Drilling system and method for preventing borehole wall instability in goaf penetrating drilling process

Abstract

The invention discloses a well drilling system and method for preventing borehole wall instability in the goaf penetrating drilling process, and belongs to the technical field of well drilling machinery. The invention aims to provide a well drilling system capable of effectively preventing well wall instability in the drilling process of penetrating through a goaf. According to the technical scheme, the device comprises a motor, a speed reducer, a clutch, a square drill rod, a rotary table, a transmission rod, a drill rod, a guide rod and a drill bit, the motor is connected with the upper end of the square drill rod through the clutch and the speed reducer, the lower end of the square drill rod penetrates through the rotary table and then is connected with a first universal coupling, and the first universal coupling is connected with the upper end of the transmission rod through a universal joint; the lower end of the transmission rod is connected with a second universal coupling through a cross rod, the lower end of the second universal coupling is connected with the upper end of a drill rod through a flange, the lower end of the drill rod is connected with a rubber sheet, a solution shock absorber is connected below the rubber sheet, the lower end of the solution shock absorber is connected with the upper end of a guide rod, and the lower end of the guide rod is connected with a drill bit. The method is used for drilling through the goaf.

Description

Technical Field

[0001] The invention discloses a drilling system and a method for preventing a well wall from becoming unstable during drilling through a goaf area, belonging to the technical field of drilling machinery. Background Art

[0002] During the drilling process through the goaf, due to the uneven geological conditions of the goaf, the drill bit is easily affected by vibration during the drilling process. In addition, due to the large shear resistance and the discontinuous rock cutting process, the rotation torque of the drill bit fluctuates greatly, which accelerates the breakage, shedding and wear of the composite piece, and also causes the drilling direction to deviate from the target trajectory, causing the well wall to become unstable. These problems all require special technical measures to solve. Therefore, the present invention proposes a drilling system and method for preventing the well wall from becoming unstable during the drilling process through the goaf, so as to further improve the mining efficiency and safety. Summary of the invention

[0003] In view of the deficiencies in the prior art, an object of the present invention is to provide a drilling system and method that can effectively prevent the well wall from becoming unstable during the drilling process through the goaf.

[0004] To achieve the above-mentioned purpose, the present invention adopts the following technical scheme: a drilling system for preventing the well wall from becoming unstable during drilling through a goaf, comprising a motor, a reducer, a clutch, a square drill rod, a turntable, a transmission rod, a drill rod, a guide rod and a drill bit, wherein the motor is connected to the upper end of the square drill rod through the clutch and the reducer, the lower end of the square drill rod is connected to the first universal coupling after passing through the turntable, the first universal coupling is connected to the upper end of the transmission rod through a universal joint, the lower end of the transmission rod is connected to the second universal coupling through a cross rod, the lower end of the second universal coupling is connected to the upper end of the drill rod through a flange, the lower end of the drill rod is connected to a rubber sheet, a liquid shock absorber is connected below the rubber sheet, the lower end of the liquid shock absorber is connected to the upper end of the guide rod through a connecting piece, and the lower end of the guide rod is connected to the drill bit;

[0005] The drill bit includes an installation section, a connecting section and a cutting section. The installation section is provided with a loading and unloading groove connected to the guide rod. The connecting section connects the installation section and the cutting section together, and a shock-proof drilling direction-guide is installed on the connecting section. A plurality of blades are evenly distributed on the periphery of the cutting section. Each blade wing is provided with cutting teeth and pointed cone teeth. A chip removal groove is provided between two adjacent blade wings. The conveying pipeline is located at the central axis of the drill bit and passes through the drill bit up and down, and an anti-backflow device is installed on the upper part of the conveying pipeline. A plurality of water holes are provided on the bottom surface of the cutting section, and each water hole is connected to the conveying pipeline through a secondary pipeline. An auxiliary water hole with an outlet inclined inward is provided on the inner side surface of each blade wing, and each auxiliary water hole is also connected to the conveying pipeline through the secondary pipeline. A nozzle is installed in each water hole and the auxiliary water hole.

[0006] Furthermore, the shockproof drilling directional device comprises a shell, a sensor, a spring shock absorber and a wireless transmission device, the shell has an inner cavity, the sensor is fixed in the inner cavity, the wireless transmission device is fixed on the top of the shell and is surrounded by a rubber protective cover on the outside, the sensor and the wireless transmission device are connected, a spring shock absorber is arranged at the bottom of the shell, the bottom of the spring shock absorber is close to the connecting section of the drill bit, and a stabilizing cover is arranged outside the spring shock absorber

[0007] Furthermore, a stabilizing cover is arranged outside the spring shock absorber, the upper end of the stabilizing cover is fixed on the outer shell of the shockproof drilling directional device, and the lower end is close to the connecting section of the drill bit.

[0008] Furthermore, the anti-backflow device includes a valve body, a valve cover, a support ring, a valve disc and a spring. The two valve covers are respectively fixed at the upper and lower ends of the valve body. The support ring is arranged inside the valve body, and its lower end is fixed on the valve cover. The upper end supports the spring. The valve disc is fixed on the spring. The top of the valve disc cooperates with the valve seat arranged inside the valve body. A guide device is also arranged between the valve disc and the support ring.

[0009] Furthermore, the number of the blades is four.

[0010] Furthermore, there are multiple cutting teeth and multiple conical teeth on the blade wing, and the multiple cutting teeth are arranged in a row from top to bottom, and the multiple conical teeth are also arranged in a row from top to bottom.

[0011] Furthermore, the cutting teeth and the pointed cone teeth both protrude 2 cm from the blade surface.

[0012] Furthermore, the blade is in a spiral shape.

[0013] Furthermore, the chip removal groove is streamlined, and its cross-sectional area gradually increases from bottom to top.

[0014] A drilling method for preventing well wall instability during drilling through a goaf area comprises the following steps:

[0015] S1. Determine the location, depth and angle of the borehole based on geological survey data. The deviation between the borehole location and the designed location shall not exceed 0.5m;

[0016] S2. Select a drilling fluid formula that meets the formation requirements according to the formation characteristics, and use the drilling system for preventing the wellbore from being unstable during drilling through the goaf as claimed in claim 1 to perform drilling, with a drilling pressure of 20-50KN and a rotation speed of 30r / min. During the drilling process, the drilling trajectory is continuously monitored by a seismic-proof while-drilling directional device. If the deviation between the drilling trajectory and the designed trajectory exceeds the allowable range, the trajectory is corrected in time until the deviation between the drilling trajectory and the designed trajectory returns to the allowable range;

[0017] S3. When drilling close to the goaf, reduce the drilling speed to 25r / min to reduce the impact and damage to the well wall;

[0018] S4. After passing through the goaf, restore the speed to 30r / min until drilling is completed;

[0019] S5. After drilling is completed, cementing operations are carried out to strengthen the well wall.

[0020] Compared with the prior art, the present invention has the following beneficial effects.

[0021] 1. In the present invention, the solution shock absorber is filled with viscous liquid. When it is impacted by external force during drilling, the fluid flows in the internal cavity of the shock absorber to generate damping force, thereby consuming vibration energy. Then, when the vibration is transmitted to the rubber sheet, the rubber sheet will deform, thereby absorbing part of the vibration energy. In this way, the solution shock absorber and the rubber sheet have a dual shock-absorbing effect, which is very effective in alleviating the vibration of the drill bit and can effectively prevent the instability of the well wall. In addition, it can also reduce the resonant frequency of the system, thereby reducing the occurrence of resonance.

[0022] 2. In the present invention, the four blades are evenly distributed on the drill bit. The four-blade design is relatively simple, which simplifies the operation process. In addition, the drill bit with a four-blade design can effectively disperse the cutting force during the drilling process, so that the load borne by each blade is more uniform, so that it can maintain high cutting efficiency and drill deeper, thereby improving the overall efficiency of the drilling operation, reducing vibration and wear, and improving the stability and service life of the drill bit. In addition, the design of four blades can usually provide a large area of ​​chip removal space, thereby improving the efficiency of cooling the drill bit.

[0023] 3. In the present invention, an auxiliary water hole is opened on the inner side surface of the blade, and a nozzle is installed in the auxiliary water hole. On the one hand, the drilling fluid can be sprayed to the bottom of the well and the surface of the drill bit more effectively, thereby enhancing the flushing effect of the drilling fluid on the bottom of the well, helping to remove the cuttings on the cutting teeth, keeping the surface of the drill bit clean, and reducing the risk of drill sticking. On the other hand, the cutting teeth can be cooled and lubricated more effectively, thereby helping to reduce the wear of the cutting teeth caused by high temperature and friction, and extending the service life of the drill bit. Thirdly, a high-pressure jet can be formed, which reduces the cutting load of the drill bit, improves the drilling efficiency, and can also change the flow direction of the drilling fluid, thereby reducing the vibration of the drill bit to a certain extent, and helping to improve the stability and cutting accuracy of the drill bit.

[0024] 4. The present invention installs an anti-backflow device in the conveying pipeline, which can prevent the backflow of drilling fluid from causing the mudstone to expand and squeeze the well wall, resulting in a shrinkage phenomenon. It also avoids the pressure difference and shear force generated by the backflow of drilling fluid acting on the well wall rock, causing formation rupture, affecting the stability of the well wall, or causing large-scale leakage of drilling fluid and formation pollution. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required to be used in the embodiments are briefly introduced below.

[0026] Figure 1 It is a structural schematic diagram of the present invention.

[0027] Figure 2 It is a schematic diagram of the structure of the drill bit in the present invention.

[0028] Figure 3 It is a schematic diagram of the structure of the shockproof while drilling directional device in the present invention.

[0029] Figure 4 It is a schematic structural diagram of the backflow prevention device in the present invention.

[0030] In the figure: 1 is a motor, 2 is a reducer, 3 is a clutch, 4 is a first universal coupling, 5 is a transmission rod, 6 is a flange, 7 is a drill rod, 8 is a connecting piece, 9 is a guide rod, 10 is a drill bit, 11 is a second universal coupling, 13 is a square drill rod, 14 is a turntable, 15 is a universal joint, 16 is a cross rod, 17 is a rubber sheet, 18 is a solution shock absorber, 101 is a loading and unloading groove, 102 is a shockproof drilling directional device, 103 is a sharp cone tooth, 104 is a cutting tooth, 106 is a blade, 109 is a conveying pipeline, 110 is a chip removal groove, 111 is a water eye, 112 is an auxiliary water eye, 113 is a connecting section, 114 is a mounting section, 115 is an anti-backflow device, 116 is a cutting section, 121 is a wireless transmission device, 122 is an inner cavity, 123 is a sensor, 124 is a spring shock absorber, 125 is a stabilizing cover, 127 is an outer shell, 128 is a rubber protective sleeve, 131 is a valve body, 132 is a valve cover, 133 is a support ring, 134 is a valve disc, 135 is a spring, 136 is a valve seat, and 137 is a guide device. DETAILED DESCRIPTION

[0031] The technical solution of the present invention is described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other implementation methods obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0032] The present invention provides the following embodiments.

[0033] like Figure 1As shown, a drilling system for preventing the instability of a well wall during drilling through a goaf area of ​​the present invention comprises a motor 1, a reducer 2, a clutch 3, a square drill rod 13, a turntable 14, a transmission rod 5, a drill rod 7, a guide rod 9 and a drill bit 10. The motor 1 is connected to the upper end of the square drill rod 13 through the reducer 2 and the clutch 3. The lower end of the square drill rod 13 passes through the turntable 14 and is connected to the first universal coupling 4. The first universal coupling 4 is connected to the upper end of the transmission rod 5 through a universal joint 15. The lower end of the transmission rod 5 is connected to the second universal coupling 11 through a cross rod 16. The lower end of the second universal coupling 11 is connected to the upper end of the drill rod 7 through a flange 6. The lower end of the drill rod 7 is connected to a rubber sheet 17. A solution damper 18 is connected below the rubber sheet 17. The lower end of the solution damper 18 is connected to the upper end of the guide rod 9 through a connector 8. The lower end of the guide rod 9 is connected to the drill bit 10.

[0034] like Figure 2 As shown, the drill bit 10 includes a mounting section 114, a connecting section 113 and a cutting section 116. The mounting section 114 is provided with a loading and unloading groove 101 connected to the guide rod 9. The connecting section 113 connects the mounting section 114 and the cutting section 116 together, and a shock-proof drilling orientation device 102 is installed on the connecting section 113. A plurality of blades 106 are evenly distributed on the periphery of the cutting section 116. Each blade wing 106 is provided with a cutting tooth 104 and a sharp cone tooth 103. A chip removal groove 110 is provided between two adjacent blade wings 106. The conveying pipe The channel 109 is located at the central axis of the drill bit 10 and passes through the drill bit 10 from top to bottom, and an anti-backflow device 115 is installed on the upper part of the delivery pipeline 109. A plurality of water holes 111 are opened on the bottom surface of the cutting section 116, and each water hole 111 is connected to the delivery pipeline 109 through a secondary pipeline. An auxiliary water hole 112 with an outlet inclined inward is opened on the inner side surface of each blade 106, and each auxiliary water hole 112 is also connected to the delivery pipeline 109 through a secondary pipeline. A nozzle is installed in each water hole 111 and the auxiliary water hole 112.

[0035] In the drill bit 10, the hydraulic system composed of the delivery pipe 109, the auxiliary pipe, the water hole 111, the auxiliary water hole 112 and the chip groove 110 can assist the drill bit 10 in breaking the rock formation after the hydraulic impact pressure reaches a certain value. The nozzles installed at the water hole 111 and the auxiliary water hole 112 are made of boron carbide material, and the interface part of the nozzle adopts a plum-tooth wrench design, which has a certain anti-loosening performance.

[0036] The nozzles in the auxiliary water hole 112 can further optimize the drilling parameters by adjusting the injection angle and flow rate.

[0037] The solution shock absorber is filled with silicone oil, which is a viscous liquid. When it is impacted by external force during drilling, the fluid flows in the internal cavity of the shock absorber to generate damping force, thereby consuming vibration energy. Then, when the vibration is transmitted to the rubber sheet, the rubber sheet will deform, thereby absorbing part of the vibration energy. In this way, the solution shock absorber and the rubber sheet have a dual shock-absorbing effect, which is very effective in alleviating drill bit vibration, effectively preventing the well wall from becoming unstable, and reducing the breakage, shedding and wear of the composite sheet on the cutting tooth 104, thereby improving rock breaking efficiency.

[0038] like Figure 3 As shown, the shockproof drilling directional device 102 includes a shell 127, a sensor 123, a spring shock absorber 124 and a wireless transmission device 121. The shell 127 has an inner cavity 122. The sensor 123 is fixed in the inner cavity 122. The wireless transmission device 121 is fixed on the top of the shell 127 and is surrounded by a rubber protective cover 128 on the outside. The sensor 123 and the wireless transmission device 121 are connected. A spring shock absorber 124 is provided at the bottom of the shell 127. The bottom of the spring shock absorber 124 is close to the connecting section 113 of the drill bit 10.

[0039] The spring shock absorber 124 is provided with a stabilizing cover 125 on the outside. The upper end of the stabilizing cover 125 is fixed on the housing 127 of the shockproof drilling directional device 102 , and the lower end is closely attached to the connecting section 113 of the drill bit 10 .

[0040] The spring damper 124 is a torsion spring damper.

[0041] The anti-seismic drilling directional device 102 adopts the electromagnetic wave wireless transmission method. The sensor 123 collects the measurement data downhole, loads the collected measurement data onto the electromagnetic wave carrier signal, and transmits the carrier signal loaded with the measurement data to the surroundings through the wireless transmission device 121, that is, the electromagnetic wave transmitter. After the receiving device on the ground or at the wellhead receives the electromagnetic wave signal, it converts it into an electrical signal, decodes the received electrical signal, and restores the original measurement data. This transmission method has the advantages of not being affected by the drilling medium, fast transmission speed, and long transmission distance. The ground receiving system monitors the downhole situation by measuring the change in the potential difference between the two points, and adjusts the drilling direction of the drill bit in real time, avoiding the problem of instability of the well wall caused by deviation of the drilling direction.

[0042] like Figure 4As shown, the anti-backflow device 115 includes a valve body 131, a valve cover 132, a support ring 133, a valve flap 134 and a spring 135. The two valve covers 132 are respectively fixed at the upper and lower ends of the valve body 131. The support ring 133 is arranged inside the valve body 131, and its lower end is fixed on the valve cover 132, and the upper end supports the spring 135. The valve flap 134 is fixed on the spring 135, and the top of the valve flap 134 cooperates with the valve seat 136 arranged inside the valve body 131. A guide device 137 is also arranged between the valve flap 134 and the support ring 133.

[0043] If the drilling fluid backflow causes formation rupture, the drilling fluid will flow into the formation cracks in large quantities, causing the pressure of the fluid column in the well to drop rapidly, thereby causing the well wall to become unstable. In addition, the stress state around the well wall will change, and the pressure support originally provided by the drilling fluid may be weakened due to formation rupture, resulting in microcracks or plastic deformation in the well wall, thereby reducing the bearing capacity of the well wall. The anti-backflow device 115 can block the backflow of drilling fluid and effectively reduce the penetration of drilling fluid into the formation.

[0044] The number of the blades 106 is four.

[0045] There are multiple cutting teeth 104 and multiple conical teeth 103 on the blade wing 106. The multiple cutting teeth 104 are arranged in a row from top to bottom, and the multiple conical teeth 103 are also arranged in a row from top to bottom.

[0046] The tooth arrangement plan should ensure that the spacing between the cutting teeth 104 is reasonable and evenly distributed to avoid poor cutting effect or excessive wear of the cutting teeth 104 due to overcrowding or oversparseness. The tooth density is suitable to be arranged from low to high. According to the formation characteristics and drilling parameters, the cutting amount that each cutting tooth 104 should bear is calculated, and then the radial arrangement formula of the cutting teeth 104 is derived according to the equal cutting principle to ensure that the spacing between the cutting teeth 104 is reasonable.

[0047] The cutting teeth 104 and the pointed cone teeth 103 both protrude 2 cm from the surface of the blade 106. The cutting teeth 104 first contact the rock or formation and perform cutting operations to reduce the torsional vibration of the drill bit caused by the cutting force. During the cutting process, the blade 106 supports and protects the cutting teeth 104. The cutting teeth 104 and the pointed cone teeth 103 cooperate with each other to complete the cutting task and effectively release the bottom hole pressure. The cutting teeth 104 are responsible for the initial cutting operation, while the pointed cone teeth 103 are responsible for the subsequent auxiliary cutting and controlling the penetration depth. The efficient cooperation helps to reduce the drilling time and improve the drilling efficiency, thereby reducing the time the well wall is exposed to unstable conditions.

[0048] The blade 106 is in a spiral shape.

[0049] The crown section of the blade 106 can be regarded as a rotating surface composed of smooth curves with the center line of the drill bit as the axis. The design method of straight line-arc-straight line is adopted. This design helps to achieve a smooth transition from the crown top to the diameter protection and avoids the early damage of the local cutting teeth caused by stress concentration. In the blade 106, the working surface is composed of an inner cone and an arc surface. The inner cone is located on the inner side of the cutting section 116 of the drill bit 10, close to the center line of the drill bit 10. The straight line segments of the crown are usually located at both ends of the blade 106 to provide stable cutting force and guiding performance. The design of the straight line segment enables the drill bit 10 to maintain a stable cutting path during the cutting process, reduce deflection and vibration, and the arc segment located between the straight line segments is the core part of the crown shape. The design of the arc segment enables the drill bit 10 to better adapt to the changes in the formation during the cutting process, improve the rock breaking efficiency and drilling speed. This contour design helps to optimize the cutting performance and drilling efficiency of the drill bit.

[0050] The chip groove 110 is streamlined, which can reduce the generation of vortices and stagnant areas on the drill bit surface. Following the law of decreasing pressure gradient in the chip groove 110, its cross-sectional area gradually increases from bottom to top. The existence of pressure difference enables the drill bit 10 to have a better cleaning effect, is also conducive to the timely discharge of rock chips, and can cool the drill bit 10 over a large area.

[0051] The width of the chip groove 110 is related to the diameter and rotation speed of the drill bit 10, and its shape is designed to be streamlined, and the calculation formula is: W = (πdn) / (60 × 1000), where W is the width of the chip groove 110, d is the diameter of the drill bit 10, and n is the rotation speed of the drill bit 10. The length of the chip groove 110 is usually related to the volume of the chips, the radius of the drill bit 10 and the drilling speed, and the calculation formula is: L = V / (πr 2 v), where: L is the length of the chip flute 110, V is the volume of the chips, r is the radius of the drill bit 10, and v is the drilling speed; the maximum cross-sectional area Smax of the chip flute 110 is determined as: Smax=Q / Va, where: Smax is the maximum cross-sectional area of ​​the chip flute 110, Q is the actual displacement, and Va is the minimum annular return speed. The parameters can be adjusted according to the actual situation to achieve the best drilling effect, avoid the appearance of mud cakes, and prevent the instability of the well wall.

[0052] The present invention discloses a drilling method for preventing the well wall from becoming unstable during drilling through a goaf area, comprising the following steps:

[0053] S1. Determine the location, depth and angle of the borehole based on geological survey data. The deviation between the borehole location and the designed location shall not exceed 0.5m;

[0054] S2. According to the formation characteristics, a drilling fluid formula that meets the formation requirements is selected, and drilling is carried out using the drilling system for preventing the well wall from becoming unstable during drilling through the goaf as described in claim 1, with a drilling pressure of 20-50KN and a rotation speed of 30r / min. During the drilling process, the drilling trajectory is continuously monitored by the anti-vibration drilling directional device 102. If the deviation between the drilling trajectory and the designed trajectory exceeds the allowable range, the trajectory is corrected in time until the deviation between the drilling trajectory and the designed trajectory returns to the allowable range; during the trajectory correction, the trajectory is corrected by using the data fed back to the ground by the anti-vibration drilling directional device 102, and the method of pulling out and drilling down is used.

[0055] S3. When drilling close to the goaf, reduce the drilling speed to 25r / min to reduce the impact and damage to the well wall;

[0056] S4. After passing through the goaf, restore the speed to 30r / min until drilling is completed;

[0057] S5. After drilling is completed, cementing operations are carried out to strengthen the well wall.

[0058] During the above drilling process, the drilling fluid flows into the drill bit 10 through the pipeline to clean the blades 106, cool the drill bit 10, carry rock cuttings upward, prevent the formation of mud cakes, and improve the stability of the well wall.

[0059] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A drilling system for preventing the well wall from becoming unstable during drilling through a goaf, comprising a motor (1), a reducer (2), a clutch (3), a square drill rod (13), a rotary table (14), a transmission rod (5), a drill rod (7), a guide rod (9) and a drill bit (10), wherein the motor (1) is connected to the upper end of the square drill rod (13) through the reducer (2) and the clutch (3), and is characterized in that: The lower end of the square drill rod (13) passes through the turntable (14) and is connected to the first universal coupling (4); the first universal coupling (4) is connected to the upper end of the transmission rod (5) through a universal joint (15); the lower end of the transmission rod (5) is connected to the second universal coupling (11) through a cross rod (16); the lower end of the second universal coupling (11) is connected to the upper end of the drill rod (7) through a flange (6); the lower end of the drill rod (7) is connected to a rubber sheet (17); a liquid shock absorber (18) is connected below the rubber sheet (17); the lower end of the liquid shock absorber (18) is connected to the upper end of the guide rod (9) through a connecting piece (8); and the lower end of the guide rod (9) is connected to the drill bit (10); The drill bit (10) comprises a mounting section (114), a connecting section (113) and a cutting section (116); the mounting section (114) is provided with a loading and unloading groove (101) connected to a guide rod (9); the connecting section (113) connects the mounting section (114) and the cutting section (116) together; and a shockproof drilling orientation device (102) is installed on the connecting section (113); a plurality of blades (106) are evenly distributed on the periphery of the cutting section (116); each blade (106) is provided with a cutting tooth (104) and a pointed cone tooth (103); a chip removal groove (110) is provided between two adjacent blades (106); and the conveying pipe (10 9) is located at the center axis of the drill bit (10) and penetrates the drill bit (10) from top to bottom, and an anti-backflow device (115) is installed in the upper part of the delivery pipeline (109), a plurality of water holes (111) are opened on the bottom surface of the cutting section (116), each water hole (111) is connected to the delivery pipeline (109) through a secondary pipeline arranged inside the cutting section (116), an auxiliary water hole (112) with an outlet inclined inward is opened on the inner side surface of each blade (106), each auxiliary water hole (112) is also connected to the delivery pipeline (109) through the secondary pipeline, and a nozzle is installed in each water hole (111) and the auxiliary water hole (112).

2. A drilling system for preventing wellbore instability during drilling through a goaf according to claim 1, characterized in that: The shockproof drilling directional device (102) comprises a housing (127), a sensor (123), a spring shock absorber (124) and a wireless transmission device (121); the housing (127) has an inner cavity (122); the sensor (123) is fixed in the inner cavity (122); the wireless transmission device (121) is fixed on the top of the housing (127) and is surrounded by a rubber protective sleeve (128); the sensor (123) and the wireless transmission device (121) are connected; a spring shock absorber (124) is arranged at the bottom of the housing (127); the bottom of the spring shock absorber (124) is in close contact with the connecting section (113) of the drill bit (10).

3. A drilling system for preventing wellbore instability during drilling through a goaf according to claim 2, characterized in that: The spring shock absorber (124) is provided with a stabilizing cover (125) outside, the upper end of the stabilizing cover (125) is fixed on the housing (127) of the shockproof drilling directional device (102), and the lower end is closely attached to the connecting section (113) of the drill bit (10).

4. A drilling system for preventing wellbore instability during drilling through a goaf according to claim 1, characterized in that: The anti-backflow device (115) comprises a valve body (131), a valve cover (132), a support ring (133), a valve flap (134) and a spring (135). The two valve covers (132) are respectively fixed at the upper and lower ends of the valve body (131). The support ring (133) is arranged inside the valve body (131). The lower end of the support ring (133) is fixed on the valve cover (132) and the upper end supports the spring (135). The valve flap (134) is fixed on the spring (135). The top of the valve flap (134) cooperates with a valve seat (136) arranged inside the valve body (131). A guide device (137) is also arranged between the valve flap (134) and the support ring (133).

5. A drilling system for preventing wellbore instability during drilling through a goaf according to claim 1, 2 or 4, characterized in that: The number of the blades (106) is four.

6. A drilling system for preventing wellbore instability during drilling through a goaf according to claim 1, characterized in that: The cutting teeth (104) and the pointed cone teeth (103) on the blade wing (106) are both multiple, and the multiple cutting teeth (104) are arranged in a row from top to bottom, and the multiple pointed cone teeth (103) are also arranged in a row from top to bottom.

7. A drilling system for preventing wellbore instability during drilling through a goaf according to claim 6, characterized in that: The cutting teeth (104) and the pointed cone teeth (103) both protrude 2 cm from the surface of the blade (106).

8. The drilling system for preventing the wellbore from becoming unstable during drilling through a goaf according to claim 1, characterized in that: The blade (106) is in the shape of a spiral line.

9. A drilling system for preventing wellbore instability during drilling through a goaf according to claim 1, characterized in that: The chip removal groove (110) is streamlined, and its cross-sectional area gradually increases from bottom to top.

10. A drilling method for preventing the well wall from becoming unstable during drilling through a goaf, characterized in that The following steps are involved: S1. Determine the location, depth and angle of the borehole based on geological survey data. The deviation between the borehole location and the designed location shall not exceed 0.5m; S2. Select a drilling fluid formula that meets the requirements of the formation according to the formation characteristics, and use the drilling system for preventing the instability of the well wall during the drilling process of the goaf as claimed in claim 1 to perform drilling, with a drilling pressure of 20-50 KN and a rotation speed of 30 r / min. During the drilling process, the drilling trajectory is continuously monitored by the anti-vibration drilling directional device (102). If the deviation between the drilling trajectory and the designed trajectory exceeds the allowable range, the trajectory is corrected in time until the deviation between the drilling trajectory and the designed trajectory returns to the allowable range; S3. When drilling close to the goaf, reduce the drilling speed to 25r / min to reduce the impact and damage to the well wall; S4. After passing through the goaf, restore the speed to 30r / min until drilling is completed; S5. After drilling is completed, cementing operations are carried out to strengthen the well wall.

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