Coal seam permeability enhancement system and method based on zeolite-assisted carbon dioxide-enhanced cavitation

Abstract

The application discloses a coal seam permeability enhancement system and method based on zeolite-assisted carbon dioxide intensification cavitation, and relates to the technical field of gas extraction. The method first segments and blocks the drilling hole through a packer, and under the premise of hydraulic fracturing, combines high-speed rotation of an impeller, heating and temperature rise, and cooperation of zeolite powder porous gas release foaming and carbon dioxide medium, breaks the micro-pore structure of the coal body by means of high-energy shock waves generated by bubble collapse, breaks the fracturing water lock effect and penetrates the coal seam fracture network, and greatly improves the overall permeability of the coal seam. Secondly, after injecting high-pressure carbon dioxide gas into the coal rock mass, the free gas and water in the pore and fracture are gradually displaced by the pressure pushing, adsorption displacement and seepage migration effect. The technical problems of limited improvement of coal seam permeability, low concentration of extracted gas methane, and inhibition of gas desorption by water lock effect after hydraulic fracturing are solved.

Description

Technical Field

[0001] This invention relates to the field of gas extraction technology, specifically to a zeolite-assisted carbon dioxide-enhanced coal seam permeability improvement system and method. Background Technology

[0002] Coal seams in my country are generally characterized by high gas content, poor permeability, and difficulty in extraction. Currently, direct negative pressure extraction via underground boreholes is the mainstream technology for controlling coal seam gas. However, due to limitations in its technical principles, this conventional extraction process has many shortcomings in field applications and is difficult to adapt to the gas control needs of complex coal seams. These problems mainly manifest in three aspects: First, the permeability enhancement effect is limited, and there are blind spots in the treatment. Conventional hydraulic fracturing relies solely on the static pressure of high-pressure water to break up coal seams, resulting in a small number of fractures with a single extension direction, which cannot effectively penetrate the microscopic pore system deep within the coal seam. At the same time, the permeability enhancement effect of fracturing is significantly constrained by in-situ stress, and large-scale "fracturing blank zones" are easily formed inside the coal seam, resulting in incomplete overall permeability enhancement and difficulty in fundamentally improving the gas permeability conditions of the coal seam.

[0003] Secondly, without enhanced gas desorption and displacement, extraction efficiency is low and resource consumption is high. Conventional fracturing fluids only have the function of creating fractures and cannot actively promote the desorption and migration of coal seam gas. The entire gas extraction process relies entirely on the natural decay of the coal seam pressure. In the early stage of extraction, the gas concentration can be maintained at a normal level, but in the later stage, the concentration will drop rapidly to below 10%. A large amount of low-concentration gas cannot be effectively utilized and can only be directly discharged into the air, which not only wastes clean energy but also exacerbates greenhouse gas emissions, resulting in poor ecological and economic benefits.

[0004] Third, water accumulation during fracturing can cause waterlock damage, blocking gas migration channels. The large amount of high-pressure water injected during fracturing operations will remain within the coal seam fractures for a long time after construction, forming a dense water film at the micropore throats of the coal seam, directly blocking the channels for gas desorption, seepage, and migration. This problem can lead to a "water-free" coal seam, significantly reducing the gas extraction concentration, and in severe cases, causing gas extraction to be interrupted, resulting in extraction failure. Summary of the Invention

[0005] This invention overcomes the shortcomings of existing technologies by proposing a carbon dioxide-enhanced coal seam permeability enhancement system and method based on zeolite-assisted permeability enhancement, which solves the technical problems of limited improvement in coal seam permeability, low methane concentration in extracted gas, and water-locking effect inhibiting gas desorption after hydraulic fracturing in existing coal seam permeability enhancement and gas extraction.

[0006] This invention is achieved through the following technical solution: The zeolite-assisted carbon dioxide-enhanced coal seam permeability improvement system includes a fracturing-cavitation composite pipeline. Packers are installed at intervals on the upper and lower sides of the pipeline's outer wall. These two sets of packers work together to form a sealed cavitation working cavity inside the borehole. Two independent, mutually isolated, and non-competitive media channels are opened within the fracturing-cavitation composite pipeline: a solid-liquid two-phase media injection channel and a carbon dioxide gas-phase displacement media injection channel. Fracturing fluid is delivered to the cavitation working cavity through the solid-liquid two-phase media injection channel, and high-pressure carbon dioxide gas is delivered to the cavitation working cavity through the carbon dioxide gas-phase displacement media injection channel. A temperature-controlled heating component is fixedly installed on the fracturing-cavitation composite pipeline, and a swirling disturbance component is coaxially mounted on the outer wall of the pipeline. Both the temperature-controlled heating component and the swirling disturbance component are located between the two packers. The temperature-controlled heating component heats the fracturing fluid within the cavitation working cavity, while the swirling disturbance component includes a rotatable impeller that disturbs the fracturing fluid to generate cavitation.

[0007] Furthermore, the impeller is provided with a porous zeolite functional coating.

[0008] Furthermore, the fracturing fluid is mixed with zeolite powder.

[0009] Furthermore, the zeolite powder has a mesh size of 300–325, an equivalent particle size of 43–74 μm, and a powder addition concentration of 5–12 g / L.

[0010] Furthermore, a solid-liquid phase booster pump is connected to the solid-liquid phase injection channel, and a carbon dioxide gas phase displacement pump is connected to the carbon dioxide gas phase booster pump in the carbon dioxide gas phase displacement channel.

[0011] The method for enhancing the permeability of coal seams by carbon dioxide-enhanced cavitation based on zeolite employs the aforementioned permeability enhancement system and includes the following steps: Step 1: Lower the fracturing-cavitation composite pipeline into the borehole, control the expansion and sealing of the two sets of packers, and form a cavitation operation cavity inside the borehole. Step 2: Inject fracturing fluid into the cavitation operation cavity through the solid-liquid two-phase medium injection channel, and introduce high-pressure carbon dioxide gas into the cavitation operation cavity through the carbon dioxide gas phase displacement medium injection channel. Step 3: Simultaneously activate the temperature-controlled heating component and the swirling disturbance component. The temperature-controlled heating component increases the temperature of the fracturing fluid and reduces the difficulty of bubble formation. The swirling disturbance component rotates at high speed to create a low-pressure cavitation zone, generating a large number of cavitation bubbles. The collapse of the cavitation bubbles generates shock waves that impact the coal body, forming dense cavitation micro-fractures in the coal seam, connecting the original coal seam fractures and eliminating the water-locking effect. This allows the cavitation fractures to fully develop, extend, and connect, forming a three-dimensional interconnected fracture network.

[0012] Furthermore, the fracturing fluid is heated and stabilized at 65–70°C using a temperature-controlled heating component. After heating, the dynamic viscosity of the fracturing fluid decreases to 0.45–0.55 MPa·s, and the saturated vapor pressure of the liquid phase increases to over 25 kPa.

[0013] Furthermore, the impeller in the swirling disturbance component rotates at 3000–6000 rpm, and the high-speed swirling shearing action generated by the impeller creates a local negative pressure zone in the cavitation operation cavity.

[0014] Furthermore, after the permeability enhancement is completed, the lowering depth of the fracturing-cavitation composite pipeline is adjusted, and steps one to three are repeated to complete the permeability enhancement operation of the entire coal seam section of the borehole.

[0015] The beneficial effects of this invention compared to the prior art are as follows: 1. This invention injects high-pressure water mixed with zeolite powder into the borehole using a solid-liquid phase booster pump. The porous structure of the zeolite continuously generates auxiliary bubbles, which, combined with the cavitation effect, breaks up the microscopic cracks in the coal seam. This effectively eliminates the water-locking effect left after traditional hydraulic fracturing, opens up the channels for gas desorption and migration inside the coal seam, fundamentally improves the problem of low permeability in coal seams, and significantly enhances the overall efficiency of coal seam gas extraction.

[0016] 2. This invention employs a temperature-controlled heating component, a swirling disturbance component, and porous zeolite powder in a synergistic coupling. By creating a low-pressure cavitation zone through swirling disturbance, reducing the resistance to bubble formation by raising the temperature, and supplementing and stabilizing the gas nuclei through the porous structure of zeolite, the energy of the shock wave generated by the collapse of cavitation bubbles is enhanced through multiple pathways. This allows for the connection of micro-fractures in the coal seam without the need for ultra-high water pressure, reducing the load on the underground high-pressure water supply equipment, resulting in higher energy utilization efficiency and a more superior coal seam permeability enhancement effect.

[0017] 3. This invention sets up two independent conveying power systems: a solid-liquid phase booster pump and a carbon dioxide gas phase booster displacement pump, along with two independent media injection channels, to achieve separate conveying of high-pressure zeolite-containing water and carbon dioxide gas, and in-situ coupling reaction within the cavity. Carbon dioxide can increase the cavitation initiation pressure and generate trace amounts of carbonic acid, slightly reducing the surface tension of the water, further enhancing the cavitation effect. At the same time, it replaces adsorbed methane on the coal surface, reducing the mixing of ineffective impurities such as air into the extraction pipeline, increasing the concentration of extracted gas, and reducing greenhouse gas pollution caused by low-concentration gas emissions.

[0018] 4. The cavitation bubbles generated by this invention can penetrate deep into the coal body through the fracturing fractures. When the bubbles collapse in the pores and micro-fractures of the coal body, they can directly impact the pore walls and disturb the pore structure, effectively promoting the desorption and release of adsorbed methane in the deep part of the coal body, and further improving the efficiency of coal seam gas extraction. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the system operation of the present invention; Figure 2 This is a schematic diagram of the system structure of the present invention; Figure 3 This is a schematic diagram of the packer of the system of the present invention in the state of not bulging; Figure 4 This is a schematic diagram illustrating the process of generating cavitation bubbles according to the present invention. Figure 5 This is a schematic diagram of the structure of the impeller and the porous zeolite functional coating of the present invention; In the diagram, 1. Coal seam, 2. Borehole, 3. Packer, 4. Temperature control heating component, 5. Solid-liquid two-phase medium injection channel, 6. Swirl disturbance component, 7. Carbon dioxide gas phase displacement medium injection channel, 8. Fracturing-cavitation composite pipeline, 9. Cavitation bubbles, 10. Coal seam fracture, 11. Solid-liquid phase booster pump, 12. Carbon dioxide gas phase booster pump, 13. Porous zeolite functional coating. Detailed Implementation

[0020] To make the technical problems to be solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail with reference to the embodiments and accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. The technical solutions of this invention are described in detail below with reference to the embodiments and accompanying drawings, but the scope of protection is not limited thereto. Example 1

[0021] Please see Figures 1 to 3 This embodiment proposes a zeolite-assisted carbon dioxide-enhanced coal seam permeability improvement system, including a fracturing-cavitation composite pipeline 8. The fracturing-cavitation composite pipeline 8 is the core load-bearing component of this system, coaxially lowered into the borehole 2 of the coal seam. The overall external structure of the fracturing-cavitation composite pipeline 8 is adapted to the inner diameter of the borehole 2, ensuring smooth vertical movement and segmented operation. The pipe wall of the fracturing-cavitation composite pipeline 8 adopts a double-layer isolated integral molding structure, with two completely independent, sealed, and non-compatible axially connected media channels inside: a solid-liquid two-phase media injection channel 5 and a carbon dioxide gas phase displacement media injection channel 7. The two channels are symmetrically arranged circumferentially along the pipe wall of the fracturing-cavitation composite pipeline 8, completely isolated by the solid pipe wall, with independent openings at the ends and no media exchange paths. This allows for the synchronous and independent delivery of solid-liquid suspended media and high-pressure gas phase media, avoiding pipeline blockage and pressure loss caused by premature media mixing.

[0022] Two sets of pressure-maintaining capsule-type packers 3 are fixedly installed axially and vertically on the outer wall of the fracturing-cavitation composite pipeline 8. The two sets of packers 3 adopt a symmetrical layout, with the spacing being the standard coal seam single-section permeability enhancement operation length, which can be flexibly adjusted according to the depth of the borehole 2. The packers 3 are rigidly fixed to the outer wall of the fracturing-cavitation composite pipeline 8 by high-strength clamps, sealing and fitting the outer wall surface of the fracturing-cavitation composite pipeline 8. After inflation, they can completely fit the borehole wall of the borehole 2. The upper and lower sets of packers 3 cooperate with each other to precisely isolate and form a closed and pressure-stabilized cavitation operation cavity inside the borehole 2 and between the two sets of packers 3, completely isolating the upper and lower spaces of the cavity, preventing pressure leakage and media loss, and ensuring the stability of the cavitation operation pressure.

[0023] Multiple temperature-controlled heating components 4 are fixedly embedded in the outer wall of the fracturing-cavitation composite pipeline 8, and these components are located within the working area between two packers 3. A wrap-around installation method is used, with the inner side of the temperature-controlled heating components 4 tightly fitted to the pipe wall of the fracturing-cavitation composite pipeline 8 without gaps. This allows for uniform heating of the fracturing fluid medium within the cavitation working cavity through both heat conduction from the pipe wall and heat radiation within the cavity. The temperature-controlled heating components 4 are entirely waterproof, pressure-resistant, and explosion-proof, suitable for humid and high-pressure underground working environments. Their installation position avoids the active area of ​​the packers 3 and the working areas of other components, preventing structural interference. In this embodiment, the temperature-controlled heating components 4 use a VCFB-DJR-ExdI mining explosion-proof encased heating sleeve.

[0024] Multiple sets of swirling disturbance components 6 are coaxially installed on the outer wall of the fracturing-cavitation composite pipeline 8. These components are concentric with the pipeline 8 and evenly distributed within the corresponding pipeline sections of the cavitation operation cavity, serving as the core components for fluid disturbance within the cavity. Each swirling disturbance component 6 includes an impeller and a motor. The impeller is connected to the motor's output shaft via a rotating shaft, allowing it to rotate at high speed under the motor's drive. The fracturing-cavitation composite pipeline 8 has a built-in fixed support, on which the motor is mounted. Sealed bearings at both ends of the rotating shaft ensure rotational stability and sealing, preventing media from seeping into the transmission structure. Furthermore, the entire impeller surface is covered with a porous zeolite functional coating 13. This coating 13 is bonded to the impeller substrate through a high-temperature sintering process, allowing it to rotate synchronously with the impeller at high speed, continuously contacting the fluid within the cavity to achieve the function of micro-peeling of fine zeolite powder and continuous replenishment of gas nuclei.

[0025] The solid-liquid two-phase medium injection channel 5 of the fracturing-cavitation composite pipeline 8 is externally connected to a solid-liquid phase booster pump 11. The channel interface uses a high-pressure sealed quick-connect joint to achieve a closed-loop pipeline connection. The solid-liquid phase booster pump 11 stably delivers high-pressure zeolite suspension fracturing fluid to the solid-liquid two-phase medium injection channel 5. The carbon dioxide gas phase displacement medium injection channel 7 is externally connected to a carbon dioxide gas phase booster pump 12, which also uses a high-pressure sealed explosion-proof joint for independent connection and is completely isolated from the solid-liquid delivery pipeline. Both pumps are independent power sources for the system, corresponding to two sets of medium delivery channels respectively. The delivery pressure and flow rate can be independently adjusted to achieve precise mixing and synchronous supply of gas-liquid two-phase media.

[0026] Specifically, in this embodiment, the operating speed of the swirling disturbance component 6 is controlled at 3000-6000 rpm, preferably 4500 rpm, and the impeller outer edge linear velocity is 15-30 m / s. When the swirling disturbance component 6 rotates, the high-speed swirling shearing effect generated by the impeller can create a local negative pressure zone in the closed cavitation operation cavity. The lowest static pressure in the region can reach 0.8 kPa, which is far lower than the saturated vapor pressure of water at the same temperature, effectively inducing the water body to generate primary cavitation bubbles. The porous zeolite functional coating 13 on the surface of the swirling disturbance component 6 is formed by high-temperature sintering of 4A synthetic zeolite, with a coating thickness of 3.5 mm, a porosity of 52.3%, and an aggregate particle size of 0.2-0.5 mm. The high-speed fluid continuously washes the porous zeolite functional coating 13, constantly peeling off trace amounts of zeolite fine powder to replenish the liquid phase gas nuclei.

[0027] The temperature-controlled heating component 4 has a rated heating power of 3 to 6 kW, preferably 4.5 kW, which can steadily raise the temperature of the zeolite suspension fracturing fluid in the cavitation operation cavity from the ambient temperature (22 to 28 ℃) of the coal seam to 65 to 70 ℃. After the temperature rises, the dynamic viscosity of the liquid decreases to 0.45 to 0.55 MPa·s, and the saturated vapor pressure of the liquid phase increases to more than 25 kPa, further reducing the bubble nucleation threshold and increasing the amount of bubble generation.

[0028] The solid-liquid phase booster pump 11 delivers pressure of 20-45 MPa, preferably 28 MPa; the zeolite suspension fracturing fluid uses 300-325 mesh 4A type zeolite powder with an equivalent particle size of 43-74 μm and a powder addition concentration of 5-12 g / L. The ultrafine zeolite powder is uniformly suspended in the conventional fracturing fluid and continuously precipitates micro-gas nuclei based on its own porous cavity.

[0029] The gas supply pressure of the carbon dioxide gas phase booster displacement pump 12 is 25-50 MPa, preferably 32 MPa. The gas phase injection pressure is moderately higher than the liquid phase injection pressure, which facilitates the slight dissolution of CO2 into the fracturing fluid to generate weak carbonic acid, thereby weakening the mineral strength of the coal body. At the same time, it relies on the pressure difference to seep into the deep pores of the coal body, replacing the adsorbed methane and driving out the water trapped in the fractures.

[0030] The working principle of the zeolite-assisted carbon dioxide-enhanced coal seam permeability improvement system is as follows: When the swirling disturbance component 6 rotates at high speed, the impeller drives the zeolite suspension fracturing fluid in the cavity to move at high speed. Through strong shearing and swirling action, a local negative pressure zone is created around the impeller, which can reduce the minimum static pressure in the area to 0.8 kPa, far below the saturated vapor pressure of water at room temperature. This forces the water to generate a large number of primary cavitation bubbles, overcoming the problem of difficult bubble nucleation under conventional water pressure conditions. At the same time, the high-speed swirling can disturb the uniform mixing of the medium in the cavity, avoid the sedimentation and accumulation of zeolite powder, and ensure the uniform and stable performance of the medium.

[0031] The porous zeolite functional coating 13 on the surface of the swirling disturbance component 6 and the ultrafine zeolite powder in the fracturing fluid have abundant micro-porous cavity structures. Under the continuous scouring of high-speed fluid and the friction of impeller rotation, the porous zeolite functional coating 13 continuously peels off trace amounts of fine zeolite powder to replenish liquid phase gas nuclei. The ultrafine zeolite powder suspended in the fracturing fluid can continuously precipitate micro gas nuclei based on its own pores, which greatly increases the nucleation density of liquid phase bubbles and solves the problems of insufficient gas nuclei and weak cavitation effect in traditional pure water media.

[0032] The temperature-controlled heating component precisely raises the temperature of the fracturing fluid inside the cavity, stabilizing it from the formation's ambient temperature of 22–28°C to 65–70°C. After the temperature rise, the dynamic viscosity of the liquid phase decreases to 0.45–0.55 MPa·s, significantly improving fluid flowability. At the same time, the saturated vapor pressure of the liquid phase rises to over 25 kPa, significantly reducing the bubble nucleation threshold and allowing more micro-nuclei to rapidly develop into effective cavitation bubbles. This multiplies the total amount of bubbles generated and their development efficiency, thus enhancing the intensity of cavitation.

[0033] High-pressure carbon dioxide gas is injected into the cavitation cavity at a pressure slightly higher than that of the liquid phase. On the one hand, a small amount dissolves into the fracturing fluid to generate weak carbonic acid, which gently dissolves clay, carbonates and other mineral impurities inside the coal body, weakens the microstructure strength of the coal body and assists in the expansion of fractures. On the other hand, relying on the pressure difference advantage, it seeps into the pores and deep fractures of the coal body, precisely replacing the adsorbed methane on the surface of the coal body, while driving out the water trapped inside the fractures, breaking the water-locking effect left by hydraulic fracturing from the root and opening up the gas seepage channel.

[0034] In summary, this system achieves multi-mechanism synergistic cavitation rock breaking. The instantaneous collapse of bubbles generates impact pressure and strong shear force in the micro-region, repeatedly impacting and scouring the pore walls and fracture surfaces at high frequency. This directly strips away coal and rock particles, fracturing primary micro-fractures, connecting isolated pore networks, and expanding fracture aperture, achieving active fracturing and directional modification of the coal and rock mass's microstructure, significantly improving the overall permeability of the coal seam. Simultaneously, temperature, carbon dioxide, and zeolite synergistically enhance cavitation: increased temperature lowers water viscosity and saturated vapor pressure, reducing the bubble formation threshold and increasing the nucleation rate and quantity; carbon dioxide increases the cavitation initiation pressure and, through dissolution, forms trace amounts of carbonic acid, slightly reducing the surface tension of water, further promoting bubble nucleation and development; carbon dioxide reduces water surface tension and promotes bubble nucleation, while simultaneously softening coal and rock minerals through weak corrosion of carbonic acid, weakening fracture structure strength, and reducing fracturing energy consumption; the porous structure of zeolite continuously precipitates stable micro-gas nuclei under disturbance and increased temperature, significantly increasing bubble density, collapse frequency, and impact intensity. The coupling of these three factors amplifies the cavitation destructive effect. Carbonic acid chemically dissolves coal and petrified minerals, which, combined with the mechanical erosion caused by bubble collapse, creates a thermo-chemical coupled cavitation effect, continuously expanding and connecting fracture channels, ultimately achieving efficient permeability enhancement of the coal seam. Example 2

[0035] Please see Figures 1 to 5 This embodiment proposes a zeolite-assisted carbon dioxide-enhanced coal seam permeability enhancement method, based on the permeability enhancement system described in Embodiment 1, and includes the following steps: Step 1: Before operation, clean and inspect the borehole 2 inside coal seam 1 to ensure it is unobstructed, free from collapse, and free from water accumulation. The assembled and debugged permeability enhancement system is then smoothly lowered into the pre-set operating depth inside borehole 2 using hoisting equipment, ensuring the fracturing-cavitation composite pipeline 8 is coaxial, centered, and without deviation. Once in place, the packer pressurization system is activated, controlling the synchronous and uniform expansion of the upper and lower sets of pressure-holding capsule packers 3. This ensures the outer wall of the packer 3 tightly adheres to the borehole wall of borehole 2, creating a completely sealed and pressure-stabilized cavitation operating cavity between the two sets of packers 3. The sealing performance of the cavitation operating cavity is then checked to ensure there is no pressure leakage or media cross-flow.

[0036] Step Two: After the sealing test is passed, the solid-liquid phase booster pump 11 and the carbon dioxide gas phase booster displacement pump 12 are started simultaneously to carry out media transportation operations according to the preset pressure and flow parameters. The solid-liquid phase booster pump 11 continuously delivers the prepared zeolite suspension fracturing fluid into the cavitation operation cavity through the solid-liquid two-phase media injection channel 5. The ultrafine zeolite powder is uniformly suspended in the liquid phase, initially replenishing the micro-gas nuclei in the liquid phase. The carbon dioxide gas phase booster displacement pump 12 introduces high-pressure carbon dioxide gas into the cavitation operation cavity through the carbon dioxide gas phase displacement media injection channel 7. The high-pressure carbon dioxide gas is moderately integrated into the zeolite suspension fracturing fluid, reducing the surface tension of the water and generating trace amounts of weak carbonic acid. At the same time, it initially replaces the adsorbed methane in the coal seam pores and fractures, pre-breaking the shallow water lock blockage. The two sets of media are transported independently and do not interfere with each other, achieving synergistic effect through pressure matching.

[0037] Step 3: After the medium is stably injected for 1-2 minutes and the cavity is filled with medium, the temperature-controlled heating component 4 and the swirling disturbance component 6 are simultaneously activated. The temperature-controlled heating component 4 continuously and precisely heats the fracturing fluid in the cavity, maintaining the medium temperature stably at 65-70℃, reducing the liquid phase viscosity, increasing the saturated vapor pressure, and significantly reducing the difficulty of bubble nucleation. The swirling disturbance component 6 rotates at a high speed of 4500 rpm, creating a large-scale local negative pressure zone in the cavity, inducing the water to generate a large number of primary cavitation bubbles. At the same time, the swirling scouring causes the porous zeolite functional coating 13 to peel off fine powder, and the liquid phase zeolite powder to continuously release gas. The superposition of multiple gas nuclei sources generates a large number of cavitation bubbles 9. A large number of cavitation bubbles 9 repeatedly expand and rapidly collapse in a closed high-pressure environment, generating high-intensity shock waves and micro-jet streams, which continuously impact the coal body around the borehole 2, breaking the dense structure of the coal body, generating dense cavitation micro-fractures, penetrating the original coal seam fractures 10, and initially eliminating the water-locking channels remaining from hydraulic fracturing.

[0038] Step 4: Maintain the continuous and stable operation of the solid-liquid phase booster pump 11, the carbon dioxide gas phase booster displacement pump 12, the temperature control heating component 4, and the swirling disturbance component 6, keeping the pressure, temperature, and swirling velocity within the cavity within the optimal operating parameter range, and continuously carry out cavitation fracture creation, carbon dioxide dissolution modification, and gas replacement water displacement operations. Through steady-state operation for a preset duration, allow the cavitation fractures to fully develop, extend, and connect, forming a three-dimensional interconnected fracture network. At the same time, thoroughly remove the water trapped deep within the fractures, weaken the coal body structural strength, and comprehensively improve the permeability of the coal seam surrounding the borehole.

[0039] Step 5: After the preset operating time is reached, following the order of "liquid phase first, then gas phase," sequentially shut down the solid-liquid phase booster pump 11 and the carbon dioxide gas phase booster displacement pump 12 to stop the external injection of gaseous and liquid media and prevent backflow of residual pressure in the pipeline. After the pressure of the medium in the cavity naturally and steadily drops to a stable value, sequentially shut down the swirling disturbance component 6 and the temperature control heating component 4 to terminate the cavity disturbance and heating operation.

[0040] Step Six: After the equipment is completely shut down, control the packer 3 to slowly depressurize and contract, releasing the seal on borehole 2. Ensure a smooth and shock-free depressurization process to prevent secondary collapse of the coal seam in the borehole. Subsequently, through a combination of reverse venting via the pipeline medium channel and natural drainage from borehole 2, thoroughly remove any remaining liquid, residual gas, coal dust detached from fissures, accumulated water, and other impurities from the cavity, ensuring the cleanliness of borehole 2 and preparing for the next stage of operation.

[0041] Step 7: After the single-section operation is completed, adjust the lowering depth of the fracturing-cavitation composite pipeline 8 as a whole, and move it to the next unoperated section according to the length of the coal seam borehole segment. Repeat all the operation processes from Step 1 to Step 6, and complete the cavitation permeability enhancement transformation of the entire depth of borehole 2 segment by segment, so as to finally achieve the overall improvement of the permeability of the whole borehole and the entire section of the coal seam.

[0042] To verify the actual permeability enhancement effect of the technical solution of this invention, a field comparative test was conducted on four adjacent boreholes with consistent geological conditions in the same working face. A raw coal blank group, a conventional pure water fracturing control group (control group 1), a pure water and CO2 fracturing control group (control group 2), and the process test group of Example 2 were set up. After 7 days of settling following construction, gas extraction and coal body physical parameters were monitored for 30 consecutive days. The test results show that, compared with raw coal and two traditional permeability enhancement processes, this invention, relying on the synergistic effects of zeolite gas release, CO2 chemical dissolution, temperature increase and viscosity reduction, and high-speed vortex cavitation, can increase coal seam permeability by up to 32.92 times. It can significantly eliminate the water-locking effect caused by fracturing, and achieve a leapfrog improvement in effective permeability radius, gas extraction concentration, and daily gas production index, fully demonstrating the advanced nature and practicality of the permeability enhancement technology of this invention. The test results of different groups in the comparative experiment are shown in Table 1: Table 1 Results of the experiment in different groups

[0043] The results show that the zeolite-assisted carbon dioxide-enhanced coal seam permeability improvement device and method described in this invention can effectively improve the permeability of coal seams. Compared with traditional hydraulic fracturing and other technologies, the method of this invention has a significant effect, which not only greatly improves the permeability of coal seams, but also effectively alleviates the water-locking effect caused by hydraulic fracturing.

[0044] Regarding the specific structure of this invention, it should be noted that the connection relationships between the various component modules used in this invention are definite and achievable. Except as specifically described in the embodiments, their specific connection relationships can bring about corresponding technical effects and solve the technical problems proposed by this invention without relying on the execution of corresponding software programs. The models of the components, modules, and specific components appearing in this invention, the connection methods between them, and the conventional usage methods and expected technical effects brought about by the above technical features, unless specifically described, are all publicly disclosed content in patents, journal articles, technical manuals, technical dictionaries, and textbooks that can be obtained by those skilled in the art before the application date, or belong to conventional technology, common knowledge, and other existing technologies in this field. There is no need to elaborate, which makes the technical solution provided in this case clear, complete, and achievable, and can reproduce or obtain corresponding physical products based on this technical means.

[0045] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A zeolite-assisted carbon dioxide-enhanced coal seam permeability improvement system, characterized in that, The system includes a fracturing-cavitation composite pipeline (8), with packers (3) installed at intervals on the upper and lower outer walls of the pipeline (8). The two sets of packers (3) work together to form a closed cavitation operation cavity inside the borehole (2). Two independent media channels, which are isolated from each other and do not cross pressure, are opened inside the fracturing-cavitation composite pipeline (8), namely a solid-liquid two-phase media injection channel (5) and a carbon dioxide gas phase displacement media injection channel (7). Fracturing fluid is delivered to the cavitation operation cavity through the solid-liquid two-phase media injection channel (5) and carbon dioxide gas phase displacement media injection channel (7). The gas phase displacement medium injection channel (7) delivers high-pressure carbon dioxide gas to the cavitation operation cavity; a temperature control heating component (4) is fixedly installed on the fracturing-cavitation composite pipeline (8), and a swirling disturbance component (6) is coaxially assembled on the outer wall of the fracturing-cavitation composite pipeline (8). The temperature control heating component (4) and the swirling disturbance component (6) are both located between two packers (3). The temperature control heating component (4) is used to heat the fracturing fluid in the cavitation operation cavity. The swirling disturbance component (6) includes a rotatable impeller, which disturbs the fracturing fluid to generate cavitation.

2. The zeolite-assisted carbon dioxide-enhanced coal seam permeability improvement system according to claim 1, characterized in that, The impeller is provided with a porous zeolite functional coating (13).

3. The zeolite-assisted carbon dioxide-enhanced coal seam permeability improvement system according to claim 1, characterized in that, The fracturing fluid is mixed with zeolite powder.

4. The zeolite-assisted carbon dioxide-enhanced coal seam permeability improvement system based on claim 3, characterized in that, The zeolite powder has a mesh size of 300–325, an equivalent particle size of 43–74 μm, and a powder addition concentration of 5–12 g / L.

5. The zeolite-assisted carbon dioxide-enhanced coal seam permeability improvement system according to claim 1, characterized in that, The solid-liquid two-phase medium injection channel (5) is connected to an external solid-liquid phase booster pump (11), and the carbon dioxide gas phase displacement medium injection channel (7) is connected to an external carbon dioxide gas phase booster pump (12).

6. A method for enhancing the permeability of coal seams through carbon dioxide-enhanced cavitation based on zeolite, characterized in that, The antireflective system described in any one of claims 1-5 is employed, and includes the following steps: Step 1: Lower the fracturing-cavitation composite pipeline (8) into the borehole (2), control the expansion and sealing of the two packers (3), and form a cavitation operation cavity inside the borehole (2); Step 2: Inject fracturing fluid into the cavitation operation cavity through the solid-liquid two-phase medium injection channel (5), and introduce high-pressure carbon dioxide gas into the cavitation operation cavity through the carbon dioxide gas phase displacement medium injection channel (7). Step 3: Simultaneously turn on the temperature control heating component (4) and the swirling disturbance component (6). The temperature control heating component (4) increases the temperature of the fracturing fluid and reduces the difficulty of bubble generation. The swirling disturbance component (6) rotates at high speed to create a low-pressure cavitation zone and generate a large number of cavitation bubbles (9). The cavitation bubbles (9) collapse and generate shock waves that impact the coal body, forming dense cavitation micro-fractures in the coal seam, connecting the original coal seam fractures (10) and eliminating the water-locking effect. This allows the cavitation fractures to fully develop, extend, and connect, forming a three-dimensional interconnected fracture network.

7. The method for enhancing coal seam permeability based on zeolite-assisted carbon dioxide-enhanced cavitation according to claim 6, characterized in that, The fracturing fluid is heated and stabilized at 65-70°C by the temperature control heating component (4). After the temperature rises, the dynamic viscosity of the fracturing fluid drops to 0.45-0.55 MPa·s, and the liquid phase saturated vapor pressure rises to more than 25 kPa.

8. The method for enhancing coal seam permeability based on zeolite-assisted carbon dioxide-enhanced cavitation according to claim 6, characterized in that, The impeller in the swirling disturbance component (6) rotates at 3000 to 6000 rpm. The high-speed swirling shearing action generated by the impeller creates a local negative pressure zone in the cavitation operation cavity.

9. The method for enhancing coal seam permeability based on zeolite-assisted carbon dioxide-enhanced cavitation according to claim 6, characterized in that, After the permeability enhancement is completed, adjust the lowering depth of the fracturing-cavitation composite pipeline (8), and repeat steps one to three to complete the permeability enhancement operation of the entire coal seam of the borehole (2).

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