Downhole borehole casing fracturing method

Abstract

The application discloses a downhole drilling casing fracturing method, which comprises the following steps: S1, drilling a surface layer, lowering a first casing and cementing a well, and forming a hole mouth wall protection; S2, performing directional pilot drilling in the first casing to form a pilot hole; S3, connecting a reaming casing drill bit with a fracturing casing, setting a pilot drill bit in the front end of the reaming casing drill bit, lowering the reaming casing drill bit into the pilot hole to perform reaming and drilling to a preset position, then lowering a fishing tool from inside the fracturing casing, unlocking and recovering the pilot drill bit, and leaving the fracturing casing in the pilot hole; S4, performing annular reverse circulation grouting and well cementing on the fracturing casing; S5, installing a tee joint injection head on the fracturing casing; S6, lowering a perforating tool through the tee joint injection head to perform a drag type hydraulic jet perforation; and S7, performing large-displacement casing fracturing through the tee joint injection head and the fracturing casing. The downhole drilling casing fracturing method has the advantages of high displacement, high efficiency and good well cementing effect.

Description

Technical Field

[0001] This invention relates to the field of downhole fracturing technology, and more specifically to a downhole drilling casing fracturing method. Background Technology

[0002] Hydraulic fracturing technology, as a key means of enhancing coal seam permeability and controlling surrounding rock in underground coal mines, has been widely used in many mining areas in my country. Currently, open-hole staged fracturing is the most common method used in underground coal mines. However, due to the special operating environment and geological conditions underground, this traditional process has many inherent defects, which seriously restricts the improvement of fracturing effect and the further promotion of the technology.

[0003] Firstly, regarding casing installation, downhole drilling is mostly upward-angled holes, and the drilling rig thrust is limited due to the constraints of roadway space. Traditional push-type casing installation methods struggle to overcome the enormous friction between the casing and the borehole wall, as well as the casing's own weight, resulting in limited casing installation depth, low success rate, and even failure to reach the designed layer.

[0004] Secondly, in terms of fracturing capabilities, traditional open-hole fracturing technology faces significant bottlenecks. Firstly, fracturing fluid is typically delivered through fracturing tubing with a narrow inner diameter, resulting in high fluid friction and severely limiting the increase in fracturing capacity. Secondly, the lack of a stable wellhead structure and a full-bore injection channel prevents direct fluid supply within the borehole, further restricting the fracturing capacity.

[0005] Third, the packers used in open-hole sections have limited pressure-bearing capacity, and increasing the discharge rate often involves an increase in pumping pressure, posing a risk of packer failure. These factors combined make it difficult for traditional processes to achieve fracturing operations with large discharge rates and high pumping pressures, thus limiting the complexity and extent of fracturing fractures.

[0006] In addition, open-hole fracturing also suffers from problems such as poor borehole wall stability leading to collapse, severe fracturing fluid leakage, difficulty in running the tool string, and unreliable packer setting effect. Therefore, there is an urgent need for a new process that can adapt to the underground coal mine environment, effectively solve the problem of casing running, and establish a stable large-diameter fracturing channel, in order to break through the existing technical bottlenecks and achieve efficient and reliable underground coal seam fracturing stimulation. Summary of the Invention

[0007] The present invention aims to at least partially solve one of the technical problems in the related art.

[0008] Therefore, embodiments of the present invention propose a method for fracturing downhole drilling casing.

[0009] The downhole casing fracturing method of this invention includes the following steps:

[0010] S1: First, surface drilling begins, then the casing is run in and cemented to form a borehole wall. S2: Perform second-stage directional pilot drilling within the first-stage casing to form a pilot hole; S3: Connect the reaming casing drill bit to the fracturing casing. A guide drill bit is set inside the front end of the reaming casing drill bit. The reaming casing drill bit is lowered into the pilot hole to ream the hole and drill to the preset position. Then, the retrieval tool is lowered from inside the fracturing casing, the guide drill bit is unlocked and retrieved, and the fracturing casing is left in the pilot hole. S4: Perform annular reverse circulation grouting cementing on the fracturing casing; S5: Install a tee injection head on the fracturing casing; S6: Dragging water jet perforation is performed using a three-way injection head and a lower injection port tool; S7: Large-volume casing fracturing is performed using a tee injection head and fracturing casing.

[0011] In some embodiments, in step S2, a pilot hole is formed by connecting a pilot sleeve to a pilot drill bit and drilling into an open sleeve.

[0012] In some embodiments, in step S3, the guide drill bit is threadedly connected to the reaming casing drill bit, and the rotational torque direction of the reaming casing drill bit is opposite to the thread direction of the guide drill bit.

[0013] In some embodiments, a retrieval drill rod is lowered into the fracturing casing, and the retrieval drill rod is aligned with and locked to the retrieval joint at the rear end of the guide drill bit. A rotational torque opposite in direction to the rotational torque of the reaming casing drill bit is applied to the guide drill bit through the retrieval drill rod, causing the guide drill bit to separate from the reaming casing drill bit. The retrieval drill rod is then lifted, and the guide drill bit is retrieved outside the pilot hole.

[0014] In some embodiments, step S4 specifically includes: injecting cement slurry from the wellhead into the annular space between the fracturing casing and the borehole wall to form a two-stage cement annulus; the cement slurry displaces the drilling fluid and gas at the bottom of the annulus into the interior of the fracturing casing and returns to the wellhead.

[0015] In some embodiments, in step S5, a hole-sweeping drill bit is placed inside the fracturing casing via a hole-sweeping sleeve to perform hole sweeping.

[0016] In some embodiments, the three-way injection head includes a first working port, a second working port, and a third working port. The first working port and the second working port are arranged opposite to each other. The first working port is in communication with the fracturing casing. The diameters of the first working port and the second working port are the same as the inner diameter of the fracturing casing. A gate valve is provided between the second working port and the third working port. The third working port is provided with a high-pressure hose for connecting to the fracturing pump.

[0017] In some embodiments, a fixing ring is provided on the tee injection head, and the ring support leg of the fixing ring abuts against the surrounding rock of the roadway to withstand the high pressure reaction force during fracturing.

[0018] In some embodiments, step S6 specifically includes: lowering a spray gun with a bridge plug packer to a preset position through a three-way injection head, setting the bridge plug packer to seal the lower well section, and then performing hydraulic jet perforation to penetrate the fracturing casing, cementing sheath and penetrate into the rock formation.

[0019] In some embodiments, in step S7, fracturing fluid is directly injected into the fracturing casing through a three-way injection head, and the fracturing casing is used as a flow channel for fracturing fluid to be pumped in large volumes.

[0020] The downhole casing fracturing method of this invention, firstly, utilizes an integrated enlarged casing-running process to ensure the casing can be smoothly run to the designed formation, solving the casing-running problem; secondly, it establishes a full-bore casing channel, significantly reducing fluid friction and enabling high-volume, high-pumping-pressure fracturing, thereby effectively increasing the complexity and spread of the fracturing fractures; thirdly, the robust casing and cementing layer provide reliable wellbore support, preventing open-hole collapse and severe fracturing fluid loss; simultaneously, using the casing instead of the open-hole packer for pressure bearing eliminates the risk of packer failure, improving the reliability and safety of the operation. This downhole casing fracturing method of this invention overcomes the bottlenecks of traditional processes in casing running, operational flow rate, wellbore stability, and tool reliability, providing a novel construction process for efficient and reliable coal seam permeability enhancement in coal mines. Attached Figure Description

[0021] Figure 1 This is a schematic flowchart of the downhole drilling casing fracturing method according to an embodiment of the present invention.

[0022] Figure 2 This is a schematic diagram of the installation of an open sleeve according to an embodiment of the present invention.

[0023] Figure 3 This is a construction schematic diagram of the pilot drill bit and pilot casing according to an embodiment of the present invention.

[0024] Figure 4 This is a construction schematic diagram of the reaming casing drill bit and the guide drill bit according to an embodiment of the present invention.

[0025] Figure 5 This is a construction schematic diagram of the two-section cement ring according to an embodiment of the present invention.

[0026] Figure 6 This is a construction diagram of the hole-sweeping drill bit and hole-sweeping sleeve according to an embodiment of the present invention.

[0027] Figure 7This is a construction diagram of the bridge plug packer and spray gun according to an embodiment of the present invention.

[0028] Figure 8 This is a schematic diagram of the connection between the three-way injection head and the fracturing casing in an embodiment of the present invention.

[0029] 1. Opening casing; 101. Opening flange; 102. Grouting port; 2. Pilot drill bit; 3. Pilot casing; 4. Pilot hole; 5. Enlarging casing drill bit; 6. Fracturing casing; 601. Opening flange; 7. T-junction injection head; 701. First working port; 702. Second working port; 703. Third working port; 8. Guide drill bit; 9. Fishing joint; 10. Opening cement ring; 11. Fixing ring clamp; 12. Ring clamp support leg; 13. Hole sweeping drill bit; 14. Hole sweeping casing; 15. Perforation; 16. Bridge plug packer; 17. Spray gun; 18. Spray gun casing; 19. Gate valve. Detailed Implementation

[0030] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0031] like Figures 1 to 8 As shown, the downhole casing fracturing method of this invention includes the following steps: S1: First, surface drilling begins, casing 1 is run in and cemented to form a borehole wall. S2: Perform second-stage directional pilot drilling within the first-stage casing 1 to form pilot hole 4; S3: Connect the reaming casing drill bit 5 to the fracturing casing 6. A guide drill bit 8 is set inside the front end of the reaming casing drill bit 5. The reaming casing drill bit 5 is lowered into the pilot hole 4 to ream the hole and drill to the preset position. Then, a retrieval tool is lowered from inside the fracturing casing 6, the guide drill bit 8 is unlocked and retrieved, and the fracturing casing 6 is left in the pilot hole 4. S4: Perform annular reverse circulation grouting and cementing on fracturing casing 6; S5: Install the tee injection head 7 on the fracturing casing 6; S6: Dragging water jet perforation 15 through the injection head 7 and the injection tool; S7: Large-volume casing fracturing is performed through the tee injection head 7 and the fracturing casing 6.

[0032] In the downhole casing fracturing method of this invention, a stable wellhead is first established by cementing an open casing 1. Then, a pilot hole 4 is formed using a directional drill bit 8 to provide precise guidance for subsequent operations. The reaming drill bit is connected to the fracturing casing 6 and run in one piece, completing the casing lowering while reaming, directly overcoming the problem of insufficient depth caused by friction and gravity components in traditional push-type casing lowering. After cementing, a full-bore, high-strength wellbore structure is formed, laying the foundation for subsequent construction. After installing the tee injection head 7, a precise perforation 15 is made inside the casing by drag-type hydraulic jetting, and finally, the casing itself is used as an injection channel for high-volume fracturing.

[0033] The downhole casing fracturing method of this invention, firstly, utilizes an integrated enlarged casing-running process to ensure the casing can be smoothly run to the designed formation, solving the casing-running problem; secondly, it establishes a full-bore casing channel, significantly reducing fluid friction and enabling high-volume, high-pumping-pressure fracturing, thereby effectively increasing the complexity and spread of the fracturing fractures; thirdly, the robust casing and cementing layer provide reliable wellbore support, preventing open-hole collapse and severe fracturing fluid loss; simultaneously, using the casing instead of the open-hole packer for pressure bearing eliminates the risk of packer failure, improving the reliability and safety of the operation. This downhole casing fracturing method of this invention overcomes the bottlenecks of traditional processes in casing running, operational flow rate, wellbore stability, and tool reliability, providing a novel construction process for efficient and reliable coal seam permeability enhancement in coal mines.

[0034] For example, in step S1, a drilling rig is used to construct an initial borehole to the designed depth (typically 10m-15m). A Φ168 initial casing 1 is then lowered into the borehole. Rapid-hardening cement is injected into the annulus from the bottom of the initial casing 1 to the borehole opening for full-length cementing, forming a stable borehole wall. An initial flange 101 is then installed on the initial casing 1.

[0035] In some embodiments, in step S2, the pilot sleeve 3 is connected to the pilot drill bit 2 and drills into an open sleeve 1 to form a pilot hole 4.

[0036] For example, such as Figure 2 As shown, within the first casing 1, a Φ89 pilot casing 3 and a Φ120 pilot drill bit 2 are lowered, and a blowout preventer is connected to the wellhead. Using measurement-while-drilling (MWD) to monitor the trajectory, the pilot drill bit 2 is controlled to directionally drill along the designed stratum, forming a Φ120 pilot hole 4. After drilling to the designed depth, the Φ89 pilot casing is removed, leaving the open-hole pilot hole 4. The use of the pilot casing 3 to assist drilling improves the quality and operational safety of the pilot hole 4, creating better conditions for casing lowering in a single-stage reaming process, fundamentally solving the core problem of difficulty in lowering the casing to the designed stratum.

[0037] In some embodiments, in step S3, the guide drill bit 8 is threadedly connected to the reaming casing drill bit 5, and the rotational torque direction of the reaming casing drill bit 5 is opposite to the thread direction of the guide drill bit 8.

[0038] For example, such as Figure 3 As shown, step S3 employs an integrated "drill-reamation-down" process, utilizing a high-torque threaded structure to solve the problems of deep-hole casing installation and inner casing retrieval. The specific steps are as follows: Assemble the guide drill bit 8 and the reaming casing drill bit 5, and lower them together into the pilot hole 4. The front end of the reaming casing drill bit 5 is a Φ120 guide drill bit 8 (used for insertion into the pilot hole 4 for alignment), and the rear end of the reaming casing drill bit 5 is connected to a Φ127 fracturing casing 6. The guide drill bit 8 and the reaming casing drill bit 5 are connected and fixed using a reverse thread (left-handed thread). The ground drilling rig applies torque through the Φ127 fracturing casing 6. The torque is transmitted to the internal guide drill bit 8 through the torque transmission housing of the reaming casing drill bit 5. At this time, due to the reverse thread connection, it ensures that the threads do not disengage during drilling, and reaming drilling is performed in the pilot hole 4 until the predetermined layer is reached.

[0039] In the downhole casing fracturing method of this invention, the directional drill bit 8 and the reaming casing bit 5 are connected by threads, and the rotational torque direction of the reaming casing bit 5 is designed to be opposite to the thread direction of the directional drill bit 8. During forward rotation for reaming drilling, this reverse thread connection tightens with each rotation, ensuring the integrity of the drill string assembly and the reliability of the transmission. When it is necessary to retrieve the directional drill bit 8, simply stopping rotation or applying a reverse torque allows the connection to be easily disengaged downhole, thus enabling the successful retrieval of the directional drill bit 8. This design directly and efficiently overcomes the problems of insufficient depth and low success rate caused by friction and gravity components in traditional push-type casing installation, and ensures the retrievability of the core tool.

[0040] In some embodiments, a retrieval drill rod (not shown in the figure) is lowered into the fracturing casing 6, and the retrieval drill rod is docked and locked with the retrieval joint 9 at the rear end of the guide drill bit 8. A rotational torque opposite to the rotational torque of the reaming casing drill bit 5 is applied to the guide drill bit 8 through the retrieval drill rod, so that the guide drill bit 8 is separated from the reaming casing drill bit 5. The retrieval drill rod is then lifted, and the guide drill bit 8 is retrieved to the outside of the pilot hole 4.

[0041] For example, such as Figure 3As shown, after drilling to the designed depth, stop rotating the fracturing casing 6. Lower the Φ89 retrieval drill rod into the Φ127 fracturing casing 6, connecting the front end of the retrieval drill rod to a dedicated retrieval tool (male taper or dedicated threaded connector). The retrieval tool contacts the retrieval connector 9 at the bottom of the hole, and the threads are locked together by forward rotation (right-hand). Continue applying forward rotation torque (right-hand). At this time, the torque is transmitted to the guide drill bit 8 via the "retrieval drill rod - retrieval connector 9". Since the guide drill bit 8 and the reaming casing drill bit 5 have reverse threads (left-hand threads), under the action of right-hand torque, the reverse threads are unscrewed. Lift the Φ89 drill rod, bringing the released guide drill bit 8 and retrieval connector 9 out of the hole together, leaving only the Φ127 fracturing casing 6 and the reaming casing drill bit 5 inside the hole.

[0042] Because the reverse threaded connection tightens as the drill string rotates forward during reaming, it ensures the integrity of the drill string assembly and the reliability of the transmission. When it is necessary to retrieve the directional drill bit 8, a retrieval drill rod is lowered into the fracturing casing 6, aligning and locking with the retrieval connector 9 at the rear end of the directional drill bit 8. A rotational torque opposite to that of the reaming casing drill bit 5 is applied to the directional drill bit 8 through the retrieval drill rod, causing the directional drill bit 8 to separate from the reaming casing drill bit 5. The retrieval drill rod is then lifted, retrieving the directional drill bit 8 outside the pilot hole 4. This operation process achieves the safe and reliable retrieval of the directional drill bit 8, completing the full "casing lowering-tool retrieval" work cycle. This design directly and efficiently overcomes the problems of insufficient depth and low success rate caused by friction and gravity components in traditional push-type casing lowering, while ensuring the retrievability of the core tool and reducing operating costs.

[0043] In some embodiments, step S4 specifically includes: injecting cement slurry from the wellhead into the annular space between the fracturing casing 6 and the borehole wall to form a two-stage cement annulus 10. The cement slurry replaces the drilling fluid and gas at the bottom of the annulus into the interior of the fracturing casing 6 and returns to the wellhead.

[0044] For example, such as Figure 5 As shown, reverse circulation grouting is performed using the grouting port 102 on the side wall of the open casing 1. The specific instructions and steps are as follows: Cement slurry is injected through the grouting port 102 of the open casing 1, entering the annular space between the Φ127 fracturing casing 6 and the Φ168 borehole wall. Under pump pressure, the slurry flows towards the bottom of the hole, forcing residual drilling fluid and gas at the bottom of the hole into the interior of the Φ127 fracturing casing 6 and back out towards the borehole opening, achieving complete slurry replacement. When fresh cement slurry returns from inside the fracturing casing 6, grouting is stopped, and borehole cleaning is performed after logging is completed.

[0045] In step S4, annular reverse circulation grouting cementing is performed on the fracturing casing 6. Specifically, cement slurry is injected from the wellhead into the annular space between the fracturing casing 6 and the borehole wall to form a two-stage cement sheath 10. The cement slurry displaces the drilling fluid and gas at the bottom of the annulus into the interior of the fracturing casing 6 and returns to the wellhead. This cementing method ensures that the cement slurry continuously and stably fills and displaces the annulus from bottom to top, effectively avoiding problems such as channeling, gas encapsulation, or displacement that may occur in conventional forward circulation cementing due to well deviation, uneven annular space, etc. This results in a dense, complete, and well-bonded cement sheath, providing uniform and high-strength external support for the fracturing casing 6.

[0046] In some embodiments, in step S5, a drilling bit 13 is placed inside the fracturing casing 6 via a drilling casing 14 to perform drilling. For example... Figure 6 As shown, in step S5, before installing the tee injection head 7, the sweeping drill bit 13 is connected to the sweeping casing 14 and placed inside the fracturing casing 6 for sweeping. This sweeping operation aims to remove cement blocks, mud, or other debris that may remain inside the casing after cementing, ensuring that the inner wall of the fracturing casing 6 is smooth and the borehole is intact, clearing obstacles for the smooth passage of subsequent perforating tools and fracturing fluid. After completing the sweeping and confirming that the inside of the casing is clean, the tee injection head 7 is then installed.

[0047] In some embodiments, the three-way injection head 7 includes a first working port 701, a second working port 702, and a third working port 703. The first working port 701 and the second working port 702 are disposed opposite to each other. The first working port 701 is connected to the fracturing casing 6. The diameter of the first working port 701 and the second working port 702 is the same as the inner diameter of the fracturing casing 6. A gate valve 19 is provided between the second working port 702 and the third working port 703. The third working port 703 is provided with a high-pressure hose for connecting to the fracturing pump.

[0048] For example, such as Figures 6 to 8 As shown, after the second-stage cement sheath 10 has solidified, the tee injection head 7 is installed. The tee injection head 7 is installed on the second-stage flange 601 of the fracturing casing. The main diameter of the tee injection head 7 is consistent with the inner diameter of the Φ127 fracturing casing 6, maintaining a full-bore state without any diameter reduction or throttling elements. The first working port 701 and the second working port 702 are of the same diameter as the fracturing casing 6, forming a full-bore, non-variable straight flow channel from the surface equipment to the target layer at the bottom of the well. The perforating tool string can be unobstructedly lowered and pulled out through the second working port 702; the third working port 703 is connected to the fracturing pump through a high-pressure hose, serving as the injection channel for fracturing fluid.

[0049] In some embodiments, a fixing ring 11 is provided on the three-way injection head 7, and the ring support leg 12 of the fixing ring 11 abuts against the surrounding rock of the roadway to withstand the high pressure reaction force during fracturing.

[0050] For example, such as Figures 6 to 8 As shown, a fixed ring 11 is provided on the three-way injection head 7, and its ring support leg 12 abuts against the surrounding rock of the roadway to withstand the high-pressure reaction force during fracturing. This design directly transmits the huge axial thrust generated during fracturing to the stable surrounding rock of the roadway through a rigid structure, rather than being borne solely by the casing thread or wellhead flange, greatly enhancing the stability and impact resistance of the entire wellhead device.

[0051] In some embodiments, step S6 specifically includes: lowering a spray gun 17 with a bridge plug packer 16 into a preset position through a three-way injection head 7, setting the bridge plug packer 16 to seal the lower well section, and then performing hydraulic jet perforation 15 to penetrate the fracturing casing 6, cementing sheath and penetrate into the rock formation.

[0052] For example, such as Figure 7 As shown, in step S6, the bridge plug packer 16 is set inside the casing, providing precise segmental isolation for the perforation 15 and subsequent fracturing, ensuring that fracturing energy is concentrated on the target coal seam. The hydraulic jet perforation 15 technology utilizes high-speed fluid to directly cut the casing, cement sheath, and rock strata, forming a clean and regular perforation channel, avoiding the compaction damage and irregular perforation problems that may be caused by traditional shaped charge perforations 15. Dragging hydraulic jet perforation 15 is performed through the perforation tool lowered by the stable three-way injection head 7, ultimately utilizing the complete channel formed by this device and the fracturing casing 6 for high-pressure, high-volume fracturing. The use of the bridge plug packer 16 in conjunction with the hydraulic jet perforation 15 method achieves precise positioning and efficient fracture creation of the target coal seam, creating a high-quality initial fracture entrance for subsequent high-volume casing fracturing, ensuring the effective transfer of fracturing energy.

[0053] In some embodiments, in step S7, fracturing fluid is directly injected into the fracturing casing 6 through the three-way injection head 7, and the fracturing casing 6 is used as the flow channel for fracturing fluid to be pumped in large volume.

[0054] For example, such as Figure 8 As shown, the wellhead gate valve 19 is closed, and the third working port 703 is connected to the fracturing pump through a high-pressure hose. The Φ127 fracturing casing 6 is used directly as the fracturing channel for pumping. Since a large-diameter channel with full bore is established, the construction discharge is no longer limited by small-diameter tools, and the full load output of the pumping capacity can be achieved, thereby obtaining a more complex fracture network and a farther range of impact.

[0055] Therefore, in step S7, the fracturing fluid is directly injected into the fracturing casing 6 through the third working port 703 of the three-way injection head 7, and the fracturing casing 6 itself serves as the flow channel for high-volume pumping of the fracturing fluid. The fracturing fluid enters the full-bore fracturing casing 6 directly through the high-pressure hose and the three-way injection head 7, flows through the orifice formed by the previously perforated hole 15, and enters the target coal seam. Because the entire injection channel (from pump to formation) has a large diameter and extremely low friction, and the wellhead is anchored to the surrounding rock by the fixed ring 11, it can withstand extremely high pumping pressure. Therefore, it is possible to safely and stably achieve high-volume, high-pressure construction that is impossible with traditional processes.

[0056] The downhole casing fracturing method of this invention firstly employs a pilot casing 3 to assist drilling, improving the quality and operational safety of the pilot hole 4. Secondly, the reverse threaded connection design and the accompanying retrieval process ensure tool stability during the integrated enlargement and casing running process, and reliably separate and retrieve the directional drill bit 8 after completion, not only solving the problem of deep casing running but also improving tool utilization and operational economy. Thirdly, the annular reverse circulation grouting cementing process forms a high-quality cement sheath, greatly enhancing the integrity and pressure-bearing capacity of the wellbore, effectively sealing off non-target formations, and preventing ineffective leakage of fracturing fluid in non-target formations. Furthermore, the post-cementing cleaning process is a crucial step in ensuring the success of subsequent operations, ensuring the absolute unobstructed flow of the internal channels of the fracturing casing 6. Most importantly, a specially designed equal-diameter tee injection head 7 is directly connected to the fracturing casing 6, constructing a bottleneck-free, high-pressure injection system from the surface pumping equipment to the bottom of the well. This structure completely solves the high friction problem caused by the use of small-diameter fracturing tubing in traditional processes and provides a stable and reliable wellhead device. Specifically, the design of the fixed ring 11 anchors the wellhead device to the surrounding rock of the roadway, enabling it to safely and reliably withstand the enormous reaction force generated by high-volume, high-pressure fracturing. This fundamentally solves the bottleneck problems of traditional open-hole fracturing, such as the lack of a stable wellhead structure and limited construction pressure. Finally, the use of a bridge plug packer 16 in conjunction with hydraulic jet perforation 15 achieves precise positioning and efficient fracture creation of the target coal seam, creating a high-quality initial fracture entrance for subsequent high-volume casing fracturing and ensuring the effective transfer of fracturing energy. Ultimately, high-volume pumping is performed using the fracturing casing 6 as a direct injection channel. The downhole casing fracturing method of this invention completely overcomes the fundamental defects of traditional open-hole fracturing processes, which cannot achieve high-volume, high-pressure construction due to high tubing friction, weak wellhead structure, and limited packer pressure resistance. This allows for the injection of higher energy into the coal seam, forming a more complex and extensive fracture network, thereby greatly improving the effectiveness of coal seam permeability enhancement and gas extraction. This technology systematically overcomes the bottlenecks of traditional processes in areas such as borehole guidance, tool reliability, casing installation, cementing quality, wellbore cleaning, wellhead fixing, perforation accuracy, drilling displacement and pressure, and wellbore stability, providing a completely new solution for achieving efficient and reliable coal seam permeability enhancement in underground coal mines.

[0057] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0058] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0059] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection, an electrical connection, or a connection that allows communication between them; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0060] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0061] In this invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0062] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for fracturing downhole drilling casing, characterized in that, Includes the following steps: S1: First, surface drilling begins, a casing (1) is installed and cemented to form a borehole wall. S2: Perform second-stage directional pilot drilling within the first-stage casing (1) to form a pilot hole (4); S3: Connect the reaming casing drill bit (5) to the fracturing casing (6). Set a guide drill bit (8) inside the front end of the reaming casing drill bit (5). Lower the reaming casing drill bit (5) into the pilot hole (4) to ream the hole and drill to the preset position. Then lower the retrieval tool from inside the fracturing casing (6), unlock and retrieve the guide drill bit (8), and leave the fracturing casing (6) in the pilot hole (4). S4: Perform annular reverse circulation grouting cementing on the fracturing casing (6); S5: Install a tee injection head (7) on the fracturing casing (6); S6: Drag-type hydraulic jet perforation (15) is performed through the injection head (7) and the injection tool. S7: Large-volume casing fracturing is performed through the three-way injection head (7) and fracturing casing (6).

2. The downhole casing fracturing method according to claim 1, characterized in that, In step S2, the pilot hole (4) is formed by connecting the pilot sleeve (3) to the pilot drill bit (2) and drilling into an open sleeve (1).

3. The downhole casing fracturing method according to claim 1, characterized in that, In step S3, the guide drill bit (8) is threadedly connected to the reaming casing drill bit (5), and the rotational torque direction of the reaming casing drill bit (5) is opposite to the thread direction of the guide drill bit (8).

4. The downhole casing fracturing method according to claim 3, characterized in that, The retrieval drill rod is lowered into the fracturing casing (6) and connected and locked with the retrieval joint (9) at the rear end of the guide drill bit (8). A rotational torque opposite to that of the reaming casing drill bit (5) is applied to the guide drill bit (8) through the retrieval drill rod, so that the guide drill bit (8) is separated from the reaming casing drill bit (5). The retrieval drill rod is then lifted to retrieve the guide drill bit (8) to the outside of the pilot hole (4).

5. The downhole casing fracturing method according to claim 1, characterized in that, In step S4, the annular reverse circulation grouting cementing specifically includes: injecting cement slurry from the wellhead into the annular space between the fracturing casing (6) and the borehole wall to form a two-stage cement annulus (10). The cement slurry replaces the drilling fluid and gas at the bottom of the annulus into the interior of the fracturing casing (6) and returns to the wellhead.

6. The downhole casing fracturing method according to claim 1, characterized in that, In step S5, the hole-sweeping drill bit (13) is connected to the hole-sweeping sleeve (14) and placed inside the fracturing sleeve (6) for hole sweeping.

7. The downhole casing fracturing method according to claim 1, characterized in that, The three-way injection head (7) includes a first working port (701), a second working port (702) and a third working port (703). The first working port (701) and the second working port (702) are arranged opposite to each other. The first working port (701) is connected to the fracturing casing (6). The diameter of the first working port (701) and the second working port (702) is the same as the inner diameter of the fracturing casing (6). A gate valve (19) is provided between the second working port (702) and the third working port (703). The third working port (703) is provided with a high-pressure hose for connecting to the fracturing pump.

8. The downhole casing fracturing method according to claim 7, characterized in that, A fixed ring hoop (11) is provided on the three-way injection head (7). The ring hoop support leg (12) of the fixed ring hoop (11) abuts against the surrounding rock of the roadway to withstand the high pressure reaction force during fracturing.

9. The downhole casing fracturing method according to claim 1, characterized in that, Step S6 specifically includes: lowering a spray gun (17) with a bridge plug packer (16) to a preset position through a three-way injection head (7), setting the bridge plug packer (16) to seal the lower well section, and then performing hydraulic jet perforation (15) to penetrate the fracturing casing (6), cementing sheath and penetrate into the rock formation.

10. The downhole casing fracturing method according to claim 7, characterized in that, In step S7, fracturing fluid is directly injected into the fracturing casing (6) through the three-way injection head (7), and the fracturing casing (6) is used as the flow channel for fracturing fluid to be pumped in large volume.

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