CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a division of U.S. application Ser. No. 10/801,458, filed Mar. 15, 2004, that claims the benefit of document disclosure No. 537417, filed Aug. 29, 2003.

FEDERALLY SPONSORED RESEARCH

Not applicable

SEQUENCE LISTING OR PROGRAM

Not applicable

BACKGROUND OF THE INVENTIONâ€”FIELD OF THE INVENTION

This invention relates to hydrocarbon extraction and processing, specifically to heating hydrocarbonaceous matter in hydrocarbon-bearing formations in situ for more efficient extraction and processing.

BACKGROUND OF THE INVENTIONâ€”DISCUSSION OF PRIOR ART

North American reserves of oil shale and tar sand contain enough hydrocarbonaceous material to be a global provider of hydrocarbons products for the foreseeable future. Large-scale commercial exploitation of certain hydrocarbon-bearing resources, available in huge deposits on the North American continent, has been impeded by a number of problems especially cost of extraction and potentially significant negative environmental impact. Oil shale is also plentiful in the United States, but the cost of useful fuel recovery has been generally noncompetitive. The same is true for tar sands, which occur in estimated vast number of problems, amounts in Western Canada. In addition, heavy or viscous oil is often left untapped in a conventionally-produced oil well, due to the extra cost of extraction. These types of hydrocarbon deposits are becoming increasingly important, as reserves of low viscosity crude petroleum are being quickly depleted.

Materials such as oil shale, tar sands, and coal are amenable to heat processing to produce gases and hydrocarboneous liquids. Generally, the heat develops the porosity, permeability, and/or mobility necessary for recovery. Oil shale is a sedimentary rock, which upon pyrolysis, or distillation, yields a condensable liquid, referred to as a shale oil, and non-condensable gaseous hydrocarbons. The condensable liquid may be refined into products that resemble petroleum products. Oil sand is an erratic mixture of sand, water, and bitumen, with the bitumen typically being present as a film around water-enveloped sand particles. Though difficult, various types of heat processing can release the bitumen, which is an asphalt-like crude oil that is highly viscous.

In the destructive distillation of oil shale or other solid or semi-solid hydrocarbonaceous materials, the solid material is heated to an appropriate temperature and the emitted products are recovered. In practice, however, the limited efficiency of this process has prevented achievement of large-scale commercial application. For example, the desired organic constituent in oil shale, known as kerogen, constitutes a relatively small percentage of the bulk shale material, so very large volumes of shale need to be heated to elevated temperatures in order to yield relatively small amounts of useful end products. The handling of the large amounts of material is, in itself, a problem, as is the disposal of wastes. Also, substantial energy is needed to heat the shale, and the efficiency of the heating process and the need for relatively uniform and rapid heating have been limiting factors on success.

In the case of tar sands, the volume of material to be handled, as compared to the amount of recovered product, is again relatively large, since bitumen typically constitutes only about ten percent of the total, by weight. Material handling of tar sands is particularly difficult even under the best of circumstances. Such processing potentially results in huge, negative environmental impacts.

A number of proposals, broadly classed as in situ methods, have been made for processing and recovering hydrocarbonaceous deposits. Such methods may involve underground heating or retorting of material in place, with little or no mining or disposal of solid material in the formation. Useful constituents of the formation, including heated liquids of reduced viscosity, may be drawn to the surface by a pumping system or forced to the surface by injection techniques. For such methods to be successful, the amount of energy required to effect the extraction must be minimized.

Proposals to use radio frequency to heat relatively large volumes of hydrocarbonaceous formations are exemplified by the disclosures of the following U.S. patents: U.S. Pat. No. 4,140,180 to Bridges et al., 1979; U.S. Pat. No. 4,135,579 to Rowland et al., 1979; U.S. Pat. No. 4,140,179 to Kasevich et al., 1979; U.S. Pat. No. 4,144,935 to Bridges et al., (1980); U.S. Pat. No. 4,193,451 to Dauphine 1980; U.S. Pat. No. 4,457,365 to Kasevich et al., 1984; U.S. Pat. No. 4,470,459 to Copland et al., 1984; U.S. Pat. No. 4,513,815 to Rundell et al., 1985; U.S. Pat. No. 5,109,927 to Supernaw et al., 1992; U.S. Pat. No. 5,236,039 to Edelstein et al., 1993; and U.S. Pat. No. 6,189,611 to Kasevich et al., 2001.

One proposed electrical in situ approach employs a set of arrays of dipole antennas located in a plastic or other dielectric casing in a formation, such as a tar sand formation. A VHF or UHF power source would energize the antennas and cause radiating fields to be emitted into the deposit. However, at these frequencies, and considering the electrical properties of the formations, the field intensity drops rapidly as distance from the antennas increases. Consequently, non-uniform heating results in inefficient overheating of portions of formations in order to obtain at least minimum average heating of the bulk of the formations.

Another past proposal utilizes in situ electrical induction heating of formations. As in other proposals, the process depends on the inherent conduction ability, which is limited even under the best of conditions, of the formations. In particular, secondary induction heating currents are induced in the formations by forming an underground toroidal induction coil and passing electrical current through the turns of the coil. Drilling vertical and horizontal boreholes forms the underground toroid, and conductors are threaded through the boreholes to form the turns of the toroid. However, as the formations are heated and water vapors are removed from it, the formations become more resistive, and greater currents are required to provide the desired heating. In general, the above-mentioned techniques are limited by the relatively low thermal and electrical conductivity of the bulk formations of interest. Thus, the inefficiencies resulting from non-uniform heating render existing techniques slow and inefficient.

Currently, the most commercially accepted method of in situ extraction of hydrocarbons from oil tar sands is the steam flood process that uses a combination of steam or other gaseous pressures along with RF to decrease the viscosity so as to force the oil through the sand to a nearby producer well. This process requires enormous amounts of high-pressure steam that is typically generated with natural gas. On the down side, as price of crude oil increases, the price of natural gas generally rises accordingly, increasing the cost of employing steam flood methods. The steam flood method has been blamed for disrupting natural gas pressures; so the gas producers want to extract their natural gases prior to bitumen recover. But, the users of steam flood bitumen recovery processes need the subterranean pressures from the natural gas reservoirs to assist the steam flood. The loss of the natural gas reservoir can make the steam flood process uneconomical.

Controlled or uniform temperature heating of a hydrocarbonaceous volume to be recovered is desirable, but current methods cannot achieve this goal. Instead, current methods generally result in non-uniform temperature distributions, which can result in the necessity of inefficient overheating of portions of the formations. Extreme temperatures in localized areas may cause damage to the producing volume such as carbonization, skinning of the paraffin waxes, and arcing between the conductors can occur. Furthermore, vaporization of water creates steam that negatively affects the passage of frequency waves to the substances that require heating.

None of the previous proposals for the extraction of hydrocarbons from these types of formations have provided a method of separating the foreign matter from the valuable hydrocarbons prior to extracting to the surface of the earth. The washing of sand from heated oils generally requires steam or other energy consuming processes. The foreign matter in tar sand may contain ten times the desired hydrocarbons. As a result, a substantial negative environmental impact, with respect to disposal of the undesirable foreign matter, would exist if enough hydrocarbons were extracted to support a North American or global demand of oil. Another problem with washing the sand from the oil is the amount of water that would be required for large-scale production. Not only would tremendous amounts of fresh water be required, but also disposal of the resulting contaminated water would be an important issue. Disposing of the undesirable organic and inorganic substances such as heavy metals, sulfur, etc that would be separated from the hydrocarbons would impose additional environmental challenges. Furthermore, extracting large amounts of heated bitumen and heavy oils to the surface of the earth can release sizable amounts of greenhouse gases and other pollutants into the atmosphere during the ensuing washing, crude storage, separating, and refining processes.

Although RF dielectric heating systems have been used for heating hydrocarbon-bearing formations in the past, there remains a need for improved apparatuses and process techniques to rapidly, efficiently, and uniformly heat specific chemical compositions that reside in bitumen, and/or individual hydrocarbon compositions. There also is a substantial need for a method of separating the undesirable matter from the hydrocarbons and leaving it generally disposed in the context of its original environment.

Disadvantages of Capacitive RF Dielectric Heating

A specific disadvantage of known capacitive RF dielectric heating methods is the potential for thermal runaway or hot spots in a heterogeneous medium since the dielectric losses are often strong functions of temperature. Another disadvantage of capacitive heating is the potential for dielectric breakdown (arcing) if the electric field strengths are too high across the sample. Thicker samples with fewer air gaps allow operation at a lower voltage.

Prior Art

FIGS. 1â€“4

(Prior Art) show an example of a known capacitive RF dielectric heating system. A high voltage RF frequency sinusoidal AC signal is applied to a set of parallel electrodes 20 and 22 on opposite sides of a dielectric medium 24. Medium 24 to be heated is located between electrodes 20 and 22, in an area defined as the product treatment zone. An AC displacement current flows through medium 24 as a result of polar molecules in the medium aligning and rotating in opposite fashion to the applied AC electric field. Direct conduction does not occur. Instead, an effective AC current flows through the capacitor due to polar molecules with effective charges rotating back and forth. Heating occurs because these polar molecules encounter interactions with neighboring molecules, resulting in lattice and frictional losses as they rotate.

The resultant electrical equivalent circuit of the device of

FIG. 1

is therefore a capacitor in parallel with a resistor, as shown in

FIG. 2A

. There is an in-phase I_Rcomponent and an out-of-phase I_Ccomponent of the current, relative to the applied RF voltage. In-phase component I_Rcorresponds to the resistive voltage loss. These losses get higher as the frequency of the applied signal is increased for a fixed electric field intensity or voltage gradient due to higher speed interactions with the neighboring molecules. The higher the frequency of the alternating field, the greater the energy imparted into medium 24 until the frequency is so high that the rotating molecules can no longer keep up with the external field due to lattice limitations.

This frequency, which is referred to as a â€œDebye resonance frequencyâ€ after the mathematician who modeled it, represents the frequency at which lattice limitations occur. Debye resonance frequency is the frequency at which the maximum energy can be imparted into a medium for a given electric field strength (and therefore the maximum heating). This high frequency limitation is inversely proportional to the complexity of the polar molecule. For example, hydrocarbons with polar side groups or chains have a slower rotation limitation, and thus lower Debye resonance, than simple polar water molecules. These Debye resonance frequencies also shift with temperature as the medium 24 is heated.

FIGS. 2A

, 2B, and 2C are equivalent circuit diagrams of the dielectric heating system of

FIG. 1

for different types of hydrocarbon-bearing formations. Resultant electrical equivalent circuits may be different from the circuit shown in

FIG. 2A

, depending on the medium 24. For example, in a medium 24 such as a hydrocarbonaceous formation with a high moisture and salt content, the electrical circuit only requires a resistor (

FIG. 2B

), because the ohmic properties dominate. For media with low salinity and moisture, however, the resultant electrical circuit is a capacitor in series with a resistor (

FIG. 2C

Various other hydrocarbons, elements, or compositions within a hydrocarbon-bearing formation may use different electrical circuit analogs. More complex models having serial and parallel aspects in combination to address second order effects are possible. Any of the components in any of the models may have temperature and frequency dependence.

An example of a conventional RF heating system is shown in

FIGS. 3 and 4

(Prior Art). In this system, a high voltage transformer/rectifier combination provides a high-rectified positive voltage (5 kV to 15 kV) to the anode of a standard triode power oscillator tube. A tuned circuit (parallel inductor and capacitor tank circuit) is connected between the anode and grounded cathode of such tube as shown in

FIG. 4

, and also is part of a positive feedback circuit inductively coupled from the cathode to the grid of the tube to enable oscillation thereby generating the RF signal. This RF signal generator circuit output then goes to the combined capacitive dielectric and resistive/ohmic heating load through an adapter network consisting of a coupling circuit and a matching system to match the impedance of the load and maximize heating power delivery to the load, as shown in

FIG. 3

. An applicator includes an electrode system that delivers the RF energy to the medium 24 to be heated, as shown in

FIG. 1

The known system of

FIGS. 1â€“4

can only operate over a narrow band and only at a fixed frequency, typically as specified by existing ISM (Industrial, Scientific, Medical) bands. Such a narrow operating band does not allow for tuning of the impedance. Any adjustment to the system parameters must be made manually and while the system is not operating. Also, the selected frequency can drift. Therefore, to the extent that the known system provides any control, such control is not precise, robust, real time or automatic.

OBJECTS AND ADVANTAGES

Accordingly, several objects and advantages of the present invention are:

- (a) to provide an improved method of hydrocarbon extraction;
- (b) to provide a method to heat specific elements, chemical compositions, and/or specific hydrocarbons within the hydrocarbon-bearing formation utilizing a variable frequency automated capacitive radio frequency dielectric heating system;
- (c) to provide in situ heat processing of hydrocarbonaceous earth formations utilizing a variable frequency automated capacitive radio frequency dielectric heating system, in such a manner that efficiently achieves substantially uniform heating of a particular bulk volume of the formations;
- (d) to provide a system and method for efficiently heat processing relatively large blocks of hydrocarbonaceous earth formations with minimal adverse environmental impacts and for yielding a high net-energy ratio of energy recovered-to-energy expended;
- (e) to provide a method to heat specific elements and compositions within a hydrocarbon-bearing formation, utilizing a variable frequency automated capacitive radio frequency dielectric heating system, while other elements and compositions within the formation are transparent to the frequencies being used to heat the targeted compositions.

Further objects and advantages are to provide a method to heat specific elements and compositions within a hydrocarbon-bearing formation, utilizing a variable frequency automated capacitive radio frequency dielectric heating system, which has the ability to heat specific elements and compositions within a formation, to separate foreign matter from desired hydrocarbons or other desirable substances within a subterranean environment, prior to above-ground extraction.

Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.

SUMMARY

In accordance with the present invention an extraction and processing method of hydrocarbonaceous formations comprises an in situ heating process that utilizes a variable frequency automated capacitive radio frequency dielectric heating system, comprising an optional fluid carrier medium (for example, water or a saline solution), which can be unaffected, when desired, by the frequencies being presented to the target elements within the formation

DRAWINGSâ€”FIGURES

FIG. 1

(Prior Art) is a schematic diagram of an existing capacitive RF dielectric heating system.

FIGS. 2A

, 2B and 2C (Prior Art) are equivalent circuit diagrams of the dielectric heating system of

FIG. 1

for different types of hydrocarbon-bearing formations.

FIG. 3

(Prior Art) is a block diagram of the dielectric heating system of

FIG. 1

FIG. 4

(Prior Art) is a block diagram showing the high power RF signal generation section of the dielectric heating system of

FIG. 3

in greater detail.

FIG. 5

is a block diagram of a capacitive RF dielectric heating system in accordance with the invention.

FIG. 6

is a flow chart illustrating steps of impedance matching methods for use in the capacitive RF dielectric heating system diagrammed in

FIG. 5

FIG. 7

is a block diagram similar to

FIG. 5

, except showing an alternative embodiment of a capacitive RF dielectric heating system.

FIG. 8

is a flow chart illustrating steps of impedance matching methods for use in the capacitive RF dielectric heating system diagrammed in

FIG. 7

FIG. 9

is a top plan view of a grid electrode, which may be used in the systems of

FIGS. 5 and 7

FIG. 10

is a sectional view taken along line 10â€”10 of

FIG. 9

FIGS. 11A through 11E

are block diagrams of five hydrocarbon heating and extraction process flows which benefit from use of a dielectric heating system.

FIG. 12

shows three frequency generating and monitoring wells with their devices activated at the bottom of a hyrdrocarbonaceous deposit.

FIG. 13

shows a cavern opening upward in the center to form a larger, cone-shaped main cavern 335.

FIG. 14

shows a main cavern expanded to include the adjacent caverns seen in

FIG. 13

FIG. 15

shows the main cavern, which will soon be limited in its outward and upward spread into the formation, and will begin to appear dome-shaped as the formation is exploited.

FIG. 16

shows a close up of the main cavern, within brackets 16â€”16 from

FIG. 15

, and several process techniques.

DRAWINGSâ€”REFERENCE NUMERALS

20 electrode
22 electrode
24 medium
26 fluid carrier medium
30 variable RF frequency signal generator
32 broadband linear power amplifier
34 tunable impedance matching network
35 voltage, current, and optional temperature measurement equipment
36 AC RF signal displacement current
38 computer
40 electrically-isolated electrode element
42 heat sensors
44 electrically-isolated electrode element
46 Switches
120 Electrode(s)
122 Electrode(s)
124 Medium
130 variable RF frequency signal generator
132 broadband linear power amplifier
133 connection between amplifier 132 and matching network 134
134 tunable impedance matching network
135 voltage, current, and optional temperature measurement equipment
136 AC RF power waveform
137a RF Current Probe
137b RF Voltage Probe
138 Computer
150 tunable directional coupler
152 forward power measurement portion
154 reverse power measurement portion
156 measurement device
158 resonant cavity
159 capacitive coupling network
170 step: set signal generator 30 to an initial frequency or frequencies
172 step: measure temperature at medium
174 step: compare frequency(ies) and temperature
176 step: decide if change in frequency is required
178 step: change frequency, if needed
181 step: automatic impedance matching process
182 step: measure actual load impedance
184 step: tune out capacitive reactance
186 step: measure impedance match.
188 sub-step: measure forward and reflected powers
190 step: compare effective load impedance
192 step: adjust effective load impedance
193 step: automatic tuning of tunable impedance matching network
194 step: compare measured temperature
196 step: end of process
200 step: set signal generator 30 to an initial frequency or frequencies
208 step: automatic impedance matching process
210 step: measure actual load impedance
212 step: tune out reactance component of impedance
213 step: measure impedance match between signal generating unit and effective load
214 sub-step: measure forward and reverse powers
220 step: compare effective load impedance to impedance of signal generating unit
222 step: adjust effective load impedance
224 sub-step: automatic tuning of impedance matching network
225 control line
226 sub-step: change frequency, or frequencies of applied power waveform
228 step: compare monitored temperature with desired temperature
229 step: continue heating process, if necessary
230 step: end of process
301 well
302 overburden
304 medium (hydrocarbon-bearing formation)
306 bedrock or soil
308 reservoir of fluid carrier medium 320
310 derrick
315 radio waves
316 monitoring devices (data input sensors)
317 data transfer
318 frequency-emitting device
319 coaxial cable
320 fluid carrier medium
330 material being pumped to surface
332 reservoir
334 medium 304 being heated
335 main cavern
338 main reservoir
340 layer
342 layer
344 sediment
346 stratified layer
348 stratified layer
350 piping
352 piping
355 satellite cavern
356 stratified layer
358 stratified layer
360 stratified layer
362 stratified layer
364 dome cap
368 high-powered frequency-emitting device
370 process
372 remote underwater vessel
374 remote underwater vessel
376 process
377 slurry
378 location

DETAILED DESCRIPTION

FIGS. 5â€“10

: Capacitive RF Dielectric Heating

The electrical heating techniques disclosed below are applicable to various types of hydrocarbon-containing formations, such as oil shale, tar sands, coal, heavy oil, partially depleted petroleum reservoirs, etc. The relatively uniform heating which results from the following techniques, even in formations having relatively low electrical conductivity and relatively low thermal conductivity, provides great flexibility in applying recovery techniques. Accordingly, as will be described, the variable frequency automated capacitive radio frequency dielectric electrical heating of the present invention can be utilized either alone or in conjunction with other in situ recovery techniques to maximize efficiency for given applications.

I have devised a technique for uniform heating of relatively large blocks of hydrocarbonaceous formations using variable frequency automated capacitive radio frequency dielectric electrical heating that is substantially confined to the volume to be heated and effects dielectric heating of the formations. An important aspect of my invention relates to the fact that certain hydrocarbonaceous earth formations, for example unheated oil shale, exhibit dielectric absorption characteristics in the radio frequency range. Unlike most prior art electrical heating in situ approaches, the use of dielectric heating as disclosed below eliminates the reliance on electrical conductivity properties of the formations.

Capacitive Dielectric vs. Ohmic

Capacitive dielectric heating differs from lower frequency ohmic heating in that capacitive heating depends on dielectric losses. Ohmic heating, on the other hand, relies on direct ohmic conduction losses in a medium and requires the electrodes to contact the medium directly. (In some applications, capacitive and ohmic heating are used together.)

Capacitive RF dielectric heating methods offer advantages over other electromagnetic heating methods. For example, such heating methods offer more uniform heating over the sample geometry than higher frequency radiative dielectric heating methods (e.g., microwaves), due to superior or deeper wave penetration into the sample and simple uniform field patterns. In addition, capacitive RF dielectric heating methods operate at frequencies low enough to use standard power grid tubes that are lower cost (for a given power level) and allow for generally much higher power generation levels than microwave tubes.

Capacitive RF dielectric heating methods also offer advantages over low frequency ohmic heating. These include the ability to heat a medium, such as medium 24, 124, or 304 shown in

FIG. 5

, 7, or 12â€“16, that is surrounded by an air or fluid barrier (i.e., the electrodes do not have to contact the medium directly). The performance of capacitive heating is therefore also less dependent on the product making a smooth contact with the electrodes. Capacitive RF dielectric heating methods are not dependent on the presence of DC electrical conductivity and can heat insulators as long as they contain polar dielectric molecules that can partially rotate and create dielectric losses. A typical existing design for a capacitive dielectric heating system is described in â€œElectric Process Heating: Technologies/Equipment/Applicationsâ€ , by Orfeuil, M., Columbus: Battelle Press (1987).

Temperature Measurement: Past vs. this Invention

Measuring of temperature in conjunction with dielectric heating in a hydrocarbon-bearing formation is not unique. However, in the past, temperature measurement was used as a more coarse form of process control, such as determining reservoir temperatures in various locations for modulation of generator power strength. In prior art, frequencies have been established with laboratory testing to determine an optimum frequency setting for the generator and even to predict frequency-setting adjustments that take into consideration changes in the environment. All prior processes using RF dielectric heating have heated the mass as a whole without the ability to manipulate the heating rates of specific chemical compositions within the formation.

Debye Frequencies

However, in a subterranean environment, it is novel to continuously measure dielectric properties, Debye frequencies in relationship to temperature, electrical conductivity of the formation, and/or electrical permittivity, and to use these measurements as parameters for near instantaneous tuning of frequency(s) to create rapid heating of specific chemical compositions within a hydrocarbon-bearing formation. The ability to rapidly heat specific elements or chemical compounds, hydrocarbon or otherwise, within a hydrocarbon-bearing formation provides a technological advance that will spawn unique hydrocarbon recovery and extraction process techniques.

The present methods and systems provide for improved overall performance and allow for more precise and robust control of the heating processes. With the new methods and systems, specific dielectric properties of hydrocarbons, elements, or chemical compositions within a bitumen deposit or other hydrocarbonaceous formation are determined and/or used in the process, either directly as process control parameters or indirectly as by reference to a model used in the process that includes relationships based on the properties. New ways of using capacitive RF dielectric heating in the various phases of heating hydrocarbon deposits and techniques to separate foreign matter prior to above surface extraction are disclosed. Two approaches are described below.

In the first approach, described in connection with the system shown in

FIG. 5

, a variable frequency RF waveform is generated. The waveform is output to an amplifier and an impedance matching network to generate an electric field to heat the hydrocarbon bearing matter. Based on at least the measured temperature of the hydrocarbons, elements, or compositions within the hydrocarbonaceous deposit and/or one or more of specific dielectric or ohmic properties of the same, the system is controlled to provide optimum heating. Multiple frequency power waveforms can be applied simultaneously.

In the second approach, which is described primarily in connection with the system of

FIG. 7

, enhanced feedback provides for automatic impedance matching. By matching the impedance, maximum power is supplied to the load, and the maximum heating rate is achieved. In general, achieving the highest possible heating rate is desirable because higher heating rates of specific hydrocarbons, elements, or compositions within a hydrocarbonaceous deposit will allow for separation techniques not currently possible. Specific implementations of each approach are discussed below, following sections on the characterization and monitoring of dielectric properties and impedance matching.

Characterization, Monitoring, and Modeling of Medium

Characterization of dielectric properties vs. frequency and temperature of medium 24, 124, or 304 assists in the design of a capacitive RF dielectric heating system to lower the viscosity of hydrocarbons, separate unwanted elements or compositions within a hydrocarbon bearing deposit, and extract the desirable hydrocarbons, elements, and/or compounds to the surface, by some methods of the present invention. Medium 24, 124, or 304 is hydrocarbonaceous material, which may include one or more of the following: hydrocarbons, kerogen, bitumen, oil shales, paraffin, waxes, and other chemical compositions such as sulfur. It is preferable to heat the hydrocarbonaceous matter at a sufficiently high temperature, while avoiding unnecessary hydrocarbon vaporization. Such heating should occur without boiling a fluid carrier medium 26 or 320 (FIGS. 5 and 12â€“16), as will be discussed elsewhere. Thus, to aid in the selection of appropriate operating conditions, tar sand bitumen, oil shale, and heavy oil samples are studied to assess the effects of RF energy on key properties of the hydrocarbons and associated elements, minerals, and other chemical compositions present in the deposit samples at various frequencies and temperatures. The results of these studies influence the design of capacitive dielectric heating systems.

An electromagnetic/heat transfer mathematical model can be used to predict the dielectric heating characteristics of various hydrocarbons and related formation substances. Such a model may involve 2-D and/or 3-D mathematical modeling programs as well as finite element methodologies to model composite materials. Best results are achieved with a model that integrates both electromagnetic and heat transfer principles.

To supply the alternating displacement current at a needed frequency, variable components of the tunable RF signal generator circuit and associated matching networks are actively tuned to change frequency, or tuned automatically, or switched with a control system. Therefore, a software control system is also provided to set up the frequency profile. A variable frequency synthesizer or generator and a broadband power amplifier and associated matching systems and electrodes are useful components of such a capacitive dielectric heating system. In some implementations, temperature monitoring of medium 24, 124, or 304 using thermal sensors such as sensors 42, 137a, 137b, and/or 316 or infrared scanners is conducted, the data is fed back into the control system, and the frequency groups from the generator are swept accordingly to track a parameter of interest, such as Debye resonances (explained below) or other dielectric property, or other temperature dependent parameters.

The key electromagnetic parameters of medium 24, 124, or 304 to be tested are defined as follows:

Ïƒ=Electrical Conductivity (S/m)
âˆˆ=Electric Permittivity (F/m)
Î¼=Magnetic Permeability (H/m)
E=RMS Electric Field Intensity (V/m)
H=RMS Magnetic Field Intensity (A/m)
B=Magnetic Flux Density (W/m ²)
The Permittivity and permeability can be divided into loss terms as follows:
âˆˆ=âˆˆâ€²âˆ’jâˆˆâ€³â€ƒâ€ƒ(1)
Î¼=Î¼â€²âˆ’jÎ¼â€³â€ƒâ€ƒ(2)
where
- j=âˆš{square root over (âˆ’1)}
- âˆˆâ€²=Energy Storage Term of the Permittivity
- âˆˆâ€³=Loss Term of the Permittivity
- Î¼â€²=Energy Storage Term of the Permeability
- Î¼â€³=Loss Term of the Permeability

When analyzing the experimental data, the magnetic losses can be assumed equal to zero and for the most part frequency can be assumed high enough that the dielectric loss factor âˆˆâ€³ dominates over losses due to electrical conductivity Ïƒ (i.e., where Ï‰âˆˆâ€³>>Ïƒ, with angular frequency Ï‰=2Ï€f, f being the frequency measured in Hz). The electrical conductivity Ïƒ is measured and accounted for where needed (mainly at the lower end of the frequency range). With those assumptions in mind, the expressions for equivalent capacitance and equivalent resistance in

FIG. 2

reduce to the following:
C=(âˆˆâ€²S)/dâ€ƒâ€ƒ(3)
R=d/(Ï‰âˆˆâ€³S),â€ƒâ€ƒ(4)
where S is the exposed area of the plates and d is the plate separation between electrodes.

As mentioned above, capacitive heating systems according to the present invention operate at frequencies in the Medium Frequency (MF: 300 kHzâ€“3 MHz) and/or High Frequency (HF: 3 MHzâ€“30 MHz) bands, and sometimes stretch into the lower portions of the Very High Frequency (VHF: 30 MHzâ€“300 MHz) band. The frequency is generally low enough that the assumption can be made that the wavelength of operation is much larger than the dimensions of the hydrocarbonaceous deposit medium 24, 124, or 304, thus resulting in highly uniform parallel electric field lines of force across the components of medium 24, 124, or 304 and/or fluid carrier medium 26 or 320 targeted for heating.

Impedance Matching

Electrical impedance is a measure of the total opposition that a circuit or a part of a circuit presents to electric current for a given applied electrical voltage, and includes both resistance and reactance. The resistance component arises from collisions of the current-carrying charged particles with the internal structure of a conductor. The reactance component is an additional opposition to the movement of electric charge that arises from the changing electric and magnetic fields in circuits carrying alternating current. With a steady direct current, impedance reduces to resistance.

As used here, input impedance is defined as the impedance looking into the input of a particular component or components, whereas output impedance is defined as the impedance looking back into the output of the component or components.

The heating load, or, more formally, the actual load, is the combination of medium 24, 124, or 304 (i.e., the hydrocarbonaceous substances, other specific compositions natural to the formation, and/or water), fluid carrier medium 26 or 320 (if used), and exposed formation, e.g., capacitive electrodes 20, 22, 318 and any electrode enclosure that may be present. Thus, as used here, the actual load impedance is the input impedance looking into the actual load. The impedance of medium 24, 124, or 304 is influenced by its ohmic and dielectric properties, which may be temperature dependent. Thus, the actual load impedance typically changes over time during the heating process because the impedance of medium 24, 124, or 304 varies as the temperature changes.

The effective adjusted load impedance, which is also an input impedance, is the actual load impedance modified by any impedance adjustments. In specific implementations, impedance adjustments include the input impedance of a tunable impedance matching network coupled to the load and/or the input impedance of a coupling network coupled to the structure surrounding the load (e.g., the electrodes and/or enclosure, if present). In these implementations, the effective load includes the impedance load of any impedance adjusting structures and the actual load. Other impedance adjustments that may assist in matching the effective adjusted load impedance to the output impedance of the signal generating unit may also be possible. The effective load impedance is the parameter of interest in the present impedance matching approach.

The signal-generating unit, as used here, refers to the component or components that generate the power waveform, amplify it (if necessary), and supply it to the load. In specific implementations, the signal-generating unit includes a signal generator, an amplifier that amplifies the signal generator output and conductors, e.g. a coaxial cable, through which the amplified signal generator output is provided to the load.

The signal generating unit's impedance that is of interest is its output impedance. In specific implementations, the output impedance of the signal generating unit is substantially constant within the operating frequency range and is not controlled. Both the input impedance and the output impedance of the power amplifier, as well as the signal generator out impedance and the conductor characteristic impedance are substantially close to 50 ohms. As a result, output impedance of the signal-generating unit is also substantially close to 50 ohms.

Thus, in specific implementations, matching the effective adjusted load impedance to the output impedance of the signal generating unit reduces to adjusting the effective adjusted load impedance such that it â€œmatchesâ€ 50 ohms. Depending upon the circumstances, a suitable impedance match is achieved where the effective adjusted load impedance can be controlled to be within 25 to 100 ohms, which translates to nearly 90% or more of the power reaching the actual load.

Impedance matching is carried out substantially real-time, with control of the process taking place based on measurements made during the process. Impedance matching can be accomplished according to several different methods. These methods may be used individually, but more typically are used in combination to provide different degrees of impedance adjustment in the overall impedance matching algorithm.

The frequency of the signal generator may be controlled. In an automated approach, the signal generator frequency is automatically changed based on feedback of a measured parameter. For example, the signal generator frequency may be changed based on the actual load temperature and predetermined relationships of frequency vs. temperature. The frequency may be changed to track Debye resonances as described above and/or to maintain an approximate impedance match. Typically, this serves as a relatively coarse control algorithm.

For more precise control, aspects of the power waveform supplied to the effective load can be measured, fed back and used to control the frequency. For example, the forward power supplied to the effective load and the reverse power reflected from the effective load can be measured, and used in conjunction with measurements of the actual voltage and current at the load to control the frequency.

A tunable matching network can be automatically tuned to adjust the effective load impedance to match the output impedance of the signal generating unit. In a first step, series inductance is used in the output portion of the impedance matching network to tune out the series capacitive component of the actual load impedance. The series inductance is set by measuring the initial capacitive component, which is determined by measuring the voltage and current at the actual load and determining their phase difference. It is also possible to measure the voltage and current within the matching network and control for a zero phase shift. For more complex models of the load, other models will be necessary. An alternative approach would be to use a shunt inductor to tune out a shunt capacitive load.

Changes in the dielectric properties with heat directly influence the intensity and phase relationship of the RF wave energy. Measurements of these two parameters during the process can be related to corresponding changes of the physical properties of the material being processed. Initially, the resulting effective load impedance will be purely resistive, but will likely differ from the desired 50-ohm level. In a second step, additional elements within the matching network are tuned to make the input impedance of the matching network, which is defined as the effective adjusted load impedance for a described implementation, match the desired 50-ohm target. The second step tuning is controlled based on the measured forward and reflected power levels.

It is possible to adjust the gap in a capacitive coupling network positioned at the load. Such adjustments could be made automatically during the heating process with a servo a motor. It is possible to physically adjust the capacitive electrodes that are included as a part of the actual load to make minor adjustments to the actual load impedance. (Other adjustments are likely more easily controlled.)

Specific implementations that incorporate impedance matching are discussed in the following sections that detail two approaches.

FIG. 5

: First Approachâ€”Matching Impedance Using Temperature Measurements

One exemplary system suitable for the first approach, in which at least the measured temperature of the hydrocarbonaceous substance(s), specific chemical compositions, and/or hydrocarbons targeted for heating is monitored, is shown in

FIG. 5

. The system of

FIG. 5

includes a variable RF frequency signal generator 30 with output voltage level control, a broadband linear power amplifier 32, and a tunable impedance-matching network 34 (for fixed or variable frequency operation) to match the power amplifier output impedance to the load impedance of the capacitive load, which includes electrodes 20 and 22 and medium 24, and may or may not contain fluid carrier medium 26 being optionally heated. Medium 24 in this application is hydrocarbonaceous material, which may include one or more of the following: hydrocarbon compositions, kerogen, crude bitumen, oil bearing shales, paraffin, waxes, and other chemical compositions that naturally reside in these deposits such as sulfur. Fluid carrier medium 26 preferably is generally a liquid such as water, a saline solution, or de-ionized water, but other fluids could be used such as natural gas, nitrogen, carbon dioxide, and flue gas.

The system is constructed to provide an alternating RF signal displacement current 36 at an RF frequency in the range of 300 kHz to 300 MHz. This range includes the MF (300 kHz to 3 MHz), HF (3 MHz to 30 MHz), and VHF (30 MHz to 300 MHz) frequencies in the lower regions of the radio frequency (RF) range.

In the specific implementation shown in

FIG. 5

, variable RF frequency signal generator 30 is a multi-RF frequency signal generator capable of simultaneously generating multiple different frequencies. Although a single frequency signal generator may be used, the multi-frequency signal generator is useful for methods in which frequency-dependent dielectric properties of specific compositions and/or hydrocarbons targeted for heating are monitored and used in controlling the heating process, such as is explained in the following section.

Debye Resonance Frequency Implementations

As one example, the energy efficiency and/or heating rate are maximized at or near the location in frequency of the â€œDebye resonanceâ€ (defined earlier) of medium 24. In other specific implementations, dielectric properties other than Debye resonances are tracked and used in controlling capacitive RF dielectric heating, e.g., when Debye resonances are not present or are not pronounced. These other dielectric properties may be dependent upon frequency and/or temperature, similar to Debye resonances, but may vary at different rates and to different extents. Examples of such other dielectric properties are electrical conductivity and electrical permitivity.

In this example, the RF signal frequency is tuned to the optimal Debye frequency or frequencies of targeted media 24 for heating hydrocarbons and/or chemical compositions that reside in hydrocarbonaceous material. Multiple Debye resonances may occur in a composite material. So, multiple composite frequency groups can be applied to handle the several Debye resonances. Also, the RF signal frequencies can be varied with temperature to track Debye frequency shifts with changes in temperature.

The RF frequency or composite signal of several RF frequencies is selected to correlate with the dominant Debye resonance frequency groups of medium 24 that is being heated. These Debye resonances are dependent on the polar molecular makeup of medium 24 and thus are researched for different types of hydrocarbon compounds, and/or specific chemical compositions or elements that reside in hydrocarbonaceous deposits, to appropriately program the heating system. The generation system, in this case variable RF frequency signal generator 30, is capable of generating more than one frequency simultaneously. The control system for this heating system is capable of being calibrated for optimal efficiency to the various hydrocarbons or chemical compositions that are targeted for heating.

The frequency or composite frequency groups of the RF signal used in the heating system will track with and change with temperature to account for the fact that the Debye resonance frequencies of the polar molecular constituents of the hydrocarbonaceous material or other targeted medium 24 also shift with temperature.

With the most preferred apparatuses, the RF signal power level and resulting electric field strength can be adjusted automatically by a computer control system which changes the load current to control heating rates and account for different hydrocarbon geometries and bitumen, oil shale, or heavy oil compositions. The power level is controlled by: (1) measuring the current and field strength across the actual load with voltage and current measurement equipment 35 (

FIG. 5

); and (2) adjusting the voltage (AC field strength), which in turn varies the current, until measurements of the current and field strength indicate that the desired power level has been achieved. As shown in

FIG. 5

, computer 38 also controls multi-frequency RF signal synthesizer 30 to change its frequency and to adjust the tunable impedance matching network 34.

FIG. 6

: Flowchart for First Approach

FIG. 6

is a flowchart showing a heating process according to the first approach in more detail. In step 170, signal generator 30 is set to an initial frequency or frequencies. For expository convenience, it is assumed in this example that a single frequency is set, but the description that follows applies equally to cases where multiple frequencies are set.

The set frequency may be selected with reference to a predetermined frequency or frequency range based on a known relationship between frequency and temperature. For example, the set frequency may be selected based on one or more Debye resonances of the medium 24 as described above.

In step 172, the temperature at medium 24 is measured. In step 174, the measured temperature and set frequency are compared to a predetermined relationship of frequency and temperature for medium 24. The relationship may be stored in computer 38, e.g., in the form of a look-up table.

If the comparison between the set frequency and the predetermined frequency indicates that the set frequency must be changed (step 176; YES), the process advances to step 178, the set frequency is automatically changed by control signals sent to signal generator 30, and step 174 is repeated. If no change in the set frequency is required (step 176; NO), the process advances.

As indicated by the dashed line, an automatic impedance matching process 181 follows step 176. For an exemplary implementation, automatic impedance matching begins with step 182. In step 182, the magnitude and phase of the actual load impedance are measured using voltage and current measurement equipment 35, and the measured values are relayed to computer 38. In step 184, the phase angle difference between the measured voltage and current is determined to tune out the reactance component of the impedance. One element of controlling impedance match is, therefore, to tune out the capacitive reactance component of the actual load resulting in zero phase shifts between the voltage and current.

In step 186, the impedance match between the signal generating unit and the effective load is measured. Optionally, impedance match can be controlled through measuring the power waveforms supplied to and reflected from the effective load (the â€œforward and reverse powersâ€ ) (optional sub-step 188), assuming the system of

FIG. 5

is configured to include a measurement instrument 156 and directional coupler 150 as shown in

FIG. 7

, which will be discussed later. (Measurement of the forward and reverse powers is described in the following section.) Following completion of step 186, the process advances to step 190. In step 190, the effective load impedance is compared to the predetermined impedance of the signal-generating unit. If the impedance match is not sufficient, the process proceeds to step 192. If the impedance match is sufficient, the process proceeds to step 194.

In step 192, the effective load impedance is adjusted. In the implementation of the approach of

FIG. 5

, the effective load impedance is adjusted by automatically tuning tunable impedance matching network 34 based on control signals sent from computer 38 (step 193). Following step 192, the process returns to step 186.

In step 194, the measured temperature is compared to a desired final temperature. If the measured temperature equals or exceeds the desired final temperature, the heating process in completed (step 196). Otherwise, heating is continued and the process returns to step 172.

Heating hydrocarbons or other targeted elements or specific chemical compositions can be rapidly achieved. The rapid heating capability is due to the same uniform heating advantage described above and the maximum power input to the heated load by the matching of generator frequency or composite of frequencies to the Debye resonance frequency groups of the targeted compositions that reside in hydrocarbon-bearing formations 304, and tracking those Debye resonance frequency groups with temperature. Power control capability of the generator/heating system allows for the ability to set heating rates to optimize heating processes.

In some implementations, higher overall energy efficiency is obtained by matching the generator frequency or composite of frequencies of the RF waveform to the Debye resonance frequency groups of the specific compositions that reside in hydrocarbonaceous formations and by tracking those resonances with temperature resulting in a shorter heating time per unit volume for a given energy input.

Complete control of the heating process is achieved by the selective heating of various constituents of medium 24, including the bitumen, hydrocarbo