Experimental Device and Method for Evaluating Fault Instability Induced by Carbon Dioxide Geological Sequestration

Abstract

An experimental device for the impact of carbon dioxide geological storage on fault stability, comprising a fault simulation specimen and a true triaxial stress loading system, a liquid carbon dioxide injection system, a temperature loading system, an acoustic emission monitoring system, a gas concentration monitoring system, a stress and deformation monitoring system, a temperature monitoring system, and a fluid pressure monitoring system, which are respectively connected to the fault simulation specimen. The experimental method for evaluating the fault instability induced by carbon dioxide geological storage includes the steps of preparing the fault simulation specimen, building the experimental equipment, loading the experimental conditions and collecting data, calculating the stress, and analyzing the mechanism of fault instability induced by carbon dioxide storage. The present invention relates to the technical field of fault stability evaluation and carbon storage experiments, and can provide data support for improving the effect of carbon dioxide geological storage through simulation experiments.

Description

Technical Field

[0001] The present invention relates to the technical field of fault stability evaluation and carbon sequestration experiments, and particularly to an experimental device and method for evaluating the instability of faults induced by geological carbon dioxide sequestration. Background Art

[0002] In the CCUS technology (Carbon Capture, Utilization and Storage), the captured CO2 can be liquefied, injected and stored in deep underground reservoirs for thousands of years, making geological CO2 sequestration a reliable option for mitigating climate change and contributing to the realization of China's carbon peak and carbon neutrality goals.

[0003] Injecting a large amount of carbon dioxide into the formation will cause a series of geomechanical problems, easily leading to fault activation, fluid leakage along the fault, and even triggering seismic activities. Conducting physical simulation experiments in the laboratory is an important method for studying the stability of faults in carbon sequestration sites.

[0004] The existing physical simulation experiments of carbon dioxide sequestration consider fewer fault factors. For example, the carbon dioxide sequestration experimental device disclosed in the Chinese invention patent "Carbon Dioxide Sequestration Experimental Device, Method and System" (Patent No.: CN 202211202870.8) can simulate the overlying formation pressure, monitor the injection carbon dioxide flow rate and pressure data, and realize the research on the carbon dioxide sequestration potential. The carbon dioxide coal seam sequestration simulation test device disclosed in the Chinese invention patent "Method for Simulating Carbon Dioxide Coal Seam Sequestration under Multi-Field Coupling Conditions" (Patent No.: CN201611123112.1) can apply a triaxial geostress field to the specimen and simulate the geostress changes caused by external disturbances. The invention patent "An Experimental Device and Method for Testing the Shear Slip Characteristics of Faults in Carbon Sequestration Sites" (Patent No.: CN202210451691.1) can simulate the shear slip of faults under the influence of carbon sequestration, study the reduction phenomenon of the friction coefficient of the structural plane and the shear slip characteristics. However, the experimental devices and methods proposed in the above existing technologies cannot simulate the high-temperature environment and fault distribution of deep reservoirs, cannot monitor and analyze the fault mechanical behavior during the carbon dioxide injection process, and it is difficult to accurately evaluate the fault stability during the carbon dioxide injection process. And currently, no geological fault specimen has been designed that can be used to monitor factors such as stress, deformation, acoustic emission rupture signal, gas concentration and fluid pressure during the carbon dioxide injection process.

[0005] Therefore, there is an urgent need to design an experimental device and method for fault instability based on geological carbon dioxide sequestration to study the mechanical behavior of geological faults under the influence of carbon dioxide injection and dynamically monitor the unstable slip of geological faults during the carbon dioxide injection process. Summary of the Invention

[0006] To overcome the deficiencies of the prior art, the present invention discloses an experimental device and method for evaluating the instability of faults induced by geological carbon dioxide sequestration. By simulating the real load environments such as in-situ stresses and temperatures of faults, and monitoring the effects of liquid carbon dioxide injection on fault stability from multiple aspects including stress, deformation, acoustic emission rupture signals, gas concentration, and fluid pressure, it is of great significance for improving the effect of geological carbon dioxide sequestration.

[0007] To achieve the above object, the technical solution of the present invention is as follows:

[0008] An experimental device for the influence of geological carbon dioxide sequestration on fault stability, comprising a fault simulation specimen, and a true triaxial stress loading system, a liquid carbon dioxide injection system, a temperature loading system, an acoustic emission monitoring system, a gas concentration monitoring system, a stress and deformation monitoring system, a temperature monitoring system, and a fluid pressure monitoring system respectively connected to the fault simulation specimen;

[0009] The true triaxial stress loading system is used to apply in-situ stresses in three directions to the fault simulation specimen, the liquid carbon dioxide injection system is used to simulate geological carbon dioxide sequestration, the temperature loading system is used to heat the fault simulation specimen to the real temperature of the simulated formation, the acoustic emission monitoring system is used to monitor the micro-rupture information of the fault simulation specimen; the gas concentration monitoring system is used to collect the concentration of permeating gas on the top surface of the fault simulation specimen during carbon dioxide sequestration; the stress and deformation monitoring system is used to monitor the strains along the fault dip and along the fault strike, and calculate the tangential stress at each point on the fault plane; the temperature monitoring system is used to obtain the temperature at the set position on the fault plane in real time; the fluid pressure monitoring system is used to obtain the fluid pressure of liquid carbon dioxide in the fault simulation specimen in real time.

[0010] Preferably, the fault simulation specimen is cast from cement mortar through a mold, the fault simulation specimen is of a cubic structure, an inclined fault plane is provided on the fault simulation specimen, the fault plane is parallel to the Y-axis direction in the true triaxial stress loading system and divides the fault simulation specimen into an upper fault block specimen and a lower fault block specimen, and an injection hole for connecting the liquid carbon dioxide injection system is provided at the bottom of the upper fault block specimen or the lower fault block specimen.

[0011] Preferably, the true triaxial stress loading system includes a triaxial loading chamber, two X-axis stress loading heads arranged in the triaxial loading chamber along the X-axis direction, two Y-axis stress loading heads arranged in the triaxial loading chamber along the Y-axis direction, one Z-axis stress loading head arranged at the top of the triaxial loading chamber along the Z-axis direction, a loading pressing plate, a hydraulic injection pipeline, and a hydraulic servo loading device. The inner ends of the X-axis stress loading head, Y-axis stress loading head, and Z-axis stress loading head are all connected with a loading pressing plate. A fault simulation specimen is arranged between the five loading pressing plates, and the five loading pressing plates are respectively connected to the four side surfaces and the top surface of the fault simulation specimen. The hydraulic servo loading device is arranged outside the triaxial loading chamber and is respectively connected to the X-axis stress loading head, Y-axis stress loading head, and Z-axis stress loading head through the hydraulic injection pipeline.

[0012] Preferably, the liquid carbon dioxide injection system includes a carbon dioxide gas cylinder, a carbon dioxide liquefaction chamber, a constant-speed and constant-pressure fluid loading device, and a liquid injection pipeline. One end of the carbon dioxide liquefaction chamber is connected to the constant-speed and constant-pressure fluid loading device, and the other end of the carbon dioxide liquefaction chamber is divided into two paths. One path is connected to the carbon dioxide gas cylinder through a booster pump, and the other path enters the injection hole through the liquid injection pipeline to be connected to the fault simulation specimen; a pressure regulating valve is also arranged between the booster pump and the carbon dioxide gas cylinder.

[0013] Preferably, the temperature loading system includes a heating plate and a heating power source. The heating plate is placed on the base of the triaxial loading chamber, the bottom end of the fault simulation specimen is in contact with the top end of the heating plate, and the heating plate is connected to the heating power source through a wire.

[0014] Preferably, the acoustic emission monitoring system includes a plurality of acoustic emission probes, a signal amplifier, and an acoustic emission receiving end. The acoustic emission probes are evenly arranged on the outer surfaces of two opposite horizontal side walls of the fault simulation specimen in the X-axis direction and are closely attached to the outer surface of the fault simulation specimen. One end of the signal amplifier is connected to each acoustic emission probe through a wire, and the other end is connected to the acoustic emission receiving end through a wire.

[0015] Preferably, the gas concentration monitoring system includes a gas collection pipeline, a gas flow meter, and a gas concentration detector. The gas collection pipeline is arranged in the Z-axis stress loading head, the bottom end of the gas collection pipeline is in clearance fit with the top surface of the fault simulation specimen, and the top end of the gas collection pipeline is provided with a gas flow meter and a gas concentration detector.

[0016] Preferably, the stress and deformation monitoring system includes a dynamic strain gauge and multiple groups of strain gauges. Each group of strain gauges is pasted on the surface of the hanging wall specimen or the footwall specimen of the fault where the fault plane is located. Each group of strain gauges includes 3 strain gauges, one of which is arranged along the dip direction of the fault, another is arranged along the strike of the fault, and the third strain gauge is located on the angular bisector of the other two strain gauges, in the form of a 45° right-angle strain rosette. Each strain gauge is respectively connected to the dynamic strain gauge through a wire.

[0017] Preferably, the temperature monitoring system includes a temperature collector and multiple thermocouples. The thermocouples are uniformly buried along the dip direction of the fault plane in the preset grooves on the outer surface of the hanging wall specimen or the footwall specimen at the location where the fault plane is located. Each thermocouple is respectively connected to the temperature collector through a wire.

[0018] Preferably, the fluid pressure monitoring system includes a fluid pressure display and a fluid pressure sensor. The fluid pressure sensor is connected to the liquid injection pipeline in the injection hole and is connected to the fluid pressure display through a wire.

[0019] Preferably, the fault simulation specimen is sleeved with a sealing rubber sleeve to prevent liquid carbon dioxide from leaking into the triaxial loading chamber during injection. A seal is provided at the upper end of the fault plane, and the gas seeping out from the fault plane is collected into the gas collection pipeline through the seal.

[0020] Using the experimental device for the influence of carbon dioxide geological storage on fault stability of the present invention, an experimental method for evaluating the instability of faults induced by carbon dioxide geological storage includes the following steps:

[0021] Step 1, prepare a fault simulation specimen;

[0022] Pour cement mortar into the preset left cavity and right cavity in the mold to cast the hanging wall specimen and the footwall specimen of the fault. The preset fault strike is parallel to the Y-axis stress loading direction of the true triaxial stress loading system, and the preset fault plane dip angle value is θ. Drill an injection hole at the center of the bottom surface of the footwall specimen of the fault.

[0023] Step 2, install the experimental equipment;

[0024] Attach each strain gauge group and thermocouple closely to the preset grooves on the surface of the upper plate specimen or the lower plate specimen of the fault where the fault plane is located. Insert the liquid injection pipe into the fault simulation specimen through the injection hole. Place the heating plate of the temperature loading system on the base of the triaxial loading chamber. Place the prepared fault simulation specimen in a sealed rubber sleeve, lead out the sensor lines, and then fix and install the fault simulation specimen on the heating plate. The loading pressure plate is closely attached to the four horizontal surfaces and the upper surface of the specimen. Place the acoustic emission probes in two of the pressure plates in the X-axis direction and closely attach them to the surface of the fault simulation specimen. Make the X-axis stress loading indenter, Y-axis stress loading indenter, and Z-axis stress loading indenter closely attach to the pressure plate. The bottom end of the gas collection pipe in the Z-axis stress loading indenter is in clearance fit with the top surface of the fault simulation specimen.

[0025] Step 3: Apply triaxial in-situ stress and temperature to the fault simulation specimen.

[0026] Preset the triaxial stress values according to the in-situ stress of the target formation. Start the true triaxial stress loading system and synchronously apply the in-situ stress σ x in the X-axis direction, σ y in the Y-axis direction, and σ z in the Z-axis direction to the fault simulation specimen. Preset the temperature value according to the temperature of the target formation. Turn on the heating power supply of the temperature loading system and heat the fault simulation specimen using the heating plate.

[0027] Step 4: Inject liquid carbon dioxide into the fault simulation specimen.

[0028] Open the carbon dioxide gas cylinder, control the outlet pressure of the carbon dioxide gas cylinder through the pressure regulating valve, turn on the booster pump to increase the pressure of the gaseous carbon dioxide in the pipeline to the condition of liquid carbon dioxide, and store the liquid carbon dioxide in the carbon dioxide liquefaction chamber. After the carbon dioxide liquefaction chamber is filled, close the injection valve, open the connecting valve between the liquefaction chamber, the injection pipe, and the constant speed and constant pressure fluid loading device, and control the injection of liquid carbon dioxide into the fault simulation specimen in a constant pressure or constant flow mode through the constant speed and constant pressure fluid loading device.

[0029] Step 5: Monitor the fault slip and gas leakage of the fault simulation specimen.

[0030] During the injection of liquid carbon dioxide, use the acoustic emission monitoring system to collect acoustic emission data and perform spatial location of the rupture points. Use the stress and deformation monitoring system to monitor the strain along the dip direction of the fault plane and the strain along the strike direction of the fault plane in real time, calculate the tangential stress at each point on the fault plane, use the gas concentration monitoring system to obtain the change in the concentration of the leaked gas on the fault plane in real time, use the fluid pressure monitoring system to monitor the fluid pressure on the fault plane in real time, and use the temperature monitoring system to obtain the temperature measured by each thermocouple in real time.

[0031] Step 6: Calculate the Coulomb stress change on the fault plane.

[0032] Calculate the normal stress according to the triaxial in-situ stress, fault dip direction and fault dip angle of the fault simulation specimen. The calculation formula is as follows:

[0033]

[0034]

[0035] Where: σ n is the normal stress on the fault plane, σ x and σ z are the in-situ stresses in the X-axis direction and the Z-axis direction respectively, θ is the fault dip angle, and τ is the shear stress on the fault plane;

[0036] Then, use the calculated normal stress and shear stress on the fault plane and the fluid pressure monitoring system to monitor the fluid pressure on the fault plane in real time to calculate the change of the Coulomb stress on the fault plane. The calculation formula is as follows:

[0037] CFS = τ + μ·(σ n + P)

[0038] Where: CFS is the Coulomb stress on the fault plane, μ is the friction coefficient of the fault plane, and P is the fluid pressure on the fault plane;

[0039] Step 7, record and analyze the relationship between the change of the Coulomb stress on the fault plane and the strain change monitored by the strain gauge on the fault plane;

[0040] Step 8, stop injecting liquid carbon dioxide and analyze the influence mechanism of carbon dioxide sequestration-induced fault instability;

[0041] Keep the temperature of the fault simulation specimen, the triaxial loading in-situ stress and the liquid carbon dioxide injection parameters unchanged, and use the acoustic emission monitoring system, the stress and deformation monitoring system and the gas concentration monitoring system to monitor the fault simulation specimen. When a rupture signal appears in the fault simulation specimen, observe the stress change difference between the acoustic emission rupture point and other positions on the fault plane. When continuous rupture signals appear in the fault simulation specimen, the Coulomb stress on the fault plane increases significantly and the concentration of the leaked gas increases greatly, the fault simulation specimen is damaged, and the fault instability experiment of the fault simulation specimen under liquid carbon dioxide injection is ended. Turn off the liquid carbon dioxide injection system, the temperature loading system and the true triaxial stress loading system; Based on the acoustic emission signal, the injected fluid pressure, the concentration of the leaked gas, and the change of the Coulomb stress on the fault, conduct an evaluation of carbon dioxide sequestration-induced fault instability.

[0042] The beneficial effects of the experimental device and method for evaluating carbon dioxide geological sequestration-induced fault instability of the present invention are as follows:

[0043] 1. The experimental device for the influence of geological storage of carbon dioxide on fault stability proposed in the present invention adopts a true triaxial stress loading system to simulate the real ground stress state of the geological fault, and adopts a temperature loading system to simulate the formation temperature of the geological fault at different depths, thereby restoring the real environment of the geological fault. Through the acoustic emission monitoring system, gas concentration monitoring system, stress and deformation monitoring system, temperature monitoring system and fluid pressure monitoring system, accurate monitoring of fault slip instability during the injection of liquid carbon dioxide is achieved.

[0044] 2. The present invention proposes an experimental method for evaluating fault instability induced by geological storage of carbon dioxide. Through the geological storage experiment of carbon dioxide on fault simulation samples in a simulated geological environment, dynamic data such as acoustic emission rupture signals, Coulomb stress, leakage gas concentration and fluid pressure on the fault surface are obtained. Combining the location of the acoustic emission rupture point and the change of Coulomb stress at various locations on the fault surface, the relationship between geological storage of carbon dioxide and fault stability is analyzed, providing a reliable means for studying the mechanism of fault instability induced by geological storage of carbon dioxide. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 , Two Schematic diagram of the experimental setup for the effect of geological storage of carbon dioxide on fault stability;

[0046] Figure 2 , schematic diagram of the location distribution of injection holes, fault planes, strain gauges and thermocouples in the fault simulation specimen;

[0047] Figure 3 , the relationship diagram between fault attitude and fault plane stress;

[0048] 1. Fault simulation specimen; 2. Fault surface; 3. Injection hole; 4. Triaxial loading chamber; 5. X-axis stress loading head; 6. Z-axis stress loading head; 7. Loading plate; 8. Hydraulic injection pipeline; 9. Hydraulic servo loading device; 10. Carbon dioxide cylinder; 11. Booster pump; 12. Pressure regulating valve; 13. Air compressor; 14. Carbon dioxide liquefaction chamber; 15. Constant speed and constant pressure fluid loading device; 16. Liquid injection pipeline; 17. Heating plate; 18. Heating power supply; 19. Acoustic emission probe; 20. Signal amplifier; 21. Acoustic emission receiving end; 22. Gas collection pipeline; 23. Gas flow meter; 24. Gas concentration detector; 25. Dynamic strain gauge; 26. Strain gauge group; 27. Temperature acquisition instrument; 28. Thermocouple; 29. Fluid pressure display; 30. Fluid pressure sensor. DETAILED DESCRIPTION

[0049] The following description is only a preferred embodiment of the present invention and is not intended to limit the scope of protection of 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.

[0050] Example 1

[0051] As Figure 1 shown, the experimental device for evaluating the induced fault instability in geological carbon dioxide storage provided by the present invention includes a fault simulation specimen, a true triaxial stress loading system, a liquid carbon dioxide injection system, a temperature loading system, an acoustic emission monitoring system, a gas concentration monitoring system, a stress and deformation monitoring system, a temperature anomaly monitoring system, and a fluid pressure monitoring system.

[0052] As Figure 2 shown, the fault simulation specimen 1 is prepared by casting cement mortar. The dip angle of the fault plane 2 is set to 60°. Cement mortar is injected into two preset cavities of the mold to form the hanging wall specimen and the footwall specimen of the fault respectively. The interface where the hanging wall specimen and the footwall specimen of the fault overlap is the fault plane 2. The strike of the fault plane is parallel to the Y-axis stress loading direction of the true triaxial stress loading system. The overall size of the fault simulation specimen 1 is 300 mm × 300 mm × 300 mm. An injection hole 3 with a diameter of 18 mm and a length of 70 mm is provided at the center of the bottom surface of the footwall specimen, which is used to connect the liquid carbon dioxide injection system.

[0053] The true triaxial stress loading system includes a triaxial loading chamber 4, an X-axis stress loading ram 5, a Y-axis stress loading ram, a Z-axis stress loading ram 6, a loading platen 7, a hydraulic injection pipeline 8, and a hydraulic servo loading device 9. Among them, 4 loading platens 7 are placed at the 4 edges of the triaxial loading chamber 4 and are connected to the X-axis stress loading ram 5 and the Y-axis stress loading ram. Another loading platen 7 is located above the fault simulation specimen 1 and is connected to the Z-axis stress loading ram 6. And the 5 platens 7 are all in close contact with the fault simulation specimen 1. The X-axis stress loading ram 5, the Y-axis stress loading ram, and the Z-axis stress loading ram 6 are respectively connected to the hydraulic servo loading device 9 through the hydraulic injection pipeline 8, which is used to apply triaxial in-situ stresses to the fault simulation specimen;

[0054] The liquid carbon dioxide injection system includes a carbon dioxide gas cylinder 10, a booster pump 11, a pressure regulating valve 12, an air compressor 13, a carbon dioxide liquefaction chamber 14, a constant speed and constant pressure liquid loading device 15, and a liquid injection pipeline 16. One end of the carbon dioxide liquefaction chamber 14 is divided into two paths. One path is connected to the carbon dioxide gas cylinder through the booster pump 11, and the other path passes through the liquid injection pipeline 16 and penetrates into the injection hole 3 to connect to the fault simulation specimen, which is used to inject liquid carbon dioxide into the fault simulation specimen. The other end of the carbon dioxide liquefaction chamber 14 is connected to the constant speed and constant pressure fluid loading device 15, which is used to maintain the pressure of the liquid carbon dioxide. The pressure regulating valve 12 controls the outlet pressure of the carbon dioxide gas cylinder 10, and then the booster pump 11 connected to the air compressor 13 converts the gaseous carbon dioxide into liquid and stores it in the carbon dioxide liquefaction chamber 14;

[0055] The temperature loading system includes a heating plate 17 and a heating power supply 18. The heating plate is placed on the base of the triaxial loading chamber and is connected to the heating power supply;

[0056] The acoustic emission monitoring system includes a plurality of acoustic emission probes 19, a signal amplifier 20 and an acoustic emission receiving end 21. The acoustic emission probes 19 are evenly arranged on two opposite horizontal sides of the fault simulation specimen 1. The acoustic emission probes 19 are fixed in the acoustic emission probe placement holes reserved on the loading pressing plate 7 and are in close contact with the surface of the fault simulation specimen 1. The wires connecting the acoustic emission probes 19 are led out of the triaxial loading chamber 4 through the holes on the loading pressing plate 7 and are connected to the acoustic emission receiving end 21 through the signal amplifier 20;

[0057] The gas concentration monitoring system includes a gas collection pipeline 22, a gas flow meter 23 and a gas concentration detector 24. The gas collection pipeline is arranged in the Z-axis stress loading indenter 6. The bottom end of the gas collection pipeline 22 has a clearance fit with the top surface of the fault simulation specimen 1. The gas flow meter 23 and the gas concentration detector 24 are arranged on the gas collection pipeline. The gas flow meter is used to monitor the gas leakage flow rate on the top surface of the fault simulation specimen during the carbon dioxide injection process, and the gas concentration monitor is used to monitor the gas leakage concentration on the top surface of the fault simulation specimen during the carbon dioxide injection process;

[0058] The stress and deformation monitoring system includes a dynamic strain gauge 25 and multiple groups of strain gauges 26. Each strain gauge is pasted on the fault surface of the hanging wall specimen of the fault. Each strain gauge group 26 includes three strain gauges. One strain gauge is arranged along the fault dip direction, another strain gauge is arranged along the fault strike direction, and the third strain gauge is located on the angular bisector of the other two strain gauges in a 45° right-angle strain rosette form. Each strain gauge is respectively connected to the dynamic strain gauge to obtain the dynamic shear stress value and shear strain value along the fault dip direction;

[0059] The temperature anomaly monitoring system includes a temperature collector 27 and multiple thermocouples 28. The thermocouples 28 are evenly buried in the grooves at the fault surface along the fault dip direction of the fault surface 2. Each thermocouple 28 is respectively connected to the temperature collector 27 to obtain the temperature at the position of each thermocouple in real time;

[0060] The fluid pressure monitoring system includes a fluid pressure display 29 and a fluid pressure sensor 30. The fluid pressure sensor 30 is connected to the liquid injection pipeline 16 and then connected to the fluid pressure display 29.

[0061] The experimental device for evaluating the instability of faults induced by geological sequestration of carbon dioxide proposed by the present invention uses a true triaxial stress loading system to simulate the true in-situ stress state of geological faults, and a temperature loading system to simulate the formation temperature of geological faults at different depths, restoring the real environment of geological faults. Through the acoustic emission monitoring system, gas concentration monitoring system, stress and deformation monitoring system, temperature monitoring system and fluid pressure monitoring system, the fault slip instability during the injection of liquid carbon dioxide is accurately monitored.

[0062] Example 2

[0063] Using the experimental device for evaluating the influence of geological sequestration of carbon dioxide on fault stability of the present invention, the experimental method for evaluating the instability of faults induced by geological sequestration of carbon dioxide includes the following steps:

[0064] Step 1, prepare a fault simulation specimen;

[0065] Pour the cement mortar into the preset left cavity and right cavity in the mold, and cast the hanging wall specimen and the footwall specimen of the fault. Preset the fault strike to be parallel to the Y-axis stress loading direction of the true triaxial stress loading system, and preset the fault plane dip angle value to be θ. Drill an injection hole with a diameter of 18 mm and a length of 70 mm at the center of the bottom surface of the footwall specimen.

[0066] Step 2, install the experimental equipment;

[0067] Attach each strain gauge group and thermocouple closely to the preset grooves on the surface of the hanging wall specimen or the footwall specimen where the fault plane is located. Insert the liquid injection pipeline into the fault simulation specimen through the injection hole, and place the heating plate of the temperature loading system on the base of the triaxial loading chamber. Place the prepared fault simulation specimen in a sealed rubber sleeve, lead out the sensor lines, and then fix and install the fault simulation specimen on the heating plate. The loading pressure plate is closely attached to the 4 horizontal surfaces and the upper surface of the specimen. Place the acoustic emission probe in the 2 pressure plates in the X-axis direction and close it to the surface of the fault simulation specimen, so that the X-axis stress loading indenter, Y-axis stress loading indenter, and Z-axis stress loading indenter are closely attached to the pressure plate. The bottom end of the gas collection pipeline in the Z-axis stress loading indenter is in clearance fit with the top surface of the fault simulation specimen;

[0068] Step 3, apply triaxial in-situ stress and temperature to the fault simulation specimen;

[0069] Preset the triaxial stress values according to the in-situ stress of the target formation, start the true triaxial stress loading system, and synchronously apply the X-axis in-situ stress σ x , Y-axis in-situ stress σ y and Z-axis in-situ stress σ z to the three directions of the fault simulation specimen. Preset the temperature value according to the temperature of the target formation, turn on the heating power supply of the temperature loading system, and use the heating plate to heat the fault simulation specimen;

[0070] Step 4: Inject liquid carbon dioxide into the fault simulation specimen;

[0071] Open the carbon dioxide gas cylinder, control the outlet pressure of the carbon dioxide gas cylinder through the pressure regulating valve, turn on the booster pump to increase the pressure of the gaseous carbon dioxide in the pipeline to the condition of liquid carbon dioxide, and store the liquid carbon dioxide in the carbon dioxide liquefaction chamber. After the carbon dioxide liquefaction chamber is full, close the injection valve, open the connection valve between the liquefaction chamber, the injection pipeline and the constant-speed and constant-pressure fluid loading device, and control the liquid carbon dioxide to be injected into the fault simulation specimen in a constant-pressure or constant-flow mode through the constant-speed and constant-pressure fluid loading device;

[0072] Step 5: Monitor the fault slip and gas leakage of the fault simulation specimen;

[0073] During the injection process of liquid carbon dioxide, use the acoustic emission monitoring system to collect acoustic emission data and perform spatial positioning of the rupture points. Use the stress and deformation monitoring system to monitor the strain along the dip direction of the fault plane and the strain along the strike direction of the fault plane in real time, calculate the shear stress at each point on the fault plane, use the gas concentration monitoring system to obtain the change in the concentration of the leaked gas on the fault plane in real time, use the fluid pressure monitoring system to monitor the fluid pressure on the fault plane in real time, and use the temperature monitoring system to obtain the temperature measured by each thermocouple in real time;

[0074] Step 6: Calculate the change in the Coulomb stress on the fault plane;

[0075] Calculate the normal stress according to the triaxial in-situ stress, fault dip and fault inclination angle of the fault simulation specimen. The calculation formula is as follows:

[0076]

[0077]

[0078] In the formula: σ n is the normal stress on the fault plane, σ x and σ z are the in-situ stress in the X-axis direction and the in-situ stress in the Z-axis direction respectively, θ is the fault inclination angle, and τ is the shear stress on the fault plane;

[0079] Then use the calculated normal stress and shear stress on the fault plane and the fluid pressure on the fault plane monitored by the fluid pressure monitoring system in real time to calculate the change in the Coulomb stress on the fault plane. The calculation formula is as follows:

[0080] CFS = τ + μ·(σ n + P)

[0081] In the formula: CFS is the Coulomb stress on the fault plane, μ is the friction coefficient of the fault plane, and P is the fluid pressure on the fault plane;

[0082] Step 7, record the relationship between the change in the Coulomb stress of the fault plane and the change in the strain monitored by the strain gauges on the fault plane;

[0083] Step 8, stop the injection of liquid carbon dioxide and analyze the influence mechanism of carbon dioxide sequestration-induced fault instability;

[0084] Keep the temperature of the fault simulation specimen, the triaxial loading in-situ stress, and the liquid carbon dioxide injection parameters unchanged, and use the acoustic emission monitoring system, the stress and deformation monitoring system, and the gas concentration monitoring system to monitor the fault simulation specimen. When a rupture signal appears in the fault simulation specimen, observe the difference in stress between the acoustic emission rupture point and other positions on the fault plane. When continuous rupture signals appear in the fault simulation specimen, the Coulomb stress on the fault plane increases significantly, and the concentration of the leaked gas increases substantially, the fault simulation specimen is damaged, and the fault instability experiment of the fault simulation specimen under liquid carbon dioxide injection is ended. Turn off the liquid carbon dioxide injection system, the temperature loading system, and the true triaxial stress loading system; Based on the acoustic emission signals, the injection fluid pressure, the concentration of the leaked gas, and the change in the Coulomb stress of the fault, conduct an evaluation of carbon dioxide sequestration-induced fault instability.

[0085] The present invention proposes an experimental method for evaluating carbon dioxide geological sequestration-induced fault instability. Through the carbon dioxide geological sequestration experiment of the fault simulation specimen in a simulated geological environment, dynamic data such as acoustic emission rupture signals, Coulomb stress, concentration of leaked gas, and fluid pressure on the fault plane are obtained. By combining the position of the acoustic emission rupture point and the change in the Coulomb stress at various positions on the fault plane, the relationship between carbon dioxide geological sequestration and fault stability is analyzed, providing a reliable means for studying the mechanism of carbon dioxide geological sequestration-induced fault instability.

Claims

1. An experimental device for the influence of carbon dioxide geological storage on fault stability, characterized in that: It includes a fault simulation specimen, and a true triaxial stress loading system, a liquid carbon dioxide injection system, a temperature loading system, an acoustic emission monitoring system, a gas concentration monitoring system, a stress and deformation monitoring system, a temperature monitoring system, and a fluid pressure monitoring system that are respectively connected to the fault simulation specimen; The true triaxial stress loading system is used to apply in-situ stresses in three directions to the fault simulation specimen. The liquid carbon dioxide injection system is used to simulate the geological storage of carbon dioxide. The temperature loading system is used to heat the fault simulation specimen to the true temperature of the simulated formation. The acoustic emission monitoring system is used to monitor the microcrack information of the fault simulation specimen. The gas concentration monitoring system is used to collect the concentration of permeating gas on the top surface of the fault simulation specimen during the carbon dioxide storage process. The stress and deformation monitoring system is used to monitor the strains along the fault dip and strike, and calculate the shear stress at each point on the fault plane. The temperature monitoring system is used to obtain the temperature at a set position on the fault plane in real time. The fluid pressure monitoring system is used to obtain the fluid pressure of liquid carbon dioxide in the fault simulation specimen in real time; The fault simulation specimen is cast from cement mortar through a mold. The fault simulation specimen is a cubic structure, and an inclined fault plane is provided on the fault simulation specimen. The fault plane is parallel to the Y-axis direction in the true triaxial stress loading system and divides the fault simulation specimen into an upper fault block specimen and a lower fault block specimen. An injection hole for connecting the liquid carbon dioxide injection system is provided at the bottom end of the upper fault block specimen or the lower fault block specimen; The true triaxial stress loading system includes a triaxial loading chamber, two X-axis stress loading heads arranged in the triaxial loading chamber along the X-axis direction, two Y-axis stress loading heads arranged in the triaxial loading chamber along the Y-axis direction, one Z-axis stress loading head arranged at the top of the triaxial loading chamber along the Z-axis direction, loading pressure plates, hydraulic injection pipelines, and a hydraulic servo loading device. The inner ends of the X-axis stress loading heads, Y-axis stress loading heads, and Z-axis stress loading heads are all connected with loading pressure plates. A fault simulation specimen is arranged between the five loading pressure plates, and the five loading pressure plates are respectively connected to the four side surfaces and the top surface of the fault simulation specimen. The hydraulic servo loading device is arranged outside the triaxial loading chamber and is respectively connected to the X-axis stress loading heads, Y-axis stress loading heads, and Z-axis stress loading heads through the hydraulic injection pipelines.

2. The experimental device for the influence of carbon dioxide geological storage on fault stability according to claim 1, characterized in that: The liquid carbon dioxide injection system includes a carbon dioxide gas cylinder, a carbon dioxide liquefaction chamber, and a liquid injection pipeline. One end of the carbon dioxide liquefaction chamber is connected to a constant-speed and constant-pressure fluid loading device. The other end of the carbon dioxide liquefaction chamber is divided into two paths. One path is connected to the carbon dioxide gas cylinder through a booster pump, and the other path enters the injection hole through the liquid injection pipeline to connect to the fault simulation specimen; A pressure regulating valve is also provided between the booster pump and the carbon dioxide gas cylinder.

3. The experimental device for the influence of carbon dioxide geological storage on fault stability according to claim 2, characterized in that: The temperature loading system includes a heating plate and a heating power supply. The heating plate is placed on the base of the triaxial loading chamber. The bottom end of the fault simulation specimen is in contact with the top end of the heating plate. The heating plate is connected to the heating power supply through a wire.

4. The experimental device for the influence of carbon dioxide geological storage on fault stability according to claim 3, characterized in that: The acoustic emission monitoring system includes multiple acoustic emission probes, a signal amplifier and an acoustic emission receiving end. The acoustic emission probes are evenly arranged on the outer surfaces of two relatively horizontal side walls of the fault simulation sample in the X-axis direction and are closely attached to the outer surface of the fault simulation sample. One end of the signal amplifier is connected to each acoustic emission probe through a wire, and the other end is connected to the acoustic emission receiving end through a wire.

5. The experimental device for the influence of carbon dioxide geological storage on fault stability according to claim 4, characterized in that: The gas concentration monitoring system includes a gas collection pipeline, a gas flow meter and a gas concentration detector. The gas collection pipeline is arranged in the Z-axis stress loading pressure head. The bottom end of the gas collection pipeline is gap-matched with the top surface of the fault simulation sample. The top end of the gas collection pipeline is provided with a gas flow meter and a gas concentration detector.

6. The experimental device for the influence of carbon dioxide geological storage on fault stability according to claim 5, characterized in that: The stress and deformation monitoring system includes a dynamic strain gauge and multiple groups of strain gauge groups. Each strain gauge group is pasted on the surface of the fault hanging plate sample or the fault footwall sample where the fault plane is located. Each strain gauge group includes 3 strain gauges, one of which is set along the fault dip, another is set along the fault strike, and the third is located on the angular bisector of the other two strain gauges in the form of a 45° right-angle strain rosette. Each strain gauge is connected to the dynamic strain gauge through a wire.

7. The experimental device for the influence of carbon dioxide geological storage on fault stability according to claim 6, characterized in that: The temperature monitoring system includes a temperature collector and multiple thermocouples. The thermocouples are evenly buried in preset grooves on the outer surface of the fault upper plate sample or the fault lower plate sample at the location of the fault plane along the inclination of the fault plane. Each thermocouple is connected to the temperature collector through a wire.

8. The experimental device for the influence of carbon dioxide geological storage on fault stability according to claim 7, characterized in that: The fluid pressure monitoring system comprises a fluid pressure display instrument and a fluid pressure sensor, wherein the fluid pressure sensor is connected to the liquid injection pipe in the injection hole and is connected to the fluid pressure display instrument through a wire; The fault simulation sample sleeve is provided with a sealing rubber sleeve to prevent liquid carbon dioxide from leaking into the triaxial loading chamber during injection. The upper end of the fault surface is provided with a sealing member, through which the gas seeping from the fault surface is collected into a gas collection pipeline.

9. Experimental method for evaluating the induced fault instability in geological carbon dioxide sequestration, characterized by The experimental device for the effect of geological storage of carbon dioxide on fault stability as claimed in claim 8 comprises the following steps: Step 1, preparing a fault simulation sample; Pour cement mortar into the preset left cavity and right cavity in the mold to cast the fault upper plate sample and the fault lower plate sample. The fault strike is preset to be parallel to the Y-axis stress loading direction of the true triaxial stress loading system. The fault plane inclination angle is preset to θ. An injection hole is drilled at the center of the bottom surface of the fault lower plate sample. Step 2, installation of experimental equipment; Attach each strain gauge group and thermocouple closely to the preset grooves on the surface of the upper or lower plate specimen of the fault where the fault plane is located. Insert the liquid injection pipe into the fault simulation specimen through the injection hole. Place the heating plate of the temperature loading system on the base of the triaxial loading chamber. Place the prepared fault simulation specimen in a sealed rubber sleeve, lead out the sensor lines, and then fix and install the fault simulation specimen on the heating plate. The loading pressure plate is closely attached to the four horizontal surfaces and the upper surface of the specimen. Place the acoustic emission probes in two of the pressure plates in the X-axis direction and closely attach them to the surface of the fault simulation specimen. Make the X-axis stress loading indenter, Y-axis stress loading indenter, and Z-axis stress loading indenter closely attach to the pressure plate. The bottom end of the gas collection pipe in the Z-axis stress loading indenter has a clearance fit with the top surface of the fault simulation specimen. Step 3: Apply triaxial in-situ stresses and temperature to the fault simulation specimen. Preset the triaxial stress values according to the in-situ stress of the target formation, start the true triaxial stress loading system, and synchronously apply the in-situ stress σ in the X-axis direction to the three directions of the fault simulation specimen x , the in-situ stress σ in the Y-axis direction y and the in-situ stress σ in the Z-axis direction z , preset the temperature value according to the target formation temperature, turn on the heating power supply of the temperature loading system, and heat the fault simulation specimen using the heating plate; Step 4: Inject liquid carbon dioxide into the fault simulation specimen. Open the carbon dioxide gas cylinder, control the outlet pressure of the carbon dioxide gas cylinder through the pressure regulating valve, turn on the booster pump to increase the pressure of the gaseous carbon dioxide in the pipeline to the condition of liquid carbon dioxide, and store the liquid carbon dioxide in the carbon dioxide liquefaction chamber. After the carbon dioxide liquefaction chamber is full, close the injection valve, open the connection valve between the liquefaction chamber, the injection pipe, and the constant-speed and constant-pressure fluid loading device, and control the liquid carbon dioxide to be injected into the fault simulation specimen in a constant-pressure or constant-flow mode through the constant-speed and constant-pressure fluid loading device. Step 5: Monitor the fault slip and gas leakage of the fault simulation specimen. During the injection of liquid carbon dioxide, use the acoustic emission monitoring system to collect acoustic emission data and perform spatial location of the rupture points. Use the stress and deformation monitoring system to monitor the strain along the dip direction of the fault plane and the strain along the strike direction of the fault plane in real time, and calculate the shear stress at each point on the fault plane. Use the gas concentration monitoring system to obtain the change in the concentration of the leaked gas on the fault plane in real time. Use the fluid pressure monitoring system to monitor the fluid pressure on the fault plane in real time. Use the temperature monitoring system to obtain the temperature measured by each thermocouple in real time. Step 6: Calculate the change in the Coulomb stress on the fault plane. Calculate the normal stress according to the triaxial in-situ stresses, fault dip, and fault rake of the fault simulation specimen. The calculation formula is as follows: Where: σ n is the normal stress on the fault plane, σ x and σ z are the in-situ stresses in the X-axis direction and the Z-axis direction respectively, θ is the fault dip angle, and τ is the shear stress on the fault plane; Then use the calculated normal stress and shear stress on the fault plane and the fluid pressure on the fault plane monitored by the fluid pressure monitoring system in real time to calculate the change in the Coulomb stress on the fault plane. The calculation formula is as follows: CFS = τ + μ·(σ n + P) In the formula: CFS is the Coulomb stress on the fault plane, μ is the friction coefficient of the fault plane, and P is the fluid pressure on the fault plane. Step 7: Record and analyze the relationship between the change in the Coulomb stress on the fault plane and the strain change monitored by the strain gauges on the fault plane. Step 8: Stop the injection of liquid carbon dioxide and analyze the influence mechanism of carbon dioxide sequestration-induced fault instability. Keep the temperature of the fault simulation specimen, the triaxial loading in-situ stress, and the liquid carbon dioxide injection parameters unchanged. Use the acoustic emission monitoring system, the stress and deformation monitoring system, and the gas concentration monitoring system to monitor the fault simulation specimen. When a fracture signal appears in the fault simulation specimen, observe the stress change difference between the acoustic emission fracture point location and other locations on the fault plane. When continuous fracture signals appear in the fault simulation specimen, the Coulomb stress on the fault plane increases significantly, and the leakage gas concentration increases substantially, the fault simulation specimen is damaged, and the fault instability experiment of the fault simulation specimen under liquid carbon dioxide injection is ended. Then, turn off the liquid carbon dioxide injection system, the temperature loading system, and the true triaxial stress loading system. Based on the acoustic emission signal, the injection fluid pressure, the leakage gas concentration, and the change of the fault Coulomb stress, carry out the evaluation of carbon dioxide sequestration-induced fault instability.

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