Patent Grant
8763692
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.
A list of possibly relevant patents and literature follows:
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.
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.
The applicator system 10 includes an electrical cable 12, which has a first conductor 14, a second conductor 16, and a shield 18. The applicator also includes a conductive well pipe 20 with insulator segments 22 and radiating segments 32, an RF source 24, connection sites 26, first conductive jumpers 28, second conductive jumpers 30, and a magnetic sleeve 34.
The electrical cable 12 can 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 region 4. As a practical matter, the electrical cable 12 resistance should be much less than the load resistance of ore region 4. Shielded cables are generally required to convey electrical power through earth at radio frequencies.
The conductive well pipe 20 can 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 in
The RF source 24 is connected to the electrical cable 12 through the first conductor 14 and the second conductor 16 and is configured to apply a signal with a frequency f to the electrical cable 12. 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 region 4. 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 source 16 can 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 source 16 can 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 source 16 can 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 source 16 can 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 source 16.
The first conductor 14 is electrically connected to the conductive well pipe 20 at one or more connection sites 26. A connection site 26 is a section of the electrical cable 12 where the shield 18 has been stripped away to allow access to the first conductor 14 and the second conductor 16, and generally occurs near an insulator segment 22. For example, the first conductor 14 can be connected to the conductive well pipe 20 through a first conductive jumper 28. The first conductive jumper 28 can be, for example, a copper wire, a copper pipe, a copper strap, or other conductive metal. The first conductive jumper 26 feeds current from the first conductor 14 onto the conductive well pipe 26 just beyond an insulator segment 22.
Similarly, the second conductor 16 is electrically connected to the shield 18 at one or more connection sites 26. For example, the second conductor 16 can be connected to the shield 18 through a second conductive jumper 30. The second conductive jumper 30 can be, for example, a copper wire, a copper pipe, a copper strap, or other conductive metal. Connecting the second conductive jumper 30 to the shield 18 completes the closed electrical circuit, as described below.
In operation, the first conductor 14, the first conductive jumper 28, the conductive well pipe 20, the second conductor 16, the second conductive jumper 30, and the shield 18 create a closed electrical circuit, which is an advantage because the combination of these features allows the applicator system 10 to 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 pipe 20 itself functions as an applicator to heat the surrounding ore region 4.
When the applicator system 10 is operated, current I flows through a radiating segment 32, which creates a circular magnetic induction field H, which expands outward radially with respect to a radiating segment 32. A magnetic field H in turn creates eddy currents Ie, which heat the ore region 4 and 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 applicator 10, 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:
The strength of the magnetic flux density Bφ generated by the well antenna derives from Ampere's law and is given by:
Bφ=μILe−jkr sin θ/4πr2
Where:
The magnetic field can reach out as required from the conductive well pipe 20, 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 segment 32, it dissipates along the length of the radiating segment 32, thereby creating a less effective magnetic field H at the far end of a radiating segment 32 with respect to the radio frequency source 24. Thus, the length of a radiating segment 32 can be about 35 meters or less for effective operation when the applicator 10 is operated at about 1 to 10 kHz. However, the length of a radiating segment 32 can be greater or smaller depending on a particular applicator 10 used to heat a particular ore region 4. A preferred length for a radiating segment 32 is approximately:
δ=√(2/σωμ)
Where:
The applicator system 10 can extend one kilometer or more horizontally through the ore region 4. Thus, in practice an applicator system 10 can consist of an array of twenty (20) or more radiating segments 32 connected by insulator segments 22, depending on the electrical conductivity of the underground formation, so the applicator system 10 provides 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 segments 32 are 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 segments 32 is excited by approximately equal amplitude and equal phase currents. The realized current distribution along the array of radiating segments 32 forming the applicator 10 can 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 source 16 is a method of certain disclosed embodiments to approximate a uniform distribution for even heating.
The magnetic sleeve 34 surrounds the electrical cable 12 and the conductive well pipe 20 in, optionally all the way through, the overburden region 2. The magnetic sleeve 34 can 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 sleeve 34 can include balanced transmission lines, isolated metal sleeves, and series inductive windings.
The magnetic sleeve 34 keeps the portion of the applicator system 10 that it covers electrically neutral. Thus, when the applicator 10 system is operated, electromagnetic radiation is concentrated within the ore region 4 because RF electric currents cannot flow over the outside of well pipe 20 due to the inductive reactance of magnetic sleeve 34. This is an advantage because it is desirable not to divert energy by heating the overburden region 2, which is typically highly conductive relative to the hydrocarbon ore region 4.
Some embodiments can include one or more electrical separations 40 in the applicator system 10. An electrical gap 42 is a section of the electrical cable where the shield has been stripped away and generally occurs near an insulator segment 22. An electrical gap 42 is similar to a connection site 26; however, no connection between the conductors and the conductive well pipe occurs at an electrical separation 40. The electrical separation 40 can be used to modify the electrical impedances obtained from the radiating segments 32. The electrical separations 40 change the load resistances provided by the radiating segments 32 and change the sign of the electrical reactance provided by radiating segments 32.
At an electrical separation 40, the radiating segments 32 are center fed, and the radiating segments become unfolded antennas that do not have DC continuity. Without the electrical separation 40, the radiating segments 32 are end fed, and the radiating segments become folded antennas having DC continuity. Thus, the radiating segments 32 can be made capacitive or inductive by including or not including electrical separations 40. Below the first resonance of the radiating segments 32, for example, at low frequencies, including electrical separations 40 can make the radiating segments capacitive. At higher frequencies, not including electrical separations 40 can make the radiating segments inductive and lower resistance, depending on the characteristics of the ore region 4. Electrical separations 40 can also be used to select between magnetic field induction and electric field induction heating modes in the ore region 4.
The applicator system 10 of
As described above with respect to
In this embodiment, the first conductor 14 is electrically connected to the conductive well pipe 20 at one or more first connection sites 36. A first connection site 36 is a section of the electrical cable 12 where the shield 18 has been stripped away to allow access to the first conductor 14 and the second conductor 16. In this embodiment, the first connection sites 36 occur at regular intervals but no corresponding insulator segment is present on the conductive well pipe 20. Again, the first conductor 14 can be connected to the conductive well pipe 20 through a first conductive jumper 28. The first conductive jumper 28 can be, for example, a copper wire, a copper pipe, a copper strap, or other conductive metal. The first conductive jumper 26 feeds current from the first conductor 14 onto the conductive well pipe 20.
Similarly, the second conductor 16 is electrically connected to the conductive well pipe 20 at one or more second connection sites 38. For example, the second conductor 16 can be connected to the conductive well pipe 20 through a second conductive jumper 30. The second conductive jumper 30 can 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 conductor 16 as it does on the first conductor 14, the second conductor removes current I from the conductive well pipe 20.
In the illustrated embodiment, although this is not a requirement for other embodiments, each connection site alternates between being a first connection site 36 or a second connection site 38. Thus, along the length of the conductive well pipe 20 current I is fed onto and then removed from the conductive well pipe in an alternating fashion. The shield 18 is also bonded to the conductive well pipe 20 at regular, frequent intervals indicated as bond sites 39.
In operation, the first conductor 14, the first conductive jumper 28, the conductive well pipe 20, the second conductor 16, the second conductive jumper 30, create a closed electrical circuit, which is an advantage because the combination of these features allows the applicator system 10 to 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 to
Simulations show that as the current I dissipates along the length of the conductive well pipe 32 as it flows, which creates a less effective magnetic field H at the far end of a radiating segment 32 with respect to the radio frequency source 24. Thus, the length of a radiating segment 32 can be about 35 meters or less for effective operation when the applicator 10 is operated at about 1 to 10 kHz. However, as described above the length of a radiating segment 32 can be greater or smaller depending on a particular applicator system 10 used to heat a particular ore region 4, and again because the applicator system 10 can extends one kilometer or more horizontally through the ore region 4, an applicator system can consist of twenty (20) or more radiating segments 32.
Once again a magnetic sleeve 34 surrounds the electrical cable 12 and the conductive well pipe 20 in, optionally throughout, the overburden region 2, which is an advantage because it is desirable not to divert energy by heating the overburden region 2, which is typically highly conductive.
Alternative embodiments to certain disclosed embodiments not shown are possible, for instance, the vertical well embodiment can be implemented without insulator segments 22, similar to that described above with respect to
At the step 41, 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 cable 12 and the conductive well pipe 20 of
At the step 42, 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 in
A representative RF heating pattern in accordance with this invention will now be described. The
Raising and lowering the transmitter frequency to adjust the electrical coupling to the ore as it desiccates causes the applicator system 10 load resistance to adjust. Operating the transmitter at a critical frequency Fc provides effective electrical coupling, so the power dissipated in the hydrocarbon ore exceeds the power lost in the antenna-applicator structure. The real dielectric permittivity ∈r of 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 segment 32 is therefore less in leaner ores and higher in rich ores.
Continuing to refer to
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/r3 rate (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 formation 4, 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/r5 to 1/r8. This advantageously limits the depth of heating penetration in certain disclosed embodiments to substantially that of the hydrocarbon formation 4.
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 source 24 can be raised as the in situ liquid water boils off the conductive well pipe 20 surfaces, 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 system 10 and 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 segments 32 forms magnetic fields around the radiating segments 32 according 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 system 10 and useful electrical load resistances are developed. The magnetic near fields curl around the axis of application system 10 in closed loops. In induction heating the equivalent circuit of the application system 10 is 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.
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.
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| 2925519 | Jun 2009 | FR |
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| WO2008011412 | Jan 2008 | WO |
| WO 2008030337 | Mar 2008 | WO |
| WO2008098850 | Aug 2008 | WO |
| WO2009027262 | Aug 2008 | WO |
| WO2009114934 | Sep 2009 | WO |
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| Number | Date | Country | |
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| 20120125607 A1 | May 2012 | US |
