Combined storage and improvement method of sand-carrying oxidant pulse injection and static fracturing in hard coal seams

Abstract

The present invention discloses a combined storage and improvement method of sand-carrying oxidant pulse injection-static fracturing for hard coal seams. The method comprises the following steps: first, a sand-carrying oxidant liquid is pulsed injected into the coal seam, and the oxidant liquid impacts the coal body under the water wedge degradation effect of the pulse wave. Simultaneously, the oxidant liquid contacts the coal body during the impact process to carry out an oxidation reaction, so that the coal body forms a fracturing weak surface and generates microcracks. Then, small quartz sand particles and the oxidant liquid are squeezed into the microcracks to expand and extend the microcracks, while increasing the contact area between the oxidant liquid and the coal body, thereby enhancing the oxidation effect and providing more fracturing weak surfaces for subsequent static fracturing. Subsequently, the sand-carrying fracturing fluid is injected into the coal seam for static fracturing, so that the microcracks in the fracturing weak surface converge, develop and expand longitudinally, and large quartz sand particles are squeezed into the expanded cracks to further expand and extend the hole. By alternately performing the oxidant pulse injection and static fracturing processes, a complex fracture network is continuously formed inside the coal seam, and finally in-situ oxidation-static fracturing progressive permeability enhancement of the coal seam is completed.

Description

Technical Field

[0001] The invention relates to a combined storage and improvement method of sand-carrying oxidant pulse injection and static fracturing in hard coal seams, belonging to the technical field of coal seam permeability enhancement and gas extraction. Background Art

[0002] my country boasts abundant coalbed methane (CBM) resources. Coal reservoirs are complex, with permeabilities generally less than 0.1 mD, making them dense. Due to the low porosity, low permeability, and poor stability of coal, as well as reservoir damage caused by fracturing, extraction from most coal seams is difficult, severely impacting gas extraction and utilization. Therefore, efficient permeability enhancement and drainage in coal seams is key to reducing gas hazards, ensuring safe coal mine production, and effectively utilizing CBM resources.

[0003] Currently, hydraulic fracturing is the primary method for increasing the production of tight coal reservoirs. Hydraulic fracturing and pulsed hydraulic fracturing have been applied in many domestic mines with promising results. However, hydraulic fracturing has the disadvantages of single, regionally focused fractures in hard coal seams, making it difficult to form a complex fracture network. While pulsed hydraulic fracturing can improve these shortcomings, and the water wedge degradation effect of pulsed waves can increase the number of coal cracks to a certain extent, it still fails to achieve ideal fracturing results in many mines. Furthermore, after fracturing, some cracks undergo varying degrees of closure under the influence of in-situ stress, reducing the effectiveness of coalbed methane extraction. Fracturing is essentially the extension and expansion of microcracks, which converge into macrocracks. Relevant research has shown that the degree of damage to the coal around the fracturing well has a significant impact on the subsequent fracturing effect. Inducing a certain degree of micro-damage to the coal around the fracturing well before fracturing can significantly enhance the subsequent fracturing effect. Sodium hypochlorite solution has strong oxidizing properties and can produce an oxidation reaction with the coal body to dissolve the organic matter around the coal seam cleats and increase the porosity of the coal body. It is the preferred reagent for generating micro-damage to the coal body before fracturing.

[0004] Based on the above reasons, how to provide a new method to solve the shortcomings of uneven and localized expansion of fracturing cracks during coalbed methane development, so that a complex fracture network is formed in the coal reservoir after fracturing, and the fractures are not easy to close, thereby ensuring subsequent efficient gas extraction, is the research direction required by the present invention. Summary of the Invention

[0005] In response to the problems existing in the above-mentioned prior art, the present invention provides a combined storage and improvement method of sand-carrying oxidant pulse injection and static fracturing in hard coal seams, which can solve the shortcomings of uneven and localized expansion of fracturing cracks during coalbed methane development, so that a complex fracture network is formed in the coal reservoir after fracturing, and the fractures are not easy to close, thereby ensuring the subsequent efficient extraction of coalbed methane.

[0006] In order to achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a combined storage and improvement method of sand-carrying oxidant pulse injection and static fracturing in hard coal seams, the specific steps of which are:

[0007] A. Assemble the pulsating injection-static fracturing equipment: Connect sand storage tank 1, sand storage tank 2, and the liquid storage tank to the inlets of the sand-liquid mixing device via pipelines. Sand storage tank 1 contains small quartz sand particles, sand storage tank 2 contains large quartz sand particles, and the liquid storage tank contains an oxidant solution. The outlet of the sand-liquid mixing device is connected to the inlet of a high-pressure plunger pump via a pipeline. The outlet of the high-pressure plunger pump is connected to one end of the fracturing tubing, and the pulse generator is connected to the side of the fracturing tubing via a pipeline. A backpressure valve is installed on the pipeline between the sand-liquid mixing device and the high-pressure plunger pump to control the on-off of the pipeline. A pressure gauge is installed on the fracturing pipe to monitor the real-time pressure in the fracturing pipe and feed it back to the digital control terminal, which controls the high-pressure plunger pump and the pulse generator; the pipelines between the sand storage tank 1, the sand storage tank 2 and the liquid storage tank and the sand-liquid mixing device are respectively equipped with a one-way valve 1, a one-way valve 2 and a one-way valve 3, which are used to control the on-off and one-way flow of the respective pipelines; the pipeline between the sand-liquid mixing device and the high-pressure plunger pump, and the pipeline between the pulse generator and the fracturing pipe are respectively equipped with a one-way valve 4 and a one-way valve 5, which are used to control the on-off and one-way flow of the corresponding pipelines;

[0008] B. Construction of fracturing well: Construct a fracturing well through the rock formation toward the target coal seam, deliver the other end of the fracturing pipe to the target coal seam, and install a packer on the fracturing pipe to seal the fracturing well in the rock formation section;

[0009] C. Pulsated injection of oxidant: Open one-way valve 1 and one-way valve 3 to transport the oxidant solution in the liquid storage tank and the quartz sand particles in the sand storage tank 1 to the sand-liquid mixing device respectively. After reaching the required delivery volume, close one-way valve 1 and one-way valve 3, start the sand-liquid mixing device to fully stir and mix the oxidant solution and the quartz sand particles to form sand-carrying oxidizing liquid; then open one-way valve 4, one-way valve 5 and the back pressure valve, control the high-pressure plunger pump and the pulse generator to start through the digital control terminal, and inject the fully mixed sand-carrying oxidizing liquid in the sand-liquid mixing device into the coal seam through the fracturing pipe in a pulsed pressure state. , so that it impacts the coal body under the water wedge degradation effect of the pulse wave, and at the same time, the oxidizing liquid contacts the coal body during the impact process to react with the oxidation liquid to form a fracturing weak surface in the coal body and produce micro cracks, thereby squeezing small quartz sand particles into the micro cracks to expand and extend the holes in the micro cracks while increasing the contact area between the oxidizing liquid and the coal body, thereby enhancing the oxidation effect; during the pulsating injection process, the pressure gauge feeds back the pressure data in the fracturing pipe in real time, and the digital control terminal adjusts the high-pressure plunger pump and the pulse generating device according to the fed-back pressure data, stops the continuous fracturing injection after a certain period of time, and closes the one-way valve V. High-pressure plunger pump and pulse generating device;

[0010] D. Static fracturing: Open one-way valve 2 and one-way valve 3 to allow the oxidant solution in the liquid storage tank and the large quartz sand particles in the sand storage tank 2 to be respectively transported to the sand-liquid mixing device. After the required transport volume is reached, close one-way valve 2 and one-way valve 3, start the sand-liquid mixing device to fully stir and mix the oxidant solution and the large quartz sand particles to form a sand-carrying fracturing fluid; then control the high-pressure plunger pump to start through the digital control terminal, and inject the fully mixed sand-carrying fracturing fluid in the sand-liquid mixing device into the target coal seam through the fracturing pipe. By maintaining the injection pressure, static fracturing is performed on the micro-cracks after the hole is expanded, so that the large quartz sand particles are squeezed into the micro-cracks after the hole is expanded in step C, further expanding and extending the hole, while increasing the contact area between the oxidant and the coal body, thereby enhancing the oxidation effect; during the static fracturing process, the pressure data in the fracturing pipe is monitored in real time by a pressure gauge. When the pressure in the fracturing pipe shows a significant pressure drop, it indicates that the static fracturing is completed. At this time, the high-pressure plunger pump is turned off;

[0011] E. Progressive permeability enhancement of coal seams: Repeat steps C and D multiple times, alternating between pulsed oxidant injection and static fracturing processes, continuously forming a complex fracture network inside the coal seam, and ultimately completing the in-situ oxidation-static fracturing progressive permeability enhancement of the target coal seam.

[0012] Furthermore, the target coal seam is a hard coal seam with a Proctor coefficient f>3.0, especially a hard coal seam with a high mineral content. After the reservoir is transformed, small and large quartz sand particles enter the cracks, which can support the cracks under the action of ground stress and make them less likely to close.

[0013] Furthermore, in step C, the pulse wave has an amplitude of 4-6 MPa and a frequency of 10-20 Hz. The pulse wave has two main effects. First, the water wedge degradation effect of the pulse wave causes fatigue damage to the coal mass and squeezes the oxidant solution into micro-cracks, expanding and extending the pores in the micro-cracks while increasing the contact area between the oxidant and the coal mass, enhancing the oxidation effect. Second, the unstable state of the pulse wave can keep small quartz sand particles in the fracturing fluid suspended, preventing accumulation, and better delivering the small quartz sand particles into the coal micro-cracks for expansion and support.

[0014] Furthermore, the oxidant solution is a 10% sodium hypochlorite solution. This solution has a strong oxidizing ability and can effectively increase the porosity of the coal body, increase the crack width, etch the cleat surface, and form a fracturing weak surface through oxidation impact on the coal body during the pulse injection process, providing a fracturing foundation for subsequent static fracturing.

[0015] Furthermore, the small quartz sand particles are quartz sand particles with a particle size range of 100 mesh to 150 mesh, and the large quartz sand particles are quartz sand particles with a particle size range of 20 mesh to 50 mesh.

[0016] Furthermore, the significant pressure drop in step D is a pressure drop of more than 10 MPa. In this case, it indicates that the fractures are well developed, allowing the fracturing fluid to penetrate into the coal seam as the fractures expand, resulting in a pressure drop, thereby achieving the effect of this static fracturing.

[0017] Compared with the existing technology, the present invention adopts a combination of oxidant pulse injection and static fracturing, which has the following advantages:

[0018] 1. The present invention first deploys a pulse injection-static fracturing device, then mixes an oxidant solution and small quartz sand particles to form a sand-carrying oxidizing liquid, and injects the sand-carrying oxidizing liquid into the target coal seam by pulse injection. The sand-carrying oxidizing liquid impacts the coal body under the water wedge degradation effect of the pulse wave. At the same time, the oxidizing liquid contacts the coal body during the impact process to undergo an oxidation reaction, thereby forming a fracturing weak surface in the coal body and generating microcracks. Then, the small quartz sand particles and the oxidizing liquid are squeezed into the microcracks to expand and extend the microcracks while increasing the contact area between the oxidizing liquid and the coal body, thereby enhancing the oxidation effect. Finally, the coal body around the fracturing well is oxidized to form a microcrack network, providing more fracturing weak surfaces for subsequent static fracturing. An oxidant solution and large particles of quartz sand are mixed to form a sand-carrying fracturing fluid, and the sand-carrying fracturing fluid is injected into the target coal seam through static fracturing for continuous static fracturing, so that the various tiny cracks in the fracturing weak surface converge, develop, and expand longitudinally, and the large particles of quartz sand are squeezed into the cracks after expansion to further expand and extend the hole, while increasing the contact area between the oxidant liquid and the coal body, thereby enhancing the oxidation effect. By alternating oxidant pulse injection and static fracturing processes, a complex fracture network is continuously formed inside the coal seam, solving the shortcomings of uneven and localized fracturing crack expansion during coalbed methane development, and finally completing the in-situ oxidation-static fracturing progressive permeability enhancement of the target coal seam.

[0019] 2. The present invention squeezes small quartz sand particles into micro-cracks during pulsating injection, and squeezes large quartz sand particles into the expanded cracks during subsequent static fracturing. The entry of large quartz sand particles will push the small quartz sand particles that entered previously to continue to penetrate deeper into the cracks as the cracks expand. In this way, when fracturing and permeability enhancement are completed for gas extraction, the small quartz sand particles and large quartz sand particles continue to remain in the cracks. When the cracks subsequently have a tendency to close under the influence of ground stress, the small quartz sand particles, large quartz sand particles and coal seams are all relatively hard. Therefore, the small quartz sand particles and large quartz sand particles can support the cracks and reduce the degree of crack closure, thereby ensuring efficient, continuous and stable gas extraction. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the overall layout of the present invention.

[0021] In the figure: 1-sand storage tank one; 2-sand storage tank two; 3-one-way valve one; 4-one-way valve two; 5-delivery pipe; 6-back pressure valve; 7-sand-liquid mixing device; 8-liquid storage tank; 9-pressure gauge; 10-high-pressure plunger pump; 11-pulse generator; 12-one-way valve five; 13-fracturing pipe; 14-rock layer; 15-target coal seam; 16-packer; 17-microcracks; 18-small quartz sand particles; 19-large quartz sand particles; 20-fracture network; 21-one-way valve three; 22-digital control terminal; 23-fracturing well; 24-data transmission line; 25-one-way valve four. DETAILED DESCRIPTION

[0022] The present invention will be further described below.

[0023] like Figure 1 As shown, the specific steps of the present invention are:

[0024] A. Assemble the pulsating injection-static fracturing equipment: Connect sand storage tank 1 (1), sand storage tank 2 (2), and liquid storage tank 8 to the respective inlets of the sand-liquid mixing device 7 via pipelines. Sand storage tank 1 (1) contains small quartz sand particles 18, while sand storage tank 2 (2) contains large quartz sand particles 19. The small quartz sand particles 18 are in the 100-150 mesh range, while the large quartz sand particles 19 are in the 20-50 mesh range. Liquid storage tank 8 contains an oxidant solution, a 10% sodium hypochlorite solution. This solution has a strong oxidizing ability and can effectively increase coal porosity, widen fracture width, and etch cleat surfaces. During the pulsating injection process, the oxidative impact on the coal creates a fracturing weak surface, providing a foundation for subsequent static fracturing. The outlet of the sand-liquid mixing device 7 is connected to the inlet of the high-pressure plunger pump 10 through the delivery pipe 5, the outlet of the high-pressure plunger pump 10 is connected to one end of the fracturing pipe 13, and the pulse generator 11 is connected to the side of the fracturing pipe 13 through the pipeline; the delivery pipe 5 between the sand-liquid mixing device 7 and the high-pressure plunger pump 10 is equipped with a back pressure valve 6 for controlling the on-off of the pipeline; the fracturing pipe 13 is equipped with a pressure gauge 9 for monitoring the real-time pressure in the fracturing pipe 13 and feeding it back to the digital control terminal 22, which controls the high-pressure plunger pump 10 and the pulse generator 11 are controlled; the pipelines between the sand storage tank 1, the sand storage tank 2, and the liquid storage tank 8 and the sand-liquid mixing device 7 are respectively equipped with a one-way valve 3, a one-way valve 2, and a one-way valve 3 21, which are used to control the on-off and one-way flow of their respective pipelines; the pipeline between the sand-liquid mixing device 7 and the high-pressure plunger pump 10, and the pipeline between the pulse generator 11 and the fracturing pipe 13 are respectively equipped with a one-way valve 4 25 and a one-way valve 5 12, which are used to control the on-off and one-way flow of the corresponding pipelines;

[0025] B. Construction of a fracturing well: A fracturing well 23 is constructed through the rock formation toward the target coal seam 15. The other end of the fracturing pipe 13 is delivered to the target coal seam 15, and a packer 16 is installed on the fracturing pipe 13 so that the packer 16 seals the fracturing well in the rock formation section. The target coal seam 15 is a hard coal seam with a Proctor coefficient f>3.0, especially a hard coal seam with a high mineral content. After reservoir reconstruction, small and large quartz sand particles enter the cracks, which can support the cracks under the action of ground stress and prevent them from closing.

[0026] C. Pulsated injection of oxidant: open the one-way valve 13 and the one-way valve 3 21 to transport the oxidant solution in the liquid storage tank 8 and the quartz sand small particles in the sand storage tank 1 to the sand-liquid mixing device 7 respectively. After reaching the required transport volume, close the one-way valve 13 and the one-way valve 3 21, start the sand-liquid mixing device 7 to fully stir and mix the oxidant solution and the quartz sand small particles to form a sand-carrying oxidizing liquid; then open the one-way valve 4 25, the one-way valve 5 12 and the back pressure valve 6, control the high-pressure plunger pump 10 and the pulse generator 11 to start through the digital control terminal 22, and inject the fully mixed sand-carrying oxidizing liquid in the sand-liquid mixing device 7 into the coal seam through the fracturing pipe 13 in a pulsed pressure state, so that it impacts the coal body under the water wedge degradation effect of the pulse wave, and at the same time, the oxidizing liquid contacts the coal body during the impact process to react with the coal body to form a fracturing weak surface and generate microcracks in the coal body. 17, thereby causing the small quartz sand particles 18 to squeeze into the micro-cracks 17 to expand and extend the micro-cracks while increasing the contact area between the oxidizing liquid and the coal body, thereby enhancing the oxidation effect; during the pulsating injection process, the pressure gauge 9 provides real-time feedback of the pressure data in the fracturing pipe, and the digital control terminal 22 adjusts the high-pressure plunger pump 10 and the pulse generator 11 according to the feedback pressure data, so that the fracturing pipe 13 maintains a stable sinusoidal wave injection, with a peak pressure constant at 10 MPa and an error of no more than ±0.5 MPa. Since the pulsating pressure is the pressure that fluctuates up and down with time, the peak pressure is the maximum value of the pulsating pressure, and the amplitude of the pulse wave is the difference between the peak pressure and the minimum pressure, the required pulsating injection requirements can be met through this parameter setting. The continuous fracturing injection is stopped after 48 hours, and the one-way valve 5 12, the high-pressure plunger pump 10 and the pulse generator 11 are closed;

[0027] D. Static fracturing: Open the one-way valve 24 and the one-way valve 3 21 to transport the oxidant solution in the liquid storage tank 8 and the large quartz sand particles 19 in the sand storage tank 22 to the sand-liquid mixing device 7 respectively. After reaching the required transport volume, close the one-way valve 24 and the one-way valve 3 21, start the sand-liquid mixing device 7 to fully stir and mix the oxidant solution and the large quartz sand particles 19 to form a sand-carrying fracturing fluid; then control the high-pressure plunger pump 10 to start through the digital control terminal 22, and inject the fully mixed sand-carrying fracturing fluid in the sand-liquid mixing device 7 into the target coal seam 15 through the fracturing pipe 13. The expanded micro-cracks 17 are subjected to static fracturing by maintaining the injection pressure, so that large quartz sand particles 19 are squeezed into the micro-cracks 17 expanded in step C, further expanding and extending the hole while increasing the contact area between the oxidizing fluid and the coal body, thereby enhancing the oxidation effect. During the static fracturing process, the pressure data in the fracturing pipe 13 is monitored in real time by the pressure gauge 9. When the pressure in the fracturing pipe 13 drops by more than 10 MPa, it indicates that the fractures are well developed, allowing the fracturing fluid to extend into the coal seam as the fractures expand, resulting in a pressure drop and completing the static fracturing. At this time, the high-pressure plunger pump 10 is turned off.

[0028] E. Progressive permeability enhancement of coal seams: Repeat steps C and D multiple times, alternating between pulsed oxidant injection and static fracturing processes, continuously forming a complex fracture network 20 inside the coal seam, and ultimately completing the in-situ oxidation-static fracturing progressive permeability enhancement of the target coal seam 15.

[0029] As an improvement to the present invention, in step C, the pulse wave has an amplitude of 4 to 6 MPa and a frequency of 10 to 20 Hz. The pulse wave has two main effects. First, the water wedge degradation effect of the pulse wave causes fatigue damage to the coal body and squeezes the oxidant solution into microcracks, expanding and extending the microcracks while increasing the contact area between the oxidant and the coal body, enhancing the oxidation effect. Second, the unstable state of the pulse wave can keep the small quartz sand particles 18 in the fracturing fluid suspended, preventing accumulation, and better delivering the small quartz sand particles 18 into the coal body microcracks 17 for expansion and support.

[0030] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A combined storage and improvement method of sand-carrying oxidant pulse injection and static fracturing in hard coal seams, characterized in that: The specific steps are: A. Assemble the pulsating injection-static fracturing equipment: Connect sand storage tank 1, sand storage tank 2, and the liquid storage tank to the inlets of the sand-liquid mixing device via pipelines. Sand storage tank 1 contains small quartz sand particles, sand storage tank 2 contains large quartz sand particles, and the liquid storage tank contains an oxidant solution. The outlet of the sand-liquid mixing device is connected to the inlet of a high-pressure plunger pump via a pipeline. The outlet of the high-pressure plunger pump is connected to one end of the fracturing tubing, and the pulse generator is connected to the side of the fracturing tubing via a pipeline. A backpressure valve is installed on the pipeline between the sand-liquid mixing device and the high-pressure plunger pump to control the on-off of the pipeline. A pressure gauge is installed on the fracturing pipe to monitor the real-time pressure in the fracturing pipe and feed it back to the digital control terminal, which controls the high-pressure plunger pump and the pulse generator; the pipelines between the sand storage tank 1, the sand storage tank 2 and the liquid storage tank and the sand-liquid mixing device are respectively equipped with a one-way valve 1, a one-way valve 2 and a one-way valve 3, which are used to control the on-off and one-way flow of the respective pipelines; the pipeline between the sand-liquid mixing device and the high-pressure plunger pump, and the pipeline between the pulse generator and the fracturing pipe are respectively equipped with a one-way valve 4 and a one-way valve 5, which are used to control the on-off and one-way flow of the corresponding pipelines; B. Construction of fracturing well: Construct a fracturing well through the rock formation toward the target coal seam, deliver the other end of the fracturing pipe to the target coal seam, and install a packer on the fracturing pipe to seal the fracturing well in the rock formation section; C. Pulsated injection of oxidant: Open one-way valve 1 and one-way valve 3 to transport the oxidant solution in the liquid storage tank and the quartz sand particles in the sand storage tank 1 to the sand-liquid mixing device respectively. After reaching the required delivery volume, close one-way valve 1 and one-way valve 3, start the sand-liquid mixing device to fully stir and mix the oxidant solution and the quartz sand particles to form sand-carrying oxidizing liquid; then open one-way valve 4, one-way valve 5 and the back pressure valve, control the high-pressure plunger pump and the pulse generator to start through the digital control terminal, and inject the fully mixed sand-carrying oxidizing liquid in the sand-liquid mixing device into the coal seam through the fracturing pipe in a pulsed pressure state. , so that it impacts the coal body under the water wedge degradation effect of the pulse wave, and at the same time, the oxidizing liquid contacts the coal body during the impact process to react with the oxidation liquid to form a fracturing weak surface in the coal body and produce micro cracks, thereby squeezing small quartz sand particles into the micro cracks to expand and extend the holes in the micro cracks while increasing the contact area between the oxidizing liquid and the coal body, thereby enhancing the oxidation effect; during the pulsating injection process, the pressure gauge feeds back the pressure data in the fracturing pipe in real time, and the digital control terminal adjusts the high-pressure plunger pump and the pulse generating device according to the fed-back pressure data, stops the continuous fracturing injection after a certain period of time, and closes the one-way valve V. High-pressure plunger pump and pulse generating device; D. Static fracturing: Open one-way valve 2 and one-way valve 3 to allow the oxidant solution in the liquid storage tank and the large quartz sand particles in the sand storage tank 2 to be respectively transported to the sand-liquid mixing device. After the required transport volume is reached, close one-way valve 2 and one-way valve 3, start the sand-liquid mixing device to fully stir and mix the oxidant solution and the large quartz sand particles to form a sand-carrying fracturing fluid; then control the high-pressure plunger pump to start through the digital control terminal, and inject the fully mixed sand-carrying fracturing fluid in the sand-liquid mixing device into the target coal seam through the fracturing pipe. By maintaining the injection pressure, static fracturing is performed on the micro-cracks after the hole is expanded, so that the large quartz sand particles are squeezed into the micro-cracks after the hole is expanded in step C, further expanding and extending the hole, while increasing the contact area between the oxidant and the coal body, thereby enhancing the oxidation effect; during the static fracturing process, the pressure data in the fracturing pipe is monitored in real time by a pressure gauge. When the pressure in the fracturing pipe shows a significant pressure drop, it indicates that the static fracturing is completed. At this time, the high-pressure plunger pump is turned off; E. Progressive permeability enhancement of coal seams: Repeat steps C and D multiple times, alternating between pulsed oxidant injection and static fracturing processes, continuously forming a complex fracture network inside the coal seam, and ultimately completing the in-situ oxidation-static fracturing progressive permeability enhancement of the target coal seam.

2. The hard coal seam sand-carrying oxidant pulse injection-static fracturing combined storage and improvement method according to claim 1 is characterized in that: The target coal seam is a hard coal seam with a Proctor coefficient f>3.

0. After the reservoir is transformed, small and large quartz sand particles enter the cracks, which can support the cracks under the action of ground stress and make them less likely to close.

3. The combined storage and improvement method of sand-carrying oxidant pulse injection and static fracturing in hard coal seams according to claim 1 is characterized in that: In the step C, the amplitude of the pulse wave is 4-6 MPa and the frequency is 10-20 Hz.

4. The hard coal seam sand-carrying oxidant pulse injection-static fracturing combined storage and improvement method according to claim 1, characterized in that: The oxidant solution is a sodium hypochlorite solution with a concentration of 10%.

5. The hard coal seam sand-carrying oxidant pulse injection-static fracturing combined storage and improvement method according to claim 1, characterized in that: The small quartz sand particles are quartz sand particles with a particle size range of 100 mesh to 150 mesh, and the large quartz sand particles are quartz sand particles with a particle size range of 20 mesh to 50 mesh.

6. The hard coal seam sand-carrying oxidant pulse injection-static fracturing combined storage and improvement method according to claim 1, characterized in that: The significant pressure drop in step D is a pressure drop of more than 10 MPa.

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