Multi-element multi-phase fluid displacement coal bed gas 4D visual simulation experiment system and multi-element multi-phase fluid displacement coal bed gas 4D visual simulation experiment method

Abstract

The invention discloses a multielement multiphase fluid displacement coalbed methane 4D visual simulation experiment system and a multielement multiphase fluid displacement coalbed methane 4D visual simulation experiment method. The system comprises a gas-liquid mixed injection system, a Kr adsorption system, a visual triaxial core holder system, a CT scanning system and a tail gas analysis and treatment system. According to the experimental method, Kr similar to CH4 in dynamic diameter and other parameters is selected as a CT tracer to replace CH4, and 4D visualization of coal body internal pore and fracture structure evolution and Kr concentration field displacement migration process can be realized through three-dimensional reconstruction of CT images at different displacement moments and registration, gray correction and normalized concentration calculation of the images. The problem that the gas migration rule in the coal body in the displacement experiment is not clear is solved; the dynamic diffusion process inversion of the coal bed gas in the coal bed difficult to extract is realized by utilizing the distribution characteristic evolution of the Kr gas in the coal body, so that the in-situ coal body fissure structure in the coal sample and the dynamic migration law of methane in the displacement process are obtained, and the method has great significance for revealing the displacement development mechanism of unconventional natural gas such as the coal bed gas.

Description

Technical Field

[0001] This invention relates to the field of visualization simulation experiment technology for coalbed methane extraction, specifically to a 4D visualization simulation experiment system and method for multi-component, multiphase fluid displacement of coalbed methane. Background Technology

[0002] China has abundant geological resources of coalbed methane, with total reserves estimated at approximately 80 trillion cubic meters. 3 Coalbed methane (CBM) buried at depths exceeding 1500 meters is generally referred to as deep CBM, accounting for approximately 75% of total reserves and possessing enormous development potential. However, deep coal seams are characterized by relatively high geothermal temperature, formation pressure, and geostress, along with relatively low permeability, significantly increasing the difficulty of CBM development. Therefore, it is essential to adopt relevant technologies to enhance CBM development. The technology of injecting multi-component, multi-phase fluids such as CO2, N2, CO2 / N2 mixtures, flue gas, and superheated steam into the coal seam to displace CBM has achieved good results. The injected media in this displacement technology are mostly multi-component, multi-phase fluids, which may be gaseous, liquid, or supercritical, and some are high-temperature fluids carrying a large amount of energy. While improving the connectivity of reservoir pores and fractures, this displacement induces the desorption and migration of methane in the coal seam, achieving the purpose of increasing permeability and extraction. This has become a research hotspot in enhanced CBM development technology and an important technology in the field of enhanced underground coalbed methane extraction. The process of displacing methane in coalbed methane using multi-component and multi-phase fluids is very complex. The efficiency of displacement and production enhancement is affected by multiple parameters such as temperature, pressure, composition, flow rate, reservoir structure, and reservoir water cut. Therefore, conducting displacement simulation experiments in the laboratory to study the intrinsic factors and mechanisms affecting displacement efficiency and obtaining corresponding index parameters is of great significance for coalbed methane extraction engineering.

[0003] Traditional displacement experiments involve placing a coal sample into a holder and changing parameters such as the injection fluid pressure, temperature, and composition, while simultaneously monitoring the outlet gas production to obtain the dynamic gas production patterns under different injection parameters. Because displacement occurs within the coal sample inside the holder, the experimental process is like a black box; it's impossible to obtain the dynamic migration patterns of the in-situ coal mass and methane within the coal sample during displacement. Traditional CT in-situ testing methods can only obtain changes in the coal sample structure, not the dynamic migration patterns of methane during displacement, significantly limiting in-depth research on the mechanism of methane displacement in coal seams by multi-component, multi-phase fluids.

[0004] Therefore, the research direction required by this invention is to provide a new displacement test system and method that can obtain the in-situ coal body fracture structure and dynamic migration law of methane inside the coal sample during the displacement process, so as to provide data support for revealing the mechanism of coalbed methane displacement technology and actual coalbed methane mining. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a 4D visualization simulation experimental system and method for multi-component, multiphase fluid displacement of coalbed methane. It utilizes the evolution of the distribution characteristics of tracer Kr gas, which has a diameter extremely similar to that of CH4 molecules, within the coal body to invert the dynamic diffusion process of coalbed methane in difficult-to-extract coal seams. This allows for the acquisition of the in-situ coal body fracture structure and the dynamic migration law of methane within the coal sample during the displacement process, thus providing data support for revealing the mechanism of coalbed methane displacement technology and for actual coalbed methane extraction.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is: a multi-component, multi-phase fluid displacement coalbed methane 4D visualization simulation experimental system, including a gas-liquid injection system, a Kr adsorption system, a visual triaxial core holder system, a CT scanning system and a gas analysis and processing system. The gas-liquid mixing system includes a multi-phase fluid mixing system and a temperature- and pressure-controlled quantitative injection system. The multi-phase fluid mixing system includes N2 cylinders, CO2 cylinders, water vapor cylinders, other displacement component gas cylinders, and a gas mixer. The N2 cylinders, CO2 cylinders, water vapor cylinders, and other displacement component gas cylinders are connected to the gas mixer via pipelines. The gas mixer is used to mix the gases from each cylinder to generate a multi-phase fluid. The temperature- and pressure-controlled quantitative injection system includes an air compressor, a temperature-controlled quantitative injector, a pressure gauge, a gas thermometer, and a gas volumetric flow meter. The air compressor's inlet is connected to a gas mixer, and its outlet is connected to a temperature-controlled quantitative injector. This injector pressurizes the multi-phase fluid and delivers it to the temperature-controlled quantitative injector. The temperature-controlled quantitative injector is connected to a visual triaxial core holder system via a pipeline to control the injection flow rate into the system. A pressure gauge, a gas thermometer, and a gas volumetric flow meter are sequentially connected to the pipeline between the temperature-controlled quantitative injector and the visual triaxial core holder system to monitor the temperature, pressure, and flow rate of the multi-phase, multi-medium gas after adjustment by the temperature-controlled quantitative injector. The visible triaxial core holder system includes an X-ray transparent triaxial core holder, a hydraulic constant-speed and constant-pressure pump, and electric heating paper. The X-ray transparent triaxial core holder has an internal space for placing the coal sample. The coal sample is placed within this space. The upper part of the X-ray transparent triaxial core holder has a slip ring joint, and the lower part is equipped with a rotating base, allowing the X-ray transparent triaxial core holder to rotate around its axis for performing micron-level CT scans of the coal sample at multiple angles. The hydraulic constant-speed and constant-pressure pump is connected to the X-ray transparent triaxial core holder and is used to pump hydraulic oil into the X-ray transparent triaxial core holder to apply confining pressure to the coal sample inside and to control the axial pressure applied by the upper and lower pressure heads. The electric heating paper is evenly wrapped around the outer wall of the X-ray transparent triaxial core holder to uniformly heat and maintain the temperature of the coal sample. The Kr adsorption system includes a Kr gas cylinder and a vacuum pump. The Kr gas cylinder is connected to the air inlet of an air compressor via a pipeline, and is used to deliver Kr gas into the X-ray transparent triaxial core holder via the air compressor, so that the coal sample in the X-ray transparent triaxial core holder reaches an adsorption state with a certain Kr content under specific temperature and pressure to simulate the in-situ coal seam occurrence conditions. The vacuum pump is connected to the X-ray transparent triaxial core holder via a pipeline, and is used to extract the gas in the X-ray transparent triaxial core holder. The CT scanning system includes an X-ray source, a receiving screen, and a computer. The X-ray source and the receiving screen are located on both sides of the main body of the X-ray transparent triaxial core holder, so that the X-rays emitted by the X-ray source pass through the coal sample and are received by the receiving screen. This is used to scan the internal spatial structure and Kr migration state of the coal sample inside the X-ray transparent triaxial core holder in real time. The computer is connected to both the X-ray source and the receiving screen. It is used to control the frequency of the X-ray emission source and automatically record the X-ray intensity at different projection angles. After calculation and reconstruction, a three-dimensional tomographic scan image is obtained. The gas analysis and processing system includes a multi-range volumetric flow meter, a high-precision gas chromatograph, and a gas collection device. One end of the high-precision gas chromatograph is connected to an X-ray transparent triaxial core holder via a pipeline, and the other end is connected to the gas collection device. It is used to monitor and analyze the gas composition discharged from the X-ray transparent triaxial core holder in real time, and the gas is discharged into the gas collection device for collection after monitoring. The multi-range volumetric flow meter is installed on the pipeline between the high-precision gas chromatograph and the X-ray transparent triaxial core holder, and is used to monitor the real-time gas flow rate discharged from the X-ray transparent triaxial core holder.

[0007] Furthermore, a first check valve is installed on the pipeline between the Kr gas cylinder and the air compressor; a second check valve is installed on the pipeline between the N2 gas cylinder and the gas mixer; a third check valve is installed on the pipeline between the CO2 gas cylinder and the gas mixer; a fourth check valve is installed on the pipeline between the other displacement component gas cylinders and the gas mixer; a fifth check valve is installed on the pipeline between the water vapor cylinder and the gas mixer; an inlet valve is installed on the pipeline between the temperature-controlled quantitative injector and the X-ray transparent triaxial core holder; a sixth check valve is installed on the pipeline between the vacuum machine and the X-ray transparent triaxial core holder; and an outlet valve is installed on the pipeline between the X-ray transparent triaxial core holder and the high-precision gas chromatograph.

[0008] Furthermore, a pressure gauge and a thermometer are installed on the pipeline between the X-ray transparent triaxial core holder and the high-precision gas chromatograph to monitor the real-time pressure and temperature of the gas discharged from the X-ray transparent triaxial core holder; the multi-range volumetric flow meter includes a low-range volumetric flow meter, a medium-range volumetric flow meter, and a high-range volumetric flow meter. The low-range volumetric flow meter and the seventh check valve are connected in series to form the first branch, the medium-range volumetric flow meter and the eighth check valve are connected in series to form the second branch, and the high-range volumetric flow meter and the ninth check valve are connected in series to form the third branch; the first branch, the second branch, and the third branch are arranged in parallel.

[0009] Furthermore, the X-ray transparent triaxial core holder is made of X-ray transparent materials, such as high-strength carbon fiber, PEEK, and other polymer materials.

[0010] Furthermore, it also includes an intelligent temperature control system, which is connected to the electric heating paper to control the electric heating paper to heat the coal sample evenly, simulating the in-situ temperature environment of deep coal seams.

[0011] The working method of the above-mentioned multi-phase fluid displacement coalbed methane 4D visualization simulation experimental system is as follows: A. Preparation of Water-Saturated Coal Samples and Construction of the Experimental System: After collecting coal from deep coal seams, the coal body is cut into several core columns of the same size, and the surface of the cores is cleaned to form individual coal samples. Then, a borehole is drilled at the center of one end face of each coal sample for subsequent injection of multi-component and multi-phase fluids. The natural water absorption rate of the coal samples is tested to study the change law of coal sample moisture content with immersion time. By controlling the immersion time, the moisture content of the coal samples is adjusted to simulate the real water content state of the coal seam in situ. Select any water-absorbing coal sample, attach a wearable rubber sleeve to the surface of the coal sample, place it in an X-ray transparent triaxial core holder, and complete the assembly of each system. B. Simulation of in-situ temperature and pressure parameters of coal seam: Based on the deep geostress and temperature environment to be simulated, set the axial pressure, confining pressure, and temperature values; start the hydraulic constant speed and constant pressure pump of the visible triaxial core holder system to pump hydraulic oil, so that the X-ray transparent triaxial core holder applies axial pressure and confining pressure to the coal sample; at the same time, start the intelligent temperature control system to uniformly heat the coal sample with electric heating paper until the set axial pressure, confining pressure, and temperature values ​​are reached, then stop heating and pressurization, and maintain the current pressure and temperature; then start the CT scanning system to perform a CT scan on the coal sample in the current state, and control the rotation of the X-ray transparent triaxial core holder during the CT scan, so as to complete a micron CT scan of the coal sample to record the original pore structure state of the coal sample before Kr adsorption injection; C. In-situ Coal Seam Gas Content Adsorption Simulation: Kr gas is used to replace CH4 to simulate the initial CH4 content in deep coal seams. Specifically, the vacuum machine is turned on and the valves of each pipeline are opened to evacuate the internal pipelines of the gas-liquid mixing system, Kr adsorption system, visual triaxial core holder system, CT scanning system, and gas analysis and processing system. After evacuation, the outlet valve of the X-ray transparent triaxial core holder is closed, while the inlet valve remains open. The Kr gas pressure and temperature values ​​are set, and based on the known Langmuir correction formula, combined with isothermal adsorption and industrial analysis data, the amount of Kr required for the test coal sample to reach the predetermined gas content under the set temperature and pressure conditions is calculated. Then, the gas is sequentially passed through air pressure... A compressor and a temperature-controlled quantitative injector inject a fixed amount of Kr gas at a constant pressure and temperature into the X-ray transparent triaxial core holder. When the set pressure value is reached, the inlet valve is closed, and a certain period of time is waited for the coal sample to adsorb the Kr gas. During the adsorption period, the internal structure of the coal sample is scanned in real time using a CT in-situ scanning system at regular intervals. Combined with the gas pressure change at the main inlet of the X-ray transparent triaxial core holder, dynamic visualization monitoring of the internal structure of the coal sample and the Kr adsorption state during the Kr adsorption process is achieved. After the Kr gas reaches adsorption equilibrium, the internal structure of the coal sample is scanned again, thus completing another micron CT scan of the coal sample to obtain the internal structure of the coal sample in the Kr adsorption state. D. Simulation of the Kr process for high-temperature multi-component gas displacement of coal samples: The proportions, temperatures, and injection pressures of various gases in the displacement medium are set. The inlet and outlet valves are kept open. CO2, N2, other displacement components, and water vapor are mixed according to the set proportions using a gas mixer and pressurized to the set injection pressure by an air compressor. Then, the mixture is heated to the set temperature by a temperature-controlled quantitative injector, thereby generating the required high-temperature multi-component multiphase fluid. This fluid is then quantitatively injected into an X-ray transparent triaxial core holder for displacement. During the high-temperature multi-component multiphase fluid displacement, the coal sample is subjected to real-time in-situ micron-CT scans from multiple angles to obtain data on the multi-component multiphase fluid. During the displacement of Kr in the coal sample, the internal flow field, pore and fracture structure characteristics, and damage patterns were observed. Simultaneously, a high-precision gas chromatograph and a multi-range volumetric flow meter were used to monitor and record the gas composition, concentration, and flow rate in real time. This allowed for real-time acquisition of the high-temperature multiphase fluid injection rate, Kr gas extraction rate, and the interaction between the high-temperature multiphase fluid and Kr gas during the displacement of Kr in the X-ray transparent triaxial core holder. The displacement process was stopped when the Kr concentration in the discharged gas reached the critical Kr concentration. Finally, a micron-scale CT scan was performed on the internal spatial structure of the coal sample at this point to obtain the internal flow field, pore and fracture structure characteristics, and damage patterns after the displacement was completed. E. Image and Data Analysis: Reconstruct each CT scan image to form a three-dimensional model of the coal sample and its internal pore and fracture structure at each CT scan time. Then, combine each three-dimensional model with its respective scan time point to intuitively characterize the pore and fracture evolution process over time, revealing the spatiotemporal evolution law of the synergistic effect of temperature and chemical reaction on the three-dimensional mineral dissolution and pore and fracture structure inside the coal body under the action of high-temperature multi-element gas. Register, grayscale correct, and normalize the concentration calculation of CT images at different displacement stages to obtain the spatiotemporal evolution law of Kr concentration field evolution inside the coal body, realizing 4D visualization of the Kr migration process inside the coal body during displacement. Subsequently, compare the CT scan results with the monitoring results of outlet gas flow rate and composition to comprehensively analyze the main controlling factors and internal mechanisms affecting the high-temperature multi-element gas displacement effect and gas production efficiency.

[0012] Furthermore, the wearable rubber sleeve is made of high-temperature resistant and corrosion-resistant materials and is used to cushion and protect the coal sample.

[0013] Compared with existing technologies, the inventors have discovered that Kr gas is an adsorbent gas very similar to methane, with the limiting adsorption capacity and adsorption rate in coal differing by no more than 10%. Furthermore, Kr and methane share very similar parameters such as molecular dynamic diameter, van der Waals radius, Lennard-Jones potential constant, and polarizability. Since gas transport and diffusion rates depend to a considerable extent on molecular dynamic diameter, Kr and methane exhibit very similar storage and transport characteristics within porous rocks such as coal. Additionally, Kr gas exhibits excellent X-ray absorption characteristics, enabling the acquisition of high-contrast CT scan images. Based on these advantages, this invention combines a gas-liquid mixing system, a Kr adsorption system, a visual triaxial core holder system, a CT scanning system, and a gas analysis and processing system, offering the following benefits: This invention selects Kr, whose physical parameters are very close to those of CH4 molecules, such as molecular dynamics diameter and van der Waals radius, to replace CH4 adsorbed coal samples in order to simulate the original gas content of coal seams. By utilizing Kr's excellent X-ray absorption characteristics in conjunction with high-precision micron-CT, high-contrast CT scan images are obtained. That is, in the images obtained by CT scan during the displacement experiment, the areas where Kr gas is present will be highlighted. In this way, combined with the subsequent three-dimensional reconstruction of the images, the evolution of pores and fractures and the methane migration process during the displacement of coalbed methane by multi-component and multi-phase fluids can be clearly and intuitively simulated. This enables 4D visualization research on the dynamic migration law of methane inside the coal body during the displacement process. Furthermore, the scheme of this invention helps to deeply reveal the intrinsic principles and mechanisms of multi-component and multi-phase fluid displacement of coalbed methane. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the overall structure of the experimental system of the present invention.

[0015] In the diagram: 1-Kr gas cylinder, 2-1-First check valve, 2-2-Second check valve, 2-3-Third check valve, 2-4-Fourth check valve, 2-5-Fifth check valve, 2-6-Inlet valve, 2-7-Sixth check valve, 2-8-Outlet valve, 2-9-Seventh check valve, 2-10-Eighth check valve, 2-11-Ninth check valve, 3-Gas mixer, 4-Temperature-controlled metering injector, 5-Air compressor, 6-Pressure gauge, 7-Thermometer, 8-Gas volumetric flow meter, 8-1-High-flow-rate gas volumetric flow meter, 8- 2-Medium flow gas volumetric flow meter, 8-3-Low flow gas volumetric flow meter, 9-Constant pressure and constant flow pump, 10-Steam injection wellhead, 11-Coal sample, 12-Outer wall, 13-Hydraulic oil, 14-Wearable rubber sleeve, 15-Heating paper, 16-Rotating base, 17-X-ray source, 18-Receiver screen, 19-Computer, 20-Gas chromatograph, 21-Gas collection device, 22-N2 gas cylinder, 23-CO2 gas cylinder, 24-Other displacement component gas cylinders, 25-Water vapor cylinder, 26-Intelligent temperature control system, 27-Switch. Detailed Implementation

[0016] The present invention will be further described below.

[0017] like Figure 1 As shown, a multi-phase fluid displacement coalbed methane 4D visualization simulation experimental system includes a gas-liquid injection system, a Kr adsorption system, a visual triaxial core holder system, a CT scanning system, and a gas analysis and processing system. The gas-liquid mixing system includes a multi-phase fluid mixing system and a temperature- and pressure-controlled quantitative injection system. The multi-phase fluid mixing system includes an N2 cylinder 22, a CO2 cylinder 23, a water vapor cylinder 25, other displacement component gas cylinders 24, and a gas mixer 3. The N2 cylinder, CO2 cylinder 23, water vapor cylinder 25, and other displacement component gas cylinders 24 are connected to the gas mixer 3 via pipelines. The gas mixer 3 is used to mix the gases from each cylinder to generate a multi-phase fluid. The temperature- and pressure-controlled quantitative injection system includes an air compressor 5, a temperature-controlled quantitative injector 4, a pressure gauge 6, a gas thermometer 7, and a gas... The volumetric flow meter 8 and the air compressor 5 are connected to the gas mixer 3 at the air inlet and the temperature-controlled quantitative injector 4 at the air outlet. The air compressor 5 is used to pressurize the multi-phase fluid and deliver it to the temperature-controlled quantitative injector 4. The temperature-controlled quantitative injector 4 is connected to the visual triaxial core holder system via a pipeline to control the injection flow rate into the visual triaxial core holder system. The pressure gauge 6, gas thermometer 7, and gas volumetric flow meter 8 are sequentially connected to the pipeline between the temperature-controlled quantitative injector 4 and the visual triaxial core holder system to monitor the temperature, pressure, and flow rate of the multi-phase, multi-medium gas after adjustment by the temperature-controlled quantitative injector 4. The visible triaxial core holder system includes an X-ray transparent triaxial core holder, a hydraulic constant-speed and constant-pressure pump 9, and an electric heating paper 15. The X-ray transparent triaxial core holder has an internal space for placing a coal sample 11. The upper part of the X-ray transparent triaxial core holder has a slip ring joint, and the lower part is equipped with a rotating base, allowing the X-ray transparent triaxial core holder to rotate around its axis for performing micron-level CT scans of the coal sample 11 at multiple angles. The hydraulic constant-speed and constant-pressure pump 9 is connected to the X-ray transparent triaxial core holder and is used to pump hydraulic oil into the X-ray transparent triaxial core holder to apply confining pressure to the coal sample 11 inside, and to control the axial pressure applied by the upper and lower pressure heads. The electric heating paper 15 is evenly wrapped around the outer wall 12 of the X-ray transparent triaxial core holder for uniformly heating and insulating the coal sample. The X-ray transparent triaxial core holder is made of X-ray transparent materials, such as high-strength carbon fiber, PEEK, and other polymer materials. It also includes an intelligent temperature control system 26, which is connected to the electric heating paper 15 and is used to control the electric heating paper 15 to heat the coal sample 11 evenly, simulating the in-situ temperature environment of deep coal seams.

[0018] The Kr adsorption system includes a Kr gas cylinder 1 and a vacuum pump. The Kr gas cylinder 1 is connected to the air inlet of an air compressor 5 via a pipeline, and is used to deliver Kr gas into the X-ray transparent triaxial core holder through the air compressor 5, so that the coal sample in the X-ray transparent triaxial core holder reaches an adsorption state with a certain Kr content under specific temperature and pressure to simulate the in-situ coal seam occurrence conditions. The vacuum pump is connected to the X-ray transparent triaxial core holder via a pipeline, and is used to extract the gas in the X-ray transparent triaxial core holder. The CT scanning system includes an X-ray source 17, a receiving screen 18, and a computer 19. The X-ray source 17 and the receiving screen 18 are located on both sides of the main body of the X-ray transparent triaxial core holder, so that the X-rays emitted by the X-ray source 17 pass through the coal sample 11 and are received by the receiving screen 18, which is used to scan the internal spatial structure and Kr migration state of the coal sample in the X-ray transparent triaxial core holder in real time. The computer 19 is connected to the X-ray source 17 and the receiving screen 18, and is used to control the frequency of the X-ray emission source and automatically record the X-ray intensity at different projection angles. After calculation and reconstruction, a three-dimensional tomographic scan image is obtained. The gas analysis and processing system includes a multi-range volumetric flow meter, a high-precision gas chromatograph 20, and a gas collection device. One end of the high-precision gas chromatograph 20 is connected to an X-ray transparent triaxial core holder via a pipeline, and the other end is connected to the gas collection device. It is used to monitor and analyze the gas composition discharged from the X-ray transparent triaxial core holder in real time, and the gas is discharged into the gas collection device for collection after monitoring. The multi-range volumetric flow meter is installed on the pipeline between the high-precision gas chromatograph 20 and the X-ray transparent triaxial core holder, and is used to monitor the real-time gas flow rate discharged from the X-ray transparent triaxial core holder.

[0019] A first check valve 2-1 is installed on the pipeline between the Kr gas cylinder 1 and the air compressor 5; a second check valve 2-2 is installed on the pipeline between the N2 gas cylinder 22 and the gas mixer 3; a third check valve 2-3 is installed on the pipeline between the CO2 gas cylinder 23 and the gas mixer 3; a fourth check valve 2-4 is installed on the pipeline between the other displacement component gas cylinder 24 and the gas mixer 3; a fifth check valve 2-5 is installed on the pipeline between the water vapor cylinder 25 and the gas mixer 3; an inlet valve 2-6 is installed on the pipeline between the temperature-controlled quantitative injector 4 and the X-ray transparent triaxial core holder; a sixth check valve 2-7 is installed on the pipeline between the vacuum machine and the X-ray transparent triaxial core holder; and an outlet valve 2-8 is installed on the pipeline between the X-ray transparent triaxial core holder and the high-precision gas chromatograph 20.

[0020] A pressure gauge and a thermometer 7 are installed on the pipeline between the X-ray transparent triaxial core holder and the high-precision gas chromatograph 20 to monitor the real-time pressure and temperature of the gas discharged from the X-ray transparent triaxial core holder. The multi-range volumetric flow meter includes a low-range volumetric flow meter 8-3, a medium-range volumetric flow meter 8-2, and a high-range volumetric flow meter 8-1. The low-range volumetric flow meter 8-3 and the seventh check valve 2-9 are connected in series to form the first branch. The medium-range volumetric flow meter 8-2 and the eighth check valve 2-10 are connected in series to form the second branch. The high-range volumetric flow meter 8-1 and the ninth check valve 2-11 are connected in series to form the third branch. The first, second, and third branches are connected in parallel.

[0021] The working method of the above-mentioned multi-phase fluid displacement coalbed methane 4D visualization simulation experimental system is as follows: A. Preparation and Testing System for Water-Saturated Coal Samples: After collecting coal from deep coal seams, the coal body is cut into several core columns of the same size, and the surfaces are cleaned to form individual coal samples. A borehole is then drilled at the center of one end face of each coal sample 11 for subsequent injection of multi-phase fluids. A natural water absorption rate test is conducted to study the change in coal sample moisture content with immersion time. By controlling the immersion time, the coal sample moisture content is adjusted to simulate the actual in-situ water content of the coal seam. Any water-absorbing coal sample is selected, and a wearable rubber sleeve 14 is attached to its surface. The sample is then placed in an X-ray transparent triaxial core holder, and the assembly of each system is completed. The wearable rubber sleeve 14 is made of high-temperature and corrosion-resistant material and is used to buffer and protect the coal sample.

[0022] B. Simulation of in-situ temperature and pressure parameters of coal seam: Based on the deep geostress and temperature environment to be simulated, set the axial pressure, confining pressure, and temperature values; start the hydraulic constant speed and constant pressure pump of the visible triaxial core holder system to pump hydraulic oil, so that the X-ray transparent triaxial core holder applies axial pressure and confining pressure to the coal sample 11; at the same time, start the intelligent temperature control system 26 to make the electric heating paper 15 uniformly heat the coal sample 11 until the set axial pressure, confining pressure, and temperature values ​​are reached, then stop heating and pressurization, and maintain the current pressure and temperature; then start the CT scanning system to perform a CT scan on the coal sample in the current state, and control the rotation of the X-ray transparent triaxial core holder during the CT scan, so as to complete a micron CT scan of the coal sample 11 to record the original pore structure state of the coal sample before Kr adsorption injection; C. In-situ Coal Seam Gas Content Adsorption Simulation: Kr gas is used to replace CH4 to simulate the initial CH4 content in deep coal seams. Specifically, the vacuum machine is turned on and the valves of each pipeline are opened to evacuate the internal pipelines of the gas-liquid mixing system, Kr adsorption system, visual triaxial core holder system, CT scanning system, and gas analysis and processing system. After evacuation, the outlet valve of the X-ray transparent triaxial core holder is closed, while the inlet valve remains open. The Kr gas pressure and temperature values ​​are set, and based on the known Langmuir correction formula, combined with isothermal adsorption and industrial analysis data, the amount of Kr required for the test coal sample to reach the predetermined gas content under the set temperature and pressure conditions is calculated. Then, the gas is sequentially passed through air compressor 5 and control... A quantitative, constant-pressure, and constant-temperature Kr gas is injected into the X-ray transparent triaxial core holder using a temperature-controlled injector 4. When the set pressure value is reached, the inlet valve is closed, and the coal sample 11 is allowed to adsorb the Kr gas for a certain period of time. During the adsorption process, the internal structure of the coal sample 11 is scanned in real time using a CT in-situ scanning system at regular intervals. Combined with the gas pressure change at the main inlet of the X-ray transparent triaxial core holder, the internal structure of the coal sample and the Kr adsorption state during the Kr adsorption process are dynamically and visually monitored. After the Kr gas reaches adsorption equilibrium, the internal structure of the coal sample 11 is scanned again, thus completing another micron-level CT scan of the coal sample to obtain the internal structure of the coal sample 11 in the state of Kr adsorption completion. D. Simulation of the Kr process for high-temperature multi-phase gas displacement of coal samples: The proportions, temperatures, and injection pressures of various gases in the displacement medium are set. The inlet and outlet valves are kept open. CO2, N2, other displacement components, and water vapor are mixed according to the set proportions using a gas mixer 3 and pressurized to the set injection pressure by an air compressor 5. Then, the mixture is passed to a temperature-controlled quantitative injector 4 and heated to the set temperature, thereby generating the required high-temperature multi-phase fluid. This fluid is then quantitatively injected into an X-ray transparent triaxial core holder for displacement. During the high-temperature multi-phase fluid displacement, the coal sample 11 is scanned in situ using micron-CT at multiple angles in real time to obtain the multi-phase fluid data. During the displacement of Kr in the coal sample, the internal flow field, pore and fracture structure characteristics, and damage patterns were observed. Simultaneously, a high-precision gas chromatograph 20 and a multi-range volumetric flow meter were used to monitor and record the gas composition, concentration, and flow rate in real time. This allowed for real-time acquisition of the high-temperature multi-phase fluid injection volume, Kr gas extraction volume, and interaction between the high-temperature multi-phase fluid and Kr gas during the displacement of Kr in the X-ray transparent triaxial core holder. The displacement process was stopped when the Kr concentration in the discharged gas reached the critical Kr concentration. Finally, a micron-scale CT scan was performed on the internal spatial structure of the coal sample 11 at this point to obtain the internal flow field, pore and fracture structure characteristics, and damage patterns after the displacement was completed. E. Image and Data Analysis: Reconstruct each CT scan image to form a three-dimensional model of the coal sample and its internal pore and fracture structure at each CT scan time. Then, combine each three-dimensional model with its respective scan time point to intuitively characterize the pore and fracture evolution process over time, revealing the spatiotemporal evolution law of the synergistic effect of temperature and chemical reaction on the three-dimensional mineral dissolution and pore and fracture structure inside the coal body under the action of high-temperature multi-element gas. Register, grayscale correct, and normalize the concentration calculation of CT images at different displacement stages to obtain the spatiotemporal evolution law of Kr concentration field evolution inside the coal body, realizing 4D visualization of the Kr migration process inside the coal body during displacement. Subsequently, compare the CT scan results with the monitoring results of outlet gas flow rate and composition to comprehensively analyze the main controlling factors and internal mechanisms affecting the high-temperature multi-element gas displacement effect and gas production efficiency.

[0023] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A 4D visualization simulation experimental system for multi-component, multi-phase fluid displacement of coalbed methane, characterized in that, This includes a gas-liquid mixing system, a Kr adsorption system, a visual triaxial core holder system, a CT scanning system, and a gas analysis and processing system; The gas-liquid mixing system includes a multi-phase fluid mixing system and a temperature- and pressure-controlled quantitative injection system. The multi-phase fluid mixing system includes N2 cylinders, CO2 cylinders, water vapor cylinders, other displacement component gas cylinders, and a gas mixer. The N2 cylinders, CO2 cylinders, water vapor cylinders, and other displacement component gas cylinders are connected to the gas mixer via pipelines. The gas mixer is used to mix the gases from each cylinder to generate a multi-phase fluid. The temperature- and pressure-controlled quantitative injection system includes an air compressor, a temperature-controlled quantitative injector, a pressure gauge, a gas thermometer, and a gas volumetric flow meter. The air compressor's inlet is connected to a gas mixer, and its outlet is connected to a temperature-controlled quantitative injector. This injector pressurizes the multi-phase fluid and delivers it to the temperature-controlled quantitative injector. The temperature-controlled quantitative injector is connected to a visual triaxial core holder system via a pipeline to control the injection flow rate into the system. A pressure gauge, a gas thermometer, and a gas volumetric flow meter are sequentially connected to the pipeline between the temperature-controlled quantitative injector and the visual triaxial core holder system to monitor the temperature, pressure, and flow rate of the multi-phase, multi-medium gas after adjustment by the temperature-controlled quantitative injector. The visible triaxial core holder system includes an X-ray transparent triaxial core holder, a hydraulic constant-speed and constant-pressure pump, and electric heating paper. The X-ray transparent triaxial core holder has an internal space for placing the coal sample. The coal sample is placed within this space. The upper part of the X-ray transparent triaxial core holder has a slip ring joint, and the lower part is equipped with a rotating base, allowing the X-ray transparent triaxial core holder to rotate around its axis for performing micron-level CT scans of the coal sample at multiple angles. The hydraulic constant-speed and constant-pressure pump is connected to the X-ray transparent triaxial core holder and is used to pump hydraulic oil into the X-ray transparent triaxial core holder to apply confining pressure to the coal sample inside and to control the axial pressure applied by the upper and lower pressure heads. The electric heating paper is evenly wrapped around the outer wall of the X-ray transparent triaxial core holder to uniformly heat and maintain the temperature of the coal sample. The Kr adsorption system includes a Kr gas cylinder and a vacuum pump. The Kr gas cylinder is connected to the air inlet of an air compressor via a pipeline, and is used to deliver Kr gas into the X-ray transparent triaxial core holder via the air compressor, so that the coal sample in the X-ray transparent triaxial core holder reaches an adsorption state with a certain Kr content under specific temperature and pressure to simulate the in-situ coal seam occurrence conditions. The vacuum pump is connected to the X-ray transparent triaxial core holder via a pipeline, and is used to extract the gas in the X-ray transparent triaxial core holder. The CT scanning system includes an X-ray source, a receiving screen, and a computer. The X-ray source and the receiving screen are located on both sides of the main body of the X-ray transparent triaxial core holder, so that the X-rays emitted by the X-ray source pass through the coal sample and are received by the receiving screen. This is used to scan the internal spatial structure and Kr migration state of the coal sample inside the X-ray transparent triaxial core holder in real time. The computer is connected to both the X-ray source and the receiving screen. It is used to control the frequency of the X-ray emission source and automatically record the X-ray intensity at different projection angles. After calculation and reconstruction, a three-dimensional tomographic scan image is obtained. The gas analysis and processing system includes a multi-range volumetric flow meter, a high-precision gas chromatograph, and a gas collection device. One end of the high-precision gas chromatograph is connected to an X-ray transparent triaxial core holder via a pipeline, and the other end is connected to the gas collection device. It is used to monitor and analyze the gas composition discharged from the X-ray transparent triaxial core holder in real time, and the gas is discharged into the gas collection device for collection after monitoring. The multi-range volumetric flow meter is installed on the pipeline between the high-precision gas chromatograph and the X-ray transparent triaxial core holder, and is used to monitor the real-time gas flow rate discharged from the X-ray transparent triaxial core holder.

2. The 4D visualization simulation experimental system for coalbed methane displacement by multi-component and multiphase fluids according to claim 1, characterized in that, A first check valve is installed on the pipeline between the Kr gas cylinder and the air compressor; a second check valve is installed on the pipeline between the N2 gas cylinder and the gas mixer; a third check valve is installed on the pipeline between the CO2 gas cylinder and the gas mixer; a fourth check valve is installed on the pipeline between the other displacement component gas cylinders and the gas mixer; a fifth check valve is installed on the pipeline between the water vapor gas cylinder and the gas mixer; an inlet valve is installed on the pipeline between the temperature-controlled quantitative injector and the X-ray transparent triaxial core holder; a sixth check valve is installed on the pipeline between the vacuum machine and the X-ray transparent triaxial core holder; and an outlet valve is installed on the pipeline between the X-ray transparent triaxial core holder and the high-precision gas chromatograph.

3. The 4D visualization simulation experimental system for coalbed methane displacement by multi-component and multiphase fluids according to claim 1, characterized in that, The pipeline between the X-ray transparent triaxial core holder and the high-precision gas chromatograph is equipped with a pressure gauge and a thermometer to monitor the real-time pressure and temperature of the gas discharged from the X-ray transparent triaxial core holder. The multi-range volumetric flow meter includes a low-range volumetric flow meter, a medium-range volumetric flow meter, and a high-range volumetric flow meter. The low-range volumetric flow meter and the seventh check valve are connected in series to form the first branch, the medium-range volumetric flow meter and the eighth check valve are connected in series to form the second branch, and the high-range volumetric flow meter and the ninth check valve are connected in series to form the third branch. The first, second, and third branches are connected in parallel.

4. The 4D visualization simulation experimental system for coalbed methane displacement by multi-component and multiphase fluids according to claim 1, characterized in that, The X-ray transparent triaxial core holder is made of X-ray transparent material.

5. The 4D visualization simulation experimental system for coalbed methane displacement by multi-component multiphase fluids according to claim 1, characterized in that, It also includes an intelligent temperature control system, which is connected to the electric heating paper to control the electric heating paper to heat the coal sample evenly, simulating the in-situ temperature environment of deep coal seams.

6. A method for operating the 4D visualization simulation experimental system for multi-component, multiphase fluid displacement of coalbed methane according to any one of claims 1 to 5, characterized in that, The specific steps are as follows: A. Preparation of Water-Saturated Coal Samples and Construction of the Experimental System: After collecting coal from deep coal seams, the coal body is cut into several core columns of the same size, and the surface of the cores is cleaned to form individual coal samples. Then, a borehole is drilled at the center of one end face of each coal sample for subsequent injection of multi-component and multi-phase fluids. The natural water absorption rate of the coal samples is tested to study the change law of coal sample moisture content with immersion time. By controlling the immersion time, the moisture content of the coal samples is adjusted to simulate the real water content state of the coal seam in situ. Select any water-absorbing coal sample, attach a wearable rubber sleeve to the surface of the coal sample, place it in an X-ray transparent triaxial core holder, and complete the assembly of each system. B. Simulation of in-situ temperature and pressure parameters of coal seam: Set the axial pressure, confining pressure and temperature values ​​according to the deep geostress and temperature environment to be simulated. The hydraulic constant-speed and constant-pressure pump of the visual triaxial core holder system is activated to pump hydraulic oil, enabling the X-ray transparent triaxial core holder to apply axial pressure and confining pressure to the coal sample. At the same time, the intelligent temperature control system is activated to uniformly heat the coal sample with electric heating paper until the set axial pressure, confining pressure, and temperature values ​​are reached. Heating and pressurization are then stopped, and the current pressure and temperature are maintained. Then, the CT scanning system is activated to perform a CT scan on the coal sample in the current state. During the CT scan, the rotation of the X-ray transparent triaxial core holder is controlled to complete a micron-level CT scan of the coal sample, which is used to record the original pore structure state of the coal sample before Kr adsorption and injection. C. In-situ Coal Seam Gas Content Adsorption Simulation: Kr gas is used to replace CH4 to simulate the initial CH4 content in deep coal seams. Specifically, the vacuum pump is turned on and the valves of each pipeline are opened to evacuate the internal pipelines of the gas-liquid mixing system, Kr adsorption system, visual triaxial core holder system, CT scanning system, and gas analysis and processing system. After evacuation, the outlet valve of the X-ray transparent triaxial core holder is closed, while the inlet valve remains open. The Kr gas pressure and temperature are set, and the amount of Kr required for the test coal sample to reach the predetermined gas content under the set temperature and pressure conditions is calculated based on the Langmuir correction formula, combined with isothermal adsorption and industrial analysis data. Then, the gas is sequentially passed through an air compressor. A quantitative, constant-pressure, and constant-temperature Kr gas is injected into the X-ray transparent triaxial core holder using a temperature-controlled quantitative injector. When the set pressure value is reached, the inlet valve is closed, and the coal sample is allowed to adsorb the Kr gas for a certain period of time. During the adsorption process, the internal structure of the coal sample is scanned in real time using a CT in-situ scanning system at regular intervals. Combined with the gas pressure change at the main inlet of the X-ray transparent triaxial core holder, dynamic visualization monitoring of the internal structure and Kr adsorption state of the coal sample during the Kr adsorption process is achieved. After the Kr gas reaches adsorption equilibrium, the internal structure of the coal sample is scanned again to complete another micron-level CT scan of the coal sample and obtain the internal structure of the coal sample in the Kr adsorption complete state. D. Simulation of the Kr process for high-temperature multi-component gas displacement of coal samples: The proportions, temperatures, and injection pressures of various gases in the displacement medium are set. The inlet and outlet valves are kept open. CO2, N2, other displacement components, and water vapor are mixed according to the set proportions using a gas mixer and pressurized to the set injection pressure by an air compressor. Then, the mixture is heated to the set temperature by a temperature-controlled quantitative injector, thereby generating the required high-temperature multi-component multiphase fluid. This fluid is then quantitatively injected into an X-ray transparent triaxial core holder for displacement. During the high-temperature multi-component multiphase fluid displacement, the coal sample is subjected to real-time in-situ micron-CT scans from multiple angles to obtain data on the multi-component multiphase fluid. During the displacement of Kr in the coal sample, the internal flow field, pore and fracture structure characteristics, and damage patterns were observed. Simultaneously, a high-precision gas chromatograph and a multi-range volumetric flow meter were used to monitor and record the gas composition, concentration, and flow rate in real time. This allowed for real-time acquisition of the high-temperature multiphase fluid injection rate, Kr gas extraction rate, and the interaction between the high-temperature multiphase fluid and Kr gas during the displacement of Kr in the X-ray transparent triaxial core holder. The displacement process was stopped when the Kr concentration in the discharged gas reached the critical Kr concentration. Finally, a micron-scale CT scan was performed on the internal spatial structure of the coal sample at this point to obtain the internal flow field, pore and fracture structure characteristics, and damage patterns after the displacement was completed. E. Image and Data Analysis: Reconstruct each CT scan image to form a three-dimensional model of the coal sample and its internal pore and fracture structure at each CT scan. Then, combine each three-dimensional model with its respective scan time point to intuitively characterize the evolution process of pores and fractures over time, revealing the spatiotemporal evolution law of the synergistic effect of temperature and chemical reaction on the three-dimensional mineral dissolution and pore and fracture structure inside the coal body under the action of high-temperature multi-element gas. Register, grayscale correct and normalize the concentration calculation of CT images at different displacement stages, and invert to obtain the spatiotemporal evolution law of Kr concentration field evolution inside the coal body, realizing 4D visualization of the Kr migration process inside the coal body during displacement.

7. The working method of the 4D visualization simulation experimental system for displacing coalbed methane by multi-component and multiphase fluids according to claim 6, characterized in that, The wearable rubber sleeve is made of high-temperature and corrosion-resistant materials and is used to cushion and protect the coal sample.

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