Modular mobile supercritical CO2 phase change synergistic plasma fracturing system and method

Abstract

The invention relates to the technical field of unconventional oil and gas exploitation, and particularly discloses a modular movable supercritical CO2 phase change synergistic plasma fracturing system and method.The system comprises a ground control center, an underground movable fracturing workstation and a multifunctional pipe cable connecting the ground control center and the underground movable fracturing workstation; the underground movable fracturing workstation comprises a CO2 phase change regulation and control cabin module and a plasma pulse fracturing module; the CO2 phase change regulation and control cabin module is used for carrying out gradient heating and pressurization on liquid CO2, so that the liquid CO2 is converted into supercritical CO2 jet flow in situ underground to fracture a coal seam; the plasma pulse fracturing module is used for exciting plasma pulse shock waves in the supercritical CO2 environment, cooperatively expanding a fracture network and achieving space-time cooperation of the two kinds of anhydrous energy. The problems that the supercritical CO2 fracturing phase state is unstable, the crack complexity is insufficient and the operation efficiency is low are solved, multi-section fracturing is completed through one-time drilling operation, and efficient, accurate and full-process waterless reservoir transformation is achieved.

Description

Technical Field

[0001] This application relates to the field of unconventional oil and gas extraction technology, and in particular to a modular mobile supercritical CO2 phase change synergistic plasma fracturing system and method. Background Technology

[0002] The economical exploitation of low-permeability coal seams (permeability coefficient typically below 0.1 mD) heavily relies on effective reservoir stimulation technologies. Traditional hydraulic fracturing suffers from problems such as enormous water consumption, water-locking damage, difficult wastewater treatment, and potential reservoir contamination. Gas fracturing, especially supercritical CO2 fracturing, is considered a promising CO2-free water-free alternative due to its advantages such as low viscosity, strong diffusivity, and promotion of gas desorption. However, existing gas fracturing equipment and technologies face the following bottlenecks: 1. Coarse-grained phase control and unstable efficiency: Existing equipment typically only pumps liquid or gaseous CO2 into the wellbore, relying on natural downhole temperature and pressure conditions to bring it to a supercritical state. This process is passive and uncontrollable, causing the CO2 fluid properties (density, viscosity) to fluctuate drastically in the wellbore and fractures, severely affecting fracture creation efficiency and morphology.

[0003] 2. Limited energy form and insufficient fracture complexity: Although pure supercritical CO2 fracturing can generate main fractures, its "static pressure" characteristics make it difficult to effectively connect natural weak surfaces and form complex fracture networks like the "water wedge" effect of hydraulic fracturing, resulting in limited modification volume.

[0004] 3. Low level of equipment integration and inflexible operation: Existing technologies mostly adopt a simple mode of surface pumping and single downhole tools. For multi-layer or long coal seams, repeated drilling and tool changes or large-scale perforation are required, resulting in long operation cycles, high costs, and difficulty in achieving differentiated and precise fracturing of each segment.

[0005] 4. Lack of real-time "perception-decision-execution" capability in the mine: The control of existing equipment relies heavily on ground telemetry and preset programs, which are slow to respond to real-time changes in coal seam geological conditions during fracturing and cannot achieve adaptive optimization fracturing. Summary of the Invention

[0006] In order to achieve active and precise control of CO2 phase state and integrate multiple energy sources for intelligent downhole movement and operation, this application provides a modular mobile supercritical CO2 phase change synergistic plasma fracturing system and method.

[0007] The modular mobile supercritical CO2 phase change synergistic plasma fracturing system provided in this application adopts the following technical solution: A modular mobile supercritical CO2 phase change synergistic plasma fracturing system includes a ground control center, a downhole mobile fracturing workstation, and a multifunctional cable connecting the two. The downhole mobile fracturing workstation includes a CO2 phase change control chamber module and a plasma pulse fracturing module. The CO2 phase change control chamber module is used to heat and pressurize liquid CO2 in stages, so that it is converted into a supercritical CO2 jet in situ downhole to fracture the coal seam. The plasma pulse fracturing module is used to generate plasma pulse shock waves in a supercritical CO2 environment to collaboratively expand the fracture network.

[0008] Furthermore, the CO2 phase change control chamber module includes a cryogenic liquid CO2 buffer chamber, a multi-stage microwave heating assembly, a supercritical state stabilizing chamber, and a pulse jet valve arranged in series.

[0009] Furthermore, the plasma pulse fracturing module includes a high-voltage pulse capacitor bank, a pulse shaping network, and a downhole plasma generator.

[0010] Furthermore, the downhole plasma generator includes a coaxial annular electrode and a directional jet electrode.

[0011] Furthermore, the multifunctional cable includes a high-voltage CO2 delivery channel, a power cable, an optical fiber data transmission channel, and a microtube, wherein the microtube is used to deliver a temporary plugging agent, a nano-support agent, or a tracer.

[0012] Furthermore, the end of the downhole mobile fracturing station is provided with a multi-functional injection sub-section, which is connected to the microtube.

[0013] Furthermore, the downhole mobile fracturing workstation also includes a power and positioning module for driving the downhole mobile fracturing workstation to move along the wellbore and position itself at the target fracturing section.

[0014] This application also provides a modular mobile supercritical CO2 phase change synergistic plasma fracturing method, which employs the aforementioned modular mobile supercritical CO2 phase change synergistic plasma fracturing system, and the method includes the following steps: S1: Lower the downhole mobile fracturing workstation to the first target formation in the well and anchor it; S2: Activate the CO2 phase change control chamber module to heat and pressurize the liquid CO2 in stages, so that it is converted into a supercritical CO2 jet in situ underground to fracturing the coal seam. S3: Activate the plasma pulse fracturing module to generate plasma pulse shock waves in a supercritical CO2 environment to collaboratively expand the crack network; S4: After completing the fracturing of the current section, move the downhole mobile fracturing workstation to the next target section and repeat steps S2-S3.

[0015] Furthermore, in step S2, the temperature and density of supercritical CO2 are controlled by adjusting the heating power to achieve stable and controllable fracturing behavior.

[0016] Furthermore, in step S3, the plasma pulse shock wave is triggered during or between supercritical CO2 jet injection, achieving synergy between static pressure fracture formation and dynamic impact fracture expansion.

[0017] In summary, this application includes at least one of the following beneficial technical effects: 1. This application moves the core process of CO2 phase change control from a "black box" guess on the surface to a "transparent" and precise execution underground; by precisely heating with microwaves underground, the physical properties of supercritical CO2 entering the coal seam are directly controlled, which greatly improves fracturing efficiency and predictability. 2. This application provides a fracturing equipment integrating plasma pulses, which miniaturizes and pressure-resistant high-energy plasma pulse generators and integrates them into a mobile workstation, realizing in-situ, spatiotemporal synchronous coordination of static supercritical CO2 fracturing and dynamic plasma shock wave fracturing. The plasma pulse, as an "energy amplifier," effectively compensates for the shortcomings of gas fracturing in terms of fracture creation complexity. Under a high-voltage electric field, the CO2 between the electrodes is broken down, generating a high-temperature plasma channel that expands rapidly, forming a strong shock wave. This shock wave propagates in the supercritical CO2 pre-fracturing coal body, and by utilizing its extremely high dynamic load stress rate, it effectively activates the natural joints and bedding in the coal seam, promoting fracture branching and the formation of complex fracture networks. 3. The modular and mobile design of this application enables a set of downhole equipment to complete multi-stage fracturing of the entire well without the need for multiple trips or coiled tubing, which greatly reduces operation time and cost and realizes multi-stage waterless fracturing in one trip. 4. The equipment provided in this application integrates a complete capability chain from sensing and analysis to execution. The downhole central controller can adjust phase change parameters and pulse energy within milliseconds to seconds based on local real-time data, thereby achieving adaptive fracturing of heterogeneous coal seams. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the overall structure of a modular mobile supercritical CO2 phase change synergistic plasma fracturing system according to an embodiment of this application; Figure 2 This is a schematic diagram of the structure of the downhole mobile fracturing workstation in the embodiments of this application; Figure 3 This is a schematic diagram of the CO2 phase change control chamber module in the embodiments of this application; Figure 4 This is a schematic diagram of the plasma pulse fracturing module in an embodiment of this application.

[0019] Reference numerals: 1. Intelligent decision-making unit; 2. Data analysis unit; 3. High-pressure CO2 supply and circulation module; 4. Power supply module; 5. Multifunctional cable; 6. Hydraulic anchoring arm; 7. Electric crawler; 8. Azimuth sensor; 9. Distributed optical fiber; 10. Microseismic acquisition sub; 11. Video probe; 12. Industrial-grade controller; 13. High-capacity battery pack; 14. CO2 phase change control chamber module; 15. Plasma pulse fracturing module; 16. Multifunctional injection sub; 17. Horizontal well section; 18. Downhole workstation; 19. Cryogenic liquid CO2 buffer chamber; 20. Multi-stage microwave heater; 21. Pulse jet valve; 22. Supercritical state stabilizing and forming chamber; 23. High-voltage pulse capacitor bank; 24. Pulse shaping network; 25. Coaxial ring electrode; 26. Directional jet electrode. Detailed Implementation

[0020] The following is in conjunction with the appendix Figure 1-4 This application will be described in further detail.

[0021] This application discloses a modular mobile supercritical CO2 phase change synergistic plasma fracturing system. (Refer to...) Figure 1 The modular mobile supercritical CO2 phase change synergistic plasma fracturing system includes a ground control center, a downhole mobile fracturing workstation, and a multi-functional cable connecting the two.

[0022] Among them, reference Figure 1 The ground control center includes a high-pressure CO2 supply and circulation module 3, a power supply module 4, a data analysis unit 2, and an intelligent decision-making unit 1. The intelligent decision-making unit 1 has a built-in fracturing optimization model based on machine learning algorithms, which can adjust operation instructions in real time according to downhole data.

[0023] The multifunctional cable 5 is a composite structure containing a high-voltage CO2 delivery channel, a power cable, an optical fiber data transmission channel, and microtubes. The microtubes are used to deliver temporary plugging agents, nano-supporting agents, or tracers. In this embodiment, the high-voltage CO2 delivery channel is located at the center of the multifunctional cable 5, and the power cable (6000V), optical fiber communication bundle, and two 5mm diameter stainless steel microtubes are arranged in the interlayer of the multifunctional cable 5.

[0024] Reference Figure 2 The core equipment is a mobile downhole fracturing workstation, which adopts a modular design and includes, from top to bottom, a power and positioning module, a sensing and communication module, a central control and energy module, a CO2 phase change control chamber module 14, a plasma pulse fracturing module 15, and a multi-functional injection sub 16.

[0025] Specifically, the power and positioning module includes an electric crawler 7, a hydraulic anchoring arm 6, and an orientation sensor 8, enabling the workstation to move autonomously and accurately position itself within horizontal or inclined boreholes.

[0026] The sensing and communication module integrates a distributed optical fiber 9, a microseismic acquisition section 10, and a video probe 11 to form a near-field crack monitoring network.

[0027] The central control and energy module coordinates the operation of the various sub-modules, including the industrial-grade controller 12, the high-capacity battery pack 13, and the power management system.

[0028] Reference Figure 3 The CO2 phase change control chamber module 14 includes a cryogenic liquid CO2 buffer chamber 19, a multi-stage microwave heating assembly, a supercritical pressure-stabilized forming chamber 22, and a pulse jet valve 21, all arranged in series. The cryogenic liquid CO2 buffer chamber 19 is connected to the high-pressure CO2 supply and circulation module 3 via a high-pressure CO2 delivery channel in the multi-functional cable 5, and is used to receive liquid CO2 from the ground. The multi-stage microwave heating assembly uses a ring-array microwave transmitter head arranged around the CO2 flow channel, with adjustable power, to rapidly and uniformly heat the liquid CO2 to the target temperature. The supercritical pressure-stabilized forming chamber 22 is a chamber with high pressure resistance and an inner wall with a turbulence-inspired structure (biomimetic spiral turbulence groove) to ensure that CO2 is fully converted into a stable supercritical fluid. The pulse jet valve 21 is a high-speed valve that can precisely control the flow rate and switching timing, used to inject supercritical CO2 jets into the coal seam.

[0029] The working mode of the CO2 phase change control chamber module 14 is as follows: the liquid CO2 on the ground flows into the low temperature liquid CO2 buffer chamber 19 through the high pressure CO2 delivery channel in the multi-functional cable 5. After the multi-stage microwave heating component heats and pressurizes the liquid CO2 in stages, the CO2 is transformed into a stable supercritical fluid in the supercritical pressure forming chamber 22. The pulse jet valve 21 sprays out the supercritical CO2 jet to fracturing the coal seam.

[0030] Reference Figure 4 The plasma pulse fracturing module 15 includes a high-voltage pulse capacitor bank 23, a pulse shaping network 24, and a downhole plasma generator. The plasma generator includes a coaxial annular electrode 25 and a directional jet electrode 26. The coaxial annular electrode 25 generates radially omnidirectional shock waves, suitable for uniformly enlarging vertical well sections. The directional jet electrode 26, using a flow guide or a special anode-cathode arrangement, guides the plasma arc to a specific orientation, suitable for horizontal wells or directional fracturing requirements. The plasma generator can excite plasma pulse shock waves in a supercritical CO2 environment, synergistically expanding the fracture network.

[0031] Reference Figure 2The CO2 phase change control chamber module 14 and the plasma pulse fracturing module 15 are installed in a rotatable chamber. Multiple through holes are arranged circumferentially on the chamber wall, with the positions of these holes corresponding to the output end of the CO2 phase change control chamber module 14 or the emission end of the plasma pulse fracturing module 15. By rotating the chamber, the orientation of the CO2 phase change control chamber module 14 and the plasma pulse fracturing module 15 can be changed, thereby controlling the direction of the supercritical CO2 jet and the emission direction of the plasma pulse shock wave to achieve omnidirectional fracturing along the wellbore circumference.

[0032] The multifunctional injection stub 16 is connected to the microtube for injecting temporary plugging agents, nanopropeptides, or tracers.

[0033] A modular, mobile supercritical CO2 phase change synergistic plasma fracturing method using the above system includes the following steps: S1: Deploy a ground control center at the well site, including power supply module 4, high-pressure CO2 supply and circulation module 3, data analysis unit 2, and intelligent decision-making unit 1. After drilling, perform well cleaning. Connect the downhole mobile fracturing workstation to the multi-functional cable 5 and lower it to the horizontal well section 17.

[0034] The intelligent decision-making unit 1 issues a command, and the electric crawler 7 moves the downhole mobile fracturing workstation along the horizontal well section 17. The video probe 11, combined with the directional locator, provides real-time position information. When the downhole mobile fracturing workstation reaches the designed first fracturing section, the hydraulic anchoring arm 6 extends, anchoring the downhole mobile fracturing workstation to the open hole wall to resist the recoil force during subsequent operations; simultaneously, the distributed optical fiber 9 begins to collect the background temperature value around the hole, and the microseismic acquisition sub 10 enters listening mode.

[0035] S2: Intelligent decision-making unit 1 sends a command to activate CO2 phase change control chamber module 14, and liquid CO2 is pumped from the ground at a pressure of 15MPa with a flow rate of 2.5m³. 3 / min. Liquid CO2 enters the cryogenic liquid CO2 buffer chamber 19, and the pressure inside the chamber stabilizes at 8MPa. The multi-stage microwave heater 20 is activated. The preset target temperature is 45℃. The three-stage magnetron (operating frequency 2.45GHz) applies power in stages according to the PID intelligent temperature control algorithm (total power approximately 15kW), and the liquid CO2 is rapidly and uniformly heated as it flows through the high-transmittance ceramic flow channel. By adjusting the heating power, the temperature and density of the supercritical CO2 are controlled to achieve stable and controllable fracturing behavior.

[0036] The heated fluid enters the supercritical pressure-stabilizing chamber 22, where it is completely converted into homogeneous supercritical CO2 under forced mixing in the internal biomimetic spiral turbulence groove. The measured temperature is 42℃, and the pressure stabilizes at 15MPa. The pulse jet valve 21 receives a command and enters a high-frequency pulse operating mode. Its piezoelectric ceramic actuator rapidly drives the valve core at a frequency of 30Hz and a pulse width of 3ms. The supercritical CO2 is accelerated through the artificial diamond Laval nozzle at the rear of the valve port, forming a supersonic jet that continuously impacts the coal wall. This process lasts approximately 2 minutes. The highly permeable supercritical CO2 jet opens and extends natural microfractures in the coal rock using a quasi-static pressure method, forming a preliminary, somewhat directional fracture system and providing a low-resistance propagation medium for subsequent plasma shock waves.

[0037] S3: During the interval or after the pulse jet valve 21 has finished operating, the intelligent decision-making unit 1 triggers the plasma pulse fracturing module 15. The high-voltage pulse capacitor bank 23 (energy storage 2kJ) discharges through the high-power gas switch in the pulse shaping network 24 (PFN), generating a high-voltage pulse with a peak voltage of 30kV and a rise time of 5μs. This high-voltage pulse is applied to the coaxial annular electrode 25 of the downhole plasma generator. At this time, the environment around the electrode is filled with high-density supercritical CO2 injected in step S2. The electrode gap is broken down by the high voltage, generating a high-temperature plasma channel, which expands explosively and generates a strong shock wave with a peak pressure exceeding 200MPa. This shock wave uses supercritical CO2 as the energy carrier medium and radiates efficiently into the coal seam. Huge dynamic tensile stress is generated at the tip of the initial fracture formed in step S2 and at the natural weak surface of the coal body, activating and expanding a large number of branch fractures. The plasma pulse shock wave is triggered during or between supercritical CO2 jet injections, achieving the synergy of static pressure fracture creation and dynamic impact fracture expansion.

[0038] Plasma pulses are continuously emitted 20 times at a repetition frequency of 10 Hz, forming a cyclic impact load that promotes further extension and connection of the fracture network. Throughout the operation, distributed optical fiber 9 and microseismic acquisition section 10 transmit temperature field change data and microseismic signals back to the ground in real time through the optical fiber data transmission channel in multifunctional cable 5. Data analysis unit 2 analyzes the degree of fracture development in this segment in real time. If the expected results are not achieved, intelligent decision-making unit 1 adjusts the supercritical CO2 injection parameters (pressure, pulse frequency) and plasma pulse energy and frequency to perform reinforced fracturing. If data analysis unit 2 shows a significant increase in the number of microseismic events and the event point cloud exhibits a network distribution characteristic, it indicates that a complex fracture network is forming.

[0039] S4: After completing the fracturing of the current section, inject a biodegradable temporary plugging agent (such as polymer gel balls) through the surface pump connected to the multi-functional injection sub 16. The temporary plugging agent enters the existing fracture and accumulates in its throat, temporarily sealing the section. The hydraulic anchoring arm 6 retracts, and the intelligent decision unit 1 drives the downhole mobile fracturing workstation to move to the next target section, repeating steps S2-S3.

[0040] After completing the six-stage fracturing, the work station was withdrawn. Subsequent gas extraction data showed that: (1) Extraction concentration and purity: The gas extraction concentration in the fracturing area increased from 15% before the operation to 65%, and the daily extraction purity increased by more than 8 times; (2) Attenuation period: The attenuation period of high-concentration extraction was significantly extended, indicating the formation of an effective long-term seepage channel; (3) Fracture network morphology: Through microseismic monitoring inversion, the fracture influence range was ellipsoidal, and the fracture network complexity index (characterized by the fractal dimension of event distribution) was about 35% higher than that of traditional hydraulic fracturing.

[0041] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A modular mobile supercritical CO2 phase change synergistic plasma fracturing system, characterized in that: It includes a ground control center, a downhole mobile fracturing workstation, and a multi-functional cable connecting the two. The downhole mobile fracturing workstation includes a CO2 phase change control chamber module and a plasma pulse fracturing module. The CO2 phase change control chamber module is used to heat and pressurize liquid CO2 in stages, so that it is converted into a supercritical CO2 jet in situ downhole to fracture the coal seam. The plasma pulse fracturing module is used to generate plasma pulse shock waves in a supercritical CO2 environment to collaboratively expand the fracture network.

2. The modular mobile supercritical CO2 phase change synergistic plasma fracturing system according to claim 1, characterized in that: The CO2 phase change control chamber module includes a cryogenic liquid CO2 buffer chamber, a multi-stage microwave heating assembly, a supercritical state stabilizing chamber, and a pulse jet valve arranged in series.

3. The modular mobile supercritical CO2 phase change synergistic plasma fracturing system according to claim 1, characterized in that: The plasma pulse fracturing module includes a high-voltage pulse capacitor bank, a pulse shaping network, and a downhole plasma generator.

4. The modular mobile supercritical CO2 phase change synergistic plasma fracturing system according to claim 3, characterized in that: The downhole plasma generator includes a coaxial annular electrode and a directional jet electrode.

5. A modular mobile supercritical CO2 phase change synergistic plasma fracturing system according to claim 1, characterized in that: The multifunctional cable includes a high-voltage CO2 delivery channel, a power cable, an optical fiber data transmission channel, and a microtube, wherein the microtube is used to deliver a temporary plugging agent, a nano-support agent, or a tracer.

6. A modular mobile supercritical CO2 phase change synergistic plasma fracturing system according to claim 5, characterized in that: The end of the downhole mobile fracturing station is equipped with a multi-functional injection sub-section, which is connected to the microtube.

7. A modular mobile supercritical CO2 phase change synergistic plasma fracturing system according to claim 1, characterized in that: The downhole mobile fracturing workstation also includes a power and positioning module for driving the downhole mobile fracturing workstation to move along the wellbore and position itself at the target fracturing section.

8. A modular mobile supercritical CO2 phase change synergistic plasma fracturing method, employing the modular mobile supercritical CO2 phase change synergistic plasma fracturing system described in any one of claims 1-7, characterized in that: Includes the following steps: S1: Lower the downhole mobile fracturing workstation to the first target formation in the well and anchor it; S2: Activate the CO2 phase change control chamber module to heat and pressurize the liquid CO2 in stages, so that it is converted into a supercritical CO2 jet in situ underground to fracturing the coal seam. S3: Activate the plasma pulse fracturing module to generate plasma pulse shock waves in a supercritical CO2 environment to collaboratively expand the crack network; S4: After completing the fracturing of the current section, move the downhole mobile fracturing workstation to the next target section and repeat steps S2-S3.

9. A modular mobile supercritical CO2 phase change synergistic plasma fracturing method according to claim 8, characterized in that: In step S2, the temperature and density of supercritical CO2 are controlled by adjusting the heating power to achieve stable and controllable fracturing behavior.

10. A modular mobile supercritical CO2 phase change synergistic plasma fracturing method according to claim 8, characterized in that: In step S3, the plasma pulse shock wave is triggered during or between supercritical CO2 jet injection, achieving synergy between static pressure fracture formation and dynamic impact fracture expansion.

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