Fracturing System and Method Based on Supercritical CO2 Phase Regulation and Multi-Energy Field Synergy

Abstract

This application relates to the field of coalbed methane extraction technology, specifically disclosing a fracturing system and method based on supercritical CO2 phase regulation and multi-energy field synergy. The system includes a surface control module, a downhole multi-functional integrated toolchain, and a CO2 phase regulation injection module. The method includes: constructing an annular temperature-controlled preheating zone using downhole microwave heating to actively regulate the injected liquid CO2 to a supercritical state to complete the initial fracturing; then injecting a chemical pressurizing agent to maintain the phase window and generate gas pulses to expand the fracture through an exothermic reaction; finally, injecting a bio-permeability enhancer to achieve long-term bio-permeability enhancement. The entire process is an intelligent closed loop composed of digital twins and fiber optic sensing, operating without water. Through the spatiotemporal synergy of physical, chemical, and biological energy, it solves the pain points of traditional waterless fracturing, such as unstable phase state, low fracture complexity, and rapid conductivity attenuation, while also possessing carbon sequestration potential.

Description

Technical Field

[0001] This application relates to the field of coalbed methane extraction technology, and in particular to a fracturing system and method based on supercritical CO2 phase regulation and multi-energy field synergy. Background Technology

[0002] Due to their complex geological conditions and low permeability, soft, low-permeability coal seams have always presented a significant technical challenge in coalbed methane extraction and coalbed gas development. Conventional hydraulic fracturing, as the primary reservoir stimulation method, faces severe challenges in practical application. On the one hand, hydraulic fracturing operations consume large amounts of water, with single-well operations requiring thousands of cubic meters of water. On the other hand, fracturing fluid flowback is difficult and easily triggers a series of negative effects, including: water-locking damage caused by capillary forces trapping water in the coal pores, severely blocking gas desorption and seepage channels; coal softening upon contact with water leading to borehole collapse; and surface environmental pollution caused by large amounts of wastewater flowback. These shortcomings limit the permeability enhancement effect and economic viability of hydraulic fracturing in soft, low-permeability coal seams.

[0003] To address the water phase damage and environmental issues associated with hydraulic fracturing, waterless fracturing technology has gradually become the mainstream research direction in this field. Currently, the main waterless fracturing technologies in the industry include high-pressure gas fracturing, liquid / supercritical carbon dioxide fracturing, and explosive fracturing, but each technology has limitations in its application. (1) High-pressure gas fracturing: Nitrogen or air is usually used as the fracturing medium. However, the gas has extremely low viscosity and poor proppant carrying capacity, making it difficult to effectively deliver the proppant to the deep part of the fracture, resulting in low fracturing efficiency. At the same time, due to the high compressibility and rapid filtration of gas, the fractures formed after fracturing are prone to close quickly after depressurization, making it difficult to form a long-term effective flow channel.

[0004] (2) Liquid / Supercritical CO2 fracturing: Supercritical CO2 (SC-CO2) has low viscosity and high diffusivity close to that of a gas, and high density close to that of a liquid, which has a good permeation and swelling effect on coal matrix. However, this technology has two core limitations: First, the fracturing mechanical behavior of supercritical CO2 is sensitive to the temperature-pressure conditions it is in. In the complex and variable geothermal gradient environment downhole, the phase state (gas, liquid or supercritical state) of CO2 is prone to uncontrollable dynamic transformation, resulting in violent fluctuations in energy release during the fracturing process and unstable fracture propagation path and morphology; Second, relying solely on the physical properties of supercritical CO2 itself, its fracture-creating ability and fracture support ability are still limited, especially in constructing a three-dimensional complex fracture network and maintaining the long-term conductivity of fractures.

[0005] (3) Blasting cracking: Cracks are created by releasing energy instantaneously through high-energy gas or explosives. However, the energy release process of this method is difficult to control precisely, which can easily cause excessive damage to the original structure of the coal and rock mass, destroy the overall stability of the coal seam, and pose a high safety risk.

[0006] In summary, the core challenges in the current field of waterless fracturing technology lie in the lack of precise means to control the physical state of fracturing media in complex downhole environments, and the absence of technical strategies for the refined coordinated operation of different waterless energy enhancement mechanisms across time and space. These deficiencies directly limit the permeability enhancement effect of coal seams, making it difficult to form a long-term stable, highly conductive, three-dimensional gas seepage network. Summary of the Invention

[0007] To address the issues of water resource dependence, water-locking damage, and the instability, low fracture complexity, and poor long-term effectiveness of existing waterless fracturing processes during permeability enhancement in low-permeability coal seams, this application provides a fracturing system and method based on supercritical CO2 phase regulation and multi-energy field synergy.

[0008] The fracturing system based on supercritical CO2 phase state control and multi-energy field synergy provided in this application adopts the following technical solution: A fracturing system based on supercritical CO2 phase regulation and multi-energy field synergy, comprising: The CO2 phase-controlled injection module is used to inject CO2 into the wellbore in a specified initial phase. A downhole multi-functional integrated tool string includes a shell with a channel for CO2 flow running through it along its length. Inside the shell, along the CO2 flow direction, are sequentially arranged a microwave preheating zone, a self-generated thermochemical pulse pressurization zone, and a bio-permeability enhancement injection zone. The microwave preheating zone heats the flowing CO2 to a supercritical state, and the supercritical CO2 is used to perform initial fracturing of the reservoir to form initial fractures. The self-generated thermochemical pulse pressurization zone controllably releases a chemical pressurizing agent into the flowing supercritical CO2. The chemical pressurizing agent, when mixed with the supercritical CO2, undergoes an exothermic chemical reaction, generating high-temperature gas and forming pulse pressurization, thus re-activating the initial fractures. The bio-permeability enhancement injection zone controllably releases a bio-permeability enhancement agent into the reservoir to achieve long-term bio-permeability enhancement of the fracture surface. The surface control module is communicatively connected to the CO2 phase-state regulation injection module and the downhole multi-functional integrated tool string, and is used to control the parameters of the fracturing process.

[0009] Furthermore, in the microwave preheating zone, a multi-frequency microwave radiation array is arranged circumferentially on the inner wall of the outer shell to form an annular temperature-controlled preheating zone in the CO2 flow path.

[0010] Furthermore, in the self-generating thermochemical pulse pressurization zone, the inner wall of the outer shell is provided with a chemical pressurizer storage chamber for storing chemical pressurizer and phase change expandable proppant along the circumferential direction, and the inner wall of the chemical pressurizer storage chamber is provided with several slug injection valves.

[0011] Furthermore, in the bio-permeability injection area, the inner wall of the outer shell is provided with a plurality of bio-permeability storage cavities for storing bio-permeability, and the bio-permeability storage cavities are provided with release valves.

[0012] Furthermore, a partition plate is provided between the microwave preheating zone and the self-generated thermochemical pulse pressurization zone, and between the self-generated thermochemical pulse pressurization zone and the bio-permeability enhancement injection zone, and a regulating valve is provided on the partition plate.

[0013] Furthermore, a chemical pressurizing agent storage tank and a biological permeation enhancer storage tank are installed on the ground. The chemical pressurizing agent storage tank is connected to the chemical pressurizing agent storage chamber via a pipeline, and the biological permeation enhancer storage tank is connected to the biological permeation enhancer storage chamber via a pipeline.

[0014] Furthermore, a distributed optical fiber sensor array is provided on the inner wall of the outer shell along the CO2 flow path.

[0015] This application also provides a fracturing method based on supercritical CO2 phase state control and multi-energy field synergy. Employing the aforementioned fracturing system based on supercritical CO2 phase state control and multi-energy field synergy, the method includes the following steps: Preliminary fracturing: After drilling, the downhole multi-functional integrated tool string is lowered to the target reservoir. CO2 is injected into the wellbore in the specified initial phase. CO2 flows in from one end of the downhole multi-functional integrated tool string and is heated to a supercritical state when it flows through the microwave preheating zone. The supercritical CO2 flows sequentially through the self-generating thermochemical pulse pressurization zone and the bio-permeability enhancer injection zone. The chemical pressurizing agent and the bio-permeability enhancer are kept from being released. After the supercritical CO2 flows out from the downhole multi-functional integrated tool string, it performs preliminary fracturing on the reservoir to form initial fractures. Secondary activation: After the initial fracturing is completed, CO2 is injected into the wellbore in the specified initial phase. CO2 flows in from one end of the downhole multi-functional integrated tool string. When the CO2 flows through the microwave preheating zone, it is heated to the supercritical state. When the supercritical CO2 flows through the self-generated thermochemical pulse pressurization zone, it releases a chemical pressurizing agent. The chemical pressurizing agent mixes with the supercritical CO2 and undergoes an exothermic chemical reaction, generating high-temperature gas and forming pulse pressurization. After the supercritical CO2 carries the high-temperature and high-pressure pulse gas out of the downhole multi-functional integrated tool string, it reactivates the initial fracture. Bio-permeability enhancement: After the secondary excitation is completed, the bio-permeability enhancer injection zone releases the bio-permeability enhancer. The microorganisms in the bio-permeability enhancer use organic matter and / or CO2 in the coal as substrates for metabolism, thereby achieving continuous expansion of the fracture scale and in-situ fixation and transformation of CO2.

[0016] Furthermore, in the secondary activation step, the chemical pressurizing agent is mixed with a phase change expandable proppant, which softens and expands at downhole temperatures to support microfractures.

[0017] Furthermore, it also includes a closed-loop control step: during the execution of the initial fracturing, secondary activation, and biological permeability enhancement steps, the ground control module adjusts the fracturing process parameters in real time according to the monitoring parameters, including temperature, pressure, acoustic data, and inverted fracture morphology, and the fracturing process parameters including CO2 injection parameters, microwave heating power, and chemical pressurizer release rate.

[0018] In summary, this application includes at least one of the following beneficial technical effects: 1. Active control of anhydrous fracturing medium phase: This application eliminates water and water-based fracturing fluids, thus avoiding water-phase-induced damage such as coal seam water lock-in and coal softening / pore collapse. Through the design of a downhole "microwave preheating zone," real-time, active, and precise control of the in-situ phase transition point and phase evolution of the injected CO2 fluid is achieved. This transforms supercritical CO2 fracturing behavior from a traditional passive dependence on the original formation temperature and pressure conditions to a mode that can be actively preset and controlled according to engineering needs, improving the technical challenge of unstable fracture propagation caused by uncontrollable phase fluctuations in existing supercritical CO2 fracturing methods.

[0019] 2. Orderly Synergy of Multiple Energy Fields in Spatiotemporal Dimensions: This application couples three anhydrous energy fields—physical energy (microwave thermal field), chemical energy (controlled exothermic reaction), and biological energy (microbial metabolism)—in a sequence of action: "physical fracturing – chemical fracturing and support – long-term biological permeability enhancement." Microwave preheating ensures the supercritical phase of CO2 and establishes a basic temperature field for subsequent chemical reactions and biological activities. The heat release and gas production effects of the chemical reactions not only enhance the fracturing and extension processes of physical fracturing but also achieve efficient proppant delivery and self-consolidation support. Subsequent metabolic activities of microorganisms further microscopically modify, clear, and maintain the fracture network. The synergistic effect of these three fields forms a mutually reinforcing and self-driven permeability enhancement ecosystem.

[0020] 3. Full-chain intelligent sensing and adaptive control: Based on digital twin technology and a distributed optical fiber real-time sensor network, this application constructs a closed-loop control system of "sensing-decision-control". This system can analyze the dynamic response of coal seam heterogeneity to the fracturing process in real time, and adaptively adjust microwave power, injection parameters, and reaction triggering timing accordingly. This enables personalized fracturing scheme optimization for different coal seam conditions, which helps improve the stability and reliability of permeability enhancement under complex geological conditions.

[0021] 4. Green Environmental Protection and Carbon Negative Emission Potential: This application achieves zero water consumption and zero wastewater discharge throughout the entire process, reducing the environmental risks of traditional fracturing. After being used to enhance permeability, the injected CO2 fluid is partially geologically stored through coal seam adsorption, mineralization reactions, and microbial transformation, demonstrating potential for carbon sequestration and carbon reduction. Simultaneously, this method can improve coal mine gas extraction efficiency and coalbed methane recovery quality, achieving a win-win situation for both environmental benefits and the economic benefits of energy development. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the overall structure of a fracturing system based on supercritical CO2 phase regulation and multi-energy field synergy, according to an embodiment of this application. Figure 2 This is a schematic diagram of the downhole multi-functional integrated tool string structure in an embodiment of this application; Figure 3 This is a schematic diagram of the working principle of the "microwave preheating zone" in the embodiments of this application; Figure 4 This is a flowchart of a fracturing method based on supercritical CO2 phase control and multi-energy field synergy according to an embodiment of this application; Figure 5 This is a schematic diagram comparing the fracture networks formed by traditional supercritical CO2 fracturing and the method of this embodiment, where (a) is traditional supercritical CO2 fracturing and (b) is the method of this embodiment.

[0023] Reference numerals: 1. Microwave preheating zone; 101. Outer shell; 102. High-frequency microwave radiation array; 103. Regulating valve; 104. Separator plate; 2. Self-generating thermochemical pulse pressurization zone; 201. Slug injection valve; 202. Chemical pressurizer storage chamber; 3. Bio-permeability enhancer injection zone; 301. Release valve; 302. Bio-permeability enhancer storage chamber; 303. Composite bio-permeability enhancer microcapsule; 4. Surface control module; 5. Liquid CO2 storage tank; 6. Chemical pressurizer storage tank; 7. Bio-permeability enhancer storage tank; 8. Replenishment port; 9. Injection pipeline; 10. Flow valve; 11. Cryogenic pump; 12. Wellbore; 13. Reservoir; 14. Distributed fiber optic sensing probe; 15. Temperature control and heating device. Detailed Implementation

[0024] The following is in conjunction with the appendix Figure 1-5This application will be described in further detail.

[0025] Example 1 This application discloses a fracturing system based on supercritical CO2 phase state control and multi-energy field synergy. (Refer to...) Figure 1 and Figure 2 The fracturing system based on supercritical CO2 phase regulation and multi-energy field synergy includes: The CO2 phase state control injection module is used to inject CO2 into the wellbore 12 in a specified initial phase state. The downhole multi-functional integrated tool string includes a shell 101. The shell 101 has a channel for CO2 flow running through it along its length. Inside the shell 101, along the CO2 flow direction, are sequentially arranged a microwave preheating zone 1, a self-generated thermochemical pulse pressurization zone 2, and a bio-permeability enhancement injection zone 3. The microwave preheating zone 1 heats the flowing CO2 to a supercritical state, and the supercritical CO2 is used to perform initial fracturing of the reservoir 13 to form initial fractures. The self-generated thermochemical pulse pressurization zone 2 controllably releases a chemical pressurizing agent into the flowing supercritical CO2. The chemical pressurizing agent mixes with the supercritical CO2 and undergoes an exothermic chemical reaction, generating high-temperature gas and forming pulse pressurization to re-excite the initial fractures. The bio-permeability enhancement injection zone 3 controllably releases a bio-permeability enhancement agent into the reservoir to achieve long-term bio-permeability enhancement of the fracture surface. The surface control module 4 is connected to the CO2 phase regulation injection module and the downhole multi-functional integrated tool string to control the fracturing process parameters.

[0026] This application couples three anhydrous energy fields—physical energy (microwave thermal field), chemical energy (controlled exothermic reaction), and biological energy (microbial metabolism)—in a sequence of action: "physical fracture creation - chemical fracture widening and support - biological long-term permeability enhancement." Microwave preheating ensures the supercritical phase of CO2 and establishes a basic temperature field for subsequent chemical reactions and biological activities. The heat release and gas production effects of the chemical reactions not only enhance the fracture creation and extension process of physical fracturing but also achieve efficient proppant delivery and self-consolidation support. Subsequent metabolic activities of microorganisms further microscopically modify, clear, and maintain the fracture network. The orderly synergy of these multiple energy fields across time and space forms a mutually reinforcing and self-driven permeability enhancement ecosystem.

[0027] Reference Figure 1 The CO2 phase control injection module includes a liquid CO2 storage tank 5 and an injection pipeline 9 connected thereto. The output end of the liquid CO2 storage tank 5 is equipped with a cryogenic pump 11 and a flow valve 10. The output end of the liquid CO2 storage tank 5 is also equipped with a temperature control heating device 15, which is used to control and heat the output liquid CO2 so that CO2 is injected into the wellbore 12 in a specified initial phase such as liquid, subcritical or supercritical.

[0028] Reference Figure 2 The outer shell 101 of the downhole multi-functional integrated tool string is a hollow cylinder made of high-temperature and high-pressure resistant material, with openings at both ends. Its outer diameter is adapted to the size of the wellbore 12, and the internal hollow cavity serves as a channel for CO2 flow. An anchoring device can be configured on the outer shell 101 to anchor the downhole multi-functional integrated tool string within the wellbore 12.

[0029] Reference Figure 2 and Figure 3 In the microwave preheating zone 1, a multi-frequency microwave radiation array 102 is arranged circumferentially on the inner wall of the outer shell 101 to form an annular temperature-controlled preheating zone (the temperature can be controlled between 80-120℃) along the CO2 flow path, so that the CO2 flowing through this zone is rapidly and uniformly heated to the supercritical state. The annular temperature-controlled preheating zone can actively regulate the phase transition point and fracturing behavior of CO2. Specifically, by adjusting the microwave power of the multi-frequency microwave radiation array 102 in real time, the temperature and density of supercritical CO2 can be precisely controlled, thereby stabilizing its viscosity, diffusion coefficient and fracturing capacity, and achieving smooth and controllable fracture extension.

[0030] Reference Figure 1 and Figure 2 In the self-generating thermochemical pulse pressurization zone 2, a chemical pressurizer storage chamber 202 is circumferentially arranged on the inner wall of the outer shell 101. In this embodiment, the chemical pressurizer storage chamber 202 is annular, with its outer diameter matching the inner diameter of the outer shell 101. The annular space enclosed by its inner wall serves as a channel for CO2 flow. Several slug injection valves 201 are provided on the inner wall of the chemical pressurizer storage chamber 202 for injecting the chemical pressurizer and phase change expandable proppant from the chemical pressurizer storage chamber 202 into the supercritical CO2 in a slug-like manner. A chemical pressurizer storage tank 6 is located on the ground. The chemical pressurizer storage tank 6 is connected to the chemical pressurizer storage chamber 202 via an injection pipe 9 and corresponding pipelines (not shown in the figure); a replenishment port 8 is provided on the chemical pressurizer storage tank 6.

[0031] When chemical pressurizing agents (such as ammonium nitrate) are mixed with supercritical CO2, they can be activated by high temperature and undergo a controllable exothermic chemical reaction. For example, ammonium nitrate decomposes: 2NH4NO3→2N2+O2+4H2O, producing high-temperature gases (nitrogen, oxygen, water vapor, etc.) and rapidly increasing pressure.

[0032] The thermal effect of the exothermic chemical reaction further maintains and expands the phase window of supercritical CO2, while the gas pressurization effect produces a high-frequency pulse-like "hammering" effect on the cracks formed by supercritical CO2 fracturing, promoting the branching and complication of the cracks.

[0033] Furthermore, the water vapor generated from the decomposition of the chemical pressurizer forms a mixed phase with supercritical CO2, carrying phase change expandable proppant (such as surface-modified polylactic acid microspheres) deep into the fracture. The phase change expandable proppant softens and expands moderately at downhole temperatures, achieving effective support for microfractures.

[0034] Reference Figure 2 In the bio-permeability enhancement injection area 3, several bio-permeability enhancement storage chambers 302 are arranged circumferentially on the inner wall of the outer shell 101. The bio-permeability enhancement storage chambers 302 store composite bio-permeability enhancement microcapsules 303, and a release valve 301 is provided on the bio-permeability enhancement storage chambers 302. A bio-permeability enhancement storage tank 7 is provided on the ground, and the bio-permeability enhancement storage tank 7 is connected to the bio-permeability enhancement storage chambers 302 through an injection pipe 9 and corresponding pipelines (not shown in the figure).

[0035] As an example, the composite bio-permeability enhancer microcapsules 303 contain two functional bacterial groups: carbon-eating bacteria and methanogens. These two groups work synergistically to achieve long-term modification and carbon sequestration of coal seam fractures. Carbon-eating bacteria use organic matter (carbon) in the coal seam as a metabolic substrate, generating trace amounts of bioacids on the fracture surface and within the coal matrix through their biological metabolic activities. These bioacids then dissolve and form biopores, continuously expanding the fracture scale, improving the properties of the gas desorption interface, and enhancing gas desorption and seepage efficiency. Methanogens, on the other hand, utilize CO2 retained in the coal seam after fracturing and hydrogen present in the environment as reactants to synthesize methane in situ through metabolic pathways. This process achieves partial biological fixation of injected CO2, demonstrating carbon sequestration potential, and directly increases the total hydrocarbon content of coalbed methane, improving gas production quality.

[0036] The aforementioned biological processes are slow and persistent, with an effective cycle that can last from several months to several years. Thus, based on the fracture network formed by physical fracturing and chemical energy enhancement, biological reinforcement and long-term maintenance are achieved, forming a self-driven and self-repairing coal seam permeability enhancement ecosystem.

[0037] Furthermore, refer to Figure 2 Separator plates 104 are installed between the microwave preheating zone 1 and the autogenous thermochemical pulse pressurization zone 2, and between the autogenous thermochemical pulse pressurization zone 2 and the bio-permeability enhancement injection zone 3. These two separator plates 104 divide the internal space of the downhole multi-functional integrated tool string into a three-chamber series structure. The separator plates 104 are made of high-temperature and high-pressure resistant material. The separator plates 104 facilitate thorough mixing of supercritical CO2 and the chemical pressurizing agent in the autogenous thermochemical pulse pressurization zone 2. To regulate the flow rate of supercritical CO2 into and out of the autogenous thermochemical pulse pressurization zone 2, regulating valves 103 are installed on both separator plates 104.

[0038] Reference Figure 2The inner wall of the outer shell 101 is equipped with a distributed fiber optic sensor array 14 along the CO2 flow path for monitoring downhole parameters, including temperature, pressure and acoustic waves; information on fracture initiation and propagation can be obtained through acoustic wave data.

[0039] The surface control module 4 incorporates a digital twin model, which couples reservoir three-dimensional geomechanical parameters, multiphase flow model, and CO2 phase equation to simulate, predict, and optimize fracturing process parameters. A distributed fiber optic sensor array 14 feeds back the monitored downhole parameters to the surface control module 4, forming a closed loop of "sensing-decision-control".

[0040] The ground control module 4 is connected to the cryogenic pump 11, flow valve 10, and temperature-controlled heating device 15 in the CO2 phase state regulation injection module. The ground control module 4 calculates the optimal initial phase state of CO2 required for the target fracturing section based on the digital twin model and the geothermal gradient, and generates control commands to adjust the operating parameters of the cryogenic pump 11, flow valve 10, and temperature-controlled heating device 15.

[0041] The surface control module 4 is connected to the multi-frequency microwave radiation array 102, slug injection valve 201, release valve 301, and regulating valve 103 in the downhole multi-functional integrated tool string. By controlling the microwave power of the multi-frequency microwave radiation array 102, the temperature and density of supercritical CO2 can be regulated; by controlling the opening and closing of the slug injection valve 201 and the release valve 301, the release timing and release rate of the chemical pressurizing agent and the bio-penetrating agent can be regulated; by controlling the opening and closing of the regulating valve 103, the flow rate of supercritical CO2 inflowing into and outflowing from the self-generated thermochemical pulse pressurization zone 2 can be regulated.

[0042] Example 2 Using the fracturing system based on supercritical CO2 phase regulation and multi-energy field synergy provided in Example 1, a fracturing method based on supercritical CO2 phase regulation and multi-energy field synergy was implemented in a mine with a burial depth of 650m and a permeability coefficient of 0.03m. 2 / (MPa 2 Tests were conducted on soft, low-permeability coal seams (d). Figure 4 As shown, the fracturing method includes the following steps: Step 1, Preliminary fracturing: After drilling, the downhole multi-functional integrated tool string was lowered and anchored to the target reservoir 13.

[0043] Based on the digital twin model and geothermal gradient, the optimal initial phase of CO2 required for the target fracturing section is calculated. The surface control module 4 generates control commands to adjust the operating parameters of the cryogenic pump 11, flow valve 10, and temperature-controlled heating device 15, injecting liquid CO2 into the wellbore 12 at a pressure of 20 MPa, with the flow rate controlled at 5 m³ / min via the flow valve 10. 3 / min.

[0044] The temperature of microwave preheating zone 1 is set to 105℃. Liquid CO2 flows in from one end of the downhole multi-functional integrated tool string and is heated to a supercritical state when it flows through microwave preheating zone 1.

[0045] The regulating valve 103 is opened, the slug injection valve 201 and the release valve 301 are closed, and the supercritical CO2 flows sequentially through the self-generated thermochemical pulse pressurization zone 2 and the bio-permeability enhancer injection zone 3. The chemical pressurizing agent and the bio-permeability enhancer are kept from being released. After the supercritical CO2 flows out from the downhole multi-functional integrated tool string, it performs preliminary fracturing on the reservoir 13 to form initial fractures.

[0046] Monitoring by distributed fiber optic sensing probe 14 showed that CO2 maintained a stable supercritical state (T=92℃, P>7.4MPa) after flowing through microwave preheating zone 1, successfully forming the main crack system. The number of microseismic events was 1.8 times that of conventional CO2 injection.

[0047] Step 2, Secondary Excitation: After the initial fracturing is completed, based on the initial fracture network formed in step S1, secondary energy excitation and fracture propagation are performed under anhydrous conditions. Specifically: Continue to inject CO2 into the wellbore 12 in the specified initial phase state. CO2 flows in from one end of the downhole multi-functional integrated tool string and is heated to the supercritical state when it flows through the microwave preheating zone 1.

[0048] Keep the regulating valve 103 open and the release valve 301 closed. Open the slug injection valve 201 according to the excitation timing. When the supercritical CO2 flows through the self-generated heat chemical pulse pressurization zone 2, the chemical pressurizer storage chamber 202 releases the chemical pressurizer and phase change expandable proppant into the supercritical CO2 in the form of a slug.

[0049] When a chemical pressurizing agent is mixed with supercritical CO2, it is activated at high temperature and undergoes a controllable exothermic chemical reaction, generating high-temperature gas and forming a pulsed pressurization. The supercritical CO2, carrying the high-temperature, high-pressure pulsed gas, flows out from the downhole multi-functional integrated tool string, further stimulating the initial fractures. The thermal effect of the exothermic chemical reaction further maintains and expands the phase window of the supercritical CO2, while the gas pressurization effect generates a high-frequency pulsed "hammering" effect on the fractures formed by supercritical CO2 fracturing, promoting fracture branching and complexity. The miscibility of water vapor and supercritical CO2 promotes the softening and expansion of the phase-change expandable proppant, thereby supporting the cracks.

[0050] Distributed fiber optic sensing probe 14 monitoring showed three controllable pulsed pressure peaks (8-12 MPa higher than baseline), and DAS signals indicated a significant increase in secondary micro-fracture events. The temperature briefly rose to 130°C, effectively maintaining the supercritical CO2 state.

[0051] Step 3, Bio-enhanced Osmosis: After the secondary activation is completed and the downhole temperature decreases, the release valve 301 is opened, and the bio-permeability enhancer storage chamber 302 releases the composite bio-permeability enhancer microcapsules 303. Among them, carbon-eating bacteria generate bio-acids and biopores, expanding the crack scale and improving gas desorption; methanogenic bacteria fix the CO2 retained after fracturing, increasing the hydrocarbon content of coalbed methane in situ.

[0052] Step 4: Closed-loop control: During the execution of steps 1-3, the ground control module 4 adjusts the fracturing process parameters such as CO2 injection parameters, microwave heating power, and chemical pressurizer release rate in real time based on the monitoring parameters such as temperature, pressure, acoustic data and inverted fracture morphology collected by the distributed fiber optic sensing probe 14, so as to achieve full-process adaptive optimization.

[0053] After completing the above steps, connection and extraction were carried out. The peak gas extraction concentration of the experimental well (using the method of this embodiment) was 120% higher than that of the control well (using the traditional supercritical CO2 fracturing method), and the decay period was extended by more than 200%. After one year of continuous monitoring, the extraction flow rate of the experimental well was still more than twice that of the control well, and the gas chromatography detected an increase in the proportion of biogenic methane, confirming the bio-permeability enhancement effect.

[0054] Fracture networks formed using traditional supercritical CO2 fracturing, such as Figure 5 As shown in (a), the fracture network is dominated by a small number of main fractures with few branch fractures; due to the lack of effective proppant, the fractures tend to close; and due to the absence of biological activity, the fracture conductivity decays rapidly. The fracture network formed using the method of this embodiment is as follows: Figure 5 As shown in (b), the cracks expand and develop extensively, forming a multi-level branched crack network; the phase change expandable proppant uniformly fills the cracks, and the crack structure is stable; the bio-infiltration forms secondary pores, and the conductivity is maintained for a long time.

[0055] 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 fracturing system based on supercritical CO2 phase state regulation and multi-energy field coordination, characterized in that: include: The CO2 phase-controlled injection module is used to inject CO2 into the wellbore in a specified initial phase. The downhole multi-functional integrated tool string includes a shell with a channel for CO2 flow running through it along its length. Inside the shell, along the direction of CO2 flow, there are sequentially arranged a microwave preheating zone, a self-generated thermochemical pulse pressurization zone, and a bio-permeability enhancement injection zone. The microwave preheating zone is used to heat the flowing CO2 to a supercritical state, and the supercritical CO2 is used to perform preliminary fracturing of the reservoir to form initial fractures. The self-generating thermochemical pulse pressurization zone is used to controllably release a chemical pressurizing agent into the flowing supercritical CO2. The chemical pressurizing agent can undergo an exothermic chemical reaction after mixing with the supercritical CO2, generating high-temperature gas and forming pulse pressurization to re-excite the initial fracture. The bio-permeability enhancement injection zone is used to controllably release a bio-permeability enhancement agent into the reservoir to achieve long-term bio-permeability enhancement of the fracture surface. The surface control module is communicatively connected to the CO2 phase regulation injection module and the downhole multi-functional integrated tool string, and is used to control the parameters of the fracturing process.

2. The system according to claim 1, wherein the system is a system for controlling fracturing based on supercritical CO2 phase state and synergized with multi-energy field. In the microwave preheating zone, a multi-frequency microwave radiation array is arranged circumferentially on the inner wall of the outer shell to form an annular temperature-controlled preheating zone along the CO2 flow path.

3. The system of claim 1, wherein the system is configured to control the phase of supercritical CO2 and the multi-energy field to fracture the subterranean formation. In the self-generated thermochemical pulse pressurization zone, a chemical pressurizer storage chamber for storing chemical pressurizer and phase change expandable proppant is provided circumferentially on the inner wall of the outer shell, and a number of slug injection valves are provided on the inner wall of the chemical pressurizer storage chamber.

4. The system according to claim 3, wherein the system is based on supercritical CO2 phase state regulation and multi-energy field synergy fracturing system. In the bio-permeability injection area, the inner wall of the outer shell is provided with several bio-permeability storage chambers along the circumference for storing bio-permeability, and each bio-permeability storage chamber is provided with a release valve.

5. A fracturing system based on supercritical CO2 phase regulation and multi-energy field synergy according to claim 4, characterized in that: Separator plates are provided between the microwave preheating zone and the autogenous thermochemical pulse pressurization zone, and between the autogenous thermochemical pulse pressurization zone and the bio-penetrating agent injection zone. Regulating valves are provided on the separator plates.

6. The fracturing system based on supercritical CO2 phase regulation and multi-energy field synergy according to claim 4, characterized in that: A chemical pressurizing agent storage tank and a biological permeation enhancer storage tank are installed on the ground. The chemical pressurizing agent storage tank is connected to the chemical pressurizing agent storage chamber through a pipeline, and the biological permeation enhancer storage tank is connected to the biological permeation enhancer storage chamber through a pipeline.

7. The fracturing system based on supercritical CO2 phase regulation and multi-energy field synergy according to claim 1, characterized in that: The inner wall of the outer shell is provided with a distributed optical fiber sensor array along the CO2 flow path.

8. A fracturing method based on supercritical CO2 phase state control and multi-energy field synergy, characterized in that: The fracturing system based on supercritical CO2 phase regulation and multi-energy field synergy as described in any one of claims 1-7 includes the following steps: Preliminary fracturing: After drilling, the downhole multi-functional integrated tool string is lowered to the target reservoir. CO2 is injected into the wellbore in the specified initial phase. CO2 flows in from one end of the downhole multi-functional integrated tool string and is heated to a supercritical state when it flows through the microwave preheating zone. The supercritical CO2 flows sequentially through the self-generating thermochemical pulse pressurization zone and the bio-permeability enhancer injection zone. The chemical pressurizing agent and the bio-permeability enhancer are kept from being released. After the supercritical CO2 flows out from the downhole multi-functional integrated tool string, it performs preliminary fracturing on the reservoir to form initial fractures. Secondary activation: After the initial fracturing is completed, CO2 is injected into the wellbore in the specified initial phase. CO2 flows in from one end of the downhole multi-functional integrated tool string. When the CO2 flows through the microwave preheating zone, it is heated to the supercritical state. When the supercritical CO2 flows through the self-generated thermochemical pulse pressurization zone, it releases a chemical pressurizing agent. The chemical pressurizing agent mixes with the supercritical CO2 and undergoes an exothermic chemical reaction, generating high-temperature gas and forming pulse pressurization. After the supercritical CO2 carries the high-temperature and high-pressure pulse gas out of the downhole multi-functional integrated tool string, it reactivates the initial fracture. Bio-permeability enhancement: After the secondary excitation is completed, the bio-permeability enhancer injection zone releases the bio-permeability enhancer. The microorganisms in the bio-permeability enhancer use organic matter and / or CO2 in the coal as substrates for metabolism, thereby achieving continuous expansion of the fracture scale and in-situ fixation and transformation of CO2.

9. The fracturing method based on supercritical CO2 phase state control and multi-energy field synergy according to claim 8, characterized in that: In the secondary activation step, the chemical pressurizing agent is mixed with a phase change expandable proppant, which softens and expands at downhole temperatures to support microfractures.

10. The fracturing method based on supercritical CO2 phase state control and multi-energy field synergy according to claim 8, characterized in that: It also includes a closed-loop control step: during the execution of the initial fracturing, secondary activation, and biological permeability enhancement steps, the ground control module adjusts the fracturing process parameters in real time according to the monitoring parameters, including temperature, pressure, acoustic data, and inverted fracture morphology, and the fracturing process parameters including CO2 injection parameters, microwave heating power, and chemical pressurizer release rate.

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