High-temperature and high-pressure multi-cycle experiment system and method for replacing natural gas with non-hydrocarbon gas

Abstract

The invention is applicable to the technical field of oil and gas storage and development, and provides a high-temperature and high-pressure multi-cycle experiment system and method for replacing natural gas with non-hydrocarbon gas, and the experiment system comprises a gas supply unit, a displacement system unit, a pump control unit, an accessory equipment unit, a data acquisition and processing unit and a power supply unit. The experiment system can continuously and dynamically monitor the pressure change and the pressure difference of the two ends of the rock core in the displacement process, and can collect multi-dimensional data such as gas components, concentration and flow at an outlet in real time. The system can simulate a multi-round cyclic injection-production process of replacing methane with non-hydrocarbon gas such as carbon dioxide, and dynamically record each round of injection-production behavior, component change and pressure response. The displacement efficiency and the storage efficiency of the non-hydrocarbon gas in the reservoir are evaluated through conjoint analysis of recovery efficiency evolution, gas mixing and storage characteristics, and an experimental basis and an engineering reference are provided for gas storage operation and enhanced gas reservoir development.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil and gas storage and development, and in particular relates to a high-temperature, high-pressure, multi-cycle experimental system and method for replacing natural gas with non-hydrocarbon gas. Background Art

[0002] In current gas storage peak-shaving operations, the use of an appropriate cushion gas can improve reservoir pressure support and natural gas recovery efficiency. Currently, most gas storage facilities use CH4 itself as a cushion gas, typically accounting for 15%-75% of the total gas volume. This CH4 remains trapped in the reservoir for long periods, making it difficult to utilize. Therefore, an additional non-hydrocarbon cushion gas is urgently needed to displace this CH4 and replace its long-term cushion gas, thereby increasing the effective recovery rate of natural gas.

[0003] CO2, as a cushion gas, is not only used to replenish formation energy during multiple cycles of injection and production, but also enables segmented displacement of residual natural gas and provides geological storage. However, the supercritical physical properties, mixing behavior, and seepage characteristics of CO2 differ significantly from those of CH4. As an alternative cushion gas, N2 also presents issues such as calorific value dilution and difficulty in recovery. Therefore, a repeatable and controllable upper and lower pressure experimental system is needed to simulate the displacement process of methane injection and production at the same outlet and CO2 injection and production at different outlets, revealing the evolution of the gas mixing front, displacement efficiency, and CO2 retention behavior under cyclic injection and production conditions, and providing experimental verification and parameter support for the flexible peak regulation and carbon sequestration coupling of gas storage. Summary of the Invention

[0004] The purpose of the embodiments of the present invention is to provide a high-temperature, high-pressure, multi-cycle experimental system and method for replacing natural gas with non-hydrocarbon gas suitable for injection-production cycle conditions, aiming to solve the problems raised in the above background technology.

[0005] The embodiment of the present invention is implemented as follows: a high-temperature and high-pressure multi-cycle experimental system for replacing natural gas with non-hydrocarbon gas, comprising a gas supply unit, a displacement system unit, a pump control unit, an auxiliary equipment unit, a data acquisition and processing unit, and a power supply unit; The gas supply unit is used to deliver methane and carbon dioxide / nitrogen to the displacement system unit; The displacement system unit includes a methane intermediate buffer container, a bottom gas intermediate buffer container, a water collection tank, a core rubber sleeve, a core sample, a core holder, and a back pressure valve; the core sample is installed in the core holder, the core rubber sleeve is covered on the core sample, the methane intermediate buffer container and the bottom gas intermediate buffer container are both connected to the gas supply unit, and the methane intermediate buffer container and the bottom gas intermediate buffer container are also respectively connected to the methane injection port and the bottom gas injection port at both ends of the core holder; the outlet end of the core holder is equipped with a back pressure valve; The pump control unit includes a hand-cranked confining pressure pump, a displacement pump, and a back-pressure pump. The displacement pump is connected to the liquid inlet at the lower end of the methane intermediate buffer container and the bottom gas intermediate buffer container through a pipeline. The back-pressure pump is connected to the outlet end of the core holder. The hand-cranked confining pressure pump is connected to the core holder. The auxiliary equipment unit includes a vacuum pump, a gas flow meter, a gas chromatograph, and a gas collection bag; the vacuum pump is connected to the pipeline between the outlet end of the core holder and the back pressure valve through a four-way valve; the gas flow meter is connected to the back pressure valve for real-time monitoring and recording of the volume flow of the produced gas; the gas chromatograph is connected to the gas flow meter for analyzing the components of the mixed gas and their changes; the gas collection bag is connected to the gas chromatograph for collecting gas; The data acquisition and processing unit is used to monitor experimental parameters in real time and perform digital storage and remote data management of multiple parameters; The power supply unit is used to continuously provide stable power to the experimental system.

[0006] A further technical solution is that the gas supply unit includes a methane cylinder, a carbon dioxide / nitrogen cylinder and a gas leak detection device, and the methane cylinder and the carbon dioxide / nitrogen cylinder are connected to the methane intermediate buffer container and the bottom gas intermediate buffer container in the displacement system unit through a high-pressure valve and a pressure regulating valve respectively.

[0007] According to a further technical solution, the displacement system unit is installed in a movable constant temperature oven, which is used to provide a stable and controlled temperature environment for the displacement system unit.

[0008] According to a further technical solution, the methane intermediate buffer container and the bottom gas intermediate buffer container are made of 316L stainless steel, with a single volume of 1000 mL and a pressure rating of 70 MPa, and are provided with a gas-liquid isolation piston structure inside; Among them, the piston structure in the bottom gas intermediate buffer container is made of CO2 corrosion-resistant material.

[0009] According to a further technical solution, the core sample is a real reservoir core with a size of φ38×100 mm; The angle adjustment range of the core holder is 0~90°; The core rubber sleeve has a specification of φ39×φ46×300 mm and is made of a material resistant to CO2 corrosion. An aluminum foil layer can be provided between the rubber sleeve and the core to enhance sealing and corrosion resistance.

[0010] According to a further technical solution, the components in the displacement system unit are connected by quick connectors.

[0011] According to a further technical solution, the pump control unit further includes a water tank and an air compressor; The water tank is connected to the displacement pump and the back-pressure pump at the same time to provide liquid supply; The air compressor is also connected to the displacement pump and the back-pressure pump to provide continuous power.

[0012] According to a further technical solution, the data acquisition and processing unit includes a pressure acquisition module, a temperature acquisition module and an intelligent acquisition system; The temperature acquisition module is arranged in the constant temperature oven to monitor the operating temperature of the system; Pressure acquisition modules are provided on the pipeline between the methane gas cylinder and the methane intermediate buffer container, the pipeline between the carbon dioxide / nitrogen gas cylinder and the bottom gas intermediate buffer container, the pipeline between the methane intermediate buffer container and the core holder, the pipeline between the bottom gas intermediate buffer container and the core holder, the pipeline between the hand-cranked confining pressure pump and the core holder, the output pipeline of the displacement pump, and the output pipeline of the back pressure pump; The gas flow meter, temperature acquisition module and each pressure acquisition module are all connected to an intelligent acquisition system, and the intelligent acquisition system is used for continuous data acquisition and digital storage.

[0013] Another object of an embodiment of the present invention is to provide a high-temperature, high-pressure, multi-cycle experimental method for replacing natural gas with a non-hydrocarbon gas, based on the above-mentioned experimental system, comprising the following steps: Step 1: Core sample preparation and pretreatment; Select core samples with the required geological conditions, cut and polish them into standard cylindrical specimens; after completion, clean, dry and cool them in sequence to ensure that there are no impurities remaining in the core pores and simulate real reservoir conditions; Step 2: Determination of core physical parameters; The processed core samples were mass measured, with dry weight, length, and diameter recorded. Gas permeability measurements were also conducted to obtain basic porosity and permeability. Step 3: Experimental system connection and leak detection; Connect each unit to form a closed loop to ensure the tightness of the pipeline. After the initial connection is completed, use N2 gas to purge the system to remove residual air and impurities. Then close the system outlet valve, pressurize it by filling it with N2 and let it stand, observing the pressure changes. If the system pressure remains basically constant within the set time, it is considered to be airtight. Step 4: System vacuuming and core loading; The pre-treated core sample is assembled into a core holder, and the core holder is used to simulate gas displacement behavior under different geological conditions. Depending on the experimental design requirements, a single core or multiple cores with different permeabilities can be selected. After the core sample is loaded, the outlet valve of the intermediate buffer container is closed and the vacuum pump is started to evacuate the system. Step 5: Constant temperature preheating and system stabilization; Start the constant temperature oven, set the target experimental temperature, and preheat the core sample and system pipelines; proceed to the next step after the system temperature stabilizes to the set value; Step 6: Filling and pressurizing the intermediate container with gas; Fill the methane intermediate buffer container and the bottom gas intermediate buffer container with the required gas in sequence, close the outlet valve, and use a constant speed and constant pressure displacement pump to pressurize the gas in the intermediate buffer container until the preset injection pressure is reached; Step 7: Saturation of core sample with CH4; Set the upper and lower pressure limits of the system, and adjust the back-pressure valve and hand-cranked confining pressure pump to maintain stable confining pressure conditions. Open the valves at the outlet of the methane intermediate buffer container and the inlet of the core holder. Use the displacement pump to push the piston and inject CH4 into the core sample until it is fully saturated. After stabilizing the pressure, open the CH4 production outlet valve to discharge CH4 gas. The discharged volume is measured using a gas flow meter to obtain the core sample saturation volume V1. Subsequently, to obtain reference data, inject CH4 into the core again until it is saturated and stabilized to the set upper pressure limit. After closing the injection valve, open the gas production outlet valve and perform simulated gas production, reducing the core pressure from the upper pressure limit to the lower pressure limit. During this process, the volume of produced gas is recorded using a gas flow meter, recorded as V2, to characterize the gas production capacity under pure CH4 conditions and serve as a reference for subsequent CO2 replacement efficiency.

[0014] Step 8: Pad gas replacement injection; Switch back to the CO2 injection path and open the outlet valve of the intermediate buffer container for the bottom gas. Ensure that all other valves in the system are closed. Use a displacement pump to inject CO2 as the bottom gas into the core sample at a constant pressure or constant flow rate, gradually increasing the core pressure from the lower limit pressure to the set upper limit pressure, providing the driving force for the subsequent methane displacement process. The volume of injected water can be indirectly converted to the injected volume of CO2, and then converted to the gas volume V3 under standard conditions using the equation of state. Step 9: Production gas collection and component analysis; A gas chromatograph and a gas collection bag are connected to the core holder outlet to perform online component analysis and sampling of the produced gas, obtaining a curve showing the CH4 / CO2 concentration changing over time. Simultaneously, a gas flow meter at the outlet records the cumulative volume of the produced gas in real time. Combined with the volume fractions of each component measured by the chromatogram, the volume of displaced CH4 (V4) and the volume of concomitantly produced CO2 (V5) are calculated. Step 10: Multiple rounds of injection and production; To simulate the multiple injection-production cycles of a gas storage facility in actual operation, after completing the first round of gas displacement experiments, subsequent cycles were repeated according to the following steps: First, the valve at the core holder outlet was closed, the CH4 injection path was reopened, and CH4 was injected into the core through the methane intermediate buffer container and displacement pump until the reservoir pressure returned to the set upper limit. After a period of quiescence, the outlet valve was opened again to produce CH4. At the same time, the outlet pressure was adjusted to the set lower limit using a backpressure pump, and the volume and flow rate of produced gas were recorded. Step 11: After the completion of multiple rounds of gas production, the CO2 injection path can be reopened based on the gas production volume and gas purity of the previous round, and CO2 can be injected into the core through the bottom gas intermediate buffer container to replace the CH4 that was not used in the previous round of gas production or the residual bottom gas; the injection process continues until the reservoir pressure recovers to the upper limit pressure, and then the next round of gas production continues to the lower limit pressure; based on the CO2 injection volume in each cycle and the component analysis results of the produced gas, the actual CO2 consumption of the round is calculated; before entering the next round of gas injection, the corresponding volume of CO2 is supplemented as needed to ensure the stability of the bottom gas content in the reservoir; at the same time, the CH4 production volume, CO2 injection volume and residual gas content of each round are recorded for comprehensive analysis of the impact of the number of cycles on the natural gas recovery rate, CO2 replacement efficiency and storage efficiency; Step 12: System exhaust and core sample unloading; After all cyclic injection and production experiments are completed, close all gas source valves and gradually release the internal pressure of the system to normal pressure. After the pressure is completely released, remove the core holder, remove the core sample, and perform subsequent characterization as needed to analyze pore structure changes or gas distribution characteristics. Step 13: Data processing and result analysis; The data collected during each injection-production phase of the experiment were collated, and the CH4 production volume, CO2 injection and consumption volume at each phase were recorded. The cumulative displacement efficiency, storage efficiency, and changing trends of the displacement effects of each round were summarized and calculated. The displacement efficiency was calculated using the formula η1 = V4 / V1, and the storage efficiency was calculated using the formula η2 = 1 - V5 / V3.

[0015] The embodiments of the present invention provide a high-temperature, high-pressure, multi-cycle experimental system and method for replacing natural gas with non-hydrocarbon gas, which has the following beneficial effects: (1) It can fully simulate the entire process of enhanced production of various bottom gases under the actual high temperature and high pressure conditions of the gas storage reservoir, and the experimental conditions are closer to the actual reservoir environment; (2) The experimental system has precise pressure, flow and temperature control capabilities, and is combined with an adjustable back-pressure valve to simulate different injection and production pressure windows, enabling real-time monitoring and dynamic adjustment of multiple parameters to ensure experimental accuracy and repeatability; (3) Through systematic research, the displacement characteristics, mixing mechanism, front advancement and retention rules of different gas pads under cyclic injection and production were clarified, providing a detailed and reliable theoretical basis for actual gas storage injection and production plans and CO2 storage strategies; (4) The system is highly modular, with good scalability and multi-cycle capabilities. It supports the comparison of multiple schemes under different types of gas storage (such as structural type, water-soluble gas reservoir type, etc.) and heterogeneous reservoir conditions. It is suitable for exploring the synergistic effects of bottom gas management and carbon sequestration under complex injection and production modes, and has broad prospects for promotion and engineering application. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A schematic structural diagram of a high-temperature, high-pressure, multi-cycle experimental system for replacing natural gas with non-hydrocarbon gas provided by an embodiment of the present invention; Figure 2 A schematic diagram of the internal structure of a constant temperature oven in a high-temperature, high-pressure, multi-cycle experimental system for replacing natural gas with non-hydrocarbon gas provided by an embodiment of the present invention; Figure 3 A flow chart of a high-temperature, high-pressure, multi-cycle experimental method for replacing natural gas with a non-hydrocarbon gas according to an embodiment of the present invention; Figure 4 This is the schematic diagram of experimental working condition control; Figure 5 A comparison of gas production behavior and total gas storage during multiple injection-production cycles at different cushion gas injection pressures (gas production behavior on the left, total gas storage on the right).

[0017] In the accompanying drawings: gas supply unit I; displacement system unit II; pump control unit III; auxiliary equipment unit IV; data acquisition and processing unit V; power supply unit VI; methane cylinder 1; carbon dioxide / nitrogen cylinder 2; high-pressure valve 3; pressure regulating valve 4; gas leak detection device 5; methane intermediate buffer container 6; bottom gas intermediate buffer container 7; water collecting tank 8; core rubber sleeve 9; core sample 10; core clamp 11; three-way valve 12; quick connector 13; four-way valve 14; back pressure valve 15; vacuum pump 16; gas flow meter 17; gas chromatograph 18; gas collection bag 19; hand-cranked confining pressure pump 20; displacement pump 21; back pressure pump 22; water tank 23; air compressor 24; pressure acquisition module 25; temperature acquisition module 26; constant temperature oven 27; intelligent acquisition system 28; master control power module 29. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0019] The specific implementation of the present invention is described in detail below with reference to specific embodiments.

[0020] like Figure 1 and Figure 2 As shown, a high-temperature, high-pressure, multi-cycle experimental system for replacing natural gas with non-hydrocarbon gas according to one embodiment of the present invention includes a gas supply unit I, a displacement system unit II, a pump control unit III, an auxiliary equipment unit IV, a data acquisition and processing unit V, and a power supply unit VI; The gas supply unit I is used to deliver methane and carbon dioxide / nitrogen to the displacement system unit II; The displacement system unit II includes a methane intermediate buffer container 6, a bottom gas intermediate buffer container 7, a water collection tank 8, a core rubber sleeve 9, a core sample 10, a core clamp 11 and a back pressure valve 15; the core sample 10 is installed in the core clamp 11 with an adjustable angle, and the core rubber sleeve 9 is coated on the core sample 10 to ensure the system sealing and chemical stability under high-pressure carbon dioxide displacement conditions; the methane intermediate buffer container 6 and the bottom gas intermediate buffer container 7 are connected to the gas supply unit I to receive methane and carbon dioxide / nitrogen from the gas supply unit I, and the methane intermediate buffer container 6 and the bottom gas intermediate buffer container 7 are also respectively connected to the methane injection port and the bottom gas injection port at both ends of the core clamp 11 to provide a stable gas source for the subsequent injection process; the outlet end of the core clamp 11 is equipped with a back pressure valve 15 with a pressure resistance of 70 MPa to simulate the back pressure of the gas production wellbore and maintain a stable displacement pressure difference; The pump control unit III includes a hand-cranked confining pressure pump 20, a displacement pump 21, and a back-pressure pump 22, which are respectively used to achieve constant pressure / constant rate gas injection, confining pressure control of the core holder 11, and back-pressure regulation at the system outlet; the displacement pump 21 is connected to the liquid inlets at the lower ends of the methane intermediate buffer container 6 and the bottom gas intermediate buffer container 7 through a pipeline, and is used to apply hydraulic pressure thereto to maintain the stability of the injected gas pressure; the back-pressure pump 22 is connected to the outlet end of the core holder 11, and is used to accurately regulate the gas production back pressure during the displacement process; the hand-cranked confining pressure pump 20 is connected to the core holder 11, and is used to apply a confining pressure higher than the injection pressure to the outer layer of the core holder 11 to simulate the formation stress state; The auxiliary equipment unit IV includes a vacuum pump 16, a gas flow meter 17, a gas chromatograph 18 and a gas collection bag 19; the vacuum pump 16 is connected to the pipeline between the outlet end of the core holder 11 and the back pressure valve 15 through a four-way valve 14, and is used for system air extraction at the beginning of the experiment, assisting in the air tightness detection of the system, and ensuring that the device is in a good initial state; the gas flow meter 17 is connected to the back pressure valve 15, and is used to monitor and record the volume flow of the produced gas in real time to ensure the flow accuracy control during the experiment; the gas chromatograph 18 is connected to the gas flow meter 17, and is used to analyze the components of the mixed gas and their changes, and to assist in the evaluation of gas migration and mixing behavior; the gas collection bag 19 is connected to the gas chromatograph 18, and is used to collect natural gas and CO2, for gas component detection, and to calculate the displacement efficiency in combination with the gas volume detected by the gas flow meter 17.

[0021] The data acquisition and processing unit V is used to monitor key experimental parameters (such as pressure, temperature, flow rate, pressure difference) in real time, and to perform digital storage and remote data management of multiple parameters; The power supply unit VI includes a master control power module 29, which is used to continuously provide stable power to the experimental system, support the long-term continuous operation of each unit, and ensure the safe and efficient conduct of the experiment.

[0022] In the embodiment of the present invention, the confining pressure applied by the hand-cranked confining pressure pump 20 to the outer layer of the core holder 11 is generally set to be 3 MPa higher than the pore pressure. The air collection bag 19 is a 100L Teflon (polytetrafluoroethylene) air collection bag.

[0023] like Figure 1 As shown, as a preferred embodiment of the present invention, the gas supply unit I includes a methane gas cylinder 1, a carbon dioxide / nitrogen gas cylinder 2 and a gas leak detection device 5, and the methane gas cylinder 1 and the carbon dioxide / nitrogen gas cylinder 2 are respectively connected to the methane intermediate buffer container 6 and the bottom gas intermediate buffer container 7 in the displacement system unit II through a high-pressure valve 3 and a pressure regulating valve 4.

[0024] In this embodiment of the present invention, a gas leak detection device 5 is used to test the tightness of the gas source system and pipelines before the experiment begins, preventing high-pressure gas leaks from affecting experimental results or equipment safety. After pressure regulation and leak detection, the gas flows to the methane intermediate buffer tank 6 and the bottom gas intermediate buffer tank 7, respectively, providing a stable gas source for the subsequent injection process.

[0025] like Figure 1 and Figure 2As shown, as a preferred embodiment of the present invention, the displacement system unit II is installed in a mobile constant temperature oven 27, which is used to provide a stable and controlled temperature environment (temperature range: room temperature ~ 80 ° C) for the displacement system unit II to ensure that the experiment is carried out under the set thermodynamic conditions.

[0026] As a preferred embodiment of the present invention, the methane intermediate buffer container 6 and the bottom gas intermediate buffer container 7 are made of 316L stainless steel, with a single volume of 1000 mL and a pressure rating of 70 MPa. A gas-liquid isolation piston structure is provided inside. Liquid pressure is injected through a high-precision constant pressure and constant speed displacement pump 21 to control the gas pressure, thereby achieving constant pressure (maximum pressure 70 MPa) or constant speed (0.01~25 mL / min) displacement; the piston structure in the bottom gas intermediate buffer container 7 is made of CO2 corrosion-resistant material.

[0027] As a preferred embodiment of the present invention, the core sample 10 is a real reservoir core with a size of φ38×100 mm; The angle of the core holder 11 can be adjusted in the range of 0 to 90°; The core rubber sleeve 9 has a specification of φ39×φ46×300 mm and is made of CO2 corrosion-resistant materials such as fluororubber. It can be wrapped with aluminum foil to further reduce CO2 penetration, thereby ensuring the system sealing and chemical stability under high-pressure carbon dioxide displacement conditions.

[0028] like Figure 1 As shown in FIG. 1 , as a preferred embodiment of the present invention, the components in the displacement system unit II are connected by quick connectors 12 to facilitate disassembly and modular replacement.

[0029] like Figure 1 As shown, as a preferred embodiment of the present invention, the pump control unit III further includes a water tank 23 and an air compressor 24; The water tank 23 is connected to both the displacement pump 21 and the back-pressure pump 22 to provide liquid supply; The air compressor 24 is also connected to the displacement pump 21 and the back-pressure pump 22 to provide continuous power to ensure that the experiment is carried out continuously and stably.

[0030] like Figure 1 As shown, as a preferred embodiment of the present invention, the data acquisition and processing unit V includes a pressure acquisition module 25, a temperature acquisition module 26 and an intelligent acquisition system 28; The temperature acquisition module 26 is set in the constant temperature oven 27 to monitor the operating temperature of the system and cooperate with the constant temperature oven 27 to perform constant temperature control on the core clamping area to ensure that the experiment is carried out stably under the set thermodynamic conditions; A pressure acquisition module 25 is provided on the pipeline between the methane cylinder 1 and the methane intermediate buffer container 6, the pipeline between the carbon dioxide / nitrogen cylinder 2 and the bottom gas intermediate buffer container 7, the pipeline between the methane intermediate buffer container 6 and the core holder 11, the pipeline between the bottom gas intermediate buffer container 7 and the core holder 11, the pipeline between the hand-cranked confining pressure pump 20 and the core holder 11, the output pipeline of the displacement pump 21, and the output pipeline of the back pressure pump 22; The gas flow meter 17, temperature acquisition module 26 and each pressure acquisition module 25 are all connected to the intelligent acquisition system 28. The intelligent acquisition system 28 is used to continuously acquire and digitally store multiple parameters such as pressure, temperature, flow rate, differential pressure, etc., providing complete and reliable data support for experimental process evaluation and subsequent result analysis.

[0031] like Figure 3 As shown, another embodiment of the present invention provides a high-temperature, high-pressure, multi-cycle experimental method for replacing natural gas with non-hydrocarbon gas, based on the above-mentioned experimental system, including the following steps: Step 1: Core sample preparation and pretreatment; Core samples with the required geological conditions are selected, cut, and polished into standard cylindrical specimens (e.g., approximately φ3.8 cm in diameter and 7.5 cm in length). After completion, pretreatment steps such as cleaning, drying, and cooling are performed to ensure that the core pores are free of impurities and simulate realistic reservoir conditions.

[0032] Step 2: Determination of core physical parameters; The mass of the processed core samples was measured, and geometric parameters such as dry weight, length, and diameter were recorded. Gas permeability was also measured to obtain physical properties such as basic porosity and permeability, which were used as a reference for subsequent experimental calculations.

[0033] Step 3: Experimental system connection and leak detection; Connect each unit to form a closed loop to ensure pipeline tightness. After the initial connection is completed, first purge the system with N2 gas to remove residual air and impurities. Then, close the system outlet valve, pressurize it by filling it with N2, and observe the pressure changes. If the system pressure remains basically constant within the set time, it is considered to be airtight.

[0034] Step 4: System vacuuming and core loading; The pre-treated core sample is assembled into the core holder 11, and the gas displacement behavior under different geological structures (such as faults, dip reservoirs, etc.) is simulated through the core holder 11. According to the experimental design requirements, a single core or different permeabilities (such as Figure 2The core is then assembled into multiple sections (k1, k2, and k3) to enhance the simulation of reservoir heterogeneity. After core sample loading is complete, the outlet valve of the intermediate buffer container is closed and the vacuum pump 16 is activated to evacuate the system. This process involves exhausting the entire system, including the interior of the core holder 11 and the connecting pipelines, to remove residual gas, reduce initial gas interference, and ensure that subsequent CH4 and CO2 injection experiments closely resemble the original pore conditions of the actual formation. The temperature acquisition module 26 and the intelligent data acquisition system 28 simultaneously record the experimental environment, improving experimental controllability and data reliability.

[0035] Step 5: Constant temperature preheating and system stabilization; Start the constant temperature oven 27 and set the target experimental temperature (e.g., 20-80°C) to preheat the core sample and system piping. Once the system temperature stabilizes to the set value, proceed to the next step to ensure the displacement experiment is conducted under constant temperature conditions.

[0036] Step 6: Filling and pressurizing the intermediate container with gas; The required gases are sequentially charged into the methane intermediate buffer container 6 and the bottom gas intermediate buffer container 7 (taking CO2 as an example), the outlet valves are closed, and the gas in the intermediate buffer container is pressurized using a constant speed and constant pressure displacement pump 21 until the preset injection pressure is reached (for experiments that require maintaining CO2 in a supercritical state, ensure that the temperature inside the intermediate container is not lower than 31.1°C and the pressure is not lower than 7.38 MPa).

[0037] Step 7: Saturation of core sample with CH4; Set the upper and lower pressure limits of the system (for example, upper limit 10 MPa, lower limit 7 MPa), such as Figure 4 As shown. Adjust the backpressure valve 15 and hand-cranked confining pressure pump 20 to maintain stable confining pressure conditions (e.g., backpressure 11 MPa, confining pressure 13 MPa). Open the valves at the outlet of the methane intermediate buffer container 6 and the inlet of the core holder 11. Use the displacement pump 21 to push the piston and inject CH4 into the core sample until it is fully saturated. After stabilizing the pressure, open the CH4 production outlet valve to discharge CH4 gas. Measure the discharged volume using the gas flowmeter 17 to obtain the core sample saturation volume V1. If necessary, repeat the vacuum and saturation operations to ensure full saturation of the core sample. Inject CH4 again to saturate the core sample and increase the pressure to the upper pressure limit. Close the valve of the methane intermediate buffer container 6. Then, open the outlet valve and perform a simulated gas production operation from the upper pressure limit to the lower pressure limit. Record the produced volume under pure CH4 conditions, denoted as V2, for reference analysis.

[0038] Step 8: Pad gas replacement injection; Switch to the CO2 injection path and open the outlet valve of the intermediate buffer container 7 for the backing gas. Ensure that the remaining valves in the system are closed. Use the displacement pump 21 to inject CO2 as the backing gas into the core sample at a constant pressure or constant flow rate, gradually increasing the core pressure from the lower limit pressure to the preset upper limit pressure, providing the driving force for the subsequent CH4 replacement process. During the injection process, the actual CO2 injection volume is recorded by the volume of the injection liquid (such as water) injected by the displacement pump and converted to the volume V3 under standard conditions using the state equation. To simulate different backing gas ratios, the upper limit pressure of the CO2 injection can be set as a variable to adjust the pressure difference between it and the initial CH4 saturation pressure. Figure 4 The experimental setup of pad bottom gas displacement under different CO2 injection pressure conditions is shown.

[0039] Step 9: Production gas collection and component analysis; Open the CH4 production valve and begin collecting produced gas. Connect a gas chromatograph 18 and a gas collection bag 19 to the outlet of the core holder 11 to perform online component analysis and sampling of the produced gas, thereby obtaining a curve showing the CH4 / CO2 concentration changes over time. Simultaneously, a gas flowmeter 17 records the cumulative outlet flow in real time and calculates the volume V4 of displaced CH4 and the volume V5 of concomitantly produced CO2, based on the component volume fractions.

[0040] Step 10: Multiple rounds of injection and production; To simulate the multiple injection and production cycles of a gas storage facility in actual operation, after completing the first round of gas displacement experiments, the subsequent cycles were repeated according to the following steps: first, the outlet valve of the core holder 11 was closed, the CH4 injection path was reopened, and CH4 was injected into the core through the methane intermediate buffer container 6 and the displacement pump 21 until the reservoir pressure returned to the set upper limit (for example, 10 MPa); then, after a period of quiescence, the outlet valve was opened again to produce CH4, and the outlet pressure was adjusted to the set lower limit (for example, 7 MPa) by the back pressure pump 22, and the volume and flow rate of the produced gas were recorded. Figure 5 The pressure control and flow rate change process shown.

[0041] Step 11: After the multiple rounds of gas production are completed, the CO2 injection path can be reopened based on the gas production volume and gas purity of the previous round, and CO2 can be injected into the core through the bottom gas intermediate buffer container 7 to replace the CH4 that was not used in the previous round of gas production or the residual bottom gas. The injection process continues until the reservoir pressure returns to the upper limit pressure, and then the next round of gas production continues to the lower limit pressure. Based on the CO2 injection volume and the component analysis results of the produced gas in each cycle, the actual CO2 consumption of that round is calculated. Before entering the next round of gas injection, the corresponding volume of CO2 can be supplemented as needed to ensure the stability of the bottom gas content in the reservoir and achieve continuity and repeatability of the replacement effect. The number of multiple cycles can be flexibly set according to the experimental plan, and the CH4 production volume, CO2 injection volume and residual gas content of each round are recorded at the same time to comprehensively analyze the impact of the number of cycles on natural gas recovery rate, CO2 replacement efficiency and storage efficiency.

[0042] Step 12: System exhaust and core sample unloading; After all cyclic injection and production experiments are completed, close all gas source valves and gradually release the system's internal pressure to atmospheric pressure. Once the pressure is completely released, remove the core holder 11, remove the core sample, and perform subsequent characterization (such as CT scanning or nuclear magnetic resonance imaging) as needed to analyze pore structure changes or gas distribution characteristics.

[0043] Step 13: Data processing and result analysis; Data on pressure, flow rate, and gas composition collected during each injection-production phase of the experiment were collated. Key parameters such as CH4 production volume and CO2 injection and consumption volumes were recorded for each phase. Cumulative displacement and storage efficiencies over multiple cycles were calculated, along with trends in displacement performance across each phase. Displacement efficiency was calculated using the formula η1 = V4 / V1, while storage efficiency was calculated using the formula η2 = 1 - V5 / V2.

[0044] In the embodiment of the present invention, the process of this method has strong adaptability and scalability. The upper and lower limit pressures, the upper limit pressure of the injected gas, the injection flow rate, the fluid physical properties (such as the state of CO2 or N2) and the number of cycles and other key variables can be flexibly adjusted according to the experimental research needs to adapt to the simulation requirements of different reservoir conditions and injection-production strategies, and further enrich the research depth and scope of application of the reservoir injection-production process and the bottom gas displacement behavior.

[0045] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A high-temperature, high-pressure, multi-cycle experimental system for replacing natural gas with non-hydrocarbon gas, characterized in that: It includes a gas supply unit, a displacement system unit, a pump control unit, ancillary equipment unit, a data acquisition and processing unit, and a power supply unit; The gas supply unit is used to deliver methane and carbon dioxide / nitrogen to the displacement system unit; The displacement system unit includes a methane intermediate buffer container, a bottom gas intermediate buffer container, a water collection tank, a core rubber sleeve, a core sample, a core holder, and a back pressure valve; the core sample is installed in the core holder, the core rubber sleeve is covered on the core sample, the methane intermediate buffer container and the bottom gas intermediate buffer container are both connected to the gas supply unit, and the methane intermediate buffer container and the bottom gas intermediate buffer container are also respectively connected to the methane injection port and the bottom gas injection port at both ends of the core holder; the outlet end of the core holder is equipped with a back pressure valve; The pump control unit includes a hand-cranked confining pressure pump, a displacement pump, and a back-pressure pump. The displacement pump is connected to the liquid inlet at the lower end of the methane intermediate buffer container and the bottom gas intermediate buffer container through a pipeline. The back-pressure pump is connected to the outlet end of the core holder. The hand-cranked confining pressure pump is connected to the core holder. The auxiliary equipment unit includes a vacuum pump, a gas flow meter, a gas chromatograph, and a gas collection bag; the vacuum pump is connected to the pipeline between the outlet end of the core holder and the back pressure valve through a three-way valve; the gas flow meter is connected to the back pressure valve for real-time monitoring and recording of the volume flow of the produced gas; the gas chromatograph is connected to the gas flow meter for analyzing the components of the mixed gas and their changes; the gas collection bag is connected to the gas chromatograph for collecting gas; The data acquisition and processing unit is used to monitor experimental parameters in real time and perform digital storage and remote data management of multiple parameters; The power supply unit is used to continuously provide stable power to the experimental system.

2. The high-temperature, high-pressure, multi-cycle experimental system for replacing natural gas with non-hydrocarbon gas according to claim 1 is characterized in that: The gas supply unit includes a methane gas cylinder, a carbon dioxide / nitrogen gas cylinder and a gas leak detection device. The methane gas cylinder and the carbon dioxide / nitrogen gas cylinder are respectively connected to the methane intermediate buffer container and the bottom gas intermediate buffer container in the displacement system unit through a high-pressure valve and a pressure regulating valve.

3. The high-temperature, high-pressure, multi-cycle experimental system for replacing natural gas with non-hydrocarbon gas according to claim 2 is characterized in that: The displacement system unit is installed in a movable constant temperature oven, which is used to provide a stable and controlled temperature environment for the displacement system unit.

4. The high-temperature, high-pressure, multi-cycle experimental system for replacing natural gas with non-hydrocarbon gas according to claim 1 is characterized in that: The methane intermediate buffer container and the bottom gas intermediate buffer container are made of 316L stainless steel, with a single volume of 1000 mL and a pressure rating of 70 MPa, and are equipped with a gas-liquid isolation piston structure; Among them, the piston structure in the bottom gas intermediate buffer container is made of CO2 corrosion-resistant material.

5. The high-temperature, high-pressure, multi-cycle experimental system for replacing natural gas with non-hydrocarbon gas according to claim 1 is characterized in that: The core samples are real reservoir cores with a size of φ38×100 mm; The angle adjustment range of the core holder is 0~90°; The core rubber sleeve has a specification of φ39×φ46×300 mm and is made of a material resistant to CO2 corrosion.

6. The high-temperature, high-pressure, multi-cycle experimental system for replacing natural gas with non-hydrocarbon gas according to claim 1 is characterized in that: The components in the displacement system unit are connected by quick connectors.

7. The high-temperature, high-pressure, multi-cycle experimental system for replacing natural gas with non-hydrocarbon gas according to claim 1 is characterized in that: The pump control unit also includes a water tank and an air compressor; The water tank is connected to the displacement pump and the back-pressure pump at the same time to provide liquid supply; The air compressor is also connected to the displacement pump and the back-pressure pump to provide continuous power.

8. The high-temperature, high-pressure, multi-cycle experimental system for replacing natural gas with non-hydrocarbon gas according to claim 3 is characterized in that: The data acquisition and processing unit includes a pressure acquisition module, a temperature acquisition module and an intelligent acquisition system; The temperature acquisition module is arranged in the constant temperature oven to monitor the operating temperature of the system; Pressure acquisition modules are provided on the pipeline between the methane gas cylinder and the methane intermediate buffer container, the pipeline between the carbon dioxide / nitrogen gas cylinder and the bottom gas intermediate buffer container, the pipeline between the methane intermediate buffer container and the core holder, the pipeline between the bottom gas intermediate buffer container and the core holder, the pipeline between the hand-cranked confining pressure pump and the core holder, the output pipeline of the displacement pump, and the output pipeline of the back pressure pump; The gas flow meter, temperature acquisition module and each pressure acquisition module are all connected to an intelligent acquisition system, and the intelligent acquisition system is used for continuous data acquisition and digital storage.

9. A high-temperature, high-pressure, multi-cycle experimental method for replacing natural gas with non-hydrocarbon gas, based on the high-temperature, high-pressure, multi-cycle experimental system for replacing natural gas with non-hydrocarbon gas according to claim 8, characterized in that: The following steps are involved: Step 1: Core sample preparation and pretreatment; Select core samples with the required geological conditions, cut and polish them into standard cylindrical specimens; after completion, clean, dry and cool them in sequence to ensure that there are no impurities remaining in the core pores and simulate real reservoir conditions; Step 2: Determination of core physical parameters; The processed core samples were mass measured, with dry weight, length, and diameter recorded. Gas permeability measurements were also conducted to obtain basic porosity and permeability. Step 3: Experimental system connection and leak detection; Connect each unit to form a closed loop to ensure the tightness of the pipeline. After the initial connection is completed, use N2 gas to purge the system to remove residual air and impurities. Then close the system outlet valve, pressurize it by filling it with N2 and let it stand, observing the pressure changes. If the system pressure remains basically constant within the set time, it is considered to be airtight. Step 4: System vacuuming and core loading; The pre-treated core samples were assembled into core holders, and the gas displacement behavior under different geological conditions was simulated through the core holders. According to the experimental design requirements, a single core or multiple cores with different permeabilities are selected. After the core sample is loaded, the outlet valve of the intermediate buffer container is closed and the vacuum pump is started to evacuate the system. Step 5: Constant temperature preheating and system stabilization; Start the constant temperature oven, set the target experimental temperature, and preheat the core sample and system pipelines; proceed to the next step after the system temperature stabilizes to the set value; Step 6: Filling and pressurizing the intermediate container with gas; Fill the methane intermediate buffer container and the bottom gas intermediate buffer container with the required gas in sequence, close the outlet valve, and use a constant speed and constant pressure displacement pump to pressurize the gas in the intermediate buffer container until the preset injection pressure is reached; Step 7: Saturation of core sample with CH4; Set the upper and lower pressure limits of the system, adjust the back pressure valve and the hand-cranked confining pressure pump to maintain stable confining pressure conditions; open the valves at the outlet of the methane intermediate buffer container and the inlet of the core holder, and push the piston with the displacement pump to inject CH4 into the core sample until it is completely saturated; after static pressure stabilization, open the CH4 production outlet valve to discharge CH4 gas, and measure the discharged volume with a gas flow meter to obtain the saturated volume V1 of the core sample; then, to obtain reference data, inject CH4 into the core again to saturate it and stabilize it to the set upper pressure limit; after closing the injection valve, open the gas production outlet valve and perform simulated gas production operations to reduce the core pressure from the upper pressure limit to the lower pressure limit; during this process, record the volume of produced gas with the gas flow meter, recorded as V2, which is used to characterize the gas production capacity under pure CH4 conditions and serves as a reference benchmark for subsequent CO2 replacement efficiency; Step 8: Pad gas replacement injection; Switch back to the CO2 injection path and open the outlet valve of the intermediate buffer container for the bottom gas. While ensuring that all other valves in the system are closed, inject CO2 as the bottom gas into the core sample at a constant pressure or constant flow rate through the displacement pump, so that the core pressure gradually increases from the lower limit pressure to the set upper limit pressure, providing driving force conditions for the subsequent methane replacement process. The injected volume of CO2 is indirectly converted from the volume of injected water and converted into the gas volume V3 under standard conditions using the state equation. Step 9: Production gas collection and component analysis; A gas chromatograph and a gas collection bag are connected to the outlet of the core holder to perform online component analysis and sampling of the produced gas, obtaining a curve showing the CH4 / CO2 concentration changing over time. Simultaneously, the cumulative volume of the produced gas is recorded in real time by a gas flow meter at the outlet. Combined with the volume fractions of each component measured by the chromatographic method, the volume of displaced CH4 (V4) and the volume of accompanying CO2 (V5) are calculated. Step 10: Multiple rounds of injection and production; To simulate the multiple injection-production cycles of a gas storage facility in actual operation, after completing the first round of gas displacement experiments, subsequent cycles were repeated according to the following steps: First, the valve at the core holder outlet was closed, the CH4 injection path was reopened, and CH4 was injected into the core through the methane intermediate buffer container and displacement pump until the reservoir pressure returned to the set upper limit. After a period of quiescence, the outlet valve was opened again to produce CH4. At the same time, the outlet pressure was adjusted to the set lower limit using a backpressure pump, and the volume and flow rate of produced gas were recorded. Step 11: After the completion of multiple rounds of gas production, the CO2 injection path is reopened based on the gas production volume and gas purity of the previous round, and CO2 is injected into the core through the bottom gas intermediate buffer container to replace the CH4 that was not used in the previous round of gas production or the residual bottom gas; the injection process continues until the reservoir pressure recovers to the upper limit pressure, and then the next round of gas production is continued to the lower limit pressure; based on the CO2 injection volume in each cycle and the component analysis results of the produced gas, the actual CO2 consumption of the round is calculated; before entering the next round of gas injection, the corresponding volume of CO2 is supplemented as needed to ensure the stability of the bottom gas content in the reservoir; at the same time, the CH4 production volume, CO2 injection volume and residual gas content of each round are recorded for comprehensive analysis of the impact of the number of cycles on the natural gas recovery rate, CO2 replacement efficiency and storage efficiency; Step 12: System exhaust and core sample unloading; After all cyclic injection and production experiments are completed, close all gas source valves and gradually release the internal pressure of the system to normal pressure. After the pressure is completely released, remove the core holder, remove the core sample, and perform subsequent characterization as needed to analyze pore structure changes or gas distribution characteristics. Step 13: Data processing and result analysis; The data collected during each injection-production phase of the experiment were collated, and the CH4 production volume, CO2 injection and consumption volume at each phase were recorded. The cumulative displacement efficiency, storage efficiency, and changing trends of the displacement effects of each round were summarized and calculated. The displacement efficiency was calculated using the formula η1 = V4 / V1, and the storage efficiency was calculated using the formula η2 = 1 - V5 / V3.

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