extracting oil using electrically enhanced oil recovery. The inflow section is separated from an adjacent one of said multiple sections by a ring-shaped opening extending about the full 360 degree circumference of the casing, and an isolation barrier mounted in the ring-shaped opening. The barrier is comprises electrically non-conductive material isolating the inflow section from the adjacent section. The barrier has an interior dimension similar to the interior dimension of the casing sections on both sides of the barrier, and an exterior dimension extending beyond the exterior circumference of the casing sections and into the formation surrounding the ring-shaped opening between said adjacent casings sections. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic view of an oil-field casing modified in accordance with the present invention to provide isolating barriers; within the oil-bearing formation; 
           [0009]      FIG. 2  is a diagrammatic view (not to scale) of the oil-field casing of  FIG. 1 , showing the current flow from the perforated or slotted inflow section of the casing when used with a remote surface electrode; 
           [0010]      FIG. 3  is a diagrammatic view (not to scale) of the oil-field casing of  FIG. 1 , showing the current flow from the perforated or slotted inflow section of the casing when used with a two-section casing, having isolated upper and lower casing sections above and below the perforated or slotted inflow section which serve as remote electrodes; 
           [0011]      FIG. 4  is a diagrammatic view similar to  FIG. 2  (not to scale), showing the casing terminating at its lower end in a lateral extension; 
           [0012]      FIG. 5  is a perspective view of an isolating barrier; with a portion broken away; 
           [0013]      FIG. 6  is a vertical sectional view of an isolating barrier insulated from the center of the casing by a layer of insulation; and 
           [0014]      FIG. 7  is an enlarged fragmentary perspective view showing the components of the isolating barrier when the invention is used in a bore hole filled with conductive fluids. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0015]    Referring now to  FIG. 1  a system for recovering subterranean oil reserves is illustrated. The system includes a well  10  extending into an oil-bearing formation  46 . A production casing  44  extending into the well  10  has a perforated inflow section  45  adjacent the oil-bearing formation that allows fluid flow from the subterranean formations into the casing. A power source  20  is operable to induce an electrical current into the oil-bearing formation and a pump  41  b pumps oil and other fluids from the casing up to the surface through production tubing  41   a.    
         [0016]    The details of the well structure will now be described in greater detail. The well  10  extends through subterranean layers  47  over the oil-bearing formation  46  and may extend into subterranean layers below the oil bearing formation, referred to as underburden. The production casing  44  is an axially elongated conduit extending into the well. The casing may be comprised of any of variety of materials, including, but not limited to electrically-conductive materials, such as metal. In the present instance, the production casing is formed of steel. 
         [0017]    The production casing  44  is in fluid communication with a wellhead  41  from which the production casing hangs. An intermediate casing  13  surrounding the production casing extends into the overburden  47  and may be retained in place by a cement sheath surrounding the intermediate casing. A surface casing  12  extending into the ground adjacent the wellhead  41  surrounds the intermediate casing  13 . The surface casing may be retained in place by a cement sheath surrounding the surface casing. 
         [0018]    A perforated inflow section  45  in the casing  44  allows fluid to flow into the casing  45  from the surrounding oil-bearing formation. The perforations extend around the circumference of the casing and may be any of a variety of perforations, such as variously-shaped holes or slots, such as longitudinally-extending slots. 
         [0019]    A pump  41  b within the casing  44  is operable to pump oil and/or other fluids through the casing  44  and/or out of the well  10 . In particular, an elongated conduit, referred to as production tubing, may extend into the production casing  44  so that the bottom edge of the production tubing is positioned adjacent the perforated inflow section  45 . The pump  41  b may be in fluid communication with the production tubing  41   a  so that the pump  41  b pumps fluid from the casing up to the surface through the production tubing. In this way, the pump  41   b  may be positioned adjacent the inflow section  45  of the casing  44 . 
         [0020]    The system further includes a power source  20  for inducing an electrical current into the subterranean layers and in particular into the oil-bearing layer  46 . By inducing an electrical current into the oil bearing formation  46 , which enhances oil production. For instance, the electrical current induced into the oil may provide one or more of the following affects on the fluid in the oil-bearing formation: reduction of oil viscosity due to temperature rise resulting from passage of electrical current; electro-kinetic processes, and/or electrochemical reactions that affect the properties of fluid and the medium matrix. 
         [0021]    The power source  20  may provide an AC or DC current and is connected to two electrically conductive elements that operate as electrodes. For instance, the production casing  44  may be formed of metal and may be connected with the power source so that the production casing operates as one of the electrodes. The second electrode is located remotely from the casing so that the oil-bearing formation  46  is between the two electrodes. For instance, the second electrode may be a metallic plate or other conductive structure positioned at the surface or the casing of an adjacent well may be formed of electrically conductive material, such as metal, so that the adjacent well casing operates as the second electrode. Regardless of whether the second electrode is on the surface or subsurface, both electrodes are connected to the power source  20  to provide a current flow between the two electrodes. 
         [0022]    In some applications, if the production casing  44  of a well  10  is used as an electrode, the induced current from the power source  20  may not pass through the oil in the oil-bearing formation  46  for a variety of reasons. One issue that may arise is due to the differing resistivities of the subterranean layers that cause only a small portion of the current to flow through the oil-bearing formation. Specifically, the majority of the induced current may bypass the oil-bearing formation  46 , due to higher electrical conductivity in the overburden and underburden  47  surrounding the oil-bearing formations. Additionally, the surface area of the electrode in contact with the oil-bearing formation  46  is substantially smaller than that in contact with the rest of the formation, which can lead to only a small portion of the induced current passing through the oil in the oil-bearing formation. 
         [0023]    To improve the current flow through the oil-bearing formation, the system includes one or more electrically-insulative barriers  49  along the length of the production casing  44 . The barrier  49  operates to substantially isolate electrically one portion of the production casing from other portion or portions of the casing. For instance, as shown in  FIG. 1 , the power source  20  is connected with the inflow section  45  of the production casing  44 . The barriers  49  substantially isolate the inflow section  45  electrically from the rest of the production casing. Accordingly, the induced current will tend to flow through the oil-bearing formation rather than along the production casing  44 . 
         [0024]    The barrier  49  is a radially extending structure that protrudes into the subterranean layers. Specifically, in the present instance, an upper barrier extends radially outwardly from the casing  44  adjacent the upper portion of the oil-bearing formation  46 . If the production casing  44  extends through the oil-bearing formation, a lower barrier  49  may also be utilized. For instance, as shown in  FIG. 1 , a second barrier extends radially outwardly into the lower portion of the oil-bearing formation. In this way, a portion of the casing  44  operating as an electrode is electrically isolated from another portion of the casing, and in particular, is isolated from the portion of the casing above and below the barriers  49 . 
         [0025]    Although the barrier  49  has been described above as an element formed of an insulative material, it should be understood that the barrier may be configured in other manners that electrically isolate the portion of the casing  44  acting as the electrode from the rest of the casing. Specifically, a short length of the casing could be removed from between the electrode portion of the casing and the reset of the casing. For instance, the casing could be severed around the entire circumference of the casing so a gap is formed between the inflow portion  45  of the casing and the portion of the casing above the inflow portion. In this way, there is a break in the conductive path of the casing between the inflow portion  45  and the upper portion of the casing. Similarly, a section of the casing can be removed from the casing adjacent the lower portion of the oil-bearing formation  46 . 
         [0026]    Although the barriers  49  may be configured so that they are installed when the production casing  44  is installed into the well, in the typical scenario, the barriers  49  are installed after the casing  44  is already installed in the well  10 . For instance, referring to FIGS.  1  and  5 - 6 , a short section of the production casing may be cut-out. In the present instance, a section of the production casing approximately one foot long or less is severed from the production casing so that the production casing is not a continuous length of conduit. The casing  44  may be severed using any of a variety of techniques, such as milling, explosive cutting, jet milling, and/or chemical dissolution of the area of the casing where the opening or window is desired. 
         [0027]    After severing the casing across the entire cross-section of the casing, the subterranean formation adjacent the severed section is also cut away to form a void or cavity  52  extending around the circumference of the production casing adjacent the opening. The cavity  52  extends radially outwardly into the subterranean layer, which in the present instance is the oil-bearing formation  46 . For instance, the cavity may extend approximately 10-20 inches away from the casing. The cavity  52  also extends axially along the length of the casing  44  so that the length of the cavity along the length of the casing is longer than the length of casing that is severed from the casing. 
         [0028]    After the casing is severed and the cavity is formed, non-conductive material is inserted into the cavity  52  and into the window formed in the casing  44 . Any of a variety of non-conductive materials can be used to fill the cavity to form the barrier element. In the present instance, the non-conductive material is a readily deformable material, such as a liquid or generally flowable viscous material that is injected into the cavity  52  and into the window in the casing. If the non-conductive material is fluid or a generally flowable material, preferably the material is curable so that the material forms a generally rigid or stable structure in the form of the barrier  49 . Although the non-conductive material can be any of a variety of generally insulative materials, such as plastic or rubber, in the present instance the material is an insulative epoxy injected into the cavity that cures to form a generally solid structure that electrically isolates and/or insulates the portion of the casing between the barriers  49  from the rest of the casing  44 . When installing barriers both above and below the oil-bearing formation, the first barrier is allowed to set and solidify, before the second circumferential opening is made in the casing, so that the casing section between the barriers is stabilized while the second opening is being formed. 
         [0029]    When the borehole will contain electrically conductive fluids, the fluid in the casing may provide an electrical pathway that will dissipate the electric flow from the power source. For instance, one of the fluids in the well may be salt water that may provide a conductive path. Therefore, an interior barrier may be provided to impede the induced current from traveling through the salt water along the length of the casing rather than through the oil-bearing formation. The interior barrier insulates the interior of the borehole, especially in the areas of the isolating barriers  49 .  FIGS. 6 and 7  illustrate an isolating barrier  52  between adjacent sections  44   a  and  44   b  of the casing  44 . An insulating liner  55  covers the interior surface of the barrier  52 . The liner  55  has a plurality of longitudinal passages  56  extending along its length to allow fluids to travel past the barrier. The passages  56  are closed by a fiberglass mandrel  57  within the liner  55 . The closed passages may serve as conduits for use during the formation of the isolating barrier  52 . 
         [0030]    Referring now to  FIG. 2 , an alternate installation is illustrated in which a well casing  120  is used in conjunction with a surface electrode  121  and a power supply  122 . The well casing  120  has an inflow section with perforations or slots forming an inflow section similar to the installation illustrated in  FIG. 1 . The inflow section in  FIG. 2  is positioned in the oil-bearing formation  146  which falls between the adjoining overburden and underburden  147 . The current flow is indicated by arrows  123 . 
         [0031]    Another alternate installation is illustrated in  FIG. 3  in which a well casing  149  is separated into an upper section  149   a,  and a lower section  149   c  by a perforated or slotted inflow section  149   b.  Isolating barriers  159   a  and  159   b  are mounted above and below the inflow section  149   b  having perforations or slots to electrically isolate the inflow section  149   b  from the upper and lower sections  149   a  and  149   c.  A power supply  152  has one side  150  connected to the inflow section and the other side connected to the upper and lower sections. The current flow generated by the power supply  152  is indicated by the arrows  153 . 
         [0032]      FIG. 4  shows an installation where a well casing  160  is used in conjunction with a surface electrode  161  and a power supply  162 . The well casing  160  terminates in a lateral extension having a first section  169   a,  a perforated or slotted inflow section  169   b,  and a terminal section  169   c  in the oil-bearing formation  166  which falls between the adjoining overburden and underburden  147 . The inflow section  169   b  is isolated from the sections  169   a  and  169   c  by isolating barriers  179   a  and  179   b.  The current flow is indicated by arrows  163 . 
         [0033]    In the embodiments illustrated in  FIGS. 2 and 3 , two electrical isolating barriers  159   a  and  159   b  are positioned about the perimeter of the casing on opposite sides of the inflow section, so as to divert the electric current flow in either direction. However if the casing terminates beyond the inflow section within the oil-bearing formation, the second barrier  159   b  may be omitted. 
         [0034]    While particular embodiments of the invention have been illustrated and described, changes or modifications may be made without departing from the inventive concepts as set forth in the following claims:

Summary:
A method and apparatus for electrically-enhanced oil recovery from an oil-bearing subterranean formation are provided. An isolation opening extends around the circumference of a casing to provide a separation of the inflow section from an adjacent section of the casing. A portion of the oil-bearing formation disposed on the outside of the isolation opening is removed to form a void in the oil-bearing formation adjacent the isolation opening. A material having less electrical conductivity than the casing is inserted into the void and the isolation opening to form an insulative barrier.