Acidification process for forming fishbone branches based on graded pore forming

Abstract

The invention discloses an acidification process for forming fishbone branches based on graded pore forming, and relates to the technical field of oil and gas field development engineering. Comprising the following steps that an injection tool set carried by an oil pipe is lowered to a target transformation position, acid liquid is injected to a stratum, and a guide hole is formed in the transformation position; pumping liquid is switched into filtrate reduction liquid, fixed-point injection is carried out on the same transformation position, and a high flow guide channel is formed; sand-carrying fluid is pumped into the stratum, sand blasting perforation is conducted on the transformation position, and fishbone-shaped branch hole channels are formed; the pump injection liquid is switched to be acid liquid, fixed-point injection is carried out on the same transformation position, the acid liquid rapidly rushes in along the fishbone-shaped branch hole channels to corrode stratum rocks to form wormholes, and meanwhile the acid liquid pushes sand grains to the deep portion of the stratum; when the acid liquid injection amount reaches a preset equivalent value, the oil pipe is dragged to the next specified target transformation position to repeat the operation; an oil-gas flow channel with high flow conductivity is formed, the segmented transformation effect is improved, the productivity of an oil well can be improved, operation is easy, and safety is high.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas field development engineering, and in particular to an acidizing process based on graded pore making to form fishbone branches. Background Art

[0002] Carbonate reservoirs are complex and highly heterogeneous, posing numerous challenges to horizontal well development, which has driven the continuous development and evolution of a range of stimulation technologies. Currently, three commonly used staged acid fracturing technologies for carbonate horizontal wells in the oil industry include perforation and packer staged acid fracturing, external packer and ball-dropping sliding sleeve staged acid fracturing, and coiled tubing drag staged acid fracturing.

[0003] The perforation + packer staged acid fracturing technique uses packers to separate a horizontal well section into several independent fracturing stages, then applies acid fracturing to each stage individually through the tubing string. Limitations of this technique include: It requires running packers and other tools into the wellbore, which places high demands on wellbore conditions and increases the complexity and cost of the operation; It also places high demands on the packer's performance; if the packer is not tightly sealed, acid crossflow may occur, compromising the acid fracturing effect; and when the horizontal well section is long, running and setting the packer is more difficult, increasing the operational risk.

[0004] The segmented acid fracturing technology, which combines external packers with ball-dropping sliding sleeves, involves pre-installing an external packer on the casing. This technology, combined with a completion string equipped with a sliding sleeve, activates the sleeves through ball-dropping, gradually opening the acid fracturing channels and achieving segmented reconstruction. Limitations of this technology include: The reliability of the sleeve tool must be guaranteed; if the sleeve malfunctions, it may not open or close properly, impacting the smooth progress of the acid fracturing operation; The sleeves must be highly resistant to pressure and acid; high-pressure, high-concentration acid can damage or corrode them; and When using a sliding sleeve driven by ball-dropping pressure to perform multi-stage reconstruction, problems with ball placement and sealing can occur, impacting the reconstruction of subsequent stages.

[0005] Coiled tubing drag-and-drag segmented acid fracturing involves running coiled tubing (CT) into the wellbore, carrying drillable bridge plugs or recoverable packers. The dragged tool isolates and applies acid fracturing section by section. After the installation, the plugs are drilled out or the packers are retrieved, restoring the full wellbore. Limitations of this technology include: low operational efficiency, the need for frequent tripping and running of the CT, and a long construction cycle. The CT is susceptible to wear and corrosion downhole, requiring regular inspection and replacement, increasing construction costs and maintenance workload. When performing segmented acid fracturing in long horizontal well sections, well depth is limited, and running and operating the CT to install the packers is challenging, requiring advanced technical skills and experience.

[0006] It can be seen that the existing conventional acid fracturing transformation technology has defects such as complex construction process, difficult operation, high performance requirements for downhole tools, and easy problems with sealing, which further increase construction risks and operating costs. These defects have greatly limited the efficient application and promotion of segmented acid fracturing technology in complex carbonate well conditions, long horizontal well sections and large-scale transformation scenarios. Summary of the Invention

[0007] The present invention aims to solve the technical problems of conventional acid fracturing reforming technology, such as complex construction process, great difficulty in operation, high performance requirements for downhole tools, and easy problems with setting, which lead to increased construction risks and construction costs. The present invention aims to provide an acidizing process based on staged pore creation to form fishbone branches. The process comprises the following four stages of pore creation: a first acid injection to form a guide hole, a filtrate reduction spray to form a high-conductivity channel, a sandblasting to form a fishbone-shaped branch hole, and a second acid injection for further dissolution. Ultimately, an oil and gas flow channel with high conductivity is formed, thereby improving the staged reforming effect and facilitating increased oil well productivity. The process is simple to operate, does not require complex tools and cumbersome construction processes, and does not cause sand jamming and other accidents that affect operation safety.

[0008] The present invention is achieved through the following technical solutions.

[0009] An acidification process for forming fishbone branches based on hierarchical pore formation comprises the following steps: S1. Lower the injection tool group carried by the tubing to the toe of the open hole horizontal section or the preset target stimulation location, and inject acid into the formation through the surface pumping system to form a pilot hole at the stimulation location; S2. Switch the pump injection liquid to filtrate reduction and implement fixed-point injection at the same transformation location to form a high-conductivity channel; S3. Pump sand-carrying fluid into the formation and perform sandblasting and perforation at the reconstructed location based on the high-conductivity channel to form fishbone-shaped branch channels. S4: Switch the pumping fluid to acid and perform targeted injection at the same transformation location. The acid rapidly advances along the fishbone-shaped branch channels, dissolving the formation rock to form wormholes. At the same time, the acid pushes the sand particles deeper into the formation. S5. When the acid injection volume reaches the preset equivalent value, drag the tubing to the next designated target transformation location; S6. Repeat steps S1 to S4 until the transformation of all preset targets in the naked eye horizontal segment is completed.

[0010] The acidizing process of this method first forms a 50-80 cm guide hole by spraying acid, and then sprays filtration reduction fluid to reduce the filtration loss of the guide hole, forming a high-conductivity channel, creating conditions for subsequent liquid breakthrough. After completing the above two steps, sand-carrying fluid is used for fixed-point sandblasting to form fishbone-shaped branches, forming a penetrating channel with a depth of 1-3 meters. Subsequently, acid is continuously sprayed on the basis of the bone-shaped branch holes. The acid liquid rapidly advances along the high-permeability channel to dissolve the formation rock to form 5-20 cm wormholes. At the same time, the acid liquid carries the sand particles to the deep part of the formation. As the formation pressure decreases, the sand particles form effective support during the formation closure process, ensuring the continuous effectiveness of the high-conductivity three-dimensional channel in the entire process, enhancing the reservoir transformation effect, expanding the horizontal well oil leakage area, and improving the oil well production capacity. It has strong adaptability to the staged transformation of open hole horizontal wells in low-permeability carbonate reservoirs.

[0011] In summary, the present invention adopts a first acid injection to form a guide hole, a fluid reduction spray to form a high-conductivity channel, a sandblasting to form a fishbone-shaped branch hole, and a second acid spray for further dissolution. Through a staged four-stage pore-making method, a high-conductivity oil and gas flow channel is finally formed, which improves the staged transformation effect and is conducive to increasing the productivity of the oil well. The operation is simple, does not require complex tools and cumbersome construction processes, and will not cause accidents such as sand sticking that affect operation safety.

[0012] Furthermore, the sand-carrying fluid refers to a linear glue with a sand ratio of 7 to 20%, and the proppant in the linear glue is abrasive or quartz sand.

[0013] Furthermore, the particle size of the abrasive is selected in accordance with the following principle: the nozzle diameter in the jetting tool set should be greater than 3 times the maximum particle size of the abrasive.

[0014] Furthermore, the quartz sand has a particle size of 40-70 mesh.

[0015] Furthermore, the sand ratio of the sand-carrying fluid is adjusted according to the formation permeability: When the formation permeability is less than 5 mD, a linear adhesive with a sand ratio of 7% is used; For formations with a permeability of 5-10 mD, a linear adhesive with a sand ratio of 10% is used; The formation permeability is 10-20 mD, and a linear glue with a sand ratio of 15% is used; The formation permeability is >20 mD, and a linear glue with a sand ratio of 20% is used.

[0016] Furthermore, steps S1-S4 are all performed under high pressure, with a pump pressure ≥ 55 MPa.

[0017] Furthermore, the acid solution used in steps S1 and S4 is hydrochloric acid with a mass concentration of 15% to 28%.

[0018] Furthermore, the mass concentration of the acid solution is adjusted according to the formation permeability: The formation permeability is less than 5 mD, and the hydrochloric acid concentration is ≥28%; The formation permeability is 5~10 mD, and the hydrochloric acid concentration is 25~28%; The formation permeability is 10-20 mD, and the hydrochloric acid concentration is 20-25%; The formation permeability is greater than 20 mD, and the hydrochloric acid concentration is 15-20%.

[0019] Furthermore, in step S5, when the acid injection volume reaches a preset equivalent value, the surface pumping system switches to a low-pressure mode with a pump pressure of 25 MPa to 35 MPa.

[0020] Furthermore, the method further includes step S7: After completing the transformation of all preset target points, the liquid in the oil pipe is replaced by clean water. After stopping the pump, the oil pipe is dragged out of the wellhead and the well is opened for flowback.

[0021] Compared with the prior art, the present invention has the following advantages and beneficial effects.

[0022] 1. The gravel acidizing process of this method first forms a 50-80 cm guide hole by spraying acid, and then sprays to reduce filtration loss, reduce the filtration loss of the guide hole, form a high-conductivity channel, and create conditions for subsequent liquid breakthrough. After completing the above two steps, sand-carrying fluid is used for fixed-point sandblasting to form fishbone-shaped branches, forming a penetrating channel with a depth of 1-3 meters. Subsequently, acid is continuously sprayed on the basis of the bone-shaped branch holes. The acid liquid rapidly advances along the high-permeability channel to dissolve the formation rock to form 5-20 cm wormholes. At the same time, the acid liquid carries the sand particles to the deep formation. As the formation pressure decreases, the sand particles form effective support during the formation closure process, ensuring the continuous effectiveness of the high-conductivity three-dimensional channel throughout the entire process, enhancing the reservoir transformation effect, expanding the horizontal well oil leakage area, and improving the oil well productivity. It has strong adaptability to the staged transformation of open hole horizontal wells in low-permeability carbonate reservoirs.

[0023] 2. The acidizing process of the present invention uses a first acid injection to form a guide hole, a filtrate reduction spray to form a high-conductivity channel, a sandblasting to form fishbone-shaped branch holes, and a second acid injection for further dissolution, ultimately forming a highly conductive oil and gas flow channel. This improves the effect of segmented transformation, does not require complex tools and cumbersome construction processes, and solves the problems of complex tools and cumbersome construction processes in the prior art. It achieves the goal of completing reservoir transformation of preset target points in all horizontal sections with a single string drag, thereby achieving the purpose of improving construction efficiency.

[0024] 3. The acidizing process of the present invention, for carbonate open-hole horizontal wells, can achieve segmented reforming through hydrodynamic isolation, without the need for additional downhole isolation tools. Actual case studies have demonstrated the effectiveness of the segmented reforming process of the present invention, preventing accidents such as sand sticking that could affect operational safety. DETAILED DESCRIPTION

[0025] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the technical solutions in the embodiments of the present invention are clearly and completely described below in conjunction with the embodiments. Obviously, the schematic implementation modes of the present invention and their descriptions are only used to explain the present invention and are not intended to limit the present invention.

[0026] The following is a detailed description of an embodiment of the present invention's acidification process for forming fishbone branches based on hierarchical pore formation. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions may be omitted. This is done to avoid unnecessary redundancy and to facilitate understanding by those skilled in the art.

[0027] The "ranges" disclosed herein are defined in terms of lower and upper limits. A given range is defined by selecting a lower limit and an upper limit, and the selected lower and upper limits define the boundaries of the particular range. Ranges defined in this manner may be inclusive or exclusive and may be arbitrarily combined, i.e., any lower limit may be combined with any upper limit to form a range.

[0028] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0029] Unless otherwise specified, all technical features and optional technical features of the present invention can be combined with each other to form a new technical solution.

[0030] Unless otherwise specified, the terms "include" and "comprising" used in the present invention may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other substances not listed may also be included or that only the listed substances are included.

[0031] Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0032] The technical solution of the present invention is further described in detail below with reference to the embodiments.

[0033] It should be noted that the experimental methods used in the examples are conventional methods unless otherwise specified. The materials, reagents, methods and instruments used are conventional materials, reagents, methods and instruments in the art, unless otherwise specified, and can be obtained from commercial channels by those skilled in the art.

[0034] Example 1 An acidification process for forming fishbone branches based on hierarchical pore formation comprises the following steps: S1. Lower the injection tool group carried by the tubing to the toe of the open hole horizontal section or the preset target stimulation location, and inject acid into the formation through the surface pumping system to form a pilot hole at the stimulation location; S2. Switch the pump injection liquid to filtrate reduction and implement fixed-point injection at the same transformation location to form a high-conductivity channel; S3. Pump sand-carrying fluid into the formation and perform sandblasting and perforation at the reconstructed location based on the high-conductivity channel to form fishbone-shaped branch channels. S4: Switch the pumping fluid to acid and perform targeted injection at the same transformation location. The acid rapidly advances along the fishbone-shaped branch channels, dissolving the formation rock to form wormholes. At the same time, the acid pushes the sand particles deeper into the formation. S5. When the acid injection volume reaches the preset equivalent value, the surface pumping system switches to low-pressure mode, with a pump pressure of 25MPa~35MPa, and drags the tubing at a low speed to the next designated target modification location; S6. Repeat steps S1 to S4 until the transformation of all preset targets in the naked eye horizontal segment is completed; S7. After completing the transformation of all preset target points, the liquid in the tubing is displaced with clean water. After stopping the pump, the tubing is dragged out of the wellhead and the well is opened for flowback. As the formation pressure decreases, the channels and micro-cracks formed by the transformation close, and the sand particles provide support, ensuring the completion of a high-conductivity three-dimensional channel throughout the entire process.

[0035] The acidizing process of this method first forms a 50-80 cm guide hole by spraying acid, and then sprays filtration reduction fluid to reduce the filtration loss of the guide hole, forming a high-conductivity channel, creating conditions for subsequent liquid breakthrough. After completing the above two steps, sand-carrying fluid is used for fixed-point sandblasting to form fishbone-shaped branches, forming a penetrating channel with a depth of 1-3 meters. Subsequently, acid is continuously sprayed on the basis of the bone-shaped branch holes. The acid liquid rapidly advances along the high-permeability channel to dissolve the formation rock to form 5-20 cm wormholes. At the same time, the acid liquid carries the sand particles to the deep part of the formation. As the formation pressure decreases, the sand particles form effective support during the formation closure process, ensuring the continuous effectiveness of the high-conductivity three-dimensional channel in the entire process, enhancing the reservoir transformation effect, expanding the horizontal well oil leakage area, and improving the oil well production capacity. It has strong adaptability to the staged transformation of open hole horizontal wells in low-permeability carbonate reservoirs.

[0036] In summary, the present invention adopts a first acid injection to form a guide hole, a fluid reduction spray to form a high-conductivity channel, a sandblasting to form a fishbone-shaped branch hole, and a second acid spray for further dissolution. Through a staged four-stage pore-making method, a high-conductivity oil and gas flow channel is finally formed, which improves the staged transformation effect and is conducive to increasing the productivity of the oil well. The operation is simple, does not require complex tools and cumbersome construction processes, and will not cause accidents such as sand sticking that affect operation safety.

[0037] The jetting tool assembly used in the present invention includes a guide shoe, a stabilizer, a shear-type safety release, a hydraulic safety release device, and a spray gun; wherein the spray gun is equipped with a Laval nozzle; the oil pipe includes a continuous oil pipe and a conventional oil pipe, and the outer diameter of the oil pipe, the outer diameter of the nozzle, and the number of nozzles are selected in accordance with the specifications in Table 1.

[0038] Table 1. Selection criteria for oil pipe and nozzle parameters

[0039] In the present invention, steps S1 to S4 are all performed under high pressure, with a pump pressure of ≥55 MPa. The pump pressure and displacement of the sand-carrying fluid pumped into the formation are selected in accordance with the specifications in Table 2.

[0040] Table 2. Selection criteria for pump pressure and displacement of sand-carrying fluid

[0041] In the present invention, the sand-carrying fluid refers to a linear glue with a sand ratio of 7 to 20%, and the proppant in the linear glue is abrasive or quartz sand.

[0042] The abrasive particle size selection follows the following principles: The nozzle diameter in the jetting tool assembly should be greater than three times the maximum abrasive particle size. For example, if the abrasive particle size is 2 mm, the nozzle diameter should be ≥ 6 mm to prevent flow channel blockage. In addition, for harder formations, the abrasive is harder and requires a wear-resistant nozzle. For example, diamond abrasive requires a wear-resistant tungsten carbide nozzle.

[0043] The quartz sand particle size specification is 40-70 mesh.

[0044] The sand ratio of the sand-carrying fluid is adjusted according to the formation permeability: When the formation permeability is less than 5 mD, a linear adhesive with a sand ratio of 7% is used; For formations with a permeability of 5-10 mD, a linear adhesive with a sand ratio of 10% is used; The formation permeability is 10-20 mD, and a linear glue with a sand ratio of 15% is used; The formation permeability is >20 mD, and a linear glue with a sand ratio of 20% is used.

[0045] The selection of the sand ratio of the sand-carrying fluid shall comply with the specifications in Table 3.

[0046] Table 3. Selection criteria for sand ratio of sand-carrying fluid

[0047] The selection of the viscosity of the sand-carrying fluid shall comply with the specifications in Table 4.

[0048] Table 4. Selection criteria for viscosity of sand-carrying fluid

[0049] In the present invention, the acid solution used in steps S1 and S4 is hydrochloric acid with a mass concentration of 15% to 28%.

[0050] The mass concentration of the acid solution is adjusted according to the formation permeability: The formation permeability is less than 5 mD, and the hydrochloric acid concentration is ≥28%; The formation permeability is 5~10 mD, and the hydrochloric acid concentration is 25~28%; The formation permeability is 10-20 mD, and the hydrochloric acid concentration is 20-25%; The formation permeability is greater than 20 mD, and the hydrochloric acid concentration is 15-20%.

[0051] The acid volume and concentration of the acid solution shall be selected in accordance with the specifications in Table 5.

[0052] Table 5. Selection criteria for acid volume and concentration of acid solution

[0053] Application Examples The gravel-packed herringbone acidizing process of the present invention is illustrated using a development well drilled to a depth of 3,700 meters and with an openhole horizontal section length of 1,190 meters. The reservoir lithology is limestone with an average formation permeability of less than 5 mD. The primary treatment fluids consisted of 28% hydrochloric acid, 15 cp linear gel, 40-70 mesh quartz sand, and clean water. A total of 25 targeted sites were treated. Except for the first three targets, the remaining 22 sites were deeply stimulated using the gravel-packed herringbone acidizing process. The downhole tool used was a 5,000-meter-long, 60.3-mm-outer-diameter coiled tubing and the jetting tool combination shown in Table 6.

[0054] Table 6. Jetting tool combination parameters

[0055] The method of Example 1 was used for construction, and the construction process was as follows: (1) Connect the coiled tubing and jetting tool assembly on the ground; (2) Lower the coiled tubing carrying the injection tool to the 3700-meter position in the open hole horizontal section, and perform only high-pressure pump acid modification at three target points at 3700 meters, 3650 meters, and 3600 meters respectively; (3) Drag the coiled tubing carrying the jetting tool to the target position at 3550 meters in the open hole horizontal section and start the acidizing construction of graded hole formation to form fishbone branches. The construction process is shown in Table 7 below.

[0056] (4) After the target position at 3550 m is transformed, repeat the above acidizing construction steps, drag the coiled tubing injection tool group to 3500 m, and then complete the reservoir transformation of the remaining 21 target points in the open hole horizontal section in turn.

[0057] (5) After completing the transformation of all open hole horizontal sections, pull the coiled tubing up and drag it to a depth of 2510 meters, using an 11m 3 Clean water replaces the acid in the coiled tubing, with a displacement of 1 m 3 / min, pump pressure 59-60 MPa, stop the pump, and pull the coiled tubing out of the wellhead.

[0058] Table 7. Acidification process for forming fishbone branches by graded pore formation according to the present invention

[0059] Finally, it should be noted that the above specific embodiments are only used to explain in detail the purpose, technical solutions and beneficial effects of the present invention. It should be understood that the above is only a specific implementation method of the present invention and is not used to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above specific embodiments, ordinary technicians in this field should understand that it is still possible to modify the technical solutions described in the above embodiments, or to replace or improve some or all of the technical features therein. These modifications, equivalent replacements, and improvements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered by the scope of the claims and description of the present invention.

Claims

1. An acidification process based on hierarchical pore formation to form fishbone branches, characterized in that: The following steps are involved: S1. Lower the injection tool group carried by the tubing to the toe of the open hole horizontal section or the preset target stimulation location, and inject acid into the formation through the surface pumping system to form a pilot hole at the stimulation location; S2. Switch the pump injection liquid to filtrate reduction and implement fixed-point injection at the same transformation location to form a high-conductivity channel; S3. Pump sand-carrying fluid into the formation and perform sandblasting and perforation at the reconstructed location based on the high-conductivity channel to form fishbone-shaped branch channels. S4: Switch the pumping fluid to acid and perform targeted injection at the same transformation location. The acid rapidly advances along the fishbone-shaped branch channels, dissolving the formation rock to form wormholes. At the same time, the acid pushes the sand particles deeper into the formation. S5. When the acid injection volume reaches the preset equivalent value, drag the tubing to the next designated target transformation location; S6. Repeat steps S1 to S4 until the transformation of all preset targets in the naked eye horizontal segment is completed.

2. The acidification process for forming fishbone branches based on hierarchical pore formation according to claim 1, characterized in that: The sand-carrying fluid refers to a linear glue with a sand ratio of 7-20%, and the proppant in the linear glue is abrasive or quartz sand.

3. The acidification process for forming fishbone branches based on hierarchical pore formation according to claim 2, characterized in that: The particle size of the abrasive is selected in accordance with the following principle: the nozzle diameter in the jet tool set should be greater than 3 times the maximum particle size of the abrasive.

4. The acidification process for forming fishbone branches based on hierarchical pore formation according to claim 2, characterized in that: The quartz sand particle size specification is 40-70 mesh.

5. The acidification process for forming fishbone branches based on hierarchical pore formation according to claim 2, characterized in that: The sand ratio of the sand-carrying fluid is adjusted according to the formation permeability: When the formation permeability is less than 5 mD, a linear adhesive with a sand ratio of 7% is used; For formations with a permeability of 5-10 mD, a linear adhesive with a sand ratio of 10% is used; The formation permeability is 10-20 mD, and a linear glue with a sand ratio of 15% is used; The formation permeability is >20 mD, and a linear glue with a sand ratio of 20% is used.

6. The acidification process for forming fishbone branches based on hierarchical pore creation according to claim 1, characterized in that: Steps S1-S4 are all carried out under high pressure, with a pump pressure ≥ 55 MPa.

7. The acidification process for forming fishbone branches based on hierarchical pore formation according to claim 1, characterized in that: The acid solution used in steps S1 and S4 is hydrochloric acid with a mass concentration of 15% to 28%.

8. The acidification process for forming fishbone branches based on hierarchical pore formation according to claim 7, characterized in that: The mass concentration of the acid solution is adjusted according to the formation permeability: The formation permeability is less than 5 mD, and the hydrochloric acid concentration is ≥28%; The formation permeability is 5~10 mD, and the hydrochloric acid concentration is 25~28%; The formation permeability is 10-20 mD, and the hydrochloric acid concentration is 20-25%; The formation permeability is greater than 20 mD, and the hydrochloric acid concentration is 15-20%.

9. The acidification process for forming fishbone branches based on hierarchical pore formation according to claim 1, characterized in that: In step S5, when the acid injection volume reaches a preset equivalent value, the surface pumping system switches to a low-pressure mode with a pump pressure of 25 MPa to 35 MPa.

10. The acidification process for forming fishbone branches based on hierarchical pore formation according to any one of claims 1 to 9, characterized in that: It also includes step S7: After completing the transformation of all preset target points, the liquid in the oil pipe is replaced by clean water. After stopping the pump, the oil pipe is dragged out of the wellhead and the well is opened for flowback.

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