A downhole variable power steam stripping device and method for heavy oil recovery

Abstract

The application provides a downhole variable-power steam stripping device and method for thick oil exploitation, and relates to the technical field of thick oil thermal recovery. The device comprises a heat insulation pipe column, a composite heating cable, a water injection unit and a frequency converter. The composite heating cable is nested in the heat insulation pipe column and has an axial space with the inner wall of the heat insulation pipe column. The cable comprises a power section and a skin heating section from top to bottom. The water injection unit injects liquid water into the axial space. The frequency converter supplies power to make the skin heating section heat based on the skin effect. The water is heated and vaporized in situ and injected into the oil layer as high dryness steam. The application also provides a corresponding method. The variable-diameter heating outer armor tube is used to realize differential power distribution along the well depth, and the temperature is fed back in real time through the temperature measuring optical fiber in the independent test pipe column to close-loop adjust the frequency conversion parameters. The application gets rid of the dependence on the ground boiler, eliminates the heat loss along the way, realizes the spatial dimension variable-power heating and the time dimension intelligent control, and significantly improves the steam dryness and the heat energy utilization rate.

Description

Technical Field

[0001] This invention relates to the field of heavy oil thermal recovery technology, and more specifically to a downhole variable power steam drying device and method for heavy oil extraction. Background Technology

[0002] Petroleum, the "lifeblood" of modern industry, is a crucial foundation for ensuring national energy security and stable economic development. With the increasing depletion of global conventional crude oil resources, the development of heavy and extra-heavy oil resources, which account for a vast majority of proven reserves, has become a major way to fill energy gaps. However, due to the extremely high viscosity and poor fluidity of heavy oil, its deep extraction is far more difficult than that of conventional crude oil. Therefore, achieving efficient, economical, and green extraction of heavy oil has become a major technological challenge facing the petroleum industry.

[0003] Currently, thermal recovery methods such as steam huff and puff and steam-assisted gravity drainage (SAGD) are widely used for heavy oil development. However, the core of these methods lies in injecting high-dryness steam downhole to reduce crude oil viscosity. Traditional steam generation and transportation processes heavily rely on large boiler clusters on the surface. During the process of transporting steam from the wellhead along the thousands of meters of wellbore to the target oil layer, a large amount of heat energy is lost through the wellbore wall to non-target layers due to the huge temperature difference between the inside and outside of the wellbore and the heat absorption of the formation. Statistics show that wellbore heat loss in kilometer-deep wells can lead to a 20% to 30% decrease in steam dryness, and the overall thermal energy utilization rate is generally below 60%. This not only makes it difficult to guarantee the dryness of the steam at the bottom of the well, failing to meet the needs of deep heavy oil extraction, but also directly restricts the final recovery rate. More importantly, this high-energy-consuming and low-efficiency traditional steam injection process is no longer suitable for the modern oilfield's green and low-carbon development trend.

[0004] To alleviate the aforementioned contradictions, the industry has attempted to use downhole electric heating technology as an auxiliary or alternative means, among which skin effect electric heating technology has attracted attention due to its high heating efficiency. However, existing skin effect electric heating for oil wells suffers from several insurmountable technical drawbacks. First, most existing technical solutions employ constant power heating, which generates a large amount of useless heat in non-target oil layers, resulting in significant energy waste. Second, these solutions generally still rely on steam generated by surface boilers as the primary downhole heat source, using downhole electric heating only as an auxiliary means of "drying" or "supplementing heat," failing to fundamentally solve the inherent problem of significant heat dissipation during long-distance wellbore transport. Third, the inability to precisely allocate power according to the differentiated heat requirements of different downhole layers results in a still relatively low overall system energy efficiency ratio. Fourth, in terms of downhole monitoring and control, traditional temperature sensing elements are mostly externally mounted, which are not only easily damaged by the high temperature and pressure and fluid erosion downhole, but also their measurement signals are easily interfered with by the strong electromagnetic field of high-power heating cables, resulting in distorted temperature feedback data, making it difficult to use for closed-loop control.

[0005] In summary, existing heavy oil thermal recovery technologies and downhole electric heating systems generally suffer from bottlenecks such as strong dependence on surface boilers, large heat loss in the wellbore, inability to achieve efficient control of thermal energy throughout the wellbore, lack of spatially differentiated power output capabilities, and poor reliability of monitoring and control systems. Summary of the Invention

[0006] The purpose of this invention is to provide a downhole variable power steam drying device and method for heavy oil extraction, so as to completely get rid of dependence on surface boilers, eliminate heat dissipation along the process from the source, and have intelligent segmented power regulation.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A downhole variable-power steam drying device for heavy oil extraction includes: Insulated tubing strings are used to extend into casing wells in oil formations; The composite heating cable is nested inside the insulated tube column, and an axial space is left between the composite heating cable and the inner wall of the insulated tube column for water and water vapor to pass through. The composite heating cable is configured from top to bottom as a power section and a skin heating section. The power section is used for power transmission, and the skin heating section is used for heating water and water vapor in the axial space. The water injection unit is connected to the upper end of the axial space and is used to inject water into the axial space. The frequency converter is electrically connected to the power section of the composite heating cable and is used to input AC power of a set frequency into the composite heating cable.

[0008] Furthermore, the main power supply core is set in the center of the composite heating cable, and the main power supply core is wrapped with an insulation layer around its periphery; The skin heating section of the composite heating cable is wrapped with a heating outer armor tube around the circumferential periphery of the insulation layer. The power section of the composite heating cable is wrapped with a power outer armor tube around the circumference of the insulation layer, and a return core wire is arranged between the insulation layer and the power outer armor tube. The heating outer sheath tube is insulated from the power outer sheath tube. The upper ends of the main power supply core wire and the return core wire are electrically connected to the frequency converter. The lower end of the main power supply core wire is electrically connected to the lower end of the heating outer sheath tube, and the lower end of the return core wire is electrically connected to the upper end of the heating outer sheath tube.

[0009] Furthermore, the distance between the inner wall of the upper section of the heating outer armor tube and the main power supply core wire is D1, and the distance between the inner wall of the lower section of the heating outer armor tube and the main power supply core wire is D2, where D1 is less than D2.

[0010] Furthermore, the outer diameter of the upper section of the heating outer armor tube is the same as the outer diameter of the lower section of the heating outer armor tube.

[0011] Furthermore, the power section and the skin-feed heating section of the composite heating cable are connected by an insulating base; The insulating bases are respectively connected to the heating outer armor tube and the power outer armor tube; The main power supply core wire connector is located in the center of the insulating base. The main power supply core wire connector is electrically connected to the main power supply core wire of the power section and the main power supply core wire of the skin heating section. The insulating base is equipped with a return core wire connector, which is electrically connected to the lower end of the return core wire and the upper end of the heating outer armor tube.

[0012] Furthermore, it also includes a cable roller and a cable pusher, the composite heating cable being wound on the cable roller, and the cable pusher being used to clamp and extend the composite heating cable into the interior of the insulation column, or to pull the composite heating cable out of the insulation column.

[0013] Furthermore, the upper end of the heat-insulating tubing is sequentially equipped with an electric shear valve, a suspension seal, and an electrical connection box from bottom to top.

[0014] Furthermore, it also includes a test string, a temperature sensor, and a controller. The test string is used to extend into the casing well of the oil layer. The lower end of the test string is aligned with the lower end of the insulation string and the test string and the insulation string are arranged in parallel. The temperature sensor is installed inside the test string. The temperature sensor signal is connected to the controller, and the controller signal is connected to the frequency converter.

[0015] Furthermore, the temperature sensor is configured as a distributed temperature-sensing optical fiber.

[0016] A downhole variable-power steam drying method for heavy oil extraction, employing the aforementioned downhole variable-power steam drying device for heavy oil extraction, the method comprising the following steps: S1. Run the tubing string: Insert the insulated tubing string into the casing well of the oil layer to the preset depth, and run the test tubing string parallel to the casing well of the oil layer to the preset depth aligned with the lower end of the insulated tubing string. S2. Insert the composite heating cable and temperature sensor: Insert the composite heating cable into the insulation column through the upper opening, ensuring that the skin heating section of the composite heating cable reaches the preset depth corresponding to the target oil layer, while the power section of the composite heating cable remains in the non-target oil layer. An axial space is formed between the composite heating cable and the inner wall of the insulation column, and the composite heating cable and insulation column are suspended and sealed. Insert the temperature sensor into the test column through the upper opening, and connect the temperature sensor signal to the controller. S3. Water injection: Start the water injection unit to inject liquid water into the axial space from the upper end of the heat insulation column. The liquid water flows from top to bottom along the axial space and flows through the power section and skin heating section of the composite heating cable in sequence. S4. Variable frequency heating: Start the frequency converter and input AC power of the set frequency to the main power supply core and return core of the composite heating cable, so that the heating outer armor tube of the skin heating section generates the skin effect, the heating outer armor tube heats up and heats the liquid water flowing in the axial space. After absorbing heat, the liquid water undergoes phase change and vaporizes into wet steam. As the wet steam continues to flow downward, it is further heated and dried into high dryness steam, which enters the target oil layer through the lower outlet of the heat-insulated tube column. S5. Closed-loop control: The temperature sensor collects downhole temperature data in real time and transmits it to the controller. The controller dynamically adjusts the output frequency and / or voltage of the frequency converter according to the temperature data to regulate the heating power of the skin heating section and keep the bottom steam dryness within the set range.

[0017] Compared with the prior art, the downhole variable power steam drying device and method for heavy oil extraction of the present invention has achieved the following significant technical effects: 1. This invention completely eliminates reliance on surface boilers, eradicating heat loss along the wellbore from the source. By employing in-situ downhole heating, liquid water undergoes phase change vaporization and dehydration directly within the target oil layer, abandoning the traditional surface steam injection process where steam is transported along thousands of meters of wellbore. Liquid water is transported in liquid form within the wellbore, resulting in a large heat capacity and minimal heat loss. Upon reaching the target layer, it is converted into high-dryness steam in situ by the skin heating section, fundamentally eliminating steam dryness loss during long-distance wellbore transport. Practical application verification shows that the bottomhole steam dryness can be stably maintained above 70%, an improvement of 20 to 30 percentage points compared to traditional surface steam injection processes, significantly increasing the overall thermal energy utilization rate.

[0018] 2. This invention achieves differentiated variable-power heating in the downhole spatial dimension. By setting a heating outer armor structure with a constant outer diameter and a variable inner diameter in the skin heating section, a natural impedance difference is formed between the high-power vaporization section and the low-power dryness maintenance section in the same series circuit. The high-power section can generate high-density Joule heat to meet the large amount of heat required for rapid heating and phase change of liquid water; the low-power section only provides the basic heat required to keep the steam from liquefying, avoiding energy waste. This structure achieves precise zonal heat distribution along the well depth without changing the smooth external shape of the cable, so that the heating power is accurately matched with the enthalpy requirements of different layers, and the overall energy consumption is reduced by 15% to 20%.

[0019] 3. This invention achieves intelligent closed-loop control of downhole heating power in the time dimension. By inserting distributed temperature-sensing optical fibers into an independent test string, physical separation of temperature measurement and heating is achieved, effectively shielding the strong electromagnetic field interference and transient thermal shock generated by high-power variable frequency AC power, ensuring the stability and accuracy of downhole temperature monitoring data. The surface frequency converter and controller dynamically adjust the frequency and voltage of the output AC power based on the real-time temperature profile data fed back from the temperature-sensing optical fibers, using variable frequency electrical parameters to change the skin depth to achieve overall control of the total heating power. Through a dual synergistic mechanism of controlling local stepped temperature differences with a "variable inner diameter structure" and controlling the global heat base with "variable frequency AC power," optimal closed-loop matching of steam dryness and energy consumption is achieved throughout the entire lifecycle of heavy oil thermal recovery.

[0020] 4. This invention simplifies the cable structure and improves the safety and reliability of downhole operations. The invention uses a metal-insulated transition joint to achieve electrical switching between the power section and the skin-heating section. This allows the power section to maintain a dual-core, low-loss power transmission state, while the skin-heating section forms a single-core, outer-armored return-flow heating state. The structure is simple and reliable, reducing manufacturing difficulty and downhole operation costs. Simultaneously, the working return current is mainly distributed near the inner wall of the heating outer armor tube; the outer surface of the outer armor, which is in contact with liquid water, does not serve as a concentrated channel for the working current. Combined with wellhead insulation and electrical protection measures, this fully ensures electrical safety under full-phase water contact heat exchange conditions. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the downhole variable power steam drying device for heavy oil extraction according to an embodiment of the present invention; Figure 2 This is a partial structural schematic diagram of the composite heating cable according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the cross-sectional structure of the power section of the composite heating cable according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the cross-sectional structure of the skin heating section of the composite heating cable according to an embodiment of the present invention; Figure 5 This is a schematic diagram of the cross-sectional structure of the upper section of the skin-feeding heating section according to an embodiment of the present invention; Figure 6 This is a schematic diagram of the cross-sectional structure of the lower section of the skin heating section in an embodiment of the present invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the invention will be further described in detail below with reference to specific embodiments and the accompanying drawings. Certain embodiments of the invention will be described more fully below with reference to the accompanying drawings, and some, but not all, of these embodiments will be shown. In fact, various embodiments of the invention can be implemented in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided to enable the invention to meet applicable legal requirements.

[0023] In the description of this invention, it should be noted that the terms "inner," "outer," "upper," "lower," "front," and "rear," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0024] In this embodiment of the invention, a downhole variable-power steam drying device and method for heavy oil extraction are provided. Please refer to [reference needed]. Figures 1 to 6 As shown.

[0025] A downhole variable power steam lifting device for heavy oil extraction includes an insulated tubing string 1, a composite heating cable 2, a water injection unit, and a frequency converter 3.

[0026] The heat-insulated tubing string 1 is used to extend into the casing well 4 of the oil layer. The composite heating cable 2 is nested inside the heat-insulated tubing string 1, and an annular axial space 5 is left between the composite heating cable 2 and the inner wall of the heat-insulated tubing string 1 for water and water vapor to pass through.

[0027] The composite heating cable 2 is wound on the cable drum 61. The cable pusher 62 is used to clamp and insert the composite heating cable 2 into the interior of the heat insulation column 1, or to pull the composite heating cable 2 out of the heat insulation column 1.

[0028] The cable pusher 62 includes a frame 621 and a pair of cable drive wheels 622 arranged on the frame 621. The pair of cable drive wheels 622 together clamp the composite heating cable 2. The cable drive wheels 622 are driven to rotate by a motor. The left cable drive wheel 622 rotates clockwise, and the right cable drive wheel 622 rotates counterclockwise, so as to move the composite heating cable 2 downward, allowing the composite heating cable 2 to extend into the interior of the heat insulation column 1. The left cable drive wheel 622 rotates counterclockwise, and the right cable drive wheel 622 rotates clockwise, so as to move the composite heating cable 2 upward, allowing the composite heating cable 2 to be pulled out of the heat insulation column 1.

[0029] The composite heating cable 2 is configured from top to bottom as a power section 201 and a skin heating section 202. The power section 201 is used for low-loss power transmission, and the skin heating section 202 is used for in-situ heating of water and water vapor in the axial space 5.

[0030] The composite heating cable 2 has a main power supply core 21 in the center, and the main power supply core 21 is wrapped with an insulation layer 22 around its periphery.

[0031] The skin heating section 202 of the composite heating cable 2 has a heating outer armor tube 23 wrapped around the circumferential periphery of the insulation layer 22.

[0032] The power section 201 of the composite heating cable 2 has a power outer armor tube 24 wrapped around the circumferential periphery of the insulation layer 22, and a return core wire 25 is arranged between the insulation layer 22 and the power outer armor tube 24.

[0033] Among them, the heating outer armor tube 23 and the power outer armor tube 24 are kept insulated. The upper ends of the main power supply core wire 21 and the return core wire 25 are electrically connected to the frequency converter 3. The lower end of the main power supply core wire 21 is electrically connected to the lower end of the heating outer armor tube 23, and the lower end of the return core wire 25 is electrically connected to the upper end of the heating outer armor tube 23.

[0034] When the frequency converter 3 inputs AC power of a set frequency to the main power supply core 21 and return core 25 of the composite heating cable 2, a circuit is formed between the main power supply core 21, the heating outer armor tube 23 and the return core 25. The power outer armor tube 24 is used for mechanical protection, pressure bearing, tensile strength and cooperation with the wellhead sealing device, and is not used as a current circuit channel.

[0035] In the skin-collecting heating section 202, alternating current flows sequentially through the main power supply core 21 and the heating outer armor tube 23, generating the skin effect, proximity effect, and electromagnetic induction. Under the influence of the skin effect, the current on the heating outer armor tube 23 is mainly concentrated in the area near the conductor surface. Since the lower end of the main power supply core 21 is short-circuited with the lower end of the heating outer armor tube 23, the skin-collecting heating section 202 forms an electrical circuit with opposite current directions. Under the influence of the proximity effect, the reverse currents attract and squeeze each other, causing the current on the heating outer armor tube 23 to be confined to a very thin layer on the inner wall of the heating outer armor tube 23. During this process, the heating outer armor tube 23 generates heat, which is transferred to the axial space 5, heating the liquid water flowing through the axial space 5. After absorbing heat, the liquid water undergoes a phase change and vaporizes into wet steam. As the wet steam continues to flow downward, it is further heated and dried into high-dryness steam. The high-dryness steam is discharged from the lower end of the heat-insulated tube column 1 and injected into the target oil layer.

[0036] In this case, since the current on the heating outer armor tube 23 flows along a very thin layer of the inner wall of the heating outer armor tube 23, the outer surface of the heating outer armor tube 23 in contact with liquid water does not serve as a concentrated channel for the working current, and the contact heat exchange safety requirements are met through insulation isolation and wellhead electrical protection measures.

[0037] The distance between the inner wall of the upper section of the heating outer armor tube 23 and the main power supply core wire 21 is D1, and the distance between the inner wall of the lower section of the heating outer armor tube 23 and the main power supply core wire 21 is D2, where D1 is less than D2. The outer diameter of the upper section of the heating outer armor tube 23 is the same as the outer diameter of the lower section to ensure consistent external dimensions and maintain a smooth external shape.

[0038] The heating outer sheath tube 23 has a closed-loop circular cross-section. Under the influence of electromagnetic induction, the intensity of the electromagnetic induction is further adjusted by changing the distance between the inner wall of the heating outer sheath tube 23 and the main power supply core wire 21, thus achieving fine-tuning of the power in the upper and lower sections of the heating outer sheath tube 23. Specifically, the power in the upper section of the heating outer sheath tube 23 is high, and the power in the lower section is low. In this way, within the same heating path, a high-power vaporization section is formed in the upper section, and a low-power sustaining section is formed in the lower section, achieving stepped variable-power heating in the target oil layer.

[0039] Liquid water first flows through the upper high-power vaporization section, where it is heated to high power. This causes the liquid water to absorb heat and undergo a phase change, vaporizing into wet steam. As the wet steam continues to flow downwards, it is further heated and dried into high-dryness steam. The steam then continues to flow through the lower low-power maintenance section, naturally reducing its heating power under the same current. This achieves energy-saving maintenance of steam dryness and reduces unnecessary energy consumption.

[0040] The length ratio of the high-power vaporization section to the low-power maintenance section can be adjusted according to the formation's steam absorption capacity, such as 1:1, 2:1, or 3:1.

[0041] The power section 201 and the skin-feeding heating section 202 of the composite heating cable 2 are connected via an insulating base 26. The insulating base 26 is made of insulating material and connects to both the heating outer armor tube 23 and the power outer armor tube 24. A main power supply core wire connector is located at the center of the insulating base 26, which is electrically connected to both the main power supply core wire 21 of the power section 201 and the main power supply core wire 21 of the skin-feeding heating section 202. A return core wire connector is located on the insulating base 26, which is electrically connected to the lower end of the return core wire 25 and the upper end of the heating outer armor tube 23.

[0042] The water injection unit is equipped with a high-pressure water pump and is connected to the upper end of the axial space 5 to inject water into the axial space 5.

[0043] The frequency converter 3 is electrically connected to the power section of the composite heating cable 2. The frequency converter 3 is used to input AC power of a set frequency to the composite heating cable 2. By changing the frequency, the skin depth of the skin heating section 202 is changed, so as to achieve the optimal matching of in-situ steam generation and energy consumption in the heavy oil thermal recovery process.

[0044] The frequency converter 3 is connected to the oilfield's on-site power grid or clean energy power supply system, adjusting the input power according to power supply conditions and production cycle. Through in-situ downhole heating, heat loss during steam transport along the long wellbore can be reduced, increasing the effective heat obtained from the target oil layer.

[0045] After the composite heating cable 2 extends into the insulation column 1 to a preset depth, the upper end of the insulation column 1 is sequentially equipped with an electric shear valve 71, a suspension seal 72, and an electrical connection box 73 from bottom to top to ensure electrical insulation isolation under water injection conditions and safety protection under contact heat exchange conditions.

[0046] The test string 8 is inserted into the casing well 4 of the oil layer. The lower end of the test string 8 is aligned with the lower end of the insulation string 1, and the test string 8 and the insulation string 1 are arranged parallel to each other. The temperature sensor 81 is installed inside the test string 8 and collects temperature profile data at different depths downhole. In this embodiment, the temperature sensor 81 is configured as a distributed temperature sensing fiber optic cable. The temperature sensor 81 is connected to the controller 9 via a signal cable 810, and the controller 9 is connected to the frequency converter 3 via a signal cable. The distributed temperature sensing fiber optic cable is located inside the test string 8, which can utilize the physical isolation of the test string 8 to shield the distributed temperature sensing fiber optic cable from the transient thermal shock and electromagnetic interference of the high-power heat source, thereby improving the stability and accuracy of downhole temperature monitoring.

[0047] A downhole variable-power steam drying method for heavy oil extraction, using the downhole variable-power steam drying device for heavy oil extraction described in this embodiment, includes the following steps: S1. Run the pipe string: Insert the heat-insulating tubing string 1 into the oil layer casing well 4 to a preset depth, and then lower the test tubing string 8 parallel into the oil layer casing well 4 to a preset depth aligned with the lower end of the heat-insulating tubing string 1.

[0048] S2, Insert composite heating cable 2 and temperature sensor 81: The composite heating cable 2 is inserted into the interior of the insulation column 1 through the upper opening, so that the skin heating section 202 of the composite heating cable 2 reaches the preset depth corresponding to the target oil layer section, while the power section 201 of the composite heating cable 2 remains in the non-target oil layer section. An axial space 5 is formed between the composite heating cable 2 and the inner wall of the insulation column 1, and the composite heating cable 2 and the insulation column 1 are suspended and sealed. The temperature sensor 81 is inserted into the test column 8 through the upper opening, and the temperature sensor 81 is connected to the controller 9 via the signal cable 810.

[0049] The upper end of the heat-insulating tube column 1 is equipped with a water injection valve 51 that communicates with the axial space 5, and the water injection unit is connected to the water injection valve 51 via a water injection pipeline.

[0050] S3, Water Injection: Open the water injection valve 51 to start the water injection unit, injecting liquid water from the upper end of the heat-insulating tube column 1 into the axial space 5. The liquid water flows from top to bottom along the axial space 5, passing sequentially through the power section 201 and the skin-feed heating section 202 of the composite heating cable 2. When the water flows through the power section 201, the power section 201 mainly performs the function of electrical energy transmission, generating very little heat and is not used as a heating area.

[0051] S4, Variable frequency heating: Start the frequency converter 3 and input AC power of a set frequency to the main power supply core 21 and return core 25 of the composite heating cable 2, so that the heating outer armor tube 23 of the skin heating section 202 generates a skin effect. The liquid water flowing in the axial space 5 contacts the heating outer armor tube 23, and the heating outer armor tube 23 generates high-density heat to heat the liquid water flowing in the axial space 5. After absorbing heat, the liquid water undergoes a phase change and vaporizes into wet steam. As the wet steam continues to flow downward, it is further heated and dried into high-dryness steam. The high-dryness steam enters the target oil layer through the lower outlet of the heat-insulated tube column 1.

[0052] In this process, liquid water first flows through the upper high-power vaporization section, where it is heated to high power. After absorbing heat, the liquid water undergoes a phase change and vaporizes into wet steam. As the wet steam continues to flow downwards, it is further heated and dried into high-dryness steam. The steam then continues to flow along the lower low-power maintenance section, naturally reducing the heating power under the same current. This achieves energy-saving maintenance of steam dryness and reduces unnecessary energy consumption.

[0053] S5, Closed-loop control: Temperature sensor 81 collects downhole temperature data in real time and transmits it to controller 9. Controller 9 dynamically adjusts the output frequency and / or voltage of inverter 3 according to the temperature data and target steam dryness to regulate the heating power of skin heating section 202 and keep the steam dryness at the bottom of the well within the set range.

[0054] The present invention has been described in detail above with reference to the accompanying drawings. Based on the above description, those skilled in the art should have a clear understanding of the downhole variable power steam drying device and method for heavy oil extraction according to the present invention. Of course, the specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A downhole variable-power steam drying device for heavy oil extraction, characterized in that, include: Insulated tubing strings are used to extend into casing wells in oil formations; The composite heating cable is nested inside the insulated tube column, and an axial space is left between the composite heating cable and the inner wall of the insulated tube column for water and water vapor to pass through. The composite heating cable is configured from top to bottom as a power section and a skin heating section. The power section is used for power transmission, and the skin heating section is used for heating water and water vapor in the axial space. The water injection unit is connected to the upper end of the axial space and is used to inject water into the axial space. The frequency converter is electrically connected to the power section of the composite heating cable and is used to input AC power of a set frequency into the composite heating cable.

2. The downhole variable power steam drying device for heavy oil extraction according to claim 1, characterized in that, The main power supply core is set in the center of the composite heating cable, and the main power supply core is wrapped with an insulation layer around its periphery. The skin heating section of the composite heating cable is wrapped with a heating outer armor tube around the circumferential periphery of the insulation layer. The power section of the composite heating cable is wrapped with a power outer armor tube around the circumference of the insulation layer, and a return core wire is arranged between the insulation layer and the power outer armor tube. The heating outer sheath tube is insulated from the power outer sheath tube. The upper ends of the main power supply core wire and the return core wire are electrically connected to the frequency converter. The lower end of the main power supply core wire is electrically connected to the lower end of the heating outer sheath tube, and the lower end of the return core wire is electrically connected to the upper end of the heating outer sheath tube.

3. A downhole variable-power steam drying device for heavy oil extraction according to claim 2, characterized in that, The distance between the inner wall of the upper section of the heating outer armor tube and the main power supply core wire is D1, and the distance between the inner wall of the lower section of the heating outer armor tube and the main power supply core wire is D2, where D1 is less than D2.

4. A downhole variable-power steam drying device for heavy oil extraction according to claim 3, characterized in that, The outer diameter of the upper section of the heating outer armor tube is the same as the outer diameter of the lower section of the heating outer armor tube.

5. A downhole variable-power steam drying device for heavy oil extraction according to claim 2, characterized in that, The power section and the skin-collecting heating section of the composite heating cable are connected by an insulating base; The insulating bases are respectively connected to the heating outer armor tube and the power outer armor tube; The main power supply core wire connector is located in the center of the insulating base. The main power supply core wire connector is electrically connected to the main power supply core wire of the power section and the main power supply core wire of the skin heating section. The insulating base is equipped with a return core wire connector, which is electrically connected to the lower end of the return core wire and the upper end of the heating outer armor tube.

6. A downhole variable-power steam drying device for heavy oil extraction according to claim 1, characterized in that, It also includes a cable roller and a cable pusher, the composite heating cable being wound on the cable roller, and the cable pusher being used to clamp and extend the composite heating cable into the interior of the insulation column, or to pull the composite heating cable out of the insulation column.

7. A downhole variable-power steam drying device for heavy oil extraction according to claim 1, characterized in that, The upper end of the insulated tubing is equipped with an electric shear valve, a suspension seal, and an electrical connection box in sequence from bottom to top.

8. A downhole variable-power steam lifting device for heavy oil extraction according to any one of claims 1 to 7, characterized in that, It also includes a test string, a temperature sensor, and a controller. The test string is used to extend into the casing well of the oil layer. The lower end of the test string is aligned with the lower end of the insulation string and the test string and the insulation string are arranged in parallel. The temperature sensor is installed inside the test string. The temperature sensor signal is connected to the controller, and the controller signal is connected to the frequency converter.

9. A downhole variable-power steam drying device for heavy oil extraction according to claim 8, characterized in that, The temperature sensor is configured as a distributed temperature-measuring optical fiber.

10. A downhole variable-power steam lifting method for heavy oil extraction, employing the downhole variable-power steam lifting device for heavy oil extraction as described in any one of claims 1 to 9, characterized in that, The method includes the following steps: S1. Run the tubing string: Insert the insulated tubing string into the casing well of the oil layer to the preset depth, and run the test tubing string parallel to the casing well of the oil layer to the preset depth aligned with the lower end of the insulated tubing string. S2. Insert the composite heating cable and temperature sensor: Insert the composite heating cable into the insulation column through the upper opening, ensuring that the skin heating section of the composite heating cable reaches the preset depth corresponding to the target oil layer, while the power section of the composite heating cable remains in the non-target oil layer. An axial space is formed between the composite heating cable and the inner wall of the insulation column, and the composite heating cable and insulation column are suspended and sealed. Insert the temperature sensor into the test column through the upper opening, and connect the temperature sensor signal to the controller. S3. Water injection: Start the water injection unit to inject liquid water into the axial space from the upper end of the heat insulation column. The liquid water flows from top to bottom along the axial space and flows through the power section and skin heating section of the composite heating cable in sequence. S4. Variable frequency heating: Start the frequency converter and input AC power of the set frequency to the main power supply core and return core of the composite heating cable, so that the heating outer armor tube of the skin heating section generates the skin effect, the heating outer armor tube heats up and heats the liquid water flowing in the axial space. After absorbing heat, the liquid water undergoes phase change and vaporizes into wet steam. As the wet steam continues to flow downward, it is further heated and dried into high dryness steam, which enters the target oil layer through the lower outlet of the heat-insulated tube column. S5. Closed-loop control: The temperature sensor collects downhole temperature data in real time and transmits it to the controller. The controller dynamically adjusts the output frequency and / or voltage of the frequency converter according to the temperature data to regulate the heating power of the skin heating section and keep the bottom hole steam dryness within the set range.

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