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

Abstract

The invention discloses an enhanced gas extraction system for a double-pulsation acid fracturing anti-reflection low-permeability coal seam. The enhanced gas extraction system comprises a pressure overshoot protection mechanism, a fracturing fluid conveying line, a gas extraction line, a liquid mixing box, a pressure adjusting and monitoring mechanism and a frequency adjusting and monitoring mechanism. The invention further discloses a double-pulsation acid fracturing anti-reflection low-permeability coal seam enhanced gas extraction method which comprises the following steps that S1, a target coal seam is investigated and researched, and fracturing fluid is obtained; s2, the system is installed; s3, the fracturing pressure and the pulsation frequency of the fracturing fluid are adjusted, and continuous periodic circulating fracturing is conducted on the fracturing drill hole; s4, the fracturing pressure and the pulsation frequency of the fracturing fluid are adjusted, and continuous periodic circulating fracturing is conducted on the fracturing drill hole; s5, the steps S3-S4 are repeated for five times, and the gas concentration is subjected to extraction detection to generate data. The method has the advantages that the connectivity of the fracture-pore structure in the coal body is improved, the coal seam permeation enhancing effect is improved, the coal seam gas extraction efficiency is improved, and the structures are linked with one another.

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 are affected by geological structure and geological metamorphism, showing "three highs and one low" (high geostress, high geotemperature, high gas pressure and low permeability) characteristics. It has been proven that coal seams with permeability less than 1mD account for 72% of the total, and there are micropores in the coal seams. Some pores and cracks are filled with minerals, and the connectivity of pores and cracks is poor. At the same time, the mining depth of my country's coal mines extends to the depth at a rate of 10m to 25m per year. The characteristics of high geostress and low permeability of coal seams are becoming more and more obvious, making it difficult for gas extraction to meet the design requirements.

[0003] Efficient gas extraction is an important guarantee for safe production in mines. At present, coal seam transformation methods include hydraulic fracturing and acid fracturing. The current problem with the on-site application of hydraulic fracturing technology is that the cracks produced by hydraulic fracturing often expand in the direction parallel to the maximum principal stress due to the large size and high viscosity of water molecules, which easily produces a single tensile crack and cannot form a complex fracture network. In addition, it is difficult to change the situation of coal seam pores and cracks being filled with minerals by using hydraulic fracturing technology alone; while acid fracturing can effectively dissolve the mineral filling structure in the pores and cracks, but 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, which are 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 holes, and the fracturing fluid must first pass through the crushing zone a where there are a large number of secondary cracks, enter the fracture zone b, and reach the primary fracture 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 permeability-enhanced low-permeability coal seam enhanced gas extraction system and method. The dual-pulsation hydraulic fracturing and acid fracturing are integrated to complement each other's advantages and improve the gas extraction efficiency. The current single hydraulic fracturing technology is difficult to change the filling of pores and cracks in coal seams with minerals, which hinders the diffusion and seepage of gas, and the crack expansion direction of single acid fracturing is significantly affected by ground stress, both of which cause low permeability of coal seams and low gas extraction efficiency.

[0005] To this end, the technical solution adopted by the present invention is: a dual-pulsation acid fracturing permeability-enhanced 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 with 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 connected to the fracturing fluid delivery line, and the ends are connected to the pulsating fracturing pump; the fracturing fluid delivery line is connected to the mixing box 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 arranged in sequence 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 arranged at the connection between the negative pressure extraction pipe and the fracturing fluid delivery line, and an observation hole extraction valve is arranged 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 connected in sequence 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 the pressure relief for protection.

[0006] As a preferred embodiment of the above scheme, the connecting sections between branch line 1, branch line 2, branch line 3 and the fracturing fluid delivery line are equipped with high-pressure casings, and the gas extraction line and the fracturing fluid delivery line are connected at a high-pressure hose. Since the pressure transmitter needs to use a thinner pipe to detect the pressure inside the main pipe, the connecting sections between branch line 1, branch line 2, branch line 3 and the fracturing fluid delivery line use high-pressure casings as further protection measures, and the weak connection between the gas extraction line and the fracturing fluid delivery line is connected with a high-pressure hose, 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] The scheme also provides a method for enhanced gas extraction in low-permeability coal seams using double pulsation acid fracturing and permeability enhancement, comprising the following steps:

[0009] Step S1, first conduct a detailed investigation of the target coal seam, complete sampling, and then conduct industrial analysis on the coal sample, element analysis and mineral content determination, and obtain the most suitable fracturing fluid for the target coal seam based on the reaction kinetics of the mineral and the acid solution;

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

[0011] Step S3, start the pulsating fracturing pump to fill the prepared fracturing fluid into the fracturing borehole through the fracturing fluid delivery line, 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 and the frequency adjustment monitoring mechanism, and perform continuous cyclic fracturing on the fracturing borehole. When backflow is observed in the borehole, stop delivering the fracturing fluid into the fracturing borehole;

[0012] Step S4, restart the pulsating fracturing pump to fill the fracturing borehole 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 and the pressure adjustment monitoring mechanism, and perform continuous cyclic fracturing on the fracturing borehole. When backflow is observed in the borehole, stop delivering the fracturing fluid to 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 days to generate data.

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

[0015]

[0016] Where: J is the reaction rate, mol / (cm 2 .s); Cs is the acid concentration on the rock surface 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] In the above formula, take the common logarithm on both sides to determine the calculation formula of acid-rock reaction kinetics:

[0018] lgJ=lgK+mlgC

[0019] Plot lgJ and lgC, and use the intercept of the straight line to obtain the reaction rate constants K and m, and then obtain the reaction kinetic equation. According to the reaction kinetic equation and the type and content of the filling material of the coal seam itself, as well as the properties of the single acid solution, the range of acid solution formula for the main different mineral contents in the coal sample can be obtained;

[0020] The dissolution of the minerals in the coal by the 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 the coal sample;

[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] The best fracturing fluid can be selected by quantitatively determining the mineral content of coal samples, according to the acid formula range of the main different mineral contents in the coal samples, and by comparing the m and n values ​​in the mineral dissolution kinetic equation of the acid solution according to the selection criteria of making the m value as large as possible and the n value as small as possible; 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 give 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, so as to ensure 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 cycles 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 broken 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 action of relatively "low pressure-low frequency", the fracturing fluid flows along the main fracture channel, constantly stimulating the secondary fractures and inducing them to produce more micro fractures. At the same time, the acidic fracturing fluid can effectively dissolve the minerals inside the pores and increase the porosity inside the coal body. After double pulsation fracturing, the coal seam can produce main fractures, secondary fractures and micro fractures, which communicate with each other to form a complex fracture network. At the same time, the acidic fracturing fluid greatly increases the connectivity of the fracture-pore structure inside the coal body, increasing the specific surface area and porosity of the coal body. In addition, the coal seam will be damaged to a certain extent under the dissolution of the acidic fracturing fluid, which can reduce the compressive strength of the coal seam, make it easier to increase the permeability of the coal seam by pulsating fracturing, and effectively reduce the risk of disasters such as rock burst. Therefore, the coupling effect of the two will greatly improve the permeability effect of the coal seam and greatly improve the efficiency of gas extraction.

[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 solenoid 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 solenoid valve 1. When the pressure transmitter 3 reaches the set pressure, the signal is transmitted to the frequency controller through the solenoid valve 2, which is compared with the design frequency, and 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 fracture-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 “three-zone” distribution map of cracks around the borehole before fracturing.

[0031] Figure 2 It is a 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 embodiments and in conjunction with the accompanying drawings:

[0034] Combination Figure 2 — Figure 3 As shown, a dual-pulsation acid fracturing permeability enhancement and low-permeability coal seam enhanced 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 box 4 containing fracturing fluid, a pressure regulating monitoring mechanism 5 for regulating the fracturing pressure, and a frequency regulating monitoring mechanism 6 for regulating the pulsation frequency.

[0035] The openings of the fracturing borehole 2 and the 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 which are sequentially connected along the branch line 55.

[0038] The frequency regulation monitoring mechanism 6 is composed of a branch line 2 64 and a pressure transmitter 3 61 , a solenoid valve 2 62 , and a frequency controller 63 which are sequentially connected along the branch line 2 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 design frequency comparison.

[0040] The first ends of the branch line 1 55 and the second branch line 64 are connected to the fracturing fluid delivery line 7 , and the ends thereof are 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 between the gas extraction line 8 and the fracturing fluid delivery line 7.

[0044] A fracturing hole extraction valve 82 is provided at the joint 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 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 of 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 by dual pulsation acid fracturing and permeability enhancement, the specific implementation steps are as follows:

[0049] Step S1, first conduct a detailed investigation on the target coal seam 1, complete sampling, and then conduct industrial analysis on the coal samples, 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 minerals and acid.

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

[0051]

[0052] Where: J is the reaction rate, mol / (cm 2 .s); Cs is the acid concentration on the rock surface 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] In the above formula, take the common logarithm on both sides to determine the calculation formula of acid-rock reaction kinetics:

[0054] lgJ=lgK+mlgC

[0055] By 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.

[0056] According to the reaction kinetics equation, the type and content of the filling material in the coal seam itself, and the properties of the single acid solution, the range of acid solution formula for the main different mineral contents in the coal sample is obtained, as shown in the following table:

[0057]

[0058]

[0059] The dissolution of the minerals in the coal by the 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 the coal sample;

[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 reaction kinetics between the mineral and the acid are as follows:

[0062]

[0063] By quantitatively determining the mineral content of coal samples, according to the acid formula 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 solutions S1 to S3.

[0064] Step S2, drilling a fracturing borehole 2 and an observation borehole 3 spaced and arranged parallel to the fracturing borehole 2 in the target coal seam 1, and then installing the above-mentioned dual pulsating acid fracturing permeability enhancement low permeability coal seam enhanced 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 periodic cyclic fracturing on the fracturing borehole 2. When the borehole 3 is observed to have water backflow, 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 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 the borehole 3 is observed to have water backflow, stop delivering the fracturing fluid to the fracturing borehole 2.

[0069] 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 days to generate data;

[0070] After comparing the gas concentrations extracted by ordinary drilling and pulsating fracturing, 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 enhancement low permeability coal seam enhanced 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 box (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 (52) 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, solenoid valve 2 (62) transmits a signal to frequency controller (63) for design frequency comparison; the head end of branch line 1 (55) and branch line 2 (64) are connected to fracturing fluid delivery line (7), and the end end is connected to pulse The fracturing fluid delivery line (7) is connected to a mixing tank (4) and a 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 fluid 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. A dual pulsating acid fracturing permeability enhancement low permeability coal seam enhanced 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 rubber hose.

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

4. A method for enhancing gas extraction in low-permeability coal seams by dual 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 conduct 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 arranged in parallel with the fracturing borehole (2) in the target coal seam (1), and then installing the dual pulsating acid fracturing permeability enhancement low permeability coal seam enhanced 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-6Mpa and the pulsating frequency to 30Hz-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), stopping the delivery of the fracturing fluid into the fracturing borehole (2); 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), and performing continuous cyclic fracturing on the fracturing borehole (2). When backflow is observed in the borehole (3), stopping the delivery of fracturing fluid into the fracturing borehole (2); 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 days to generate data.

5. A method for enhanced gas extraction in low-permeability coal seams with dual pulsation acid fracturing and permeability enhancement according to claim 4, characterized in that: In step S1, the interface reaction model is used to solve the acid solution formula range under different conditions for the main mineral content in the coal sample to obtain the fracturing fluid most suitable for the target coal seam, and the solution is firstly performed according to the following reaction rate expression; Where: J is the reaction rate, mol / (cm 2 .s); Cs is the acid concentration on the rock surface at reaction time t, mol / L; m is the number of reactions; K is the microreaction rate constant, mol / [cm2·s·(mol / mL) m ]; In the above formula, take the common logarithm on both sides to determine the calculation formula of acid-rock reaction kinetics: lgJ=lgK+mlgC Plot lgJ and lgC, and use the intercept of the straight line to obtain the reaction rate constants K and m, and then obtain the reaction kinetic equation. According to the reaction kinetic equation and the type and content of the filling material of the coal seam itself, as well as the properties of the single acid solution, the range of acid solution formula for the main different mineral contents in the coal sample can be obtained; The dissolution of the minerals in the coal by the 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 the coal sample; 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 ability to dissolve 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 determining the mineral content of coal samples, according to the acid formula 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 kinetic equation of acid dissolution of minerals.

6. A method for enhanced gas extraction in low-permeability coal seams with 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 minutes to 45 minutes.

7. A method for enhanced gas extraction in low-permeability coal seams with 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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