A casing collar type downhole plugging device for a malignant lost circulation formation and a construction method thereof

Abstract

This invention provides a casing-short-section downhole plugging device and its construction method for severely lost circulation formations. The device integrates a drop-out connector, packer, casing short section, and drilling structure into a single tool string. Through the combined action of the upper packer, rigid casing short section, and lower packer structure, a stable, sealed zone completely isolated from the wellbore is formed around the lost circulation section. The rigid casing short section's rigid wall then directly severs the fluid channel between the lost circulation formation and the wellbore, thereby achieving permanent structural plugging of the severely lost circulation formation. This invention employs a ball-drop pressure differential sequential start-up method, allowing drilling, packing, and drop-out operations to be completed sequentially in a single downhole operation. The process is simple, improving construction safety and reliability. Furthermore, the device is not dependent on the specific morphology of the lost circulation channel and exhibits good plugging effects on fractured, cavernous, and highly heterogeneous lost circulation formations, showing broad application prospects.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas well drilling engineering technology, specifically relating to a casing short-section downhole plugging device for severe lost circulation formations and its construction method. Background Technology

[0002] In oil and gas well drilling, downhole leakage is a common and serious engineering problem that significantly impacts drilling safety and efficiency, especially in formations with well-developed fractures, strong dissolution, or significant heterogeneity, where severe leakage or even total leakage accidents are prone to occur. Such leakage is often characterized by poor wellbore stability, large-scale leakage channels, and strong connectivity. Conventional plugging materials struggle to form an effective retention within these channels, leading to low plugging success rates, numerous rework attempts, and in severe cases, even well abandonment.

[0003] Existing technologies for handling severe leakage mainly include (1) chemical plugging materials, (2) packer-assisted plugging technology, and (3) expansion tube and liner-type structural plugging technology. Among them, (1) chemical plugging technology mainly relies on the bridging and consolidation ability of plugging materials in cracks or pores, which is difficult to deal with large-scale, highly interconnected leakage channels; (2) packer-assisted plugging usually isolates the leakage section with a single packer to provide a space for the plugging agent to stay, but it is still a plugging idea for filling the plugging agent, and the sealing reliability is poor, making it difficult to permanently seal the leakage channel; (3) expansion tube or liner-type technology can achieve well section isolation to a certain extent, but the construction process is complicated, the requirements for wellbore conditions are high, and it is difficult to ensure smooth running and reliable setting under severe leakage conditions.

[0004] In addition, existing plugging operations require multiple well runs and multiple process switching, resulting in long construction cycles and high operational risks, making it difficult to meet the engineering requirements of "rapid plugging, reliable isolation, and continuous drilling" under complex well conditions.

[0005] In summary, there is an urgent need for a downhole plugging device that can complete drilling, plugging, isolation, and detachment operations in a single well run, in order to improve the efficiency and success rate of treating severe lost circulation formations. Summary of the Invention

[0006] To address the problems of low success rate, complex construction process, and high downhole operation risk in existing technologies for plugging leaks in severe leak formations, this invention provides a casing short-section downhole plugging device and its construction method for severe leak formations. Specifically, it integrates drilling, sealing, and release functions into the same tool string. The upper packer, rigid casing short section, and lower sealing structure work together to form a stable and sealed zone completely isolated from the wellbore around the leaking section. Then, the rigid pipe wall of the rigid casing short section directly cuts off the fluid channel between the leaking formation and the wellbore, thereby achieving permanent structural sealing of the severe leak formation.

[0007] To achieve the above objectives, the present invention provides a casing short section downhole plugging device for severely lost circulation formations, characterized in that, with the end near the wellhead as the upper part and the end near the bottom of the well as the lower part, the device comprises, from top to bottom, a release connector, an upper packer, a rigid casing short section, and a drilling end structure.

[0008] The release connector is a hydraulic release connector, which adopts a ball-throwing pressure differential start method. Its start pressure is higher than the setting pressure of the upper packer, and it is used to achieve release after packing is completed.

[0009] The upper packer is located above the target stratum with severe leakage. It is a packer activated by ball drop pressure differential and has a shearable ball seat assembly inside. After ball drop, it closes the central flow channel of the device to establish the pressure differential between the inside and outside of the tube required for setting.

[0010] The rigid casing section completely spans and covers the entire severe leakage formation, with its top and bottom ends connected to the upper packer and the drill end structure, respectively.

[0011] Below the target severely lost formation and above the drill end structure, a lower packer structure is provided along the axial direction of the rigid casing short section. The lower packer structure has a pressure-holding and locking function after forming, and is used to cooperate with the upper packer to form a bidirectional pressure-bearing packer boundary above and below the lost well section.

[0012] After the lower packer structure is formed, it, together with the upper packer and the rigid casing short section, forms a stable closed zone that is completely isolated from the wellbore around the lost circulation section. The fluid channel between the lost circulation formation and the wellbore is directly cut off by the rigid pipe wall of the rigid casing short section, thereby achieving permanent structural sealing of the severely lost circulation formation.

[0013] In a preferred embodiment, the release connector includes a shear pin and a sliding sleeve structure. When the pressure inside the pipe reaches the designed release pressure, the shear pin is sheared off, and the connection between the upper drilling tool and the lower well-keeping and plugging device is released through the sliding sleeve structure.

[0014] In a preferred embodiment, after the upper packer is set, the shearable ball seat assembly is sheared by applying pressure inside the tube to restore the central diameter of the device, so that the pressure inside the tube can continue to be transmitted downward to the lower packer structure.

[0015] In a preferred embodiment, the lower sealing structure is a hydraulically expandable sleeve external packer, the inner cavity of which is connected to the central diameter of the device, and the sealing sleeve is radially expanded by pressurizing inside the pipe to complete the setting seal;

[0016] Preferably, the hydraulic expansion sleeve packer is provided with a liquid locking mechanism inside. The liquid locking mechanism is a one-way valve pressure-holding structure. After the setting is completed and the pressure is released, it can lock the pressure inside the sleeve cavity, so that the packer can continuously maintain a sealed state.

[0017] In a preferred embodiment, the lower sealing structure is a cement sealing section formed by injecting cement slurry into the lower end of a rigid casing sub and solidifying it. The cement sealing section completely fills the wellbore annulus and central channel below the target lost formation, forming a permanent lower sealing boundary. Preferably, the lower end of the rigid casing sub is provided with a one-way cement injection hole for one-way injection of cement slurry to prevent backflow of cement slurry.

[0018] In a preferred embodiment, the casing short section is a casing short section conforming to API standards, and its length is designed based on the thickness of the toxic leakage formation and the lengths of its upper and lower safety sections.

[0019] In a preferred embodiment, the drilling end structure is used to drill or disturb the formation at the bottom of the well, so that the rigid casing sub is run down to the designed well depth;

[0020] Preferably, the drilling end structure is a cemented carbide welded integrated drilling structure or a drillable drill bit.

[0021] In a preferred embodiment, the method is applicable to fractured, pore-cavity, and highly heterogeneous loss formations.

[0022] This invention also provides a construction method for sealing severely leaking formations using any of the above-mentioned devices, comprising the following steps:

[0023] S1 matches the corresponding length of rigid casing short sections according to the length of the target lost formation section. The casing short section with the drilling end structure, the upper packer and the release joint are sealed and connected in sequence. The prefabrication and installation of the lower packer structure are completed. Then, the release joint is connected to the drilling tool to form a complete drilling tool string for plugging operations.

[0024] S2 completes the drilling operation of the target well section through the drilling end structure, ensuring that the rigid casing short section completely crosses and covers the entire target severe leakage formation, the lower packer structure is located below the target severe leakage formation, and the upper packer is located above the target severe leakage formation;

[0025] S3, based on the type of the lower packer structure, completes the forming of the lower packer structure and the setting of the upper packer according to the corresponding process, forming a stable closed zone with bidirectional pressure above and below the lost well section;

[0026] After S4 completes the full section isolation, the release operation is completed by pressurizing the pipe to trigger the release joint, thus separating the upper drilling tools from the lower well-keeping and plugging device.

[0027] S5 pulls out the upper drill string and directly cuts off the fluid channel between the lost formation and the wellbore through the rigid wall of the rigid casing short section, completing the permanent structural sealing of the severe lost formation.

[0028] In a preferred embodiment, the method is used for real-time plugging of leaks encountered during drilling when serious leakage occurs, or for remedial permanent plugging of leaked sections encountered after well completion.

[0029] In a preferred embodiment, in step S3, when the lower sealing structure is a hydraulically expandable casing external packer, the specific construction steps are as follows:

[0030] S3-1a: Insert a sealing ball that matches the shearable ball seat assembly into the drill string, so that the sealing ball sits in the central flow channel of the sealing device, and pressurize the ground to the setting pressure differential designed for the upper packer to complete the setting;

[0031] S3-2a: Continue to increase the pressure inside the pipe to the designed shear pressure, shear the shearable ball seat assembly, the ball seat body falls off, restore the center diameter of the drill string, so that the pressure inside the pipe can be transmitted downward to the lower sealing structure.

[0032] S3-3a: The pressure inside the pipe continues to increase to the design setting pressure difference of the lower packer structure, which drives the sealing rubber sleeve of the hydraulic expansion type casing external packer to expand radially and stick tightly to the well wall, completing the setting and forming. After depressurization, the pressure inside the rubber sleeve cavity is locked by the internal liquid locking mechanism to maintain the sealing state continuously.

[0033] Preferably, the setting pressure differential of the upper packer, the shear pressure differential of the shearable ball seat assembly, the setting pressure differential of the lower packer structure, and the release pressure differential of the release joint are set in a gradient increasing manner.

[0034] In a preferred embodiment, in step S3, when the lower sealing structure is a cement sealing section, the specific construction steps are as follows:

[0035] S3-1b: Inject cement slurry into the target well section at the lower end of the rigid casing sub. After solidification to form a cement plug section, a permanent sealing boundary is formed.

[0036] S3-2b: Insert a sealing ball that matches the shearable ball seat assembly into the drill bit, so that the sealing ball sits in the central flow channel of the sealing device. The ground is pressurized to the setting pressure differential designed for the upper packer to complete the setting and seal. It works in conjunction with the lower cement sealing section to form a stable closed zone with bidirectional pressure.

[0037] Compared with the prior art, the technical solution of the present invention has the following advantages:

[0038] 1. This invention breaks through the traditional technical approach of relying on bridging and consolidation of plugging materials, and proposes a new approach for structural plugging of severe leakage formations using a casing short section plus upper and lower sealing boundaries. By constructing a local rigid liner structure that spans the severe leakage formation within the well, long-term isolation of the leakage channel is achieved at the structural level, significantly improving the success rate of treating severe leakage formations.

[0039] 2. This invention integrates the drilling end structure, packer, and release connector into the same tool string, enabling drilling, setting, and release operations to be completed continuously in a single well run, reducing the engineering risks associated with multiple well runs and complex process switching.

[0040] 3. For fractured, severely lost circulation formations, this invention employs a tiered design for the activation pressure of the upper packer, lower packer, and release joint to achieve orderly triggering of various downhole functional units, ensuring packer reliability and release safety, and improving the device's adaptability to complex well conditions. For cavernous and highly heterogeneous lost circulation formations, this invention uses a cement-sealed section for the lower packer structure, filling it into the wellbore annulus and central channel below the target lost circulation formation to form a permanent lower packer boundary. Therefore, this invention's device is independent of specific lost circulation channel morphology and exhibits good adaptability to fractured, cavernous, and highly heterogeneous lost circulation formations, making it particularly suitable for severe lost circulation conditions where conventional plugging techniques are ineffective. Attached Figure Description

[0041] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0042] Figure 1 This is a schematic diagram of the casing short section downhole plugging device according to Embodiment 1 of the present invention;

[0043] Figure 2 This is a construction flowchart of the casing short-section downhole plugging device according to Embodiment 1 of the present invention;

[0044] Figure 3 This is a schematic diagram of the casing short section downhole plugging device according to Embodiment 2 of the present invention;

[0045] Figure 4 This is a construction flowchart of the casing short-section downhole plugging device according to Embodiment 2 of the present invention.

[0046] Key reference numerals: 1. Drill pipe; 2. Drop joint; 3. Upper packer; 4. Casing sub; 5. External packer; 6. Carbide welded integrated drilling structure; 7. Drillable drill bit. Detailed Implementation

[0047] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. However, it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0048] To address the problems of large-scale, highly interconnected leakage channels in severely leaky formations, and the difficulty in effectively retaining conventional plugging materials, this invention provides a casing short-section downhole plugging device and its construction method for severely leaky formations.

[0049] The device integrates the drop-out connector, packer, casing sub and drilling structure into the same tool string, and completes drilling, packing, isolation and detachment operations in a single well run. At the same time, it constructs a local rigid liner structure that crosses the severely leaky formation in the well to achieve structural sealing of the leakage channel.

[0050] Unlike conventional plugging techniques that rely on the bridging and retention capabilities of plugging materials, this invention uses upper and lower packers to achieve cross-sectional plugging and isolation of the lost formation, and utilizes casing short sections to form stable mechanical support and physical barriers, fundamentally sealing the connectivity of malignant leakage channels. It is suitable for fractured, cavernous, and highly heterogeneous lost formations. To address the difficulties in plugging malignant lost formations, this invention specifically employs the following technical means:

[0051] (1) By setting an upper packer above the severe lost circulation formation and a lower packer structure below the lost circulation formation, a closed isolation section is formed in the well, which effectively prevents the lost circulation formation from communicating with the upper and lower well sections;

[0052] (2) By setting a casing short section between the upper and lower packers, it crosses the formation with severe leakage and forms a local rigid liner structure in the well, which plays a supporting and isolation role for the broken, fractured or dissolved formations.

[0053] (3) By setting a drilling end structure at the bottom of the device and selecting a cemented carbide welded integrated drilling structure or a drillable drill bit as the drilling end according to the formation conditions, short-distance drilling or bottom disturbance can be achieved during the lowering process, thereby improving the device's ability to pass through and its success rate in severe leakage formations.

[0054] (4) By setting a release joint, the device can be controlled to be released, so that the lower casing section and packer can remain in the well for a long time, while the upper drilling tools can be pulled out smoothly, and subsequent drilling operations can be repeated.

[0055] In response to the aforementioned device design, this invention also provides a suitable downhole plugging method. This method allows for the rapid treatment of severely leaking formations by completing the device installation, partial drilling, isolation, and release operations in a single well run. Specifically:

[0056] The device of the present invention adopts a ball-dropping pressure differential sequential start-up method, which realizes the orderly operation of each functional unit downhole through the start-up pressure difference of different components.

[0057] During construction, firstly, drilling of the target well section is completed using the drill-end structure, ensuring that the rigid casing sub completely spans and covers the entire target severely leaking formation, the lower packer is positioned below the target severely leaking formation, and the upper packer is positioned above the target severely leaking formation. Then, depending on the type of lower packer, the lower packer is formed and the upper packer is set according to the corresponding process, creating a stable, bidirectionally pressurized sealed area above and below the leaking well section. When the lower packer is a packer, it and the upper packer are set sequentially using different starting pressures. When the lower packer uses cement injection, the lower packer forms a sealing boundary through cement curing. After ensuring successful sealing above and below the target severely leaking formation, further pressurization is applied to the starting pressure of the release joint, achieving separation of the upper drill string from the lower wellhead tool, thus completing the permanent structural sealing of the severely leaking formation.

[0058] The aforementioned sequential start-up mechanism avoids multiple well runs and complex procedure switching, enabling drilling, isolation, and unloading processes to be completed continuously in the same operation, thus improving the safety and reliability of the construction. Furthermore, the overall construction method is simple in process and tightly integrated in procedures, making it suitable for deep wells, highly deviated wells, and complex well conditions.

[0059] The following detailed description of the technical solution of the present invention, combined with actual working conditions in oil and gas fields, provides a complete and clear explanation. The described embodiments are merely preferred implementations of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the protection scope of the present invention.

[0060] The upper and lower positions mentioned in this invention are based on the vertical orientation of the oil and gas well during normal operation, with the end closer to the wellhead being upper and the end closer to the bottom of the well being lower; the drill pipe refers to the API standard conventional drill pipe that is matched with the surface drilling rig and used to transport this device into the well, and is not part of the main body of the device that is protected by this invention.

[0061] Example 1

[0062] This embodiment is applicable to severely fractured formations with high levels of leakage. These formations are often located in fault zones or areas of intense tectonic activity, characterized by large-scale, highly interconnected fractures. During drilling, drilling fluid is rapidly lost along the fracture system, often resulting in total or near-total leakage. Because conventional plugging materials cannot effectively bridge the fracture channels, the wellbore and formation remain connected for extended periods, rendering conventional plugging measures ineffective.

[0063] For highly fractured and malignant lost-flow formations, refer to Figure 1 The casing short section downhole plugging device structure shown and Figure 2 The construction method shown includes the following steps:

[0064] S1 matches a rigid casing sub of the corresponding length based on the length of the target lost formation section. The casing sub with integrated cemented carbide welding, the hydraulic expansion casing packer, the upper packer with a shearable ball seat assembly, and the hydraulic release joint are sequentially and sealed together. The release joint is then connected to the drilling tool to form a complete plugging drill string. The drill string is subjected to borehole and water pressure sealing tests to ensure that the seal is qualified.

[0065] S2 lowers the working tubing into the well and completes the drilling operation of the target well section through the drilling end structure, ensuring that the rigid casing sub completely crosses and covers the entire target severe leakage formation, the lower packer structure is located below the target severe leakage formation, and the upper packer is located above the target severe leakage formation; remove rock cuttings and mud cake in the wellbore annulus and ensure that the packer seat cover is clean;

[0066] S3-1a: Insert a sealing ball that matches the shearable ball seat assembly into the drill string, so that the sealing ball sits on the ball seat body of the upper packer, completely sealing the central flow channel of the device. Pressurize the ground to the setting pressure differential designed for the upper packer, and confirm that the upper packer is set and sealed successfully.

[0067] S3-2a: Continue to increase the pressure inside the pipe to the designed shear pressure. The pressure drops instantly and shears off the shearable ball seat assembly. The ball seat body falls to the bottom of the well, restoring the center diameter of the drill string, so that the pressure inside the pipe can be transmitted downward to the lower sealing structure.

[0068] S3-3a: Continue pressurizing inside the pipe until the design setting pressure differential of the lower packer structure is reached, driving the sealing rubber sleeve of the hydraulic expansion type casing packer to expand radially and adhere tightly to the well wall, completing the setting and forming; depressurize, and lock the pressure inside the rubber sleeve through the built-in one-way valve type liquid locking mechanism, so that the rubber sleeve does not shrink back and continues to maintain the sealing state; confirm that the two-way closed interval formed by the upper and lower packer structures is qualified, with no crossflow and no leakage.

[0069] After S4 completes the full section isolation, the pressure inside the pipe is increased to the designed release pressure of the release joint. The pressure drops instantly, causing the shear pin of the release joint to be sheared, the sliding sleeve mechanism to be unlocked, and the release is completed, realizing the separation of the upper drilling tool from the lower well-keeping and plugging device.

[0070] S5 slowly pulls out the upper drill string, while the rigid casing sub with the drill end structure, upper packer, and lower packer structure are permanently left in the well. The fluid channel between the lost formation and the wellbore is directly cut off by the rigid wall of the rigid casing sub, completing the permanent structural sealing of the severely lost formation.

[0071] In this embodiment, the setting pressure difference of the upper packer in step S3-1a, the shear pressure difference of the shearable ball seat assembly in step S3-2a, the setting pressure difference of the lower packer structure in step S3-3a, and the release pressure difference of the release joint in step S4 are set in a gradient increasing manner.

[0072] Example 2

[0073] This embodiment is applicable to carbonate rock formations with highly developed cavities and vaults, which are prone to severe leakage. These formations contain large-scale dissolution spaces, and drilling fluid leaks rapidly after entering the vault system during drilling, often resulting in instantaneous total leakage and rapid drop in fluid level. Due to the large scale and complex shape of the vault spaces, conventional plugging materials are unable to form a stable seal within the vaults.

[0074] For carbonate rock formations with highly developed solution pores and caverns, refer to... Figure 3 The casing short section downhole plugging device structure shown and Figure 4 The construction method shown includes the following steps:

[0075] S1 matches a rigid casing sub of the corresponding length based on the length of the target lost formation section. The casing sub with a drillable bit, the hydraulically expandable outer casing packer, the upper packer with a shearable ball seat assembly inside, and the hydraulic release joint are sequentially and sealed together. The release joint is then connected to the drilling tool to form a complete plugging drill string. The drill string is subjected to borehole and water pressure sealing tests to ensure that the seal is qualified.

[0076] S2 lowers the working tubing into the well and completes the drilling operation of the target well section through the drilling end structure, ensuring that the rigid casing sub completely crosses and covers the entire target severe leakage formation, the lower packer structure is located below the target severe leakage formation, and the upper packer is located above the target severe leakage formation; remove rock cuttings and mud cake in the wellbore annulus and ensure that the packer seat cover is clean;

[0077] S3-1b: Inject cement slurry into the target section at the lower end of the rigid casing short section to ensure that all the cement slurry enters the target annulus section. Shut down the well and wait for it to solidify. After the cement slurry has completely solidified and formed a cement plug section, increase the pressure and test to ensure that a permanent sealing boundary is formed.

[0078] S3-2b: Insert a sealing ball that matches the shearable ball seat assembly into the drill bit, so that the sealing ball sits on the ball seat body of the upper packer, completely sealing the central flow channel of the device. Pressurize the ground to the setting pressure differential designed for the upper packer, confirm that the upper packer is set and sealed properly, and cooperate with the lower cement sealing section to form a stable closed area under bidirectional pressure.

[0079] After S4 completes the full section isolation, the pressure inside the pipe is increased to the designed release pressure of the release joint. The pressure drops instantly, causing the shear pin of the release joint to be sheared, the sliding sleeve mechanism to be unlocked, and the release is completed, realizing the separation of the upper drilling tool from the lower well-keeping and plugging device.

[0080] S5 slowly pulls out the upper drill string, while the rigid casing sub with the drill end structure, upper packer, and lower packer structure are permanently left in the well. The fluid channel between the lost formation and the wellbore is directly cut off by the rigid wall of the rigid casing sub, completing the permanent structural sealing of the severely lost formation.

[0081] In summary, as can be seen from Examples 1 and 2, the device provided by the present invention integrates the drilling end structure, packer, and release connector into the same tool string. It adopts a ball-dropping pressure differential sequential start-up method, and completes drilling, packing, and release operations sequentially in one well run. The process is simple and improves the safety and reliability of construction.

[0082] Moreover, through the combined action of the upper packer, rigid casing sub, and lower packer structure, a stable and closed zone completely isolated from the wellbore is formed around the lost circulation section. Then, a local rigid liner structure spanning the severe lost circulation formation is constructed inside the well using the rigid casing sub, which can achieve permanent structural sealing of the severe lost circulation formation, with remarkable results.

[0083] In addition, the device provided by this invention does not depend on the specific leakage channel morphology and has a good sealing effect on fracture-type, cave-type and highly heterogeneous leakage formations, and has broad application prospects.

[0084] The foregoing description of specific exemplary embodiments of the invention is for illustrative and explanatory purposes. These descriptions are not intended to limit the invention to the precise forms disclosed, and it will be apparent that many changes and variations can be made in accordance with the foregoing teachings. The exemplary embodiments were chosen and described in order to explain the specific principles of the invention and its practical application, thereby enabling those skilled in the art to implement and utilize various different exemplary embodiments of the invention, as well as various different choices and variations. The scope of the invention is intended to be defined by the claims and their equivalents.

Claims

1. A casing short-section downhole plugging device for severely lost circulation formations, characterized in that, With the end closest to the wellhead as the top and the end closest to the bottom of the well as the bottom, the device, from top to bottom, includes: a release connector, an upper packer, a rigid casing sub, and a drilling end structure. The release connector is a hydraulic release connector, which adopts a ball-throwing pressure differential start method. Its start pressure is higher than the setting pressure of the upper packer, and it is used to achieve release after packing is completed. The upper packer is located above the target stratum with severe leakage. It is a packer activated by ball drop pressure differential and has a shearable ball seat assembly inside. After ball drop, it closes the central flow channel of the device to establish the pressure differential between the inside and outside of the tube required for setting. The rigid casing section completely spans and covers the entire severe leakage formation, with its top and bottom ends connected to the upper packer and the drill end structure, respectively. Below the target severely lost formation and above the drill end structure, a lower packer structure is provided along the axial direction of the rigid casing short section. The lower packer structure has a pressure-holding and locking function after forming, and is used to cooperate with the upper packer to form a bidirectional pressure-bearing packer boundary above and below the lost well section. After the lower packer structure is formed, it, together with the upper packer and the rigid casing short section, forms a stable closed zone that is completely isolated from the wellbore around the lost circulation section. The fluid channel between the lost circulation formation and the wellbore is directly cut off by the rigid pipe wall of the rigid casing short section, thereby achieving permanent structural sealing of the severely lost circulation formation.

2. The casing short-section downhole plugging device according to claim 1, characterized in that, The release connector includes a shear pin and a sliding sleeve structure. When the pressure inside the pipe reaches the designed release pressure, the shear pin is sheared off, and the connection between the upper drilling tool and the lower well-keeping and plugging device is released through the sliding sleeve structure.

3. The casing short-section downhole plugging device according to claim 1, characterized in that, After the upper packer is set, the shearable ball seat assembly is cut by applying pressure inside the tube to restore the central diameter of the device, so that the pressure inside the tube can continue to be transmitted downward to the lower packer structure.

4. The casing short-section downhole plugging device according to claim 1, characterized in that, The lower sealing structure is a hydraulically expandable casing external packer. The inner cavity of the hydraulically expandable casing external packer is connected to the central diameter of the device. The sealing sleeve is radially expanded by pressurizing inside the pipe to complete the setting seal.

5. The casing short-section downhole plugging device according to claim 4, characterized in that, The hydraulic expansion sleeve packer is equipped with a liquid locking mechanism inside. The liquid locking mechanism is a one-way valve pressure-holding structure. After the setting is completed and the pressure is released, it can lock the pressure inside the sleeve cavity, so that the packer can continuously maintain a sealed state.

6. The casing short-section downhole plugging device according to claim 1, characterized in that, The lower sealing structure is a cement sealing section formed by injecting cement slurry into the lower end of a rigid casing short section and solidifying it. The cement sealing section completely fills the wellbore annulus and central channel below the target lost formation, forming a permanent lower sealing boundary.

7. The casing short-section downhole plugging device according to claim 1, characterized in that, The casing short section is a casing short section that conforms to API standards, and its length is designed based on the thickness of the severe leakage formation and the length of its upper and lower safe sections; The drilling end structure is used to drill or disturb the formation at the bottom of the well, so that the rigid casing sub is run down to the designed well depth.

8. A construction method for plugging severely leaking formations using the apparatus according to any one of claims 1-7, characterized in that, Includes the following steps: S1 matches the corresponding length of rigid casing short sections according to the length of the target lost formation section. The casing short section with the drilling end structure, the upper packer and the release joint are sealed and connected in sequence. The prefabrication and installation of the lower packer structure are completed. Then, the release joint is connected to the drilling tool to form a complete drilling tool string for plugging operations. S2 completes the drilling operation of the target well section through the drilling end structure, ensuring that the rigid casing short section completely crosses and covers the entire target severe leakage formation, the lower packer structure is located below the target severe leakage formation, and the upper packer is located above the target severe leakage formation; S3, based on the type of the lower packer structure, completes the forming of the lower packer structure and the setting of the upper packer according to the corresponding process, forming a stable closed zone with bidirectional pressure above and below the lost well section; After S4 completes the full section isolation, the release operation is completed by pressurizing the pipe to trigger the release joint, thus separating the upper drilling tools from the lower well-keeping and plugging device. S5 pulls out the upper drill string and directly cuts off the fluid channel between the lost formation and the wellbore through the rigid wall of the rigid casing short section, completing the permanent structural sealing of the severe lost formation.

9. The construction method for plugging severely leaking formations according to claim 8, characterized in that, In step S3, when the lower sealing structure is a hydraulically expandable casing external packer, the specific construction steps are as follows: S3-1a: Insert a sealing ball that matches the shearable ball seat assembly into the drill string, so that the sealing ball sits in the central flow channel of the sealing device, and pressurize the ground to the setting pressure differential designed for the upper packer to complete the setting; S3-2a: Continue to increase the pressure inside the pipe to the designed shear pressure, shear the shearable ball seat assembly, the ball seat body falls off, restore the center diameter of the drill string, so that the pressure inside the pipe can be transmitted downward to the lower sealing structure. S3-3a: The pressure inside the pipe continues to increase to the design setting pressure differential of the lower packer structure, driving the sealing rubber sleeve of the hydraulic expansion type casing packer to expand radially and adhere tightly to the well wall, completing the setting and forming. After depressurization, the pressure inside the rubber sleeve cavity is locked by the internal liquid locking mechanism to maintain the sealing state continuously.

10. The construction method for plugging severely leaking formations according to claim 8, characterized in that, In step S3, when the lower sealing structure is a cement sealing section, the specific construction steps are as follows: S3-1b: Inject cement slurry into the target well section at the lower end of the rigid casing sub. After solidification to form a cement plug section, a permanent sealing boundary is formed. S3-2b: Insert a sealing ball that matches the shearable ball seat assembly into the drill bit, so that the sealing ball sits in the central flow channel of the sealing device. The ground is pressurized to the setting pressure differential designed for the upper packer to complete the setting and seal. It works in conjunction with the lower cement sealing section to form a stable closed zone with bidirectional pressure.

Images