A device and method for constructing an experiment of coal powder invasion and retention in a coal reservoir propped fracture

Abstract

The application discloses a kind of experimental device and method of constructing coal reservoir support fracture coal powder invasion and retention, belong to coalbed gas development technical field.The application provides a kind of experimental device of constructing coal reservoir support fracture coal powder invasion and retention, comprising: mixing unit, core holder, support fracture simulation unit and monitoring equipment;The output end of mixing unit is used to output gas-liquid two-phase flow;Cylindrical tectonic coal sample is placed in the inside of core holder, and the inlet end of core holder is communicated with the output end of mixing unit;The periphery of core holder is communicated with the pressurizing equipment for applying confining pressure to core holder;Support fracture simulation unit includes a container filled with proppant inside, and the internal space of the container is used to simulate the fracture.The experimental process of the present support fracture coal powder invasion and retention experimental device is more scientific and rigorous, the simulation is high in fidelity, and the management basis can be more conveniently provided for the support fracture coal powder invasion and retention phenomenon.

Description

Technical Field

[0001] This invention relates to the field of coalbed methane development technology, specifically to an experimental apparatus and method for constructing coal dust intrusion and retention within supported fractures in coal reservoirs. Background Technology

[0002] my country has well-developed tectonic coal reservoirs, accounting for approximately 60% of the country's total coal resources. Due to the soft and fractured structure of tectonic coal, the amount of primary / secondary coal dust within these reservoirs is far greater than that of primary structural coal, making it the primary source of coal dust produced by coalbed methane wells. During coalbed methane well drainage, tectonic coal dust easily intrudes into the proppant fractures with the fluid flow, causing retention and blockage, resulting in a significant reduction in the conductivity of the proppant fractures. Therefore, studying the characteristics of coal dust intrusion and retention within the proppant fractures of tectonic coal reservoirs is of great significance for controlling coal dust at the source of production, inhibiting its damage to the conductivity of the proppant fractures, and ultimately achieving economical and efficient coalbed methane well drainage.

[0003] Currently, experimental studies on the intrusion and retention of coal dust in support fractures have not thoroughly investigated the true source and intrusion potential of the coal dust within the fractures. The content of intruded coal dust in experiments is mostly determined based on the coal dust content in the produced fluid at the site. However, the coal dust content in the produced fluid at the site is the remaining content of coal dust after it has passed through the support fractures and shaft, and is not equivalent to the content of coal dust intruded into the support fractures. Therefore, the experimental process is not rigorous and lacks scientific basis. Summary of the Invention

[0004] The purpose of this invention is to overcome the problems existing in the prior art and provide an experimental device and method for constructing coal dust intrusion and retention in supported fractures of coal reservoirs. The experimental device of this invention is more scientific and rigorous, with high simulation realism, and can fully reflect the real source of coal dust intrusion in supported fractures as well as the intrusion potential and rate of coal dust, thereby accurately evaluating the characteristics of coal dust retention and blockage in supported fractures and the degree of permeability damage to supported fractures induced by coal dust retention and blockage.

[0005] This invention provides an experimental device for the intrusion and retention of coal dust in supported fractures of a structural coal reservoir, comprising: a mixing unit, a core holder, a supported fracture simulation unit, and monitoring equipment; the output end of the mixing unit is used to output a gas-liquid two-phase flow; a columnar structural coal sample is placed inside the core holder, and the inlet end of the core holder is connected to the output end of the mixing unit; the periphery of the core holder is connected to a pressurizing device for applying confining pressure to the core holder; the supported fracture simulation unit includes a container filled with proppant, the internal space of the container is used to simulate fractures, the proppant is filled inside the container, all proppant and the inner cavity of the container are used to simulate supported fractures, the inlet end of the container is connected to the output end of the mixing unit and the outlet end of the core holder, and the container is provided with an outlet for discharging the gas-liquid two-phase flow, wherein the gas-liquid two-phase flow mixed with coal dust flows through the pores between the proppant in the supported fracture; the monitoring equipment is used to monitor the intrusion and retention of coal dust in the supported fracture and its induced permeability evolution process.

[0006] Preferably, the mixing unit includes a first injection pump, a gas source, and a gas flow controller. The output ends of the first injection pump and the gas source are both connected to the inlet ends of the core holder and the container. The first injection pump is used to output background fluid at a constant flow rate, and the gas source and the gas flow controller are combined to output gas at a constant flow rate.

[0007] Preferably, the inlet end of the core holder is provided with a first on / off valve, and the outlet end is provided with a second on / off valve. The core holder, the first on / off valve, and the second on / off valve are all connected in parallel with a third on / off valve. One end of the third on / off valve is connected to the output end of the mixing unit, and the other end of the third on / off valve is connected to the inlet end of the container.

[0008] Preferably, the container is provided with a fourth shut-off valve at its inlet end, the container's outlet is connected to the inlet end of the gas-liquid separator, the gas-liquid separator's outlet end is connected to the fraction collector, the gas-liquid separator is provided with an exhaust port, a fifth shut-off valve is provided on the pipeline between the container and the gas-liquid separator, and a sixth shut-off valve is provided on the pipeline between the gas-liquid separator and the fraction collector.

[0009] Preferably, the pressurization device includes a second injection pump and a seventh on / off valve, the output end of the second injection pump being connected to the confining pressure application port of the core holder via the seventh on / off valve.

[0010] Preferably, the monitoring device includes a laser scanning confocal microscope, a pressure sensor, a gas flow meter, and a processor. The laser scanning confocal microscope is used to observe and characterize the coal dust intrusion and retention process in the support fracture in real time. The pressure sensor is used to detect the pressure at the inlet of the container. The gas flow meter is used to detect the gas flow rate discharged from the gas-liquid separator. At the same time, the laser scanning confocal microscope, the pressure sensor, and the gas flow meter can respectively send the measured image, pressure, and flow signals to the processor. The processor can obtain the coal dust intrusion and retention characteristics in the support fracture by processing the image signals, including the coal dust retention form, retention location, and the evolution law of pore throat blockage induced by coal dust. The processor can obtain the gas phase and liquid phase permeability of the support fracture by processing the pressure and flow signals.

[0011] Preferably, when no pulverized coal intrudes, the gas-liquid two-phase flow output from the mixing unit sequentially passes through the third shut-off valve, the fourth shut-off valve, the container, and the fifth shut-off valve, and finally enters the gas-liquid separator to achieve gas-liquid phase separation. At this time, the gas-phase and liquid-phase permeabilities of the supporting fracture are respectively:

[0012]

[0013] In the formula: K ge and K we These represent the gas-phase and liquid-phase permeabilities, respectively, without coal powder intrusion, in m. 2 ; p 1 and p a These are the inlet and outlet pressures of the container when there is no coal dust intrusion, respectively, in Pa; q g and q w The values ​​are the gas flow rate measured by the gas flow meter when there is no coal powder intrusion and the liquid flow rate output by the first injection pump, respectively, in m. 3 / s; μ g and μ w The viscosity of the gas phase and liquid phase are respectively, in Pa·s; L To support the crack length, m; A To support the cross-sectional area of ​​the crack, m 2 ;

[0014] When pulverized coal intrudes, the gas-liquid two-phase flow output from the mixing unit sequentially passes through the first shut-off valve, core holder, second shut-off valve, fourth shut-off valve, container, and fifth shut-off valve, and finally enters the gas-liquid separator to achieve gas-liquid phase separation. At this point, the gas and liquid phase permeabilities of the supporting fracture are respectively:

[0015]

[0016] In the formula: and These represent the gas-phase and liquid-phase permeability when pulverized coal has intruded, in m. 2 ; and These are the inlet and outlet pressures of the container when pulverized coal has entered, respectively, in Pa; and The values ​​are the gas flow rate measured by the gas flow meter when pulverized coal intrusion occurs and the liquid flow rate output by the first injection pump, respectively, in m. 3 / s; μ g and μ w The viscosity of the gas phase and liquid phase are respectively, in Pa·s; L To support the crack length, m; A To support the cross-sectional area of ​​the crack, m 2 ;

[0017] Meanwhile, the gas phase and liquid phase permeability damage rates of the supporting fractures were obtained as follows:

[0018]

[0019] In the formula: D g and D w These are the gas phase and liquid phase permeability damage rates, respectively, dimensionless; K ge and K we These represent the gas-phase and liquid-phase permeabilities, respectively, without coal powder intrusion, in m. 2 ; and These represent the gas-phase and liquid-phase permeability when pulverized coal has intruded, in m. 2 .

[0020] Preferably, the supporting crack simulation unit further includes a transparent plate, a sealing ring, two filters, and multiple screws. The container has an opening, and a sealing ring surrounds the opening on the end face of the opening. The transparent plate is fixed to the end face of the container opening by the multiple screws to seal the container. At the same time, a laser scanning confocal microscope observes the process of coal dust intrusion and retention in the supporting crack through the transparent plate. The two filters are respectively set at the inlet and outlet ends on the inner wall of the container, and the filter size allows coal dust to pass through freely.

[0021] Preferably, the mixing unit further includes a first one-way valve and a second one-way valve. The output end of the first injection pump is provided with a first one-way valve, which is connected to the inlet end of the core holder and the container. The first one-way valve is used to prevent the gas-liquid two-phase flow from flowing back into the first injection pump. The output end of the gas source is provided with a gas flow controller and a second one-way valve that are connected to each other. The second one-way valve is connected to the inlet end of the core holder and the container. The second one-way valve is used to prevent the gas-liquid two-phase flow from flowing back into the gas flow controller and the gas source.

[0022] A method for experimentally constructing coal dust intrusion and retention within supported fractures in coal reservoirs includes the following steps:

[0023] S100. Prepare gas and background fluid treated with fluorescent agent, and mix the gas and background fluid into a stable gas-liquid two-phase flow at a certain flow rate ratio; construct coal sample by pressing coal powder by cold pressing, and the coal powder is treated with fluorescent agent, and the color of the fluorescent agent used in the background fluid can be distinguished from the color of the coal powder.

[0024] S200. The gas-liquid two-phase flow in step S100 is introduced into a container containing a proppant. The pressure difference between the container inlet and outlet is continuously monitored until the pressure difference remains stable. The changes in the flow pattern of the gas-liquid two-phase flow in the proppant and the container simulated prop fracture are observed, and the gas and liquid phase permeability of the prop fracture is calculated.

[0025] S300. After saturating the columnar structural coal sample with untreated background fluid, place the structural coal sample into the core holder and apply a certain confining pressure to the coal sample in the core holder.

[0026] S400. The gas-liquid two-phase flow from step S100 is introduced into the core holder. After passing through the core holder, the gas-liquid two-phase flow is introduced into the container in step S200. The pressure difference between the inlet and outlet of the container is continuously monitored until the pressure difference remains stable. The changes in the flow pattern of the two-phase flow in the support fracture and the process of coal powder intrusion and retention are observed. The gas and liquid permeability of the support fracture is calculated.

[0027] S500. Change the gas phase and liquid phase flow ratio in step S100, and repeat the above steps S100~S400. By comparing the characteristics of coal powder intrusion and retention in the support fracture and the induced permeability damage rate under different gas phase and liquid phase flow ratios, analyze the influence law of gas phase and liquid phase flow ratio on coal powder intrusion and retention in the support fracture, and then clarify the optimal gas phase and liquid phase flow ratio for inhibiting coal powder intrusion and retention in the support fracture.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] After saturating the coal sample with background fluid, it was placed in a core holder. A pressurizing device was used to apply pressure to the core holder to simulate the geostress environment. First, all ports of the core holder were closed, and a gas-liquid two-phase flow was output through the mixing unit. The gas-liquid two-phase flow entered the container until a stable gas-liquid two-phase flow was discharged from the container outlet. The evolution of permeability of the prop fracture under the condition of no coal dust was monitored. Then, all ports of the core holder were opened, and the path from the mixing unit directly into the container was closed. The gas-liquid two-phase flow was introduced into the core holder. After passing through the core holder, the gas-liquid two-phase flow entered the container. When the gas-liquid two-phase flow passed through the coal sample in the core holder, it eroded the coal dust in the coal sample, carrying the coal dust out of the core holder and directly into the container. The process of coal dust retention and blockage and permeability evolution in the prop fracture was monitored.

[0030] This experimental device for the intrusion and retention of coal dust in supported fractures of structural coal reservoirs adopts a series connection mode between structural coal samples and supported fracture simulation units. By using structural coal samples as the material source of intruding coal dust, it conducts simulation experiments on the intrusion and retention of coal dust in supported fractures. This avoids the problem that current methods, which only artificially set the concentration of intruding coal dust, cannot simulate the process of coal dust intruding into supported fractures from the source (structural coal). The experimental process based on this device is more scientific and rigorous, with high simulation realism. It can fully reflect the potential and rate of coal dust intrusion into supported fractures under fluid action, and accurately evaluate the characteristics and degree of supported fracture retention and blockage and coal dust-induced permeability damage. Thus, it provides a basis for the control of coal dust retention and blockage in supported fractures and the prediction of coalbed methane well production capacity. Attached Figure Description

[0031] Figure 1 This is a diagram of the experimental apparatus for the intrusion and retention of coal dust in the coal support cracks of the present invention.

[0032] Figure 2 This is a cross-sectional view of the support crack simulation unit of the present invention.

[0033] Figure 3 This is a top view of the support crack simulation unit of the present invention.

[0034] Figure 4 This is a flowchart of the experimental procedure for coal dust intrusion and retention in the coal support cracks of the present invention.

[0035] Explanation of reference numerals in the attached figures:

[0036] 1. First injection pump, 2. Gas source, 3. Second injection pump, 4. Gas flow controller, 5. First check valve, 6. Second check valve, 7. Core holder, 8. Support fracture simulation unit, 801. Transparent plate, 802. Screw, 803. Container, 804. Sealing ring, 805. Filter screen, 9. Laser scanning confocal microscope, 10. Gas-liquid separator, 11. Distillate collector, 12. Gas flow meter, 13. Processor, 14. Pressure sensor, 15. First on / off valve, 16. Third on / off valve, 17. Seventh on / off valve, 18. Second on / off valve, 19. Fourth on / off valve, 20. Fifth on / off valve, 21. Sixth on / off valve. Detailed Implementation

[0037] The specific embodiments of the present invention are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0038] like Figure 1-3 As shown, an experimental device for the intrusion and retention of coal dust in supported fractures of a structural coal reservoir includes: a mixing unit, a core holder 7, a supported fracture simulation unit 8, and monitoring equipment; the output end of the mixing unit is used to output a gas-liquid two-phase flow; a columnar structural coal sample is placed inside the core holder 7, and the inlet end of the core holder 7 is connected to the output end of the mixing unit; the periphery of the core holder 7 is connected to a pressurizing device for applying confining pressure to the core holder 7; the supported fracture simulation unit 8 includes a container 80 filled with proppant. 3. The internal space of container 803 is used to simulate fractures. Propionate is filled inside container 803. All proppant and the inner cavity of container 803 are used to simulate supporting fractures. The inlet end of container 803 is connected to the output end of the mixing unit and the outlet end of the core holder 7. Container 803 is provided with an outlet for discharging gas-liquid two-phase flow. The gas-liquid two-phase flow mixed with coal powder flows through the pores between proppant in the supporting fractures. The monitoring equipment is used to monitor the coal powder intrusion and retention process in the supporting fractures and its induced permeability evolution process.

[0039] After saturating the tectonic coal sample with background fluid for 48 hours, it was placed in the core holder 7. All ports of the core holder 7 were closed, and the mixing unit output a gas-liquid two-phase flow. The gas-liquid two-phase flow entered container 803 until a stable gas-liquid two-phase flow was discharged from the outlet of container 803. The changes in the flow pattern of the gas-liquid two-phase flow within the supporting fracture were observed, and the gas and liquid permeabilities of the supporting fracture were calculated. Then, all ports of the core holder 7 were opened, and a pressurizing device applied confining pressure to the tectonic coal sample within the core holder 7 to simulate the formation stress environment. Finally, the mixing unit was closed. The gas-liquid two-phase flow directly enters the container 803, and then enters the container 803 through the core holder 7. When the gas-liquid two-phase flow passes through the coal sample in the core holder 7, it will erode the coal powder in the coal sample, thereby carrying the coal powder out of the core holder 7 and then directly into the container 803. The changes in the flow pattern of the gas-liquid two-phase flow in the support fracture and the process of coal powder intrusion and retention are observed. The gas and liquid phase permeability of the support fracture is calculated, and the gas and liquid phase permeability damage rate is determined.

[0040] This experimental device for the intrusion and retention of coal dust in supported fractures of structural coal reservoirs employs a series connection between structural coal samples and a simulated support fracture unit. By using the structural coal samples as the material source for intruding coal dust, it conducts simulation experiments on the intrusion and retention of coal dust in supported fractures. This avoids the problem of current methods that rely solely on artificially setting the amount of coal dust, making it difficult to realistically simulate the coal dust intrusion process. The experimental process of this device is more scientific and rigorous, with high simulation realism, and can fully reflect the potential and rate of coal dust intrusion into supported fractures under fluid action. Furthermore, this invention uses visualization observation methods to present the microscopic process of coal dust intrusion and retention in supported fractures in a three-dimensional and intuitive manner. It systematically reveals the forms and mechanisms of coal dust retention and the evolution of pore throat blockage induced by it. Through real-time monitoring of the permeability of supported fractures, it quantitatively evaluates the damage characteristics and degree of damage caused by coal dust intrusion to the conductivity of supported fractures, thus providing guidance for the prevention and control of coal dust in coalbed methane wells.

[0041] Preferably, such as Figure 1-3The mixing unit includes a first injection pump 1, a gas source 2, and a gas flow controller 4. The output ends of the first injection pump 1 and the gas source 2 are both connected to the inlet end of the core holder 7 and the inlet end of the container 803. The first injection pump 1 is used to output background fluid at a constant flow rate, and the gas source 2, combined with the gas flow controller 4, is used to output gas at a constant flow rate. The mixing unit also includes a first one-way valve 5 and a second one-way valve 6. The output end of the first injection pump 1 is provided with the first one-way valve 5, which is connected to the inlet end of the core holder 7 and the container 803. The first one-way valve 5 is used to prevent the gas-liquid two-phase flow from flowing backward into the first injection pump 1. The output end of the gas source 2 is provided with the gas flow controller 4 and the second one-way valve 6, which are connected to each other. The second one-way valve 6 is connected to the inlet end of the core holder 7 and the inlet end of the container 803. The second one-way valve 6 is used to prevent the gas-liquid two-phase flow from flowing backward into the gas flow controller 4 and the gas source 2.

[0042] The first injection pump 1 outputs a stable background fluid through the first check valve 5, and the gas source 2 outputs a stable airflow through the gas flow controller 4 and the second check valve 6. During this process, the gas flow rate can be changed by adjusting the gas flow controller 4. The background fluid and gas mix to form a gas-liquid two-phase flow and enter the core holder 7 or container 803. The first check valve 5 and the second check valve 6 can prevent the gas-liquid two-phase flow from flowing backward.

[0043] Preferably, the constructed coal sample is a columnar coal sample with a diameter of ϕ25 mm × 50 mm, which is formed by cold pressing of coal powder with a particle size of less than 0.18 mm, and the coal powder is treated with a fluorescent agent for fluorescence. The background fluid is the on-site produced liquid or a 2% KCl solution. The on-site produced liquid is filtered and degassed by a filter membrane. The 2% KCl solution is prepared by deionized water and KCl. The background fluid is treated with a fluorescent agent for fluorescence, and the color of the fluorescent agent used in the background fluid can be distinguished from the color of the coal powder. The gas output from the gas source 2 is helium or nitrogen. The proppant is quartz sand or ceramsite, and the proppant particle size range is 16~20 mesh, 20~40 mesh, or 40~70 mesh.

[0044] Preferably, such as Figure 1 The pressurization device includes a second injection pump 3 and a seventh shut-off valve 17. The output end of the second injection pump 3 is connected to the confining pressure application port of the core holder 7 through the seventh shut-off valve 17.

[0045] Opening the seventh shut-off valve 17 allows pressure to be pumped into the core holder 7 via the second injection pump 3, which is used to apply confining pressure to the structural coal sample inside the core holder 7.

[0046] Preferably, such as Figure 1The core holder 7 has a first on / off valve 15 at its inlet end and a second on / off valve 18 at its outlet end. The core holder 7, the first on / off valve 15, and the second on / off valve 18 are all connected in parallel with a third on / off valve 16. One end of the third on / off valve 16 is connected to the output end of the mixing unit, and the other end of the third on / off valve 16 is connected to the inlet end of the container 803.

[0047] When the gas-liquid two-phase flow passes through the third shut-off valve 16, the first shut-off valve 15 and the second shut-off valve 18 are closed, and then the gas-liquid two-phase flow directly enters the container 803; when the gas-liquid two-phase flow passes through the core holder 7, the third shut-off valve 16 is closed, and the first shut-off valve 15 and the second shut-off valve 18 are opened at the same time, so that the tectonic coal powder stored in the core holder 7 enters the support fracture in the container 803, thereby characterizing the retention and blockage phenomenon of coal powder in the support fracture.

[0048] like Figure 1 The container 803 has a fourth shut-off valve 19 at its inlet end, the outlet of the container 803 is connected to the inlet end of the gas-liquid separator 10, the outlet of the gas-liquid separator 10 is connected to the distillation collector 11, the gas-liquid separator 10 has an exhaust port, a fifth shut-off valve 20 is provided on the pipeline between the container 803 and the gas-liquid separator 10, and a sixth shut-off valve 21 is provided on the pipeline between the gas-liquid separator 10 and the distillation collector 11.

[0049] The gas-liquid two-phase flow enters the gas-liquid separator 10 through the fifth shut-off valve 20, and the liquid phase enters the fraction collector 11 through the sixth shut-off valve 21, while the gas phase flows out through the exhaust port.

[0050] Preferably, such as Figure 1 The monitoring equipment includes a laser scanning confocal microscope 9, a pressure sensor 14, a gas flow meter 12, and a processor 13. The laser scanning confocal microscope 9 is used to observe and characterize the coal dust intrusion and retention process in the support fracture in real time. The pressure sensor 14 is used to detect the inlet pressure of the container 803. The gas flow meter 12 is used to detect the gas flow rate discharged from the gas-liquid separator 10. At the same time, the laser scanning confocal microscope 9, the pressure sensor 14, and the gas flow meter 12 send the measured images, pressure, and flow signals to the processor 13, respectively. The processor 13 can obtain the coal dust intrusion and retention characteristics in the support fracture by processing the image signals, including the coal dust retention form, retention location, and the evolution law of coal dust-induced pore throat blockage. The processor 13 can obtain the gas phase and liquid phase permeability of the support fracture by processing the pressure and flow signals.

[0051] A laser scanning confocal microscope 9 was used to perform three-dimensional imaging at different locations along the flow direction of the support crack. Specifically, 100 images were acquired vertically at different locations, with an interval of 3-5 μm between each image. The processor 13 received the image signals continuously captured by the laser scanning confocal microscope 9 at different locations. Based on the different fluorescence characteristics of the background solution, coal powder, and proppant, the pore structure and the gas-water-coal powder three-phase distribution inside the support crack at different locations were reconstructed in three dimensions. Then, the coal powder retention form, retention location, and the evolution law of pore throat blockage induced by coal powder retention in the support crack were analyzed.

[0052] When no coal dust intrudes into the supporting fracture, the gas-liquid two-phase flow output from the mixing unit passes sequentially through the third shut-off valve 16, the fourth shut-off valve 19, container 803, and the fifth shut-off valve 20, and finally enters the gas-liquid separator 10 to achieve separation of the gas and liquid phases. At this time, the gas phase permeability and liquid phase permeability of the supporting fracture in container 803 are respectively:

[0053]

[0054] In the formula: K ge and K we These represent the gas-phase and liquid-phase permeabilities, respectively, without coal powder intrusion, in m. 2 ; p 1 and p a These are the inlet and outlet pressures of container 803 when there is no coal dust intrusion, respectively, in Pa; q g and q w The values ​​are the gas flow rate measured by gas flow meter 12 and the liquid flow rate output by the first injection pump 1 when there is no coal powder intrusion, respectively, in m. 3 / s; μ g and μ w The viscosity of the gas phase and liquid phase are respectively, in Pa·s; L To support the crack length, m; A To support the cross-sectional area of ​​the crack, m 2 ;

[0055] When pulverized coal intrudes, the gas-liquid two-phase flow output from the mixing unit sequentially passes through the first shut-off valve 15, the core holder 7, the second shut-off valve 18, the fourth shut-off valve 19, the container 803, and the fifth shut-off valve 20, and finally enters the gas-liquid separator 10 to achieve gas-liquid phase separation. At this time, the gas phase permeability and liquid phase permeability of the supporting fractures in the container 803 are respectively:

[0056]

[0057] In the formula: and These represent the gas-phase and liquid-phase permeability when pulverized coal has intruded, in m. 2 ; and These are the inlet and outlet pressures (in Pa) of container 803 when pulverized coal has entered; and The values ​​are the gas flow rate measured by gas flow meter 12 when pulverized coal intrusion occurs and the liquid flow rate output by the first injection pump 1, respectively, in m. 3 / s; μ g and μ w The viscosity of the gas phase and liquid phase are respectively, in Pa·s; L To support the crack length, m; A To support the cross-sectional area of ​​the crack, m 2 ;

[0058] Meanwhile, the gas phase and liquid phase permeability damage rates of the supporting fractures were obtained as follows:

[0059]

[0060] In the formula: D g and D w These are the gas phase and liquid phase permeability damage rates, respectively, dimensionless; K ge and K we These represent the gas-phase and liquid-phase permeabilities, respectively, without coal powder intrusion, in m. 2 ; and These represent the gas-phase and liquid-phase permeability when pulverized coal has intruded, in m. 2 .

[0061] During the experiment, the flow ratio of the gas phase to the liquid phase was changed by adjusting the first displacement pump 1, the gas source 2, and the gas flow controller 4. By injecting two-phase flows with different gas-liquid flow ratios into the core holder 7, coal dust of different degrees was induced to infiltrate into the container 803, thereby observing and verifying the coal dust retention and blockage phenomenon in the support fracture. At the same time, based on the coal dust intrusion and retention experiments under different gas-liquid flow ratios, the relationship curves of gas and liquid phase permeability damage rates with the gas-liquid flow ratio of the support fracture can also be obtained. The two-phase flow conditions corresponding to the lowest gas and liquid phase permeability damage rates are the seepage conditions when coal dust is least likely to be retained in the support fracture. The optimal seepage conditions obtained from the experiment can guide the optimization of coal dust control and drainage system in the two-phase flow stage of coalbed methane wells, thereby improving the gas production of coalbed methane wells.

[0062] This invention studies the intrusion and retention behavior of pulverized coal in propped fractures under different gas-liquid two-phase flow conditions, revealing the influence of gas-liquid ratio, apparent gas-liquid velocity, and two-phase flow pattern on pulverized coal intrusion and retention. Using the gas and liquid phase permeability damage rate in propped fractures as evaluation indicators, the optimal combination of gas and liquid flow velocities for inhibiting pulverized coal retention and blockage in propped fractures is determined.

[0063] Preferably, the support crack simulation unit 8 further includes a transparent plate 801, a sealing ring 804, two filters 805, and multiple screws 802. The container 803 has an opening, and a sealing ring 804 surrounds the opening on the end face of the opening. The transparent plate 801 is fixed to the opening end face of the container 803 by multiple screws 802 to seal the container 803. At the same time, the process of coal dust intrusion and retention in the support crack is observed through the transparent plate 801 using a laser scanning confocal microscope 9. The two filters 805 are respectively set at the inlet end and outlet end on the inner wall of the container 803. The size of the filters allows coal dust to pass freely while inhibiting the passage of proppant.

[0064] An experimental method for the retention of coal dust in supported fractures in a structural coal reservoir, with reference to Figure 4 Includes the following steps:

[0065] S100. Prepare gas and background fluid treated with fluorescent agent, and mix the gas and background fluid into a stable gas-liquid two-phase flow at a certain flow rate ratio; construct coal sample by pressing coal powder by cold pressing, and the coal powder is treated with fluorescent agent, and the color of the fluorescent agent used in the background fluid can be distinguished from the color of the coal powder.

[0066] S200. The gas-liquid two-phase flow from step S100 is introduced into a container 803 containing a proppant. The pressure difference between the inlet and outlet of the container 803 is continuously monitored until the pressure difference remains stable. The changes in the flow pattern of the gas-liquid two-phase flow in the proppant and the simulated prop fracture in the container 803 are observed by laser scanning confocal microscope, and the gas phase permeability and liquid phase permeability of the prop fracture are calculated.

[0067] S300. After saturating the columnar structural coal sample with untreated background fluid, place the structural coal sample into the core holder 7 and apply a certain confining pressure to the coal sample in the core holder 7.

[0068] S400. The gas-liquid two-phase flow from step S100 is introduced into the core holder 7. After passing through the core holder 7, the gas-liquid two-phase flow is introduced into the container 803 in step S200. The pressure difference between the inlet and outlet of container 803 is monitored until the pressure difference remains stable. The changes in the flow pattern of the gas-liquid two-phase flow and the coal powder intrusion and retention process in the support fracture are observed by laser scanning confocal microscope, and the gas phase permeability and liquid phase permeability of the support fracture are calculated.

[0069] S500. Change the gas phase and liquid phase flow ratio in step S100, and repeat steps S100 to S400. By comparing the characteristics of coal powder intrusion and retention in the propped fractures and the induced permeability damage rate under different gas phase and liquid phase flow ratios, analyze the influence of the gas phase and liquid phase flow ratio on coal powder intrusion and retention in the propped fractures, and then clarify the optimal gas phase and liquid phase flow ratio to suppress coal powder intrusion and retention in the propped fractures. Use the experimental results to guide the drainage of coalbed methane wells.

[0070] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A device for constructing an experiment for investigating the invasion of coal fines into a propped fracture in a coal reservoir, characterized in that, The application relates to a device for simulating the invasion and retention of coal powder in a propped fracture, which comprises the following parts: a mixing unit with an output end for outputting a gas-liquid two-phase flow; a core holder (7) for placing a columnar tectonic coal sample inside, wherein the inlet end of the core holder (7) is communicated with the output end of the mixing unit, the gas-liquid two-phase flow flushes the tectonic coal sample to generate coal powder, and the periphery of the core holder (7) is communicated with a pressurizing device for applying a confining pressure to the core holder (7); a propped fracture simulation unit (8) comprising a container (803) filled with proppants inside, wherein the internal space of the container (803) is used for simulating a fracture, the proppants are filled in the container (803), all the proppants and the inner cavity of the container (803) are used for simulating a propped fracture, the inlet end of the container (803) is communicated with the output end of the mixing unit and the outlet end of the core holder (7), the inlet end of the core holder (7) is provided with a first on-off valve (15), the outlet end of the core holder (7) is provided with a second on-off valve (18), the core holder (7), the first on-off valve (15) and the second on-off valve (18) are all connected in parallel with a third on-off valve (16), one end of the third on-off valve (16) is communicated with the output end of the mixing unit, the other end of the third on-off valve (16) is communicated with the inlet end of the container (803), the container (803) is provided with an outlet for discharging the gas-liquid two-phase flow, and the gas-liquid two-phase flow mixed with the coal powder flows through the pores between the proppants in the propped fracture; a monitoring device for monitoring the invasion and retention of the coal powder in the propped fracture and the evolution process of the permeability, wherein the monitoring device comprises a laser scanning confocal microscope (9), a pressure sensor (14), a gas flow meter (12) and a processor (13), the laser scanning confocal microscope (9) is used for observing the invasion and retention process of the coal powder in the propped fracture and continuously taking pictures of the invasion and retention process to obtain images, the pressure sensor (14) is used for detecting the pressure of the inlet end of the container (803), the gas flow meter (12) is used for detecting the flow of the discharged gas in a gas-liquid separator (10), and the laser scanning confocal microscope (9), the pressure sensor (14) and the gas flow meter (12) can respectively send the measured image, pressure and flow signals to the processor (13), the processor (13) can obtain the invasion and retention characteristics of the coal powder in the propped fracture by processing the image signals, the invasion and retention characteristics include the retention form, retention position and evolution law of the pore throat blockage induced by the coal powder, and the processor (13) can obtain the gas phase and liquid phase permeabilities of the propped fracture by processing the pressure and flow signals.

2. The apparatus of claim 1, wherein the coal reservoir constructed with the coal powder invasion and retention experiment device is characterized by, The mixing unit comprises a first injection pump (1), a gas source (2) and a gas flow controller (4), the output ends of the first injection pump (1) and the gas source (2) are communicated with the inlet ends of the core holder (7) and the container (803), the first injection pump (1) is used for outputting a background fluid at a constant flow, and the gas source (2) is combined with the gas flow controller (4) and used for outputting gas at a constant flow.

3. The apparatus of claim 1, wherein the coal reservoir constructed with the coal powder invasion and retention experiment device is characterized by, The inlet end of the container (803) is provided with a fourth on-off valve (19), the outlet end of the container (803) is communicated with the inlet end of the gas-liquid separator (10), the liquid outlet end of the gas-liquid separator (10) is communicated with the fraction collector (11), the gas-liquid separator (10) is provided with a gas outlet, the pipeline between the container (803) and the gas-liquid separator (10) is provided with a fifth on-off valve (20), and the pipeline between the gas-liquid separator (10) and the fraction collector (11) is provided with a sixth on-off valve (21).

4. The apparatus of claim 1, wherein the coal reservoir constructed with the coal powder invasion and retention experiment device is characterized by, The pressurizing device comprises a second injection pump (3) and a seventh on-off valve (17), and the output end of the second injection pump (3) is communicated with the confining pressure application port of the core holder (7) through the seventh on-off valve (17).

5. The apparatus of claim 3, wherein the coal reservoir constructed with the support fractures of the coal powder invasion and retention experiment device is characterized by, When there is no coal powder invasion, the gas-liquid two-phase flow output by the mixing unit successively passes through the third on-off valve (16), the fourth on-off valve (19), the container (803) and the fifth on-off valve (20), and finally enters the gas-liquid separator (10) to realize the separation of the gas phase and the liquid phase, and at this time, the gas phase and the liquid phase permeability of the supporting fracture are: wherein: K ge and K we are the gas and liquid phase permeability without coal fines invasion, m 2 ; p 1and p a are the inlet and outlet pressure of the vessel (803) without coal fines invasion, Pa; q g and q w are the gas flow measured by the gas flow meter (12) and the liquid phase flow output by the first injection pump (1) without coal fines invasion, m 3 / s; μ g and μ w are the gas and liquid phase viscosity, Pa s; L is the propped fracture length, m; A to support the fracture cross-sectional area, m 2 ; When there is coal powder invasion, the gas-liquid two-phase flow output by the mixing unit successively passes through the first on-off valve (15), the core holder (7), the second on-off valve (18), the fourth on-off valve (19), the container (803) and the fifth on-off valve (20), and finally enters the gas-liquid separator (10) to realize the separation of the gas phase and the liquid phase, and at this time, the gas phase and the liquid phase permeability of the supporting fracture are: wherein: and are the gas and liquid phase permeability with coal powder invasion, m 2 ; and are the inlet and outlet pressure of the vessel (803) with coal powder invasion, Pa; and are the gas flow measured by the gas flow meter (12) and the liquid phase flow output by the first injection pump (1) with coal powder invasion, m 3 / s; μ g and μ w are the gas and liquid phase viscosity, Pa·s; L is the propped fracture length, m; A to support the fracture cross-sectional area, m 2 ; At the same time, the gas phase and the liquid phase permeability damage rates of the supporting fracture are respectively: where: D g and D w are the gas and liquid phase permeability impairment ratios, dimensionless, respectively; K ge and K we are the gas and liquid phase permeability without coal powder invasion, m 2 . and are the gas and liquid phase permeability with coal powder invasion, m 2 .

6. The apparatus of claim 1, wherein the coal reservoir constructed fracture coal fines invasion retention experiment device is characterized by, The supporting fracture simulation unit (8) further comprises a transparent plate (801), a sealing ring (804), two filter screens (805) and a plurality of screws (802), the container (803) is provided with an opening, the end face of the opening is provided with the sealing ring (804) surrounding the opening, the transparent plate (801) is fixed at the opening end face of the container (803) through the plurality of screws (802), and the container (803) is sealed; meanwhile, the coal powder invasion and retention process in the supporting fracture is observed by using a laser scanning confocal microscope (9) through the transparent plate (801), and the two filter screens (805) are arranged at the inlet end and the outlet end on the inner wall of the container (803).

7. The apparatus of claim 2, wherein the coal reservoir constructed fracture coal fines invasion retention experiment device is characterized by, The mixing unit further comprises a first one-way valve (5) and a second one-way valve (6), the output end of the first injection pump (1) is provided with the first one-way valve (5), the first one-way valve (5) is communicated with the inlet end of the core holder (7) and the container (803), and the first one-way valve (5) is used for preventing the gas-liquid two-phase flow from flowing reversely into the first injection pump (1); the output end of the gas source (2) is sequentially provided with a gas flow controller (4) and the second one-way valve (6) which are communicated with each other, the second one-way valve (6) is communicated with the inlet end of the core holder (7) and the container (803), and the second one-way valve (6) is used for preventing the gas-liquid two-phase flow from flowing reversely into the gas flow controller (4) and the gas source (2).

8. A method for constructing a coal reservoir proppant fines invasion retention experiment, using the experimental device according to any one of claims 1 to 7, characterized in that, The method comprises the following steps: S100, prepare gas and background fluid treated by fluorescent agent, mix the gas and background fluid into stable gas-liquid two-phase flow at a certain flow ratio; construct the coal sample by cold pressing the coal powder, and the coal powder is treated by fluorescent agent, and the color of the fluorescent agent used in the background fluid can be distinguished from the color of the coal powder; S200, pass the gas-liquid two-phase flow in step S100 into the container (803) with proppant, continuously monitor the pressure difference between the inlet end and the outlet of the container (803) until the pressure difference remains stable, observe the change of the gas-liquid two-phase flow pattern in the propped fracture simulated by the container (803), and calculate the gas phase and liquid phase permeability of the propped fracture; S300, after saturating the columnar tectonic coal sample with background fluid not treated by fluorescent agent, place the tectonic coal sample into the core holder (7), and apply a certain confining pressure to the tectonic coal sample in the core holder (7); S400, pass the gas-liquid two-phase flow in step S100 into the core holder (7), and pass the gas-liquid two-phase flow after passing through the core holder (7) into the container (803) in step S200, monitor the pressure difference between the inlet end and the outlet of the container (803) until the pressure difference remains stable, observe the change of the gas-liquid two-phase flow pattern in the propped fracture and the coal powder invasion and retention process, and calculate the gas phase and liquid phase permeability of the propped fracture; S500, change the gas phase and liquid phase flow ratio in step S100, repeat steps S100-S400, compare the coal powder invasion and retention characteristics and induced permeability damage rate in the propped fracture under different gas phase and liquid phase flow ratios, analyze the influence law of the gas phase and liquid phase flow ratio on the coal powder invasion and retention in the propped fracture, and then determine the optimal gas phase and liquid phase flow ratio to inhibit the coal powder invasion and retention in the propped fracture.

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