A method for preventing rock burst by water and nitrogen injection and freeze fracturing in fault area

Abstract

This invention relates to the field of coal mine safety mining and rockburst prevention technology, specifically a rockburst prevention method using water-nitrogen injection for freezing and fracturing in fault areas. The method includes: determining the stress core zone within the fault area through active and passive CT inversion, and delineating the target range; arranging boreholes within the target range, extending the boreholes into the stress core zone; injecting high-pressure water into the boreholes after pretreatment; injecting cryogenic liquid nitrogen into the boreholes after water injection, utilizing the freezing and vaporization effects of liquid nitrogen to expand and connect rock fractures, thus achieving pressure relief; using active and passive CT inversion to detect the pressure relief effect and determine whether the pressure relief qualification standard has been met; if not, cyclic water-nitrogen injection for freezing and fracturing is performed until the standard is met; after successful pressure relief, the boreholes are sealed and continuous monitoring is conducted. This invention offers precise positioning, significant pressure relief effect, and scientific detection, requires no blasting operations, has good construction safety, and can achieve long-term prevention of rockbursts in fault areas.

Description

Technical Field

[0001] This invention relates to the field of coal mine safety mining and rockburst prevention technology, specifically a rockburst prevention method using water-nitrogen injection for freezing and pressure relief in fault areas. Background Technology

[0002] Rockburst is a common and major disaster in deep coal mining. Essentially, it occurs when coal and rock masses undergo sudden and severe deformation and failure under high stress, releasing a large amount of elastic potential energy. This poses a serious threat to mine safety and can easily lead to roadway collapse, equipment damage, and even casualties. Fault areas, being weak points in coal mining, are high-risk areas for rockbursts due to poor rock mass integrity and significant stress concentration; their rockburst hazard is far greater than in ordinary coal and rock mass areas.

[0003] Currently, methods for preventing and controlling rockbursts in fault areas suffer from problems such as weak targeting of stress relief, poor effectiveness, and imperfect detection methods. It is difficult to accurately locate the core area of ​​stress concentration in the fault, and the effectiveness cannot be verified by efficient detection methods after stress relief. Furthermore, there is a lack of cyclic optimization mechanisms, which makes it impossible to completely eliminate the hidden dangers of rockbursts in fault areas and make it difficult to meet the prevention and control needs of complex fault areas.

[0004] Therefore, there is an urgent need for a method for preventing rockbursts that can accurately locate the core area of ​​fault stress, achieve efficient pressure relief, verify the effectiveness through scientific testing, and be cyclically optimized, in order to overcome the shortcomings in current practical applications. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preventing rockbursts by water-nitrogen injection for freezing and depressurization in fault areas, so as to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A method for preventing rockbursts in fault areas by water-nitrogen injection for freezing and depressurization includes the following steps:

[0008] S1. Determine the stress core area within the fault region and delineate the target range through active and passive CT inversion;

[0009] S2. Drill holes are arranged within the target area, extending into the stress core area. After drilling pretreatment, high-pressure water is injected into the drill holes.

[0010] S3. After water injection is completed, cryogenic liquid nitrogen is injected into the borehole. The freezing and vaporization expansion effects of liquid nitrogen are used to expand and connect the rock mass fissures, thereby relieving pressure.

[0011] S4. Use active and passive CT inversion to detect the pressure relief effect and determine whether the pressure relief qualification standard is met. If the standard is not met, repeat steps S2 and S3 until it is qualified.

[0012] S5. After the pressure relief is qualified, the borehole is sealed and continuous monitoring is carried out.

[0013] As a further aspect of the present invention: the stress core area mentioned in step S1 is: within the fault influence zone, the wave velocity anomaly area obtained by seismic wave CT inversion, and the area where the passively monitored stress value exceeds a predetermined multiple (e.g., 1.3-1.5 times) of the original rock stress.

[0014] As a further aspect of the present invention: the active and passive CT inversion in step S1 includes active detection and passive monitoring; the active detection adopts seismic wave detection or electromagnetic radiation detection, and the passive monitoring adopts at least one of stress monitoring, rock mass deformation monitoring or acoustic emission monitoring.

[0015] As a further aspect of the present invention: the active and passive CT inversion includes:

[0016] Active detection: Excitation points are set up in the tunnel, seismic waves are excited by the source, and geophones are set up along the tunnel. The wave velocity distribution is inverted by the travel-time tomography algorithm.

[0017] Passive monitoring: Pre-embed borehole stress gauges and acoustic emission probes in the fault area for continuous monitoring to obtain stress changes and microseismic event distribution;

[0018] Joint inversion: The active wave velocity structure is fused with the passively located microseismic events to delineate the region with abnormal wave velocity, concentrated microseismic events, and stress values ​​exceeding 1.3-1.5 times the original rock stress, which is referred to as the stress core region.

[0019] As a further aspect of the present invention: the injection pressure of the high-pressure water in step S2 is 8-25 MPa, and the water injection time is not less than 10 minutes per meter of borehole until the return water pressure at the borehole opening stabilizes.

[0020] As a further aspect of the present invention, the injection pressure of the high-pressure water is adjusted according to the integrity of the rock mass: for fractured rock mass or soft coal, the injection pressure is 8-15 MPa; for hard rock with uniaxial compressive strength ≥60 MPa or deep high-stress core area, the injection pressure is 15-25 MPa.

[0021] As a further aspect of the present invention: the borehole pretreatment in step S2 includes cleaning the borehole and installing a sealing device; the sealing device is installed at the borehole opening section to achieve a seal between the borehole and the rock wall.

[0022] As a further aspect of the present invention: after water injection in step S3, the water is left to stand for 12-24 hours, and after draining or blowing away the water accumulated in the borehole, the cryogenic liquid nitrogen is injected; the temperature of the cryogenic liquid nitrogen is -196℃, the injection pressure is 2-8MPa, and the injection rate does not exceed 50L / min; when injecting cryogenic liquid nitrogen into the borehole, a vacuum-insulated double-layer structure delivery pipe is used, and the inner and outer layers of the vacuum-insulated double-layer structure form a closed vacuum cavity, with an insulation coating on the outer wall of the inner pipe.

[0023] As a further aspect of the present invention: in step S3, temperature measuring holes are arranged within the stress core region, and the injection of cryogenic liquid nitrogen is stopped when the average temperature monitored by the temperature measuring holes drops below -2°C.

[0024] As a further aspect of the present invention: the standard for qualified decompression in step S4 is: the average value of the maximum principal stress in the stress core area is reduced to less than 1.2 times the original rock stress, and at least one of the following conditions is met: the area of ​​the wave velocity anomaly zone shown by seismic wave CT inversion is reduced by more than 70%, or the fracture density is increased to more than 3 times the original rock mass.

[0025] Compared with the prior art, the beneficial effects of the present invention are:

[0026] 1. Precise positioning and strong targeting: Through active and passive CT inversion technology, the spatial distribution of fault planes and the location of stress core areas can be accurately determined, so that subsequent water injection and nitrogen injection for freeze cracking can directly reach the stress concentration core, avoid blind construction, and greatly improve the targeting and efficiency of pressure relief.

[0027] 2. Significant pressure relief effect: The combined effect of borehole water injection softening and nitrogen injection freezing cracking is achieved. Water injection first softens the rock mass and reduces its strength, while nitrogen injection freezing cracking utilizes the effects of frost heave and vaporization expansion to expand rock mass fissures and release stress, resulting in more thorough pressure relief and effectively eliminating stress concentration in the stress core area.

[0028] 3. Scientific testing and cyclic optimization: The active and passive CT inversion technology is used to accurately verify the decompression effect, which can intuitively reflect the development of rock mass fractures and stress distribution. Combined with the cyclic freeze-thaw mechanism, decompression can be repeated in areas that do not meet the standards to ensure that the decompression effect meets the safety requirements.

[0029] 4. Construction safety and high operability: No blasting operation is required during the construction process, avoiding the risk of severe disturbance to the rock mass and secondary impact caused by blasting; the entire method and steps are clear, making it easy for on-site construction personnel to operate and promote its application.

[0030] 5. Significant long-term stress relief effect: After stress relief is completed, a large number of through fractures are formed in the rock mass, which can release the internal stress of the rock mass for a long time. Combined with subsequent continuous monitoring and borehole sealing, long-term prevention and control of rockburst in fault areas can be achieved. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the overall process of the method of the present invention.

[0032] Figure 2 This is a diagram showing the borehole layout in the fault stress core area.

[0033] Figure 3 This is a schematic diagram of the drilling layout and the water-nitrogen injection structure for freezing cracking.

[0034] In the diagram: 1. Fault region; 2. Stress core region; 3. Large-diameter borehole; 4. Shared water / nitrogen injection channel; 5. Stress gauge; 6. Sealing device. Detailed Implementation

[0035] The technical solution of this application will be further described in detail below with reference to specific embodiments.

[0036] The embodiments of this application are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application.

[0037]

Example

[0038] This embodiment takes a fault in a deep mining area of ​​a mine as the implementation object, and illustrates the specific application of the present invention under two different working conditions.

[0039] Working Condition 1: Intact Rock Mass

[0040] A reverse fault in the deep mining area of ​​a certain mine (i.e. Figure 2 The fault zone 1 has a fault drop of about 15m, a fault influence zone width of about 30m, a tunnel burial depth of about 800m, and an original rock stress of about 25MPa.

[0041] like Figure 1 As shown, the overall process of this method is as follows: First, the stress core area in the fault region is determined by active and passive CT inversion, and the target range is delineated; then, boreholes are arranged within the target range and high-pressure water is injected for softening; next, cryogenic liquid nitrogen is injected for freezing and decompression; then, active and passive CT inversion is used again to check the decompression effect; if the effect is qualified, borehole sealing and subsequent monitoring are carried out; if the effect is unqualified, the water injection and nitrogen injection steps are returned to perform cyclic freeze-thaw.

[0042] The following is in accordance with Figure 1 The process shown, combined with Figure 2 and Figure 3 This embodiment will be described in detail.

[0043] Step 1: Determine the stress core region using active and passive CT inversion.

[0044] like Figure 1 and Figure 2 As shown, active and passive CT inversion is carried out by combining active seismic wave detection and passive stress monitoring to determine the stress core zone 2 within fault region 1.

[0045] Active detection: Fifteen excitation points were set up in the tunnel near the fault, and seismic waves were excited by a controllable source. Geophones with a spacing of 8m were set up along the tunnel, and the wave velocity distribution was inverted by the travel-time tomography algorithm.

[0046] Passive monitoring: Pre-embed borehole stress gauges and acoustic emission probes in the fault area and continuously monitor for 10 days to obtain stress changes and the distribution of microseismic events.

[0047] Joint inversion: The active wave velocity structure is fused with the passively located microseismic events to delineate the area with wave velocity anomalies (wave velocity 15% higher than the surrounding area), concentrated microseismic events, and stress values ​​exceeding 32 MPa (1.28 times the original rock stress of 25 MPa). This area is designated as stress core zone 2, and the target range is delineated.

[0048] Step 2: Drilling and Injecting Water

[0049] like Figures 1 to 3 As shown, six sets of large-diameter boreholes 3 are arranged within the designated target area. The borehole diameter is φ133mm, and the borehole depth extends into the stress core zone 2. All large-diameter boreholes 3 are cleaned, and rubber expansion packers 6 are installed at the borehole openings as sealing devices (e.g., ...). Figure 3 As shown in the figure, this achieves a seal between the borehole and the rock wall.

[0050] High-pressure water is injected into the large-diameter borehole 3 through a shared water / nitrogen injection channel 4. This shared channel 4 is a vacuum-insulated double-layer pipe. The water injection pressure is determined to be 18 MPa based on the integrity of the rock mass. The injection time is 15 minutes per meter of borehole until the return pressure at the borehole opening stabilizes. The water fully penetrates into the rock fissures, softening the coal and rock mass and initially alleviating stress concentration. Figure 3 The stress gauge 5 in the middle is used to monitor stress changes in real time during water injection and subsequent processes.

[0051] Step 3: Nitrogen injection for freezing and pressure relief

[0052] After water injection, allow the mixture to stand for 18 hours to allow the moisture to fully distribute in the rock fissures. After purging the borehole of accumulated water with compressed air, inject cryogenic liquid nitrogen into the large-diameter borehole 3 through a vacuum-insulated double-layer pipe. The liquid nitrogen temperature is -196℃, the injection pressure is controlled at 5MPa, and the injection rate is 40L / min.

[0053] Temperature measuring holes were arranged within the stress core zone 2. Nitrogen injection was stopped when the average temperature monitored by the temperature measuring holes dropped below -2℃ (nitrogen injection continued for about 30 minutes). The freezing and expansion effect of liquid nitrogen was used to freeze and expand the water in the fissures, forcing the rock mass fissures to expand. At the same time, the vaporization and expansion of liquid nitrogen further impacted and relieved the pressure, releasing the elastic potential energy inside the rock mass.

[0054] Step 4: Effectiveness verification and cyclic freeze-thaw cycles

[0055] After nitrogen injection for freezing cracking is completed, as follows Figure 1 and Figure 2 As shown, active and passive CT inversion was used again to detect fault region 1 and stress core region 2. The detection results showed that the average value of the maximum principal stress in stress core region 2 decreased to 22 MPa (0.88 times the original rock stress), the area of ​​the wave velocity anomaly zone decreased by 75%, and the fracture density increased to 3.5 times that of the original rock mass, meeting the stress relief qualification standard. No cyclic freeze-thaw was required, and the process directly proceeded to step five.

[0056] Step 5: Subsequent Maintenance

[0057] All large-diameter boreholes 3 were sealed with cement grout to prevent the inflow of mine water, gas, etc. Simultaneously, stress gauges 5 were continuously installed for stress monitoring, and potential rockburst hazards were regularly investigated. Long-term monitoring showed that the stress in stress core area 2 remained stable, with no stress concentration observed, and the risk of rockburst was significantly reduced.

[0058] Working Condition 2: Fractured Rock Mass

[0059] The fault area in a certain mine is an extensional fault, with relatively fractured rock mass and an original rock stress of approximately 20 MPa. The procedure is implemented using the same method as in working condition one, with the following difference:

[0060] In step one, the stress value of stress core zone 2 was determined to be 28 MPa (1.4 times the original rock stress); in step two, the water injection pressure was adjusted to 12 MPa (for fractured rock mass); in step three, the nitrogen injection pressure was controlled at 4 MPa, and the nitrogen injection rate was 30 L / min; in step four, the initial test results showed that the area of ​​the wave velocity anomaly zone only decreased by 55%, which did not meet the 70% qualification standard, therefore... Figure 1 As shown, the process returns to the water and nitrogen injection steps for a second freeze-thaw cycle. After retesting, the area of ​​the abnormal wave velocity zone decreased by 72%, and the stress dropped to 23.5 MPa (1.175 times the original rock stress), meeting the pressure relief qualification standard, and proceeding to step five.

[0061] Both of the above operating conditions have verified the effectiveness and adaptability of the method of the present invention.

[0062] The above are merely preferred embodiments of the present invention. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention, and these should also be considered within the scope of protection of the present invention. These will not affect the effectiveness of the implementation of the present invention or the practicality of the patent.

Claims

1. A method for preventing rock burst by water-nitrogen freezing fracturing and pressure relief in a fault zone, characterized in that, Includes the following steps: S1. Determine the stress core area within the fault region and delineate the target range through active and passive CT inversion; S2. Drill holes are arranged within the target area, extending into the stress core area. After drilling pretreatment, high-pressure water is injected into the drill holes. S3. After water injection is completed, cryogenic liquid nitrogen is injected into the borehole. The freezing and vaporization expansion effects of liquid nitrogen are used to expand and connect the rock mass fissures, thereby relieving pressure. S4. Use active and passive CT inversion to detect the pressure relief effect and determine whether the pressure relief qualification standard is met. If the standard is not met, repeat steps S2 and S3 until it is qualified. S5. After the pressure relief is qualified, the borehole is sealed and continuous monitoring is carried out.

2. The method according to claim 1, wherein, The stress core area mentioned in step S1 is: within the fault influence zone, the area with abnormal wave velocity obtained by seismic wave CT inversion, and the area where the passively monitored stress value exceeds a predetermined multiple of the original rock stress.

3. The method according to claim 1, wherein the method is characterized by, The active and passive CT inversion in step S1 includes active detection and passive monitoring; the active detection adopts geophysical detection methods, and the passive monitoring adopts at least one of stress or deformation monitoring methods.

4. The method for preventing rockbursts in fault areas by water-nitrogen injection for freezing and depressurization as described in claim 3, characterized in that, The active and passive CT inversion includes: Active detection: Excitation points are set up in the tunnel, seismic waves are excited by a source, and detectors are set up along the tunnel. Wave velocity distribution is inverted by tomographic imaging algorithm. Passive monitoring: Pre-embed borehole stress gauges and acoustic emission probes in the fault area for continuous monitoring to obtain stress changes and microseismic event distribution; Joint inversion: The active wave velocity structure is fused with the passively located microseismic events to delineate the region with abnormal wave velocity, concentrated microseismic events, and stress values ​​exceeding a predetermined multiple of the original rock stress, which is referred to as the stress core region.

5. The method according to claim 1, wherein the method is characterized by, The injection pressure of the high-pressure water in step S2 is 8-25 MPa, and the water injection time is no less than 10 minutes per meter of borehole until the return water pressure at the borehole opening stabilizes.

6. The rock burst prevention method of fault zone water-nitrogen injection freeze fracturing pressure relief according to claim 5, characterized in that, The injection pressure of the high-pressure water is adjusted according to the integrity of the rock mass: for fractured rock mass or soft coal, the injection pressure is 8-15 MPa; for hard rock with uniaxial compressive strength ≥60 MPa or deep high-stress core area, the injection pressure is 15-25 MPa.

7. The method according to claim 1, wherein the method is characterized by, The borehole pretreatment in step S2 includes cleaning the borehole and installing a sealing device; the sealing device is installed at the borehole opening to achieve a seal between the borehole and the rock wall.

8. The method according to claim 1, wherein the method is characterized by, After water injection is completed in step S3, the water is allowed to stand for a predetermined time. After draining or blowing away the water in the borehole, the cryogenic liquid nitrogen is injected. The temperature of the cryogenic liquid nitrogen is -196℃, the injection pressure is 2-8MPa, and the injection rate does not exceed 50L / min. When injecting cryogenic liquid nitrogen into the borehole, a vacuum-insulated double-layered delivery pipe is used. The inner and outer pipes of the vacuum-insulated double-layered structure are in a closed vacuum cavity, and the outer wall of the inner pipe is provided with a heat-insulating coating.

9. The method according to claim 1, wherein the method is characterized by, In step S3, temperature measuring holes are arranged within the stress core area. When the average temperature monitored by the temperature measuring holes drops below the predetermined temperature threshold, the injection of cryogenic liquid nitrogen is stopped.

10. The method according to claim 1, wherein the method is characterized by, The standard for qualified stress relief in step S4 is: the average value of the maximum principal stress in the stress core area is reduced to below a predetermined multiple of the original rock stress, and at least one of the following conditions is met: the area of ​​the wave velocity anomaly zone shown by seismic wave CT inversion is reduced by more than a predetermined proportion, or the fracture density is increased to more than a predetermined multiple of the original rock mass.

Images