Method for creating microfractures in a subterranean formation using a slotted tubular string

Abstract

This invention relates to the field of petroleum technology and discloses a downhole slotted tubing string and a method for creating microfractures in the formation. The downhole slotted tubing string includes an outer cylinder assembly (22), a spray gun assembly (30), and a piston assembly (21) slidably inserted within the outer cylinder assembly. The upper end of the outer cylinder assembly (22) is connected to a first packer (1), the top end of the spray gun assembly is connected to the bottom of the piston assembly, and the bottom end of the spray gun assembly is connected to a second packer (4). The injection fluid entering through the first packer causes the piston assembly to drive the spray gun assembly downward, and simultaneously sprays out from the nozzle (32) of the spray gun assembly through the injection fluid flow channel (50) to create longitudinal slots in the sidewall of the wellbore and the formation. These longitudinal slots enable direct communication between the wellbore and the deep formation area, and promote the connection and continuous expansion of formation capillary channels, thereby achieving the purpose of creating microfractures in the formation.

Description

Technical Field

[0001] This invention relates to the field of petroleum technology, specifically to a downhole slotted tubing string. Based on this, it also relates to a method for creating microfractures in formations. Background Technology

[0002] During the oil extraction process, after years of production and development, a large number of oil and water wells accumulate a large amount of dead oil, cement, dirt, chemical residues and other foreign organic impurities in the pores of the rock formations near the wellbore in the production zone. Over time, these deposits and blockages cause a decrease in permeability in the near-wellbore zone, resulting in a decrease in the production capacity and injection capacity of a large number of oil and water wells.

[0003] Commonly used methods for unblocking near-wellbore areas of mid-to-late-stage oil and water wells include: deep perforation with shells, acidizing, sidetracking, and small-scale fracturing. However, these conventional unblocking measures all have several problems. For example, deep perforation with shells is simple and easy to implement, and it is effective for unblocking new wells. However, for wells that have been producing for many years, repeated perforation can cause severe damage to the production zone and severe collapse and breakage of the cement sheath outside the wellbore. Furthermore, shell perforation still generates new compaction zones and contamination zones, and its ability to clear formation blockages caused by later production is limited. Acidizing is a conventional unblocking measure in oilfields, but indiscriminate acid injection can lead to uneven acid etching due to the uneven distribution of formation pores. Some severely blocked layers have low acid resistance and poor unblocking effectiveness. Sidetracking and small-scale fracturing are quick to take effect, but they are expensive. For many ordinary production wells and injection wells, due to cost and capacity limitations, using sidetracking and fracturing is uneconomical.

[0004] Therefore, there is an urgent need for an apparatus and method to solve the problem of low-cost measures being ineffective and effective measures being costly in the aforementioned production processes. Summary of the Invention

[0005] The purpose of this invention is to overcome the problem of declining production capacity of oil and water wells in the prior art, and to provide a downhole slotted tubing string that has the advantage of creating microfractures in the formation.

[0006] To achieve the above objectives, the present invention provides a downhole slotting string, which includes an outer cylinder assembly, a spray gun assembly, and a piston assembly slidably disposed within the outer cylinder assembly. A first packer is connected to the upper end of the outer cylinder assembly, the top end of the spray gun assembly is connected to the bottom of the piston assembly, and a second packer is connected to the bottom end of the spray gun assembly. A jet fluid flow channel is formed within the piston assembly, communicating with both the first packer and the spray gun assembly. Jet fluid entering through the first packer causes the piston assembly to drive the spray gun assembly downwards, and simultaneously ejects from the nozzle of the spray gun assembly through the jet fluid flow channel, thereby creating longitudinal slots in the wellbore sidewall and formation.

[0007] Optionally, the piston assembly includes an upper piston rod, a piston, and a lower piston rod that are sequentially and sealed together, and the injection fluid flow channel includes a first fluid channel formed inside the upper piston rod, a piston fluid channel formed on the piston, and a second fluid channel formed inside the lower piston rod.

[0008] Optionally, the oil chamber outer cylinder assembly includes an upper piston sealing end, an oil chamber outer cylinder, and a lower piston sealing end connected in sequence. The upper piston sealing end is sealed and fitted on the outside of the upper piston rod, the oil chamber outer cylinder is sealed and fitted on the outside of the piston, and the lower piston sealing end is sealed and fitted on the outside of the lower piston rod.

[0009] Optionally, the outer wall of the upper piston rod, the inner wall of the outer cylinder of the oil chamber, the bottom wall of the upper piston sealing end, and the top surface of the piston are arranged to form an upper oil chamber, and the outer wall of the lower piston rod, the inner wall of the outer cylinder of the oil chamber, the top wall of the lower piston sealing end, and the bottom surface of the piston are arranged to form a lower oil chamber.

[0010] Optionally, the piston has a first liquid passage and a second liquid passage spaced circumferentially on its side wall, the first liquid passage and the second liquid passage connecting the upper oil chamber and the lower oil chamber.

[0011] Optionally, a damping orifice is provided at the top of the first fluid passage, which allows hydraulic oil to flow from the lower oil chamber to the upper oil chamber, and a one-way outlet valve is provided at the bottom of the second fluid passage, which allows hydraulic oil to flow from the upper oil chamber to the lower oil chamber.

[0012] Optionally, a spring is provided in the lower oil chamber, with one end of the spring connected to the bottom surface of the piston and the other end connected to the top wall of the sealing end of the lower piston.

[0013] Optionally, a first limiting member is connected to the top of the upper piston rod. The outer diameter of the first limiting member is larger than the inner diameter of the upper piston sealing end, so as to restrict the downward movement of the first limiting member when it contacts the upper piston sealing end.

[0014] Optionally, a second limiting member is provided between the top surface of the piston and the sealing end of the upper piston to restrict the upward movement of the piston.

[0015] Optionally, the outer walls of the upper piston sealing end and the lower piston sealing end are respectively formed with oil injection holes for injecting hydraulic oil into the upper oil chamber and the lower oil chamber.

[0016] Optionally, a telescopic joint assembly is connected between the spray gun assembly and the second packer. The telescopic joint assembly includes an inner telescopic section and an outer telescopic section that are sealed to each other. The spray gun assembly is sleeved on the outside of the outer telescopic section and can drive the outer telescopic section to move up and down relative to the inner telescopic section.

[0017] Optionally, a check valve is connected to the bottom of the second packer to allow liquid in the wellbore to flow unidirectionally into the second packer.

[0018] Optionally, the first packer and the second packer each include a packer sleeve that can expand under a preset pressure to sealably connect to the inner wall of the wellbore.

[0019] Optionally, the spray gun assembly includes a spray gun body with a mounting hole, the nozzle being mounted in the mounting hole and configured to slope downwards from the inside out.

[0020] A second aspect of the present invention provides a method for creating microfractures in a formation, the method employing the aforementioned downhole slotted tubing string and comprising the following steps:

[0021] S1: Connect the first packer to one end of the oil pipe, and connect the other end of the oil pipe to the pump set on the ground;

[0022] S2: The downhole slotted tubing string is lowered into the target formation within the wellbore via the tubing;

[0023] S3: The pump unit on the surface injects the injection fluid into the downhole slotted string through the tubing, and enables the injection fluid to be ejected from the spray gun assembly;

[0024] S4: Increase the pressure inside the downhole slotted tubing to the preset pressure of the first packer and the second packer, so that the first packer and the second packer begin to expand and seal the gap between them and the wellbore;

[0025] S5: The pressure inside the downhole slotted tubing continues to rise, and the upper end of the piston assembly moves downward under pressure, which in turn moves the spray gun assembly downward, so that the injection fluid can cut a longitudinal slot in the sidewall of the wellbore.

[0026] Optionally, the method for creating microfractures in the formation further includes the following steps:

[0027] S6: After the injection fluid completes the slotting, the displacement fluid is pumped into the downhole slotting string through the pump group on the ground to squeeze the injection fluid into the formation, and at the same time the wellbore gate valve of the high-pressure wellhead on the ground is closed.

[0028] S7: Shut down the pump group on the ground. After the pressure in the tubing drops to 0MPa, the pressure in the downhole slotted tubing drops, the first packer and the second packer retract, and the piston assembly drives the spray gun assembly to move upward.

[0029] S8: After the pressure inside the wellbore drops to a safe value, open the wellbore gate valve;

[0030] S9: After the pressure inside the wellbore drops to 0MPa, lift the downhole slotted tubing string upwards.

[0031] In this invention, a piston assembly drives a spray gun assembly to cut longitudinal slots in the sidewall of the wellbore and the formation, creating a staggered channel between the injected fluid and the return fluid in the wellbore. This reduces the offsetting factors between the incoming and outgoing fluids, achieving direct communication between the wellbore and the deep formation. After high-pressure, high-volume injection of the injected fluid, the fluid used for cutting the slots can continue to fill the deep formation along the original pores at the end of the cut, promoting the continuous expansion, growth, and deepening of the formation capillary channels. This also allows multiple pores in more distant formations to further connect, merge, and expand, generating more micro-fractures. This achieves the goal of creating micro-fractures in the formation. Furthermore, a single high-pressure pump injection system on the surface can independently complete the implementation, resulting in low operating costs, good performance, and simplified downhole operations. It can be widely applied in fields such as oil extraction. Attached Figure Description

[0032] Figure 1 This is a cross-sectional view of the downhole slotted tubing string in this invention;

[0033] Figure 2 yes Figure 1 Enlarged schematic diagram of point I in the middle.

[0034] Explanation of reference numerals in the attached figures

[0035] 1-First packer; 21-Piston assembly; 211-Upper piston rod; 212-Piston; 213-Lower piston rod; 214-First limiting member; 215-Second limiting member; 22-Oil chamber outer cylinder assembly; 221-Upper piston sealing end; 222-Oil chamber outer cylinder; 223-Lower piston sealing end; 224-Oil injection hole; 30-Spray gun assembly; 31-Spray gun body; 32-Nozzle; 4 - Second packer; 41- One-way valve; 42- Packer sleeve; 50- Injection fluid flow channel; 51- First fluid channel; 52- Piston fluid channel; 53- Second fluid channel; 61- Upper oil chamber; 62- Lower oil chamber; 71- First fluid passage; 72- Second fluid passage; 81- Damping orifice; 82- One-way outlet valve; 9- Spring; 10- Telescopic joint assembly; 11- Telescopic inner section; 12- Telescopic outer section. Detailed Implementation

[0036] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention. In the invention, unless otherwise stated, directional terms such as "upper" and "lower" generally refer to the orientation shown in the accompanying drawings, and "inner" and "outer" refer to the inner and outer sides relative to the outline of each component itself.

[0037] like Figure 1 As shown, this invention provides a downhole slotting string, which includes an outer casing assembly 22, a spray gun assembly 30, and a piston assembly 21 slidably inserted within the outer casing assembly 22. A first packer 1 is connected to the upper end of the outer casing assembly 22. The top end of the spray gun assembly 30 is connected to the bottom of the piston assembly 21, and a second packer 4 is connected to the bottom end of the spray gun assembly 30. A jet fluid flow channel 50 is formed within the piston assembly 21, communicating with both the first packer 1 and the spray gun assembly 30. The jet fluid entering through the first packer 1 causes the piston assembly 21 to drive the spray gun assembly 30 downwards, and simultaneously ejects from the nozzle 32 of the spray gun assembly 30 through the jet fluid flow channel 50, thereby creating longitudinal slots in the wellbore sidewall and formation. The wellbore can also be referred to as casing, and the jet fluid can be jet slurry liquid.

[0038] In this invention, the piston assembly 21 drives the spray gun assembly 30 to cut longitudinal slots in the sidewall of the wellbore and the formation, creating a staggered channel between the injected fluid and the return fluid in the wellbore. This reduces the offsetting factors between the incoming and outgoing fluids, achieving direct communication between the wellbore and the deep formation. After high-pressure, high-volume injection of the injected fluid, the fluid used for cutting the slots can continue to fill the deep formation along the original pores at the end of the cut, promoting the continuous expansion, growth, and deepening of the formation capillary channels. It also promotes further communication, fusion, and expansion of multiple pores in more distant formations, generating more microfractures. This achieves the goal of creating microfractures in the formation. Furthermore, a single high-pressure pump injection system on the surface can independently complete the implementation, resulting in low operating costs, good performance, and simplified downhole operations. It can be widely applied in fields such as oil extraction. Specifically, the injected fluid enters the formation from the bottom of the longitudinal slot, while the return fluid in the wellbore enters the wellbore from the top of the longitudinal slot.

[0039] Furthermore, both the first packer 1 and the second packer 4 include a packer sleeve 42, which can expand under a preset pressure to sealably connect to the inner wall of the wellbore. Specifically, after the packer sleeve 42 expands, it seals the wellbore section, causing pressure build-up in the formation area where the wellbore section is sealed, and its vertical position is fixed, so that the injection fluid sprayed by the spray gun assembly 30 can only continue to fill the deep formation along the end of the fractured formation and the original pores. The first packer 1 and the second packer 4 are preferably expandable K344 packers.

[0040] Reference Figure 2 The piston assembly 21 includes an upper piston rod 211, a piston 212, and a lower piston rod 213, which are sequentially and sealed together. The injection fluid flow channel 50 includes a first fluid channel 51 formed inside the upper piston rod 211, a piston fluid channel 52 formed on the piston 212, and a second fluid channel 53 formed inside the lower piston rod 213. The upper piston rod 211 and the lower piston rod 213 can be threaded to both ends of the piston 212, or they can be connected to both ends of the piston 212 via a connecting structure.

[0041] Further, the oil chamber outer cylinder assembly 22 includes an upper piston sealing end 221, an oil chamber outer cylinder 222, and a lower piston sealing end 223 connected in sequence. The upper piston sealing end 221 is sealed and fitted onto the outside of the upper piston rod 211, the oil chamber outer cylinder 222 is sealed and fitted onto the outside of the piston 212, and the lower piston sealing end 223 is sealed and fitted onto the outside of the lower piston rod 213. The outer wall of the upper piston rod 211, the inner wall of the oil chamber outer cylinder 222, the bottom wall of the upper piston sealing end 221, and the top surface of the piston 212 enclose to form an upper oil chamber 61. The outer wall of the lower piston rod 213, the inner wall of the oil chamber outer cylinder 222, and the top surface of the piston 212 together form an upper oil chamber 61. The top wall of the lower piston sealing end 223 and the bottom surface of the piston 212 enclose a lower oil chamber 62. A first fluid passage 71 and a second fluid passage 72 are circumferentially spaced on the side wall of the piston 212, communicating with the upper oil chamber 61 and the lower oil chamber 62. A damping hole 81 is provided at the top of the first fluid passage 71, allowing hydraulic oil to flow from the lower oil chamber 62 to the upper oil chamber 61. A one-way outlet valve 82 is provided at the bottom of the second fluid passage 72, allowing hydraulic oil to flow from the upper oil chamber 61 to the lower oil chamber 62. The inner surfaces of the upper piston sealing end 221 and the lower piston sealing end 223 are provided with grooves for accommodating sealing rings. The upper piston rod 211 and the lower piston rod 213 are sealed by the sealing rings within these grooves. Furthermore, the piston 212 is also provided with a groove for accommodating a sealing ring, and the piston 212 achieves sealing with the outer cylinder 222 of the oil chamber through the sealing ring in the groove. In one embodiment, the stroke of the piston assembly 21 is preferably 50mm to 70mm. It is understood that the first packer 1 can be sealed to the upper end of the upper piston sealing end 221 through an adapter and a connector. Moreover, the upper piston sealing end 221 and the lower piston sealing end 223 can be threaded to both ends of the outer cylinder 222 of the oil chamber, or they can be connected to both ends of the outer cylinder 222 of the oil chamber through a connecting structure.

[0042] Furthermore, a spring 9 is installed inside the lower oil chamber 62. One end of the spring 9 is connected to the bottom surface of the piston 212, and the other end is connected to the top wall of the lower piston sealing end 223. When the pressure at the top of the upper piston rod 211 is less than the pressure at the bottom surface of the piston 212, the spring 9 enables the piston assembly 21 to rise and reset quickly, while the upper oil chamber 61 flows into the lower oil chamber 62 through the one-way oil outlet valve 82. In addition, since the damping orifice 81 is a specially designed fluid channel, the piston assembly 21 can achieve extremely slow and uniform linear movement under a pressure of 30MPa to 50MPa, thereby driving the spray gun assembly 30 to cut longitudinal slots in the sidewall of the wellbore and the formation.

[0043] In one embodiment, a first limiting member 214 is connected to the top of the upper piston rod 211. The outer diameter of the first limiting member 214 is larger than the inner diameter of the upper piston sealing end 221, so as to restrict the downward movement of the first limiting member 214 when it contacts the upper piston sealing end 221. The first limiting member 214 is threadedly connected to the top of the upper piston rod 211. It is understood that in other embodiments, the upper piston rod 211 and the first limiting member 214 may be an integral structure. One stroke of operation ends when the first limiting member 214 moves to the upper surface of the upper piston sealing end 221.

[0044] Furthermore, a second limiting member 215 is provided between the top surface of the piston 212 and the upper piston sealing end 221 to restrict the upward movement of the piston 212. In one embodiment, the second limiting member 215 can be threaded into the interior of the upper piston sealing end 221.

[0045] In one embodiment of the present invention, the outer walls of the upper piston sealing end 221 and the lower piston sealing end 223 are respectively formed with oil injection holes 224 for injecting hydraulic oil into the upper oil chamber 61 and the lower oil chamber 62.

[0046] Furthermore, in one embodiment, the spray gun assembly 30 includes a spray gun body 31 with mounting holes and a nozzle 32 installed in the mounting holes. The nozzle 32 is configured to slope downwards from the inside out. This allows the spray liquid to be sprayed in a downward direction, thereby directly agitating the sediment between the nozzle 32 and the second packer 4, making it less likely for abrasive sand on the formation to deposit on the expanded packer sleeve 42, which facilitates the easy recovery of the second packer 4. If the nozzle 32 is sloped upwards, this function is weakened, and the potential risks are greater than if it were sloped downwards. The spray gun body 31 can have 2-4 mounting holes evenly formed, which are also configured to slope downwards from the inside out. The nozzle 32 is a straight nozzle made of an ultra-hard material. In addition, the spray gun assembly 30 may include a conversion head. The outer surface of the conversion head is provided with a sealing external thread, and the inner surface of the upper half of the spray gun body 31 is provided with an internal thread that mates with the external thread, thereby achieving a sealed connection between the conversion head and the spray gun body 31.

[0047] Furthermore, a telescopic joint assembly 10 is connected between the spray gun assembly 30 and the second packer 4. This telescopic joint assembly 10 includes a telescopic inner section 11 and a telescopic outer section 12 that are sealed to each other. The spray gun assembly 30 is sleeved on the outside of the telescopic outer section 12 and can drive the telescopic outer section 12 to move up and down relative to the telescopic inner section 11. The outer surface of the telescopic outer section 12 is provided with a sealing external thread, and the inner surface of the lower half of the spray gun body 31 is provided with an internal thread that mates with the external thread, thereby achieving a sealed connection between the telescopic outer section 12 and the spray gun body 31. In addition, in one embodiment, the inner surface of the telescopic outer section 12 is provided with a groove for accommodating a sealing ring. The telescopic outer section 12 achieves a seal with the telescopic inner section 11 through the sealing ring located in the groove. Furthermore, the connection to the second packer 4 can be achieved through an adapter that is sealed to the outside of the telescopic inner section 11. When the pressure at the top of the upper piston rod 211 is less than the pressure at the bottom of the piston 212, the spring 9 allows the piston assembly 21 to quickly rise and return to its original position. Simultaneously, oil flows from the upper oil chamber 61 to the lower oil chamber 62 through the one-way outlet valve 82, and the telescopic outer section 12 also returns to its original position. By providing telescopic inner section 11 and telescopic outer section 12 that can move relative to each other, the up-and-down movement of the spray gun assembly 30 does not affect the operation of the second packer 4.

[0048] Reference Figure 1 The bottom of the second packer 4 can be connected to a one-way valve 41 to allow the liquid in the wellbore to flow into the second packer 4 in one direction, thereby venting the gas in the injection fluid flow channel 50 and allowing the injection fluid to enter the injection fluid flow channel 50.

[0049] A second aspect of the present invention provides a method for creating microfractures in a formation, the method employing the aforementioned downhole slotted tubing string and comprising the following steps:

[0050] S1: Connect the first packer 1 to one end of the oil pipe, and connect the other end of the oil pipe to the pump set on the ground;

[0051] S2: The downhole slotted tubing string is lowered into the target formation within the wellbore via the tubing;

[0052] S3: The pump unit on the surface injects the injection fluid into the downhole slotted string through the tubing, and enables the injection fluid to be ejected from the spray gun assembly 30;

[0053] S4: Increase the pressure inside the downhole slotted tubing to the preset pressure of the first packer 1 and the second packer 4, so that the first packer 1 and the second packer 4 begin to expand and seal the gap between them and the wellbore;

[0054] S5: The pressure inside the downhole slotted tubing continues to rise, and the upper end of the piston assembly 21 moves downward under pressure, which in turn moves the spray gun assembly 30 downward, so that the sprayed fluid can cut a longitudinal slot in the side wall of the wellbore.

[0055] Specifically, when high-pressure injection fluid is pumped into the tubing, the injection fluid is transmitted to the spray gun assembly 30 via the first packer 1 and the injection fluid flow channel 50. A portion of the injection fluid is ejected from multiple nozzles 32 of the spray gun assembly 30, while the remaining injection fluid is transmitted to the check valve 41 via the expansion joint assembly 10 and the second packer 4. The valve ball inside the check valve 41 is then pressed, closing the fluid flow channel within the downhole slotted tubing. All injection fluid can only flow out through the nozzles 32. As the pressure within the downhole slotted tubing rises to the preset pressure of the first packer 1 and the second packer 4, the packer sleeve 42 begins to expand and extend, sealing the gap between the downhole slotted tubing and the wellbore. Furthermore, the upper end of the piston assembly 21 moves downward under pressure, driving the spray gun assembly 30 downward, allowing the injection fluid to cut a longitudinal slot in the sidewall of the wellbore.

[0056] The hydraulic fracturing capacity of this method for creating microfractures in formations is less than that of hydraulic fracturing. However, because the longitudinal fractures cut out can serve as guide fractures for formation fracturing, the pressure-pressurizing fracturing capacity is stronger than that of hydraulic jetting perforation or slotting alone. Furthermore, this method can be implemented independently by a single high-pressure pump injection system on the surface, resulting in low operating costs, good performance, and simplified downhole operations. It can be widely applied in fields such as oil extraction.

[0057] In addition, the method for creating microfractures in this formation also includes the following steps:

[0058] S6: After the injection fluid completes the slotting, the displacement fluid is pumped into the downhole slotting string through the pump group on the ground to squeeze the injection fluid into the formation, and at the same time the wellbore gate valve of the high-pressure wellhead on the ground is closed.

[0059] S7: Shut down the pump group on the ground. After the pressure in the tubing drops to 0MPa, the pressure in the downhole slotted tubing drops, the first packer 1 and the second packer 4 retract, and the piston assembly 21 drives the spray gun assembly 30 to move upward.

[0060] S8: After the pressure inside the wellbore drops to a safe value, open the wellbore gate valve;

[0061] S9: After the pressure inside the wellbore drops to 0MPa, lift the downhole slotted tubing string upwards.

[0062] Even after the gate valve at the surface high-pressure wellhead is closed, there is still a certain pressure inside the wellbore. As one implementation method, after the gate valve has been closed for one hour and the pressure inside the wellbore has returned to a safe level, the gate valve is opened to control the discharge of fluid from the wellbore until the pressure inside the wellbore drops to 0 MPa. Then, the downhole slotted tubing is lifted upwards, ending the operation.

[0063] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various specific technical features in any suitable manner. To avoid unnecessary repetition, the present invention will not describe the various possible combinations separately. However, these simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A downhole slotted tubing string, characterized in that, The downhole slotting string includes an oil chamber outer cylinder assembly (22), a spray gun assembly (30), and a piston assembly (21) that slides through the oil chamber outer cylinder assembly (22). The upper end of the oil chamber outer cylinder assembly (22) is connected to a first packer (1), and the oil chamber outer cylinder assembly (22) includes an upper piston sealing end (221), an oil chamber outer cylinder (222), and a lower piston sealing end (223) connected in sequence. The top of the spray gun assembly (30) is connected to the bottom of the piston assembly (21), and the bottom of the spray gun assembly (30) is connected to a second packer (4). A telescopic joint assembly (10) is connected between the spray gun assembly (30) and the second packer (4). The telescopic joint assembly (10) includes a telescopic inner section (11) and a telescopic outer section (12) that are sealed to each other. The spray gun assembly (30) is sleeved on the outside of the telescopic outer section (12) and can drive the telescopic outer section (12) to move up and down relative to the telescopic inner section (11). The piston assembly (21) has a jet fluid flow channel (50) that communicates with the first packer (1) and the spray gun assembly (30) respectively. The piston assembly (21) includes an upper piston rod (211), a piston (212), and a lower piston rod (213) that are sequentially and sealed together. The upper piston sealing end (221) is sealed on the outside of the upper piston rod (211), the oil chamber outer cylinder (222) is sealed on the outside of the piston (212), and the lower piston sealing end (223) is sealed on the outside of the lower piston rod (213). The jet fluid flow channel (50) includes a first fluid channel (51) formed inside the upper piston rod (211), a piston fluid channel (52) formed on the piston (212), and a second fluid channel (53) formed inside the lower piston rod (213). The injection fluid entering through the first packer (1) enables the piston assembly (21) to drive the spray gun assembly (30) downward, and simultaneously sprays out from the nozzle (32) of the spray gun assembly (30) through the injection fluid flow channel (50) to cut longitudinal slots in the sidewall of the wellbore and the formation. The outer wall of the upper piston rod (211), the inner wall of the oil chamber outer cylinder (222), the bottom wall of the upper piston sealing end (221), and the top surface of the piston (212) enclose to form the upper oil chamber (61). The outer wall of the lower piston rod (213), the inner wall of the oil chamber outer cylinder (222), the top wall of the lower piston sealing end (223), and the bottom surface of the piston (212) enclose to form the lower oil chamber (62). A spring (9) is provided in the lower oil chamber (62). One end of the spring (9) is connected to the bottom surface of the piston (212), and the other end is connected to the top wall of the sealing end (223) of the lower piston. The piston (212) has a first fluid passage (71) and a second fluid passage (72) axially spaced along the side wall. The first fluid passage (71) and the second fluid passage (72) connect the upper oil chamber (61) and the lower oil chamber (62). The top of the first fluid passage (71) is provided with a damping hole (81), which allows hydraulic oil to flow from the lower oil chamber (62) to the upper oil chamber (61). The bottom of the second fluid passage (72) is provided with a one-way outlet valve (82), which allows hydraulic oil to flow from the upper oil chamber (61) to the lower oil chamber (62).

2. The downhole slotted tubing string according to claim 1, characterized in that, The top of the upper piston rod (211) is connected to a first limiting member (214), the outer diameter of which is larger than the inner diameter of the upper piston sealing end (221), so as to restrict the first limiting member (214) from moving downward when it contacts the upper piston sealing end (221).

3. The downhole slotted tubing string according to claim 1, characterized in that, A second limiting member (215) is provided between the top surface of the piston (212) and the sealing end (221) of the upper piston to restrict the upward movement of the piston (212).

4. The downhole slotted tubing string according to claim 1, characterized in that, The outer walls of the upper piston sealing end (221) and the lower piston sealing end (223) are respectively provided with oil injection holes (224) for injecting hydraulic oil into the upper oil chamber (61) and the lower oil chamber (62).

5. The downhole slotted tubing string according to claim 1, characterized in that, The bottom of the second packer (4) is connected to a one-way valve (41) to allow liquid in the wellbore to flow unidirectionally into the second packer (4).

6. The downhole slotted tubing string according to claim 1, characterized in that, The first packer (1) and the second packer (4) each include a packer sleeve (42), which can expand under a preset pressure to seal against the inner wall of the wellbore.

7. The downhole slotted tubing string according to claim 1, characterized in that, The spray gun assembly (30) includes a spray gun body (31) with a mounting hole, and the nozzle (32) is mounted in the mounting hole and is configured to be inclined downward from the inside to the outside.

8. A method for creating microfractures in a formation, characterized in that, The method for creating microfractures in the formation employs a downhole slotted tubing string as described in any one of claims 1-7, and includes the following steps: S1: Connect the first packer (1) to one end of the oil pipe, and connect the other end of the oil pipe to the pump set on the ground; S2: The downhole slotted tubing string is lowered into the target formation within the wellbore via the tubing; S3: The pump unit on the ground injects the injection fluid into the downhole slotted string through the tubing, and the injection fluid is ejected from the nozzle (32) of the spray gun assembly (30); S4: Increase the pressure inside the downhole slotted tubing to the preset pressure of the first packer (1) and the second packer (4), so that the first packer (1) and the second packer (4) begin to expand and seal the gap between them and the wellbore; S5: The pressure inside the downhole slotted tubing continues to rise, and the upper end of the piston assembly (21) moves downward under pressure, which in turn drives the spray gun assembly (30) to move downward, so that the sprayed fluid can cut a longitudinal slot in the side wall of the wellbore.

9. The method for creating microfractures in formation according to claim 8, characterized in that, The method for creating microfractures in the formation further includes the following steps: S6: After the injection fluid completes the slotting, the displacement fluid is pumped into the downhole slotting string through the pump group on the ground to squeeze the injection fluid into the formation, and at the same time the wellbore gate valve of the high-pressure wellhead on the ground is closed. S7: The pump group on the ground is shut down. After the pressure in the tubing drops to 0MPa, the pressure in the downhole slotted tubing drops, the first packer (1) and the second packer (4) retract, and the piston assembly (21) drives the spray gun assembly (30) to move upward. S8: After the pressure inside the wellbore drops to a safe value, open the wellbore gate valve; S9: After the pressure inside the wellbore drops to 0MPa, lift the downhole slotted tubing string upwards.

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