Coal seam transformation experimental device based on CO2-N2 combined displacement by fracturing

Abstract

The present invention relates to a coal seam transformation experimental device for CO2-N2 combined displacement based on fracturing, comprising an experimental chamber, wherein the upstream side of the experimental chamber is connected to a gas supply unit, a fracturing fluid unit and a water supply unit, and CO2, N2 and fracturing fluid are respectively injected into a plurality of coal samples in the experimental chamber, and the water supply unit transports water into the experimental chamber to maintain the pressure in the chamber; the downstream side of the experimental chamber is connected to a gas analysis unit, a permeability detection unit and a drain pipe; the experimental chamber is made of a transparent material, and a CT probe is provided on the top for real-time photography of internal cracks in the coal sample; a main rotating shaft is provided in the experimental chamber, and a plurality of support rods are connected to the main rotating shaft, and a coal sample is provided at the free end of each support rod, the outer side of the coal sample is wrapped in a jacket, and the upstream end is connected to the gas supply unit and the fracturing fluid unit through a pipeline, and the downstream end is connected to the gas analysis unit and the permeability detection unit through a pipeline; the coal sample rotates with the main rotating shaft, moves to the bottom of the CT probe in turn, and is tested.

Description

Technical Field

[0001] The invention belongs to the technical field of coalbed methane well transformation and production increase, and in particular relates to a coalbed transformation experimental device based on CO2-N2 combined displacement by fracturing. Background Art

[0002] Coalbed methane (CBM) is a high-quality, clean energy source and a vital supplement to natural gas. It is widely found in medium- to deep-lying coal seams, which are characterized by high geostress, low porosity, and low permeability. Currently, large-scale hydraulic fracturing is used to open coal seam fractures, establish gas production pathways, and improve coal reservoir permeability to increase CBM production per well. Techniques for transforming medium- to deep-lying coal reservoirs are also gradually improving, and CO2 displacement of CBM is rapidly developing. Coal seams have a CO2 adsorption capacity 2-4 times greater than that of CH4. Injecting CO2 can displace CH4 from the coal seam through competitive adsorption, significantly increasing recovery rates. Furthermore, CO2 can be stored geologically.

[0003] Currently, CO2 displacement of coalbed methane (CBM) is an emerging technology primarily in the laboratory research phase. Laboratory research equipment typically consists of a sealed chamber containing a coal sample. Mechanical pressure is applied to the sealed chamber to simulate coalbed pressure. CO2 is then introduced into the chamber, and the resulting gas is collected and tested to study CO2 displacement of CBM and the effects of fracturing. However, existing experimental equipment typically tests one sample at a time; a single set of equipment cannot process multiple samples simultaneously, resulting in low experimental efficiency and inability to directly and effectively observe the fracturing of the coal sample in real time. Summary of the Invention

[0004] To address the above problems, the present invention provides a coal seam transformation experimental device based on fracturing CO2-N2 combined displacement, comprising an experimental chamber, the upstream side of which is connected to a gas supply unit, a fracturing fluid unit, and a water supply unit, for injecting methane, CO2, N2, and fracturing fluid into a plurality of coal samples in the experimental chamber, respectively; the water supply unit supplies water to the experimental chamber to maintain the pressure in the chamber; the downstream side of the experimental chamber is connected to a gas analysis unit, a permeability detection unit, and a drain pipe;

[0005] The experimental chamber is made of transparent material and is equipped with a CT probe on the top for real-time photography of internal cracks in coal samples. A main rotating shaft is provided in the experimental chamber, and several support rods are connected to the main rotating shaft. A coal sample is set at the free end of each support rod. The outside of the coal sample is wrapped with an outer jacket, and the upstream end is connected to the gas supply unit and the fracturing fluid unit through a pipeline, and the downstream end is connected to the gas analysis unit and the permeability detection unit through a pipeline. The coal sample rotates with the main rotating shaft, moves to the bottom of the CT probe in turn, and is tested.

[0006] Optionally, the experimental chamber is made of glass and has a cubic shape, which is convenient for placement on a laboratory table or bench; the interior of the experimental chamber has a spherical or ellipsoidal hollow inner cavity, the inner wall of the inner cavity and the outer wall of the experimental chamber are solid glass, and a horizontal main rotating shaft is set at the center line of the inner cavity;

[0007] The upstream side of the experimental chamber is connected to the upstream through-pipe, and the downstream side is connected to the downstream through-pipe, which is used for inputting and outputting water, and accommodating the pipelines connecting the front and rear ends of the coal samples.

[0008] Further optionally, the upstream through-tube is truncated cone-shaped, and the central axis is horizontal, one end of the upstream through-tube connected to the experimental chamber is the end, and the other end is the front end, and the diameter of the end is larger than the diameter of the front end; the pipes connected to the front ends of each coal sample converge into the upstream through-tube and are connected into a bundle, and then pass through the front end of the upstream through-tube and are connected to the corresponding air supply unit or fracturing fluid unit; a branch pipe is provided on the side of the upstream through-tube for connecting to the water supply unit to provide water with a certain flow rate for the experimental chamber.

[0009] Further optionally, the structure of the downstream through pipe is the same as that of the upstream through pipe. Specifically, the downstream through pipe is frustum-shaped, and the central axis is horizontal. One end of the downstream through pipe connected to the experimental chamber is the front end, and the other end is the end. The diameter of the front end is larger than the diameter of the end. The pipelines connected to the rear ends of each coal sample converge into the downstream through pipe and are connected into a bundle. Then, they pass through the end of the downstream through pipe and are connected to the corresponding gas analysis unit or permeability detection unit. A branch pipe is provided on the side of the downstream through pipe for connecting to a drainage pipe.

[0010] Further optionally, the main shaft coincides with the center line of the inner cavity, and both ends of the main shaft are connected to a vertical support rod through a bearing, and the bottom end of the support rod is fixedly connected to the corresponding bottom surface of the inner cavity.

[0011] The middle of the main shaft is connected to a plurality of support rods, and the plurality of support rods are evenly arranged along the circumference of the main shaft;

[0012] A driven gear is sleeved on the outer side of the main shaft, and a driving gear is provided in the inner cavity. The driving gear is engaged with the driven gear. A motor is provided outside the experimental chamber. The motor's shaft penetrates the inner cavity and is connected to the center of the driving gear to drive the main shaft and several support rods to rotate.

[0013] Optionally, a bracket is provided at the top of the support rod, and the bracket is concave in shape. The horizontal bar at the bottom of the bracket is connected to the top of the support rod, and a fixing ring is provided on the top of the vertical bars on both sides of the bracket. The pipelines at both ends of the coal sample pass through the corresponding fixing rings respectively, and the fixing rings tighten the corresponding pipelines so that the coal sample is suspended above the horizontal bar, that is, the coal sample is suspended between the two vertical bars.

[0014] Optionally, a fixed chamber is sleeved on the outer side of the middle portion of the main rotating shaft. The fixed chamber is annular and hollow inside. The inner ring side of the fixed chamber is fixedly connected to the main rotating shaft, and the outer ring side extends outward from the main rotating shaft.

[0015] The fixed chamber is equipped with several control assemblies, each corresponding to a support rod. These assemblies include three telescopic devices arranged side by side, arranged along the circumference of the main shaft: the first telescopic device, the second telescopic device, and the third telescopic device. Due to the limited space in the fixed chamber, two or three support rods are generally sufficient on the main shaft.

[0016] Further optionally, the interior of the support rod is hollow, the bottom end of the support rod is fixedly connected to the outer ring side of the fixed bin, and the top end is provided with a retractable and movable pipe fitting; the outer diameter of the pipe fitting is slightly smaller than the inner diameter of the support rod, and a soft sleeve is connected to the bottom end of the pipe fitting and the top end of the support rod to ensure that water does not enter the support rod and the pipe fitting;

[0017] The telescopic component of the second telescopic device is inside the support rod, the top of the telescopic component is connected to the top inside the pipe, and the cross bar of the bracket is connected to the top outside the pipe to control the lifting of the pipe, bracket and coal sample.

[0018] Further optionally, a detection chamber is provided on the outer side of the support rod, the top surface of the detection chamber is open, and a through hole is provided in the center of the bottom surface of the detection chamber, through which the support rod passes and extends through the detection chamber (i.e., the support rod extends from the top of the detection chamber); the telescopic end of the first telescopic device and the telescopic end of the third telescopic device are respectively connected to the bottom of both ends of the detection chamber to control the lifting and lowering of the detection chamber;

[0019] When the coal sample needs to be detected by a CT probe, the first telescopic device pushes the pipe to drive the coal sample to rise, and at the same time the first telescopic device and the third telescopic device push the detection chamber to rise. The detection chamber can cover the outside of the coal sample and the bracket, and the top surface of the detection chamber is tightly fitted into the top surface of the inner cavity. The part of the top surface of the inner cavity corresponding to the interior of the detection chamber is provided with an air vent, and the air vent is connected to the air supply unit through an air path, which is used to ventilate the detection chamber, so that the water in the detection chamber is discharged from the through hole on the bottom surface, so that the coal sample is out of the water environment, but at the same time is still in a gas pressure environment, and then is subjected to CT probe detection.

[0020] Optionally, the gas supply unit includes a nitrogen cylinder, a carbon dioxide cylinder, a methane cylinder, a mixing device, and a matching gas flow meter and flow control device. Methane can be directly input into each coal sample. The nitrogen cylinder and the carbon dioxide cylinder are connected to the mixing device. Nitrogen and carbon dioxide are mixed in a certain proportion and then input into each coal sample, or nitrogen or carbon dioxide is input into each coal sample separately. The outlet of the mixing device is connected to several pipelines in parallel, which respectively correspond to the front end of the coal sample.

[0021] Optionally, the fracturing fluid unit includes a fracturing fluid storage tank and a fracturing pump. The fracturing fluid storage tank is used to prepare the fracturing fluid required for the experiment, which is then delivered into the outer jacket of the corresponding coal sample by the delivery pump.

[0022] Optionally, the water supply unit includes a water pool, a water pump and a flow meter. The two ends of the water pool are connected to the branches of the upstream and downstream pipes through the water inlet pipe and the drainage pipe respectively. The water pump controls the water flow to provide the inner cavity with the water pressure required for the experiment to simulate the ground stress conditions.

[0023] Optionally, the gas analysis unit includes a gas chromatograph or an infrared spectrometer, which is used to detect the concentrations of CH4, CO2, and N2 in the exhaust gas of the coal sample and monitor the displacement gas production dynamics in real time.

[0024] Optionally, the permeability detection unit includes a permeability meter. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a schematic diagram of the structure of the coal seam transformation experimental device based on fracturing CO2-N2 combined displacement;

[0026] Figure 2 This is a schematic diagram of the interior of the experimental chamber;

[0027] Figure 3 Schematic diagram of the bracket and the detection chamber;

[0028] Figure 4 This is a schematic diagram of the detection chamber and the top wall of the inner cavity being closed.

[0029] In the accompanying drawings, 1-experimental chamber, 2-gas supply unit, 3-fracturing fluid unit, 4-water supply unit, 5-gas analysis unit, 6-permeability detection unit, 7-CT probe, 8-main shaft, 9-inner cavity, 10-support rod, 11-coal sample, 12-upstream through pipe, 13-downstream through pipe, 14-support rod, 15-driven gear, 16-detection chamber, 17-air vent, 18-cross bar, 19-vertical bar, 20-fixed ring, 21-fixed chamber, 22-first telescopic device, 23-second telescopic device, 24-third telescopic device. DETAILED DESCRIPTION

[0030] This embodiment provides a coal seam transformation experimental device based on CO2-N2 combined displacement by fracturing, such as Figure 1-Figure 4 As shown, it includes an experimental chamber 1, the upstream side of which is connected to a gas supply unit 2, a fracturing fluid unit 3, and a water supply unit 4. Methane, CO2, N2, and fracturing fluid are injected into the coal samples in the experimental chamber 1 respectively. The water supply unit 4 transports water into the experimental chamber 1 to maintain the pressure in the chamber. The downstream side of the experimental chamber 1 is connected to a gas analysis unit 5, a permeability detection unit 6, and a drain pipe.

[0031] The experimental chamber 1 is made of transparent material and is provided with a CT probe 7 on the top for real-time photography of the internal cracks of the coal sample; a main rotating shaft 8 is provided in the experimental chamber 1, and a number of support rods 10 are connected to the main rotating shaft 8. A coal sample 11 is set at the free end of each support rod 10. The outer side of the coal sample is wrapped with a jacket, and the upstream end is connected to the gas supply unit 2 and the fracturing fluid unit 3 through a pipeline, and the downstream end is connected to the gas analysis unit 5 and the permeability detection unit 6 through a pipeline; the coal sample rotates with the main rotating shaft 8, moves to the bottom of the CT probe 7 in turn, and is tested.

[0032] The experimental device of the present invention features an experimental chamber 1 for accommodating multiple coal samples. A water supply unit 4 provides water at a constant flow rate within the chamber 1, maintaining a constant water pressure within the chamber to simulate formation pressure. Conventional experimental devices often use mechanical pressure to compress coal samples to simulate formation pressure, requiring the chamber to be designed with movable sides in three dimensions. However, CO2 needs to be input and output within the samples, necessitating airtightness within the chamber, a difficult feat that increases the chamber's complexity and requires consideration of the chamber's material to ensure it can withstand mechanical compression. The present invention, however, uses flowing water to provide pressure within the chamber 1. Ordinary glass can withstand experimental conditions with water pressures below 10 MPa, significantly reducing the cost of the chamber 1. The chamber 1 can also be manufactured in various shapes to meet experimental requirements. The transparent glass also allows for imaging of coal samples within the chamber using a CT probe 7. If the required water pressure is higher, higher-performance glass can be used. (A wide variety of glass types, including bulletproof glass, are available, ensuring it can meet the experimental needs of this field.) The combination of water pressure and glass chamber can not only meet the high-pressure experimental needs in this field, but also reduce costs. The required equipment is all conventional equipment and is easily available.

[0033] The main shaft 8 within the experimental chamber 1 rotates several connected coal samples, which are then inspected in turn by the CT probe 7. Coal samples are enclosed in a jacket, shielded from water flow. Methane, CO2, N2, and fracturing fluid can be injected into the jacket, and gases and fluids can also be output, allowing each coal sample to be tested individually. The CT probe 7 also monitors the coal samples in real time, visualizing the cracks within the coal samples.

[0034] Optionally, the experimental chamber 1 is made of glass and has a cubic shape, which is convenient for placement on a laboratory table or bench; the interior of the experimental chamber 1 has a spherical or ellipsoidal hollow inner cavity 9, and the space between the inner wall of the inner cavity 9 and the outer wall of the experimental chamber 1 is solid glass, and a horizontal main rotating shaft 8 is provided at the center line of the inner cavity 9;

[0035] The upstream side of the experimental chamber 1 is connected to the upstream through pipe 12, and the downstream side is connected to the downstream through pipe 13, which are used for inputting and outputting water, and accommodating the pipelines connecting the front and rear ends of the coal samples.

[0036] Alternatively, the inner cavity 9 of the experimental chamber 1 is centered and preferably ellipsoidal. The thickness of the glass between the top of the inner cavity 9 and the outer surface of the top of the experimental chamber 1 is appropriately determined based on the imaging requirements of the CT probe 7. The radiation emitted by the CT probe 7 passes through the glass of the experimental chamber 1 and the outer jacket of the coal sample, irradiating the coal sample and passing through the coal sample to form an image of the internal cracks of the coal sample.

[0037] Further optionally, the upstream through-tube 12 is truncated cone-shaped, and the central axis is horizontal, one end of the upstream through-tube 12 connected to the experimental chamber 1 is the end, and the other end is the front end, and the diameter of the end is larger than the diameter of the front end; the pipelines connected to the front ends of each coal sample converge into the upstream through-tube 12 and are connected into a bundle, and then pass through the front end of the upstream through-tube 12 and are connected to the corresponding air supply unit 2 or fracturing fluid unit 3; a branch pipe is provided on the side of the upstream through-tube 12 for connecting to the water supply unit 4 to provide water with a certain flow rate to the experimental chamber 1.

[0038] The pipes connected to each coal sample are constrained into a bundle in the upstream pipe 12, and a seal is set at the position where it passes through the upstream pipe 12. If necessary, high-sealing glue is used to prevent water leakage at the front end of the upstream pipe 12.

[0039] Further optionally, the structure of the downstream through pipe 13 is the same as that of the upstream through pipe 12. Specifically, the downstream through pipe 13 is truncated cone-shaped, and the central axis is horizontal. One end of the downstream through pipe 13 connected to the experimental chamber 1 is the front end, and the other end is the end. The diameter of the front end is larger than the diameter of the end; the pipelines connected to the rear ends of each coal sample converge into the downstream through pipe 13 and are connected into a bundle, and then pass through the end of the downstream through pipe 13 and are connected to the gas analysis unit 5 and the permeability detection unit 6; a branch pipe is provided on the side of the downstream through pipe 13 for connecting to a drainage pipe.

[0040] The pipes connecting the coal samples are bundled together within the downstream pipe 13, and a seal is installed at the point where they pass through the downstream pipe 13. If necessary, high-sealing glue is used to prevent water leakage at the end of the downstream pipe 13. The upstream pipe 12, downstream pipe 13, experimental chamber 1, and two branch pipes can be integrally formed to improve sealing.

[0041] The water providing the ambient pressure is inputted into the upstream through-pipe 12 by the branch pipe, and then into the inner cavity 9. This is a process of increasing the cross-section of the water flow channel. In general, the water pipes are connected to the cavity in a manner that has the same diameter. When the water flows into the cavity, the cross-section suddenly increases, the water flow velocity suddenly decreases, and the pressure suddenly decreases, which is not conducive to the control of the flow rate and pressure of the overall system. The inner diameter of the upstream through-pipe 12 of the present invention gradually increases, and the spherical or ellipsoidal inner cavity 9 cooperates so that the water flow inside it can gradually adapt to the gradually increasing channel and finally enter the inner cavity 9. The water pressure changes smoothly and is easy to control, which also makes the water pressure in the inner cavity 9 more stable and better controlled. Several pressure gauges are set at various locations inside the inner cavity 9 to continuously measure the water pressure.

[0042] Further optionally, the center line of the main shaft 8 coincides with the center line of the inner cavity 9, and both ends of the main shaft 8 are connected to a vertical support rod 14 through a bearing, and the bottom end of the support rod 14 is fixedly connected to the bottom surface of the corresponding inner cavity 9.

[0043] The middle portion of the main shaft 8 is connected to a plurality of support rods 10. The plurality of support rods 10 are evenly arranged along the circumference of the main shaft 8. The positions where the plurality of support rods 10 are connected to the main shaft 8 are on a vertical circle, that is, the connection positions of the support rods 10 are at the same position in the length direction of the main shaft 8.

[0044] A driven gear 15 is sleeved on the outer side of the main shaft 8, and a driving gear is provided in the inner cavity 9. The driving gear meshes with the driven gear 15. A motor is provided outside the experimental chamber 1. The shaft of the motor penetrates the inner cavity 9 and is connected to the center of the driving gear to drive the main shaft 8 and several support rods 10 to rotate.

[0045] The length of the main rotating shaft 8 is reasonably set according to the experimental needs and is generally not longer than the center line of the inner cavity 9. The driven gear 15 can be arranged at one end of the main rotating shaft 8 close to the upstream through pipe 12 or close to the downstream through pipe 13, and the driven gear 15 is fixedly connected to the main rotating shaft 8. The driving gear is on one side of the main rotating shaft 8, and the motor is arranged on the outside of the side of the experimental chamber 1 connected to the upstream through pipe 12 or the downstream through pipe 13, and is close to the driven gear 15 and the driving gear. A rotating seal is provided at the position where the motor shaft passes through the experimental chamber 1 to prevent water leakage. The motor drives the main rotating shaft 8 to rotate through the driving gear and the driven gear 15, thereby driving the support rod 10 and the coal sample to rotate.

[0046] Optionally, a bracket is provided at the top of the support rod 10, and the bracket is concave in shape. The horizontal bar 18 at the bottom of the bracket is connected to the top of the support rod 10, and a fixing ring 20 is provided on the top of the vertical bars 19 on both sides of the bracket. The pipelines at both ends of the coal sample pass through the corresponding fixing rings 20 respectively, and the fixing rings 20 tighten the corresponding pipelines so that the coal sample is suspended above the horizontal bar 18, that is, the coal sample is suspended between the two vertical bars 19.

[0047] Optionally, the coal sample is in the shape of a long and thin strip, and the outer jacket of the coal sample is made of flexible rubber or other waterproof and airtight flexible material, which can tightly wrap the coal sample, and the water pressure in the inner cavity 9 can act on the coal sample through the jacket; the jacket is provided with an interface at the positions corresponding to the two ends of the coal sample, for connecting pipelines; the length direction of the coal sample and the cross bar 18 is parallel to the main rotating shaft 8.

[0048] The interface can be made of hard plastic and fixedly connected to the outer jacket, ensuring a tight seal between the outer jacket and the interface. The coal sample is completely isolated from the water environment in the inner chamber 9, while being able to withstand the pressure of the water environment. Since the outer jacket is subjected to water pressure, the inner jacket is fed with gas or liquid at a certain pressure that meets the experimental requirements, ensuring that the outer jacket will not be ruptured.

[0049] The coal sample is a cylindrical or rectangular fixed coal block with a certain thickness and hardness. The front end of the coal sample points to the upstream through pipe 12, and the rear end points to the downstream through pipe 13. The interface at the front end of the coal sample is connected to a pipeline for inputting gas and / or fracturing fluid into the coal sample, and the interface at the rear end of the coal sample is connected to another pipeline for outputting gas and fracturing fluid. The fixing ring 20 clamps the corresponding pipeline to prevent the pipeline from falling out of the fixing ring 20, and does not affect the transportation of gas and liquid inside the pipeline. In the inner cavity 9, the coal sample can be suspended above the horizontal bar 18 (that is, suspended between the two vertical bars 19) by utilizing the effect of buoyancy. Neither the horizontal bar 18 nor the vertical bar 19 contacts the coal sample, and does not affect the coal sample's ability to withstand water pressure.

[0050] Optionally, a fixed chamber 21 is sleeved on the outer side of the middle portion of the main rotating shaft 8. The fixed chamber 21 is annular and hollow inside. The inner ring side of the fixed chamber 21 is fixedly connected to the main rotating shaft 8, and the outer ring side extends outward from the main rotating shaft 8.

[0051] Several control components are provided in the fixed bin 21, and the control components correspond one-to-one to the support rods 10; the control components include three telescopic devices arranged side by side, and the three telescopic devices are arranged along the circumference of the main rotating shaft 8, namely the first telescopic device 22, the second telescopic device 23 and the third telescopic device 24.

[0052] Further optionally, the interior of the support rod 10 is hollow, the bottom end of the support rod 10 is fixedly connected to the outer ring side of the fixed chamber 21, and the top end is provided with a retractable and movable pipe fitting; the outer diameter of the pipe fitting is slightly smaller than the inner diameter of the support rod 10, and the bottom end of the pipe fitting and the top end of the support rod 10 are connected with a soft sleeve to ensure that water does not enter the support rod 10 and the pipe fitting;

[0053] The telescopic component of the second telescopic device 23 is located inside the support rod 10, the top of the telescopic component is connected to the top inside the pipe, and the cross bar 18 of the bracket is connected to the top outside the pipe to control the lifting of the pipe, bracket and coal sample.

[0054] The support rod 10 and the pipe are made of a hard plastic material that can withstand water flow within the inner cavity 9. The heights of the inner cavity 9 and the support rod 10 are strategically designed so that when the pipe extends partially beyond the support rod 10 (it does not need to be fully extended), the coal sample can be raised to a height suitable for CT testing (the coal sample does not need to reach the top of the inner cavity 9). A soft sleeve is connected between the pipe and the support rod 10 to prevent water from entering the support rod 10 and protect the second telescopic device 23. Furthermore, when CT testing is not being performed, the soft sleeve fills the narrow gap between the pipe and the support rod 10, making the pipe virtually motionless. Combined with the support provided by the telescopic components of the second telescopic device 23, this ensures the stability of the support and the coal sample.

[0055] Further optionally, a detection chamber 16 is provided on the outer side of the support rod 10. The top surface of the detection chamber 16 is open, and a through hole is provided in the center of the bottom surface of the detection chamber 16. The support rod 10 passes through the through hole and penetrates the detection chamber 16 (i.e., the support rod 10 extends from the top of the detection chamber 16). The telescopic ends of the first telescopic device 22 and the telescopic ends of the third telescopic device 24 are respectively connected to the bottoms of the two ends of the detection chamber 16 to control the lifting and lowering of the detection chamber 16.

[0056] When the coal sample needs to be detected by the CT probe 7, the first telescopic device 22 pushes the pipe to drive the coal sample to rise, and at the same time the first telescopic device 22 and the third telescopic device 24 push the detection chamber 16 to rise. The detection chamber 16 can cover the outside of the coal sample and the bracket, and the top surface of the detection chamber 16 is tightly fitted to the top surface of the inner cavity 9. The part of the top surface of the inner cavity 9 corresponding to the inside of the detection chamber 16 is provided with an air vent 17. The air vent 17 is connected to the air supply unit 2 through an air path, and is used to ventilate the detection chamber 16, so that the water in the detection chamber 16 is discharged from the bottom hole, so that the coal sample is out of the water environment, but at the same time is still in the gas pressure environment, and then is detected by the CT probe 7.

[0057] The present invention utilizes a glass experimental chamber 1 and inner cavity 9, coupled with a constant flow rate of water to provide pressure, significantly simplifying experimental operations and reducing costs. However, due to the characteristics of the CT probe 7, the CT probe 7 images the coal sample through the glass wall and outer jacket, which can produce artifacts. Therefore, before the experiment begins, the coal sample is first imaged through the glass wall and outer jacket to obtain a base image, pre-determining the artifacts of the glass wall and outer jacket. Subsequent images of the coal sample taken during the experiment are then compared with the base image, subtracting the artifacts from the base image, allowing for clearer definition and observation of cracks and changes in the coal sample resulting from the experiment.

[0058] However, there is flowing water between the coal sample and the top wall of the inner cavity 9, which will also produce slight artifacts during shooting. However, the flowing water changes from time to time, and the base artifacts cannot be subtracted like the above method. In order to solve the water flow problem, the present invention designs a liftable detection chamber 16. The initial position of the detection chamber 16 is lower than the bracket, and the detection chamber 16 does not affect the coal sample. The second telescopic device 23 controls the coal sample and the bracket to rise into place. At the same time, the other two telescopic devices extend synchronously and push the detection chamber 16 to rise. The support rod 10 passes through the detection chamber 16, so the support rod 10 acts as a guide rail, guiding the detection chamber 16 to rise vertically along the length direction of the support rod 10. After the coal sample and the bracket rise into place, there is a gap between the coal sample and the top wall of the inner cavity 9. The detection chamber 16 continues to rise until the top surface of the detection chamber 16 is close to the top wall of the inner cavity 9. The position of the CT probe 7 corresponds to the position of the coal sample below, which is convenient for capturing the full picture of the coal sample.

[0059] The detection chamber 16 can be cubical in shape, with the corresponding top wall of the inner cavity 9 being flat. A sealing edge is provided on the top edge of the detection chamber 16. The first and third telescopic devices 22, 24 simultaneously press upward against the detection chamber 16, pressing the top edge of the detection chamber 16 against the top wall of the inner cavity 9 to form a tight seal, ensuring airtightness within the detection chamber 16. Because the inner cavity is a water environment, the sealing edge is slightly wider. In the compressive direction between the detection chamber and the inner cavity top wall, the sealing edge can tightly adhere to the inner cavity top wall (similar to the principle of a suction cup on glass).

[0060] The telescopic ends of the first and third telescopic devices 22, 24 are conventional telescopic rods. The coal sample is parallel to the main rotating shaft 8, and the line connecting the telescopic ends of the first and third telescopic devices 24 is perpendicular to the coal sample. Therefore, the two telescopic ends are located on either side of the coal sample and outside the imaging range of the CT probe 7, thus not affecting the imaging of the CT probe 7. The control circuitry of the three telescopic devices corresponding to each support rod 10 extends into the main rotating shaft 8, along the main rotating shaft 8 to the downstream side of the experimental chamber 1, then out of the main rotating shaft 8, merging with the coal sample pipeline, and out of the downstream passage 13.

[0061] Next, the nitrogen cylinder in the gas supply unit 2 supplies gas to the vent 17 (compressed air can also be supplied to the vent), displacing the water in the test chamber 16. The water then drains into the inner cavity 9 through the through-holes in the bottom of the test chamber 16. The liquid level in the test chamber 16 only needs to drop below the coal sample; it is not necessary to completely drain the water. A pressure sensor probe is located above the test chamber 16 to monitor the pressure in the upper middle portion of the test chamber 16 in real time. Nitrogen or air displaces the water at a pressure slightly greater than the water pressure in the inner cavity, and then the air pressure is restored to equal the water pressure in the inner cavity, ensuring that the coal sample remains at the test pressure. With air between the CT probe 7 and the coal sample, the CT probe 7 then captures an image of the coal sample, eliminating the influence of the flowing water and capturing the crack morphology and propagation direction in real time.

[0062] After the shooting is completed, air is extracted from the vent 17, and the water outside the detection chamber 16 enters the detection chamber 16 again from the through hole. After the water refills the detection chamber 16, the first and third telescopic devices 24 drive the detection chamber 16 to descend, and the second telescopic device 23 drives the coal sample to descend. After the detection chamber 16 and the coal sample are reset, the main shaft 8 continues to rotate to detect the coal sample of the next support rod 10.

[0063] A section of the pipes connected to both ends of each coal sample is reserved in the inner cavity 9, so that the pipes are redundant and will not be pulled too tight when the coal sample rises. After the coal sample rises to its place, the detection chamber 16 continues to rise. The top edge of the detection chamber 16 is provided with docking grooves corresponding to the positions of the pipes at both ends of the coal sample. When the top edge of the detection chamber 16 presses against the top wall of the inner cavity 9, the pipes pass through the corresponding grooves. There are also sealing gaskets at the grooves. The rubber material of the pipes is relatively hard and will not be flattened. The redundant amount of the pipes means that when the detection chamber 16 rises and presses the pipes, it will not pull the pipes connected to the coal sample side, but will pull the pipes in the direction of the upstream through pipe 12 and the downstream through pipe 13, so that part of the pipes enter the detection chamber 16.

[0064] The position where the bottom end of the support rod 10 is connected to the fixed warehouse 21, the position where the bottom of the telescopic end of the first / third telescopic device 24 is connected to the fixed warehouse 21, and the position where the line passes through the main shaft 8 are all sealed to prevent water leakage from the fixed warehouse 21 and the main shaft 8.

[0065] The motor drives the main shaft 8 to rotate at a relatively slow speed, allowing for CT testing of individual coal samples. Compared to the water flow rate within the inner chamber 9, the rotation and elevation of the coal samples are very slow. To avoid excessive rotation or revolutions of the pipe bundle, the main shaft 8 can be rotated in both forward and reverse directions. Because the pipes within the experimental chamber have sufficient clearance, they can rotate with the main shaft to a certain degree.

[0066] Optionally, the gas supply unit 2 includes a nitrogen cylinder, a carbon dioxide cylinder, a methane cylinder, a mixing device, and a matching gas flow meter and flow control device, and methane can be directly input into each coal sample; the nitrogen cylinder and the carbon dioxide cylinder are connected to the mixing device, and the nitrogen and carbon dioxide are mixed in a certain proportion and then input into each coal sample, or nitrogen or carbon dioxide is input into each coal sample separately, and the outlet of the mixing device is connected to several pipelines in parallel, which are respectively connected to the front end of the coal sample; the outlet pipe of the methane cylinder is connected to several branches in parallel, and each branch is connected to the pipeline at the front end of a coal sample, so that methane can be input into the coal sample separately.

[0067] The nitrogen cylinder is further provided with a branch connection vent 17, and the vent 17 is also connected in parallel with an exhaust pipeline for extracting gas.

[0068] The gas supply unit 2 can provide mixed gases with different ratios of carbon dioxide and nitrogen for the coal sample, so as to study the effects of different ratios of carbon dioxide and nitrogen, or carbon dioxide alone, or nitrogen alone, on the transformation of the coal sample.

[0069] Before the fracturing experiment, methane was introduced into all coal samples. After the output gas was tested by a gas analysis unit, methane was discontinued and the fracturing experiment was resumed after the coal sample was saturated with methane. Nitrogen and carbon dioxide were then injected. Nitrogen or carbon dioxide could be injected alone, or mixed in a specific ratio to study the competitive adsorption relationship between nitrogen, carbon dioxide, and methane. When the methane content in the input gas equaled the methane content in the output gas, the coal sample was considered saturated with adsorption, and the methane adsorption capacity of the coal sample could be measured.

[0070] Optionally, the fracturing fluid unit 3 includes a fracturing fluid storage tank and a fracturing pump. The fracturing fluid storage tank is used to prepare the fracturing fluid required for the experiment, which is then delivered to the outer jacket of the corresponding coal sample by a delivery pump. The fracturing fluid unit can adjust the injection pressure (0-30 MPa), flow rate (0-500 mL / min), and pulse frequency. During the experiment, the fracturing pump is activated to inject fracturing fluid, and the pressure / flow rate is adjusted to simulate initial or repeated fracturing. The CT probe 7 records the changes in fracture propagation morphology during the fracturing process. The CT probe 7 is connected to the data integration and control center and transmits the fracturing crack change data to the data integration and control center for capture and rendering.

[0071] The gas pipe of the gas supply unit 2 and the delivery pipe of the fracturing unit for the same coal sample can be connected in parallel to form a single pipeline, which is then passed through the upstream pipe 12 and connected to the coal sample. The coal sample can be directly fractured using carbon dioxide and / or nitrogen; alternatively, fracturing fluid can be added to the carbon dioxide and / or nitrogen before fracturing the coal sample.

[0072] Optionally, the water supply unit 4 includes a water pool, a water pump and a flow meter. The two ends of the water pool are connected to the branches of the upstream and downstream pipes 13 through the water inlet pipe and the drainage pipe respectively. The water pump controls the water flow to provide the inner cavity 9 with the water pressure required for the experiment to simulate the ground stress conditions.

[0073] The fracturing and gas in the coal sample are discharged from the pipeline at the rear end of the coal sample, and then connected to a separation device to separate the gas and then input it into the gas analysis unit.

[0074] Optionally, the gas analysis unit 5 includes a gas chromatograph or an infrared spectrometer, which is used to detect the concentrations of CH4, CO2, and N2 in the exhaust gas of the coal sample and monitor the displacement gas production dynamics in real time.

[0075] Optionally, the permeability detection unit 6 includes a permeability meter, the flow meter of the permeability meter is installed on the pipelines at the front and rear ends of the coal sample, and during the process of gas entering and exiting the coal sample, the gas flow entering and exiting the coal sample is detected, and the data and time are recorded to detect the permeability of the coal sample before and after the fracturing experiment.

[0076] According to existing methods, permeability detection generally sets multiple pressure conditions (water pressure conditions in the inner cavity). Under different pressure conditions, methane gas is purged from the front end of the coal sample, and the gas flow change at the rear end of the coal sample is detected to calculate the permeability.

Claims

1. A coal seam transformation experimental device based on CO2-N2 combined displacement by fracturing, characterized in that: The experimental chamber comprises an experimental chamber, the upstream side of which is connected to an air supply unit, a fracturing fluid unit and a water supply unit. The air supply unit injects methane, CO2 and N2 into the coal samples in the experimental chamber, the fracturing fluid unit provides fracturing fluid for the coal samples in the experimental chamber, and the water supply unit transports water into the experimental chamber to maintain the pressure in the chamber. The downstream side of the experimental chamber is connected to a gas analysis unit, a permeability detection unit and a drainage pipe. The experimental chamber is made of transparent material and is equipped with a CT probe on the top for real-time photography of internal cracks in coal samples. A main rotating shaft is provided in the experimental chamber, and several support rods are connected to the main rotating shaft. A coal sample is set at the free end of each support rod. The outside of the coal sample is wrapped with an outer jacket, and the upstream end is connected to the gas supply unit and the fracturing fluid unit through a pipeline, and the downstream end is connected to the gas analysis unit and the permeability detection unit through a pipeline. The coal sample rotates with the main rotating shaft, moves to the bottom of the CT probe in turn, and is tested.

2. The coal seam transformation experimental device according to claim 1, characterized in that: The experimental chamber is made of glass and has a cubic shape. The interior of the experimental chamber has a spherical or ellipsoidal hollow inner cavity. The inner wall of the inner cavity and the outer wall of the experimental chamber are made of solid glass. A horizontal main rotating shaft is set at the center line of the inner cavity. The upstream side of the experimental chamber is connected to the upstream through-pipe, and the downstream side is connected to the downstream through-pipe, which is used for inputting and outputting water, and accommodating the pipelines connecting the front and rear ends of the coal samples.

3. The coal seam transformation experimental device according to claim 2, characterized in that: The upstream and downstream pipes are both truncated cone-shaped, and their central axes are both horizontal. One end of the upstream pipe connected to the experimental chamber is the end, and the other end is the front end. The diameter of the end is larger than the diameter of the front end. The pipes connected to the front ends of the coal samples are merged into the upstream pipe and connected into a bundle. Then, they pass through the front end of the upstream pipe and are connected to the corresponding air supply unit or fracturing fluid unit. A branch pipe is provided on the side of the upstream pipe for connecting to the water supply unit. One end of the downstream through-pipe connecting the experimental chamber is the front end, and the other end is the end. The diameter of the front end is larger than the diameter of the end. The pipes connected to the rear ends of each coal sample converge into the downstream through-pipe and are connected into a bundle. Then, they pass through the end of the downstream through-pipe and are connected to the corresponding gas analysis unit or permeability detection unit. A branch pipe is provided on the side of the downstream through-pipe for connecting to a drainage pipe.

4. The coal seam transformation experimental device according to claim 2, characterized in that: The two ends of the main shaft are connected to a vertical support rod through a bearing, and the bottom end of the support rod is fixedly connected to the bottom surface of the corresponding inner cavity. The middle of the main shaft is connected to a plurality of support rods, and the plurality of support rods are evenly arranged along the circumference of the main shaft; A driven gear is sleeved on the outer side of the main shaft, and a driving gear is provided in the inner cavity. The driving gear is engaged with the driven gear. A motor is provided outside the experimental chamber. The motor's shaft penetrates the inner cavity and is connected to the center of the driving gear to drive the main shaft and several support rods to rotate.

5. The coal seam transformation experimental device according to claim 4, characterized in that: A bracket is provided at the top of the support rod, and the bracket is concave in shape. The horizontal bar at the bottom of the bracket is connected to the top of the support rod. A fixing ring is provided on the top of the vertical bars on both sides of the bracket. The pipelines at both ends of the coal sample pass through the corresponding fixing rings respectively. The fixing rings tighten the corresponding pipelines so that the coal sample is suspended above the horizontal bar.

6. The coal seam transformation experimental device according to claim 1, characterized in that: The outer side surface of the middle part of the main rotating shaft is sleeved with a fixed chamber, which is annular and hollow inside. The inner ring side of the fixed chamber is fixedly connected to the main rotating shaft, and the outer ring side extends outward from the main rotating shaft. Several control components are provided in the fixed bin, and the control components correspond to the support rods one by one; the control components include three telescopic devices arranged side by side, and the three telescopic devices are arranged along the circumference of the main rotating shaft, namely the first telescopic device, the second telescopic device and the third telescopic device.

7. The coal seam transformation experimental device according to claim 6, characterized in that: The interior of the support rod is hollow, the bottom end of the support rod is fixedly connected to the outer ring side of the fixed bin, and the top end is provided with a retractable and movable pipe fitting; The telescopic component of the second telescopic device is inside the support rod, the top of the telescopic component is connected to the top inside the pipe, and the cross bar of the bracket is connected to the top outside the pipe to control the lifting of the pipe, bracket and coal sample.

8. The coal seam transformation experimental device according to claim 7, characterized in that: A detection chamber is provided on the outside of the support rod, the top surface of the detection chamber is open, and a through hole is provided in the center of the bottom surface of the detection chamber. The support rod passes through the through hole and penetrates the detection chamber; the telescopic end of the first telescopic device and the telescopic end of the third telescopic device are respectively connected to the bottom of both ends of the detection chamber to control the lifting and lowering of the detection chamber; When the coal sample needs to be detected by a CT probe, the second telescopic device pushes the pipe to drive the coal sample to rise, and at the same time the first telescopic device and the third telescopic device push the detection chamber to rise. The detection chamber can cover the outside of the coal sample and the bracket, and the top surface of the detection chamber is tightly fitted into the top surface of the inner cavity. The part of the top surface of the inner cavity corresponding to the interior of the detection chamber is provided with an air vent, and the air vent is connected to the air supply unit through an air path, which is used to ventilate the detection chamber, so that the water in the detection chamber is discharged from the through hole on the bottom surface, so that the coal sample is out of the water environment, but at the same time is still in a gas pressure environment, and then is subjected to CT probe detection.

9. The coal seam transformation experimental device according to claim 1, characterized in that: The gas supply unit includes a nitrogen cylinder, a carbon dioxide cylinder, a methane cylinder, a mixing device, and a matching gas flow meter and flow control device. Methane can be directly input into each coal sample; the nitrogen cylinder and the carbon dioxide cylinder are connected to the mixing device, and nitrogen and carbon dioxide are mixed in a certain proportion and then input into each coal sample, or nitrogen or carbon dioxide can be input into each coal sample separately. The outlet of the mixing device is connected to several pipelines in parallel, which are respectively connected to the front end of the coal sample; The fracturing fluid unit includes a fracturing fluid storage tank and a fracturing pump. The fracturing fluid storage tank is used to prepare the fracturing fluid required for the experiment, which is then pumped into the outer jacket of the corresponding coal sample by the fracturing pump.

10. The coal seam transformation experimental device according to claim 1, characterized in that: The water supply unit includes a water pool, a water pump and a flow meter. The two ends of the water pool are connected to the branches of the upstream and downstream pipes through the water inlet pipe and the drain pipe respectively. The water pump controls the water flow to provide the water pressure required for the experiment to simulate the ground stress conditions. The gas analysis unit includes a gas chromatograph or an infrared spectrometer, which is used to detect the concentrations of CH4, CO2, and N2 in the exhaust gas of the coal sample and monitor the displacement gas output dynamics in real time; The permeability detection unit includes a permeability meter.

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