Parallel fed well antenna array for increased heavy oil recovery

A parallel fed well antenna array and method for heating a hydrocarbon formation is disclosed. An aspect of at least one embodiment is a parallel fed well antenna array. It includes an electrically conductive pipe having radiating segments and insulator segments. It also includes a two conductor shielded electrical cable where the shield has discontinuities such that the first conductor and the second conductor are exposed. The first conductor is electrically connected to the conductive pipe and the second conductor is electrically connected to the shield of the electrical cable just beyond an insulator segment of the conductive well pipe A radio frequency source is configured to apply a signal to the electrical cable.

BACKGROUND OF THE INVENTION

The present invention relates to heating a geological formation for the extraction of hydrocarbons, which is a method of well stimulation. In particular, the present invention relates to an advantageous radio frequency (RF) applicator and method that can be used to heat a geological formation to extract heavy hydrocarbons.

As the world's standard crude oil reserves are depleted, and the continued demand for oil causes oil prices to rise, oil producers are attempting to process hydrocarbons from bituminous ore, oil sands, tar sands, oil shale, and heavy oil deposits. These materials are often found in naturally occurring mixtures of sand or clay. Because of the extremely high viscosity of bituminous ore, oil sands, oil shale, tar sands, and heavy oil, the drilling and refinement methods used in extracting standard crude oil are typically not available. Therefore, recovery of oil from these deposits requires heating to separate hydrocarbons from other geologic materials and to maintain hydrocarbons at temperatures at which they will flow.

Current technology heats the hydrocarbon formations through the use of steam and sometimes through the use of RF energy to heat or preheat the formation. Steam has been used to provide heat in-situ, such as through a steam assisted gravity drainage (SAGD) system. Steam enhanced oil recovery can not be suitable for permafrost regions due to surface melting, in stratified and thin pay reservoirs with rock layers, where there is insufficient caprock, where there are insufficient water resources to make steam, and steam plant deployment can delay production. At well start up, for example, the initiation of the steam convection can be slow and unreliable, as conductive heating in hydrocarbon ores is slow. Radio frequency electromagnetic heating is known for speed and penetration so unlike steam, conducted heating to initiate convection can not be required. The increased speed of production can increase profits. RF heating can be used to initiate convection for steam heated wells or used alone.

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SUMMARY OF THE INVENTION

A parallel fed well antenna array and method for heating a hydrocarbon formation is disclosed. The array includes an electrically conductive pipe having radiating segments and insulator segments. It also includes a two conductor shielded electrical cable where the shield has discontinuities to expose the first conductor and the second conductor. The first conductor is electrically connected to the conductive pipe and the second conductor is electrically connected to the shield of the electrical cable just beyond an insulator segment of the conductive well pipe A radio frequency source is configured to apply a signal to the electrical cable. A nonconductive sleeve covers a portion of the electrically conductive pipe and the electrical cable to keep that section of the device electrically neutral.

Another aspect of at least one embodiment is an alternative parallel fed antenna array that can be retrofit to existing well pipes because it doesn't require insulator segments on the well pipe. Rather, it includes an electrically conductive pipe and a two conductor shielded electrical cable where the shield has discontinuities such that the first conductor and the second conductor are exposed. Both the first conductor and the second conductor are electrically connected to the conductive pipe. A radio frequency source is configured to apply a signal to the electrical cable. A nonconductive sleeve covers a portion of the electrically conductive pipe and the electrical cable to keep that section of the device electrically neutral.

Yet another aspect of at least one embodiment involves a method for heating a hydrocarbon formation. In the first step a two conductor shielded electrical cable is coupled to a conductive well pipe. A radio frequency signal is then applied to the electrical cable that is sufficient to create a circular magnetic field relative to the axis of the conductive well pipe.

Other aspects of certain disclosed embodiments will be apparent from this disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The subject matter of this disclosure will now be described more fully, and one or more embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are examples of the invention, which has the full scope indicated by the language of the claims.

Radio frequency (RF) heating is heating using one or more of three energy forms: electric currents, electric fields, and magnetic fields at radio frequencies. Depending on operating parameters, the heating mechanism can be resistive by Joule effect or dielectric by molecular moment. Resistive heating by Joule effect is often described as electric heating, where electric current flows through a resistive material. Dielectric heating occurs where polar molecules, such as water, change orientation when immersed in an electric field. Magnetic fields also heat electrically conductive materials through induction of eddy currents, which heat resistively by joule effect.

RF heating can use electrically conductive antennas to function as heating applicators. The antenna is a passive device that converts applied electrical current into electric fields, magnetic fields, and electrical current fields in the target material, without having to heat the structure to a specific threshold level. Preferred antenna shapes can be Euclidian geometries, such as lines and circles. Line shaped antennas can fit the linear geometry of hydrocarbon wells and the line shaped antenna can supply magnetic fields for induction of eddy currents, source electric currents by electrode contact for resistive heating, and supply electric fields for electric induction of displacement currents. Additional background information on linear antennas can be found at S. K. Schelkunoff & H. T. Friis,Antennas: Theory and Practice, pp 229-244, 351-353 (Wiley New York 1952). The radiation patterns of antennas can be calculated by taking the Fourier transforms of the antennas' electric current flows. Modern techniques for antenna field characterization can employ digital computers and provide for precise RF heat mapping.

Susceptors are materials that heat in the presence of RF energy. Salt water is a particularly good susceptor for RF heating; it can respond to all three types of RF energy. Oil sands and heavy oil formations commonly contain connate liquid water and salt in sufficient quantities to serve as an RF heating susceptor. For instance, in the Athabasca region of Canada and at 1 KHz frequency, rich oil sand (15% bitumen) can have about 0.5-2% water by weight, an electrical conductivity of about 0.01 s/m (siemens/meter), and a relative dielectric permittivity of about 120. As bitumen melts below the boiling point of water at reservoir conditions, liquid water can be a used as an RF heating susceptor during bitumen extraction, permitting well stimulation by the application of RF energy. In general, RF heating can have superior penetration to conductive heating in hydrocarbon formations and superior speed. It might require months for conducted heat to penetrate 10 meters in hydrocarbon ore while RF heating energy can penetrate the same distance in microseconds.

RF heating can also have properties of thermal regulation because steam is a not an RF heating susceptor. Thus, electromagnetic energy can be used to heat the water in place in the hydrocarbon ore and the water can then heat the hydrocarbons by conduction. Electromagnetic energy generally heats liquid water much faster than hydrocarbons by a factor of 100 or more. The microstructure of Athabasca oil sand consists of bitumen films covering pores of water with sand cores. In other words, each sand grain is in water drop, and the water drop is covered with bitumen. RF heating the core water mobilizes the oil by reducing its viscosity. The RF stimulated well generally produces the oil and water together, which are then separated at the surface. Heating subsurface heavy oil bearing formations by prior RF systems has been inefficient, in part, because prior systems use resistive heating techniques, which require the RF applicator to be in contact with water in order to heat the formation. Liquid water contact can be unreliable because live oil can deposit nonconductive asphaltines on the electrode surfaces and because the water can boil off the surfaces. Heating an ore region through primarily inductive heating, both electric and magnetic, is an advantage of certain disclosed embodiments.

FIG. 1shows a diagrammatic representation of an embodiment. An aspect of the invention is a parallel fed well antenna array, which creates an RF applicator that can be used, for example, to heat a hydrocarbon formation. The applicator system generally indicated at10extends through an overburden region2and into an ore region4. Throughout the ore region4the applicator is generally linear and can extend horizontally over one kilometer in length. In accordance with this invention, electromagnetic radiation provides heat to the hydrocarbon formation, which allows heavy hydrocarbons to flow. The hydrocarbons can then be captured by one or more extraction pipes (not shown) located within or adjacent to the ore region4, or the system can include pumps or other mechanisms to drain the heated hydrocarbons.

The applicator system10includes an electrical cable12, which has a first conductor14, a second conductor16, and a shield18. The applicator also includes a conductive well pipe20with insulator segments22and radiating segments32, an RF source24, connection sites26, first conductive jumpers28, second conductive jumpers30, and a magnetic sleeve34.

The electrical cable12can be any known two conductor shielded electrical cable. The shield prevents unwanted heating of the overburden and allows the electrical currents to be distributed to any number and length of well pipe segments in the ore region4. As a practical matter, the electrical cable12resistance should be much less than the load resistance of ore region4. Shielded cables are generally required to convey electrical power through earth at radio frequencies.

The conductive well pipe20can be made of any conductive metal, but in most instances will be a typical steel well pipe. The conductive well pipe can include a highly conductive coating, such as copper. In the embodiment shown inFIG. 1, the well pipe has several insulator segments22. The insulator segments22can be comprised of any electrically nonconductive material, such as, for example, plastic or fiberglass pipe. The insulator segments22can also be formed by installing or positioning a ferrite bead over sections of the outside of the conductive well pipe20. The insulator segments22function to separate different sections of the well pipe20, which form the radiating segments32, so as to provide electrical discontinuities along the length of the pipe20.

The RF source24is connected to the electrical cable12through the first conductor14and the second conductor16and is configured to apply a signal with a frequency f to the electrical cable12. In practice, frequencies between 1 kHz and 10 MHz can be effective to heat a hydrocarbon formation, although the most efficient frequency at which to heat a particular formation can be affected by the composition of the ore region4. It is contemplated that the frequency can be adjusted according to well known electromagnetic principles in order to heat a particular hydrocarbon formation more efficiently. Simulation software indicates that the RF source16can be operated effectively at 2 Megawatts to 10 Megawatts power for a 1 km long well, so an example of a metric for a formation in the Athabasca region of Canada can be to apply about 2 to 10 kilowatts of RF power per meter of well length initially and to do so for 1 to 4 months to start up the well. Production power levels can be reduced to about ten percent to twenty percent of this amount or steam can be used after RF startup. The RF source16can include a transmitter and an impedance matching coupler including devices such as transformers, resonating capacitors, inductors, and other well known components to conjugate match, correct power factor, and manage the dynamic impedance changes of the ore load as it heats. The RF source16can also be an electromechanical device such as a multiple pole alternator or a variable reluctance alternator with a slotted rotor that modulates coupling between two inductors. The rim of the slotted rotor can rotate at supersonic speeds to produce radio frequency alternating current at frequencies between 1 and 100 KHz. The RF source16can also be a vacuum tube device, such as an Eimac 8974/X-2159 power tetrode or an array of solid state devices. Thus, there are many options to realize RF source16.

The first conductor14is electrically connected to the conductive well pipe20at one or more connection sites26. A connection site26is a section of the electrical cable12where the shield18has been stripped away to allow access to the first conductor14and the second conductor16, and generally occurs near an insulator segment22. For example, the first conductor14can be connected to the conductive well pipe20through a first conductive jumper28. The first conductive jumper28can be, for example, a copper wire, a copper pipe, a copper strap, or other conductive metal. The first conductive jumper26feeds current from the first conductor14onto the conductive well pipe26just beyond an insulator segment22.

Similarly, the second conductor16is electrically connected to the shield18at one or more connection sites26. For example, the second conductor16can be connected to the shield18through a second conductive jumper30. The second conductive jumper30can be, for example, a copper wire, a copper pipe, a copper strap, or other conductive metal. Connecting the second conductive jumper30to the shield18completes the closed electrical circuit, as described below.

In operation, the first conductor14, the first conductive jumper28, the conductive well pipe20, the second conductor16, the second conductive jumper30, and the shield18create a closed electrical circuit, which is an advantage because the combination of these features allows the applicator system10to generate magnetic near fields so the antenna need not to have conductive electrical contact with the ore. The closed electrical circuit provides a loop antenna circuit in the linear shape of a dipole. The linear dipole antenna is practical to install in the long, linear geometry of oil well holes whereas circular loop antennas can be impractical or nearly so. The conductive well pipe20itself functions as an applicator to heat the surrounding ore region4.

When the applicator system10is operated, current I flows through a radiating segment32, which creates a circular magnetic induction field H, which expands outward radially with respect to a radiating segment32. A magnetic field H in turn creates eddy currents Ie, which heat the ore region4and cause heavy hydrocarbons to flow. The operative mechanisms are Ampere's Circuital Law:
∫B·dl
and Lentz's Law
δW=W·B
to form the magnetic near field and the eddy current respectively. The magnetic field can reach out as required from the applicator10, through electrically nonconductive steam saturation areas, to reach the hydrocarbon face at the heating front.

For certain embodiments and formations, the strength of the heating in the ore due to the magnetic fields and eddy currents is proportional to:
P=π2B2d2f2/12ρD

Where:P=power delivered to the ore in wattsB=magnetic flux density generated by the well antenna in Teslasd=the diameter of the well pipe antenna in metersρ=the resistivity of the hydrocarbon ore in ohms=1/σf=the frequency in HertzD=the magnetic permeability of the hydrocarbon ore

The strength of the magnetic flux density Bφgenerated by the well antenna derives from Ampere's law and is given by:
Bφ=μILe−jkrsin θ/4πr2

Where:B=magnetic flux density generated by the well antenna in Teslasμ=magnetic permeability of the oreI=the current along the well antenna in amperesL=length of antenna in meterse−jkr=Euler's formula for complex analysis=cos(kr)+j sin(kr)θ=the angle measured from the well antenna axis (normal to well is 90 degrees)r=the radial distance outwards from the well antenna in meters

The magnetic field can reach out as required from the conductive well pipe20, through electrically nonconductive steam saturation areas, to reach the hydrocarbon face at the heating front. Simulations have shown that as the current I flows along a radiating segment32, it dissipates along the length of the radiating segment32, thereby creating a less effective magnetic field H at the far end of a radiating segment32with respect to the radio frequency source24. Thus, the length of a radiating segment32can be about 35 meters or less for effective operation when the applicator10is operated at about 1 to 10 kHz. However, the length of a radiating segment32can be greater or smaller depending on a particular applicator10used to heat a particular ore region4. A preferred length for a radiating segment32is approximately:
δ=√(2/σωμ)

Where:δ=the RF skin depthσ=the electrical conductivity of the underground ore in mhos/meterω=the angular frequency of the RF current source16in radians=2π(frequency in hertz)μ=the absolute magnetic permeability of the conductor=μoμr

The applicator system10can extend one kilometer or more horizontally through the ore region4. Thus, in practice an applicator system10can consist of an array of twenty (20) or more radiating segments32connected by insulator segments22, depending on the electrical conductivity of the underground formation, so the applicator system10provides a modular method of construction. The conductivity of Athabasca oil sand bitumen ores can be between 0.002 and 0.2 mhos per meter depending on hydrocarbon content. The richer ores are less electrically conductive. In general, the radiating segments32are electrically small, for example, they are much shorter than both the free space wavelength and the wavelength in the media they are heating. The array formed by the radiating segments32is excited by approximately equal amplitude and equal phase currents. The realized current distribution along the array of radiating segments32forming the applicator10can initially approximate a shallow serrasoid (sawtooth), and a binomial distribution after steam saturation temperatures is reached in the formation. Varying the frequency of the RF source16is a method of certain disclosed embodiments to approximate a uniform distribution for even heating.

The magnetic sleeve34surrounds the electrical cable12and the conductive well pipe20in, optionally all the way through, the overburden region2. The magnetic sleeve34can be made up of a variety of materials, and it preferentially is bulk electrically nonconductive (or nearly so) and it has a high magnetic permeability. For example, it can be comprised of a bulk nonconductive magnetic grout. A bulk nonconductive magnetic grout can be composed of, for example, a magnetic material and a vehicle. The magnetic material can be, for example, nickel zinc ferrite powder, pentacarbonyl E iron powder, powdered magnetite, iron filings, or any other magnetic material. The particles of magnetic material can have an electrically insulative coating such as FePO4(Iron Phosphate) to eliminate eddy currents. The vehicle can be, for example, silicone rubber, vinyl chloride, epoxy resin, or any other binding substance. The vehicle can also be a cement, such as Portland cement, which can additionally seal the well casings into the underground formations while simultaneously containing the magnetic medium. At sufficiently low frequencies, the nonconductive sleeve can also use lamination techniques to control eddy currents therein. The laminations can comprise layers of magnetic sheet metal with electrical insulation between them such as silicon steel sheets with insulating varnishes. Other laminations can include windings of magnetic wire or magnetic strip with electrical insulation. Alternatives to the magnetic sleeve34can include balanced transmission lines, isolated metal sleeves, and series inductive windings.

The magnetic sleeve34keeps the portion of the applicator system10that it covers electrically neutral. Thus, when the applicator10system is operated, electromagnetic radiation is concentrated within the ore region4because RF electric currents cannot flow over the outside of well pipe20due to the inductive reactance of magnetic sleeve34. This is an advantage because it is desirable not to divert energy by heating the overburden region2, which is typically highly conductive relative to the hydrocarbon ore region4.

Some embodiments can include one or more electrical separations40in the applicator system10. An electrical gap42is a section of the electrical cable where the shield has been stripped away and generally occurs near an insulator segment22. An electrical gap42is similar to a connection site26; however, no connection between the conductors and the conductive well pipe occurs at an electrical separation40. The electrical separation40can be used to modify the electrical impedances obtained from the radiating segments32. The electrical separations40change the load resistances provided by the radiating segments32and change the sign of the electrical reactance provided by radiating segments32.

At an electrical separation40, the radiating segments32are center fed, and the radiating segments become unfolded antennas that do not have DC continuity. Without the electrical separation40, the radiating segments32are end fed, and the radiating segments become folded antennas having DC continuity. Thus, the radiating segments32can be made capacitive or inductive by including or not including electrical separations40. Below the first resonance of the radiating segments32, for example, at low frequencies, including electrical separations40can make the radiating segments capacitive. At higher frequencies, not including electrical separations40can make the radiating segments inductive and lower resistance, depending on the characteristics of the ore region4. Electrical separations40can also be used to select between magnetic field induction and electric field induction heating modes in the ore region4.

FIG. 2shows an alternative embodiment of certain disclosed embodiments. In this embodiment, no insulator sections are installed in the conductive well pipe20. Although this embodiment can allow for retrofitting existing oil wells, it is also less efficient and leads to more conductor loss.

The applicator system10ofFIG. 2includes an electrical cable12, which has a first conductor14, a second conductor16, and a shield18. The applicator also includes a conductive well pipe20, an RF source24, first connection sites36, second connection sites38, first conductive jumpers28, second conductive jumpers30, magnetic sleeve34, and bond sites36.

As described above with respect toFIG. 1, the electrical cable12has a first conductor14, a second conductor16, and a shield18and can be any known two conductor shielded cable. The conductive well pipe20can be made of any conductive metal, but in most instances will be a typical steel well pipe. The conductive well pipe20can include a highly conductive coating, such as copper. The RF source24also operates as explained above with respect toFIG. 1.

In this embodiment, the first conductor14is electrically connected to the conductive well pipe20at one or more first connection sites36. A first connection site36is a section of the electrical cable12where the shield18has been stripped away to allow access to the first conductor14and the second conductor16. In this embodiment, the first connection sites36occur at regular intervals but no corresponding insulator segment is present on the conductive well pipe20. Again, the first conductor14can be connected to the conductive well pipe20through a first conductive jumper28. The first conductive jumper28can be, for example, a copper wire, a copper pipe, a copper strap, or other conductive metal. The first conductive jumper26feeds current from the first conductor14onto the conductive well pipe20.

Similarly, the second conductor16is electrically connected to the conductive well pipe20at one or more second connection sites38. For example, the second conductor16can be connected to the conductive well pipe20through a second conductive jumper30. The second conductive jumper30can be, for example, a copper wire, a copper pipe, a copper strap, or other conductive metal. Because current I flows in the opposite direction on the second conductor16as it does on the first conductor14, the second conductor removes current I from the conductive well pipe20.

In the illustrated embodiment, although this is not a requirement for other embodiments, each connection site alternates between being a first connection site36or a second connection site38. Thus, along the length of the conductive well pipe20current I is fed onto and then removed from the conductive well pipe in an alternating fashion. The shield18is also bonded to the conductive well pipe20at regular, frequent intervals indicated as bond sites39.

In operation, the first conductor14, the first conductive jumper28, the conductive well pipe20, the second conductor16, the second conductive jumper30, create a closed electrical circuit, which is an advantage because the combination of these features allows the applicator system10to generate magnetic near fields so the antenna need not have conductive electrical contact with the ore. The closed electrical circuit provides benefits as described above with respect toFIG. 1. Moreover, the applicator system10operates in substantially the same manner as described above, and an array of radiating segments32is formed.

Simulations show that as the current I dissipates along the length of the conductive well pipe32as it flows, which creates a less effective magnetic field H at the far end of a radiating segment32with respect to the radio frequency source24. Thus, the length of a radiating segment32can be about 35 meters or less for effective operation when the applicator10is operated at about 1 to 10 kHz. However, as described above the length of a radiating segment32can be greater or smaller depending on a particular applicator system10used to heat a particular ore region4, and again because the applicator system10can extends one kilometer or more horizontally through the ore region4, an applicator system can consist of twenty (20) or more radiating segments32.

Once again a magnetic sleeve34surrounds the electrical cable12and the conductive well pipe20in, optionally throughout, the overburden region2, which is an advantage because it is desirable not to divert energy by heating the overburden region2, which is typically highly conductive.

FIG. 3depicts yet another alternative embodiment. In this embodiment the applicator system10extends into a vertical well rather than a substantially horizontal well. This embodiment heats the ore region4in substantially the same manner as described above, however, because the well is vertical rather than horizontal, the effect will be slightly different because the magnetic fields will still expand radially from the conductive well pipe20, and as such the magnetic fields will be generally oriented at a right angle to the magnetic field described above. The hydrocarbons can then be captured by one or more extraction pipes (not shown) located within or adjacent to the ore region4, or the system can include pumps or other mechanisms to drain heated hydrocarbons.

Alternative embodiments to certain disclosed embodiments not shown are possible, for instance, the vertical well embodiment can be implemented without insulator segments22, similar to that described above with respect toFIG. 2.

FIG. 4depicts an embodiment of a method for heating a hydrocarbon formation40. At the step41, a two conductor shielded electrical cable is coupled to a conductive well pipe. At the step42, a radio frequency signal is applied to the electrical cable, which is sufficient to create a circular magnetic field relative to the radial axis of the conductive well pipe.

At the step41, a two conductor shielded electrical cable is coupled to a conductive well pipe. For instance, the electrical cable and the conductive well pipe can be the same or similar to the electrical cable12and the conductive well pipe20ofFIG. 1,2, or3. Furthermore, the electrical cable is electrically coupled to the conductive well pipe. For instance, conductive jumpers can be used as described above with respect toFIG. 1,2, or3. The conductive well pipe is preferably located in the ore region of a hydrocarbon formation.

At the step42, a radio frequency signal is applied to the electrical cable sufficient to create a circular magnetic field relative to the radial axis of the conductive well pipe. For instance, for the applicator systems depicted inFIGS. 1,2, and3, a 1 to 10 kilohertz signal having about 1 Watt to 5 Megawatts power can be sufficient to create a circular magnetic field penetrating about 10 to 15 meters half power depth radially from the conductive well pipe into the hydrocarbon formation, however, the prompt penetration depth and the signal applied can vary based on the composition of a particular hydrocarbon formation. The signal applied can also be adjusted over time to heat the hydrocarbon formation more effectively as susceptors within the formation are desiccated or replenished. The circular magnetic field creates eddy currents in the hydrocarbon formation, which will cause heavy hydrocarbons to flow.

A representative RF heating pattern in accordance with this invention will now be described. TheFIG. 5well dimensions are as follows: the horizontal well section is 1 kilometers long and at a depth of 30 meters, applied power is 1 Watt and the heat scale is the specific absorption rate in Watts/kilogram. The heating pattern shown is for time t=0, for example, when the RF power is first applied. The frequency is 1 kilohertz (which is sufficient for penetrating many hydrocarbon formations). Formation electrical parameters were permittivity=500 farads/meter and conductivity=0.0055 mhos/meter, which can be typical of rich Canadian oil sands at 1 kilohertz.

FIG. 5depicts an isometric or overhead view of an RF heating pattern for a heating portion of two element array twinaxial linear applicator in accordance with this invention, which can be the same or similar to that described above with respect toFIG. 1. The heating pattern depicted shows RF heating rate of a representative hydrocarbon formation for the parameters described below at time t=0 or just when the power is turned on. 1 Watt of power was applied to the antenna applicator to normalize the data. As can be seen, the heating rate is smooth and linear along the conductive well pipe20because current is fed onto the conductive well pipe at regular intervals. The realized temperatures (not shown) are a function of the duration of the heating and the applied power, as well as the specific heat of the ore. Rich Athabasca oil sand ore was used in the model, and the ore conductivities used were from an induction resistivity log. A frequency of 1 kHz was applied. Raising the frequency increases the ore load electrical resistance reducing wiring gauge requirements, decreasing the frequency reduces the number of radiating segments22required. The heating is reliable as liquid water contact to the applicator system is not required. Radiation of waves was not occurring in theFIG. 5example and the heating was by magnetic induction. The instantaneous half power radial penetration depth from the applicator system10can be 5 meters for lean Athabasca ores and 9 meters for rich Athabasca ores as the dissipation rate that provides the heating is increased with increased conductivity. Any heating radius can be accomplished over time by growing a steam bubble/steam saturation zone around the applicator system or by allowing for conduction and/or convection to occur. As the thermal conductivity of bitumen is low the speed of heating with certain disclosed embodiments can be much faster than steam at start up. The electromagnetic fields readily penetrate rock strata to heat beyond them, whereas steam will not. Thus at least two modes of heat propagation occur: prompt heating by electromagnetic fields and gradual heating by conduction and convection from the dissipated electromagnetic fields.

FIG. 6depicts a cross sectional view of an RF heating pattern for an applicator system10according to the same parameters.FIG. 6maps the contours of the rate of heat application in watts per meter cubed at time t=0, for example, when the electric power has just been turned on. The antenna is being supplied 1 Watt of power to normalize the data. The ore is rich Athabasca oil sand 20 meters thick. Both induction heating by circular magnetic near field and displacement current heating by near electric field are evident. Numerical electromagnetic methods were used to perform the analysis which physical scale model test validated. Underground propagation constants for electromagnetic fields include the combination of a dissipation rate and a field expansion rate, as the fields are both turning to heat and the flux lines are being stretched with increasing radial distance and circumference. In certain disclosed embodiments, the radial field expansion/spreading rate is 1/r2. The radial dissipation rate is a function of the ore conductivity and it can be 1/r3to 1/r5. The half power depth of the prompt RF heating energy, axially from the applicator10can be 10 meters or more depending on formation conductivity. The prompt effective heating length, axially along a single radiating segment32is about one radio frequency skin depth, although gradual heating modes can occur, which allows for any segment length. Precision of heat application corresponds with the number of applicator systems10and multiple applicator systems10can be utilized to form an underground array to control the range and shape of the heating.

FIG. 7shows the load resistance in ohms versus the length in meters of a center fed bare well pipe dipole immersed in rich Athabasca oil sand. The oil sand had a conductivity of 0.002 mhos per meter. In certain embodiments,FIG. 7can be representative of the circuit properties of a single radiating segment32. The electric current has just initially been applied and the well pipe conductor losses are not included in the figure. A typical length for radiating segment32in the rich Athabasca ore can be one (1) RF skin depth or 18 meters at 400 KHz. Thus, as depicted, a single dipole antenna element can deliver about 54 ohms of resistance. As the heating progresses, the salinity of the in situ water increases, the ore conductivity increases, and the antenna load resistance decreases (not shown). Finally, an underground saturation zone (“steam bubble”) forms around the applicator system10, the ore conductivity drops rapidly and the load resistance of the antenna rises rapidly by a factor of about 3 (not shown). The ending resistance of the single radiating segment32is about 162 ohms. The loss of liquid water contact with the applicator system10is not problematic due to the radio frequency and the inductive coupling of the single radiating segment32to the ore.

Raising and lowering the transmitter frequency to adjust the electrical coupling to the ore as it desiccates causes the applicator system10load resistance to adjust. Operating the transmitter at a critical frequency Fcprovides effective electrical coupling, so the power dissipated in the hydrocarbon ore exceeds the power lost in the antenna-applicator structure. The real dielectric permittivity ∈rof the ore is much less important than the ore conductivity in determining antenna load resistance. This is because dielectric heating is negligible at relatively low radio frequencies in hydrocarbon ore, and there are no radio waves, just near fields. The electrical conductivity of Athabasca oil sand is inversely related to the oil content, so the richer (high oil content) ores have lower ore electrical conductivity. The electrical load resistance of the single radiating segment32is therefore less in leaner ores and higher in rich ores.

FIG. 8is the driving point reactance in ohms versus length in meters of a center fed dipole of bare well pipe immersed in rich Athabasca oil sand having a conductivity of 0.002 mhos per meter. The electric current has just initially been applied and the well pipe is assumed to be a perfect electric conductor for simplicity. In certain embodiments,FIG. 8can be representative of the circuit properties of a single radiating segment32. A typical length for radiating segment32in the rich Athabasca ore can be one (1) RF skin depth, which is 18 meters at 400 KHz, so single dipole antenna element can deliver about 9 ohms of capacitive reactance. A method of the present invention is to operate the radiating segments32at their resonance frequency in the formation, for example, at a frequency where reactance of the radiating segments32is at zero (0) ohms. Operation at resonance advantageously reduces the power factor to minimize reactive power in the electrical cable12allowing for smaller conductor gauges to be used. A bare 35 meter long radiating segment32is resonant at 400 KHz and many other frequencies in rich oil sand.

Continuing to refer toFIG. 8and for operation in rich Athabasca oil sand on 0.002 mhos/meter conductivity, the resonant length (35 meters) for radiating segments32is independent of frequency over a wide frequency range. Because of the dissipative nature of the oil sand media, the free space wavelength does not apply. A half wave resonant dipole in free space would be 367 meters long at 400 kHz yet in the oil sand 400 KHz resonance occurs at 35 meters length. The velocity factor in the oil sand is therefore about 1/10 that of free space at 400 KHz.

Although not so limited, heating from certain disclosed embodiments might primarily occur from reactive near fields rather than from radiated far fields. The heating patterns of electrically small antennas in uniform media can be simple trigonometric functions associated with canonical near field distributions. For instance, a single line shaped antenna, for example, a dipole, can produce a two petal shaped heating pattern cut due the cosine distribution of radial electric fields as displacement currents (see, for example,Antenna Theory Analysis and Design, Constantine Balanis, Harper and Roe, 1982, equation 4-20a, pp 106). In practice, however, hydrocarbon formations are generally inhomogeneous and anisotropic such that realized heating patterns are substantially modified by formation geometry. Multiple RF energy forms including electric current, electric fields, and magnetic fields interact as well, such that canonical solutions or hand calculation of heating patterns might not be practical or desirable.

Far field radiation of radio waves (as is typical in wireless communications involving antennas) does not significantly occur in antennas immersed in hydrocarbon formations. Rather the antenna fields are generally of the near field type so the electric flux lines begin and terminate on or near the antenna structure and the magnetic flux lines curl around the antenna. In free space, near field energy rolls off at a 1/r3rate (where r is the range from the antenna conductor) and for antennas small relative wavelength it extends from there to λ/2π (lambda/2 pi) distance, where the radiated field can then predominate. In the hydrocarbon formation4, however, the antenna near field behaves much differently from free space. Analysis and testing has shown that heating dissipation causes the roll off to be much higher, about 1/r5to 1/r8. This advantageously limits the depth of heating penetration in certain disclosed embodiments to substantially that of the hydrocarbon formation4.

Several methods of heating are possible with the various embodiments. Conductive, contact electrode type resistive heating in the strata can be accomplished at frequencies below about 100 Hertz initially. In this method the antenna conductors comprise electrodes to directly supply electric current. Later, the frequency of the radio frequency source24can be raised as the in situ liquid water boils off the conductive well pipe20surfaces, which can continue heating which could otherwise stop as electrical contact with the formation opens. A method of certain disclosed embodiments is therefore to inject electric currents initially, and then to elevate the radio frequency to maintain energy transfer into the formation by using electric fields and magnetic fields, neither of which requires conductive contact with in situ water in the formation.

Another method of heating is by displacement current by the application of electric near fields into the underground formation, for example, through capacitive coupling. In this method the capacitance reactance between the applicator system10and the formation couples the electric currents without conductive electrode contact. The coupled electric currents then heat by Joule effect.

Another method of heating with certain disclosed embodiments is the application of magnetic near fields (H) into the underground strata to accomplish the flow of electric currents by inductive coupling and eddy currents. Induction heating is a compound process. The flow of electric currents through the radiating segments32forms magnetic fields around the radiating segments32according to Ampere's law, these magnetic fields form eddy electric currents in the ore by Lentz's Law, and the flow of these electric currents in the ore then heat the ore by Joule effect. The magnetic near field mode of heating is reliable as it does not require liquid water contact to the applicator system10and useful electrical load resistances are developed. The magnetic near fields curl around the axis of application system10in closed loops. In induction heating the equivalent circuit of the application system10is akin to a transformer primary winding and the hydrocarbon ore akin to the transformer secondary winding, although physical windings do not exist. Linear straight electrical conductors such as the present embodiments can be effective at producing magnetic fields.

Generally, in underground heating the real permittivity ∈′ of the hydrocarbon ores is of secondary importance to the ore conductivity σ. Dielectric heating, as is common for microwaves, is not pronounced. Imaginary permittivity ∈″ relates directly to the conductivity a according to the relation ∈′=j2πfσ where f is the frequency in Hertz.

Thus, the present invention can accomplish stimulated or alternative well production by application of RF electromagnetic energy in one or all of three forms: electric fields, magnetic fields and electric current for increased heat penetration and heating speed. The antenna is practical for installation in conventional well holes and useful for where steam can not be used or to start steam enhanced wells. The RF heating can be used alone or in conjunction with other methods and the applicator antenna is provided in situ by the well tubes through devices and methods described.

FIG. 9is a contour plot mapping realized temperatures produced by certain embodiments.FIG. 9is merely exemplary: realized temperatures will vary from reservoir to reservoir depending on formation characteristics such as depth, the applied RF power, and the duration of the heating. Only the right half space is shown for efficiency in analysis and the left half space (not shown) is similar to the right. The units are in degrees Celsius and the time is 6 years after startup so the well system is in production. Start up might require weeks depending on the RF power. The view is a cross sectional view of a two hole embodiment well system using the applicator system10. The upper hole contains the applicator system10. The bottom hole is a producer well to drain the hydrocarbons and it can contain slits, pumps, and the like to drain the produced oil and lift it to the surface. The use of two holes is similar to the injector well-producer well geometry of a steam assisted gravity drainage (SAGD) system. A steam saturation zone (“steam bubble”) can form around the applicator system10in the upper hole. This “steam bubble” grows to form an inverted triangle shaped region in which the liquid water is desiccated and the RF electromagnetic fields are free to expand because steam and sand are not lossy to electromagnetic fields. The realized temperatures do not exceed the boiling point of the liquid water at reservoir pressure, coking of the ore does not occur, and in practice the realized temperatures are sufficient to melt the bitumen for extraction.FIG. 9relates to operation in a North American bitumen formation. In heavy oil formation, lower temperatures can be used.

FIG. 10depicts the oil saturation contours of a well system implementing certain embodiments after 6 years of production. Units of 1.0 mean all the original oil is in place and 0.0 unit regions contain no hydrocarbons. The bitumen drains at or ahead of the steam front, and the bitumen and connate water are produced together. About 80 percent of the bitumen in the formation was produced in the example and more or less bitumen can be produced depending on the rate of heating used, formation characteristics, co-injection of steam with RF, and many other factors. RF heated wells can produce faster than steam heated wells. As can be appreciated, increased speed can increase profits. With RF there is no need to wait for heat conduction to start heat convection, and thus, start up can be reliable. The propagation speed of RF heating energy in the ore is about the speed of light, so ore that is 10 or more meters from the applicator system10receives heating energy microseconds after RF power is turned on. The water saturation contours (not shown) for the heating example are somewhat similar to theFIG. 10oil saturation contours, although the dry zone tends to grow more vertically.

Although preferred embodiments have been described using specific terms, devices, and methods, such description is for illustrative purposes only. The words used are words of description rather than of limitation. It is to be understood that changes and variations can be made by those of ordinary skill in the art without departing from the spirit or the scope of the present invention, which is set forth in the following claims. In addition, it should be understood that aspects of the various embodiments can be interchanged either in whole or in part. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.