Dual-pulsation acid fracturing and permeability-enhanced low-permeability coal seam gas extraction system and method

Abstract

The present invention discloses a dual-pulsation acidic fracturing and permeability-enhanced low-permeability coal seam gas extraction system, comprising a pressure overshoot protection mechanism, a fracturing fluid transmission line, a gas extraction line, a mixing liquid box, a pressure regulating and monitoring mechanism, and a frequency regulating and monitoring mechanism; the present invention also discloses a dual-pulsation acidic fracturing and permeability-enhanced low-permeability coal seam gas extraction method, comprising the following steps: step S1, investigating the target coal seam and obtaining fracturing fluid; step S2, installing the above-mentioned system; step S3, adjusting the fracturing pressure and pulsating frequency of the fracturing fluid, and performing continuous cyclic fracturing on the fracturing borehole; step S4, adjusting the fracturing pressure and pulsating frequency of the fracturing fluid to, and performing continuous cyclic fracturing on the fracturing borehole; step S5, repeating steps S3-S4 five times, and extracting and detecting gas concentration to generate data. The present invention has the advantages of increasing the connectivity of the internal fracture-pore structure of the coal body, improving the permeability enhancement effect of the coal seam, improving the coal seam gas extraction efficiency, and having an interlocking structure.

Description

Technical Field

[0001] The present invention belongs to the technical field of permeability enhancement and low-permeability coal seam extraction, and in particular relates to a dual-pulsation acid fracturing permeability enhancement and low-permeability coal seam gas extraction system and method. Background Art

[0002] my country's coal seams, influenced by geological structure and metamorphism, exhibit "three highs and one low" characteristics: high geostress, high geotemperature, high gas pressure, and low permeability. Currently, 72% of coal seams have proven permeability less than 1 mD. Furthermore, micropores are well-developed within these seams, with some pores and fissures filled with minerals, resulting in poor connectivity between these pores and fissures. Furthermore, as coal mining depth in my country continues to increase at a rate of 10 to 25 meters per year, the high geostress and low permeability characteristics of coal seams are becoming increasingly pronounced, making it difficult to achieve gas extraction results that meet design requirements.

[0003] Efficient gas extraction is an important guarantee for safe production in mines. Currently, coal seam transformation methods include hydraulic fracturing and acid fracturing. The current problems with the on-site application of hydraulic fracturing technology are: due to the large size and high viscosity of water molecules, the cracks generated by hydraulic fracturing often expand in the direction parallel to the maximum principal stress, which easily produces a single tensile crack and cannot form a complex fracture network. In addition, it is difficult to change the situation of mineral filling in the pores and cracks in the coal seam by using hydraulic fracturing technology alone; while acid fracturing can effectively dissolve the mineral filling structure in the pores and cracks, the crack expansion direction is significantly affected by the ground stress; Figure 1 The distribution diagram of the "three zones" of cracks around the borehole before fracturing is as follows: the broken zone a, the crack zone b, and the original crack zone c. Figure 1 It can be seen that before the fracturing operation, there are a large number of secondary cracks around the fracturing hole, and the fracturing fluid must first pass through the crushing zone a with a large number of secondary cracks, enter the crack zone b, and reach the primary crack zone c. The gas diffusion and seepage capabilities of the two methods are poor, the coal seam permeability is low, and the gas extraction efficiency is low. Summary of the Invention

[0004] The present invention aims to provide a dual-pulsation acid fracturing system and method for enhancing gas extraction in low-permeability coal seams. The dual-pulsation hydraulic fracturing and acid fracturing are integrated to complement each other's advantages and improve gas extraction efficiency. It solves the problems that the current single hydraulic fracturing technology is difficult to change the filling of pores and cracks in coal seams with minerals, hindering the diffusion and seepage of gas, and the crack expansion direction of single acid fracturing is significantly affected by ground stress, both of which result in low coal seam permeability and low gas extraction efficiency.

[0005] To this end, the technical solution adopted by the present invention is: a dual-pulsation acid fracturing permeability-enhancing low-permeability coal seam gas extraction system, including a pressure overshoot protection mechanism, a fracturing fluid delivery line with the infusion end extending into the fracturing borehole, a gas extraction line with the extraction end extending into the observation borehole, a mixing box containing fracturing fluid, a pressure regulating monitoring mechanism for regulating the fracturing pressure, and a frequency regulating monitoring mechanism for regulating the pulsating frequency; the pressure regulating monitoring mechanism includes a branch line 1 and a pressure transmitter 1, a pressure data collector, and an electromagnetic valve 1 connected in sequence along the branch line 1; the frequency regulating monitoring mechanism includes a branch line 2 and a pressure transmitter 3, an electromagnetic valve 2, and a frequency controller connected in sequence along the branch line 2. When the pressure transmitter 3 reaches the set pressure, the signal is transmitted to the frequency controller through the electromagnetic valve 2 for design frequency comparison. The head ends of branch line one and branch line two are both connected to the fracturing fluid delivery line, and the tail ends are both connected to the pulsating fracturing pump; the fracturing fluid delivery line is connected to the mixing tank and the fracturing borehole, and a pulsating fracturing pump, a filter, a pressure gauge, a flow meter, a throttle valve, and a liquid injection pipeline valve are sequentially arranged along the fracturing fluid delivery direction; the gas extraction line includes a negative pressure extraction pipe connected to the output end of the fracturing fluid delivery line and the observation borehole, a fracturing hole extraction valve is provided at the junction of the negative pressure extraction pipe and the fracturing fluid delivery line, and an observation hole extraction valve is provided at the connection between the negative pressure extraction pipe and the observation borehole; the pressure overshoot protection mechanism includes branch line three connected to the fracturing fluid delivery line and pressure transmitter two and solenoid valve three sequentially connected along branch line three. When the pressure of the fracturing fluid delivery line is higher than the set pressure, solenoid valve three is energized to open and release the pressure for protection.

[0006] As a preferred embodiment of the above scheme, the connection sections between branch line 1, branch line 2, branch line 3 and the fracturing fluid delivery line are equipped with high-pressure casings, and high-pressure hoses are used at the joints between the gas extraction line and the fracturing fluid delivery line. Since the pressure transmitter needs to use a thinner pipe to detect the pressure inside the main pipe, high-pressure casings are used as further protection measures at the connection sections between branch line 1, branch line 2, branch line 3 and the fracturing fluid delivery line. High-pressure hoses are used to connect the weak connections between the gas extraction line and the fracturing fluid delivery line, which is a reasonable design.

[0007] Further preferably, the fracturing borehole and observation borehole openings are both provided with high-pressure capsules for sealing, and the fracturing borehole opening corresponding to the infusion end of the fracturing fluid delivery line is equipped with a high-pressure sealing water braid to ensure the sealing of the connection, and the design structure is reasonable.

[0008] This solution also provides a method for enhanced gas extraction in low-permeability coal seams using dual-pulsation acid fracturing and permeability enhancement, including the following steps:

[0009] Step S1: First, conduct a detailed investigation of the target coal seam and complete sampling. Then, perform industrial analysis on the coal samples, determine elemental analysis and mineral content, and obtain the most suitable fracturing fluid for the target coal seam based on the reaction kinetics of the minerals and the acid solution.

[0010] Step S2: drilling a fracturing borehole and an observation borehole spaced parallel to the fracturing borehole in the target coal seam, and then installing the above-mentioned dual-pulsation acid fracturing and permeability-enhanced low-permeability coal seam gas extraction system;

[0011] Step S3, starting the pulsating fracturing pump to fill the prepared fracturing fluid into the fracturing borehole through the fracturing fluid delivery line, and adjusting the fracturing pressure of the fracturing fluid to 4Mpa-6Mpa and the pulsating frequency to 30Hz-40Hz through the pressure adjustment monitoring mechanism and the frequency adjustment monitoring mechanism, and performing continuous cyclic fracturing on the fracturing borehole. When backflow is observed in the borehole, stopping the delivery of fracturing fluid into the fracturing borehole;

[0012] Step S4, restarting the pulsating fracturing pump to fill the fracturing borehole with fracturing fluid, and adjusting the fracturing pressure of the fracturing fluid to 2MPa-3MPa and the pulsating frequency to 15Hz-20Hz through the frequency adjustment monitoring mechanism and the pressure adjustment monitoring mechanism, and performing continuous cyclic fracturing on the fracturing borehole. When backflow is observed in the borehole, stopping the delivery of fracturing fluid into the fracturing borehole;

[0013] Step S5, repeat steps S3-S4 five times, close the injection pipeline valve, open the fracturing hole extraction valve and the observation hole extraction valve, and perform extraction detection on the gas concentration in the fracturing borehole and the observation borehole at intervals of several days to generate data.

[0014] As a preferred embodiment of the above scheme, in step S1, the interfacial reaction model is used to solve the acid solution formula range under different conditions based on the main mineral content in the coal sample to obtain the fracturing fluid most suitable for the target coal seam, and the solution is first solved according to the following reaction rate expression;

[0015]

[0016] Where: J is the reaction rate, mol / (cm 2 .s); Cs is the rock surface acid concentration at reaction time t, mol / L; m is the number of reactions; K is the microreaction rate constant, mol / [cm2·s·(mol / mL) m ];

[0017] Taking the common logarithm on both sides of the above formula, the calculation formula for the acid-rock reaction kinetics is determined as:

[0018] lgJ=lgK+mlgC

[0019] Plotting lgJ and lgC, the intercept of the straight line can be used to obtain the reaction rate constants K and m, and then the reaction kinetic equation can be derived. Based on the reaction kinetic equation, the type and content of the coal seam's own fillings, and the properties of the single acid solution, the range of acid solution formulas for the main different mineral contents in the coal sample can be obtained;

[0020] The dissolution of minerals in coal by acidic fracturing fluid is a solid-liquid phase reaction. The following solid-liquid reaction kinetic model formula is used to solve the acid formula under different conditions for the main mineral content in coal samples;

[0021]

[0022] In the formula, C is the dissolution rate, %; t is the reaction time, s; m is the maximum dissolution amount of the acid on the mineral, which can reflect the dissolution ability of the acid on the mineral; n is the reaction time when the acid dissolution rate of the mineral reaches half, which can reflect the dissolution rate of the acid on the mineral.

[0023] By quantitatively determining the mineral content of coal samples, according to the acid formulation range of the main different mineral contents in the coal samples, and based on the selection criteria of m value as large as possible and n value as small as possible, the best fracturing fluid can be selected by comparing the m and n values ​​in the mineral dissolution kinetic equation of acid solution; the steps are reasonable.

[0024] More preferably, in the steps S2 and S3, when water backflow is observed in the borehole, the delivery of the fracturing fluid into the fracturing borehole is stopped for 30 to 45 minutes, so as to allow the fracturing fluid and the minerals inside the pores and fissures sufficient time to dissolve.

[0025] More preferably, the fracturing pressure and pulsation frequency in step S3 are twice the fracturing pressure and pulsation frequency in step S4, ensuring dual pulsation of relatively high pressure and high frequency and relatively low pressure and low frequency.

[0026] Beneficial effects of the present invention:

[0027] (1) Compared with the existing pores and fissures filled with some minerals, which hinder the diffusion and seepage of gas, this scheme adopts a combination of dual-pulsation hydraulic fracturing and acid fracturing, cleverly performing dual pulsation through relatively high and low pressures and relatively high and low frequencies. Under the action of relatively "high pressure-high frequency", it can quickly pass through the crushing area around the hole to form a main fissure channel and secondary fissures. The fissures continue to expand and are connected and communicated with each other. The acid fracturing fluid can effectively dissolve the minerals and obstructions between the fissures. On this basis, under the relatively low-pressure, low-frequency conditions, the fracturing fluid flows along the primary fracture channels, continuously stimulating secondary fractures and inducing the formation of more microfractures. Simultaneously, the acidic fracturing fluid effectively dissolves minerals within the pores, increasing the porosity within the coal. Through dual-pulsation fracturing, primary, secondary, and microfractures are generated in the coal seam, interconnected by these fractures to form a complex fracture network. Simultaneously, the acidic fracturing fluid significantly increases the connectivity of the fracture-pore structure within the coal, increasing its specific surface area and porosity. Furthermore, the acidic fracturing fluid causes a certain degree of damage to the coal seam, reducing its compressive strength. This makes pulsating fracturing more resistant to permeability enhancement and effectively reduces the risk of disasters such as rock bursts. Therefore, the coupled effect of these two factors significantly enhances the permeability enhancement of the coal seam and significantly improves gas extraction efficiency.

[0028] (2) The design has a pressure overshoot protection mechanism. When the pressure in the high-pressure pipeline is higher than the set pressure, the electromagnetic valve 3 is energized to open and release the pressure, which serves the purpose of protection. The pressure transmitter 1 is the control source for the output and feedback of the high-pressure pipeline pressure signal and is connected to the pressure data collector. The pulsating fracturing pump is also adjusted through the electromagnetic valve 1. When the pressure transmitter 3 reaches the set pressure, the signal is transmitted to the frequency controller through the electromagnetic valve 2, and compared with the design frequency. The frequency can be adjusted to the design frequency through the frequency controller. The design structure is closely linked and the concept is novel.

[0029] In summary, the present invention has the advantages of coupling dual pulsating hydraulic fracturing and acid fracturing, increasing the connectivity of the internal crack-pore structure of the coal body, improving the permeability of the coal seam, improving the efficiency of coal seam gas extraction, and having an interlocking structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is the distribution map of the "three zones" of cracks around the borehole before fracturing.

[0031] Figure 2 Schematic diagram of the system structure of the present invention.

[0032] Figure 3 This is a comparison curve of extracted gas concentration. DETAILED DESCRIPTION

[0033] The present invention will be further described below by way of examples and in conjunction with the accompanying drawings:

[0034] Combine Figure 2 — Figure 3 As shown, a dual-pulsation acid fracturing and permeability-enhanced low-permeability coal seam gas extraction system consists of a pressure overshoot protection mechanism 9, a fracturing fluid delivery line 7 with the delivery end extending into the fracturing borehole 2, a gas extraction line 8 with the extraction end extending into the observation borehole 3, a mixing tank 4 containing fracturing fluid, a pressure regulating and monitoring mechanism 5 for regulating the fracturing pressure, and a frequency regulating and monitoring mechanism 6 for regulating the pulsation frequency.

[0035] The openings of fracturing borehole 2 and observation borehole 3 are both equipped with high-pressure capsules for airtight sealing.

[0036] The orifice of the fracturing borehole 2 corresponds to the infusion end of the fracturing fluid delivery line 7 and is equipped with a high-pressure sealing water braid 21 .

[0037] The pressure regulating and monitoring mechanism 5 is composed of a branch line 55 and a pressure transmitter 51, a pressure data collector 52, and a solenoid valve 53 connected in sequence along the branch line 55.

[0038] The frequency adjustment and monitoring mechanism 6 is composed of a second branch line 64 and a third pressure transmitter 61 , a second solenoid valve 62 , and a frequency controller 63 sequentially connected along the second branch line 64 .

[0039] When the pressure transmitter 3 61 reaches the set pressure, the signal is transmitted to the frequency controller 63 through the solenoid valve 2 62 for comparison with the designed frequency.

[0040] The first end of branch line 1 55 and the second end of branch line 2 64 are both connected to the fracturing fluid delivery line 7 , and the second end of branch line 64 are both connected to the pulsating fracturing pump 71 .

[0041] The fracturing fluid delivery line 7 connects the mixing tank 4 and the fracturing borehole 2, and is provided with a pulsating fracturing pump 71, a filter 72, a pressure gauge 73, a flow meter 74, a throttle valve 75, and a liquid injection pipeline valve 76 in sequence along the fracturing fluid delivery direction.

[0042] The gas extraction line 8 is composed of a negative pressure extraction pipe 81 connected to the output end of the fracturing fluid delivery line 7 and the observation borehole 3.

[0043] A high-pressure hose is preferably used at the junction of the gas extraction line 8 and the fracturing fluid delivery line 7.

[0044] A fracturing hole extraction valve 82 is provided at the connection point between the negative pressure extraction pipe 81 and the fracturing fluid delivery line 7 , and an observation hole extraction valve 83 is provided at the connection point between the negative pressure extraction pipe 81 and the observation borehole 3 .

[0045] The pressure overshoot protection mechanism 9 is composed of a branch line 3 93 connected to the fracturing fluid delivery line 7 , and a pressure transmitter 2 91 and a solenoid valve 3 92 sequentially connected along the branch line 3 93 .

[0046] When the pressure in the fracturing fluid delivery line 7 is higher than the set pressure, the solenoid valve 3 92 is energized to open and release the pressure for protection.

[0047] The connection sections between branch line 1 55 , branch line 2 64 , branch line 3 93 and the fracturing fluid delivery line 7 are equipped with high-pressure casings.

[0048] A method for enhancing gas extraction in low-permeability coal seams using dual-pulsation acid fracturing and permeability enhancement is described. The specific implementation steps are as follows:

[0049] Step S1: First, conduct a detailed investigation of the target coal seam 1 and complete sampling. Then, perform industrial analysis on the coal samples, determine elemental analysis and mineral content, and obtain the most suitable fracturing fluid for the target coal seam 1 based on the reaction kinetics of minerals and acid.

[0050] In step S1, the interfacial reaction model is used to solve the acid solution formula range under different conditions based on the main mineral content in the coal sample to obtain the fracturing fluid most suitable for the target coal seam. The solution is first performed according to the following reaction rate expression;

[0051]

[0052] Where: J is the reaction rate, mol / (cm 2 .s); Cs is the rock surface acid concentration at reaction time t, mol / L; m is the number of reactions; K is the microreaction rate constant, mol / [cm2·s·(mol / mL) m ];

[0053] Taking the common logarithm on both sides of the above formula, the calculation formula for the acid-rock reaction kinetics is determined as:

[0054] lgJ=lgK+mlgC

[0055] Plot lgJ and lgC, and use the intercept of the straight line to obtain the reaction rate constants K and m, and then derive the reaction kinetic equation.

[0056] Based on the reaction kinetics equation, the type and content of the coal seam's own fillings, and the properties of a single acid solution, the range of acid solution formulas for the main different mineral contents in the coal sample is obtained, as shown in the following table:

[0057]

[0058]

[0059] The dissolution of minerals in coal by acidic fracturing fluid is a solid-liquid phase reaction. The following solid-liquid reaction kinetic model formula is used to solve the acid formula under different conditions for the main mineral content in coal samples;

[0060]

[0061] Where C is the dissolution rate, %; t is the reaction time, s; m is the maximum dissolution amount of the acid on the mineral, which can reflect the acid's dissolution ability on the mineral; n is the reaction time when the acid's dissolution rate on the mineral reaches half, which can reflect the acid's dissolution rate on the mineral. The results of the mineral and acid reaction kinetics are shown in the following table:

[0062]

[0063] By quantitatively measuring the mineral content of coal samples, according to the acid formulation range of the main different mineral contents in the coal samples, and based on the selection criteria of making the m value as large as possible and the n value as small as possible, the best fracturing fluid can be selected by comparing the m and n values ​​in the mineral dissolution kinetic equation of the acid solutions S1 to S3.

[0064] Step S2: drill a fracturing borehole 2 and an observation borehole 3 spaced apart and parallel to the fracturing borehole 2 in the target coal seam 1, and then install the above-mentioned dual-pulsation acid fracturing permeability-enhanced low-permeability coal seam gas extraction system.

[0065] Step S3, start the pulsating fracturing pump 71 to fill the prepared fracturing fluid into the fracturing borehole 2 through the fracturing fluid delivery line 7, and adjust the fracturing pressure of the fracturing fluid to 4Mpa~6Mpa and the pulsating frequency to 30Hz~40Hz through the pressure adjustment monitoring mechanism 5 and the frequency adjustment monitoring mechanism 6, and perform continuous cyclic fracturing on the fracturing borehole 2. When backflow is observed in the borehole 3, stop delivering the fracturing fluid to the fracturing borehole 2.

[0066] In step S2 and step S3, when water backflow is observed in the borehole 3, the delivery of the fracturing fluid into the fracturing borehole 2 is stopped for 30 minutes to 45 minutes.

[0067] The fracturing pressure and pulsation frequency in step S3 are twice the fracturing pressure and pulsation frequency in step S4.

[0068] Step S4, restart the pulsating fracturing pump 71 to fill the fracturing borehole 2 with fracturing fluid, and adjust the fracturing pressure of the fracturing fluid to 2MPa~3Mpa and the pulsating frequency to 15Hz~20Hz through the frequency adjustment monitoring mechanism 6 and the pressure adjustment monitoring mechanism 5, and perform continuous cyclic fracturing on the fracturing borehole 2. When backflow phenomenon is observed in the borehole 3, stop delivering fracturing fluid to the fracturing borehole 2.

[0069] Step S5: Repeat steps S3-S4 five times, close the injection line valve 76, open the fracturing hole extraction valve 82 and the observation hole extraction valve 83, and perform extraction detection on the gas concentration in the fracturing borehole 2 and the observation borehole 3 at intervals of several days to generate data;

[0070] After comparing the gas concentrations of conventional drilling and pulsating fracturing extraction, the data are summarized in the following table:

[0071]

[0072]

[0073] According to the above table, generate Figure 3 The comparison curve of extracted gas concentration is shown.

Claims

1. A dual-pulsation acid fracturing and permeability-enhancing low-permeability coal seam gas extraction system, characterized by: The invention comprises a pressure overshoot protection mechanism (9), a fracturing fluid delivery line (7) with a delivery end extending into a fracturing borehole (2), a gas extraction line (8) with an extraction end extending into an observation borehole (3), a mixing tank (4) containing fracturing fluid, a pressure regulating monitoring mechanism (5) for regulating fracturing pressure, and a frequency regulating monitoring mechanism (6) for regulating pulsation frequency; the pressure regulating monitoring mechanism (5) comprises a branch line (55) and a pressure transmitter (51) and a pressure data collector (51) connected in sequence along the branch line (55). 2), solenoid valve 1 (53); the frequency adjustment monitoring mechanism (6) includes branch line 2 (64) and pressure transmitter 3 (61), solenoid valve 2 (62), and frequency controller (63) connected in sequence along branch line 2 (64). When pressure transmitter 3 (61) reaches the set pressure, the signal is transmitted to frequency controller (63) through solenoid valve 2 (62) for design frequency comparison. The head ends of branch line 1 (55) and branch line 2 (64) are connected to fracturing fluid delivery line (7), and the end ends are connected to pulse The fracturing fluid delivery line (7) is connected to the mixing tank (4) and the fracturing borehole (2), and a pulsating fracturing pump (71), a filter (72), a pressure gauge (73), a flow meter (74), a throttle valve (75), and a liquid injection pipeline valve (76) are sequentially arranged along the fracturing fluid delivery direction; the gas extraction line (8) includes a negative pressure extraction pipe (81) connected to the output end of the fracturing fluid delivery line (7) and the observation borehole (3), and the negative pressure extraction pipe (81) is connected to the fracturing fluid delivery line (7). A fracturing hole extraction valve (82) is provided at the joint of the liquid delivery line (7), and an observation hole extraction valve (83) is provided at the connection between the negative pressure extraction pipe (81) and the observation borehole (3); the pressure overshoot protection mechanism (9) comprises a branch line three (93) connected to the fracturing fluid delivery line (7) and a pressure transmitter two (91) and a solenoid valve three (92) connected in sequence along the branch line three (93); when the pressure of the fracturing fluid delivery line (7) is higher than the set pressure, the solenoid valve three (92) is energized to open and release the pressure for protection.

2. The dual-pulsation acid fracturing and permeability-enhanced low-permeability coal seam gas extraction system according to claim 1, characterized in that: The connection sections between the branch line 1 (55), the branch line 2 (64), the branch line 3 (93) and the fracturing fluid delivery line (7) are equipped with high-pressure casings, and the connection between the gas extraction line (8) and the fracturing fluid delivery line (7) uses a high-pressure hose.

3. The dual-pulsation acid fracturing and permeability-enhanced low-permeability coal seam gas extraction system according to claim 1 is characterized by: The openings of the fracturing borehole (2) and the observation borehole (3) are both provided with high-pressure capsules for airtight sealing, and the opening of the fracturing borehole (2) corresponds to the infusion end of the fracturing fluid delivery line (7) equipped with a high-pressure sealing water braid (21).

4. A method for enhancing gas extraction in low-permeability coal seams by using double-pulsation acid fracturing and permeability enhancement, characterized in that: The following steps are involved: Step S1: First, conduct a detailed investigation of the target coal seam (1), complete sampling, and then perform industrial analysis on the coal sample, element analysis and mineral content determination, and obtain the most suitable fracturing fluid for the target coal seam (1) based on the reaction kinetics of the mineral and the acid solution; Step S2, drilling a fracturing borehole (2) and an observation borehole (3) spaced and parallel to the fracturing borehole (2) in the target coal seam (1), and then installing the dual-pulsation acid fracturing permeability-enhancing low-permeability coal seam gas extraction system according to any one of claims 1 to 3; Step S3, starting the pulsating fracturing pump (71) to fill the prepared fracturing fluid into the fracturing borehole (2) through the fracturing fluid delivery line (7), and adjusting the fracturing pressure of the fracturing fluid to 4Mpa to 6Mpa and the pulsating frequency to 30Hz to 40Hz through the pressure regulating monitoring mechanism (5) and the frequency regulating monitoring mechanism (6), and performing continuous cyclic fracturing on the fracturing borehole (2). When backflow is observed in the borehole (3), the delivery of the fracturing fluid into the fracturing borehole (2) is stopped; Step S4, restarting the pulsating fracturing pump (71) to fill the fracturing borehole (2) with fracturing fluid, and adjusting the fracturing pressure of the fracturing fluid to 2MPa-3MPa and the pulsating frequency to 15Hz-20Hz through the frequency adjustment monitoring mechanism (6) and the pressure adjustment monitoring mechanism (5), performing continuous cyclic fracturing on the fracturing borehole (2), and stopping the delivery of fracturing fluid into the fracturing borehole (2) when backflow is observed in the borehole (3); Step S5, repeat steps S3-S4 five times, close the injection pipeline valve (76), open the fracturing hole extraction valve (82) and the observation hole extraction valve (83), and perform extraction detection on the gas concentration in the fracturing borehole (2) and the observation borehole (3) at intervals of several days to generate data.

5. The method for enhanced gas extraction from low-permeability coal seams using dual-pulsation acid fracturing and permeability enhancement according to claim 4, characterized in that: In step S1, the interfacial reaction model is used to solve the acid solution formula range under different conditions based on the main mineral content in the coal sample to obtain the fracturing fluid most suitable for the target coal seam, and the solution is first solved according to the following reaction rate expression; Where: J is the reaction rate, mol / (cm 2 .s); Cs is the rock surface acid concentration at reaction time t, mol / L; m is the number of reactions; K is the microreaction rate constant, mol / [cm2·s·(mol / mL) m ]; Taking the common logarithm on both sides of the above formula, the calculation formula for the acid-rock reaction kinetics is determined as: lgJ=lgK+mlgC Plotting lgJ and lgC, the intercept of the straight line can be used to obtain the reaction rate constants K and m, and then the reaction kinetic equation can be derived. Based on the reaction kinetic equation, the type and content of the coal seam's own fillings, and the properties of the single acid solution, the range of acid solution formulas for the main different mineral contents in the coal sample can be obtained; The dissolution of minerals in coal by acidic fracturing fluid is a solid-liquid phase reaction. The following solid-liquid reaction kinetic model formula is used to solve the acid formula under different conditions for the main mineral content in coal samples; Where C is the dissolution rate, %; t is the reaction time, s; m is the maximum dissolution amount of the acid on the mineral, which can reflect the acid's dissolution ability on the mineral; n is the reaction time when the acid's dissolution rate on the mineral reaches half, which can reflect the acid's dissolution rate on the mineral; By quantitatively measuring the mineral content of coal samples, according to the acid formulation range of the main different mineral contents in the coal samples, and based on the selection criteria of making the m value as large as possible and the n value as small as possible, the best fracturing fluid can be selected by comparing the m and n values ​​in the mineral dissolution kinetic equation of the acid.

6. The method for enhanced gas extraction from low-permeability coal seams using dual-pulsation acid fracturing and permeability enhancement according to claim 4, characterized in that: In the steps S2 and S3, when water backflow is observed in the borehole (3), the delivery of the fracturing fluid into the fracturing borehole (2) is stopped for 30 to 45 minutes.

7. The method for enhanced gas extraction from low-permeability coal seams using dual-pulsation acid fracturing and permeability enhancement according to claim 4, characterized in that: The fracturing pressure and pulsation frequency in step S3 are twice the fracturing pressure and pulsation frequency in step S4.

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