Abstract:
The present invention relates generally to a radiofrequency reactor for use in thermally recovering oil and related materials. The radiofrequency reactor includes a radiofrequency antenna configured to be positioned within a well, where the well is provided within an area in which crude oil exists in the ground. The radiofrequency antenna includes a cylindrically-shaped radiating element for radiating radiofrequency energy into the area in which crude oil exists. The cylindrically-shaped radiating element is configured to allow passage of fluids there through. The radiofrequency reactor also includes a radiofrequency generator electrically coupled to the radiofrequency antenna. The radiofrequency reactor is operable to control the radiofrequency energy generated.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims priority from U.S. provisional patent application No. 60/692,112, which was filed on Jun. 20, 2005, and which is incorporated herein by reference in its entirety. This application is a continuation-in-part application of, and claims priority to, U.S. application Ser. No. 11/471,276, filed Jun. 20, 2006, and now allowed, and which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to the use of radiofrequency energy to heat heavy crude oil or both heavy crude oil and subsurface water in situ, thereby enhancing the recovery and handling of such oil. The present invention further relates to methods for applying radiofrequency energy to heavy oils in the reservoir to promote in situ upgrading to facilitate recovery. This invention also relates to systems to apply radiofrequency energy to heavy oils in situ. 
       BACKGROUND OF THE INVENTION 
       [0003]    Heavy crude oil presents problems in oil recovery and production. Crude oils of low API gravity and crude oils having a high pour point present production problems both in and out of the reservoir. Extracting and refining such oils is difficult and expensive. In particular, it is difficult to pump heavy crude oil or move it via pipelines. 
         [0004]    Recovery of heavy crude oils may be enhanced by heating the oil in situ to reduce its viscosity and assist in its movement. The most commonly used process today for enhanced oil recovery is steam injection, where the steam condensation increases the oil temperature and reduces its viscosity. Steam in the temperature range of 150 to 300 degrees Fahrenheit may decrease the heavy oil viscosity by several orders of magnitude. Cyclic steam simulation (CCS) is a method that consists of injecting steam into a well for a period of time and then returning the well to production. A recently developed commercial process for heavy oil recovery is steam assisted gravity drainage (SAGD), which finds its use in high permeability reservoirs such as those encountered in the oil sands of Western Canada. SAGD has resulted recovery of up to 65% of the original oil in places, but requires water processing. All such methods tend to be expensive and require the use of external water sources. 
         [0005]    Other methods in current use do not require the use of water or steam. For example, processes such as the Vapex process, which uses propane gas, and naphtha assisted gravity drainage (NAGD) use solvents to assist in the recovery of heavy crude oils. The drawback to these processes is that the solvents—propane or naphtha—are high value products and must be fully recovered at the end of the process for it to be economical. 
         [0006]    Yet another potential method to enhance the recovery of heavy crude oils is the Toe-To-Heel Injection (THAI) process proposed by the University of Bath. THAI involves both vertical wells and a pair of horizontal wells similar to that used in the SAGD configuration, and uses combustion as the thermal source. Thermal cracking of heavy oil in the porous media is realized, and the high temperature in the mobile oil zone provides efficient thermal sweeping of the lighter oil to the production well. 
         [0007]    Even when they are recovered, heavy crude oils present problems in refinement. Heavy and light crude oil processing will give the same range of refined products but in very different proportions and quantities. Heavy oils give much more vacuum residues than lighter oils. These residues have an API between one and five and very high sulfur and metals content, which makes treatment difficult. Several processes exist to convert vacuum residues. They are thermal, catalytic, chemical, or combinations of these methods. Thermal processes include visbreaking, aquathermolysis and coking. 
         [0008]    Solvent deasphalting (SDA) is a proven process which separates vacuum residues into low metal/carbon deasphalted oil and a heavy pitch containing most of the contaminants, especially metals. Various types of hydrotreating processes have been developed as well. The principle is to lower the carbon to hydrogen ratio by adding hydrogen, catalysis such as tetralin. The goal is to desulfurize and remove nitrogen and heavy metals. These processes may require temperature control, pressure control, and some form of reactor technology such as fixed bed, ebullated bed, or slurry reactor. 
         [0009]    Recent concepts associate different processes to optimize the heavy crude conversion. For example, the combination of hydrotreating and solvent deasphalting in refineries or on site for partial upgrading of heavy crude may be used. 
         [0010]    Finally, the process of gasification for upgrading heavy oil is used. It consists of conversion by partial oxidation of feed, liquid, or solid into synthesis gas in which the major components are hydrogen and carbon monoxide. 
         [0011]    There is a need for an apparatus and method to enhance the recovery of heavy crude oils that does not suffer from the drawbacks associated with current methods. In particular, there is a need for a method that does not use steam or water from external sources, solvents that must be recovered, or combustion. Ideally, such an apparatus and method would at the same time assist in the in situ refinement of the heavy oil. 
         [0012]    The present invention provides just such a method and apparatus. It utilizes radiofrequency energy to combine enhanced oil recovery with physical upgrading of the heavy oil. 
       BRIEF SUMMARY OF THE INVENTION 
       [0013]    The present invention provides a system and method to apply radiofrequency energy to in-situ heavy crude oil to heat the oil and other materials in its vicinity. This system and method enhance the recovery of the heavy crude oil. At the same time, it may be used to upgrade the heavy crude oil in situ. 
         [0014]    This system enhances the recovery of oil through a thermal method. Heavy crude oils have high viscosities and pour points, making them difficult to recover and transport. Heating the oil, however, lowers the viscosity, pour point, and specific gravity of the oil, rendering it easier to recover and handle. Thus, in the present invention, directed radiofrequency radiation and absorption are used to heat heavy oil and reduce its viscosity, thus enhancing recovery. This dielectric heating also tends to generate fissures and controlled fracture zones in the formation for enhanced permeability and improved flow recovery of fluids and gases. 
         [0015]    The system of the present invention is an in-situ radiofrequency reactor (RFR) to apply radiofrequency energy to heavy crude oil in situ. The RFR incorporates an in-situ configuration of horizontal and vertical wells in a heavy crude oil field. Using these wells, the RFR creates a subterranean reactor for the optimum production and surface recovery of the heavy crude oil. The RFR will provide an oil/hydrocarbon vapor front that will optimize recovery of the oil. 
         [0016]    In it simplest form, the RFR may consist of two wells in the oil field, one a radiofrequency well and the second an oil/gas producing well. At least a portion of both wells are horizontal in the oil field, and the horizontal portion of the radiofrequency well is above the horizontal portion of the oil/gas producing well. A radiofrequency transmission line and antenna are placed in the horizontal radiofrequency well and used to apply radiofrequency energy to the oil, thereby heating it. The resulting reduction in the viscosity of the oil and mild cracking of the oil causes the oil to drain due to gravity. It is then recovered through the horizontal oil/gas producing well. Naturally, any number of radiofrequency and oil/gas producing wells can be used to create an RFR for the recovery of heavy crude oils. 
         [0017]    The invention also has the capability of further enhancing recovery through the directed upgrading of the heavy oil in situ. The horizontal radiofrequency well may be strongly electromagnetically coupled to the horizontal oil/gas producing well so that the temperature of the horizontal oil/gas producing well may be precisely controlled, thereby allowing for upgrading of the heavy oil in the producing well over a wide range of temperatures. The oil/gas producing well may be embedded in a fixed bed of material, such as a catalyst bed, selected to provide upgrading of the crude oil draining from above. The upgrading can be based on several different known technologies, such as visbreaking, coking, aquathermolysis, or catalytic bed reactor technology. 
         [0018]    The present invention has several promising advantages over present methods used to enhance recovery of heavy oil. In particular, the RFR does not require the use of water from external sources. This reduces expense and makes the recovery more economical and efficient. Furthermore, the present invention does not require the use of expensive solvents. Through the use of the present invention, enhanced recovery of heavy crude oil can be achieved more efficiently and cost-effectively. 
         [0019]    Furthermore, in situ processing of crude oil has several advantages over conventional oil surface upgrading technology. First, in situ upgrading can be applied on a well to well basis, so that large volumes of production needed for surface processes are not required. Large, costly pressure vessels are not required since the reservoir formation serves as a reactor vessel. It can be applied in remote locations where a surface refinery would be inappropriate. Some of the required gases and possibly water can be generated in situ by the radiofrequency energy absorption. Finally, full range whole crude oils are treated by RFR and not specific boiling range fractions as is commonly done in refineries. This is made possible by the ability of radiofrequency absorption to provide precise temperature control throughout the reactor volume. The proposed reactor provides large quantities of heat through radiofrequency absorption close to the production well where the catalyst bed is placed. No heat carrying fluids are necessary with radiofrequency heating. 
         [0020]    In one embodiment of the invention, an in situ radiofrequency reactor for use in thermally recovering oil and related materials may be provided. The reactor may comprise at least one radiofrequency heating well in an area in which crude oil exists in the ground, a radiofrequency antenna positioned within each radiofrequency heating well in the vicinity of the crude oil, a cable attached to each radiofrequency antenna to supply radiofrequency energy to such radiofrequency antenna, a radiofrequency generator attached to the cables to generate radiofrequency energy to be supplied to each radiofrequency antenna, and at least one production well in proximity to and below the radiofrequency wells for the collection and recovery of crude oil. 
         [0021]    In another embodiment of the invention, an in situ radiofrequency reactor for use in thermally recovering oil and related materials and refining heavy crude oil in situ may be provided. The reactor may comprise at least one radiofrequency heating well in an area in which crude oil exists in the ground, a radiofrequency antenna positioned within each radiofrequency heating well in the vicinity of the crude oil, a cable attached to each radiofrequency antenna to supply radiofrequency energy to such radiofrequency antenna, a radiofrequency generator attached to the cables to generate radiofrequency energy to be supplied to each radiofrequency antenna, at least one production well in proximity to and below the radiofrequency wells and coupled magnetically to the radiofrequency wells for the collection and recovery of crude oil, and at least one catalytic bed in which the production well is embedded. 
         [0022]    In yet another embodiment of the invention, a method for recovering heavy crude oil is provided. The method comprises the steps of positioning a radiofrequency antenna in a well in the vicinity of heavy crude oil, generating radiofrequency energy, applying the radiofrequency energy to the heavy crude oil with the radiofrequency antenna to heat the oil, and recovering the heavy crude oil through production well. 
         [0023]    In one aspect, in general, a radiofrequency reactor for use in thermally recovering oil and related materials. The radiofrequency reactor includes a radiofrequency antenna configured to be positioned within a well, where the well is provided within an area in which crude oil exists in the ground. The radiofrequency antenna includes a cylindrically-shaped radiating element for radiating radiofrequency energy into the area in which crude oil exists. The cylindrically-shaped radiating element is configured to allow passage of fluids there through. The radiofrequency reactor also includes a radiofrequency generator electrically coupled to the radiofrequency antenna. The radiofrequency reactor is operable to control the radiofrequency energy generated. 
         [0024]    Aspects may include one or more of the following. 
         [0025]    The cylindrically-shaped radiating element in the radiofrequency reactor includes a plurality of apertures for allowing passage of the fluids. In some examples, the plurality of apertures have dimensions selected on the basis of the frequency of the radiofrequency energy. 
         [0026]    The radiofrequency reactor includes a coaxial cable for coupling the radiofrequency antenna to the radiofrequency generator. 
         [0027]    The radiofrequency reactor includes a choke assembly positioned between the radiofrequency antenna and radiofrequency generator to maximize transmission of the radiofrequency energy to the radiofrequency antenna. In some examples, the choke assembly includes an inner conductive casing surrounded by a dielectric portion, the assembly running at least one-quarter of a maximal frequency to be emitted, and the inner casing is connected to a cable for coupling the radiofrequency antenna to the radiofrequency generator. 
         [0028]    The radiofrequency reactor may be one of a plurality of reactors. In such a situation, the radiofrequency generator of each reactor is operable to control the radiofrequency energy generated and is configured to work in conjunction with the radiofrequency generators of the plurality of reactors. 
         [0029]    The radiofrequency generator operable to control the radiofrequency energy generated is configured to control the phase of the radiofrequency energy emitted. 
         [0030]    In another aspect, in general, a method of retrofitting an oil well for extracting crude oil. The method includes electrically coupling a radiofrequency generator to a radiofrequency antenna, where the radiofrequency antenna includes a cylindrically-shaped radiating element for radiating radiofrequency energy into the crude oil. The method also includes controlling the radiofrequency generator to provide radiofrequency energy to the radiofrequency antenna. 
         [0031]    Aspects may include one or more of the following. 
         [0032]    Positioning the radiofrequency generator proximally to the well surface and electrically coupling the radiofrequency generator to the cylindrically-shaped radiating element via a coaxial cable. 
         [0033]    Connecting a choke assembly between the radiofrequency generator and the cylindrically-shaped radiating element. 
         [0034]    Controlling the radiofrequency generator to provide radiofrequency energy to the radiofrequency antenna, including controlling the phasing of the radiofrequency energy emitted. 
         [0035]    While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative embodiments of the invention. As will be realized, the invention is capable of modifications in various obvious aspects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]      FIG. 1  is a perspective view of a basic in situ radiofrequency reactor. 
           [0037]      FIG. 2  is a perspective view of an alternative arrangement of an in situ radiofrequency reactor. 
           [0038]      FIG. 3  is a top view of an arrangement for an in situ radiofrequency reactor for use in large oil fields. 
           [0039]      FIG. 4  is a perspective view of a single borehole radiation type applicator that may be used in the radiofrequency reactor of the present invention. 
           [0040]      FIG. 5  is a diagram of a prior art steam assisted gravity drainage (SAGD) system. 
           [0041]      FIG. 6  is a diagram of a well retrofitted as an in situ radiofrequency reactor. 
           [0042]      FIG. 7  is a diagram of a slotted liner protruding from a well shaft. 
       
    
    
     DETAILED DESCRIPTION 
       [0043]    A variety of different arrangements of wells and antennae may be employed to apply radiofrequency energy to heavy crude oil in situ, thereby enhancing oil recovery and achieving in situ upgrading of the oil. The proper structure and arrangement for any particular application depends on a variety of factors, including size of field, depth, uniformity, and nature and amount of water and gases in the field. 
         [0044]      FIG. 1  is a perspective view of a basic in situ radiofrequency reactor. Heavy oil is present in oil field  10 . Oil/gas production well  20  is drilled into the oil field for recovery of heavy oil and other materials. At least a portion of oil/gas production well  20  is drilled horizontally through the oil field. Horizontal oil/gas production well  21  is positioned to receive oil and other gas that are moved or generated by the action of the radiofrequency reactor. A second well, radiofrequency well  30 , is drilled into the oil field in proximity to oil/gas production well  20 . At least a portion of radiofrequency well  30  is drilled horizontally through the oil field in proximity to and above horizontal oil/gas production well  21 . Horizontal radiofrequency well  31  is used to apply radiofrequency energy to the surrounding heavy crude oil field, thereby heating the oil and reducing its viscosity. Due to gravity, the reduced heated heavy crude oil drains, where it may be captured by and pumped out through oil/gas production well  20  to storage or processing equipment. 
         [0045]    Radiofrequency energy is generated by a radiofrequency generator. It is transmitted via radiofrequency transmission line  40  through radiofrequency well  30  and horizontal radiofrequency well  31  to radiofrequency antenna  41 . Radiofrequency antenna  41  applies radiofrequency energy to the surrounding heavy crude oil, thereby heating it and reducing its viscosity so that it may be collected by and recovered through oil/gas production well  20 . The oil/gas production well  20  may also act as a parasitic antenna to redirect radiation in an upward direction toward the formation to be heated by the radiofrequency energy, thereby increasing efficiency. 
         [0046]    For purposes of in situ processing and upgrading of the heavy crude oil, horizontal oil/gas production well  21  may be embedded in catalytic bed  50 . Horizontal radiofrequency well  31  may be strongly electromagnetically coupled to horizontal oil/gas producing well  21  so that the temperature of horizontal oil/gas producing well  21  may be precisely controlled, thereby allowing for upgrading of the heavy oil in horizontal oil/gas production well  21  over a wide range of temperatures. The upgrading can be based on several different known technologies, such as visbreaking, coking, aquathermolysis, or catalytic bed reactor technology. 
         [0047]    Radiofrequency antennae may be placed in an oil field in numerous configurations to maximize oil recovery and efficiency.  FIG. 2  shows a perspective view of an alternative arrangement of an in situ radiofrequency reactor. Radiofrequency antennae  41  may be placed in proximity to one another in oil field  10 . Radiofrequency energy is supplied to the antennae  41  by a radiofrequency generator and then applied to the oil field  10 . The resulting heating reduces the viscosity of the oil, which drains due to gravity. Horizontal oil/gas production well  21  is positioned below the antennae  41  to collect and recover the heated oil. 
         [0048]    As with the RFR in  FIG. 1 , this arrangement may also be used to process the heavy oil in situ. A horizontal radiofrequency well  31  with horizontal radiofrequency antenna  42  may be placed in proximity to horizontal oil/gas producing well  21  below antennae  41  to control the temperature of the oil. Horizontal oil/gas production well  21  may be embedded in catalytic bed reactor  50 . The oil may thereby be upgraded in situ. 
         [0049]      FIG. 3  shows a top view of another arrangement for an in situ radiofrequency reactor for use in large oil fields. In this radial configuration, one central and vertical radiofrequency heating well  32  with radiofrequency antenna  41  is used for larger volumes of oil. Radiofrequency antenna  41  applies radiofrequency energy to area  11 , thereby heating the oil in that area. The heated oil drains to horizontal oil/gas production wells  21  for collection and recovery. Parallel horizontal radiofrequency wells  31  may also be used to heat the oil. In addition, radiofrequency antennae  43  may be placed in vertical radiofrequency wells  33  to assist with in situ upgrading of the heavy crude oil. 
         [0050]    The radiofrequency antennae used in the RFR system of the present invention may be any of those known in the art.  FIG. 4  shows a perspective view of a radiofrequency applicator that may be used with the RFR of the invention. Applicator system  45  is positioned within radiofrequency well  30 . Applicator system  45  is then used to apply electromagnetic energy to heavy crude oil in the vicinity of radiofrequency well  30 . 
         [0051]    Applicator structure  46  is a transmission line retort. Radiofrequency energy is supplied to applicator  46  by an RF generator (not shown). The radiofrequency generator is connected to applicator  46  via radiofrequency transmission line  40 . The radiofrequency transmission line  40  may or may not be supported by ceramic beads, which are desirable at higher temperatures. By this means, the radiofrequency generator supplies radiofrequency energy to applicator  46 , which in turn applies radiofrequency energy to the target volume of oil. 
         [0052]    Although one specific examples of an applicator structure is given, it is understood that other arrangements known in the art could be used as well. Uniform heating may be achieved using antenna array techniques, such as those disclosed in U.S. Pat. No. 5,065,819. 
         [0053]    The present invention also has application in oil shale fields, such as those present in the Western United States. Large oil molecules that exist in such oil shale have been heated in a series of experiments to evaluate the dielectric frequency response with temperature. The response at low temperatures is always dictated by the connate water until this water is removed as a vapor. Following the water vapor state, the minerals control the degree of energy absorption until temperatures of about 300-350 degrees centigrade are reached. In this temperature range, the radiofrequency energy begins to be preferentially absorbed by the heavy oil. The onset of this selective absorption is rapid and requires power control to insure that excessive temperatures with attendant coking do not occur. 
         [0054]    Because of the high temperature selective energy absorption capability of heavy oil, it is therefore possible to very carefully control the bulk temperature of crude oil heated by radiofrequency energy. The energy requirement is minimized once the connate water is removed by steaming. It takes much less energy to reach mild cracking temperatures with radiofrequency energy than any other thermal means. 
         [0055]    Kasevich has published a molecular theory that relates to the specific heating of heavy of oil molecules. He found that by comparing cable insulating oils with kerogen (oil) from oil shale, a statistical distribution of relaxation times in the kerogen dielectric gave the best theoretical description of how radiofrequency energy is absorbed in oil through dielectric properties. With higher temperatures and lowering of potential energy barriers within the molecular complex a rapid rise in selective energy absorption occurs. 
         [0056]    In use, a user of an embodiment of the present invention would drill oil/gas production wells and radiofrequency wells into a heavy crude oil field. At least a portion of the wells would be horizontal. The radiofrequency wells would be placed in proximity to and above the oil/gas production wells. The user would install a radiofrequency antenna in each radiofrequency well and supply such antennae with radiofrequency energy from a radiofrequency generator via a radiofrequency transmission cable. The user would then apply radiofrequency energy using the radiofrequency generator to the antenna, thereby applying the radiofrequency energy to the heavy crude oil in situ. The radiofrequency energy would be controlled to minimize coking and achieve the desired cracking and upgrading of the heavy crude oil. The resulting products would then be recovered via the oil/gas production well and transferred to a storage or processing facility. 
         [0057]    Referring again to  FIG. 4 , the applicator structure  46  is a vertical monopole antenna within a non-metallic production pipe (shown as a radiofrequency well  30 ). The production pipe extension below the applicator or antenna may be used to enhance the radiation efficiency by adjusting the length of the pipe. The pipe may extend into or below the subterranean oil or gas. 
         [0058]    As described in the above background section, steam assisted gravity drainage (SAGD), is an existing commercial process for heavy oil recovery, used especially in high permeability reservoirs such as those encountered in the oil sands of Western Canada. Referring to  FIG. 5 , in the SAGD process, two parallel horizontal oil wells  520  &amp;  550  are drilled in the formation, one above the other (in some examples, roughly 10 meters apart). The upper well acts as a steam injector  520  and typically includes a slotted liner  522  (in some examples, roughly 300 meters long) for allowing steam to be released through the slots  530 . The steam increases the temperature of the crude oil in the oil sand formation  512 , reducing the crude oil&#39;s viscosity and allowing it to be collected by gravity drainage via the lower well, referred to as an oil producer  550 . The slotted liner  522  is typically made of conductive materials. 
         [0059]    Referring to  FIG. 6 , in some embodiments, the SAGD configuration is retrofitted to use one or both wells (or portions thereof, e.g., the liners) as an antenna for emitting RF energy into the oil sand formation. The RF energy increases the temperature of the crude oil, reducing its viscosity and allowing it to be collected. In some embodiments the oil is collected using a pipe (not shown) within the same well as the well  600  configured to host an antenna. 
         [0060]    A coaxial cable  630  connects a power source (not shown), for example, a radiofrequency generator stationed on the surface, to the slotted liner  622 . The coaxial cable  630  has a central conductor  632  surrounded by a dielectric insulating portion and an outer conductive shield  634 . In some embodiments, the outer conductor  634  is also wrapped in an external insulating layer. 
         [0061]    At the distal end of the well, the coaxial cable&#39;s central conductor  632  is electrically connected to the well&#39;s slotted liner  622 . In some embodiments, the connection to the liner  622  is achieved using a metal contact ring  660  to which the central conductor  632  is electrically connected  664  (e.g., welded). The contact ring  660  is mated with the liner  622 . 
         [0062]    In some embodiments, an insulating section  650  is used, for example, to separate the slotted liner  622  from the well wall  620 . The insulating section  650  is a hollow cylinder that allows the coaxial cable  630  and any other cables or pipes (e.g., an oil collection pipe) to pass through it. In some examples, the insulating section  650  is ceramic. 
         [0063]    As shown if  FIG. 6 , the well  600  is supported in the earth  616  by a cement casing  614 . The cement  614  is susceptible to cracking if subjected to excessive heat. In such embodiments, it may be desirable to restrict the level of RF energy returning up the well  600 , for example, to reduce the risk of the cement  614  cracking. Therefore, a high impedance block is created. 
         [0064]    In the embodiment shown in  FIG. 6 , the outer conductor  634  of the coaxial cable  630  is electrically connected  648  to a quarter-wave choke assembly  640 . The optimal length of the choke assembly is an odd multiple of quarter-wavelengths (¼, ¾, 5/4, etc.). That is, the choke assembly  640  extends back from the insulator  650  at least one quarter of the maximum wavelength for the energy to be emitted from the antenna. The choke assembly  640  may extend further back, in some examples, extending all of the way back to the surface. 
         [0065]    The quarter-wave choke assembly  640  includes an inner conductor  642 , which is separated from either the well wall  620  or an outer assembly casing  644  by either air or a dielectric layer  646 . The outer conductor  634  of the coaxial cable  630  is electrically connected  648  to the inner conductor  642  of the choke assembly  640 . The inner conductor  642  is shorted  654  to the inner side of the well wall  620  at the proximal end of the choke assembly  640 . 
         [0066]    The quarter-wave choke assembly  640  creates a high impedance block restricting the flow of energy back up the well  600 . Alternatively, in some embodiments, the outer conductor  634  is electrically connected directly to the inside of the well wall  620 . 
         [0067]    Referring again to  FIG. 5 , in certain embodiments, multiple wells (e.g., both the steam injector  520  and the oil producer  550 ) are retrofitted as RF antennas. In such embodiments, the multiple antennas are powered in a manner to boost the RF energy, for example, by emitting energy in phase. In other embodiments, the phase of the energy emitted by each of the multiple antennas can be tuned to control the energy levels within the oil sand formation by controlling the antennas to emit out of phase. 
         [0068]    In certain applications, the slots in the slotted liner are sized in a manner to increase the efficacy of subsequent RF retrofit. Referring to  FIG. 7 , in some embodiments, a well  700  is configured with two slotted liners—an inner liner  710  and an outer liner  720 . Each liner includes slots  730 . At least one liner, e.g., the inner liner  710 , is configured to be adjusted, acting as a telescoping sleeve. By telescoping the liner, the size of the slots  730  are adjusted. The liner overlap  740  therefore creates variably sized slots. Using this approach, the slots in the slotted liner are dynamically sized as needed. 
         [0069]    In some embodiments, the presence of the RF retrofit does not preclude the contemporary use of steam or other oil recovery methods. For example, the RF energy is used to initiate the process of oil recovery by alternative means. 
         [0070]    Although the present invention has been described with reference to preferred embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.