A method and device for plugging a surface relief well before overburden layer mining

Abstract

This invention discloses a method and device for sealing surface pressure relief wells before upper protective layer mining, belonging to the field of coal mine gas control. The method includes: determining the target sealing section based on the theoretical mining influence boundary and the actual wellbore leakage transition boundary; lowering a mechanical anchor base to the lower part of the target sealing section and radially expanding and anchoring it; injecting a high-viscosity flow-resisting medium through the peripheral injection channel to fill the annulus and fissures outside the screen pipe to form a diffusion constraint zone; then injecting a permanently solidified sealing grout to form a peripheral sealing body bridging the screen pipe, annulus, and well wall; judging the peripheral sealing status based on grouting pressure and flow feedback, and switching to the central injection channel after reaching the target, forming an axial sealing body inside the wellbore, which, together with the peripheral sealing body, constitutes a radial-axial composite sealing. This invention can accurately seal multiple leakage paths and has in-situ verification and compensation functions, effectively preventing air leakage and spontaneous combustion in goaf areas.

Description

Technical Field

[0001] This invention relates to the field of coal mine gas control and mine safety technology, specifically to a method and device for sealing surface pressure relief wells before the upper protective layer is mined. Background Technology

[0002] In coal mine gas control, a combination of protective layer mining and surface depressurization well extraction is typically used. After the lower protective layer is mined, the overlying strata form caving and fracture zones, allowing depressurized gas from the overlying coal seam to be extracted through mining-induced fractures into the surface depressurization well. However, there is usually a long interval between the end of the lower protective layer mining and the mining of the overlying coal seam. As the overlying working face gradually approaches the original surface depressurization well, subsequent mining activities may cause deformation, damage, or breakage of the screen pipes inside the well, creating interconnected leakage channels within the wellbore, the annulus outside the screen pipes, and the surrounding fractures. Under the negative pressure of extraction, air entering the extraction system reduces the gas extraction concentration. Simultaneously, air entering the goaf along the wellbore and fractures provides oxygen for the oxidation of residual coal, increasing the risk of spontaneous combustion in the goaf.

[0003] Existing surface decompression well plugging methods typically involve directly sealing the wellbore using mechanical plugs, cement grout, or other sealing materials. The plugging location is primarily determined based on theoretical mining height or construction experience, making it difficult to adapt to complex leakage paths formed by residual screen deformation and connections between the screen's outer annulus and surrounding fractures. Especially when residual screens remain in the well, simply sealing the inside of the screen may still result in bypass leakage along the screen's outer annulus, bypassing the plugging body. Directly injecting grout into the fracture area can easily lead to grout leakage along open fractures and insufficient filling of the target area. Furthermore, existing plugging methods lack identification of actual leakage locations, status feedback during the grouting process, and post-plugging airtightness verification and compensation measures. Therefore, it is necessary to propose a directional composite plugging method and device suitable for surface decompression wells before overlying coal seam mining. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a directional composite plugging method and device for surface decompression wells before overlying coal seams are mined. This method can achieve a complete plugging process of precise positioning, directional plugging and in-situ verification for complex gas leakage paths formed by residual screen pipes, annulus outside the screen pipes and fractures around the well.

[0005] The technical solution of the present invention to solve the above-mentioned technical problems is as follows:

[0006] A directional composite plugging method for surface decompression wells before overlying coal seam mining includes the following steps:

[0007] The mining parameters of the overlying coal seam to be mined and the wellbore structure parameters of the surface decompression well are obtained. The mining influence boundary formed by the mining of the overlying coal seam is determined based on the mining parameters. The gas flow state at different depths is obtained along the surface decompression well. The wellbore leakage state transition boundary is determined based on the change in gas flow state between adjacent depths. The target sealing section is determined by the mining influence boundary and the wellbore leakage state transition boundary.

[0008] The mechanical anchor base is lowered to the lower part of the target sealing section, and the mechanical anchor base is driven to expand radially, so that the mechanical anchor base is anchored to the well wall of the surface decompression well or the residual screen pipe in the well, and a supporting sealing interface is formed at the lower part of the target sealing section.

[0009] High-viscosity flow-blocking medium is injected into the annulus outside the reserved screen pipe and the well perimeter fractures connected to the annulus through the injection channel of the mechanical anchor base. The high-viscosity flow-blocking medium fills the annulus and well perimeter fractures and forms a diffusion constraint zone that restricts the axial escape of the permanent consolidation sealing slurry.

[0010] After the diffusion confinement zone is formed, permanent consolidation sealing slurry is injected through the peripheral injection channel, so that the permanent consolidation sealing slurry expands from the inside of the residual screen pipe to the outside within the range defined by the diffusion confinement zone, and solidifies to form a peripheral sealing body that bridges the residual screen pipe, the annulus outside the screen pipe and the well wall.

[0011] The peripheral sealing state is determined based on the changes in grouting pressure and grouting flow rate during the peripheral injection process. Once the peripheral sealing state reaches the preset conditions, the peripheral injection is stopped. Permanent consolidation sealing slurry is injected into the wellbore above the mechanical anchor base through a central injection channel that is independent of the peripheral injection channel, forming an axial sealing body connected to the peripheral sealing body, so as to cut off the bypass leakage channel outside the screen pipe and the axial leakage channel inside the screen pipe respectively.

[0012] After the permanent consolidation sealing grout has solidified, a test negative pressure is applied to the surface depressurization well to obtain the negative pressure maintenance status and the changes in the gas composition in the well. The sealing airtightness is determined based on the negative pressure maintenance status and the changes in the gas composition in the well. When the sealing airtightness does not meet the preset airtightness conditions, compensation grouting is carried out on the unsealed area through the compensation grouting channel retained in the mechanical anchor seal base until the sealing airtightness meets the preset airtightness conditions.

[0013] Further proposed solution: The mining-induced impact boundary is determined based on the mining height, coal seam dip angle, overlying lithology, and the development height of the caving zone and fracture zone of the overlying coal seam to be mined; at least three parameters among wellbore pressure, gas flow rate, methane concentration, and oxygen concentration are obtained at preset depth intervals along the surface decompression well, and the leakage state transition position is determined based on the parameter changes between adjacent detection positions; the leakage state transition position is spatially correlated with the mining-induced impact boundary, and a sealing safety distance is set between the two to determine the bottom position and axial length of the target sealing section.

[0014] Further solution: The mechanical anchor base includes a central support body, a conical drive body sleeved on the outside of the central support body, multiple expansion sealing petals arranged circumferentially along the central support body, a flexible sealing layer covering the outside of the expansion sealing petals, and a backstop anchoring component linked with the expansion sealing petals; during the lowering of the mechanical anchor base, the expansion sealing petals remain in a radially contracted state; after reaching the target sealing section, the conical drive body and the expansion sealing petals generate axial relative displacement, and the conical drive body drives the expansion sealing petals to move radially outward, so that the flexible sealing layer presses against the residual screen pipe or well wall, and at the same time, the backstop anchoring component forms an axial backstop fit with the residual screen pipe or well wall.

[0015] Further solution: The mechanical anchor base is provided with a central injection channel and a peripheral injection channel that are fluidly isolated from each other. The peripheral injection channel is connected to radial injection ports distributed circumferentially along the mechanical anchor base. After the mechanical anchor base is anchored, the peripheral injection channel is connected to the corresponding screen holes on the reserved screen pipe. The high-viscosity flow-blocking medium and the permanent solidified sealing slurry forming the peripheral sealing body enter the annulus outside the reserved screen pipe in sequence through the radial injection ports and screen holes. The central injection channel is connected to the inside of the wellbore above the mechanical anchor base, so that the peripheral sealing body and the axial sealing body are formed through independent fluid paths.

[0016] Further solution: The high-viscosity flow-blocking medium is a degradable fluid with both a flowing state and a viscosity-enhancing state. It maintains its flowing state as it enters the annulus outside the screen tube and the peri-well fractures through the peripheral injection channel and radial injection port, and forms a viscosity-enhancing state after entering these areas. This viscosity-enhancing state increases the flow resistance in the annulus outside the screen tube and the peri-well fractures, allowing the subsequently injected permanent consolidation sealing slurry to fill the annulus outside the screen tube radially and restricting axial dispersion of the permanent consolidation sealing slurry along the fractures connected to the target sealing section. After the permanent consolidation sealing slurry solidifies, the high-viscosity flow-blocking medium degrades, and the peripheral sealing body assumes the permanent sealing function of the target sealing section.

[0017] Further proposed solution: During the process of injecting permanent consolidation and sealing grout, continuously obtain the grouting pressure and grouting flow rate per unit time, and obtain the changing trends of grouting pressure and grouting flow rate per unit time over time, respectively; when the grouting pressure continues to rise and the grouting flow rate per unit time continues to decrease, and the corresponding peripheral sealing judgment conditions are reached within the preset duration, it is determined that the annulus outside the screen pipe and the well perimeter fracture connected to it have reached the peripheral sealing state, the peripheral injection is stopped and the process is switched to the central injection channel.

[0018] Further proposed solution: The peripheral sealing body is formed by the permanent consolidation sealing slurry passing through the screen holes of the residual screen pipe and radially extending outward into the annulus of the screen pipe, and is solidified and connected to the well wall and the residual screen pipe; the axial sealing body is formed by the permanent consolidation sealing slurry extending upward from the mechanical anchor base along the interior of the residual screen pipe and the wellbore, and the lower part of the axial sealing body is solidified and connected to the peripheral sealing body, so that the peripheral sealing body and the axial sealing body together constitute a radial-axial composite sealing structure.

[0019] Further proposed solution: After the permanent consolidation grout reaches the preset consolidation state, a test negative pressure is established above the plugging body through a surface extraction pipeline. After extraction is stopped, wellbore pressure, methane concentration, and oxygen concentration are obtained at preset time intervals. The airtightness of the plugging is determined based on the wellbore negative pressure attenuation, oxygen concentration increment, and methane concentration change during the test period. When the wellbore negative pressure attenuation, oxygen concentration increment, and methane concentration change meet the corresponding airtightness criteria, the plugging is deemed qualified.

[0020] Further solution: When the airtightness of the plugging does not meet the preset airtightness conditions, determine the compensation plugging requirements based on the wellbore negative pressure decay state and the changes in oxygen and methane concentrations; open the compensation grouting channel retained in the mechanical anchor seal base, and transport the compensation plugging grout to the unsealed area between the outer plugging body and the well wall and / or between the outer plugging body and the residual screen pipe; after the compensation plugging grout solidifies, re-establish the test negative pressure and conduct airtightness testing until the airtightness of the plugging reaches the preset airtightness conditions.

[0021] A surface decompression well plugging device for implementing the above-mentioned directional composite plugging method includes a surface operation unit, a composite lowering string, a wellbore status detection component, a mechanical anchoring base, and a grouting control component.

[0022] The wellbore status detection component is mounted on the composite lowering string and is used to obtain the gas flow status at different well depths.

[0023] The mechanical anchor base is connected to the lower part of the composite lowering column and includes a central bearing body, a conical driving body, multiple expansion sealing petals arranged circumferentially along the central bearing body, a flexible sealing layer arranged on the outside of the expansion sealing petals, and a backlash anchor. The conical driving body and the expansion sealing petals form a driving cooperation that converts axial displacement into radial expansion displacement.

[0024] The mechanical anchor base is equipped with a central injection channel, a peripheral injection channel and a compensation grouting channel that are isolated from each other. The peripheral injection channel is connected to multiple radial injection ports facing the screen holes of the residual screen pipe. The central injection channel is connected to the inside of the well barrel above the mechanical anchor base.

[0025] The grouting control component is connected to the central injection channel, the peripheral injection channel and the compensation grouting channel respectively, and controls the grouting switching between the peripheral injection channel and the central injection channel according to the grouting pressure and grouting flow rate during the peripheral injection process;

[0026] The mechanical anchor base is also equipped with a grout-stopping component to seal the corresponding grouting channel after the mechanical anchor base is separated from the composite lowering string and to retain at least the compensation grouting channel in the wellbore.

[0027] The present invention has the following beneficial effects:

[0028] This invention determines the target sealing section by combining the theoretical mining influence boundary formed by the overlying coal seam mining with the actual gas leakage transition boundary at different depths of the surface decompression well. Compared with methods that determine the sealing location solely based on the height of the collapse zone, fracture zone, or construction experience, this invention ensures that the sealing location matches the actual gas leakage channel in the wellbore and the subsequent mining influence area. Simultaneously, a radially expandable mechanical anchoring base forms a support, sealing, and axial anchoring interface at the bottom of the target sealing section, which can adapt to the working conditions of residual screen tube deformation, local diameter reduction, and irregular cross-section in the well, providing a stable construction foundation for subsequent staged grouting and improving the reliability of the sealing structure.

[0029] This invention employs a graded composite sealing method consisting of high-viscosity flow-blocking demarcation, peripheral radial sealing, and internal axial sealing. The high-viscosity flow-blocking medium enters the annulus outside the screen tube and the peri-well fractures through the radial injection port and the screen holes of the residual screen tube, pre-increasing the flow resistance of the open fractures and thus limiting the disorderly diffusion of the subsequent permanent consolidation sealing slurry along the distal fractures. Subsequently, the permanent consolidation sealing slurry forms a peripheral sealing body bridging the residual screen tube, the annulus outside the screen tube, and the well wall within the controlled area. After the peripheral sealing state reaches the preset conditions, it forms an axial sealing body connected to it. This cuts off the bypass leakage channels formed by the annulus outside the screen tube and the peri-well fractures, as well as the axial leakage channels formed inside the screen tube, thereby reducing the risk of gas concentration decrease caused by air entering the extraction system and the risk of residual coal oxidation and spontaneous combustion caused by air migration to the goaf.

[0030] This invention utilizes changes in grouting pressure and flow rate during the peripheral grouting process to determine the peripheral sealing status and accordingly controls the phase switching from peripheral to central grouting, establishing a state feedback relationship between different sealing stages. After the permanent sealing body solidifies, a test negative pressure is established using the original surface depressurization well extraction system, and the airtightness of the sealing is verified in situ by combining negative pressure decay, changes in methane concentration, and changes in oxygen concentration. When the test results do not meet the preset airtightness conditions, directional grouting can be directly applied to the unsealed area of ​​the original sealing interface through the reserved compensation grouting channel without damaging the already formed main sealing structure. This forms a closed-loop construction system of "positioning, anchoring, flow obstruction, peripheral sealing, axial sealing, airtightness verification, and compensation sealing," improving the sealing quality of the surface depressurization well and the gas control and fire prevention safety assurance capabilities during subsequent overlying coal seam mining. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the overall structure of the surface decompression well sealing device of the present invention;

[0032] Figure 2 This is a schematic diagram of the composite lowering column structure;

[0033] Figure 3 This is a schematic diagram comparing the retracted and expanded states of the mechanical anchor base.

[0034] Figure 4 A schematic diagram of the three-channel flow path for a mechanical anchor seal base;

[0035] Figure 5 A schematic diagram of the wellbore structure and leakage channels;

[0036] Figure 6 A schematic diagram showing the entry of a high-viscosity, flow-blocking medium into the annulus outside the screen tube;

[0037] Figure 7 This is a schematic diagram of the structure after the outer radial sealing body and the axial sealing body are formed;

[0038] Figure 8 The grouting pressure-flow feedback control curve during the peripheral grouting process;

[0039] Figure 9 A schematic diagram showing the retention of grouting channels for compensation after the construction pipe column is detached;

[0040] Figure 10 This is a schematic diagram of the final structure after the sealing is completed.

[0041] In the diagram: 1—Surface operation unit; 2—Composite lowering string; 21—Bearing pipe; 22—Central grout delivery pipe; 23—Outer grout delivery pipe; 24—Compensating grout delivery pipe; 3—Wellbore condition detection assembly; 31—Pressure detection unit; 32—Flow rate detection unit; 33—Methane detection unit; 34—Oxygen detection unit; 4—Mechanical anchor seal base; 41—Central bearing body; 42—Conical drive body; 43—Expansion sealing flap; 44—Flexible sealing layer ; 45—Anti-reverse anchor; 46—Central injection channel; 47—Peripheral injection channel; 471—Radial injection port; 48—Compensation grouting channel; 481—Grouting outlet; 482—One-way grout stop valve; 49—Detachable connection assembly; 5—Grouting control assembly; 6—Surface pressure relief well; 61—Residual screen pipe; 62—Well wall; 63—Outer annulus of the screen pipe; 64—Screen hole; 65—Well perimeter fracture; 7—Peripheral plugging body; 8—Axial plugging body. Detailed Implementation

[0042] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0043] The following description, in conjunction with the structural composition and construction process of the present invention, further illustrates a directional composite plugging method and device for surface decompression wells before overlying coal seams are mined.

[0044] refer to Figures 1-10 As shown, this implementation method is for the transitional situation where the lower protective layer has been mined, the surface depressurization well has completed the corresponding stage of depressurization gas extraction, and the overlying coal seam has not yet been mined. After the initial mining and long-term extraction, there may be residual screen pipes, screen pipe fractures, local deformation of the casing, and communication between the annulus outside the screen pipe and the wellbore fractures in the surface depressurization well. This forms axial gas leakage channels inside the screen pipe, gas leakage channels in the annulus outside the screen pipe, and bypass channels in the wellbore fractures. This implementation method does not grout the inside of the wellbore separately, but sequentially completes the determination of the target sealing section, mechanical anchoring, high viscosity flow resistance, peripheral permanent sealing, internal permanent sealing, and sealing quality verification, and performs directional compensation sealing when the sealing is unqualified.

[0045] The surface decompression well plugging device used in this embodiment mainly includes: a surface operation unit 1, a composite lowering string 2, a wellbore status detection component 3, a mechanical anchor sealing base 4, and a grouting control component 5.

[0046] Among them, the ground operation unit 1 is set at the wellhead of the ground decompression well 6, and is used to complete the lowering and lifting of the composite lowering string 2, the driving of the mechanical anchor sealing base 4, and the injection of different sealing media.

[0047] The surface decompression well 6 retains a residual screen pipe 61 used in the previous gas extraction. The residual screen pipe 61 and the well wall 62 form an outer annulus 63 of the screen pipe. Screen holes 64 are distributed on the residual screen pipe 61. The outer annulus 63 of the screen pipe is connected to part of the well perimeter fracture 65.

[0048] The composite lowering string 2 includes at least a supporting pipeline 21, a central grout delivery pipe 22, an outer grout delivery pipe 23, and a compensating grout delivery pipe 24. The three grout delivery pipes form mutually isolated fluid transport paths.

[0049] The mechanical anchor base 4 is installed at the lower end of the composite lowering column 2. It serves as both a mechanical support structure at the bottom of the target sealing section and a flow path switching structure for the high-viscosity flow-blocking medium and the permanently solidified sealing slurry.

[0050] The mechanical anchor base 4 includes a central bearing 41, a conical drive body 42, an expansion sealing flap 43, a flexible sealing layer 44, a backlash anchor 45, a central injection channel 46, a peripheral injection channel 47, a compensation grouting channel 48, and a detachable connection component 49.

[0051] The central support 41 has a hollow column structure, and its upper end is connected to the composite lowering column 2 through a detachable connection component 49.

[0052] Multiple expansion sealing petals 43 are arranged circumferentially along the central support body 41, and their inner sides form an inclined driving surface adapted to the conical surface of the conical drive body 42. The conical drive body 42 can move axially relative to the central support body 41, converting the axial displacement of the conical drive body into the radial displacement of the expansion sealing petals.

[0053] In the lowered state, the conical drive body 42 is in the first axial position, and each expansion sealing petal 43 is in a radially contracted state. At this time, the maximum outer diameter of the mechanical anchor base 4 is smaller than the effective inner diameter of the well section to be passed. After reaching the target sealing position, the conical drive body 42 is driven by the composite lowering string 2, causing it to enter the internal space enclosed by the multiple expansion sealing petals 43. As the conical drive body 42 continues to move axially, each expansion sealing petal 43 expands outward synchronously.

[0054] The flexible sealing layer 44 is disposed on the outside of the expansion sealing petal 43. When the expansion sealing petal 43 moves outward, the flexible sealing layer 44 first contacts the inner wall of the reserved tube 61. As the expansion amount further increases, the flexible sealing layer 44 undergoes local compression deformation, thereby compensating for the elliptical deformation, local depression and inner diameter difference of the residual tube 61.

[0055] The anti-retraction anchor 45 is located on the outside or below the expansion sealing petal 43 and has an engaging portion inclined towards the wellhead. When the mechanical anchor base 4 is lowered, the anti-retraction anchor 45 remains closed, not affecting the passage of the device through the wellbore. After the expansion sealing petal 43 expands radially, the anti-retraction anchor 45 moves outward with the expansion sealing petal 43, and its engaging portion contacts the inner wall of the residual screen pipe 61. When the mechanical anchor base 4 is subjected to an upward grouting reaction force or axial load, the anti-retraction anchor 45 tends to engage further outward, thereby restricting the mechanical anchor base 4 from moving towards the wellhead.

[0056] Therefore, the mechanical anchor base simultaneously provides radial sealing, axial anchoring, and slurry support. For well sections where the residual screen pipe has been completely destroyed, the expansion sealing flap 43 and the anti-reverse anchor 45 can also directly fit with the inner wall of the intact casing or the cleaned well wall.

[0057] The specific arrangement of the three grouting channels: The mechanical anchor base 4 has three mutually isolated grouting channels inside.

[0058] Central injection channel 46: It is arranged along the axial direction of the central support body 41, with its upper end connected to the central grout delivery pipe 22 and its lower end outlet located on the upper end face of the mechanical anchor seal base 4. Therefore, the permanent consolidation sealing grout injected by the central grout delivery pipe 22 enters the interior of the residual screen pipe 61 from the upper end of the mechanical anchor seal base 4 and accumulates along the wellbore axial direction, eventually forming the axial sealing body 8.

[0059] The peripheral injection channel 47 is independently located outside the central injection channel 46. The upper end of the peripheral injection channel 47 connects to the peripheral grout delivery pipe 23, and the lower part connects to multiple radially distributed injection ports 471. After the mechanical anchor base 4 is anchored, the radial injection ports 471 are located within the height range of the screen holes 64 of the residual screen pipe 61. The medium transported by the peripheral grout delivery pipe 23 sequentially passes through the peripheral grout delivery pipe 23 → peripheral injection channel 47 → radial injection ports 471 → screen holes 64 → outside the screen pipe, entering the outside of the screen pipe. Therefore, peripheral grouting does not simply leave the grout inside the screen pipe, but actively utilizes the original screen holes of the residual screen pipe to establish a radial transport path to the outside of the screen pipe.

[0060] The compensating grouting channel 48 is isolated from the aforementioned two channels, and its upper end is connected to the compensating grout delivery pipe 24 during the construction phase. At least one set of grout outlets 481 are provided at the lower part of the compensating grouting channel 48, with the grout outlets 481 facing the edge of the predetermined area formed by the outer sealing body 7. One-way grout stop valves 482 are provided at the grout outlets 481. The one-way grout stop valves 482 allow the compensating grout to flow outward from the compensating grouting channel 48 and restrict the outer grout from reversing into the compensating grouting channel 48 after grouting is completed.

[0061] Step S1: Determine the theoretical mining influence boundary:

[0062] Before construction, obtain the mining height M and the dip angle of the overlying coal seam to be mined. Lithology of the immediate top, lithology of the older top, thickness of the strata and corresponding mechanical parameters.

[0063] The caving zone height Hk and fracture zone height Hl are determined based on existing geological data of the mine and the development pattern of mining-induced fractures. For example, the following can be used:

[0064]

[0065] Where M is the coal seam mining height, The coefficient of mass expansion of the collapsed rock mass. The angle of dip of the coal seam.

[0066] The height of the fracture zone is determined based on the lithology of the overlying strata, mining height, and corresponding specifications or actual measured parameters of the mine.

[0067] The theoretical boundary of mining impact is obtained based on the spatial extent of the collapse zone and fracture zone. It should be noted that this invention does not directly use the theoretically calculated boundary as the final sealing location, but rather as the first boundary.

[0068] Step S2: Detect the actual leakage state transition boundary:

[0069] The extraction capacity of the original extraction system of the surface depressurization well is retained, and the set test negative pressure is maintained at the wellhead.

[0070] The wellbore condition detection component 3 is lowered step by step along the wellbore using the composite lowering string 2. The wellbore condition detection component 3 includes a pressure detection unit 31, a flow detection unit 32, a methane detection unit 33, and an oxygen detection unit 34.

[0071] The pressure at each detection location is acquired sequentially according to a preset depth interval. ,flow methane volume fraction and oxygen volume fraction .

[0072] When a combined transition in pressure response, flow rate, methane concentration, and oxygen concentration occurs between two adjacent detection locations, the corresponding well depth range is identified as a candidate leakage state transition zone. The candidate leakage state transition zone is then re-measured by reducing the detection interval to obtain the actual leakage state transition boundary.

[0073] Ultimately, the target sealing section is determined by combining the theoretical impact boundary of mining, the actual leakage transition boundary, and the safe sealing distance. This avoids the problem of determining the sealing location solely based on the theoretical fracture zone height, which fails to adapt to the differences in actual wellbore fracture development.

[0074] Step S3: Lower and anchor the mechanical anchor base:

[0075] Based on the bottom position of the target sealing section, the mechanical anchor base 4 is lowered to the designed depth.

[0076] Once in place, the ground operation unit 1 applies an axial driving force to the composite lowering column 2, causing the conical drive body 42 to move axially relative to the central support body 41, driving the expansion sealing petal 43 to expand radially in sync.

[0077] The flexible sealing layer 44 presses against the inner wall of the residual screen tube 61 to form an initial seal. The anti-reverse anchor 45 moves outward with the expanding sealing flap and engages with the inner wall of the residual screen tube, restricting the upward movement of the mechanical anchor base.

[0078] At this time, the mechanical anchor base simultaneously forms three functions at the lower part of the target sealing section: radial sealing, axial anchoring, and slurry support.

[0079] Step S4: Inject a biodegradable, high-viscosity flow-blocking medium:

[0080] After completing the mechanical anchor sealing, do not inject cement grout first. Open the external grout delivery pipe 23.

[0081] The biodegradable high-viscosity flow-blocking medium enters the annulus 63 outside the screen tube through the peripheral injection channel 47 and the radial injection port 471, and then through the screen hole 64. Since the annulus 63 outside the screen tube is connected to the peri-well fracture 65, a portion of the high-viscosity flow-blocking medium continues to enter the peri-well fracture 65.

[0082] In one embodiment, the high-viscosity flow-blocking medium can be a biodegradable polymer system with shear-thinning properties. During pumping, it maintains a viscosity suitable for transport under shear stress; upon entering the annulus outside the screen and the peri-well fractures, the shear stress weakens, and the system viscosity increases.

[0083] Thus, the original low-resistivity open fractures gradually transform into high-flow-resistance fractures. The high-viscosity, flow-resistant medium does not serve as the final permanent sealing medium; its core function is to pre-establish a controlled diffusion boundary for the subsequent permanent consolidation slurry, essentially acting as a downhole "slurry flow field shaping layer."

[0084] Step S5: Form a permanent outer sealing structure:

[0085] After the high-viscosity flow-blocking medium completes the pre-sealing, keep the external injection path unchanged and switch the injection medium to a permanent consolidation sealing grout.

[0086] The permanent consolidation and sealing grout is still transported outward along the path of peripheral injection channel → radial injection port → sieve hole → annular space outside the sieve tube.

[0087] Because the flow resistance of the distal fracture has been increased by the high-viscosity flow-blocking medium, the permanently consolidated sealing grout no longer preferentially escapes along the large fracture over long distances, but mainly diffuses radially within the annulus 63 on the outside of the screen tube near the target sealing section.

[0088] The slurry gradually fills the screen holes 64, the annulus 63 outside the screen tube, and the near-wellbore fracture inlet. After consolidation, it forms an outer sealing body 7. The outer sealing body 7 spans the remaining screen tube 61, the annulus 63 outside the screen tube, and the well wall 62, thus forming a radial sealing disc. This structure is specifically designed to solve the problem of traditional methods that "only seal the inside of the screen tube, but air still leaks out from the annulus outside the screen tube."

[0089] Step S6: Determine the status of external blockage through pressure-flow feedback:

[0090] During the permanent grouting process, the grouting control component 5 collects the grouting pressure in real time. and grouting flow rate .

[0091] When there are open channels in the periphery, it usually manifests as an increase in grouting flow rate and a decrease in grouting pressure, or difficulty in maintaining pressure. As the annulus outside the screen pipe and the well periphery fracture inlet are gradually filled, the flow resistance increases, which manifests as an increase in grouting pressure and a decrease in grouting flow rate.

[0092] Therefore, this implementation method does not simply end the peripheral grouting according to a fixed grouting volume. When the grouting pressure reaches the design pressure range, and the grouting volume per unit time decreases to a set range, and this state continues for a preset time, the grouting control component 5 determines that the peripheral sealing body has formed a sealing state that meets the requirements.

[0093] Subsequently, the peripheral injection channel 47 was closed, peripheral grouting was stopped, and the central injection channel 46 was opened. Therefore, the central sealing action was triggered by the peripheral sealing state.

[0094] Step S7: Forming an axial permanent seal:

[0095] After the outer sealing is completed, permanent consolidation sealing grout is delivered to the central injection channel 46 through the central grout delivery pipe 22.

[0096] The slurry enters the wellbore from the upper end of the mechanical anchor base 4. Since the bottom mechanical anchor base 4 has formed a supporting interface, the slurry will not continue to flow downwards, but will gradually accumulate upwards from above the mechanical anchor base.

[0097] The slurry fills the interior of the residual screen tube 61 and the wellbore space connected to it, ultimately forming an axial plug body 8 with the designed axial length. The lower part of the axial plug body 8 is connected to the previously formed peripheral plug body 7.

[0098] Thus, a three-layer composite sealing system composed of the peripheral radial sealing body 7, the axial sealing body 8 and the bottom mechanical anchoring sealing base 4 is finally formed. The mechanical anchoring sealing base 4 is responsible for supporting, initial sealing and anchoring; the peripheral sealing body 7 is responsible for blocking the annulus outside the screen pipe and the near-well fracture bypass; the axial sealing body 8 is responsible for blocking the axial gas leakage paths inside the screen pipe and the wellbore. These three do not repeatedly seal the same channel, but respectively function for different gas leakage paths.

[0099] Separation of the mechanical anchoring sealing base from the working string: After the central grouting reaches the designed grouting amount and sealing height, stop grouting.

[0100] The detachable connection assembly 49 adopts a releasable axial connection structure. A predetermined rotation and / or axial action is applied to the composite lowering string 2 through the ground operation unit 1, so that the composite lowering string 2 is disconnected from the mechanical anchoring sealing base 4.

[0101] After the connection is released, the mechanical anchoring sealing base 4, the peripheral sealing body 7 and the axial sealing body 8 are left in the well, and the composite lowering string 2 can be recovered upward.

[0102] The grout stop structure at each grouting outlet is automatically closed to prevent the grout that has not been completely consolidated from flowing back into the corresponding flow channel. In the embodiment where long-term grout replenishment capability needs to be retained, the compensation grouting channel 48 is connected to the wellhead through an independent small-diameter grout replenishment pipe, so that the grout replenishment pipe remains after the composite lowering string is recovered.

[0103] In-situ negative pressure verification after sealing and consolidation: After the permanent consolidated sealing grout reaches the preset consolidation state, the construction is not directly recognized as completed.

[0104] The original ground pressure relief well extraction system is used to re-establish the test negative pressure above the sealing body. When the preset initial negative pressure is reached , close the extraction valve.

[0105] Record different test times corresponding pressures , methane concentrations and oxygen concentrations , and calculate the negative pressure attenuation in the test period , where is the end point of the test period. At the same time, the oxygen concentration increment and the methane concentration variation .

[0106] If the negative pressure retention state, oxygen concentration change and methane concentration change all meet the set air tightness judgment conditions, the sealing is judged to be qualified. This verification process simultaneously judges "whether the air pressure leaks" and "whether external air actually enters", so it is more targeted than a simple static pressure test.

[0107] Directional compensation grouting after sealing failure: If the detection shows that the negative pressure attenuation exceeds the set range, the oxygen concentration continues to rise, or the methane concentration decreases accordingly, it is determined that there is still a residual leakage path in the composite sealing structure. In this case, the original sealing body is not re-drilled out. Connect the ground grouting equipment to the reserved compensation grouting pipe 24.

[0108] The compensating grout enters the edge area of ​​the outer sealing body 7 via the compensating grout delivery pipe 24 → compensating grouting channel 48 → one-way grout stop valve 482 → grouting outlet 481.

[0109] Since the axial plugging body 8 has been solidified, the compensating grout is restricted by the axial plugging body and the main body of the peripheral plugging body, and tends to enter the area between the peripheral plugging body and the well wall, the area between the peripheral plugging body and the residual screen pipe, and the near-well fractures that were not completely filled by the first grouting.

[0110] Therefore, this is not about "re-pouring cement," but rather a process of initial sealing result detection → determining residual leakage → returning to the original sealing interface via a pre-reserved channel → directional resealing. After the grouting is completed and solidified, negative pressure and gas composition testing are performed again according to step S9 until the airtightness criteria are met.

[0111] Specific engineering examples: For instance, a certain mine uses a combination of lower protective layer mining and surface decompression well extraction of gas from the overlying coal seam. The lower protective layer working face has been mined out, and the overlying target coal seam is expected to be mined out in the future.

[0112] The surface decompression well has a diameter of 216 mm, and the outer diameter of the residual screen pipe inside the well is 139.7 mm.

[0113] The theoretical mining influence boundary was determined based on the target coal seam mining height, overlying lithology, and existing mining-induced fracture data. Subsequently, segmented detection was conducted under the original depressurization and drainage negative pressure conditions. As the detection module moved from the shallow part towards the target area, a significant decrease in oxygen concentration and an increase in methane concentration were detected within a certain well depth range, along with synchronous changes in pressure and flow response. This determined the actual leakage state transition boundary. The target sealing section was then determined by combining the theoretical mining influence boundary and the preset safety distance.

[0114] A mechanical anchor base with an outer diameter smaller than the effective inner diameter of the residual screen tube is lowered to the target position. Once in place, the expansion sealing flap unfolds, causing the flexible sealing layer to press against the inner wall of the screen tube, and is axially locked by the anti-reverse anchor.

[0115] First, a biodegradable, high-viscosity flow-blocking medium is pumped into the peripheral injection channel, allowing it to enter the annulus outside the screen tube and near-wellbore fractures through radial injection ports and screen holes. After the flow-blocking pretreatment is completed, the medium is switched to a cement-based permanent consolidation slurry.

[0116] During the peripheral grouting process, the grouting pressure and flow rate are continuously monitored. Once a stable pressure increase and grout volume decrease are observed and maintained for a set time, peripheral grouting is stopped. Subsequently, the central injection channel is opened, and cement-based permanent consolidation grout is injected above the mechanical anchor base to form an axial sealing column.

[0117] After the grout has solidified, a negative pressure maintenance test and CH4 / O2 component detection are performed. If the test results meet the set airtight conditions, the compensation grouting pipe is closed and sealed; if the test results do not meet the airtight conditions, secondary grouting is performed through the compensation grouting pipe, and the test is repeated after the secondary grout has solidified.

[0118] Working principle summary: The key to this invention is not simply using a combination of materials such as "mechanical plug, high viscosity fluid and cement slurry", but rather using different structures and media to establish a continuous sealing action chain according to the formation mechanism of the leakage path.

[0119] First, the spatial range that may be affected by subsequent mining is determined by using the theoretical mining influence boundary. Then, the actual gas state transition position in the wellbore is detected by using the negative pressure conditions of the original depressurization well, thereby solving the problem of "where to plug". Subsequently, a stable support interface is established inside the residual screen pipe by using a retractable and expandable mechanical plugging base, which solves the problem of "how to reliably position the plugging structure" under the conditions of deformed screen pipe and irregular cross-section.

[0120] Based on this, the high-viscosity flow-resistant medium enters the annulus outside the screen tube and the peri-well fractures through the radial injection port and screen holes, first increasing the flow resistance of the distal fractures to limit the diffusion area for subsequent permanent grout. The permanently consolidated grout then forms an outer sealing body along the same radial path. Only after confirming that the outer bypass is under control based on the increase in grouting pressure and the decrease in grout intake is the center grouting switched to form an axial sealing body. Therefore, the process is as follows: high-viscosity flow-resistant demarcation → outer bypass cutoff → status feedback confirmation → internal axial cutoff.

[0121] Finally, the original pressure relief and extraction system was used to establish a test negative pressure. The actual leakage status after sealing was judged by the negative pressure maintenance state and the change of CH4 / O2 composition. For the sealing body that did not meet the airtightness requirements, directional grouting was carried out by re-entering the edge area of ​​the original sealing interface through the reserved compensation grouting channel. This formed a complete sealing closed loop: double boundary positioning, mechanical anchoring, flow obstruction and boundary, peripheral sealing, feedback switching, axial sealing, negative pressure verification, and compensation sealing.

[0122] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A directional composite plugging method for surface decompression wells before overlying coal seam mining, characterized in that, Includes the following steps: S1. Obtain the mining parameters of the overlying coal seam to be mined and the wellbore parameters of the surface decompression well, and determine the mining influence boundary; obtain the gas flow state at different depths along the surface decompression well, determine the leakage state transition boundary based on the changes in the gas flow state, and determine the target sealing section in combination with the mining influence boundary; S2. Lower the mechanical anchor base to the lower part of the target plugging section and expand it radially so that the mechanical anchor base is anchored to the residual screen pipe or well wall in the well, forming the support and plugging interface of the target plugging section; S3. Inject a high-viscosity flow-blocking medium into the annulus outside the residual screen pipe and the well perimeter fractures connected to it through the mechanical anchor sealing base to form a diffusion constraint zone for limiting the escape of the permanent consolidation sealing slurry; inject the permanent consolidation sealing slurry into the diffusion constraint zone to form an outer sealing body that bridges the residual screen pipe, the annulus outside the screen pipe and the well wall. S4. After the outer sealing is completed, a permanent consolidation sealing slurry is injected into the wellbore above the mechanical anchor base to form an axial sealing body connected to the outer sealing body. The outer sealing body cuts off the bypass gas leakage channel on the outside of the screen pipe, and the axial sealing body cuts off the axial gas leakage channel inside the wellbore. S5. After the permanent consolidation sealing grout has solidified, a test negative pressure is applied to the surface decompression well. The airtightness of the sealing is determined based on the wellbore condition under the test negative pressure, and whether to implement compensation grouting is determined based on the airtightness of the sealing.

2. The directional composite plugging method according to claim 1, characterized in that: The mining impact boundary is determined based on the mining height, coal seam dip angle, overlying lithology, and the development height of the caving zone and fracture zone of the overlying coal seam to be mined. The wellbore pressure, gas flow rate, methane concentration, and oxygen concentration are obtained at preset depth intervals along the surface decompression well. The leakage state transition position is determined based on the changes of at least three parameters between adjacent detection positions. The bottom position and axial length of the target sealing section are determined based on the leakage state transition position, the mining impact boundary, and the preset sealing safety distance.

3. The directional composite plugging method according to claim 1, characterized in that: The mechanical anchor base includes a central bearing body, a conical drive body, multiple expansion sealing petals arranged circumferentially along the central bearing body, a flexible sealing layer disposed on the outside of the expansion sealing petals, and a backlash anchor. During the lowering process, the expansion sealing petals are kept in a radially contracted state. After reaching the target sealing section, the conical drive body is axially displaced relative to the expansion sealing petals. The conical drive body pushes the expansion sealing petals radially outward, causing the flexible sealing layer to press against the residual screen pipe or well wall, and causing the backlash anchor to form an axial backlash prevention engagement with the residual screen pipe or well wall.

4. The directional composite plugging method according to claim 3, characterized in that: The mechanical anchor base is provided with a central injection channel, a peripheral injection channel and a compensating grouting channel that are isolated from each other. The peripheral injection channel is connected to a radial injection port arranged around the periphery of the mechanical anchor base. The radial injection port corresponds to the screen holes of the residual screen tube, so that the high-viscosity flow-blocking medium and the permanently solidified sealing grout forming the peripheral sealing body enter the annulus outside the screen tube in sequence through the peripheral injection channel, the radial injection port and the screen holes. The central injection channel is connected to the inside of the wellbore above the mechanical anchor base.

5. The directional composite plugging method according to claim 4, characterized in that: The high-viscosity flow-blocking medium is a degradable fluid with both a flowing state and a viscosity-enhancing state. After entering the annulus outside the screen pipe and the peri-well fracture through the peripheral injection channel, the high-viscosity flow-blocking medium forms a viscosity-enhancing state, thereby increasing the flow resistance of the annulus outside the screen pipe and the peri-well fracture and restricting the subsequent injection of permanent consolidation sealing slurry from escaping along the peri-well fracture to the outside of the target sealing section. After the permanent consolidation sealing slurry solidifies, the peripheral sealing body undertakes the permanent sealing function.

6. The directional composite plugging method according to claim 4, characterized in that: During the process of injecting permanent consolidation and sealing grout in the periphery, the grouting pressure and grouting flow rate are obtained. When the change in grouting pressure and the change in grouting flow rate per unit time reach the preset periphery sealing judgment conditions, grouting through the periphery injection channel is stopped, and the process is switched to the central injection channel. Permanent consolidation and sealing grout is then injected into the mechanical anchor base through the central injection channel.

7. The directional composite plugging method according to claim 6, characterized in that: The peripheral sealing body is formed by the permanent consolidation sealing slurry passing through the screen holes of the residual screen pipe and extending radially outward into the annulus of the screen pipe, and is solidified and connected with the residual screen pipe and the well wall; the axial sealing body is formed by the permanent consolidation sealing slurry extending upward from the mechanical anchor seal base along the inside of the wellbore, and the lower part of the axial sealing body is solidified and connected with the peripheral sealing body, so that the peripheral sealing body and the axial sealing body form a radial-axial composite sealing structure.

8. The directional composite plugging method according to claim 1, characterized in that: After the permanent consolidation sealing slurry reaches the preset consolidation state, a test negative pressure is established above the sealing body through the extraction pipeline of the surface decompression well. After extraction is stopped, the wellbore pressure, methane concentration and oxygen concentration are obtained at preset time intervals. The sealing airtightness is determined based on the amount of negative pressure decay, oxygen concentration increment and methane concentration change during the test.

9. The directional composite plugging method according to claim 8, characterized in that: When the airtight sealing condition does not meet the preset airtight conditions, compensation sealing slurry is injected into the unsealed area between the outer sealing body and the well wall, between the outer sealing body and the residual screen pipe, and / or the area connected to the outer sealing body through the compensation grouting channel retained in the well. After the compensating sealing grout has solidified, a test negative pressure is established again to determine the airtightness of the sealing until the preset airtightness conditions are met.

10. A surface decompression well plugging device for implementing the directional composite plugging method according to any one of claims 1-9, characterized in that, This includes a composite tubing string, wellbore status detection components, mechanical anchor base, and grouting control components; The wellbore status detection component is mounted on the composite lowering string and is used to obtain the gas flow status at different well depths. The mechanical anchor base is connected to the lower part of the composite lowering column and includes a central bearing body, a conical driving body, multiple expansion sealing petals arranged circumferentially along the central bearing body, a flexible sealing layer arranged on the outside of the expansion sealing petals, and a backlash anchor. The conical driving body and the expansion sealing petals form a driving cooperation that converts axial displacement into radial expansion displacement. The mechanical anchor base is equipped with a central injection channel, a peripheral injection channel and a compensation grouting channel that are isolated from each other. The peripheral injection channel is connected to multiple radial injection ports facing the screen holes of the residual screen pipe. The central injection channel is connected to the inside of the well barrel above the mechanical anchor base. The grouting control component is connected to the central injection channel, the peripheral injection channel and the compensation grouting channel respectively, and controls the grouting switching between the peripheral injection channel and the central injection channel according to the grouting pressure and grouting flow rate during the peripheral injection process; The mechanical anchor base is also equipped with a grout-stopping component to seal the corresponding grouting channel after the mechanical anchor base is separated from the composite lowering string and to retain at least the compensation grouting channel in the wellbore.

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