Low-permeability coal seam combined fracturing method with slotted, foam pulse, phase change and acidification

Abstract

The application discloses a low-permeability coal seam slotted-foam pulsation-phase change-acidification synergic fracturing method, which comprises the following steps: forming a three-dimensional slot through hydraulic slotting, injecting CO2 foam fracturing fluid in an alternating pulsation mode, forming an initial crack network by using pulse fatigue effect, forming a supercritical state of CO2 under the temperature and pressure conditions of the stratum, stopping pumping and building pressure, and controlling the pressure to drop at a set rate, simultaneously exerting the expansion impact of the supercritical CO2 gasification phase change and the mineral dissolution of the self-generated CO2 carbonic acid, and synergically expanding the crack network, and after the segmented repeated process, realizing CO2 capture and liquefaction recycling by using a separation and recovery device. The method has the advantages of synergic multiple mechanisms of pulsation fatigue damage, phase change expansion impact and self-generated acidification dissolution, can effectively reduce crack initiation pressure and improve crack network complexity, has the advantages of low water consumption and recyclable CO2, and is suitable for efficient reconstruction of deep low-permeability coal seams, and can improve gas extraction efficiency and mine gas disaster control effect.

Description

Technical Field

[0001] This invention belongs to the field of mine gas disaster prevention and control and coalbed methane development technology, specifically involving a low-permeability coal seam fracturing method of synergistic fracturing of fracturing-foam pulsation-phase change-acidification, which is particularly suitable for efficient permeability enhancement of deep, high-stress, low-porosity, and low-permeability coal seams. Background Technology

[0002] Methane, the main component of coal mine gas, is both a major hazard threatening safe coal mine production and a valuable unconventional natural gas resource. Low-permeability coal seams are widely distributed in my country, especially deep coal seams, which are generally characterized by high ground stress, low porosity, low permeability, and strong heterogeneity. Gas is mostly present in an adsorbed state within the coal matrix, with poor connectivity of primary fractures. Conventional underground directional long-borehole gas extraction suffers from problems such as a small effective impact range, rapid decline in extraction flow rate, and long extraction cycles to meet standards, making it difficult to meet the needs of efficient gas control and coalbed methane development. Therefore, artificial fracturing stimulation measures are needed to improve coal seam permeability.

[0003] Current downhole coal seam fracturing technology mainly relies on hydraulic fracturing, which has three main shortcomings in practical applications: First, under high geostress conditions, the initiation pressure of continuous constant-pressure injection is high, and the fractures tend to extend unidirectionally along the direction of geostress dominance, resulting in poor uniformity of fracturing at the far end of long directional boreholes and between segments, making it difficult to form complex fracture networks. Second, the water consumption during construction is large, and the retention of fracturing fluid can easily lead to water-locking effects, hydration expansion of clay minerals, and pore blockage, damaging the reservoir and inhibiting gas desorption and seepage. Third, conventional fracturing often forms a single main fracture, with limited fracture network complexity and long-term conductivity, resulting in a short duration of permeability enhancement.

[0004] CO2 exhibits strong competitive adsorption on coal seams, effectively displacing adsorbed methane and promoting gas desorption. Existing pure CO2 fracturing technology suffers from low fluid viscosity and weak filtration control. While CO2 foam fracturing can improve fluid rheological properties and enhance filtration control, conventional continuous injection methods do not fully leverage the fatigue damage effect of pulsating pressure. Furthermore, some pulsating fracturing schemes fail to organically combine pre-cutting for pressure relief, pulsating fracturing, pressure-induced phase change fracturing, and CO2 recovery and utilization, thus hindering the synergistic effect of multiple mechanisms and limiting the fracturing effectiveness and economic benefits of low-permeability coal seams.

[0005] Therefore, there is an urgent need to develop a synergistic fracturing process suitable for directional long boreholes in low-permeability coal seams, which can enhance the effects of gas control and coalbed methane development, reduce water consumption and reservoir damage, achieve CO2 recycling, and balance environmental and economic benefits. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a synergistic fracturing method for low-permeability coal seams, which combines fracturing, foam pulsation, phase transformation, and acidizing. This method solves the problems of high initiation pressure, simple fracture network morphology, large water consumption, severe reservoir damage, and low stimulation efficiency of conventional fracturing techniques. At the same time, it enables CO2 recycling and improves the overall benefits of the project.

[0007] To achieve the above objectives, the technical solution adopted by this invention is: a synergistic fracturing method for low-permeability coal seams involving fracturing, foam pulsation, phase change, and acidizing, employing a foam-based fracturing fluid injection device, a liquid CO2 injection device, a foamer, a pulsation fracturing pump, fracturing pipelines, a drag-type packer, and a CO2 foam fracturing fluid separation and recovery device, specifically including the following steps: S1. Construction of directional long boreholes: Select the target fracturing coal seam, build a fracturing operation platform in the roadway, construct directional long boreholes for fracturing, and install CO2 foam fracturing fluid separation and recovery devices in the roadway.

[0008] S2. Preparatory work: Push the slotted fracturing string with a drag packer into the fracturing borehole and precisely drag it to the target fracturing section; connect the pipelines in the roadway according to the fluid flow direction: the foaming base fluid injection device and the liquid CO2 injection device are respectively connected to the foamer, the foamer outlet is connected to the pulse fracturing pump through the pipeline, and the pulse fracturing pump is connected to the borehole fracturing pipeline through the fracturing pipe.

[0009] S3. Sealing test: Check the sealing of pipeline joints, flanges and elbows. Pre-pressurize the borehole with a pulse fracturing pump to test the sealing of the packer. After confirming that there is no leakage, proceed to the fracturing process.

[0010] S4. CO2 Foam Pulsating Fracturing: Using a slotted fracturing pipeline, slots are created in the target section, forming a three-dimensional distribution of fractures. The packer is then dragged backward and water is injected to expand it, achieving initial setting. The foaming base fluid pump and liquid CO2 pump are activated, injecting foaming base fluid and liquid CO2 into the foamer in proportion to generate stable CO2 foam fracturing fluid. The foaming base fluid is an aqueous solution of surfactant with added foam stabilizer. The orifice shut-off valve is opened, and the pulsating fracturing pump is activated to inject the foam fracturing fluid into the coal seam in an alternating pulsating manner. Utilizing the fatigue damage effect and water wedge effect generated by the periodic pulse pressure, the fracturing initiation pressure is reduced, natural fractures are activated, and an initial complex fracture network is formed. Furthermore, after injection into the directional long borehole, the CO2 in the CO2 foam fracturing fluid undergoes a phase transition under high temperature and high pressure conditions in the formation, forming supercritical CO2.

[0011] S5. Synergistic Expansion of Fractures by Phase Change Expansion and Acid Dissolution: After continuous injection for a set time, the pulsating fracturing pump is stopped, and the borehole shut-off valve is closed to pressurize the borehole. The borehole pressure is controlled to decrease slowly at a rate of 1~3MPa / day. Under the temperature and pressure conditions of the deep formation, supercritical CO2 gradually vaporizes and expands as the pressure decreases. The impact force generated by the rapid volume expansion continues to act on the fracture wall and tip, promoting fracture extension and branching. At the same time, CO2 dissolves in water in the fracturing fluid under high pressure to form carbonic acid. Carbonic acid reacts with carbonate minerals and clay minerals in the coal seam, weakening the mechanical strength of the coal body and expanding porosity and microfractures. The synergistic effect of the physical impact of phase change expansion and the chemical weakening effect of carbonic acid dissolution further expands the scale of the fracture network.

[0012] S6. Directional long borehole segmented fracturing: Unseal the packer, drag the tubing string to the next target layer, and set the layer. Repeat the pulsating fracture creation-pressure expansion process of steps S4 to S5 to complete the fracturing transformation of all layers in the directional long borehole in sequence.

[0013] S7. Fracturing Fluid Backflow and Recovery: After all sections have been modified, open the orifice shut-off valve and backflow valve, and connect the CO2 foam fracturing fluid separation and recovery device to the fracturing pipe. Through extraction, the fracturing fluid carrying gas and CO2 is backflowed to the CO2 foam fracturing fluid separation and recovery device. After the separation and recovery device completes the gas-liquid two-phase separation, it separates CO2 and CH4. The recovered CO2 is compressed and liquefied and then transported to the liquid CO2 pumping device for recycling. CH4 is recovered as associated gas, and the waste liquid is centrally treated.

[0014] Further, in step S4, by adjusting the injection flow ratio of foaming base liquid to liquid CO2, a CO2 foam fracturing fluid with a foam mass of 70% to 90% is prepared, wherein the foam mass refers to the percentage of CO2 gas volume to the total foam volume; the pulsation frequency range of the pulsation fracturing pump is 0.1 to 20 Hz, and the pressure fluctuation amplitude is 2 to 4 MPa.

[0015] Furthermore, in step S5, if the daily rate of decrease in borehole pressure is detected to exceed 3 MPa / day, the pulse fracturing pump is turned on to replenish the borehole pressure, and then the borehole pressure is turned off again to achieve multiple rounds of cyclic phase change impact and acid erosion.

[0016] Furthermore, in step S5, the pressure holding time for a single borehole is 2 to 4 days.

[0017] Furthermore, the foamer adopts a combined stirring-blowing foaming structure: a rotary motor is installed at the bottom, and stirring blades are installed inside. The liquid CO2 inlet is located at the bottom of the foamer. During foaming, the foaming base liquid is injected first, and then liquid CO2 is introduced. At the same time, the motor is started to drive the fan blades to shear at high speed. Through the dual action of blowing air and mechanical stirring, uniform and stable CO2 foam is generated.

[0018] Furthermore, the pipelines for conveying liquid CO2 and CO2 foam are made of high-pressure resistant, corrosion resistant, and heat-insulating materials, and all pipelines and equipment are connected by flange sealing.

[0019] Furthermore, in step S7, the CO2 foam fracturing fluid separation and recovery device achieves gas-liquid two-phase separation through staged sedimentation, and then separates CO2 and CH4 gas through selective adsorption materials; the recovered CO2 is compressed and liquefied and then fed into the liquid CO2 pump injection device for reuse, while CH4 is recycled as a by-product.

[0020] Compared with the prior art, the present invention has the following advantages: 1. This invention sequentially couples four mechanisms—hydraulic fracturing pre-decompression, pulsating pressure fatigue fracturing, CO2 phase change expansion impact, and autogenous carbonic acid dissolution weakening—to form an integrated process of "pre-decompression - initial fracturing - synergistic fracturing expansion - enhanced permeability and efficiency": hydraulic fracturing creates three-dimensional grooves to generate stress concentration points, lowering the fracturing initiation threshold; pulsating injection activates natural fractures through alternating fatigue effects, forming an initial complex fracture network; during the pressure buildup stage, precise control of the pressure drop rate simultaneously achieves the physical impact of continuous supercritical CO2 phase change expansion and the chemical weakening of slow autogenous carbonic acid dissolution, with multiple mechanisms sequentially and synergistically expanding the fracture scale. Compared to conventional CO2 foam fracturing, this invention can reduce the fracturing initiation pressure by more than 15%, significantly improve the complexity of the fracture network and the volume of fracturing, and in field applications, the initial gas production of the coal seam is increased by 35% compared to conventional hydraulic fracturing.

[0021] 2. This invention uses CO2 foam fracturing fluid with 70%~90% high foam quality, and the water consumption is only 10%~30% of that of conventional hydraulic fracturing, which greatly reduces water consumption and the risk of groundwater pollution. At the same time, it avoids reservoir damage such as clay expansion and water-locking effect caused by water-based fracturing fluid. Moreover, the competitive adsorption of methane by CO2 can effectively displace adsorbed gas and promote gas desorption and seepage.

[0022] 3. This invention uses a CO2 foam fracturing fluid separation and recovery device to perform gas-liquid separation and gas fractionation on the return fluid, thereby achieving CO2 capture, liquefaction and recycling, reducing operating costs; at the same time, some CO2 is geologically preserved through physical adsorption of coal and mineral dissolution and mineralization, combining the dual value of coalbed methane development and carbon emission reduction, thus achieving both economic and environmental benefits. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the overall layout of the system of the present invention.

[0024] Figure 2 This is a schematic diagram of the directional long borehole segmented slotting fracturing process of the present invention.

[0025] Figure 3 This is a schematic diagram of the internal structure of the foamer of the present invention.

[0026] Figure 4 This is a schematic diagram of the CO2 foam fracturing fluid separation and recovery device of the present invention.

[0027] In the diagram: 1-Road of the roadway, 2-Roadway floor, 3-Low-permeability coal seam, 4-Roadway, 5-Foaming base liquid pumping device, 6-Liquid CO2 pumping device, 7-Foamer, 7-1-Foaming base liquid inlet, 7-2-Liquid CO2 inlet, 7-3-Agitator blade, 7-4-Rotary motor, 7-5-Foam outlet, 8-Pulsating fracturing pump, 9-Fracturing pipe, 10-Orifice shut-off valve, 11-Directional long borehole, 12-Pressure monitoring device, 1 3-Slotted fracturing pipeline, 14-Drag packer, 15-Three-dimensional slotted groove, 16-Fracturing network, 17-Flowback valve, 18-Flowback pipeline, 19-CO2 foam fracturing fluid separation and recovery device, 19-1-Stage settling chamber, 19-2-Liquid collection tank, 19-3-Gas recovery pipeline, 19-4-Selective adsorption unit, 19-5-CH4 storage tank, 19-6-CO2 storage tank, 19-7-CO2 circulation pipeline. Detailed Implementation

[0028] The present invention will be further described below.

[0029] like Figure 1 As shown, the fracturing system upon which this invention is based is arranged in the underground roadway 4, which is located between the roadway roof 1 and the roadway floor 2, with the low-permeability coal seam 3 located on one side of the roadway 4. During construction, a directional long borehole 11 is drilled from the roadway 4 into the low-permeability coal seam 3, extending along the coal seam strike or the predetermined modification direction. The foaming base fluid injection device 5, the liquid CO2 injection device 6, the foamer 7, the pulse fracturing pump 8, and the CO2 foam fracturing fluid separation and recovery device 19 are all arranged in the roadway 4 to shorten the length of the high-pressure pipeline and facilitate centralized underground operation and control.

[0030] The foaming base liquid injection device 5 is connected to the foaming base liquid inlet 7-1 of the foamer 7 via a foaming base liquid delivery pipeline. The liquid CO2 injection device 6 is connected to the liquid CO2 inlet 7-2 of the foamer 7 via a high-pressure liquid CO2 delivery pipeline. The foam outlet 7-5 of the foamer 7 is connected to the pulse fracturing pump 8 via a high-pressure CO2 foam delivery pipeline. The pulse fracturing pump 8 is connected to the slotted fracturing pipeline 13 via fracturing pipe 9 and orifice shut-off valve 10. All high-pressure pipelines and equipment interfaces are sealed with flanges to ensure the safety of high-pressure operations. The pressure monitoring device 12 is located near the orifice and connected to the directional long borehole 11 to continuously monitor the in-hole pressure during the injection, pressurization, and backflow stages.

[0031] like Figure 2As shown, the slotted fracturing pipeline 13 is arranged inside the directional long borehole 11, and the towable packer 14 is fitted on the outside of the slotted fracturing pipeline 13. The construction adopts a segment-by-segment dragging method from the distal end to the borehole opening: first, the slotted fracturing pipeline 13 and the towable packer 14 are pushed to the distal target segment of the directional long borehole 11. Upon reaching each target segment, hydraulic slotting is performed using the slotting nozzle at the front end of the slotted fracturing pipeline 13 to form a three-dimensional slotted groove 15. Subsequently, the packer is set and pulsating fracturing is performed on that segment. The periodic alternating pressure generated by pulsating fracturing first causes fatigue damage at the tip of the three-dimensional slotted groove 15, which then promotes the extension of the main fracture and activates surrounding natural fractures to form branch fractures, constituting a fracture network 16. After one fracturing segment is completed, the towable packer 14 is released from its setting state and dragged towards the borehole opening to the next target segment. The slotting, setting, and pulsating fracturing process is repeated until all target segments of the directional long borehole 11 are modified.

[0032] like Figure 3 As shown, the foamer 7 is a closed pressure-bearing shell structure. The foaming base liquid inlet 7-1 is located on the side of the shell, the liquid CO2 inlet 7-2 is located at the bottom of the shell, and the foam outlet 7-5 is located at the top of the shell. The rotary motor 7-4 is installed at the bottom of the foamer 7, and its output shaft passes through the shell and extends into the shell. Multi-stage stirring blades 7-3 are arranged axially on the stirring shaft. During foaming operation, an aqueous solution of surfactant with added foam stabilizer is first injected as the foaming base liquid through the foaming base liquid inlet 7-1, and then liquid CO2 is introduced through the liquid CO2 inlet 7-2. At the same time, the rotary motor 7-4 is started to drive the multi-stage stirring blades 7-3 to shear at high speed, so that the injected CO2 is dispersed into fine and uniform bubbles, forming a highly stable CO2 foam fracturing fluid, which is discharged from the foam outlet 7-5 to the pulse fracturing pump.

[0033] like Figure 4 As shown, the CO2 foam fracturing fluid separation and recovery device 19 mainly includes a staged settling chamber 19-1, a liquid collection tank 19-2, a gas recovery pipeline 19-3, a selective adsorption unit 19-4, a CH4 storage tank 19-5, a CO2 storage tank 19-6, and a CO2 circulation pipeline 19-7. The backflow fluid enters the staged settling chamber 19-1 via the backflow pipeline 18, where defoaming and gas-liquid separation are completed. The liquid phase settles and enters the liquid collection tank 19-2 for centralized treatment; the gas phase enters the selective adsorption unit 19-4 via the gas recovery pipeline 19-3. The selective adsorption unit 19-4 is filled with selective adsorption material to separate and purify CO2 and CH4. The separated CH4 is transported to the CH4 storage tank 19-5 for recycling; the separated CO2 is transported to the CO2 storage tank 19-6, and after compression and liquefaction, it is returned to the liquid CO2 pumping device 6 through the CO2 circulation pipeline 19-7 for reuse in subsequent fracturing sections or the next batch of drilling.

[0034] Based on the above system, the specific construction steps of this invention are as follows: S1. Construction of directional long borehole: Select the target low-permeability coal seam 3, build a fracturing operation platform in roadway 4, and use directional drilling equipment to construct fracturing directional long borehole 11. The borehole extends along the coal seam direction. At the same time, a CO2 foam fracturing fluid separation and recovery device 19 is installed in roadway 4 to complete the pre-connection of pipelines.

[0035] S2. Preparatory work: Push the slotted fracturing string equipped with the draggable packer 14 into the directional long borehole 11, and accurately position it to the first target fracturing section using the draggable device; Connect the ground pipelines according to the fluid flow direction: The foaming base fluid injection device 5 and the liquid CO2 injection device 6 are respectively connected to the corresponding inlets of the foamer 7, and the outlet of the foamer 7 is connected to the pulse fracturing pump 8 through the high-pressure pipeline, and the outlet of the pulse fracturing pump 8 is connected to the slotted fracturing pipeline 13 at the borehole opening through the fracturing pipe 9.

[0036] S3. Sealing test: Conduct a comprehensive inspection of the tightness of pipeline joints, flanges, and elbows. Start the pulse fracturing pump 8 and slowly inject clean water into the borehole for pre-pressurization. Monitor the pressure changes through the pressure monitoring device 12 to verify the sealing performance of the drag packer 14 and the absence of leakage in the pipeline system. After confirming that it is qualified, proceed to the formal fracturing process.

[0037] S4. CO2 Foam Pulsating Fracturing: First, high-pressure water jets from the fracturing pipeline 13 are used to hydraulically cut the target section, forming three-dimensionally distributed slots 15 in the coal seam and creating stress concentration areas. After cutting, the packer 14 is dragged backward to the set position, and water is injected to expand and achieve sealing. Then, the foaming base fluid pumping device 5 and the liquid CO2 pumping device 6 are started, and the injection flow ratio of the two is adjusted so that the foamer 7 outputs CO2 foam fracturing fluid with a foam mass of 70%~90%. In this embodiment, the foaming base fluid is an aqueous solution of anionic surfactant with 0.3% foam stabilizer added. The orifice shut-off valve 10 is opened, the pulsating fracturing pump 8 is started, and the pulsation frequency is set to 0.1~20Hz and the pressure fluctuation amplitude is 2~4MPa. The CO2 foam fracturing fluid is injected into the target coal seam in an alternating pulsating manner. Periodic alternating pressure, through pulse fatigue and water wedge effects, initiates fracturing from the stress concentration point of the three-dimensional slotted groove 15, while simultaneously activating natural fractures in the coal seam, forming an initial complex fracture network 16. During the injection process, after CO2 foam enters the deep borehole, under temperature and pressure conditions exceeding 31.3℃ and 7.38MPa, CO2 undergoes a phase transition to a supercritical state.

[0038] S5. Synergistic Fracturing Expansion through Phase Change Expansion and Acid Dissolution: After a set time of continuous pulsed injection, the pulsed fracturing pump 8 is stopped, and the borehole shut-off valve 10 is closed to allow for pressure buildup. The pressure buildup time for a single stage is controlled to be 2-4 days. During the pressure buildup period, the borehole pressure is monitored in real time through the pressure monitoring device 12, and the borehole pressure is controlled to decrease slowly at a rate of 1-3 MPa / day. As the pressure gradually decreases, supercritical CO2 gradually vaporizes and expands. The continuous impact force generated by the rapid volume expansion acts on the fracture wall and tip, promoting the extension of the main fracture and catalyzing the generation of branch fractures. At the same time, under high pressure, CO2 dissolves in the water in the fracturing fluid to generate carbonic acid. The carbonic acid reacts with carbonate minerals such as calcite and clay minerals such as kaolinite in the coal seam, weakening the mechanical strength of the coal body and expanding the pore and microfracture scale. The synergistic effect of the physical impact of phase change expansion and the chemical weakening effect of carbonic acid dissolution further expands the scale and connectivity of the fracture network 16.

[0039] If the daily rate of decrease in borehole pressure exceeds 3 MPa / day, it indicates that the fracture is expanding too fast and the pressure is decaying abnormally. At this time, the pulse fracturing pump 8 is briefly turned on to inject foam fracturing fluid to restore the borehole pressure to the set range. Then, the borehole shut-off valve 10 is closed again to continue pressurizing, thereby achieving multiple cycles of phase change impact and acid erosion to enhance the fracture expansion effect.

[0040] S6. Directional long borehole segmented fracturing: After completing the fracturing of the current segment, depressurize and release the drag packer 14, drag the slotted fracturing string to the next target layer, and repeat the slotting-pulsating fracture-pressure expansion process of steps S4~S5. The fracturing and modification of all segments of the directional long borehole 11 is completed sequentially from the far end to the borehole opening.

[0041] S7. Fracturing Fluid Flowback and Recovery: After the entire target section is modified, open the orifice shut-off valve 10 and the flowback valve 17, and connect the flowback pipeline 18 to the CO2 foam fracturing fluid separation and recovery device 19. The flowback fluid carrying gas and CO2 is transported to the separation and recovery device via negative pressure extraction. The flowback fluid first enters the staged settling chamber 19-1 for defoaming and stratification. The liquid phase is discharged into the liquid collection tank 19-2 for centralized treatment; the gas phase enters the selective adsorption unit 19-4 via the gas recovery pipeline 19-3, where CH4 and CO2 are separated by molecular sieve adsorption material. The separated CH4 is input into the CH4 storage tank 19-5 for recycling; the separated CO2 is compressed and liquefied by a compressor and then input into the CO2 storage tank 19-6, and then returned to the liquid CO2 pumping device 6 via the CO2 circulation pipeline 19-7, achieving recycling.

[0042] The extraction test of the coal seam after fracturing in this embodiment showed that the initial gas production of the coal seam was 35% higher than that of conventional hydraulically fracturing coal seams in the same block, the fracture initiation pressure was 18% lower than that of conventional CO2 foam fracturing, and the water consumption per hole was only 22% of that of conventional hydraulic fracturing. The transformation effect was significant, and the development benefits and environmental benefits were unified.

[0043] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A synergistic fracturing method for low-permeability coal seams involving fracturing, foaming, pulsating, phase change, and acidizing, comprising a foaming base fluid injection device, a liquid CO2 injection device, a foamer, a pulsating fracturing pump, fracturing pipelines, a drag-type packer, and a CO2 foam fracturing fluid separation and recovery device, characterized in that... Includes the following steps: S1. Construction of directional long boreholes: Select the target deep low-permeability coal seam and construct directional long boreholes underground; S2. Preparatory work: Insert packers and slotted fracturing pipelines into the borehole to complete the segmentation. Connect the foaming base liquid injection device and liquid CO2 injection device to the foamer in the roadway. The outlet of the foamer is connected to the slotted fracturing pipeline via the pulsed fracturing pump. S3. Sealing test: Tests the sealing performance of the fracturing fluid flow line and the packer; S4. CO2 foam pulsed fracturing: hydraulically cuts fractures in the target section to form a three-dimensional fracture groove; foaming base liquid and liquid CO2 are mixed in a foamer to generate stable CO2 foam fracturing fluid; the pulsed fracturing pump is started to inject the CO2 foam fracturing fluid into the coal seam in an alternating pulsed manner, forming an initial complex fracture network through the pulse fatigue effect; and after the CO2 in the CO2 foam fracturing fluid is injected into the directional long borehole, it undergoes a phase change under the action of high temperature and high pressure of the formation to form supercritical CO2. S5. Synergistic expansion of fractures through phase transformation expansion and acid erosion: After continuous injection for a set time, injection is stopped and borehole pressure is shut off, and the borehole pressure is gradually reduced at a rate of 1~3MPa / day; supercritical CO2 gradually gasifies and expands as the pressure decreases, continuously impacting the walls and tips of the fracture network; at the same time, under high pressure, CO2 reacts with water in the fracturing fluid to generate carbonic acid, continuously dissolving carbonates and clay minerals in the coal seam, weakening the mechanical strength of the coal body, and further expanding the fracture network under the synergistic effect of phase transformation expansion force and acid erosion. S6. Directional long borehole segmented fracturing: Repeat steps S4~S5 to complete the fracturing and modification of all segments of the borehole in sequence; S7. Fracturing fluid flowback and recovery: Open the borehole for flowback. The fracturing fluid carrying gas and CO2 enters the CO2 foam fracturing fluid separation and recovery device to complete the gas-liquid two-phase separation. The separated CO2 is compressed and liquefied for recycling.

2. The method according to claim 1, characterized in that, In step S4, by adjusting the injection flow ratio of foaming base liquid to liquid CO2, a CO2 foam fracturing fluid with a foam mass of 70% to 90% is generated; the pulsation frequency range of the pulsation fracturing pump is 0.1 to 20 Hz, and the pressure fluctuation amplitude is 2 to 4 MPa.

3. The method according to claim 1, characterized in that, In step S5, when the daily rate of decrease in borehole pressure exceeds 3 MPa / day, the pulse fracturing pump is turned on to replenish the pressure in the borehole, and then the borehole pressure is turned off again to achieve multiple rounds of cyclic phase change impact and acid erosion.

4. The method according to claim 1, characterized in that, In step S4, the foamer adopts a combined foaming process of stirring and blowing: first, foaming base liquid is injected into the foamer, then liquid CO2 is introduced from the bottom of the foamer, and at the same time, the lower rotating motor is started to drive the internal stirring fan blades to shear at high speed to generate uniform and stable CO2 foam.

5. The method according to claim 1, characterized in that, The pipelines for conveying liquid CO2 and CO2 foam are made of high-pressure resistant, corrosion resistant, and heat-insulating materials; all pipelines and equipment are connected by flange sealing.

6. The method according to claim 1, characterized in that, In step S7, the CO2 foam fracturing fluid separation and recovery device achieves gas-liquid two-phase separation through staged sedimentation, and then separates CO2 and CH4 gas through selective adsorption materials; the recovered CO2 is compressed and liquefied and then fed into the liquid CO2 pump injection device for reuse, while CH4 is recycled as a by-product.

7. The method according to claim 1, characterized in that, In step S5, the pressure holding time for a single borehole is 2 to 4 days.

8. The method according to claim 1, characterized in that, The foaming base liquid is an aqueous solution of a surfactant with added foam stabilizer.

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