Self-excited low-frequency high-pressure pulsating hydraulic fracturing device and method

Abstract

The application relates to a self-excitation type downhole low-frequency high-pressure pulsating hydraulic fracturing device and method, which comprises a cylinder body in a hollow cylindrical structure, a piston arranged in the cavity of the cylinder body, the piston being in a whole conical frustum structure, a flow guide channel being arranged in the piston and extending along the axial direction of the piston and penetrating through the upper and lower ends of the piston, a supporting seat and a plug arranged on the supporting seat, the supporting seat also being arranged in the cavity of the cylinder body and located at the lower end of the piston, the plug being in a conical body structure and timely contacting with the flow guide channel, a plurality of flow guide holes being uniformly arranged along the circumferential direction of the supporting seat, and a reset member being sleeved on the plug. The device promotes the forward propulsion of fracturing sand, reduces the settlement of the fracturing sand and reduces energy consumption and ground operation cost through the working liquid to push the mechanical piston, so that the working liquid generates pulsation.

Description

TECHNICAL FIELD

[0001] The present application relates to a self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing device and method, and belongs to the technical field of downhole tools for fracturing operations. BACKGROUND

[0002] Fracturing is the most aggressive stimulation measure for low-permeability oil and gas reservoirs, but the success rate and efficiency of fracturing reconstruction have been a world-class problem, which has plagued oil exploration and development at home and abroad, and this problem has not been completely solved so far. During the hydraulic fracturing operation, it is difficult to seal the hole at high pressure, the fracture pore is not easy to control and the success rate is low, and the sand plugging problem occurs.

[0003] At present, a large number of researchers have studied the plugging and fracture of low-permeability fracture reservoirs, and have proposed various measures and methods. However, the power device of the pulsating hydraulic fracturing technology in the prior art is a pulsating pump placed on the ground, which converts electrical energy into mechanical energy to make the working fluid obtain a certain pulsating frequency and act on the reservoir rock. This method will cause a lot of energy loss, and the fracture effect in ultra-low permeability reservoirs is not very significant, and the fracturing sand cannot reach the tip of the fracture, which may cause sand plugging. SUMMARY

[0004] In view of the above technical problems, the present application provides a self-excited low-frequency high-pressure pulsating hydraulic fracturing device completely in the downhole, which drives the mechanical piston by the working fluid to cause the working fluid to pulsate, promotes the forward advancement of the fracturing sand, reduces the settlement of the fracturing sand, and reduces the energy consumption and ground operation cost.

[0005] To achieve the above purpose, the present application adopts the following technical scheme:

[0006] A self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing device, comprising:

[0007] A cylinder body having a hollow cylindrical structure;

[0008] A piston arranged in the cavity of the cylinder body, the piston has a whole conical frustum structure, and a flow guide channel extending along the axial direction and penetrating through the upper and lower ends of the piston is formed in the piston;

[0009] A supporting seat and a plug arranged on the supporting seat, the supporting seat is also arranged in the cavity of the cylinder body and located at the lower end of the piston, the plug has a conical structure and is in timely contact with the flow guide channel, and a plurality of flow guide holes are uniformly arranged along the circumferential direction of the supporting seat;

[0010] A reset member sleeved on the plug.

[0011] Preferably, the self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing device further comprises an upper joint and a lower joint, both of which are hollow cylindrical structures, the upper joint is connected with the upper end of the cylinder body, the lower joint is connected with the lower end of the cylinder body, and the upper joint, the flow guide channel, the flow guide hole and the lower joint are in communication with each other.

[0012] Preferably, a groove is formed in the bottom periphery of the piston, and a rubber ring is sleeved in the groove.

[0013] Preferably, the cavity of the cylinder body is composed of three cavities, a frustum cavity and small-cylinder cavities and large-cylinder cavities located at both ends of the frustum cavity, the piston is located in the frustum cavity, and the supporting seat is located in the large-cylinder cavity.

[0014] Preferably, the taper of the plug is 2:(10-12), the height is 120mm-180mm, and the diameter ranges from 0 to 20mm.

[0015] Preferably, the matching taper of the cylinder body and the piston ranges from 1:(20-25).

[0016] Preferably, the diameter of the flow guide channel is 15mm-20mm, and the length is 180mm-200mm.

[0017] Preferably, the flow guide hole is an arc-shaped hole, and the arc length is 20-40mm.

[0018] Preferably, the length of the small-cylinder cavity accounts for one fourth of the total length of the piston, and the taper of the lower part of the piston is 1:(10-12).

[0019] The second aspect of the present application provides a generating method of the self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing device, comprising the following steps:

[0020] The upper end of the self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing device is connected to the lower part of the fracturing pipe column, and the lower end is connected to the upper part of the sandblaster. In the fracturing pre-fluid injection stage, the overflow channel at the upper part of the piston and the flow guide channel play a throttling role. The piston is subjected to water pressure and goes down, compressing the reset member. The compression of the reset member causes the plug to enter the flow guide channel. When the piston compresses the reset member to the maximum compression position, the plug completely enters the flow guide channel. At this time, the pressure generated by the fluid at the upper part of the piston is less than the reset force of the reset member. At this time, the reset member starts to push the piston to go up. Until the pressure generated by the fluid at the upper part of the piston is greater than the reset force of the reset member again, the piston goes down, thereby generating reciprocating pulsating water power, and low-frequency pulsating water power in the small displacement hydraulic fracturing working fluid.

[0021] The present application has the following advantages due to the above technical scheme:

[0022] 1. The low-frequency pulsating water power generated by the device can strengthen the plugging of old fractures in the reservoir during the plugging agent injection stage, avoid the old fractures being pressed open in the subsequent hydraulic fracturing engineering, and create conditions for the generation of new fractures.

[0023] 2. Based on the rock mechanics damage theory and the simulation test research results, it is found that the pulsating action produces pulsating waves, which will have reflection, superposition and other effects, and alternating stress is generated at the end of the fracture, which causes rock fatigue damage and destruction to the reservoir. In hydraulic fracturing, the rock initiation pressure is significantly reduced, which can effectively reduce the formation breakdown pressure, is beneficial to the generation of artificial fractures, and is easy to reopen new fractures on the basis of old fractures.

[0024] 3. In the process of hydraulic fracturing, the working fluid carries the fracturing sand to advance in the fracture. During the advancing process, sedimentation is easy to occur, causing sand plugging. Under the pulsating action, the fracturing sand will be subjected to low-frequency pulsating water power, which can inhibit the sedimentation of the fracturing sand, expand the advancing range of the fracturing sand, achieve uniform sand paving, enhance the sand carrying capacity, and effectively support the far end of the fracture, thereby effectively improving the fracture conductivity.

[0025] 4. The device is a self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing device determined based on the low-frequency pulsating hydraulic fracturing mechanism research. The device can realize low-frequency pulsating hydraulic output in a water-driven mode without changing the original fracturing construction design, can press open multiple branch fractures around the main fracture, and can improve the overall hydraulic fracturing effect and reduce the fracture conductivity.

[0026] 5. The low-frequency pulsating action generated by the device will also have a slight pulsating effect on the downhole drilling tool. In the drilling process, the dynamic friction between the drilling tool and the well wall is reduced, which can effectively reduce the frictional resistance along the way BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The sectional view of the self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing device provided by the embodiment of the present application is shown in the figure.

[0028] Figure 2 The installation sectional view of the hydraulic fracturing device and other equipment provided by the embodiment of the present application is shown in the figure.

[0029] Figure 3 The sectional view of the piston provided by the embodiment of the present application is shown in the figure.

[0030] Figure 4 The sectional view of the cylinder provided by the embodiment of the present application is shown in the figure.

[0031] Figure 5 The perspective view of the supporting seat and the plug provided by the embodiment of the present application is shown in the figure.

[0032] Figure 6 The top view of the hydraulic fracturing device provided by the embodiment of the present application is shown in the figure.

[0033] The various marks in the figure are as follows:

[0034] 1 - upper joint; 2 - cylinder; 3 - piston; 4 - flow guide channel; 5 - rubber ring; 6 - plug; 7 - return spring; 8 - supporting seat; 9 - lower joint; 10 - fracturing pipe column; 11 - sand blaster; 12 - flow guide hole. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical solutions and advantages of the present application clearer, the technical solutions in the present application are described clearly and completely below. Obviously, the described embodiments are some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0036] Unless otherwise defined, the technical terms or scientific terms used in the present application should be understood as the usual meanings understood by those skilled in the art. The words "first", "second", "third", "fourth" and similar words used in the present application do not represent any order, number or importance, but are only used to distinguish different components. The words "include" or "contain" and similar words mean that the elements or objects before the words cover the elements or objects listed after the words and their equivalents, and do not exclude other elements or objects. The words "connect" or "connected" and similar words are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect.

[0037] For the convenience of description, spatial relative terms can be used in the specification to describe the relationship of one element or feature to another element or feature as shown in the drawings, such as "inner", "outer", "inside", "outside", "below", "above", etc. Such spatial relative terms are intended to include different orientations of the device in use or operation in addition to the orientation depicted in the figures.

[0038] At present, a large number of researchers have studied the plugging and fracture forming of low permeability fracture reservoirs, and various measures and methods have been proposed. However, the power device of the pulsating hydraulic fracturing technology in the prior art is a pulsating pump placed on the ground, which converts electrical energy into mechanical energy to make the working fluid obtain a certain pulsating frequency and act on the reservoir rock. This method will cause a large amount of energy loss, and the fracture forming effect is not very significant in ultra-low permeability reservoirs, and the fracturing sand cannot reach the tip of the fracture, which may cause sand plugging.

[0039] In order to overcome the defects of the prior art, in view of the problems existing in the conventional hydraulic fracturing technology construction, the self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing generating device is formed on the basis of not changing the original hydraulic fracturing construction design, which is driven by the flow of hydraulic fracturing working fluid, and the fracturing fluid acting on the reservoir fracture generates low-frequency pulsating hydraulic action through the device.

[0040] As shown in Figure 1 , the self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing generating device relates to the self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing generating device, which comprises a cylinder body 2, an upper joint 1 arranged at the upper end of the cylinder body 2 and a lower joint 9 (screwed) arranged at the lower end of the cylinder body 2, a composite conical piston 3 matched with the cylinder body 2 is arranged in the cylinder body 2, a flow guide channel 4 is arranged in the inside of the piston 3, the flow guide channel 4 can reduce the influence of water hammer effect generated at the top of the piston 3 when the piston 3 goes up, and at the same time ensure that a large displacement is suitable for the hydraulic fracturing process, and the fracturing sand in the high-sand-ratio fracturing fluid and the high-viscosity thick plugging agent in the repeated diverting fracturing can pass through smoothly. Referring to Figure 3 , a rubber ring 5 is sleeved on the outer periphery of the bottom of the piston 3 to prevent pressure from being released from the outer periphery, a reset spring 7 is arranged at the lower part of the piston 3, a plug 6 is sleeved in the reset spring 7, and a supporting seat 8 is arranged at the lower end of the plug 6.

[0041] Specifically, as shown in 5, a plurality of flow guide holes 12 are uniformly arranged in the circumferential direction of the supporting seat 8, and the upper joint 1, the flow guide channel 4, the flow guide hole 12 and the lower joint 9 are in communication with each other.

[0042] As shown in Figure 2As shown, the application also provides a generation method of the downhole low-frequency pulsating hydraulic fracturing device. The upper end of the low-frequency high-pressure pulsating hydraulic fracturing device is connected to the lower part of the fracturing pipe column 10 and the upper part of the sand blaster 11. In the fracturing pre-fluid injection stage, the overflow passage of the upper part of the piston 3 and the flow guide passage 4 in the piston 3 play a throttling role. The piston 3 is lowered under the water pressure, and the compression reset spring 7 is compressed. The compression of the reset spring 7 causes the conical plug 6 in the center of the support seat 8 to enter the flow guide passage 4 in the piston 3. When the piston 3 compresses the reset spring 7 to the maximum compression position, the plug 6 completely enters the flow guide passage 4 of the piston 3. At this time, the pressure generated by the fluid in the upper part of the piston 3 is less than the reset force of the reset spring 7. At this time, the reset spring 7 starts to push the piston 3 upward. Until the pressure generated by the fluid in the upper part of the piston 3 is greater than the reset force of the reset spring 7 again, the piston 3 is lowered. Thus, reciprocating pulsating water is generated, which can generate low-frequency pulsating water in the small displacement hydraulic fracturing working fluid.

[0043] The self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing device of the application generates low-frequency pulsating water in the sand-carrying fluid injection stage. The piston 3 compresses the reset spring 7 to move downward and the reset spring 7 pushes the piston 3 to move upward, forming a complete reciprocating mechanical pulsation system. The low-frequency pulsating water generated by the system can generate pulsating frequency by pushing the working fluid. Through the superposition effect of the pulsating wave, it is beneficial to the extension and expansion of the crack. Under a certain displacement of the fracturing fluid, the low-frequency pulsation generated by the pulsation system propagates in the fractured surface of the formation. The propagation effect will gradually decay (usually the propagation decay is proportional to the square or higher power of the pulsation frequency). In the crack surface, the generated pulsating hydraulic action acts on the reservoir rock or the fracturing sand through the working fluid, which can effectively inhibit the settlement of the fracturing sand, prevent the occurrence of sand plugging, effectively carry sand, and achieve the purpose of uniform sand distribution.

[0044] Based on the rock mechanics damage theory and the results of simulation test research, it is found that the pulsating action produces pulsating waves, which will have reflection, superposition and other effects, and alternating stress is generated at the end of the crack, which causes rock fatigue damage to the reservoir. In hydraulic fracturing, the rock initiation pressure is significantly reduced, which can effectively reduce the formation fracture pressure, is beneficial to the generation of artificial cracks, and is easy to reopen new cracks on the basis of old cracks. The low-frequency pulsating hydraulic action generated by the device can strengthen the plugging of old cracks in the plugging agent injection stage, avoid the reopening of old cracks in subsequent hydraulic fracturing engineering, and create conditions for the generation of new cracks.

[0045] In another aspect, in the hydraulic fracturing process, the working fluid carries the fracturing sand to advance in the fracture, and the settlement is prone to occur in the advancing process, causing sand blockage; however, the low-frequency pulsating hydraulic action inhibits the settlement of the fracturing sand, expands the advancing range of the fracturing sand, achieves uniform sand paving, enhances the sand carrying capacity, and effectively supports the far end of the fracture, thereby effectively improving the fracture conductivity.

[0046] The self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing device of the application can reduce the rock rupture pressure under the pulsating action of the working fluid of the hydraulic fracturing, is beneficial to reducing the rock cracking pressure, and can achieve the purpose of cracking; in the low-frequency pulsating process, it is found that the initial propagation of the pulsating pressure wave is basically a cosine wave, is relatively stable, but the amplitude has been expanded by 3 times, and under certain conditions, the reflection, superposition and reciprocation of the pulsating pressure wave are regular, and the amplitude reaches 5 times at most, thereby expanding the amplitude of the pulsating pressure wave and increasing the pressure. In the field construction, it is shown that the pulsating pressure amplitude can be increased by 3-5 times under the low-frequency pulsating action.

[0047] Therefore, the pulsating hydraulic pressure generated by the self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing device of the application can propagate and extend in the original fracture surface, the pulsating action gradually decays in the propagation process, the fractures around the original fracture are pressed, the sand carrying of the fracturing fluid is effectively controlled under the pulsating hydraulic action, sand blockage is avoided, the settlement of the fracturing sand is inhibited, the artificial hydraulic fracturing fracture conductivity is improved, and the effect of increasing production is achieved.

[0048] The generation and structure size of the low-frequency pulsating hydraulic fracturing pulsating action generated by the device are based on the calculation of the theory of hydraulics and the simulation calculation on site.

[0049] The specific structure size of the cylinder body 2 and the piston 3, the size of the flow guide channel 4, the elastic coefficient of the reset spring 7, and the size of the rubber ring 5 are all related fluid performance parameters, elastic coefficients and stiffnesses obtained by combining the theory with the field test.

[0050] Let the mass of the piston 3 be m, the effective cross-sectional area be S L , the stiffness coefficient of the reset spring 7 be k, the working fluid density be p, the working fluid viscous resistance coefficient be d, the average pressure difference of the upper and lower sections of the piston 3 be P1 and P2 respectively, the upward and downward displacement of the piston 3 be x, the upward and downward stroke of the piston 3 be L, the time be t, and the reciprocating vibration frequency be f, then when the piston 3 goes down:

[0051] P1S L = mx'' + kx + dS L p(x') 2(1-1)

[0052] x| t＝0 = 0 (1-2)

[0053] x'| t＝0 = 0 (1-3)

[0054] In the formula, x" is the up-and-down movement acceleration of the piston 3; x' is the up-and-down movement speed of the piston 3;

[0055] When the piston 3 is going up:

[0056] P2S L = mx" + kx + δS L ρ(x') 2 (1-4)

[0057] x| t＝0 = h (1-5)

[0058] x'| t＝0 = 0 (1-6)

[0059] In the formula, h is the stroke of the piston;

[0060] Here, the average pressure difference P1 and P2 borne by the up-and-down cross sections of the piston 3 in the stroke are calculated according to the size and stroke position of the piston 3 by using the principle of fluid mechanics. The outer peripheral flow passage cross section is different when the stroke position of the piston 3 is different, and the up-and-down pressure difference of the piston 3 is also different. In actual calculation, the flow distribution of the through hole of the piston 3 and the outer peripheral flow passage cross section of the piston 3 needs to be considered, and the viscous resistance coefficient of the outer peripheral flow passage of the piston 3 changes, so that the analytical algorithm is not easy to obtain. Therefore, the formula is modified in the test of this experiment, and the numerical analysis algorithm of fluid mechanics is used to determine P1 and P2.

[0061] The stroke of the piston 3 is:

[0062]

[0063] The stiffness coefficient of the reset spring 7 is represented as:

[0064]

[0065] The reciprocating vibration frequency is:

[0066]

[0067] In the above formula: when the formula is calculated by using the above formula, P a is the pressure difference borne by the up-and-down cross sections of the piston 3 when the piston 3 is driven downward by the working fluid, which is about 2.6 MPa; P bIn order to make the piston 3 go down, the outer peripheral flow passage section of the piston 3 is gradually increased, and when the outer peripheral flow passage section of the piston 3 is increased to a certain extent, the pressure difference between the upper and lower sections of the piston 3 is about 0.5 MPa. However, due to the different concentrations of the fracturing sand, the actual calculation values are different.

[0068] In the test calculation, the MATLAB is used to compile the calculation program to calculate the pressure difference between the upper and lower sections of the piston 3 at different stroke positions, and the injection and discharge capacity of the fracturing fluid is designed to be 1.2-4.5 m 3 / min, so as to calculate other parameters when the assumed pulsation frequency is 15 Hz.

[0069] In the actual engineering design calculation of the present application, the structure size of the cylinder body 2 and the piston 3 and the stiffness coefficient of the reset spring 7 are calculated and determined on the basis of the calculation of the variable cross-section flow passage fluid parameters in the piston 3 in combination with the simulation test.

[0070] In order to further realize the assumed pulsation frequency, the matched cylinder body 2, piston 3 and plug 6 on the support seat 8 are used to increase the outer peripheral flow passage section of the piston 3 when the piston 3 goes down, so as to realize the unloading pressure and the subsequent flow discharge, the reciprocating movement of the piston 3 and the guide passage 4 in the piston 3 can adapt to the large discharge capacity in the hydraulic fracturing construction, the piston 3 stroke is controlled in a certain range, so as to better realize the low-frequency pulsation output of the low-frequency pulsation hydraulic fracturing generating device.

[0071] According to the rock fatigue damage mechanics research theory, under the action of low-frequency pulsation, the rock crack initiation pressure is reduced, which promotes the hydraulic fracturing fracture; when the low-frequency pulsation fluctuation propagates in the crack, the low-frequency pulsation wave will theoretically superimpose, which causes the expansion of the pulsation pressure wave amplitude. The numerical analysis method simulation calculation shows that under certain conditions, the pulsation pressure wave amplitude can be expanded by about 1.0-5.0 times; the physical simulation test comparative analysis shows that the pulsation pressure amplitude can be expanded by about 2.0-4.0 times. The simulation of the field test pulsation hydraulic fracturing comparative analysis shows that the pulsation pressure amplitude can be expanded by 1.5-3.0 times.

[0072] The self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing device is a self-excited pulsating device completely in the downhole, and the fracturing sand is contacted with the plug 6 through the flow channel 4, and the plug 6 divides the fracturing sand into four flow holes 12 on the bearing seat 8, so as to avoid the water hammer effect; because under the action of the working hydraulic pressure, the piston 3 goes down, the plug 6 on the composite bearing seat 8 gradually enters the flow channel 4, so that the flow channel 4 is gradually closed, until the pressure difference between the upper and lower pistons 3 is less than the spring restoring force, so that the restoring spring 7 starts to push the piston 3 to go up, until the pressure generated by the fluid in the upper piston 3 is greater than the restoring force of the restoring spring 7 again, the piston 3 goes down again; when the piston 3 goes up, the high hydraulic impact force is generated on the rock in the opening moment of the flow channel 4 in the piston 3; through the intermittent opening and closing of the piston 3, the low-frequency pulsating hydraulic wave of the working fluid can act on the bottom hole rock, and the high-efficiency fracturing effect can be achieved; at the same time, under the action of the low-frequency pulsating hydraulic wave, the settlement of the fracturing sand is effectively inhibited.

[0073] Therefore, by using the device, the artificial fracture initiation pressure in hydraulic fracturing can be effectively reduced, the fracture conductivity can be improved, and the settlement of the fracturing sand in the fracture can be reduced, the sand distribution can be promoted, and the fracturing stimulation effect can be improved.

[0074] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing generating device, characterized in that, It comprises: a cylinder (2) which is a hollow cylindrical structure; a piston (3) arranged in the cavity of the cylinder (2), the piston (3) is generally frustum-shaped, a flow guide channel (4) is arranged in the piston (3) and extends along the axial direction of the piston (3) and penetrates the upper and lower ends of the piston (3); a supporting seat (8) and a plug (6) arranged on the supporting seat (8), the supporting seat (8) is also arranged in the cavity of the cylinder (2) and located at the lower end of the piston (3), the plug (6) is conical in shape and in contact with the flow guide channel (4), a plurality of flow guide holes (12) are uniformly arranged along the circumferential direction of the supporting seat (8); a sealing ring is sleeved on the outer periphery of the bottom of the piston (3) to prevent pressure from being released from the outer periphery, a reset member is arranged at the lower part of the piston (3), and the plug (6) is sleeved in the reset member; the cavity of the cylinder (2) is composed of three cavities, including a frustum cavity and small and large cylindrical cavities located at the two ends of the frustum cavity, the piston (3) is located in the frustum cavity, and the supporting seat (8) is located in the large cylindrical cavity; the taper of the plug (6) is 2: (10-12), the height is 120mm-180mm, and the diameter range is 0-20mm; the diameter of the flow guide channel (4) is 15mm-20mm, and the length is 180mm-200mm.

2. The self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing generating device according to claim 1, characterized in that, It also comprises an upper connector (1) and a lower connector (9), both of which are hollow cylindrical structures, the upper connector (1) is connected with the upper end of the cylinder (2), the lower connector (9) is connected with the lower end of the cylinder (2), and the upper connector (1), the flow guide channel (4), the flow guide hole (12) and the lower connector (9) are in communication with each other.

3. The self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing generating device according to claim 1, characterized in that, A recess is arranged on the outer periphery of the bottom of the piston (3), and a rubber ring (5) is sleeved in the recess.

4. The self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing generating device according to claim 1, characterized in that, The matching taper range of the cylinder (2) and the piston (3) is 1: (20-25).

5. The self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing generating device according to claim 1, characterized in that, The flow guide hole (12) is an arc-shaped hole with an arc length of 20-40mm.

6. The self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing generating device according to claim 1, characterized in that, The length of the small cylindrical cavity accounts for one fourth of the total length of the piston (3), and the taper of the lower part of the piston (3) is 1: (10-12).

7. A method for generating the self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing device according to any one of claims 1-6, characterized in that, It comprises the following steps: The upper end of the self-excited downhole low-frequency high-pressure pulsating hydraulic fracturing device is connected to the lower part of the fracturing pipe column (10), and the lower end is connected to the upper part of the sandblaster (11). In the fracturing pre-fluid injection stage, the overflow channel at the upper part of the piston (3) and the flow guide channel (4) play a throttling role. The piston (3) is subjected to water pressure and goes down, compressing the reset member. The compression of the reset member causes the plug (6) to enter the flow guide channel (4). When the piston (3) compresses the reset member to the maximum compression position, the plug (6) completely enters the flow guide channel (4). At this time, the pressure generated by the fluid at the upper part of the piston (3) is less than the reset force of the reset member. At this time, the reset member starts to push the piston (3) to go up until the pressure generated by the fluid at the upper part of the piston (3) is greater than the reset force of the reset member again. The piston (3) goes down, thereby generating reciprocating pulsating water power and low-frequency pulsating water power in the small-displacement hydraulic fracturing working fluid.

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