Coal bed gas well composite fracturing and permeability improvement construction method

Abstract

The coal bed gas well composite fracturing and permeability improvement construction method comprises a carbon dioxide injection pipe, a nitrogen injection pipe, two blocking and sealing devices, a fracturing fluid preparation device, a power generation assembly and a PLC controller, the nitrogen injection pipe is coaxially sleeved outside the carbon dioxide injection pipe, the nitrogen injection pipe and the carbon dioxide injection pipe are coaxially lowered into the coal bed gas well, the two blocking and sealing devices are coaxially sleeved and fixed on the nitrogen injection pipe at intervals, the fracturing fluid preparation device is coaxially sleeved and fixed on the nitrogen injection pipe and located between the two blocking and sealing devices, the bottom of the fracturing fluid preparation device is provided with a liquid booster pump, the top of the fracturing fluid preparation device is provided with a gas booster pump, and the liquid inlet of the liquid booster pump is connected with the fracturing fluid outlet of the fracturing fluid preparation device. The present application has the advantages of scientific design, reasonable structure, good composite fracturing and permeability improvement effect, low requirement on equipment performance and energy saving and consumption reduction.

Description

Technical Field

[0001] This invention relates to the field of coalbed methane well fracturing technology, and more specifically, to a composite fracturing and permeability enhancement construction method for coalbed methane wells. Background Technology

[0002] Coal seam fracturing effectively connects the wellbore with the natural fractures of the coal seam, thereby more rationally distributing the pressure drop around the wellbore during drainage and gas production, increasing production capacity and gas desorption rate, and improving recovery rate. Therefore, fracturing is widely used in coalbed methane extraction as an important enhanced production measure. Since carbon dioxide has a critical temperature of 31.1 degrees Celsius and a critical pressure of 73 atmospheres, it is easily liquefied under pressure. Typically, liquid carbon dioxide can be obtained at 20 degrees Celsius by pressurizing to 60 atmospheres. Therefore, liquid carbon dioxide has become a commonly used fracturing fluid for fracturing coal seams.

[0003] Existing fracturing systems typically place the fracturing pipe inside the coal seam well, with a surface booster pump delivering fracturing fluid through the pipe into the fracturing section of the well. This method requires high operating pressure from the booster pump, placing high demands on equipment performance and consuming significant energy. Furthermore, if liquid carbon dioxide is used as the fracturing fluid, injecting it through fracturing pipes on the surface can lead to some of the liquid carbon dioxide turning into a gaseous state before reaching the fracturing coal seam due to the long pipe length, thus affecting the fracturing effect. Summary of the Invention

[0004] The purpose of this invention is to provide a composite fracturing and permeability enhancement construction method for coalbed methane wells. This invention is scientifically designed, has a reasonable structure, provides good composite fracturing and permeability enhancement effect, has low requirements for equipment performance, and saves energy and reduces consumption.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] The composite fracturing and permeability enhancement construction method for coalbed methane wells specifically includes the following steps:

[0007] (1) The composite fracturing and permeability enhancement device for coalbed methane wells is lowered into the coalbed methane well to a section where fracturing is required. The composite fracturing and permeability enhancement device for coalbed methane wells includes two barrier sealing devices, a fracturing fluid preparation device, a liquid booster pump and a gas booster pump.

[0008] (2) Seal the wellbore of the fracturing section of the coalbed methane well using a barrier sealing device;

[0009] (3) Carbon dioxide gas is converted into liquid carbon dioxide through a fracturing fluid preparation device and used as fracturing fluid;

[0010] (4) Liquid carbon dioxide is injected into the fracturing section coal seam by a liquid booster pump, and nitrogen is injected into the fracturing section coal seam by a gas booster pump to perform composite fracturing and permeability enhancement on the fracturing section coal seam.

[0011] (5) After the fracturing and permeability enhancement of the coal seam in the fracturing section is completed, the composite fracturing and permeability enhancement device of the coalbed gas well is then lowered into the coalbed gas well to another section that needs fracturing.

[0012] (6) Repeat steps (2)-(5) to complete the composite fracturing and permeability enhancement of the coal seam in the multi-stage fracturing section of the coalbed methane well.

[0013] The coalbed methane well composite fracturing and permeability enhancement device also includes a carbon dioxide injection pipe, a nitrogen injection pipe, a power generation component, and a PLC controller. The nitrogen injection pipe is coaxially sleeved outside the carbon dioxide injection pipe. The lower ends of the nitrogen and carbon dioxide injection pipes are flush and fixedly connected by a flange seal. The nitrogen and carbon dioxide injection pipes are inserted into the coalbed methane well concentrically. The upper end of the carbon dioxide injection pipe is connected to a carbon dioxide supply device outside the well, and the upper end of the nitrogen injection pipe is connected to a nitrogen supply device outside the well. The supply pressure of the nitrogen supply device is greater than that of the carbon dioxide supply device. Two barrier sealing devices are coaxially and fixedly fitted onto the nitrogen injection pipe at an interval. The fracturing fluid preparation device is concentrically fitted and fixed onto the nitrogen injection pipe. Located between two barrier sealing devices, the gas booster pump, power generation unit, and PLC controller are all located at the top of the fracturing fluid preparer, while the liquid booster pump is located at the bottom. The inlet of the liquid booster pump is connected to the fracturing fluid outlet of the fracturing fluid preparer, and the outlet of the liquid booster pump is connected to a hydraulic fracturing pipe. The inlet of the gas booster pump is connected to the gas outlet of the fracturing fluid preparer, and the outlet of the gas booster pump is connected to a pneumatic fracturing pipe. The power generation unit provides power to the PLC controller, liquid booster pump, and gas booster pump, respectively. The PLC controller is connected to the fracturing fluid preparer, liquid booster pump, and gas booster pump, respectively, and is also connected to the remote control system in the wellbore control room.

[0014] The two barrier sealing devices have the same structure. The lower barrier sealing device includes a circular airbag and two circular discs. The two circular discs are fixedly mounted on the lower part of the nitrogen injection pipe with a vertical gap and the same center. The diameter of the circular discs is smaller than the inner diameter of the coalbed methane well. The circular airbag is mounted on the nitrogen injection pipe with the same center and is located between the two circular discs. The upper side of the circular airbag is glued to the lower side of the upper circular disc, and the lower side of the circular airbag is glued to the upper side of the lower circular disc. The nitrogen injection pipe is provided with a first inflation port that communicates with the inside of the circular airbag. A first explosion-proof solenoid valve is provided on the first inflation port. The upper circular disc is equipped with a first exhaust port that communicates with the inside of the circular airbag. A second explosion-proof solenoid valve is provided on the first exhaust port. A pressure sensor is provided inside the circular airbag. The PLC controller is connected to the first explosion-proof solenoid valve, the second explosion-proof solenoid valve and the pressure sensor respectively.

[0015] The fracturing fluid preparation device includes a fracturing tank, which is a cylindrical structure with a central hole. The fracturing tank is centrally and sealed and fixedly mounted on a nitrogen injection pipe. Inside the fracturing tank are two spaced-apart partitions, also centrally and sealed and fixedly mounted on the nitrogen injection pipe. These two partitions divide the inner cavity of the fracturing tank into a nitrogen storage chamber, a preparation chamber, and a fracturing fluid storage chamber. A piston is slidably mounted on the nitrogen injection pipe within the preparation chamber. The outer circumference of the piston is in sealed sliding contact with the inner wall of the preparation chamber. The nitrogen injection pipe has a second inflation port connected to the preparation chamber above the piston, and a third explosion-proof solenoid valve is installed on the second inflation port. A gas injection pipe is radially connected to a carbon dioxide injection pipe. The inner end of the gas injection pipe communicates with the interior of the carbon dioxide injection pipe, and the outer end of the gas injection pipe is connected to the nitrogen injection pipe and communicates with the preparation chamber below the piston. A fourth explosion-proof solenoid valve is installed on the outer end of the gas injection pipe. The lower surface of the upper partition is equipped with a device for measuring the activity. The laser rangefinder sensor for measuring the movement distance has a third gas inlet on the upper partition connecting the nitrogen storage chamber and the preparation chamber, and a fifth explosion-proof solenoid valve on the third gas inlet. The lower partition has a liquid injection port connecting the preparation chamber and the fracturing fluid storage chamber, and a sixth explosion-proof solenoid valve on the liquid injection port. The gas booster pump and PLC controller are both installed on the top of the fracturing tank. The top of the fracturing tank has a second exhaust port connected to the nitrogen storage chamber. The second exhaust port is connected to the gas booster pump inlet and is equipped with a seventh explosion-proof solenoid valve. The liquid booster pump is installed at the bottom of the fracturing tank. The bottom of the fracturing tank has a drain port connected to the fracturing fluid storage chamber. The drain port is connected to the liquid booster pump inlet and is equipped with an eighth explosion-proof solenoid valve. The PLC controller is connected to the third, fourth, laser rangefinder sensor, fifth, sixth, seventh, and eighth explosion-proof solenoid valves respectively.

[0016] The power generation components include a small wind turbine and a ring-shaped battery pack. Both the small wind turbine and the ring-shaped battery pack are installed on the top of the fracturing tank. The ring-shaped battery pack is fitted onto a nitrogen injection pipe. The top of the fracturing tank is equipped with a third exhaust port that communicates with the nitrogen storage chamber. The third exhaust port is connected to the air inlet of the small wind turbine and is equipped with a ninth explosion-proof solenoid valve. The small wind turbine is electrically connected to the ring-shaped battery pack. The ring-shaped battery pack is electrically connected to a PLC controller, a liquid booster pump, and a gas booster pump, respectively.

[0017] Step (1) is as follows: Initially, the two annular gasbags are in an uninflated state. The carbon dioxide injection pipe and the nitrogen injection pipe are lowered into the coalbed methane well. Then, the two barrier sealing devices, the fracturing fluid preparation device, the power generation component and the PLC controller are lowered into the coalbed methane well along with the nitrogen injection pipe. When the lower end of the nitrogen injection pipe is lowered into the coalbed methane well to a section that needs fracturing, the fracturing fluid preparation device is located in the wellbore of the corresponding fracturing section of the coalbed methane well. The two barrier sealing devices are located at the upper and lower parts of the wellbore of the fracturing section of the coalbed methane well, respectively. Then, the upper end of the carbon dioxide injection pipe is connected to the carbon dioxide supply device outside the well, and the upper end of the nitrogen injection pipe is connected to the nitrogen supply device outside the well.

[0018] Step (2) is as follows: Start the nitrogen supply device. The staff operates the PLC controller in the control room outside the well through the remote control system. The PLC controller controls the two first explosion-proof solenoid valves to open. Then the nitrogen supply device pumps the high-pressure low-temperature nitrogen downward through the annular cavity between the nitrogen injection pipe and the carbon dioxide injection pipe. The high-pressure low-temperature nitrogen is injected into the two annular air bags through the two first air inlets respectively, so that the two annular air bags expand and come into close contact with the inner wall of the coalbed methane well. When the pressure sensor reaches the specified value, the PLC controller controls the corresponding first explosion-proof solenoid valve to close and stop the inflation into the corresponding annular air bag. The two annular air bags then seal the well of the fracturing section of the coalbed methane well.

[0019] Step (3) is as follows: Start the carbon dioxide supply device. The PLC controller controls the fourth explosion-proof solenoid valve to open first. Then the carbon dioxide supply device pumps carbon dioxide gas at a certain pressure downward through the carbon dioxide injection pipe. The carbon dioxide gas is injected into the preparation chamber at the bottom of the piston through the injection pipe, which causes the piston to slide upward. The laser range sensor measures the distance of the piston in real time and transmits it to the PLC controller. When the piston slides upward to the set height, the PLC controller controls the fourth explosion-proof solenoid valve to close and simultaneously controls the third explosion-proof solenoid valve to open. Then, high-pressure low-temperature nitrogen gas is injected into the preparation chamber at the top of the piston through the second gas filling port. Since the gas supply pressure of the nitrogen supply device is greater than that of the carbon dioxide supply device, when the high-pressure low-temperature nitrogen gas continues to be injected into the preparation chamber at the top of the piston, it will press down the piston, causing the piston to slide downward and squeeze the second... Carbon dioxide gas is cooled by high-pressure, low-temperature nitrogen gas. As the pressure of the carbon dioxide gas increases and its temperature decreases, when the piston slides down to a set height (calculated and designed according to the critical conditions for carbon dioxide liquefaction, using conventional technology), the carbon dioxide gas is pressurized and cooled to convert into liquid carbon dioxide. The PLC controller controls the third explosion-proof solenoid valve to close, while simultaneously controlling the fourth, fifth, and sixth explosion-proof solenoid valves to open. The liquid carbon dioxide then enters the fracturing fluid storage chamber through the injection port. At the same time, carbon dioxide gas is injected again into the preparation chamber at the bottom of the piston through the gas injection pipe, causing the piston to slide upward. Nitrogen gas in the preparation chamber at the top of the piston enters the nitrogen storage chamber through the third gas filling port. Following the above working process, carbon dioxide gas is continuously converted into liquid carbon dioxide and stored in the fracturing fluid storage chamber.

[0020] Step (4) is as follows: After the process of converting carbon dioxide gas into liquid carbon dioxide in step (3) lasts for ten minutes, the PLC controller controls the liquid booster pump and the gas booster pump to start, and controls the seventh and eighth explosion-proof solenoid valves to open. The liquid booster pump extracts liquid carbon dioxide from the fracturing fluid storage chamber and pressurizes it, then injects the liquid carbon dioxide into the coal seam of the fracturing section through the hydraulic fracturing pipe. At the same time, the gas booster pump extracts nitrogen from the nitrogen storage chamber and pressurizes it, then injects the high-pressure nitrogen into the coal seam of the fracturing section through the pneumatic fracturing pipe. The liquid carbon dioxide and high-pressure nitrogen penetrate into the coal seam of the fracturing section for composite fracturing and permeation. During the whole process, if the power of the ring battery pack is insufficient, the PLC controller controls the ninth explosion-proof solenoid valve to open, and the nitrogen in the nitrogen storage chamber enters the small wind turbine through the third exhaust port to drive the small wind turbine to work and charge the ring battery pack until the ring battery pack is fully charged.

[0021] Step (5) is as follows: After a certain period of time, the coal seam in the fracturing section is completed and the fracturing is enhanced. The PLC controller controls the liquid booster pump and the gas booster pump to shut down. At the same time, the third, fourth, fifth, sixth, seventh and eighth explosion-proof solenoid valves are closed, and the two second explosion-proof solenoid valves are opened. The nitrogen in the two annular airbags is discharged through the corresponding first exhaust port. The two annular airbags then contract and separate from the inner wall of the coalbed methane well. Then, following the operation in step (1), the coalbed methane well composite fracturing enhancement device is continued to be lowered into the coalbed methane well to another section that needs fracturing.

[0022] This invention represents a significant advancement and substantial improvement over existing technologies. Specifically, it involves lowering a fracturing fluid preparation device into the wellbore of a coalbed methane well in the fracturing section. High-pressure, low-temperature nitrogen and carbon dioxide gases are injected into the fracturing fluid preparation device. The high-pressure, low-temperature nitrogen pressurizes and cools the carbon dioxide gas, converting it into liquid carbon dioxide downhole. This liquid carbon dioxide, used as the fracturing fluid, is then pressurized by a liquid booster pump and injected into the coal seam in the fracturing section. Simultaneously, a gas booster pump injects high-pressure nitrogen into the coal seam through a pneumatic fracturing pipe. The liquid carbon dioxide and high-pressure nitrogen permeate into the coal seam in the fracturing section, resulting in combined fracturing and permeability enhancement. This combined fracturing and permeability enhancement has a good effect, lower equipment performance requirements, and saves energy. Furthermore, preparing liquid carbon dioxide downhole and directly injecting it into the coal seam in the fracturing section via a liquid booster pump prevents some of the liquid carbon dioxide from turning into gas before fracturing the coal seam, ensuring the fracturing effect.

[0023] In summary, this invention is scientifically designed, structurally sound, and has a good composite fracturing and permeability enhancement effect. It also has low requirements for equipment performance and saves energy and reduces consumption. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the working state of the present invention.

[0025] Figure 2 This is a schematic diagram of the structure of the present invention.

[0026] Figure 3 This is a top view of the barrier sealing device of the present invention. Detailed Implementation

[0027] The embodiments of the present invention are further described below with reference to the accompanying drawings.

[0028] like Figure 1-3 As shown, the composite fracturing and permeability enhancement construction method for coalbed methane wells specifically includes the following steps:

[0029] (1) The composite fracturing and permeability enhancement device for coalbed methane wells is lowered into the coalbed methane well 6 to a section where fracturing is required. The composite fracturing and permeability enhancement device for coalbed methane wells includes two barrier sealing devices, a fracturing fluid preparation device, a liquid booster pump 7 and a gas booster pump 8.

[0030] (2) The wellbore of coalbed methane well No. 6 in the fracturing section was sealed by a barrier sealing device;

[0031] (3) Carbon dioxide gas is converted into liquid carbon dioxide through a fracturing fluid preparation device and used as fracturing fluid;

[0032] (4) Liquid carbon dioxide is injected into the fracturing section coal seam through liquid booster pump 7, and nitrogen is injected into the fracturing section coal seam through gas booster pump 8 to perform composite fracturing and permeability enhancement on the fracturing section coal seam.

[0033] (5) After the fracturing and permeability enhancement of the coal seam in the fracturing section is completed, the composite fracturing and permeability enhancement device of the coalbed gas well is then lowered into the coalbed gas well 6 to another section that needs fracturing.

[0034] (6) Repeat steps (2)-(5) to complete the composite fracturing and permeability enhancement of the coal seam in the multi-stage fracturing section of the coalbed methane well 6.

[0035] The coalbed methane well composite fracturing and permeability enhancement device also includes a carbon dioxide injection pipe 1, a nitrogen injection pipe 2, a power generation component, and a PLC controller 4. The nitrogen injection pipe 2 is coaxially sleeved outside the carbon dioxide injection pipe 1. The lower ends of the nitrogen injection pipe 2 and the carbon dioxide injection pipe 1 are flush and sealed and fixedly connected by a flange 5. The nitrogen injection pipe 2 and the carbon dioxide injection pipe 1 are inserted into the coalbed methane well 6 concentrically. The upper end of the carbon dioxide injection pipe 1 is connected to the carbon dioxide supply device outside the well, and the upper end of the nitrogen injection pipe 2 is connected to the nitrogen supply device outside the well. The supply pressure of the nitrogen supply device is greater than that of the carbon dioxide supply device. Two barrier sealing devices are coaxially and fixedly mounted on the nitrogen injection pipe 2 at intervals. The fracturing fluid preparation device is concentrically mounted and fixed on the nitrogen injection pipe 2. Between the two barrier sealing devices, the gas booster pump 8, the power generation unit, and the PLC controller 4 are all located at the top of the fracturing fluid preparer, and the liquid booster pump 7 is located at the bottom of the fracturing fluid preparer. The inlet of the liquid booster pump 7 is connected to the fracturing fluid outlet of the fracturing fluid preparer, and the outlet of the liquid booster pump 7 is connected to the hydraulic fracturing pipe 9. The inlet of the gas booster pump 8 is connected to the gas outlet of the fracturing fluid preparer, and the outlet of the gas booster pump 8 is connected to the pneumatic fracturing pipe 10. The power generation unit provides power to the PLC controller 4, the liquid booster pump 7, and the gas booster pump 8 respectively. The PLC controller 4 is connected to the fracturing fluid preparer, the liquid booster pump 7, and the gas booster pump 8 respectively. The PLC controller 4 is also connected to the remote control system (not shown) in the well control room.

[0036] The two barrier sealing devices have the same structure. The lower barrier sealing device includes a circular airbag 11 and two circular discs 12. The two circular discs 12 are fixedly fitted onto the lower part of the nitrogen injection pipe 2 with an upper and lower gap and the same center. The diameter of the circular discs 12 is smaller than the inner diameter of the wellbore of the coalbed methane well 6. The circular airbag 11 is fitted onto the nitrogen injection pipe 2 with the same center and is located between the two circular discs 12. The upper side of the circular airbag 11 is in adhesive contact with the lower side of the upper circular disc 12, and the lower side of the circular airbag 11 is in contact with the lower circular disc. The upper side of 12 is glued in contact with the nitrogen injection pipe 2, which is provided with a first inflation port that communicates with the inside of the annular airbag 11. A first explosion-proof solenoid valve 13 is provided on the first inflation port. A first exhaust port that communicates with the inside of the annular airbag 11 is installed on the upper annular disc 12. A second explosion-proof solenoid valve 14 is provided on the first exhaust port. A pressure sensor (not shown in the figure) is provided inside the annular airbag 11. The PLC controller 4 is connected to the first explosion-proof solenoid valve 13, the second explosion-proof solenoid valve 14 and the pressure sensor respectively.

[0037] The fracturing fluid preparation device includes a fracturing tank 15, which is a cylindrical structure with a central hole. The fracturing tank 15 is centrally and sealed and fixedly mounted on a nitrogen injection pipe 2. Inside the fracturing tank 15, there are two spaced-apart partitions 16 that are centrally and sealed and fixedly mounted on the nitrogen injection pipe 2. The two partitions 16 divide the inner cavity of the fracturing tank 15 into a nitrogen storage chamber 17, a preparation chamber 18, and a fracturing fluid storage chamber 19. A piston 20 is slidably mounted on the nitrogen injection pipe 2 inside the preparation chamber 18. The outer circle of the piston 20 is flush with the inner wall of the preparation chamber 18. The system features a sealed sliding contact. The nitrogen injection pipe 2 has a second inflation port connected to the preparation chamber 18 at the top of the piston 20. A third explosion-proof solenoid valve 21 is installed on the second inflation port. A gas injection pipe 22 is radially connected to the carbon dioxide injection pipe 1. The inner end of the gas injection pipe 22 communicates with the interior of the carbon dioxide injection pipe 1, and the outer end of the gas injection pipe 22 is connected to the nitrogen injection pipe 2 and communicates with the preparation chamber 18 at the bottom of the piston 20. A fourth explosion-proof solenoid valve 23 is installed at the outer end of the gas injection pipe 22. The lower surface of the upper partition 16 is equipped with a device for measuring the dynamic... The laser rangefinder 24, which measures the movement distance of the fracturing tank 15, has a third gas inlet on the upper partition 16 connecting the nitrogen storage chamber 17 and the preparation chamber 18. A fifth explosion-proof solenoid valve 25 is installed on the third gas inlet. A liquid injection port on the lower partition 16 connects the preparation chamber 18 and the fracturing fluid storage chamber 19. A sixth explosion-proof solenoid valve 26 is installed on the liquid injection port. The gas booster pump 8 and the PLC controller 4 are both installed on the top of the fracturing tank 15. The top of the fracturing tank 15 has a second exhaust port connected to the nitrogen storage chamber 17. The second exhaust port is connected to the gas booster pump. The air inlet of pump 8 is connected to and equipped with a seventh explosion-proof solenoid valve 27. The liquid booster pump 7 is installed at the bottom of the fracturing tank 15. The bottom of the fracturing tank 15 is provided with a drain port that communicates with the fracturing fluid storage chamber 19. The drain port is connected to the inlet of the liquid booster pump 7 and equipped with an eighth explosion-proof solenoid valve 28. The PLC controller 4 is connected to the third explosion-proof solenoid valve 21, the fourth explosion-proof solenoid valve 23, the laser rangefinder 24, the fifth explosion-proof solenoid valve 25, the sixth explosion-proof solenoid valve 26, the seventh explosion-proof solenoid valve 27 and the eighth explosion-proof solenoid valve 28 respectively.

[0038] The power generation components include a small wind turbine 29 and a ring-shaped battery pack 30. Both the small wind turbine 29 and the ring-shaped battery pack 30 are installed on the top of the fracturing tank 15. The ring-shaped battery pack 30 is fitted onto the nitrogen injection pipe 2. The top of the fracturing tank 15 is provided with a third exhaust port that communicates with the nitrogen storage chamber 17. The third exhaust port is connected to the air inlet of the small wind turbine 29 and is equipped with a ninth explosion-proof solenoid valve 3. The small wind turbine 29 is electrically connected to the ring-shaped battery pack 30. The ring-shaped battery pack 30 is electrically connected to the PLC controller 4, the liquid booster pump 7, and the gas booster pump 8, respectively.

[0039] Step (1) is as follows: Initially, the two annular airbags 11 are in an uninflated state. The carbon dioxide injection pipe 1 and the nitrogen injection pipe 2 are lowered into the coalbed methane well 6. Then, the two barrier sealing devices, the fracturing fluid preparation device, the power generation component and the PLC controller 4 are lowered into the coalbed methane well 6 together with the nitrogen injection pipe 2. When the lower end of the nitrogen injection pipe 2 is lowered into the coalbed methane well 6 to a section that needs fracturing, the fracturing fluid preparation device is located in the wellbore of the corresponding fracturing section of the coalbed methane well 6. The two barrier sealing devices are located at the upper and lower parts of the wellbore of the fracturing section of the coalbed methane well 6, respectively. Then, the upper end of the carbon dioxide injection pipe 1 is connected to the carbon dioxide supply device outside the well, and the upper end of the nitrogen injection pipe 2 is connected to the nitrogen supply device outside the well.

[0040] Step (2) is as follows: Start the nitrogen supply device. The staff operates the PLC controller 4 through the remote control system in the control room outside the well. The PLC controller 4 controls the two first explosion-proof solenoid valves 13 to open. Then the nitrogen supply device pumps the high-pressure low-temperature nitrogen downward through the annular cavity between the nitrogen injection pipe 2 and the carbon dioxide injection pipe 1. The high-pressure low-temperature nitrogen is injected into the two annular air bags 11 through the two first air inlets respectively, so that the two annular air bags 11 expand and come into close contact with the inner wall of the coalbed methane well 6. When the pressure sensor reaches the specified value, the PLC controller 4 controls the corresponding first explosion-proof solenoid valve 13 to close and stop the inflation into the corresponding annular air bag 11. The two annular air bags 11 then seal the well of the fracturing section of the coalbed methane well 6.

[0041] Step (3) is as follows: Start the carbon dioxide supply device. The PLC controller 4 controls the fourth explosion-proof solenoid valve 23 to open first. Then the carbon dioxide supply device pumps carbon dioxide gas at a certain pressure downward through the carbon dioxide injection pipe 1. The carbon dioxide gas is injected into the preparation chamber 18 at the bottom of the piston 20 through the injection pipe 22, which causes the piston 20 to slide upward. The laser ranging sensor 24 measures the distance of the piston 20 in real time and transmits it to the PLC controller 4. When the piston 20 slides upward to the set height, the PLC controller 4 controls the fourth explosion-proof solenoid valve 23 to close and controls the third explosion-proof solenoid valve 21 to open. Then the high-pressure low-temperature nitrogen gas is injected into the preparation chamber 18 at the top of the piston 20 through the second gas filling port. Since the gas supply pressure of the nitrogen supply device is greater than the gas supply pressure of the carbon dioxide supply device, when the high-pressure low-temperature nitrogen gas continues to be injected into the preparation chamber 18 at the top of the piston 20, it will press down the piston 20 and cause the piston 20 to slide downward. The piston 20 compresses carbon dioxide gas while simultaneously cooling it with high-pressure, low-temperature nitrogen. This increases the pressure and decreases the temperature of the carbon dioxide gas. When the piston 20 slides downwards to a set height (calculated and designed according to the critical conditions for carbon dioxide liquefaction, using conventional technology), the carbon dioxide gas is pressurized and cooled to convert into liquid carbon dioxide. The PLC controller 4 controls the third explosion-proof solenoid valve 21 to close, while simultaneously controlling the fourth, fifth, and sixth explosion-proof solenoid valves 23, 25, and 26 to open. Liquid carbon dioxide then enters the fracturing fluid storage chamber 19 through the injection port. Simultaneously, carbon dioxide gas is injected again through the injection pipe 22 into the preparation chamber 18 below the piston 20, causing the piston 20 to slide upwards. Nitrogen gas in the preparation chamber 18 above the piston 20 enters the nitrogen storage chamber 17 through the third inflation port. Following this process, carbon dioxide gas is continuously converted into liquid carbon dioxide and stored in the fracturing fluid storage chamber 19.

[0042] Step (4) is as follows: After the process of converting carbon dioxide gas into liquid carbon dioxide in step (3) continues for ten minutes, the PLC controller 4 controls the liquid booster pump 7 and the gas booster pump 8 to start, and controls the seventh explosion-proof solenoid valve 27 and the eighth explosion-proof solenoid valve 28 to open. After the liquid booster pump 7 draws liquid carbon dioxide from the fracturing fluid storage chamber 19 for pressurization, it injects the liquid carbon dioxide into the coal seam of the fracturing section through the hydraulic fracturing pipe 9. At the same time, the gas booster pump 8 draws nitrogen from the nitrogen storage chamber 17 for pressurization. Then, high-pressure nitrogen is injected into the coal seam of the fracturing section through the pneumatic fracturing pipe 10. Liquid carbon dioxide and high-pressure nitrogen permeate into the coal seam of the fracturing section for composite fracturing and permeation enhancement. During the whole process, if the power of the ring battery pack 30 is insufficient, the PLC controller 4 controls the ninth explosion-proof solenoid valve 3 to open. Then, the nitrogen in the nitrogen storage chamber 17 enters the small wind turbine 29 through the third exhaust port, driving the small wind turbine 29 to work and charge the ring battery pack 30 until the ring battery pack 30 is fully charged.

[0043] Step (5) is as follows: After a certain period of time, the coal seam in the fracturing section is completed and the fracturing is enhanced. The PLC controller 4 controls the liquid booster pump 7 and the gas booster pump 8 to close. At the same time, the third explosion-proof solenoid valve 21, the fourth explosion-proof solenoid valve 23, the fifth explosion-proof solenoid valve 25, the sixth explosion-proof solenoid valve 26, the seventh explosion-proof solenoid valve 27 and the eighth explosion-proof solenoid valve 28 are closed. The two second explosion-proof solenoid valves 14 are opened. The nitrogen in the two annular airbags 11 is discharged through the corresponding first exhaust port. The two annular airbags 11 then contract and separate from the inner wall of the coalbed methane well 6. Then, according to the operation of step (1), the coalbed methane well composite fracturing enhancement device is continued to be lowered into the coalbed methane well 6 to another section that needs fracturing.

[0044] The PLC controller 4, carbon dioxide supply device, nitrogen supply device, remote control system, liquid booster pump 7, gas booster pump 8, first explosion-proof solenoid valve 13, second explosion-proof solenoid valve 14, pressure sensor, third explosion-proof solenoid valve 21, fourth explosion-proof solenoid valve 23, laser rangefinder sensor 24, fifth explosion-proof solenoid valve 25, sixth explosion-proof solenoid valve 26, seventh explosion-proof solenoid valve 27, eighth explosion-proof solenoid valve 28, small wind turbine generator 29, toroidal battery pack 30, and ninth explosion-proof solenoid valve 3 are all existing technologies and can be purchased on the market. Their specific structures and working principles will not be described in detail. The control part in this invention uses conventional control technology and does not involve new computer programs.

[0045] This invention involves lowering a fracturing fluid preparation device into the wellbore of a coalbed methane well 6 in the fracturing section. High-pressure, low-temperature nitrogen and carbon dioxide gases are injected into the fracturing fluid preparation device. The high-pressure, low-temperature nitrogen pressurizes and cools the carbon dioxide gas, converting it into liquid carbon dioxide downhole. The liquid carbon dioxide, used as fracturing fluid, is pressurized by a liquid booster pump 7 and injected into the fracturing section coal seam via a hydraulic fracturing pipe 9. Simultaneously, a gas booster pump 8 injects high-pressure nitrogen into the fracturing section coal seam through a pneumatic fracturing pipe 10. The liquid carbon dioxide and high-pressure nitrogen permeate into the fracturing section coal seam for combined fracturing and permeability enhancement. This combined fracturing and permeability enhancement has a good effect, low equipment performance requirements, and energy savings. Furthermore, preparing liquid carbon dioxide downhole and directly injecting it into the fracturing section coal seam via the liquid booster pump 7 prevents some of the liquid carbon dioxide from turning into gas before fracturing the coal seam, ensuring the fracturing effect.

[0046] The above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for composite fracturing and permeability enhancement in coalbed methane wells, characterized in that: Specifically, the following steps are included: (1) The composite fracturing and permeability enhancement device for coalbed methane wells is lowered into the coalbed methane well to a section where fracturing is required. The composite fracturing and permeability enhancement device for coalbed methane wells includes two barrier sealing devices, a fracturing fluid preparation device, a liquid booster pump and a gas booster pump. (2) Seal the wellbore of the fracturing section of the coalbed methane well using a barrier sealing device; (3) Carbon dioxide gas is converted into liquid carbon dioxide through a fracturing fluid preparation device and used as fracturing fluid; (4) Liquid carbon dioxide is injected into the fracturing section coal seam by a liquid booster pump, and nitrogen is injected into the fracturing section coal seam by a gas booster pump to perform composite fracturing and permeability enhancement on the fracturing section coal seam. (5) After the fracturing and permeability enhancement of the coal seam in the fracturing section is completed, the composite fracturing and permeability enhancement device of the coalbed gas well is then continued to be lowered into the coalbed gas well to another section that needs fracturing. (6) Repeat steps (2)-(5) to complete the composite fracturing and permeability enhancement of the coal seam in the multi-stage fracturing section within the coalbed methane well; The coalbed methane well composite fracturing and permeability enhancement device also includes a carbon dioxide injection pipe, a nitrogen injection pipe, a power generation component, and a PLC controller. The nitrogen injection pipe is coaxially sleeved outside the carbon dioxide injection pipe. The lower ends of the nitrogen and carbon dioxide injection pipes are flush and fixedly connected by a flange seal. The nitrogen and carbon dioxide injection pipes are inserted into the coalbed methane well concentrically. The upper end of the carbon dioxide injection pipe is connected to a carbon dioxide supply device outside the well, and the upper end of the nitrogen injection pipe is connected to a nitrogen supply device outside the well. The supply pressure of the nitrogen supply device is greater than that of the carbon dioxide supply device. Two barrier sealing devices are coaxially and fixedly fitted onto the nitrogen injection pipe at an interval. The fracturing fluid preparation device is concentrically fitted and fixed onto the nitrogen injection pipe. Located between two barrier sealing devices, the gas booster pump, power generation unit, and PLC controller are all located at the top of the fracturing fluid preparer, while the liquid booster pump is located at the bottom. The inlet of the liquid booster pump is connected to the fracturing fluid outlet of the fracturing fluid preparer, and the outlet of the liquid booster pump is connected to a hydraulic fracturing pipe. The inlet of the gas booster pump is connected to the gas outlet of the fracturing fluid preparer, and the outlet of the gas booster pump is connected to a pneumatic fracturing pipe. The power generation unit provides power to the PLC controller, liquid booster pump, and gas booster pump, respectively. The PLC controller is connected to the fracturing fluid preparer, liquid booster pump, and gas booster pump, respectively, and is also connected to the remote control system in the wellbore control room.

2. The method for composite fracturing and permeability enhancement of coalbed methane wells according to claim 1, characterized in that: The two barrier sealing devices have the same structure. The lower barrier sealing device includes a circular airbag and two circular discs. The two circular discs are fixedly mounted on the lower part of the nitrogen injection pipe with a vertical gap and the same center. The diameter of the circular discs is smaller than the inner diameter of the coalbed methane well. The circular airbag is mounted on the nitrogen injection pipe with the same center and is located between the two circular discs. The upper side of the circular airbag is glued to the lower side of the upper circular disc, and the lower side of the circular airbag is glued to the upper side of the lower circular disc. The nitrogen injection pipe is provided with a first inflation port that communicates with the inside of the circular airbag. A first explosion-proof solenoid valve is provided on the first inflation port. The upper circular disc is equipped with a first exhaust port that communicates with the inside of the circular airbag. A second explosion-proof solenoid valve is provided on the first exhaust port. A pressure sensor is provided inside the circular airbag. The PLC controller is connected to the first explosion-proof solenoid valve, the second explosion-proof solenoid valve and the pressure sensor respectively.

3. The method for composite fracturing and permeability enhancement of coalbed methane wells according to claim 2, characterized in that: The fracturing fluid preparation device includes a fracturing tank, which is a cylindrical structure with a central hole. The fracturing tank is centrally and sealed and fixedly mounted on a nitrogen injection pipe. Inside the fracturing tank are two spaced-apart partitions, also centrally and sealed and fixedly mounted on the nitrogen injection pipe. These two partitions divide the inner cavity of the fracturing tank into a nitrogen storage chamber, a preparation chamber, and a fracturing fluid storage chamber. A piston is slidably mounted on the nitrogen injection pipe within the preparation chamber. The outer circumference of the piston is in sealed sliding contact with the inner wall of the preparation chamber. The nitrogen injection pipe has a second inflation port connected to the preparation chamber above the piston, and a third explosion-proof solenoid valve is installed on the second inflation port. A gas injection pipe is radially connected to a carbon dioxide injection pipe. The inner end of the gas injection pipe communicates with the interior of the carbon dioxide injection pipe, and the outer end of the gas injection pipe is connected to the nitrogen injection pipe and communicates with the preparation chamber below the piston. A fourth explosion-proof solenoid valve is installed on the outer end of the gas injection pipe. The lower surface of the upper partition is equipped with a device for measuring the activity. The laser rangefinder sensor for measuring the movement distance has a third gas inlet on the upper partition connecting the nitrogen storage chamber and the preparation chamber, and a fifth explosion-proof solenoid valve on the third gas inlet. The lower partition has a liquid injection port connecting the preparation chamber and the fracturing fluid storage chamber, and a sixth explosion-proof solenoid valve on the liquid injection port. The gas booster pump and PLC controller are both installed on the top of the fracturing tank. The top of the fracturing tank has a second exhaust port connected to the nitrogen storage chamber. The second exhaust port is connected to the gas booster pump inlet and is equipped with a seventh explosion-proof solenoid valve. The liquid booster pump is installed at the bottom of the fracturing tank. The bottom of the fracturing tank has a drain port connected to the fracturing fluid storage chamber. The drain port is connected to the liquid booster pump inlet and is equipped with an eighth explosion-proof solenoid valve. The PLC controller is connected to the third, fourth, laser rangefinder sensor, fifth, sixth, seventh, and eighth explosion-proof solenoid valves respectively.

4. The method for composite fracturing and permeability enhancement of coalbed methane wells according to claim 3, characterized in that: The power generation components include a small wind turbine and a ring-shaped battery pack. Both the small wind turbine and the ring-shaped battery pack are installed on the top of the fracturing tank. The ring-shaped battery pack is fitted onto a nitrogen injection pipe. The top of the fracturing tank is equipped with a third exhaust port that communicates with the nitrogen storage chamber. The third exhaust port is connected to the air inlet of the small wind turbine and is equipped with a ninth explosion-proof solenoid valve. The small wind turbine is electrically connected to the ring-shaped battery pack. The ring-shaped battery pack is electrically connected to a PLC controller, a liquid booster pump, and a gas booster pump, respectively.

5. The method for composite fracturing and permeability enhancement of coalbed methane wells according to claim 4, characterized in that: Step (1) is as follows: Initially, the two annular gasbags are in an uninflated state. The carbon dioxide injection pipe and the nitrogen injection pipe are lowered into the coalbed methane well. Then, the two barrier sealing devices, the fracturing fluid preparation device, the power generation component and the PLC controller are lowered into the coalbed methane well along with the nitrogen injection pipe. When the lower end of the nitrogen injection pipe is lowered into the coalbed methane well to a section that needs fracturing, the fracturing fluid preparation device is located in the wellbore of the corresponding fracturing section of the coalbed methane well. The two barrier sealing devices are located at the upper and lower parts of the wellbore of the fracturing section of the coalbed methane well, respectively. Then, the upper end of the carbon dioxide injection pipe is connected to the carbon dioxide supply device outside the well, and the upper end of the nitrogen injection pipe is connected to the nitrogen supply device outside the well.

6. The method for composite fracturing and permeability enhancement of coalbed methane wells according to claim 5, characterized in that: Step (2) is as follows: Start the nitrogen supply device. The staff operates the PLC controller in the control room outside the well through the remote control system. The PLC controller controls the two first explosion-proof solenoid valves to open. Then the nitrogen supply device pumps the high-pressure low-temperature nitrogen downward through the annular cavity between the nitrogen injection pipe and the carbon dioxide injection pipe. The high-pressure low-temperature nitrogen is injected into the two annular air bags through the two first air inlets respectively, so that the two annular air bags expand and come into close contact with the inner wall of the coalbed methane well. When the pressure sensor reaches the specified value, the PLC controller controls the corresponding first explosion-proof solenoid valve to close and stop the inflation into the corresponding annular air bag. The two annular air bags then seal the well of the fracturing section of the coalbed methane well.

7. The method for composite fracturing and permeability enhancement of coalbed methane wells according to claim 6, characterized in that: Step (3) is as follows: Start the carbon dioxide supply device. The PLC controller controls the fourth explosion-proof solenoid valve to open first. Then the carbon dioxide supply device pumps carbon dioxide gas at a certain pressure downward through the carbon dioxide injection pipe. The carbon dioxide gas is injected into the preparation chamber at the bottom of the piston through the injection pipe, which causes the piston to slide upward. The laser range sensor measures the distance of the piston in real time and transmits it to the PLC controller. When the piston slides upward to the set height, the PLC controller controls the fourth explosion-proof solenoid valve to close and at the same time controls the third explosion-proof solenoid valve to open. Then the high-pressure low-temperature nitrogen gas is injected into the preparation chamber at the top of the piston through the second filling port. Since the supply pressure of the nitrogen gas supply device is greater than the supply pressure of the carbon dioxide gas supply device, when the high-pressure low-temperature nitrogen gas is continuously injected into the preparation chamber at the top of the piston, it will press down. The piston slides downwards, compressing carbon dioxide gas. Simultaneously, high-pressure, low-temperature nitrogen gas cools the carbon dioxide gas, increasing its pressure and decreasing its temperature. When the piston slides down to a set height, the carbon dioxide gas is pressurized and cooled, converting it into liquid carbon dioxide. The PLC controller closes the third explosion-proof solenoid valve and simultaneously opens the fourth, fifth, and sixth explosion-proof solenoid valves. The liquid carbon dioxide then enters the fracturing fluid storage chamber through the injection port. At the same time, carbon dioxide gas is injected again into the preparation chamber below the piston through the gas injection pipe, causing the piston to slide upwards. Nitrogen gas in the preparation chamber above the piston enters the nitrogen storage chamber through the third gas filling port. Following this process, carbon dioxide gas is continuously converted into liquid carbon dioxide and stored in the fracturing fluid storage chamber.

8. The method for composite fracturing and permeability enhancement of coalbed methane wells according to claim 7, characterized in that: Step (4) is as follows: After the process of converting carbon dioxide gas into liquid carbon dioxide in step (3) lasts for ten minutes, the PLC controller controls the liquid booster pump and the gas booster pump to start, and controls the seventh and eighth explosion-proof solenoid valves to open. The liquid booster pump extracts liquid carbon dioxide from the fracturing fluid storage chamber and pressurizes it, then injects the liquid carbon dioxide into the coal seam of the fracturing section through the hydraulic fracturing pipe. At the same time, the gas booster pump extracts nitrogen from the nitrogen storage chamber and pressurizes it, then injects the high-pressure nitrogen into the coal seam of the fracturing section through the pneumatic fracturing pipe. The liquid carbon dioxide and high-pressure nitrogen penetrate into the coal seam of the fracturing section for composite fracturing and permeation. During the whole process, if the power of the ring battery pack is insufficient, the PLC controller controls the ninth explosion-proof solenoid valve to open, and the nitrogen in the nitrogen storage chamber enters the small wind turbine through the third exhaust port to drive the small wind turbine to work and charge the ring battery pack until the ring battery pack is fully charged.

9. The method for composite fracturing and permeability enhancement of coalbed methane wells according to claim 8, characterized in that: Step (5) is as follows: After a certain period of time, the coal seam in the fracturing section is completed and the fracturing is enhanced. The PLC controller controls the liquid booster pump and the gas booster pump to shut down. At the same time, the third, fourth, fifth, sixth, seventh and eighth explosion-proof solenoid valves are closed, and the two second explosion-proof solenoid valves are opened. The nitrogen in the two annular airbags is discharged through the corresponding first exhaust port. The two annular airbags then contract and separate from the inner wall of the coalbed methane well. Then, following the operation in step (1), the coalbed methane well composite fracturing enhancement device is continued to be lowered into the coalbed methane well to another section that needs fracturing.

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